Abstract
Animal wastes from high-density farming have severe impacts on the nitrogen cycle. According to current regulations, the disposal of manure on cropland is constrained by nitrogen content in the agricultural soils. On the contrary, anaerobic digestion (AD) of these wastes can produce energy and a digestate, which is easier to handle than manure and can be applied for agronomic uses. When herbaceous crops are co-digested with manure to increase the efficiency of biogas production, the nitrogen content in the digestate further increases, unless these larger plants are equipped with nitrogen stripping technologies. We propose a model to compare larger (cooperative) and smaller (single parcel) AD conversion plants. The whole process is modeled: from the collection of manures, to the cultivation of energy crops, to the disposal of the digestate. The model maximizes the energy produced on the basis of available biomass, road network, local heat demand and local availability of land for digestate disposal. Results are the optimal size and location of the plants, their technology and collection basins. The environmental performances of such plants are also evaluated. The study has been applied to the province of Forlì-Cesena, an Italian district where animal farming is particularly relevant.
Highlights
The agricultural-zootechnical sector has several impacts on the environment, due to all the wastes generated by animals
Assuming a greenhouse gases (GHG) reduction equal to the national target established within the Kyoto Protocol (−6.5% of 1990 level), the district of Forlì-Cesena has to cut its emissions of about 80 Tg of CO2eq (1990 emissions being 1384 Tg of CO2eq) [15]
We assume that corn silage needed to co-digest manure in anaerobic digesters (AD) plants is available in the quantity needed for the AD process
Summary
The agricultural-zootechnical sector has several impacts on the environment, due to all the wastes generated by animals. Tao and Mancl [6] presented a method to derive the amount of manure produced from the number of animals on-farm and the amount of land needed to dispose of it with respect to its nitrogen, phosphorus and potassium contents. As the plant capacity increases, economies of scale in capital equipment are realized, but energy costs of transportation increase as manure and other substrates must be conveyed over longer distances to the plant site This is relevant for manure because of its high humidity and low energy content. The model allows comparing cooperative (or community) and individual conversion plants, by maximizing the energy produced on the basis of available biomass, road network, local heat demand, and availability of land for digestate disposal. The consequences on the local agriculture system are evaluated comparing the land required to grow the corn silage needed for co-digestion to current land use
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