Experimental Analysis of the Stability and Combustion Characteristics

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Combustion

Experimental Analysis of the Stability and Combustion Characteristics of Propane-Oxyfuel and Propane-Air Flames in a Non-Premixed, Swirl-Stabilized Combustor Zubairu Abubakar, Yinka S. Sanusi, and Esmail M.A. Mokheimer Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01819 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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Experimental Analysis of the Stability and Combustion Characteristics of Propane-Oxyfuel and Propane-Air Flames in a Non-Premixed, Swirl-Stabilized Combustor Zubairu Abubakara, S. Y. Sanusia and Esmail M. A. Mokheimera,b a

Mechanical Engineering Department, College of Engineering, King Fahd University of Petro;eum and Minerals, Dhahran 31261, Saudi Arabia b Corresponding Author Mailing Address: P.O. Bix: 279, Dhahran 31261, Saudi Arabia , e-mail: [email protected], Tel.: +966138602959, Fax: +9668602949

Abstract Carbon-Dioxide (CO2) emission forms the biggest portion of greenhouse gases emissions known to cause global warming which can lead to climate change. One of the most widely recommended means of tackling CO2 emission is carbon capture technique which includes oxyfuel combustion. In oxyfuel combustion, O2/CO2 oxidizer mixtures are utilized to lower the oxy-combustion temperatures in order to make it suitable for the components of the combustion systems. These oxidizer mixtures, depending on the relative concentrations of the species, exhibit distinct combustion characteristics. In this study, flame stability of propane-air and propane-oxyfuel combustion is studied in a non-premixed, swirl-stabilized combustor. The combustion of air was compared to two oxyfuel mixtures namely oxyfuel I and II in terms of lean blowout limits. Oxyfuel II and air combustion were also compared in terms of temperature. Furthermore, the effects of CO2 dilution level, equivalence ratio, swirl number, and combustor firing rate on oxyfuel flame stability were studied. Results show that, for lean mixtures, the propane-air flame transits from attached-flame to lifted-flame before subsequent flame extinction. This is contrary to oxyfuel I and II flames that transit directly from attached-flame to no-flame regime at all firing rates studied. Near stoichiometry, however, the oxyfuel flames display distinct flame transitions including liftup before extinction as a consequence of CO2 dilution at high firing rates. These flame transitions before blowout were observed to be flowinduced. NOx and CO emissions were seen to depend strongly on air and oxyfuel combustion temperatures. The amount of CO2 required in the oxidizer at blowout was observed to decrease significantly as the equivalence ratio decreases from 1 to 0.9 signifying an enhanced stability at stoichiometric conditions. Further studies revealed that the oxyfuel flames are more stable at swirl number of 1.0 when compared to 0.6 and 1.5.

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1.

Introduction

The environmental concern from fossil fuels utilization in energy generation is primarily the results of emissions from their combustion. These emissions include greenhouse gases like Carbon-Dioxide (CO2) known to cause global warming and ultimately a potential for climate change. It is these environmental concerns, coupled with an ever growing demand for energy among other things, that gave rise to the renewed attention in more efficient and eco-friendlier combustion of fossil fuels.1 Oxyfuel combustion as one of the means for tackling CO2 emission is a branch of Carbon Capture and Sequestration (CCS) technology that uses pure oxygen in combustion of fossil fuels, so that the products will be mainly H2O and CO2. The CO2 in the combustion products, therefore, can be captured and sequestered after condensation of H2O from the gas stream.2 There are many challenges associated with CCS in terms of capture, transportation, and storage of the CO2 from both technical and economic standpoints. However, available practices like the utilization of the captured CO2 in enhanced oil recovery (EOR) is a promising step towards improved CCS.3 Subsequently, with oxyfuel combustion, cleaner combustion of fossil fuels including economically-under-utilized, highly-carbonaceous fuels like residual oil, coal and other heavy fuels becomes more viable.4 Pilot scale retrofitted plants utilizing oxyfuel combustion, like Doosan Babcock in the United Kingdom are already in operation, and are reporting CO2 rich flue gas that could be a very good candidate for CCS.5 Because of high temperatures associated with oxyfuel combustion in a stream of pure oxygen, use of diluent is necessary in order to lower these excessive temperatures to limits suitable for components in combustion systems. An obvious choice of such diluent is recycled CO2 from the flue gases to form O2/CO2 oxidizer employed in oxyfuel combustion.6 Wei Luo et.al7 explored the use of a control system in flue gas recirculation in order to control the effect of quality and characteristics of the flue gas using dynamic exergy evaluation technique. Oxyfuel flame stability is greatly dependent upon the concentration of CO2 vis-à-vis O2 in the O2/CO2 oxidizer mixture. As a result of differences in optical and thermodynamic properties of CO2 and N2, the amount by volume of oxygen in O2/CO2 oxidizer should be higher than 21% known for air.8,9 Literatures reported up to 30% oxygen in O2/CO2 oxidizer required for similar characteristics between air and oxyfuel combustion.10,11


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Flame instability is a consequence of the competition between rate of reaction and the rate of diffusion of species and energy.12-14 Static stability limit of combustors is characterized by their blowout propensity in which flame goes extinct in the combustor. Flame blowout is sometimes preceded by flame liftup when a flame gets detached from its anchoring position and stabilized at a distance downstream. The mechanism driving blowout phenomena is when the time required for the chemical reaction in combustion to occur is longer than the propagation rate of the combustion species.15 Critical Damkohler number also known as mixing time scale can be used to predict the range of static stability for a certain range of combustion conditions. It is defined as the ratio of residence time to chemical time at blowout. Damkohler number can be expressed as in equation (1):

Damkohler number, Da =

( )

!" (#$% )

=

&'()

*+,- .

(1)

The residence time is quantified by the ratio of a length scale (L) to velocity scale (reference velocity; Uref). Chemical time, on the other hand, is quantified by the ratio of thermal diffusivity (α) to the square of laminar flame speed (/&0 ). The expression in equation (1) can also be written using the Arrhenius kinetic rate formula as:

12 ~

4+,5 .

exp 8

9:;

(2)

It is important to operate combustors at stoichiometry when employing oxyfuel combustion to avoid the use of excess oxygen produced via costly separation techniques. Also, at stoichiometry, concern related to instabilities resulting from lean combustion is eliminated. Hence the most important parameter related to oxyfuel flame stability is the CO2 dilution level in O2/CO2 oxidizer mixtures. Kutne et.al9 reported a huge role played by CO2 diluent on oxyfuel flame stability in their study focusing on O2 percentage between 20-40%. Also, oxygen concentration in O2/CO2 oxidizer was reported to affect significantly the production and consumption rates of CO in oxyfuel combustion products.16,17 Higher CO2 concentration in the oxidizer was reported to achieve in-combustor desulphurization. This fact can be exploited in oxyfuel combustion of oil shale having high sulfur contents.18 Other oxyfuel combustion literatures include numerical work by Krieger et.al19, gas turbine application of oxyfuel combustion by Liu et.al20, as well as


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the thermodynamic cycle analysis for oxyfuel combustion conducted by Hammer et.al21. It is remarkable, however, that most of the available literatures on oxyfuel combustion focuses on coal, apparently, because of its comparatively higher carbon content, making its combustion CO2-rich, and because of coal’s potential in IGCC plants. This includes comprehensive review on the state of the art oxy-fuel combustion fundamentals, their modeling, and models implementation22 and other studies dealing with oxyfuel combustion of coal23-28. Some literatures are also available on oxyfuel combustion of methane, with other higher carbon fuels like propane receiving little attention. This study aims to add to oxyfuel combustion literature of propane fuel. It is worth mentioning here that propane fuel has been used in the present study due to its higher carbon contents compared with that of methane. Selecting a fuel with high carbon contents is intended in this study of oxyfuel combustion, which is one of the carbon capture and sequestration (CCS) technologies that are usually recommended for fuels with high carbon contents (such as coal) since the higher carbon content in the fuel increases the amount of CO2 to be captured from oxyfuel combustion exhausts, which justifies the extra cost of using oxyfuel combustion. This is also particularly desirable in oxyfuel combustion with carbon capture that targets the utilization of the captured CO2 in processes like enhanced oil recovery. Moreover, propane as a gaseous fuel was easy to avail and handle in our experiment compared with coal, which is not used for power production and industry in the middle east countries, which have abundance in oil and gas. The study was conducted experimentally in a swirl-stabilized, nonpremixed combustor. Different O2/CO2 oxidizer mixture was employed to study propane-oxyfuel flame stability under different combustor firing rates and different swirl numbers. The lean stability limits, combustor temperature distribution, as well as emissions from propane-oxyfuel and propane-air combustion are also compared and discussed.

2.

Experiment

The experimental set-up used in this study is shown in Figure 1 (a, b). The combustion system uses an atmospheric, non-premixed, swirl-stabilized cylindrical combustor of 70 mm diameter quartz glass tube, which is 300 mm long. The gases having purity of 99.99% used throughout the experiments were supplied from cylinders via mass flow controllers with an uncertainty of ± 0.5% manufactured by Bronkhorst High-Tech. Images of the flames were taken using a NIKON digital camera and were characterized as time averaged flames macrostructures. The emissions 4 ACS Paragon Plus Environment

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were measured with “TESTO 350 combustion and emission analyzer” having a dilution factor of ×5 for CO measurements. Temperature was measured using an R-type thermocouple. Details of the combustor head depicting the swirler and the fuel supply nozzle is shown in Figure 2. The axial swirler is located upstream of the combustor. Three different swirlers having estimated swirl numbers of 0.6, 1.0, and 1.5 based on their vane angles (30, 45, and 55 degrees, respectively) were used in this study for flame stabilization. The fuel nozzle is a 6.35 mm diameter pipe having 16 fuel channels (0.13 mm × 0.45 mm) around a centered bluff body of 5 mm. The two other oxidizer mixtures used in the study for comparison with air were named Oxyfuel I and Oxyfuel II. Oxyfuel I was fixed to have 21% oxygen concentration by volume like air with the remaining 79% being CO2. Oxyfuel II on the other hand has 31% oxygen concentration by volume in the O2/CO2 oxidizer mixture was chosen because at that ratio, the mixture has similar adiabatic temperature with air-propane combustion.


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Figure 1(a): Schematic of experimental set-up

Figure 1(b): The combustor cross-section, dimensions are in mm

Figure 2: Details of the combustor head, dimensions are in mm


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3.

Results and Discussions

3.1

Air versus Oxyfuel Combustion

3.1.1

Flame stability

Flame transitions images depicting flames macrostructure for air as well as Oxyfuel II combustion are shown in Figures 3 and 4, respectively. The images were obtained for swirl number 0.6 at a fixed firing rate of 5 MW/m3 by decreasing gradually (by 0.01), the global equivalence ratio (Φ) until flame extinction, while monitoring the flame transitions. This equivalence ratio sweep test was conducted from stoichiometric conditions to equivalence ratio at flame blowout for the two oxidizers. For propane-air, at stoichiometry, the flame’s primary reaction zone manifested as a deep blue flame anchored on the fuel nozzle around which a reddish swirling plume was observed as seen in Figure 3. The reddish plume has been previously reported to be resulting from carbon present as a consequence of incomplete combustion29. As more oxidizer was supplied by decreasing the equivalence ratio, more oxygen is made available for combustion, and consequently the plume was observed to vanish completely around Φ = 0.8 forming a v-shaped flame at Φ = 0.7 as shown. Further decrease in Φ weakens the flame base leading to its liftup at Φ = 0.62, forming an M-shaped flame, suggestively stabilized in outer recirculation zone formed by the swirling flow. Subsequent flame transitions occurs at even lower equivalence ratio forming oscillating, cone-shaped flame at Φ = 0.5 that further elongates and continue to thin out in the base at Φ = 0.48, until its eventual blowout recorded at Φ = 0.47. The macrostructure of the Oxyfuel II flame does not change with equivalence ratio as it remains compact and v-shaped irrespective of the equivalence ratio as shown in Figure 4. Also, unlike propane-air flames, oxyfuel II flames do not undergo any transition in macrostructure until blowout. It is worth noting here that despite having the same adiabatic flame temperatures, air and oxyfuel II combustion display different flame dynamics. This could be attributable to differences in heat loss characteristics from the two flames. Moreover, the reddish luminescence observed for airpropane combustion was also observed for oxyfuel case having pure oxygen without dilution. However, with CO2 dilution, e.g. oxyfuel II case, no reddish luminescence was observed even at


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stoichiometric conditions. This is attributed to the fact that the added CO2 has the effect of enhancing the mixing and consequently helps achieving complete combustion in the combustor.

Figure 3: Flame transitions of propane-air at firing rate of 5MW/m3 and 0.6 swirl number

Figure 4: Flame transitions of propane-oxyfuel II at firing rate of 5MW/m3 and 0.6 swirl number

In Figure 5, the stability map depicting the blowout limits for the three different oxidizers (Air, Oxyfuel I, and Oxyfuel II) is shown with an inset showing the flame-no-flame regions of the profiles. For a swirl number of 0.6, and for each firing rate studied, the blowout points were obtained by increasing continuously the oxidizer flow rate until flame blowout occurs. The 8 ACS Paragon Plus Environment

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blowout points for propane-air and Oxyfuel II flames were observed to be very close to each other compared to those of Oxyfuel I particularly at low firing rate. This is expected as Oxyfuel II mixture has similar adiabatic flame temperature with propane-air combustion. As can be seen in the figure, for propane-air, the blowout points are nearly constant irrespective of the firing rate. This suggests its strong dependence on heat loss from the reaction zone such that the flame goes extinct at nearly the same adiabatic flame temperature. For Oxyfuel II flames, however, equivalence ratio at blowout increases marginally as the firing rate increases indicating a departure from the propane-air cases. This suggests that heat loss from the flames is different resulting from higher amount of CO2 in the oxyfuel II cases. In other words, while the two flames might have the same rate of liberation of heat by chemical reactions, their respective rate of heat loss is not the same. The same trend is observed for oxyfuel I flames, with the blowout equivalence ratio, however, being far higher than those of oxyfuel II flames. This is consistent with the results reported by Kutne et.al9 and is a consequence of higher amount of CO2 diluent in oxyfuel I flame. In Figure 6 the stability map for propane-air combustion is shown depicting the blowout as well as liftup points at different firing rates. The liftup phenomenon was observed to occur at a constant velocity ratio of 0.58 (ratio of fuel velocity to oxidizer velocity) irrespective of the combustor firing rate suggesting that it is flow induced. Figure 7 depicts the blowout points for Oxyfuel I and II cases as a function of the oxidizer Reynold’s number for a swirl number of 0.6. Increase in fuel Reynold’s number is accompanied by a corresponding increase in oxidizer Reynold’s number at blowout.


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Figure 5: Blowout points for air and oxyfuel flames at 0.6 swirl number

Figure 6: Blowout and Liftup point for propane-air combustion at swirl number 0.6

Figure 7: Fuel and oxidizer Reynold’s number at blowout for oxyfuel flames at 0.6 swirl number

The superior stability window exhibited by air combustion over the oxyfuel cases results from differences in properties of N2 and CO2 in the oxidizers respectively. This is because flame speed of O2/CO2 mixtures combustion is lower than that of air combustion at the same level of oxygen concentration and within certain range. The lower flame speed is due to lower values of thermal 10 ACS Paragon Plus Environment

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diffusivity (?) and flame temperature (@A ) in the O2/CO2 mixtures compared to those in air (O2/N2) mixture, all of which affects the Damkohler number as evident in equations (1) and (2) 30

. Also at high concentrations, CO2 was reported by Heil et.al16 to participate in the reaction

with the effect of lowering burning rates. This effect, however, dampens at high O2 concentrations in the mixture. This direct link between the stability limits and flame speed explains the extended stability limits exhibited by both air and oxyfuel flames with addition of hydrogen as shown in Figures 8 and 9, respectively. The figures were obtained for air at a fixed firing rate of 3.5 MW/m3 with hydrogen content in the fuel varied from zero (100% propane) to 40% H2 by volume, and oxyfuel at a fixed firing rate of 2.5 MW/m3 and stoichiometric equivalence ratio (at fixed equivalence ratio, only CO2 was added until flame blowout). The higher flame speed of hydrogen helps enhance the flames stability when hydrogen was added in the fuel as evident from higher oxidizer Reynold’s number at flame blowout observed in the figures.

Figure 8: Stability map for air combustion with hydrogen addition at firing rate of 3.5MW/m3 and 1.0 swirl number


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Figure 9: Stability map for oxyfuel combustion with hydrogen addition at firing rate of 2.5MW/m3, 1.0 swirl number, and stoichiometric equivalence ratio

3.1.2

Temperature Profiles and Emissions

Figures 10 and 11 depict the axial temperature distributions for air and Oxyfuel II combustion, respectively, measured along the centerline of the combustor at the distance of 150-250 mm from its dump plane. The profiles were obtained at a fixed firing rate of 5 MW/m3, swirl number of 0.6, and at different equivalence ratios as shown. For all the profiles, decreasing the equivalence ratio decreases the temperature as expected, owing to the cooling effect resulting from supplying more oxidizer at lower equivalence ratio. Also, the temperature in both cases decreases downstream of the combustor, signifying heat loss by convection and radiation from the combustion gases as they move towards the exhaust. Figure 12 gives a comparison between air and oxyfuel II axial temperature profiles at a fixed firing rate of 5MW/m3, 0.6 swirl number, and 0.8 global equivalence ratio. The temperatures were measured along the centerline of the combustor at the distance of 150-250 mm from the combustor’s dump plane. Although, oxygen concentration in oxyfuel II oxidizer mixture was chosen specifically to have the same adiabatic flame temperature with propane-air combustion, the air combustion resulted in higher temperatures downstream in the combustor as shown. This is attributable mainly to high thermal 12 ACS Paragon Plus Environment

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radiation of CO2 keeping in mind the higher CO2 concentrations in the oxyfuel gases. Subsequently, more heat loss by radiation and hence lower temperature is recorded for oxyfuel II case compared to the propane-air case.

Figure 10: Axial temperature distribution for air combustion at firing rate of 5MW/m3 and 0.6 swirl number


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Figure 11: Axial temperature distribution for oxyfuel II combustion at firing rate of 5MW/m3 and 0.6 swirl number

Figure 12: Axial temperature distribution comparison air and for oxyfuel II combustion at firing rate of 5MW/m3, 0.6 swirl number, and Φ of 0.8.

In Figure 13 and 14, NOx and CO emissions are shown as a function of equivalence ratio for air and oxyfuel combustion respectively. For the air combustion case, the NOx emission was obtained close to the combustor exit at a fixed firing rate of 4 MW/m3 and swirl number of 1.0. As can be seen in Figure 13, NOx emission increases with increase in equivalence ratio. This is because increasing the equivalence ratio increases the temperature as we have seen earlier in this section. Consequently, higher air combustion temperatures resulted into production of more thermal NOx via Zeldovich mechanism. Figure 14 shows CO emission from oxyfuel II at a fixed firing rate of 2 MW/m3 and swirl number of 1.0 recorded near the combustor exit. As expected, lower temperatures corresponding to lower equivalence ratios are associated with high CO emission. This is because at lower temperatures, the conversion of CO into CO2 gets dampened due to cooling. For oxyfuel combustion therefore, operating the combustor near stoichiometry is beneficial not only in minimizing the costly, excessive oxygen usage in the process, but also in targeting exhaust gases suitable for carbon capture and sequestration.


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Figure 13: NOx emission from propane-air combustion at firing rate of 4MW/m3 and swirl number of 1.0

Figure 14: CO emission from oxyfuel II combustion at firing rate of 2MW/m3 and swirl number of 1.0

3.2

Effect of CO2 dilution on oxyfuel combustion stability

In the previous section, we have discussed the stability of propane-air and propane-oxyfuel flames at lean global equivalence ratios (lean blowout limits). There is a strong motivation in 15 ACS Paragon Plus Environment

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lean combustion involving air oxidizer for NOx reduction. This motivation, however, does not extend to oxyfuel combustion utilizing high-purity oxygen since there is almost no nitrogen present in the oxidizer, and more importantly, because of the penalty to be incurred for excessive usage of oxygen due to cost of its production. Consequently, for oxyfuel combustion, operating at stoichiometric equivalence ratio is beneficial in reducing oxygen usage as well as eliminating lean-combustion-related instabilities. Oxyfuel combustion, however, is associated with excessive temperatures, posing material and structural safety concerns in combustion systems. This concern for excessive combustion temperatures can be mitigated by employing CO2 as a diluent, so that O2/CO2 oxidizer mixture is formed for oxyfuel combustion. Therefore, in this section, we investigate the effect of CO2 concentration in O2/CO2 oxidizer mixture on oxyfuel flame stability near stoichiometric equivalence ratio. In the experiments carried out, starting with 100% oxygen at a certain equivalence ratio and firing rate, CO2 was added continuously while monitoring the flame transitions until flame blowout. Figure 15 shows transitions in flame macrostructures represented by images obtained at different CO2 dilution level, fixed firing rate of 5 MW/m3, swirl number of 0.6, and stoichiometric equivalence ratio. Unlike the abrupt flame extinction observed earlier for oxyfuel I and II ensuing from low equivalence ratios, at high firing rates and around stoichiometric equivalence ratios CO2 dilution resulted into distinct flame transition, leading to blowout at high CO2 concentrations, as can be seen in the figure. At zero CO2 dilution level (100% oxygen), an elongated, annular, jet-like flame having reddish luminescence was observed. As the CO2 dilution level increases, the flame becomes shorter and more compact with a V-shape. This is because the addition of CO2 increases the oxidizer flow rate resulting into enhanced turbulent mixing in the combustor, thereby enhancing the combustion process. The CO2 dilution is known to have the effect of decreasing flame temperature and burning velocity31. Reduced temperature and burning velocities in flames, as well as high strain rates imposed by the flow on the flame can lead to reduction in the flame’s resistance to extinction. Consequently, at higher CO2 dilution level, the flame resistance gets weakened causing it to liftup and stabilized at a distance away from the fuel nozzle at 76% CO2 dilution level. Further increase in the CO2 causes the elongation of the lifted flame, forming an oscillating, swirling, cone-like flame, that blowout at 79% CO2 in the (O2/CO2) oxidizer mixture.


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Figure 15: Propane-oxyfuel flame transitions at different CO2 dilution levels, firing rate of 5MW/m3, swirl number of 0.6, and stoichiometric equivalence ratio Figure 16 shows the stability map in terms of Reynold’s number obtained at stoichiometric equivalence ratio and swirl number of 0.6 for propane oxyfuel flame. From Figure 16, it can be observed that the stability window widens (high oxidizer Reynold’s numbers at flame blowout) at high fuel Reynold’s number. This can be attributed to greater heat release rates at higher firing rates. Also, it was observed that the flame transit directly from attached-flame to no-flame regime at combustor firing rate of

Experimental Analysis of the Stability and Combustion Characteristics

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