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Communications Reducing N2O Emission by Co-Combustion of Coal and Biomass D. C. Liu,* T. Mi, B. X. Shen, and B. Feng National Lab of Coal Combustion, Huazhong University of Science & Technology, Wuhan, 430074, China
Franz Winter Technical University of Vienna, Austria Received May 1, 2001. Revised Manuscript Received October 30, 2001 1. Introduction N2O is a sharp greenhouse gas. It is responsible for the destruction of ozone in the atmosphere. The concentration of N2O in atmosphere increases at the rate of 0.2-0.4% every year.1 The emission of N2O in fluidized bed coal combustion is about 50-200 ppmv, with the highest being 400 ppmv.2 During recent years, the intensive researches on the mechanisms of formation and decomposition of N2O have been carried out at home and abroad. Moritomi et al.,3 Amand, and Andersson4 correlated the N2O emission to the fuel ratio of various fuels. Boemer et al.5 considered factors such as coal rank, excess air ratio, and temperature as the most important parameters influencing N2O and NOx emissions. Leckner et al.6 showed that the present publications agree with each other concerning the influence of the main parameters, except that of limestone feed. Co-combustion is defined as the combustion of a renewable fuel (i.e., biomass) along with the primary fuel (coal, natural gas and so on). Recent studies burning biomass with fossil fuels in Europe and the United State have a positive impact on the environment and the economics of power generation. The emissions of SO2 and NOx were reduced in the most co-combustion tests (depending on the biomass fuel used). The use of biomass to reduce NOx is attractive for several reasons. First, biomass contains little nitrogen compared to coal; this results in lower nitrogen oxide formation. In addition, biomass contains virtually no sulfur, so SO2 emissions are reduced in direct proportion to the coal replacement. There are few papers about N2O emission under fluidized bed combustion by co-combustion of coal and biomass. In this paper, the principle of N2O emission by co-combustion will be discussed in detail. 2. Experimental Section Apparatus and Measurement. Each of the fuels was tested in the range of 975-1125 K in a bench-scale fluidized bed reactor system, shown schematically in Figure 1. The reactor is a small bubble fluidized bed made of a quartz glass tube, with a bed * Corresponding author. E-mail:
[email protected]. (1) Weiss, R. F. J. Geophys, Res. Lett. 1981, 86, 7282. (2) Yong, B. C.; et al. Proceedings of the 5th International Workshop on N2O Emission, Tuskuba, Japan, 1992. (3) Moritomi, H.; Suzuki, Y.; et al. 5th International ASME Conference on Fluidized Bed Combustion, New York, 1991; pp 1005-1012 (4) Amand, L. E.; Andersson, S. 10th International ASME Coference On Fluidized Bed Combustion, 1989. (5) Boemer, A.; Braun, A.; et al. 12th International Conference on Fluidized Bed Combustion, 1993; pp 585-597 (6) Leckner, B.; Amand, L. E. 5th International NIRE workshop on Nitrous Oxide Emissions, Tsukuba, Japan, 1992.
Figure 1. Schematic diagram of the experimental apparatus. diameter of 20 mm and a freeboard section height of 600 mm. The fluidized bed reactor and the gas preheater are situated in an electrically heating oven. The gas flow rate was 10-3 m3/min. The height of bed material is maintained at height of about 20 mm. The fluidizing gas is preheated to about 100 °C at the lower part of the electrically heating furnace. The upper part of the electrically heating furnace produces heat to maintain and control the temperature of the bed. Thermocouple probe were inserted vertically into bed material from the top of the reactor. Fuel is fed into the bed by a fluidization feeder. The composition of the flue gas passes through a dust catcher and NaOH water solution first before being measured. Concentrations of NOx, CO/ CO2, and O2 are measured with infrared technology apparatuses (model EQUINOX 55 Bruker Inc. German). The concentration of N2O is measured by the gas chromatograph with electron capture detector (HP 5890GC). The silicon sand is used as bed material, and the particle size of coal and the bed material is 180-210 µm. After the 30 min of steady operation, the sample collection began and lasted 45 min. The samples were collected at every 15 min interval; the every data in the paper is average value of three samples at different time. The overall experiment time lasted 2 h. The approximate and ultimate analyses of the coals and other fuels are presented in Table 1. Coals such as GL and CF came from different places in China.
3. Results and Discussions 3.1. Influences of Co-Combustion of Coal and Biomass on N2O Emission. The influence of co-combustion of coal and biomass on N2O emission in the range of 975-1225 K is presented in Figures 2 and 3. It can be seen from Figures 2 and 3 that both co-combustion of coal and wood chip and co-combustion of coal and rice husk can decrease the emission of N2O. The emission of N2O by co-combustion of coal and biomass decreases with increasing the ratio of the biomass to coal. When the ratio of biomass to coal increases from 5% to 15%, the reduction of N2O emission is from 24.7% to 36.7% for co-combustion of GL coal and rice husk. The emission of N2O decreases with temperature increasing. At the same temperature,
10.1021/ef010108f CCC: $22.00 Published 2002 by the American Chemical Society Published on Web 01/03/2002
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Table 1. Ultimate and Proximate Analyses of the Coals and Biomass ultimate analysis (%) fuels
C
H
N
GL coal CF coal wood chip rice husk
88.9 83.7 43.6 35.2
4.2 5.4 5.61 5.8
1.4 1.3 0.21 0.19
S
O
proximate analysis (%) water volatile
1.3 4.2 1.1 1.0 8.6 4.9 0.15 40.13 6.49 0.03 38.9 6.64
10.9 16.3 81.13 55.12
ash
fixed C
16.9 71.1 31.3 45.5 3.86 8.52 18.87 19.37
Figure 5. Influences of co-combustion of coal and biomass on e NOx mission: (1) no R, (2) R/C ) 5%, (3) R/C ) 10%, and (4) R/C ) 15%. R denotes rice husk; C denotes coal.
Figure 2. Influences of co-combustion of GL coal and biomass on N2O emission: (1) no W, (2) W/C ) 5%, (3) W/C ) 10%, (4) W/C ) 15%, and (5) W/C ) 25%. W denotes wood chip; C denotes coal.
oxygen-lean zone in the lower part to restrict N2O and NOx formation, and they decrease concentrations of N2O and NOx. Volatile from biomass contain above 60% in weight, whose compositions are CO, H2, CH4, C2H4, and C3H6 mostly7,8. Under high temperature, the pyrolysis gas will decompose into a lot of reductive radicals such as hydrocarbon, hydrogen, and oxyhydrogen. The radicals will reduce N2O and NOx by reactions as follows:9,10 N2O + H f N2 + OH N2O + OH f N2 + HO2 NO + CH2 f H + HNCO NO + CH f HCN + O NO + CHi f HCN f NCO f NH f N f N2
Figure 3. Influences of co-combustion of CF coal and biomass on N2O emission: (1) no R, (2) R/C ) 5%, (3) R/C ) 10%, and (4) R/C ) 15%. R denotes rice husk; C denotes coal.
3.3.2. Heterogeneous Reactions of the Reduction of N2O and NOx Emissions. The quick devolatilization of wood chip and rice husk results in the formation of char with higher porosity and higher reactivity.11 This encourages the decompositions of N2O and NOx.13 The reactions are: N2O + C f N2 + CO N2O + CO f N2 + CO2 NO + (-C) f N2 + (-CO)
Figure 4. Influences of co-combustion of coal and biomass on NOx emission: (1) no W, (2) W/C ) 5%, (3) W/C ) 10%, (4) W/C ) 15%, and (5) W/C ) 25%. W denotes wood chip; C denotes coal.
with increasing of the ratio of biomass to coal, the N2O emissions decrease. 3.2. Influences of Co-Combustion of Coal and Biomass on NOx Emission. As for NOx emission, the influences of co-combustion of coal and biomass on NOx emission in the range of 975-1225 K are presented in Figures 4 and 5. Seen from Figures 4 and 5, the concentration of NOx emission increases as the ratio of biomass to coal increases, while concentration of NOx emission increases with temperature increasing. At the same temperature, NOx emission decreases with increasing the ratio of biomass to coal. 3.3. Analyses of the Decreases of N2O and NOx Emissions. The decreases of N2O and NOx emissions may be explained as follows: 3.3.1. Homogeneous Reactions of the Reduction of N2O and NOx Emissions. From Table 1, it is known that wood chip and rice husk contains very high volatiles. Adding wood chip or rice husk to coal results in a larger release of volatiles in the lower part of the fluidized bed. The volatiles will consume most of the oxygen and form
Li12 found that the pyrolysis of biomass produces NH3 and HCN simultaneously, but the formation of HCN went to completion much more rapidly than that of NH3. HCN can interact with the nascent char significantly to form soot or N2. It indicates that in the lower part of the fluidized bed, the pyrolysis of biomass produces HCN mostly, which can be reduced by the char quickly. Because HCN is the main precursor of nitrogen oxide,14 N2O and NOx emissions from co-combustion of coal and biomass may be less than that from coal combustion. 4. Conclusions The co-combustion of biomass and coal can reduce the emissions of N2O and NOx. The emissions of N2O and NOx decrease with increase of the ratio of biomass to coal. The mechanism explanation of the reduction of the emissions of N2O and NOx by co-combustion of biomass and coal may be as follows: Quick release of volatile in biomass in the lower part of the fluidized bed produces a lot of radicals, which will consume the local oxygen and deoxidize N2O and NOx. EF010108F (7) Williams, Paul T.; Nugranad, N. Energy 2000, 25, 493-513. (8) Lede, J. Sol. Energy 1999, 65 (1), 3-13. (9) Donghee, H.; Mungal, M. G.; et al. Combust. Flame 1999, 119, 483493. (10) Larfeldt, J.; Leckner, B. Fuel 2000, 79, 1637-1643. (11) Ndiema, C. K. W.; Mpendazoe, F. M.; et al. Energy Convers. Mgmt. 1998, 39 (13), 1357-1367. (12) Tan, L. L.; Li, C.-Z. Fuel 2000, 79, 1883-1889 (13) Tan, L. L.; Li, C.-Z. Fuel 2000, 79, 1891-1897 (14) Schafer, S.; Bonn, B. Fuel 2000, 79, 1239-1246
