• Energy Research

The displacement and eventual replacement of fossil-derived fuel gases with biomass-derived alternatives can help the energy sector to achieve net zero by 2050. Decarboxylation of butyric acid, which can be obtained from biomass, can produce high yields of propane, a component of liquefied petroleum gases. The use of different gaseous reaction atmospheres of nitrogen, hydrogen, and compressed air during the catalytic hydrothermal conversion of butyric acid to propane have been investigated in a batch reactor within a temperature range of 200–350 °C. The experimental results were statistically evaluated to find the optimum conditions to produce propane via decarboxylation while minimizing other potential side reactions. The results revealed that nitrogen gas was the most appropriate atmosphere to control propane production under the test conditions between 250 °C and 300 °C, during which the highest hydrocarbon selectivity for propane of up to 97% was achieved. Below this temperature range, butyric acid conversion remained low under the three reaction atmospheres. Above 300 °C, competing reactions became more significant. Under compressed air atmosphere, oxidation to CO2 became dominant, and under nitrogen, thermal cracking of propane became significant, producing both ethane and methane as side products. Interestingly, under a hydrogen atmosphere, hydrogenolytic cracking propane became dominant, leading to multiple C–C bond cleavages to produce methane as the main side product at 350 °C.

Conventional biomass gasification involves a complex set of chemical reactions leading to the production of a product gas mainly composed on carbon monoxide, hydrogen, carbon dioxide and methane.

The production of low-carbon gaseous fuels from biomass has the potential to reduce greenhouse gas emissions and promote energy sustainability, stability and affordability around the world. Glycerol, a large-volume by-product of biodiesel production, is a potential feedstock for the production of low-carbon energy vectors. In this present work, an aqueous solution of pure glycerol was reacted under hydrothermal conditions using a total of 10 types of heterogeneous catalysts to evaluate its conversion to gas products (hydrogen, methane, CO, CO2 and C2–C4 hydrocarbon gases). Two bimetallic Ni-Fe and Ni-Cu catalysts, three Pt-based catalysts and physical mixtures of the five catalysts were tested. The reactions were carried out in a batch reactor for 1 h reaction time, using a 9:1 mass ratio of water/glycerol (10 wt%) and the reaction temperatures ranged between 250–350 °C using and without using 1 g of catalyst. The effects of the catalysts and reaction conditions on the conversion of glycerol in terms of carbon and hydrogen gasification efficiencies, selectivity and yields of components in the gas products were investigated. CO2 remained the most dominant gas product in all experiments. The results indicated that increasing the reaction temperature favoured gas formation and both carbon and hydrogen gasification efficiencies. The combination of Ni-Cu and Pt/C catalysts was the most selective catalyst for gas formation at 350 °C, giving carbon gasification efficiency of 95.6 wt%. Individually, the catalyst with the highest hydrogen production was Pt/C and the highest propane yield was obtained with the Ni-Cu bimetallic catalyst. Some catalysts showed good structural stability in hydrothermal media but need improvements towards better yields of desired fuel gases.

Catalytic supercritical water gasification of heavy (dewatered) bio-oil has been investigated in a batch reactor in the presence of ruthenium catalysts in the form of RuO2 on γ-Al2O3 support. The reactions were carried out at temperatures of 400 °C, 450 °C and 500 °C and reaction times of up to 60 min using 15 wt% of bio-oil feed. Increased ruthenium oxide loading led to increased carbon gasification efficiencies (CGE) and bio-methane production. Hence, using the 20 wt% RuO2/γ-Al2O3 catalyst, CGE was 97.4 wt% at 500 °C and methane yield reached nearly 30 wt% of the bio-oil feed, which gave a CH4/CO2 molar ratio of 1.28. There was evidence that the RuO2 was involved in the initial conversion of the bio-oil to carbon oxides and hydrogen as well as the reduction of the CO2 to methane via CO methanation. However, competition for CO consumption via the water-gas shift reaction was also possible due to the large presence of water as the reaction medium. This work therefore demonstrates that high concentrations of heavy fraction of bio-oil can be catalytically converted to a methane-rich gas product under hydrothermal conditions at moderate temperatures. The calorific values of the gas product reached up to 54 MJ kg−1, which is nearly 3 times the HHV of the bio-oil feed.

Abstract The production of hydrogen via subcritical water gasification of model food waste compounds, glucose and glutamic acid representing carbohydrate and protein has been studied. The influence of NaOH additive, Ni/Al 2 O 3 and Ni/SiO 2 catalysts and combinations of these catalysts in relation to hydrogen production was investigated at a temperature of 330 °C and 13.5 MPa pressure. Generally, glucose produced more hydrogen gas than glutamic acid even with NaOH. Hydrogen production from the model food waste compounds in the presence of NaOH was superior to that of the investigated nickel catalysts supported on SiO 2 and Al 2 O 3 . The production of hydrogen gas increased when NaOH was added, whereas other gases, CO, CO 2 and hydrocarbons (C 1 –C 4 ) decreased with increasing concentration of NaOH. Combination of the catalysts Ni/SiO 2 or Ni/Al 2 O 3 with NaOH alkali only slightly increased hydrogen production. The addition of NaOH reduced the amount of CO 2 , and tar produced during subcritical water gasification of the model compounds, unlike when Ni/SiO 2 and Ni/Al 2 O 3 catalysts were used. Furthermore NaOH decreased the amount of carbon deposited on the Ni/SiO 2 and Ni/Al 2 O 3 catalyst surface. However the formation of dawsonite was confirmed and provided experimental evidence of the reaction of sodium hydroxide with alumina in Ni/Al 2 O 3 , with a potential for decreased catalytic activity.

Chlorella vulgaris, Spirulina platensis and Saccharina latissima were processed under supercritical water gasification conditions at 500 °C, 36 MPa in an Inconel batch reactor for 30 min in the presence/absence of NaOH and/or Ni-Al(2)O(3). Hydrogen gas yields were more than two times higher in the presence of NaOH than in its absence and tar yields were reduced by up to 71%. Saccharina, a carbohydrate-rich macro-alga, gave the highest hydrogen gas yields of 15.1 mol/kg. The tars from all three algae contained aromatic compounds, including phenols, alkyl benzenes and polycyclic aromatic hydrocarbons as well as heterocyclic nitrogen compounds. Tars from Chlorella and Spirulina contained high yields of pyridines, pyrroles, indoles and pyrimidines. Up to 97% TOC removal were achieved in the process waters from the gasification of the algae. Analyses for specific nutrients in the process waters indicated that the process waters from Saccharina could potentially be used for microalgae cultivation.

Abstract The compositions of the pyrolysis products of pure low-density polyethylene (LDPE) and polystyrene (PS) and their mixtures have been investigated over a temperature range from 300 to 500 °C. The pyrolysis experiments were carried out in a closed batch reactor under inert nitrogen atmosphere to study the effects of reaction temperature and residence time. LDPE was thermally degraded to oil at 425 °C however, beyond this temperature the proportion of oil product decreased as a result of its conversion to char and hydrocarbon gas. Compositional analysis of the oil products showed that aliphatic hydrocarbons were the major components, but the proportion of aromatic compounds increased at higher temperatures and residence times. On the other hand, PS degraded at around 350 °C, mainly into a viscous dark-coloured oil. The formation of char only increased marginally until 425 °C, but was dramatically enhanced at 450 and 500 °C, reaching up to 30 wt.%. The oil product from PS even at 350 °C consisted almost entirely of aromatic compounds especially toluene, ethylbenzene and styrene. Under increasing temperatures and residence times, the oil product from PS was preferentially converted to char, while gas formation was preferred for the oil from LDPE. For instance at 500 °C, PS produced about twice the amount of char obtained from LDPE indicating the role of aromatic compounds in char formation via condensation of the aromatic ring structure. During the co-pyrolysis of a 7:3 mixture of LDPE and PS, wax product was observed at 350 °C leading to oil at 400 °C, indicating that the presence of PS influenced the conversion of LDPE by lowering its degradation temperature. The mixture produced more oil and less char than the individual plastics at 450 °C.

The high-temperature pyrolysis behaviour of a sample of refuse derived fuel (RDF) as a model of municipal solid waste (MSW) was investigated in a horizontal tubular reactor between 700 and 900 °C, at varying heating rates, and at an extended vapour residence time. Experiments were designed to evaluate the influence of process conditions on gas yields as well as gas and oil compositions. Pyrolysis of RDF at 800 °C and at rapid heating rate resulted in the gas yield with the highest CV of 24.8 MJ m-3 while pyrolysis to 900 °C at the rapid heating rate generated the highest gas yield but with a lower CV of 21.3 MJ m-3. A comparison of the effect of heating rates on oil products revealed that the oil from slow pyrolysis, contained higher yields of more oxygenates, alkanes (C8-C39) and alkenes (C8-C20), while the oil from rapid pyrolysis contained more aromatics, possibly due to the promotion of Diels-Alder-type reactions.

Abstract Two major organic reactions in water under high temperature and high pressure, hydrothermal oxidation and gasification, possess great potential towards the thermochemical treatment of high moisture content organic wastes of different varieties and sources. Essentially, hydrothermal oxidation converts organic liquid and/or solid wastes to mainly carbon dioxide and water. Environmentally important and refractory organic pollutants such as polycyclic aromatic compounds, e.g. pyrene, naphthalene, phenanthrene, fluorene and biphenyl, and di-n-butyl phthalate, have been decomposed under oxidative hydrothermal conditions, with conversions of up to 99·9% obtained. Real world environmental wastes and pollutants have also been oxidised. The hydrothermal gasification process offers a new route for 'green' fuel production from biomass and biowastes resulting in a CO2 neutral energy technology. Generally, the main products of hydrothermal gasification are carbon dioxide, carbon monoxide, hydrogen, C1–C4 hydro...