Simulation of gas-solid flow characteristics of the circulating fluidized bed boiler under pure-oxygen combustion conditions

Under the pressure of carbon neutrality,many carbon capture,utilization and storage technologies have witnessed rapid development in the recent years,including oxy-fuel combustion(OFC)technology.However,the conventional OFC technology usually depends on the flue gas recirculation system,which faces significant investment,high energy consumption,and potential low-temperature corrosion problem.Considering these deficiencies,the direct utilization of pure oxygen to achieve particle fluidization and fuel combustion may reduce the overall energy consumption and CO_(2)-capture costs.In this paper,the fundamental structure of a self-designed 130 t·h^(-1) pure-oxygen combustion circulating fluidized bed(CFB)boiler was provided,and the computational particle fluid dynamics method was used to analyze the gas-solid flow characteristics of this new-concept boiler under different working conditions.The results indicate that through the careful selection of design or operational parameters,such as average bed-material size and fluidization velocity,the pure-oxygen combustion CFB system can maintain the ideal fluidization state,namely significant internal and external particle circulation.Besides,the contraction section of the boiler leads to the particle backflow in the lower furnace,resulting in the particle suspension concentration near the wall region being higher than that in the center region.Conversely,the upper furnace still retains the classic core-annulus flow structure.In addition to increasing solid circulation rate by reducing the average bed-material size,altering primary gas ratio and bed inventory can also exert varying degrees of influence on the gas-solid flow characteristics of the pure-oxygen combustion CFB boiler.

Recovery of alumina from circulating fluidized bed combustion Al-rich fly ash using mild hydrochemical process

To utilize CFBC Al-rich fly ash, a mild hydrochemical extraction process was investigated for recovery of alumina. An alumina extraction efficiency of 92.31%was attained using a 45%NaOH solution, an original caustic ratio （molar ratio of Na2O to Al2O3 in the sodium aluminate solution） of 25, a molar ratio of CaO to SiO2 in the fly ash of 1.1, a liquid volume to solid mass ratio of 9, a reaction temperature of 280 ℃, and a residence time of 1 h when treating fly ash with an alumina to silica mass ratio （A/S） of 0.78 and an alumina content of 32.43%. Additionally, the alumina leaching mechanism was explored via structural and chemical analysis, which revealed that after alkaline digestion, the main solid phase containing silica was NaCaHSiO4 with a theoretical A/S of zero.

I MWth Test Facilities for Circulating Fluidized Bed Combustion

This paper presents the technical parameters and features of 1 MWth test facilities for circulating fluidized bed combustion (CFBC) at Thermal Power Research Institute (TPRI) of State Power Corporation (SP), introduces the test items that can be proceeded and trial combustion projects completed. The development status of CFBC technologies abroad and the level of China in this field are also introduced in the paper.

Strength and thermal behavior of low weight foam geopolymer using circulating fluidized bed combustion fly ash

A comparative study of the influence of elevated temperature on foam geopolymer using circulating fluidized bed combustion fly ash（CFA） was reported. Foam geopoymers were prepared with different amounts of foam agent and different Si O2/Al2O3 molar ratios of 3.1, 3.4, and 3.8. The mechanical, thermo-physical properties and microstructure of the foam geopolymers before and after exposure to elevated temperature of 800, 1000, and 1200 ℃ were investigated. The specimen with Si O2/Al2O3 molar ratio of 3.8 exhibits the highest compressive strength, better microstructure and dimension stability before and after firing. Carnegeite, nepheline, and zeolite crystalline phases appearing after exposure may contribute to the good post-exposure strength. Low weight foam geopolymer using CFA can increase strength and maintain higher stability as high as 1000 ℃.

Properties of Concrete Incorporating Bed Ash from Circulating Fluidized Bed Combustion and Ground Granulates Blast-furnace Slag

The properties of concrete incorporating circulating fluidized bed combustion （CFBC） bed ash and ground granulates blast-furnace slag （GGBS） were studied.Compressive strength,drying shrinkage,mercury intrusion porosimetry （MIP）,scanning electronic microscopy （SEM）,and X-ray diffraction （XRD） of concrete samples containing CFBC bed ash and GGBS were used.This work used initial surface absorption test （ISAT） and rapid chloride penetration test （RCPT） on concrete to measure the absorption and the ability of concrete to resist chloride ion characteristics for different concrete samples containing CFBC bed ash and GGBS.Open circuit potential （OCP）,direct current polarization resistance were obtained to evaluate rebar corrosion.The CFBC bed ash was X-ray amorphous and consist of SiO2,Al2O3 and CaO compounds.As the replacement of CFBC for sand increases,the rate of initial surface absorption （ISA） increases but compressive strength decreases.When the content of CFBC bed ash replacement for sand maintains constant,the replacement of GGBS for cement increases,compressive strength increases but the rate of ISA decreases.Chloride and corrosion resistance of rebar significantly improve by utilizing a proper amount of CFBC bed ash and GGBS in concrete.

Synergetic NO reduction by biomass pyrolysis products simulating their reburning in circulating fluidized bed decoupling combustion

The present work investigated the synergetic effect of pyrolysis-derived char,tar and gas(py-gas)on NO reduction,which may occur in circulating fluidized-bed decoupling combustion(CFBDC)system treating N-rich fuel.Experiments were carried out in a lab-scale drop-tube reactor for NO reduction by some binary mixtures of reagents including char/py-gas,tar/py-gas and tar/char.At a specified total mass rate of0.15 g·min^-1 for NO-reduction reagent,the char/py-gas(binary reagent)enabled the best synergetic NO reduction in comparison with the others.There existed effective interactions between char and some species in py-gas(i.e.,H2,CxHy)during NO reduction by pyrolysis products,meanwhile the tar/py-gas or tar/char mixture only caused a positive effect when tar proportion was necessarily lowered to about 26%.On the other hand,the synergetic effects were not improved for all tested binary reagents by increasing the reaction temperature and residence time.

Emissions of SO_2, NO and N_2O in a circulating fluidized bed combustor during co-firing coal and biomass

This paper presents the experimental investigations of the emissions of SO2, NO and N20 in a bench scale circulating fluidized bed combustor for coal combustion and co-firing coal and biomass. The thermal capacity of the combustor is 30 kW. The setup is electrically heated during startup. The infuence of the excess air, the degree of the air staging, the biomass share and the feeding position of the fuels on the emissions of SO2, NO and N2O were studied. The results showed that an increase in the biomass shares resulted in an increase of the CO concentration in the flue gas, probably due to the high volatile content of the biomass. In co-firing, the emission of SO2 increased with increasing biomass share slightly, however, non-linear increase relationship between SO2 emission and fuel sulfur content was observed. Air staging significantly decreased the NO emission without raising the SO2 level. Although the change of the fuel feeding position from riser to downer resulted in a decrease in the NO emission level, no obvious change was observed for the SO2 level. Taking the coal feeding position R as a reference, the relative NO emission could significantly decrease during co-firing coal and biomass when feeding fuel at position D and keeping the first stage stoichiometry greater than 0.95. The possible mechanisms of the sulfur and nitrogen chemistry at these conditions were discussed and the ways of simultaneous reduction of SO2, NO and N2O were proposed.

Advances in reduction of NO_x and N_2O emission formation in an oxy-fired fluidized bed boiler

Fossil fuel combustion is one of the major means to meet the mounting global energy demand. However, the increasing NO_x and N_2 O emissions arising from fossil fuel combustion process have hazardous effects. Thus, mitigating these gases is vital to attain a sustainable environment. Interestingly, oxy-fuel combustion in fluidized bed for carbon capture and minimized NO_x emissions is strongly sustainable compare to the other approaches. It was assessed that NO_x formation and fuel-N conversion have significant limitation under oxy-fluidized bed compared to air mode and the mechanism of NO_x formation is still deficient and requires further development. In addition, this review paper discussed the potential of primary measure as low emission process with others supplementary techniques for feasible NO_x reduction. The influences of combustion mode, operating parameters, and reduction techniques such as flue gas recirculation, oxygen staging, biomass co-firing, catalyst, influence of fluidized bed design and structure, decoupling combustion and their merges are respectively evaluated. Findings show that significant minimization of NO_x emission can be achieved through combination of primary and secondary reduction techniques.

Performance with Respect to Flue Gas Composition of a Combined Desulfurization/Denitration Process Using Powder-Particle Fluidized Bed

A new combined desulfurization/denitration (DeSOx/DeNOx,) process was tested in this study. The process uses the so-called powder-particle fluidized bed (PPFB) as the major reactor in which a coarse DeNOx catalyst, several hundred micrometers in size, is fluidized by flue gas as the fluidization medium particles, while a continuously supplied fine DeSOx sorbent, several to tens of micrometers in diameter, is entrained with the flue gas. Ammonia for NOx reduction is fed to the bottom of the bed, thus, SOX and NOX are simultaneously removed in the single reactor. By adopting a model gas, SO2-NO-H2O-N2-air, to simulate actual flue gas in a laboratory-scale PPFB, simultaneous SO2 and NO removals were explored with respect to various gas components of flue gas. It was found that the variations of SO2 removal with concentrations (fractions) of oxygen, water vapor, SO2 and NO in flue gas are little affected by the simultaneous NOx reduction. However, the dependencies of NO removal upon such gas components are closely related to the inter-actions between DeSOx sorbent and DeNOx catalyst.

The Simulation of Influence of Different Coals on the Circulating Fluidized Bed Boiler's Combustion Performance

The combustion performance of the boiler largely depends on the coal type. Lots of experimental research shows that different fuels have different combustion characteristics. It is obvious that fuel will change the whole operating performance of Circulating Fluidized Bed Combustion (CFBC). We know even in a pilot-scale running boiler, the measurement of some parameters is difficult and costly. Therefore, we developed the way of simulation to evaluate the combustion performance of Chinese coals in CFB. The simulation results show that, different coals will result in different coal particle diameter and comminution depending on their mineral component and the change will affect the distribution of ash in CFBC system. In a word, the computational results are in accordance with experimental results qualitatively but there are some differences quantitatively.

Combustion Model For Staged Circulating Fluidized Bed Boiler

A mathematical model for atmospheric staged circulating fluidized bed combustion, which takes fluiddynamics, combustion, heat transfer, pollutants formation and retention, into account was developedin the Institute of Engineering Thermophysics (IET) recently. The model of gas solid flow at the bot-tom of the combustor was treated by the tworphase theory of fluidized bed and in the upper regionas a core-sannulus flow structure. The chemical species CO, CO2, H2, H2O, CH4, O2 and N2 wereconsidered in the reaction process. The mathematical model consisted of sub-models of fluid namics,coal heterogeneous and gas homogeneous chemical reactions, heat transfer, particle fragmentation andattrition, mass and energy balance etc. The developed code was applied to simulate an operating stagedcirculating fluidized bed combustion boiler of early design and the results were in good agreement withthe operating data. The main submodels and simulation results are given in this paper.

Energy Efficiency Analysis of Oxy-Fuel Circulating Fluidized Bed Combustion System with High Oxygen Concentration

The low net efficiency of oxy-fuel circulating fluidized bed(CFB)combustion is mainly due to the addition of air separation unit(ASU)and carbon dioxide compression and purification unit(CPU).High oxygen concentration is one of the effective methods to improve the net efficiency of oxy-fuel combustion technology in CFB.In this research,a series of calculation and simulation were carried out based on Aspen Plus platform to provide valuable information for further investigation on the CFB oxy-fuel combustion system with high oxygen concentration(40%,50%).A CFB oxy-fuel combustion system model with high oxygen concentration was established including ASU,CPU and CFB oxy-fuel combustion and heat exchange unit.Based on the simulation data,energy and exergy efficiency were analyzed to obtain the following results.The cross-sectional area of furnace and tail flue of 50%CFB oxy-fuel combustion boiler are 43%and 56%of the original size respectively,reducing the construction and investment cost effectively.With the increase of oxygen concentration,the net efficiency of power generation increased significantly,reaching 24.85%and increasing by 6.09%under the condition of 50%oxy-fuel combustion.The total exergy loss increases with the increase of oxygen concentration.In addition,the exergy loss of radiation heat transfer is far higher than convection heat transfer.

Reactive gas-solids flows in large volumes—3D modeling of industrial circulating fluidized bed combustors

A model is presented for the simulation of reactive gas-solids flows in large industrial reactors. Circulating fluidized bed （CFB） combustors with several thousands of cubic meters reaction volume are probably the largest reactors of this type. A semi-empirical modeling approach has been chosen to model the three-dimensional concentration distributions of gas and solids components and temperatures inside the combustion chamber of such boilers. Two industrial CFB boilers are investigated in detail： the 105 MWe Duisburg combustor in Germany and the 235 MWe Turow combustor in Poland. The semi-empirical model approach is described first. Then the model is used to show how the three-dimensional concentration and temperature fields are formed by the interaction of several local phenomena. Good agreement between simulation and measurements has been achieved.

活化方式对CFBC脱硫灰自硬化性能的影响

通过物理粉磨、化学激发和物理-化学联合激发3种方式系统研究了循环流化床锅炉燃烧脱硫灰(CFBC脱硫灰)物理力学性能的变化规律,并采用水化热,FTIR,XRD和SEM测试方法分析了活化方式对CFBC脱硫灰自硬化水化历程及性能的影响机理.结果表明:物理粉磨、化学激发和联合活化均提高了脱硫灰的自硬强度及水化反应程度,且降低了需水量比.与原状CFBC脱硫灰相比,粉磨后试样需水量比降低了13%(质量分数),7,28d抗压强度分别提高了210.0%,25.4%;改性剂A,B加入后混磨试样需水量比降低了19%,7,28d抗压强度各自提高了337.0%,96.5%.在相同的A掺量下,CFBC脱硫灰与A混合后混磨试样的强度效果优于粉磨后A的外加.CFBC脱硫灰的主要水化产物为AFt,Ca(OH)2,CaSO4.2H2O,C-S-H凝胶.

Experimental Study on the Effect of Bed Material Amount and Fuel Particle Size on Load Change of Circulating Fluidized Bed

The large-scale integration of new energy generation has put forward higher requirements for the peak-shaving capability of thermal power.The circulating fluidized bed(CFB)depends on the advantages of a wide load adjustment range and low cost of pollutant control to become a good peak shaving power supply.However,the large delay and inertia caused by its unique combustion mode make it very difficult to change the load quickly.To further understand the factors that affect the load change of CFB,and explore the method of increasing CFB load change rate,the load change experiment on the combustion side was carried out in the 0.1 MW CFB experiment platform.The influence law of bed material amount and fuel particle size on load change of CFB combustion side was revealed for the first time.The results indicated that the increase of bed material amount was beneficial to improve the load change rate on the combustion side of CFB and reduce the carbon content of fly ash,but had no obvious effect on NO_(x)emission.When the bed height at rest increased from 200 mm to 400 mm,the load change rate of the CFB combustion side load from 50%to 75%increased from 0.78%/min to 1.14%/min,and the carbon content of fly ash at 75%load decreased from 26.6%to 24.9%.In addition,the reduction of fuel particle size positively improved the load change rate on the combustion side of the CFB and reduced NO_(x)emission but had a negative effect on reducing the carbon content of fly ash.When the fuel particle size decreased from 0-1 mm to 0-0.12 mm,the load change rate of CFB combustion side load from 50%to 75%increased from 0.78%/min to 1.09%/min,and the NO_(x)emission and carbon content of fly ash at 75%load decreased from 349.5 mg/m^(3)to 194.1 mg/m^(3)and increased from 26.6%to 31.8%,respectively.

CFBC锅炉炉内多重内循环燃烧技术研究与应用

本文论述了由ＣＦＢＣ锅炉炉膛下部密相区内循环燃烧技术与炉膛上部稀相区空间循环燃烧技术所构成的炉内多重内循环燃烧技术的原理与结构．应用本技术所设计完成的循环流化床锅炉，具有效率高、体积小．成本低、磨损轻、动力消耗低、运行稳定、操作方使、负荷调节范围广、煤种适应性强等优点．是工业循环流化床锅炉的理想炉型．长期的运行实践证明，炉内多重内循环燃烧技术是适用于矮小燃烧空间的工业循环流化床锅炉的理想技术之一。

富钒石煤与生物质在CFBC试验台上的混烧实验

针对石煤富钒的特点,利用实验室规模的循环流化床燃烧装置(CFBC),进行了生物质与富钒石煤的混烧实验研究,以期通过富集钒和回收热量达到资源综合利用的目的。实验结果表明:所实验的富钒石煤容易着火,燃烧稳定,燃尽性好,包括与生物质混烧也是如此;经过燃烧后,石煤中的钒大部分转换为五氧化二钒,并在灰渣中富集;石煤颗粒尺度大小、燃烧温度对钒的迁移特性没有显著影响;通过向炉内加入石灰石进行炉内脱硫,当Ca/S摩尔比为3时,烟气中的二氧化硫排放浓度低于国标。

Effect of Curing Conditions on the Hydration and Performance of CFBC Ash Cementitious System

Circulating fluidized bed combustion （CFBC） ash can be used as supplementary cementitious material for concrete production for its high pozzolanic activity. We investigated the effect of curing conditions on the hydration and performance of CFBC ash-Portland cement system （30： 70, by mass） including hydration products, paste microstructure, linear expansion ratio, chemically combined water content and compressive strength. The results show that tobermorite rather than ettringite is generated under the condition of autoclaved curing. The expansion and mortar strength of the system cured in water is higher than those cured in air at a given age, and the strength and bulk volume may retract under the condition of air curing. In addition, autoclaved curing facilitates the increase of strength gain at early curing ages （the increase rate lowers down in the following ages） and the improvement of system volume stability. It is suggested that sufficient water is necessary for the curing of CFBC ash cementitious system, and autoclaved curing may be considered where volume stability is a primary concern.

燃煤锅炉改燃兰炭燃烧工艺及效益分析

兰炭因热值高、硫含量低的特点被广泛应用于散煤替代,燃煤锅炉改燃兰炭是实现资源合理利用、降低污染物排放和改善企业经济效益的有效途径。以河北某循环流化床锅炉改燃兰炭工程为例,燃烧系统采用掺烧灰渣的方法,解决改燃后机械未完全燃烧导致损失大和物料循环不稳定问题,确定了兰炭和灰渣的最佳掺混比为9∶1;配风系统采用控制总量并降低一次风率的方法,解决兰炭着火困难的问题,确定了最佳一次风比例为55%;换热系统采用增加锅炉尾部受热面的方法,解决因燃料热值提升较大产生的蒸汽超温和锅炉效率下降问题。改燃扩容后锅炉烟尘、SO2和NOx排放分别比原锅炉降低了73%、94%和55%,以原锅炉35 t/h的额定功率运行,每小时节约燃料成本5.51%,产吨蒸汽所需燃料成本降低6.47元。

燃煤循环流化床锅炉掺烧污泥应用研究进展

利用循环流化床锅炉合理处置产量大且含有有害物质的污泥,可使其烟气中污染物排放浓度达标,因而燃煤循环流化床锅炉掺烧污泥具有广阔的应用前景。从污泥的掺烧现状、掺烧机理、燃烧工况、存在问题以及优化方案等方面对燃煤循环流化床锅炉掺烧污泥进行综述研究。煤炭与污泥混燃可实现燃料特性互补,消除单一燃料的负面特性。结合国内外燃煤循环流化床锅炉掺烧污泥相关工艺,指出现阶段我国污泥掺烧对燃烧效率、锅炉运行以及烟气污染物排放的影响,并提出优化工况的具体措施。综合认为,燃煤循环流化床锅炉掺烧污泥可实现污泥的减量化、无害化、资源化处置,需从污泥预处理、锅炉系统优化、数值模拟等多角度出发,进行系统优化设计和改造,以确保混合燃料在炉内高效燃烧、达标排放。