Simulation of Hydrogen Production through the Gasification of Peanut Shells under Supercritical Water conditions: Investigating effects of Ca(OH)2 Catalyst, Temperature, Pressure, Residence Time and Economic Variability

Abstract

Peanut (Arachis hypogaea) is a plant from the Fabaceae family (legumes). Peanut is one of the most important food products grown in countries with tropical climates like Senegal and is a valuable crop for the agro-industrial sector. Peanuts are edible, but their shells are generally discarded as waste after the harvesting and processing stages. However, the management of waste generated by the harvesting and processing of peanuts represents a major challenge worldwide. This study proposes a conceptual design for the catalytic supercritical water gasification of peanut shells. A detailed Power Law kinetic simulation model was developed using ASPEN Plus V14 software to analyse, optimise, and evaluate the efficiency of the peanut shell supercritical water gasification process. The developed model comprises three process units for pretreatment, gasification, separation and purification. The economic analysis of the optimised process was evaluated using hydrogen obtained from the gasification of peanut shells, under supercritical conditions, based on a comprehensive discounted cash flow analysis (DCF). The simulation results were validated by comparing them with experimental data found in the existing literature. The comparison showed that the results predicted by the model agreed well with those reported in the literature. The main effects, as well as interaction effects of four process parameters, including temperature, pressure, catalyst loading, and residence time, on the yield of syngas, were investigated using a sensitivity analysis. According to these results, increasing the temperature from subcritical (300 0C) to supercritical (1000 0C) increased the production of H2 and CO while reducing the production of CH4 and CO2. Furthermore, H2 and CO2 yields improved when the pressure was increased from 220 to 350 bar, reducing the production of CH4 and CO at the same time. However, the change in pressure did not show a significant effect on hydrogen yield. More importantly, the effect of Ca(OH)2 catalyst was investigated, and the findings demonstrated that it has a positive influence on H2 yield. The Ca(OH)2 catalyst amplifies the yield of hydrogen by 16.308 %. Moreover, to optimise the hydrogen production of the process, the simultaneous effect of different process parameters on the hydrogen yield was studied using a sensitivity analysis. According to the model’s best prediction, the hydrogen yield can reach 193.993 kg/h when the reaction conditions are temperature = 750 0C, pressure = 220 bar, biomass to water of 1:4, and residence time of one hour. Based on the economic analysis, the Levelized cost of hydrogen (LCOH) is estimated at $ 1.30/kg, which is relatively low compared to hydrogen produced from other biomass conversion processes due to the ready viii availability of the feedstock. In addition, an internal rate of return of 12%, a payback period of 4.6 years, and a return on investment of 113.30% were obtained with a net present value of $ 11,839,892.99. The results from the profitability analysis indicate that the SCWG project for hydrogen production is viable from an economic standpoint.