By Theo van Kempen, Swine Nutrition Specialist, North Carolina State University - Animal Science swine waste researchers at North Carolina State University has been evaluating gasification technology. The primary purpose was conversion of animal manure into an energy source and mineral ash that could be used as a feed ingredient for pigs.

Dr Theo van Kempen
Swine Nutrition Specialist

At first, a complicated gasifier was tested that turned out to be a poor fit for the animal industry. However, recently, a Canadian company, BGP, has provided us with a simpler gasifier that seems to fit agricultural applications much better.

The design of the BGP gasifier is based on years of experience, with the primary objective being the destruction of waste without causing air pollution. The original design contained no moving parts other than a door for loading and unloading and a downdraft blower. However, to better fit this technology to animal agriculture, the unit is being re-engineered to allow simple but automatic loading and unloading, thus allowing for unattended processing of waste. A diagram of the re-engineered unit is provided in Figure 1.


Figure 1. Diagram of the BGP gasifier

In the BGP gasifier, a downdraft burner is used to heat the L-shaped combustion chamber to 800?C. Heat transfers from the combustion chamber to the gasification chamber through heat-conducting tiles (the rest of the unit is lined with insulating fire-bricks). Feces are introduced in batches into the gasification chamber through a hatch on the top of the unit. Inside, the high heat causes the fecal material to gasify. Gases formed during this process escape from the gasification chamber into the combustion chamber, where they are burned and, in turn, fuel the system. Ashes remaining after gasification of the feces are dumped into an ash chamber using a tilting floor in the gasification chamber. There, any remaining carbon is burned off. Ash is removed from the bottom of the ash chamber by means of an auger.

Experiments with the BGP gasifier have shown that the unit is indeed easy to operate and that it delivers fail-safe operation. Batches of manure can be gasified in approximately 4 hours. The remaining ash is of high quality and should be well suited for use as a feed ingredient, resulting in the complete recycling of phosphorus.

Using such a system on a farm with 5,000 grow/finish pigs would result in a total heat output of approximately 0.55 megawatts (of which approximately 15 to 30 percent is from the fuel source used to fuel the system). A portion of this heat can be harvested in the form of hot water, or it can be converted to electricity.

The latter, although technically possible, is not the best solution as such systems increase in efficiency with size, with an on-farm system probably not being economical. Using the heat as hot water would allow for heating of buildings, which can result in cost savings. Other uses of heat may exist as well. One possibility is to use it to evaporate water from the liquid waste stream and concentrate the waste so it can be used as a concentrated fertilizer.

Another possibility is to gasify the feces in a central location so that a much larger operation can be built. In that case, the heat could be used more efficiently for production of electricity. If a profitable market for electricity does not exist, then the heat could be used to produce steam for a feed mill or rendering plant.

One of the reasons gasification technology is appealing is that it is more environmentally friendly than regular combustion processes. Although gasification is a combustion process, during gasification both temperature and oxygen availability can be controlled. This control is responsible for the lower levels of pollutants. For example, NOx emissions are temperature-dependent, with production becoming pronounced at temperatures over 700?C and becoming of concern over 1,000?C.

One major benefit of gasification of fecal material is that any bioactive compounds in it will be destroyed. This includes antibiotic residues, bacteria, viruses, and prions. Although there is little proof now that any of these form a real concern for public health, such destruction would be welcomed by the general public.

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