C.J. Lim

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.

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.

Torrefaction of sawdust in a fluidized bed reactor

In the present work, stable fluidization of sawdust was achieved in a bench fluidized bed with an inclined orifice distributor without inert bed materials. A solids circulation pattern was established in the bed without the presence of slugging and channeling. The effects of treatment severity and weight loss on the solid product properties were identified. The decomposition of hemicelluloses was found to be responsible for the significant changes of chemical, physical and mechanical properties of the torrefied sawdust, including energy content, particle size distribution and moisture absorption capacity. The hydrophobicity of the torrefied sawdust was improved over the raw sawdust with a reduction of around 40 wt.% in saturated water uptake rate, and enhanced with increasing the treatment severity due to the decomposition of hemicelluloses which are rich in hydroxyl groups. The results in this study provided the basis for torrefaction in fluidized bed reactors.

Particle velocity profiles in a circulating fluidized bed riser of square cross-section

Abstract A five-fibre optical particle velocity measuring system was used to determine vertical velocities and the fractions of rising and falling particles in a circulating fluidized bed riser of 146 mm × 146 mm square cross-section. The superficial gas velocity and solids circulation rate are shown to affect lateral and axial profiles of particle velocity and the fraction of particles moving upwards. The thickness of the outer annulus downflow region first decreased with height until a minimum was reached and then became thicker towards the top of the riser. Because of the exit effect, lateral profiles of particle velocity were asymmetric at the top of the riser and the wall downflow layer was thicker on the side opposite to the exit than on the exit side. The corners were also found to be regions of greater downflow.

The fluidized bed membrane reactor system: a pilot scale experimental study

An experimental investigation has been performed to examine a novel reactor which combines advantages of fluidized beds as catalytic reactors, in particular catalyst bed uniformity, improved heat transfer and virtual elimination of diffusional limitations, with advantages offered by permselective membrane technology, in particular shifting the conventional thermodynamic equilibrium and thein situ separation and removal of a desirable reaction product. A pilot scale reforming plant was commissioned to study the new fluidized bed membrane reactor (FBMR) system for steam reforming of natural gas. The reactor has a hydrogen production capacity of 6 m3/h (STP) and a main body diameter of 97 mm, designed to withstand temperatures up to 1023°C and pressures up to 1.5 MPa. Thin-walled palladium-based tubes are provided as hydrogen permselective membranes. The experiments validated the FBMR concept, showed the relative importance of different operating variables influencing the reactor performance and allowed the effectiveness of hydrogen permeation through palladium-based membranes in a fluidized bed environment to be evaluated.

Oxidative torrefaction of biomass residues and densification of torrefied sawdust to pellets

Oxidative torrefaction of sawdust with a carrier gas containing 3-6% O(2) was investigated in a TG and a fluidized bed reactor, with the properties of the torrefied sawdust and pellets compared with traditional torrefaction without any O(2), as well as the dry raw material. It is found that the oxidative torrefaction process produced torrefied sawdust and pellets of similar properties as normally torrefied sawdust and corresponding pellets, especially on the density, energy consumption for pelletization, higher heating value and energy yield. For moisture absorption and hardness of the torrefied pellets, the oxidative torrefaction process showed slightly poor but negligible performance. Therefore, it is feasible to use oxygen laden combustion flue gases as the carrier gas for torrefaction of biomass. Besides, torrefied sawdust can be made into dense and strong pellets of high hydrophobicity at a higher die temperature than normally used in the production of traditional control pellets.

A prediction of performance of commercial coal gasifiers

Abstract A simple equilibrium model has been formulated for gas composition and yield from coal gasifiers. Mass and energy balances are combined with equilibrium relationships to calculate major and minor species in the produced gas. Gas compositions reported in the literature for nine commercial and semicommercial gasifier designs are found to be in reasonable agreement with predictions of the model.

Voidage profiles in a circulating fluidized bed of square cross-section

Abstract Using an optical fiber particle concentration measuring system, an extensive study was carried out of voidage profiles in a circulating fluidized bed riser of square cross-section. Both lateral and axial voidage profiles were obtained. Particle concentration is not necessarily lowest at the axis of the riser, but may go through a minimum between the wall and the axis. The study also reveals the influence of the corners on the voidage profile. Because of the exit effect, profiles of voidage at the top of the riser are asymmetric. Bimodal and even trimodal probability distributions of particle concentration are found. The intermittency index is used to characterize the heterogeneity/homogeneity of the flow.

A model for heat transfer in circulating fluidized beds

Abstract A heat transfer model is proposed for predicting transfer coefficients in circulating fluidizeds beds. Based on observations and on hydrodynamic measurements of the core—annulus flow structure in low- and high-temperature risers, the model assumes that particles in the annulus congregate into strands with different voidages. These strands are assumed to undergo a repeating process of formation, downward movement along the wall, and disintegration. Heat is transferred between the wall and the strands by transient heat conduction with a wall contact resistance. The resulting model gives reasonable agreement with published data for smooth surfaces. It accounts successfully for the effects of heat transfer surface length and particle size. However, it is shown that the effect of the geometry of the heat transfer surface on the flow pattern of particles must be taken into account in future models for membrane or other non-smooth geometrics.