Life cycle analysis of an integrated biogas-based agriculture energy system

• Main Author: Zhang, Siduo
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/43993

Large quantity of manure is generated in the livestock industry in British Columbia (BC) and natural gas is being consumed intensively in BC’s agriculture sector. We proposed to integrate the livestock farms and the greenhouses to promote waste-to-energy and waste-to-material exchanges following the principles of Industrial Ecology (IE). Natural gas consumptions on farms are replaced by renewable biogas generated from anaerobic digestion (AD) of farm wastes (mainly livestock manure). CO₂ for plant enrichment in greenhouses is supplied by biogas combustion flue gases and the residues (digestate) from digesters are used as animal bedding materials, plant growing media, and liquid fertilizers. An integrated dairy farm and greenhouse was first modeled. Co-digestion of manure with a variety of organic farm wastes was further evaluated with an aim to enhance the biogas production. To address the problems of too much digestate surplus and high CO₂ demand for greenhouse CO2 enrichment, the mushroom farm was further introduced into the integrated system. In this way, the digestate surplus can be used as a growing media for growing mushrooms and the CO₂–rich ventilation air from the mushroom can be directed to the greenhouse for CO₂ enrichment. A Life Cycle Analysis (LCA) was conducted to quantify the environmental impacts of each of the proposed cases in comparison to the conventional agriculture practices. The LCA results showed that the integrated dairy farm-greenhouse system reduces non-renewable energy consumption, climate change, acidification, respiratory effects from organic emissions, and human toxicity by more than 50% compared to conventional operations; among which the reductions in non-renewable energy consumption, climate change, and human toxicity are the most significant. If the digestate surplus is treated as a waste, the integrated system has a ~20% increase in eutrophication and respiratory effects from inorganic emissions. When other organic wastes are codigested with dairy manure, all the impacts can be further reduced in all cases. If a mushroom farm is introduced to form an integrated dairy farm-greenhouse-mushroom farm system, a large greenhouse can be facilitated and the digestate can be largely reused; thus all the analyzed impacts are significantly reduced compared to the base scenario.