Biomass gasification and power generation technology

Chuangzhi Wu, Longlong Ma, Yong Chen, Shunpeng Zheng, Xiuli Yin, Haibin Li
People Republic of China

1. Introduction
Biomass Gasification and Power Generation (BGPG) technology can convert biomass energy to electricity effectively. It has the potential in alleviating environmental pollution, slowing global warming and reducing our dependence on limited fossil fuels. To establish decentralized, independent power station incorporated in power network or off-network by utilizing abundant biomass resource based on local conditions can not only offset the shortage of electricity, but also alleviate environmental pollution and green house gas emission. So the technology can be reckoned as both promising renewable energy technology and environmental engineering.

Biomass resources are highly dispersed over a wide area and their energy content is comparatively low, which requires a cost for their collection and transportation. This suggests that the decentralized utilization, such as mid-scale biomass gasification and power generation technology with the electricity capacity of 400-6000kW, may be more feasible than large-scale combustion technology and has the potential to approach market in developing countries.

The mid-scale biomass gasification and power generation technology developed by Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences has achieved advanced world standard both in integrated technical indicator and cost performance as compared with same kind of technologies in other countries. The technology has been granted 9 patents and has been applying in China and several Southeast Asia countries. Beyond energy, obvious benefits in economy, society and environment have been obtained. The technology was thus awarded three prizes in recent five years, i.e. Second prize for advance of science and technology in Guangdong, silver award in Shanghai international industry fair and outstanding patent award of Guangdong province.

2. Technical characteristics, technical indicator and levels
Biomass gasification and power generation technology employs advanced circulating fluidized bed gasifiers for converting biomass waste such as wood, stalk, straw and bagasse to fuel gases and multilevel gas cleaning devices for removing tar and dust. Cleaned fuel gas enters into gas engine to generate electricity. Further, waste heat recovery boiler and steam turbine are combined to improve system efficiency, resulting combined cycle generation. Main characteristics of this technology are as follows:
- Circulating fluidized bed gasifier is adopted for generating fuel gas. Simple and reliable multilevel gas cleaning method is used for removing tar and dust effectively, which are combined with several 500kW gas engines to replace gas turbine. The whole system possesses good biomass adaptability, high handle capacity, good load adaptability and high generating efficiency.
- Difficult problems in gas cleaning process are solved through a new patent technology. A special bacteria, which combines aerobe and anaerobe, is used to treat sewage produced by tar washing, resulting in cyclic utilization of the tar sewage in gasification and power generation system.
- The established 5MW biomass gasification and combined cycle generation system overcomes the limitation of conventional IGCC technology developed in other countries. Its technical difficulty and system cost can be reduced greatly and all the devices used in the system can be domestically produced. It is thus feasible technically and economically to apply this technology in most developing countries.
As mentioned, the combination property of the whole system is stable and feasible with relatively low unit investment and operation cost. Its technical and economic indicators are shown as follows:

*Based on the biomass price of 25 USD/ton

At relatively low investment and cost, the maximum generating efficiency of the system can achieve 28%, which is higher than that of small-scale coal based power generation system, moreover, all the devices used are approved products with low manufacturing cost. So the technology is competitive in market and has great popularization potential. The established 5MW biomass gasification and combined cycle generation system has achieved international advanced level in the aspects of cost performance, which is suitable to the application of 3-10MW BGPG system and can provide technical support for establishing dispersed biomass electricity system in developing countries with abundant agricultural waste.

3. Application
Aiming at the demand of biomass electricity market, serial products with the capacity of 400-5000kW had been designed successfully, which lays a foundation for the development of BGPG technology. Since 2000, the technology has been generalized in China and several Southeast Asia countries. There are 31 project contracts have been signed, with total installed capacity of 34MW and accumulated contract value of 9.9 million USD. It is estimated that the application of this technology can obtain indirect benefit of 4.2 million USD and added production value of 12.6 million USD, and simultaneously reduce 275 thousand ton of CO2 emission per year. At present, it has been the most applied biomass power technology in the world and has obtained obvious economic and environmental benefit.

The application of BGPG technology can provide an effective way to solve "stalk problem" which occurs most rural areas and is difficult to be handled. For example, a 3000kW BGPG station can treat 20 thousand ton of agricultural waste per year. We assume that the power station buy stalk waste from local farmers in the price of 25 USD/ton, the output of 125-250 thousand USD can be added accordingly. The station can also provide employment opportunities for 50 persons. The direct cost, including stalk cost and labor cost, is about 0.044-0.056 USD/kW.h, which is then turned into the earning of local farmers. If BGPG station with total capacity of 50 thousand kW can be established in every county, more than 12.5 million USD can be obtained, which shows obvious social efficiency.

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