Hydrogen-rich Combustible Gas Production By In-situ Catalytic Gasification Of MSW With CaO-based Catalysts

The municipal solid waste(MSW)generation and global energy demand are increasing at an alarming rate due to population growth,urbanization and industrialization raising concerns over environmental issues and natural resources depletion.These issues diverted the attention to develop renewable and sustainable energies to meet future demands.Among all sustainable and clean energy options,hydrogen has been considered as one of the most favorable.MSW is mostly composed of hydrocarbons,so it has the potential to be an attractive alternative feedstock for energy recovery via gasification.Moreover,most of the MSW is being disposed of in landfills or open dumpsites,which contributes to greenhouse gas(GHG)emissions in the form of methane release.Thus,utilizing MSW as energy feedstock for gasification process would eventually reduce the GHG emissions.Furthermore,the MSW gasification process is normally considered to be more environmentally viable option compared to MSW incineration in terms of less toxic emissions as well as easy to control and abate pollutants within the process.The gasification technology for hydrogen-rich syngas production is still in the development phase.The literature review suggested that tar formation,syngas contamination,high moisture content of MSW and catalyst deactivation are the main challenges in the development and applicability of gasification technology for hydrogen-rich syngas production.So,to produce syngas for high value-added applications,these issues and challenges need to be resolved.Meanwhile,the application of catalysts is considered highly effective in addressing these issues.Post gasification catalytic reforming has been extensively studied however limited work is available regarding in-situ catalytic gasification of MSW.The attractiveness of in-situ catalytic gasification is related to its low cost and less process complexity.Therefore,for the development of in-situ catalytic gasification strategies,the CaO-based waste material(waste marble)and CaO-based catalysts were developed and evaluated for their performance regarding in-situ catalytic gasification of MSW.Furthermore,the catalysts were also modified by Ni doping and different promoter/stabilizer materials to enhance catalyst activity and stability.The key objective of this thesis is to understand the effect of different catalysts on in-situ catalytic gasification of MSW and resulting products.Motivating from the Ca-based catalyst for in-situ catalytic gasification coupled with in-situ CO2 capture,the waste marble powder(WMP)was used as the in-situ catalyst for steam gasification MSW in a fixed-bed reactor.The H2 yield increased from 202 mL/g to 489 mL/g,and tar content reduced from 12.4 wt.%to wt.5.2%with WMP along with improvement in carbon conversion efficiency(CCE)and dry gas yield(DGY).Moreover,WMP addition had a significant influence not only on tar reduction but also on its chemical composition,by transforming heavy aromatic tar compounds into light aromatic tar compounds.The results affirmed that WMP has the potential to be an alternative to CaO-based catalyst.Meanwhile,WMP is a waste byproduct of marble processing industry,and its reuse will lead to waste minimization.Later,the WMP was upgraded with Ni precursor and different transition metals(M)as promoters(i.e.Fe,Cu,Co and Zn)to develop a series of bimetallic catalysts(Ni-M-WMP).These catalysts were studied for their in-situ catalytic gasification of wet MSW in a fixed-bed reactor.The results revealed that with Ni-WMP catalyst,the DGY,H2 yield and CCE greatly enhanced compared to without catalyst.In contrast to the Ni-WMP catalyst,the bimetallic promoted catalysts showed higher catalytic activity towards higher yield,DGY,CCE and lower tar content.In terms of H2 yield and tar removal,the modified WMP catalysts can be ranked as Ni-Co-WMP>Ni-Cu-WMP>Ni-Fe-WMP>Ni-Zn-WMP.Further,a new approach(i.e.DC arc discharge plasma melting)was used to develop Ni-CaO-TiO2 catalyst for gasification of MSW.The stability was assessed in terms of cyclic CO2 capture,and results revealed that the Ni-CaO-TiO2 catalyst stability was enhanced by TiO2 incorporation due to its polymorphic transition property.The gasification results showed that H2 content increased from 263 mL/g to 1003 mL/g while DGY from 0.75 Nm3/kg to 1.74 Nm3/kg with Ni-CaO-TiO2 catalyst.Significant tar elimination was also achieved(from 9.38 to 2.55 wt.%)with Ni-CaO-TiO2 catalyst along with the transformation of tar compounds from heavy to lighter aromatic compounds.Finally,a novel Ni-CaO-based catalyst promoted by HfO2 was developed via impregnation technique.The stability of the catalyst was investigated in terms of cyclic CO2 capture and found that addition of HfO2 into catalyst greatly upgraded its stability for cyclic CO2 capture up to 20 cycles.While the catalytic gasification experiments with wet MSW revealed that the H2 yield was improved with Ni-CaO catalyst from 212 mL/g to 442 mL/g and further improved to 597 mL/g with HfO2 promoted catalyst(Ni-CaO-HfO2).It was found that HfO2 can promote catalytic activity towards methane reforming,hydrocarbon reforming,and tar cracking,which resulted in hydrogen-rich syngas.Meanwhile,a significant tar reduction was recorded with HfO2 promoted catalyst from 8.97 wt.%to wt.2.81%,while transforming tar composition from heavier to lighter aromatic hydrocarbon compounds.The results presented in this thesis suggested that the catalysts developed and used performed well,and significant improvement in syngas composition and tar reduction was observed.Moreover,the significant enhancement in stability of Ni-CaO-based catalyst was also achieved in terms of cyclic CO2 capture enabling potential utilization of these catalysts in chemical loop gasification(CLG)process for in-situ catalytic gasification coupled with in-situ CO2 capture.