Investigation of Coke Quality Variation between Heat-Recovery and Byproduct Cokemaking Technology

Abstract

Coke structural properties that lead to coke quality difference between heat recovery and byproduct cokemaking technology are investigated. Coke fingers collected from two zones (bottom center and top center) of a commercial heat recovery oven were analyzed and compared to the coke produced from the same coal blend using a moveable-wall slot oven. Coke fingers were studied at set intervals, starting from the floor of the heat recovery oven through the top of the bed, and from the slot oven wall to wall. Coke strength after reaction (CSR) test, gas adsorption techniques, optical microscopy, and X-ray diffraction analyses were used to study the coke samples. Although exhibiting lower overall CSR compared to the heat-recovery oven coke, the slot oven produced a more uniform coke quality along the coke cake width, because of the narrow width and bilateral heating of the slot oven. The top-section coke of the heat-recovery oven displayed unique structural characteristics ascribed to the availability of free space atop this coke and the use of radiant heat to drive its coking process. The heat-recovery oven bottom center and slot-oven coke displayed low surface area and total porosity, indicating restrictive coke cake expansion. Limited swelling is attributed to the coal charge weight overlying the heat-recovery oven bottom coke and the constrained nature of the slot-oven chamber. However, the shorter coking time used in the slot oven disadvantageously deprives the slot-oven coke of sufficient time to fully develop its carbon structure; therefore, the heat-recovery oven cokes demonstrated better carbon structural development. For the slot-oven coke, carbon structural development seems to have a stronger impact on CSR than surface area does, while for the heat-recovery oven top center coke, surface area appears to be the parameter that most affects CSR.

Comparison of a laboratory-scale coke and a pilot-scale coke from matched coal

“…Each oven has a different design, resulting in different heating rates during the plastic stage of coking [4]. The heating rate during the formation of the plastic layer influences the development of the resulting coke microstructure [4, 17] and microtexture [18]. The difference in confining pressure also has an impact on coke microstructure development [7], and the combination of these parameters influence the coke strength after reaction (CSR) value of the coke generated [4].…”

“…The majority of studies in the literature have demonstrated that their laboratory-scale ovens are able to produce replicate cokes to those generated in pilot-scale ovens, with the CSR of the cokes produced matching (within measurement error) their pilot-scale counterparts [1–3, 5]. Other studies echo the common knowledge that the CSR of a coke is strongly dependent on coking conditions [4, 18], and suggest that given the dependence of heating rate on coke oven design, it is not straightforward to produce a small-scale coke with an analogous CSR to the equivalent pilot-scale coke.…”