Material and energy conversion efficiency of anaerobic fermentation process of Penisetum hybrid for biogas

Abstract: Energy grass, as the second generation of new energy crop, has a bright development prospect. Hybrid penisetum was selected as the raw material. Silage is a well-developed technology, which can preserve the nutrient component to a large extent. Ensiling is a process in which water-solution carbohydrates (WSC) are converted into lactic acid by lactic acid bacteria (LAB) in anaerobic condition. We investigated the physicochemical characteristics of silage grass raw material, and explored the biogas production performances by laboratory batch anaerobic digestion at mesophilic temperature (35±1)℃. The results showed that the biogas production of silage grass with the C/N ratios of 137 and 25 (adjusted by urea and ammonium bicarbonate) was 327.02 and 295.43 mL/g respectively, and the corresponding methane content (volume fraction) was 58.49% and 61.05%, while the biogas production of untreated energy grass was 280.02 and 242.33 mL/g, and the methane was 59.96% and 61.46%, respectively. The volatile solid (VS) removal rate of substrate was in the range of 51.68%-57.56%, and it mainly attributed to the degradation of cellulose and hemicellulose. Therefore, the energy grass with high cellulose and hemicellulose content should be selected as the feedstock in the development of energy grass biogas project. In addition, lignin, and organic matter difficult to degrade should be used to enhance the comprehensive benefits of energy grass biogas project. We also established an open material flow analysis system by the anaerobic digestion of silage grass in the LAB-2 500 mL reactor and analyzed the distribution of carbon and nitrogen elements in different material throughout all the digestion process. According to the material flow analysis of silage grass in the anaerobic digestion process, 33.1% and 8% carbon in the raw material entered into the biogas and fermentation broth, respectively, and the other was still in the raw material. The distribution of nitrogen was that 69.4% was in the fermentation broth, and 30.7% was still in the raw material. Moreover, according to the energy flow analysis of silage grass in the anaerobic digestion process, the energy yield of 33.1% entered into the biogas, while the energy yield of 61.3% was still unused in the raw material. The mass conversion rate of energy biomass was low, 60% carbon, 80% nitrogen and 60% energy were still in the digestion residue. Therefore, the use of carbon and nitrogen elements in the energy grass biomass was greatly important in the anaerobic digestion process, and it can be beneficial to improve the efficiency of energy grass biogas project in the future. This study provides a reference for the development and utilization of different energy grasses.