Review of full-scale anaerobic digestion in China
Post on 21-Jun-2016
Agricultural Wastes 18 (1986) 197-205
Review of Full-Scale Anaerobic Digestion in China
D. L. Hawkes
Department of Mechanical and Production Engineering, The Polytechnic of Wales, Pontypridd, Mid-Glamorgan, South Wales, Great Britain
(Received 5 March 1986; accepted 12 March 1986)
Varying reports of the numbers of digesters in China, and their effectiveness, have been circulating in the last few years and only recently have visitors been able to see the situation at first hand. The following review of full- scale working digesters is based on information gained from the Fourth International Symposium on Anaerobic Digestion, AD85, held at Guangzhou, Guangdong Province, at which 450 delegates were present, overl80 of them from outside China.
In November, 1985, the Fourth International Symposium on Anaerobic Digestion, AD85, was held in Guangzhou, Guandong Province, China. With millions of working digesters, China has probably more experience than any other country in the practice of digestion in a rural setting. The holding of this major conference in that country was recognition of this.
Although there have been digesters working in China since the 1920s, there were problems with the early designs which were simply constructed at very low cost; many of them leaked. The major developments have taken place since the early 1970s when the policy of the government encouraged the development of more and better systems. This policy has now resulted in a total of 4.48 million domestic biogas digesters working well and mainly situated in rural areas (Cao, 1985).
197 Agricultural Wastes 0141-4607/86/$03.50 Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
198 D. L. Hawkes
The Chinese Imperial Qing dynasty was brought down by the 1911 revolution of Sun Yat-sen. In 1921 the Chinese Communist Party was founded and, in October, 1934, the famous Long March began from the province of Jiangxi in the south to the northern province of Shaanxi. The present Chinese leader, Deng Xiaoping, who took part in the Long March, led the Communist forces against the Japanese, who were in control of the northeastern provinces. The Communists were fighting in a large part of the north of the country and, in 1943, Deng launched a campaign amongst the rural population called the 'Great Production Movement' aimed at boosting the local harvests by 'rewarding the hardworking and punishing the lazy' by bonus payments and other incentives to good workers. The post-war struggle with the Nationalist leader, Chaing Kai- shek, resulted in the Communists coming to power in 1949 and the Nationalists retreating to Taiwan. The Great Leap Forward, in 1958, initiated by the then Chairman, Mao Tse-tung, called for enforced nationwide farm collectives and increased steel production in backyard furnaces. His campaign is now admitted to have been a disaster producing a series of prolonged famines, which starved as many as 27 million people in the succeeding 4 years (Doerner, 1986). In 1961, Deng Xiaoping and President Liu Shaoqi set about correcting the situation and reinstituted incentives. The relationships between Deng and Mao deteriorated until, in 1966, Mao and the so-called radicals around him, known as the 'Gang of Four ~, Mao's wife Jiang Qing, Zhang Chunqiao, Yao Wenyuan and Wang Hongwen, plunged the country into the Great Proletarian Cultural Revolution conducted by the fanatical Red Guards. The death of Mao Tse-tung in September, 1976, and the arrest of the 'Gang of Four' cleared the way for the return to power of Deng Xiaoping in the following year, where he has remained until now.
Early research work in the 1920s on practical digesters in China was carried out by Lo Guo-rui, his first working digester being at his home in Swatow, Kuangtong (now Guangdong) Province. After building a number of successful digesters he established a company to market his designs and was granted a patent in 1930. His company, later based in Shanghai, not only built a large number of successful digesters, some of which remain to this day, but also set up training courses to solve the problem of a lack of suitable technical back-up. In 1930 Tiang Li-fang improved on the Lo
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design with a saving of materials and consequent cost reduction. Even in those early days there was a variety of uses for the biogas, from the lighting of a photographer's studio to the baking of biscuits (Cui & Xie, 1985).
At the time of the Great Leap Forward in 1958 many rural digesters were built with communities vying with each other for the highest number built at the lowest cost. The quality of these was not always good, since it was numbers that counted, and, consequently, many of them developed serious problems and failed to operate. In the 1970s, because of the serious shortage of fuel in the rural areas, there was a revival of interest in digester building, but later, after some initial difficulties, there was. a policy, begun in 1979, to build better quality and better managed digesters. During this period the Guangzhou Energy Institute of the China Academy of Science, Guangdong Province, began researching a PVC bag to store gas, the result being the development of the well-known red mud plastic now extensively used. Emphasis was also placed on developing leak-proof digesters and successful designs were established by the Biogas Extension Office of Pinfu County, Zhejiang Province, and the biogas test station of Guzhen County, Anhui Province. Another institute well known for its digester designs is the Chengdu Biogas Scientific Research Institute of the Ministry of Agriculture, Animal Husbandry and Fishery (MAAF).
Government training policy
The government policy of 'consolidation and improvement, actively promoting the development, paying equal attention to construction and management of digesters, and seeking for practical results' (Bian, 1985), has resulted in the setting up of special agencies in charge of biogas development. There are now, as a result, more than 1700 service stations with 7000 trained construction teams who have been examined in the subject (Bian, 1985). These service stations or companies are self-managing and have independent accounting, and undertake contracts for digester building and management and produce and sell digester building materials and accessories (Cui & Xie, 1985). Altogether, 40 000 people have obtained, after about a 1-month course, certificates of technical competence. Some training courses are internationally famous; for example, the course held at the Asian-Pacific Regional Biogas Research-Training Centre, set up in 1981 at the Chengdu Biogas Institute, Chengdu, Sichuan Province (Anon., 1983). In addition to this impressive technical organisation, biogas experi- ment stations have been set up in seven provinces and fifty-six prefectures and counties. There are over 1000 people engaged in scientific research on anaerobic digestion in more than fifty research institutions and universities.
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The agricultural sector dominates the Chinese economy, providing about 75% of employment and about half the national income. Of the land area of China, about 9.3 million km 2 (compared to 244005 km 2 for the UK), two-thirds of it is either desert or mountain and only 10% is cultivated. Rural incomes have tripled since 1979, but still the per capita income is amongst the lowest in the world and China's GNP per head is now estimated at 171 (1984), (5699 for UK) (National Westminster Bank, 1985). Recent agricultural reforms have concentrated on the more efficient use of existing resources and the commune system has been completely reorganised. Now land is allocated to individual families who are contracted to fulfil a specific output quota, any surplus being available for sale in the free market. In addition, prices for the quota crops have increased, giving a further incentive for greater output. One problem that still remains is that of a rural energy shortage with direct burning of biomass, mainly as straw and firewood, accounting for 80% of the energy consumed. It is estimated that 70 million m a of firewood is burnt each year with consequent problems of deforestation. It is because of this and the need to dispose hygienically of animal and human wastes that anaerobic digestion is so popular. Pigs are the main meat animals and are kept in large numbers and the common human sewage disposal system is by means of nightsoil buckets which are collected each morning and their contents distributed on the land. An anaerobic digester reduces the pollution and helps prevent the spread of disease (Zhao, 1985).
A typical unheated rural household digester is in the order of 6-10m 3 capacity and can produce about 250-300 m 3 biogas per year in the southern region. In the colder northern region this amount falls to about 150- 210m 3 per year. The gas is used mainly for cooking and lighting.
The early simple biogas pits were often of poor quality, with major maintenance needed after 5 years or so, and cost only 3~40 yuan. More modern digesters are better constructed and last much longer, although the investment is higher at about 150-200 yuan (Huang, 1985) (1 = 4.3 yuan (October, 1985)). To appreciate the local value of 150 yuan it should be noted that the eight grade monthly salary range for a factory worker is between about 35 yuan and 110 yuan with an average of about 65 yuan; 10%-12% more can be earned with bonuses. Husbands and wives both work. Out of this wage the urban worker will pay about 5 yuan per month for rent for the family apartment which will include water and electricity. Two meals each day at work cost up to 15 yuan per month and, after other expenses, the worker will probably have left about 10 yuan per month as savings to spend on items like furniture or a bicycle. A bicycle
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will cost 150-200 yuan. Agricultural workers can earn more, but usually for longer hours, and a family income of 10 470 yuan per year is considered very high (Lou, 1986).
Digesters are now frequently made using moulding techniques. The steel moulds used in Sichuan Province allow the concrete to be cast in situ; when the concrete has set the moulds are taken apart and withdrawn to be used again (Lai, 1985). Alkaline resisting, glass fibre reinforced, low- alkaline cement known as GRC (Zang et al., 1985) is a material which has allowed digesters to be made at lower cost with high strength and thin, but impermeable, sections. There is a wide variety of these digesters (Cao, 1985), each claiming advantages for the cost or for the ease of handling of the manure.
Floating gas-holders have been used in some types of Chinese digesters from the early 1930s, many of the designs being for gas-holders separate from the digester and made of concrete or plastic. This arrangement is especially the case for medium and larger scale digesters. The advantage of a floating gas-holder is that a constant gas pressure can be maintained at the burner. Where plastic gas-holders are used the material is red mud plastic, a PVC with a clay filler which was designed to resist the effects of sunlight and for a low cost.
Digesters using this material have been operated successfully (Fig. 1), although they have a low gas pressure which can be a problem for the burners used, are easily damaged and, for larger digesters, usually have a building over them which increases their cost and uses land. Most of the rural digesters are of the fixed dome type and only a few are of the floating gas-holder or plastic type. Often, the rural digester will be buried underground for convenience and strength. Medium-size digesters, such as those on livestock farms for power generation, are more commonly horizontal arch digesters and are also built underground.
Centralised biogas digesters are becoming more common, although the investment per household at about 300-600 yuan (Huang, 1985) is much higher than for individual units. The largest centralised system at present is on Qianjin Farm, Chongming County, where there are 65 digesters, each of 50 m 3, to supply gas to 720 households.
Uses of the gas and digester residues
The gas from a rural digester is usually used as a fuel for cooking, either in the communal kitchens or individual households, or for lighting. The
202 D. L. Hawkes
Fig. I. Red mud plastic digester of 50m 3 capacity installed at the Sihui County Xiabu Livestock Farm.
development of appropriate devices, including chicken brooders, catalytic radiators and biogas engines for the efficient use of the gas, has kept pace with digester development (Jiang & Cao, 1985). Electrical generation is also practised to provide power and lighting. The residue from the digester is used as a fertiliser and to improve the soil structure. It is also used to feed fish in the many fish ponds in each village or to grow mushrooms. In Haian County of Jiangsu Province 530000m a were grown in 1984 after extensive experimental trials (Peng, 1985).
AD 85 technical visit
Delegates to the Fourth International Symposium on Anaerobic Digestion had the opportunity to visit the Zhaoqing Prefecture Foreign Trade and Xiabu Joint-operated Livestock Farm, about 2-h coach journey from Guangzhou (Fig. 2). The farm, which was established in 1975, raises 10 000 pigs and 20000 chickens each year with a citrus crop of over 115 tons. The total investment cost of the digesters so far installed is about 60 000 yuan and the total savings in fuel costs over the nine years of operation is 267 000 yuan. As well as a 50 m 3 red mud plastic type of digester there are two large ones of the 'Dingfansui' tunnel type, one being of 481 m 3 (Fig. 3) and the other of 388 m 3 capacity. The farm has five digesters with
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Fig. 2. A banner of welcome at the gate for delegates visiting the Xiabu Livestock Farm is a typical demonstration of the hospitality offered.
Fig. 3. Delegates at the Fourth International Symposium on Anaerobic Digestion examining the 481 m a tunnel digester working on animal manure.
204 D. L. Hawkes
a total capacity of 1059m 3. The daily gas production of 190m 3 is used as fuel for a communal dining room for 120 people, as cooking fuel for 49 households, and for other processes. Electricity for lighting is generated from two 12 kW generator sets in the farm's 'power station'. The digester effluent is used as fertiliser on the agricultural land and some excess is used in the 12000m 2 of fish ponds.
Anaerobic digestion is now being practised in towns and cities where there are abundant raw materials; distilleries, food factories and similar processes. There are believed to be in the order of l0 000 large and medium digesters with a combined capacity of about 3 l0 000 m a. The biogas is either burnt directly or used to generate electricity; a total of 7836 kW is now supplied. Distilleries are perhaps the largest source of material for digestion; for example, the one at Louzhi, Sichuan, has two 1000 m a digesters producing 3000m a per day (Cui & Xie, 1985). At the Nanyang Alcohol Plant in Hunan Province, two 2000 m 3 plants are treating the residues of the process which uses sweet potato as the raw material, the 8000 m a of biogas being used as a fuel and a chemical feedstock. In 1983 a 2700m a digester, with a working volume of 2500 m 3, was built at Penglai Brewery in Shandong Province and produces about two volumes of gas per volume of digester per day. The digester operates at 55C with a retention time of 11-13 days. The production of the solvent, butanol, at the Hui An Chemical Plant in Shaanxi Province includes two digesters of 3600 m a each for treating the waste and the plant will eventually produce 18 200 m 3 of gas per day.
Anaerobic filters are used, the largest known being of 150 m a and 185 m 3 capacity of Tongcheng Brewery in Hubei Province with a production of about four volumes of biogas per volume of digester. Many wastes are, of course, being studied at the experimental level (Zheng & Wu, 1985; Wang etal., 1985) but, as yet, the application at full scale is limited. Similarly, the UASB type has limited widespread acceptance, the larger ones being combinations of UASB and filters, an example of which is that at the Guangzhou Institute of Energy Conversion where a 130 m 3 digester is treating molasses-alcohol waste from the Pingsha Sugar Refinery and producing 546 m a of biogas per day on an experimental basis.
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