By Ronna T. Ungs, P.E., Harris Group Inc.
Use of biogas from manure for power production offers various benefits related to reduced atmospheric methane and carbon dioxide emissions, reduced local odor problems, and diminished potential for groundwater and stream contamination. Biogas generation systems, however, are costly from both capital and operating standpoints. To weigh these costs against potential benefits, a study was conducted analyzing manure biodigestion as the primary fuel source for a 10 MW power generation plant at an ethanol production facility. The study established that small applications (1 MW to 10 MW) provide the best economics, and results may be dependent on one or a combination of the following: waste codigestion, economic incentives, and heavy public support.
Biogas systems capitalize on the ability of anaerobic digestion processes to break down volatile solids in manure and generate methane. Biogas is composed of methane, carbon dioxide, and trace amounts of hydrogen sulfide and hydrogen gas. Differing ranges (50-80%) of methane content have been reported for different manures and digestion systems, although a typical mixture is approximately 60% methane.
Available manure biodigestion options include covered lagoons, complete mix digesters, and plug flow digesters. Complete mix biodigestion systems are suitable for animal manure containing 3-10% solids, and are generally composed of a digestion tank with a cone-shaped bottom to facilitate sludge removal. Digestion tanks use mechanical mixers or recirculate gas collected from the top of the tank, and are typically heated via a coil inside the digester. Complete mix digesters operate with a typical hydraulic retention time of 10 to 20 days. Plug flow digesters employ rectangular, in-ground engineered tanks (or troughs) with air-tight expandable covers. Manure is added daily to one end of the tank and slowly pushes the resident volume of manure through the trough. Methane is collected in the expandable cover. Plug flow is suitable only for dairy manure with a solids content of 11-13%. These systems usually operate with a hydraulic retention time of 20 to 30 days. Slurry digesters are a variant of the plug flow digester, and can operate at lower solids concentrations. Covered lagoons typically apply to liquid manure with less than 3% solids, and require hydraulic retention times of 40 to 60 days.
The energy potential estimated per animal for this study equated to requirements for manure from approximately 90,000 dairy cattle for a 10 MW generating plant, or 67,000 beef cattle. The actual number of animals required is highly dependent upon feedlot manure collection efficiency, which varies from 25- 85%. Dairy manure tends to be highly available because the amount of time spent by milking cows in barns or on hard surfaces is relatively high. In contrast, livestock animals typically spend a greater portion of the time outside and are not as frequently confined to barns or hard surfaces where manure can be collected.
Manure feed must be collected regularly, and be free of large quantities of bedding and other materials. Beyond two weeks, biogas loss from the manure is approximately 15%. After 4 weeks, manure is unsuitable for use. Manure feed cannot contain high levels of salts, heavy metals, ammonia, and antibiotics. These compounds are toxic to methanogenic bacteria. A common feed additive (Rumensin) is also toxic to methanogens. Equipment cleaning and maintenance compounds may also harm bacterial action. Disruption of bacterial populations from feedstock contamination can require several months to restabilize the bacteria. Some bedding materials may cause other unwanted operational problems for a digester. Clumps of bedding, particularly sand, can clog influent and effluent pipes of the digester and invite operational problems, and will also reduce digester efficiency.
Sludge from anaerobic digesters is weed seed free and odorless, and is generally nearly completely stabilized. Anaerobic digestion processes significantly reduce biochemical oxygen demand and pathogen levels of the influent manure. Fertilizer nutrients (nitrogen, phosphorous, potassium) present in raw manure are nearly completely retained in digester effluent. In addition, anaerobic digestion also converts much of the organic nitrogen present in influent manure to inorganic nitrogen (ammonium), producing a higher quality, more available fertilizer than undigested manure. Liquid digester effluent is typically returned to participating farms for land application as fertilizer. Dissolved solids can be removed from liquid digester effluent and processed into saleable soil amendments.
Power generating options for biogas systems include internal combustion engines, gas turbines, and microturbines. Internal combustion engines coupled with alternating-current generators are more commonly employed because of lower installed capital costs, lack of compression and drying requirements, higher efficiency, and significantly more experience. With these units, approximately one-third of the available digester gas energy is used for power production, one-third is used to maintain an acceptable digester temperature, and one-third is lost in inefficiencies (in both the internal combustion engine and the generator). Gas turbines, while more efficient, require gas compression, drying, and H2S removal. Microturbines are commonly used for biogas power production systems, and offer generating capabilities in the range of 3 to 200 kW.
A gross total digester capacity of up to 16 million gallons in a series of reactors could be required to fuel a 10 MW power generating facility. Ultimately, digester volume would depend upon manure characteristics and required hydraulic retention time. Manure equalization and mixing facilities would also likely be required. It was assumed that 11 internal combustion engines with individual gross capacities of 0.9 MW would be necessary to generate the required power when burning biogas (data for similar units operating on landfill gas were readily available). Effluent storage would be required to provide surge volume when resultant fertilizer cannot be applied (wintertime).
Digester gas is generally burned as it is produced. Digester parameters including gas production rate, digester pH, gas methane/carbon dioxide ratio, and volatile acid content must be monitored and maintained. Manure pretreatment may be required, including water addition, solids separation, mixing, and heating. The primary operating costs for a biodigestion system include feedstock transportation and internal combustion engine maintenance. Transportation economics depend heavily on proximity of feedstock to the plant; transportation costs are reported to account for 35-50% of the overall operating cost for community-based biogas plants in Europe.
Efforts associated with anaerobic digestion have focused primarily on manures from dairy and swine operations, although some attention has been focused on poultry manures. Complete mix digester volumes ranging from 3,500 to 70,000 cf (representing daily capacities of 25,000 to 500,000 gal of manure/digester) have been constructed. At the time of this study, the largest operating digester in the U.S. treated manure from only 4,000 cattle (larger plants to treat manure from up to 30,000 cattle were under design or construction). A number of larger community-based biogas systems have been installed in Europe. The first European biogas plants were constructed to utilize only livestock manure, although economic considerations required that the plants add industrial and household waste.
Key issues that must be addressed with respect to implementation of anaerobic biodigestion for mid-scale power generation applications include:
- Lack of current applications at the scale studied (10 MW)
- High capital investment and operating costs
- Availability of feedstock within reasonable distance
- Quality of available feedstock
- Requirements for contract negotiations with manure suppliers
The size of a biodigestion facility required to generate power for the ethanol plant studied (8-10 MW) presents unique physical and operating challenges. Options to generate the required power utilizing biogas as a primary fuel source could include:
- Codigestion with other organic wastes to increase energy potential of feedstock
- Codigestion with municipal waste to improve economics by adding tipping fees
- Development of distributed digestion facilities to reduce hauling distance and create manageably sized facilities
- Development of numerous on-farm digesters with agreements for maintenance and power purchase
*Implementation of limited-size facility and purchase of remaining required power
Construction of a full-scale power generating facility that utilizes biogas fuel may not be economically feasible. However, renewable energy technologies that minimize emissions of greenhouse gases are receiving increased federal and state attention. Potential biomass projects may be capable of capitalizing on tax credits and subsidized financing issued by federal and state governments. The Production Tax Credit may be expanded in upcoming legislation to address energy produced from all biomass, including biogas. NRCS’s Environmental Quality Incentives Program provides guidance to states for rewarding biomass projects. Reduced methane emissions can also generate carbon credits that may ultimately be used in greenhouse gas trading programs. As a result, the potential for application of federal energy subsidies and tax incentives to improve project economics is high.