RELATIVE REACTIVITIES OF COAL BLEND COMPONENTS; PARTIAL COMBUSTION IN A LABORATORY-SCALE SUSPENSION-FIRING REACTOR

摘要

The relative reactivities of coal blend components have been investigated by partial combustion experiments in a novel, laboratory-scale, suspension-firing reactor. The batch of sample is injected into the combustion zone, where particles circulate in a preheated stream of air. The oxygen concentration in the combustion zone has been limited to 3% vol (balance nitrogen) to simulate the environment prevailing in the devolatilisation zone of a pulverised-fuel burner. Devolatilisation takes place in suspension mode, followed by the partial combustion of volatiles and of char particles. Combustion is then interrupted by a flow of nitrogen at pre-set times and unbumed char particles are swept out of the reaction zone, usually several seconds after injection. Extents of reaction of the blends are then compared with those of pure components. Results from the suspension-firing reactor are encouraging, The trends found for coal blend reactivities as a function of blend composition were consistent those previously found in a 135-MW unit at the Tocopilla power station in Chile ― whilst using the same set of coal blend samples. The compositions of residual chars were examined by non-isothermal combustion tests in a thermogravimetric balance. These experiments allowed the quantification of levels of preferential combustion in suspension firing, of the higher-reactivity coal in the blend. Char reactivity appears to be progressively reduced by enrichment with the lower-reactivity component, with obvious implications for carbon bum-out in the tail of a pulverised coal flame. The coupled use of the suspension-firing reactor with non-isothermal thermogravimetric analysis thus promises to provide a powerful tool for assessing the combustion performance of unfamiliar coal blends on commercial scale plant.