Impact of coal quality on equipment lifetime at coal-fired power stations

Abstract

With the export of coal being more lucrative than selling coal to South African power producers, power station operators might consider accepting lower-quality coal. While the impact lower quality coal has on cycle efficiency is understood, the influence it has on equipment reliability and lifetime is often not understood. This study focusses on addressing the question of how different characteristics of coal influences different damage mechanisms of common power station equipment. The results are translated into a reference framework that can be used when coal quality variation is expected. The influence of coal calorific value and ash content has on air-heater element erosion was evaluated. This was accomplished by establishing a correlation between calorific value and ash content of coal from a specific colliery; this was then used to calculate the mass of fly ash and flue gas produced when burning enough coal to satisfy the boiler load. An erosion model was then used along with historical coal quality and air heater erosion history to develop and fit a model for full boiler load. The model was verified against data not used during the development of the model, and a seemingly good prediction was made when compared to the measured result. The calorific value of the coal in the model was varied for a hypothetical situation; this indicated that as calorific value decreases the erosion of air heater elements increases. The influence abrasiveness index has on mill liners was also investigated as part of this study. Historical liner ultrasonic thickness and coal abrasiveness index results were used to fit a mathematical formula. The results indicate that for the ball mills at the power station used in the case study, the abrasiveness index did not have a significant influence on the wear rate of mill liners. The relationship was established to be directly proportional to increased abrasiveness index resulting in an increased wear rate. The final two case studies that form part of this overall study were focussed on boiler temperature variations as a result of variation in coal calorific value and establishing the impact coal “hang-ups” have on the lifetime of a drum reclaimer. The first of these two case studies was completed by creating a mathematical thermo-hydraulic model of a hypothetical boiler and calculating the effect calorific value would have on the boiler temperature distribution. The results were then compared to temperature-related damage mechanisms; the comparison indicated that a variation in calorific value, whether up or down from the designed value would be negative for overall boiler health. The final case study was not completed due to the unavailability of related equipment. A full description of the envisaged study is provided.