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FUSION-LIKE BEHAVIOUR OF BIOMASS PYROLYSIS Jacques LEDE, Huai Zhi LI and Jacques VILLERMAUX
Laboratoire des Sciences du Genie Chimique, CNRS-ENSIC 1, rue Grandville 54042 NANCY (FRANCE)
Considering a thermal reaction of a Solid +Fluid type, the appa- rent rate of reaction can be controlled by chemistry, thermal and mass trans- fer resistances. If the chemical processes are very fast, and if the fluid products are easily eliminated from the medium, the overall rate of reaction is controlled by heat transfer resistances. This is the case of the ablation regime [I], characterized by a steep temperature gradient at the wood sur- face and consequently by a thin superficial layer e of reacting solid moving a t a constant velocity v towards the cold unreacted parts of the solid.
Suppose now that heat is provided by a surface at TW.A theoretical increase of the surface temperature Td of the solid (by increa- sing TW)would lead to a subsequent increase of the heat flux demand. Such a demand would be satisfied by an equal external heat flux supply, a condition fullfilled only with large temperature gradients (TW- Td). The consequence would be a stagnation of Td, leading to a fusion like behaviour of,the reac- tion.
Wood pyrolysis carried out in conditions of high available heat fluxes and efficient elimination of products occurs in ablation regime with production of very low fractions of char [2,3,4] and could therefore behave as a simple fusion. This paper presents a brief outline of the main ideas and results obtained to this effect and issued from different approaches. More details can be found in related papers c5,6,7,8].
The reaction has been carried out i n three different conditions : heating against a hot spinning disk ; against a fixed heated surface ; i n a continuous cyclone reactor. In the f i r s t two cases, the behaviour of the
reaction is compared t o that of solids undergoing simple fusion i n the same conditions.
SPINNING DISK EXPERIMENTS
The melting of ice, paraffin and "rilsan" (polyamide 11) and the pyrolysis of wood have been. carried out by applying under known pressures p, rods of the corresponding solids against a hot spinning stainless steel disk
(-temperature Tw) [5,6]. I n wood experiments, the reaction produces almost exclusively gases and liquids, the solids being mainly ashes deposited on
the disk. Thqliquids produced are rapidly extracted from the wood surface and eliminated by the fast moving disk on which they undergo further decompo- sition to gases at a rate depending on Tw. The presence of the thin liquid layer acts as a kind of lubricant.
Figure 1 reveals that under comparable values of p, the behaviour of v as a function of vR is similar, the orders of magnitude of v being the same for the four types of solids. For v > 2 m s-I, v increases with p following :
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