| The Bio-SNG (biomass-based synthetic natural gas) production from interconnected fluidized beds and pressurized fluidized beds methanation reactor is proposed in this dissertation and comprehensive research about it is conducted with respects of experiments, simulation and environmental benefits analysis.An experimental study about bio-syngas production from biomass gasification via a 25kWth interconnected fluidized beds system is carried out with the wheat straw as the feedstock. The influences of gasification temperature, the ratio of steam to biomass (SIB) and bed material types on the bio-syngas content, yield and the ratio of H2 to CO (H2:CO) in the product gas are analyzed. The results show that suitable gasification temperature and SIB may promote the rise of bio-syngas content, while higher gasification temperature goes against the increase of H2:CO, which benefits from the higher SIB. However, with the rise of gasification temperature and SIB, the bio-syngas yield increases. With respect to bed materials, the H2:CO and bio-syngas yield from Ca-based catalyst are larger than that from inert bed material.A pressurized fluidized bed methanation reactor system was designed and constructed. An experimental study of methane production from bio-syngas was carried out on this methantion reactor system to obtain the suitable operating conditions with the two Ni-based catalysts. In the meanwhile, the modern characterizing methods are used to study the catalytic mechanisms and carbon deposition of the methanation catalysts. The results show that the methane are effectively produced from the bio-syngas in the pressurized fluidized bed reactor and the typical methane formation rate is higher than 3.2 mol/(L·h) and CO conversion rate is larger than 80%. After the comparison of the two experimental results, these two catalysts have the same influences on the methanation reactions despite of their different surface compositions and morphologies. Moreover, the operating conditions, i.e. methanation temperature, pressure, space velocity and ratio of H2 to CO, have significant influences on the performance indexes (i.e. methane formation rate and CO conversion rate) of methanation processes. Higher methanation temperature is favored to the methanation process and the methane formation rate and CO conversion rate achieve the maximum values at the methanation temperature about 350℃. The methane formation rate and CO conversion rate increase with the rise of methanation pressure, especially when the methanation pressure is higher than 0.3 MPa. With the increase of space velocity, the methane formation rate increases while the CO conversion rate declines accordingly. With the rise of H2:CO, the methane formation rate increases accordingly, while the CO conversion rate rises and reaches the highest values when the ratio is about 3 and then the CO conversion rate maintains a high value after that. The produced methane needs to be discharged from the system since more methane in the product gas may cause the methane formation rate and CO conversion rate to decline. Carbon deposition is observed on these two catalyst samples since free carbon is generated on its surface. With the increase of reaction temperature, the phenomenon of carbon deposition becomes more serious. Moreover, the BET analysis shows that the surface morphologies are changed due to the carbon deposition.Based on the interconnected fluidized beds gasification system and pressurized fluidized bed methanation system, the simulating model of the Bio-SNG production process is set up and the correctness and feasibility of the model is validated through the comparison between the simulating results and experimental ones. Then, the influences of operating conditions (gasification temperature and pressure, SIB, methanation temperature and pressure, biomass categories) on the system performance indexes are studied. The results show that the operating conditions have significant effects on the composition of crude SNG, methane yield and energy efficiency. Higher gasification temperature may cause the methane content to increase slowly in the crude SNG, but too high gasification temperature may result in the decline of methane yield and energy efficiency. Therefore, there exists a suitable gasification temperature for the highest methane yield and energy efficiency. Higher gasification pressure gives rise to the methane content in the crude SNG and there is an optimum gasification pressure for the maximum methane yield and energy efficiency. With the rise of SIB, the methane content in the crude SNG decreases accordingly and there also exists an optimum SIB. Higher methanation temperature may cause the methane content reduction in the crude SNG and the methane yield and energy efficiency rise on the contrary. However, with the increase of methanation pressure, the methane content rises and the methane yield and energy efficiency decline. The biomass categories have less influence on the methane content but have large influence on the methane yield and energy efficiency. That is, the methane yield and energy efficiency from sawdust is the largest and that from the rice straw is the least.A cradle-to-gate life cycle assessment of Bio-SNG production has been carried out from the perspective of China localization, and the inventories of biomass cultivating and harvesting, feedstock transportation, Bio-SNG facility construction and dismantling, and power consumption (i.e. purchased electricity (PE) scheme and self-supply power (SP) scheme) stages have been analyzed. Then, the inventory data is characterized and normalized taken four types of environmental impact indicators into account, i.e. CADP, GWP, AP and RI. The characterized results show that the power consumption stage and Bio-SNG production stage have great influence on these four environmental impact indicators, while the normalized results show that the contribution of the environmental impact indicators to the Bio-SNG life cycle assessment is as following:AP> CADP> RI> GWP, among which the GWP is negative. The SP scheme can reduce the environmental impacts of Bio-SNG product significantly since its environmental impact indicators are all less than that of the PE scheme. After comparing the Bio-SNG, natural gas (NG) and coal-based synthetic natural gas (C-SNG) from the environmental impact perspective, the amount of CO2 emission from C-SNG is the largest, followed by that from NG, while the amount of CO2 emission from Bio-SNG is negative. In short, Bio-SNG shows great advantage in the abiotic resources saving, greenhouse gases emission reduction, acidification potential and respiratory inorganic. Finally, a sensitivity analysis about allocation principles (i.e. economic allocation, mass allocation, HHV allocation and exergy allocation) about the rice straw and rough rice is carried out to evaluate the allocation principles on the environmental impacts. The result shows that AP and RI indicators are more sensitive to the allocation principles, followed by the CADP and GWP. The reduction of acid gas emission and the abiotic depletion should be focused on when optimizing the Bio-SNG clean production since Bio-SNG shows great environmental benefits in the greenhouse gases emission reduction. In conclusion, from the perspective of LCA environmental impacts, Bio-SNG production shows a great promising future in China. |
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