Review on heat-utilization processes and heat-exchange equipment in biogas engineering

Abstract

With the increasing demand for environmental protection and renewable energy, bioenergy technology has been attracting considerable attention. Anaerobic digestion (AD) is the process to convert the low-grade biomass into bioenergy, in which both heat-recovery and -recycling should be treated carefully in order to improve the process efficiency. In this work, the heat-recovery and its utilization processes were reviewed, and different types of heat exchangers as well as their advantages in biogas engineering were surveyed. It shows that the recovery and utilization of the waste heat from biogas plants with an internal system, such as slurry effluent unit, the combined heat and power unit, the sanitation unit, and the internal recycle unit, are important for improving the AD efficiency of biogas production. For example, the recovery and recycling of waste heat from the effluent can result in a 2–3 °C temperature increase for the inlet manure slurry. For thermophilic AD, the heat recovery from effluent can save about 50% of the total heat requirement. The external heating process is more suitable for large- and medium-scale biogas plants, and the heat transfer coefficient of external heating (850–1000 W/m2 K−1) is almost two-times higher than that of the internal heating (300–400 W/m2 K−1). To utilize the waste heat in biogas plants, heat exchangers have been designed for biogas slurry. However, further improvement on the heat exchangers with anti-blockage, anti-fouling, high efficiency, and low investment is still needed. Moreover, the heat exchanger suitable for a low-temperature-difference system is specially needed in China, but the development is still in its infancy. Therefore, to tailor to the Chinese national conditions, special external heating processes should be designed and reoriented to the diversity of biomass, the climatic environmental conditions, and the renewable Chinese policies.