Clean synthesis gas production from municipal solid waste via catalytic gasification and reforming technology
Graphical abstract
Introduction
Municipal solid waste (MSW) contains a large number of hydrocarbons and abundant energy. The municipal solid waste can be converted into available oil, gas, carbon and other energy sources via appropriate utilization of the waste, which can realize resource utilization of MSW and has great development potential and market prospects [[1], [2], [3], [4]]. With the progress and optimization of technologies, MSW treatment technologies for reducing amount have gradually changed to the processing technologies for resource utilization [[2], [5], [6], [7]]. Among them, the MSW gasification technology has attracted much attention [[8], [9], [10]], and is considered as a highly potential MSW recycling technology.
However, due to the complicated composition of MSW, the ratio of H2/CO in crude synthesis gas generated via MSW gasification is not reasonable [[11], [12], [13]]. The syngas product is often accompanied by tar and other by-products [[14], [15]], and the add-value of direct utilization of crude syngas is lower. In addition, in the process of MSW gasification, the N and S elements in MSW are mostly transformed into SO2 and NOx, and heavy metals such as Cr, Pb, Zn, Cu and Hg are volatilized in different degrees existing in the synthesis gas [[16], [17]], which directly impede the safe use of synthesis gas. Therefore, it is necessary to reform the crude synthesis gas produced by gasification to obtain clean and reasonable syngas [[12], [18]].
Steam reforming of synthesis gas using catalysts during gasification [19], namely catalytic gasification and reforming, is the most developed process for generation of H2 from methane and adjusting ratio of H2/CO in syngas. Compared with the noble metal catalysts, nickel based catalysts are most commonly used for industrial applications due to its comparable high activity toward CH bond activation, better availability, and low cost. Nevertheless, the coke formation and impurities in the syngas can lead to a decrease in the active surface area, lower catalytic activity and thus cause Ni-based catalysts deactivation. How to avoid catalyst deactivation are the major challenges with steam reforming of crude synthesis gas using Ni-based catalysts [[20], [21]]. On the other hand, alkali metals can promote the activity and selectivity of Ni-based catalysts. Alkaline earth metals can increase the adsorption of CO2 on the catalyst [22]. With the special oxygen storage capacity, rare earth metal oxides can reduce the probability of coke formation on the catalyst surface, and play a positive role in the carbon/steam gasification reforming of the catalyst surface. Among the rare metal oxides, ceria (CeO2) has been intensively studied as an effective promoter in recent years [[23], [24]], because of the unique function of ceria in redox behaviors of Ce4+/Ce3+ and the interaction between metal and ceria, which can greatly improve the activity and stability of the catalyst and resistance to coke deposition. Improving the disadvantages of Ni-based catalysts via promotion with alkaline metals, alkaline earth metals or rare earth metals have been explored as potential solutions to this challenge [25].
Based on this background, this paper carried out experiments on catalytic gasification and reforming of MSW, and prepared clean synthesis gas with gas purification unit. Three kinds of nickel-based catalysts with different CeO2 contents were prepared by impregnation method for the catalytic reforming of municipal solid waste gasification. The effects of operating parameters on the generation characteristics of synthesis gas and conversion characteristics of environmental load substances were studied. Combined with the gas purification unit for the removal of pollutants [[26], [27]], the potential pollution characteristics of MSW gasification and reforming technology were analyzed, in order to provide the necessary technical support for synthesis gas clean-up.
Section snippets
Raw materials
Table 1 below showed the main components of the collected municipal solid wastes (MSW). Municipal solid wastes were shredded and screened to remove metal and inorganic material components, and then extruded into pellets with diameter of 8 mm and length of 20–30 mm. The proximate and ultimate analysis of shaped MSW are shown in Table 2.
Catalyst preparation
Ni-based catalysts with NiO mass fraction of 20% were prepared by impregnation method. Mg, Ca and Ce were used as promoters. The activated alumina pellets (γ-Al2O3
BET specific surface areas characterization
The BET surface areas for the support and catalysts before and after experiments were obtained by N2 physisorphtion acquired at 77 K using a NOVA 1200e apparatus. The results were shown in Table 4.
γ-Al2O3 is a porous material with a specific surface area of 342 cm2/g, which is beneficial to the adsorption of the active component of catalysts. The specific surface area and pore volume of catalysts prepared by impregnation method were lower than those of Al2O3 support. The loading of Ni and
Conclusions
Clean synthesis gas was successfully produced from municipal solid waste via gasification catalytic reforming. The energy utilization of gasification catalytic reforming system became more reasonable and the content of environmental load substances was greatly reduced and can be well controlled. At the temperature of 850 °C, equivalence ratio = 0.06, steam/carbon ratio = 3.6 and the action of nickel based catalysts, the clean synthesis gas was produced with hydrogen concentration of 36.78 vol.%
Acknowledgement
This work was supported by the National Natural Science Foundation of China [Grant number 51478092].
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