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Characteristics of oxy-fuel combustion in gas turbines
Applied Energy 89 (2012) 387–394
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Characteristics of oxy-fuel combustion in gas turbines C.Y. Liu a,b, G. Chen b, N. Sipöcz c, M. Assadi c, X.S. Bai a,⇑ a
Division of Fluid Mechanics, Lund University, 221 00 Lund, Sweden Faculty of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China c Faculty of Technology and Natural Science, University of Stavanger, Norway b
a r t i c l e
i n f o
Article history: Received 26 April 2011 Received in revised form 27 July 2011 Accepted 2 August 2011 Available online 7 September 2011 Keywords: Non-premixed oxy-fuel flames Flame quenching Oxy-fuel/CO2 cycles Gas turbines
a b s t r a c t This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH4, O2, CO2 and H2O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Oxy-fuel combustion has attracted the attention in power production industry and academic society due to its potential of having practically zero emissions of NOx and CO2, thus presenting an important option to address the environmental issues, in particular green house gas CO2 emissions. Oxy-fuel processes use oxygen instead of air for combustion of fuels, and use dilution agents such as CO2 or steam for flame temperature control and material cooling. Oxygen may be obtained via air separation units, e.g. cryogenic or membrane based processes. The combustion process takes place in nitrogen free or low-nitrogen environment resulting in a flue gas composed mainly of CO2 and H2O, as well as a low concentration of impurities such as argon and oxygen. Therefore, a simplified flue gas processing by means of condensation of H2O to capture CO2, without using costly separation methods such as chemical absorption, can be possible. There are several proposed combined cycle concepts in oxy-fuel gas turbine processes with natural gas combustion in oxygen and CO2, for example, the O2/CO2 cycle [1–3], the COOLENERG cycle [4], the COOPERATE cycle [5], and the MATIANT cycle [6]. These cy⇑ Corresponding author. E-mail address: [email protected] (X.S. Bai). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.08.004
cles belong to the group also known as Semi-Closed Oxy-Fuel Combustion Combined Cycles (SCOF-CC). Recent studies within the European Union funded research project ENCAP (Enhanced Capture of CO2) have concluded that SCOF-CC has good potential with limited techno-economical hinders for realization [7,8]. Besides SCOF-CC a number of other oxy-fuel cycles using steam/water as working fluids have been proposed including the Graz cycle [9], and the Water cycle [10] developed by Clean Energy Systems (CES). These cycles may require high temperature turbines and new design for the turbomachinery. Oxy-fuel combustion technology is still in the research and development stage. The European project ENCAP sets up goals to prepare technologies for power companies to be able to launch new design projects by 2010 aiming at large demonstration plants with the potential for wide commercial exploitation in 2015–2020 [11]. For oxy-fuel gas turbine cycles, researches have been focused on thermodynamic studies of system performance. The combustion behavior, e.g. the flame dynamics and reaction zone structures in the gas turbine combustors, is less addressed. From thermodynamic studies it has been shown that a small amount of trace species in the combustion products can have a great impact on the CO2 capture, storage and transportation. Li et al. [12] demonstrated that the purification process of the flue gas stream of oxy-fuel combustion is highly influenced by the existence of impurities such as
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the small amount of N2 resulted from the air separation units, and the remaining O2 in the flue gas due to incomplete combustion. The presence of non-condensable gases results in increased condensation duty for the recovery of the CO2. This in turn leads to lower system efficiency and increased cost for separation. To minimize the oxygen concentration in the flue gas and meanwhile achieve complete combustion of fuel, stoichiometric mixture is preferred in oxy-fuel combustion. CO2 and/or steam are used to control the flue gas temperature. Jericha and Göttlich [13] outlined a burner and combustor configuration, in which fuel, oxygen and steam were supplied separately in different inlets. The steam was supplied through an annular outer swirler inlet to form a swirling flow motion to wrap the flames and to cool down the flue gases. Such combustor configuration would likely generate rather high flame temperature locally in the reaction zones that will enhance the dissociation of H2O and CO2 and thus affect the composition of the flue gas such that the un-consumed oxygen can be high in the flue gas. To reduce the flame temperature and thereby the remaining oxygen in the flue gas it can be beneficial to premix the oxygen and CO2 or steam before injecting them to the combustor. There are several possibilities that need to be explored for example, different levels of premixing of the fuel/oxygen/steam/CO2 prior to their injection into the combustor, and different mixing patterns inside the combustor. The thermodynamic studies will give the same answer for the flue gas in the postflame zone if the inlet temperature, combustor pressure and the overall mass flows of fuel, oxygen, steam, and CO2 streams are kept the same. However, the flame dynamics and reaction zone structures are dependent on combustor configurations as they are dictated by the detailed inflow conditions for the fuel/oxygen/ steam and CO2 supplies. The aim of the present paper is to investigate the thermodynamic and combustion characteristics of oxy-fuel combustion under conditions related to gas turbine operation, by performing chemical kinetic simulations for different combustion conditions, including various levels of oxygen/diluent ratio in the oxidizer stream. Under typical gas turbine combustion conditions the chemical reaction zones may be considered as local laminar flamelets, whereas under extremely high turbulence conditions, the entire combustion chamber may be modeled as a well-stirred reactor. Both combustion modes are simulated using detailed chemical kinetic mechanisms, GRI-Mech 3.0 [14], and detailed transport modeling.
Table 1 Fuel and oxidizer compositions (volume basis) and baseline operation conditions. Parameters
Baseline reference conditions Case 1
Case 2
Case 3
Oxidizer stream CO2 (mol%) O2 (mol%) H2O (mol%) Mass flow (kg/s) Temperature (K)
87.95 8.54 3.51 10 520
81.09 15.47 3.44 8 520
84.59 15.41 0 560 600
Fuel stream CH4 (mol%) Mass flow (kg/s) Temperature (K)
100 0.16 288
100 0.235 288
100 16 423
Combustion chamber Pressure (bar) Overall equivalence ratio
17 1
17 1
40 1
The chemical kinetic mechanism used in this study is GRI-Mech 3.0 mechanism [14]. The GRI mechanism has been tested and validated comprehensively for methane and natural gas combustion over a wide range of pressure and temperature conditions. It contains 53 species and 325 reactions. Three combustion modes are considered in this study. The first is the thermodynamic equilibrium mode. This corresponds to the ideal case of chemical equilibrium combustion in the post-flame zone far downstream in the combustion chamber. Calculations are carried out for the adiabatic temperature and equilibrium species concentrations. The results can be of importance for determination of turbine inlet temperature. The second mode corresponds to high intensity and small-scale turbulence in the combustion chamber, with low Damköhler number and distributed reaction zones. The combustion process is simulated using the well-stirred reactor concept. The third mode is the classical non-premixed combustion with fuel and oxidizer supplied separately to the combustion chamber through two different streams. Only non-premixed flames are considered here as in oxyfuel combustion NOx is not a problem thanks to the absence of nitrogen in the oxidizer. A counter-flow flame configuration is employed to simulate the local reaction zone, and local flamelet extinctions. The numerical simulations are performed using the Cantera simulation package, which is open-source, object-oriented software for problems involving chemical kinetics, thermodynamics and transport processes [15].
2. Problem description and simulation methods 3. Results and discussions In the oxy-fuel gas turbine combustor studied in this work, the fuel stream supplies pure methane (a model fuel for natural gas), and the oxidizer stream is comprised of oxygen, carbon dioxide and water vapor. Three baseline conditions are considered as shown in Table 1. The main differences between these conditions are the O2 concentration in the oxidizer, the oxidizer inlet temperature, the combustor pressure, and the mass flow rate. The fuel flow and oxygen flow in all three cases are set to be in the exact stoichiometric ratio. The amount of CO2/steam is chosen so that optimal flue gas temperatures are obtained. In present industrial gas turbines the operating combustor pressure is typically between 10 and 40 bars. High combustor pressure in combination with high turbine inlet temperature results in high engine efficiency. However, the turbine inlet temperature is limited by turbine material temperature. Case 1 and case 2 correspond to the medium pressure gas turbines. Case 3 corresponds to high pressure and high efficiency gas turbines. The data of case 3 are taken from ENCAP studies of oxy-fuel SCOC-CC plant with 400 MW net power output and a net efficiency about 50% [8].
3.1. Thermodynamic characteristics First, the adiabatic combustion temperature and species compositions for the fuel/oxidizer mixture in chemical equilibrium are computed for different oxidizer conditions. The computations are based on the GRI 3.0 chemical reaction mechanism and the involved species, under the constant pressure and constant enthalpy condition. Fig. 1 shows the adiabatic temperature of the equilibrium combustion products with different volume fraction of O2 in the oxidizer. The combustor pressure is set to 17 and 40 bar, corresponding to the baseline cases given in Table 1. For the 17 bar case (CC17), the volume fraction of H2O in the oxidizer stream is kept constant at 3.51%; the sum of oxygen and CO2 volume fractions is constant at 96.49%. The initial temperature of the fuel/oxidizer mixture is 515 K, corresponding to baseline cases 1 and 2. For the 40 bar cases (CC40), the oxidizer is made up of oxygen and CO2 only, with the initial temperature of the fuel/oxidizer mixture of 595 K, corresponding to the baseline case 3. The fuel and oxygen
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Fig. 1. (Left) adiabatic temperature and (right) equilibrium CO concentrations, of oxy-fuel combustion and CH4/N2/O2 combustion, as a function of O2 volume fraction in the oxidizer. CC17: oxy-fuel combustion at 17 bar; CC40: oxy-fuel combustion at 40 bar; CA17: CH4/N2/O2 combustion at 17 bar.
are kept at stoichiometric ratio for all conditions. These test conditions are chosen in order to cover the baseline cases in Table 1. As shown in Fig. 1, the adiabatic temperature in all cases increases monotonically as the volume fraction of O2 in the oxidizer increases, owing to the decrease of diluents in the mixture. For oxy-fuel combustion, with the volume fraction of O2 at 21% in the oxidizer, the adiabatic temperature is about 1956 K for the 17 bar case, and 2000 K for the 40 bar case; with 5% O2 in the oxidizer the adiabatic temperatures for oxy-fuel combustion are about 942 K (for 17 bar) and 1000 K (for 40 bar). Since the turbine inlet temperature cannot be set too high due to the material limitation of turbine blades, the oxygen fraction in the oxidizer cannot be too high. The baseline case 1 (with a volume fraction of oxygen of 8.54% in the oxidizer) has an adiabatic temperature about 1200 K; for this temperature the turbine blades can withstand the heat. For cases 2 and 3 with a higher volume fraction of oxygen about 15% in the oxidizer, the adiabatic temperatures are about 1600 K to 1700 K, which requires advanced cooling to protect the turbine blades. The equilibrium species concentrations in the flue gas are shown in Figs. 1 and 2. With higher oxygen volume fractions in
the oxidizer stream the mole fraction of CO increases. The oxygen concentration in the equilibrium mixture increases as well, owing to the dissociation reactions of combustion products (e.g. CO2 and H2O) at high temperatures that tend to shift the chemical equilibrium towards reactants. To compare with the combustion condition of natural gas in air, we have carried out equilibrium calculations with pressure at 17 bar and at different oxygen level in a mixture of N2 and O2 (denoted as ‘‘N2–O2 oxidizer’’, CA17 in the figures). Other conditions of CA17 are similar to those of CC17. Significant difference between the oxy-fuel and the N2/O2/fuel combustion can be seen in the adiabatic flame temperature and the equilibrium compositions. The temperature of oxy-fuel combustion is about 200–400 K lower than the corresponding CH4/N2/O2 combustion since the heat capacity of CO2 is higher than that of N2. NO increases exponentially as the oxygen concentration in the N2–O2 oxidizer increases, owing to the increase in the combustion temperature, cf. Figs. 1 and 2. In oxy-fuel combustion NO is absent since nitrogen is not available in the mixture. The mole fractions of O2 and CO are lower in the flue gas of oxy-fuel combustion, owing to the low combustion temperature, which suppresses the dissociation reactions.
Fig. 2. (Left) Equilibrium CO2 concentration, and (right) equilibrium O2 and NO concentration, of oxy-fuel combustion and CH4/N2/O2 combustion, as a function of O2 volume fraction in the oxidizer. CC17: oxy-fuel combustion at 17 bar; CC40: oxy-fuel combustion at 40 bar; CA17: CH4/N2/O2 combustion at 17 bar.
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At high combustor pressure and high oxidizer temperature conditions, case CC40, it is seen that the adiabatic combustion temperature is about 40–60 K higher than the corresponding 17 bar case. This difference is mainly due to the difference in the oxidizer stream temperature (and to a much less degree due to the elevated fuel stream temperature, see Table 1), which causes the initial temperature of the unburned fuel and oxidizer mixture of case CC40 to increase by 80 K as compared to the 17 bar cases. As a result of the increase in combustion temperature, the equilibrium CO mole fraction for the 40 bar case at 15% oxygen volume fraction in the oxidizer stream is about 475 ppm, compared to the 348 ppm at 17 bar. The equilibrium oxygen in the products for the 40 bar case is 242 ppm, whereas it is 180 ppm at 17 bar. 3.2. Well-stirred reactor In well-stirred reactors the combustion process is assumed to be at low Damköhler numbers. Equivalently, the mixing time for the fuel and oxidizer is assumed to be shorter than the chemical reaction time. The combustion process depends on chemical kinetics and the flow residence time in the gas turbine combustion chamber. Well-stirred reactor model has been used in earlier combustor design [16]. Numerical simulations show that when the flow residence time is small, the fuel/oxidizer mixture in the reactor is not ignitable. There is a critical residence time beyond which the mixture starts to ignite rapidly. This critical residence time is physically related to the ignition delay time of the mixture. Fig. 3 shows the critical residence time for combustion and the adiabatic temperature of the combustion products under different initial temperature of the fuel/oxidizer mixture. Cases 1, 2 and 3 in the figures correspond to the baseline cases 1, 2 and 3 respectively, but with an elevated range of initial temperature of the mixture (baseline cases 1, 2 and 3 have an initial temperature of 517 K, 514 K and 595 K, respectively). The product temperature is evaluated after sufficiently long residence time when the product temperature reaches to the steady-state (independent of flow residence time). As the initial temperature of the mixture increases, the product temperature increases rapidly at low initial temperatures, followed by a slower increase in product temperature at high initial temperatures. This is an effect of higher heat capacity of the mixture at high product temperatures. The product temperature for the mixture of the baseline case 1 (oxygen volume fraction of 8.54%) is about 500 K lower than that of case 2, which has a higher oxygen volume fraction in the oxidizer (15.47%).
The mole fractions of O2, and CO in the flue gases are shown in Fig. 3. With higher initial temperature un-consumed oxygen in the flue gas is higher, and the emission of CO is also higher. This can be attributed to the increased importance of the dissociation reactions of H2O and CO2 at high combustion temperatures. This observation is consistent with the equilibrium results shown in Figs. 1 and 2. The influence of oxygen/diluent ratio in the oxidizer on the flue gas composition can be found in the results of case 1 and case 2. It appears that increase in the oxygen level in the oxidizer stream results in lower un-consumed oxygen in the flue gas, whereas less significant change is observed in the CO volume fractions. This can be explained by the fact that with high oxygen in the combustion products the oxidation of CO is enhanced; but the higher combustion temperature with high oxygen level in the oxidizer leads to higher dissociation reactions of H2O and CO2, which results in higher emission of CO and oxygen. These two effects counter-balance each other, yielding a less pronounced effect on CO emission, but higher oxygen in the flue gas. The effect of pressure on the combustion products can be identified by comparing the results of case 2 and case 3 where the oxygen levels in the oxidizer streams are rather similar. Both CO emissions and un-consumed oxygen level in the flue gas are higher in the 40 bar case than those in the corresponding 17 bar case. The product temperature for case 3 is fairly identical to that of case 2 at low initial temperature conditions. At higher initial temperatures, e.g. 1400 K, the product temperature for case 3 is about 40 K lower than that for case 2. It appears that with higher combustor pressure the combustion process is less complete with a higher amount of oxygen and CO in the flue gas, leading to lower combustion temperature. For the above investigated cases, the critical residence time shows exponential increase as the initial temperature of the mixture is decreased from 1400 K to 800 K. Below 750 K, the residence time is more than 2 s. Compared to case 1, case 2 has a higher oxygen level in the mixture, which leads to a higher combustion temperature as well as lower critical residence time. It is interesting to note that although the combustion temperature for case 2 and case 3 are similar, the critical residence times are significantly different, with the 40 bar case having a 2–3 times lower critical residence time than the 17 bar cases. This demonstrates the high sensitivity of the critical residence time to the combustor pressure, showing the influence of pressure on the chemical kinetics occurring in the process. From Fig. 3 and Table 1, it appears that the three baseline cases given in Table 1 would not be practically feasible if they were in
Fig. 3. Critical residence time (Tau) of combustion, combustion product temperature under adiabatic condition (Tf) and mole fractions of CO and oxygen in the flue gases at different initial temperature of fuel/oxidizer mixture.
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well-stirred reactors: for the initial temperature of the mixture lower than 600 K, the critical flow residence time required to have complete combustion in the combustion chamber is more than 10 s. For the baseline case 1 and 2, this would require a combustor volume larger than 20 m3, and for case 3, this would require a combustor volume larger than 50 m3. According to the well-stirred reactor result, to have a critical residence time less than 1 ms for the three baseline cases, the initial temperature of the mixture would be higher than 1200 K. If the fuel inlet temperature is kept as that in Table 1, the inlet temperature of the oxidizer stream has to be higher than 1200–1400 K. 3.3. Non-premixed combustion Non-premixed combustion mode is likely employed in the oxyfuel based gas turbine processes [8]. In practical gas turbine combustion chambers the mixing time for the fuel and oxidizer is typically larger than the chemical reaction time. The process is mixing controlled, i.e. the reaction zone structure is a thin diffusion flamelet. To study the oxy-fuel flamelet structures we adopted a counter-flow diffusion flame configuration [17] in the detailed chemical kinetic simulations. This flame configuration is often used to generate the flamelet library in turbulent non-premixed flame modeling [17]. Fig. 4 shows typical flame structures of non-premixed flames, where profiles for temperature and several key species across the reaction zones are shown for four different flame conditions. The fuel (CH4) and oxygen diffuse towards each other and react in the reaction zones where chemical energy is released and the temperature is high. The reaction zones can be characterized using OH radicals. For all four flames OH radicals are seen in very narrow zones of thickness less than 0.2 mm. CO is an intermediate species formed during fuel oxidization, and consumed typically by reaction with OH radicals to form CO2. These reactions are in the reaction zones where OH radicals exist. As seen in the figure, CO exists in much wider zones than the OH radicals, owing to molecular diffusion between the fuel/oxidizer streams and the reaction zones. Under the same combustor pressure and inlet stream temperature conditions, it is seen that the flame with oxygen/steam as oxidizer (WC17) has higher flame temperature (owing to lower heat capacity of H2O as compared to CO2), much higher OH radicals and wider reaction zones, and much lower CO than the oxygen/CO2 as oxidizer. By increasing pressure from 17 bar (CC17) to 40 bar (CC40a) it is seen that the flame becomes thinner. However, the peak mole fractions of CO and OH radicals remain nearly the same. The peak flame temperature for CC40a is slightly higher than that in CC17. In case CC40 the fuel stream temperature is set to that in the baseline case 3 with a fuel stream temperature 135 K higher than that in cases 1 and 2. The peak flame temperature of CC40 remains nearly identical to that of the low fuel stream temperature case, CC40a, due to the fact that the peak flame temperature is found at near-stoichiometric mixture where the species are mainly originated from the oxidizer stream. The profiles of species mole fractions and the reaction zone thickness in case CC40 are almost identical to those in the case CC40a, showing the negligible influence of the fuel stream temperature on the oxy-fuel flames. Non-premixed flames can be quenched by lowering the temperatures of the oxidizer stream, or by increasing the flow strain rate (equivalently scalar dissipation rate). Previous theoretical and computational studies [17–19] of non-premixed flames at different strain/scalar dissipation rates show that at small and moderate strain/scalar dissipation rate the flame temperature is not sensitive to the variations in the strain/scalar dissipation rate. At very high strain rate (close to quenching) conditions, however, the flame temperature is very sensitive to the variations in strain/scalar dissipation rate. At very low strain rate, radiative heat loss from the
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flames to environment can also lead to flame extinction for both non-premixed [17] and premixed flames [20]. In oxy-fuel combustion, variation of oxygen volume fraction in the oxidizer stream can also cause flame quenching. This aspect is examined below. To limit our discussion, we have chosen moderate strain/scalar dissipation rates (the strain rate is kept about 600 s 1) and adiabatic combustion in the following studies, focusing only on the effect of diluent in the oxidizer stream and inlet temperature of the oxidizer stream on the flames. Fig. 5a shows the peak flame temperature in the flamelet under different oxidizer compositions and oxidizer inlet temperature conditions, with the combustor pressure 17 bar. The fuel stream temperatures are kept the same as that in baseline cases 1 and 2. The volume fractions of the oxygen and CO2 in the oxidizer stream are varied along with the oxidizer temperature. The extinction behavior of oxy-fuel flames can be identified in Fig. 5a. With high oxygen/diluent ratio in the oxidizer, the peak flame temperature is high. The flames are stable flamelets. As the oxygen/diluent ratio decreases the peak flame temperature also decreases. This decrease is slow at high oxygen/diluent ratios and very rapid at low oxygen/diluent ratios. There is a critical oxygen/diluent ratio in the oxidizer stream, below which no flame would exist. With higher oxidizer inlet temperature the critical oxygen/diluent ratio for flame extinction is lower. The low part of Fig. 5a corresponds to flame quenching whereas no flame is predicted in the computations. It is seen that quenching occurs for all test cases when the peak temperature in the flamelet is about 1800–1900 K. These quenching temperatures are determined numerically by decreasing successively the oxygen volume fraction in the oxidizer from a stable flame condition. Once the flame is close to quenching an infinitesimal small decrease of the oxygen volume fraction in the oxidizer would cause quenching of the flame. These quenching peak flame temperatures are independent of the inlet temperature of the oxidizer stream. When the peak temperature in the flamelet is lower than these values, the chain reactions are no longer possible to maintain rapid production of radicals and the flames become very sensitive to the change of flow conditions and eventually it causes flame extinction. Fig. 5b shows calculations with water/steam oxy-fuel cycles under different oxygen volume fraction. All conditions, except the composition of the diluents, are kept the same as those in the CO2 cycles (shown in Fig. 5a). As seen, the combustion behavior with water as the major diluent in the oxidizer is similar to that with CO2 as the major diluent. Flame quenching occurs when the peak temperature in the flamelet is about 1700–1900 K. Under the same oxygen volume fraction in the oxidizer stream, the peak temperature in the flamelet with water as the major diluent in the oxidizer is about 100 K higher than that with CO2 as the major diluent. Fig. 6a shows the results with CO2 oxy-fuel cycle at 40 bar (with the fuel stream corresponding to case 3 in Table 1). The volume fractions of oxygen and CO2 are varied. Compared to the 17 bar case (Fig. 5a), the peak flame temperature at 40 bar is slightly higher (by about 100 K). Consequently, the critical oxygen/diluent ratios for flame quenching at different oxidizer temperatures are slightly lower than that for the 17 bar case. Flame quenching occurs when the peak flame temperature is lower than a critical peak flame temperature (around 1800–1900 K). One may use the data shown in Figs. 5 and 6a to characterize regimes of stable flames and extinction in oxy-fuel combustion. Fig. 6b shows the iso-contours of peak flame temperature of 1900 K in the parameter space of oxidizer composition and oxidizer stream temperature. Peak flame temperature of 1900 K may be a plausible critical temperature to characterize flame extinction, as shown Figs. 5 and 6a. The iso-contours divide the parameter domain into two parts; the upper-right part has peak
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Fig. 4. Distributions of mole fractions of species and temperature across the reaction zones of counter-flow non-premixed flames. CC17: 20% oxygen/80% CO2 in the oxidizer (on volume basis) with combustor pressure 17 bar; WC17: 20% oxygen/80% H2O in the oxidizer with combustor pressure 17 bar; CC40: 20% oxygen/80% CO2 in the oxidizer with combustor pressure 40 bar. Fuel stream temperatures: 288 for CC17, CC40a, and WC17; 423 K for CC40. Oxidizer stream temperature: 1100 K for all cases. The strain rates are about 600 s 1 for all cases.
Fig. 5. Peak flame temperature in non-premixed oxy-fuel flames under different oxidizer stream temperature and oxidizer composition with a combustor pressure 17 bar and fuel stream condition set to be the same as that in baseline case 1. (a) Oxygen/CO2 mixture in the oxidizer; (b) oxygen/steam mixture in the oxidizer.
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Fig. 6. (a) Peak flame temperature in non-premixed flames under different oxidizer stream temperature and O2 concentration in O2/CO2 oxidizer mixture, and with combustor pressure of 40 bar. (b) Iso-contours of peak flame temperature of 1900 K under different oxidizer compositions, temperature, and combustor pressure. CC17: O2/CO2 in the oxidizer with combustor pressure 17 bar; WC17: O2/H2O in the oxidizer with combustor pressure 17 bar; CC40: O2/CO2 in the oxidizer with combustor pressure 40 bar.
flame temperature higher than 1900 K, corresponding to the stable combustion regime. The low-left part of the domain has peak flame temperature lower than 1900 K or quenched flames, corresponding to the blowout regime. As seen, the water/steam oxy-fuel flames have larger stable flame domain than the CO2 oxy-fuel flames, owing to the higher flame temperature in water/steam oxy-fuel flames. With oxygen/CO2 as oxidizer the 40 bar case has a slightly larger stable flame domain than the 17 bar case. The baseline case 1 and case 2 shown in Table 1 have oxidizer temperature about 520 K, close to the dot-dashed line in Fig. 5. From Figs. 5a and 6b it is seen that the required minimal volume fraction of oxygen in the oxidizer stream is about 24%. Below this value the diffusion flamelets are quenched, i.e. no stable combustion is feasible. For baseline case 1 to have stable diffusion flamelets, the oxidizer has to be preheated above 1700 K; for case 2 to have stable diffusion flamelets, the oxidizer has to be preheated above 1200 K. For water/steam oxy-fuel flames with oxidizer temperature of about 520 K, the minimal oxygen volume fraction in the oxidizer is 21%, about 3% lower than that with CO2 as diluent. For the baseline case 3 to have sustainable flames, the oxidizer inlet temperature has to be higher than 1150 K, or the volume fraction of oxygen in the oxidizer stream has to be higher than 23%, slightly lower than that in case 2.
lation of hot gases to the flame [22]. On the other hand, in the IFRF oxy-natural gas flame rigs, e.g. Oxyflame-1 and Oxyflame-2 [23,24], pure oxygen was supplied through the oxidizer inlets resulting in a flame temperature as high as 2500 K and stable diffusion flame. With high level oxygen in the oxidizer the combustion products however become hot and this may lead to high level of oxygen in the flue gas due to the dissociation reactions at high temperatures. There is an optimal ‘‘window’’ of oxygen/ diluent ratio in the oxidizer stream. Based on the results shown in Figs. 5 and 6 we may be able to optimize the oxidizer supplies to the gas turbine combustors. The oxygen and diluent (such as CO2) should be premixed to different oxygen levels in different oxidizer flow streams. For example for baseline case 1, the primary oxidizer which is supplied in the upstream through the dome of the combustion chamber should have minimal oxygen level of 24% under the oxidizer temperature 520 K condition. The excessive CO2 shall be supplied through the liner holes downstream of the primary reaction zones to have a suitable temperature when the flue gas enters to the turbines. This will cool down the combustion products generated in the primary reaction zones. Stable combustion and low turbine inlet temperature can be obtained simultaneously by optimizing the oxygen and CO2 supplies to the combustion chamber.
3.4. Optimal supply of oxygen and diluent to oxy-fuel combustion 4. Conclusions As demonstrated above, to generate stable combustion in gas turbine combustion chambers with oxy-fuel combustion, certain minimal oxygen level in the oxidizer or elevated oxidizer temperature has to be maintained. The fundamental reason for this is the need to have sufficiently high temperature in the reaction zones for the chain reactions to proceed. This result is consistent with the previous experiments and simulations of oxy-fuel flames. Flame instability and poor burnout have been experienced when oxygen/CO2 are premixed and supplied together to the flame as the oxidizer [21]. For example, in the recent experiments of Heil et al. [22] it was shown that poor burnout and lifted dark flames appeared when the oxygen mole fraction in the O2/CO2 stream was set to 21%; when the oxygen volume fraction was increased to 27% and 34%, full burnout and stable flames were obtained. In order to burn the fuel with lower oxygen level in the oxidizer (O2/CO2) stream the burner had to be modified to allow for recircu-
Detailed thermodynamic and chemical kinetic simulations were carried out to study oxy-fuel combustion in gas turbines. Three baseline conditions are studied. In the first and second case, the oxygen level in the oxidizer is set to 8.54% and 15.47% (on volume basis), respectively, with a combustor pressure of 17 bar, whereas in the third case an elevated combustor pressure of 40 bar is used along with the oxygen level of 15.41%. Chemical equilibrium calculations indicate that the three cases can generate a theoretical turbine inlet temperature of about 1200–1700 K, corresponding respectively to non-cooling and cooling turbine blade conditions. Higher oxygen/diluent ratio in the oxidizer stream results in higher combustion temperature of stoichiometric mixture. The remaining oxygen in the flue gas and the emissions of unburned hydrocarbons and CO are also found to be higher. Chemical equilibrium calculations show a low sensitivity of the combustion products and
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temperature to the combustor pressure under the investigated 17 bar and 40 bar conditions. Detailed chemical kinetic studies under well-stirred reactor conditions show that the baseline cases may not be practically feasible since the critical flow residence time for the well-stirred reactors can be as high as 10 s, which requires very large combustor and low flow speed. Chemical kinetic calculations show high sensitivity of the critical flow residence time to the oxygen/diluent ratio in the oxidizer stream, the oxidizer stream temperature and the combustor pressure. Chemical kinetic studies for the non-premixed thin flamelet conditions show that the flames may not be sustainable under the baseline conditions due to the low peak flame temperature that results in elevated radical recombination consuming radicals and low chain branching reactions producing radicals. The minimal oxygen/diluent ratio in the oxidizer and the minimum temperature for the oxidizer stream are determined from the chemical kinetic simulations for the non-premixed combustion mode. Below this minimum oxygen/diluent ratio in the oxidizer flow the flames cannot be sustainable. The reaction zone structures are sensitive to the oxygen/diluent ratio in the oxidizer, the oxidizer stream temperature and the combustor pressure. The reaction zone in the 40 bar case is significantly thinner than that in the 17 bar case. The concentration of OH radicals in the 40 bar case is lower than the 17 bar case. The peak flame temperature is moderately sensitive to the combustor pressure but highly sensitive to the oxygen/diluent ratio and the oxidizer stream temperature. The results suggest the existence of a window for optimal oxygen/diluent ratio in the oxidizer flow streams for oxy-fuel combustion in gas turbines. Excessively high oxygen fraction in the oxidizer stream is helpful for sustainable flames but it can lead to excessively high flame temperature and elevated dissociation reactions of CO2 and H2O that yields higher oxygen in the flue gas. Based on these data, optimal oxy-fuel combustion in gas turbines can be designed with different oxygen/diluent ratios in different inlets. Acknowledgements The work at Tianjin University was sponsored by China Natural Science Foundation (Grant No. 50428605) and CSC (China Scholarship Council). At Lund University it was sponsored by the Swedish Research Council VR, SSF and STEM through CeCOST. At University of Stavanger it was supported by the Norwegian Ministry for Higher Education. References [1] Kvamsdal HM, Jordal K, Bolland O. A quantitative comparison of gas turbine cycles with CO2 capture. Energy 2007;32:10–24.
[2] Bolland O, Mathieu P. Comparison of two CO2 removal options in combined cycle power plants. Energy Convers Manage 1998;39(16–18):1653–63. [3] Dillon DJ, Panesar RS, Wall RA, Allam RJ, White V, Gibbins J, et al. Oxycombustion processes for CO2 capture from advanced supercritical PF and NGCC power plant. In: Proceedings of the seventh international conference on greenhouse gas control technologies – GHGT7. Vancouver, Canada; September 2004. [4] Staicovici MD. Further research zero CO2 emission power production: the COOLENERG process. Energy 2002;27:831–44. [5] Yantovski EI. Stack downward zero emission fuel-fired power plants concept. Energy Convers Manage 1996;37:867–77. [6] Mathieu P, Nihart R. Sensitivity analysis of the MATIANT cycle. Energy Convers Manage 1999;40:1687–700. [7] The European Technology Platform for Zero Emission Fossil Fuel Power Plants (ZEP). The final report from working group 1 power plant and carbon dioxide capture; 13th October, 2006. [8] Sanz W, Jericha H, Bauer B, Göttlich E. Qualitative and quantitative comparison of two promising oxy-fuel power cycles for CO2 capture. Paper GT2007-27375, ASME Turbo Expo, Montreal, Canada; 2007. [9] Jericha H, Sanz W, Göttlich E. Design concept for large output graz cycle gas turbines. ASME Paper GT2006-90032, ASME Turbo Expo 2006, Barcelona, Spain; 2006. [10] Anderson RE, MacAdam S, Viteri F, Davies DO, Downs JP, Paliszewski A. Adapting gas turbines to zero emission oxy-fuel power plants. Paper GT 200851377, ASME Turbo Expo, Berlin, Germany; 2008. [11] http://www.encapco2.org/index.htm [accessed 02.06.09]. [12] Li H, Yan J, Yan J, Anheden M. Impurity impacts on the purification process on oxy-fuel combustion based CO2 capture and storage system. Appl Energy 2009;86:202–13. [13] Jericha H, Göttlich E. Conceptual design for an industrial prototype graz cycle power plant. Paper GT 2002-30118, ASME Turbo Expo, Amsterdam, Netherlands; 2002. [14] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M, et al. GRI 3.0.
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Characteristics of oxy-fuel combustion in gas turbines C.Y. Liu a,b, G. Chen b, N. Sipöcz c, M. Assadi c, X.S. Bai a,⇑ a
Division of Fluid Mechanics, Lund University, 221 00 Lund, Sweden Faculty of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China c Faculty of Technology and Natural Science, University of Stavanger, Norway b
a r t i c l e
i n f o
Article history: Received 26 April 2011 Received in revised form 27 July 2011 Accepted 2 August 2011 Available online 7 September 2011 Keywords: Non-premixed oxy-fuel flames Flame quenching Oxy-fuel/CO2 cycles Gas turbines
a b s t r a c t This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH4, O2, CO2 and H2O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Oxy-fuel combustion has attracted the attention in power production industry and academic society due to its potential of having practically zero emissions of NOx and CO2, thus presenting an important option to address the environmental issues, in particular green house gas CO2 emissions. Oxy-fuel processes use oxygen instead of air for combustion of fuels, and use dilution agents such as CO2 or steam for flame temperature control and material cooling. Oxygen may be obtained via air separation units, e.g. cryogenic or membrane based processes. The combustion process takes place in nitrogen free or low-nitrogen environment resulting in a flue gas composed mainly of CO2 and H2O, as well as a low concentration of impurities such as argon and oxygen. Therefore, a simplified flue gas processing by means of condensation of H2O to capture CO2, without using costly separation methods such as chemical absorption, can be possible. There are several proposed combined cycle concepts in oxy-fuel gas turbine processes with natural gas combustion in oxygen and CO2, for example, the O2/CO2 cycle [1–3], the COOLENERG cycle [4], the COOPERATE cycle [5], and the MATIANT cycle [6]. These cy⇑ Corresponding author. E-mail address: [email protected] (X.S. Bai). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.08.004
cles belong to the group also known as Semi-Closed Oxy-Fuel Combustion Combined Cycles (SCOF-CC). Recent studies within the European Union funded research project ENCAP (Enhanced Capture of CO2) have concluded that SCOF-CC has good potential with limited techno-economical hinders for realization [7,8]. Besides SCOF-CC a number of other oxy-fuel cycles using steam/water as working fluids have been proposed including the Graz cycle [9], and the Water cycle [10] developed by Clean Energy Systems (CES). These cycles may require high temperature turbines and new design for the turbomachinery. Oxy-fuel combustion technology is still in the research and development stage. The European project ENCAP sets up goals to prepare technologies for power companies to be able to launch new design projects by 2010 aiming at large demonstration plants with the potential for wide commercial exploitation in 2015–2020 [11]. For oxy-fuel gas turbine cycles, researches have been focused on thermodynamic studies of system performance. The combustion behavior, e.g. the flame dynamics and reaction zone structures in the gas turbine combustors, is less addressed. From thermodynamic studies it has been shown that a small amount of trace species in the combustion products can have a great impact on the CO2 capture, storage and transportation. Li et al. [12] demonstrated that the purification process of the flue gas stream of oxy-fuel combustion is highly influenced by the existence of impurities such as
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the small amount of N2 resulted from the air separation units, and the remaining O2 in the flue gas due to incomplete combustion. The presence of non-condensable gases results in increased condensation duty for the recovery of the CO2. This in turn leads to lower system efficiency and increased cost for separation. To minimize the oxygen concentration in the flue gas and meanwhile achieve complete combustion of fuel, stoichiometric mixture is preferred in oxy-fuel combustion. CO2 and/or steam are used to control the flue gas temperature. Jericha and Göttlich [13] outlined a burner and combustor configuration, in which fuel, oxygen and steam were supplied separately in different inlets. The steam was supplied through an annular outer swirler inlet to form a swirling flow motion to wrap the flames and to cool down the flue gases. Such combustor configuration would likely generate rather high flame temperature locally in the reaction zones that will enhance the dissociation of H2O and CO2 and thus affect the composition of the flue gas such that the un-consumed oxygen can be high in the flue gas. To reduce the flame temperature and thereby the remaining oxygen in the flue gas it can be beneficial to premix the oxygen and CO2 or steam before injecting them to the combustor. There are several possibilities that need to be explored for example, different levels of premixing of the fuel/oxygen/steam/CO2 prior to their injection into the combustor, and different mixing patterns inside the combustor. The thermodynamic studies will give the same answer for the flue gas in the postflame zone if the inlet temperature, combustor pressure and the overall mass flows of fuel, oxygen, steam, and CO2 streams are kept the same. However, the flame dynamics and reaction zone structures are dependent on combustor configurations as they are dictated by the detailed inflow conditions for the fuel/oxygen/ steam and CO2 supplies. The aim of the present paper is to investigate the thermodynamic and combustion characteristics of oxy-fuel combustion under conditions related to gas turbine operation, by performing chemical kinetic simulations for different combustion conditions, including various levels of oxygen/diluent ratio in the oxidizer stream. Under typical gas turbine combustion conditions the chemical reaction zones may be considered as local laminar flamelets, whereas under extremely high turbulence conditions, the entire combustion chamber may be modeled as a well-stirred reactor. Both combustion modes are simulated using detailed chemical kinetic mechanisms, GRI-Mech 3.0 [14], and detailed transport modeling.
Table 1 Fuel and oxidizer compositions (volume basis) and baseline operation conditions. Parameters
Baseline reference conditions Case 1
Case 2
Case 3
Oxidizer stream CO2 (mol%) O2 (mol%) H2O (mol%) Mass flow (kg/s) Temperature (K)
87.95 8.54 3.51 10 520
81.09 15.47 3.44 8 520
84.59 15.41 0 560 600
Fuel stream CH4 (mol%) Mass flow (kg/s) Temperature (K)
100 0.16 288
100 0.235 288
100 16 423
Combustion chamber Pressure (bar) Overall equivalence ratio
17 1
17 1
40 1
The chemical kinetic mechanism used in this study is GRI-Mech 3.0 mechanism [14]. The GRI mechanism has been tested and validated comprehensively for methane and natural gas combustion over a wide range of pressure and temperature conditions. It contains 53 species and 325 reactions. Three combustion modes are considered in this study. The first is the thermodynamic equilibrium mode. This corresponds to the ideal case of chemical equilibrium combustion in the post-flame zone far downstream in the combustion chamber. Calculations are carried out for the adiabatic temperature and equilibrium species concentrations. The results can be of importance for determination of turbine inlet temperature. The second mode corresponds to high intensity and small-scale turbulence in the combustion chamber, with low Damköhler number and distributed reaction zones. The combustion process is simulated using the well-stirred reactor concept. The third mode is the classical non-premixed combustion with fuel and oxidizer supplied separately to the combustion chamber through two different streams. Only non-premixed flames are considered here as in oxyfuel combustion NOx is not a problem thanks to the absence of nitrogen in the oxidizer. A counter-flow flame configuration is employed to simulate the local reaction zone, and local flamelet extinctions. The numerical simulations are performed using the Cantera simulation package, which is open-source, object-oriented software for problems involving chemical kinetics, thermodynamics and transport processes [15].
2. Problem description and simulation methods 3. Results and discussions In the oxy-fuel gas turbine combustor studied in this work, the fuel stream supplies pure methane (a model fuel for natural gas), and the oxidizer stream is comprised of oxygen, carbon dioxide and water vapor. Three baseline conditions are considered as shown in Table 1. The main differences between these conditions are the O2 concentration in the oxidizer, the oxidizer inlet temperature, the combustor pressure, and the mass flow rate. The fuel flow and oxygen flow in all three cases are set to be in the exact stoichiometric ratio. The amount of CO2/steam is chosen so that optimal flue gas temperatures are obtained. In present industrial gas turbines the operating combustor pressure is typically between 10 and 40 bars. High combustor pressure in combination with high turbine inlet temperature results in high engine efficiency. However, the turbine inlet temperature is limited by turbine material temperature. Case 1 and case 2 correspond to the medium pressure gas turbines. Case 3 corresponds to high pressure and high efficiency gas turbines. The data of case 3 are taken from ENCAP studies of oxy-fuel SCOC-CC plant with 400 MW net power output and a net efficiency about 50% [8].
3.1. Thermodynamic characteristics First, the adiabatic combustion temperature and species compositions for the fuel/oxidizer mixture in chemical equilibrium are computed for different oxidizer conditions. The computations are based on the GRI 3.0 chemical reaction mechanism and the involved species, under the constant pressure and constant enthalpy condition. Fig. 1 shows the adiabatic temperature of the equilibrium combustion products with different volume fraction of O2 in the oxidizer. The combustor pressure is set to 17 and 40 bar, corresponding to the baseline cases given in Table 1. For the 17 bar case (CC17), the volume fraction of H2O in the oxidizer stream is kept constant at 3.51%; the sum of oxygen and CO2 volume fractions is constant at 96.49%. The initial temperature of the fuel/oxidizer mixture is 515 K, corresponding to baseline cases 1 and 2. For the 40 bar cases (CC40), the oxidizer is made up of oxygen and CO2 only, with the initial temperature of the fuel/oxidizer mixture of 595 K, corresponding to the baseline case 3. The fuel and oxygen
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Fig. 1. (Left) adiabatic temperature and (right) equilibrium CO concentrations, of oxy-fuel combustion and CH4/N2/O2 combustion, as a function of O2 volume fraction in the oxidizer. CC17: oxy-fuel combustion at 17 bar; CC40: oxy-fuel combustion at 40 bar; CA17: CH4/N2/O2 combustion at 17 bar.
are kept at stoichiometric ratio for all conditions. These test conditions are chosen in order to cover the baseline cases in Table 1. As shown in Fig. 1, the adiabatic temperature in all cases increases monotonically as the volume fraction of O2 in the oxidizer increases, owing to the decrease of diluents in the mixture. For oxy-fuel combustion, with the volume fraction of O2 at 21% in the oxidizer, the adiabatic temperature is about 1956 K for the 17 bar case, and 2000 K for the 40 bar case; with 5% O2 in the oxidizer the adiabatic temperatures for oxy-fuel combustion are about 942 K (for 17 bar) and 1000 K (for 40 bar). Since the turbine inlet temperature cannot be set too high due to the material limitation of turbine blades, the oxygen fraction in the oxidizer cannot be too high. The baseline case 1 (with a volume fraction of oxygen of 8.54% in the oxidizer) has an adiabatic temperature about 1200 K; for this temperature the turbine blades can withstand the heat. For cases 2 and 3 with a higher volume fraction of oxygen about 15% in the oxidizer, the adiabatic temperatures are about 1600 K to 1700 K, which requires advanced cooling to protect the turbine blades. The equilibrium species concentrations in the flue gas are shown in Figs. 1 and 2. With higher oxygen volume fractions in
the oxidizer stream the mole fraction of CO increases. The oxygen concentration in the equilibrium mixture increases as well, owing to the dissociation reactions of combustion products (e.g. CO2 and H2O) at high temperatures that tend to shift the chemical equilibrium towards reactants. To compare with the combustion condition of natural gas in air, we have carried out equilibrium calculations with pressure at 17 bar and at different oxygen level in a mixture of N2 and O2 (denoted as ‘‘N2–O2 oxidizer’’, CA17 in the figures). Other conditions of CA17 are similar to those of CC17. Significant difference between the oxy-fuel and the N2/O2/fuel combustion can be seen in the adiabatic flame temperature and the equilibrium compositions. The temperature of oxy-fuel combustion is about 200–400 K lower than the corresponding CH4/N2/O2 combustion since the heat capacity of CO2 is higher than that of N2. NO increases exponentially as the oxygen concentration in the N2–O2 oxidizer increases, owing to the increase in the combustion temperature, cf. Figs. 1 and 2. In oxy-fuel combustion NO is absent since nitrogen is not available in the mixture. The mole fractions of O2 and CO are lower in the flue gas of oxy-fuel combustion, owing to the low combustion temperature, which suppresses the dissociation reactions.
Fig. 2. (Left) Equilibrium CO2 concentration, and (right) equilibrium O2 and NO concentration, of oxy-fuel combustion and CH4/N2/O2 combustion, as a function of O2 volume fraction in the oxidizer. CC17: oxy-fuel combustion at 17 bar; CC40: oxy-fuel combustion at 40 bar; CA17: CH4/N2/O2 combustion at 17 bar.
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At high combustor pressure and high oxidizer temperature conditions, case CC40, it is seen that the adiabatic combustion temperature is about 40–60 K higher than the corresponding 17 bar case. This difference is mainly due to the difference in the oxidizer stream temperature (and to a much less degree due to the elevated fuel stream temperature, see Table 1), which causes the initial temperature of the unburned fuel and oxidizer mixture of case CC40 to increase by 80 K as compared to the 17 bar cases. As a result of the increase in combustion temperature, the equilibrium CO mole fraction for the 40 bar case at 15% oxygen volume fraction in the oxidizer stream is about 475 ppm, compared to the 348 ppm at 17 bar. The equilibrium oxygen in the products for the 40 bar case is 242 ppm, whereas it is 180 ppm at 17 bar. 3.2. Well-stirred reactor In well-stirred reactors the combustion process is assumed to be at low Damköhler numbers. Equivalently, the mixing time for the fuel and oxidizer is assumed to be shorter than the chemical reaction time. The combustion process depends on chemical kinetics and the flow residence time in the gas turbine combustion chamber. Well-stirred reactor model has been used in earlier combustor design [16]. Numerical simulations show that when the flow residence time is small, the fuel/oxidizer mixture in the reactor is not ignitable. There is a critical residence time beyond which the mixture starts to ignite rapidly. This critical residence time is physically related to the ignition delay time of the mixture. Fig. 3 shows the critical residence time for combustion and the adiabatic temperature of the combustion products under different initial temperature of the fuel/oxidizer mixture. Cases 1, 2 and 3 in the figures correspond to the baseline cases 1, 2 and 3 respectively, but with an elevated range of initial temperature of the mixture (baseline cases 1, 2 and 3 have an initial temperature of 517 K, 514 K and 595 K, respectively). The product temperature is evaluated after sufficiently long residence time when the product temperature reaches to the steady-state (independent of flow residence time). As the initial temperature of the mixture increases, the product temperature increases rapidly at low initial temperatures, followed by a slower increase in product temperature at high initial temperatures. This is an effect of higher heat capacity of the mixture at high product temperatures. The product temperature for the mixture of the baseline case 1 (oxygen volume fraction of 8.54%) is about 500 K lower than that of case 2, which has a higher oxygen volume fraction in the oxidizer (15.47%).
The mole fractions of O2, and CO in the flue gases are shown in Fig. 3. With higher initial temperature un-consumed oxygen in the flue gas is higher, and the emission of CO is also higher. This can be attributed to the increased importance of the dissociation reactions of H2O and CO2 at high combustion temperatures. This observation is consistent with the equilibrium results shown in Figs. 1 and 2. The influence of oxygen/diluent ratio in the oxidizer on the flue gas composition can be found in the results of case 1 and case 2. It appears that increase in the oxygen level in the oxidizer stream results in lower un-consumed oxygen in the flue gas, whereas less significant change is observed in the CO volume fractions. This can be explained by the fact that with high oxygen in the combustion products the oxidation of CO is enhanced; but the higher combustion temperature with high oxygen level in the oxidizer leads to higher dissociation reactions of H2O and CO2, which results in higher emission of CO and oxygen. These two effects counter-balance each other, yielding a less pronounced effect on CO emission, but higher oxygen in the flue gas. The effect of pressure on the combustion products can be identified by comparing the results of case 2 and case 3 where the oxygen levels in the oxidizer streams are rather similar. Both CO emissions and un-consumed oxygen level in the flue gas are higher in the 40 bar case than those in the corresponding 17 bar case. The product temperature for case 3 is fairly identical to that of case 2 at low initial temperature conditions. At higher initial temperatures, e.g. 1400 K, the product temperature for case 3 is about 40 K lower than that for case 2. It appears that with higher combustor pressure the combustion process is less complete with a higher amount of oxygen and CO in the flue gas, leading to lower combustion temperature. For the above investigated cases, the critical residence time shows exponential increase as the initial temperature of the mixture is decreased from 1400 K to 800 K. Below 750 K, the residence time is more than 2 s. Compared to case 1, case 2 has a higher oxygen level in the mixture, which leads to a higher combustion temperature as well as lower critical residence time. It is interesting to note that although the combustion temperature for case 2 and case 3 are similar, the critical residence times are significantly different, with the 40 bar case having a 2–3 times lower critical residence time than the 17 bar cases. This demonstrates the high sensitivity of the critical residence time to the combustor pressure, showing the influence of pressure on the chemical kinetics occurring in the process. From Fig. 3 and Table 1, it appears that the three baseline cases given in Table 1 would not be practically feasible if they were in
Fig. 3. Critical residence time (Tau) of combustion, combustion product temperature under adiabatic condition (Tf) and mole fractions of CO and oxygen in the flue gases at different initial temperature of fuel/oxidizer mixture.
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well-stirred reactors: for the initial temperature of the mixture lower than 600 K, the critical flow residence time required to have complete combustion in the combustion chamber is more than 10 s. For the baseline case 1 and 2, this would require a combustor volume larger than 20 m3, and for case 3, this would require a combustor volume larger than 50 m3. According to the well-stirred reactor result, to have a critical residence time less than 1 ms for the three baseline cases, the initial temperature of the mixture would be higher than 1200 K. If the fuel inlet temperature is kept as that in Table 1, the inlet temperature of the oxidizer stream has to be higher than 1200–1400 K. 3.3. Non-premixed combustion Non-premixed combustion mode is likely employed in the oxyfuel based gas turbine processes [8]. In practical gas turbine combustion chambers the mixing time for the fuel and oxidizer is typically larger than the chemical reaction time. The process is mixing controlled, i.e. the reaction zone structure is a thin diffusion flamelet. To study the oxy-fuel flamelet structures we adopted a counter-flow diffusion flame configuration [17] in the detailed chemical kinetic simulations. This flame configuration is often used to generate the flamelet library in turbulent non-premixed flame modeling [17]. Fig. 4 shows typical flame structures of non-premixed flames, where profiles for temperature and several key species across the reaction zones are shown for four different flame conditions. The fuel (CH4) and oxygen diffuse towards each other and react in the reaction zones where chemical energy is released and the temperature is high. The reaction zones can be characterized using OH radicals. For all four flames OH radicals are seen in very narrow zones of thickness less than 0.2 mm. CO is an intermediate species formed during fuel oxidization, and consumed typically by reaction with OH radicals to form CO2. These reactions are in the reaction zones where OH radicals exist. As seen in the figure, CO exists in much wider zones than the OH radicals, owing to molecular diffusion between the fuel/oxidizer streams and the reaction zones. Under the same combustor pressure and inlet stream temperature conditions, it is seen that the flame with oxygen/steam as oxidizer (WC17) has higher flame temperature (owing to lower heat capacity of H2O as compared to CO2), much higher OH radicals and wider reaction zones, and much lower CO than the oxygen/CO2 as oxidizer. By increasing pressure from 17 bar (CC17) to 40 bar (CC40a) it is seen that the flame becomes thinner. However, the peak mole fractions of CO and OH radicals remain nearly the same. The peak flame temperature for CC40a is slightly higher than that in CC17. In case CC40 the fuel stream temperature is set to that in the baseline case 3 with a fuel stream temperature 135 K higher than that in cases 1 and 2. The peak flame temperature of CC40 remains nearly identical to that of the low fuel stream temperature case, CC40a, due to the fact that the peak flame temperature is found at near-stoichiometric mixture where the species are mainly originated from the oxidizer stream. The profiles of species mole fractions and the reaction zone thickness in case CC40 are almost identical to those in the case CC40a, showing the negligible influence of the fuel stream temperature on the oxy-fuel flames. Non-premixed flames can be quenched by lowering the temperatures of the oxidizer stream, or by increasing the flow strain rate (equivalently scalar dissipation rate). Previous theoretical and computational studies [17–19] of non-premixed flames at different strain/scalar dissipation rates show that at small and moderate strain/scalar dissipation rate the flame temperature is not sensitive to the variations in the strain/scalar dissipation rate. At very high strain rate (close to quenching) conditions, however, the flame temperature is very sensitive to the variations in strain/scalar dissipation rate. At very low strain rate, radiative heat loss from the
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flames to environment can also lead to flame extinction for both non-premixed [17] and premixed flames [20]. In oxy-fuel combustion, variation of oxygen volume fraction in the oxidizer stream can also cause flame quenching. This aspect is examined below. To limit our discussion, we have chosen moderate strain/scalar dissipation rates (the strain rate is kept about 600 s 1) and adiabatic combustion in the following studies, focusing only on the effect of diluent in the oxidizer stream and inlet temperature of the oxidizer stream on the flames. Fig. 5a shows the peak flame temperature in the flamelet under different oxidizer compositions and oxidizer inlet temperature conditions, with the combustor pressure 17 bar. The fuel stream temperatures are kept the same as that in baseline cases 1 and 2. The volume fractions of the oxygen and CO2 in the oxidizer stream are varied along with the oxidizer temperature. The extinction behavior of oxy-fuel flames can be identified in Fig. 5a. With high oxygen/diluent ratio in the oxidizer, the peak flame temperature is high. The flames are stable flamelets. As the oxygen/diluent ratio decreases the peak flame temperature also decreases. This decrease is slow at high oxygen/diluent ratios and very rapid at low oxygen/diluent ratios. There is a critical oxygen/diluent ratio in the oxidizer stream, below which no flame would exist. With higher oxidizer inlet temperature the critical oxygen/diluent ratio for flame extinction is lower. The low part of Fig. 5a corresponds to flame quenching whereas no flame is predicted in the computations. It is seen that quenching occurs for all test cases when the peak temperature in the flamelet is about 1800–1900 K. These quenching temperatures are determined numerically by decreasing successively the oxygen volume fraction in the oxidizer from a stable flame condition. Once the flame is close to quenching an infinitesimal small decrease of the oxygen volume fraction in the oxidizer would cause quenching of the flame. These quenching peak flame temperatures are independent of the inlet temperature of the oxidizer stream. When the peak temperature in the flamelet is lower than these values, the chain reactions are no longer possible to maintain rapid production of radicals and the flames become very sensitive to the change of flow conditions and eventually it causes flame extinction. Fig. 5b shows calculations with water/steam oxy-fuel cycles under different oxygen volume fraction. All conditions, except the composition of the diluents, are kept the same as those in the CO2 cycles (shown in Fig. 5a). As seen, the combustion behavior with water as the major diluent in the oxidizer is similar to that with CO2 as the major diluent. Flame quenching occurs when the peak temperature in the flamelet is about 1700–1900 K. Under the same oxygen volume fraction in the oxidizer stream, the peak temperature in the flamelet with water as the major diluent in the oxidizer is about 100 K higher than that with CO2 as the major diluent. Fig. 6a shows the results with CO2 oxy-fuel cycle at 40 bar (with the fuel stream corresponding to case 3 in Table 1). The volume fractions of oxygen and CO2 are varied. Compared to the 17 bar case (Fig. 5a), the peak flame temperature at 40 bar is slightly higher (by about 100 K). Consequently, the critical oxygen/diluent ratios for flame quenching at different oxidizer temperatures are slightly lower than that for the 17 bar case. Flame quenching occurs when the peak flame temperature is lower than a critical peak flame temperature (around 1800–1900 K). One may use the data shown in Figs. 5 and 6a to characterize regimes of stable flames and extinction in oxy-fuel combustion. Fig. 6b shows the iso-contours of peak flame temperature of 1900 K in the parameter space of oxidizer composition and oxidizer stream temperature. Peak flame temperature of 1900 K may be a plausible critical temperature to characterize flame extinction, as shown Figs. 5 and 6a. The iso-contours divide the parameter domain into two parts; the upper-right part has peak
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Fig. 4. Distributions of mole fractions of species and temperature across the reaction zones of counter-flow non-premixed flames. CC17: 20% oxygen/80% CO2 in the oxidizer (on volume basis) with combustor pressure 17 bar; WC17: 20% oxygen/80% H2O in the oxidizer with combustor pressure 17 bar; CC40: 20% oxygen/80% CO2 in the oxidizer with combustor pressure 40 bar. Fuel stream temperatures: 288 for CC17, CC40a, and WC17; 423 K for CC40. Oxidizer stream temperature: 1100 K for all cases. The strain rates are about 600 s 1 for all cases.
Fig. 5. Peak flame temperature in non-premixed oxy-fuel flames under different oxidizer stream temperature and oxidizer composition with a combustor pressure 17 bar and fuel stream condition set to be the same as that in baseline case 1. (a) Oxygen/CO2 mixture in the oxidizer; (b) oxygen/steam mixture in the oxidizer.
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Fig. 6. (a) Peak flame temperature in non-premixed flames under different oxidizer stream temperature and O2 concentration in O2/CO2 oxidizer mixture, and with combustor pressure of 40 bar. (b) Iso-contours of peak flame temperature of 1900 K under different oxidizer compositions, temperature, and combustor pressure. CC17: O2/CO2 in the oxidizer with combustor pressure 17 bar; WC17: O2/H2O in the oxidizer with combustor pressure 17 bar; CC40: O2/CO2 in the oxidizer with combustor pressure 40 bar.
flame temperature higher than 1900 K, corresponding to the stable combustion regime. The low-left part of the domain has peak flame temperature lower than 1900 K or quenched flames, corresponding to the blowout regime. As seen, the water/steam oxy-fuel flames have larger stable flame domain than the CO2 oxy-fuel flames, owing to the higher flame temperature in water/steam oxy-fuel flames. With oxygen/CO2 as oxidizer the 40 bar case has a slightly larger stable flame domain than the 17 bar case. The baseline case 1 and case 2 shown in Table 1 have oxidizer temperature about 520 K, close to the dot-dashed line in Fig. 5. From Figs. 5a and 6b it is seen that the required minimal volume fraction of oxygen in the oxidizer stream is about 24%. Below this value the diffusion flamelets are quenched, i.e. no stable combustion is feasible. For baseline case 1 to have stable diffusion flamelets, the oxidizer has to be preheated above 1700 K; for case 2 to have stable diffusion flamelets, the oxidizer has to be preheated above 1200 K. For water/steam oxy-fuel flames with oxidizer temperature of about 520 K, the minimal oxygen volume fraction in the oxidizer is 21%, about 3% lower than that with CO2 as diluent. For the baseline case 3 to have sustainable flames, the oxidizer inlet temperature has to be higher than 1150 K, or the volume fraction of oxygen in the oxidizer stream has to be higher than 23%, slightly lower than that in case 2.
lation of hot gases to the flame [22]. On the other hand, in the IFRF oxy-natural gas flame rigs, e.g. Oxyflame-1 and Oxyflame-2 [23,24], pure oxygen was supplied through the oxidizer inlets resulting in a flame temperature as high as 2500 K and stable diffusion flame. With high level oxygen in the oxidizer the combustion products however become hot and this may lead to high level of oxygen in the flue gas due to the dissociation reactions at high temperatures. There is an optimal ‘‘window’’ of oxygen/ diluent ratio in the oxidizer stream. Based on the results shown in Figs. 5 and 6 we may be able to optimize the oxidizer supplies to the gas turbine combustors. The oxygen and diluent (such as CO2) should be premixed to different oxygen levels in different oxidizer flow streams. For example for baseline case 1, the primary oxidizer which is supplied in the upstream through the dome of the combustion chamber should have minimal oxygen level of 24% under the oxidizer temperature 520 K condition. The excessive CO2 shall be supplied through the liner holes downstream of the primary reaction zones to have a suitable temperature when the flue gas enters to the turbines. This will cool down the combustion products generated in the primary reaction zones. Stable combustion and low turbine inlet temperature can be obtained simultaneously by optimizing the oxygen and CO2 supplies to the combustion chamber.
3.4. Optimal supply of oxygen and diluent to oxy-fuel combustion 4. Conclusions As demonstrated above, to generate stable combustion in gas turbine combustion chambers with oxy-fuel combustion, certain minimal oxygen level in the oxidizer or elevated oxidizer temperature has to be maintained. The fundamental reason for this is the need to have sufficiently high temperature in the reaction zones for the chain reactions to proceed. This result is consistent with the previous experiments and simulations of oxy-fuel flames. Flame instability and poor burnout have been experienced when oxygen/CO2 are premixed and supplied together to the flame as the oxidizer [21]. For example, in the recent experiments of Heil et al. [22] it was shown that poor burnout and lifted dark flames appeared when the oxygen mole fraction in the O2/CO2 stream was set to 21%; when the oxygen volume fraction was increased to 27% and 34%, full burnout and stable flames were obtained. In order to burn the fuel with lower oxygen level in the oxidizer (O2/CO2) stream the burner had to be modified to allow for recircu-
Detailed thermodynamic and chemical kinetic simulations were carried out to study oxy-fuel combustion in gas turbines. Three baseline conditions are studied. In the first and second case, the oxygen level in the oxidizer is set to 8.54% and 15.47% (on volume basis), respectively, with a combustor pressure of 17 bar, whereas in the third case an elevated combustor pressure of 40 bar is used along with the oxygen level of 15.41%. Chemical equilibrium calculations indicate that the three cases can generate a theoretical turbine inlet temperature of about 1200–1700 K, corresponding respectively to non-cooling and cooling turbine blade conditions. Higher oxygen/diluent ratio in the oxidizer stream results in higher combustion temperature of stoichiometric mixture. The remaining oxygen in the flue gas and the emissions of unburned hydrocarbons and CO are also found to be higher. Chemical equilibrium calculations show a low sensitivity of the combustion products and
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temperature to the combustor pressure under the investigated 17 bar and 40 bar conditions. Detailed chemical kinetic studies under well-stirred reactor conditions show that the baseline cases may not be practically feasible since the critical flow residence time for the well-stirred reactors can be as high as 10 s, which requires very large combustor and low flow speed. Chemical kinetic calculations show high sensitivity of the critical flow residence time to the oxygen/diluent ratio in the oxidizer stream, the oxidizer stream temperature and the combustor pressure. Chemical kinetic studies for the non-premixed thin flamelet conditions show that the flames may not be sustainable under the baseline conditions due to the low peak flame temperature that results in elevated radical recombination consuming radicals and low chain branching reactions producing radicals. The minimal oxygen/diluent ratio in the oxidizer and the minimum temperature for the oxidizer stream are determined from the chemical kinetic simulations for the non-premixed combustion mode. Below this minimum oxygen/diluent ratio in the oxidizer flow the flames cannot be sustainable. The reaction zone structures are sensitive to the oxygen/diluent ratio in the oxidizer, the oxidizer stream temperature and the combustor pressure. The reaction zone in the 40 bar case is significantly thinner than that in the 17 bar case. The concentration of OH radicals in the 40 bar case is lower than the 17 bar case. The peak flame temperature is moderately sensitive to the combustor pressure but highly sensitive to the oxygen/diluent ratio and the oxidizer stream temperature. The results suggest the existence of a window for optimal oxygen/diluent ratio in the oxidizer flow streams for oxy-fuel combustion in gas turbines. Excessively high oxygen fraction in the oxidizer stream is helpful for sustainable flames but it can lead to excessively high flame temperature and elevated dissociation reactions of CO2 and H2O that yields higher oxygen in the flue gas. Based on these data, optimal oxy-fuel combustion in gas turbines can be designed with different oxygen/diluent ratios in different inlets. Acknowledgements The work at Tianjin University was sponsored by China Natural Science Foundation (Grant No. 50428605) and CSC (China Scholarship Council). At Lund University it was sponsored by the Swedish Research Council VR, SSF and STEM through CeCOST. At University of Stavanger it was supported by the Norwegian Ministry for Higher Education. References [1] Kvamsdal HM, Jordal K, Bolland O. A quantitative comparison of gas turbine cycles with CO2 capture. Energy 2007;32:10–24.
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