A Simplified Kinetic Rate Expression for Describing the Oxidation of

The following rate expressions were obtained: r1 = 1.21 .... In the reactions above, the H2O and the 1/2H2 are only included to balance the reactions...
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Energy & Fuels 2001, 15, 1094-1099

A Simplified Kinetic Rate Expression for Describing the Oxidation of Volatile Fuel-N in Biomass Combustion Anders Brink,* Pia Kilpinen, and Mikko Hupa Process Chemistry Group, Åbo Akademi University, Lemminka¨ isenkatu 14-18B, FIN-20520 Turku, Finland Received November 29, 2000

A new model for describing the fuel-N oxidation to NO and N2 in biomass combustion is presented. The formulation is based on the assumption that in biomass combustion the fuel-N is released as ammonia. The model consists of two formal reactions describing the oxidation of volatilized fuel-N: NH3 + O2 ) NO + H2O + 1/2H2, and NH3 + NO ) N2 + H2O + 1/2H2. The rate expressions were extracted from perfectly stirred reactor simulations using a comprehensive mechanism. The rate of NH3 oxidation to NO was determined by adding the net rate of all reactions involving NH3. The rate was determined at conditions where the formation of NO was dominating. The rate of the reaction between NH3 and NO was obtained by adding the net reaction rate of all reactions involving N2. The following rate expressions were obtained: r1 ) 1.21 × 108T2e-8000/T[NH3][O2]0.5[H2]0.5, and r2 ) 8.73 × 1017T-1e-8000/T[NH3][NO]. The rates are given in mole‚cm-3‚s-1, the temperature in K and the concentrations in mole‚cm-3. The model is developed for use in CFD modeling of full-scale combustion devises. It describes the fuel-N chemistry well in flame-like conditions. In flue gas it predicts faster conversion than expected by a comprehensive mechanism.

Introduction Emission of nitrogen oxides has been of great concern since the 70’s. Today, even emission of NOx from smallscale combustion of biomass has become an issue. Fitting such combustion devices with NOx abatement systems would usually be far too expensive. A more attractive way to reduce the NOx emissions is to design the combustion devise in such a way that the NOx formation is minimized. In such a design process computational fluid dynamics (CFD) modeling can be of great importance. Biofuels have a lower heating value than most fossil fuels. In many cases the moisture content of the fuel is also high. As a consequence, the temperature in the furnace is relatively low and the formation of thermalNO is usually not a severe problem. Although these fuels often contain less nitrogen than fossil ones, the formation of NOx from fuel-bound nitrogen is the main source of NOx emissions. NOx can be formed from fuelbound nitrogen both though homogeneous as well as through heterogeneous reactions. Biomass is more volatile than coal, for example, which results in homogeneous chemistry dominating. Therefore design optimization with CFD mainly relies on a proper description of the gas-phase reactions of the volatile fuel-N components. In the literature a number of simplified models for fuel-N oxidation have been presented.1-5 Probably the * Corresponding author. Fax: +358 2 215 4780. E-mail: anders. [email protected]. (1) De Soete, G. G. Proceedings of the Fifteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1974; pp 1093-1102. (2) Mitchell, J. W.; Tarbell, J. M. AIChE J. 1982, 28 (2), 302-311.

most well-known ones are the model of de Soete 1 and that of Mitchell and Tabell2. De Soete’s1 overall reactions for NO and N2 formation from fuel-N consists of a number of expressions for the reaction rates depending on the local conditions. In CFD simulations, usually only four of these are used. In the experiments de Soete used when deriving his rate expressions, the temperature varied between 1824 K and 2490 K. In fact, in only one experiment was the temperature below 2100 K. Mitchell and Tarbell2 published a set of simplified reactions to describe the nitric oxide formation during pulverized coal combustion. In their model there are twelve reactions altogether, including heterogeneous reactions for the description of char combustion and reduction of NO by char. The rate constants of the two reactions describing oxidation of NH3 and formation of N2 from NH3 and NO were fitted from experimental data of the selective noncatalytic reduction (SNCR) process. The temperature range of the experiments was approximately 11001300 K. Mereb and Wendt3 have proposed a simplified mechanism for reburning chemistry. However, the validity of their model is limited to what they call “the slow kinetic regime” in reburning. There are also a number of models describing the nitrogen chemistry under SNCR conditions. Duo4 proposed a two-step mechanism for the SNCR mechanism that was developed at a constant oxygen concentration. A more recent model for the SNCR chemistry has been proposed by (3) Mereb J. B.; Wendt, O. L. Proceedings of the Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 1273-1279. (4) Duo, W. Ph.D. Thesis, Technical University of Denmark, 1990. (5) Brouwer, J.; Heap, M. P.; Pershing, D. W.; Smith, P. J. Proceedings of the Twenty-Sixth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1996; pp 2117-2124.

10.1021/ef0002748 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001

Oxidation of Volatile Fuel-N in Biomass Combustion

Brouwer et al..5 They proposed a seven-step mechanism that in addition to NH3 and NO also included HNCO, NCO, and N2O. There are several alternative procedures to test the reliability of the existing models for prediction of NOx emissions in CFD modeling. One alternative is to include them in the CFD and evaluate them directly simulating relevant applications. In the literature, a number of studies of NOx emissions from biomass combustion have been reported. Vakkilainen et al.6 have compared CFD predictions of NOx from a boiler burning spent pulping liquor with emission measurements. In their study, the nitrogen chemistry was described with the model of Brouwer et al.5 They found that the model was not very accurate at the condition occurring in a recovery boiler, although the temperature inside such a boiler is not very different from that in the SNCR process. Kja¨ldman7 modeled a peat-fired laboratory furnace and compared computed NOx emissions to measured ones. He compared three different models for the nitrogen chemistry. He found that the NH3 reactions in the model of Mitchell and Tarbell2 work well under these conditions and that the predictions agreed reasonably well with measurements. Coda Zabetta et al.8 calculated the fuel-N conversion in a bubbling fluidized bed combustor with four different simplified fuel-N mechanisms. In their study, the conversion of NH3 varied from almost total conversion to NO, to almost total conversion to N2, depending on which model was used. The difficulty of assessing the reaction mechanism in a CFD code is that there are numerous assumptions that must be made that will affect the results. The way the turbulence-chemistry interaction is modeled is also of great importance. Another approach to evaluate the reaction mechanisms is to apply them to simpler, but well-defined configurations such as calculations of ideal reactors. Using this approach, it is easier to eliminate the influence of other critical assumptions that may have to be made in CFD calculations. In a recent numerical study Norstro¨m et al.9 studied the behavior of available simplified mechanism for the description of NH3 oxidation under well-defined conditions. In their study a total of six different simplified models were investigated. The conclusion was that none of the existing ones could predict the NH3 oxidation reliably, and that a more accurate simplified mechanism for the description of NH3 oxidation at biomass combustion conditions is needed. On the basis of these studies one can state that there is a need for a new simplified model for the description of the gas-phase fuel-N conversion developed especially for those conditions prevailing in biomass combustion processes. In the present work new kinetic expressions for the conversion of fuel-N in biomass combustion is put forth. The global reactions are simple, being identical to those used in models already presented in the (6) Vakkilainen, E.; Kja¨ldman, L.; Taivassalo, V.; Kilpinen, P.; Norstro¨m, T. Proceedings of the International Chemical Recovery Conference, Tampa, FL, 1998. (7) Kja¨ldman, L. Proceedings of 4th International Conference on Technologies and Combustion for a Clean Environment (Lisbon, Portugal), 1997. (8) Coda Zabetta, E.; Norstro¨m, T.; Kilpinen, P.; Hupa, M. Proceedings of the Swedish-Finnish Flame Day (Va¨xjo¨, Sweden), 1999. (9) Norstro¨m, T.; Kilpinen, P.; Brink, A.; Vakkilainen, E.; Hupa, M. Energy Fuels 2000, 14 (5), 947-952.

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literature. In this way they are easy to use in CFD modeling. When estimating the new kinetic rate expressions it was assumed that the fuel-N in biomass combustion is released as NH3. The limitation to NH3 is supported by several studies of biomass gasification and pyrolysis. Nieminen et al.10 have reported 8001000 mg/m3 NH3 and only 25-45 mg/m3 HCN in the gasification gas from biofuel that mainly consisted of wood and paper. Leppa¨lahti et al.11 reported 3170-4600 ppm NH3 and 520-930 ppm HCN in their peat gasification experiments. Coda Zabetta et al.12 state that the conversion of fuel-N to NH3 varies from 70% to 95% for peat and wood. Aho et al.13 measured the composition of nitrogen-containing species in the pyrolysis gas of black liquor. They found that the main fixed-nitrogencontaining species was NH3 and that no HCN could be detected. Further, it was assumed that the pyrolysis gas mainly consists of CO2, CO, H2O, and H2, and that hydrocarbons could be neglected. The main reason for neglecting hydrocarbons is that this simplifies the nitrogen chemistry considerably, which facilitates the determination of rate constants. It also makes the use of only two reactions possible. The drawback is that,for example, reburning effects cannot be modeled. Determination of Reaction Rate Expressions. As a starting point it was assumed that a similar global two-step mechanism as that used by, for example, Mitchell and Tarbell2 should be used. The formal reactions in such a mechanism can be written:

NH3 + O2 ) NO + H2O + 1/2H2

(R1)

NH3 + NO ) N2 + H2O + 1/2H2

(R2)

where the first reaction describes the oxidation of NH3 to NO, and the second reaction the destruction of NO by NH3. None of these reactions are real ones, but they describe the overall behavior of a more complex chemistry. In the reactions above, the H2O and the 1/2H2 are only included to balance the reactions. To provide data for determining the rate constants of reactions R1 and R2 perfectly stirred reactor (PSR) calculations using a comprehensive mechanism were carried out. PSR conditions are believed to be most relevant when considering the final use of the model, i.e., CFD studies of combustion processes: some turbulence-chemistry interaction models even include the PSR as a building block.14 All calculations were carried out using a comprehensive mechanism consisting of elementary reactions. The mechanism is a subset of a mechanism referred to as KILPINEN97 used by Coda Zabetta et al.12 It corresponds to the first 138 reactions in their reaction mechanism. Six different temperatures were investigated: 900 K, 1100 K, 1300 K, 1500 K, 1700 K, and 1900 K. At each temperature three different residence times were used: 0.1, 1, and 10 ms. The (10) Nieminen, J.; Palonen, J.; Kivela¨, M. VGB PowerTech 1999, 79 (10), 69-74. (11) Leppa¨lahti, J.; Kurkela, E. Fuel 1991, 70, 491-497. (12) Coda Zabetta, E.; Kilpinen, P.; Hupa, M.; Ståhl, K.; Leppa¨lahti, J.; Cannon, M.; Nieminen, J. Energy Fuels 2000, 14 (4), 751-761. (13) Aho, K.; Vakkilainen, E.; Hupa, M. Tappi J. 1994, 77 (5), 121127. (14) Magnussen, B. F. Proceedings of Eighteenth International Congress on Combustion Engines (Tianjin, China), 1989.

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Figure 1. Rate of NH3 oxidation at 1100 K. The symbols represent the reaction rates obtained by adding the net rate of the reactions involving NH3. The squares represent results obtained with a residence time of 0.1 ms, the circles with 1 ms, and the triangles with 10 ms. The dashed line shows the assumed relation between the reaction rate and concentrations used for determining the rate expressions for reaction R1.

mixture consisted of a presumed pyrolysis gas mixed with different amounts of oxygen. The oxygen content of the mixtures varied from 1 vol % to 10 vol %. In all these cases it was assumed that the mixtures contained 1000 ppm NH3. Also a number of flue gas-like mixtures were calculated. Here too the temperature, residence time, and the oxygen content varied in the same way. In contrast to the first cases, referred to as flame-like conditions, where the sum of CO and H2 was 8.0 vol %, in these mixtures they were only 1.0 vol %. It was assumed that the flue gas either contained 1000 ppm NH3 and 0 ppm NO or 500 ppm of NH3 and 500 ppm of NO. The rate of the NH3 oxidation reaction, i.e., R1, was determined by adding the net rates of all elementary reactions in which NH3 takes part. It was found that under those conditions where the NO formation is favored three reactions were responsible for almost all consumption of NH3. These are NH3 + H ) NH2 + H2; NH3 + O ) NH2 + OH; and NH3 + OH ) NH2 + H2O. Of these, the last reaction was the most important one. Since the aim of this work was to develop a model for use in CFD modeling of full-scale combustion devices, preferably not using a more complex scheme than that given by R1 and R2, the classical steady-state assumption was used to relate the OH radical to the main components. This is done by assuming that the following reactions are in partial equilibrium: H2 + OH ) H2O + H; H + O2 ) OH + O; and O + H2 ) OH + H. The outcome of this analysis is that the OH radical is proportional to the square root of the product of the O2 and the H2 concentrations. Next, for each temperature the reaction rate was 0.5 0.5 XH2 where X stands plotted against the term XNH3XO 2 for the mole fraction. Figure 1 shows an example of such a plot for a temperature of 1100 K. There is a considerable scatter in this plot, even after removing those points representing results of calculations where the mixture was not ignited. However, basing the estimates of the reaction rate coefficients only on those points corresponding to the largest reaction rates, a clear correlation could be found. These points represent mainly results obtained at the shortest residence times

Brink et al.

Figure 2. Rate coefficient for the NH3 oxidation reaction (reaction R1) as a function of temperature. The line shows the rate given by the modified Arrhenius expression. The squares represent the rate coefficients extracted from the PSR calculations with the comprehensive mechanism.

Figure 3. Rate of N2 formation at 1100 K. The symbols represent the reaction rates obtained by adding the net rate of the reactions involving N2. The squares represent results obtained with a residence time of 0.1 ms, the circles with 1 ms, and the triangles with 10 ms. The dashed line shows the assumed relation between the reaction rate and concentrations used for determining the rate expressions for reaction R2.

Figure 4. Rate coefficient of reaction R2 as a function of temperature. The line shows the rate given by the modified Arrhenius expression. The squares represent the rate coefficients extracted from the PSR calculations with the comprehensive mechanism.

in the calculations. At such a residence time the conversion of NH3 proceeds almost according to the stoichiometry of reaction R1 and the influence of route R2 can be neglected. From the rate coefficients determined in the temperature interval 900 K to 1900 K the following Arrhenius-

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Figure 5. Comparison between results obtained with a comprehensive mechanism, the model of Mitchell and Tarbell2 and the model developed in this work. The figures show results for a typical biomass pyrolysis gas containing 1000 ppm NH3. The results in the left column are for λ ) 0.7 and the right for λ ) 1.3, where λ is the stoichiometric ratio. In both cases the temperature was held constant at 1100 K.

Figure 6. Comparison between results obtained with a comprehensive mechanism, the model of Mitchell and Tarbell2 and the model developed in this work. The figures show results for a typical biomass pyrolysis gas containing 1000 ppm NH3. The results in the left column are for λ ) 0.7 and the right for λ ) 1.3, where λ is the stoichiometric ratio. In both cases the temperature was held constant at 1500 K.

like expression was obtained:

involved in. Here too, the elementary reactions forming N2 were identified. It was found that the final step of N2 formation mainly included very fast reactions of radicals that have been formed in reactions between NHi and NO. To keep the model simple, it was assumed that the reaction rate of this formal reaction is first order in NH3 and NO. In Figure 3 the reaction rate of N2 formation at 1100 K calculated by adding all reactions N2 is involved in is plotted together with the estimated dependence on the level of NH3 and NO. The suggested rate expression for this reaction is

r1 ) 1.21 × 108T2e-8000/T[NH3][O2]0.5[H2]0.5 where the reaction rate r1 is given in mol cm-3 s-1, the temperature T in Kelvin, and the concentration in mol cm-3. Figure 2 shows the estimated rate coefficient as a function of temperature as well as the values obtained by proposed expression. While determining the reaction coefficients it was also noticed that the rate coefficient of this reaction was lower at the flue gaslike conditions than at flame-like conditions, although in both situations the reaction NH3 + OH ) NH2 + H2O largely dominated the consumption of NH3. An analysis of this elementary reaction revealed that at flue gaslike conditions, the backward rate was significant, whereas at flame-like conditions the backward rate was negligible. This points to the difficulties in determining a simple reaction rate expression valid for every condition. It can be noted that Mereb and Wendt3 in their development of a reburning mechanism calculated the NHi concentrations assuming that the reactions NHi + OH ) NHi-1 + H2O where i ) 1, 2, 3 were in partial equilibrium. Following the same procedure as for reaction R1, the rate of reaction R2 was determined by analyzing the formation rate of N2. The formation rate of N2 was calculated analogously to the destruction rate of NH3, i.e., by adding all elementary reactions that N2 is

r2 ) 8.73 × 1017T-1e-8000/T[NH3][NO] where r2 is given in mol cm-3 s-1, the temperature in K, and the concentrations in mol cm-3. When estimating the rate expression for R2, points corresponding to nonreacting mixtures have been excluded. Figure 4 shows the rate coefficients obtained with the suggested expression compared to those based on the results from the PSR calculations. Evaluation of the New Two-Step Model. In the calculations done for evaluation purpose the CO/H2/O2 chemistry was described with the same 29 elementary reactions as in the calculations performed for obtaining data, but the 108 reactions describing the nitrogen chemistry in the comprehensive mechanism were replaced by the two global reactions. To be able to use

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Figure 7. Comparison between results obtained with a comprehensive mechanism, the model of Mitchell and Tarbell2 and the model developed in this work. The figures show results for typical biomass flue gas containing 500 ppm NH3 and 500 ppm NO. The results in the left column are for a flue gas obtained from burning the pyrolysis gas with λ ) 0.9 and the right with λ ) 1.1, where λ is the stoichiometric ratio. In both cases the temperature was held constant at 1100 K.

Figure 8. Comparison between results obtained with a comprehensive mechanism, the model of Mitchell and Tarbell2 and the model developed in this work. The figures show results for typical biomass flue gas containing 500 ppm NH3 and 500 ppm NO. The results in the left column are for a flue gas obtained from burning the pyrolysis gas with λ ) 0.9 and the right with λ ) 1.1, where λ is the stoichiometric ratio. In both cases the temperature was held constant at 1500 K.

the PSR code15 the subroutine CKWYP from the CHEMKIN library16 returning the molar reaction rates was modified such that the reaction rates of the two reactions describing the nitrogen chemistry were calculated according to the suggested expression. In these calculations the formal reactions were expressed in slightly different way: here it was assumed that H was formed instead of 1/2 H2. Figures 5 and 6 show that in combustion under fuelrich conditions the new model underestimates the rate of the reaction between NH3 and NO. Under fuel-lean conditions the agreement between the results obtained with the new mechanism and the results obtained with the comprehensive mechanism are good. A probable reason for this is that in the data on which the estimates of the rate coefficients are based, fuel-lean conditions are better represented since some of the fuel-rich mixtures did not react. Especially at the lower end of the temperature range that was investigated was this a problem. At 1100 K the model of Mitchell and Tarbell2 predicts too slow a conversion, particularly the oxidation of NH3 to NO is significantly underestimated. At 1500 K and fuel-rich conditions the model of Mitchell and Tarbell2 is more accurate than the one proposed in

this work for long residence times. Here the model of Mitchell and Tarbell2 is able to predict the formation of N2 well. However, under fuel-lean conditions the importance of the N2 formation is underestimated, whereas the new model under these conditions represents the chemistry nicely. A large part of the inaccuracies in the predictions with the new two-step model can be attributed to problems associated with the OH partial equilibrium assumption. At very short residence times in particular there are almost no reactions according to the comprehensive mechanism and the OH radical level is very low. However, assuming that the OH radical is proportional to the square root of the product of the O2 concentration and the H2 concentration yields the highest estimates of the OH radical level under these conditions. For this reason, the largest disagreement between the different predictions is found under these conditions. If the fresh mixture did not contain H2, there would not be this type of problem. Under flue gas-like conditions the situation is different. Figure 7 shows the result for a temperature of 1100 K. Here, the reaction rates are almost negligible according to the comprehensive mechanism. The model of Mitchell and Tarbell2 also predicts low reaction rates. With the two-step model much higher reaction rates are predicted. From Figures 7 and 8 it can be concluded that under these conditions the new mechanism overestimates the NH3 oxidation rate. Under these conditions

(15) Glarborg, P.; Kee, R. J.; Grcar, J. F.; Miller J. A. Report No. SAND86-8209, Sandia National Laboratories, Livermore, CA, 1986. (16) Kee, R. J.; Rupley, F. M.; Miller J. A. Report No. SAND898009B, Sandia National Laboratories, Livermore, CA, 1991.

Oxidation of Volatile Fuel-N in Biomass Combustion

too the errors originate to a large extent from the assumptions of partial equilibrium for the OH radical. Conclusions A two-step reaction model with rate expressions developed especially for the description of fuel-N chemistry in combustion of biomass is presented. The reaction rates are based on simulations of biomass pyrolysis gas combustion in a PSR with a comprehensive elementary mechanism. The rate expression for the NH3 oxidation is assumed to be proportional to the square root of the product of the O2 and the H2 concentrations. This is a consequence of the fact that in the calculations with the comprehensive mechanism the most important reaction for destruction of NH3 was NH3 + OH ) NH2 + H2O. The dependence on the O2 and H2 concentrations are obtained when making a conventional partial equilibrium assumption for the OH radical. The rate expression of the global reaction between NH3 and NO leading to N2 is assumed to be first order in NH3 and NO. Analysis with the comprehensive mechanism showed that N2 mainly was formed through reactions where radicals produced in reactions between NHi and NO take part. The nitrogen chemistry is well represented in flamelike conditions. Under these conditions the new model

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better represents the chemistry than the one of Mitchell and Tarbell2 that was used for comparison. In flue gaslike conditions, on the other hand, the model of Mitchell and Tarbell2 is often better. This trend is in agreement with the conditions used when determining the reaction rates. For the new model flame-like conditions were prioritized, whereas the rate expressions in the one of Mitchell and Tarbell2 are extracted from experimental data of the SNCR process. When using overall reaction rates it is important to apply them to conditions they are intended for. The new kinetic expressions reported in this study are suitable for modeling of the chemistry of volatile-nitrogen in the combustion zone. When modeling the SNCR process, other simplified mechanisms could be used. Acknowledgment. This work is a part of the activities at the Åbo Akademi Process Chemistry Group within the Finnish Centre of Excellence Program (20002005) by the Academy of Finland. The financial support by the Finnish National Technology Agency, AndritzAhlstrom, Fortum Power and Heat Oy, and Kvaerner Pulping Oy is gratefully acknowledged. We also thank Dr.-Ing. Christian Mueller and Edgardo Coda Zabetta for their support in analyzing the reaction rates. EF0002748