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Conversion of CO2 into organic chemicals offers a promising route for advancing the circularity of carbon capture, utilisation, and storage in line with the international 2050 Net Zero agenda. The widely known commercialised chemical fixation of CO2 into organic chemicals is the century-old Kolbe-Schmitt reaction, which carboxylates phenol (via sodium phenoxide) into salicylic acid. The carboxylation reaction is normally carried out between the gas-solid phases in a batch reactor. The mass and heat transfer limitations of such systems require rather long reaction times and a high pressure of CO2 and are often characterised by the low formation of undesirable side products. To address these drawbacks, a novel suspension-based carboxylation method has been designed and carried out in this present study, where sodium phenoxide is dispersed in toluene to react with CO2. Importantly, the addition of phenol played a critical role in promoting the stoichiometric conversion of phenoxide to salicylic acid. Under the optimal conditions of a phenol/phenoxide molar ratio of 2:1 in toluene, a reaction temperature of 225 °C, a CO2 pressure of 30 bar, a reaction time of 2 h, and stirring at 1000 rpm, an impressive salicylic acid molar yield of 92.68% has been achieved. The reaction mechanism behind this has been discussed. This development provides us with the potential to achieve a carboxylation reaction of phenoxide with CO2 more effectively in a continuous reactor. It can also facilitate the large-scale fixing of CO2 into hydroxy aromatic carboxylic acids, which can be used as green organic chemical feedstocks for making various products, including long-lived polymeric materials.

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In this present work, the hydrolysis of a sample of rapeseed oil (RSO) and two waste cooking oil (WCO) samples in subcritical water has been carried out in a stirred batch stainless-steel reactor to produce fatty acids. Using RSO as a model triglyceride, the effects of reaction parameters on the yields of fatty acids were investigated to determine the optimum set of hydrolysis conditions to be a temperature of 300 °C, a reaction time of 60 min, and a vegetable oil-water mass ratio of 1:2. The set of optimum conditions was applied to the hydrolysis of the WCOs. Oleic acid was the dominant fatty acid with yields of 74.4 wt % from RSO and 57.5 and 72.4 wt % from the two WCOs, respectively, while palmitic acid was the second most abundant fatty acid with yields of up to 31 wt %. The feedstocks and fatty acid products were characterized by acid-base titration and thermogravimetric analysis (TGA). Thereafter, the hydrolysis products from the optimum conditions were esterified to their fatty acid methyl esters (FAMEs), which were further characterized by gas chromatography-mass spectrometry (GC/MS) and TGA. With RSO at the optimum hydrolysis conditions, acid-base titration gave a fatty acid yield of 97.2 wt %, while TGA gave 86 wt %. Under the same conditions, the yield of FAMEs from GC/MS analysis was 88.6 wt %, while TGA gave a FAMEs' yield of 91 wt %. This study showed that the simple TGA method provided detailed and complete characterization of lipid feedstocks and their conversion products. In addition, subcritical water hydrolysis can be used to valorize WCOs to fatty acids, with little or no extensive feedstock purification, for various applications including biodiesel production.

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Trommel fines are a high-ash aggregate waste derived from material recycling facilities (MRFs) and are usually disposed in landfill. Increasing UK landfill tax and environmental concerns, however, calls for a flexible technology that can effectively process and extract valuable energy from trommel fines at high efficiencies. One possible technology is fast pyrolysis coupled to a combined heat and power (pyro-CHP) plant. To determine the feasibility of such technology, an understanding of its economic characteristics is required, in addition to the technical details. This study presents an economic evaluation of a pyro-CHP plant processing three pre-treated trommel fines feedstocks for energy recovery over a 20-year period. The three feedstocks were designated as DPT (from initial separation/size reduction), AW (from ash reduction by washing DPT with water) and AWS (from ash reduction by washing DPT with aqueous surfactant solution). Under all processing capacities (200 kg/h to 2000 kg/h) total revenues from the pyro-CHP system were higher than landfill costs, but only became profitable at 2000 kg/h processing capacity for the DPT feedstock. Further analysis showed positive net present values (NPV) only for AW and AWS, e.g. at 2000 kg/h capacities, with payback periods of about 14 years compared to 35 years for DPT at a fixed CHP efficiency of 60% and 20% internal rate of return (IRR). Sensitivity analysis carried out using different values of IRR and CHP efficiencies, confirmed the superior economic performance of the washed feedstocks over DPT, with payback periods reducing to about 6 years in some cases.

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Ex-situ co-pyrolysis of sugarcane bagasse pith and polyethylene terephthalate (PET) was investigated over zeolite-based catalysts using a tandem micro-reactor at an optimised temperature of 700 °C. A combination of zeolite (HZSM-5) and sodium carbonate/gamma-alumina served as effective catalysts for 18% more oxygen removal than HZSM-5 alone. The combined catalysts led to improved yields of aromatic (8.7%) and olefinic (6.9%) compounds. Carbon yields of 20.3% total aromatics, 18.3% BTXE (benzene, toluene, xylenes and ethylbenzene), 17% olefins, and 7% phenols were achieved under optimal conditions of 700 °C, a pith (biomass) to PET ratio of 4 and an HZSM-5 to sodium carbonate/gamma-alumina ratio of 5. The catalytic presence of sodium prevented coke formation, which has been a major cause of deactivation of zeolite catalysts during co-pyrolysis of biomass and plastics. This finding indicates that the catalyst combination as well as biomass/plastic mixtures used in this work can lead to both high yields of valuable aromatic chemicals and potentially, extended catalyst life time.

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The effect of heating rates ranging from 5 °C min-1 to 350 °C min-1 on the yields of pyrolysis products of wood and its main pseudo-components (cellulose, hemicellulose and lignin) have been investigated at a temperature of 800 °C in a horizontal fixed bed reactor. Results showed a successive dramatic increase and decrease in gas and liquid yields, respectively, while the yields of solid products showed a gradual decrease as heating rates increased. Increased gas formation and an increasingly aromatic oil/tar support the theory of rapid devolatilization of degradation products with increasing heating rate, leading to extensive cracking of primary pyrolysis vapours. Solid products with coal-like calorific value and large surface areas were obtained. CO became the dominant gas both on a mass and volume basis, at the heating rate of 350 °C min-1 for all samples except xylan, which also produced a significant yield of CO2 (20.3 wt% and 25.4 vol%) compared to the other samples. Cellulose produced a gas product with highest calorific value of 35 MJ kg-1 at the highest heating rate. Results also indicate that the three main pseudo-components of biomass each exert a different influence on the products of high temperature pyrolysis of woody biomass.

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Energy from Waste (EfW) technologies such as fluidized bed fast pyrolysis, are beneficial for both energy generation and waste management. Such technologies, however face significant challenges due to the heterogeneous nature, particularly the high ash contents of some municipal solid waste types e.g. trommel fines. A study of the physical/mechanical and thermal characteristics of these complex wastes is important for two main reasons; (a) to inform the design and operation of pyrolysis systems to handle the characteristics of such waste; (b) to control/modify the characteristics of the waste to fit with existing EFW technologies via appropriate feedstock preparation methods. In this study, the preparation and detailed characterisation of a sample of biogenic-rich trommel fines has been carried out with a view to making the feedstock suitable for fast pyrolysis based on an existing fluidized bed reactor. Results indicate that control of feed particle size was very important to prevent problems of dust entrainment in the fluidizing gas as well as to prevent feeder hardware problems caused by large stones and aggregates. After physical separation and size reduction, nearly 70wt% of the trommel fines was obtained within the size range suitable for energy recovery using the existing fast pyrolysis system. This pyrolyzable fraction could account for about 83% of the energy content of the 'as received' trommel fines sample. Therefore there was no significant differences in the thermochemical properties of the raw and pre-treated feedstocks, indicating that suitably prepared trommel fines samples can be used for energy recovery, with significant reduction in mass and volume of the original waste. Consequently, this can lead to more than 90% reduction in the present costs of disposal of trommel fines in landfills. In addition, the recovered plastics and textile materials could be used as refuse derived fuel.

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Chars produced from the pyrolysis of different waste materials have been investigated in terms of their use as a catalyst for the catalytic cracking of biomass pyrolysis gases during the two-stage pyrolysis-gasification of biomass. The chars were produced from the pyrolysis of waste tyres, refused derived fuel and biomass in the form of date stones. The results showed that the hydrocarbon tar yields decreased significantly with all the char materials used in comparison to the non-char catalytic experiments. For example, at a cracking temperature of 800°C, the total product hydrocarbon tar yield decreased by 70% with tyre char, 50% with RDF char and 9% with biomass date stones char compared to that without char. There was a consequent increase in total gas yield. Analysis of the tar composition showed that the content of phenolic compounds decreased and polycyclic aromatic hydrocarbons increased in the product tar at higher char temperatures.

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The influence of reaction conditions on the composition of liquid products during two-stage hydrothermal conversion of alkali lignin has been investigated in a batch reactor. Reactions were carried out in the presence of formic acid (FA) and Pt/Al2O3 catalyst. The two different sets of reaction conditions involved alternative reaction times of 1h and 5h at 265°C and 350°C, respectively. These provided different contributions to reaction severity, which affected the compositions of liquid products. Yields of liquid products reached up to 40wt% (on lignin feed basis) in the presence of FA under the less severe reaction condition. With 5h reaction time at 350°C, alkylphenols, alkylguaiacols and hydrocarbons were the dominant liquid products. However, with 5h reaction time at 265°C, phenol and methanol became dominant. The two-stage hydrothermal process led to improved lignin conversion, with the potential to manipulate the liquid product range.

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The formation of 2-4 ring polycyclic aromatic hydrocarbons (PAH) from the pyrolysis of nine different municipal solid waste fractions (xylan, cellulose, lignin, pectin, starch, polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET)) were investigated in a fixed bed furnace at 800 °C. The mass distribution of pyrolysis was also reported. The results showed that PS generated the most total PAH, followed by PVC, PET, and lignin. More PAH were detected from the pyrolysis of plastics than the pyrolysis of biomass. In the biomass group, lignin generated more PAH than others. Naphthalene was the most abundant PAH, and the amount of 1-methynaphthalene and 2-methynaphthalene was also notable. Phenanthrene and fluorene were the most abundant 3-ring PAH, while benzo[a]anthracene and chrysene were notable in the tar of PS, PVC, and PET. 2-ring PAH dominated all tar samples, and varied from 40 wt.% to 70 wt.%. For PS, PET and lignin, PAH may be generated directly from the aromatic structure of the feedstock.

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The potential of supercritical water gasification (SCWG) of macroalgae for hydrogen and methane production has been investigated in view of the growing interest in a future macroalgae biorefinery concept. The compositions of syngas from the catalytic SCWG of Laminaria hyperborea under varying parameters including catalyst loading, feed concentration, hold time and temperature have been investigated. Their effects on gas yields, gasification efficiency and energy recovery are presented. Results show that the carbon gasification efficiencies increased with reaction temperature, reaction hold time and catalyst loading but decreased with increasing feed concentrations. In addition, the selectivity towards hydrogen and/or methane production from the SCWG tests could be controlled by the combination of catalysts and varying reaction conditions. For instance, Ru/Al2O3 gave highest carbon conversion and highest methane yield of up to 11 mol/kg, whilst NaOH produced highest hydrogen yield of nearly 30 mol/kg under certain gasification conditions.

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Three environmental samples containing dioxins and furans have been oxidized in the presence of hydrogen peroxide under supercritical water oxidation conditions. The samples consisted of a waste incinerator fly ash, sewage sludge and contaminated industrial soil. The reactor system was a batch, autoclave reactor operated at temperatures between 350 degrees C and 450degrees C, corresponding to pressures of approximately 20-33.5 MPa and with hydrogen peroxide concentrations from 0.0 to 11.25 vol%. Hydrogen peroxide concentration and temperature/pressure had a strong positive effect on the oxidation of dioxins and furans. At the highest temperatures and pressure of supercritical water oxidation of 4500C and 33.5 MPa and with 11.25 vol% of hydrogen peroxide, the destruction efficiencies of the individual polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/PCDF) isomers were between 90% and 99%. There did not appear to be any significant differences in the PCDD/PCDF destruction efficiencies in relation to the different sample matrices of the waste incinerator fly ash, sewage sludge and contaminated industrial soil.

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Hydrothermal treatment involving the oxidation and gasification of sewage sludge and pharmaceutical wastewater samples has been carried out in batch autoclave reactor. The efficiency of the oxidation processes measured in terms of total organic carbon (TOC) and total solids (TS) removal showed that effective sample treatments were achieved at moderately low temperatures of 350-450 degrees C. More than 90% TOC removal was achieved in all samples, with samples with lower organic contents reaching above 99% TOC removal. A maximum of 78% TS removal was achieved due to the high initial ash contents of the samples. Alkaline hydrothermal gasification of the samples with very high TOC contents yielded gas products rich in hydrogen gas and methane. Due to high concentrations of acetonitrile in some of the samples, the yields of methane were high, giving calorific values of the product gases of up to 35 MJ/m3 in some samples; however, the gasification process resulted in lower TOC destruction compared to the oxidation process.

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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.

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The influence of gaseous ammonia and sulphur dioxide on the formation of 2378-substituted PCDD/F on a reference flyash from a municipal waste incinerator has been investigated using a laboratory scale fixed-bed reactor. The reference flyash samples (BCR-490) was reacted under a simulated flue gas stream at temperatures of 225 and 375°C for 96h. The experiments were carried out in two series: first with simulated flue gas alone, and then with injection of NH(3) or SO(2) gas into the flue gas just before the reactor inlet. It was found that the injection of gaseous ammonia into the flue gas could decrease the concentration of both PCDD and PCDF by 34-75% from the solid phase and by 21-40% from the gas phase. Converting the results to I-TEQ values, it could reduce the total I-TEQ values of PCDD and PCDF in the sum of the flyash and exhaust flue gas by 42-75% and 24-57% respectively. The application of SO(2) led to 99% and 93% reductions in the PCDD and PCDF average congener concentrations, respectively in the solid phase. In the gas phase, the total reductions were 89% and 76% for PCDD and PCDF, respectively. Moreover, addition of SO(2) reduced the total I-TEQ value of PCDD and PCDF in the flyash and exhaust flue gas together by 60-86% and 72-82% respectively. Sulphur dioxide was more effective than ammonia in suppressing PCDD/F formation in flyash under the conditions investigated.

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The seventeen 2378-substituted polychlorinated dibenzo-p-dioxins and dibenzo-p-furans (PCDD/Fs) congeners have been separated and analyzed in sewage sludge and incinerator flyash samples using a CP-Sil 88 column (50 m × 0.25 mm I.D., 0.25 µm film thickness) operating at a maximum oven temperature of 240°C. The column was used on a Varian 450-GC with a Varian 320-MS Triple Quadrupole. Calibration standards were used to determine the transition chemistries of the 2378-substituted PCDD/F congeners in the gas chromatography/mass spectrometry (GC-MS/MS) system. The five-point calibration curve for each of the congeners showed very good linearity with R(2) values greater than 0.999. The recovery of labelled compounds ranged from 50% to 120%. Analytical results from a reference flyash (BCR-490) and a reference sewage sludge (BCR-677) compared very well with the certified values, giving percentage deviations in I-TEQ (international toxic equivalents) of 4.93% and 0.53%, respectively. Results from 'real' flyash samples underscored the level of progress made in the abatement of dioxin emissions from incinerators; the old incinerator flyash contained much higher PCDD/F concentrations than the modern one. In addition, the concentrations profiles of PCDD/Fs in the 'real' sewage sludge from two UK wastewater treatment plants (WWTPs) showed that one contained a total PCDD/Fs content of 314 ng I-TEQ kg(-1), while the other gave a total of 53 ng I-TEQ kg(-1). Over an 18-month period of operation, no significant loss of analytical performance was observed from the low-temperature column.

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The yield of PCDD/F in relation to the presence of oxygenated PAH in model waste incinerator flyash has been investigated in a fixed bed laboratory scale reactor. Experiments were undertaken by thermal treatment of the model flyash at 250 and 350°C under a simulated flue gas stream for 2 h. After reaction, the PCDD/F content of the reacted flyash and the PCDD/F released into the exhaust gas, and subsequently trapped by XAD-II resin in a down-stream condensation system were analyzed. The PAHs investigated were, dibenzofuran and benzo[b]naphtho[2,3-d]furan and were spiked onto the model flyash as reactant precursors for PCDD/F formation. The results showed significant formation of furans from both of the PAH investigated, however except from some highly chlorinated dioxin congeners, the formation of dioxins was not so common. Benzonaphthofuran was significantly more reactive than dibenzofuran in PCDD/F formation, in spite of the fact that dibenzofuran is structurally more similar to that of PCDD/F. Thus, there was no clear attribution between the chemical structure of PAH used and the formation of PCDD/F. There were considerable differences between the yields of PCDD/F congeners in the gaseous species and those in the reacted flyash under the same operational conditions. The concentration of PCDD/Fs was reduced at the higher reaction temperature of 350°C; however, the higher temperature resulted in the majority of the PCDD/F formed on the flyash being released into the gas phase.

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The near-critical water gasification of dairy industry waste in the form of Whey, a product composed of mixtures of carbohydrates (mainly lactose) and amino acids such as glycine and glutamic acid, has been studied. The gasification process involved partial oxidation with hydrogen peroxide in the presence of NaOH. The reactions were studied over the temperature range from 300°C to 390°C, corresponding pressures of 9.5-24.5 MPa and reaction times from 0 min to 120 min. Hydrogen production was affected by the presence of NaOH, the concentration of H(2)O(2), temperature, reaction time and feed concentration. Up to 40% of the theoretical hydrogen gas production was achieved at 390°C. Over 80% of the Whey nitrogen content was found as ammonia, mainly in the liquid effluent.

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The application of activated carbons has become a commonly used emission control protocol for the removal or adsorption of persistent organic pollutants from the flue gas streams of waste incinerators. In this study, the 2378-substituted PCDD/F removal efficiency of three types of activated carbons derived from the pyrolysis of refuse derived fuel, textile waste and scrap tyre was investigated and compared with that of a commercial carbon. Experiments were carried out in a laboratory scale fixed-bed reactor under a simulated flue gas at 275°C with a reaction period of four days. The PCDD/F in the solid matrices and exhaust gas, were analyzed using gas chromatography coupled with a triple quadrupole mass spectrometer. In the absence of activated carbon adsorbent, there was a significant increase in the concentration of toxic PCDD/F produced in the reacted flyash, reaching up to 6.6 times higher than in the raw flyash. In addition, there was a substantial release of PCDD/F into the gas phase, which was found in the flue gas trapping system. By application of the different commercial, refuse derived fuel, textile and tyre activated carbons the total PCDD/F toxic equivalent removal efficiencies in the exhaust gas stream were 58%, 57%, 64% and 52%, respectively. In general, the removal of the PCDDs was much higher with an average of 85% compared to PCDFs at 41%. Analysis of the reacted activated carbons showed that there was some formation of PCDD/F, for instance, a total of 60.6 µg I-TEQ kg(-1) toxic PCDD/F was formed in the refuse derived fuel activated carbon compared to 34 µg I-TEQ kg(-1) in the commercial activated carbon. The activated carbons derived from the pyrolysis of waste, therefore, showed good potential as a control material for PCDD/F emissions in waste incinerator flue gases.

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The gasification of some selected components of food wastes using H(2)O(2) as the oxidant and in the presence of NaOH has been investigated under subcritical water conditions. Hydrogen production was enhanced when both NaOH and H(2)O(2) were used compared to when either NaOH or H(2)O(2) alone was used or in their absence. Results indicated that the H(2)O(2) acted to partially oxidize the samples while NaOH significantly increased hydrogen gas yields by promoting the water-gas shift reaction with subsequent CO(2) capture. In the presence of NaOH, the main components were Na(2)CO(3), CH(3)COONa and CH(3)COONa.3H(2)O. Char and tar production were suppressed in the presence of NaOH.

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High-impact polystyrene (HIPS) flame retarded with decabromodiphenyl ether (DDE), has been reacted in supercritical water from 380 to 450 degrees C and 21.5 to 31.0 MPa pressure in a batch reactor. Different concentrations of sodium hydroxide additive were used in situ to neutralize the corrosive inorganic bromine species released during the reactions. It appeared that supercritical water conditions lowered the decomposition temperature of both the fire-retardant DDE and HIPS. The reaction products included oils (up to 76 wt%), char (up to 18 wt%) and gas (up to 2.4 wt%) which was mainly methane. The presence of the alkaline water led to up to 97 wt% debromination of the product oil, producing virtually bromine-free oil feedstock. The removal of antimony from the oil product during processing was of the order of 98 wt%. The oil consisted of many single- and multiple-ringed aromatic compounds, many of which had alkyl substituents and/or aliphatic C(n)-bridges (n=1-4). The major single-ringed compounds included toluene, xylenes, ethylbenzene, propylbenzene and alpha-methylstyrene. Bibenzyl (diphenylethane), stilbene, diphenylmethane, diphenylpropane, diphenylcyclopropane, diphenylpropene, diphenylbutane, diphenylbutene and diphenylbuta-1,3-diene were the major C(n)-bridged compounds. Diphenyl ether and acetophenone were the major oxygenated compounds found. The process thus has the potential to produce bromine-free and antimony-free oils from fire-retardant plastics.

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