CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

Abstract

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

Effect of Fluidizing Particle on Drying Characteristics of Porous Materials in Superheated Steam Fluidized Bed under Reduced Pressure

The hygroscopic porous particle was used as the fluidizing particle for the superheated steam fluidized bed drying under reduced pressure. A relatively large material was immersed in the fluidized bed as the drying sample. The drying characteristics of the sample were examined experimentally and the results were compared with those in the case of inert particle fluidized bed.

The water transfer from the sample to the fluidizing particle bed in the case of hygroscopic porous particle facilitated the drying regardless of pressure and temperature in the drying chamber. The increment degree of the sample temperature at the earlier period of drying was smaller in the case of hygroscopic porous particle than in the case of inert particle, and the phenomenon was more remarkable in the case of superheated steam than in the case of hot air.     

A descriptive model of carbon attrition in the fluidized combustion of a coal char

A model is proposed which takes into account the interaction between combustion in the pores, weakening of the mechanical structure of the particle and attrition during the fluidized combustion of a char. A constitutive relationship between the degree of enfeeblement of the outer region of particles and the rate of regression of the surface is introduced to characterize the behaviour of the char in respect to combustion assisted attrition. An orthogonal collocation technique is used to solve model equations.Results obtained in the batch fluidized bed combustion of the char of a bituminous coal, by means of a 40 mm ID laboratory unit especially equipped for time resolved collection of attrited carbon, are used to test the model. Attrited carbon rates and carbon combustion efficiencies measured in experiments carried out with oxygen concentration in fluidizing gas changing from 21 to 0.75% show a fair agreement with values given by model calculations.     

Attrition of spherical electrode carbon particles during batch fluidized combustion

Spherical electrode carbon particles prepared from carbon rods of dry cell batteries have been used to study the attrition behaviour in a bubbling fluidized bed combustor. Experiments have been conducted in a 40 mm I.D. and 1 m high fluidized bed combustor operated at 1 m/s superficial velocity. The bed was operated with nitrogen and with two different oxygen concentrations at 850°C to study the effect of combustion on attrition of these particles. The experimental technique used allowed the time resolution of attrited fines generation, providing detailed curves of attrition rates as a function of time. Attrition rate constants have been evaluated. Results show an enhancement of attrition due to combustion even for spherical, homogeneous and smooth particles.     

Modelling of circulating fluidized bed combustion of a char

The combustion of a char in the 41 mm ID riser of a laboratory circulating fluidized bed combustor has been investigated at different air excesses and rates of solids (char and sand) circulating in the loop. Riser performance was characterized by an axial oxygen concentration profile as well as by the overall carbon content and particle size distribution. The proposed model accounts for carbon surface reaction, intraparticle and external diffusion, and attrition. External diffusion effects were relevant in the riser dense region where char was potentially entrapped in large clusters of inert solids. Experimental data and results of the model calculations are in satisfactory agreement.     

In-bed char combustion of Australian coals in PFBC. 2. Char combustion without secondary fragmentation

Char combustion parameters that significantly affect the in-bed combustion of char in PFBC were determined experimentally using a batch-fed PFBC. The ratio of carbon to oxygen consumed on the surface of a burning char particle was determined and it was concluded that CO was the only product of char combustion in PFBC.

Model simulations revealed that, for PFBC, mass transfer controlled the combustion of large char particles ≥2 mm, whereas the combustion of small char particles below 0.9–2 mm was controlled by both mass transfer and chemical kinetics.

System pressure influenced the char combustion via the interaction between chemical kinetics and the mass transfer of oxygen to the char. Char particle temperature varied markedly with oxygen partial pressure in the particulate phase, indicating a distribution of char particle combustion rates in PFBC. In modelling char combustion in PFBC, the temperature of char particles in the bed should be calculated at different locations based on a heat balance around the burning char particle taking into account the local bed oxygen concentration.     

Prediction of burning rate of coal in fluidized-bed combustion

A theory has been proposed to evaluate the burning rate of a single carbon particle in a continuously operated coal-fired fluidized bed. Experimental verification was carried out in a laboratory scale 200 mm × 200 mm combustor. The burning rate increases with the increase of the fluidization velocity and the size of the bed material. The predicted data on burning rate agree fairly well with the experimental values. The gas concentration in the bed and freeboard has also been measured and it is seen that the consumption of oxygen mostly takes place in the bed.     

Flow regimes and combustion behaviour in coal-burning spouted and spout-fluid beds

A study of coal combustion was carried out in a 0.15-m diameter half-column spout-fluid bed combustor, in which a sub-bituminous Alberta coal was burned in inert beds of sand. High temperatures promoted spout instability leading to earlier appearance of pulsatory spouting, the jet-in-fluidized-bed regime, and slugging than for room temperature operation. While radial temperature profiles below the bed surface were quite uniform, axial profiles showed a significant temperature jump in the fountain region above the bed surface. Axial profiles of oxygen concentration fell abruptly at the bed surface due to rapid burning there. The size of inert particles and the amount of auxiliary air were found to influence the hydrodynamics and combustion behaviour.     

PERFORMANCE OF A PRESSURIZED TWO-STAGE FLUIDIZED GASIFICATION PROCESS FOR PRODUCTION OF LOW-BTU GAS FROM COAL CHAR

A two-stage pressurized fluidized-bed gasification process has been developed to produce low-heating value gases from coal char. The reactor was 0.075 m id. and 1.4 m long, and gasification experiments were conducted under pressures up to 790 kPa and at temperatures up to 1323 K. A partition disc was used to divide the fluidized bed into two stages, using the first stage as a partial combuster and gasifier and the second stage as a gasifier. The disc was designed to control compositions of coal char particles in both stages so that the heat required for the endothermic gasification reaction in the second stage can be provided by the heat of combustion in the first

For conditions examined here, the disc with an opening ratio of 40° was found to give optimum distribution of the char particles in both stages without ash agglomeration. It was also shown that all oxygen gas was completely consumed within the first stage

The heating value of the product gas increased with the char feed rale. However, there may be an oplimum Teed ratio of char and sand-particles since the higher char feed rate causes more frequent ash agglomeration as well as less carbon conversion     

Fluidized combustion of char and volatiles from coal

This paper reviews the literature about fluidized combustion of char and volatiles from coal. While the rate of combustion of particles bigger than about 2 mm is mainly limited by the rate of diffusion of oxygen, there is usually an effect of chemical rate. As particle diameter decreases, the influence of chemical rate increases. Char particles are porous and combustion occurs in pores near their exterior. Char reactivity can be modelled by an effective pore area for combustion. At high bed temperatures (>1,150 K), the rate of combustion of volatiles is limited by the rate of mixing of fuel and oxygen. At low bed temperatures (<1,000 K), combustion reactions are inhibited by the inert particles in the bed.     

Combustion characteristics of a two-stage swirl-flow fluidized bed combustor

In order to investigate the combustion characteristics of a two-stage swirl-flow fluidized bed combustor, combustion experiments of low-grade anthracite coal were performed. Experimental parameters were the fluidizing air velocity, coal feed rates, bed temperature, stoichiometric air ratio, swirl nozzle diameter and rotational diameter. The experimental results showed that, due to the swirl flow, the elutriation rates of fines were lower than those of the single-stage fluidized bed combustor. The combustible contents of the ash in the outflow streams were also reduced. Therefore, the combustion efficiency of the two-stage swirl-flow fluidized bed combustor was 20% greater than that of the single-stage fluidized bed combustor under the same operating conditions.     

海藻生物质颗粒流化床燃烧试验研究

在小型流化床试验台上研究了海藻颗粒(条浒苔与马尾藻)的流化床燃烧。海藻在流化床内的挥发分析出燃烧时间都在1 min左右。条浒苔颗粒在流化床中燃烧先进行脱水和挥发分的燃烧,接着发生焦炭燃烧,其燃烧过程符合缩核模型,炭核由外向内逐层燃烧,而灰层半径几乎不变。但马尾藻颗粒由于挥发分的大量快速释放而迅速膨胀破碎成屑。另外通过对条浒苔颗粒及不同燃烧时间后收集的焦炭颗粒剖面的SEM扫描电镜观察,发现随着燃烧的进行,颗粒内孔隙增大,微孔表面粗糙。进一步详细研究了两种海藻颗粒(条浒苔与马尾藻)在流化床内单次投料下的燃烧。随着床温的升高,条浒苔释放NO_x相对浓度增加,CO相对浓度减少。而马尾藻释放气体中SO₂与NO_x含量相对条浒苔有所增加;随着床温的升高,CO相对浓度减少。床温的升高使得床内传热速率加快,两种海藻挥发分的析出提前,燃尽时间缩短。风速、床高的升高使得两种海藻燃烧容易,燃尽时间缩短。     

Combustion of single char particles in a turbulent fluidized bed

The combustion of single bituminous char particles (4-12 mm diameter) was studied in a turbulent fluidized bed operated at 1098 K using air as the fluidising medium. Results indicated that particles burn with constant density following a shrinking sphere model. Burning rates are much higher than those observed in a bubbling fluidized bed. The rate of transfer of oxygen to the particle surface is also higher than that observed in bubbling beds. A model is proposed to calculate the Sherwood numbers of the burning carbon particles. Experimental values of the Sherwood numbers agree well with those predicted from the model.     

ANALYSIS OF SIMULTANEOUS RADIATIVE AND CONDUCTIVE HEAT TRANSFER IN FLUIDIZED BEDS

In the bubbling regime of operation for fluidized beds, the major mechanism for heat transfer is transient conduction to periodic packets of densely packed particles at the heat transfer surface. The well known Mickley and Fairbanks model, with various subsequently proposed modifications, adequately describes this transient conduction mechanism. However, no adequate theory exists for heat transfer in high-temperature fluidized beds where radiative contribution becomes significant.

Analysis of the radiative contribution is complicated by the nonlinear interaction of radiation with conduction/convection. This paper describes a differential formulation of the combined radiative/ conductive heat transfer process. The discrete flux method used by Churchill et al. for radiative transport in heterogeneous media is applied here to the problem of transient heat transfer to packets in fluidized beds. Packets are modeled as radiatively participating media with absorption, scattering, and emission of radiation. Simultaneous solution of the governing differential equations for temperature and forward and backward radiation fluxes permits calculation of instantaneous heat flux at the heat-transfer surface. Radiative transfer during bubble contact is added as a time-weighted contribution.

Using experimental data on radiative cross sections (from packed media experiments) and experimental data on packet residence times (from fluidized bed experiments), the combined conductive/radiative heat transfer to packets was obtained for examples of fluidized beds at different fluidizing velocities and wall temperatures. The analytical results indicate that the relative importance of radiation is affected by particle size, average packet residence time, and the radiative attenuation cross sections. For operating conditions representative of fluidized bed combustion, the model estimates a 10 to 20 percent contribution by radiation to the total heat transfer. Comparison to limited experimental data from the literature shows reasonable agreement.     

Internal burning of petroleum coke particles in a fluidized bed

Physical properties of small irregular petroleum coke particles (3.08 mm) have been measured at various stages of combustion in a fluidized bed of sand operating in the range 973–1173 K. Internal burning took place during the particle heating period even at bed temperatures of 1173 K. However, at this temperature, the particles burnt predominantly under mass transfer control subsequent to ignition and particle heating. Combustion behaviour at 973 K was typical of internal burning characteristics, i.e. pore enlargement and decrease in particle density continued as burning progressed.     

Optical probe measurements of the temperature of burning particles in fluidized beds

The temperatures of freely moving petroleum coke particles burning in the interior of a fluidized bed of sand have been measured. An optical probe technique, employing two-wavelength pyrometry with wavelengths centred at 800 and 1000 nm, was used for the measurements over a wide range of operating conditions. Measurements were conducted in bubbling fluidized beds of 1.82 × 10⁻⁴ m and 1.09 × 10⁻³m sand particles maintained at bed temperatures of 973 and 1200 K. Batches of petroleum coke particles having average diameters in the range 7.5 × 10⁻⁴ to 8.1 × 10⁻³ m were charged into the hot fluidized bed combustor, and upon the passage of a burning particle across the optical probe's field of view, a pulsed signal was generated. Signals from the interior of the bed were continuously monitored by a pair of radiometers interfaced to a high speed data acquisition system and were processed off-line for particle temperature calculations. The average particle temperatures exceeded the bed temperature by 60–360 K for the operating conditions considered. It was found that particle temperatures in the bubble phase were on average, higher than in the emulsion phase. Further, the temperatures of particles were lower closer to the distributor as a consequence of particle circulation and combustion-assisted attrition within the bed.     

KINETICS OF RICE HULL CHAR BURNOUT IN A BENCH-SCALE FLUIDIZED-BED REACTOR

Rice hull, also known as husk, is a by-product of the rice milling process; it is a significant alternative energy resource for the milling industries of rice producing countries. Apart from its high energy content, the residual ash from complete combustion of rice hull at moderate temperatures in a combustor, e.g., a fluidized-bed reactor, is also a potential cement extender. The regeneration of steam for parboil rice and for other process activities makes combustion an attractive means of disposing rice hull in the rice milling industries, which otherwise is a menace to the rice miller.

In the present work, the combustion kinetics of Guyana rice hull char was studied in a bench-scale fluidized-bed reactor. The salient features of the combustion of the char were observed, and the temporal history of burnout was traced at temperatures of 973 and 1173 K. The oxygen concentrations of the inlet fluidizing gas were maintained at 2.3 and 4.6% by regulating the mixing of nitrogen and air.

The rates of combustion of the residual char generated from rice hull through rapid devolatilization were determined to range from 3.18 × 10^-6 to 9.04 × 10^-6kg/s. By fitting the conversion data with various heterogeneous reaction models, it appears that the data could be described, to some extent, by the unreacted core model with ash layer diffusion control. However, the complexity of the carbon/silica arrangement in the residual char results in uncertainties in model selection.     

ECONOMIC CONSIDERATIONS IN FLUIDIZED BED DRYING OF PASTES USING INERT PARTICLES

A fluidized bed of inert particles lpar;packing)--> can be used advantageously for the drying of paste-like materials of high moisture content. Wet pasty material is fed into a fluidized bed of chemically inert coarse particles. The wet material coats the surface of the inert particles. Drying takes place mainly in the thin layer formed on the surface of particles. After reaching a certain moisture content, the dried material film breaks off the surface of the packing particles, and leave the fluidized bed as a fine powder in the exit gas stream.

Experiments were performed using different organic and inorganic materials e.g. raw materials from human and veterinary     

Depth of combustion inside char from evidence of oxidized pyrite

In coal combustion processes the rate controlling mechanism varies from external diffusion and pore diffusion to chemical kinetics. The particle zone where combustion takes place therefore changes from the outer layer, for large particles at high temperatures, to the total internal pore volume, for small particles at low temperatures. Partial penetration of oxygen occurs in intermediate cases. Recent publications report on fluidized bed experiments where, according to the model used, combustion takes place at or near the outer surface of the particle (‘shrinking particle model’) in a narrow boundary layer. Knowledge of the depth of this layer could contribute to combustion modelling. This paper shows that during fluidized bed combustion at 900 °C oxygen penetrates into the char to a depth of 50–100 μm. This is concluded from the width of the zone where pyrite particles in the char are oxidized. The presence of open pores may increase the depth of the internal combustion layer up to several hundreds of micrometres.