Optimization of light olefin production system from agricultural and forestry residues and analysis of its mass and energy conversion

Light olefins, represented by ethylene and propylene, are important platform compounds. At present, naphtha and natural gas are the main raw materials for the production of light olefins. The use of agricultural and forestry waste from lignocellulosic biomass can be an alternative supplement to produce renewable light olefins, considering the limited fossil resources and restricted environmental legislation. In this paper, the process simulation of light olefin production by gasification of agricultural and forestry waste biomass, methanol synthesis and the following step of methanol to olefins was modeling. And the material and energy consumption of the process was analyzed and optimized in this context. Aspen Plus software was applied for process modeling. The integrated process included several main sections, of which were O2-steam gasification of wood chips, reforming of raw fuel gas, adjustment of fuel gas composition (sulfur removal, water-gas shift reaction, pressure-swing CO2 adsorption), methanol synthesis and separation, light olefin production from methanol, waste heat utilization, boiler and steam turbine, cooling tower. The effect of main operation parameters on process performance was investigated, including the weight ratios of steam to biomass and O2-rich gas to biomass (S/B and O/B), H2 to CO ratio of syngas (H2/COsyngas), and reaction temperature of methanol synthesis. The evaluation index were feedstock usage (RF), water usage(RH2O), electricity usage(Relec), light olefin efficiency (ηole) and total energy efficiency(ηT), et al. Potential energy method, based on low heating values of the streams, was adapted for the analysis and evaluation of energy conversion of the process under different operation conditions. The results show that RF of 7.86 t/t, RH2O of 15.9 t/t, Relec of 4.12 MWh/t, ηole of 40.7% and ηT of 43.0% were obtained under the optimized process parameters, which was at S/B=0.26, O/B=0.14, H2/COsyngas and methanol synthesis temperature of 245 ℃. Self-supply of electricity was realized in this integrated process. Fresh water was supplied mainly to compensate the water loss in the cooling tower, due to water evaporation into air via cooling fan to decrease the temperature of recycling water. The energy loss of the system was mainly made up of air-cooling heat, evaporation heat from cooling tower and exhaust gas, accounting for 24.1% of the energy of biomass feedstocks. It can be concluded that the integrated process for bio-light olefin production from biomass was proved to be theoretically feasible in this context. High S/B ratio, proper O2 amount, syngas with H2/CO ratio of 2.0 and low temperature of methanol synthesis is favorable for the yield of syngas and energy efficiency of light olefin. However, the total energy efficiency of the process was rarely affected due to the co-effect of electricity consumption and light olefin synthesis, which was in a compensated pattern for the system. The consumption ratios of biomass, water and electricity for per tonne of light olefin production were relatively higher than the results in some reference, due to the conservative conversion parameters designed in this context. Still the process performance of this simulation was still higher than the actual industrial results. And there are still techniques to be solved in future, such as the development of highly active catalysts for methanol synthesis at low temperature and heat integration to enhance energy efficiency.