The Study And Process Design Of Producing Fuel Alcohols From Biomass Synthesis Gas

There are many ways of using biomass as energy, converting them into fuels by thermo-chemical process is one of them, In this paper, converting biomass into alcohols by thermo-chemical process is studied, and the following process is mainly considered:biomass is converted into crude synthesis gas by gasification, after the process of gas cleanup and conditions, with the appropriate mixed alcohols synthesis catalyst, the synthesis gas is converted into mixed alcohols.In the process development, the following several aspects should be noted:the natures of biomass are dispersion, seasonal and have low energy density, so it is needed to create complete collection system; the synthesis gas composition is different, which is produced by different gasifiers, so it need to select a reasonable gasifier; the hydrogen to carbon ratio requirements by different catalysts are different, and it should take into account the gas composition in the export of the gasifier; at present, using ethanol as transport fuel is more mature in alcohol fuels market; producing alcohols can be co-generation with power or other chemical products; more standards and improvement of alcohol fuels transport system are better for alcohol fuels going into the market.The technical review of making alcohols from synthesis gas is emphasis on the following:catalyst, catalytic synthesis process and separation technology of products. At present, the existing catalyst have low selectivity, and have some issues, such as producing hydrocarbons as by-product. Slurry reactor has a good effect to remove the heat of reaction; two reactors in series helps to improve alcohols synthesis effects. At present, the mixed alcohols separation methods are mainly distillation and molecular sieve dehydration.The following impacts are considered in the conceptual process design and study of making alcohols from biomass by thermo-chemical process:reform fractions of recycle synthesis gas, methane conversion, BCL gasifier and GTI gasifier in parallel, the fractions of methanol recycle. Through the process simulation of making alcohols by BCL gasifier, it is found that:the process has high methane content, and it has separation problems of alcohols- water-hydrocarbons-synthesis gas system. Through analysis of the alcohols production models, it is found that:the reform fractions of recycle synthesis gas is a significant factor; the conversion ratio of methane is needed to be improved, which can lower the methane content and improves the process H2/CO molar ratio, but it have disadvantages of reducing the reform fractions of recycle synthesis gas. GTI gasifier only take the role of easing the process condition, and it can not lower the water and methane content effectively; also, GTI gasifier is still needed to be improved in energy utilization. Increasing the fractions of methanol recycle is better to reduce the molar ratio of H2/CO. Overall, the high circulation rate of methanol and the high reform fractions of recycle synthesis gas are better for mixed alcohols producing.In order to reduce the methane content and improve material utilization of the process, a new process design model is established. The products of new designed model are fuel ethanol, SNG, with sulfur and C3+ alcohols as by-products. The new model does not use the process of water electrolysis which can provide H2; It also does not use the process of producing synthetic natural gas by CO shift to methane.The feasibility of directly remove methane from process and added CO shift device are studied. Before the establishment of the model of producing fuel ethanol and SNG using BCL gasifier. It is better to lower the methane content by adding methane adsorption device after the acid gas removal unit. More than 90% of the methane can be removed, if CO shift device added.Through the further analysis of the established co-production model, it can be found that:at the new process conditions, the methane content of the synthesis gas can be maintained at lower content, and the mixed alcohols synthesis conditions is effectively meet. If alcohols steam reforming reactions are not considered,1000kg poplar (50% moisture content) can produce 72.95kg ethanol and 44.24kg SNG; only 34.06% C of the poplar is converted into products, the remaining C is emission in the form of carbon dioxide; CO2 emissions can not be lower by adding alcohols steam reforming and CO2 reforming reactions, but alcohols production can be higher.