Production of biogas from switchgrass energy plant in semi-continuous stirred-tank reactor

The aim of this master of science thesis is to determine the effect of organic loading rate on the batch process as Biochemical Methane Potential (BMP) and semi-continuous stirred tank reactor (SSTR) process, along with the reactor working conditions, system responses and performance. Characterization analysis of the switchgrass plant was performed within the scope of master of science thesis. It has been determined that switchgrass contains 41.95 % carbon (C). The amount of hemicellulose and cellulose was determined as 36.24% and 35.94%, respectively. High carbon, high cellulose and hemicellulose amount (72.18%) of switchgrass proves its preferability for biogas production by anaerobic digestion. The theoretical methane potential (TBMP) of switchgrass, whose extended chemical formula is C140H244O109N1, was found as 439.5 mLCH4/gVS. The biochemical methane potential (BMP) test of switchgrass was carried out at different organic loading rates (0.75, 1.0 and 1.5 gVSswitchgrass/L.Day) and substrate/inoculum ratios (1.1, 1.5 and 2.2 gVSswitchgrass/gVSinoculum). The specific methane production amount obtained after 60 days of SRT were found as 203.9, 190.5 and 181.0 mLCH4/gVS, respectively. It was observed that the amount of specific methane production decreased when the amount of organic loading rate was increased. These values correspond to 46%, 43% and 41% of the switchgrass TBMP value, respectively. 80% of the methane generation of 203.9 mLCH4/gVS at 0.75 OLR was obtained in the first 24 days. Although the batch BMP test was continued for 60 days, this finding also reveals that the retention time can be taken 24-25 days in terms of economic methane production from switchgrass. In order to predict the kinetic coefficients, the batch BMP test methane production datas are modeled with Modified Gompertz, Cone, Reaction Curve and First Order Kinetic model. Regression coefficients (R2) in all models were calculated between 0.992 and 0.999. It was found that whenthe organic loading rate was increased, hydrolysis rate constants (k) and methane production rate values (Rm) were decreased and the lag phase (λ) time was increased. Anaerobic fermentation in the SSTR system of switchgrass was performed under the same conditions (feed amount, SRT and temperature) with BMP, and the effect of organic loading rate on specific methane production in the semi-continuous anaerobic process was investigated. Specific methane production values of switchgrass were calculated as 147.6, 156.9 and 60.11 mLCH4/gVS, respectively, for the organic loading rates of 0.75, 1.0 and 1.5 gVS/L.Day. When the organic loading rate was increased to 1.5 gVS/L.Day, specific methane production decreased critically. The reason behind this phenomenon is thought to be due to the break down of anaerobic fermentation related to the slow hydrolysis of lignocellulosic structure of switchgrass and the lack of alkalinity source to buffer the system in the acidogenesis stage even if very high volatile organic acid concentrations were not observed. Molecular genetic tools as DNA isolation, polymerase chain reaction, denature gradient gel electrophoresis and sequencing were performed for the samples taken from the semi-continuous anaerobic reactor in order to determine the microorganisms involved in the anaerobic digestion process of switchgrass. Methanosarcina, Methanosaeta, Methanothrix and Metanometilovorans as archaea and Chloroflexi, Firmicutes, Spirochaetes and Actinobacteria as bacteria were observed during the 270 days of semi-continuous anaerobic reactor operation. Although the methane production obtained at 1.0 gVS/L.Day OLR is 6% more than the methane production obtained at 0.75 gVS/L.Day OLR, considering the total economic benefit, it was concluded that it would be more efficient to operate the low organic matter loading rate (0.75 gVS/L.Day) in both the batch reactor and the semi-continuous feed anaerobic reactor (SSTR), since switchgrass is not a waste and it is a cultivated plant for energy production.