Progress 10/01/05 to 09/30/06
Outputs
A heat transfer model that predicts thermal energy requirements for plug-flow anaerobic digester operation and a biogas production model are combined to determine heat available for beneficial uses on the farm. Both models are extensively validated against data measured over a one-year period from an operating anaerobic digester located in Upstate New York, and the predictions agree very well with measured data. Annual thermal energy requirements for operation of an example plug-flow anaerobic digester that has an influent manure flow rate of 60 meter cubed per day was simulated. The digester consumed 18 percent of the total thermal energy for its operation, that is, maintenance heat and heat required to bring the temperature of the influent to an operating temperature, leaving theoretically 82 percent of the heat for other beneficial uses. The available heat will be, however, reduced by the efficiency of the system used for conversion. Biogas production is sensitive
to the kinetic parameter and more so to influent total volatile solids, in a linear fashion. An increase in the kinetic parameter by 0.01 resulted in a decrease of biogas production by 7.6 percent, and for every 1 kg per meter cubed increase in total volatile solids resulted in 9.5 percent increase of daily biogas production. Influent manure, 60 meter cubed per day flow rate, preheated to an increase of 1 C resulted in a saving of 224.3 MJ per day of heat. A preliminary economic feasibility model for centralization of anaerobic digestion of dairy manure mixed with food waste was also developed. The model was converted into a user-friendly, web-based computer program and allows users to input different variables and computes profits or costs of a defined centralized digestion system. Default values are given in the program and can be modified by the user. The model accounts for various costs including: equipment, transportation, manure, labor, insurance and maintenance, and accounts
for revenues from tipping fees, sales of electricity, sales of effluent solids and liquid manure, and sales of excess heat. Tipping fees and sales of liquid manure were the major contributors to the overall revenue with 2,000 or more cows in the system. The largest cost was the price of manure followed by labor costs. Sensitivity analyses were performed to determine the total number of cows required for the enterprise to remain profitable and the percent change in profit, when changes in parameter values were made, one-at-a-time. The model does not account for time-value of money.
Impacts
One major impact that results from an anaerobic digestion process is reduction of offensive odors, thus improve neighbor relations. The second major impact is biogas production and the energy in the biogas could be converted into electricity and heat that could be used on the farm. The captured heat could be used for heating water for the farm use, drying manure solids, chilling through evaporative coolers and drying of agricultural products, greenhouse operation, or sale of hot water to the community. This way, the farmer generates some income and potentially employs more people. Developing a tool (a model) that quantifies the energy demands involved helps anaerobic digester designers in selecting construction materials and insulation type and thickness appropriate for the location and environmental conditions. Furthermore, manure from a cluster of farms could be processed at a centralized location to maximize economic benefit. With centralization, small farms that
cannot afford to install digesters could sell their manure to a processing plant and generate income. In order to generate additional income from tipping fees, a centralized digestion facility would need to be located where large volumes of food waste can be obtained. This translates into an additional revenue source and makes the investment more profitable.
Publications
- Gebremedhin, K.G. and S. F. Inglis. 2006. Validation of a biogas production model and determination of thermal energy from plug-flow anaerobic digesters. Trans. of ASABE (accepted).
- Minchoff, CJ. and K.G. Gebremedhin. 2006. Economic feasibility study for a centralized digestion system. ASABE Annual International Meeting Oregon Convention Center Portland, Oregon 9 - 12 July, Paper No. 064198, St. Joseph, MI.
- Gebremedhin, K.G. and S. F. Inglis. 2006. Biogas production model for plug-flow anaerobic digesters. ASABE Annual International Meeting Oregon Convention Center Portland, Oregon 9 - 12 July, Paper No. 064172, St. Joseph, MI.
- Binxin W., E. L. Bibeau and K. G. Gebremedhin. 2006. Three-Dimensional Numerical Simulation Model of Biogas Production for Anaerobic Digesters ASABE Annual International Meeting Oregon Convention Center Portland, Oregon 9 - 12 July, Paper No. 064060, St. Joseph, MI.
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Progress 10/01/05 to 12/31/05
Outputs
A comprehensive fundamentally based mathematical model that predicts energy requirements to operate an anaerobic digester at a specified temperature and the resulting gas production is developed. This information is vital to designers as they determine heating requirements and energy use for the digester system. The model accounts for heat loss/gain through influent and effluent, the digester floor, top cover material and walls, including predicting when the ground surrounding the digester walls could potentially freeze, thus increasing heat loss through the walls. Solar energy transmitted through the cover material into the digester is also accounted for in the model. The heat and gas production model is validated against measured heat requirement and gas production data and the results agree reasonably well. The model can be used to calculate energy requirements to operate a digester on a daily, monthly or yearly basis, and determine biogas produced. The lowest
energy requirement to operate the digester occurs in July and the highest in December and January. Significant energy could be saved if the influent manure can be pre-heated before entering into the digester using the heat in the effluent manure. The next tasks will be: 1. do sensitivity analysis of the critical parameters to minimize energy loss and maximize gas production, and 2. develop an economic model that will be an integral part of the heat loss and gas production model. The economic model will consider a centralized location of operation for few farms. The last task will be to convert the comprehensive model into an interactive and friendly web-based model. The model will then be available in the Cornell Manure Management Program's web site to be used by extension agents, extension engineers, designers/consultants and producers.
Impacts
When designing an anaerobic digester at a specific site, it is important to understand the total energy demand required for digester function. Knowing the energy demand allows you to estimate the net energy available for other uses. There are many factors that affect heat exchange between an anaerobic digester and its surroundings. The primary factors include soil characteristics and conditions such as density, thermal conductivity, specific heat of the soil, temperature and moisture conditions of the ground. For example, a digester placed in dry sand will lose heat at a considerably slower rate than a digester placed in saturated sand. Ground surface temperature is influenced by weather conditions and duration of exposure. The ground could be frozen to a certain depth depending on the severity of the weather, soil type and moisture content. Understanding these factors would help designers/contractors in selecting an appropriate site for the system.
Publications
- Gebremedhin, K.G., B. Wu, C. Gooch and S. Inglis. 2005. Heat transfer model for plug-flow anaerobic digesters. Transactions of ASAE, 48(2):777-785.
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