Solid Pyrolysis Modelling by a Lagrangian and Dimensionless Approach--Application to Cellulose Fast Pyrolysis

Abstract

Many kinds of solids (e.g., biomass, thermoplastic and coal) thermally decompose according to similar types of kinetic pathways. They usually include a first step giving rise to more or less stable solid or liquid species followed by competitive reactions with formation of the final products (solids, vapours, and/or gases). In order to be extended to several types of solids, the present pyrolysis model relies on an original dimensionless Lagrangian approach. Mathematical equations of mass and heat balances are written in the assumptions that non-volatiles products form distinct layers which are separated by each other by moving interfaces that propagate towards the inner parts of the sample. The conditions required to apply this Lagrangian approach to solid thermal decomposition are discussed. The time evolution of pyrolysis products and sample internal temperature profiles are obtained by numerical solving of equations written in a reduced form. The first results of simulations performed in a large range of dimensionless parameters values (thermal Thiele, Biot and thermicity criteria) are reported. It is pointed out that variations of reduced masses as a function of reduced time significantly depend on operating conditions (particularly thermal Thiele and Biot numbers). The results of the model are finally compared to experimental data reported in the literature for cellulose fast pyrolysis. The agreement is quite good considering the uncertainties with which the physicochemical parameters are known from the literature.

Characterization of biomass fast pyrolysis

“…In addition, Lédé et al [31] have shown that cellulose kinetic constants can be used for representing the behavior of wood FP. Other published models (see, e.g., [9,12,32,33,60]) report similar qualitative results for the variations of T and T′. Moreover, nondimensional models show that our conclusions are valid for any type of solid undergoing a thermal decomposition.…”

“…They can likely been extended to any other types of processes implying solids thermal decompositions reactions [60].…”

Temperature and heating rate of solid particles undergoing a thermal decomposition. Which criteria for characterizing fast pyrolysis?

“…[2] shows also that any reaction of type (2) gives similar trends, inside extraordinary large domains of operating parameters, natures of solids, as well as values of enthalpies and chemical kinetics constants. These observations are also the same in the case of any solid undergoing more sophisticated kinetics (taking into account secondary reactions) as shown in a non dimensional modeling approach [26].…”

“…In most of practical situations, the reactions with thick particles are carried out in conditions of fixed reactor temperature T H . Such cases have been the subject of numerous works (modelling and experiments) [12,15,16,24,25,26].…”

“…Eq. (26) and (27) show that for given T H and T S , the heating time is proportional to t T and hence to L 0 /h. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 10 agreement with those of Narayan and Antal [17].…”