Biogas on-farm: energy and material flow

Summary

Objectives:
European countries are committed to reduce CO2 emission originating from fossil fuels. On-farm produced biogas may replace energy produced from fossil fuels and so contribute to achieve the target. Up to now only in Germany a greater number of on-farm biogas plants has been established. The data of these plants can be used to evaluate cost and benefit of on-farm biogas production in other European countries. This paper concerns the following questions: Which parameters of biogas plant construction and operation have an influence on profit and sustainability of biogas production on-farm?
Hypothesis:
A biogas plant integrated within a self-contained farm organism is economically more competitive and more sustainable than an industrial biogas production unit of a mainstream farm.
Method:
First, a model is established that describes energy and material flow of two farm types. Farm type one produces biogas from slurry of 100 adult bovine units (ABU) and 10% co-ferment. Biogas powers a diesel engine of 26 kW electric power capacity using 10% ignition diesel fuel. Electric power production covers farm consumption and the surplus is supplied to the main grid. Heat is used for the farm estate surplus remains unused. Farm type two produces the same amount of biogas, but uses a gas motor. Additionally the farm includes a glasshouse of 1000 m2 to make use of electric power surplus and heat surplus. Further, the exhaust of the gas motor substitutes CO2 fertiliser procurement to the glasshouse.
Second, cost and benefit analysis of biogas production and application is done using empirical data of the most recent biogas plant survey in Germany. These data are adjusted to Finnish conditions where necessary.
Third, parameter variation is employed to find out the sensibility of the most important variables in terms of marginal profit and interest yield of investment for the biogas plant.
Results:
Farm type one delivers a positive interest yield of 2,6 % under German conditions. Under Finnish conditions there is no profit possible. Concerning methane production the marginal profit sensibility decreases in the following order: dry matter of slurry > quantity of co-substrate > reactor efficiency in terms of CH4 kg-1 organic dry matter (oDM) > fermentation period > number of ABU. Concerning electric power and heat production the marginal profit sensibility decreases in the following order: Level of electric power compensation > efficiency of energy conversion methane to electric power > price level of fuel oil and electric power.
Farm type two delivers a positive interest yield of 8 % under Finnish conditions. Concerning methane production the marginal profit sensibility does not differ from farm type one except with regard to CO2 fertiliser costs. Concerning electric power and heat production the marginal profit sensibility decreases in the following order: price level of light and heavy fuel oil and electric power > efficiency of energy conversion of methane to electric power > process energy > heating energy and heating period of the farm estate > operating time in h d-1 of the gas motor.
Discussion:
The better economic performance of farm type two under Finnish conditions mainly bases on substitution of CO2 fertiliser by the gas motor exhaust gas. The interest yield is very sensitive on energy input prices; however less sensitive than the interest yield of farm type one in respect of electric power compensation level. Further use of reactor digestion residues as organic fertiliser may improve sustainability of farm type two.