For over 50 years the Chinese have struggled to develop and diffuse biogas technology. At present, China has about 5 million family biogas plants in working order. Although over 7 million have been constructed in the past, many of them were poorly built with inadequate mixtures of earth, sand and lime, This was mainly because during the 1950s and 1970s "quality" was sacrificed at the altar of "quantity" which has left a lasting impression in farmers' minds of digesters never producing much gas. Today, about 25 million Chinese people use biogas mainly for cooking and lighting. A further 10,000 largeand medium-size biogas digesters are working in food factories, wineries, livestock farms etc. Biogas produced in large enterprises is transferred to centralized biogas supply stations, biogas motive power stations (422 with an installed capacity of 5849 HP) or biogas electric power stations (822 with a total of 7836 kW).
The Bureau of Environment Protection and Energy (1991) estimates that about 54 per cent of the energy requirements (equivalent to about 282 Mtcoe) of the 900 million rural population comes from over 560 Mt of biomass, mainly in the form of straw and firewood although this is probably a conservative estimate. Continued reliance on traditional patterns of biomass supply and use to meet rural household fuel needs may result not only in greater disparity between supply and demand, but also in greater disruption of local agricultural and ecological systems. Improved efficiency of biomass production and use thus has become imperative (World Bank, 1985). Additionally, energy shortages may be one of the fundamental motives for the continued development of biogas. The use of human excrement in digesters rather than spreading it on the field (as is usual in China) can destroy more than 90 per cent of the intestinal parasites and other pathogens thus making the nutrient recycling process far more hygienic. This is because the Chinese digestion plants have a long retention time of about six months (Rajabapaiah et al, 1992). In addition to animal and human dung, about 2 Mt straw is digested each year (Daxiong et al, 1990; Zong 1989).
Daxiong et al (1990) carried out an economic analysis of 58 biogas plants in Tongliang, Sichuan, and compared this with data produced by other researchers in 242 biogas plants in Hubei. Their analysis shows a high rate of return on investment in biogas and short payback periods of between one and four years (see table 10). Capital costs vary from 15 to 40 yuan per m³ of digester capacity, and the annual gas output varies from about 30 to 40 m³ per yr for each m³ of digester. The annual value of this biogas in terms of savings in coal, kerosene, burned biomass, labour and fertilizer varies from about 7 to 16 yuan If operating costs are included, the internal rate of return (IRR) varies from 59 per cent to 114 per cent.
Biogas plants in China have been subsidized and/or received low-interest loans. Since 1980, the State has allocated more than 10 million yuan every year for the development of biogas, which represents 200 yuan for each plant constructed every year. This money is spent in improving biogas equipment, promotion, standardization, servicing, training, research on new technology etc. Standardized production techniques and equipment have significantly improved the reliability and quality of biogas supply; there are 116 biogas research centres in China today (Daxiong et al, 1990; Anon., 1991).
The more recent opening-up of the economy to financial incentives is beginning to have a major effect on the biogas programme. Until 1983, when a peasant built a digester the State and local government provided two thirds of the money and also guaranteed the supply of building materials. Since 1983, there has been a move towards financial selfreliance which has resulted in a reduction in subsidies from two thirds to one third which, in turn, has led to a decrease in the construction of biodigesters. Although the rate of return remains high, the increase in the initial capital outlay is a disincentive to users. For example, Sichuan province which before 1983 built an average of 250,000 biodigesters annually, constructed only 81,000 in 1983 (Daxiong et al, 1990).
Socio-economic changes in China are also affecting biogas production. For example, rural migration to the more-economically-developed areas has resulted in peasants not having sufficient organic matter to fill their biogas digesters (since they produce fewer pigs); also, since biogas production is labour-intensive, there are not enough people (particularly young people) to take care of the biodigesters. Additionally, due to improving living standards, a growing number of peasants prefer to buy privately-sold coal than to use biogas, because it saves time which can be spent on more lucrative work (Daxiong et al, 1990).
Notes: a Life = 15 years
The costs of construction of biogas plants are difficult to compare between countries and over time due to the importance of controlled prices in China. The capital costs in Tongliang and Hubei vary from 15 to 40 yuan per 1 m³ of digester capacity, and the annual gas output vanes from about 30 to 45 m³ yr from each m³ of digester.
The value of this gas in terms of savings in coal, kerosene, burned biomass, labour and fertilizer varies from about 7 to 16 yuan /m³ digester capacity. This represents a paybach period of between about I and 5 years. If the daily operating costs are included, the internal rate of return vanes from about 59 to 14.2 per cent.
In the 1975-1978 period, each biogas plant cost about 80 yuan as compared with 156 yuan for a 6.6 m³ plant in 1983, although this involved twice as much labour (80 work days/plant in 1988) and the output from each digester averaged only 151 m³ compared with nearly 300 m in the current plants. Source: Daxiong et a. 1990.
This changing situation seems to indicate that production and management of biogas will become more centralized and industrialized in both rural and urban areas and will be used as part of an integrated production system. In this way, advanced technology could be used to increase production and financial returns and thus will have greater appeal to peasants. As living standards improve so will the demand for energy. Since commercial energy production in China cannot keep pace with the expected rise in consumption, adequate energy supplies in rural areas cannot be guaranteed. Hence, biogas appears to be a very viable energy source.