Study On Thermal Characteristics Of Low Energy Anaerobic Fermentation Reactor In Cold Area

The high energy cost of heating compensation in winter for biogas projects in cold areas of north China restricts the development of biogas projects.The core problem of low energy consumption anaerobic fermentation reactor is that the reactor can reasonably and effectively use energy from the"quantity"and"quality"of energy,so as to ensure the anaerobic fermentation performance of the reactor and make it run with energy saving and high efficiency.Therefore,energy saving analysis and research were carried out in this paper on the physical size,insulation layer setting and exergy loss of the above-ground anaerobic fermentation reactor.Based on the theory of unsteady heat transfer,the heat transfer model of the underground embedded anaerobic reactor is established,and it is analyzed,simplified and solved by the finite element method.The main work and research results of this paper are as follows:（1）Based on the concept of building shape coefficient in the field of civil architecture,the model of building shape coefficient of three kinds of anaerobic fermentation reactors（flat top,cone top and vault top）is established.According to the principle that the body shape coefficient is proportional to the heat consumption,MATLAB is used to solve the body shape coefficient models of the three reactors.The shape coefficients of the three reactors were obtained as S_flat=3（π/V）^1/3、S_conical≈2.72（π/V）^1/3、S_arch≈2.62（π/V）^1/3.According to the analysis,the shape and size of the cone-top and vault-top reactors do not meet the requirements of the actual fermentation process under the constraint of the minimum body size coefficient.Therefore,the low-energy anaerobic fermentation reactors should be cylindrical with the optimal ratio of height to diameter of 1:2.（2）On the basis of the above research results,based on the heat dissipation model of anaerobic fermentation reactor,the method of economic insulation layer thickness,the method of maximum allowable temperature drop and the method of control heat transfer gradient were adopted respectively.The thermal insulation layer thickness and thermal resistance of anaerobic fermentation reactors in different regions were compared and calculated.The results show that the method of economic insulation thickness and the method of maximum allowable temperature drop have some defects in the practical application.The method of control heat transfer gradient can basically meet the actual engineering requirements.Based on this method,the thermal conductivity thresholds of the top and side wall envelope structures of energy-saving reactors of different sizes in Harbin,Shenyang and Beijing in winter were calculated.（3）Through exergy analysis of the heating system in the medium temperature anaerobic fermentation reactor,it was determined that the water supply temperature of the heating system was 47℃and the return water temperature was 40℃,the exergic efficiency of the heating system was 86%,and the energy level difference between the energy supplied and used in the reactor was 0.018.The above shows that the energy efficiency of heating system is high and the energy level matching between supply and demand is reasonable.（4）Taking the size of a large 1000 m³methane engineering anaerobic fermentation reactor as an example,the heat transfer model of an underground embedded anaerobic fermentation reactor was established,and the two-dimensional unsteady partial differential equation was simplified into a standard elliptic partial differential equation.The finite element method was used to solve the model with the help of MATLAB software.The temperature field,heat flow distribution,heat dissipation path and temperature gradient around the reactor tank are obtained.The heat transfer consumption of the underground embedded reactor without insulation layer is 203321W,which is42.6%of that of the above-ground reactor of the same volume.