Supercritical water gasification of RDF and its components over RuO2/γ-Al2O3 catalyst: New insights into RuO2 catalytic reaction mechanisms

Abstract

Five samples, including a composite refuse derived fuel (RDF) and four combustible components of municipal solid wastes (MSW) have been reacted under supercritical water conditions in a batch reactor. The reactions have been carried out at 450 °C for 60 min reaction time, with or without 20 wt% RuO2/gamma-alumina catalyst. The reactivities of the samples depended on their compositions; with the plastic-rich samples, RDF and mixed waste plastics (MWP), giving similar product yields and compositions, while the biogenic samples including mixed waste wood (MWW) and textile waste (TXT) also gave similar reaction products. The use of the heterogeneous ruthenium-based catalyst gave carbon gasification efficiencies (CGE) of up to 99 wt%, which was up by at least 83% compared to the non-catalytic tests. In the presence of RuO2 catalyst, methane, hydrogen and carbon dioxide became the dominant gas products for all five samples. The higher heating values (HHV) of the gas products increased at least two-fold in the presence of the catalyst compared to non-catalytic tests. Results show that the ruthenium-based catalyst was active in feedstock steam reforming, methanation and possible direct hydrogenolysis of C-C bonds. This work provides new insights into the catalytic mechanisms of RuO2 during SCWG of carbonaceous materials, along with the possibility of producing high yields of methane from MSW fractions.Keywords: MSW, supercritical water gasification, ruthenium catalysis, mechanisms, methane * Corresponding Author. Tel.: +44 121 204 4703 Email address: j.onwudili@aston.ac.uk 2 IntroductionRapid urbanization and technological developments are largely responsible for the increasing generation of millions of tonnes of municipal solid wastes (MSW) in major cities around the world. MSW can be broadly classified into organic and inorganic fractions. Among the organic components of MSW, further classifications can be made into biodegradable and nonbiodegradable fractions. By far, food and garden wastes occupy a huge proportion of MSW but these are biodegradable and are often treated by anaerobic digestion (AD) to produce biogas, leading to complete mineralization. After the separation of recyclables, the mixture of nonreadily biodegradable organic components make up a combustible fraction of MSW, which can be made into solid recovered fuels (SRF) and refuse derived fuels (RDF), depending on the specification. The stringent regulations concerning the production and utilization of SRF and RDF indicate that many components of MSW cannot be directly burned as fuels [1][2]. Massproduced synthetic polymers such as plastics and textile materials fall into the category of combustible MSW fractions [3]. Other organic components of MSW that do not hold huge attractions for AD operators include waste wood and reinforced cardboards.Advanced thermochemical technologies suitable for treating plastics and other combustible organic wastes and materials include incineration, pyrolysis and gasification. These technologies convert organic ...

Catalytic Properties and Recycling of NiFe2O4 Catalyst for Hydrogen Production by Supercritical Water Gasification of Eucalyptus Wood Chips

“…The high conversion of eucalyptus wood chips at 450 and 500 • C indicates that efficient interactions occurred between the NiFe 2 O 4 catalytic surface and reactant the molecules at these temperatures. According to Azadi et al [42] the use of smaller catalyst particles, as in the present work (18 nm), can reduce the diffusion path and consequently enhance the catalyst performance, as previously reported by other researchers [43][44][45]. Our results confirmed that the NiFe 2 O 4 powder prepared by the combustion reaction was active as a catalyst for SCWG and may be useful for moderate temperature processes.…”

“…The increase in methane concentration could be due to both methanation (exothermic) of carbon oxides (CO x ) and demethylation (endothermic) of larger molecules in the liquid products. The latter is supported by the overall increase in the biomass gasification at the higher temperature, 500 • C. In the presence of the catalyst, the increase in the overall biomass gasification also showed the influence of the catalyst in converting the liquid and solid products to gases [4,32,[43][44][45]. Table 2 provides good evidence that both the water-gas shift reaction and methane reforming occurred, forming increased concentrations of CO 2 and H 2 .…”

“…from the GC/MS analysis of the oil products obtained at 450 °C with and without the NiFe2O4 as shown in Figure 7. The bio-oil components have been classified as aliphatic (alcohols, aldehydes, carboxylic acids, and ketones,), phenols (phenol, alkylated phenols), alkylbenzenes (xylenes, ethylbenzene, and various methyl benzenes), and polycyclic aromatic hydrocarbons (PAH) (naphthalene and alkylated naphthalenes, biphenyls, fluorene), and others (cyclopropane, cyclopropene, and cycloheptatriene) according to Onwudili and Williams [44]. The results showed appreciable formation of cyclic aliphatic and aromatic organic compounds (cyclopropanes, phenols, indenes, toluene, xylene).…”

“…These results may indicate that the catalyst was effective in promoting decarboxylation of the biomass or the intermediate liquid products to produce hydrocarbons and CO 2 . The results in Figure 7 also indicate that high amounts of phenolic compounds, such as light alkyl phenols, were obtained at 450 and 500 • C. Complex mixtures of phenolic compounds are usually obtained as products of lignin degradation by flash pyrolysis of lignocellulosic biomass [9,44]. Phenol derivatives are important chemicals and there is a growing interest in producing cost-effective phenols from lignocellulosic materials, such as eucalyptus wood residues.…”

Research progress on catalytic pyrolysis and reuse of waste plastics and petroleum sludge

“…During the reaction, heavy oil generally begins to decompose at around 370 °C, and the condensation reaction accelerates as increase of pyrolysis temperature. [33] The pyrolysis of alkanes in petroleum sludge mainly includes the formation of small molecular alkenes and secondary alkanes after the C-C bond is broken and the formation of alkenes with a constant carbon number after the C-H bond is broken. The general formula can be expressed as: [34] R-CH2CH2-R' → R-CH=CH2 + R'H…”