循环流化床燃烧在高过剩空气下的NOx排放

提出了借助循环流化床在高过剩空气系数下燃烧的技术提供高温空气的新构思。搭建了循环流化床燃烧热态试验台,完成了循环流化床燃烧在高过剩空气系数下的NOx排放特性试验,结果表明:循环流化床在高过剩空气系数下燃烧温度分布均匀,燃烧稳定性好;过剩空气系数增大,氮氧化物排放增加;提升管二次风高度的增加和还原区系数的减小有利于控制氮氧化物的排放水平和减少煤中的N向NOx的转化比。在过剩空气系数为1.6、还原区系数为0.72和二次风高度为1 500 mm时,循环流化床NOx排放为339 mg/m3,煤中的N向NOx转化比为21%。循环流化床高温空气NOx的浓度对燃料高温燃烧NOx排放的影响需要进一步研究。     

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.     

Devolatilization and combustion of large coal particles in a fluidized bed

Devolatilization and combustion of large particles of Eastern Canadian coals (Evans and Minto), 5-50 mm dia., were studied in a bench-scale atmospheric fluidized bed reactor at 1023-1173 K with 0.5 mm sand particles as the bed material. The devolatilization time, mass loss history, changes in proximate volatiles content and C/H mass ratio, and temperature history at the centre of the particle during devolatilization were determined. The mass loss during devolatilization is correlated with the proximate volatiles content of the parent coal. The devolatilization time is correlated with the initial particle diameter by a power-law relation with an exponent of 1.54-1.64. The results show insignificant effect of superficial velocity on devolatilization.     

Multiple reactor testing of a synthetic sorbent for regenerative sulfur capture in fluidized bed combustion of coal

A new synthetic regenerable sorbent was developed as an alternative to limestone and dolomite for regenerative sulfur capture in fluidized bed combustion (FBC of coal. The sorbent consists of CaO (8-9 wt%) on a spherical 3 mm diameter γ-Al₂O₃ support. A shrinking unreacted core model (SURE2) is proposed to describe the sulfation process. Experiments have been carried out in a 12 mm internal diameter fixed bed reactor, a 0.05 m ID fluidized bed reactor and a thermobalance. The results obtained for all three reactors are in good agreement with the predictions of the SURE2 model.     

Study on the emission characteristics of nitrogen oxides with coal combustion in pressurized fluidized bed

Nitrogen oxides are one of the most significant pollution sources during coal combustion. This experimental study was conducted in a 15 kWthlab-scale pressurized fluidized bed(inner diameter = 81–100 mm, H =2100 mm) firing with bituminous coals. The effects of operating parameters, including bed temperature(800 ℃–900 ℃), operating pressure(0.1–0.4 MPa), excess air level(16%–30%) and flow pattern on NOx and N_2 O emissions were systematically studied during the tests. During each test the interaction effects of all the operating parameters were properly controlled. The results show that most operating parameters have an opposite effect on NOxand N_2 O emissions, and the N_2 O emissions mainly depend on the bed temperature. Increasing the operating pressure can significantly suppress the fuel-N conversion to NOxbut enhance its conversion to N_2 O. With the rise of the excess air level and fluidization number, NOxemissions grow distinctly while N_2 O emissions remain almost unchanged. Total nitrogen oxide emissions increase with the bed temperature while decrease with the operating pressure.     

A method of evaluating limestone reactivity with SO2 under fluidized bed combustion conditions

The reactivity of limestone with sulphur dioxide has been evaluated during conditions similar to those existing in a fluidized bed combustor using a fixed-bed quartz reactor. The reactivity of up to 11 particle sizes of 2 limestones were evaluated, and an exponential decay function was found to best describe the rate behaviour versus conversion of CaO to CaSO₄. The two constants in the exponential decay approximation could both be expressed as functions of particle size. Subsequently, the limestone reactivity as a function of both size and conversion could be reasonably well described by a total of 4 to 6 constants. An analytical sulphur capture model for fluidized bed boilers (FBB) that incorporates this type of reactivity function is proposed.     

A novel dual circulating fluidized bed system for chemical looping processes

A fluidized bed system combining two circulating fluidized bed reactors is proposed and investigated for chemical looping combustion. Direct hydraulic communication of the two circulating fluidized bed reactors via a fluidized loop seal allows for high rates of global solids circulation and results in a stable solids distribution in the system. A 120 kW fuel power bench scale unit was designed, built, and operated. Experimental results are presented for natural gas as fuel using a nickel‐based oxygen carrier. No carbon was lost to the air reactor under any conditions operated. It is shown from fuel power variations that a turbulent/fast fluidized bed regime in the fuel reactor is advantageous. Despite the relatively low riser heights (air reactor: 4.1 m, fuel reactor: 3.0 m), high CH₄ conversion and CO₂ yield of up to 98% and 94%, respectively, can be reported for the material tested. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

流化床煤燃烧过程不同气氛下的气态氮释放特征

利用微型流化床反应装置,结合快速过程质谱仪,在850~940℃操作温度下,研究了三种不同粒度分布烟煤和无烟煤在热解、气化和燃烧反应条件下四种主要气态氮产物HCN、NH₃、NO和NO₂的释放规律。结果表明,微型流化床可以实时检测挥发分氮和焦炭氮的动态释放序和类型,热解、气化和燃烧反应气氛的改变主要影响HCN和NH₃的释放量。热解产物的气态氮主要是来自于挥发分,燃烧反应的HCN和NH₃的释放量与温度有明显关系,而气化反应的各类气态氮释放量随温度变化波动不大。煤颗粒尺寸和温度变化对烟煤和无烟煤中各类气态氮释放量产生影响比较复杂,其中NH₃的释放特性是区分挥发分N释放和半焦N释放的重要特征。     

Release of volatiles from large coal particles in a hot fluidized bed

Coal particles with diameters of 3–11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores.     

Mixing and combustion in a shallow coal-limestone fluidized bed combustor

A model based on the Monte Carlo approach was developed to simulate the mixing and combustion behavior of a shallow coal-limestone fluidized bed combustor. The model involved the coupling of two sub-models: a combustion sub-model based on the two-phase concept of fluidization and a mixing sub-model based on our previously developed dynamic mixing model. The combustion sub-model considered both the volatile and char combustion. It assumed that the combustor consisted of three distinct phases, i.e., jet, bubble and emulsion, with combustion occurring only in the emulsion phase. The mixing sub-model considered the upward or downward movement of a coal particle in the bed as being governed by certain probability laws; these laws were, in turn, affected by the bubbling hydrodynamics. In all, the combustor simulation model took into consideration the effects of coal feed rate, coal size distribution, limestone size, air flow rate and combustor temperature on the combustor behavior. The simulation results included the dynamic response of coal concentration profile, coal size distribution, coal particle elutriation rate as well as the mixing status between the coal and limestone particles.     

Methods for measuring the concentrations of SO2, and of gaseous reduced sulphur compounds in the combustion chamber of a circulating fluidized bed boiler

The present work was aimed at developing and improving methods for measurement of gaseous sulphur compounds in the combustion chamber of a fluidized bed boiler (FBB). The sampling of SO₂ was improved by removing NH₃ and H₂O with a sorbent immediately after the probe. The concentration of reduced sulphur species was determined by means of two conventional SO₂ analyzers and an intermediate converter, where the reduced species are oxidized to SO₂. Gas phase sulphides were also sampled with a gas quenching probe by means of a basic solution which was subsequently analysed by wet chemistry. The methods were tested during coal combustion in a 12 MW circulating FBB without limestone for two cases of air‐staging.     

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.     

循环流化床回流物料循环的特性

为了研究循环物料在流化床内回流并参与循环的过程,进一步揭示流化床内物料的循环特性,文中搭建了冷态循环流化床顶部回流试验台。利用激光多普勒粒子动态分析仪(PDA)对塔内颗粒回流时床内气固二相流场和颗粒浓度分布进行了测量。同时对物料回流量和床内表观气速等操作条件对回流过程的影响进行了考察。研究表明,回流颗粒在流化床内的返混可以分为3个区域:回流区、返混加速区和主流区,回流颗粒造成床内流场的不均匀性。回流长度主要受物料回流量和床内表观气速影响。     

The axial hydrodynamic behavior in a liquid-solid circulating fluidized bed

Based on experimental results from a 7.6 cm I. D. and 3 m high liquid–solid circulating fluidized bed, the liquid–solid circulating fluidization regime has been separated into two zones: the initial circulating fluidization zone and the fully developed circulating fluidization zone. The distinct hydrodynamic behavior and the influence of particle properties in the two circulating fluidization zones have been studied. The overall flow structure in LSCFB, although still somewhat non–uniform, is much more uniform than that in a gas-solid CFB. Our experimental results also show that the axial flow characteristics and the regime transition can be strongly affected by the particle density. The stable operation range of the circulating fluidization system and the influences of some associated factors, such as the solids inventory and the particle density, were also investigated for the first time.     

煤在富氧流化床燃烧条件下汞的析出及形态分布

目前有关于富氧气氛下流化床燃烧汞的形态转化特性的报道还不是很多,因此本文开展了富氧气氛下煤种对汞形态转化特性的影响的实验研究。采用流化床作为实验设备,选用徐州烟煤和淮北烟煤作为实验燃料,研究了空气气氛下不同温度和不同煤种对汞析出规律的影响,富氧气氛下煤种对汞形态转化规律的影响,并深入分析了相应的汞氧化机理。研究结果表明：在空气气氛下,温度的增加会促进汞的氧化,煤中的含硫量对汞的氧化也有影响;在富氧气氛下,徐州烟煤燃烧产生的气态总汞浓度高于淮北烟煤燃烧产生的气态总汞浓度,徐州烟煤的Hg²⁺（g）的分布率也比淮北烟煤的Hg²⁺（g）的分布率高出16%左右,因为徐州烟煤中高含硫量会影响Hg²⁺（g）的分布率;富氧气氛下徐州烟煤的Hg²⁺的分布率低于空气气氛下的,而淮北烟煤的Hg²⁺（g）分布率则与之相反,这与两种煤中硫含量的不同有关。     

加压循环流化床富氧燃烧中试研究

二氧化碳捕集和封存技术(CCUS)是减少温室气体排放,实现全球环境可持续发展的有效技术手段。加压富氧燃烧技术是一种低成本CCUS技术。循环流化床燃烧技术(CFB)是目前商业化程度最好的清洁煤燃烧技术之一,加压循环流化床富氧燃烧耦合了加压富氧燃烧和循环流化床燃烧的诸多优点,具有很强的工业应用前景。但加压循环流化床富氧燃烧系统结构复杂,燃烧工况的切换和烟气再循环导致其在启动、控制、运行等方面面临巨大挑战。目前对于加压循环流化床富氧燃烧的研究大多处于理论建模、机理研究和小试试验阶段。为了更深入地探究加压循环流化床富氧燃烧的启动和运行方法,中国科学院工程热物理研究所在MW级加压循环流化床富氧燃烧中试试验平台上进行了中试研究,实现了加压富氧燃烧的稳定运行,获得了中试尺度加压富氧燃烧运行模式,以及启动和运行过程中温度、压力、给煤量和风量的变化曲线。加压富氧燃烧工况运行中整体O₂体积分数为29%,压力为0.30 MPa,功率为0.84 MW,尾部烟气中CO₂体积分数达91%,可较好地实现CO₂产品的捕集和压缩纯化。中试尺度加压富氧燃烧启动和运行的主要流程为:启动阶段-常压O₂/N₂燃烧阶段-常压富氧燃烧-加压富氧燃烧阶段,各阶段切换平稳。     

On the timing of primary fragmentation during bituminous coal particle devolatilisation in a fluidized bed combustor

The times at which devolatilising coal particles fragmented were compared with their devolatilisation times in a fluidized bed combustor. Whereas the devolatilisation times were similar, the time to first fragmentation varied markedly from coal to coal. Immediate fragmentation was attributed to thermal shock. Fragmentation during the first 1/3 of the devolatilisation time was attributed to the internal pressure generated from restricted transport of volatiles through the pores in the coal structure. Fragmentation near the end of devolatilisation may occur due to weakening of the coal structure by loss of volatiles. Primary fragmentation did not affect the devolatilisation time, even for coals which fragmented early in their devolatilisation time.     

Calcium sulphide formation in fluidized bed boilers

Analyses of samples of bed ash from a stationary fluidized bed boiler show the presence of calcium sulphide. In some samples, half of the total sulphur was present as sulphide. The samples containing CaS were obtained under unstaged conditions and with a high excess air ratio, 1.3 to 1.4. The samples were taken after a stop in the limestone addition, i.e. at high SO₂ emissions of about 1000 mL/m³ (ppm). No CaS was found during limestone addition when the SO₂ emission was 300–400 mL/m³. This indicates that formation of large amounts of CaS may be initiated as the SO₂ concentration exceeds some critical level.     

A simplified model of nitric oxide emission from a circulating fluidized bed combustor

A simplified mathematical model leading to a closed form of solution is developed for estimation of nitric oxide emission from a coal fired circulating fluidized bed (CFB) furnace. The furnace is divided into two sections: a lower section below and an upper section above the secondary air injection level. Reactions in the cyclone and the return leg are neglected. Furnace dimensions, coal feed rate, coal composition and furnace temperature are inputs to the model which was validated against several pilot scale and commercial units. Experimental results from two pilot plants and two commercial power plants agree with model predictions. A sensitivity analysis was carried out using the model to examine the effect of different operating parameters and coal properties on the overall NO emission from the furnace. It was found that excess air and furnace temperature are most important factors influencing the NO emission level. The primary to secondary air ratio influences the NO emission level reasonably. Properties of coal are other factors which affect the NO emission to a large extent. The model, though it invovles some simplification, predicts the overall emission of NO with a level of accuracy accepted in commercial operation.