Gasification of municipal solid wastes with gypsum wastes under different gasifying environments

Abstract

The global increase in population and rapid urbanization has been followed by a growing generation of municipal solid wastes and construction and demolition sector wastes. This has put increasing pressure on countries around the world to come up with sustainable ways to deal with issues related to waste generation, climate change, and social and environmental degradation. This paper investigates a novel means of co-utilization of municipal solid wastes (MSW) with gypsum wastes from the construction and demolition sector for energy recovery and materials recycling and explores synergistic improvement in their processed value. We investigated high-temperature pyrolysis and gasification under different gasifying environments of inert (N2), 10 % O2, 20 % CO2, and 70 % H2O at two different temperatures of 800 and 900 °C, in a semi-batch fixed bed reactor. Syngas obtained was analyzed for its composition and yield, while the solid residue obtained was analyzed for the crystalline phases of the inorganic content using powder-XRD (X-ray diffraction) diagnostics. These analyses revealed that increasing temperature as well as the addition of gypsum can significantly improve the carbon conversion and overall product gas yields.When comparing the gasifying environments, steam gasification provided increased syngas mass yields by up to 184 %, 425 % and 377 % respectively, as compared to CO2 gasification, O2 gasification, and pyrolysis environments. Adding gypsum at equal mass proportions (1:1) to MSW pyrolysis at 900 °C increased the syngas yield by 10 % compared to the non-catalytic case, while adding it at 800 °C reduced syngas yield by 6 % compared to the non-catalyzed pyrolytic case at 800 °C. Adding gypsum during O2 gasification increased the syngas yield by 25 % compared to the non-catalytic cases at 900 °C and increased by 12.5 % at 800 °C. However, the syngas yield was reduced by 84 % when gypsum was added at equal mass proportions (1:1) to steam gasification of MSW at 900 °C and 50 % for 800 °C. Adding gypsum to CO2 gasification also reduced the cumulative yield of syngas for both temperatures under investigation. However, the syngas quality was improved with gypsum addition as the yields of hydrocarbon were reduced. The char/solid residue was found to contain significant portions of CaS suggesting sulfur fixation capability of this process into the ash which can improve the ash characteristics in terms of metal recovery along with the improved waste conversion.