STUDY OF THE PROBLEMS OF ENERGY PRODUCTION FROM ANAEROBIC FERMENTATION IN AGRICULTURE

Marco Negri Ph. D. Thesis SCUOLA DI DOTTORATO TERRA, AMBIENTE E BIODIVERSITÀ Department of Agricultural and Environmental Science Production, Landscape, Agroenergy University of Milan Ph.D. in Agricultural Ecology XXVII Cycle Academic year 2013-2014 Study of the problems of energy production from anaerobic fermentation in agriculture Global warming is linked to the reduction of greenhouse gas emissions (GHG). The anaerobic digestion of animal manure and energy crops is a promising way of reducing GHG emissions. The increasing number of biogas plants involves a high consumption of energy crops and the need of big agricultural area. In Italy, cereals silages are the main feedstock for biogas production and are commonly grown under two different crop systems: single crop (only maize) and double crop (maize after a winter cereal). This work presents the results of experimental field tests carried out by monitoring the anaerobic biomethane potential (ABP) of different cereals silages commonly grown in the Padanian Plan: mmmaize silage FAO Class 300, 400 and 500 as second crop following triticale or wheat, and ear maize silage FAO Class 600 and whole plant maize silage FAO Class 700 as single crop. In addition to these studies, the biological degradability in anaerobic conditions was evaluated. The anaerobic conditions were similar to those of a real scale biogas plant and the digestate mass attributable to the single biomass was calculated as well. An anaerobic digestion (AD) plant located in Piedmont was also studied. consists of a single stage process made of two CSTR digesters (total volume of 5340 m3) operating in mesophilic conditions. The electric and thermal power are 998 kWEE (ηEE = 40.9%) and 577 kWTE (ηTE = 23.6%), respectively. The comparison between single and double crop systems was drafted in terms of biogas and biomethane production per hectare. The double crop system was carried out with triticale and maize class 500 and it achieved that highest biogas and biomethane productions per hectare. Nevertheless the results of the comparison between the single crop and the double crop systems are variable and slightly unambiguous. Regarding the methane production from maize silage, the tests results are in agreement with other related studies carried out previously; the methane production by harvesting and ensiling the whole maize plant is higher than the one achievable by the other plant portions. The harvest of the ear only can be very interesting as it is characterized by considerably high specific productions. The achieved results concerning the corn stover and leavesare interesting , especially when considering the future possibility to use the byproduct of corn production (stover) for energy purpose. The biomass composition conditions the degradability of the biomass itself and of the digestate production. Biomasses with a low or null fiber content have high degradation rates and low digestate productions. Ear maize silage is the most degradable biomass (>97%). As a consequence, it generates the lowest digestate mass per mass unit (38%). The higher degradability of ear maize silage is due to the high starch content in the biomass (>60%) and to the low fiber content. Whole plant maize silage is less degradable (about 90%) than ear maize silage. Triticale and wheat silages, because of their strongly similar composition, show degradation rates and kinetics very similar to each other. Moreover, because of the higher fiber content if compared to maize and ear maize silages, they are less degradable. The biogas produced by the AD plant during the monitoring represents 96% of the potential biogas production; in these conditions, the plant produced 8378 MWhEE, with an average electrical power of 968 kWEE. The overall electric self-consumption has been equal to 653 MWhEE (7.79% of the gross electricity production). Each day, on average, 10782 m3N of biogas are produced.