Development of a micro burner for the experimental determination of the net calorific value at constant pressure of pulverised coal

Abstract

Coal combustion remains the major resource of power generation internationally. Coal consists of different substances, which influence its quality and properties. One such property is the calorific value (CV), which specifies the amount of energy per mass contained in a substance after it is released in the form of heat after complete combustion. There are basically two types of CV, the gross calorific value at constant volume (GCVv), and the net calorific value at constant pressure (NCVp). These values differ owing to the different processes that are followed to determine the CV. When considering the method of determining the overall efficiency of a power station, the CV of coal is the greatest contributor to inaccuracy; therefore, it is very important to determine the correct CV of coal. The Bomb calorimeter is the norm used to determine the CV on power stations in South Africa. The Bomb calorimeter utilises a direct method to determine the GCVv. However, the GCVv does not represent an accurate CV on a power station. The constant volume process followed by the Bomb calorimeter enables it to recover latent heat formed during the combustion process. This latent heat is, however, not recovered in the constant pressure process by a burner on a power station. Therefore, the NCVp is a more representative CV, since it utilises the same process as on a power station. The only method currently used to determine the NCVp of a solid fuel such as coal is from derived calculations. Different methods exist to determine the NCVp by derived calculations; however, no international standard is available for these derived calculations and these calculations differ to some degree. This is still a theoretical approach that does not include losses. The development of a direct method was thus required to determine the NCVp of one type of coal as a proof of concept. The literature survey indicated that the NCVp coal analyser must be based on the principle of a flow calorimeter that is used to determine the NCVp of gaseous fuels on a mass-energy balance. To develop this device, it was necessary to design a burner and combustion chamber, as well as to select and size associated auxiliaries. The burner was designed by using cold flow computational fluid dynamics (CFD) modelling in conjunction with burner and combustion principles to attain a self-sustained coal flame on a laboratory-sized scale. The device was then commissioned by assembling the different components and by doing preliminary tests, such as liquid petroleum gas (LPG) combustion, pulverised coal settling flow and calibration. The calibration test showed that the NCVp coal analyser's result differed by 1.4% from the known NCVp of LPG. After the commissioning of the device had been completed, one type of coal was tested to attain a self-sustaining flame, and to determine the NCVp. The test was deemed successful, since a self-sustaining flame was indeed achieved, and the NCVp of the tested coal was determined. The burner achieved 99.78% combustion efficiency based on unburnt carbon content in ash. The profile of the self-sustaining flame correlated well with the cold flow CFD streamlines used to design the burner. The NCVp of the tested coal determined by the NCVp coal analyser differed to some extent from that of the different calculated NCVp of different methods. However, since the calibration showed difference of a mere 1.4% of LPG, the NCVp of the tested coal is considered acceptable.