John R. Grace

Biomass gasification in a circulating fluidized bed

Abstract This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45 kg/h . Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.

Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier

Abstract A non-stoichiometric equilibrium model based on free energy minimization is developed to predict the performance of gasifiers. The model considers five elements and 44 species in both the gas and solid phases. The gas composition and heating values vary primarily with temperature and the relative abundance of key elements, especially carbon, hydrogen and oxygen. Pressure only influences the result significantly over a limited temperature range. The model predicts the onset of formation of solid carbon where the gas composition becomes insensitive to additional carbon. The carbon formation boundary is plotted in C–H–O ternary diagrams as a function of temperature and pressure. When the experimental carbon conversion is introduced, the kinetically modified equilibrium model gives good predictions of the gas composition from an air-blown pressurized circulating fluidized bed gasifier for two coals. The role of water, including both fuel moisture and steam injection, is examined based on a water balance on the feed and product gas to evaluate the steam demand.

A state-of-the-art review of gas–solid turbulent fluidization

Abstract Turbulent fluidization has only been widely recognized as a distinct flow regime for the past two decades, even though it is commonly utilized in industrial fluidized-bed reactors due to vigorous gas–solids contacting, favourable bed-to-surface heat transfer, high solids hold-ups (typically 25–35% by volume), and limited axial mixing of gas. Despite its practical importance, turbulent fluidization has received much less attention than the adjacent flow regimes of bubbling, slugging and fast fluidization, due to the challenges of experimental and theoretical work related to this flow regime. However, recent years have seen an upsurge in interest in turbulent fluidization. Various methods – pressure fluctuations, visual observations, capacitance signals, optical fibre probes and bed expansion – have been used to determine the transition velocity, usually denoted U c , at which turbulent fluidization begins. Different methods tend to give different results. There appear to be as many as three different types of turbulent fluidization, depending on such factors as mean particle size, particle size distribution, column diameter and internal baffles, if any. When turbulent fluidization is preceded by bubbling, U c denotes a change from closed laminar bubble wakes to open turbulent wakes. The upper boundary of turbulent fluidization occurs when a distinct upper bed surface disappears due to substantial entrainment. Much of the literature regarding the turbulent fluidization flow regime adopts the terminology of the bubbling regime, ascribing such properties as bubble diameter and bubble rising velocity, despite the transitory and distorted nature of the voids. Turbulent beds exhibit non-uniform radial voidage distributions, with lower time-mean voidages near the wall than in the interior of the column. Axial mixing of both gas and solids is usually characterized by axial dispersion coefficients and Peclet numbers which depend on the column dimensions, as well as the gas and particle properties. Empirical equations are presented for prediction of these quantities for both gas and solids. Surface-to-bed convective heat transfer coefficients tend to reach a maximum in the turbulent fluidization regime. When turbulent beds are represented by two-phase models, interphase mass exchange is rapid. Reactor models vary widely, some treating the turbulent bed as a single phase homogeneous suspension subject to axial dispersion, while others assume two-phase behaviour. A probabilistic approach that merges these approaches as the gas velocity increases shows promise. While considerable progress has been made, substantial challenges remain in understanding and characterizing the turbulent fluidization flow regime.

Flow regime diagrams for gas-solid fluidization and upward transport

Abstract Flow regime maps are presented for gas-solids fluidized beds and gas-solids upward transport lines. For conventional gas solids fluidization, the flow regimes include the fixed bed, bubbling fluidization, slugging fluidization and turbulent fluidization. For gas solids vertical transport operation, solids flux must be incorporated in the flow regime diagrams. The flow regimes then include dilute-phase transport, fast fluidization or turbulent flow, slug/bubbly flow, bubble-free dense-phase flow and packed bed flow. In practical circulating fluidized beds and transport risers, operation below the fast fluidization regime is commonly impossible due to equipment limitations. Practical flow regime maps are proposed with the flow regimes, including homogeneous dilute-phase flow, core-annular dilute-phase flow (where there are appreciable lateral gradients but small axial gradients) and fast fluidization (where there are both lateral and axial gradients). The boundary between fast fluidization and dilute-phase pneumatic transport is set by the type A choking velocity, at which the uniform suspension collapses and particles start to accumulate in the bottom region of the transport line, while the mechanism of transition from fast fluidization to dense-phase flow depends on the column and particle diameters.

Hydrodynamics of gas-solid fluidization

Work published on gas-solid fluidization since 1986 is reviewed, with emphasis on findings that appear to be new or to represent significant steps forward in advancing the understanding of fluidization phenomena, or which have potential practical implications. Hydrodynamic regimes ranging from bubbling to fast fluidization are addressed. Mixing phenomena and circulating fluidized beds are given special attention.

Effect of bubble interaction on interphase mass transfer in gas fluidized beds

Abstract A non-interfering technique has been used to measure the concentration of ozone in pairs of bubbles injected into a bed of inactive 390 μm glass beads fluidized by ozone-free air. The transfer of the ozone tracer from the bubble phase to the dense phase is enhanced when compared to the transfer from isolated bubbles in the same particles and the same column. Bubble growth is also greater for the case where pairs of bubbles are introduced than when bubbles are present in isolation. Enhancement of interphase mass transfer for interacting bubbles in the present work and in previous studies incr with particle size and can be explained in terms of enhancement of the throughflow (or convective) component of transfer while the diffusive component unaltered. This mechanism leads to new equations for estimating interphase mass transfer in freely bubbling fluidized beds.

Microstructural aspects of the behaviour of circulating fluidized beds

Abstract Local instantaneous and time-averaging suspension densities were determined in a 152 mm diameter by 9.3 m tall circulating fluidized-bed riser using a needle capacitance probe inserted from the side. Radial profiles were obtained at three different heights and for three different solids circulation fluxes for sand particles of mean diameter 148 μm. The results confirm that local densities are greater near the wall than in the interior of the column. The fast-fluidization regime is shown to consist of a developing flow with transfer of particles to and from the wall region. An intermittency index, which would be zero for perfect core—annulus flow and one for perfect cluster flow, is used to characterize the flow; it shows that the behaviour is always between these two limits but tending towards the former with increasing height up the column.

High-velocity fluidized bed reactors

Abstract There is increasing interest in high-velocity fluidized bed reactors operated in the turbulent and fast fluidization regimes. Understanding of the hydrodynamics of these fluidization regimes has improved greatly in recent years, and there are prospects for applications beyond those practiced in industry at this time. Reactors operated in these regimes offer some unique features for gas-solid contacting. However, considerable work is required to achieve a better understanding of these high-velocity systems and to allow them to be optimized with respect to reactor geometry and operating conditions.

The fluidized-bed membrane reactor for steam methane reforming : model verification and parametric study

A new approach is presented for the modelling of a fluidized-bed membrane reactor (FBMR). The model considers the two-phase nature of the fluidized-bed reactor system and the parallel reactions taking place in stream methane reforming, as well as selective permeation through the walls of membrane tubes immersed in the bed. The model is based on the two-phase bubbling bed model with allowance for some gas flow in the dense phase. Plug flow is assumed for the combined sweep gas and permeating hydrogen flowing through the membrane tubes. Freeboard non-isothermal effects and reactions are also taken into account. The coupled differential equations for the fluidized bed and membrane tubes are solved numerically. The model is in very good agreement with experimental data, both with and without permeation, obtained in a pilot-scale reactor system. Parametric investigations demonstrate the effect of key operating variables and design parameters over a wide range. The model is also tested for its sensitivity to changes in hydrodynamic parameters. Increasing the permeation of hydrogen through the membrane tubes is of key importance in achieving high methane conversions and in minimizing adverse reactions in the freeboard region. Hydrodynamic and kinetic properties have limited influence for the conditions studied.

Sulfation and reactivation characteristics of nine limestones

The sulfation and steam hydration reactivation characteristics of nine limestones were evaluated based on laboratory sulfation and hydration tests and scanning electron microscope analyses. The calcium utilization and the sulfation pattern of the limestones were found to depend on the morphology and microstructure of the calcined limestones. Three sulfation patterns were observed in the limestones: unreacted core, uniformly sulfated and network. The steam hydration behavior of the sulfated limestones was dependent on the sulfation pattern. Unreacted core particles appeared to be easier to reactivate, thereby giving higher overall calcium utilization, compared to network-sulfated particles. Uniformly sulfated particles did not react upon hydration and could not be reactivated with steam at 250 or 450°C.