K. Østergaard

High-temperature reaction between sulphur dioxide and limestone. V: The effect of periodically changing oxidizing and reducing conditions

Abstract Sulphur capture by limestone has been studied in a laboratory reactor developed to simulate the periodically changing oxidizing and reducing conditions experienced by limestone particles in a fluidized-bed combustor. Under oxidizing conditions, sulphur is captures ad CaSO 4 . Under reducing conditions, and in the presence of CO, sulphur is captured as CaS. Transformation of CaSO 4 to CaS and vice versa appears to proceed via CaO. Substituting CO with H 2 reducing agent causes an increase in the rate of reductive decomposition of CaSO 4 , and no formation of CaS is observed. Using CH 4 neither reductive decomposition of CaSO 4 nor formation of CaS is observed. The sulphur capacity of 14 European limestones was studied under constant oxidizing as well as under alternating oxidizing and reducing conditions. The relative ranking of the limestones appears to be little influenced by the reaction conditions. Generally, a slight reduction in the sulphur capacity is observed under alternating conditions. The exceptions are limestones with a high content of Fe 2 O 3 , which lowers the sulphur capacity significantly, presumably due to reduced stability of the sulphated limestone under reducing conditions. Rates of CaS formation and of reductive decomposition of CaSO 4 differ greatly for different limestones. Reduction of particle size increases the SO 2 release due to CaS oxidation but decreases the release of SO 2 due to reductive decomposition. Both CaS oxidation and reductive decomposition of CaSO 4 may lead to a diminished degree of desulphurization in real combustors. A temperature optimum observed for desulphurization in fluidized-bed combustors appears to be caused primarily by the competition between sulphur capture and sulphur release, the latter of which becomes increasingly important at high temperatures.

High-temperature reaction between sulphur dioxide and limestone—I. Comparison of limestones in two laboratory reactors and a pilot plant

Abstract Twenty-three different limestones were studied with respect to their capacity for reaction with sulphur dioxide, and were found to differ markedly. Geologically young limestones have the highest capacity, and geological old limestones the lowest. The presence of ferric oxide affected the sulphur dioxide capacity of limestones adversely, but otherwise no general relationship was observed between chemical composition and sulphur dioxide capacity. A negative correlation exists between the time required for calcination and the sulphur dioxide capacity. Experiments were carried out in three different reactor systems: a laboratory recycle reactor, a laboratory fluidized-bed reactor, and a coal-fired fluidized-bed pilot plant. Qualitatively, the rankings of limestones with respect to sulphur dioxide capacity were identical for the three reactor systems, and laboratory experiments may thus be used for the determination of such a ranking. The pilot plant was less efficient with respect to limestone utilization than the other reactor systems, elutriation from the bed or locally reducing conditions in the bed being assumed to be the major cause of this difference.

High-temperature reaction between sulphur dioxide and limestone—III. A grain-micrograin model and its verification

Abstract A modified grain model of the reaction between calcined chalk, sulphur dioxide and oxygen has been developed, and the model has been verified by comparison with a large volume of experimental data. A chalk particle is constituted of grains that are non-porous in the uncalcined state. The pore volume of calcined chalk is distributed between macropores corresponding to the interstices between grains and micropores formed in the grains during calcination. The porous grains are assumed to be constituted of non-porous micrograins. Mass transfer in the pores takes place by molecular diffusion and Knudsen diffusion, and micrograins react with sulphur dioxide and oxygen according to a shrinking, unreacted-core mechanism. Since calcium sulphate formed by the reaction has a significantly higher molar volume than calcium oxide, micrograins will grow in volume with increasing degree of sulphation, eventually filling the micropores at a degree of sulphation of approximately 50%. Further reaction in the grains takes place according to a shrinking, partially-reacted-core mechanism, accompanied by an increase in grain volume. The only unknown parameters in the model are the tortuosity factor and the diffusion coefficient in the solid product layer encasing micrograins and grains.

The use of residence time distribution data for estimation of parameters in the axial dispersion model

Abstract In this paper are presented four convenient methods for determination of the mean residence time and the axial dispersion coefficient of a flow system by analysis of data obtained by means of the imperfect tracer pulse method. The analysis is based upon numerical evaluation of the transfer function and its derivatives for a number of values of the Laplace transform parameter, Sτ .

High-temperature reaction between sulphur dioxide and limestone—II. An improved experimental basis for a mathematical model

Abstract Most experimental data available in the literature on the high-temperature reaction between sulphur dioxide and limestone have been obtained in the form of an average degree of conversion (sulphation) of the solid limestone particle. Such data have been widely used as a basis for mathematical models of the reaction. It is, however, well known that a sulphated limestone particle is characterized by a highly non-uniform intraparticle conversion profile. Information on the intraparticle reaction is therefore desirable in order to improve the basis for mathematical models. Such information is presented in the following for a limestone constituted of non-porous grains. The product of the sulphation reaction under oxidizing conditions is demonstrated to be anhydrite II, not anhydrite III as commonly assumed in the literature. These two forms of calcium sulphate are of different molar volume, an important parameter in any detailed mathematical model. Pore size distributions have been determined before and after calcination and after sulphation. Calcination causes formation of a micropore structure and development of a bimodal pore size distribution. During sulphation the micropores become filled, or their entrances become blocked, but the macropore structure present in the uncalcined material remains. Intraparticle conversion profiles were determined in sulphated particles by energy-dispersive X-ray analysis. The degree of conversion decreases linearly from the particle surface inwards to a certain distance from the surface where it decreases to zero, forming a sharp front that moves inwards with increasing reaction time.

High-temperature reaction between sulphur dioxide and limestone—IV. A discussion of chemical reaction mechanisms and kinetics

Abstract A model describing the chemical kinetics of the sulphation of calcium oxide has been developed in terms of elementary chemical reaction mechanisms. The model predicts that the rate-determining reaction at low tempeatures is the disproportionation of calcium sulphite and that the rate-determining reaction at high temperatures is the direct oxidation of calcium sulphite and/or the reaction between calcium oxide and sulphur trioxide. The model is in good agreement with the limited amount of experimental measurements of initial rates of the sulphation reaction available in the literature.

The effect of particle size and bed height on the expansion of mixed phase (gas—liquid) fluidized beds

Abstract The expansion of gas—liquid fluidized beds has been measured for beds of glass ballotini fluidized by air/water. Particle diameters varied from 0.28 to 2.2 mm, and settled bed heights from 14–107 cm. The flow rates of air and water were varied independently. The expansion of a liquid fluidized bed is often reduced when a swarm of gas bubbles is injected into the bed. This bed contraction is largest for beds of small particles; a contraction of 48 per cent has been observed in a highly expanded bed of 0.28 mm ballotini. For small particles the largest contraction was observed in beds of high height to diameter ratio. For larger particles the contraction is less pronounced, and no significant effect of the ratio between bed height and diameter was observed.

An investigation of mass transfer in a countercurrent three-phase fluidized bed

The rate of absorption with chemical reaction has been measured in a bed of solid spheres fluidized by upward flowing gas and irrigated by downward flowing liquid for different reactant concentrations in the liquid phase and different values of the liquid superficial velocity. Values of the effective inerfacial area per unit volume of a static bed, a, and the gas-film mass-transfer coefficient, kg, are reported as functions of the liquid superficial velocity.

The thermal instability of solid catalysts and the temperature dependence of heterogeneous catalytic process rates

Abstract The influence of simultaneous heat and mass transfer upon the rate of a heterogeneous catalytic process has been analysed with particular regard to the effect upon the temperature dependence of the process rate. The results are shown in dimensionless Arrhenius diagrams. These diagrams differ very markedly from diagrams for isothermal catalyst particles. The catalyst may exist in several steady states, the apparent activation energy may assume infinite values, and small changes in temperature may result in very significant changes in the process rate and in the apparent activation energy. It is suggested that “hot spots” in catalytic reactors can be caused by such effects. The effects are more pronounced for a zero-order reaction in flat catalyst plates than for a first-order reaction in spherical catalyst particles.