Effect of entrainment on combustion efficiency high ash anthracites in fluidized bed combustors

The characteristics of the freeboard combustion of Korean low grade anthracites were studied in two fluidized bed combustors, which employed over-bed feeding and non-recycle of cyclone captured solids. The total combustion efficiency was ruled mainly by entrainment of fed coal particles and more or less by the freeboard combustion of entrained coal particles. The conversion of entrained coal particles increased with the bed temperature and decreased with the higher coal feed rate per bed area.     

流化床与颗粒剂

流化床造粒融合了设备、辅料和颗粒设计等多项技术。本文分析了造粒机理 ,并从造粒芯、辅料及颗粒设计等方面介绍了流化床造粒技术的最新进展     

Fluidized bed combustion of refuse-derived fuel in presence of protective coal ash

Combustion of refuse-derived fuel (RDF) alone or together with other biomass leads to superheater fouling and corrosion in efficient power plants (with high steam values) due to vaporization and condensation of alkali chlorides. In this study, means were found to raise the portion of RDF to 40% enb without risk to boilers. This was done by co-firing RDF with coal and optimizing coal quality. Free aluminum silicate in coal captured alkalies from vaporized alkali chlorides preventing Cl condensation to superheaters. Strong fouling and corrosion were simultaneously averted. Results from 100 kW and 4 MW CFB reactors are reported.     

Fluidized bed combustion of refuse-derived fuel in presence of protective coal ash

Combustion of refuse-derived fuel (RDF) alone or together with other biomass leads to superheater fouling and corrosion in efficient power plants (with high steam values) due to vaporization and condensation of alkali chlorides. In this study, means were found to raise the portion of RDF to 40% enb without risk to boilers. This was done by co-firing RDF with coal and optimizing coal quality. Free aluminum silicate in coal captured alkalies from vaporized alkali chlorides preventing Cl condensation to superheaters. Strong fouling and corrosion were simultaneously averted. Results from 100 kW and 4 MW CFB reactors are reported.     

Enhancement of combustion efficiency with mixing ratio during fluidized bed combustion of anthracite and bituminous blended coal

In order to investigate the effect of mixing ratio of bituminous coal to blended coal on the enhancement of combustion efficiency, combustion experiments of blended coal with anthracite and bituminous are done in a laboratory scale fluidized bed combustor (10.8 cm ID and 170 cm height). The gross heating values of anthracite and bituminous coal used in this study are 2,810 cal/g and 6,572 cal/g, respectively. Experimental parameters are fuel feed rate, superficial gas velocity and mixing ratio of bituminous coal to blended coal. The combustion efficiency increases with the mixing ratio of bituminous coal due to the lower unburned carbon losses and higher burning velocity of bituminous coal. The rate of combustion in the combustor was increased with mixing ratio resulted from a higher burning velocity of bituminous coal. The measured combustion efficiency experimentally is about 3.5-12.4% higher than that of the calculated value based on the individual combustion of anthracite and bituminous coal under the same operating conditions. The optimum mixing ratio (MR) of bituminous coal determined is around 0.75 in this study. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Fluidized bed combustion of coal

To conserve oil an alternative start-up procedure for the fluidized-bed combustor has been established which uses charcoal. The elutriated carbon loss has been measured for different grades of coal, and the variation of carbon loss with fluidization velocity is also reported. The results could be useful in the design of fluidized-bed combustion systems.     

Effect of attrition on particle size distribution and SO2 capture in fluidized bed combustion under high CO2 partial pressure conditions

In both pressurized and oxygen-enriched fluidized bed combustion the partial pressure of CO₂ in the reactor becomes high, which affects SO₂ capture by limestone. Both of these technologies are also applicable to decreasing greenhouse gas emissions; the first one by increasing the efficiency of electric energy production and the latter by enabling capture of carbon dioxide for storage.Attrition increases the reaction rate by removing the sulphated layer on the particle, thus reducing the diffusion resistance. In the well-known solution for the shrinking core model the reaction time can be presented as the sum of the contributions of the kinetics and diffusion. It is shown that the effect of attrition can be expressed as an auxiliary term in this expression. A method to extract the diffusivity of the product layer from the SO₂ response in a bench-scale fluidized bed test using a limestone sample with a wide particle size distribution is presented. Based on a population balance model, a method to estimate the particle-size-dependent attrition rate from measured particle size distributions of the feed and bed material is illustrated for a 71-MW_e pressurized power plant. In addition attrition and its effect on the optimization of the limestone particle size for sulphur capture in oxygen-enriched combustion are discussed.     

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.     

Fluidized bed combustion of pre-dried Thai bagasse

Sugar cane bagasse is one of the most viable biomass fuels in Thailand. However, because of high moisture content in the fuel (of 45–55%), combustion of “as-received” bagasse is rather ineffective and unstable process.In this experimental work, the bagasse was preliminarily dried before conducting combustion tests on a fluidized bed combustor with a conical bed. Silica sand was used as the inert bed material for ensuring sustainable fuel ignition and combustion in this reactor. Effects of operating variables (fuel feed rate and excess air) as well as static bed height on the axial temperature and gas concentration profiles (for O₂, CO₂, CO and NO_x) in the combustor were investigated. For estimating the environmental impact by this bagasse-fuelled system, CO and NO_x emissions from the combustor were quantified. The combustion efficiency was found to be in the range of 96 to 99.7% for firing the pre-dried sugar cane bagasse in wide ranges of the operating variables.     

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。     

粉煤灰粒度分布对水泥性能的影响

主要研究了不同粉磨时间的粉煤灰密度、比表面积、粒度分布等颗粒特性的变化规律,以及研究了将不同粉磨时间的粉煤灰按30％比例掺人硅酸盐水泥中的水泥性能变化情况.结果表明：随着粉磨时间的增加,粉煤灰颗粒的密度、比表面积和粒度分布都呈有规律的变化;其中粉磨60 min时的粉煤灰水泥性能达到最佳.     

Identification of the initial particle size distribution for coal combustion simulations

Pulverized coal generally features wide particle-size distributions (PSD). How to set an initial PSD in coal combustion simulation remains an open question. To answer this, the Gaussian-quadrature theory was applied to provide a discrete reconstruction of the PSD described by Rosin-Rammler and Upper-Limit functions. From the perspective of the moments of PSD, a theoretical analysis was conducted for mass, momentum, and energy exchange terms between gas and particle phases in pulverized coal jet flame. The analysis presented that the first, the second and the third moments of PSD were the most important to capture the flame ignition behavior. The large eddy simulations showed that the proposed method with two or more particle sizes could provide good predictions of flame ignition distances. And Sauter mean diameter (D₃₂) could represent the complete PSD when predicting the ignition behavior of pulverized coal jet flame.     

酵母悬浮液进料时膨胀床空隙率和粒径分布的动态变化

引言 膨胀床吸附技术可以直接从细胞破碎液(未澄清液)中提取目标产物[1-3],而无需预先进行固液分离操作,具有快速、高效和集成化的特点.在实际应用中,料液中细胞(碎片)的存在将会影响到膨胀床的床层稳定性和操作性能[4-6].     

Limestone particle attrition and size distribution in a small circulating fluidized bed

Limestone particle attrition was investigated in a small circulating fluidized bed reactor at temperatures from 25 to 850 °C, 1 atm pressure and superficial gas velocities from 4.8 to 6.2 m/s. The effects of operating time, superficial gas velocity and temperature were studied with fresh limestone. No calcination or sulfation occurred at temperatures ?580 °C, whereas calcination and sulfation affected attrition at 850 °C. Increasing the temperature (while maintaining the same superficial gas velocity) reduced attrition if there was negligible calcination. Attrition was high initially, but after ∼24 h, the rate of mass change became constant. The ratio of initial mean particle diameter to that at later times increased linearly with time and with (U_g − U_mf)², while decreasing exponentially with temperature, with an activation energy for fresh limestone of −4.3 kJ/mol. The attrition followed Rittinger’s surface theory [Beke B. Comminution. Budapest: Akademiai Kiado, 1964; Ray YC, Jiang TS, Wen CY. Particle attrition phenomena in a fluidized bed. Powder Technol 1987a; 49:193-206]. The change of surface area of limestone particles was proportional to the total excess kinetic energy consumed and to the total attrition time, whereas the change of surface area decreased exponentially with increasing temperature. At 850 °C, the attrition rate of calcined lime was highest, whereas the attrition rate was lowest for sulfated particles. When online impact attrition was introduced, the attrition rate was about an order of magnitude higher than without impacts.     

Effect of coal particle size distribution on packed bed pressure drop and gas flow distribution

This paper focuses on the role of coal particle size distribution on pressure drop and gas flow distribution through packed coal beds. This fundamental knowledge is helpful in better understanding the operational behaviour of fixed bed dry bottom gasifiers. The Sasol synfuels plants in South Africa use 80 such gasifiers to convert more than 26 million tons of coal per annum to synthesis gas, and ensuring stable operation is of primary importance to ensure high synthesis gas production rates and gasifier availability. Pressure drop measurements on laboratory scale equipment were conducted to investigate the effect of particle size distribution on packed bed pressure drop. The well-known Ergun equation for pressure drop does not accommodate the effect of size distribution on pressure drop. A novel approach was followed to model pressure drop through simulated coal bed structures using Computational Fluid Dynamics (CFD). The coal bed structures were simulated by assuming that the coal particles are represented by randomised convex polyhedra in three-dimensional space. The computational space was divided into polyhedra using statistical Voronoi tessellation technique, which have been shown to be versatile in modelling problems in many fields, e.g. filtration, molecular physics, metallurgy, geology, forestry and astrophysics. This approach of flow modelling through packed coal beds is able to accommodate size distribution effects on pressure drop and gas flow distribution. The modelling work shows large deviations from plug flow with broad size distributions. The lowest bed pressure drop with the closest approximation to plug flow is obtained with the narrowest particle size distribution. Low gas flow rates are also beneficial for reducing excessive channel flow. Combustion profiles for different particle size distributions were studied using a pilot scale combustor. The combustion profiles provide confirmation of the CFD modelling results, namely that narrow particle size distributions and low gas flow rates reduce channel burning. Excessive channel burning was observed for broad particle size distributions, and is enhanced by high gas flow rates. The experimental and modelling work which was conducted, clearly indicate that narrow coal particle size distributions are desirable for optimum gas flow distribution and lowest packed bed pressure drop.     

Packed bed combustion of char particles: experiments and an ash model

This paper presents the results of a series of experiments on the packed bed combustion of coke particles, including temperature and gas concentration profiles in the bed and measurements of particle size, void fraction and sphericity on both fuel and ash. An earlier numerical model is extended to include the effects of ash on the bed void fraction and other properties as well as on heat conduction within the bed and heat and mass transfer between particulate and gas. The model is shown to give good agreement with experimental data. It is demonstrated that the physical properties of ash—particle size, void, and sphericity—can have a considerable influence on other bed processes.     

Control of particle size distribution for the synthesis of small particle size high solids content latexes

A polymerization process to synthesize bimodal latexes with maximum particle diameters below 350 nm and solids content above 65 wt% has been developed.The process is based on an iterative strategy to determine the optimal particle size distribution that gives the maximum packing factor for a given range of particle sizes and at a given solids content. The calculated optimal bimodal PSD was experimentally obtained in a seeded semi-continuous emulsion polymerization reaction as follows: in the first step, a polymer seed latex was loaded in the reactor and grown, under monomer starved conditions, until a given particle size. At this point a fraction of the same seed was added to the reactor and the feed was continued until the desired particle size distribution and solids content were achieved. The point at which the seed was added again to the reactor and the amount of seed required were determined by the iterative strategy and depended on the competitive growth rate ratio of large and small particles that is an input for the iterative strategy.Implementation of the solution obtained from the iterative strategy, and for the first time in the open literature, led to the production of a coagulum free and stable bimodal latex with 70 wt% of solids content and particle sizes below 350 nm.     

Fluidized bed ash cooler used in a circulating fluidized bed boiler: An experimental study and application

The performance of a fluidized bed ash cooler was studied using a cold experiment bench. The air flow rate, particle size of the solids and air distributor type are considered to be the key parameters of the operation of a fluidized bed ash cooler (FBAC). It was found that the amount of fluidizing air directly affects the conveying of ash and that they have a quadratic relationship. An acceptable particle size distribution for the solids is d₅₀ < 450 μm. Furthermore, the experiment reveals that the height of the weir in the FBAC does not affect the conveying of the ash flow. The influence of injected air on the transportation of ash is not significant. Optimal air nozzle dimensions are presented.