•Efficient char conversion into hydrogen-rich gas is achieved at lower temperature.•Hydrogen concentration in the product gas is able to attain 70%.•Parameters of biomass pyrolysis and char gasification are optimized together by RSM.•Different effects of potassium during pyrolysis and gasification are investigated.•Optimized pyrolysis temperature is favorable to reduce char regularity.In order to achieve the efficient conversion of biomass char into hydrogen-rich gas at lower temperatures and investigate the different effects of potassium during biomass pyrolysis and char conversion, gasification experiments of the chars derived from wheat straw were carried out in steam atmosphere based on Response Surface Methodology. The response surface was set up with three parameters (pyrolysis temperature, K2CO3 loading concentration and the content of K-based catalyst in bed materials) for char gasification performances of carbon conversion efficiency, hydrogen yield and reaction rate to make analysis and optimization about the experimental conditions. The results showed that suggested conditions of char steam gasification were 650 °C for pyrolysis temperature, 22% for K2CO3 loading concentration and 70% for the content of K-based catalyst in bed materials. In these conditions, the confirmatory experiment indicated that carbon conversion efficiency was 95.1% while hydrogen yield and reaction rate reached 182.5 mol/kg-char and 1.55%/min, which was in good agreement with the model prediction. In addition, H2 concentrations were able to attain 70%. The X-ray diffractometry (XRD), scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy were employed to draw conclusions of char characterizations and account for the optimization results: i) chars obtained by the pyrolysis around 650 °C were liable to be gasified due to the cracked structures without carbonization occurring, and ii) properly loaded K2CO3 tended to cause the formation of porous sponge-like textures on char surfaces and contributed to maintaining the variety of functional groups with H and O, resulting in enhanced char reactivity.

•We establish response models to optimize biomass steam gasification performance.•Significance of different operating variables are analyzed and compared.•Appropriate catalyst ratios in the double-bed reactor vary with temperature changing.•Reforming bed is favorable to enhancing conversion efficiency and hydrogen yield.Based on Response Surface Methodology, the experiments of biomass catalytic gasification designed by Design-Expert software were carried out in steam atmosphere and double-bed reactor. The response surface was set up with three parameters (gasification temperature, the content of K-based catalyst in gasification bed and the content of Ni-based catalyst in reforming bed) for biomass gasification performance of carbon conversion efficiency and hydrogen yield to make analysis and optimization about the reaction characteristics and gasification conditions. Results showed that gasification temperature and the content of K-based catalyst in gasification bed had significant influence on carbon conversion efficiency and hydrogen yield, whilst the content of Ni-based catalyst in reforming bed affected the gasification reactions to a large extent. Furthermore, appropriate conditions of biomass steam gasification were 800 °C for gasification temperature, 82% for the content of K-based catalyst in gasification bed and 74% for the content of Ni-based catalyst in reforming bed by the optimization model. In these conditions, the steam gasification experiments using wheat straw showed that carbon conversion efficiency was 96.9% while hydrogen yield reached 64.5 mol/kg, which was in good agreement with the model prediction. The role of the reforming bed was also analyzed and evaluated, which provided important insight that the employment of reforming bed made carbon conversion efficiency raised by 4.8%, while hydrogen yield achieved a relative growth of 50.5%.

Simulation of methanol synthesis via H2-rich biomass-derived syngas from biomass gasification in interconnected fluidized beds is carried out, using Aspen Plus software to establish this model. In the case of CaCO3 catalysis, the effects of operating parameters, including gasification temperature and pressure, steam /biomass ratio (S/B), and liquefaction temperature and pressure, on the methanol yield are analyzed. The results are as follows: In the case of CaCO3 catalysis, biomass steam gasification can obtain 82.13% hydrogen-rich gas and when the Cu-based catalyst in China which is composed of 58.1% CuO + 30.06% Al2O3 + 31.7% ZnO + 4.0% H2O is adopted as the methanol synthesis catalyst; the gasification temperature is suggested to be controlled at about 750 °C in the interconnected fluidized beds biomass gasification system with the purpose of methanol production. Furthermore, the gasification pressure is proposed to approach to the ambient pressure and the S/B ratio of 0.4−0.5 operates better. On the optimal operating condition, the maximum of 12.19 mol/(kg biomass (daf)) of methanol yield may be obtained. The research provides useful results for the further study of biomass gasification and methanol synthesis from biomass syngas.

This paper focuses on a biomethanol from the rice straw process involving the thermodynamic, economic, and environmental performance in China. Based on the simulation of methanol synthesis via biomass gasification in interconnected fluidized beds using Aspen Plus software, the method of LCA (Life Cycle Assessment) is applied to evaluate the impact of pollutant emissions in the full life cycles of biomethanol. The integrated performance of biomethanol system is analyzed combining with energy utilization, economic cost, and environmental impact. The results show that the methanol yield can reach 0.308 kg/(kg rice straw), i.e., the energy efficiency of rice straw conversion to biomethanol is approximately 42.7%. For a biomethanol plant with an annual production of 50,000 tons, the real cost of biomethanol is evaluated at 2685 RMB/t, in which the economic cost is 2347 RMB/t, and the environmental cost is 337.6 RMB/t. Because of its high investment cost, presently the economic cost of biomethanol is higher than that of coal-based methanol in China. Nevertheless biomethanol will be becoming more competitive with the shortage of fossil fuel in the future. In the whole life cycle, the main pollutant emissions come from the biomethanol production process and biomethanol end-use by automobiles, whereas the net environmental effect is negative during the rice cultivation. Global warming is the most influential factor of the different impact categories. However, 1910 kg of CO2 can be fixed for one ton of methanol by photosynthesis in the growth of rice, thus the effect of global warming is significantly reduced by biomass utilization compared with coal-based methanol. The integrated performance indicates that producing methanol from rice straw is beneficial to both the utilization of agriculture waste and in the improvement of environment.