Coal pellets as an alternative fuel for a semi-continuous coal stove

Abstract

Around four million South African households depend on coal for fuel. Despite gaining electricity access, many continue using coal due to its affordability and cultural significance. Low-income households often rely on damaged or homemade stoves, leading to inefficient combustion and higher indoor emissions. In 2016, household pollutants contributed to roughly 3.8 million deaths globally. Approximately 11% of South Africa's run-of-mine coal consists of fines, often discarded due to costly disposal processes. While coal agglomeration (e.g., pellets) can add value to these fines by addressing storage and transport issues, studies have revealed that they are seldom adopted in low-income South African communities.

This study aims to determine the thermal performance and emissions quality of coal pellets, in order to evaluate the suitability thereof as an alternative to current domestic fuel sources. To achieve this aim, lump coal and coal pellets with different characteristics were combusted in asemi-continuous coal stove developed by New Dawn Engineering in collaboration with the NorthWest University. Six fuel batches were tested: lump coal (C), binderless pellets (P-B), and four variations of PVA binder pellets, both with and without waterproofing (P-2.5, P-5, P-2.5-W, and P-5-W).

Key findings indicate that combustion behaviour differed significantly between lump coal and coal pellets. The lump coal run exhibited the highest power output of 8.0 kW, and an overall thermal efficiency of 71.2%. In comparison, the binderless pellets displayed the lowest power rating of 4.2 kW, whereas the P-2.5 pellet run displayed the highest overall thermal efficiency of 77.5%. In terms of emissions, the semi-continuous coal stove achieved lower emission levels than a typical domestic coal stove burning both lump coal and low smoke fuel. The lump coal exhibited the lowest carbon monoxide CO emission factor of 0.67 ± 0.04 g/MJ, while most pellet runs, particularly P-2.5, produced CO emission levels more than double that of lump coal. The high CO emission factors of the coal pellets were attributed to the combustion in oxygen deficient conditions, which was observed during all the pellet fuel experiments. The SOX emission factors remained relatively comparable between the different fuel batches with no significant correlation to operating parameters. Variations in NOx emissions was mainly attributed to combustion in a reductive environment, with lump coal producing the highest NOx emission factor of 120 ± 6mg/MJ. Particulate matter emissions showed no significant correlation with fuel type or characteristics. The P-5 run produced the lowest PM levels with an emission factor of 15 ± 1 mg/MJ. Overall, the lump coal and P-5 stove-fuel combinations exhibited superior performance relative to the other combinations evaluated across both low- and high-power settings. The P-5 pellets is therefore identified as a suitable alternative fuel for the semi-continuous coal stove. The fuel PSD of -19 mm +13.2 mm is also recommended for domestic use.