W. Prins

Flash pyrolysis kinetics of pine wood

The kinetics of sawdust pyrolysis in the temperature range from 300 to 600 °C has been measured. A thermogravimetric analyser was applied for the temperature range from 300 to 450 °C while for measurements in the temperature range from 450 to 600 °C, an entrained flow reactor has been used. The kinetic expression that describes the mass loss of sawdust due to pyrolysis is assumed to be of a single first-order in the unconverted wood. The first-order rate constant obtained from measurements in both the thermogravimetric analyser and the entrained flow reactor can be described by an Arrhenius equation with k0 = 1.4 1010 kg.kg?1.s?1 and Ea = 150 KJ.mol?1.

Pyrolysis of biomass in the rotating cone reactor: modelling and experimental justification

In the rotating cone reactor, wood particles fed to the bottom of the rotating cone, together with an excess of inert heat carrier particles, are converted while being transported spirally upwards along the cone wall. The cone geometry is specified by a top angle of π/2 radians and a maximum diameter of 650 mm. Products obtained from the flash pyrolysis of wood dust in a rotating cone reactor are non-condensable gases, bio-oil and char. This paper reports on results of computations and measurements to determine the influence of process parameters like the cone rotational speed (6–15 Hz), the reactor volume (3–200 l), the wood-dust feed rate (1–3.5 g s−1) and the reactor temperature (550−700°C) on the product composition. The experimental results are compared with predictions of an integrated reactor model which accounts for: (i) the type of particle flow in the reactor; (ii) the wood decomposition kinetics; (iii) the rate of heat transfer to the wood particles; (iv) the kinetics of gas phase reaction (tar cracking); and (v) gas exchange with the space in which ash, char and partially unconverted wood is collected. For the conditions applied, the difference between predicted and measured weight fractions of gas, tar and char produced was always less than 10%. If further appeared that the wood particles were always completely converted inside the reactor and that the product distribution is only affected by the gas-phase reaction kinetics and residence time. The gas-phase residence time is determined by the available reactor volume and the feed rate of the wood particles. At optimal reactor conditions, the tar yield is almost maximal (70% d.a.f. wood base).

Platinum catalyzed oxidation of carbon monoxide as a model reaction in mass transfer measurements

The oxidation of CO with oxygen over a Pt/γ-alumina catalyst is proposed as a model reaction to be used for the determination of mass transfer coefficients in packed and fluidized beds. It is applicable at relatively low temperatures (<800 K) and for very small particles (<100 μm). In the present work, the kinetics of this reaction have been verified in a small fixed bed facility (average particle diameter 54 μm), for various reactant concentrations, temperatures and superficial gas velocities. As a result, Langmuir–Hinshelwood kinetics (Ea=75.4 kJ mol-1) appeared to describe the experimental results better than a power law expression (Ea=90.6 kJ mol-1). Three temperature regimes can be identified upon interpretation of experimental results with a suitable single particle model: a reaction rate controlled regime (I) at relatively low temperatures, characterized by a reaction order for oxygen of plus one and a carbon monoxide reaction order of minus one, an intermediate temperature interval (regime II) for which the reactions rate is influenced by both mass transfer and kinetics, and where the apparent reaction order in CO and O2 change to values lower than minus one and higher than one, respectively, and the high temperature regime (III) where mass transfer resistances are dominant, and the apparent reaction orders in O2 and CO are changed to values of zero and plus one, respectively. In case of carbon monoxide oxidation over a platinum catalyst, the observed orders in O2 and CO provide an extra instrument to recognize the prevailing conversion rate controlling phenomenon, apart from known indicators like the observed activation energy value or the influence of hydrodynamic conditions. As a consequence, the reliability of mass transfer measurements is significantly improved. This has been verified by the application in mass transfer measurements for a packed bed and for a riser system.

Mass transfer from a freely moving single sphere to the dense phase of a gas fluidized bed of inert particles

Naphthalene spheres (particle diameter 2 < dn < 20 mm) were vaporized in beds fluidized by air at a temperature of about 65°C. The bed material consisted of inert glass beads or alumina in the size range 100 < dp < 700 μm. Mass transfer coefficients were measured by determining weight loss with time, as a function of dn, dp and the fluidization velocity U. The ratio dn/dp has been varied from 3 to 200. An interesting conclusion might be that there is no influence of the fluidization velocity on these transfer coefficients; they only depend on the minimum fluidization velocity Umf. The empirical correlation mf(jD)mfRemfm=0.105 + 1.505 (dn/dp)−1.05 with m = 0.35 + 0.29 (dn/dp)−0.50, as a best fit of all the results, is accurate within 15%. At dn/dp = 1 it links up very well with the results of Hsiung and Thodos. For large values of dn/dp agreement with known results of mass transfer measurements between a fluidized bed and a wall or an object is good.

Devolatilization and ignition of coal particles in a two-dimensional fluidized bed

In a two-dimensional (15 × 200 × 400 mm) high-temperature fluidized bed, devolatilization ignition and combustion phenomena of single coal particles have been studied. The particles, with diameters of 4–9 mm, were selected from three coal types of widely different rank: brown coal, bituminous coal, and anthracite. The bed consisted in most cases of porous alumina particles (0.6 mm diameter), and was fluidized by O2/N2 gas mixtures. At constant bed temperatures ranging from 200 to 850°C, the various stages prior to the eventual combustion of the residual char particle were recorded on videotape. This paper gives an account of visual observations on the release, ignition, and combustion of volatiles as well as on the ignition of char. Results of measurements of the temperature and delay time of both volatiles and char ignition are also reported. Finally, the period over which flames of volatiles are visible in the bed has been measured for each coal particle; at sufficiently high bed temperature they are indicative for the total devolatilization time.

Principles of a novel multistage circulating fluidized bed reactor for biomass gasification

In this paper a novel multistage circulating fluidized bed reactor has been introduced. The riser of this multistage circulating fluidized bed consists of several segments (seven in the base-case design) in series each built-up out of two opposite cones. Due to the specific shape, a fluidized bed arises in the bottom cone of each riser segment. Back-mixing of gas and solids between the segments is prevented effectively. The absence of back-mixing combined with the enlarged solids residence time in each segment (each segment is a fluidized bed) creates the opportunity to operate, spatially divided, separate process steps in a single reactor. The benefit of a concept in which different processes are carried out in separate segments of the same reactor has been demonstrated for the specific case of biomass gasification. In the novel reactor it is possible to create oxidation segments in which O2 reacts exclusively with char (carbon). This results in an increased carbon conversion and consequently improved gasification efficiency. Creating an exclusive char combustion zone, aimed at improving both the carbon conversion and the thermal efficiency, has also been applied successfully in a conventional CFB biomass gasifier (ECNs CFB 100 kg wood/h) by building a flow restriction in the riser between the primary air nozzles and the biomass feed point.

The reductive decomposition of calcium sulphate I. Kinetics of the apparent solid-solid reaction

The reductive decomposition of calcium sulphate by hydrogen is used for the regeneration of calcium-based atmospheric fluidized bed combustion (AFBC) SO2 sorbents. The apparent solid?solid reaction between CaS and CaSO4, one of the steps involved in the reaction mechanism of the reductive decomposition of CaSO4, has been investigated in a thermobalance construction provided with a coupled system for the analysis of product gases. By creating suitable reaction conditions in the experimental set-up, it is possible to study this step separately. The apparent solid?solid reaction between CaS and CaSO4 takes place when a mixture of these compounds is heated in an inert atmosphere to temperatures above 1100 K: 3/4 CaSO4 + 1/4CaS - CaO + SO2 The fractional production rate of CaO (or SO2) appears to depend on the ratio in which CaSO4 and CaS are present initially. A simple model is formulated to calculate the fractional production rate as a function of the fractional degree of production and the initial composition of the reactants. One of the principal assumptions of the model is the presence of a liquid phase formed by mixtures of CaS and CaSO4. Fractional production rates calculated after determining the required model parameters (kinetic constants and volume plus composition of the liquid phase) show close agreement with experimentally obtained values.

A grain size distribution model for non-catalytic gas-solid reactions

A new model to describe the non-catalytic conversion of a solid by a reactant gas is proposed. This so-called grain size distribution (GSD) model presumes the porous particle to be a collection of grains of various sizes. The size distribution of the grains is derived from mercury porosimetry measurements. The measured pore size distribution is converted into a grain size distribution through a so-called pore-tosphere factor whose value is also derived from the porosimetry measurements. The grains are divided into a number of size classes. For each class the conversion rate is calculated either according to the shrinking core model, involving core reaction and product layer diffusion as rate-determining steps or according to a new model in which some reaction at the grain surface is assumed to be limiting. The GSD model accounts for the phenomenon of pore blocking by calculating the maximum attainable conversion degree for each size class. In order to verify the model, two types of precalcined limestone particles with quite different microstructures were sulphided as well as sulphated. Furthermore, a single sample of sulphided dolomite was regenerated with a mixture of carbon dioxide and steam. For each reaction good agreement was attained between measured and simulated conversion vs. time behaviour.

Performance of silica-supported copper oxide sorbents for SOx/NOx-removal from flue gas: II. Selective catalytic reduction of nitric oxide by ammonia

The selective catalytic reduction (SCR) of nitric oxide by ammonia was studied for silica-supported copper oxide particles to be used as a sorbent/catalyst in a continuous process for the simultaneous removal of SOx and NOx from flue gases. The SCR-behaviour was determined as a function of the sulphation degree, i.e. the fraction of copper oxide converted to copper sulphate, at temperatures ranging from 20 to 450°C. Up to 350°C, the fresh catalyst with 0% CuSO4 showed a high selectivity towards production of nitrogen and water by the reaction of nitric oxide with ammonia and oxygen. At higher temperature, nitric oxide removal efficiencies decreased due to the oxidation of ammonia by oxygen. With an increase of the sulphation degree, the maximum temperature for selective catalytic reduction of nitric oxide gradually increased up to 420°C for a sulphation degree of 80%. In addition, the maximum nitric oxide removal efficiency increased as well. After regeneration of catalyst particles with a sulphation degree of 80%, realised by reduction with hydrogen and subsequent re-oxidation, the catalytic behaviour was similar to that of fresh catalyst particles with a sulphation degree of 5%. This is ascribed to the formation of some Cu2S during the reduction, which is oxidised to CuSO4 in the subsequent oxidation step. Since the selectivity towards the reduction of nitric oxide with ammonia is maintained up to about 375°C, a temperature which is very suitable for SOx removal as well, the silica-supported CuO investigated can be applied as a sorbent/catalyst for the simultaneous removal of SOx and NOx from flue gases. The reaction rate constants for SOx and NOx removal appeared to be of the same order of magnitude provided that the reduced sorbent/catalyst enters the absorber directly, i.e. without a separate pre-oxidation.

Performance of silica-supported copper oxide sorbents for SOx/NOx-removal from flue gas I. Sulphur dioxide absorption and regeneration kinetics

Sulphur dioxide absorption and regeneration kinetics of several silica-supported copper oxide (CuO) sorbents were studied in a microbalance over a temperature range of 300 to 450°C. The porous silica support was prepared according to a sol-gel technique, and CuO was deposited on this support through an ion-exchange technique to achieve a uniform, highly dispersed CuO deposition. During up to 75 cycles of oxidation, sulphation, and reduction, the ion-exchanged sorbents did not show a significant loss in chemical activity except for some deactivation in the first 1?3 cycles. The sulphation kinetics of the pre-oxidised ion-exchanged sorbents were found to be in agreement with literature data for impregnated alumina-supported CuO sorbents. In case of direct contact of reduced ion-exchanged sorbents with simulated flue gas, the simultaneous and fast oxidation was determined to have a large positive effect on the sulphation rate up to approximately 60% conversion to copper sulphate. This was mainly attributed to structural effects inside the CuO deposits. For the sulphated ion-exchanged sorbents, the reduction by hydrogen was identified as an autocatalytic reaction. The autocatalytic effect was also observed during the (much slower) reduction by methane, but there it was preceded by a period in which a second autocatalytic effect appeared. The reaction kinetics of the ion-exchanged sorbents developed were furthermore compared with experimental results of other silica-supported CuO sorbents prepared by vacuum impregnation and homogeneous deposition-precipitation.