Experimental Study of Oxy-fuel Combustion under Gas Turbine

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Experimental Study of Oxy-Fuel Combustion under Gas Turbine Conditions Inge Saanum, and Mario Ditaranto Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03114 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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Experimental Study of Oxy-Fuel Combustion under Gas Turbine Conditions Inge Saanum*‡ and Mario Ditaranto‡ SINTEF Energy Research, 7465 Trondheim, Norway KEYWORDS: oxy-fuel high pressure combustion, gas turbine combustion, CO2 capture. * Corresponding Author ‡These authors contributed equally. All authors have given approval to the final version of the manuscript.

ABSTRACT

Oxy-fuel combustion is one of the main routes for Carbon Dioxide (CO2) capture in power plants. The technology is well developed for coal fired power plants, but is less explored for natural gas fired gas turbine cycles. Implementing oxy-fuel CO2 capture in gas turbines is more complex than in boilers as the power density is larger and the working fluid of the power cycle is changed from air to CO2. The combustion system must then handle the combustion of the fuel with pure oxygen (O2) and the recirculated exhaust gas composed of mainly CO2. In this study, the pressurized combustion of methane in O2/CO2 atmospheres in a pressurized oxy-fuel combustion facility (HIPROX) is presented. The experiments focused on flame stability and CO

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emissions which are potentially challenging in this type of combustion. The experimental set up is based on an in-house axial swirl stabilized burner placed inside an optical combustion chamber made of quartz. Flame stability and CO emissions were studied at different O2 concentrations, excess O2 ratios, fuel power loads up to 100 kW and pressures up to 10 bar. Concerning stability, the trade-off between stability and excessive temperature and overheating was clearly evidenced and quantified for that burner. Depending on pressure and power loads, an O2 concentration of about 30 % resulted in a stable safe flame for most conditions while blow-off could occur at O2 concentration from 23 – 29 % depending on power and pressure. The experimental results show that special considerations have to be taken with respect the CO formation when implementing oxy-fuel combustion in gas turbine conditions. High equilibrium CO concentrations at flame temperature combined with short residence times at high and intermediate temperatures can lead to high emissions of CO. The CO emissions were found to be highly dependent on the excess O2, and although there is a strong decreasing trend with increasing O2 excess, even with 10% O2 excess the CO values were excessively high. It was found that the CO emissions are partly controlled by the equilibrium CO concentration and therefore increased with increasing O2 concentration in the oxidizer as a result of increasing adiabatic flame temperature.


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INTRODUCTION Increased concerns in the global climate evolution due to anthropogenic Greenhouse gases emissions, has challenged the power generation and industrial sectors (responsible for almost half of the global CO2 emissions) to find technical solutions for strongly reducing these emissions to the atmosphere. These technologies are gathered under the appellation of Carbon Capture and Storage (CCS), where three main CO2 capture pathways are possible: the postcombustion, the pre-combustion, and the oxy-fuel combustion routes. All those have pros and cons which must be considered at local conditions, such as retrofit or new plants, fuel quality, flexibility requirement, water management, regulations, industrial risks, etc. For coal fired boilers, the technology is proven by several pilot projects1-3 and plans for demonstration projects are made4. All capture technologies require extra energy from the power process to generate the pure CO2 for separation and compression. In the case of oxy-fuel, this energy penalty originates from the air separation to produce oxygen (O2) for the nitrogen (N2) free combustion. However, the focus on development of cryogenic air separation unit (ASU) has led to improved efficiencies both because of improvements in the technology and better integration to the processes5. In Perrin et al.5, between 2000 and 2013 a 20% improvement in separation energy is reported for commercial ASU. Although the cryogenic ASU is today the only technology capable of delivering large scale quantities of oxygen compatible with power plants size, other promising technologies based on membranes6-8 would provide a further step change in separation energy requirement, thus for the overall efficiency of the power cycle with CO2 capture too. Oxy-fuel combustion capture is not limited to coal and can be applied to natural gas with an adapted technology for gas turbines (a.k.a. semi-closed oxy-fuel gas turbine cycle). The maturity of this technology is much less advanced than for oxy-coal combustion and available literature is


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mainly limited to fundamental combustion studies and thermodynamic process analysis. Several variations of semi-closed oxy-fuel gas turbine cycles have been proposed4,

9-11

. Barba et al.11

assess 19 different configurations and IEAGHG12 made an assessment of some promising ones selected according to the working fluid composition. The major reason for the lack of development of these cycles is that since combustion in a gas turbine occurs in direct contact with the cycle working fluid, CO2 must be used as working fluid and a total re-design of the turbomachinery must be made. This large R&D investment in new oxy-fuel gas turbines combined with a wish to cope first with the most abundant fossil fuel with larger CO2 footprint (i.e. coal) has made oxy-fuel capture from natural gas fuel a down prioritized area. Oxy-fuel capture technologies for natural gas is characterized by pressurized oxy-fuel combustion and few pilot projects have been reported in that domain. Lee et al.13 at KIMM have built an oxy-fuel gas turbine pilot plant integrated with a waste heat recovery system, where steam is used as dilution medium in a 5 bar pressure chamber. Kutne et al.14 investigated the stability behavior of a pressurized air based burner in oxy-fuel conditions and showed that re-design of existing burners was necessary. In the OXYGT project15 an oxy-fuel burner based on Siemens swirl stabilized burner technology was developed and characterized up to 4.2 bar. The study established operational maps as a function of oxygen concentration in the oxidizer mixture and quantified CO emissions. There has been another type of oxy-fuel process proposed based on supercritical working fluid. The most advanced are those of Clean Energy Systems (CES)16 and NetPower17. The former uses CO2 in a 300 bar combustion chamber while the latter uses steam as working fluid with a maximum pressure of 150 bar. These novel cycles, although technically challenging by their innovative aspects, hold promises of higher energy efficiencies as shown in the comparative


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study of IEAGHG12. As these processes are developed by private consortium, little detailed information is publicly available about the combustion characteristics. In a semi-closed oxy-fuel gas turbine cycles, which is the focus in the current work, the working fluid is mainly CO2. The amount of steam in the working fluid depends on the exhaust condenser temperature. Because the chemical and physical properties of CO2, the combustion process is different than in traditional air combustion strategies. One chemical effect is due to CO2 that participates in the combustion reactions and can form high local concentrations of CO at flame temperature18-19. The equilibrium reaction of CO2 and atomic hydrogen forming CO and OH is the dominant reaction causing the high CO concentration20. Since CO oxidation is relatively slow and limiting, to obtain low CO emission at the turbine exhaust, the residence time at temperatures lower than the flame temperature has to be long enough to at least partly obtain equilibrium at a lower temperature where the equilibrium concentration is lower. Another chemical implication of the combustion process occurring in an oxy-fuel gas turbine mode, is that the amount of oxygen should be kept as low – i.e. as close to the stoichiometric ratio - as possible in order to first, minimize the energy penalty of separating oxygen in the ASU, and secondly to minimize the need for excess oxygen cleaning of the end CO2 stream to match the quality requirement for transport and storage. Due to the large mass flow of working fluid in the engine as compared to the stoichiometric flow rate of oxygen, the latter cannot be mixed upstream with the whole working fluid, but only with the part that goes through the burner and into the flame zone of the combustion chamber. This results in very limited amounts of oxygen available for post-oxidation downstream the flame zone as opposed to ordinary gas turbines where a typical oxygen concentration in the exhaust is around 14%. A common challenge in gas turbine combustion is the NOx emissions which must comply to stringent regulations. In the


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context of CCS and the oxy-fuel technology, the limitation is set by the CO2 quality requirement for its transport, injection and storage in geological formation, and recirculation in the engine. Although in principle there is no nitrogen in an oxy-fuel process, small concentrations are practically present in both the oxygen stream delivered by ASU and the natural gas. Typical concentrations of nitrogen reaching the combustor are in Sundkvist et al.9 0.7%, 2%, and 2.7% in the fuel, oxygen stream, and working fluid respectively. Although small, these concentrations can give rise to important emissions depending on the local temperatures achieved in the flame zone21. From a thermodynamic perspective, CO2 has different properties compared to N2 and air, the largest difference being a higher volumetric heat capacity. This implies that a higher concentration of oxygen in the CO2/O2 mixture than that in air is required to achieve the same adiabatic flame temperature. Another consequence is concretely seen on the laminar flame speed, a combustion characteristic depend on several gas properties, where the CO2/O2 oxidizer mixture giving the same flame speed as in air, yields a higher adiabatic flame temperature than in air combustion9. One secondary effect, highly relevant in gas turbine combustor development is the more complex thermo-acoustic instability schemes as observed in Ditaranto and Hals22 where oxygen concentration in the oxidizer is as important as overall flow velocity and equivalence ratio for instability mapping. Due to the high temperatures achieved in gas turbine, the thermal management of the material combustor is critical. As the oxygen stream is injected directly at the burner station, the potential for larger temperature variations as a function of operational conditions is higher. Indeed, the radiative heat flux characteristics of oxy-fuel flames were found to depend greatly on oxygen concentration23, as unlike N2, CO2 and H2O which are present in


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high concentrations have radiative properties24. Consequently the design of liner cooling and temperature control in gas turbine combustor in oxy-fuel mode is more challenging. The combustion system presented in this study is directly relevant to oxy-fuel power cycles based on high CO2 concentrations and moderate high pressure such as in Sundkvist et al.9, Mathieu25, and Sanz et al.26, where the working fluid is not in supercritical conditions. The working fluid in our study is for practical reason taken to be pure CO2 whereas in realistic power cycle minor concentrations of N2, Ar and H2O can be found, typical concentrations could be 2.7%, 5.1%, and 1.8% respectively as given in Sundkvist et al.9. It is therefore not expected that such low concentrations would affect considerably the stability or the CO formation properties investigated in this study, because H2O is known to affect both the flame speed, although in a lesser extent than CO227, and the CO oxidation through its interplay with the pools of H, O and OH radicals28. EXPERIMENTAL SETUP The High Pressure Oxy-fuel Combustion facility (HIPROX) used in the study is shown in Figure 1 and aims at simulating the combustion conditions in a semi-closed oxy-fuel combustion gas turbine cycles. Methane and oxygen of industrial quality are supplied from cylinder packs. Both gas streams are controlled by thermal type mass flow controllers and fed to the burner. The working fluid (recirculated dry exhaust gas in an oxy-fuel power cycle) is simulated by industrial quality pure CO2 from a 6 m3 liquid CO2 tank which is evaporated in an electrical evaporator. The CO2 is divided into two lines that can both be independently regulated in mass flow rate and temperature by electric preheaters. One line, referred to as oxidizer CO2, feeds directly into the burner, the other, referred to as dilution CO2, in the annular section formed by the pressure vessel and the combustor. In this study, only the oxidizer CO2 line is pre-heated to a temperature of


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200°C. The dilution CO2 gets heated prior to injection in the combustion products by cooling the combustor elements as explained later.

FTIR Gas analyzer Backpressure valve

CO2

CO2 for cooling Evaporator O2

CH4

Heater (not used)

Pressurized combustion chamber

Heater

Fuel

Figure 1. Sketch of the HIPROX facility

Figure 2. High pressure oxy-fuel combustion facility (HIPROX)

The oxy-burner, shown in Figure 3, is an in-house designed axial swirl stabilized burner with a converging outlet of 20 mm throat diameter. The swirling flow is generated by axial vanes establishing a flow of oxidizer characterized by a swirl number of 0.74. It must be noted that the swirl number given is calculated based on the vanes geometry following the methodology given in Lefebvre29. However the placement of the swirl module, being well upstream the burner exit plane, makes that the effective swirl at the flame stabilization point is not known and is certainly lower due to the developing flow in the channel. The swirl number is however only a characterization of the cold flow, the combustion occurring at varying temperature depending on O2C affects greatly the flame shapes. The burner has a cylindrical center body conveying the


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gaseous fuel (methane) and ending a few millimeters before the burner exit plane. Fuel is injected near the tip of the center body through multiple holes. Oxygen is injected in the CO2 stream upstream the swirl generator. In this configuration the fuel and oxidizer are rapidly mixed in the high velocity region leading to the burner exit plane. This zone acts as a barrier to flashback and even though the flame cannot travel further upstream the tip of the centre body where the fuel injection holes are situated, it is not desired that the flame be anchored on the center body due to potential overheating.

Figure 3. Burner - optical combustor – dilution section assembly

The oxy-burner sits at the bottom end of an optical combustor made of two concentric quartz tubes with an inner diameter of 50 mm and length of 200 mm. The exit section of the optical combustor is connected to a square section double walled metal channel through a converging transition section as shown in Figure 3. The combustor and metal channel is placed in a vertically arranged pressure vessel with 10 bar pressure at 200°C wall temperature capability. The pressure vessel has optical accesses through four perpendicular flat windows. The dilution


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CO2 stream injected in the pressure vessel serves as a cooling medium of the outer pressure shell and optical accesses. It then flows in the volume formed by the double wall arrangement of the metal dilution channel in reverse direction as compared to the flame flow. The total cooling CO2 flow is then introduced in the volume formed by the double wall arrangement of optical combustor. The cooling CO2 flow is finally injected into the combustion product gases downstream the optical combustor through multiple holes placed all around the transition section as indicated in Figure 3. Just before the CO2 cooling flow meets the flame products, its temperature has raised to approximately 300 °C, corresponding to an extracted heat of approximately 20% of the input fuel power. This arrangement would correspond to the secondary dilution holes in gas turbine combustor technology. The metal channel is instrumented with pressure and temperature probes. The combustor pressure is controlled by a water cooled back-pressure valve placed at the up end of the metal channel.

RESULTS Flame stability Flame stability of a burner is generally characterized as a function of its operating parameters, that are fuel power input and equivalence ratio, as representative of bulk flow velocity through the burner and combustion chemistry respectively. Maps showing limits between stable operation, blow-off, and flashback are traditionally built according to these parameters. In oxyfuel systems, the oxygen and the working fluid (CO2) are controlled independently from one another. Consequently equivalence ratio and adiabatic flame temperature are not linked and can vary independently through variation of the CO2 flow rate in the oxidizer mixture. This added degree of freedom affects the burner operation through the decoupled dynamic and chemical effects and adds complexity in the definition of stability behavior of oxy-fuel flames. To


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characterize stability in oxy-fuel mode, it is therefore necessary to define other parameters in addition to the fuel power input, P. Two variables are defined as follows: ̇ 𝐸𝐸𝐸𝐸𝐸𝐸 (%) = ��𝑞𝑞̇ 𝑂𝑂2 − 𝑞𝑞𝑂𝑂2,𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠ℎ ��𝑞𝑞̇ 𝑂𝑂2,𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠ℎ )� ∙ 100 𝑂𝑂2𝐶𝐶 (%) = �𝑞𝑞̇ 𝑂𝑂2⁄�𝑞𝑞̇ 𝑂𝑂2 +̇ 𝑞𝑞̇ 𝐶𝐶𝐶𝐶2 �� ∙ 100

where EXO is the oxygen excess relative to stoichiometric conditions, O2C is the oxygen concentration in the oxidizer mixture composed of O2 and CO2, and 𝑞𝑞̇ 𝑖𝑖 the flow rate of gas i.

Note that in an oxy-fuel power cycle it is highly desirable to hold EXO as close to zero as possible in order to minimize the energy penalty incurred by the air separation unit.

The oxy-burner used in this study is swirl stabilized and was found to exhibit different stabilization patterns. The blow-off limit can easily be determined, but for this burner the flashback could not be detected straightforwardly as temperature is not monitored at the burner tip and tip is slightly retracted with regard to the throat. Three sustained flame modes were observed as shown in Figure 4. These modes are categorized as follows: 1.

V-shaped (attached flame)

2.

M-shaped (lifted flame with combustion in outer recirculation zones)

3.

Lifted V-shaped a)

b)

c)

Figure 4. The flame modes: a) Vshaped mode; b) M-shaped mode; c) Lifted V-shaped mode


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The V- and M-shaped modes are commonly seen in swirl flames30, where the latter is characterized by a strong internal recirculation zone (IRZ) very near the burner outlet and outer recirculation zones (ORZ) above which the flame is partially stabilized. V-shaped flames exhibit longer IRZ localized further downstream the burner exit. These modes represent the general flow patterns often found in swirl flows, however there exist several variations depending on the burner configuration and swirl strength. One important difference as shown in Oberleithner et al.30 is that the M-shaped flame contains a precessing vortex core which on one hand can improve mixing, on the other hand potentially be source of thermo-acoustic instability. The Vshaped mode is a very stable flame, but is dangerously close to flashback and can lead to overheating of the burner. The M-shaped mode is in that respect more stable and safer in terms of burner integrity. The lifted-V mode is a more unstable flame which is prone to blow-off. Both M and lifted-V modes have lifted flames, but in the latter, the ORZ do not accomplish flame stabilization as the levels of strain must be too high with regard to the local combustion strength. Figure 5 shows the regions of the modes observed during experiments in the fuel power – O2C space at atmospheric pressure. The borders were identified by running with a stable M-shaped flame and gradually vary O2C by adjusting the CO2 flow to the oxidizer until changes and blowoff was observed. In the atmospheric case of Figure 5, this was performed at 10, 20, 30 and 40 kW. The lowest border represents the blow-off limit, where the flame could not be stabilized under this oxygen concentrations limit. This limit increases naturally with power as the bulk velocity increases. At atmospheric pressure and within this range of power, an oxygen concentration of about 31.7 % is appropriate to obtain a stable flame. Further increasing O2C in the oxidizer mixture further strengthen the combustion through higher flame temperature and


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flame speed, and eventually leads to attachment of the flame inside the burner throat. The transition is accompanied by a modification of the flow pattern and distribution of the IRZ and ORZ. This further effect is amplified when O2C increases as a result of decreasing CO2 volume flow, hence bulk velocity of the oxidizer.

40 V-shaped

35 O2C (%-vol)

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M-shaped

30

Lifted V-shaped

25 Blow-off

20 10

15

20

25 30 Fuel power (kW)

35

40

Figure 5. Flame stability map in oxygen concentration – fuel power space at P = 1 bar, EXO close to stoichiometry

Figure 6 shows the same limits, but at a constant fuel power of 40 kW and pressures from 1 to 5 bar. The borders in this graph were obtained from experiments run at 1, 2, 3, 4 and 5 bar. It can be noticed that the high flame without outer recirculation (lifted-V mode) disappears at pressures above 2 bar, and that blow-off takes place directly from the M-shaped mode. The lower part of this mode can therefore not be considered as a stable flame at these conditions. The green area narrows down at higher pressure and at 5 bar it seems that 40 kW is about the minimum power the burner can handle. If the power was increased, the green area would be expected to broaden as the border towards the V-shaped area should move upwards with increased velocity. The bulk velocity at the burner throat decreases from about 40 m/s at 1 bar to


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about 8 m/s at 5 bar pressure. Although the flame speed tends to decrease with pressure9, the rate of decrease is not strong enough to compensate for the decrease in velocity due to density change. The behavior shown in Figure 6 of the lifted-V mode highlights that this is a inherently unstable mode. The maps shown in Figure 5 and Figure 6 are needed to characterize the operation of the burner at all engine conditions, but should also be made at different EXO. However, the stability sensitivity to EXO is less than the corresponding equivalence ratio in air supported combustion. 40 V-shaped

35

O2C (%-vol)

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M-shaped

30

Lifted V-shaped

25 Blow-off

20 1

1.5

2

2.5

3

3.5

4

4.5

5

Pressure (bar)

Figure 6. Flame stability map in oxygen concentration – pressure space at 40 kW, EXO close to stoichiometry

CO formation In oxy-fuel combustion, the way CO emissions in the exhaust gases are expressed must be clearly defined in order to avoid confusion when comparing values with that obtained in conventional air supported combustion. In oxy-fuel mode there is, in the simplest of configurations, only oxygen present in the primary zone of the combustor, and not in any downstream dilution streams. Specific hardware concepts can of course involve oxygen staging strategy throughout the combustor. The amount of CO2 fed at the burner is a design and


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operational choice dependent on the oxygen concentration in the oxidizer (O2C) desired in the flame stabilization zone. Emissions correction to a reference O2 value in the exhaust is therefore not relevant in oxy-fuel mode because the information of equivalence ratio and inert dilution are not linked, as discussed previously in the section Flame Stability. In this study we propose to express CO emissions as the measured CO volume fraction corrected to a reference exhaust gas obtained at a given stoichiometry and global O2/CO2 volume ratio, where the total CO2 volume flowing through the reference gas turbine, i.e. the sum of CO2 flowing through the burner and that in the dilution zone. Note that this global ratio is not related to O2C previously defined as O2C is a design selected burner parameter, independent of the power cycle. 3 20 kW, 1 bar 30 kW, 1 bar

Corrected CO [% wet]

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40 kW, 1 bar

2

100 kW, 10 bar

1

0 -10

0

10

20

30

40

EXO[%]

Figure 7. CO emissions vs. excess O2 (EXO) at O2C = 34% for 1 bar and O2C = 27 – 30% for 10 bar case

The reference chosen is arbitrarily set at a stoichiometric amount of O2 (i.e. zero excess O2) and a global reactant O2/CO2 volume ratio of 14/86. This arbitrary reference is motivated by the facts that an ideal oxy-fuel combustor would operate with no O2 excess and the 14% of O2 in non-fuel reactants corresponds to a turbine inlet temperature close to that of the cycle described in Sundkvist et al.9. In that way, the CO emissions relate to a reference oxy-fuel gas turbine cycle


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exhaust gas composition, which is useful for comparison purposes and for the engineering of CCS downstream processes such as CO2 cleaning, compression and transport. This global O2/CO2 volume ratio is lower in oxy-fuel mode than in the corresponding O2/N2 ratio in air. As a result emission values will always appear larger in oxy-fuel mode with a typical factor of 1.5 for a given mass of emitted species per mass fuel. For this simple inlet volume flows reason the CO emissions are expected to be higher than for air combustion. In addition, true chemical effects due to the role of CO2 in the kinetic chemistry9, 20 and the low excess oxygen requirement for optimal cycle efficiency, can play unfavorably to the CO emissions. The CO emissions in the exhaust gases as a function of excess oxygen ratio are shown in Figure 7. The experiments shown at 20, 30 and 40 kW were measured at 1 bar, while the 100 kW experiments were obtained at 10 bar. CO formation is very sensitive to the oxygen stoichiometry and is naturally very high in under-stoichiometric conditions (EXO < 0). At EXO = 10%, which is considered to be high with regards to the efficiency penalty that would incur to the power cycle, CO emissions are still significant even though the 100 kW case shows better result. In terms of heating value, a corrected CO concentration of 1% corresponds to about 5 % of the LHV input power. Such a loss in combustion efficiency would not be acceptable. There are however a number of reasons why these values are over-estimated as compared to a real combustor. First, the optical flame tube made of quartz give rise to heat transfer characteristics which are quite different than in a real metal combustor. But more importantly, the dilution CO2 is introduced into the combustion products soon after the flame zone (cf. Figure 3) through multiple injection holes, aggressively cooling the gases down to 500 - 600°C (measured in the exhaust gases near the back pressure valve). This effectively quenches most chemical reactions such as the concentrations measured in the diluted exhaust gas are very close to the CO formed


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in the exhaust gas leaving the optical flame tube. In fact, for the 100 kW experiments, there was a lower CO2 dilution flow rate as compared to the lower power cases and the temperature in the diluted exhaust gases was higher (850 – 900°C). The higher decreasing rate of CO concentrations for the 100 kW case are in agreement with a lower quenching and better oxidation of CO occurring in the dilution section. Another favorable condition in the 10 bar case is the significant increase in residence time, also favorable to CO oxidation.

3

Corrected CO [% wet]

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2

20 kW

1

30 kW 40 kW Equilibrium at Tad. 0 25

30

35 O2C [%]

40

45

Figure 8. CO emissions vs. O2C at EXO = 0 (stoichiometric), P = 1 bar The dependence of CO on the oxygen concentration in the oxidizer flow can be seen in Figure 8, together with the equilibrium CO concentration at the corresponding adiabatic flame temperature. The equilibrium lines are calculated with the GASEQ chemical equilibrium program31. CO emissions first drop with increasing oxygen concentration from O2C = 27 % which is the lowest O2C achievable with a stable flame (lifted V-shaped) before blow-off. Some ppm unburnt methane were measured at these conditions, indicating low temperature and slow combustion. At higher O2C however, an increasing trend can be observed and no unburnt methane could be detected, following the same trend as the equilibrium concentration at


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Energy & Fuels

corresponding adiabatic flame temperature. The CO formation is therefore at least partly controlled by the equilibrium concentration of CO at flame temperature explaining the high values measured. 3 CO measured

Corrected CO (% vol)

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2.5

Equilibrium CO

2 1.5 1 0.5 0 0

1

2

3 Pressure (bar)

4

5

6

Figure 9. Corrected measured and equilibrium CO concentrations vs. combustor pressure. Power = 40 kW, EXO = 1.7%, O2C = 30%

Figure 9 shows the CO emissions and equilibrium values at adiabatic flame temperature as a function of pressure at a constant fuel power of 40 kW, O2C = 30 %, and EXO = 1.7%. At 1 bar the flame is in the lifted V-shaped mode and the high CO formation is probably a result of poor and incomplete combustion. Above 2 bar, a slight increasing trend with pressure can be observed while the equilibrium concentration indicates a slight decreasing trend. The residence time in the flame tube increases by a factor of about 5 (5.7 ms to 27.2 ms) from the lowest to highest pressure respectively. Increased residence time does not seem to lower the CO emission however. This may be an indication that the kinetics are close to equilibrium and lowering of CO levels can only be achieved by reducing the temperature through for example O2C. The experimental and calculation results in this discussion does take into account the influence of the flame structure inside the flame tube such as the recirculation rates in the near flame zones and


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Energy & Fuels

quantifications of the mixing quality, which would require more sophisticated in-flame measurement technique and better defined reactor network for the calculations. It brings however clear indications of the challenges oxy-burners in gas turbine conditions are faced to.

CONCLUSIONS The study presented focuses on the characterization of a swirl stabilized oxy-fuel gas turbine burner in a high pressure facility where stability and CO emissions have been analyzed. CO2 diluted O2 is introduced at the burner to create a swirling flow where the flame stabilizes. Depending on the degree of dilution (O2C) and input fuel power load (equivalently flow velocity), the stabilization pattern takes different modes depending on the location of internal and outer recirculation zones. The degree of stability of the flames has been mapped as a function of O2C, fuel power load, and pressures up to 10 bar. A critical aspect is the trade-off between stabilization and excessive temperature at high O2C with potential flashback and burner overheating. The levels of CO emissions found in these experiments are found to be quite high, and although it is partly due to the experimental configuration consisting of an optical flame tube made of quartz and aggressive dilution quenching downstream the flame zone, CO is an issue that need to be addressed at the early design stage of oxy-fuel burner - combustor systems. In a gas turbine combustor, the flow pathways and residence time will have to be managed as to allow for the CO kinetic chemistry to move against lower equilibrium concentrations. ACKNOWLEDGMENTS This publication has been produced with support from the BIGCCS Centre, performed under the Norwegian research program Centers for Environment-friendly Energy Research (FME). The


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authors acknowledge the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE and the Research Council of Norway (193816/S60). Funding Sources Research Council of Norway grant 193816/S60 (Centers for Environment-friendly Energy Research), Gassco, Shell, Statoil, TOTAL, ENGIE. REFERENCES 1. Anheden, M.; Burchhardt, U.; Ecke, H.; Faber, R.; Jidinger, O.; Giering, R.; Kass, H.; Lysk, S.; Ramstrom, E.; Yan, J., Overview of Operational Experience and Results from Test Activities in Vattenfall's 30 MWth Oxyfuel Pilot Plant in Schwarze Pumpe. Enrgy Proced 2011, 4, 941-950. 2. Komaki, A.; Gotou, T.; Uchida, T.; Yamada, T.; Kiga, T.; Spero, C., Operation Experiences of Oxyfuel Power Plant in Callide Oxyfuel Project. 12th International Conference on Greenhouse Gas Control Technologies, Ghgt-12 2014, 63, 490-496. 3. Perrin, N.; Dubettier, R.; Lockwood, F.; Tranier, J. P.; Bourhy-Weber, C.; Terrien, P., Oxycombustion for coal power plants: Advantages, solutions and projects. Appl Therm Eng 2015, 74, 75-82. 4. Stanger, R.; Wall, T.; Sporl, R.; Paneru, M.; Grathwohl, S.; Weidmann, M.; Schefflmecht, G.; McDonald, D.; Myohanen, K.; Ritvanen, J.; Rahiala, S.; Hyppanen, T.; Mletzko, J.; Kather, A.; Santos, S., Oxyfuel combustion for CO2 capture in power plants. Int J Greenh Gas Con 2015, 40, 55-125. 5. Perrin, N.; Paufique, C.; Leclerc, M., Latest Performances and Improvement Perspective of Oxycombustion for Carbon Capture on Coal Power Plants. Energy Procedia 2014, 63, 524531. 6. Stadler, H.; Beggel, F.; Habermehl, M.; Persigehl, B.; Kneer, R.; Modigell, M.; Jeschke, P., Oxyfuel coal combustion by efficient integration of oxygen transport membranes. Int J Greenh Gas Con 2011, 5 (1), 7-15. 7. Habib, M. A.; Nemitallah, M.; Ben-Mansour, R., Recent Development in OxyCombustion Technology and Its Applications to Gas Turbine Combustors and ITM Reactors. Energy & Fuels 2013, 27 (1), 2-19. 8. Sundkvist, S. G.; Julsrud, S.; Viyeland, B.; Naas, T.; Budd, M.; Leistner, H.; Winkler, D., Development and testing of AZEP reactor components. Int J Greenh Gas Con 2007, 1 (2), 180187. 9. Sundkvist, S. G.; Dahlquist, A.; Janczewski, J.; Sjodin, M.; Bysveen, M.; Ditaranto, M.; Langorgen, O.; Seljeskog, M.; Siljan, M., Concept for a Combustion System in Oxyfuel Gas Turbine Combined Cycles. J Eng Gas Turb Power 2014, 136 (10). 10. Kvamsdal, H. M.; Jordal, K.; Bolland, O., A quantitative comparison of gas turbine cycles with CO2 capture. Energy 2007, 32 (1), 10-24. 11. Climent Barba, F.; Martínez-Denegri Sánchez, G.; Soler Seguí, B.; Gohari Darabkhani, H.; Anthony, E. J., A technical evaluation, performance analysis and risk assessment of multiple


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29. Lefebvre, A. H., Gas turbine combustion. 2nd ed.; Taylor & Francis: Philadelphia, 1999; p xv, 400 p. 30. Oberleithner, K.; Stohr, M.; Im, S. H.; Arndt, C. M.; Steinberg, A. M., Formation and flame-induced suppression of the precessing vortex core in a swirl combustor: Experiments and linear stability analysis. Combustion and Flame 2015, 162 (8), 3100-3114. 31. Morley, C. GASEQ - A Chemical Equilibrium Program for Windows, 2005.


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Experimental Study of Oxy-fuel Combustion under Gas Turbine

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