A critical review on microwave, fluidised-bed and convective air-drying of low rank coals: The water diffusivity on coals

A critical review on drying of low rank coal drying using conventional air-drying, fluidised bed drying and microwave drying methods. The parameters of different drying methods that affected the water effective diffusivity values were identified and critically discussed in details including hot air temperatures, flow rates, particle sizes, particle weights, and microwave powers. The microwave and fluidised bed drying methods are more effective in drying of low rank coal compared to the conventional hot air drying method. This is because the microwave drying method can heat up the internal part of the coal, thus leads to a higher drying rate and effective diffusivity values whereas the fluidised bed causes an uniform distribution of heating medium to heat up the low rank coal and the fluidisation resulted in a better mixing performance and a higher heat and mass transfer compared to the conventional hot air drying. Moreover, Midilli-Kucuk model was the best and most commonly used drying model for drying of the low rank coal. In the fluidised bed drying, the Midilli-Kucuk was the best-fitted drying model to dry the low rank coal whereas Wang & Singh model was the best drying model in the fixed-bed drying for the coarse particle of low rank coal. The microwave drying kinetics can be modeled using either the Page Model or Midilli-Kucuk model. Further to this, it was found that few parameters that significantly affected the effective diffusivity of the low rank coal are increasing temperature and flow rates of the convective air-drying meanwhile increasing the particle size and particle weight results in a decrease in effective diffusivity value for the fluidized-bed drying.