The influence of various potassium compound additions on the plasticity of a high-swelling South African coal under pyrolyzing conditions

Abstract

The influence of the anion attached to a potassium cation on the decrease of plastic properties of a high swelling vitrinite-rich medium rank B South African coal was investigated by adding potassium hydroxide (KOH), potassium carbonate (K2CO3), potassium acetate (KCH3CO2) and potassium chloride (KCl) to a partially demineralized coal sample. 5 and 10 weight percentages (determined as the amount of potassium in the potassium salt) were added to the demineralized coal and FSI, TMA and dilatometry of the blends measured. Thermogravimetric analyses of the added compounds indicated that all of the potassium salts evaporate to some extent at different temperatures. It was found that the addition of KCl does not decrease the swelling or plastic properties of the coal. However, with the addition of the oxygen – containing potassium compounds to the demineralized coal, a decrease in the swelling was observed; with the addition of KCH3CO2 showing the largest influence, followed by KOH and K2CO3. Dilatation results indicated that not all of the plastic properties of the coal were eliminated in the case of addition of KCH3CO2 (as was for K2CO3), although K2CO3 is also the decomposition product of KCH3CO2. K2CO3 starts to form from KCH3CO2 at temperatures higher that the softening temperature of the coal. The main influence on the plastic properties of the added compounds thus seems to be crucial in the temperature range from 380 to 480 °C. The large decrease in the FSI value for the KCH3CO2 loaded demineralized coal sample seems to be due to a physical effect of forming a large amount of acetone vapour at the softening point of the coal, which ruptures the fluid coal pores, leading to a decrease in the FSI value. K2CO3 decreases the plastic behaviour of the vitrinite-rich medium rank B coal to a larger extent than KCH3CO2