Author(s): Susanne WoessGallasch, Neil Bird, Peter Enzinger, Gerfried Jungmeier, Reinhard Padinger, Naomi Pena, Giuliana Zanchi
The goal of this study is to quantify the greenhouse gas (GHG) and energy impacts of a biogas plant with closed storage of digested materials. The plant of „NegH Biostrom KEG“ in Paldau in the state of Styria, Austria, was analysed. Results are based on measurements of methane produced in the closed storage tank and Life Cycle Assessment (LCA). Feedstocks for the biogas plant are primarily crops and secondarily animal manure. The plant has two main digesters and two secondary digesters in addition to the closed storage tank for digested biomass. Methane produced in the digesters and storage tank are used to produce approximately 4 GWh electric energy and 7 GWh heat per year. Only 17 % of heat produced is currently used. Total Biogas production and methane concentration from the closed storage was measured during the period May – October 2006. Based on these measurements, an annual production of 15.6 tons of CH4 per year was estimated assuming tank cleanings were done without opening the tank, which represents best practice operation. Using these results, a theoretical case was constructed of a biogas plant using the same feedstocks but storing digested biomass in an open tank. It was assumed that open storage would result in methane emissions to the atmosphere equal to those produced in the closed storage tank. A comparison of these two cases shows the increased energy production and GHG emissions avoided by using closed storage. The LCA analysis covered carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions due to construction, operation and dismantling of the energy systems; and the cultivation (including fertilizer use and land use change), harvest and transport of biomass raw materials as well as use of by-products. GHG impacts of the two biogas plants were compared to a reference system (reference system I) which delivered equivalent amounts electricity and heat. In the reference system, electricity is assumed to come from a natural gas plant, and heat from a combination of oil and wood, which was the case prior to operation of the Paldau biogas plant. The results show that: • Open storage results in total LCA GHG emissions 29 % higher than closed storage. • Due to increased CH4 production and higher CH4 concentration in the biogas coming from closed storage, open storage produces 1.9 % less energy than closed storage. • Closed storage plant results in 1,409 tons of CO2-eq per year, which is 1,094 tons or 44 % less GHG emissions than the reference system. Open storage reduces emission by 685 tons per year, or 27 %. • This equates to emissions savings of 292 kg CO2-eq per t dry biomass for the closed storage system and 183 kg for the open storage. Direct land use changes (onsite) sequester 48 tons of CO2 per year, reducing total GHG emissions in the Paldau plant by 3.4 %. A sensitivity analysis showed that if even relatively small amounts of CH4 (e.g. 5 %) escape from the storage tanks or digesters, the GHG benefits of biogas plants are substantially reduced. If all heat produced by the biogas plant Paldau can be used, GHG emission benefits increase significantly.