Hydrogen-rich syngas from wet municipal solid waste gasification using Ni/Waste marble powder catalyst promoted by transition metals
Introduction
The energy and natural resources demands are rapidly increasing due to expeditious population growth, industrialization, and urbanization. Currently, most of the energy needs around the globe are being solely fulfilled by fossil fuels, resulting in natural resources depletion and greenhouse gases (GHG) emissions (Newell et al., 2019). Therefore, renewable energy resources are continually being studied by many researchers. Hydrogen (H2) has been considered promising options due to its high calorific value and almost pollution-free, and has the potential to be used for power generation, heating, and even as transportation fuel (Zheng et al., 2018). Presently, H2 is mostly being commercially produced via unsustainable and less efficient techniques such as methane steam reforming and coal gasification (Zhang et al., 2014).
The municipal solid waste (MSW) generation is rapidly increasing at an alarming rate raising concerns over environmental and MSW disposal issues (Kaza et al., 2018). Meanwhile, MSW is mostly composed of high proportion of hydrocarbons (Mian et al., 2017). As such, MSW becomes highly attractive alternative feedstock for energy recovery or the production of liquid and gaseous fuels (such as hydrogen) (Ahmad et al., 2016, Korai et al., 2016). Gasification of MSW is highly acknowledged to be a favorable approach for producing value added syngas (Bhoi et al., 2018). However, there are barriers in attaining high purity syngas, which includes tar formation, unreformed volatile contents, CO2 production, and carbon deposition. The CO2 formation reduces the syngas energy density, subsequently increasing the cost of production. Besides, the tar content have tendency to block the heat-exchangers tubes, pipeline and auxiliary equipment, which contribute to surge in cost of maintenance and overall process efficiency (Anis and Zainal, 2011). Tar can transform to more complex structures (soot or coke) in filters, pipes, heat exchangers or other auxiliary equipment installed downstream of the gasifier, which could be problematic for continuous process operations (He et al., 2021). Furthermore, tars can deactivate catalysts through coke deposition. Since, syngas application is entirely dependent of purity therefore tar removal is essential and obligatory for various downstream applications, for example, utilization of syngas in the fuel cell or other chemical synthesis reactions (i.e., Fisher-Tropsch) demands tar free and high purity syngas (Heidenreich and Foscolo, 2015, Lopez et al., 2018). To overcome these barriers and challenges, catalysts/sorbents have been considered quite effective by several researchers (Lee et al., 2017, Soomro et al., 2018). As, catalytic gasification has the ability to improve gas composition, tar reduction and gasification efficiency (Bai et al., 2018).
Although noble metal-based catalysts (Pt, Ru, Rh, Ir, and Pd) are highly active, stable, and consume less energy, yet the drawback is scarce resources and pretty expensive (Anis and Zainal, 2011). Therefore, transition metal-based metals have been investigated extensively and found an economically viable option compare to noble metal-based catalysts (Ngo et al., 2021). Ni-based catalysts are known to be capable alternative to noble metals based catalysts (Shi et al., 2019). Despite being highly active and cost-efficient, Ni catalyst experienced deactivation due to sintering and carbon formation (Lu et al., 2017, Soomro et al., 2018). In order to enhance catalyst activity and suppress carbon deposition, the Ni-based catalyst modified by alkaline earth metal oxides such as CaO has been highly recognized. Along with promoting catalyst activity and stability, the CaO based sorbents can also promote char gasification, tar cracking, and in-situ CO2 sorption (Irfan et al., 2019a, Ji et al., 2017, Xu et al., 2018). Waste marble powder (WMP), owing to its high proportion of Ca species, can be used as a substitute to pure-CaO for catalyst modification. Meanwhile, our previous study showed that WMP has the catalytic capability towards MSW gasification. As WMP is a waste product from marble processing industries and its utilization will help to overcome loses due to generation of huge amount of waste (approximately 5 to 6 million tons per annum) which is primarily being dumped and encountered as environmental hazard (Irfan et al., 2019b). Different studies have shown that Ni-based catalyst systems can be promoted via incorporation of the second metal (such as Fe, Cu, Co, Ce), which enhance the catalytic ability as well as stability of the mono-metal catalysts (Baidya and Cattolica, 2015, Ngo et al., 2021, Wang et al., 2017, Zhang et al., 2019, Zhang et al., 2019). This enhancement could be attributed to the oxidation of coke species by virtue of mild redox capacity of the second metal oxides, therefore, suppresses coke formation along with enhanced catalytic activity (Baidya and Cattolica, 2015, Hosseini and Wahid, 2016, Soomro et al., 2018). Abu El-Rub et al. [46] demonstrated successfully that the transition metals can perform good catalytic activity towards gasification process.
A number of catalysts with different promoters and supports have been reported by many researchers for gasification and pyrolysis processes (Gao et al., 2021, Islam, 2020, Li et al., 2021). Although Ni-CaO-based catalysts with transition metals as their promoters for steam methane reforming and catalytic cracking of tar model compounds have been documented, the effect of transition metals (such as Fe, Cu, Co, and Zn) incorporation on Ni-CaO-based catalysts for gasification/pyrolysis processes is still not explored. Meanwhile, a combination of Ni-based catalyst and Ca-based sorbent along with transition metals promoters for catalytic conversion of MSW could serve multiple purposes. For instance, during catalytic MSW conversion with a Ni-CaO-based catalyst, tar cracking/reforming, CO2 sorption, and product gas composition adjustment occur, simultaneously. This can not only enhance the energy density and efficiency of the process but can also boost hydrogen and overall syngas yield. Since CaO can be used as CO2 sorbent, the catalysts study hereby will offer useful insights towards the Ni-CaO based catalysts promoted by transition metals as potential catalysts/sorbents for the chemical looping gasification process (Darmawan et al., 2018, Hu et al., 2015, Wang et al., 2021).
In this study, waste marble is considered as a substitute to CaO that utilizes waste material for catalyst development and promotes an environment-friendly and sustainable approach towards an economical catalyst development. Moreover, the transition metals as promoters in Ni-doped WMP catalysts (Ni-WMP) catalyst system may affect the efficiency of the system. Therefore, different Ni-WMP catalysts promoted by Fe, Co, Cu, and Zn were prepared by wet impregnation, and the synergetic effect of the transitions metals incorporation was investigated on the properties and performance of the catalyst. The prepared catalyst samples (Ni-M−WMP, M = Fe, Co, Cu and Zn) were characterized by X-ray diffraction (XRD), BET (Brunner Emmet Teller) analysis, and scanning electron microscopy (SEM), while the catalytic ability of prepared catalyst samples was investigated in a laboratory-scale fixed bed reactor for gasification of wet MSW. The catalytic ability of Ni-M−WMP catalyst samples was evaluated based on syngas composition, H2-yield, DGY (dry gas yield), tar content, and CCE (carbon conversion efficiency). Many studies have been carried out to produce hydrogen-rich syngas from the different feedstock, however, to the best of our knowledge, no study has been found on the catalytic gasification of wet MSW using bimetallic Ni-doped waste marble powder catalyst.
Section snippets
Materials
The MSW feedstock sampling was carried out from a transfer station responsible to collect and transfer MSW from residential area nearby Dalian University of Technology, Dalian, P.R. China. Only organic fraction of MSW was considered in this study and the inorganic fraction (i.e. glass, cans, stone and metals) was removed due to their inert and non-combustible nature. The organic fractions composed of food waste (48.5%), paper (13.3%), textile (9.9%), wood/grass (7.2%), and plastic (21.1%) were
Catalyst characterization
The XRD results presented in Fig. 2 indicated that the impregnated WMP with Ni precursor (Ni-WMP) and with Ni and second metal (M = Fe, Cu, Co, and Zn) were composed of NiO, CaO, MgO and second metal oxides (MOx). The weak peaks of Ca(OH)2 were also observed at 18.07, 28.67, 34.10, 47.12, and 50.81, which indicated that CaO absorbed the moisture and resulted in a small amount of Ca(OH)2. The diffraction peaks at 37.24, 43.27, 62.87, 75.41, and 79.40 represented the NiO, while that of CaO at
Conclusion
In this study, a series of Ni-WMP catalysts promoted by transition metals (i.e., Fe, Cu, Co, and Zn) were developed, and their performance was evaluated for wet MSW gasification. The characterization of prepared catalysts was carried via XRD, SEM, BET analysis, and the gasification experiments were performed in a fixed-bed reactor. Gasification experiments without catalyst were conducted to find optimal gasification temperature (850 °C) and MC (50%) of MSW for catalyst performance evaluation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financially supported by the Liaoning Natural Science Foundation (NSFC) (No. 51978123) and National Water Pollution Control and Management Technology Major Projects (No. 2018ZX07601-004).
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