Investigation of the impacts of alkali hydrogen peroxide (AHP) pre-treatment process on ethanol and biogas production efficiency from switchgrass

This thesis focuses on process optimization for the development of a biofuel production process from an energy crop, switchgrass that combines chemical pretreatment (Alkaline Hydrogen Peroxide) and biotechnological processes (ethanol and biogas fermentation). Within the scope of this thesis; AHP pre-treatment process was investigated and process optimization was carried out in order to obtain the sugar from the lignocellulosic switchgrass and to increase the amount of ethanol and methane production by the enhancement of fermentation processes. Energy crop switchgrass cultivated by Selçuk University, Faculty of Agriculture, Agricultural Machinery and Technology Engineering and Field Crops Departments for research purposes was used in this thesis. Total solids (TS), volatile solids (VS), total and soluble chemical oxygen demand (sCOD), soluble reducing sugar (sRedSugar), Van Soest Fraction (cellulose, hemicellulose, lignin, soluble matter) and elemental composition (C-H-N-S) analyses were performed to determine the characterization of switchgrass. Response Surface Methodology (RSM), Central Composite Design (CCD) was used for the AHP pretreatment process optimization. The experimental design of the AHP pretreatment process was planned with the Design Expert Trial 7.1.5 program and the pretreatment experiments proposed by the program were performed. The AHP process was optimized for the maximum methane production and the optimum process cost with the maximum methane production. Conditions for both optimization were determined. In the CCD experimental design, ANOVA was applied for the determination of the adequacy of the proposed models. In addition, after the pretreatment experiments, Fourier Transform Infrared Spectroscopy (FTIR), scanning electron microscopy (SEM) and Van Soest method were used to examine the changes in bond characterization, surface characteristics and composition of cellulose, hemicellulose, lignin and soluble fraction of the switchgrass. AHP pretreatment experiments were carried out in order to increase the availability of cellulose and hemicellulose to the enzymes by damaging the lignin structure of switchgrass samples. In this context, the AHP pretreatment experiments as proven efficient in delignification was evaluated with the process variables of reaction temperature (50-100°C), reaction time (6-24 hours), hydrogen peroxide (H2O2) concentration (1-3%) and biomass solid matter content (3-7%) and optimum AHP pretreatment conditions for delignification were identified. To determine the AHP pretreatment efficiency, sCOD, soluble reducing sugar and ethanol in the liquid fraction and biochemical methane potential (BMP) parameter in the solid fraction were investigated as response variables. As a result of the AHP pretreatment experiments, the maximum values of sRedSugar and sCOD values were obtained as 43.35 mgsugar/gVS and 1463.51 mgCOD/gVS, respectively under the reaction conditions of 100°C reaction temperature, 3% H2O2 concentration, 24 hour reaction time and 3% solid content. Increasing the amount of solid matter in the AHP pretreatment adversely affected methane production and the minimum amount of methane (233.9 mLCH4/gVS) was obtained under the conditions of 100°C reaction temperature, 3% H2O2 concentration, 24 hours reaction time and 3% solid content which are the conditions where maximum sRedSugar and sCOD were obtained. However, in the pretreatment experiments in which the maximum amount of solid content and H2O2 concentration are applied, the reduction of the reaction temperature and reaction time to the minimum (50°C reaction temperature, 3% H2O2 concentration, 6 hour reaction time and 7% solid content concentration) increased the amount of methane produced and the maximum amount of methane (367 mLCH4/gVS) was achieved under these conditions. No ethanol was detected in the samples at the end of ethanol fermentation of samples containing maximum sugars treated with AHP pretreatment. The AHP process has been optimized from the point of the production of sCOD, sRedSugar and Methane (BMP), since no ethanol can be detected in the samples pretreated with AHP. Linear, 2FI, and modified (backward) models were proposed by the Design Expert® 7.1.5 program for the response variables of sRedSugar, sCOD and BMP, respectively. The regression coefficients of the developed models for the sRedSugar, sCOD and BMP response variables were determined as 0.7620, 0.7345 and 0.4409, respectively. Two different approaches were used in the evaluation of AHP pretreatment process and in the optimization of the BMP independent variable. Considering the cost in the first approach; reaction temperature, reaction time, and H2O2 concentration were minimized and sCOD and sRedSugar increase were left in range. In the second approach, maximum BMP production was considered and therefore, reaction temperature and reaction time were minimized , while H2O2 concentration, sCOD and sSugar increase were left in the used value in range. In both approaches, the amount of solid matter and the amount of BMP is preferred maximum and therefore, maximized. Optimization for minimum process cost revealed 1.03% H2O2 concentration, 50°C reaction temperature, 6 hour reaction time and 7% solid content as optimum conditions. The estimated values of the increase of sCOD and sRedSugar by Design Expert® Trial 7.1.5 program are 580.75% and 248.47%, respectively for the minimum process cost conditions. As a result of the validation experiment performed under the recommended experimental conditions, the values of sCOD and sRedSugar were determined as 542.59% and 240.31%, respectively. After the validation experiment, the BMP value was determined as 344 mLCH4/gVS and an increase of 27.94% compared to the raw sample BMP was observed. It has been determined that the optimum pretreatment conditions for maximum BMP production without the consideration of cost is achieved at 2.79% H2O2 concentration, 50°C reaction temperature, 6 hour reaction time and 6.96% solid content. The estimated increase of sCOD and sRedSugar by the Design Expert® Trial 7.1.5 program are 923.88% and 8.87%, respectively. As a result of the validation experiment performed under the recommended experimental conditions, the increase of sCOD and sRedSugar were measured as 874.74% and 8.98% respectively. When the estimated values by the model are compared with values obtained from the validation test, the error for the sCOD and sRedSugar are calculated as 5.32% and 122%, respectively. In addition, the BMP value was determined as 383 mLCH4/gVS after the validation experiment and an increase of 35.27% was observed when compared with the raw sample. It was determined from the SEM images that the raw sample had a constant, invariant, and continuous surface, while observed damages on the surface of the sample was considerably high indicating that the AHP pretreatment is effective. FTIR spectrum results showed that the peaks observed were lignin and hemicellulose disintegration fragments, with very low shifts in wave length. Van Soest analysis results showed increase in cellulose and decrease in hemicellulose and lignin and these results were found to be compatible with the literature. In the literature, there is no study of the optimization of AHP pretreatment process to the energy crop switchgrass for methane production. Within the scope of this thesis, with the aim of increasing the methane production potential, AHP pretreatment process has been optimized and methane production potential of the energy crop switchgrass has been determined. Since there is no literature studying the methane production potential of the AHP pretreatment process using energy crop switchgrass, the determination of optimum process conditions and the examination of surface properties, the results obtained from this master thesis have the value that can be used as a reference point in the works to be carried out.