Evaluation Methods of Fertilizer Produced in Biogas Plants and Its Effect on Feasibility

Energy is a major demand of the globalization world. For this reason, energy production from biogas plants as a renewable energy source, is increasing day by day in the world and in our country. After the production of biogas from raw material of animal origin, various outputs can be obtained. These outputs are solid and liquid fertilizers. The use of the fertilizer produced in these plants in agriculture is important because of its possible negative impact on both the investment feasibility and the environment. In this study, biogas production was dimensioned in a wet fermentation plant with 3 MW electricity production capacity operating under mesophilic conditions. 30 days was chosen as the hydraulic retention time of the biogas plant. Fermentation tanks are continuous stirred tank reactors. The biogas engine was used for energy generation. In the biogas plant, solid and liquid fertilizers are produced as a product in addition to energy production. Enrichment processes are listed in order to convert the solid fertilizer produced in the biogas plant into salable form and to be more efficient when applied to the soil. Considering that the produced fertilizer can be evaluated in 3 different ways, feasibility analysis was examined. It is thought that the fertilizer produced in the first option is used directly in agriculture without being sold, in the second option, it is sold as solid organic fertilizer, and in the other option, it is converted into an organomineral fertilizer form by making the necessary additions. Within the scope of the results obtained, the effects of energy production as well as fertilizer sales on the feasibility of the facility were revealed. Considering the calculations made, the costs per unit kW of the facilities are respectively: 2,090 $/kWe, 3,726 $/kWe, 6,225 kWe. It has been determined that the simple payback period of 6.01 years for a biogas plant that generates income only from electricity production is reduced to 1.69 years with the sale of additional organomineral fertilizers. If the fertilizer was sold as organic fertilizer, the simple payback period was 4.24 years. However, it was observed that the cost of diammonium phosphate to be added to the system, in addition to the decrease in the payback period, increased the biogas plant costs by approximately 92.5%.