Biogas: Technology Overview
Anaerobic Digestion
Anaerobic digestion is the process that occurs when bacteria decompose organic materials in the absence of oxygen. The byproducts include gas with 60 to 80 percent methane content. The most common applications of anaerobic digestion use wastewater, animal manure, or human sewage as the organic resource. The most common types of digesters are plug flow, covered lagoon and complete mix digesters.
According to the California Energy Commission the world wide deployment of anaerobic digestion is approximately 6,300 MWth for agricultural and municipal wastes. This is estimated to increase to 9,000 MWth in 2010 with the majority of that growth being in municipal wastewater digestion.
Applications
Anaerobic digestion is commonly used in municipal wastewater treatment as a first stage treatment process for sewage sludge. Digesters convert the organic material or sewage sludge into safe and stable biosolids and methane gas. The use of anaerobic digestion technologies in wastewater treatment applications is increasing because it results in a much smaller quantity of biosolids residue compared to aerobic technologies.
In agricultural applications, anaerobic digesters can be installed anywhere that there is a clean, continuous source of manure. Dairy, and hog farms both fit this description. Dairy farms use all three types of digesters depending upon the type of manure handling system in place at the farm and the land area available for the digester. Hog farms typically use slurry technologies because the manures have low solid contents and are more easily handled as slurries.
Along with direct heat applications, the biogas produced by anaerobic digestion can be used for power generation. Reciprocating engines are the most common conversion device, although trials with micro-turbines are underway. A 300 to 500 head dairy farm generally produces sufficient manure to generate about 85 kW. Hog farms typically generate approximately 50 kW for every 500 swine. Digesters frequently satisfy the power demands for the farm on which they are installed.
Resource Availability
For on-farm anaerobic digestion of livestock manure the resource is readily accessible and only minor modifications are required to existing manure management techniques. For central plant anaerobic digestion of livestock manure the availability of a large enough number of livestock operations within a close proximity is necessary to provide a sufficient flow of manure to the facility. For anaerobic digestion of municipal wastes the resource is readily available at the waste treatment plant.
Environmental Impacts
Anaerobic digesters have multiple positive environmental impacts. First, they provide a dependable containment system for manure and organic wastes, thereby preventing groundwater contamination. Secondly, they eliminate odor problems. Thirdly, they reduce methane emissions form atmospheric decomposition of manure. Fourth, the nitrogen in the manure is converted to ammonia that is more easily converted by plants to nitrites and nitrates, thereby eliminating nitrogen overloading in the soil due to manure spreading. Finally, biogas used for power production replaces the use of fossil fuels for the same purpose.
Landfill Gas
Landfill gas (LFG) is produced by the decomposition of the organic portion of the waste stored in landfills. Landfill gas typically has a methane content between 45 and 55 percent and is considered to be an environmental risk. Political and public pressure is rising to reduce air and groundwater pollution and the risk of explosion associated with LFG. From an energy generation perspective, LFG is a valuable resource that can be burned as fuel by reciprocating engines or small gas turbines.
LFG was first used as a fuel in the late 1970s. Since then, there has been a steady development of the technology for its collection and use. LFG energy recovery is now regarded as one of the more mature and successful of the waste to energy technologies. There are more than 600 LFG energy recovery systems in 20 countries.
Applications
LFG can be used directly for process heat or may be upgraded for pipeline sales. The major constituents released from landfill wells are carbon dioxide and methane. LFG contains trace contaminants such as hydrogen sulfide and siloxanes that should be removed prior to combustion.
Power production from LFG facilities is typically less than 10 MW. As discussed earlier, several types of conversion devices can be employed to generate electricity from LFG. Typically the equipment requires only minor modification so long as the gas is properly cleaned and prepared. Internal combustion engines are by far the most common generating technology choice. About 75 percent of landfills that generate electricity use engines.
Depending on the scale of the gas collection facility, it may be feasible to generate power via a combustion turbine generator. Testing with microturbines and fuel cells is also underway.
Resource Availability
Gas production in a landfill is dependent upon the depth of trash in place and amount of water received by the landfill. Each landfill is unique because each has a different volume, receives a different amount of water, and has a different material composition. This variability makes it important to take measurements of quantity and quality of gas at a landfill before deciding to install a power generation system.
In general, LFG recovery may be economically feasible at sites that have over one million tons of waste in place, more than 30 acres available for gas recovery, a waste depth greater than 40 feet, and the equivalent of 25+ inches of annual precipitation. There are methods of changing both the quantity and quality of the LFG, if required, but doing so will affect the life span of the LFG supply. It is particularly important to understand that every landfill will reach a point in its life at which time the LFG production will decrease and eventually diminish below economically viable levels.
Many existing landfills have collection systems to remove leachate and LFG from the landfill to prevent it from infiltrating ground water supplies and causing other nuisance problems. These systems are usually connected to a flare system if there is not a power generation system installed. The flares combust the methane in the LFG. Such sites are attractive to LFG developers because the resource is generally well known and accessible.
In many cases, the payback period of LFG energy facilities is between 2 and 5 years, especially when emission credits are available. Capital costs are highly dependent on the conversion technology and landfill characteristics. The cost of installing a gas collection system can be economically prohibitive.
Environmental Impacts
Combustion of landfill gas, as with nearly any other fuel sources, does release some environmental pollutants. However, on the whole, landfill gas to energy systems are widely viewed in a positive light from environmentalists. This is because landfill gas that is otherwise released to the atmosphere is a significant source of greenhouse gas emissions due to its methane content. Collecting the gas and converting the methane to carbon dioxide greatly reduces the potency of LFG as a source of greenhouse gas emissions.