Jacques Lédé

Radiant flash pyrolysis of cellulose—Evidence for the formation of short life time intermediate liquid species

Abstract This paper reports the first results of experiments and modelling of the radiant flash pyrolysis of cellulose. Small samples are exposed to brief flashes of a concentrated radiation, at the focus of an image furnace operating with a 5 kW xenon lamp associated to two elliptical mirrors. The mean heat flux densities may be higher than 107 W m−2. The microscopic observations of the sample after the flash reveal the presence of short life time liquid species formed for flash durations lower than about 1 s. These products which are liquid in pyrolysis conditions are solid at room temperature, where they show a good stability. They are soluble in water. For longer flashes, they give rise to vapours escaping in the gas phase, while practically no char is formed. These results show that, if in biomass pyrolysis, lignin is known to give rise to a liquid phase, it is also the case for cellulose. A first simple modelling of these experiments is proposed. It relies on heat and mass balances at the sample level, on the Broido–Shafizadeh (BS) model and on experimentally estimated values of some of the optical characteristics of cellulose (reflectivity and absorptivity). Indeed, cellulose is a highly reflecting and weakly absorbing (semi-transparent) material. These properties must be necessarily taken into account in any predictive calculation (only a small fraction of the incoming flux is effectively absorbed by cellulose). The calculated values of the times of beginning and end of the reaction are compared with the results of the experiments. The good agreement confirms that the intermediate products have life times shorter than about 1 s at the reaction temperature, predicted to be close to 750 K.

Flash pyrolysis of cellulose pellets submitted to a concentrated radiation: experiments and modelling

Abstract The image furnace technology has been applied to the study of the first steps of biomass flash pyrolysis. The experiments performed with small pellets of cellulose show that the reaction primarily passes through the intermediate of short lifetime liquid species (ILC). The quantitative study of the variations of the sample mass loss and of the mass of ILC reveals the existence of a transient period followed by a steady-state regime resulting from an equilibrium between cellulose decomposition into ILC and ILC vaporization. A mathematical model has been solved in parallel. The results agree very well with the experimental measurements and yield additional information on the temperatures of cellulose pyrolysis and of ILC vaporization.

Fast pyrolysis of wood: direct measurement and study of ablation rate☆

Abstract Quantitative measurements of the apparent rate of reaction of wood rods undergoing fast pyrolysis by contact with a hot spinning steel disc are reported. The variations in the apparent volatilization rate and in the thickness of the reacting layer were studied as a function of disc velocity, rod diameter, contact pressure and disc temperature. The results show that the rate of reaction is mainly limited by heat transfer outside and inside the wood (ablation conditions). The external heat transfer coefficient, studied as a function of contact pressure, is much more important than the heat transfer coefficient calculated on the basis of simple radiation transfer. The conclusion is that fast pyrolysis of wood in the ablation regime can be observed if two necessary conditions are fulfilled : high available heat flux and efficient removal of the primary products of the reaction.

SOLAR THERMOCHEMICAL CONVERSION OF BIOMASS

Abstract The purpose of this paper is first to briefly describe the usual routes of biomass thermochemical conversion and then to discuss the possibility of using concentrated solar energy to provide the necessary heat for the processes. Gasification, fast and slow pyrolysis are more particularly described. They can be carried out for the preparation of a vast range of possible products that can be used as energy carriers and/or as a source of chemical commodities. The gasification processes are intended for the preparation of gas mixtures (CO, H 2 , etc.) for chemical synthesis, heat or electricity generation. The fast pyrolysis formerly carried out for gas production (CO, H 2 , light hydrocarbons, etc.) is now mainly studied with the objective to produce liquids (bio-oils). Slow pyrolysis is in use for a long time for the preparation of solids (charcoal). The nature and quality of the products depend mainly on the experimental conditions of the process (temperature, heating rates, residence times, etc.). The possibility of a solar entry in the gasification and pyrolysis processes is then discussed. The technical and scientific benefits, as well as the difficulties, are underlined, showing the necessity to design new types of specific reactors. From a fundamental point of view the advantages are also underlined of using a concentrated radiation as a laboratory tool for studying the very fast primary steps of biomass thermal decomposition as well as the possible existence of intermediate short life time species that are still not well known.

A new model for thermal volatilization of solid particles undergoing fast pyrolysis

A new model is proposed for representing the thermal volatilization of solid particles under the conditions of fast pyrolysis. The solid, initially at low temperature, is suddenly immersed into a high-temperature medium where it decomposes to gaseous products, the external surface of the particle being exposed to a high heat flux from the medium. As heat penetrates into the solid by conduction, the decomposition reaction occurs at a rate which increases with temperature. Equations for simultaneous heat and mass balances are derived and numerically solved, yielding the internal temperature profile and the rate of shrinkage of the particle, from which the time for total consumption is easily deduced. It is shown that the regime of volatilization depends on two parameters: a thermal Thiele and a thermal Biot modulus. The ablation regime is achieved if both M and B are large (⪢ 100). In this regime, the shrinking velocity is constant and the reaction takes place only in a thin layer at the solid surface.

Fusion-like behaviour of wood pyrolysis

Abstract The results of an experimental study of the melting rates of rods of ice, paraffin and Rilsan (polyamide 11) pressed against a spinning hot disk are reported. The observations are similar to those made with wood rods, the “fusion temperature” of which has been found close to 739 K. These results are in agreement with a theoretical study of the competition between the kinetics of thermal wood decomposition and heat transfer resistances inside and outside the wood. A second conclusion is that because of the heat fluxes that can be transferred in most of the experimental devices, it seems almost impossible to raise wood to temperatures greater than about 800 K; the direct experimental determination of wood pyrolysis reaction rate constants at temperatures above 800 K then appears impossible.

Comparison of contact and radiant ablative pyrolysis of biomass

Abstract Ablation characterizes the phenomena occurring when a solid, submitted to a high external heat flux density, gives rise to solids, liquids and/or gases that can be rapidly and continuously eliminated. Ablation can be exploited for carrying out the fast pyrolysis of materials such as biomass. This paper describes and compares, on a fundamental point of view, two methods of biomass ablative pyrolysis. In the first one, biomass is pressed against a hot surface (contact ablative pyrolysis). In the second one, biomass intercepts a concentrated radiation (radiant ablative pyrolysis). The comparison is made on the basis of the values of ablation thickness and velocity (derived from experiments and modelling), and of product fractions and compositions. The results can be very different in spite of the fact that biomass may be subjected to similar heat flux densities in both cases. The paper shows the advantages, drawbacks and complementarities of each technology.

Production of hydrogen by direct thermal decomposition of water

Abstract This paper reports the first results of a study on the feasibility of producing hydrogen by direct thermal splitting of water by use of concentrated radiation. Relative amounts of H, O, OH, H 2 , O 2 and H 2 O have been computed between 1500 and 4000 K at thermodynamic equilibrium. In addition, the rate of the overall reaction has been estimated assuming a kinetic model: 90% of equilibrium concentrations are reached after about 10 −2 , 10 −3 and 10 −4 s at 2200, 2500 and 3000 K respectively. The dissociation experiments have been carried out in an image furnace, simulating the future use of a solar furnace. Water is injected through a zirconia nozzle heated at the focus. The hot jet containing active species is then quenched by turbulent cold jets in order to minimize recombinations. Several shapes of zirconia nozzles and quenching devices have been tested in order to maximize the net production of hydrogen, which reaches 1.7 STP l. h −1 in continuous operation. Modelling of the hot nozzle allows the calculation of the gas temperature and shows that thermodynamic equilibrium conditions are reached under certain conditions. Parallel experiments show that cooling rates up to 10 6 Ks −1 can be obtained by quenching.

Flash pyrolysis of wood in a cyclone reactor

Abstract This paper reports the first results of an experimental study of the continuous flash pyrolysis of wood sawdust in a cyclone reactor between 893 and 1330 K. The reaction produces low fractions of char (4%) and the gasification yield increases from 0% at about 800 K to 90% at around 1330 K with a constant volume fraction of CO and H2 (≈73%) and an increasing fraction of light hydrocarbons (up to 50% mass fraction). The heating value of the gas reaches 19 000 kJ m−3 STP for the highest temperatures. The wood particles mainly heated by radiation and solid convection react in less than 1 s while the carrier gas (residence time of the order of 0.05 s) seems to be only weakly heated. The 46.2 × 10−6 m3 cyclone reactor can operate with excellent stability for wood flow rates up to 0.35 kg h−1 at a wall temperature of 1330 K. The cyclone seems to be very efficient for carrying out reactions of the solid → fluids type but more accurate determination of process parameters such as gas and solid residence times and heat transfer efficiencies are required to gain a better understanding of the behaviour of such a high temperature reactor.

Ablative melting of a solid cylinder perpendicularly pressed against a heated wall

Abstract This paper reports an analytical and experimental study on ablative melting of a solid cylinder perpendicularly pressed against a stationary heated surface. An explicit analytic solution is found for the rate of ablation in terms of temperature difference and pressure applied and of geometrical as well as physical properties of the solid and liquid. Data obtained in a limited number of rather crude experiments with rods of melting solids (ice, paraffin) and with rods of wood under flash pyrolysis conditions show a fair agreement with the predictions of the theoretical study thus confirming the “fusion model” of flash pyrolysis of wood in ablation regime.