Fluidized bed combustion of wet brown coal

The behaviour of very wet Victorian brown coal was examined in a bed of sand fluidized, at temperatures around 1000 K, with either air or nitrogen. Small batches of coal with a narrow particle size range were added to the 76 mm diameter bed and the times required for devolatilization and total combustion were recorded. Changes in particle water content, volatiles level and particle size distribution were also measured. All the particles tested, up to 8.4 mm in diameter, dried rapidly and remained substantially intact throughout carbonization and combustion. Devolatilization was complete after about 60 s but extensive freeboard combustion of volatiles was evident. The water content of the coal had very little influence on burnout time. Char combustion dominated the overall combustion process and took place under kinetic control with significant pore burning.     

Fluidization characteristics and charging behavior of fly ash in a vibro-fluidized bed

Fluidization characteristics of fly ash were investigated in a fluidized bed subjected to mechanical vibrations. In the presence of mechanical vibrations, the minimum fluidization velocity decreased from 4.0 to 1.0 mm/s, and the pressure drop increased slightly. The charge-to-mass ratios of fly ash along the fluidized column revealed that charging behavior of fly ash in the freeboard region significantly differed from that in the dense bed region. In the dense bed region, the net charges of fly ash were slightly negative or almost neutral, whereas fly ash in the freeboard region tended to carry positive charges. Loss-on-ignition (LOI) of fly ash was examined in different regions of the fluidized bed, and the results showed that LOI varied considerably inside the column, generally being lower in the freeboard than in the dense bed since finer particles were entrained into the freeboard. In the dense bed, particle–wall collisions resulted in positively charged carbon and negatively charged ash particles, whereas carbon particles in the freeboard region tended to carry negative charges. The findings demonstrate the potential for separating unburned carbon from fly ash by utilizing a vibro-fluidized bed as a tribocharger.     

An analytical model for freeboard and in-bed limestone sulfation in fluidized-bed coal combustors

A new model, which combines in-bed and freeboard sulfation, significantly improves the ability to predict sulfur capture by limestone sorbents in fluidized-bed coal combustors. In this model, the in-bed hydrodynamics are described in terms of a bubble phase and an emulsion phase while the freeboard region has only a diluted emulsion phase. The solids, which are in the emulsion phases, are considered to be completely back-mixed; the gaseous bubble phase travels in plug-flow but exchanges with the emulsion phase. The sulfation reaction occurs principally in the emulsion phase and the reaction rate is a direct function of the sulfur dioxide concentration, the extent of the calcium oxide conversion (as measured by a thermogravimetric analyzer), and the amount of limestone present in the bed and in the freeboard. The amount present, or holdup, in the free-board is calculated from empirical correlations for elutriation and from particle-time trajectories as predicted from equations of motion.The model indicates that a significant amount of sulfur capture can occur in the freeboard region, especially with high superficial gas velocities and small particle sizes, both of which lead to increased freeboard holdup. For an overall sulfur retention of 90%, approximately 9% of the sulfur is predicted to be captured within the freeboard above a combustor bed 1.8 m by 1.8 m by 1.2 m high of 1200 μm diameter limestone particles that are fluidized at 2.4 m/s. The model also predicts that the sulfur captured by the entrained particles is negligible. These predictions compare favorably with the actual sulfur retentions experienced in similar sized pilot-scale combustors.     

Height and structure of the fountain region above spouted beds

The height and structure of the fountain which forms in the freeboard region above the annular surface of a spouted bed is predicted based on equations of motion for particles entrained in the spout. Predicted fountain heights are in good agreement with a wide range of experimental values covering polystyrene particles, wheat, millet, glass beads and ammonium nitrate spouted in columns of diameter up to 0.292 m. A simplified procedure in which drag is ignored gives good results also under most circumstances of practical importance. The theoretical niodel proposed can be used to predict the voidage profile in the fountain core and the holdup of particles ill the freeboard, both of which may be useful in estimating the performance of spouted bed reactors.     

Mass transfer in rolling rotary kilns: a novel approach

A novel approach to modeling mass transfer in rotary kilns or rotating cylinders is explored. The movement of gas in the interparticle voids in the bed of the kiln is considered, where particles move concentrically with the geometry of the kiln and gas is entrained by these particles. The approach considers a differential section along the length of a rotary kiln where the gas concentration in the freeboard is assumed to be uniform in that section. A reactor modelling approach has been used to derive effectiveness factors for the bed as a function of bed fill, reaction kinetics and rotation speed. In many cases, the entrained gas becomes depleted within the bed, leading to a simplified model for the bed effectiveness factor. Experimental data confirms the validity of this model for slower rates. At faster rates, mass transfer can be much higher than the model predicts, indicating that other mechanisms, such as dispersion or diffusion are also important in these conditions.     

Modeling combustion of straw–bitumen pellets in a fluidized bed

Recently, straw–bitumen pellets have been proposed as an alternative fuel. In this paper, a comprehensive model for steady state combustion of straw–bitumen pellets in a bubbling fluidized bed is presented. The combustor is divided into three sectors: a dense bed zone, a splashing zone and a freeboard zone. Relevant phenomena including volatile release and segregation, char comminution and elutriation, bed particles ejection, and post-combustion above the bed have been taken into consideration. Energy equations for the splashing and freeboard zones have been established.     

化肥造气技术的现状和前景

评述了近年来煤气化领域的最新进展;提出了对改造或新建合成氨厂的具体建议,即对于小型合成氨厂, 可以采用连续气化代替间歇气化、气化型煤代替无烟煤 （或焦炭） 或选用循环流化床气化炉; 对于中型合成氨厂, 可以采用连续气化或气流床气化技术; 对于大型合成氨厂,则采用气流床气化技术、流化床气化燃烧一体化技术或者采取Ｌｕｒｇｉ炉和湿法气流床（如Ｔｅｘａｃｏ炉） 联用的方法．     

Modeling a pilot-scale fluidized bed coal gasification reactor

A steady-state model has been developed to simulate the North Carolina State University pilot-scale fluidized bed coal gasification reactor. The model involves instantaneous devolatilization of coal at the top of the gasifier (freeboard region) and char combustion and gasification in the fluidized bed. A two-phase (emulsion-dilute gas) representation of the fluidized bed incorporates the phenomena of jetting, bubbling, slugging, and mass and heat transfer between phases, and enables the prediction of individual species flow rates and temperature profiles within the bed. The model has been successfully used to simulate the gasification of a devolatilized Western Kentucky bituminous coal and a New Mexico subbituminous coal and to predict effects of various operating parameters on key gasifier performance variables.     

Modelling and experimental validation of a fluidized-bed reactor freeboard region: Application to natural gas combustion

A theoretical and experimental study of natural gas–air mixture combustion in a fluidized bed of sand particles is presented. The operating temperatures are lower than a critical temperature of 800 °C above which the combustion occurs in the vicinity of the fluidized bed. Our study focusses on the freeboard zone where most of the methane combustion takes place at such temperatures. Experimental results show the essential role of the projection zone in determining the global thermal efficiency of the reactor. The dense bed temperature, the fluidizing velocity and the mean particle diameter significantly affect the thermal behaviours.A model for natural gas–air mixture combustion in fluidized beds is proposed, counting for interactions between dense and dilute regions of the reactor [P. Pré, M. Hemati, B. Marchand, Study of natural gas combustion in fluidised beds: modelling and experimental validation, Chem. Eng. Sci. 53 (1998) (16), 2871] supplemented with the freeboard region modelling of Kunii–Levenspiel [D. Kunii, O. Levenspiel, Fluidized reactor models: 1. For bubbling beds of fines, intermediate and large particles. 2. For the lean phase: freeboard and fast fluidization, Ind. Eng. Chem. Res. 29 (1990) 1226–1234]. Thermal exchanges due to the convection between gas and particles, and due to the conduction and radiation phenomena between the gas-particle suspension and the reactor walls are counted. The kinetic scheme for the methane conversion is that proposed by Dryer and Glassman [F.L. Dryer, I. Glassman, High-temperature oxidation of CO and CH₄, Proceedings of the 14th International Symposium on Combustion, The Combustion Institute, Pittsburg (1973) 987]. Model predictions are in good agreement with the measurements.     

Elutriation from fluidized beds—II. Disengagement of particles from gas in the freeboard

The freeboard above a fluidized bed is the dilute phase region in which the gas and particles disengage. The freeboard container is normally cylindrical and usually of the same diameter as the bed but sometimes larger.Theory is given to describe horizontal turbulent diffusion of fine particles towards the freeboard walls. On reaching the walls, the fine particles descend as a falling film which may drag down gas and thus generate a gas circulation current in the freeboard. This theory gives predictions of (1) upward flux of fine particles and (2) particle concentration, both as functions of distance above the bed surface. These predictions are in reasonable agreement with (1) measurements, using isokinetic sampling, of upward particle flux above an 0.6 m diameter bed of polymer particles of mean diameter 760 μm, and (2) measurements of particle concentration at five levels above a 0.3 m square bed containing a mixture of 73 and 370 μm particles, using entrapment of particles between horizontal shutters in the freeboard.The theory gives a working formula to predict the transport disengaging height (TDH) in reasonable agreement with published data. The theory predicts that the TDH increases with freeboard diameter. The theory predicts, and experiments confirm, that the TDH can be reduced by inserting vertical baffles into the freeboard.The circulation of freeboard gas, generated by the fine particle motion, may explain the published observations (Geldart, D., Cullinan, J., Georghiades, S., Gilvray, D. and Pope, D.J., 1979, Trans. Inst. Chem. Engrs57, 269) that adding fine particles increases the elutriation of coarse particles.     

Peach and apricot stone combustion in a bubbling fluidized bed

In this study, a bubbling fluidized bed combustor (BFBC) of 102 mm inside diameter and 900 mm height was used to investigate the combustion characteristics of peach and apricot stones produced as a waste from the fruit juice industry. A lignite coal was also burned in the same combustor. The combustion characteristics of the wastes were compared with that of a lignite coal that is most widely used in Turkey. On-line concentrations of O₂, CO, CO₂, SO₂, NO_X and total hydrocarbons (C_mH_n) were measured in the flue gas during combustion experiments. By changing the operating parameters (excess air ratio, fluidization velocity, and fuel feed rate), the variation of emissions of various pollutants was studied. Temperature distribution along the bed was measured with thermocouples.During the combustion tests, it was observed that the volatile matter from peach and apricot stones quickly volatilizes and mostly burn in the freeboard. The temperature profiles along the bed and the freeboard also confirmed this phenomenon. It was found that as the volatile matter of fruit stones increases, the combustion takes place more in the freeboard region.The results of this study have shown that the combustion efficiencies ranged between 98.8% and 99.1% for coal, 96.0% and 97.5% for peach stone and 93.4% and 96.3% for apricot stones. The coal has zero CO emission, but biomass fuels have very high CO emission which indicates that a secondary air addition is required for the system. SO₂ emission of the coal is around 2400–2800 mg/Nm³, whereas the biomass fuels have zero SO₂ emission. NO_X emissions are all below the limits set by the Turkish Air Quality Control Regulation of 1986 (TAQCR) for all tests. As the results of combustion of two biomass fuels are compared with each other, peach stones gave lower CO and NO_X emissions but the SO₂ emissions are a little higher than for apricot stones. These results suggest that peach and apricot stones are potential fuels that can be utilized for clean energy production in small-scale fruit juice industries by using BFBC.     

Carbon conversion of solid fuels in the freeboard of a laboratory-scale fluidized bed combustor—application of in situ laser spectroscopy

The carbon conversion of different solid fuels (i.e. beech wood, fir wood, bituminous coal) was investigated in the freeboard of a laboratory-scale fluidized bed combustor by in situ tunable diode laser absorption spectroscopy. A room temperature continuous wave InGaAsSb/AlGaAsSb quantum well ridge diode laser emitting at 2.3-2.35 μm was wavelength tuned at 300 Hz to determine simultaneously CH₄ and CO during devolatilization and char combustion in situ 10 mm above the fuel particles. The lower detection limit was 0.2 vol% (5000 ppm m) for both species. In addition, CO, CO₂ and O₂ were determined ex situ by conventional methods.The experimental results obtained for the bituminous coal were compared to a detailed chemical kinetic model.The in situ measurements proved to be advantageous compared to conventional ex situ concentration measurements. The calculations confirm the determination of the primary products of solid fuel combustion during devolatilization and char combustion. A rather simple model for the devolatilization products was proven to describe well the release rates of CH₄ and CO for the bituminous coal.     

A Practical Method to Estimate the Bed Height of a Fluidized Bed of Fine Particles

Knowledge of both dense bed expansion and freeboard solids inventory are required for the determination of bed height in fluidized beds of fine particles, e.g., Fluidized Catalytic Cracking (FCC) catalysts. A more accurate estimation of the solids inventory in the freeboard is achieved based on a modified model for the freeboard particle concentration profile. Using the experimentally determined dense bed expansion and the modified freeboard model, a more practical method with improved accuracy is provided to determine the bed height both in laboratory and industrial fluidized beds of FCC particles. The bed height in a fluidized bed can exhibit different trends as the superficial gas velocity increases, depending on the different characteristics of the dense bed expansion and solids entrainment in the freeboard. The factors that influence the bed height are discussed, showing the complexity of bed height and demonstrating that it is not realistic to determine the bed height by a generalized model that can accurately predict the dense bed expansion and freeboard solids inventory simultaneously. Moreover, a method to determine the bed height, based on axial pressure fluctuation profiles, is proposed in this study for laboratory fluidized beds, which provides improved accuracy compared to observation alone or determining the turning points in the axial pressure profiles, especially in high‐velocity fluidized beds.     

A model for volatiles release into a bubbling fluidized-bed combustor

A volatiles release model has been developed to predict the location and quantity of coal volatiles that are released into a bubbling fluidized-bed combustor, using overbed, in-bed or underbed feed systems. This model not only considers time resolution of the simultaneous processes of coal devolatilization and coal particle movement after injection, but also takes account of the stochastic nature of this particle movement. Volatiles release is nonuniform, occurring throughout an industrial-scale bed, but only in restricted parts of a laboratory-scale bed. For the industrial-scale bed, 32% of volatiles are released directly into the freeboard while the particle is devolatilizing at the surface, whereas 24% are released there for the laboratory scale bed. The model predicts the formation of numerous discrete volatiles regions within the bed, in agreement with a new interpretation of experimental measurements from an in-bed oxygen probe. The model is shown to be consistent with other experimental in-bed measurements obtained during combustion of “simulated” volatiles.     

Measurements and numerical predictions of gas vortices formed by single bubble eruptions in the freeboard of a fluidised bed

Gas vortices generated in the freeboard of a bubbling fluidised bed have become the centre of increasingly more research due to the advances in experimental technology. The behaviour of gas flow in the freeboard of a bubbling fluidised bed is of interest for applications such as the gasification of coal where reactions of gas mixtures, as well as gas–particle heat and mass transfer take place. Knowledge of the hydrodynamics of the gas within the freeboard can be hard to characterise, especially the detailed behaviour of gases escaping from bubbles that erupt at the bed surface. In the present study, experiments were conducted on a rectangular three-dimensional gas–solid fluidised bed. The experiments used a particle imaging velocimetry (PIV) measurement technique to visualise and measure the gas flow within the freeboard after a single bubble eruption. A computational study was carried out using Eulerian–Eulerian, kinetic theory of granular flow approach with a quasi-static flow model and with LES used to account for gas turbulence. Results from a three dimensional simulation of the experimental fluidised bed were compared with experimental velocity profiles of gas flow in the freeboard of the gas–solid fluidised bed after a bubble eruption. The CFD simulations showed a qualitative agreement with the formation of the gas vortices as the bubble erupted. Consistent with experimental findings the CFD simulations showed the generation of a pair of vortices. However, the simulations were unable to demonstrate downward flow at the centre of the freeboard due to particles in free fall after a bubble eruption event was observed in the experiments. Velocity profiles from the CFD data are in reasonably good agreement with the characteristic trends observed in the experiments, whereas the CFD model was able to predict the gas vortices phenomena and the velocity magnitudes were over-predicted.     

Modelling fluidized bed combustion of high-volatile solid fuels

A model of an atmospheric bubbling fluidized bed combustor operated with high-volatile solid fuel feedings is presented. It aims at the assessment of axial burning profiles along the reactor and of the associated temperature profiles, relevant to combustor performance and operability. The combustor is divided into three sections: the dense bed, the splashing region and the freeboard. Three combustible phases are considered: volatile matter, relatively large non-elutriable char particles and fine char particles of elutriable size. The model takes into account phenomena that assume particular importance with high-volatile solid fuels, namely fuel particle fragmentation and attrition in the bed and volatile matter segregation and postcombustion above the bed. An energy balance on the splashing zone is set up, taking into account volatile matter and elutriated fines postcombustion and radiative and convective heat fluxes to the bed and the freeboard.Results from calculations with a high-volatile biomass fuel indicate that combustion occurs to comparable extents in the bed and in the splashing region of the combustor. Due to volatile matter segregation with respect to the bed, a significant fraction of the heat is released into the splashing region of the combustor and this results in an increase of the temperature in this region. Extensive bed solids recirculation associated to solids ejection/falling back due to bubbles bursting at bed surface promotes thermal feedback from this region to the bed of as much as 80-90% of the heat released by afterburning of volatile matter and elutriated fines. Depending on the operating conditions a significant fraction of the volatile matter may burn in the freeboard or in the cyclone.     

Study on the entrainment of FCC particles from a fluidized bed

The entrainment of fine FCC particles from a three-dimensional fluidized bed was studied. The velocities of particles leaving the bed surface and moving in the freeboard, those of bubbles rising in the dense bubbling zone, and the hold-up of solid particles in the freeboard were measured continuously by optical probes. Photographs of the freeboard were also taken continually and were compared with the information obtained by those probes.It was found that the coalescence of bubbles has an important effect on the ejection of solid particles and the relationship between the bubble eruption and the particle swarm ejection was clarified. Based on these results, a mechanistic model to account for the frequency of the ejected particle swarms was proposed.     

Optical image analysis of fine coal particles as an aid in optimizing combustion in a fluidized bed

The performance of a fluidized bed combustor is adversely affected by the entrainment of fine coal and char particles. The size distribution of the entrained particles is an indicator of the combustor's performance. Size analysis data may be helpful for optimizing the operating conditions.

An optical image analysis system (TAS, Leitz) was used to analyze the size distribution of char particles elutriated from a small fluidized bed after one single pass. The TAS system discriminates between highly reflecting char particles and low-reflectance ash. The maximum size of the chars produced in these experiments was about 50 μm, almost independent of the type of coal.     

Coal combustion characteristics in a pressurized fluidized bed

The characteristics of emission and heat transfer coefficient in a pressurized fluidized bed combustor are investigated. The pressure of the combustor is fixed at 6 atm. and the combustion temperatures are set to 850, 900, and 950 °C. The gas velocities are 0.9, 1.1, and 1.3 m/s and the excess air ratios are 5, 10, and 20%. The desulfurization experiment is performed with limestone and dolomite and Ca/S mole ratios are 1,2, and 4. The coal used in the experiment is Cumnock coal from Australia. All experiments are executed at 2 m bed height. In this study, the combustion efficiency is higher than 99.8% through the experiments. The heat transfer coefficient affected by gas velocity, bed temperature and coal feed rate is between 550-800 W/m² °C, which is higher than those of AFBC and CFBC. CO concentration with increasing freeboard temperature decreases from 100 ppm to 20 ppm. NO_x concentration in flue gas is in the range of 5-130 ppm and increases with increasing excess air ratio. N₂O concentration in flue gas decreases from 90 to 10 ppm when the bed temperature increases from 850 to 950 °C.     

Numerical simulation of the bubbling fluidized bed coal gasification by the kinetic theory of granular flow (KTGF)

A new numerical model based on the two-fluid model (TFM) including the kinetic theory of granular flow (KTGF) and complicated reactions has been developed to simulate coal gasification in a bubbling fluidized bed gasifier (BFBG). The collision between particles is described by KTGF. The coal gasification rates are determined by combining Arrhenius rate and diffusion rate for heterogeneous reactions or turbulent mixing rate for homogeneous reactions. The flow behaviors of gas and solid phases in the bed and freeboard can be predicted, which are not easy to be measured through the experiments. The calculated exit values of gas composition are agreed well with the experimental data. The relationship between gas composition profiles with the height of gasifier and the distributions of temperature, gas and solid velocity and solid volume fraction were discussed.