Research On The Mechanism And Process Optimization Of Biological Methanation Of Biomass Pyrolysis Gas

Pyrolysis gas, as a kind of combustible gas whose main components including H2, CO, CH4 and CO2 has so many characteristics like low calorific value, toxic, easy combustion, etc. which makes it easy to lead to accidents frequently in practical application process and also exists the problem of low efficiency at the same time due to its low calorific value As the depleting of fossil fuels, the demands of natural gas is growing, using pyrolysis gas to synthesize biological natural gas with anaerobic digestion system and upgrading biogas at the same time is extensively attentioned because of its efficient utilization. In current work, we studied the biological methanation conversion efficiency of pyrolysis gas under different conditions and examined the possible influence factors in the transformation process.The biological methanation conversion efficiency of pyrolysis gas and the system stability is studied under the conditions of different gas load and inlet gas cycling mode in the pilot scale firstly. It was found that the conversation of biological methanation is good under the running conditions of small ventilation volume and systemic circulation quantity (inlet gas volume 1 m, ventilation finished in an hour, and maintain the gas cycling all day long in the ventilation speed). In the meantime, the absolute conversion rate of H2 and CO reached the highest 87.7% and 84.3% respectively and it would lower the conversation efficiency if increased gas volume of ventilation and reduced the volume of circulation. But in process of the ventilation experiment, the acidification phenomenon appears along with time. Considering the tar and oxygen content in pyrolysis gas, we investigated emphatically the influence of tar on the biological methanation system with the limitation of the oxygen content in the next study.The influence of the compositions and contents of biomass tar and tar model compounds on the biological methanation anaerobic system was studied. (1) It was found that adding a lower content of actual tar semi-continuously (adding concentration is less than 100 mg/L) does not has obvious effects on the anaerobic system. Under the condition of moderate levels (500 mg/L to 1250 mg/L), biogas production increased, and the largest gas production can increase more than 40%, but the change of methane content is not obvious. But after adding high tar content (more than 1250 mg/L), the anaerobic system quickly collapse, biogas production volume and methane content decreased significantly. (2) It was found that the change of methane content of the biogas produced in the anaerobic system is not obvious when adding a low content of tar model compounds semi-continuously (adding concentration is less than 100 mg/L), but the biogas production volume has increased gradually, and may changes in a stable range. But when the adding concentration of tar model compounds is greater than 750 mg/L, serious inhibition phenomena appeares in the anaerobic digestion reaction system after a certain incubation time.In comprehensive consideration, the tolerance ability of anaerobic digestion system to the pyrolytic tar and tar model compounds is about 1000 mg/L and 600 mg/L respectively in this experiment case.The conversion efficiency of biological methanation and system stability of the syngas (contains only CO and H2) is studied under different gas load and inlet gas cycling mode in the laboratory scale (reaction volume of 37.5 L). It was found that the transformation capability of biological methanation system can be enhanced constantly in the long run process along with the increasing gas cycling speed and gas load. In laboratory scale, the absolute conversion rate of H2 and CO can reached 100% and 95% respectively under the running conditions of daily glucose processing load is 500 mg/L, daily inlet gas volume rate of reactor is 1 and the gas circulating speed is 4 L/min.