Biogas is a mixture of methane, carbon dioxide and various other gases and comes from the controlled anaerobic decomposition of organic materials such as manures. It provides a convenient source of cooking fuel that produces less smoke and air pollution than straw, sticks or coal and also reduces damage to land from overharvesting of fuel sources. The Chinese have excellent programs of research, development and implementation for small and medium scale biogas digesters. During August, I traveled with a team of agriculturalists, including three farmers from Pennsylvania to observe the Chinese biogas program. A study grant from the USDA office of International Cooperation and Development (OICD) and travel funds provided by the Center for Rural Pennsylvania made the trip possible.
The purpose of the trip was to observe construction and operation of small- and medium-sized biogas digesters and interact with biogas researchers, technicians and educators. During the 20 days in China we visited 34 operating digesters in four provinces and also had discussions with personnel at the Chengdu Biogas Research and Education Center and other government agencies.
Controlled anaerobic decomposition of manures not only produce biogas but also reduces objectionable odors that often result from storage of manure. The cost and complexity of typical U.S. farm digester systems has limited their use to larger farms. Most Chinese digesters are unheated and rather simple in construction. It is not clear how an unheated digester will operate under typical Pennsylvania temperatures.
Six to 10 cubic meter (1600-2600 gallons) in-ground ambient-temperature hydraulic digesters are typical. They are loaded with pig and human manure and some crop residues. Digesters are built with local materials including concrete, bricks, and bamboo. Inlet and outlet assemblies are designed to provide air seals and also to regulate gas storage and pressure. Biogas is stored in the top portion of the digester vessel. As biogas production increases, digester contents are displaced into the outlet chamber. As gas is used, material from the outlet chamber flows back into the digester vessel, maintaining pressure on the gas and also the air seal. Size and location of the loading and unloading pipes allows convenient access for removal of bottom contents or agitation by a simple pitcher type pump, a bucket or a long handled ladle.
Effluent from the digesters is used for fertilizer, as a supplement in fish-raising ponds or discharged to drainage ditches. Most of the biogas is used for cooking. Simple gas burners are available that can hold a teakettle, pot or wok or be used as a heat source in traditional masonry stoves. Biogas is also used for lighting, to fumigate stored grains and seeds, and for controlled atmosphere storage of fruits and vegetables. Ambient-temperature digesters of 100 cubic meters and larger can be found on larger community farms. The primary use of gas on these farms is also for cooking. On some farms, extra biogas is used to supplement diesel powered engine generator units.
The team believes that some ideas used by the Chinese can be applied to Pennsylvania conditions. Low cost digesters may be applicable to provide cooking gas and light for farms that do not use electricity. It may also be economical to supplement diesel power plants with biogas when available. Perhaps simple unheated digesters can be used to provide sufficient control of the manure decomposition process to minimize odors even if gas production is not optimum. Future cooperation with the Chengdu Biogas Institute is also being investigated.
Prepared by Robert E. Graves, Professor of Agricultural Engineering