Optimization of the cocoa shell, inoculum, and water mixture for biogas production with 40-70% methane concentrations

Abstract

Biomass has been a valuable source of energy over the years and its application includes many areas, whether for cooking, heating, or electricity generation. In Honduras, especially in the rural sector, biomass, mainly firewood, became the main source of energy, which has led to a high consumption of firewood and, consequently, to an increase in the rate of deforestation. In this study, we proposed to use a more specific biomass, cocoa husk, as feedstock for biogas generation. It is important to note that biogas does not only consist of methane; it also contains carbon dioxide (CO2), nitrogen (N2), hydrogen sulfide (H2S) and oxygen (O2). However, to be considered of quality, the concentration of methane in the biogas must range between 40% and 70%. The main objective of this research has been to optimize the mixture of raw materials, including cocoa husk, water and inoculum, with the purpose of obtaining a methane (CH4) concentration in the biogas produced, located in the range of 40%-70%. To achieve biogas production, several tests were carried out in four biodigesters, each with different proportions of water, inoculum, cocoa husk and grinding level. This made it possible to obtain varying amounts of biogas and to measure the methane gas concentration in each test. During the operation of the biodigesters, the temperature and pH of the mixture were constantly monitored. This was possible because methanogenic bacteria, responsible for methane generation, thrive at temperatures between 30 and 40 degrees Celsius and in a pH range of 6 to 8. The results obtained have the potential to demonstrate that cocoa shells can be a viable feedstock for biogas generation in Honduras. Furthermore, these findings could serve as a basis for future research in other cities around the world, thus contributing to the search for energy generation solutions.