Primary and secondary reactions of cellulose pyrolysis are modeled through a semi-global, multi-step scheme based on lumping the different products into three groups: gas, tar and char. The description of chemical processes is coupled to an unsteady, one-dimensional, variable property model of transport phenomena including heat convection, conduction and radiation, volatile tar and gas transport
... [Show full abstract] by diffusion and convection and momentum transfer (reference model). In order to understand the role played by assumptions, usually made in the mathematical description of large sample pyrolysis, models of different complexity are compared. The following assumptions (and simplified models) are considered: (a) constant pressure, (b) constant properties, (c) quasi-steady gas-phase processes, and (d) liquid-phase tar species. Simulations indicate that, for typical cellulosic material permeabilities, product distribution and process dynamics are not significantly influenced by pressure variations and assumption (a) can be made. The assumption of constant properties does not affect predictions qualitatively, however, quantitative differences become large at high reaction temperatures. In general, even qualitative agreement is lost for the predictions of the quasi-steady and the liquid-phase tar models. These assumptions could be made only under conditions of negligible secondary reaction rates (very small samples and/or very low reaction temperatures).