Fluidizing Medium Research Articles

Pyrolysis of polystyrene waste in a fluidized-bed reactor to obtain styrene monomer and gasoline fraction

A specially designed laboratory fluidized-bed reactor apparatus was developed for the pyrolysis of polystyrene (PS) waste in the range 450–700°C with nitrogen as the carrier gas and 20–40 mesh quartz sand as the fluidization medium, operating isothermally at atmospheric pressure. The yield of styrene monomer reached a maximum of 78.7 wt.% at a pyrolysis temperature of 600°C. Some monoaromatics with boiling point lower than 200°C could also be obtained as high-octane gasoline fraction. Styrene monomer with 99.6 wt.% purity was obtained after vacuum distillation of the liquid products, which could be used as the raw material to produce high-quality PS. The attempt to increase the yields of styrene monomer and other monoaromatics by recycling the higher boiling point fraction to the reactor during pyrolysis is also discussed.

MIXING/SEGREGATION POTENTIAL IN BINARY LIQUID FLUIDIZED BEDS

The mechanism underlying mixing/segregation of binary particles in liquid fluidized beds is reviewed and investigated in this paper. Binary mixtures of particles, when fluidized, sometimes segregate and display a behaviour called layer inversion, At low fluid velocities, one of the components is primarily found in a discrete layer at the bottom of the bed, while the other is predominantly at the top. At higher fluid velocities, the order of arrangement is reversed. The literature provides a variety of explanations for this phenomenon, derived from quite different theoretical bases. A comparative analysis of these different approaches is presented here together with the experimental results available in the literature. Based on the best model, further experimental investigation is carried out to provide; (i) comprehensive criteria to predict whether a given binary mixture of any type (both size and density variant, size variant only, density variant only) will mix/segregate or show layer inversion, and (ii) mixing/segregation regime map in terms of size ratio and density ratio of the particles for a given fluidizing medium. Therefore, knowing the properties of given particles, a second type of particles can be chosen in order to avoid or to promote segregation according to the particular process requirement.

99/01371 Optimization of an integrated gas turbine—geothermal power plant : Bidini, G. et al. Energy Convers. Manage., 1998, 39, (16–18), 1945–1956

Dedicated bioenergy crops grown on marginal lands such as reclaimed mine lands and other non-arable lands are auspicious cellulosic sources for sustainable bioenergy production. In this study, characterization of pyrolysis products were documented for two dedicated bioenergy crops grown on reclaimed coal-mine lands in West Virginia. Two clonal varieties (Private and Public varieties) of Miscanthus (Miscanthus x giganteus) and two varieties (Bomaster and Kanlow varieties) of Switchgrass (Panicum virgatum L) were used in this study. The oven-dried samples were pyrolyzed in a fixed-bed batch reactor under an inert condition and at a temperature of 500 °C to produce biochar and bio-oil (vapor condensate). The pyrolysis product, energy and carbon conversion yields were calculated. Additionally, biochar and bio-oil were characterized for elemental composition, volatile, ash and calorific values. Results showed that there was no significant difference in the biochar yields (about 30.00% yield) obtained from the clonal varieties of Switchgrass and Miscanthus. The bio-oil yields were in the range of 45.00–51.00% with the Miscanthus (Private variety) having the highest bio-oil yield of 50.61%. The carbon content of the biomass, biochar and bio-oil were found to be 46.80–48.02%, 77.72–80.23% and 54.68–59.68%, respectively. Average calorific values were found to be 19.49 MJ/kg, 29.01 MJ/kg and 24.98 MJ/kg for the biomass, biochar and bio-oil, respectively. Results for the surface chemistry of the biochars showed carbon and oxygen to be the major chemical constituents of the surfaces of the biochars. Carbon dioxide (CO2), nitrogen (N2) and carbon monoxide (CO) were found to be the major components of the non-condensable gas and the CO:CO2 ratio showed that the non-condensable gas cannot be used as a fluidizing medium.

An Innovative Combined Desulphurization/Denitration Process Using a Powder-Particle Fluidized Bed

An innovative combined desulphurization/denitration (DeSOx/DeNOx) process was tested in this study. It utilizes the so-called powder-particle fluidized bed (PPFB) to co-fluidize continuously supplied fine DeSOx sorbent (several to tens of micrometers) and coarse DeNOx catalyst (several hundreds of micrometers), the fluidization medium particles. Ammonia for NOx reduction is fed to the system from the bottom of the bed. Thus, SOx and NOx are removed simultaneously from flue gas, used as fluidizing gas, in a single reactor. By using model gases containing SO2 and NO, the removal efficiency of this process was examined in a laboratory PPFB reactor with respect to choices of DeSOx sorbents and DeNOx catalysts, reaction temperature, and major gas components. At a reaction temperature of 623 K and a static height of the catalyst bed of 0.1 m, removals greater than 90% for SO2 and about 80% for NO were achieved with the sorbent CuO·V2O5·K2SO4/Al2O3 (at Cu/S = 1) and the catalyst V2O5·WO3/TiO2 or WO3/TiO2 (at NH3/NO = 1) for a simulated flue gas SO2-NO-H2O-N2-air consisting of oxygen 2vol.%, water vapour 0–10vol.%, SO2 500 ppm and NO 500 ppm. These removals are expected to be further improved by avoiding poisoning by such a copper oxide sorbent on NOx reduction. The possible industrial application of this process is also discussed.

Removal Efficiency of the Combined Desulfurization/Denitration Process Using Powder-Particle Fluidized Bed.

A combined desulfurization/denitration (DeSOx/DeNOx) process has recently been proposed by using the powder-particle fluidized bed (PPFB). In the process, SOx is removed by a fine DeSOx sorbent continuously supplied and NOx is reduced to N2 by ammonia under the catalysis of a coarse DeNOx catalyst, the fluidization medium particles. By using the simplest model gas, SO2-NO-Air, in a laboratory scale reactor, SO2 and NO removals in the process are evaluated in this paper by testing the possible side reactions related to SO2 and NO. Then, investigations are extended to the removal efficiency for the simulated flue gas SO2-NO-N2-H2O-Air and to the effects of operating conditions, both gas velocity and the static height of catalyst bed, upon the SO2 and NO abatements. The DeSOx sorbents used were CuO·V2O5·K2SO4/Al2O3 and NaHCO3, and the DeNOx catalysts were V2O5·WO3/TiO2 and WO3/TiO2. It was found that this process suffers little from SO3 exhaust at the reactor exit. SO2 may be chemically adsorbed onto DeNOx catalyst, but it can be suppressed by using a highly efficient sorbent such as CuO·V2O5·K2SO4/Al2O3. Under NH3/NO = 1, both NO and NO2 can be similarly reduced into N2 and the possible side reactions between sorbent and NOx can also be completely abated by the NOx reduction. SO2 and NO removals in the process are more or less affected by water vapor and oxygen fractions, but the removals greater than 90% for SO2 and no less than 80% for NO can still be maintained for actual flue gas, even in a shallow bed with a static catalyst load height of 0.1 m.

Coal combustion with simulated gas turbine exhaust gas and catalytic oxidation of the unburnt fuel

One method of improving the overall efficiency of a power plant or increasing the power output is to re-power the existing boilers, using gas turbines in a combined cycle system. For the case of coal-fired plants, this option known as gas turbine topping is limited by the difficulties in coal burnout due to the low oxygen content (12 vol%) in the exhaust gas of modern gas turbines. A possible option to resolve the problem could be the introduction of an oxidation catalyst in order, first, to avoid the incomplete combustion of the coal and, second, to decrease the CO and C x H y emissions. This paper presents the results of coal combustion tests in an existing atmospheric fluidized bed with simulated gas turbine exhaust flue gas. The purposes of the trials were the examination of the possibility to achieve the combustion of low-grade lignite with about 12–14 vol% O 2 content in the turbine exhaust gas and the experimental investigation of the catalytic oxidation on the burnout behaviour as well as the resulting CO, SO 2, N 2O, C x H y and NO x emissions. Two series of measurements were carried out, with and without catalysts. The influence of the catalysts on the burnout of coal and the combustion temperature is examined and discussed. The main conclusion of this experimental investigation is that it is possible to achieve combustion of the low-grade lignite in combined cycle applications, using turbine exhaust gas as a fluidizing and oxidant medium with the support of the catalysts examined in this study, as is shown by the drastic reduction of CO and C x H y emissions.

Chaos suppression in gas-solid fluidization.

Fluidization in granular materials occurs primarily as a result of a dynamic balance between gravitational forces and forces resulting from the flow of a fluid through a bed of discrete particles. For systems where the fluidizing medium and the particles have significantly different densities, density wave instabilities create local pockets of very high void fraction termed bubbles. The fluidization regime is termed the bubbling regime. Such a system is appropriately termed a self-excited nonlinear system. The present study examines chaos suppression resulting from an opposing oscillatory flow in gas-solid fluidization. Time series data representing local, instantaneous pressure were acquired at the surface of a horizontal cylinder submerged in a bubbling fluidized bed. The particles had a weight mean diameter of 345 &mgr;m and a narrow size distribution. The state of fluidization corresponded to the bubbling regime and total air flow rates employed in the present study ranged from 10% to 40% greater than that required for minimum fluidization. The behavior of time-varying local pressure in fluidized beds in the absence of a secondary flow is consistent with deterministic chaos. Kolmogorov entropy estimates from local, instantaneous pressure suggest that the degree of chaotic behavior can be substantially suppressed by the presence of an opposing, oscillatory secondary flow. Pressure signals clearly show a "phase-locking" phenomenon coincident with the imposed frequency. In the present study, the greatest degree of suppression occurred for operating conditions with low primary and secondary flow rates, and a secondary flow oscillation frequency of 15 Hz. (c) 1998 American Institute of Physics.

Steady state of a fluidized granular medium between two walls at the same temperature

The steady state of a low-density gas of inelastic hard spheres confined between two parallel walls at the same temperature is studied. Because of the dissipation in collisions, the state is not uniform but highly inhomogeneous with a nonlinear temperature profile. Direct Monte Carlo simulations show that in the nearly elastic limit the pressure is uniform, but the state exhibits anisotropy of the diagonal terms of the pressure tensor, contrary to the predictions of the Navier-Stokes equations. For larger inelasticity, the pressure becomes nonuniform. These rheological effects, peculiar to granular systems, are explained by means of a model kinetic equation based on the Boltzmann equation. The equation is solved by constructing a systematic perturbative expansion in the square root of the degree of inelasticity. The theoretical predictions compare well with the simulation results for small inelasticity, but they are in conflict for larger values of the degree of inelasticity. The analysis provides strong evidence that this is due to the asymptotic but divergent character of the expansion, similarly to what happens when the usual Chapman-Enskog expansion is applied to molecular fluids.

Biomass Gasification: Produced Gas Upgrading by In-Bed Use of Dolomite

When some calcined dolomite (OCa·OMg) is used in the bed of a biomass gasifier of fluidized bed type the raw gas produced is cleaner than when only silica sand is used in it as fluidizing medium. In-bed dolomite changes the product distribution at the gasifier exit because of in-situ catalytic reactions promoted by the calcined dolomite. Gasifying with steam−O2 mixtures causes the tar content in the exit gas to decrease from 12 to 2−3 g tar/m3n, the H2 content to increase from 25−28 to 43 vol %, and the CO content to decrease from 45 to 27 vol % when the gas and char yields, heating value of the gas, and other main variables also undergo important changes because of the in-bed dolomite. The experimental work here reported is carried out at small pilot plant scale in a 15 cm i.d. atmospheric and bubbling fluidized bed gasifier fed by 10 kg biomass/h. Dolomite is continuously fed to the gasifier, mixed with the biomass in percentages of 2−3 wt % of the total mass flow fed. A 10 wt % of calcined dolomite in the gasifier bed is enough to significantly improve the product distribution and gas quality.

Incineration of Wastes in Novel High-Efficiency Tumbling and Rotating Fluidized Bed Incinerator

At the present time, the sewage treatment plants in the United Kingdom produce about 25,000,000 t of sewage sludge each year at a concentration of 4 solids. New regulations in the United Kingdom forbid sea dumping, and in the near future new incinerators will be required to dispose of about 5,000,000 t per year. The research program at the Sheffield University consisted of design and construction of a small-scale cylindrical rotating fluidized bed incinerator with an inside diameter of 200 mm and an axial length of up to 200 mm. The fluidizing medium was silica sand (0.6–0.7 mm) and the bed depth 30–50 mm. The rotational speed of the bed varied between 100 and 1,000 rpm (which produced 1–100 g loading). Coal, oil, sewage sludge, and chicken manure (biomass) were burnt in the bed at temperatures between 700° and 900°C. Measurements of heat transfer rate, bed temperature, and exhaust gas composition were carried out over a range of operating conditions. The influence of rotational forces on basic fluidizati...

Modelling coal gasification with a hybrid neural network

Gasification of two coals was carried out in a batch feed fluidized bed reactor at atmospheric pressure using steam as fluidizing medium. A model of coal gasification was developed, incorporating a first-principles model with a neural network parameter estimator. The hybrid neural network was trained with experimental data for the two coals and gave good performance in process modelling. A parameter for the overall reactivity of char, namely ‘active char ratio’ (ACR), was identified by the neural network, as a function of gasification time and temperature. The ACR profile showed a strong dependence on coal type. Other parameters estimated by the neural network also reflected distinct characteristics of the two coals.

Production of Activated Carbon from Coconut Shell Char in a Fluidized Bed Reactor

Activated carbon is produced from coconut shell char using steam or carbon dioxide as the reacting gas in a 100 mm diameter fluidized bed reactor. The effect of process parameters such as reaction time, fluidizing velocity, particle size, static bed height, temperature of activation, fluidizing medium, and solid raw material on activation is studied. The product is characterized by determination of iodine number and BET surface area. The product obtained in the fluidized bed reactor is much superior in quality to the activated carbons produced by conventional processes. Based on the experimental observations, the optimum values of process parameters are identified.

A Comparative Study on the Thermal Decomposition of Ammonium p-Tungstate in Batch and Fluidized-Bed Reactors

Thermal decomposition of ammonium p-tungstate (APT) to tungsten trioxide (WO 3 ) was carried out in both batch and fluidized-bed reactors. In a batch reactor, the temperature and holding time for achieving complete decomposition were found to be much higher as compared to the respective values obtained in a fluidized-bed reactor using air as the fluidizing medium. The present studies have shown that fluidized-bed decomposition of APT offers a number of distinct advantages such as lower decomposition temperature as well as time, faster kinetics, and better product morphology. The decomposition kinetics, in the fluidized-bed reactor, was found to fit well into the Power Law model, dα/dt=k(1/r)α 1-r , where α=fraction decomposed, k=rate constant, and r (a constant) is 1/2. The activation energy of the decomposition reaction suggests the overall process to be surface controlled.

Evaluation of the combustion efficiency and emission of pollutants by coal particles in a vortexing fluidized bed

Experiments were carried out in a pilot-scale (a0.45 m i.d.) vortexing fluidized bed combustor (VFBC), with coal particles as the fuel, limestone as sorbent, and river sand as the fluidizing medium. The effects of various operating parameters on the combustion efficiency and the pollutants emitted were investigated. Different formulae for the combustion efficiency are compared. A combustion efficiency of 96% can be easily obtained in a VFBC. Both the NO x and SO x emissions, based on an oxygen content of 6 vol.% in the flue gas, are found to be below 100 ppm. The retention of sulfur by limestone displays a maximum temperature in the bed at 850–880°C.

Energy flux into a fluidized granular medium at a vibrating wall

We study the power input of a vibrating wall into a fluidized granular medium, using event driven simulations of a model granular system. The system consists of inelastic hard disks contained between a stationary and a vibrating elastic wall, in the absence of gravity. Two scaling relations for the power input are found, both involving the pressure. The transition between the two occurs when waves generated at the moving wall can propagate across the system. Choosing an appropriate waveform for the vibrating wall removes one of these scalings and renders the second very simple.

Local fluctuations in a fluidized granular medium

Molecular dynamics simulations have been carried out on a two-dimensional model granular material made up of inelastically colliding discs subjected to gravity. The granular system is fluidized by a constant energy input from a vibrating base, and a stationary non-equilibrium state is achieved. Density and granular temperature profiles are determined; while the former goes through a maximum at intermediate altitudes, the latter exhibits a minimum at a higher altitude, beyond which the granular temperature is found to increase. Local density fluctuations are characterized by the time-displaced density autocorrelation function. Longitudinal compression (sound) waves propagating in the horizontal direction are found to be strongly damped, while they are overdamped in the vertical direction at the longest accessible wavelength.

97/02177 The research on the fragmentation of coal particles in a fluidized bed

This paper reports the determination of fragmentation of coal particles in a fluidized bed, and the quantitative index put forward to describe the fragmentation of coal particles in the fluidized bed. The influences of a variety of factors such as bed temperature, the time the coal stays in the fluidized bed, size of coal, coal rank and fluidizing medium on the index of the fragmentation of coal were studied at Zhejiang University. From the research results, one can draw a conclusion: the fragmentation of coal particles has different characteristics for varieties of coal ranks, and the main reasons for the fragmentation of coal particles are the first class fragmentation and the heat stress.

Gas jets in fluidized media, turbulent diffusion flames, and condensing vapor jets in liquids

Dimensional analysis and similitude is used to examine the integral characteristics of turbulent diffusion flames, jets in fluidized media, and condensing vapor jets. Data such as flame length, momentum flux and mass entrainment for these disparate jets are described in a common dimensionless frame. This, entrainment in turbulent diffusion flames permits an estimate of the particle entrainment in gas jets in fluidized media. Conversely, since the penetration length of gas jets in fluidized media is affected by the gas to solid density ratio, similitude permits an assessment of flame to ambient density ratio upon turbulent diffusion flame length.

Thermal decomposition of ammonium polymolybdate in a fluidized bed reactor

Studies on thermal decomposition of ammonium polymolybdate were carried out in a fluidized bed reactor using air as a fluidizing medium. The ammonia vapour released during the decomposition process was passed directly into hydrochloric acid solution. The change in pH of the solution was monitored with respect to time and temperature and the degree α of decomposition was calculated at intermediate stages of the process. The kinetics of the overall decomposition process were found to fit well into a ‘power law’ model. The overall activation energy Ea and the rate constant k were determined to be 16.3 kJ mol−1 and 1.24 × 10−3 s−1 respectively. The value of the activation energy suggests the process to be surface controlled. The preparation of MoO3 from ammonium polymolybdate in a fluidized bed reactor was found to be accomplished at a lower temperature and in a lesser time than that needed in a static bed reactor.

Study of Immersed Heat Exchange Surface for High Efficiency Heat Recovery from Wire Rim Tires in a Fluidized Bed Boiler.

In an internally circulating fluidized-bed (ICFE) boiler, the fluidized bed is separated by a partition into main combustion and heat recovery chambers. The flows in these chambers are generated by using silica sand as the fluidizing medium. To determine the overall heat transfer coefficient (HTC) of the boiler's panel-type immersed heat transfer tubes, combustion tests were performed with wire-rim tires. The overall (HTC) of a panel tube array was lower than that of a zigzag tube arrangement. In practice, the heat absorbed by the fins makes the coefficients of either type of tube array almost identical. The air flowrate in the circulating bed at the bottom of the heat recovery chamber can be changed to control the overall HTC to a value virtually identical with that of a zigzag tube array. The combustion of wire-rim tires leads to a buildup of wires in the zigzag array, hampering the transfer of heat. Yet, the panel-type array showed no buildup so that it was possible to maintain steady operation with this type of tube arrangement. © 1997 Scripta Technica. Inc. Heat Trans Jpn Res. 25 (2): 120–134, 1996