| Biogas production by anaerobic fermentation is an important means of straw and other agricultural waste treatment,which can solve the problem of rural energy supply and environmental pollution at the same time.The reactor in our country mostly adopts medium temperature fermentation(about 35?),and the ambient temperature in most months of the year in each region is far lower than the fermentation temperature,which affects the stability of biogas production.The outdoor reactor has a long fermentation period,so it needs to overcome the heat dissipation to the surrounding environment.At present,solar coil heaters are mostly used.When they are running,they need to use water pumps to realize the heat exchange process,which consumes a lot of electric energy.Using high-grade energy to deal with waste will seriously reduce the economy.To popularize this way of producing biogas is bound to put forward higher requirements for energy efficiency,and it is urgent to find a new and efficient heating way.Direct absorption heating technology has the advantages of simple equipment and high energy utilization,which is widely used in desalination and other industrial production.However,when it is applied to the heating anaerobic reactor,the complex photo-chemical conversion process inside should be considered.At the same time,the heat insulation of glass envelope and the internal temperature control problem are also the bottleneck of its development.In view of this situation,this paper studies the heat and mass transport mechanism in this kind of reactor by the method of experiment and numerical simulation,and increases its heat insulation and thermal storage performance by filling paraffin material in the glass enclosure.Aiming at the simulation analysis of the photo-chemical transformation in direct absorption reactor,Monte Carlo method,finite volume and band model were used to solve the coupling problem of the photo-chemical heat transfer and the biochemical reaction process of anaerobic fermentation.The ADM1 model was improved by reducing the variables,and the constant of reaction rate was modified by the coefficient of heat exchange to realize the numerical calculation of the non-isothermal temperature fermentation process.Fortran language was used to compile the corresponding calculation program.The reliability of the model and calculation method was verified from the three aspects of sunlight transmission,photo-thermal transformation and biochemical reaction experiment in the medium,and the influences of solar radiation intensity,TS concentration and other factors on the fermentation process were analyzed.The acquisition of optical parameters is the study basis of the photochemical conversion process in the direct absorption reactor.In view of slurry optical characteristics containing particles,two methods,i.e.double thickness,double thickness and function approximation,were selected to perform the absorption coefficient and refractive index of the slurry using the experimental values of optical cavity permeability.Based on mie theory,the positive model of solar transmission in slurry with particles were established,and the appropriate particle radius,concentration and scattering direction were set up.The scattering coefficient of slurry was obtained by comparing the calculated results with the experimental values of the transmissivity for optical cavity.The results show that the absorption coefficient of slurry in visible band is smaller than that in infrared band,and the refractive index of slurry in visible band is similar to that in infrared band,but its fluctuation is smaller than that in infrared band.The experimental measurement platform of heat and mass transfer process in direct absorption reactor under solar irradiation was designed and built.The relationship between the temperature gradient,volume average temperature of slurry and the parameters in anaerobic fermentation process was studied,including substrate concentration,volatile fatty acid content,p H of slurry,gas production,etc.The relationship between the index term of heat exchange coefficient and the temperature fluctuation of slurry was established,and the variable value of the relationship model was determined by using the outdoor experimental data.Two coefficients of solar absorption efficiency and relative energy saving efficiency were defined to evaluate the energy-saving effect of direct absorption reactor.Considering the evaporation loss on the surface of slurry,environmental heat loss,energy consumption of water pump and residual methane combustion value and other factors,it was analyzed that the energy-saving effect of the reactor is affected by the TS concentration of slurry,seasonal change of outdoor environment and other factors.The results show that compared with the traditional coil heating method,the direct absorption heating method can effectively save electric energy.However,the solar energy absorption efficiency of the reactor is less than 50%,and even less than 30% in spring and autumn.For the direct absorption reactor,it is necessary to further add corresponding devices to improve its solar energy utilization.In order to further improve the energy-saving efficiency of direct absorption reactor,the fermentation effect of phase change material filled in the glass envelope of reactor was studied.A separate experimental platform was designed and built to analyze the changes of biogas liquid slurry temperature,substrate concentration,gas production and two energy-saving efficiency coefficients.It was proposed to solve the problem of complex reactor structure and high ratio of height to diameter by the method of phased and regional solution.Based on the reliability of model by the experimental data,the operation of the reactor in spring,summer and autumn in Northeast China was simulated,and the corresponding parameters such as the thickness and phase change temperature of PCM layer were determined.The results show that the PCM filled in the interlayer of direct absorption reactor can effectively improve its thermal insulation and thermal storage performance,reduce the evaporation loss of the slurry surface,so the maximum solar energy absorption efficiency can be increased to 0.66,and its energy-saving efficiency can also be increased by 29.7%.Through the research,the feasibility of applying the direct absorption technology to heat the fermentation process of anaerobic reactor was verified,the mechanism of the photo-thermal transmission and biochemical transformation in anaerobic reactor under solar irradiation was deeply understood,the experimental data and numerical simulation method of the operation process of reactor under the new heating mode were obtained,and the internal temperature of the slurry under solar irradiation was mastered.The change rule of biochemical reaction rate and the influence rule of related factors in the process of non-isothermal temperature fermentation caused by the temperature gradient inside slurry under solar irradiation were mastered.The breakthrough of using PCM to improve the energy-saving efficiency of this heating method has been realized.The results provide theoretical basis and reference for the design,energy-saving efficiency evaluation and optimization of direct absorption reactor. |
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