Development of a micro scale pulverised coal test burner
Storm, Andre John
MetadataShow full item record
The power demand in South Africa continues to grow which makes it more important to understand different input parameters in power stations. One of these input parameters is the calorific value (CV) of coal. The current type of CV used in power stations is the gross calorific value at constant volume (GCVv). The GCVv is determined with a bomb calorimeter, which does not represent an accurate CV for coal used in a power station, because of the constant volume process of the bomb calorimeter. The constant volume process enables the bomb calorimeter to recover the latent heat of the moisture, which does not happen in a burner of a power station. The net calorific value at constant pressure (NCVp) represents a more accurate CV for coal used in a power station since it does not recover the latent heat of the moisture. To determine the NCVp, a flow calorimeter is required, which is essentially a coal burner that operates at constant pressure. Currently, no small-scale NCVp analysers exist. The problem is to devise a functioning pulverised coal (PC) micro-burner that discharges into a sized single combustion chamber and complies with the Fossil Fuel Firing Regulations (FFFR) of Eskom. This burner must function as an independent system without the flame support of burners positioned adjacent to or opposite it and without the positive influence of a large common furnace. The burner must be able to sustain stable combustion for such a period of time that all applicable parameters can be measured representatively. This micro coal burner was designed to operate on different types of coal and to be as small as possible in rating and physical dimensions. Because a minimum of 1.6 g/s PC is fed to this burner, the burner was designed to be 40 kW in thermal rating. To mimic combustion taking place in a full-scale burner on a power station, the micro coal burner is scaled down and operates in a similar manner. A gas burner was designed and manufactured to act as an igniter to the coal and as an experiment to predict the behaviour of the coal. The gas burner will operate in the same manner as the micro coal burner only on an even smaller scale. Different factors affecting combustion were investigated to be implemented on the design of the burner. These factors were swirl, residence time, recirculation zone, fineness and burnout time of PC. The settling of PC inside the transporting pipes was also taken into account. Firstly, combustion calculations were done to determine the air-to-fuel ratio (A:F) with 20% excess air for minimum, nominal and maximum CV coals to enable proper combustion. Because of the gas igniter, these combustion calculations were also done for liquid petroleum gas (LPG). A safe A:F inside the pulverised fuel/primary air (PF/PA) and primary core air (PCA) tubes was determined for safety purposes to comply with the FFFR. Taking the burning velocity of 0.4 m/s of coal into account, the dimensions of the tubes were calculated accordingly. For proper drying of coal and better combustion, pre-heating of air was applied for the different airflows in the calculations. The gas burner was 15% in thermal rating compared to the micro coal burner. At first, it was decided to supply the air to the burner via tangentially placed holes. The gas burner experiment showed how the combustion of gas reacted to these tangentially placed holes. In summary, the tangential placed holes provided sufficient swirl as well as a large recirculation zone. The gas burner achieved complete combustion, however, intense heat was forced back into the tubes by the vortex, which resulted in metal temperature excursions. Computational fluid dynamics (CFD) was used to investigate this problem further. To prevent this and other problems occurring in the micro coal burner, CFD was used to design swirl generators. These swirl generators have been designed to adequately create swirl, a recirculation zone and ensure safety. The swirl generators were also designed to be dependant upon one another ensuring equal angular and axial velocities upon exiting the swirl generator holes. After the designing process, the micro coal burner was manufactured and assembled. The burner indicated proper combustion for gas, but not for coal. The gas flame’s behaviour and shape correlated well with the CFD done on the design. A portion of the coal burns, but a stable flame cannot be sustained without gas support. This occurs because of improper mixing of the gas flame and coal as well as a small heat zone. To solve these problems for future modifications, an extension of the PF/PA tube is implemented after the PF/PA swirl generator. This is to contain the PF and to mix it properly with the gas flame. The hot PF will then exit the extended PF/PA tube into an extended secondary air (SA) tube that is required to be attached to the SA swirl generator. The above-mentioned assumptions indicate that the PF will now have adequate air and heat to combust. The modifications have been implemented on a new design and a CFD analysis was done which correlated well with the assumptions. These modifications will be implemented as the next stage in the quest to develop a micro burner that can ensure sustained combustion for a small-scale NCVp analyser.
- Engineering