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Experimental Study of NOReduction through Reburning of Biogas Fan Zhi-lin,†,‡ Zhang Jun,*,‡ Sheng Chang-dong,‡ Lin Xiao-feng,‡ and Xu Yi-qian‡ Department of Material Science & Engineering, Hohai UniVersity, Nanjing 210098, China, and Education Ministry Key Laboratory on Clean Coal Power Generation and Combustion Technology, Southeast UniVersity, Nanjing 210096, China ReceiVed July 4, 2005. ReVised Manuscript ReceiVed January 18, 2006
Reburning pyrolysis gas from biomass (i.e., biogas) for NO reduction was investigated experimentally. All experiments were performed on a bench-scale test facility. Both the flue gas and the biogas used in the experiments were simulated gases. In the experiments, the influences of individual components of biogas, reburning temperature, residence time, SO2 concentration, initial NO concentration, moisture, and equivalence ratio on the NO reduction efficiency were studied. Experimental results show that biogas reburning can effectively reduce NO in simulated flue gas and that the components of biogas affect the NO reduction efficiency greatly. In particular, cyclopropane plays an important role. Among all those considered, the temperature and the residence time are favorable factors, while the SO2 concentration and the initial NO concentration are adverse parameters. Increasing the equivalence ratio benefits NO reduction initially and reaches a maximum value at about 1.15, after which the impact on NO reduction is reversed. The effect of moisture on the NO reduction is quite weak.
1. Introduction Nitrogen oxides (NOx) is one of the most important air pollutants generated from coal combustion, which has been drawing wide attention because of its particular environmental hazard and difficulty of reduction. Many technologies have been developed to reduce the NOx emissions from pulverized coalfired power plants. Among them, reburning technology is recognized as one of the most promising methods because of its economy and efficiency.1 Actually, reburning means a staged combustion. From bottom to top, the combustion volume in a pulverized-coal fired furnace can be divided into three zones: primary zone, reburning zone, and burnout zone. In the primary zone, about 80% fuel is combusted under a fuel-lean condition; then in the reburning zone about 20% reburning fuel is added to generate a fuel-rich condition, and as a result, NOx from the primary zone is converted into N2 in this zone; finally, in the burnout zone, additional air is provided for the burnout of the fuel.2 Many kinds of fuel have been investigated as reburning fuels,3-9 among which natural gas proved to be of prime interest * Corresponding author: fax +86 025 83793612; e-mail
[email protected]. † Hohai University. ‡ Southeast University. (1) Dagaut, P.; Lecomte, F. Fuel 2003, 82, 1033-1040. (2) McCahey, S.; McMullan, J. T.; Williams, B. C. Fuel 1999, 78, 17711778. (3) Liu, H.; Hampartsoumian, E.; Gibbs, B. M. Fuel 1997, 76, 985993. (4) Bertran, C. A.; C. Marquesa, S. T.; Filho, R. V. Fuel 2004, 83, 109121. (5) Zhong, B.; Fu, W. J. Eng. Thermal Energy Power 1999, 14 (6), 419423. (6) Dagaut, P.; Lecomte, F.; Chevailler, S.; et al. Fuel 1999, 78, 12451252. (7) Dagaut, P.; Luche, J.; Cathonnet, M. Fuel 2001, 80, 979-986. (8) Dagaut, P.; Luche, J.; Cathonnet, M. Combust. Flame. 2000, 121, 651-661. (9) Harding, N. S.; Adams, B. R. Biomass. Bioenergy 2000, 19, 429445.
for NOx reduction.10 Recent studies showed that, when used as a reburning fuel, biomass almost matched natural gas in NOx reduction efficiency.11-13 Compared with other fuels, biomass as reburning fuel has several obvious advantages: biomass fuels are significantly lower in N and S contents, which means their production of NOx and SO2 is also significantly lower; they have a higher content of volatile matter, leading to a high NOx reduction efficiency; furthermore, they are renewable and CO2neutral fuels. However, it should be noticed that biomass needs to be pulverized appropriately for easy transporting to the furnace and for achieving a high burnout, which increases the complexity of the fuel handling system if biomass is directly used as a reburning fuel. Moreover, the contents of active alkali metals and chlorine in biomass fuels are higher,14 which increases the potential of ash deposition and corrosion on heat exchanger surfaces of metal tubes. Although the fly ash may capture some of the in-flame harmful alkali species, it is known that there is also a certain amount of alkali metal in the coal. Once the total amount of the active alkali metals reaches a certain value, dangerous deposition and corrosion will happen, which surely affects the operational performance of pulverized coal-fired boiler. For these reasons, it seems that reburning biomass directly should be avoided. It might be a better approach to pyrolyze biomass separately and then use the pyrolysis gas (i.e., biogas) as a reburning fuel. In this circumstance, a separate reactor can be operated at a much lower temperature for the pyrolysis of biomass so as to decrease the release of alkali (10) Bilbao, R.; Millera, A.; Alzueta, M. U.; et al. Fuel 1997, 76, 14011407. (11) Maly, P. M.; Zamansky, V. M.; Ho, L.; et al. Fuel 1999, 78, 327334. (12) Adams, B. R.; Harding, N. S. Fuel Process. Technol. 1998, 54, 249263. (13) Smoot, L. D.; Hill, S. C.; Xu, H. Prog. Energy Combust. Sci. 1998, 24, 385-408. (14) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; et al. Prog. Energy Combust. Sci. 2000, 26, 283-298.
10.1021/ef050198e CCC: $33.50 © 2006 American Chemical Society Published on Web 02/24/2006
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Figure 2. Influence of C3H6 on NO reduction ([NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05). Figure 1. Schematic of the experimental system.
chlorides and thereby to low the risk of the deposit and corrosion on the heat exchanger surfaces of the boiler.15 In addition, the solid product of biomass pyrolysis (i.e., biochar) is a valuable material, which can be used as a sorbent for flue gas cleaning. It is well-known that hydrocarbons are the main constituents in biogas. There have been numerous studies on NO reduction with different hydrocarbon species as reburning fuels.1,6-8,10,16-18 Compared with other hydrocarbon gaseous fuels, such as natural gas, biogas usually has a lower heating value, which may cause the changes in the operation and efficiency of NOx reduction. However, up till now, only little work has been concerned with biogas as a reburning fuel.19 In the present work, experiments were carried out with a model gas in order to understand the effects of reburning biogas on the reduction of NOx. 2. Experimental Section All experiments were performed on a bench-scale test facility. The whole system is schematically shown in Figure 1. An annular electric furnace with a temperature control unit is used in order to simulate the reaction conditions in the reburning zone. The temperature of the furnace is measured by a platinum-rhodium thermocouple. The precision of the temperature control is (1 K. The reactor is a tube with an inside diameter of 20 mm. Two embranchments with inside diameters of 8 mm are installed on the top of the reactor, through which biogas and flue gas are fed to the reactor, respectively. Several rotameter flowmeters are used to measure and control the flow rates of reaction gases. The concentrations of the treated gases are measured through an automobile exhaust analysis apparatus (FGA-4015), whose measurement precision of NO was (1 ppm. Both the flue gas and the biogas used in the experiments were model gases. The flue gas was synthesized by adjusting the flux of individual gases used during experimental process to have a composition of 16.8% CO2, 1.98% O2, 600-1200 ppm NO, 0-3000 ppm SO2, and pure N2. According to our measurements on the products of fast pyrolysis of seven kinds of biomass (i.e., sawdust, cornstalk, rice chaff, beanstalk, cotton stalk, metasequoia bark, and miscellaneous leaf),20 biogas was found to be mainly composed of H2, CO, CH4, C3H6, and CO2. Therefore, these five gas species were used to synthesize the biogas. Nine containers of the biogas consisting of 9-11.75% H2, 30-37.4% CO, 10-18.35% (15) Michelsen, H. P.; Frandsen, F.; Dam-Johansen, K.; et al. Fuel Process. Technol. 1998, 54, 95-108. (16) Han, X.; Ruckert, F.; Schnell, U.; Hein, K. R. G.; Koger, S.; Bockhorn, H. Combust. Sci. Technol. 2003, 175, 523-544. (17) Chagger, H. K.; Goddard, P. R.; Murdoch, P.; et al. Fuel 1991, 70, 1137-1142. (18) Bilbao, R.; Millera, A.; Alzueta, M. U.; et al. Fuel 1997, 76, 14011407. (19) Dagaut, P.; Lecomte, F. Energy Fuels 2003, 17, 608-613. (20) Jun, Z.; Zhilin, F.; Xiaofen, L.; et al. Dongnan Daxue Xuebao [J. Southeast UniV. (Nat. Sci. Ed.)] 2005, 35 (1), 16-19 (in Chinese).
CH4, 1-3% C3H6, and 27.23% CO2 were synthesized in advance, balanced by pure N2. In the experiments, the influence of each component of the biogas, the furnace temperature (T), the residence time (τ), the SO2 concentration ([SO2]), the initial NO concentration ([NO]), the presence of moisture ([H2O]), and the equivalence ratio (φ) on NO reduction was investigated. Here the equivalence ratio was defined following Dagaut and Lecomte:19 φ ) (fuel %/O2 %)/(fuel %/O2 %)stoic. The efficiency of NO reduction (η) was defined as η)
[NO]inlet - [NO]outlet [NO]inlet
× 100
where [NO]outlet stands for the concentration of NO in the exhaust, which was obtained by direct measurement; [NO]inlet stands for the concentration of NO in the inlet mixed streams, which was gotten by calculation as follows: [NO]inlet )
Qflue [NO]flue Qflue + Qbio
where [NO]flue is the measured concentration at the outlet of the mix container (see Figure 1), and Qflue and Qbio represent the fluxes of the flue gas and the biogas, respectively. The experimental process is described as follows. First, each flux from gas sources was regulated through flowmeters according to a certain value calculated beforehand. In this way those controlling parameters mentioned previously could be maintained as the considered experimental case. Second, the flue gas concentration was measured at the outlet of mix container to check the model flue gas (see Figure 1). Third, the exhaust concentration was measured at the outlet of reactor. Finally, the efficiency of NO reduction was calculated as its definition.
3. Results and Discussion 3.1. Effects of Biogas Components. The biogas composed of 11.75% H2, 37.4% CO, 18.35% CH4, 2.0% C3H6, 27.23% CO2, and the balance N2 was used as a base case (benchmark biogas). The influence of individual biogas components on the NO reduction was studied by varying their proportions in the model biogas, respectively, keeping the proportions of the other components constant and balancing the gas by N2. In this set of experimental cases, the furnace temperature, residence time, SO2 concentration, and initial NO concentration of flue gas and the moisture and the equivalence ratio were fixed (i.e., T ) 1273 K, τ ) 1 s, [SO2] ) 0 ppm, [NO] ) 1000 ppm, [H2O] ) 0%, and φ ) 1.05). The experimental results are shown in Figures 2-5. The effects of the components studied on NO reduction efficiency are shown in Figures 2-5. As can be seen from Figures 2-5, the different components of biogas have different effects on the NO reduction. The NO reduction efficiency increases when the proportion of CH4 or C3H6 increases; this effect is especially prominent for C3H6. In contrast, increasing
NO Reduction
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Figure 3. Influence of H2 on NO reduction ([NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05).
Figure 6. Temperature influence curve (benchmark biogas, [NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, φ ) 1.05).
Figure 4. Influence of CH4 on NO reduction ([NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05).
Figure 7. Residence time influence curve (benchmark biogas, [NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, T ) 1273 K, φ ) 1.05).
Figure 5. Influence of CO on NO reduction ([NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05).
Figure 8. SO2 concentration influence curve (benchmark biogas, [NO] ) 1000 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05).
the proportion of H2 or CO leads to a decrease in the NO reduction efficiency. CO may be converted into CO2, which does not favor NO reduction according to the mechanism mentioned in the reference,19 in which reaction 1 might happen:
that, to achieve a higher NO reduction efficiency in practice, one can optimize pyrolysis conditions to produce a biogas with a higher hydrocarbon production. 3.2. Influences of Reburning Temperature. The influence of the operating temperature on the NO reduction efficiency is shown in Figure 6. It can be seen that the efficiency increases with an increase in the reburning temperature. From 1173 to 1273 K, a sharp increase in NO reduction efficiency exists. It is well-known that, to maintain a reaction process, the energy of the reactant must be high enough to overcome the activation energy. When the temperature is increased, the energy of the reactants becomes higher and the reactants are more easily converted to products. This is the reason the NO reduction efficiency increases with increasing reburning temperature. The sharp increase in the curve indicates that some key reactions in the reburning process are of higher activation energy, and only when the temperature reaches a certain value are those key reactions active, leading to a significant improvement of the NO reduction efficiency. 3.3. Influence of Residence Time. Figure 7 indicates that increasing the residence time favors the NO reduction, which is consistent with the results of Harding et al.9 and Maly et al.11 In a shorter time, biogas is not fully consumed, and extending the residence time will certainly enhance the chance for unburned biogas to reduce NO, which results in improvement of the total NO reduction efficiency. This result implies that the inject point of the reburning biogas should be kept enough
CO2 + N ) CO + NO
(1)
Shen et al.21 also found that increasing the CO concentration in the reburning fuel feedstock was not beneficial to NO reduction. H2 can be converted into H and OH, which can affect NO reduction through reactions 2 and 3:
HNO + OH ) NO + H2O
(2)
HNO + H ) NO + H2
(3)
CH4 and C3H6 can be converted into CH3, which can accelerate NO reduction through the following reactions:
CH3 + NO ) HCN + H2O
(4)
CH3 + NO ) H2CN + OH
(5)
According to the above result, it could be concluded that the hydrocarbon species are more efficient as reducers for NO reduction than hydrogen and carbon monoxide. This implies (21) Shen, B.; Yao, Q. Huanjing Kexue Xuebao [Acta Sci. Circumstantiae] 2002, 22, 677-682 (in Chinese).
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Figure 9. Initial NO concentration influence curve (benchmark biogas, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K, φ ) 1.05).
Figure 10. Equivalence ratio influence curve (benchmark biogas, [NO] ) 1000 ppm, [SO2] ) 0 ppm, [H2O] ) 0%, τ ) 1 s, T ) 1273 K).
distance away from the outlet of the furnace in order to maintain enough residence time in practical applications. 3.4. Effect of SO2. Figure 8 shows that the presence of SO2 in the flue gas has a negative effect on the NO reduction, as a higher SO2 concentration obviously from the figure results in a lower NO reduction efficiency. The same inhibition behavior of SO2 was observed in the experiments carried out by Dagaut and Lecomte1 using a mixture of methane, ethylene, and acetylene as reburning gas. According to the kinetic modeling result of Dagaut and Lecomte,1 the inhibition of SO2 to NO reduction is due to the following sequence of reactions:
3.7. Influences of Moisture in Flue Gas. It should be mentioned that dry flue gas was used in all of the above experiments. However, the real flue gas always contains moisture. To understand the effect of moisture on the NO reduction, the experiments were performed without and with H2O (i.e., 8.35% H2O added into the dry flue gas). In both cases, other parameters are same as follows: benchmark biogas, T ) 1273 K, τ ) 1 s, [SO2] ) 0 ppm, [NO] ) 1000 ppm, and φ ) 1.05. It was found that the effect was not significant and NO reduction efficiency decreased from 52.19% to 51.91% when 8.35% H2O was added. This slight effect can be attributed to two reasons: one reason might be that the added moisture can absorb a part of the heat from surrounding gases, leading to a decrease in the actual temperature of the reactive gas to some extent, and another might be that the residence time of the flue gas in the reactor may decrease when H2O is added, because of the expansion of the moisture at the high temperature. Therefore, it can be concluded that the effect of moisture in actual flue gas on the NO reduction is very weak.
H + SO2 + M ) HOSO + M
(6)
HOSO + H ) SO2 + H2
(7)
H + H + M ) H2 + M
(8)
where reaction 8 acts as a termination process. Though the reburning gas used here is different from that in the reference,1 as a kind of middle product, H was produced in both studies. So the same mechanism can be applied here to explain the experimental results. 3.5. Influence of Initial NO Concentration. The initial NO concentration in the flue gas affects NO reduction greatly, as shown in Figure 9. At the temperature of 1273 K, NO reduction efficiency decreases nearly linearly with increasing initial NO concentration (see Figure 9). With the same equivalence ratio, the quantity of biogas is fixed, and correspondingly the NO reduction ability is limited. So the change in the equivalence ratio is necessary in order to keep a higher NO reduction efficiency, when the amount of NO produced is changed. 3.6. Influences of Equivalence Ratio. Figure 10 presents the effect of the equivalence ratio on the NO reduction. With the increasing of the equivalence ratio, the NO reduction efficiency increases gradually to reach a maximum value of about 65% at about 1.15 and then decreases with a further increase of the equivalent ratio. The trend is quite similar to the results obtained by Dagaut et al. using acetylene6 and propane7 as reburning fuel, in which a maximum NO reduction was observed only slightly above the stoichiometric condition. The trend of the curve indicates that, in practical applications, the biogas should be injected into the reburning zone with a suitable flux maintaining an equivalence ratio of about 1.15 so as to get an ideal NO reduction efficiency.
4. Conclusion An experimental study on the NO reduction efficiency of reburning biogas under simulated conditions in a reburning zone was performed. The study shows that the components of biogas affect NO reduction efficiency greatly. Higher proportions of methane and cyclopropane are beneficial for the NO reduction efficiency and the effect is especially prominent for cyclopropane, while higher CO and H2 proportions have a negative effect on the NO reduction. The presence of SO2 restrains the NO reduction, and the higher the SO2 concentration in the flue gas, the greater the effect is. The experimental results also indicate that the moisture in flue gas affects the NO reduction weakly. With increasing equivalence ratio, the NO reduction efficiency increases gradually to reach a maximum value of about 65% at about 1.15 and then decreases with further increase of the equivalent ratio. NO reduction efficiency decreases nearly linearly with the increase of the initial NO concentration. Increasing residence time or reburning temperature favors the NO reduction. Acknowledgment. This work is funded by National Nature Science Foundation of China (50276012). EF050198E
