An evaluation of coal briquettes using various binders for application in fixed- bed gasification

Abstract

South Africa continues to rely considerably on coal as a source of energy and carbon derived chemicals. The rigorous coal mining processes result in the production of over 28 Mt of coal fines per annum. Depleting coal reserves coupled with the dependency of the South African economy on coal utilisation and exportation initiated explorations into fine discard coal utilisation. Binderless agglomeration of vitrinite-rich coal has previously shown great potential, producing mechanically stronger and more water resistant briquettes as compared to briquettes produced from inertinite-rich coal. Fine coal briquetting, while making use of a suitable binder, enhances agglomeration and therefore reduces briquetting (pressing) temperatures and pressures, paving the way for producing durable products to be utilised in industrial applications. In this study, inertinite-rich, low grade coal was used along with 12 binders: clays (attapulgite and bentonite), bio char, cow dung, granulated medium tar pitch, coal tar sludge, flocculant, fly ash, lignosulphonates, polyester resin and 2 South African coal tar pitches in order to produce mechanically strong and water resistant briquettes. The binders were added in various concentrations, and the compressive strength, friability and water resistance of the resultant briquettes were determined. The briquettes manufactured using lignosulphonate and resin as binders resulted in the strongest briquettes, with compressive strengths of 16 and 12 MPa respectively at a 7.5 wt% binder concentration. Cured and uncured, with and without binder addition, the briquettes all retained their shape and size during drop tests, but none proved to be water resistant. Paraffin and wax were therefore used as waterproofing agents after pressing and curing. The reactivity of the lignosulphonate and resin briquettes was compared to that of run of mine coal (lump coal) from the same colliery. Run of mine coal and briquette chars were prepared by devolatilising samples non-isothermally up to 1000°C with a hold time of 15 min. Carbon dioxide gasification was subsequently performed at 875, 900, 925, 950 and 1000°C for the lump coal, binderless, lignosulphonate and resin briquette chars. During the gasification process, the chars exhibited Arrhenius-type dependency on temperature with the initial reactivity increasing with increasing reaction temperature. The addition of the two binders brought no significant change to the reactivity of the chars, but significant reactivity differences were observed between the manufactured briquettes and the run of mine coal chars. Surface area analysis by means of CO2 adsorption indicated an increase in micropore surface area development of the briquettes post devolatilisation, which was postulated to be the major contributor to the increased CO2 reactivity of the briquettes when compared to the ROM coal char. Using structural models, the reactivity constants for CO2 gasification of the run of mine coal, binderless, lignosulphonate, and resin briquette chars were determined. The mechanical and thermal analyses of the briquettes showed promising results for industrial application, meriting a techno-economic study prior to implementation.