Non-catalytic ash effect on char reactivity

This investigation reviews non-catalytic ash effects on char reactivity based on several biomass chars. Biomass ash contents vary substantially among fuel types and are typified by the three fuels compared in this investigation. These fuels have ash-content ratios of about 40:1, which should allow experimental observation of ash effects. In addition, the experiments explore char gasification rates under kinetically controlled conditions that should highlight the effects of ash. By contrast, diffusion controlled burning rates may mask the types of ash effects observed here. The experimental and theoretical results indicate that refractory material such as ash reduces kinetic reaction rates by rendering portions of the surface area inert as char burnout proceeds and ash content increases. Ash also affects particle size in a somewhat compensatory manner. A theoretical model has been developed to quantify the ash effect. This model indicates that all chars from ash-forming fuels should experience similar behavior, which manifests itself as a decrease in the kinetic burning rate in late stages of burnout even though the kinetic reactivity of the char does not change.