Selective removal of sodium and calcium from low-rank coal: Process integration, simulation and experimental investigation

This paper addresses the selective removal of sodium and calcium from low-rank sub-bituminous coal, thereby improving fusion temperatures of ash to reduce its slagging and fouling propensity in the pulverised coal-fired boilers. Process simulation has been conducted based on the lab-scale experimental results. Three potential low-rank coal demineralisation processes have been simulated, a single-stage water washing process incorporating evaporator to recycle the waste water, a three-stage counter-current water washing process combined with an evaporator for water recovery, and a three-stage water washing process using reverse osmosis to demineralise the recycling water. The washing experiments were implemented to determine the optimum leaching conditions. Optimum condition was found to be residence time of 30 minutes; particle size of 1-4 mm and a liquid to solid mass ratio of 1.25. The process simulation was conducted using Aspen Plus. The simulation result sindicated that, the total removal extent of sodium increased stably with the increase in the recycle number. In contrast, the mobilisation of calcium was marginal and was mainly achieved in the first stage, which can be attributed to the low solubility of CaSO₄ and organic calcium in water. For three-stage counter-current water washing, a total of 60% of sodium and 0.7% of calcium were removed, resulting in a significant reduction in ash content and increase in its fusion temperatures. Additionally, low-rank coal washing processes could produce maximum 0.14 t water/t raw coal, due to the high moisture content in the parent coal, making it economically viable. However, such a conclusion needs to be further validated via pilot-scale demonstration experiments. In particular, the water recovery from low-rank coal-water slurry through the use of the industry-scale centrifugal needs to be confirmed.