Structure-Activity Relationships in Biomass Chars Formed in An Entrained-Flow Reactor
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Pine and switchgrass (sieved to 180-250 µm) were pyrolyzed in an entrained-flow reactor at high temperatures (600-1000 °C) and high pressures (1-20 bar). Heating rates as high as 103-104 K/s were achieved with solids residence time from 3-40 s. Morphological and chemical changes in these chars are expected to simulate those in industrial reactors. Thus , the gasification reactivity of these chars can be used to model commercial gasifier operations. Morphological characterization of chars was performed using SEM , N2 and CO2 adsorption. CO2 adsorption was found to measure accurately the total micropore area , which is critical in gasification reactivity measurements. Ultimate and ICP-MS analyses were used to determine the elemental composition. Gasification reactivity of char was measured using TGA at various partial pressures of CO2 , under conditions free of any internal and external mass transport limitations. Initial gasification reactivity of chars was found to decrease with an increase in pyrolysis temperature and pressure. Although a correlation between surface area as well as ash content with reactivity is observed , it is inadequate to explain the reactivity changes. This is because the drop in reactivity with increase in temperature was much more pronounced than the decrease in surface area or ash content measured over the same temperature range. The intrinsic reactivity may also be impacted by the type of carbon (carbidic vs graphitic) in the char matrix and the active catalytic sites in the char. The type of carbon structure and active sites evolve over the entire conversion range making the reactivity versus conversion profile wavy in appearance. Attempts have been made to explain the reactivity profiles using Avicel char. Chars were ground to various particle sizes and their reactivity was measured using TGA. The initial reactivity of chars of different particle sizes correlates well with its ash content. This reveals that ash is heterogeneously distributed in the carbon matrix of char. It also reveals that the conventional procedure to measure the pore-diffusion effects by crushing chars can provide misleading results because of the changes in intrinsic reactivity caused by heterogeneous distribution of inorganic content within the different sized char particles. It can be thus seen , that the intrinsic char gasification reactivity is a complex function of the type of carbon , the ash content , ash composition and ash distribution in carbon matrix over the course of the reaction.