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Energy & Fuels 2009, 23, 224–228
NO Reduction in Decoupling Combustion of Biomass and Biomass-Coal Blend Li Dong,* Shiqiu Gao, Wenli Song, Jinghai Li, and Guangwen Xu State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ReceiVed July 25, 2008. ReVised Manuscript ReceiVed October 20, 2008
Biomass is a form of energy that is CO2-neutral. However, NOx emissions in biomass combustion are often more than that of coal on equal heating-value basis. In this study, a technology called decoupling combustion was investigated to demonstrate how it reduces NO emissions in biomass and biomass-coal blend combustion. The decoupling combustion refers to a two-step combustion method, in which fuel pyrolysis and the burning of char and pyrolysis gas are separated and the gas burns out during its passage through the burning-char bed. Tests in a quartz dual-bed reactor demonstrated that, in decoupling combustion, NO emissions from biomass and biomass-coal blends were both less than those in traditional combustion and that NO emission from combustion of blends of biomass and coal decreased with increasing biomass percentage in the blend. Cofiring rice husk and coal in a 10 kW stove manufactured according to the decoupling combustion technology further confirmed that the decoupling combustion technology allows for truly low NO emission as well as high efficiency for burning biomass and biomass-coal blends, even in small-scale stoves and boilers.
1. Introduction Biomass is a kind of renewable energy. It is the fourth largest source of energy following coal, petroleum, and natural gas, and it accounts for about 14% of the total world energy consumption, even up to 40% for developing countries.1 Biomass is CO2-neutral because its growth consumes CO2 through photosynthesis. However, the supply of biomass energy is generally seasonal, and it is uneconomical to transport biomass too far away from the producing area; therefore, biomass-coal co-firing is a quite practical way to use biomass energy. Nonetheless, biomass contains N, which can form NOx during combustion. NOx emissions per unit energy input from biomass combustion are even higher than those from coal combustion.2-4 Therefore, it is necessary to devise new biomass combustion technologies to reduce NOx emissions. Various technologies have been tested to reduce NOx emissions from biomass combustion in previous work. Staiger et al.5 and Salzmann et al.6 reported that NOx emissions from biomass combustion can be reduced about 10-20% via air- and fuel-staging technologies. Selective catalytic reduction (SCR) of NOx was also applied to biomass combustion.7,8 Nonetheless, air- or fuel-staging technology often makes the system complex and reduces the temperature in the furnace; therefore, they are * To whom correspondence should be addressed. Telephone: +86-1062621607. Fax: +86-10-62550073. E-mail:
[email protected]. (1) Chen, G. Y.; Fang, M. X.; Luo, Z. Y.; Yu, C. J. Sol. Energy 1999, 2, 122–129. (2) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Fuel Process. Technol. 1998, 54, 17–46. (3) Spliethoff, H. G.; Hein, K. R. Fuel Process. Technol. 1998, 54, 189– 205. (4) Nussbaumer, T. Energy Fuels 2003, 17, 1510–1521. (5) Staiger, B.; Unterberger, S.; Berger, R.; Hein, K. R. G. Energy 2005, 30, 1429–1438. (6) Salzmann, R.; Nussbaumer, T. Energy Fuels 2001, 15, 575–582. (7) Jensen, P. A.; Sander, B.; Dam-Johansen, K. Biomass Bioenergy 2001, 20, 431–446. (8) Baxter, L. Fuel 2005, 84, 1295–1302.
difficult to be used in small-scale stoves or boilers. For SCR, its catalyst is easily deactivated by alkali metals present in biomass ash.9 Therefore, there is still no effective technology available for controlling NOx emissions in small-scale biomass stoves or boilers, which are actually the major tools for converting and using biomass energy in the countryside. Decoupling combustion (DC) technology was originally developed to burn coal with suppressed smoke and low NO emission.10 It separates the combustion process into pyrolysis of fuel and combustion of char and pyrolysis gas, while the pyrolysis gas may be burnt out during its passage through the combusting char bed. Previous studies demonstrated that suppressed smoke in DC is mainly attributed to the complete combustion of pyrolysis gas in the combustion zone downstream of the pyrolysis zone, while low NO emission results mainly from the catalytic reduction of NO over hot char particles.11,12 Complete combustion of the pyrolysis gas in DC also led to low emissions of CO and soot, thus resulting in the higher combustion efficiency. The present work is devoted to investigating the DC of biomass, especially the biomass combustion in the small-scale biomass stoves designed according to the DC technology. In the study, three kinds of biomass fuels and their blends with coal were first tested in a quartz dual-bed reactor to clarify the effect and mechanism of low NO emission via DC. Then, a 10 kW stove adopting the DC technology for co-firing rice (9) Zheng, Y. J.; Jensen, A. D.; Johnsson, J. E. Appl. Catal., B 2005, 60, 261–272. (10) Li, J. H.; Xu, G. W.; Yang, L. D. Chinese Patent ZL 95102081.1, 1998. (11) Li, J. H.; Bai, Y. R.; Song, W. L. NOx-Suppressed smokeless coal combustion technique. Presented at the International Symposium on Clean Coal Technology, Xiamen, China, 1997. (12) He, J. D.; Song, W. L.; Gao, S. Q.; Dong, L.; Barz, M.; Li, J. H. Fuel Process. Technol. 2006, 87, 803–810.
10.1021/ef800589c CCC: $40.75 2009 American Chemical Society Published on Web 12/09/2008
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Table 1. Proximate and Ultimate Analyses of Fuels sawdust
rice husk
corn straw
coal
moisture volatile fixed carbon ash
Proximate Analysis (wt %) 8.2 7.5 8.8 78.1 64.2 74.0 12.2 13.9 12.5 1.6 14.5 4.7
4.4 25.3 57.9 12.5
C H N S Oa VNb
Ultimate Analysis (Dry, wt %) 49.4 39.6 43.8 6.05 5.68 5.43 0.35 0.55 0.84
