Gasification of high ash coal and chars from South African coals

This study seeks to explore the behaviour of several different South African bituminous coals currently used as feed in local power stations and to establish their technical performance and structural changes in a fluidised bed gasifier. It is anticipated that this would also assist in optimising gasifier operations for the gasification of fine high ash coals for power generation. The research was conducted by correlating the gasification performance of the selected coals and their derived chars against a range of chemical, physical and optical characteristics including mineral and maceral (and specifically inertinite) contents after testing in a pilot scale fluidised bed gasifier. Of specific interest were the changes in chemical microstructures during the transformation of the various coal macerals to their relevant chars following gasification. Raman spectroscopy and XRD analyses were used to examine the chemical carbon structures and the minerals associated in the coal. The relationship between the organic components and their gasified products (macerals-to-char) and the inorganic components to their gasified products (mineralsto- ash) including their physical structure and behaviour was determined by petrographic analysis. A higher loss of coal reactivity was obtained from vitrinites-rich coals due to a higher degree of structural transformation of carbon in the coal. Inertinite-rich coals experienced a lower loss of coal reactivity and lower degree of structural transformation even with longer residence time. The structural transformation of the macerals is due to realignment of the carbon molecules leading to substantial swelling (enhanced plasticity) in some macerals. Further modification was found to be due to proximity to melted minerals. Furthermore, the gasification performance of low grade coals can be optimised by varying the oxygen content used for coal gasification

Equilibria and isosteric heat of adsorption of methane on activated carbons derived from South African coal discards

Abstract: Isosteric heat of adsorption (Hst) is critical for evaluating the thermal effects of adsorption-based storage systems. Poor management of the thermal effects of an adsorptive storage system often alters the overall performance of the storage system. In this study, methane equilibrium uptake on activated carbons derived from coal discards and isosteric heat of adsorption were evaluated. The methane adsorption capacity of the produced activated carbons was measured using a high-pressure volumetric analyzer. The isotherm results in temperature ranges of 0−50 °C and pressure of up to 40 bar are analyzed using the Langmuir, Tóth, and Dubinin−Astakhov (DA) isotherm models. The results showed that, for the two activated carbons, the DA model was the best fit. In addition, we evaluated the isosteric heat of adsorption using two theoretical frameworks, Maxwell’s thermodynamic relations and the modified Polanyi potential function. The Tóth potential function and Clausius−Clapeyron equations were applied to the Dubinin−Astakhov adsorption model to obtain an analytical expression of Hst. Both methods were compared, and the result showed an overall error margin between 6 and 12%. The values of Hst obtained are over a range of 10−17 kJ/mol. It was observed that Hst decreases with an increase in methane fractional load. The Hst values obtained are useful in designing an efficient thermodynamic scheme for the ANG storage system

Multi-criteria decision analysis for the evaluation and screening of sustainable aviation fuel production pathways

The aviation sector, a significant greenhouse gas emitter, must lower its emissions to alleviate the climate change impact. Decarbonization can be achieved by converting low-carbon feedstock to sustainable aviation fuel (SAF). This study reviews SAF production pathways like hydroprocessed esters and fatty acids (HEFA), gasification and Fischer–Tropsch Process (GFT), Alcohol to Jet (ATJ), direct sugar to hydrocarbon (DSHC), and fast pyrolysis (FP). Each pathway's advantages, limitations, cost-effectiveness, and environmental impact are detailed, with reaction pathways, feedstock, and catalyst requirements. A multi-criteria decision framework (MCDS) was used to rank the most promising SAF production pathways. The results show the performance ranking order as HEFA > DSHC > FP > ATJ > GFT, assuming equal weight for all criteria

A process investigation of the biosolubilisation of low rank coal in slurry system

Includes bibliographical references (leaves 127-136).The coal biosolubilisation processes may be used to convert low rank coal to either a clean, cost-effective energy source or to value-added products. This can lead to increased utilisation of low-rank coal. Low-rank coal is currently under-utilised because of its low calorific value, high moisture and sulphur content. Most research on coal biosolubilisation has centred on pre-treated coal. Little work is reported on naive coal. Low yields of solubilised coal products are currently reported in the literature. This may be due to further degradation of the soluble processes or to limitation of solubilisation step. These products have potential as starting materials for biotransformation to value-added products. However, to date, small volumes of solubilised coal products are available to assess their potential for further biotransformation owing to current biosolubilisation of low-rank coal being widely carried out as a small scale Petri dishes or Erlenmeyer flask of volume. This dissertation presents the results of the investigation of biosolubilisation of low-rank coal in slurry systems using Trichoderma alroviride. Its main objectives were to investigate key operating variables influencing untreated low rank coal biosolubilisation and degradation of soluble products, and to study different reactor configurations for coal biosolubilisatio

Process modelling of chemical looping combustion of paper, plastics, paper/plastic blend waste, and coal

Abstract: Chemical looping combustion (CLC) is a novel carbon capture and storage technology that can be used in the proper disposal of municipal solid waste when used as a solid fuel. In this study, the results of the CLC of paper, plastics, and paper/ plastic blends were compared with CLC of South African coal using Chemcad software. The simulation was done for two different CLC processes, namely, chemical looping oxygen uncoupling (CLOU) and in situ gasification CLC (IG-CLC). The results demonstrated that coal at 66% had a lower CO2 yield than paper (86%) but a higher yield than all the plastic samples in CLOU (3356%) and an equal CO2 yield in paper and all plastic samples in IG-CLC. Furthermore, coal had a lower CO2 gas yield than all the optimum blends (72−85%) for CLOU and an equal yield with the entire paper/plastic blend in IG-CLC. On combustion efficiency, coal has a lower combustion efficiency at 80% than paper and polyvinyl chloride (PVC) at 90 and 96%, respectively, but a higher efficiency than other plastic samples that are between 30 and 70% in CLOU while in IG-CLC, it had a lower efficiency than paper, PVC, and polyethylene terephthalate and higher efficiency than high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. For paper/plastic blends, coal has higher combustion efficiency than all the paper/plastic blends in both CLOU and IG-CLC processes except for the paper/PVC where the combustion efficiency was higher than coal

Chemical looping combustion (CLC) of municipal solid waste (MSW)

Chemical Looping Combustion (CLC) has been found to be a better alternative in converting Municipal Solid Waste (MSW) to energy and has the potential to reduce the generation of dioxins due to the inhibition of the de-novo synthesis of dioxins. This study comprehensively reviews the experimental studies of CLC of MSW, the oxygen carriers, reactor types, performance evaluation, and ash interaction studies. Modeling and simulation studies of CLC of MSW were also critically presented. Plastic waste is MSW’s most studied non-biomass component in MSW under CLC conditions. This is because CLC has been shown to reduce the emission of dioxins and furans, which are normally emitted during the conventional combustion of plastics. From the several oxygen carriers tested with MSW’s CLC, alkaline earth metals (AEM) modified iron ore was the most effective for reducing dioxin emissions, improving combustion efficiency and carbon conversion. Also, oxygen carriers with supports were more reactive than single carriers and CaSO4/Fe2O3 and CaSO4 in silica sol had the highest oxygen transport ability. Though XRD analysis and thermodynamic calculations of the reacted oxygen carriers yielded diverse results due to software computation constraints, modified iron ore produced less HCl and heavy metal chlorides compared to iron ore and ilmenite. However, alkali silicates, a significant cause of fouling, were observed instead. The best reactor configuration for the CLC of MSW is the fluidized bed reactor, because it is easy to obtain high and homogeneous solid–gas mass transfer. Future research should focus on the development of improved oxygen carriers that can sustain reactivity after several cycles, as well as the system’s techno-economic feasibility

Experimental Evaluation Using Plastic Waste, Paper Waste, and Coal as Fuel in\ua0a\ua0Chemical Looping Combustion Batch Reactor

A comparative study of chemical looping combustion (CLC) with paper, plastic, and coal as fuel was carried out. Experiments were performed in a laboratory fluidized-bed reactor by alternating between reduction and oxidation cycles. The results obtained indicated that a higher temperature leads to an increase in the CO yield and carbon conversion for all fuels. Paper had the highest fractional conversion of CO to CO followed by polyvinyl chloride (PVC) and coal. This was due to the higher fraction of volatiles in paper compared to PVC and coal. Scanning electron microscopy (SEM) analysis of the oxygen carrier particle after each of the solid fuel experiment was carried out. For the used ilmenite, there was a slight difference in the morphology for the three different fuels

Experimental Evaluation Using Plastic Waste, Paper Waste, and Coal as Fuel in a Chemical Looping Combustion Batch Reactor

A comparative study of chemical looping combustion (CLC) with paper, plastic, and coal as fuel was carried out. Experiments were performed in a laboratory fluidized-bed reactor by alternating between reduction and oxidation cycles. The results obtained indicated that a higher temperature leads to an increase in the CO yield and carbon conversion for all fuels. Paper had the highest fractional conversion of CO to CO followed by polyvinyl chloride (PVC) and coal. This was due to the higher fraction of volatiles in paper compared to PVC and coal. Scanning electron microscopy (SEM) analysis of the oxygen carrier particle after each of the solid fuel experiment was carried out. For the used ilmenite, there was a slight difference in the morphology for the three different fuels

Enhanced reactivity of geopolymers produced from fluidized bed combustion bottom ash

Abstract: In this study, waste bottom ash obtained from fluidized bed combustion of low-grade South African coal in a bubbling fluidized bed reactor was used to produce geopolymers. The geopolymers were cured using both a conventional oven and a household microwave. An alkaline solution of Na2SiO3/NaOH (1:0.5) was mixed on a 1:1 ratio with a mixture of bottom ash/kaolin (1:1) mixture. Thereafter the resulting mixture was mixed with sand on a 1:0.5 ratio. Characterization of the geopolymers carried out using the following techniques, scanning electron microscope (SEM-EDX) analysis, compressive strength test, Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis. A household microwave and a conventional oven were used to enhance the geopolymerisation process and strength of the geopolymers. The results showed that the microwave curing enhanced the reactivity and compressive strength of the geopolymers. The microwave and oven cured geopolymer had the highest Si/Al ratio of 4.42 and reached a reasonable high compressive strength test of 31 MPa in 7 days compared to geopolymers cured with a conventional oven only or with a microwave only. The microwave radiation followed by conventional oven curing reduced the heat curing time and energy but improved the reactivity and strength of the geopolymer

Micostructural and Mechanical Properties of Geopolymers Synthesized from Three Coal Fly Ashes from South Africa

In this study, coal fly ashes (CFAs) from three different boiler sites in South Africa, Eskom (E coal fly ash), George Mukhari Academic Hospital (GMH coal fly ash), and KarboChem (KBC coal fly ash), were used to produce geopolymers. The coal fly ashes were pretreated with a mixed alkali activator solution of sodium silicate (NaSiO₃) and sodium hydroxide (NaOH). The geopolymer pastes were cured in an oven at 60 °C for 10 days and further cured at room temperature for 18 days. The microstructure and mechanical properties of the geopolymers were evaluated by scanning electron microscopy energy dispersive spectroscopy (SEM-EDS), X-ray diffractions (XRD), Fourier transform infrared (FTIR), thermogravimetric (TG), and compressive strength analyses. The compressive test results obtained showed that the E and KBC geopolymers have higher strength than GMH-CFA geopolymers. Similar results were obtained in the FTIR and SEM-EDS analyses. This indicates that E-CFA and KBC-CFA are more reactive and hence they have a higher degree of geopolymerization when compared to GMH-CFA