Review on Premixed Combustion Technology: Stability, Emission

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Review on premixed combustion technology: Stability, emission control, applications and numerical case study Sherif S. Rashwan, Medhat Ahmed Nemitallah, and Mohamed A. Habib Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02386 • Publication Date (Web): 16 Nov 2016 Downloaded from http://pubs.acs.org on November 20, 2016

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Review on premixed combustion technology: Stability, emission control, applications and numerical case study Sherif S. Rashwan1,2, Medhat A. Nemitallah1,3 and Mohamed A. Habib1 1

KACST Technology Innovation Center on CCS and Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia 2 Mechanical Power Department, Faculty of Engineering, Cairo University, Giza, 12613, Egypt 3 Mechanical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt

Abstract Recently, premixed combustion dominated the field of combustion research worldwide. The current work is a state of the art review that addresses the stability, approaches and emission control of premixed flames in different applications. The study addresses the recent developments made to overcome the combustor operability issues, including flame stability and emission control. The influences of oxidizer and fuel flexibility using oxy-fuel combustion and hydrogen enrichment on combustion efficiency and flame stability are investigated. Furthermore, the influences of operating and combustor design conditions on flame characteristics are discussed. Recent developments in swirl-stabilized combustor are analysed and summarized. The effect of premixing on emissions is investigated considering a variety of design and operating conditions in different applications. As per this survey, the application of fully premixed combustion in the industrial area is even so defined; all the same, promising designs are under development. The challenges regarding the application of the premixed combustion technology in the industrial field are discussed. The role of numerical CFD techniques to predict the reacting flow field and heat release in premixed combustion mode are addressed. A numerical case study is presented was addressed the premixed combustion characteristics in a swirl stabilizer gas turbine combustor. Keywords: Flame stability; combustion acoustics; oxy-combustion; lean premixed combustion; emission control; turbulent premixed combustion. Corresponding author: M.A. Nemitallah, E-Mail: [email protected] and [email protected] , Tel: +966138604959.

1. Introduction Power generation industries rely mainly on combined cycle steam power plants and gas turbine engines for energy production from gaseous fuels. Thanks to the new heat recovery technologies which utilize the heat rejected to the atmosphere and converting it into power which, in turns, increase the overall thermal efficiency of combined cycles to up to 60% 1. Hence, the importance of durable and low emissions gas turbine combustion systems is significantly developing. Many combustion technologies have been introduced to industries for the sake of the environmentalfriendly perspectives. For instance, a recent technology of the integrated gasification combined cycle (IGCC) encourages the utilization of hydrogen enrichment for many power generation

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strategies. The IGCC systems are commonly utilizing syngas with different compositions from solid fuels such as coal, waste products or biomass with different gasification processes 2–4. These IGCC systems have lower CO2 emissions from power generation. During the last three decades, lean premixed combustion (LPM) technology became the power generation standard in many applications. This can be ascribed to the fact that it enables lower NOx and CO emissions. Flame instability in premixed combustion is one of the recent major issues, especially in gas turbine applications. Therefore, homogenous mixing of air with methane under extra lean condition with the aid of hydrogen enrichment, as an approach for enhancing flame stability, will result in improvement in flame stability and reduction in NOx emissions. However, operation under hydrogen enrichment brings its own challenges in terms of flashback phenomenon due to its higher flame temperature that causes overheating of the burner components 5–7. Accordingly, combustion operability issues are too significant and compose the focus of the current review paper. Gas turbine engines for power generation have used combustors operated with diffusion flames due to their reasonable performance and higher stability characteristics. Unfortunately, this kind of combustors is no longer preferred due to the associated unacceptable concentrations of NOx. The increase in the restricted environmental regulations encouraged researchers to develop combustors that can meet such restrictions 8,9. Hence, new technologies and concepts have been introduced in the last two decades to the stationary gas turbines for power generation industry such as, LPM and catalytic combustion 10,11. The catalytic combustion is found to be highly expensive and less durable with low safety as well. Whilst in LPM, the air and fuel are introduced upstream to form homogenous mixture. In gas turbines, the combustion chambers are operated under excess air dilution to decrease the flame temperature, then consequently, the thermal NOx emissions are significantly reduced and in some application it could be virtually eliminated. However, the LPM is associated with combustion instabilities as result of the unsteady flow oscillations. This is a common problem while operating the gas turbines under LPM 12. At certain levels, these oscillations could cause operation interfere with extreme oscillations. This can lead to system failure as shown in Figure 1. The figure compares a damaged gas turbine burner due to combustion instabilities to a new burner 13.

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Figure 1: Comparison of two gas turbine burners: damaged burner (left), new assembly burner (right) 13. The instabilities associated with the oscillation of combustion are not only confined for gas turbines engines but also it has been observed for many applications. Such applications include liquid rocket engines 14,15, solid rocket motors 16, ramjet and scramjet engines 17,18 and also boilers and furnaces 19 as well. Combustion instability could be simply defined as the unsteady flow oscillations that can produce high amplitude pressure waves that interfere with the heat release field. There are key parameters affecting the combustion instabilities such as the combustor geometry and the method at which the fuel and air are introduced. Culick and Yang 20,21 discussed the main reason beyond combustion instabilities and they concluded that these instabilities could be mainly attributed to two reasons; The first reason is that combustion chambers are almost confined and the chamber walls are tending to sustain those flow oscillations for a certain time. Moreover, the second reason is that the heat required to drive these oscillations could be extracted from the heat released by the combustion as it is representing a very small fraction as compared to the heat released from combustion. Such issues are not only associated with the LPM flames, but also, with the diffusion flames as well 22. In contrast, combustion instability in LPM combustor have several characteristics that can make them more tending to unsteady oscillations of the flow. Firstly, the LPM combustion systems are operating very close to the lean blow-off limits. At these limits, a tiny change in equivalence ratio could cause a significant change in the heat release rate and, consequently, the combustor could be more tending to oscillation if these waves combined with the acoustic wave 23,24. Secondly, the lean premixed combustors are less likely to have a cooling dilution in a huge manner as compared to the diffusion combustor. In diffusion combustors, thin film of cold air is supplied to the combustor-cooling jacket, which acts as an acoustic suppression method to suppress the combustion oscillations 25,26. Thirdly, in premixed combustors, the flame is too short as compared to the acoustic wavelength which facilitates the interactions between the combustion heat release rate and flow waves 27. These issues are going to be illustrated later in the next upcoming sections of the present review. In the last two decades, many research studies and worldwide efforts have been made trying to understand the stability characteristics of lean premixed and low emissions in gas turbine engines. One of the main purposes of the current review is to understand the instabilities associated with the premixed combustion and approaches to overcome these instabilities. Recent technologies in experimental investigations, numerical modeling and simulations will be discussed. Recent approaches to overcome the combustor operability issues and compile the known results and their contribution in reducing the addressed issues are discussed. The present study aims at providing the readers/researchers with a further understanding of the considerations that are significantly affecting the combustor operability. The present review is organized in the following manner. Section 2 provides an intensive review about combustor operability issues associated with the premixed combustion, including the static and dynamic instabilities. Section 3 discuses in details the different approaches for overcoming all kinds of combustion instabilities, including static and dynamic instabilities, to improve the combustion process and reduce the exhaust emissions. The review in section 3 discusses the issue of combustion stability

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in general and more focus is made on premixed combustion. Different approaches are considered in this section including fuel and oxidizer flexibility approach (including oxy-combustion and hydrogen enrichment technologies); variable operating conditions approach and variable flame type approach (including diffusion and premixed flames). We are concerned mainly in this review with the application of premixed combustion technology in swirl stabilized combustors, mainly for gas turbine combustion applications. Based on that, section 4 discusses the application of premixed combustion technology in swirl stabilized combustors including gas turbine combustion applications. The effect of swirl on flame stability and emissions is investigated as a one of the well-known approaches to overcome the combustion instabilities and operability issues associated with the application of premixed combustion technology in gas turbine combustors. Toward better understanding of the characteristics of premixed combustion flames in gas turbine combustor, section 5 presents a numerical case study on turbulent premixed combustion in a gas turbine model combustor. Temperature, species concentrations and flow field are investigated over a range of operating conditions. Section 6 represents the concluding remarks out of the present review.

2. Combustor Operability Issues Stable combustion process could surprisingly become insecure due to underestimated changes in the operating conditions. Therefore, in this section, focus is made on the operability issues taking into consideration the dynamic instabilities and flashback. The combustion stabilization is affected by many critical factors namely, burner design, geometry, material, fuel composition, turbulence level and even the manner at which the air and fuel are introduced into the combustor. Many studies were conducted to investigate the most intelligent way to tackle the combustion instabilities using methane as a fuel. As per the open literature, we are going to review the stability and emission issues associated with the premixed combustion. Combustion instability issues are separated into two sections, static and dynamic instabilities 28,29. Static instabilities are related to blow-off and flashback phenomena, while the dynamic instabilities are characterized mainly due to thermo-acoustics and we will illustrate both of them in the next following subsections. 2.1 Static instabilities Static instabilities are separated into two categories, blow-off and flashback. In this subsection, a comprehensive literature is presented for both categories. 2.1.1 Blow-off Blow-off occurs when the flame becomes detached from the burner and propagates at a distance from the burner and is physically blown-off. It is well known that blow-off occurs when the flow velocity of the combustible mixture is higher than the flame burning velocity. In other words, blow-off occurs when the time required for chemical reaction is higher than the combustion residence time. As a result, this becomes an issue because the rates of chemical kinetics/reactions and burning flame speed are varying not only with the fuel chemical composition, but also with the fuel physical transport properties. Recently, several methods having different theories for

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developing the blow-off correlations have been studied extensively 30–33. As a matter of fact, all of these correlations for modeling the blow-off mechanisms are based on the concept that the flame is stabilized in the recirculation zone, and the condition for blow-off is defined as function of the chemical timescale defined as follows 34,35:

=

(1)

Where SL and α are the laminar flame speed and the thermal diffusivity respectively 36,37. Furthermore, many researchers suggested the use of the Damkohler number when defining the condition of blow-off. In one approach, the Damkohler number is refereeing to the position of the flame by determining the balance between the flow of combustible mixture and the burning velocity. This reflects the idea of that any unbalance between the flow scale and chemical reaction time scale will lead to change in the flame position. Using the expression of the chemical time scale of equation (1) and putting it together with the residence time (which is generally defined as d/Uref, where d and Uref refer to the characteristic length scale and velocity scale respectively) in the definition of Damkohler number one gets the following expression 38– 41 :

=

.

=

(2)

In another approach, the Damkohler number is governed by the ratio between the combustible mixture flow speeds to the flame burning speed. This is based on the well-known concept of blow-off, which states that the flame tends to blow-off when the flame burning velocity is lower than the combustible mixture flow velocity. In other words, if the flow rate of the combustible mixture is too high as compared to the chemical reaction rate, the flame is going to propagate downstream and the residence time becomes too short for chemical reaction to take place. This situation is well knowingly by blow-off. Otherwise, if the flow rate is slower than the chemical reaction rate, the flame is going to travel upstream, this situation is well knowingly by flashback. This subsection presents a compilation of previous blow-off experimental and numerical data by calculating the chemical times. Lieuwen et al. 42 studied the effect of fuel composition by investigating different fuel blends on each of the following; (1) combustor blow-off, (2) flashback and (3) dynamic stability. Damkohler number has been found to obtain the blow-off characteristics for different fuel compositions. In this study, the fuel composition was CO/H2/CH4. The data obtained depend on changing the fuel composition by changing the percentage of H2 introduced in a range from 0% to 60 % as shown in Figure 2. For clear plotting, each gas in the mixture of H2/CO/CH4 is presented in different color. The dark blue color represents a higher percentage of CH4, the orange color represents a higher percentage of hydrogen, and the yellow color represents a higher percentage of CO in the syngas blends. The results showed that the blow-off occurs at almost constant Damkohler number of 0.6 as shown in Figure 2. For illustration, the percentage of error between the calculated equivalence ratio at which blow-off occurs is less than 3% as compared to the measured values. However, the results also revealed that a flame with higher hydrogen concentration blows-off at leaner equivalences ratios as shown in Figure 3. For the sake of comparison, at hydrogen concentration of 0%, blow-

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off occurs at 0.35 equivalences ratio, while at hydrogen concentration of 60% the average blowoff occurs at 0.25 equivalence ratio, achieving a reduction in lean blow-off limits by approximately 28.5%.

Figure 2: Damkohler number versus H2% at Figure 3: Lean blow-off in terms of combustor pressure and temperature of 1.7 atm equivalence ratio versus H2% at combustor and 300 K, respectively 42 pressure and temperature of 4.4 atm, 460 K, respectively 42. In a further experiment, Shanbhogue et al. 43 reviewed the lean blow-off mechanisms of bluffbody stabilized flames, described the phenomenon of blow-off, and made a very useful comparison of the computed chemical time scales over range of equivalence ratios, from 0.6 to 1.0, utilizing propane-air flames. Zhang et al. 44 studied the lean blow-off limits for different H2/CO/CH4 fuel mixtures as an extension of the experiments conducted by Lieuwen et al 42. They have compared the blow-off residence time for well-stirred reactor to the chemical time that is calculated from equation (1) for different H2/CO/CH4 mixtures. The two time scales are related with a curve fitting line indicated by a solid line as shown in Figure 4. In addition, Figure 5 compares the anticipated and actual blow-off equivalence ratios for all low temperature data. It can be seen that the error in the predicted blow-off equivalence ratio is less than 5%. They have reported that the highest error percent was associated with the highest CO mixture.

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Figure 4: Relation between chemical time Figure 5: Comparison of the predicted and calculated using equation (1) and blow-off measured blow-off equivalence ratio at T=300K, P=1.7 atm and circle: U =59 m/s, residence time 44. square: U = 39 m/s 44. Hudak and Alberto 45,46 studied the blow-off measurements and modeling of methane oxycombustion for low CO cycles. They calculated the chemical time scale depending on the adiabatic flame temperature. It can be seen from Figure 6 that the air-fuel flames have the lowest chemical time (the fastest reaction), the oxy-fuel (CO2/O2) flames have the higher chemical time (the slowest reaction) while the oxygen-enriched air (N2/O2) flames are moderate. These results clearly demonstrate the adverse effect of CO2 dilution in the combustion process. For a further explanation to the blow-off condition, Figure 7 clearly illustrates that higher flame temperature is required for the CH4/O2/CO2 mixture to yield the same chemical time as for the CH4/air mixture. A summary of existing studies on blow-off static instability is presented in Table 1.

Figure 6: Time scale with the adiabatic flame Figure 7: Time scale with the adiabatic flame temperature for different substrates 45,46. temperature for air and oxy-fuel flames 45,46.

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Table 1: Summary of existing studies on blow-off static instability. Combustible mixture Application References Remarks/main findings Experimental/Numerical Syngas fuel Experimental lean premixed Lieuwen et -The effect of fuel CO/H2/CH4 blends gas turbine model al. [42] composition/mixtures on blow-out, flashback, dynamic stability. Propane/Air Experimental Lean blow-off Shanbhogue -Comparison of of bluff body stabilizer et al. 43 computed chemical time scales Syngas fuel Experimental lean premixed Zhang et al. -Blow-off occurs at CO/H2/CH4 blends gas turbine model [44] almost constant Damkohler number of 0.6 for hydrogen enriched from 0% to 60%. -Blow-off limit reduction was about 28.5% Experimental and Numerical Hudak et al. -Higher CH4/Air flame investigation on the effect of 45 temperature is required & CH4/O2/CO2 CO2 dilution in the chemical Alberto et for the CH4/O2/CO2 time scale as compared to al 46. mixture to yield the same air-fuel. chemical time as for the CH4/air mixture. 2.1.2 Flashback Flashback is literally the opposite phenomena of blow-off. Flashback occurs when the flame propagates upstream of the combustor because the flow velocity is lower than the burning or flame velocity, which makes the flame, propagates backward. As similar to blow-off, flashback becomes an issue because the flame speed is significantly affected by the fuel and oxidizer compositions. In another words, the minimum fuel concentration in a homogenously mixed stream of air and fuel at which a flame can propagate upstream from an ignition source against the flow direction. Studying flashback limits is very critical because it gives an indication about the safely of operation in any combustion system. Recently, there is an increasing interest on the LPM with swirl stabilizer because of its advantage in reduction of NOx emissions. This is combined with the hydrogen enrichment technique, which offers a further reduction in greenhouse gases and wider flammability limits. When hydrogen blends with methane, it can produce several forms of alternative fuels that can be used to reduce pollutant emissions. These considerations obviously arise because of its ability to reduce the CO2 emissions 47–50. However, this brings its own challenges for gas turbine operation because flashback concern arises while operation at high hydrogen concentration. This may be attributed to the improvement in the turbulent flame speed with hydrogen-fuel mixtures. One approach to shift or delay flashback is to use a swirl stabilizer with a high swirl number. This is a well-known technology, which produces a very good stability because it can generate a central recirculation zone, and outer recirculation zone which recycles the hot gases containing active

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reactants to the flame core. In turn, this can maintain better stability and wider flammability limits. The flashback phenomena could be caused by: I. Low flow velocities of the combustible mixture allow flame propagation in the boundary layer; consequently, this allows upstream flame propagation 51. II. Turbulent flame propagation at the center of the flow; flashback can occur when the flame core/center velocity becomes higher than flow velocity. This allows for upstream flame propagation. Flashback is commonly associated with oxygen or hydrogen enrichment because of their fast chemical reactions 52,53. III. Combustion instabilities due to the coupling between the pressure oscillations and heat release rate which cause unstable combustion and consequently, flashback occurs 54,55. IV. Combustion induced vortex breakdown (CIVB); due to variation of swirl number or due to operation with different fuel types, different heat release rates are generated, which can cause the CRZ to expand into a rose-tulip structure and, consequently, enhancing the occurrence of flashback 56,57. Flashback is a critical issue when operating with hydrogen enrichment under LPM. There are significant changes in the flame shape and dynamics associated with hydrogen-enrichment of more than 60%, and it becomes more difficult to accommodate/sustain the flame. These issues have encouraged the researchers to study the effect of fuel blends on flashback occurrence in swirl-stabilized combustors. Dam et al. 58 shows in Figure 8 the effect of fuel composition on the flashback limits at a swirl number of 0.97. Three different fuel compositions are considered, namely, 10%H2/90CO, 15%H2/85%CO and 20%H2/80%CO and the results are compared with the air-methane combustion. Actually, hydrogen addition increases the rates of chemical reactions and chemical kinetics and in turns increases the flame propagation speed making the flame more prone to flashback. The results revealed that at certain air mass flow rate, fuel composition with higher hydrogen fraction tends to flashback faster.

Figure 8: Flashback limits for different fuel- Figure 9: Flashback limits for different syngas blends at swirl number of 0.97 58. compositions at swirl number of 0.97 58.

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Chomiak et al. 59 conducted another study considering different coal compositions as shown in Figure 9. They have studied different compositions of syngas, namely, brown coal, bituminous, lignite and coke as listed in 59. These syngas compositions consist of certain amounts of CO2 to retard flashback occurrence. This can be attributed to the fact that CO2 slow down the chemical kinetics and in turns decrease the flame propagation upstream causing a delay in the flashback. Shelil et al. 60 studied the flashback phenomenon, which is a common issue while applying the LPM in gas turbines. CFD simulations of flashback were performed for air-methane flames stabilized over a swirler under premixed conditions. The flashback limits are plotted in Figure 10 characterized by two different regions. The first region is the area above the curve at which the flame is stable, while the area under the curve represents the second region at which the flame is unstable and flashback occurs. The study concluded that the most favorable operation to prevent flashback is at the lean mixture and the worst condition is at stoichiometry. They also studied the effect of increasing the combustor pressure on flashback, and the results are quantified in Figure 11. It was found that the total mass flow rate required to avoid flashback increases as the pressure increases.

Figure 10: Flashback determination for air- Figure 11: Effects of pressure variations on methane combustion at atmospheric pressure flashback limits at different equivalence for different equivalence ratios60. ratios 60. Abdulsada et al. 61 studied the flashback limits for different fuel types in a swirl stabilized combustor. They studied different fuel compositions started with pure methane 100% CH4 and 0% H2 up to 0% CH4 and 100% H2, in addition to the coke-oven gas (65% H2, 25% CH4, 6% CO and 4% N2). The study revealed that at swirl number of 1.47, the CRZ is extended for all fuels. They concluded that the flashback occurs when the radial velocity drops to a certain level causing propagation of the flame front upstream. In closing the discussion of static instabilities and from the above observations based on experimental data, we concluded that the higher the hydrogen enrichment level, the lower the blow-off equivalence ratio. Furthermore, the hydrogen has higher flashback tendency. Both of these findings result in narrow static stability limits in terms of the upper and lower flammability limits 62,63. Lots of studies confirmed this statement for gas turbine applications 64,65. A summary of existing studies on flashback static instability is presented in Table 2.

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Table 2: Summary of existing studies on flashback static instability. Combustion Application Reference Remarks/main technique/ Approach Study findings Experimental/Numerical The effect of fuel Dam et al. 58 -Hydrogen addition composition on the increases the rates of flashback limits. chemical reactions and chemical kinetics and in turns increases the flame propagation speed making the flame more prone to flashback Conducted a study Chomiak et al. 59 -These syngas considering different compositions consist coal compositions. of certain amounts of Flash back to retard CO2 flashback occurrence. 60 Numerical CFD Shelil et al. -The most favorable Simulations of operation to prevent flashback flashback is at the lean mixture and the worst condition is at stoichiometry 61 The flashback limits for Abdulsada et al. -At swirl number of different fuel types in a 1.47, the CRZ is swirl stabilized extended for all fuels combustor.

2.2 Dynamic instabilities In this section, the main causes of the dynamics instabilities are discussed as well as methods for suppressing the dynamic instabilities are surveyed. 2.2.1 Thermo-acoustics Dynamic instabilities refer to thermos-acoustics and identified by the state when the heat release rate combines with the acoustic pressure fluctuations leading to high sound pressure waves. These coupled oscillations could wear or damage the combustor components and, in a further extreme case, small pieces of the combustor could path through the flue gases downstream that may damage the downstream gas turbine components [68–70]. Practical combustion devices are always associated with flow oscillations, even under stable operating conditions. A Stable combustion operation is obtained when the amplitude of pressure fluctuation is approximately less than 5 % of the mean combustor pressure 27. Combustion process at high amplitude of

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pressure-oscillations is considered as unstable combustion and considered as combustion dynamic instability 69–71. These dynamic issues have been raised because of operating the power generation engines at the lean premixed conditions, as this is a very promising technique to satisfy low NOx emissions 72,73 . To accommodate the premixed technology in the combustor, the diluted air that is used downstream in the diffusion mode is dispensed because the air is mixed with the fuel upstream. This diluted air was acting as damper for the pressure oscillation that reduces pressure fluctuations. Nevertheless, a minor change in the air/fuel flow rates will change the equivalence ratio and will produce a significant heat release rate. If these oscillations are combined together, that will lead to oscillating combustion, and a dynamic instability is then established. Recently, Speth et al. 74 investigated the dynamic instabilities for different fuel compositions in terms of overall sound pressure levels in dB. As shown in Figure 12, the author investigated the sound pressure levels while varying the fuel composition (CO and H2). It was found that the sound pressure level recorded is the same for all mixtures studied, but the only difference is that the curves are shifted to lower equivalence ratios when operating at leaner conditions when hydrogen percentage increases.

Figure 12: Effect of fuel composition on the overall sound pressure levels in terms of equivalence ratio (left) and consumption speed (right) 74. There are several factors influencing the relation between pressure oscillations and the heat release rate. One of these factors is the chemical reaction time scale, which plays an important role in demonstrating the effect of changing fuel type or composition on the dynamics and it is well demonstrated by Kushari et al. 75 and Selim 76. Associated authors reported that this is true for a low frequency of 50 Hz and an acoustic period of 20 ms. For a high frequency of 500 Hz, the acoustic period is 2 ms, which illustrates that any change of fuel type will combine the pressure oscillation and heat release rate together unpredictable and may generate or dump the combined oscillations. Also, it can change the flame shape, dynamics and location. Therefore, many researchers have been encouraged to study the coupling between those parameters 77–80.

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2.2.2 Dynamic Instability suppression methods It is mandatory to damp the pressure oscillations in combustion engines because in the worst case, sufficient amplitude of these oscillations may lead to damage of the combustor walls. To avoid damaging the combustion system, the pressure oscillations should be damped by a means of active instability controllers that will act to interrupt these sorts of coupling between the acoustic waves and unsteady heat release. One approach to dump these oscillations is to add the fuel to the pilot flame but this technique brings its own drawbacks in terms of NOx penalty. Methods for damping combustion oscillations are investigated in recent studies by active 81,82 and passive 83 control as well. Passive control involves changes of the following; fuel, reactants compositions, fuel injection devices, combustion chamber design, as well as the installation of acoustics dampers. The importance of all of these suppression methods of passive control is to reduce the rate of heat released from the combustion or to increase the energy losses. In contrast, the active control method involves control of combustion systems by injecting a source of energy to the system. This technique is working as follows; first, sensing the combustion instabilities amplitude, then using a feedback loop to modify the input parameters to interrupt the coupling between unsteady heat release and the acoustic waves. Both active and passive control techniques have been successfully implemented in combustion instability control systems 84,85. Richards and Janus 86 presented mapping for the dynamic regimes to confirm that a given engine cab be well operated while changing the fuel type. Another approach is the catalytic combustion, which has shown some preferences for stable combustion. The catalytic combustion provides an acoustic damping to reduce the changes in heat release rate; however, catalyst applications must have a stable pre-burner, which counteract its advantage. Currently, the passive control methods are the dominant acoustics control of the combustion systems. This may be attributed to the complex control methods associated with the active control. One of the most effective passive control methods is the perforated liner. Lei et al. 87 studied the passive control method to control combustion instabilities with a perforated liner. They examined both the pressure sound and frequency for different cases including rigid walls and perforated wall with different bias volume flow rate. It can be seen from Figure 13 that the pressure levels and frequencies significantly decreased. They also reported that these dampers are not able to fit the large scale combustion systems because the unstable energy are very large to accommodate, so the dumping bias flow could be ignored.

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Figure 13: Frequency and sound pressure with respect to equivalence ratio and different bias flow 87. In the other context of active control of combustion instabilities, Lee et al. 88 studied experimentally the active control by modification of liquid fuel spray properties. The main objective of this study is to break up the coupling between the combustion process heat release rate and combustor pressure oscillations that drive the instability by changing spray characteristics. Changing the spray characteristics by modulating the fuel injection rate at the frequency of the instability with proper control phase to interrupt the heat release and pressure oscillations to significantly damp the combustion instabilities. This technique is considered one of the most promising approaches that could damp combustion instabilities effectively. More studies and concepts for active and passive control could be found in the literature 89–92. Furthermore, the impact of equivalence ratio oscillation on combustion dynamics in the backward-facing step combustion studied by Altay et al. 93 and Nemitallah et al. 94. The next section discuses in details the different approaches for overcoming all kinds of combustion instabilities, including static and dynamic instabilities, to improve the combustion process and reduce the exhaust emissions. Different approaches are considered in this section including fuel flexibility approach; variable operating conditions approach and variable flame type approach (including diffusion and premixed flames).

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3. Approaches for efficient combustion Robust, low emission and highly efficient power generation are the key parameters motivating the world interest in developing efficient combustors. Many approaches have been introduced and developed to achieve these goals including: (1) Fuel Flexibility approach, which in turns shall include hydrogen enrichment, oxy-fuel combustion and syngas approaches, (2) Lean premixed approach and (3) Swirl stabilizer approach. 3.1 Fuel flexibility Approach The fuel flexibility is clearly defined as how much the flame characteristics and the operability of gas turbine combustor are affected physically by changing the fuel composition. Currently, this issue has been raised to the public research due to the interest of using different fuel types and blends. The aim of this section is to investigate the impact of fuel composition on combustion characteristics such as, stability limits and exhaust emissions. The review criterion depends mainly on the analysis of recently measured experimental data including for flame stability, blow-off, flashback, dynamic stability and emissions. In spite that gas turbine engines are mainly fueled by natural gas, there is a worldwide interest in burning alternative fuels that may compensate natural gas, due to the increase in the natural gas prices and degradation of fossil fuels. These alternative fuels include syngas, biomass and liquefied petroleum gas. These fuels are different from natural gas in both physical and chemical transport properties. The change in chemical composition of the fuel results in severe change in the chemical time scale and, consequently, the operability characteristics in terms of blow-off, flashback and ignition time are affected 38,78,95–98. Richards et al. 99 comprehensively reviewed the fuel variability and stability in premixed hydrogen enriched combustion and the effect of fuel composition on premixed combustion stability and emissions. They illustrated that there is a rising interest in switching the operation of combined cycle power plants from natural gas to hydrogen enriched-methane fuel and syngas to increase the stability limits of operation and it can contribute in decreasing the NOx formation as well. Adding hydrogen significantly increase the operability of the gas turbine combustion systems. They also discussed recent technologies affecting the design and development of fuel combustor to avoid flashback, auto ignition and combustion dynamics associated with hydrogen enrichment 99. They reported that, adding H2 to the combustion process results in lower lean flammability limits. On the other hand, the flash back availability increased while adding hydrogen due to the increase in the flame speed as an adverse effect of adding H2 to the combustion process 51,100. Nevertheless, in order to maintain the same power output from the combustor, an amount of supplementary air should be introduced to the combustor. This can be attributed to the decrease in equivalence ratio as well as the lower heating value of hydrogen. They revealed that the swirl combustor is the most commonly used combustor; however, further research studies and efforts are needed to minimize combustion instabilities while varying the fuel composition. Lieuwen et al. 42 also discussed the impact of varying fuel chemical composition on the performance of LPM combustors. They studied the effect of fuel composition in terms of laminar

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and turbulent flame velocities and chemical time scale as well. One of the main findings is that the combustion behavior of fuel mixtures, such as laminar and turbulent flame speed could be tremendously changed as compared to the fuel with a single element composition. In the same context, Kido et al. 101 studied numerically the reliance of the turbulent flame speed (ST) on the turbulence intensity and equivalence ratio using different fuel types. Three different fuels including CH4, C3H8 and H2 were studied at similar laminar flame speed and the study revealed that they have an essential variation in the turbulent flame speeds as shown in Figure 14. This can be attributed to the difference transport diffusion properties.

Figure 14: Reliance of turbulent velocity on turbulence intensity for different fuel types 101. The combined effect of fuel blending and heat release rate on combustion instabilities have been investigated by Ferguson et al. 96. The effect of fuel Wobbe index (WI) was studied by mixing the natural gas with variable concentrations of ethane (WI is defined as the higher heating value of the fuel gas divided by the square root of the corresponding specific gravity; where, the specific gravity is the ratio of fuel density to air density). The study revealed that, at a fixed Wobbe index, the produced dynamic response would not be constant. 3.1.1 Oxy-fuel combustion Approach The oxy-fuel combustion approach is considered as one of the key approaches under the fuel variability technique. In this section, we are going to intensively describe this technique and comprehensively review its recent development. The most dangerous pollutants as a result of burning natural gas are the nitrogen oxides, NOx, which are formed due to combustion of air in the presence of nitrogen. Interestingly, there are many different methods for controlling or eliminating NOx including pretreatment, process modifications, combustion modification and post treatment. Other pollutants that should be highly taken into consideration are CO2 and CO emissions. Another approach to completely eliminate NOx emissions is through the application 16


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of oxy-combustion technology 102,103. However, the combustion and radiative heat transfer characteristics in case of oxy-fuel combustion differ from those of air-fuel combustion. This may be attributed to the significant differences in the physical properties of CO2 and N2 104–107. The replacement of N2 by CO2 in the oxidizer mixture impacts the flame in four issues: changes in mixture density, volumetric heat capacity and transport properties including thermal conductivity, mass diffusivity and dynamic viscosity. In the oxy-combustion technology, N2 is removed from air using an air separation unit. The remaining gas (mainly O2) is used as oxidizer and the combustion process results in flue gas mixture consisting mainly of CO2 and H2O. However, the combustion of the fuel using pure oxygen results in high flame temperature 108. In order to prevent excessively high temperatures, some of the CO2 in the flue gases is recirculated and mixed with the oxidizer 109. This technology makes the exhaust gases consisting primarily of CO2 and, thus, facilitates its capture and sequestration to eliminate its release into the atmosphere. This technology also reduces the volume and mass of the flue gases significantly with corresponding benefits of reduced heat losses and reduced size of the flue gas treatment equipment 110. However, the increased concentrations of CO2 in the oxidizer mixture results in decrease in chemical kinetics which in turn, decrease the laminar burning velocity and combustion efficiency 39,40,111. This may be attributed to the adverse kinetic interactions and additional preheating due to high concentrations of CO2. Consequently, combustion in CO2-diluted systems requires higher oxygen levels than 21% (air concentration level) in order to obtain a stable flame at the same equivalence ratio 112. Rashwan et al. 113,114 illustrated the effect of carbon dioxide addition in terms of thermal conductivity, kinematic viscosity and mass diffusivity. Carbon dioxide has higher density than N2, which affects gas volume, flame shape and pressure drop. The density of air is 0.43 kg/m3 at 800 K while the densities of CO2/O2 mixture are 0.62, 0.61, 0.60 kg/m3 at oxygen fractions of 29%, 32%, 36%, respectively. The higher density of CO2 also leads to higher heat capacity of CO2/O2 mixtures with respect to air (at 800 K, the volumetric heat capacity of air is 480 J/m3/K while those of CO2/O2 mixtures at oxygen fractions of 29%, 32%, and 36% are 480, 705, 700 and 690 J/m3/K, respectively. The high volumetric heat capacity of CO2/O2 mixtures with respect to air directly reduces the temperature level and causes less flame speeds and lower flame stability. In addition to the changes in densities and in volumetric heat capacities, flame speed is also affected by gas transport properties. The thermal conductivity and the dynamic viscosity of N2 and CO2 gases at different temperatures as well as the mass diffusivity of O2 in both N2 and CO2 are available in 114.

3.1.1.1 Oxy-combustion degrees of freedom Oxy-fuel combustion technology has different degrees of freedom that confined the operational space from the air-fuel combustion. Such oxy-fuel combustion flames are more likely to be operated near to stoichiometric to reduce both the fuel and O2 consumption with a controlled ratio of O2/CO2, which in turns control the combustion temperature. It is well known that there are three combustion issues associated with oxy-fuel combustion that must be addressed for CH4/O2/CO2 combustion systems as summarized in Figure 15. It can be seen that the operation at

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higher equivalence ratios or rich mixtures is associated with higher CO emissions, while the operation at leaner conditions is associated with higher O2 flow in the exhaust stream. However, the operation at low temperatures for low NOx emissions is most likely to blow-off. The rest of this section reviews the operating condition which can maintain the best scenario in terms of flammability limits, emissions and cost effective.

Figure 15: The operational space in terms of equivalence ratio and flame temperature [19]. 3.1.1.2 Effect of oxy-combustion on stability map From the open literature, it is well known that the combustion of CH4/O2/CO2 mixture has slower chemical kinetics than those of air-methane combustion. Consequently, the flame stability becomes more challengeable as the oxy-fuel combustion flames are more prone to blow-off. Ditaranto et al. 115,116 reported that the process of oxy-fuel combustion requires at least 30% oxygen fraction to perform in a stable manner comparable to air-fuel combustion. This 9% increase in oxygen as compared to air is compensating for the lower heat capacity of nitrogen. Liu et al. 117 studied numerically the oxy-methane combustion characteristics in a gas turbine model combustor. They concluded that high oxygen fraction in the oxidizer mixture results in high flammability limits. Joo et al. 118 compared flame structures and soot concentrations of laminar air-methane and oxygen-enriched methane diffusion flames within a range of operating pressure up to 60 bar. It is concluded that the soot concentrations of air-methane combustion are higher than those of oxygen-methane combustion over the whole operating pressure range. Nemitallah and Habib 119 studied experimentally and numerically oxy-fuel combustion characteristics of diffusion flame in a gas turbine co-axial combustor. They reported that the stability of oxy-fuel combustion adversely affected when the controlled oxygen fraction is reduced below 25%. Alberto et al. 46 studied the extinction mechanisms under premixed conditions. They showed that using CO2 as a diluent reduces the operability limits, due to the slower chemical reactions of this system relative to the case of burning in air. Rashwan et al. 113,114 studied the effect of partial premixing on oxy-fuel combustion and they compared the

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results with the cases of air-fuel combustion. They studied three oxygen fractions namely 29%, 32% and 36%. They reported that the range of oxygen fraction to obtain a stable flame is from 29% to 40 %. Also, they reported that the air-fuel combustion has larger stability range than oxyfuel combustion due to the adverse effect of introducing the CO2 into the combustion zone as shown in Figure 16.

Figure 16: Flammability limits of oxy-fuel combustion at different oxygen fraction and comparison with air-fuel combustion 114. Ramadan et al. 120 investigated the effects of oxidizer flexibility on stability maps by comparing effect of using different oxidizer while burning natural gas in a confined space and using a bluff body stabilizer with different blockage ratios. Kutne et al. 121 studied the impact of oxygen fraction on oxy-fuel combustion characteristics. They reported that operation with oxygen fractions lower than 22% was not possible even at stoichiometric conditions. Baolu et al. 122 investigated experimentally the effect of carbon dioxide dilution in oxygen-methane combustion on a mixed tubular burner. They reported that the stable flame is obtained for a reasonable range of equivalence ratio within a range of volumetric oxygen fraction from 21% to 50%. Hu et al. 123 studied the effects of equivalence ratio (from 0.8 to 1.2), oxygen concentrations (from 25% to 35%) and dilutions (N2 and CO2) on the laminar flame speed of CH4 under atmospheric conditions. They reported that the laminar flame speed of the premixed oxy-methane mixture reaches its maximum value at stoichiometric conditions. Li et al.124 investigated numerically the effects of exhaust gas recirculation on the characteristics of premixed oxy-methane flames under atmospheric pressure. The results showed shift in the flame location and changes in the species concentrations for the cases of oxy-methane combustion as compared to the case of aircombustion. A recent study conducted by Jerzak and Kuznia 125 investigated the upper and lower flammability limits of the combustion of natural gas with different oxidizers including air, oxygen-enriched air and oxy-mixture of O2 and CO2. The experiments were conducted under two 19


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different swirl numbers, 0.69 and 1.35. Figure 17 represents the flammability limits of the oxyfuel combustion flames at 25% O2, 15% CO2 and 60% N2. The study revealed that the CO2 dilution experiments in the hydrogen-enriched air improved the flashback limits as compared to the air and oxygen-enriched air. However, in comparison of the total stability limits, the oxy-fuel combustion recorded the lowest flammability limits for both swirl numbers as shown in Figure 18. The most favorable stable combustion range was recorded for the enriched air with 25% of oxygen at swirl number of 1.35. They reported that addition of 25% O2 to the air-combustion adversely affect the flashback limits and enhance blow-off limits.

Figure 17: Stability map for oxy-fuel Figure 18: Flammability limits comparison combustion at 25% O2, 15% CO2 and 60% N2 between air, oxygen-enriched air and oxy125 . mixture 125. Shi et al. 126 studied experimentally the oxy-fuel combustion of methane under different oxygen fractions. The effect of varying the oxygen fraction from 0.21 to 0.86 is analyzed and reported. The study confirmed some well-known observations regarding the oxy-fuel flames behaviors. They reported that, the flame stabilization range is very small and confined by flash-back at higher oxygen fractions and by blow-off at lower oxygen fractions. They also reported the results in terms of the flame temperature and burning velocity with respect to the oxygen fraction. As it could be seen from Figure 19, as the oxygen fraction increases, both burning velocity and flame temperature increases. To obtain stable flame operation, the lowest possible oxygen fraction is around 0.50, while for the upper stable limit, the highest oxygen fraction is about 0.86.

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Figure 19: Adiabatic flame temperature and laminar flame speed verses the oxygen fraction 126. 3.1.1.3 Effect of oxy-combustion on NO emissions Kim et al. 127 studied flame stability, NO emissions and visual flame appearance in a 0.03-0.2 MW oxy-fuel combustor using flue gas recirculation technology. They reported upon their experiment that, while operating at 40% FGR ratio at output power of 0.03 MW, NO emissions level recorded reduction of 93%. Meanwhile, operating at output power of 0.2 MW at the same FGR ratio, the NOx reduction level was approximately 85%. Figure 20 represents the NO emissions in ppm at different oxygen fractions in the 0.03-MW combustor. As shown in the figure, the lower the oxygen fraction the higher the amount of NO emissions. This is can be attributed to the lower turbulence intensity and the higher resident time associated with the lower oxygen contents. They also reported that NO emissions are significantly reduced while increasing the FGR ratio. This can be imputed to the increase in dilution effects as shown in Figure 21. The study recommended that the reduction in NO emissions in the FGR oxy-fuel combustor could be quite effective at FGR ratios higher than 40%, taking into considerations that the higher FGR ratios, the lower the energy and combustion efficiencies.

Figure 20: NO emission versus CO2 ratio at Figure 21: NO emissions versus FGR ratio output power of 0.03 MW 127. and CO2 ratio at output power of 0.2 MW 127.

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Many investigations were made for the sake of exploring the design and configuration differences between the oxy-fuel and air-fuel burners. Information on the effect of CO2 additions is very important and will bring to our mind a better understanding of how to switch the existing burners from air-operated to be operated with a controlled O2/CO2 mixture. CFD models have been established to investigate the ability of burners to accommodate the new oxidizers. Another approach used in the oxy-fuel combustion technique is the recirculation of flue gases. This method ensures a significant reduction in the exhaust emissions. A new burner was designed by Grathwohl et al. 128 to perform a CFD investigation of the oxy-fuel combustion with different oxygen fractions and compare the results with air-fuel combustion. This burner is designed to accommodate wide range of direct injected oxygen up to 100%. They reported the temperature distribution considering air, oxy-fuel with 33% O2 mixed with RFG and oxy-fuel with 21% O2 direct injected. Figure 22 represents comparison between the studied three cases in terms of temperature distribution; the study revealed that, the maximum temperature was recorded for oxygen fraction of 33%. Furthermore, the burner with direct injection revealed more stable flame.

Figure 22: Comparison of air and oxy fuel combustion 128. The summary of the conducted researches on fuel flexibility and oxy-fuel combustion approaches are summarized in Table 3. For the sake of comparison, the major findings of the recent studies are summarized in Table 3 below according to the applied approaches and applications.

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Table 3: Summary of the conducted researches on fuel flexibility and oxy-fuel combustion approaches. Combustion Technique/ Approaches

Application References Remarks/ Major findings Study/ Review/ Experimental or Numerical Review on Premixed hydrogen Richards et -The effect of fuel composition on enriched combustion. al. 96; premixed combustion stability and emissions. Experimental of lean combustion performance

Fuel flexibility

Oxidizer flexibility

Oxy combustion

premixed Lieuwen et -The impact of varying fuel al. 42 chemical composition on the performance of LPM combustors Numerically investigation Kido et al. -The reliance of the turbulent flame 101 speed (ST) on the turbulence intensity and equivalence ratio using different fuel types have been investigated. The effect of fuel Wobbe index (WI) Ferguson -By mixing the natural gas with was studied et al. 96. variable concentrations of ethane. Experimental investigation of 120 diffusion oxy-fuel combustion characteristics in a bluff body swirl stabilizer. Experimental investigation of Nemitallah diffusion oxy-fuel combustion and Habib 119 characteristics Experimental investigation of partially premixed oxy-fuel combustion Experimental investigation of oxyfuel combustion as compared to airfuel combustion

Numerically the oxy-methane combustion characteristics in a gas turbine model combustor Oxygen-enriched methane diffusion flames Oxy-fuel combustion characteristics

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-Investigated the effects of oxidizer flexibility on stability maps by comparing effect of using different oxidizer -The stability of oxy-fuel combustion adversely affected when the controlled oxygen fraction is reduced below 25% Rashwan -They reported that the range of et al. 113,114 oxygen fraction to obtain a stable flame is from 29% to 40 %. Ditaranto -Process of oxy-fuel combustion et al. 115,116 requires at least 30% oxygen fraction to perform in a stable manner comparable to air-fuel combustion Liu et al. -They concluded that high oxygen 117 fraction in the oxidizer mixture results in high flammability limits Joo et al. -The soot concentrations of air118 methane combustion are higher than those of oxygen-methane Kutne et -Operation with oxygen fractions al. 121 lower than 22% was not possible even at stoichiometric conditions


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Oxy-fuel combustion investigation Hu et al. -laminar flame speed of the oxygen concentrations (from 25% to 129 premixed oxy-methane mixture 35%) reaches its maximum value at stoichiometric conditions Numerically investigation Li et al.124 -The results showed shift in the flame location and changes in the species concentrations for the cases of oxy-methane combustion as compared to the case of aircombustion. Experimental Investigation Jerzak and -The study revealed that the CO2 Kuznia 125 dilution experiments in the hydrogen-enriched air improved the flashback limits as compared to the air and oxygen-enriched air Experimental The effect of varying the oxygen fraction from 0.21 to 0.86 Oxy-fuel combustor using flue gas recirculation technology CFD investigation of the oxy-fuel combustion

Shi et al. -To obtain stable flame operation, 126 the lowest possible oxygen fraction is around 0.50 Kim et al. -NO emissions level recorded 127 reduction of 93% Grathwohl -They reported the temperature et al. 128 distribution considering air, oxyfuel with 33% O2 mixed with RFG and oxy-fuel with 21% O2 direct injected

3.1.2 Hydrogen-enrichment approach Hydrogen energy is a key parameter for green and clean energy production and renewable and sustainable source of energy. Hydrogen is one of the most powerful and very efficient fuels because of its high energy content per kg of the fuel 130–132. It is well known that many countries around the world are mainly depending on fossil fuels for energy generation. Consequently, energy production results in huge amounts of greenhouse gases (GHG). Therefore, hydrogen enrichment technology is highly recommended for further clean energy production in the coming decades. Hydrogen energy is considered as a source of clean energy that requires lot of research studies and still has lot of mysteries phenomena that should be explored. The benefits of the application of hydrogen enrichment approach are: (1) the industrial based application, (2) environmental friendly, (3) efficient fuel and (4) renewable. Hydrogen is a flammable species that have great source of energy. Recently, it has been used in the IGCC because it encourages the use of hydrogen enrichment for many power generation applications and characterized by low CO2 emissions. Regarding the environmental consideration, hydrogen is non-toxic and produces less harmful emissions. Nevertheless, Hydrogen energy also is very powerful and very efficient. The most important benefit is that hydrogen is a renewable source of energy, as it could be produced via steam reforming of natural gas 133–135. Unlike the other sources of energy, which are non-renewable, hydrogen can be produced again and again by simple means of separation 24


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from water molecules. Actually, the hydrogen enrichment approach could be implemented on any power generation system including internal combustion engines, steam power cycles, jet engines and gas turbines 91,136 . Hydrogen technology is one of the promising techniques to tackle many problems associated with the combustion process such as the flame stability at leaner premixed conditions and NOx formation. 3.1.2.1 Effect of hydrogen enrichment on stability map Schefer 137 conducted an experimental study to investigate the stability characteristics of lean premixed flame with hydrogen enrichment. The mixture is passed through a 45-degree swirler with 5 vanes before being introduced to the burner. The study revealed that, under the rich mixture, the flame is lifted away from the burner surface. The maximum blow-off velocity occurred at stoichiometric conditions and decreased while moving towards the lean mixture. The experiments were carried out within a certain range of equivalence ratio by stating a stable flame at a certain equivalence ratio. Then by maintaining a constant fuel flow rate and gradually increase the air flow rate step by step to get the upper flammability limits and afterwards by gradually decrease the air flow rate to get the lower flammability limits. As shown in Figure 23, stable flame could be obtained within the upper and the lower flammability limits and outside these limits, the blow-off will occur. The sequence of points located in the figure represents sequence of flames obtained from F1 to F6 starting from the rich mixture (F1) and ended up with F6 while increasing the air flow rate towards the lean mixture. As the air flow rate is increased, the flame is detached by a certain distance from the burner. Then, further increase in the air flow velocity leads to reattachment to the burner surface and the blow-off occurs when the equivalence ration become sufficiently lean.

Figure 23: Flammability limits of premixed Figure 24: Blow off limits of air-methane with air-methane without hydrogen enrichment 137. hydrogen-enrichment fractions of 0%, 10% and 20% 137. The author’s main aim is to study the effect of hydrogen-enrichment on the characteristics of LPM. For this sake, a couple of experiments were conducted for a range of hydrogen enrichment including 10% and 20 % and a comparison is made with air-methane combustion. As shown in Figure 24, hydrogen addition significantly increases the lean blow-off limits. At velocity of 20

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m/s, the blow off occurs at equivalence ratio of 0.48 for air-methane combustion; meanwhile, the lean blow-off limits extended to equivalence ratios of 0.43 and 0.40 for hydrogen fractions of 10% and 20% respectively. They concluded that adding more than 20% of hydrogen to airmethane combustion results in significant increase in the OH concentration and extended the lean blow-off limits of the combustor. Zhen et al. 138 studied the combustion characteristics and performance of stoichiometric mixture of biogas mixed with hydrogen. They investigated the performance and flame behavior of three compositions of biogas fuels including BG60, BG50 and BG40; where, BG refers to biogas and the subscript refers to the CH4% in the fuel mixture. They analyzed the effect of adding hydrogen and carbon dioxide on the flame stability at different mixture Reynolds number. They showed the upper and lower flammability limits while burning these three fuels at different Reynolds number in terms of the range of the hydrogen enrichment level. The flame is stabilized when a certain amount of hydrogen is introduced in the fraction of combustible mixture, which consists mainly of CH4 and CO2 at different percentages. While increasing hydrogen enrichment level, the flame speed increases and the possibility of flashback increases. If hydrogen content is increased above a certain limit, flashback occurs indicating the upper flammability limit. On the other hand, the lower flammability limit occurs when hydrogen content decreases below a certain limit the flame speed decreases and the blow-off occurs. For the sake of comparison, the flammability limits experiments were performed for different biogases at different three values of Reynolds number including 400, 600 and 800. For instance, at Reynolds of 400, it was found that the BG60 has a stable flame ranged from α=10 to 40% (Where α denotes the mole fraction of hydrogen in the fuel mixture) and the BG50 has a stable flamed ranged from (α=20 to 40%). Apparently, the biogas BG50 has higher CO2 concentration that leads to higher hydrogen content required for flame stabilization. Rashwan et al. 113,114 pointed out that the increased level of carbon dioxide in the combustible mixture results in less stability of the flame, and at a certain level the flame blows-off. They reported that, for each fuel mixture, as the hydrogen fraction increases, the flame length decreases and the flow speed increases. This can be attributed to the increase in the flame speed due to increase the hydrogen fraction.

Figure 25: Stability map for stoichiometric biogas-H2 air flames 138.

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Other studies conducted by Lieuwen et al. 44,139 acclaimed that the H2 percentage significantly affect the blow-off limits. The LBO limits are plotted in Figure 3 in terms of equivalence ratio verses the percentage of enriched hydrogen fraction, and at different pressures. It can be seen that while increasing the pressure, the lean blow-off limits make further expansion giving the gas turbine more flexible operating range. In a further investigations performed by Strakey et al. 41,140 on laboratory scale, the effect of hydrogen enrichment on lean blow-off mechanisms in a swirl combustor is analyzed and quantified. Figure 26 represents the stability map for pressure range from 1 atm to 8 atm. Meanwhile, the hydrogen concentration ranged from 0% to 80% in the fuel. The studies revealed that while increasing H2% in the fuel, the lower equivalence ratio limit expanded from 0.46 to 0.30. Furthermore, the combustor pressure is also investigated to see its impact on the flammability limits; however, it showed limited effect.

Figure 26: Lean extinction experimental data at T=580 K and V=40 m/s 140. Nguyen and Samuelson 141 studied experimentally the effect of hydrogen enrichment level in a swirl combustor burning premixed mixture of natural gas and air under atmospheric conditions. The study revealed that hydrogen enrichment results in wider lean flammability limits of the combustor. Jackson et al. 142 studied numerically and experimentally the influence of hydrogen enrichment on lean premixed methane-air combustion. The combustible mixture fuel/air preheated up to 400 and introduced to the burner. They examined different percentages of hydrogen enrichment (0%, 5%, 10% and 20%). They reported that the improvement of lean flammability limits is attributed to the increase in the flame speed. Meanwhile, they cited a reduction on Lewis number, which is defined as the ratio between the mass diffusion rate and the thermal diffusion rate. The thermos-physical transport properties of fuel (CH4) and oxidizers (air, oxygen, nitrogen) are very critical in determining the behavior of the combustion process and visual flame appearance. Prandtl, Schmidt and Lewis numbers are very important dimensionless fluid-property groups that give an indication of relative rates of diffusion as presented 1. Ghoniem et al. 143 studied the stability and emissions using air-injection with H2 enrichment in premixed combustion. They experimentally investigated the LPM stabilized on a backward-facing step combustor for air-

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propane combustion. They concluded that the lean blow-off operation is associated with a strong pressure oscillation, which can be coupled with the heat release rate to make a strong thermosacoustics field. As shown in Figure 27, hydrogen addition is associated with higher NOx concentrations because of the temperature rise due to hydrogen addition; however, it is not compromised as it enables extending the flammability limits.

Figure 27: NO emissions and adiabatic flame temperature for different air-hydrogen flow rates at constant propane flow rate of 0.66 kg/s 143. 3.1.2.2 Effect of hydrogen on laminar burning velocity Tuncer 144 carried out lean blow-off measurements on a gas turbine combustor for different hydrogen compositions. Hydrogen enrichment considerably extended the lean blow-off limits; it depends mainly on two parameters including hydrogen enrichment level and volume flow rate. While increasing the volume flow rate, the stability is improved and the blow-off point is shifted because of the increased turbulence levels. The blow-off occurs when the flame velocity fails below the combustible mixture flow velocity. In Figure 28, it can be seen that the blow out limits are extended while increasing the hydrogen concentration in the fuel mixture.

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Figure 28: Blow-off laminar flame speed for different hydrogenfuel blends 144. Actually, the use of hydrogen-enrichment technique is very promising for gas turbines and internal combustion engines. Halter et al. 145 discussed the effect of hydrogen addition in airmethane combustion using a high pressure and laminar flame facilities. The study revealed that a small hydrogen fraction considerably changes the instantaneous and average flame characteristics. For all equivalence ratios, a reduction in the reliance of laminar flame speed against stretch occurs when hydrogen is added to the substrates. Due to lower temperature associated with the lean flames, thermal NOx is significantly decreased while using hydrogen enriched combustion technique. Coppens et al. 146 studied the effects of adding hydrogen on the burning velocity and NO emissions of CH4/air flames. The hydrogen percentage in the introduced fuel changed from 0% to 35%. The flames were stabilized over a perforated plate combustor. The influence of hydrogen fraction in the mixture on both the laminar flame velocity and the NO concentration were quantified, analyzed and reported over a range equivalence ratio as presented in Figure 29 and Figure 30. It was found in the lean mixture that the hydrogen enrichment has a weak effect on NO concentration, whilst, at rich mixture, the NO concentration significantly decreased.

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Figure 29: Laminar burning velocities for Figure 30: The measured and modeled values hydrogen enrichment levels of 35%, 25%, of the NO concentration for hydrogen 0.15%, 5% and 0% 146. enrichment levels of 35%, 25%, 0.15%, 5% and 0% 146. Ilbas et al. 147–149 conducted experiments to compare the laminar burning velocities of two different fuel mixture including hydrogen-air and hydrogen-air-methane mixture. They used different hydrogen fractions in the methane ranging from 100% hydrogen to 100% methane in the range of equivalence ratio from 0.8 to 3.2. The study revealed that increasing in hydrogen fraction in the fuel mixture brought about an increase in the flammability limits and an increase in the laminar flame speed. Figure 31 represents flame speed versus equivalence ratio for different fuels. While Figure 32 illustrates the effect of hydrogen fraction on flame speed. The authors recommended the hydrogen-methane fuel mixture of 30%H2 and 70%CH4 as an alternative fuel driving power generation plants. This fuel mixture gives good flame stability, wide flammability ranges and relatively high burning velocity.

Figure 31: Flame speed of fuel-blends versus Figure 32: Flame speed for different hydrogen fractions in methane at equivalence ratio 147. equivalence ratio of unity 147.

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3.1.2.3 Effect of Hydrogen on NOx and CO emissions Hydrogen enrichment is a promising technique to increase the flammability limits of air-methane combustion and reduce NOx emissions significantly. Griebel et al. 150, studied the lean blow-off flammability limits and NOx emissions of hydrogen-enriched air-methane combustion at different combustor pressures. They investigated the perfect premixing conditions of H2-enriched flames, lower blowout limits and NOx concentration as well. They discussed the effect of adding 20% of H2 to 80% methane by Vol% and resulted in extending the lean blow out limits by approximately 10% as compared to air-methane combustion. The study revealed that the NOx concentrations were significantly reduced by approximately 35% at reduction in the flame temperature by 60 K. The presence of hydrogen and methane mixture can extend the lean flammability limits significantly because of the higher flame speed as compared to normal airmethane combustion. Lounici et al. 151 studied the improvement of NG dual fueled engine by hydrogen enriched. They raised the problem of decreasing dual fuel engine efficiency when operating at low loads, and suggested an effective method to overcome this problem. They used an effective method to possess the flammability limits and increasing the burning velocity by adding H2 into the combustion process. They reported that the break specific fuel consumption was decreased. In addition, the emissions of hydrocarbons, CO and CO2 were decreased. Schefer et al. 152 investigated the effect of hydrogen enrichment in a lean premixed swirl combustor fueled by natural gas. The premixing was achieved by injecting the fuel 40.0 cm upstream and the flame was stabilized over a 45-degree swirler supported by seven vanes. The combustion chamber was 30.4 cm long and the quartz tube diameter was 8.3 cm. They studied the flame stability and the blow-off mechanisms for different levels of hydrogen at different fuel to air ratios. The study revealed that adding an amount of H2 to the combustion process results in significant reduction in CO concentration and increase in OH radical concentrations. Thus, the stability is improved and the flame length is decreased as shown in Figure 33. Furthermore, H2 enrichment leads to increase in the flammability limits under lean-premixing conditions.

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Figure 33: Effect of hydrogen enrichment on CO and NO Emissions at different axial positions: (a) and (c) at Z= 5.1 cm and (b) and (d) at Z= 20.3 cm 152 . Hu et al. 129,153–155 conducted a numerical study on laminar burning velocity and NO concentrations of premixed hydrogen-air-methane flames. The most important finding from their study is that operating with hydrogen fraction below 40% results in slight increase in flame speed, while when the hydrogen fraction is more than 40%, the flame speed exponentially increases. Figure 34 describes the effect of hydrogen enrichment on NO concentrations at different hydrogen fraction. Two peaks were recorded for the distribution of NO versus equivalence ratio as shown in Figure 35. The first peak occurs at the stoichiometric condition; this is can be attributed to thermal NO mechanism. While, the second peak occurs at equivalence ratio of 1.30, which can be attributed to the prompt NO mechanism. At stoichiometric condition, the effect of hydrogen enrichment on NO emissions is very weak as compared to the effect of it on rich mixtures.

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Figure 34: NO mole fractions versus hydrogen Figure 35: NO mole fractions 153. equivalence ratio 153.

fractions

versus

A study conducted by York et al. 156 on a heavy-duty gas turbine engine showed a possibility for operation at low NOx emissions. They used premixed hydrogen-enriched fuel mixture and tested it on a 10 MW combustor. They studied different fuel compositions including natural gas and two hydrogen enriched mixtures, 60%H2/40%N2, and 60%H2/30%N2/10%CO, in addition to pure hydrogen at elevated pressure of 17 atm. Figure 36 represents a comparison between these fuels in terms of NOx concentration and flame temperature.

Figure 36: NOx concentrations for different hydrogen enriched fuels 156. Hydrogen enrichment to the fuel is considered as a way to reduce the main pollutant that’s emitted by burning natural gas with air from power generation industries. This only happens when operating near the lean blow-off region. Ter-Maatha et al. 157 performed a feasibility study to compare the cost effectiveness of adding hydrogen to the combustion process. They suggested that this method is very competitive to the other new methods/techniques of NOx reduction. Daniele et al. 158 investigated experimentally lean premixed syngas combustion characteristics in

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gas turbine. In this study, the lean blow-off characteristics, the exhaust emissions and the turbulent flame speed were discussed. Different syngas fuel mixtures were considered and compared with pure natural gas. The experiments were conducted at high combustor pressure in the range of 30 bars with a maximum air flow rate of 0.3 kg/s. Flames were stabilized by establishing of recirculation zones of the flue gases due to the sudden enlargement of the combustor. The operating range of equivalence ratio ranged from 0.24 to 0.5. The experiments were carried out starting from a stable flame at a certain air/fuel ratio and, then, the fuel mass flow rate is gradually decreased until lower blow-off is obtained. Three syngas mixtures were considered including the natural gas co-firing mixture consisting of 20%H2, 20%CO and 60%CH4 in volume basis, a mixture of 50%H2 and 50%CO, and a mixture of 33%H2 and 67% CO.

Figure 37: CO and NOx emissions in ppm versus equivalence ratio 158. The results revealed that achieving low emission from syngas-combustion is very challenging because of the difficulty of reaching sufficient mixing before combustion. Furthermore, the risk of facing flashback is raised and it should be taken into consideration and precautions should be established prior to study. Many recent studies on syngas combustion highlighted that syngas combustion is commonly adopted for diffusion flames with a highly dilution of N2 or steam. The results showed that increasing the H2 fraction in the fuel mixture results in wider flammability limits at leaner operating conditions. There are significant differences in the lower blow off limits between the air-methane combustion and its correspondence with H2 enrichment. This can be attributed to the higher flame speed associated with hydrogen enrichment as compared to natural gas combustion. Ghenai159 performed a numerical investigation of syngas-combustion in a gas turbine combustor. The effect of fuel species composition and its corresponding heating values were studied to measure the emissions. The syngas fuel was obtained from a gasification process of solid fuel such as coal. He also studied the effect of shifting the fuel from natural gas to syngas on the gas

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turbine performance. Syngas fuel has hydrogen to carbon monoxide ratio H2/CO ranges from 0.63 to 2.36 and its heating values ranges from 8224 KJ/m3 to 12492 KJ/m3. The study revealed that gas turbines can burn syngas at higher efficiency than air-methane combustion and also reduces the NO emissions and the wall temperature as well. While for syngas fuel combustion, the burnt gas temperature is relatively lower than this in case of air-methane combustion. This can be attributed to its lower heating value. The results showed that the CO2 concentrations are reduced by 30% to 49% when the methane is shifted to syngas and the power was reduced by 20% to 55%. On the other hand, NO concentrations are significantly reduced within a reduction range from 11.5% to 97.5 % at the same mass flow rate of fuel but at less power generated due to lower heating value of syngas. The above literature review considering hydrogen enrichment approach is summarized in the following table. For the sake of comparison, the major findings of the recent studies are summarized in the table 4 below according to the applied approaches and applications. Table 4: Summary the hydrogen enrichment section for efficient combustion. Combustion Technique/ Approaches

Application Review/Experimental or Numerical

References

Remarks/ Major findings

Experimental study the effect of hydrogen-enrichment on the characteristics of lean premixed combustion

Schefer 137

-Adding more than 20% of hydrogen to airmethane combustion results in significant increase in the OH concentration and extended the lean blowoff limits of the combustor -On the other hand, the lower flammability limit occurs when hydrogen content decreases below a certain limit the flame speed decreases and the blow-off occurs -The H2 percentage significantly affects the blow-off limits. -The studies revealed that while increasing H2% in the fuel, the lower equivalence ratio limit expanded from 0.46 to 0.30.

Combustion Zhen et al. 138 characteristics and performance of Hydrogen Enrichment stoichiometric mixture of biogas mixed with hydrogen Experimental Study

Lieuwen et al. 44,139

laboratory scale, the Strakey et al. 41,140 effect of hydrogen enrichment on lean blow-off mechanisms

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Experimentally the Nguyen effect of hydrogen Samuelson 141 enrichment Numerically and Jackson et al. 142 experimentally, the influence of hydrogen enrichment on lean premixed methane-air combustion The stability and Ghoniem et al. 143 emissions using airinjection with H2 enrichment in premixed combustion Lean blow-off Tuncer 144 measurements on a gas turbine combustor for different hydrogen compositions Effect of hydrogen Halter et al. 145 addition in airmethane combustion

Effect of hydrogen on laminar burning velocity

Effects of adding Coppens et al. 146 hydrogen on the burning velocity

Experiments to Ilbas et al. 147–149 compare the laminar burning velocities

Hydrogen-enriched air-methane combustion

Griebel et al. 150

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and -Wider lean flammability limits of the combustor

-The improvement of lean flammability limits is attributed to the increase in the flame speed

-Hydrogen addition is associated with higher NOx concentrations because of the temperature rise due to hydrogen addition -It can be seen that the blow out limits are extended while increasing the hydrogen concentration in the fuel mixture -A reduction in the reliance of laminar flame speed against stretch occurs when hydrogen is added to the substrates. -In the lean mixture that the hydrogen enrichment has a weak effect on NO concentration, whilst, at rich mixture, the NO concentration significantly decreased. -Increasing in hydrogen fraction in the fuel mixture brought about an increase in the flammability limits and an increase in the laminar flame speed -The NOx concentrations were significantly reduced by approximately

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35% Improvement of NG Lounici et al. 151 dual fueled engine by hydrogen enriched

-They reported that the break specific fuel consumption was decreased

Effect of hydrogen Schefer et al. 152 enrichment in a lean premixed swirl combustor

-That adding an amount of H2 to the combustion process results in significant reduction in CO concentration and increase in OH radical concentrations -The effect of hydrogen enrichment on NO emissions is very weak as compared to the effect of it on rich mixtures. -Premixed hydrogenenriched fuel mixture and tested it on a 10 MW combustor -They suggested that this method is very competitive to the other new methods/techniques of NOx reduction.

Numerical study on Hu et al. 129,153–155 laminar burning velocity Heavy-duty gas York et al. 156 Effect of Hydrogen on turbine engine NOx and CO emissions Feasibility study to TerMaatha et al. 157 compare the cost effectiveness of adding hydrogen to the combustion process Experimentally lean Daniele et al. 158 premixed syngas combustion

Numerical Ghenai 159 investigation of syngas-combustion

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-Achieving low emission from syngas-combustion is very challenging because of the difficulty of reaching sufficient mixing before combustion -Gas turbines can burn syngas at higher efficiency than airmethane combustion and also reduces the NO emissions

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3.2 Variable operating conditions approach The operating conditions of gas turbine combustors such as air inlet temperature, combustion chamber pressure, equivalence ratio and swirl strength are the main influencers affecting combustion stability. Many recent studies were performed on the variability of the operating condition of gas turbine combustion systems 83,95,160–163. Lee et al. 164 illustrated the effect of combustor’s operating parameters on the LPM methane-air combustion in a swirl-stabilized combustor. The study revealed that the combustion instabilities were observed only when the inlet air temperature lies in the range from 650 K to 670 K. Similarly, operation in the range of equivalence ratio from 0.5 to 0.7 leads to combustion instability. In addition, it was found that as the swirler angle increases, the combustion instability decreases. They attributed this behavior to the modification of stream dynamics at the surfaces of the swirler. A parametric study was performed by Venkataraman et al. 165 on the effect of the equivalence ratio, the substrates inlet velocity and the swirler angles on the combustion instabilities using a bluff-body stabilizer fueled by natural gas. The study revealed that, while increasing the inlet velocity, the combustion instability increases. Figure 38 (left) represents the stability limits in terms of equivalence ratio and the oscillating pressure amplitude at different inlet velocities ranged from 2 m/s to 6 m/s. At inlet velocity of 6 m/s, as the equivalence ratio increases over 0.55, the flame tends to have some instabilities and, then, it returns back to be stable at equivalence ratio around 0.8 and, finally, it losses its stability again at stoichiometric condition. They also revealed that, while increasing the swirler degree, the stability increases at high equivalence ratios, while it adversely affects the stability at lean conditions as shown in Figure 38 (right).

Figure 38: Stability maps in terms of inlet velocity (left) and swirler degree (right) as a function of equivalence ratio 165.

3.3 Variable flame characteristics 3.3.1 Diffusion flame Diffusion flame is defined as; when the fuel and oxidizer are combined to form a combustible mixture, then the ignition starts once the mixture is created. In this case, a reaction sheet is created forming the flame border as shown in Figure 39. The Figure represents the co-axial flow jet, which is the most commonly flow configuration supporting diffusion flames. The diffusion flame starts at laminar mode, but it shifts to turbulent diffusion flame when a further increase of

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flow rate occurs. This could form flickers at the top of the flame. The flame length is also increased due to the increase in the turbulence level. The diffusion flame structure is also shown below; the fuel concentration is highest on the centerline as the fuel is introduced from a centerline pipe and slightly decreases to reach zero at the flame reaction sheet. The concentration of oxidizer is highest at the wall and tends to zero at the flame reaction sheet. In the industrial combustors, changing fuel types can affect the length of flame reaction zone. For instance, adding CO or H2 to the reactants brings the flame shorter because the flame requires less oxidant to complete the combustion. In the reciprocating engine applications, the fuel is directly injected to the cylinder, the reaction may start at any point where mixing occurs. In these cases, auto ignition is a concern and further investigations were conducted about auto ignition time scales 166,167 .

Figure 39: Reaction zones for a jet diffusion flame in a co-axial burner. 3.3.2 Premixed flames The lean premixed approach is defined as that the fuel should be mixed with air upstream before introducing the mixture to the burner as shown in Figure 40 As a matter of fact, homogeneous mixing between air and fuel under ultra-lean operation results in significant reduction in flame temperature and, accordingly, reduction in NOx emissions. This approach allows for significant emission reduction but creates an operability/stability issues that could be reduced or eliminated through fuel variability.

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12" 3" Quartz Tube

Swirler

Premixing Length

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2" Pipe for Premixed Mixture

Figure 40: Sketch of premixed gas turbine model combustor. Currently, LPM is the most effective techniques for low emission power generation from natural gas. Many recent studies reported that the application of the partially premixed or fully premixed technique is one of the most promising solutions that can significantly decrease the emissions of NOx 168–170. However, the conversion of the flame from the diffusion type (non-premixed) to the premixed type results in increased levels of flame instabilities, which brings its own challenges to burner designers and manufacturer. The instability of laminar flame is one of the classic subjects in flame dynamics, which indicate the dynamics response of premixed flame jets to the weak fluctuations/perturbations 91,136. For premixed gas turbine combustion systems, the homogeneous premixing between fuel and oxidizer is desired. Therefore, many research studies investigated the effect of air/fuel degree of premixing on combustion instabilities 171,172. Rashwan et al. 113,114 investigated in series of experiments the effect of the degree of premixing on flammability limits, visual flame appearance, extinction mechanisms and exhaust NOx and CO emissions. The flame was anchored over a perforated plate burner. They examined five different degree of premixing (the mixing length divided by the burner pipe diameter), namely 7, 25, 45, 67 and 128. It was concluded that, as the degree of premixing increases, the flammability limits decreases as shown in Figure 44; hence, the combustion instability decreases. However, increasing the degree of premixing resulted in reduction in NOx and CO emissions by approximately 48%. They showed that the flame color changed from pure blue at the higher degree of premixing at which a fully premixed flame is achieved to reddish at the lower premixing ratio at which a partially premixed flame is achieved (near to diffusion flame). This was attributed to the high soot concentrations in the partially premixed flames as compared to the fully premixed flames.

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Figure 41: Flammability limits at different degrees of premixing of air-fuel combustion 113,114. Shih et al. 172 investigated experimentally the influence of the degree of premixing of air-fuel combustion on flame stability and NOx emissions. The fuel was introduced in to the mixinglength section at two different locations (namely, 5.5 and 6 times of the tube diameter upstream of the mixing length). They reported that, incomplete mixing of the combustible mixture leads to significant reduction in the stability range of the combustor including the pressure oscillation limits and the lean blow-off equivalence ratio limits. Combustion instabilities and flame behavior significantly changed due to varying the main operability parameters such as equivalence ratio and the degree of premixing. Venkataraman et al. 165 studied the effect of degree of premixing on stability in a co-axial burner, where the flame stabilized over a bluff-body. The results are presented in Figure 42 in terms of peak-to-peak pressure amplitude versus equivalence ratio over a range of degree of premixing. They examined three premixing conditions including 50%, 75% and 100%. The results revealed that, as the degree of premixing decreases, there is an observed increase in stability strength along the considered range of equivalence ratio. Another important consideration in premixed combustion is the auto-ignition. The time needed for initiating combustion is known as the auto-ignition time. For reciprocating engines operating under premixing conditions, the auto-ignition time is recognized as a critical design parameter. It is a matter of fact that the auto-ignition should be avoided so as not to affect the combustion stability. The auto-ignition time depends mainly on the ambient pressure and the fuel composition. Fuels with higher hydrogen content have shorter ignition times 173,174. We are concerned mainly in this review with the application of premixed combustion technology in swirl stabilized combustors, mainly for gas turbine combustion applications. Based on that, the next section, section 4, discusses the application of premixed combustion technology in swirl stabilized combustors including gas turbine combustion applications. The effect of swirl on flame stability and emissions is investigated as a one of the well-known approaches to overcome 41


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the combustion instabilities and operability issues associated with the application of premixed combustion technology in gas turbine combustors.

Figure 42: Peak to peak pressure amplitude versus equivalence ratio over a range of degree of premixing165.

4. Swirl Stabilizer Approach for stability and emission enhancement 4.1 Stabilization mechanisms and swirl number Swirl stabilizer is considered as one of the easiest and most efficient techniques to maintain the flame robust and to keep it attached to the burner, which in turns enhances the combustor stability 175–179. There are many alternative ways to generate swirl such as tangential supply of combustible mixture to the burner 180,181, co-axial vane swirler 182 and the mechanical spinner 183,184 . The principle beyond swirl stabilizer is that, a swirl velocity is introduced to the inlet mixture using the vanes of the swirlers. There are two stabilization mechanisms associated with swirler. First is the central recirculation zone (CRZ), which is established due to an area created inside the stream. While the other mechanism is called external recirculation zone (ERZ), which is formed near to the burner as clarified in Figure 43.

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Figure 43: Drawing of the flow field inside a swirl stabilizer combustor 41. The swirler degree is identified by a dimensionless parameter, which is well known as swirl number Sn. It can be defined as the ratio of the axial flux of angular momentum to the axial thrust. Different formulas have been established 185,186; however, there is a well-known formula, which can be expressed as follows 187: =

!

(3)

" !

Where, U and W are the axial and tangential velocities respectively and R is the radius of swirler. Figure 44 and Figure 45 describe the axial and radial swirler geometries, respectively; upstream and downstream as well. The swirler should be mounted at a certain distance from the combustor. This axial swirler was used in a previous study by Palies et al. 188 and consists of eight vanes periodically spaced by 45 degrees. This swirler made of NASA 8411 airfoil with inner and outer vane angles of 30 and 58 degrees respectively. On the other hand, the radial swirlers made of 18 vanes spaced by 20 degrees, NASA manufactured as well. For the radial swirlers, there are two parameters that specify them including the thickness of the circular inlet and the trailing edge angle of the vanes.

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Figure 44: Axial swirler geometry: (a) upstream and (b) downstream.

Figure 45: Radial Swirler geometry: (a) upstream and (b) downstream 188 . El-Drainy et al. 187 introduced a new swirler geometry/concept. The inlet flow to this new swirler is divided into two stream; one passes axially and the other passes tangentially. The axial flow stream is intersected with the tangential flow stream making a turbulent swirl region, which in turns enhances the stability. The principle beyond this swirler is to vary the swirl number for one configuration, which is not applicable for a single type swirler. This could be done by changing the intensity of the swirling flow, which is governed by the ratio of the tangential to axial air velocity. Flows inside the axial and tangential vane swirler are described in Figure 46. This study revealed that this new concept ought to enhance the combustion efficiency because of its ability to produce high swirl number at low load which, in turn, could overcome the main issue associated with the swirl combustor which is the deficiency happened at the low load.

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Figure 46: Axial (left) and tangential (right) swirlers passing through the axial and tangential vanes respectively 187. 4.2 Effect of swirl number on flame stability In LPM approach, the flame is usually stabilized over a swirler-combustor. Swirler can provide a better mixing of the combustible mixture, reduce the fuel consumption, degrade the exhaust pollutant emissions and enhance the flame stability. On the other hand, the swirler brings its disadvantage in terms of raising the combustion dynamics problems. During the LPM operation, the flame is very sensitive to the flow fluctuations, which in turns can lead to oscillations, blowoff and extinction. There are two recirculation zones associated with swirling the stream, the inner and the outer recirculation zones. The inner zone is creased inside the central region of the stream as a result of the pressure drop associated with the rotation of stream and in turns, causes the outer limits of flame to expand. It is well known that the inner recalculation zone is strong or well established for swirl number exceeding 0.6 189. While the second stabilization mechanism is related to the ERZ, this is established by the recirculation of the hot-reactive gases between the flames and walls. Jerzak and Kuznia 125 studied experimentally the effect of swirl number on flame stability ranges limited by flashback and blow-off. They used a tangential type swirler 190–193, and the substrates were supplied tangentially using three different swirl numbers (Sg) 0.69, 1.16 and 1.35. Figure 47 and Figure 48 show the upper and the lower flammablity limits respectievly, in terms of equivalance ratio and mass flow rate. The study revealed that, the higher the swirl number, the better the flammability limits.

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Figure 47: Upper flammability limits versus Figure 48: Lower flammability limits blow total mass flow rate for different swirl off versus mass flow rate for different swirl numbers 125. numbers 125. Vaned swirlers are commonly used in gas turbine combustors. Syred et al. 180 studied the effect of swirl number (from 0.8 to 4) on flashback and blow-off. They reported that as the swirl number increases from 0.8 to 1.5, the blow-off is slightly worsened, while the flashback is improved. Therefore, using higher swirl number is preferable for flashback protection during combustion process associated with hydrogen or oxygen enrichment. Figure 49 shows the data of flashback and blow off for coke oven gas for three different swirl numbers (namely, 0.8, 1.04 and 1.46) in terms of the tangential velocity for a range of equivalence ratio.

Figure 49: Blow off (left), flashback (right) for a coke oven gas (65%H2/25%CH4) for different swirl numbers 180. In a further experiment by Syred et al.180, they studied the effect of different swirl van angle for vaned swirlers on the tangential velocity leaving the swirl while burning natural gas. They

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reported that as the swirl angle increases, the tangential velocity leaving the swirler increases as shown in Figure 53.

Figure 50: Tangential velocity blow-off for vanned swirlers 180 .

Dam et al. 58 studied the impact of swirl number on flashback limits for syngas fuel. The presented results in Figure 51 show the flashback limits for swirl numbers of 0.71 and 0.97. The swirler of swirl number of 0.71 is more tending to flashback. Therefore, at a certain oxidizer mass flow rate (air), the tendency of flashback decreases as the swirl number increases.

Figure 51: Flashback limits for different swirl number 58.

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4.3 Effect of swirl on NOx and CO emissions Continue studying the optimal gas turbine design associated with modifying the burner geometry that achieves higher degree of stability, surprisingly, these studies come out with a further reduction in the amount of NOx emissions. Lim et al. 194 performed an experimental study on a small-scale gas turbine combustor. They investigated the effect of combustor design on the exhaust emissions, especially the NOx concentration. They used two different burner designs including the block and the cone types. They found that the cone type recorded better stability and NOx reduction as compared to the block type. Along with optimization of the combustor design, lots of devices have been developed to enhance the combustion efficiency through enhancing the flow mixing. A vortex generator is equipment that generates vortices in the flow field and it has been used in a several applications. Kim et al. 195 conducted an experimental study to investigate the premixed flame behavior using a vortex generator in a gas turbine combustor. For the sake of comparison, they performed the exhaust gas measurements for NOx and CO emissions with and without the use of vortex generator. The study revealed that the NOx can be reduced by 21.2%, whilst, the CO emissions can be reduced by 13.3% while using the vortex generator. Figure 52 compares the results of CO and NOx emissions with and without the vortex generator. Claypole and Syred 196 studied the effect of swirl burner aerodynamics on NOx formation under different swirl numbers in the range from 0.63 to 3.04. They concluded that the formation of NOx occurs in the flame front. Furthermore, the recirculation zone does not affect the NOx formation in spite of the long residence times and the temperatures.

Figure 52: Distributions of NOx emissions (left) and CO emissions (right) with respect to the equivalence ratio at exit velocity of 20 m/s 196. The above literature review considering the swirl stabilizer approach is summarized in the following table. For the sake of comparison, the major findings of the recent studies are summarized in Table 5 below according to the applied approaches and applications. Toward better understanding of the characteristics of premixed combustion flames in gas turbine

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Energy & Fuels

combustor, the next section (section 5) presents a numerical case study on turbulent premixed combustion in a gas turbine model combustor. Temperature, species concentrations and flow field are investigated over a range of operating conditions.

Table 5: Summary the swirl-stabilizer section for efficient combustion. Combustion Technique/ Approaches

Application Review/Experimental or Numerical

References

Designed and axial Palies et al. 188 swirler

Introduced a new El-Drainy et al. 187 swirler geometry/concept

A Review in swirl Syred N. 189 combustion systems. Swirl Stabilizer Approache and its effects on flame stability

Experimentally the Jerzak and Kuznia 125 effect of swirl number on flame stability Experimentally Syred et al. 180 investigated the effect of swirl number (from 0.8 to 4) on flashback and blow-off The effect of different Syred et al. 180 swirl van angle

Studied the impact of Dam et al. 58 swirl number on

49


Remarks/ findings

Major

-This swirler made of NASA 8411 airfoil with an inner and outer vane angles of 30 and 58 degrees respectively -The principle beyond this swirler is to vary the swirl number for one configuration, which is not applicable for a single type swirler -The inner recalculation zone is strong or well established for swirl number exceeding 0.6 -The higher the swirl number, the better the flammability limits. -As the swirl number increases from 0.8 to 1.5, the blow-off is slightly worsened, while the flashback is improved -The swirl angle increases, the tangential velocity leaving the swirler increases -The swirler of swirl number of 0.71 is

Energy & Fuels

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Effect of swirl on NOx and CO emissions

Page 50 of 70

flashback limits for syngas fuel

more tending flashback

Experimental study on Lim et al. 194 a small-scale gas turbine combustor.

-The cone type burner recorded better stability and NOx reduction as compared to the block type. -The NOx can be reduced by 21.2%, whilst, the CO emissions reduced by 13.3%.

Experimental study to Kim et al. 195 investigate the premixed flame by vortex generator in a gas turbine combustor

to

Studied the effect of Claypole and Syred -The formation of swirl burner 196 NOx occurs in the aerodynamics on NOx flame front formation

5. Numerical modeling of premixed combustion 5.1 Turbulent premixed combustion In most of combustion applications, the flow is usually turbulent. Also, the most common classification of the premixed combustion is based on the degree of premixing between the reactants. For the premixing combustion, there are three well-known classifications including fully premixed, partially premixed and diffusion (non-premixed). Rashwan et al 114 precisely differentiate between these three classifications of premixed combustion in their experiments in terms of flammability limits, visual flame appearance/color and associated exhaust emissions. Turbulent premixed combustion can be applied in gas turbines, spark ignition engines and jet engines. This technique involves the interaction between each of the following; turbulent flow, chemical reactions and kinetics, heat and mass transfer and radiation as well. Therefore, understanding of this technique is very challengeable. 5.2 Turbulent combustion modeling schemes Numerical tools such as CFD are very important to predict the combustor operability under variety of operating conditions. These tools provide very important recommendations towards the development of the new burner designs. In addition, CFD tools are cost effective in investigating complex geometries and complex applications since they provide data that are difficult to be measured. One important factor affecting the computational efficiency is the accuracy of the numerical models used for the discretizing the set of the governing equations associated with the combustion modeling. The aim of this section is to intensively review the available modeling techniques for turbulent premixed combustion. Whilst, the precise focus is

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Energy & Fuels

made on the CFD studies performed on hydrogen-fuel blends (CH4/H2) using large eddy simulation (LES). The CFD approaches for solving turbulent reacting flow are: (1) K-epsilon model, (2) direct numerical simulation (DNS), (3) Reynolds averaged Navier-Stokes (RANS) and (4) large-eddy simulation (LES) 197,198. DNS is the most accurate approach as compared to other approaches in solving turbulent flow field. In this approach, all the scales of the turbulent flow field are completely resolved; however, it is highly expensive in terms of solution time to obtain accurate solutions for complex engineering configuration. RANS method is the most common method for engineering CFD applications. The principle beyond this method is that, the governing equations are averaged, and the flow is neither solved for large nor small eddies. The accuracy of this approach is highly affected; however, it can give average values of the flow field 199. Veynante and Egolfopoulos 200 acclaimed that the Reynolds averaged Navier-Stokes model is not reasonable enough for solving the reacting flows accurately. This is can be attributed to the unsteady nature associated with this phenomenon which makes the RANS not accurate. LES approach is mainly based on separation of scales by filtering procedure and is considered as an efficient approach. Furthermore, LES offers a computational cost effective as compared to DNS. As per the open literature, the LES is considered as a promising tool and has many advantages for modeling industrial applications. Figure 53, represents a summary of how these computational models are solving the problems. First, the DNS method is the most accurate for solving such problems because it depends on computing both of the large, intermediate and small scale of energy. Whilst, the RANS solve only the large-scale energy eddies and model the rest. On the intermediate methods of LES, it solves up till the intermediate energy and models the rest.

Figure 53: Common turbulent schemes and prediction methods for turbulent flows.

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5.3 LES governing equations As mentioned above, the LES is based on separation of scales. The large scale is directly computed, whilst the effects of small scales are modeled. The conservation equations could be filtered to reach the filtered or the Favre-filtered form of the conservation equations for continuity 201–206: $ # #%

+

$ ( *) #( )

(4)

Momentum equation:

•

=0

#,-

$ ( *) #( / #%

+

4) $ ( *( #( ) *01 / 2- 3 #,-

#6 /)

#:2-

-

2

− #, = 7 $ 89 + #, }< +

4444> #= /) 6 /) ? #,-

}
XY J K

}

Review on Premixed Combustion Technology: Stability, Emission

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