Solid Suspension Density Research Articles

Numerical Simulation Study on the Gas–Solid Flow Characteristics of a Large-Scale Dual Fluidized Bed Reactor: Verification and Extension

Dual fluidized bed (DFB) reactor systems are widely used in gas–solid two-phase flow applications, whose gas–solid flow characteristics have a significant effect on the performance of many kinds of technologies. A numerical simulation model was established on the basis of a large-scale DFB reactor with a maximum height of 21.6 m, and numerical simulations focused on gas–solid flow characteristics were carried out. The effects of the superficial gas velocity of both beds and the static bed height and particle size on the distribution of the pressure and solid suspension density and the solid circulation rate were studied. The simulation results were in good agreement with the experimental data. With the strong support of the experimental data, the gas–solid flow characteristics of large-scale DFB reactors were innovatively evaluated in this numerical simulation study, which effectively makes up for the shortcomings of the current research. The results showed that the superficial gas velocity of both beds and the static bed height have different degrees of influence on the gas–solid flow characteristics. Specifically, for 282 μm particles, when the superficial gas velocity of both beds and the static bed height were 4.5 m/s, 2.5 m/s, and 0.65 m, respectively, under typical working conditions, the bottom pressure of the two furnaces was 3412.42 Pa and 2812.86 Pa, respectively, and the solid suspension density was 409.44 kg/m3 and 427.89 kg/m3, respectively. Based on the simulation results, the empirical formulas of the solid circulation rate were fitted according to different particle sizes. Under similar conditions, the solid circulation rates of particles with a particle size of 100 μm, 282 μm, 641 μm, and 1000 μm were 2.84–13.28, 0.73–4.91, 0.024–0.216, and 0.0026–0.0095 kg/(m2s), respectively. It can be found that the influence of the particle size on the solid circulation rate is the most significant among all parameters.

Experimental Research on the Gas-Solid Flow Characteristics in Large-Scale Dual Fluidized Bed Reactor

A dual fluidized bed (DFB) reactor is the main operating system of various energy-efficient and clean utilization technologies. The gas-solid flow characteristics of the DFB reactor greatly affect the efficiency of various technologies. A large-scale DFB reactor with a maximum height of 21.6 m was built and relevant cold mode tests were carried out in this study. The effects of the superficial gas velocity of both beds, static bed height and particle size on the distribution of both pressure and solid suspension density, solid circulation rate, solid inventory distribution ratio and other characteristics were studied. For 282 μm-particles, the solid suspension density in the dense phase zone of the two beds was 100–400 and 400–800 kg/m3, respectively, when the static bed height was 0.65 m; the solid circulation rate was about 0.87–1.75, 1.04–3.04 and 1.13–3.69 kg/(m2s) when the static bed height was 0.65, 0.95 and 1.25 m, respectively. The solid circulation rate was positively correlated with the static bed height and the superficial gas velocity of both beds, yet negatively correlated with the particle size. Additionally, the empirical equation of solid circulation rate and the empirical equation of solid inventory distribution ratio were proposed, respectively. The material control method of the DFB reactor is put forward.

Operational optimization of air staging and flue gas recirculation for NOx reduction in biomass circulating fluidized bed combustion

Cost effective nitrogen oxide (NOx) reduction technology is crucial for industries and power plants owing to the stringent environmental regulations to which industries and power plants must adhere. Herein, the aim was to optimize the operational conditions for NOx reduction in forest biomass using circulating fluidized bed combustion (CFBC) combined with air staging and flue gas recirculation (FGR). The profiles of temperature and solid suspension density in the combustor, NO and CO emissions, fuel-N conversion, slagging and fouling tendencies, and combustion efficiency were comprehensively observed during combustion. The main operating parameters are the FGR ratios, optimal ratios of air staging [Primary air (PA):Secondary air (SA):Tertiary air (TA)] and FGR, and supply height of TA in the combustor. Notably, under the optimal operational conditions (air staging ratio; PA:SA:TA = 70:10:30, FGR ratio; 8%, and TA supply height; 8.1 m), the NO and CO emissions were simultaneously reduced to 25.9% and 18.2%, respectively, compared with that for the base scenario which excluded air staging and FGR. Furthermore, a high combustion efficiency of 99.72% was maintained without any ash-related problems at well-controlled bed temperatures owing to the constant solid circulation afforded by FGR. A thorough analysis of the this literature has been performed to ensure that the commercialization of the combination of air staging and FGR in biomass CFBC plants is feasible, which could be an inexpensive option for NO reduction.

Enhancing oxygen savings and carbon dioxide purity in biomass oxy-circulating fluidized bed combustion with an oxygen carrier

A pure oxygen (O2) supply for oxy-fuel combustion causes excessive power consumption and decreased net plant efficiency. The oxygen carrier-aided oxy-circulating fluidized-bed combustion (OCA-Oxy-CFBC) process can facilitate reduction in the O2 supply requirement. Moreover, the use of an oxygen carrier, instead of silica sand, as bed materials may improve oxygen transport in the combustor, leading to a reduction in the pure O2 supply required. This study aimed to investigate the effect of oxygen carrier addition on Oxy-CFBC characteristics, including temperature, pressure, solid suspension density, pollutant emissions, combustion efficiency, and O2 reduction for stable operation. As an oxygen carrier, iron ore was introduced into the downcomer pipeline using a dedicated hopper during the biomass Oxy-CFBC process under low excess O2 conditions. Although the O2 concentration in the flue gas decreased by 1.7 vol%, the CO emissions decreased from 6.8 mg/MJ to 2.8 mg/MJ when the silica sand was replaced with 33 wt% iron ore. Furthermore, the CO2 purity in the flue gas improved from 94.3 vol% to 95.7–97.4 vol% under a lower equivalence ratio (1.01–1.07) during stable OCA-Oxy-CFBC operation. Consequently, oxygen can be transported and supplied by adding an oxygen carrier during the biomass Oxy-CFBC operation, indicating O2 savings of 4.86 % for the total amount of O2 required.

Prediction and minimization of NOx emission in a circulating fluidized bed combustor: Improvement of bed quality by optimizing cyclone performance and coal particle size

• Improvement of cyclone performance contributes to reducing the NO x emission. • Reducing coal particle size can decrease the NO x emission. • NO x emission may be further reduced by reducing the height of the coal inlet. • Effects of cyclone efficiency and coal size on combustion efficiency are discussed. • A comprehensive CFB mathematical model is applied to further analyze the mechanism. The NO x emission of circulating fluidized bed (CFB) boilers can be evidently reduced via optimization of the cyclone performance and the coal particle size, which has been proved in industrial practices, while there are few reasonable interpretations for these phenomena so far. In this paper, a comprehensive 1-dimensional/1.5-dimensional CFB mathematical model was applied to investigate the effect of these two important factors on NO x emission. Simulation results on two commercial CFB boilers indicate that increasing the cyclone efficiency and reducing the coal particle size can significantly decrease the average size of bed materials and increase the solid suspension density in the freeboard zone; namely, the bed quality is improved. The local gas–solid flow behaviors inside the CFB furnace will be accordingly changed, such as the increasing volume fraction of bubble phase in dense bed, shortening of secondary air penetration depth, enrichment of particles in the downward-flow annulus or clusters, etc. Consequently, the reducing atmosphere will be formed or enhanced in local areas, leading to the effective inhibition of NO x formation. Meanwhile, the increase in the bed inventory of fine char particles also facilitates the reduction of NO x . Moreover, if appropriately reducing the height of the coal inlet or feeding fine coal particles from the solid recycle system, the NO x emission may be further reduced due to the adjustment of the volatiles release pattern. In addition, the reduction of coal particle size can contribute to the increase in fuel combustion efficiency, while the unburned carbon content in fly/bottom ash may increase or decrease with the improvement of cyclone efficiency. This study further analyzes the influence of gas–solid flow characteristics on the chemical reaction process and provides a new solution to the low-cost pollutant emission control for CFB boilers.

Anti-wear beam effects on water wall wear in a CFB boiler

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one method to protect against water wall wear. Beam effects from superficial gas velocity, solid suspension density, bed material size and beam size were investigated on water wall wear in a CFB test rig experimentally using a measurement system consisting of thin-film resistor sensors and high-precision resistance probers. Multiple sensors were set up around an anti-wear beam vertically to obtain the axial profile of the wear rate of water wall.Results show that installed anti-wear beams may reduce the wear rate of water wall. Along the water wall around the beam vertically, the maximum wear rate occurs at the upper surface of the beam while the minimum rate occurs at the bottom surface. In general, the wear rate of the water wall around an anti-wear beam increased with the superficial gas velocity, initial bed material (static bed height) and particle size, while the wear rate just below the beam was not notably influenced by those parameters. An optimum anti-beam width in the range of 6–9mm is recommended under the test condition.

Effect of furnace pressure drop on heat transfer in a 135 MW CFB boiler

The effect of the pressure drop across the furnace on heat transfer inside large scale CFB boilers was investigated with a 1-D CFB combustion model developed at Tsinghua University. The results from the 1-D model agree well with the field test data carried out in a 135MWe CFB boiler. It was found that, at a certain load and with fixed size distribution of the coal particles fed to the boiler, a decrease in furnace pressure drop yields an increase in furnace temperature and reduced heat transfer coefficient. According to the heat balance, when the pressure drop decreases by 1kPa, the heat transfer coefficient is expected to decrease approximately by 2%. The increase in temperature with decreasing pressure drop is less steep, when the boiler load increases or smaller sized coal particles are fed to the boiler. In both cases, more fine particles are carried to the upper parts of the furnace which results in a more uniform solid volume fraction distribution along the furnace height. The influence of the pressure drop on heat transfer by changing the solid suspension density weakens. Operation of the bed at relatively low pressure drop by reducing the size of coal particles will ensure adequate heat transfer.

Mathematical model for reactive recovery of invertase by chemical permeabilization of baker's yeast

BACKGROUND Cell permeabilization techniques allow selective release of an enzyme and specific reactions without breaking the cells. A mathematical model has been developed for selective permeabilization of Saccharomyces cerevisiae to recover invertase at high purity. The effect of thiols on the cell walls of S. cerevisiae was studied under different conditions of pH, cysteine concentration, speed of agitation and solid suspension density. RESULT The rate of release of invertase was a function of pH and cysteine (or thiolate ion) concentration.The maximum recovery of invertase was obtained at pH 10.0, 10 mmol dm−3 cysteine concentration and 15% (w/v) yeast loading. CONCLUSION Thiols are capable of permeabilizing the cell walls of S. cerevisiae and releasing the invertase under basic conditions. The insignificant effect of agitation shows that the external mass transfer resistance does not play any role in the release of invertase. © 2015 Society of Chemical Industry

Heat transfer behavior inside a furnace chamber of large-scale supercritical CFB reactor

In this paper, heat transfer experiments were conducted in order to evaluate the contributions of particle convection, gas convection, cluster convection and also radiation from cluster and dispersed phases to the overall heat transfer coefficient in a furnace chamber of a large-scale supercritical CFB reactor. Heat transfer behavior in a circulating fluidized bed between the water membrane wall and bed inventory has been analyzed for Geldart B particles. Bed inventory used in the tests had particle density in the range of 2650–2750kg/m3. The experimental CFB facility has the dimension of 27.6×5.3m in bed rectangular cross-section and 48m in height of the furnace. The performance tests have been carried out at three different sizes of the bed material with Sauter mean particle diameters of 0.246, 0.272 and 0.444mm. Above heat transfer studies have been conducted for the operating parameters covering the range of 1082–1183K for bed temperature, 2.92–5.25m/s for superficial velocity, 6.86–8.25kPa for pressure drop, 23–25.6kg/(ms2) for the circulation rate of solids and 0.4–0.44 for the bed voidage at minimum fluidization velocity. In order to predict the overall heat transfer coefficient, a cluster renewal approach was used. The bed-to-wall heat transfer coefficients obtained in these operating ranges were about 78–226W/(m2K). The contributions of convective and radiative heat transfer to the overall heat transfer coefficient were estimated. The variation in contributions depended on solid suspension density and bed temperature. The average contribution of convective and radiative heat transfer components varied between 13.5–54% and 46–86.5%, respectively.

Experimental study of gas–solid flow over a novel heating surface in a CFB furnace

As the capacity of a circulating fluidized bed (CFB) boiler is increased, the surface area of the wing walls must be increased to maintain the heat balance in the furnace. The minimum spacing requirement between adjacent wing walls limits their total area. A novel suspended heat transfer surface is proposed to address this problem. The suspended surface is designed with a “U” shape and is referred to as U-wall. U-walls are suspended from the top of the furnace and may provide more than 25% greater heat transfer surface area. In this study, the flow of solids over U-walls in a CFB was measured using a fiber optical probe. The effects of the superficial gas velocity, the bed material inventory, and the width of the U-walls on the solid volume fraction and particle velocity were investigated. The solid volume fraction had a U-shaped profile inside the U-wall except in the upper section, where the profile had an approximate straight-line shape. Their values increased with the superficial gas velocity and the bed material inventory. The solid vertical velocity had an inverted-U-shaped profile inside the U-wall. The solids were found to flow downward at the U-wall surface in the upper section, where the velocity increased with the superficial gas velocity and the solid suspension density. The flow of solids at the U-wall surface in the middle and lower sections changed from downward to upward as the superficial gas velocity increased. In a CFB furnace, there is an optimal width for U-walls that provides the best heat absorption characteristics; larger or smaller widths are both undesirable.

Experimental investigation and model validation of the heat flux profile in a 300MW CFB boiler

In this paper, systematic experimental investigation on the heat flux distribution inside the furnace of a 300 MW CFB boiler was presented. Detailed experimental setup and measurement techniques were presented and a finite element method approach was applied to determine the heat flux. The heat flux profile on the rear wall along the horizontal direction shows a significant imbalance at different boiler loads. As a result of the non-uniform layout of the heating surfaces, which is the essential reason, as well as the imbalance and deviation of the temperature field, solid suspension density and solid flow rate, the central section of the furnace possesses higher heat flux distribution compared to the side sections. The heat flux is also found to increase with the increasing boiler load and decrease as the height increases. Heat flux near the roof, where the solid suspension density is rather small, is found to decrease remarkably revealing less heat absorption in this area. In addition, an empirical model of heat transfer coefficient is revised using the average data at different boiler loads. A mechanism heat transfer model based on the membrane water-wall configuration is proposed and validated with the heat flux profile obtained from the measurement. The model provides good accuracy for correlating 85% of the data within ± 10%.

Effect of Bed Pressure Drop on Performance of a CFB Boiler

The effect of bed pressure drop and bed inventory on the performances of a circulating fluidized bed (CFB) boiler was studied. By using the state specification design theory, the fluidization state of the gas−solids flow in the furnace of conventional CFB boilers was reconstructed to operate at a much lower bed pressure drop by reducing bed inventory and control bed quality. Through theoretical analysis, it was suggested that there would exist a theoretical optimal value of bed pressure drop, around which the boiler operation can achieve the maximal combustion efficiency and with significant reduction of the wear of the heating surface and fan energy consumption. The analysis was validated by field tests carried out in a 75 t/h CFB boiler. At full boiler load, when bed pressure drop was reduced from 7.3 to 3.2 kPa, the height of the dense zone in the lower furnace decreased, but the solid suspension density profile in the upper furnace and solid flow rate were barely influenced. Consequently, the average heat transfer coefficient in the furnace was kept nearly the same and the furnace temperature increment was less than 17 °C. It was also found that the carbon content in the fly ash decreased first with decreasing bed pressure drop and then increased with further increasing bed pressure drop. The turning point with minimal carbon content was referred to as the point with optimal bed pressure drop. For this boiler, at the optimum point the bed pressure was around 5.7 kPa with the overall excess air ratio of 1.06. When the boiler was operated around this optimal point, not only the combustion efficiency was improved, but also fan energy consumption and wear of heating surface were reduced.

Modeling and Simulation of Crystal Size Distribution in Imperfectly Mixed Suspension Crystallization.

A model for simulating crystallization in imperfectly mixed suspensions was developed based on a population balance equation that takes into account the solid suspension density distributions of different sizes of particles in the crystallizer. A simulation method was developed based on mass balance and population balance. With this method, the particle size distributions in the product and in the crystallizer were predicted. The simulated particle size distributions in the product were verified with experimental data.

Evaluation of the gas–solid suspension density in CFB risers with exit effects

In order to model the gas–solid hydrodynamic flow structure in circulating fluidized bed (CFB) riser reactors, there is a need to predict the average and local suspension density and flow direction as functions of both axial and radial coordinates, gas and solids physical properties, and operating conditions (gas superficial velocity and imposed solids circulation rate). Existing hydrodynamic models fail to properly and reliably describe the flow characteristics typical of CFB catalytic reactors operating at high superficial gas velocities and solids mass fluxes. In the present work, we have compiled an extensive experimental database to encompass published laboratory, pilot, and available industrial scale hydrodynamic data in a concise and useable form. Thorough analysis of the database followed by an in-depth multiple regression analysis yielded a new correlation for calculating the “slip factor”, defined as the ratio of the interstitial gas velocity to the particle velocity, to describe Geldart Group A powder systems in the fully developed flow (FDF) region of the riser. The correlation allows the evaluation of the gas–solid suspension density in the riser in both the fast fluidization (FF) and pneumatic transport (PTR) flow regimes. In addition, a comparative study has been carried out on the database to investigate the effects of the geometrical configuration of the top exit of the riser on the suspension density and flow structure. Two reflection coefficient correlations are proposed to account for exit effects for both Groups A and B solids systems. The slip factor and reflection coefficient correlations have been further compared to existing correlations available in the literature for discussion and validation and shown to successfully characterize the large majority of data available in the literature.

Changing market realities enhance the appeal of ASICs : Iwao Sagara. J. Electron. Engng, 28 (October 1986)

Anti-wear beams installed on water walls of circulating fluidized bed (CFB) boilers are one method to protect against water wall wear. Beam effects from superficial gas velocity, solid suspension density, bed material size and beam size were investigated on water wall wear in a CFB test rig experimentally using a measurement system consisting of thin-film resistor sensors and high-precision resistance probers. Multiple sensors were set up around an anti-wear beam vertically to obtain the axial profile of the wear rate of water wall.Results show that installed anti-wear beams may reduce the wear rate of water wall. Along the water wall around the beam vertically, the maximum wear rate occurs at the upper surface of the beam while the minimum rate occurs at the bottom surface. In general, the wear rate of the water wall around an anti-wear beam increased with the superficial gas velocity, initial bed material (static bed height) and particle size, while the wear rate just below the beam was not notably influenced by those parameters. An optimum anti-beam width in the range of 6–9 mm is recommended under the test condition.