循环流化床锅炉飞灰含碳量的研究

循环流化床锅炉具有高效、低污染、煤种适应性广等优点。但我国流化床锅炉普遍存在着飞灰含碳量高,锅炉燃烧效率达不到设计值的问题。概述了影响飞灰含碳量的主要因素:如煤种、燃煤的粒径及风量等,重点探讨了燃煤的粒径对飞灰含碳量的影响,提出了维持锅炉稳定,降低飞灰含碳量,提高燃烧效率的一些措施。     

粒径对福建无烟煤在CFB锅炉中燃尽的影响

建立了福建无烟煤细颗粒燃烧模型,计算了其在容量35 t/h循环流化床锅炉炉膛内的燃尽时间和一次通过炉膛的停留时间,分析了不同粒径煤颗粒在不同燃烧温度和不同烟气流速时在CFB锅炉内的燃尽时间和停留时间的变化差异. 实验研究了福建无烟煤粒径对飞灰碳含量的影响及燃尽的影响. 结果表明,细煤颗粒的燃尽时间与停留时间均随粒径增大而增长,但燃尽时间增幅更明显,颗粒一次通过炉膛完全燃尽的临界粒径约为0.15 mm;粒径越大的颗粒其停留时间和燃尽时间对烟气流速和燃烧温度变化越敏感;无烟煤入炉粒径明显影响CFB锅炉飞灰含碳量,选用粒度为3~8 mm的偏粗颗粒为宜.     

Process characteristics investigation of simulated circulating fluidized bed combustion combined with coal pyrolysis

A poly-generation process of simulated circulating fluidized bed (CFB) combustion combined with coal pyrolysis was developed in a laboratory scale. Pyrolysis characteristics of three bituminous coals with high volatile contents were investigated in a fixed bed with capacity of 10 kg solid samples. The effects of initial temperature of solid heat carrier, pyrolysis holding time, blending (ash/coal) ratio and coal particle size on gas and tar yields were studied experimentally. The results indicate that the initial temperature of the heat carrier is the key factor that affects the gas and tar yield, and the gas composition. Most of the gas and the tar are released during the first few minutes of the pyrolysis holding time. For caking coal, the amount of char agglomerating on the pyrolyzer inner wall is reduced by enhancing the blending ratio. The experimental results may provide basic engineering data or information for the process design of CFB combustion combined with coal pyrolysis in a large scale.     

Performance evaluation of a 220t/h CFB boiler with water-cooled square cyclones

The first Chinese 220 t/h CFB boiler was successfully demonstrated. The boiler was compactly designed with Tsinghua-patented, water-cooled square cyclones with curved inlet based on previous experience of similar CFB boilers of smaller capacities. The demonstration showed that the boiler possesses excellent performance in start-up, fuel flexibility, flexibility to frequent turn-down ratio variation, facility availability and reliability. The performance of the water-cooled square cyclone was compared with that of other cyclones through fly ash analysis. The results showed that the overall performance of the square cyclone in such capacity is compatible to that of the round cyclone, meeting the requirements for material balance and efficient combustion in CFB boilers. The demonstration was a milestone for CFB boiler scaling-up in China.     

粉煤灰中的残余碳

粉煤灰中的残余碳是燃煤锅炉中煤未完全燃烧所产生的固体废弃物。残余碳的形成主要与燃料煤的组成和性质、锅炉特点、燃烧温度、空气量（燃烧时的氧气供给量）及燃烧时间（Ｏ工在锅炉内的停留时间）等因素有关。残余碳通常以单体半焦或焦炭、粘结在粉煤灰颗粒表面或包裹在粉煤灰颗粒中等几种形式存在，残余碳的粒度分布、比表面积、Ｈ和Ｏ的含量都有其特殊性。粉煤灰中的残余碳，不仅能反映锅炉的运行情况、影响锅炉烟气中ＣＯＸ的含     

降低循环流化床锅炉飞灰含碳量的因素分析

循环流化床锅炉的飞灰含碳量是锅炉燃烧工况好坏的直接反映 ,其对锅炉的热效率的影响是很大的 ,它还直接影响着粉煤灰的综合开发和利用 .在对云天化循环流化床锅炉的生产实验研究中 ,分析了影响循环流化床锅炉飞灰含碳量的各种因素 (床温、煤质煤粒、一次风、二次风、床压和旋风分离器效率等 ) ,通过实验研究找到了降低循环流化床锅炉飞灰含碳量的具体而又行之有效的操作方法 ,以指导实际生产 .     

KINETICS OF RICE HULL CHAR BURNOUT IN A BENCH-SCALE FLUIDIZED-BED REACTOR

Rice hull, also known as husk, is a by-product of the rice milling process; it is a significant alternative energy resource for the milling industries of rice producing countries. Apart from its high energy content, the residual ash from complete combustion of rice hull at moderate temperatures in a combustor, e.g., a fluidized-bed reactor, is also a potential cement extender. The regeneration of steam for parboil rice and for other process activities makes combustion an attractive means of disposing rice hull in the rice milling industries, which otherwise is a menace to the rice miller.

In the present work, the combustion kinetics of Guyana rice hull char was studied in a bench-scale fluidized-bed reactor. The salient features of the combustion of the char were observed, and the temporal history of burnout was traced at temperatures of 973 and 1173 K. The oxygen concentrations of the inlet fluidizing gas were maintained at 2.3 and 4.6% by regulating the mixing of nitrogen and air.

The rates of combustion of the residual char generated from rice hull through rapid devolatilization were determined to range from 3.18 × 10^-6 to 9.04 × 10^-6kg/s. By fitting the conversion data with various heterogeneous reaction models, it appears that the data could be described, to some extent, by the unreacted core model with ash layer diffusion control. However, the complexity of the carbon/silica arrangement in the residual char results in uncertainties in model selection.     

Single char particle combustion at moderate temperature: effects of ash

The effects of ash content, ash layer heat and mass transfer on a single granulated char particle (10–18 mm in diameter) combustion in an air stream (12 cm s⁻¹ in cold base) are studied. The transient temperature of various ash content char particle burning in the surrounding gas temperatures of 900 to 1200 K is simulated. Agreement between simulated and experiment results is obtained by adjusting the ash layer diffusion coefficient and heat conductivity, surface emissivity and the reaction rate constant. The reaction rate constant plays an important role in modeling the initial stage of char particle combustion even when the overall rate is ash layer diffusion controlled. It determines the particle heating rate in the initial stage of combustion, and then indirectly influences the peak temperature. The ash layer diffusion resistance affects the rate controlling processes and the pattern of the time-temperature profile. The higher ash content char particle burns with a lower peak temperature and earlier temperature decrease due to the lower ash layer porosity and lower ash layer diffusion coefficient. It is concluded that the high ash particle combustion is controlled by ash layer diffusion except in the initial stage of combustion. As for the lower ash content char particle, it is controlled mainly by reaction at lower ambient temperature and by film diffusion at a higher temperature in the earlier stage. However, in the last stage, it is controlled by ash layer diffusion. The transition occurs when the ash layer is formed and the diffusion resistance is significant, and it is at that time that the particle reaches its peak temperature.     

Modeling and experimental studies on single particle coal devolatilization and residual char combustion in fluidized bed

Single particle devolatilization followed by combustion of the residual coal char particle has been analyzed in a batch-fluidized bed. The kinetic scheme with distributed activation energy is used for coal devolatilization while multiple chemical reactions with volume reaction mechanism are considered for residual char combustion. Both the models couple kinetics with heat transfer. Finite Volume Method (FVM) is employed to solve fully transient partial differential equations coupled with reaction kinetics. The devolatilization model is used to predict the devolatilization time along with residual mass and particle temperature, while the combined devolatilization and char combustion model is used to predict the overall mass loss and temperature profile of coal. The computed results are compared with the experimental results of the present authors for combustion of Indian sub-bituminous coal (15% ash) in a fluidized bed combustor as well as with published experimental results for coal with low ash high volatile matter. The effects of various operating parameters like bed temperature, oxygen mole fraction in bulk phase on devolatilization time and burn-out time of coal particle in bubbling fluidized bed have been examined through simulation.     

Ash deposition behavior of upgraded brown coal in pulverized coal combustion boiler

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The purpose of this study is to evaluate the ash deposition for Upgraded Brown Coal (UBC) and bituminous coal in a 145 MW practical coal combustion boiler. The UBC stands for Upgraded Brown Coal. The melting temperature of UBC ash is relatively lower than that of bituminous coal ashes. Combustion tests were conducted on blended coal consisting 20 wt.% of UBC and 80 wt.% of bituminous coal. Before actual ash deposition tests, the molten slag fractions in those coal ashes were estimated by means of chemical equilibrium calculations. The calculation results showed the molten slag fraction for UBC ash reached approximately 90% at 1523 K. However, that for blended coal ash decreased to 50%. These calculation results mean that blending UBC with bituminous coal played a role in decreasing the molten slag fraction. This phenomenon occurred because the coal blending led to the formation of alumino-silicates compounds as a solid phase. Next, ash deposition tests were conducted using a practical pulverized coal combustion boiler. A water-cooled stainless-steel tube was inserted in locations at both 1523 K and 1273 K in the boiler to measure the amount of ash deposits. The results showed that the mass of ash deposition for blended coal did not greatly increase, compared with that for bituminous coal alone. Therefore, appropriately blending UBC with bituminous coal enabled the use of UBC without any ash deposition problems in practical boilers.     

The modeling of the combustion of high-ash coal-char particles suitable for pressurised fluidized bed combustion: shrinking reacted core model

An investigation was undertaken involving the combustion of high-ash coal/char particles under conditions suitable for pressurised fluidised bed combustion, in order to evaluate an overall combustion model. The use of very poor quality feedstocks (greater than 40% ash, low calorific value and high sulphur content) in conventional pulverised fuel combustors (PFC) could be technically difficult and un-economical, and has the associated disadvantage of generating gaseous pollutants. Pressurised fluidised bed combustion (PFBC) which is an attractive alternative process and which uses millimetre-sized coal particles is increasing in use on a commercial scale and is the basis for several clean coal technology processes. A Thermogravimetic Analyser (TGA) was used for the experimentation, which was capable of handling relatively large coal/char particles at high pressures and temperatures. Experimentation with prepared coal/chars particles with a diameter of 3 mm at a pressure of 487 kPa and temperatures between 750 and 950 °C was carried out. For the determination of the overall kinetics of combustion it was found necessary to deviate from the established methods (surface-based reaction) and that it was essential to incorporate diffusion in the overall reaction model. Also, the concept of carbon concentration variation in the particle is introduced to account for the effect of high ash content (a mixture of carbon and minerals), instead of assuming pure carbon. This model, which consists essentially of a shrinking reactive core, was found to agree very well with experimental results and all relevant parameters required for an overall rate equation were evaluated. It is also shown that at high temperatures the shrinking reacted core model results approached the results obtained from the conventional shrinking unreacted core model.     

Fine ash formation during combustion of pulverised coal-coal property impacts

In many countries, legislation has been enacted to set guidelines for ambient concentrations and to limit the emission of fine particulates with an aerodynamic diameter less than 10 μm (PM₁₀) and less than 2.5 μm (PM_2.5). Ash particles are formed during the combustion of coal in pf boilers and fine ash particulates may potentially pass collection devices. The ash size fractions of legislative interest formed during coal combustion are the result of several ash formation mechanisms; however, the contribution of each of the mechanisms to the fine ash remains unclear. This study provides insight into the mechanisms and coal characteristics responsible for the formation of fine ash. Five well characterized Australian bituminous coals have been burned in a laminar flow drop tube furnace in two oxygen environments to determine the amount and composition of the fine ash (PM₁₀, PM_2.5 and PM₁) formed. Coal characteristics have been identified that correlate with the formation of fine ash during coal combustion. The results indicate that coal selection based on (1) char characterization and (2) ash fusion temperature could play an important role in the minimization of the fine ash formed. The implications of these findings for coal selection for use in pf-fired boilers are discussed.     

Cotton Stalk Combustion in a Circulating Fluidized Bed

The basic properties of cotton stalk (CS) of length 10–100 mm and its ash were investigated. Studies concerning the combustion characteristics of CS were performed in a circulating fluidized bed (CFB) test facility with a heat input of 0.5 MW. According to previous cold tests, there is very little segregation during the mixing of CS with this size profile and bed material at a fluidization number of N > 7, but the hot experimental results indicate that slight segregation has a small effect on the steady combustion of the dense region. Experiments were carried out with the aim of investigating the effects of operating conditions on the axial temperature and gas concentration profiles along the combustor height, as well as the emission performance and combustion efficiency of the CFB. The experimental results indicate it is difficult to react alumina bed material with alkali metals from CS ash following 26 h of combustion. The overall conclusions appear to indicate that the application of circulating fluidized bed boilers to fire pure CS of length 10–100 mm, is feasible.     

Circulating fluidised bed co-combustion of coal and biomass

Circulating fluidised bed combustion (CFBC) is receiving wide research attention in view its potential as an economic and environmentally acceptable technology for burning low-grade coals, biomass and organic wastes, and thereby mixtures of them. Designs of the existing fluidised bed boilers for biomass combustion are mainly based on experience from coal combustion because the mechanism of combustion of biomass in fluidised beds is still not well understood. A good understanding of the combustion and pollutant formation processes and the modelling of the combustor can greatly avoid costly upsets of the plants.In this paper, the performance of CFBC burning coal and biomass mixtures was analysed. Experimental results were obtained from the combustion of two kinds of coal with a forest residue (Pine bark) in two CFB pilot plants (0.1 and 0.3 MW_th). The effect of the main operating conditions on carbon combustion efficiency was analysed. Moreover, a mathematical model to predict the behaviour of the co-combustion of coal and biomass wastes in CFB boilers has been developed and validated. The developed model can predict the different gas concentrations along the riser (O₂, CO, CH₄, etc.), and the carbon combustion efficiency. The experimental results of carbon combustion efficiencies were compared with those predicted by the model and a good correlation was found for all the conditions used.     

Properties of flash hydrated and agglomerated particles of CFB fly ashes

The fly ash (high carbon content and high unreacted CaO) recirculation in CFB is a typical method to improve the carbon burnout efficiency and the calcium utilization ratio. While the effectiveness of it is limited by the resident time and the reactivity of the re-injected fly ash particles. In the present research, an improved fly ash recirculation method is suggested in which the CFB fly ash is mixed with water or the mixtures of additives (such as waste water of paper mill, cement, sodium silicate, and carbide slag) and water in a blender. Then, this mixture is re-injected into the combustion chamber of CFB by a sludge pump. Because the temperature in CFB is higher, the fly ash was flash hydrated. At the same time, it was dehydrated and agglomerated. The size of agglomerates is bigger than that of original particle and their attrition rate is lower. Therefore the resident time of agglomerates is much longer than that of fine fly ash particles. The absorption of SO₂ is higher than that of original particles, too. This results in high carbon burnout efficiency. The hydrated lime also improves the calcium utilization.     

煤粉炉和循环流化床锅炉飞灰特性对其汞吸附能力的影响

通过分析两台容量相近的循环流化床锅炉和煤粉锅炉飞灰样品的粒径分布、表面结构特性、未燃尽碳含量、反应性和汞含量，探究两种类型锅炉飞灰特性差异及其与飞灰汞吸附能力的关系。结果表明：循环流化床和煤粉锅炉尾端除尘设备排灰口飞灰汞的含量分别为1584.0 ng/g和503.7 ng/g，其原因与飞灰粒径、未燃尽碳含量和表面特性相关。对于循环流化床锅炉，飞灰中汞含量随其粒径和反应性温度的减小而增加，随未燃尽碳含量增加而增加，且与比表面积和吸附量呈正相关关系。对于煤粉锅炉，粒径为75～53 μm的飞灰对汞吸附能力较强，未燃尽碳含量明显小于循环流化床所产生飞灰的含量，飞灰比表面积随粒径变化不大，由此导致煤粉锅炉除尘设备排灰口所取样品对汞的吸附能力远低于循环流化床锅炉相对应位置飞灰对汞的吸附能力。     

Flow structure and combustion characteristic of 65 t/h oil shale-fired circulating fluidized bed riser—2: Dilute phase

In this paper, a 65 t/h oil shale-fired circulating fluidized bed (CFB) boiler was put into commercial operation in Huadian of China in 1996 and industrial hot experiment was done for studying the flow structure and combustion characteristic of CFB dilute phase regime. Flow structure and solid flux of dilute phase regime were investigated along both vertical direction and horizontal direction under different boiler loads. Particle-size distribution and carbon content of oil shale combustion residues were obtained. Experimental investigation shows that SO₂ concentration decreases with increasing the furnace height, and NO_X emission can be reduced by adopting staged combustion technology, low-temperature combustion and circulating combustion. Experimental results are valuable for adjusting the 65 t/h boiler and designing large-scale oil shale-fired CFB boiler. At the same time, the experiment lays a foundation for modeling the flow structure, combustion, heat transfer and abrasion of oil shale-fired CFB dilute phase regime.     

Modelling of NO and N2O emissions from biomass-fired circulating fluidized bed combustors

A model for NO and N₂O emissions from biomass-fired circulating fluidized bed (CFB) combustors has been developed and evaluated in this study. All the model parameters were chosen for a typical woody biomass-pinewood chips. Both drying and devolatilization of biomass particles were modelled with limited rates, which were selected from the literature based on woody biomass fuels. The partition of fuel-nitrogen between volatiles and char was also specifically chosen for pinewood based on available experimental data from the literature. Volatile nitrogen was assumed to consist of NH₃, HCN and N₂ with the distribution between three species as input parameters to the model. Twenty-five homogenous and heterogeneous global chemical reactions were included in the model, of which 20 reactions represents the global fuel-nitrogen reactions. Both gaseous and solid phase were assumed to be in plug flow. The model has been applied to the modelling of a 12 MW_th CFB boiler. The predicted N₂O emissions were always less than 5 ppmv for pinewood combustion, which was consistent with the experimental results. The predicted NO emissions increased with the total excess air of the riser and the fuel-N content while the predicted percentage conversion of fuel-N to NO decreased with increasing fuel-N content. The NO emissions were also predicted to decrease with increasing primary zone stoichiometry. These predictions agree with the experimental results. The predicted NO emissions decreased slightly with increasing bed temperature, whereas experiments showed that NO emissions slightly increased with bed temperature for birch chips combustion and did not change with bed temperature for fir chips combustion. Sensitivity analyses reveal that the reaction between NO and char is the key reaction to determine the NO emissions.     

Modelling of combustion characteristics of high ash coal char particles at high pressure: Shrinking reactive core model

Combustion of a single-particle high ash coal char at elevated pressure has been analyzed. A fully transient shrinking reactive core model incorporating a simple mechanistic kinetic scheme is used to study the combustion characteristics of high ash coal char. The model includes heat and mass transfer phenomena, reaction kinetics and intra-particle details. Finite volume method (FVM) has been used to solve partial differential equations representing fully transient conservation equations. The char combustion model predicts the mass-loss profile and burnout time of the char particle at different temperature and oxygen concentration. The computed results are found to agree well with the published experimental findings of pressurized combustion of high ash coal char. The effects of bulk temperature, total pressure and initial particle size on combustion characteristic and burnout time have been examined through model simulation.     

Burning and physico-chemical characteristics of carbon in ash from a coal fired power plant

The paper addresses the relationship between the chemico-physical properties and the residual combustion reactivity of fly ashes from a full-scale front fired PF coal boiler. Ashes collected at different rows of electrostatic precipitators (EP) have been characterized for their particle size distribution, morphology, chemical composition and combustion reactivity. The combustion time of carbon in ash has been estimated for a wide range of temperatures using a thermobalance and a heated strip reactor.Results showed the existence of marked differences in the content of both carbon and inorganic elements according to the row of EP and the granulometric size of the samples. In contrast with this, the combustion reactivity of all ash samples was similar regardless of their collection point and particle size. Ash reactivity resulted to be approximately 100 times lower than that of the parent coal.The role of thermal annealing on the low reactivity of fly ashes and their propensity to undergo additional thermodeactivation upon further heat treatment has also been investigated. To this end coal and fly ashes have been heated under inert conditions up to 2000 °C and then characterised for their residual combustion reactivity. These tests showed that heat treatment does reduce the reactivity of coal but does not reduce any further the already low reactivity of fly ashes.