The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O₂ to the burning solid and of the products CO and CO₂ away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation

     

Experimental research of radiative heat transfer in fluidized beds

A new method to measure the radiative heat transfer in fluidized beds was presented. Experiments were carried out on a 0.8 th⁻¹ fluidized bed combustion boiler. The residual slag of fired coal was operated in a fluidized bed at room temperature. As the radiative heat transfer at room temperature is insignificant, its contribution at high temperatures might be obtained by the comparison of experimental results at high and low temperatures. On experimental study, a radiative contribution was given as a function of bed temperature and particle size. The results were compared with those in other references.     

A simple model for explosive combustion of premixed natural gas with air in a bubbling fluidized bed of inert sand

The combustion of premixed natural gas and air has been studied in a bubbling fluidized bed of inert particles. The temperature of the solids was carefully monitored, using 8 thermocouples, immersed in the bed at different heights. The observed temperature profiles were used to find the height above the distributor at which most of the combustion occurred and on this basis a clear distinction could be made between combustion above the bed and inside the bed. The region where most of the heat of combustion is evolved depends on the average bed temperature. If this temperature is low, the gases burn above the bed or just under its upper surface, but at higher temperatures the process is located close to the distributor. Rapid fluctuations in the measured temperature and pressure indicate that the process inside the bed is not a steady one. The model developed here assumes that combustion takes place inside bubbles of premixed gases, as they move through the bed. A detailed chemical kinetic model was used to calculate the induction period for ignition. The model can predict the height above the distributor at which bubbles should ignite and explode. Comparison of the experimental results with the modeling calculations indicates that the course taken by the process depends on temperature. At the lowest temperatures, the gases burn above the bed. In the high temperature range, where the bubbles ignite is determined by the induction period. At intermediate temperatures the location of the reaction is determined by the depth of the bed and bubble size, with ignition spreading from above the bed to bubbles, which are about to leave, but are still in the bed. That bubbles explode at different heights up the bed is reflected in the acoustic signals registered above and below the bed. The associated changes in the composition of the flue gases are also very characteristic.     

Effects of entrainment and agglomeration of particles on combustion of nano-aluminum and water mixtures

The combustion of nano-aluminum and water mixtures is studied theoretically for a particle size of 80 nm and over a pressure range of 1–10 MPa. Emphasis is placed on the effects of entrainment and agglomeration of particles on the burning rate and its dependence on pressure. The flame thickness increases by a factor of ∼10, when particle entrainment is considered. This lowers the conductive heat flux at the ignition front, thereby reducing the burning rate. The pressure dependence of the burning rate is attributed to the changes in the burning time and velocity of particles with pressure. In the diffusion limit, the pressure exponent increases from 0 to 0.5, when the entrainment index increases from 0 to 1.0. A similar trend is observed in the kinetics-controlled regime, although the corresponding value exceeds the diffusion counterpart by 0.5. The kinetics-controlled model significantly over-predicts the burning rate and its pressure exponent, depending on the entrainment index. The present analysis suggests that nano-particles formed closely-packed agglomerates of diameter 3–5 μm, which may burn under diffusion-controlled conditions at high pressures.     

Experimental study on coal multi-generation in dual fluidized beds

An atmospheric test system of dual fluidized beds for coal multi-generation was built.One bubbling fluidized bedis for gasification and a circulating fluidized bed for combustion.The two beds are combined with two valves:one valve to send high temperature ash from combustion bed to the gasification bed and another valve to sendchar and ash from gasification bed to combustion bed.Experiments on Shenhua coal multi-generation were madeat temperatures from 1112 K to 1191 K in the dual fluidized beds.The temperatures of the combustor are stableand the char combustion efficiency is about 98%.Increasing air/coal ratio to the fluidized bed leads to theincrease of temperature and gasification efficiency.The maximum gasification efficiency is 36.7% and thecalorific value of fuel gas is 10.7 MJ/Nm3.The tar yield in this work is 1.5%,much lower than that of pyrolysis.Carbon conversion efficiency to fuel gas and flue gas is about 90%.     

Agglomeration in fluidized beds at high temperatures: Mechanisms,detection and prevention

Fluidized-bed conversion of solid fuels is a well-established and widely used technology. Yet, operational problems are encountered in industrial practice. One of the most important problems is the occurrence of agglomeration at high temperature, meaning that bed particles adhere to each other to form larger entities (agglomerates). This process is often not recognized until sudden defluidization and often leads to a costly shutdown of the whole installation. In particular, the thermal conversion of certain biomass fuels, which is becoming increasingly popular, increases the risk of agglomeration.     

Description,applications and numerical modelling of bubbling fluidized bed combustion in waste-to-energy plants

The use of the fluidized bed combustor (FBC) has increased. It began in the 20th century as coal combustion and gasification, which then developed into catalytic reactions. Only recently, the application field has been extended to the incineration of biomass and pre-treated waste, for either power generation or waste disposal. The success of fluidized bed combustion is due to high combustion efficiency, great flexibility when it comes to the heating value of the fuel and reduction in pollutants emitted with the flue gas.     

Aluminum agglomeration reduction in a composite propellant using tailored Al/PTFE particles

In solid propellants, aluminum is widely used to improve performance, yet theoretical specific impulse is still not achieved largely because of two-phase flow losses. Losses could be reduced if aluminum particles quickly ignited, more gaseous products were produced and if upon combustion, aluminum particle breakup occurred. To explore this, tailored, fuel-rich, mechanically activated composite particles (aluminum/polytetrafluoroethylene, Al/PTFE 90/10 and 70/30 wt.%) are considered as replacements for reference aluminum powders (spherical, flake, or nanoscale) in a composite solid propellant. The effects on burning rate, pressure dependence, and aluminum ignition, combustion, and agglomeration are quantified. Using microscopic imaging, it is observed that tailored particles promptly ignite at the burning surface and appear to breakup into smaller particles, which can increase the heat feedback to the burning surface. Replacement of spherical aluminum with Al/PTFE 90/10 wt.% does not significantly affect propellant burning rate. However, Al/PTFE 70/30 wt.% increases the pressure exponent from 0.36 to 0.58, which results in a 50% increase in propellant burning rate at 13.8 MPa. This increased pressure sensitivity is consistent with more kinetically controlled combustion that occurs from smaller burning metal particles near the surface. Combustion products were quench collected using a new, liquid-free technique at 2.1 and 6.9 MPa and were measured. Both Al/PTFE 90/10 and 70/30 wt.% composite particles reduce the coarse product fraction and diameter. The most significant reduction occurs from 70/30 wt.% particle use, where average coarse product diameter is 25 μm, which is smaller than the original, average particle size and is also smaller than the 76 μm products collected from reference spherical aluminized propellant. This is a 66% decrease in agglomerate diameter or a 96% decrease in volume compared to agglomerates formed from reference spherical aluminum. Smaller diameter condensed phase products and more gaseous products will likely decrease two-phase flow loss and reduce slag accumulation.     

Effect of agglomeration/defluidization on hydrogen generation during fluidized bed air gasification of modified biomass

This study presents the effect of particle agglomeration on syngas emission during the biomass air gasification process. Various operating conditions such as operating temperature, equivalence ratio (ER), and amount of bed materials are employed. The concentrations of H₂ and CO increase along with the operating time as agglomeration begins, while CO₂ decreases at the same time. However, there is no significant change in the emission concentration of CH₄ during the defluidization process. The lower heating value increases while the system reaches the agglomeration/defluidization under various operating parameters. When the system reaches the agglomeration/defluidization process, the LHV value sharply increases. The results are obtained when the system reaches agglomeration/defluidization. The temperature increases while bed agglomeration occurs. A higher temperature increases the production of H₂ and CO, contributing to the LHV calculation.     

皇竹草流态化燃烧粘结特性试验研究

以能源植物皇竹草作为研究对象,利用5 kW鼓泡流化床实验装置,研究床层温度、燃料形状以及燃料中钙元素含量对流化床粘结失流现象的影响.试验结果表明:失流时间随着床层温度上升而缩短;燃料形状对失流时间影响不明显;钙元素具有延长失流时间的作用.床料和飞灰的ICP-OES分析结果表明,床层温度较高时,在床料中K元素较少的情况下也能发生失流现象.     

Two-phase biomass air-steam gasification model for fluidized bed reactors: Part III—model validation

The developed two-phase air-steam gasification model (Biomass Bioenergy, xxxxx) was validated using the experimental results obtained from a dual distributor fluidized bed reactor. The reactor was operated on wheat straw at various fluidization velocities, steam flow rates and biomass to steam ratios. A good agreement between the model predictions and experimental data was obtained under all operating conditions studied. The model predicted the temperatures of the bubble, emulsion and solid phases, the mole fractions of methane, hydrogen, carbon monoxide, carbon dioxide and nitrogen, and the higher heating value of the producer gas with high accuracy (R²=0.88–0.98). The correlation coefficient (R²) for the gas production was somewhat lower (0.75), which could be attributed to the assumption that the gases behaved ideally.     

Two phase biomass air-steam gasification model for fluidized bed reactors: Part II—model sensitivity

A sensitivity analysis was performed on the two phase biomass gasification model developed by Sadaka et al. (Biomass Bioenergy) to test its response to variations in three operating parameters (fluidization velocity, steam flow rate and biomass to steam ratio). The model performance criteria included bed temperature, gas compositions, higher heating value and gas production rate. The results showed that the model was sensitive to changes in all operating parameters. The temperatures of the reactor were more influenced by changes in the steam flow rate than those of fluidization velocity and biomass to steam ratio. The steam flow rate has the most effect on the mole fractions of CH₄, and CO₂ followed by the biomass to steam ratio and then the fluidization velocity. In the case of H₂, and CO the biomass to steam ratio has the most effect on their mole fractions followed by the steam flow rate and then fluidization velocity. The fluidization velocity has the most effect on the mole fraction of N₂. The biomass to steam ratio has the most effect on the gas higher heating value and the gas production rate.     

Recent Chinese heat-transfer research on bubbling and circulating fluidized beds

Recent research work conducted in China on heat transfer to immersed surfaces in bubbling and circulating fluidized beds, including coal combustors and boilers, is reviewed. Studies include measurements of heat-transfer coefficients (local and total) from horizontal and vertical tubes in bubbling fluidized beds and their variations with operating and system parameters. Special designs developed for heat-transfer probes and theoretical models for the computation of heat-transfer coefficients are discussed. Recent work is presented on circulating fluidized beds with emphasis on hydrodynamics and heat transfer. Proposed correlations and novel designs of thin-film heat-transfer and capacitance probes are included.     

Two phase biomass air-steam gasification model for fluidized bed reactors: Part I—model development

A two-phase model capable of predicting the performance of fluidized bed biomass air-steam gasification reactor during dynamic and steady state operations was developed based on the two phase theory of fluidization. Material and energy balances were taken into consideration and the minimization of free energy technique was used to calculate the gas mole fractions. The fluidized bed was divided into three zones (jetting, bubbling and slugging) and the mass and heat transfer coefficients were calculated for each zone in both bubble and emulsion phases. The model includes the hydrodynamics, transport and thermodynamic properties of fluidized bed. The finite element method was used to solve the partial differential equations. The input variables of the computer program included fluidization velocity, steam flow rate and biomass to steam ratio. The model is capable of predicting the bed temperature, gas mole fractions, higher heating value and production rate.     

Emission characteristics of organic and heavy metal pollutants in fluidized bed incineration during the agglomeration/defluidization process

The accumulation of adhesive materials may generate agglomerates during incineration. These agglomerates affect fluidization behavior and cause the formation of secondary pollutants. However, the impact of agglomeration on the emission of organics and heavy metals has seldom been investigated. Accordingly, this work focuses on the preparation of different synthetic wastes to simulate the generation of agglomerates, as well as the effects of various alkali metals, alkaline earth metals, and operating temperatures on the formation of pollutants.The experimental results indicate that defluidization time declines as the concentration of sodium increases, and that alkaline earth metals (Mg and Ca) inhibit agglomeration. Concentrations of organic pollutants gradually increase with operating time, indicating that the size of the agglomerate gradually increases, reducing the quality of fluidization and the efficiency of combustion. After defluidization, the temperature on the surface of the sand bed increases, thereby reducing the concentrations of organics, but it remains higher than that of blank operation (without Na addition). Furthermore, concentrations of three volatile metals (Cd, Pb, Cr) follow similar trends with operating time. Independent of whether the agglomerate is formed, the concentrations of emitted heavy metals are similar. After defluidization, the emitted concentration increases, because the system is transformed to the phase of the fixed bed. The abundant silica sand does not capture the heavy metals released from waste combustion. The increasing temperature of the surface of the sand bed significantly enhances the emission of heavy metals. Accordingly, the emitted organic and heavy metal pollutants behave differently during agglomeration and defluidization.     

Experimental study of wood char gasification kinetics in fluidized beds

During gasification two steps take place. The first one is pyrolysis and the second one is gasification of the char that remains back after pyrolysis. The second step is slower than the first one, so this step is the limiting factor in designing fluidized beds. Kinetic data for designing fluidized beds are necessary. The paper describes gravimetric measurements directly applied to fluidized bed with large sample sizes. The samples are char of 6 mm wood pellets and 10–40 mm wood cubes in order to directly measure ”apparent kinetics”. The parameters examined in this paper are particle size, product gases (= hydrogen) in the gasification medium, type of wood and differences in CO₂/steam gasification. The results are presented as Arrhenius diagrams and half-value period diagrams. The most important parameters are the temperature and product gases (hydrogen) in the gasification agent. The particle size seems to be less important for large wood particles as the measurements do not show significant differences for gasifying char of wood cubes 10–40 mm. The half-value periods for gasification of char from wood cubes (10 mm - 40 mm) with 100% steam at atmospheric pressure lie between 1000 s at 1023 K and 300 s at 1173 K. For char of 6 mm wood pellets the half-value periods lie between 1900 s at 1023 K and 250 s at 1173 K. The reaction is most likely in pore diffusion regime.     

Experimental studies on cotton stalk combustion in a fluidized bed

The present work reports studies on the mixing and combustion characteristics of cotton stalk (CS) with 10–100 mm in length in a fluidized bed. Effects of length and initial weight percentage of CS, diameter of alumina bed material as well as gas velocity on the mixing characteristics of CS with alumina were investigated. CS can mix well with 0.6–1 mm alumina at fluidization number N=3–8.     

Bench-scale bubbling fluidized bed systems around the world - Bed agglomeration and collapse: A comprehensive review

Thermochemical conversion by gasification process is one of the most relevant technologies for energy recovery from solid fuel, with an energy conversion efficiency better than other alternatives like combustion and pyrolysis. Nevertheless, the most common technology used in the last decades for thermochemical conversion of solid fuel through gasification process, such as coal, agriculture residues or biomass residues are the fluidized bed or bubbling fluidized bed system. For these gasification technologies, an inert bed material is fed into reactor to improve the homogenization of the particles mixture and increase the heat transfer between solid fuel particles and the bed material. The fluidized bed reactors usually operate at isothermal bed temperatures in the range of 700–1000 °C, providing a suitable contact between solid and gas phases. In this way, chemical reactions with high conversion yield, as well as an intense circulation and mixing of the solid particles are encouraged. Moreover, a high gasification temperature favours carbon conversion efficiency, increasing the syngas production and energy performance of the gasifier. However, the risk of eutectic mixtures formation and its subsequent melting process are increased, and hence the probability of bed agglomeration and the system collapse could be increased, mainly when alkali and alkaline earth metals-rich biomasses are considered. Generally, bed agglomeration occurs when biomass-derived ash reacts with bed material, and the lower melting temperature of ash components promotes the formation of highly viscous layers, which encourages the progressive agglomerates creation, and consequently, the bed collapse and system de-fluidization. Taking into account the relevance of this topic to ensure the normal gasification process operating, this paper provides several aspects about bed agglomeration, mostly for biomass gasification systems. In this way, chemistry and mechanism of bed agglomeration, as well as, some methods for in-situ detection and prediction of the bed agglomeration phenomenon are reviewed and discussed.     

Experimental study on cotton stalk combustion in a circulating fluidized bed

This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO₂ and NO) in flue (stack) gas were measured.     

褐煤在鼓泡流化床和循环流化床燃烧的汞迁移试验研究

对褐煤在小型电加热鼓泡流化床和小型电加热循环流化床中燃烧时的汞迁移特性进行了对比试验研究,重点考察了不同燃烧工况对汞迁移特性的影响。试验结果表明,炉膛温度和给煤量增加,鼓泡流化床和循环流化床的烟气总汞HgT均增加,飞灰颗粒汞含量Hg(p)均减少,并且循环流化床的烟气总汞HgT值均低于相同燃烧工况的鼓泡流化床值,循环流化床的飞灰颗粒汞含量Hg(p)值均高于相同燃烧工况的鼓泡流化床的值;流化风速增加,循环流化床的烟气总汞HgT减少,飞灰颗粒汞含量Hg(p)增加,鼓泡流化床烟气总汞HgT增加,飞灰颗粒汞含量Hg(p)减少。