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The swelling propensity of some coals may restrict their use in fixed- and fluidized-bed gasification operations, and effective reduction of swelling can widen the applicability of these coals. Extensive research has been published on the influence of additives on the swelling of pulverized coal (<500 μm), but limited knowledge exists on the influence of additives on large coal particle swelling behavior. This paper presents an investigation on the influence of K2CO3 on the extent of swelling of 5, 10, and 20 mm particles, which are suitable sizes for fixed- and fluidized-bed operations. Three South African coals were selected: TSH coal (FSI 9), GG coal (FSI 5.5–6.5), and TWD coal (FSI 0). Large coal particles (5, 10, and 20 mm) were impregnated with a 5.0 M K2CO3 solution, and the K-loading increased by a factor 9–33. Maximum K-loadings of 3.3, 3.0, and 1.4 wt % K (coal basis) were obtained for the 5, 10, and 20 mm particles, respectively. The volumetric swelling ratio (SRV) of the 20 mm GG particles was reduced from 3.0 to 1.8, and for the TSH particles from 3.1 to 2.1. The TWD coal particles showed SRV values up to 1.7, in contrast to the nonswelling behavior of the pulverized (−212 μm) coal, and were not influenced by the K2CO3 addition. It was concluded that K2CO3 only influences the volumetric transformation of coals that undergo significant plastic deformation, such as GG and TSH. Comparison of the maximum swelling coefficients showed that K2CO3 impregnation reduces the k A from 0.025 to 0.015 °C–1 for GG and from 0.013 to 0.009 °C–1 for TSH. The results show the viability of using an additive for swelling reduction of large coal particles, and together with further development, may be a suitable method for reducing unwanted swelling in relevant coal utilization processes.

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 fluidizedbed 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 K 2 CO 3 addition. Particles were impregnated with an excess 5.0 M K 2 CO 3 solution, and devolatilized in batch samples to 700°C. The K 2 CO 3 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 K 2 CO 3 impregnation reduced the bridging neck size of GG coal by up to 50%. The K 2 CO 3 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 K 2 CO 3 has utility in expanding gasification feedstock suitability, but reactivity and ash behavior should further be investigated to monitor the progress of this approach.

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