Research progress on biomass aerobic fermentation heat production and recycling

Energy demand is ever-increasing with the rapid development of society. Chemical energy has also resulted in a variety of atmospheric pollutants in traditional energy reserves, such as coal, oil, and natural gas. Biomass energy is a promising renewable energy to convert solar into chemical energy and then store it inside biomass. The available biomass resources in the world are as high as 170 billion tons at present. But there is a low utilization rate of biomass resources. The rest is burned or abandoned as waste, leading to the waste of resources, as well as serious air, water, and soil pollution. Among them, aerobic fermentation is an effective treatment for organic solid wastes. Both heat energy and organic fertilizer can be produced in a simple process without size restrictions. The fermentation heat energy can be recycled and then used to heat rural winter residential houses, vegetable greenhouses, farms, and processing plants. Organic fertilizers are used to replenish the soil fertility in the field. As a result, the heat-fertilizer combination production using aerobic fermentation is an environmentally friendly energy consumption suitable for rural areas. Although highly effective organic fertilizers have been the primary goal of aerobic fermentation, it is important to consider the heat produced during fermentation. Fermentation heat can serve as a kind of "zero-carbon" energy to replace the traditional fossil energy in heating and bio-drying, particularly for carbon peaking and carbon neutrality. Three stages are also included in the production of and recovering heat through aerobic fermentation: heat production, recovery, and usage. These stages interact with the heat acting as a medium. Some research has focused on the recycling of fermentation heat, but it is still lacking in heat recovery. This study aims to clarify the systematic relationship among the three stages of heat production, recovery, and utilization. The principle of heat production was described in biomass aerobic fermentation, the influencing factors of heat production, recovery, and utilization. The aerobic fermentation of biomass was attributed to the oxidative decomposition of organic matter under the microorganisms and the continuous release of heat. The completely oxidized substances were transformed into carbon dioxide and water, while the partially oxidized microorganisms were oxidized into humus. A systematic investigation was carried out to explore the effects of three factors on the heat production of biomass aerobic fermentation, including bacteriological agents, physicochemical properties of raw materials (particle size, pH, carbon to nitrogen ratio, and moisture), and fermentation (temperature and oxygen content). Furthermore, the current systems were summarized for heat recovery and utilization. Three types were categorized into direct utilization, sensible heat recovery, and exhaust gas heat recovery. Currently, fermentation heat recovery has been explored at lab- and pilot-scale, and commercial systems, where the heat recovery rate varied from 13.4% to 73.0%. The heat recovery rate of the fermentation system depended on the type and scale of fermentation feedstock, the type and mode of heat recovery, the fermentation, and the ambient temperature. In general, the larger the fermentation heat production was, the higher the heat recovery rate was. The average recovery rate for lab-scale systems was 1.90 MJ/h (1.16 MJ/kg DM), for pilot-scale systems 20.04 MJ/h (4.30 MJ/kg DM), for commercial-scale systems 204.91 MJ/h (7.08 MJ/kg DM). The direct utilization of fermentation heat is inexpensive and suitable for self-consumption on farms. The recovering internal heat from the fermentation system with the buried pipe is simple to operate and suitable for domestic use. The heat recovery system for exhaust heat recovery is highly efficient and suitable for commercial environments. Finally, the research direction was also given to provide support for the heat utilization of biomass aerobic fermentation.