Resource Recovery and Conservation最新文献

A mathematical model description of anaerobic digestion of animal residues was developed, taking into account material and energy balances, kinetics, and economics of the process. The model had the flexibility to be applicable to residues from any size or type of animal husbandry operation. A computer program was written for this model incorporating a procedure for process optimization, based on minimum unit gas cost, with the optimization variables being digester temperature, retention time, and influent volatile solids concentration. The computer program was used to determine the optimum baseline process conditions and economics for the fuel gas production via anaerobic digestion of residues from a 10,000 head environmental beef feedlot. This feedlot - at the conditions for minimum unit gas cost - was calculated to produce 8500 m³ (300 MCF)/day of methane at a cost of $4.90/GJ or $0.183/m' ($5.17/MCF [CH₄]), with a total capital requirement of $1,165,000, a total capital investment of $694,000, and an annual average net operating cost of $370,000. No credit for possible refeed of digester effluent was included but a cost of $2.78/Mg ($2.50/ton) for the raw manure was incorporated. The major contributions to this unit gas cost were due to labor (37%), raw manure (11%), power for gas compression (10%), and digester cost (13%). A sensitivity analysis of the unit gas cost to changes in the major contributions to unit gas cost was performed, and the results of this analysis indicated areas in the anaerobic digestion system design where reasonable improvements may be expected so as to produce gas at a more economically feasible cost. This sensitivity analysis included the effects on unit gas cost of feedlot size and type, digester type, digester operating conditions, and economic input data. For example, a 40,000 head environmental feedlot was computed to produce methane for $1.42/GJ or $0.050/m³ ($1.50/MCF), using a continuously stirred tank reactor digester and taking a credit for the digester effluent as refeed.

The quantities of sewage utilized in both the urban and rural sectors totals 36 Mm³/y. This represents only about 17% of the potential (16.5% for agriculture and 0.5% for industry).

The production of waste dust is a major problem in the cement industry in terms of monetary, energy, environmental and resource costs. This is particularly so for wet process plants. The monetary and energy costs of returning the dust are more than outweighed by the savings attained by recovering the partially processed raw materials. Proper handling and return of dust can save a typical wet process plant approximately $16 to $32 and 13 to 14 GJ per hour for each kiln.

A new technique, briquetting of the dust before its return, shows promise for plants which formerly had little success in dust recovery. This technique might also be used to process the piles of dust generated by some plants over the years. These piles are a large potential source of partially processed raw materials.

The object of this paper is to perform an analysis of the economic efficiency of methane generation on a typical 65-cow dairy farm, juxtaposed against prices and costs of auxiliary energy supplied by rural electrification. The most efficiently sized methane generation option examined is the use of methane to fuel a 30 kW generator with sales of surplus energy fed back to the utility. Whereas this option is still more expensive than present prices for electricity, this would not be the case under assumptions of escalations in relative fuel prices. On an individual farm basis, the economy is made better off by methane generation under this option by $195 per year, assuming electricity is priced at its marginal opportunity costs. The utility would incur $734 in revenue losses, but this figure represents the commensurate decrease in utility capacity and fuel. The merits of setting electricity tariffs equal to marginal costs are evidently part of the incentive for farmers to install this option. Given several scenarios of differently sized methane generators, the utility would promote the smallest facility for the farm, which in turn may be the least efficacious for the economy as a whole. This may conflict with national efficiency criteria so, therefore, regulation at the interface between the farmer and utility would have to be exercised.

Changing fuel prices have an impact to varying degrees on prices of materials. The Engineering Experiment Station, University of Wisconsin, in cooperation with the U.S. Forest Products Laboratory, Madison, Wis., is developing a computer model to simulate impacts of changing fuel prices on wood products and competing materials. In comparing energy inputs for various materials, valid and consistent methods must be used for energy accounting. Specific examples are discussed, and five criteria are included that relate to reliable energy accounting.