PROSPECTS OF BIOGAS OBTAINING FROM SWEET SORGHUM IN UKRAINE
Authors: Kulichkova G. , Savytska N. , Volodko O. , Ivanova T. , Tsygankov S. 
Published: December 2022

The development of energy has a decisive influence on the state of the economy in the country and the standard of living of the population. The production of biogas from renewable sources meets the goals of sustainable development for the countries of the world, approved at the meetings of the UN General Assembly in September 2015. With the help of biogas technologies, it is possible to increase the share of energy from renewable sources in the national energy balance, reduce the volume of waste generation and increase the volume of their processing and reuse based on innovative technologies and productions, as well as limit greenhouse gas emissions. Biogas is a flammable natural mixture of gases, which is formed as a result of the decomposition of organic substances under anaerobic conditions, is a mixture of 50-70% methane and 30-50% carbon dioxide, as well as a small amount of hydrogen sulfide, nitrogen, hydrogen and other impurities. The ratio of methane to carbon dioxide depends on the nature of the substrate and the pH during the process. It is possible to regulate the performance of anaerobic fermentation using macronutrients and micronutrients. They are stimulants, as well as more economically and environmentally safe catalysts compared to chemical reagents, which often require significant energy costs. When using such by-products of agro-industrial production as straw or bagasse and bard, in addition to the main product (alcohol with an energy potential of 22.56 MW•h/ha•year), almost twice as much energy can be obtained – 45.06 MW•h/ha• year from by-products. Sweet sorghum is a grassy plant of the cereal family, the stems of which contain juice with 12-20% sugars, it is characterized by good adaptability to arid conditions, temperature fluctuations, salty and acidic soils, and therefore it is grown as an energy crop for the production of bioethanol and biogas, in including on marginal lands. Sweet sorghum juice is a raw material for the production of bioethanol, and bagasse, as a by-product, can be used at the same enterprise to generate biogas. Thus, the enterprise produces a commercial product, solves the problem of disposal of production waste and provides itself with electricity and heat through the operation of a biogas cogeneration plant. Sorghum bagasse as a raw material for biogas has a high biological value, rich in trace elements, so it is a good candidate for anaerobic fermentation. Sweet sorghum bagasse (Botanical variety) has a high content of organic dry matter (87.85%), is rich in trace elements, therefore it is definitely a promising raw material for anaerobic fermentation. The theoretical yield of biogas from sorghum bagasse is 476.38 l/kg of dry matter, with a methane content of 52.90%. The expected energy value of methane is 9.02 MJ/kg dry matter of sorghum bagasse. As part of the study, a biochemical analysis of sweet sorghum bagasse was carried out. The moisture content of dried sorghum bagasse was less than 10%, which ensured its long-term storage as a substrate for anaerobic fermentation. The energy potential of sorghum bagasse was calculated. Since carbohydrates make up more than half of the dry matter of sorghum bagasse, they mainly provide the potential for biogas formation. At the same time, the content of proteins and fats increases the level of the target product methane in biogas. For further methane fermentation, the macro- and microelement composition of sorghum bagasse is of great importance. Ni stimulates biogas production and methane content in biogas. Addition of Ca and Mg salts as energy additives can increase CH4 production and prevent foaming. The optimal C:N:P ratio for increasing methane yield is 200:5:1. Analyzing the obtained results, we can see that such a macroelement as calcium was found to be the most in sorghum bagasse, which indicates the potential for obtaining a high yield of methane. An important indicator is also that the content of heavy metals cadmium and lead is very small. This indicates that the soil on which the sweet sorghum was grown is not contaminated with heavy metals and that the digestate after fermentation can be used as a fertilizer because it is safe. Sorghum is one of the crops with the highest C/N ratio, which determines the prospect of co-fermentation with a substrate that has a low value of this parameter (sugar beet vinasse). Sweet sorghum is a promising candidate for biogas production due to its good chemical composition, relatively high methane yield and the possibility of exploiting marginal lands in Ukraine for its cultivation.