Reduction of caking propensity in large (millimeter-sized) South African coal particles with potassium carbonate impregnation to expand fixed- and fluidized-bed gasification feedstock suitability

Abstract

The caking propensity of coal causes challenges in combustion, gasification, and fluidized-bed applications, where this property limits coal selection. Certain gasifiers are fitted with mitigation technologies such as stirrers to overcome operational problems associated with caking. However, additive addition is a possible alternative method with benefits such as reduction of caking and swelling and increased reaction rates. It is known that select additives alter the coal’s thermoplastic behavior, where studies have predominantly focused on powdered coal. Here, large coal particles (5, 10, and 20 mm) suitable for select fluidized-bed and gasification applications, from three South African coal fields, were examined. The coals have different thermal swelling characteristics: Highveld (TWD) (FSI 0), Grootegeluk (GG) (FSI 5.5–6.5), and Tshikondeni (TSH) (FSI 9) (where FSI is the free swelling index). Digital photography, SEM, and X-ray computed tomography were used to evaluate the caking propensity during devolatilization, and its reduction with K2CO3 addition. Particles were impregnated with an excess 5.0 M K2CO3 solution, and devolatilized in batch samples to 700 °C. The K2CO3 impregnation resulted in brittle and fragmented TWD char, while it decreased the caking tendency of GG coke particles and decreased the apparent surface fluidity of the TSH coke. Impregnation before devolatilization decreased the fluidlike appearance of the GG coke and decreased the amount and size of the blowholes of the TSH coke. The pore structure of the devolatilized GG particles were mostly influenced by impregnation, showing little to no signs of fusion/caking with smaller internal voids, in comparison to the raw sample. The TWD char was classified as group III char, while GG and TSH were classified as group II and I coke, respectively. Bridging neck size measurements quantitatively indicated that K2CO3 impregnation reduced the bridging neck size of GG coal by up to 50%. The K2CO3 can be used to alter the caking propensity of a moderately fluid coal such as GG, and make it suitable for use in fixed- and fluidized-bed gasifiers. Using an additive such as K2CO3 has utility in expanding gasification feedstock suitability, but reactivity and ash behavior should further be investigated to monitor the progress of this approach