There exist some problems in anaerobic digestion of crop straw, including hard biodegradation and easy to be affected by temperature in methane production process. This study improved the hydrolysis-acidification of straw by micro-aerobic bio-pretreatment before anaerobic digestion. At the same time, this study utilized the nutrients complementarily and synergistic effect between straw and other biomass wastes, built a two-phase anaerobic digestion (TPAD) system which used hard-degraded straw as main substrate. On that basis, this study built a pilot system which was composed by the solar thermal-phase change thermal storage (PCTS) and TPAD. It has succeeded in applying mesophilic anaerobic digestion of biogas production system in winter. The main research works and results include the following content.
Concerning the by-products accompanied with traditional physical-chemical pretreatment method, and cost of microbial strains of biological pretreatment. The micro-aerobic pretreatment which used hydrolysis-acidogenesis liquid from TPAD as inoculum were studied. The results show that, the degradation rate of the lignocellulose in rape straw was increased with improvement of oxygen load. However, excess oxygen loads may decrease hydrolysis-acidification performance of pretreated straw. The optimal pretreatment conditions were as following: oxygen load 20 mL/ gVS/d, VFAs concentration of inoculum liquid 4 g/L, pretreat time 10 d. The VFAs and sCOD concentration of hydrolysis-acidification fermentation of the pretreated straw were improved 88.43% and 75.50%, respectively, compared with un-treated straw. After investigating the morphology, functional groups and crystal structure variations of the rape straw, it is found that the regular structure was destroyed, the crystallinity of cellulose was decreased and the functional groups related to lignin was decreased after micro-aerobic pretreatment. In addition, the microbial community structures have varied, especially the microbial flora closely related to lignin degradation. After the pretreatment, the abundance of Actinobacteria was increased 84.5 times, and the abundance of Aspergillus niger and Penicillium citrinum were increased 4.4 fold and 1.2 times, respectively.
There are kitchen waste and livestock manure existing in rural area besides crop straw, which can be mixed together. The study on TPAD based on synergistic effects between different substrates was conducted. The results show that（1）The VFAs production was promoted when rape straw was mixed with other two substrates in hydrolysis-acidification process. The acetic acid content in VFAs showed increasing trend when the DD content increased and KW content decreased. While the content of propionic acid showed a contrary tendency.（2）The promoting synergistic effects of co-substrates anaerobic digestion can be found in research on methane producing process. The total methane yields of co-digestions were increased 29.3-183.1% compared with mono-digestion of rape straw. The promotion of methane yield was on account of two reasons. On the one hand, it was due to the increasing of easily degradable organic matter and the decreasing of C/N ratio. On the other hand, it was due to the synergistic effects between different substrates. The experimental results also revealed that the biodegradation rate of rape straw was improved when kitchen waste and duck dung was mixed with the rape straw in two-phase anaerobic digestion. The optimal mixture ratio of the rape straw, kitchen waste and duck dung was 50:25:25.（3）The influence of fermentation temperature on hydrolytic acidification and methanogenesis were studied. The results showed that, the VFAs production capacity was improved with the fermentation temperature in 20°C~35°C range. The VFAs concentration of the fermentation was reached 9270.1mg/L at 35°C. Additionally, it can be found that the methanogenesis was very sensitive to the temperature variation. The methane production capability was reduced when fermentation temperature reduced 2°C~5°C from 35°C. The cumulative biogas yield of the anaerobic digestion in 35°C was improved 31.07% and 169.88%, respectively, compared with that in 33°C and 30°C. Considering the perspective of biogas production rate, the fermentation temperature reduced 2°C from 35°C in a short time is considered acceptable.
In this study, a pilot system of two-phase anaerobic digester heated by solar energy has been constructed based on two-phase anaerobic co-substrates digestion. The solar thermal and PCTS system can collect and store solar energy to heat the two-phase anaerobic digester, and reduce the fluctuation of fermentation temperature which was caused by solar radiation fluctuation. A set of comparative field studies were carried out on different heating mode in winter. The comparison shows that, the PCTS system can reduce the fluctuation of fermentation temperature and maintain the fermentation temperature within the mesophilic range. The total biogas yield reached 20.48 m3, the energy conversion rate of the substrate increased by 1.01 times and 5.65 times, respectively, than that of TPAD heated by only solar thermal collector and without heating, respectively. The modified Gompertz model model had a good precision and fitting with the methane production rate changes of two-phase anaerobic digestion heated by solar thermal-PCTS system. The errors between model predictions and measured results was 1.49%。
The thermal performances and heat utilizing efficiency of subsystems under typical working condition in winter were analyzed. The results show that, heat utilizing efficiency of solar heated two-phase anaerobic biogas production system is 42.0% in winter. The heat utilizing efficiency could potentially be improved by optimizing the external insulation layer or improving heat transfer coefficient of the heat exchange coil. This study use computational fluid dynamics software to build the physical and mathematical models of the methanogenic reactor. Simulation results show that the heating rates increased with the increasing heating temperature, but also intense the temperature stratification in reactor. Considering overall the temperature distribution, heat exchange efficiency and cost of phase change material, this study can choose 55°C as heating temperature of solar auxiliary heating device. The uniformity degree of temperature distribution can be improved by hydraulic agitation. The maximum temperature difference inside reactor reduced from 2.3°C to 0.2°C when stirring velocity increased to 0.27m/s.