Comparison of industrial wastes as binder in the agglomeration of coal fines

Abstract

The industries that consumed the most coal in South Africa, in 2015, were electricity generation, 53%, basic iron and steel manufacturing, 20%, and synthetic fuel and chemical industries, 10%, with the rest being used in various other smaller industries, 17%. Coal fines (-1 mm) that are discarded due to handling issues provide an alternate source of fuel, which can be utilised through agglomeration. Briquetting is a large-scale agglomeration method where coal fines, with or without a binder, can be used to produce fuel briquettes. Binders are usually needed to produce briquettes that are strong and water resistant and should therefore also increase the briquette reactivity, be environmentally friendly and be cost effective. Hazardous wastes are expensive to dispose of and harmful to the environment, which is why they should be evaluated as binders for coal briquetting. The binding properties of five industrial wastes were evaluated to determine their efficacy in the agglomeration of Highveld inertinite-rich coal fines: (A) waterworks bio-sludge from a petrochemical industry, (B) acrylic acid containing a hydrocarbon by-product from a petrochemical industry, (C) pitch from a petrochemical industry, (D) wax emulsion from a petrochemical industry, and (E) recycled low-density polyethylene (LDPE) from a North-West plastic recycling company. The compressive strength, CS (MPa), impact resistance index, IRI (-), friability index, FR (%), abrasion resistance, AR (%), and water resistance index, WRI (%) of the briquettes were determined to evaluate the mechanical strength of the briquettes. The binderless briquettes had a maximum CS of 3.4 MPa, which is higher than the minimum CS of bituminous coal (2.1 MPa). The addition of Binders A–E was therefore investigated to use the briquettes as carriers for the reduction of hazardous wastes. The optimal concentration for each binder, based on mechanical strength (CS, IRI, FR, AR), was determined as 15% Binder A (4.1 MPa; 68; 80%; 99.3%), 20% Binder B (2.3 MPa, 87, 71%, 99.5%), 20% Binder C (8.2 MPa, 1000, 100%, 99.2%), 5% Binder D (2.4 MPa, 21, 46%, 95.3%) and 5% Binder E (5.3 MPa, 97, 52%, 98.3%). Briquettes bound with 20% Binder B, and 5–10% Binder E were water resistant. The reactivity of the optimal briquettes during pyrolysis (900°C), combustion (850, 875 and 900°C) and gasification (1000 and 1025°C) were tested to evaluate the effect of the binders. The binders had a small effect on the pyrolysis yields and no observable effect on the combustion and gasification reactivity of the briquettes due to the binders decomposing below 500°C. The briquettes were matched with suitable industrial processes based on the mechanical strength and reactivity results that were obtained. The binderless as well as the Binder B and D bound briquettes may be suitable for use in PCC boilers due to their strength being high enough to

enable transportation and low enough not to inhibit pulverising. Binders B, C and E bound briquettes may be more suited for use in a Lurgi-gasifier. None of the binders are recommended for use in blast furnaces due to the high ash yield, low volatile matter content and lack of swelling properties.