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NOx reburning in oxy-fuel combustion: A comparison between solid and gaseous fuels
International Journal of Greenhouse Gas Control 5S (2011) S120–S126
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
NOx reburning in oxy-fuel combustion: A comparison between solid and gaseous fuels Fredrik Normann ∗ , Klas Andersson, Filip Johnsson, Bo Leckner Chalmers University of Technology, Department of Energy and Environment, SE-412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 10 November 2010 Received in revised form 6 April 2011 Accepted 3 May 2011 Available online 28 May 2011 Keywords: NOx Nitrogen oxides Emission Reburning Oxy-fuel Combustion
a b s t r a c t In the development of oxy-fuel combustion, reburning of nitrogen oxides, recycled with the flue gases, has been investigated for primary NOx control. Reduction of between 50 and 80% of the recycled nitrogen oxides has been measured. The present work evaluates the performance of gaseous and solid fuels as agents for reduction of nitrogen oxides by comparing experimental and modelling work performed at Chalmers University of Technology with different fuels. It is shown that the reduction is similar during propane and lignite firing, but that the lignite has slightly higher reduction efficiency. Differences in combustion temperature and heterogeneous effects are possible explanations. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The reason for developing oxy-fuel power plants is to control the emission of carbon dioxide in carbon capture and storage systems. However, conventional pollutants, such as nitrogen oxides (NOx ), discussed in the present paper, must still be considered. The first emission regulations to control the emission of nitrogen oxides were established in the early 1970s, and since then regulations have become increasingly stringent and emissions have decreased. Today, the emission of nitrogen oxides is a critical design parameter in all power plants. Controlling NOx emissions into the atmosphere is required, but it is potentially important also for CO2 storage. In addition, it might be necessary to limit the level of NOx in oxy-fuel power plants because of technical limitations, such as avoiding corrosion of materials. The most common way to limit the emission of NOx in state-of-the-art power plants is through primary measures, which delay the mixing of fuel and oxidizer and limit combustion temperatures, but oxy-fuel combustion offers new options for secondary NOx control that could be beneficial (Normann et al., 2009). The conditions inherent in oxy-fuel combustion differ from those of air combustion and include low concentration of N2 , recycling of flue gases (containing NOx ), high concentration of combustion products, long gas residence time, and different temperatures.
∗ Corresponding author. Tel.: +46 31 772 1000; fax: +46 31 772 3592. E-mail address: [email protected] (F. Normann). 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.05.003
The combustion process can be divided into four stages, as shown in Fig. 1a, in which different NOx formation/reduction mechanisms are dominant. The following defines the end of each stage: I. Complete oxidation of hydrocarbon volatiles. II. Introduction of secondary oxygen; switching from reducing to oxidizing conditions. III. Complete oxidation of CO. IV. Frozen concentrations. In Fig. 1a the four stages are shown in sequence, as they would appear in plug-flow. Devolatilization and oxidation of the evolved species occur at the initial part of the turbulent flame, Stages I and II. During combustion of hydrocarbons, there is an accumulation of CO, which starts to oxidize after the completion of hydrocarbon oxidation. Stage III is at the boundary of Stages I and II where the secondary oxygen mixes with the primary stream and creates an oxidizing environment. Stage IV is the latter part of the flame where the peak temperature has been reached and the reaction activity starts to decline. It consists of char burnout and cooling of the flame until the concentrations are frozen. During all four stages of the combustion process, the nitrogen chemistry is active and includes numerous reactions, which form and reduce NOx . Different routes will dominate depending on the location in the flame and the associated combustion conditions. Fig. 1b summarizes the main reaction paths. There are two sources of nitrogen during combustion, molecular nitrogen present in the air and nitrogen chemically bound in the fuel (Fuel-N). Typically, in
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Fig. 1. (a) Definition of the four stages of the combustion process. Dashed vertical lines indicate the shift between stages and double arrows define zones with different stoichiometric ratios (). (b) Global mechanisms of NO formation and reduction in combustion. Vol-N denotes volatile nitrogen-containing compounds, typically HCN or NH3 . The roman numbers in brackets indicate in which stage each process is active.
pulverized coal combustion, more than 80% of the NO formed originates from Fuel-N (Pershing and Wendt, 1979). In Stage I, the fuel is decomposed, or pyrolised, and the nitrogen chemistry is dominated by the evolution of volatile nitrogen compounds in the fuel. This process is fast and, generally, the main devolatilization lasts some tens of milliseconds. The amount of nitrogen released as volatiles is highly dependent on coal type but also on temperature and can be up to 100% of the fuel-bound nitrogen. There are different opinions on the type and relative amount of nitrogen volatiles-release (Glarborg et al., 2003), but HCN together with NH3 are the most important species. The reburning and prompt routes which involve reactions with hydrocarbon radicals are active during Stage I. The overall contribution to NOx formation by the prompt mechanism is already insignificant in air-fired coal combustion, and it has, therefore, not gained any attention under oxy-fuel combustion where the low concentration of N2 can be expected to further decrease its importance. The reburning route, on the other hand, has achieved a great deal of attention in connection with oxy-fuel combustion as it may reduce the NOx recycled through the burner. In the second stage, the hydrocarbons are oxidized and the reburning and prompt routes are no longer active. Instead, the volatile nitrogen compounds formed in the first stage start to oxidize. This oxidation is critical to NOx formation as it governs the split between NO and N2 formation. The oxidation of volatile nitrogen compounds occurs in several stages and, eventually, either NO or N2 is formed. In Stage II, the environment is oxygen lean and N2 is typically the favoured product. However, in Stage III where the secondary oxygen is introduced to complete oxidation of the fuel, NO is the favoured product. In Stage IV, the activity is low. However, the oxidation of char is slow (it requires a few seconds for completion) compared to the gas-phase reactions and it may, therefore, be active also during this final stage. Both the evolution of NO and N2 from char and the reduction of NO by char are complex processes that are not fully understood (Molina et al., 2000). Furthermore, if the temperature is sufficiently high, the Zeldovich mechanism could be of importance to the emission also in the late stage of combustion. This mechanism is slow relative other gas phase reactions, but it does not depend on the presence of hydrocarbons and volatiles. During oxy-fuel combustion the nitrogen concentration is lower and less thermal NO is formed than during air-firing. Under certain conditions the Zeldovich mechanism may even be reversed to reduce NO (Normann et al., 2008). The homogeneous combustion chemistry is generally considered to be most important for the formation and reduction of NOx (Pershing and Wendt, 1979; Bose et al., 1988). However, because of the slow combustion of char, the possibility to influence the NO yield evolving from char-bound nitrogen by conventional combustion measures is limited. Therefore, the relative contribution to
the formation of NOx by char increases while the formation from volatile nitrogen is decreased. Furthermore, the reduction of NO by char is enhanced by the presence of CO, and the reduction could, thus, be of special interest in oxy-fuel combustion where the CO concentration is locally increased. The authors of this work have previously performed a series of investigations on the emission of nitrogen oxides from oxy-fuel combustion (Andersson et al., 2008; Kühnemuth et al., 2011; Normann et al., 2008, 2009, 2010). From the results it was concluded that the emission of nitrogen oxides, related to the amount of fuel supplied, is significantly reduced in oxy-fuel combustion compared to air-firing, because of increased reduction of formed and recycled NOx . The investigations have included experimental as well as modelling studies of coal and gas-fired oxy-fuel combustion and the aim has been to understand and optimize the oxy-fuel combustion process with respect to NOx emission. In the present paper, an overview of the reburning conditions in oxy-fuel combustion is presented, including a discussion on the influence from the oxy-fuel combustion atmosphere, involving the combustion parameters as well as different fuels. The analysis also includes a comparison between homogeneous and heterogeneous atmospheres with respect to their capability to reduce nitrogen oxides. 2. Methodology Experimental data obtained in the Chalmers 100 kW test facility are examined. These data originate from published studies (Andersson et al., 2008; Kühnemuth et al., 2011), and only a brief summary of the experimental unit, measurement equipment, operating conditions, and models used for evaluation is given here. The furnace is top-fired and operates with air or a mixture of oxygen and recycled flue gas (dry or wet). In oxy-fuel operation, oxygen (99.5% purity) is mixed into the recycled flue gas to achieve the desired concentration in the oxidizer. Furthermore, NO can be injected into the oxidizer in order to design experiments targeted on NO reduction. It is also possible to introduce oxygen downstream of the burner (800 mm from the furnace inlet), comparable to over-fire air (OFA), making it possible to operate the burner under sub-stoichiometric conditions. Measurements were performed inside the furnace through seven measurement ports located alongside the reactor from 46 to 1400 mm from the furnace inlet, as well as in the recycle loop. The concentrations of CO2 , CO, O2 , NO and total hydrocarbons were continuously measured during the experiments. Furnace temperatures were measured by suction pyrometers. Two measurement campaigns have been performed to study NOx emission: one with lignite and one with propane as fuel with conditions as given in Table 1. Since propane does not contain any nitrogen, NO is added to the recycled flue gases to represent
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Table 1 Summary of experimental conditions of the investigations on NOx in the Chalmers 100 kW oxy-fuel unit.
Fuel input Global stoichiometric ratio Burner stoichiometric ratio Oxygen concentration in oxidizer Fuel-bound nitrogen (dry, ash free) NO in oxidizer
Lignite
Propane
80 kW 1.18, (1.30, 1.41)a
81 kW 1.15
Same as global
1.15, (0.7–1.15)a
25, 27, 29%
27, 30, (25–37)%a
0.6 wt.%
0%
Depending on outlet (∼250 ppm)
Constant 500 ppm
a
For values within brackets measurements are only performed in the recycle loop.
recycled NOx , corresponding to a constant inlet NO concentration of 500 ppm. Thereby, the reburning reduction was isolated from NOx formation from fuel-bound nitrogen and gas-solid reactions. Furthermore, the residence time at high temperature (above 1600 ◦ C) was limited to minimize the influence of the Zeldovich mechanism. The lignite measurement campaign focused on varying the inlet oxygen concentration, while a broader parameter study, including also the stoichiometric ratio of the burner, was performed during the propane-firing tests. The gas composition and temperature profile in the furnace were measured during operation with both fuels. Combustion modelling was performed to evaluate and interpret the experimental results. The modelling focuses on combustion chemistry, and neither fluid dynamics nor heat transfer are modelled. Instead mixing is treated, assuming perfect plug flow, and the temperature is represented by a pre-defined profile. These assumptions are useful, as it is the nitrogen chemistry that is important to NOx formation, and several uncertainties in the combustion calculation are eliminated. However, when interpreting the experiments, imperfect mixing in the combustion process need to be considered. In the model, the mixing process is described by a continuous introduction of the oxidizer to the reactor in the first 0.5 m, according to Fig. 2. This mixing rate has been shown to give a fair representation of the furnace concentrations (Kühnemuth et al., 2011). The combustion chemistry is described by a detailed reaction mechanism, which is based on the work of Mendiara and Glarborg (2009) on C1, C2 and nitrogen chemistry under oxy-fuel conditions updated with a C3 subset from Frassoldati et al. (2003).
3. Results and discussion The reburning reduction is discussed in three sections. Firstly, the chemical effects of the CO2 -rich atmosphere in oxy-fuel combustion are presented. Secondly, the influence of the combustion conditions on reburning reduction is identified. These results are based on gas-fired flames and allow isolating the NOx reduction, because of the absence of fuel-bound nitrogen. Finally, the effect of fuel on the reburning efficiency is discussed, including a comparison between modelling and experiments. 3.1. Chemical effects on reburning in oxy-fuel combustion On a global basis, the reburning reduction can be described by the following three reactions, NO + CH1−3 → HCN + · · ·
(1)
HCN + OH, O, O2 → NO + · · ·
(2)
HCN + NO → N2 + · · ·
(3)
Thus, the reduction of NO depends on hydrocarbon radicals, and the oxidation of the HCN formed depends on the composition of the radical pool of chain-carrying radicals. During combustion, CO2 influences the important radical chemistry mainly through the CO oxidation reaction, CO + OH ↔ CO2 + H
(4)
and its importance to the chain-branching reaction, H + O2 ↔ OH + O
(5)
The chemical effects of CO2 in combustion have been investigated by means of modelling. Fig. 3 compares the concentration of the major reducing agents (CH3 and the sum of CH2 and CH, denoted CH1,2 ; Fig. 3a) and the major nitrogen species (NO and HCN; Fig. 3b) calculated with N2 and CO2 as bulk gas (Normann et al., 2010). The oxidation rate of the fuel is lowered by CO2 ; the peak concentration of hydrocarbon radicals (CH1–3 ) is lower and the residence time in the reducing zone (Stage I) is longer. This enables alternative reaction routes and changes the composition of the hydrocarbon radicals in oxy-fuel combustion to lower the concentration of CH1,2 (the peak is almost 500 times lower) and raises the CH3 concentration (the peak concentration is similar, but the reaction time is longer in CO2 ). The reburning reduction depends on the hydrocarbon radicals, and the NO reduction rate is lower in CO2 , as indicated by the slower increase in HCN compared to air firing. A difference in composition of hydrocarbon radicals is important when controlling the combustion process, because it affects the sensitivity of NOx reduction to combustion temperature and stoichiometric ratio. Furthermore, according to Reaction (4) and (5) the elevated concentration of CO2 in oxy-fuel combustion favours the formation of OH radicals and the consumption of H radicals. This alters the oxidation of HCN (Reaction (2) and (3)) formed during the reburning reduction (Reaction (1)). The oxidation of HCN to NO and N2 has been examined by detailed chemistry modelling (Normann et al., 2010; Mendiara and Glarborg, 2009; Giménez-López et al., 2010). The modelling shows that the split between NO and N2 depends on similar reactants in both atmospheres. The higher CO2 concentration in oxy-fuel combustion may increase the selectivity towards NO; however, the impact depends on the local stoichiometry or oxygen concentration. 3.2. Influence of combustion parameters
Fig. 2. Conditions for the plug-flow reactor modelling. The oxidizer is assumed to be continuously introduced to the fuel during the first 0.5 m from the furnace inlet.
In a previous work (Kühnemuth et al., 2011), the sensitivity of the reburning reduction to the combustion conditions was
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Fig. 4. The relative increase in NOx reduction when lowering the burner’s stoichiometric from 1.15 (The reduction at 1.15 is 16% and 27% during oxy-fuel and air combustion, respectively). Bars to the left represent oxy-fuel combustion (30% O2 in the oxidizer) and bars to the right correspond to air firing. The effect of temperature is indicated by the shaded area of the bars. Based on measurements by Kühnemuth et al. (2011).
Fig. 3. Concentration profile of (a) CH3 and CH1,2 , and (b) NO and HCN during combustion with the same oxygen concentration using either N2 or CO2 as bulk gas. The temperature is 1400 ◦ C and the stoichiometric ratio is 0.7. Ref.: (Normann et al., 2010).
quantified by propane experiments with NO injection in the oxidizer. The influence of temperature variations in these experiments was separated from the combustion parameters investigated by means of modelling. Fig. 4 shows the impact of the stoichiometry of the burner on NOx reduction (the increase in reduction at lower stoichiometries is shown relative to a stoichiometric ratio of 1.15). Besides achieving reducing conditions in the flame, lower burner stoichiometry also reduces the combustion temperature. Both of these effects enhance the NOx reduction by reburning; the relative importance of the decrease in combustion temperature is given in Fig. 4 as a shaded height of the bars. For example, during oxyfuel combustion with a stoichiometric ratio of 0.7 the reduction is about five times higher than the reduction at 1.15 and about half of the reduction increase is caused by the decrease in temperature. Reducing conditions make NOx decrease, but deep staging does not affect the reduction much further because of the reducing conditions (the white fields in Fig. 4 are relatively constant from case to case). Instead, the lower combustion temperature makes a further decrease in burner stoichiometric ratio beneficial for NOx reduction under oxy-fuel conditions (the shaded height in Fig. 4 increases as the ratio decreases). Furthermore, Fig. 4 compares the sensitivity to stoichiometric ratio of oxy-fuel combustion to air-firing (similar flame temperatures). In general, the reburning reduction is more sensitive to stoichiometric ratio under oxy-fuel conditions than
under air-firing. This is mainly because of an increased sensitivity to combustion temperature under oxy-fuel conditions, which may be explained by the changes in radical chemistry, as discussed above. The influence of inlet oxygen concentration (that also influences combustion temperature) was investigated in a similar way. The direct effect of the inlet oxygen concentration on NOx reduction is shown to be low, as the O2 concentration in the furnace is relatively independent of the oxygen concentration in the oxidizer, but the indirect effect of temperature has some importance. Furthermore, both the inlet oxygen concentration and the burner stoichiometry relate to the amount of flue gas recycled to the flame (higher stoichiometry or lower inlet oxygen concentration increase the recycling flow). The importance of the flue gas recycle can be illustrated by comparing the once-through reduction (excluding the recycle) to the total reduction (including the recycle). Fig. 5 shows the theoretical relation between these two reduction dimensions as a function of the amount of recycle and once-through reduction efficiency (at a ratio of unity there is no difference between once-through and total reduction). By definition, without recycle (0%) the once-through reduction equals the total reduction. Obviously, the total reduction is favoured by high gas recycle. However, the efficiency of the once-through reduction affects the importance of the recycle, and, at a high once-through reduction, the difference between the once-through and total reduction is small, independent of the amount of recycle. Therefore, the benefit of recycling flue gases in terms of NOx reduction is higher for processes of low once-through reduction efficiency. 3.3. Influence of fuel In literature, a series of fuels including gas, oil, biomass, and coal have been tested for NOx reburning in air combustion. Researchers disagree on the sensitivity of the reburning reduction to type of fuel, but there is a consensus that the hydrocarbon and nitrogen content are the main influencing factors (Zabetta et al., 2005). Fig. 6a compares the measured NO concentration at the centreline of the
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Fig. 7. Comparison of reburning efficiency between methane and propane.
Fig. 5. The theoretical relation between once-through and total reduction as a function of once-through reduction efficiency and the amount of flue gas recycle.
flame during combustion of propane doped with NO (to represent recycled NO) (Kühnemuth et al., 2011) and lignite oxy-fuel firing (Andersson et al., 2008). Fig. 6b compares two NO profiles calculated by the detailed reaction model (Kühnemuth et al., 2011) under conditions corresponding to the two fuels with the purpose of evaluating the course of events during the experiments in Fig. 6a. The
Fig. 6. a and b. Comparison between modelling and experiments with propane (30% O2 and 500 ppm NO in oxidizer) and lignite (29% O2 and 250 ppm NO in oxidizer from recycle). In the model methane and methane/HCN replaces lignite and propane as fuel. Other conditions are similar to those of the experiments.
model is a plug-flow reactor, see Fig. 2, where the fuel is introduced at the reactor inlet, while the oxidizer is gradually added during the first 0.5 m. Temperature profiles and inlet NO concentration corresponds to the two cases in Fig. 6a. Furthermore, HCN is added to the modelled fuel to represent the amount of fuel-bound nitrogen in the lignite investigated. In both cases the model predicts an initial accumulation of recycled NO in the near-burner region upstream of the first measurement position (the first peak in Fig. 6b). When combustion is initiated there is an extensive reduction of NO by hydrocarbon radicals (reburning), and a minimum in NO concentration is reached. Downstream of the minimum, the hydrocarbons are oxidized and the reducing reactions are terminated. The intermediate nitrogen species formed are oxidized, which, together with continued introduction of the NO-rich oxidizer, increases the NO concentration at the centreline of the furnace. In the lignite-fired case, the presence of fuel-bound nitrogen (HCN in the model) increases the formation rate of NO compared to the propane-fired case. In the model of Fig. 6, the mixing is completed at 0.5 m from the furnace inlet, but in the experiments the mixing is continued throughout the investigated combustion zone (1 m from furnace inlet). In the propane-fired case this is expressed by a slight increase in the NO concentration throughout the combustion zone, as the oxidizer contains 500 ppm of NO. In the lignite-fired case, the NO formation from fuel-bound nitrogen overshoots the NO concentration of the oxidizer (250 ppm) and the NO concentration at the centreline therefore decreases during the downstream parts of the flame. Overall, the centreline profiles of the two fuels behave in a similar way. The influence of type of hydrocarbon fuel on NO reburning is illustrated in Fig. 7. The same simulation as in Fig. 6b is performed with methane, propane, and a methane–propane mixture of a 9 to 1 ratio. Methane and propane behave differently in the nearburner region with methane yielding a higher NO peak before initiation of NO reduction. This behaviour could be attributed to a delay in initiation of methane combustion, which delays the introduction of hydrocarbon radicals to the flame zone. When a small amount of propane, which can initiate the combustion, is mixed with methane, the methane flame has an even lower peak than the pure propane flame in the near-burner region. The concentration profile in the near-burner region is also affected by the assumptions on mixing and temperature in this region (Kühnemuth et al., 2011). After complete oxidation of the hydrocarbon radicals (around 0.2 m from furnace inlet) the difference between the fuels is negligible; the oxidation of intermediates is the same for all fuels/fuel-mixtures. In coal combustion, small hydrocarbons
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Fig. 8. Radial temperature and NO profiles measured 215, 384, and 553 mm from the burner inlet during oxy-propane (30% O2 in oxidizer) and oxy-lignite (29% O2 in oxidizer) combustion.
(mainly C1 and C2) are released as volatiles and during the conversion of the tars. According to Fig. 7, the type of volatiles released should be of little importance to the reburning reduction compared to the point of ignition, which will depend on fuel properties and combustion conditions. Fig. 8 compares the radial profiles of temperature and NO measured at 215, 384, and 553 mm from the furnace inlet in the oxy-propane (30% O2 in oxidizer) and oxy-lignite (29% O2 in oxidizer) experiments. The peak temperature at the centre of the propane flame (Fig. 8a) is about 200–300 ◦ C higher than in the lignite flame (Fig. 8b). However, already at a distance of 100 mm from the centreline the temperatures are similar for the two fuels on all measurement levels. In contrast to the centreline NO profile (Fig. 6), there are obvious differences between the radial NO profiles of the fuels. In the propane-fired case (Fig. 8c), there is only a slight decrease in the NO concentration (500–470 ppm) from the inlet of the oxidizer to the flue gas, and the concentration of NO in the outer parts of the flame is similar throughout the furnace. The lignite-fired case (Fig. 8d) is more complex with formation of NO at the centre of the flame and increasing NO concentration at the outer flame boundaries (from 250 to 410 ppm). However, at 553 mm from the furnace inlet, both the propane and lignite profiles are well mixed in the radial direction with relatively uniform NO concentrations. There are, thus, systematic differences in temperature and furnace concentration profiles between the propane and the lignite cases that influence the reduction efficiency of NO (although the centreline concentration profiles are similar). Furthermore, there are no heterogeneous reactions in the propane experiments. Fig. 9 shows an estimate
of the reduction of NO present in the recycle gas for three inlet oxygen concentrations from the lignite and propane experiments. The reduction in the propane-fired experiments (Propane ) is calculated as the ratio of emitted and added NO in mg/MJ of fuel supplied, Propane = 1 −
NOemit NOadd
(6)
Fig. 9. Comparison between the reduction of recycled NO during propane (Kühnemuth et al., 2011) and lignite (Andersson et al., 2008) firing in the same experimental unit.
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4. Conclusion This article summarizes the experimental and modelling work performed at Chalmers University of Technology on reburning reduction in oxy-fuel combustion. It is shown that the reburning reduction is favoured by high recycle ratios, sub-stoichiometric conditions, and controlled combustion temperatures; compared to air-firing, reburning during oxy-combustion is more sensitive to temperature. The differences between solid and gaseous fuels are of particular interest. The reduction of NO is slightly higher during the oxy-lignite experiments compared to the oxy-propane experiments. The difference in reduction could be explained by the minor difference in combustion temperature observed in burning the two fuels; however, heterogeneous reduction could also contribute to the explanation. This statement requires verification by further investigation. Fig. 10. Reduction of NO during oxy-fuel (27% oxygen in oxidizer) with natural gas (black bars) and char of brown coal at different stoichiometric ratios taken from the work of Dhungel (2009).
The reduction in the lignite-fired experiments (Lignite ) is instead estimated as the ratio of emitted NO (mg/MJ fuel supplied) in air and oxy-fuel combustion, Lignite = 1 −
NOemit OXY NOemit AIR
(7)
Eq. (7) assumes that the difference between the emission of NO in air and oxy-fuel combustion originates entirely from the reduction of recycled NO (which is a reasonable assumption (Andersson et al., 2008)). As seen in Fig. 9, the reduction during propane-firing is consistently lower than that during lignite-firing. As discussed above, the difference in reduction between coal and propane may relate to heterogeneous reduction of NO as well as to the way the fuel-bound nitrogen is released to the gas-phase in coal combustion. For example, Dhungel (2009) showed in an experimental investigation that heterogeneous reactions could play an important role for the reduction of NO during oxygen-rich oxy-fuel combustion. Fig. 10 presents the results of Dhungel (2009) that compares the reduction of recycled NO during combustion of natural gas and char from lignite. The natural gas experiments indicate the reduction by homogeneous reactions, whereas the char gives an indication of reduction by heterogeneous reactions. High concentration levels of CO and poor performance of homogeneous reburning under oxygen-rich conditions explained the relatively high reduction by the char. The present investigation does not show heterogeneous reduction to be of such magnitude, even though the lignite-fired experiments indicate that the reduction of NOx possibly is enhanced, which might be attributed to heterogeneous NOx reduction. However, systematic differences between propane and lignite-firing, such as differences in combustion temperature, also influence the homogeneous reduction of NO. In fact, the temperature difference between the gas and coal-fired cases (around 300 ◦ C in peak temperature), may well explain the deviation in the reduction of recycled NO in Fig. 9 (c.f. Fig. 4).
Acknowledgements This work has been financed by Vattenfall AB. References Andersson, K., Normann, F., Johnsson, F., Leckner, B., 2008. NO emission during oxyfuel combustion of lignite. Industrial and Engineering Chemistry Research 47 (6), 1835–1845. Bose, A.C., Dannecker, K.M., Wendt, J.O.L., 1988. Coal composition effects on mechanisms governing the destruction of NO and other nitrogenous species during fuel-rich combustion. Energy and Fuels 2 (3), 301–308. Dhungel, B., 2009. Experimental investigations on combustion and emission behaviour during oxy-coal combustion. In IFK, Stuttgart University: Stuttgart, Germany. Frassoldati, A., Faravelli, T., Ranzi, E., 2003. Kinetic modeling of the interactions between NO and hydrocarbons at high temperature. Combustion and Flame 135 (1–2), 97–112. Giménez-López, J., Millera, A., Bilbao, R., Alzueta, M.U., 2010. HCN oxidation in an O2 /CO2 atmosphere: an experimental and kinetic modeling study. Combustion and Flame 157, 267–276. Glarborg, P., Jensen, A.D., Johnsson, J.E., 2003. Fuel nitrogen conversion in solid fuel fired systems. Progress in Energy and Combustion Science 29 (2), 89–113. Kühnemuth, D., Normann, F., Andersson, K., Johnsson, F., Leckner, B., 2011. Reburning of nitric oxide in oxy-fuel firing – the influence of combustion conditions. Energy and Fuels 25 (2), 624–631. Mendiara, T., Glarborg, P., 2009. Ammonia chemistry in oxy-fuel combustion of methane. Combustion and Flame 156 (10), 1937–1949. Molina, A., Eddings, E.G., Pershing, D.W., Sarofim, A.F., 2000. Char nitrogen conversion: implications to emissions from coal-fired utility boilers. Progress in Energy and Combustion Science 26 (4), 507–531. Normann, F., Andersson, K., Leckner, B., Johnsson, F., 2008. High-temperature reduction of nitrogen oxides in oxy-fuel combustion. Fuel 87 (17–18), 3579–3585. Normann, F., Andersson, K., Leckner, B., Johnsson, F., 2009. Emission control of nitrogen oxides in the oxy-fuel process. Progress in Energy and Combustion Science 35 (5), 385–397. Normann, F., Andersson, K., Johnsson, F., Leckner, B., 2010. Reburning in oxy-fuel combustion: a parametric study of the combustion chemistry. Industrial and Engineering Chemistry Research 49 (19), 9088–9094. Pershing, D.W., Wendt, J.O.L., 1979. Relative contributions of volatile nitrogen and char nitrogen to NOx emissions from pulverized coal flames. Industrial & Engineering Chemistry Process Design and Development 18 (1), 60–67. Zabetta, E.C., Hupa, M., Saviharju, K., 2005. Reducing NOx emissions using fuel staging, air staging, and selective noncatalytic reduction in synergy. Industrial and Engineering Chemistry Research 44 (13), 4552–4561.
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
NOx reburning in oxy-fuel combustion: A comparison between solid and gaseous fuels Fredrik Normann ∗ , Klas Andersson, Filip Johnsson, Bo Leckner Chalmers University of Technology, Department of Energy and Environment, SE-412 96 Göteborg, Sweden
a r t i c l e
i n f o
Article history: Received 10 November 2010 Received in revised form 6 April 2011 Accepted 3 May 2011 Available online 28 May 2011 Keywords: NOx Nitrogen oxides Emission Reburning Oxy-fuel Combustion
a b s t r a c t In the development of oxy-fuel combustion, reburning of nitrogen oxides, recycled with the flue gases, has been investigated for primary NOx control. Reduction of between 50 and 80% of the recycled nitrogen oxides has been measured. The present work evaluates the performance of gaseous and solid fuels as agents for reduction of nitrogen oxides by comparing experimental and modelling work performed at Chalmers University of Technology with different fuels. It is shown that the reduction is similar during propane and lignite firing, but that the lignite has slightly higher reduction efficiency. Differences in combustion temperature and heterogeneous effects are possible explanations. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The reason for developing oxy-fuel power plants is to control the emission of carbon dioxide in carbon capture and storage systems. However, conventional pollutants, such as nitrogen oxides (NOx ), discussed in the present paper, must still be considered. The first emission regulations to control the emission of nitrogen oxides were established in the early 1970s, and since then regulations have become increasingly stringent and emissions have decreased. Today, the emission of nitrogen oxides is a critical design parameter in all power plants. Controlling NOx emissions into the atmosphere is required, but it is potentially important also for CO2 storage. In addition, it might be necessary to limit the level of NOx in oxy-fuel power plants because of technical limitations, such as avoiding corrosion of materials. The most common way to limit the emission of NOx in state-of-the-art power plants is through primary measures, which delay the mixing of fuel and oxidizer and limit combustion temperatures, but oxy-fuel combustion offers new options for secondary NOx control that could be beneficial (Normann et al., 2009). The conditions inherent in oxy-fuel combustion differ from those of air combustion and include low concentration of N2 , recycling of flue gases (containing NOx ), high concentration of combustion products, long gas residence time, and different temperatures.
∗ Corresponding author. Tel.: +46 31 772 1000; fax: +46 31 772 3592. E-mail address: [email protected] (F. Normann). 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.05.003
The combustion process can be divided into four stages, as shown in Fig. 1a, in which different NOx formation/reduction mechanisms are dominant. The following defines the end of each stage: I. Complete oxidation of hydrocarbon volatiles. II. Introduction of secondary oxygen; switching from reducing to oxidizing conditions. III. Complete oxidation of CO. IV. Frozen concentrations. In Fig. 1a the four stages are shown in sequence, as they would appear in plug-flow. Devolatilization and oxidation of the evolved species occur at the initial part of the turbulent flame, Stages I and II. During combustion of hydrocarbons, there is an accumulation of CO, which starts to oxidize after the completion of hydrocarbon oxidation. Stage III is at the boundary of Stages I and II where the secondary oxygen mixes with the primary stream and creates an oxidizing environment. Stage IV is the latter part of the flame where the peak temperature has been reached and the reaction activity starts to decline. It consists of char burnout and cooling of the flame until the concentrations are frozen. During all four stages of the combustion process, the nitrogen chemistry is active and includes numerous reactions, which form and reduce NOx . Different routes will dominate depending on the location in the flame and the associated combustion conditions. Fig. 1b summarizes the main reaction paths. There are two sources of nitrogen during combustion, molecular nitrogen present in the air and nitrogen chemically bound in the fuel (Fuel-N). Typically, in
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Fig. 1. (a) Definition of the four stages of the combustion process. Dashed vertical lines indicate the shift between stages and double arrows define zones with different stoichiometric ratios (). (b) Global mechanisms of NO formation and reduction in combustion. Vol-N denotes volatile nitrogen-containing compounds, typically HCN or NH3 . The roman numbers in brackets indicate in which stage each process is active.
pulverized coal combustion, more than 80% of the NO formed originates from Fuel-N (Pershing and Wendt, 1979). In Stage I, the fuel is decomposed, or pyrolised, and the nitrogen chemistry is dominated by the evolution of volatile nitrogen compounds in the fuel. This process is fast and, generally, the main devolatilization lasts some tens of milliseconds. The amount of nitrogen released as volatiles is highly dependent on coal type but also on temperature and can be up to 100% of the fuel-bound nitrogen. There are different opinions on the type and relative amount of nitrogen volatiles-release (Glarborg et al., 2003), but HCN together with NH3 are the most important species. The reburning and prompt routes which involve reactions with hydrocarbon radicals are active during Stage I. The overall contribution to NOx formation by the prompt mechanism is already insignificant in air-fired coal combustion, and it has, therefore, not gained any attention under oxy-fuel combustion where the low concentration of N2 can be expected to further decrease its importance. The reburning route, on the other hand, has achieved a great deal of attention in connection with oxy-fuel combustion as it may reduce the NOx recycled through the burner. In the second stage, the hydrocarbons are oxidized and the reburning and prompt routes are no longer active. Instead, the volatile nitrogen compounds formed in the first stage start to oxidize. This oxidation is critical to NOx formation as it governs the split between NO and N2 formation. The oxidation of volatile nitrogen compounds occurs in several stages and, eventually, either NO or N2 is formed. In Stage II, the environment is oxygen lean and N2 is typically the favoured product. However, in Stage III where the secondary oxygen is introduced to complete oxidation of the fuel, NO is the favoured product. In Stage IV, the activity is low. However, the oxidation of char is slow (it requires a few seconds for completion) compared to the gas-phase reactions and it may, therefore, be active also during this final stage. Both the evolution of NO and N2 from char and the reduction of NO by char are complex processes that are not fully understood (Molina et al., 2000). Furthermore, if the temperature is sufficiently high, the Zeldovich mechanism could be of importance to the emission also in the late stage of combustion. This mechanism is slow relative other gas phase reactions, but it does not depend on the presence of hydrocarbons and volatiles. During oxy-fuel combustion the nitrogen concentration is lower and less thermal NO is formed than during air-firing. Under certain conditions the Zeldovich mechanism may even be reversed to reduce NO (Normann et al., 2008). The homogeneous combustion chemistry is generally considered to be most important for the formation and reduction of NOx (Pershing and Wendt, 1979; Bose et al., 1988). However, because of the slow combustion of char, the possibility to influence the NO yield evolving from char-bound nitrogen by conventional combustion measures is limited. Therefore, the relative contribution to
the formation of NOx by char increases while the formation from volatile nitrogen is decreased. Furthermore, the reduction of NO by char is enhanced by the presence of CO, and the reduction could, thus, be of special interest in oxy-fuel combustion where the CO concentration is locally increased. The authors of this work have previously performed a series of investigations on the emission of nitrogen oxides from oxy-fuel combustion (Andersson et al., 2008; Kühnemuth et al., 2011; Normann et al., 2008, 2009, 2010). From the results it was concluded that the emission of nitrogen oxides, related to the amount of fuel supplied, is significantly reduced in oxy-fuel combustion compared to air-firing, because of increased reduction of formed and recycled NOx . The investigations have included experimental as well as modelling studies of coal and gas-fired oxy-fuel combustion and the aim has been to understand and optimize the oxy-fuel combustion process with respect to NOx emission. In the present paper, an overview of the reburning conditions in oxy-fuel combustion is presented, including a discussion on the influence from the oxy-fuel combustion atmosphere, involving the combustion parameters as well as different fuels. The analysis also includes a comparison between homogeneous and heterogeneous atmospheres with respect to their capability to reduce nitrogen oxides. 2. Methodology Experimental data obtained in the Chalmers 100 kW test facility are examined. These data originate from published studies (Andersson et al., 2008; Kühnemuth et al., 2011), and only a brief summary of the experimental unit, measurement equipment, operating conditions, and models used for evaluation is given here. The furnace is top-fired and operates with air or a mixture of oxygen and recycled flue gas (dry or wet). In oxy-fuel operation, oxygen (99.5% purity) is mixed into the recycled flue gas to achieve the desired concentration in the oxidizer. Furthermore, NO can be injected into the oxidizer in order to design experiments targeted on NO reduction. It is also possible to introduce oxygen downstream of the burner (800 mm from the furnace inlet), comparable to over-fire air (OFA), making it possible to operate the burner under sub-stoichiometric conditions. Measurements were performed inside the furnace through seven measurement ports located alongside the reactor from 46 to 1400 mm from the furnace inlet, as well as in the recycle loop. The concentrations of CO2 , CO, O2 , NO and total hydrocarbons were continuously measured during the experiments. Furnace temperatures were measured by suction pyrometers. Two measurement campaigns have been performed to study NOx emission: one with lignite and one with propane as fuel with conditions as given in Table 1. Since propane does not contain any nitrogen, NO is added to the recycled flue gases to represent
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Table 1 Summary of experimental conditions of the investigations on NOx in the Chalmers 100 kW oxy-fuel unit.
Fuel input Global stoichiometric ratio Burner stoichiometric ratio Oxygen concentration in oxidizer Fuel-bound nitrogen (dry, ash free) NO in oxidizer
Lignite
Propane
80 kW 1.18, (1.30, 1.41)a
81 kW 1.15
Same as global
1.15, (0.7–1.15)a
25, 27, 29%
27, 30, (25–37)%a
0.6 wt.%
0%
Depending on outlet (∼250 ppm)
Constant 500 ppm
a
For values within brackets measurements are only performed in the recycle loop.
recycled NOx , corresponding to a constant inlet NO concentration of 500 ppm. Thereby, the reburning reduction was isolated from NOx formation from fuel-bound nitrogen and gas-solid reactions. Furthermore, the residence time at high temperature (above 1600 ◦ C) was limited to minimize the influence of the Zeldovich mechanism. The lignite measurement campaign focused on varying the inlet oxygen concentration, while a broader parameter study, including also the stoichiometric ratio of the burner, was performed during the propane-firing tests. The gas composition and temperature profile in the furnace were measured during operation with both fuels. Combustion modelling was performed to evaluate and interpret the experimental results. The modelling focuses on combustion chemistry, and neither fluid dynamics nor heat transfer are modelled. Instead mixing is treated, assuming perfect plug flow, and the temperature is represented by a pre-defined profile. These assumptions are useful, as it is the nitrogen chemistry that is important to NOx formation, and several uncertainties in the combustion calculation are eliminated. However, when interpreting the experiments, imperfect mixing in the combustion process need to be considered. In the model, the mixing process is described by a continuous introduction of the oxidizer to the reactor in the first 0.5 m, according to Fig. 2. This mixing rate has been shown to give a fair representation of the furnace concentrations (Kühnemuth et al., 2011). The combustion chemistry is described by a detailed reaction mechanism, which is based on the work of Mendiara and Glarborg (2009) on C1, C2 and nitrogen chemistry under oxy-fuel conditions updated with a C3 subset from Frassoldati et al. (2003).
3. Results and discussion The reburning reduction is discussed in three sections. Firstly, the chemical effects of the CO2 -rich atmosphere in oxy-fuel combustion are presented. Secondly, the influence of the combustion conditions on reburning reduction is identified. These results are based on gas-fired flames and allow isolating the NOx reduction, because of the absence of fuel-bound nitrogen. Finally, the effect of fuel on the reburning efficiency is discussed, including a comparison between modelling and experiments. 3.1. Chemical effects on reburning in oxy-fuel combustion On a global basis, the reburning reduction can be described by the following three reactions, NO + CH1−3 → HCN + · · ·
(1)
HCN + OH, O, O2 → NO + · · ·
(2)
HCN + NO → N2 + · · ·
(3)
Thus, the reduction of NO depends on hydrocarbon radicals, and the oxidation of the HCN formed depends on the composition of the radical pool of chain-carrying radicals. During combustion, CO2 influences the important radical chemistry mainly through the CO oxidation reaction, CO + OH ↔ CO2 + H
(4)
and its importance to the chain-branching reaction, H + O2 ↔ OH + O
(5)
The chemical effects of CO2 in combustion have been investigated by means of modelling. Fig. 3 compares the concentration of the major reducing agents (CH3 and the sum of CH2 and CH, denoted CH1,2 ; Fig. 3a) and the major nitrogen species (NO and HCN; Fig. 3b) calculated with N2 and CO2 as bulk gas (Normann et al., 2010). The oxidation rate of the fuel is lowered by CO2 ; the peak concentration of hydrocarbon radicals (CH1–3 ) is lower and the residence time in the reducing zone (Stage I) is longer. This enables alternative reaction routes and changes the composition of the hydrocarbon radicals in oxy-fuel combustion to lower the concentration of CH1,2 (the peak is almost 500 times lower) and raises the CH3 concentration (the peak concentration is similar, but the reaction time is longer in CO2 ). The reburning reduction depends on the hydrocarbon radicals, and the NO reduction rate is lower in CO2 , as indicated by the slower increase in HCN compared to air firing. A difference in composition of hydrocarbon radicals is important when controlling the combustion process, because it affects the sensitivity of NOx reduction to combustion temperature and stoichiometric ratio. Furthermore, according to Reaction (4) and (5) the elevated concentration of CO2 in oxy-fuel combustion favours the formation of OH radicals and the consumption of H radicals. This alters the oxidation of HCN (Reaction (2) and (3)) formed during the reburning reduction (Reaction (1)). The oxidation of HCN to NO and N2 has been examined by detailed chemistry modelling (Normann et al., 2010; Mendiara and Glarborg, 2009; Giménez-López et al., 2010). The modelling shows that the split between NO and N2 depends on similar reactants in both atmospheres. The higher CO2 concentration in oxy-fuel combustion may increase the selectivity towards NO; however, the impact depends on the local stoichiometry or oxygen concentration. 3.2. Influence of combustion parameters
Fig. 2. Conditions for the plug-flow reactor modelling. The oxidizer is assumed to be continuously introduced to the fuel during the first 0.5 m from the furnace inlet.
In a previous work (Kühnemuth et al., 2011), the sensitivity of the reburning reduction to the combustion conditions was
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Fig. 4. The relative increase in NOx reduction when lowering the burner’s stoichiometric from 1.15 (The reduction at 1.15 is 16% and 27% during oxy-fuel and air combustion, respectively). Bars to the left represent oxy-fuel combustion (30% O2 in the oxidizer) and bars to the right correspond to air firing. The effect of temperature is indicated by the shaded area of the bars. Based on measurements by Kühnemuth et al. (2011).
Fig. 3. Concentration profile of (a) CH3 and CH1,2 , and (b) NO and HCN during combustion with the same oxygen concentration using either N2 or CO2 as bulk gas. The temperature is 1400 ◦ C and the stoichiometric ratio is 0.7. Ref.: (Normann et al., 2010).
quantified by propane experiments with NO injection in the oxidizer. The influence of temperature variations in these experiments was separated from the combustion parameters investigated by means of modelling. Fig. 4 shows the impact of the stoichiometry of the burner on NOx reduction (the increase in reduction at lower stoichiometries is shown relative to a stoichiometric ratio of 1.15). Besides achieving reducing conditions in the flame, lower burner stoichiometry also reduces the combustion temperature. Both of these effects enhance the NOx reduction by reburning; the relative importance of the decrease in combustion temperature is given in Fig. 4 as a shaded height of the bars. For example, during oxyfuel combustion with a stoichiometric ratio of 0.7 the reduction is about five times higher than the reduction at 1.15 and about half of the reduction increase is caused by the decrease in temperature. Reducing conditions make NOx decrease, but deep staging does not affect the reduction much further because of the reducing conditions (the white fields in Fig. 4 are relatively constant from case to case). Instead, the lower combustion temperature makes a further decrease in burner stoichiometric ratio beneficial for NOx reduction under oxy-fuel conditions (the shaded height in Fig. 4 increases as the ratio decreases). Furthermore, Fig. 4 compares the sensitivity to stoichiometric ratio of oxy-fuel combustion to air-firing (similar flame temperatures). In general, the reburning reduction is more sensitive to stoichiometric ratio under oxy-fuel conditions than
under air-firing. This is mainly because of an increased sensitivity to combustion temperature under oxy-fuel conditions, which may be explained by the changes in radical chemistry, as discussed above. The influence of inlet oxygen concentration (that also influences combustion temperature) was investigated in a similar way. The direct effect of the inlet oxygen concentration on NOx reduction is shown to be low, as the O2 concentration in the furnace is relatively independent of the oxygen concentration in the oxidizer, but the indirect effect of temperature has some importance. Furthermore, both the inlet oxygen concentration and the burner stoichiometry relate to the amount of flue gas recycled to the flame (higher stoichiometry or lower inlet oxygen concentration increase the recycling flow). The importance of the flue gas recycle can be illustrated by comparing the once-through reduction (excluding the recycle) to the total reduction (including the recycle). Fig. 5 shows the theoretical relation between these two reduction dimensions as a function of the amount of recycle and once-through reduction efficiency (at a ratio of unity there is no difference between once-through and total reduction). By definition, without recycle (0%) the once-through reduction equals the total reduction. Obviously, the total reduction is favoured by high gas recycle. However, the efficiency of the once-through reduction affects the importance of the recycle, and, at a high once-through reduction, the difference between the once-through and total reduction is small, independent of the amount of recycle. Therefore, the benefit of recycling flue gases in terms of NOx reduction is higher for processes of low once-through reduction efficiency. 3.3. Influence of fuel In literature, a series of fuels including gas, oil, biomass, and coal have been tested for NOx reburning in air combustion. Researchers disagree on the sensitivity of the reburning reduction to type of fuel, but there is a consensus that the hydrocarbon and nitrogen content are the main influencing factors (Zabetta et al., 2005). Fig. 6a compares the measured NO concentration at the centreline of the
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Fig. 7. Comparison of reburning efficiency between methane and propane.
Fig. 5. The theoretical relation between once-through and total reduction as a function of once-through reduction efficiency and the amount of flue gas recycle.
flame during combustion of propane doped with NO (to represent recycled NO) (Kühnemuth et al., 2011) and lignite oxy-fuel firing (Andersson et al., 2008). Fig. 6b compares two NO profiles calculated by the detailed reaction model (Kühnemuth et al., 2011) under conditions corresponding to the two fuels with the purpose of evaluating the course of events during the experiments in Fig. 6a. The
Fig. 6. a and b. Comparison between modelling and experiments with propane (30% O2 and 500 ppm NO in oxidizer) and lignite (29% O2 and 250 ppm NO in oxidizer from recycle). In the model methane and methane/HCN replaces lignite and propane as fuel. Other conditions are similar to those of the experiments.
model is a plug-flow reactor, see Fig. 2, where the fuel is introduced at the reactor inlet, while the oxidizer is gradually added during the first 0.5 m. Temperature profiles and inlet NO concentration corresponds to the two cases in Fig. 6a. Furthermore, HCN is added to the modelled fuel to represent the amount of fuel-bound nitrogen in the lignite investigated. In both cases the model predicts an initial accumulation of recycled NO in the near-burner region upstream of the first measurement position (the first peak in Fig. 6b). When combustion is initiated there is an extensive reduction of NO by hydrocarbon radicals (reburning), and a minimum in NO concentration is reached. Downstream of the minimum, the hydrocarbons are oxidized and the reducing reactions are terminated. The intermediate nitrogen species formed are oxidized, which, together with continued introduction of the NO-rich oxidizer, increases the NO concentration at the centreline of the furnace. In the lignite-fired case, the presence of fuel-bound nitrogen (HCN in the model) increases the formation rate of NO compared to the propane-fired case. In the model of Fig. 6, the mixing is completed at 0.5 m from the furnace inlet, but in the experiments the mixing is continued throughout the investigated combustion zone (1 m from furnace inlet). In the propane-fired case this is expressed by a slight increase in the NO concentration throughout the combustion zone, as the oxidizer contains 500 ppm of NO. In the lignite-fired case, the NO formation from fuel-bound nitrogen overshoots the NO concentration of the oxidizer (250 ppm) and the NO concentration at the centreline therefore decreases during the downstream parts of the flame. Overall, the centreline profiles of the two fuels behave in a similar way. The influence of type of hydrocarbon fuel on NO reburning is illustrated in Fig. 7. The same simulation as in Fig. 6b is performed with methane, propane, and a methane–propane mixture of a 9 to 1 ratio. Methane and propane behave differently in the nearburner region with methane yielding a higher NO peak before initiation of NO reduction. This behaviour could be attributed to a delay in initiation of methane combustion, which delays the introduction of hydrocarbon radicals to the flame zone. When a small amount of propane, which can initiate the combustion, is mixed with methane, the methane flame has an even lower peak than the pure propane flame in the near-burner region. The concentration profile in the near-burner region is also affected by the assumptions on mixing and temperature in this region (Kühnemuth et al., 2011). After complete oxidation of the hydrocarbon radicals (around 0.2 m from furnace inlet) the difference between the fuels is negligible; the oxidation of intermediates is the same for all fuels/fuel-mixtures. In coal combustion, small hydrocarbons
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Fig. 8. Radial temperature and NO profiles measured 215, 384, and 553 mm from the burner inlet during oxy-propane (30% O2 in oxidizer) and oxy-lignite (29% O2 in oxidizer) combustion.
(mainly C1 and C2) are released as volatiles and during the conversion of the tars. According to Fig. 7, the type of volatiles released should be of little importance to the reburning reduction compared to the point of ignition, which will depend on fuel properties and combustion conditions. Fig. 8 compares the radial profiles of temperature and NO measured at 215, 384, and 553 mm from the furnace inlet in the oxy-propane (30% O2 in oxidizer) and oxy-lignite (29% O2 in oxidizer) experiments. The peak temperature at the centre of the propane flame (Fig. 8a) is about 200–300 ◦ C higher than in the lignite flame (Fig. 8b). However, already at a distance of 100 mm from the centreline the temperatures are similar for the two fuels on all measurement levels. In contrast to the centreline NO profile (Fig. 6), there are obvious differences between the radial NO profiles of the fuels. In the propane-fired case (Fig. 8c), there is only a slight decrease in the NO concentration (500–470 ppm) from the inlet of the oxidizer to the flue gas, and the concentration of NO in the outer parts of the flame is similar throughout the furnace. The lignite-fired case (Fig. 8d) is more complex with formation of NO at the centre of the flame and increasing NO concentration at the outer flame boundaries (from 250 to 410 ppm). However, at 553 mm from the furnace inlet, both the propane and lignite profiles are well mixed in the radial direction with relatively uniform NO concentrations. There are, thus, systematic differences in temperature and furnace concentration profiles between the propane and the lignite cases that influence the reduction efficiency of NO (although the centreline concentration profiles are similar). Furthermore, there are no heterogeneous reactions in the propane experiments. Fig. 9 shows an estimate
of the reduction of NO present in the recycle gas for three inlet oxygen concentrations from the lignite and propane experiments. The reduction in the propane-fired experiments (Propane ) is calculated as the ratio of emitted and added NO in mg/MJ of fuel supplied, Propane = 1 −
NOemit NOadd
(6)
Fig. 9. Comparison between the reduction of recycled NO during propane (Kühnemuth et al., 2011) and lignite (Andersson et al., 2008) firing in the same experimental unit.
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4. Conclusion This article summarizes the experimental and modelling work performed at Chalmers University of Technology on reburning reduction in oxy-fuel combustion. It is shown that the reburning reduction is favoured by high recycle ratios, sub-stoichiometric conditions, and controlled combustion temperatures; compared to air-firing, reburning during oxy-combustion is more sensitive to temperature. The differences between solid and gaseous fuels are of particular interest. The reduction of NO is slightly higher during the oxy-lignite experiments compared to the oxy-propane experiments. The difference in reduction could be explained by the minor difference in combustion temperature observed in burning the two fuels; however, heterogeneous reduction could also contribute to the explanation. This statement requires verification by further investigation. Fig. 10. Reduction of NO during oxy-fuel (27% oxygen in oxidizer) with natural gas (black bars) and char of brown coal at different stoichiometric ratios taken from the work of Dhungel (2009).
The reduction in the lignite-fired experiments (Lignite ) is instead estimated as the ratio of emitted NO (mg/MJ fuel supplied) in air and oxy-fuel combustion, Lignite = 1 −
NOemit OXY NOemit AIR
(7)
Eq. (7) assumes that the difference between the emission of NO in air and oxy-fuel combustion originates entirely from the reduction of recycled NO (which is a reasonable assumption (Andersson et al., 2008)). As seen in Fig. 9, the reduction during propane-firing is consistently lower than that during lignite-firing. As discussed above, the difference in reduction between coal and propane may relate to heterogeneous reduction of NO as well as to the way the fuel-bound nitrogen is released to the gas-phase in coal combustion. For example, Dhungel (2009) showed in an experimental investigation that heterogeneous reactions could play an important role for the reduction of NO during oxygen-rich oxy-fuel combustion. Fig. 10 presents the results of Dhungel (2009) that compares the reduction of recycled NO during combustion of natural gas and char from lignite. The natural gas experiments indicate the reduction by homogeneous reactions, whereas the char gives an indication of reduction by heterogeneous reactions. High concentration levels of CO and poor performance of homogeneous reburning under oxygen-rich conditions explained the relatively high reduction by the char. The present investigation does not show heterogeneous reduction to be of such magnitude, even though the lignite-fired experiments indicate that the reduction of NOx possibly is enhanced, which might be attributed to heterogeneous NOx reduction. However, systematic differences between propane and lignite-firing, such as differences in combustion temperature, also influence the homogeneous reduction of NO. In fact, the temperature difference between the gas and coal-fired cases (around 300 ◦ C in peak temperature), may well explain the deviation in the reduction of recycled NO in Fig. 9 (c.f. Fig. 4).
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