Biogas Technology

Status of the technology

After more than a decade's development, by the end of 1996, 460 large- and medium-scale biogas projects had been built all over the country, of which the total volumetric capacity reached 0.13 million m³ . The annual treatment of rural discard was 30 million tons and the annual biogas production came to 20 million m³ which supplied 56,000 households with biogas. These projects are mostly located in the eastern area and in the suburbs of big cities. The large- and medium-scale projects in Jiangsu, Zhejiang, Jiangxi, Shanghai, and Beijing take up half of the projects that are in operation of the whole country, i.e. 238 digesters. This is because of the fast growth of concentrated breeding industry in these areas (see Table 1).

Table 1 Status of the biogas projects on domestic husbandry farms in China

Source: ¡°Collection of the statistic data of the rural energy in 1996,¡± compiled by the Department of Environment and Energy, Ministry of Agriculture, P.R. China in 1997.

Feature and level of the technology

In recent years biogas technology in China has significantly developed. Many breakthroughs have been made in both the research on the materials for the biogas project and the technology for digestion, and the research on the system and its counterpart equipment. The skill has been improved quite a lot.

1) Various highly efficient digestion technologies and equipment have been invented and some of them have been successfully put into practical use.

Hydro-pressure biogas digesters of 50-100 m³ were mostly adopted for the early biogas projects in China. Each project consisted of several or tens of tanks. The advantage of this kind of digesters is its digestion under normal temperature. The materials for digestion need to be strictly preprocessed. They could be built under pig sties or cattle pens, which saves land. So they are still of some practical value in areas with high temperature. Their disadvantages can be seen in that it is difficult to remove the effluent, as they are underground, and it is difficult to preserve the heat; the production is low with only 0.2-0.3 m³/ m³/d when the density of the influent is 6-7%; and moreover, the technology brings about a severe loss of sludge; there are few microorganisms in the tank and the density of the residue is uneven and difficult to control, which does not meet the modern need for a fast treatment of massive waste. Since the 1980s, digestion technology and equipment have improved in light of the weakness of the former technology. Highly efficient devices for digestion have been designed and manufactured.

2) Gradual perfection of the technological process

With the development of the research on the technology for biogas projects and its extension and application, in recent years a comparatively perfect set of technological process has been abstracted. It includes the pretreatment of various raw materials, choice of technical parameters for digestion, after-treatment of the leftover, and the purification, measurement, storage, and use of biogas.

3) Great improvement of the counterpart equipment

With the perfection of the technological process of digestion, the counterpart equipment has been greatly improved. The improvement can be seen in the following:

¡¤ First, a solid and liquid separator is widely used. In recent years, researchers have come up with or chosen some separation devices in accordance with the properties of biogas projects to meet the need for separation machines of comprehensive utilization, digestion technology, and after-treatment technology. For instance, vibration sieve, slope sieve, revolving sieve, and set bed filter have been researched for the separation of animal excrement. The dehydration rate is generally 10%-40%, which can basically meet the technical requirements and the requirements of comprehensive utilization.

¡¤ Second, the slurry conveyance devices are based on the homemade ones. Several types of conveyance devices for animal excrement have been successfully researched in light of the characteristics of the diversity in types and the different physiochemical properties of the materials for biogas digestion. Among these devices the efficiency of the homemade 75YE-10 pumps reaches 65% or so, and they provide better performance than those of the same type from abroad.

¡¤ Third, further development of the equipment for the purification, storage, conveyance and use of biogas has been made. Some feasible desulfurization technologies have been worked out and desulfurization has reached the standard for the H2S content in city gas. A steel wet tank is widely employed to store biogas. Besides Rootz blower, air compressor, and nitrogen-hydrogen compressor, which are used convey biogas, special compressors for biogas have been invented as pressure-raising devices. Material for the construction of conveyance pipe has been diversified. In some projects, plastics and casts have taken the place of steel. And at the same time, new advancement has been realized in the research on the equipment for the use of biogas such as gas furnace, large stove, and biogas electricity generating unit.

4) A major change has taken place in the equipment and materials for the construction

At present, reinforced concrete is commonly used to construct large- and medium-scale biogas projects. The construction period lasts a long time and much area is occupied and the quality is difficult to be guaranteed, which leads to the unqualified construction of some projects and thus the projects fail in operation. In recent years, Hangzhou Energy and Environment Technology Co. cooperated with Lip of Germany. By using their special equipment, they employed the Lip's two-edge folding, edge-biting technology to make Lip tank or jar of 100-5000 m³ from compound steel plate 2-4mm thick. The net weight of the tank is only 10% of that of the reinforced concrete. Compared with the ordinary tank made from steel plate, it saves 30% steel. The construction period is only half of the concrete tank of the same scale. The cost for construction is more than 15% lower. Still this kind of tanks are corrosion-resisting, need no maintenance and thus have a long life.

5 ) The basic characteristics of the materials for digestion have been made clear

The prerequisite for a proper choice of technology and devices for the digestion is to make known and thus make better use of the basic physiochemical characteristics of the raw materials for digestion. At present the raw materials for the biogas projects on animal farms in our country are mainly excrement of pigs, cattle and chickens. After several years¡¯ research and practice, the composition of the elements and the gas production capacity have been made known.