Heat transfer mechanism due to particle convection in circulating fluidized bed

The nature of heat transfer in circulating fluidized beds (CFBs) by particle convection was revealed from the experimental results of the particle flux in the lateral direction. As particles contact and rebound from the heat transfer surface because of their lateral motion, they establish a large local temperature gradient. Heat from particles is then transferred primarily through the gas to the heat transfer surface. The data of the lateral particle flux provide a basis to establish a model of heat transfer in CFBs.     

不同负荷下循环流化床锅炉粉煤灰的理化性质研究

为拓展循环流化床锅炉粉煤灰的利用途径,开发粉煤灰综合利用技术,研究了不同负荷下循环流化床(CFB)锅炉粉煤灰的粒径分布、化学组成、物相组成、Al_2O_3溶出特性和微观形貌等理化性质,考察了锅炉负荷对粉煤灰理化性质的影响。结果显示,CFB锅炉负荷对粉煤灰的粒径分布、化学组成和物相组成等性质影响较小,对粉煤灰中Al_2O_3溶出率影响较大,Al_2O_3溶出率与样品颗粒的微观形貌有关,颗粒表面孔隙和裂缝越多,Al_2O_3溶出率越高。CFB锅炉粉煤灰中的无定形相含量较高,在70%以上;CaO在粉煤灰中的主要存在形式有3种:硬石膏、生石灰和无定形CaO。Al_2O_3均是以无定形氧化物的形式存在,煤样中的Al_2O_3在燃烧过程中更容易以飞灰的形式排出;石英和硬石膏更容易以底渣的形式排出。     

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.     

Numerical analysis of particle clustering effects on desulphurization and NO emission in a circulating fluidized bed combustor

Desulphurization by a calcium oxide particle cluster and an isolated calcium oxide particle in a circulating fluidized bed (CFB) combustor was numerically analyzed. The gas flow field, the sulphur retention and the nitrogen oxide emission of the cluster were predicted. Computed results showed that the SO₂ capture rate by a calcium oxide particle in the cluster is less than that of the isolated calcium oxide particle out of the cluster. The captured SO₂ and NO emissions decrease with the decrease of the cluster porosity. The maximum SO₂ capture rate by the cluster is at a temperature between 1025 and 1055 K, whereas more NO emissions were found with the increase of the gas temperature. The sulphur removal and the NO emission increase with the increase of the inlet gas velocity.     

燃煤循环流化床锅炉N_2O排放影响因素分析

阐述了CFB锅炉燃烧过程中N_2O的产生、消解机理,分析了燃料特性、床温、过量空气系数和煤中的矿物质成分等诸多因素对CFB锅炉N_2O排放的影响。分析了生物质与煤混合燃烧,炉膛后再燃,在CFB锅炉中添加催化剂以及反向分级燃烧等新型低N_2O燃烧技术,为CFB锅炉低N_2O排放技术指明了研究方向。     

燃煤增压流化床氧化亚氮排放特性试验

在小型增压流化床内研究N2 O和NO的排放特性 ,研究运行条件包括压力、氧浓度以及床层温度对N2 O和NO排放量的影响 .结果表明 ,在增压燃烧的条件下 ,随着床温的增加 ,N2 O的排放量减少得很快 ;而床温对NO的排放影响很小 ,此结果与常压下的结果不同 .随着氧浓度的增加 ,N2 O和NO的排放量均增加 ,但N2 O增幅不如NO强 .NO的排放量随着压力的增高有明显的降低 ,在氧浓度较低的条件下 ,压力越高 ,NO的降低幅度越显著 ;而压力对N2 O排放影响则相反     

流化床燃烧石油焦N_2O排放特性

通过在一小型流化床试验台上进行石油焦的燃烧试验 ,阐述了N2 O和NO形成与分解机理 ,模拟研究了N2 O的排放特性 .采用不同程度脱去挥发分的石油焦颗粒 ,研究脱挥发分的程度对N2 O形成的影响 ,脱挥发分的温度越高 ,即脱挥发分的程度越高 ,石油焦氮形成N2 O的量越少 ,这表明石油焦挥发分氮形成N2 O量高于相应石油焦焦炭氮燃烧产生的N2 O量 .燃料燃烧过程中 ,NO形成比较均匀 ,而N2 O形成比较复杂 ,燃料氮向NO的转化率随脱挥发分温度升高而增加 ,而向N2 O的转化率则有一临界脱挥发分温度点 .     

The internally circulating fluidized bed (ICFB): A novel solution to gas bypassing in spouted beds

The bypassing of a portion of the inlet spout gas to the annular region of spouted and spout-fluid beds can have a critical effect on processes requiring accurate control of the spout gas residence time even when a draft tube is utilized. A modification to the spout-fluid bed with draft tube called the Internally Circulating Fluidized Bed or ICFB is proposed which eliminates this bypassing problem. Experiments were conducted on a cold 20 cm diameter semi-cylindrical unit to define the operational characteristics of the ICFB with particular emphasis on the gas and solids flow behavior. The solids and gas flow rates in the unit were successfully related by a published correlation which was developed for the cocurrent flow of gases and solids through orifices.     

循环流化床燃煤过程NOx和N2O产生-控制研究进展

循环流化床(CFB)燃烧技术因其燃料适应范围广、污染物排放低等优点,近几十年得到广泛应用. 随着当前环保要求的日益提高,CFB燃煤过程N2O排放浓度较高成为其应用的瓶颈问题. 因此系统总结CFB燃煤过程中NOx和N2O排放的研究现状对开发新型CFB燃煤技术具有重要意义. 本工作首先讨论了CFB燃煤过程中NOx和N2O的均相和异相反应机理,然后应用这些机理分析了床温、过剩空气系数、分级燃烧,以及煤种对CFB燃煤过程NOx和N2O排放的影响. 在此基础上,对常见的抑制NOx和N2O排放的工艺从机理角度进行了归纳总结. 最后,对2种本工作认为有应用前景的CFB燃煤减排NOx和N2O新技术?反向分级燃烧技术及CFB解耦燃烧技术进行了简要论述.     

Sulphation of limestones in a fluidized bed combustor: The relationship between particle attrition and microstructure

Sulphation of two limestones in a fluidized bed combustor has been investigated. One limestone (coarse‐grained) was characterized by a significant population of relatively large pores after calcination; the other (fine‐grained) presented a finer and fairly unimodal pore size distribution. Differences in the microstructure were reflected by different thickness of the sulphate shell formed upon sulphation and ultimate calcium conversion degree. Particle attrition/fragmentation were fairly small under moderately bubbling fluidization conditions. Fragmentation upon impact was significant. The fine‐grained limestone, characterized by a thinner sulphate shell, was more susceptible to fragmentation than the other. Particle fragmentation discloses unreacted CaO enabling secondary sulphation of exhausted particles.     

以煤焦混合物为燃料的循环流化床锅炉SO2排放特性

在工业运行的410 t·h^-1循环流化床锅炉上进行烟煤、70％烟煤＋30％石油焦和50％无烟煤＋50％石油焦的燃烧试验,研究了运行参数对SO₂排放特性的影响。结果表明,3种燃料均能达到良好的燃烧效果,炉内温度场分布均匀。在相同燃烧条件下,不同燃料SO₂排放量与其中的含硫量呈正相关关系。SO₂排放量随温度的升高先减小后增大,存在最佳脱硫温度;随钙硫比的增大而减小;随过量空气系数的增大而减小;随飞灰再循环量的增大而减小。对于不同种类的石灰石,大比表面积和高比孔容积的石灰石对SO₂有较好的脱除效果。考察了燃用不同燃料的最佳温度、钙硫比和过量空气系数,阐述了飞灰再循环和石灰石微观结构在循环流化床锅炉脱硫中的机理和作用,以期对循环流化床的设计和运行工作提供指导。     

A novel method to synthesize pure-phase Si2N2O powders in a fluidized bed reactor

Si₂N₂O ceramic, an emerging functional and structural material, has a wide range of applications. However, the preparation of pure-phase Si₂N₂O powder remains challenging due to the mass transfer resistance and undesirable side reactions in the conventional methods. Herein, a novel molecular approach combined with the decomposition process has been developed to synthesize pure-phase Si₂N₂O powders. The hydrated Si(NH)₂ precursors were synthesized through the chemical vapor deposition (CVD) of SiCl₄, NH_3, and humidified N₂ in a fluidized bed reactor (FBR) in two steps. Then, the hydrated Si(NH)₂ precursors were decomposed into amorphous and subsequently transformed into crystalline powders under different temperatures and time. It was found that the molar ratio of N/O of the hydrolyzed Si(NH)₂ can be controlled by N₂ ventilation time and played an important role in synthesizing high pure Si₂N₂O powder. When it varied from 2.5:1 to 2:1, pure-phase Si₂N₂O powder was obtained after heat treatment at 1300–1500 °C, which features a big tolerance for N/O ratios. This newly developed method offered a chance for the preparation of high-quality Si₂N₂O powder with high efficiency and low cost.     

Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

The ignition and burnout of the volatiles in fluidized bed combustor are essential for its performance and emissions. NO_x are known to sensitize the oxidation of hydrocarbons, CO, and H₂. This effect is relevant especially for fluidized bed combustors, which are operated at relatively low temperatures (i.e. about 850 °C). Different reaction mechanisms and modifications to existing mechanisms have been proposed in the literature to account for these low temperature interactions of NO_x and hydrocarbons. In this work, an existing widely used reaction mechanism is adapted and tested for its capability to describe the NO sensitized oxidation of CH₄ under conditions relevant to fluidized bed combustion. NO lowers the ignition temperature to about 300 °C under the conditions investigated. Three different oxidation paths for the oxidation of CH₄ have been identified and discussed. Their relative importance strongly depends on combustion temperature, indicating that the presence of NO_x significantly affects the oxidation of the volatiles in fluidized bed combustion.     

流化床锅炉旋风分离器中心筒的运行和检修

针对75t／h循环流化床锅炉旋风分离器中心筒多次损坏（开裂、变形、倾斜、脱落、挂钩脱落等）的原因进行了认真分析,有针对性的提出防范措施,并且不断完善修复方案,经检修改造后,该炉一直运行稳定。     

Conversion of Fuel-N to N2O and NOx during Coal Combustion in Combustors of Different Scale

With focus on investigating the effect of combustor scale on the conversion of fuel-N to NOx and N2O, experiments are carried out in three combustors, including single coal particle combustion test rig...     

不同规模的煤燃烧装置中燃料-N向N2O和NOx的转化（英文）

With focus on investigating the effect of combustor scale on the conversion of fuel-N to NOx and N20, experiments are carried out in three combustors, including single coal particle combustion test rig, laboratory scale circulating fluidized-bed boiler （CFB） and full scale CFB in this work. For single coal particle combustion, the majority of f-uel-N （65%-82%） is released as NOx, while only a little （less than 8%） fuel-N yields N20. But in labora- tory scale CFB, the conversion of fuel-N to N20 is increases, but the conversion of fuel-N to NOx is quite less than that of single coal particle combustion. This is because much char in CFB can promote the NOx reduction by in- creasing N20 formation. In full scale CFB, both of the conversion of fuel-N to NOx and the conversion of fuel-N to N20 are smaller than laboratory scale CFB.     

Catalytic effect of biomass ash on CO, CH4 and HCN oxidation under fluidised bed combustor conditions

In fluidised bed combustion heterogeneous reactions catalysed by the bed material, CaO, and char are significant for the emission levels for instance of NO, N₂O, and CO. The catalysts present in the bed affect significantly the selectivity of HCN and NH₃ oxidation, which are known as precursors of NO_x (i.e. NO and NO₂) and N₂O emissions from solid fuel combustion. Thus the catalytic activity of biomass ashes may also be responsible for the negligible N₂O emissions from biomass combustion due to the presence of a large amount of solids in fluidised bed combustion, homogeneous oxidation may be suppressed within the bed by the quenching of the radicals. For this reason the catalytic oxidation of hydrocarbons and CO on the bed material may be of significance for the total burnout within the fluidised bed combustor.Within this study the effect of different ashes from spruce wood, peat, and for comparison bituminous coal on the oxidation of CH₄, CO, and HCN was studied. The different ashes were shown to have a strong catalytic activity for the oxidation of CH₄, CO, and HCN. In HCN oxidation the selectivity towards NO is high, whereas very little N₂O is formed. The activity of the ashes is strongly dependent on the fuel, which may be explained by their composition.The kinetics of the oxidation of CO and HCN in the temperature range relevant for fluidised bed combustion, i.e. 800-900 °C, has been evaluated for spruce wood ash.     

The influence of particle size and density on the radial gas mixing in the dilute region of the circulating fluidized bed

A cold model of a CFB, having a 0.120m ID, 5.75m high riser, was operated at a superficial gas velocity of 6m/s and at average solids mass fluxes varying in the range between 16 and 55kg/sm². A stream of carbon dioxide was injected continuously at a fixed position along the riser axis and used as tracer gas. Three solids, belonging to groups A and B of Geldart's classification, were selected in order to investigate separately, the effect of particle size and density on the radial mixing of the gas phase. The intensity of radial gas dispersion reduced when larger or heavier particles were used. The differences in the values of radial dispersion coefficient and Peclet number were explained, taking into account the role of the particle size with respect to the scale of gas turbulence.     

Conversions of fuel-N,volatile-N,and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N₂O during combustion of a single coal particle in O₂/N₂ and O₂/H₂O were studied in a tube reactor at low temperature. In O₂/N₂, NO reaches the maximum value in the devolatilization stage and N₂O reaches the maximum value in the char combustion stage. In O₂/H₂O, both NO and N₂O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N₂O in O₂/N₂ are obviously higher than those in O₂/H₂O, due to the reduction of H₂O on NO and N₂O. Temperature changes the trade-off between NO and N₂O. In O₂/N₂ and O₂/H₂O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N₂O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at < O₂ > = 30 vol% in O₂/N₂. In O₂/H₂O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at < O₂ > = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N₂O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N₂O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N₂O in both atmospheres. This work is to develop efficient ways to understand and control NO and N₂O emissions for a clean and sustainable atmosphere.     

不同NO3-浓度An/A/O-SBR系统PAOs-GAOs竞争及N2O释放特性

采用厌氧/缺氧/好氧运行的序批式生物反应器（An/A/O-SBR）,经不同NO₃^-浓度（10,20,30和40 mg/L,以氮计）长期驯化,考察了不同NO₃^-条件下An/A/O-SBR脱氮除磷及N₂O释放特性,基于不同微生物降解特性分析,确定了不同NO₃^-浓度下SBR系统内反硝化聚磷菌（denitrifying phosphorus accumulating organisms,DPAOs）和聚糖菌（glycogen-accumulating organisms, GAOs）竞争关系。结果表明：随NO₃^-浓度增加,总氮（TN）去除率由90%以上降至41.3%,TP去除率呈先增高后降低的趋势,N₂O产率（N₂O_emission/NO_x^-_removal）分别为1.68%、4.17%、8.92%和14.28%。An/A/O-SBR内微生物呈PAOs和GAOs共存的污染物降解特性,高浓度NO₃^-缺氧吸磷过程出现NO₂^-积累,抑制DPAOs活性,GAOs碳源竞争能力增强,NO₃^--N由10 mg/L增至40 mg/L,厌氧阶段PAOs的COD耗量比例由33.5%降至25.1%,相应GAOs的COD耗量由59.3%增至74.1%。DPAOs-GAOs共生体系内,反硝化过程NO₂^-/HNO₂积累耦合反硝化聚糖菌比例增加,加剧了高NO₃^-下An/A/O-SBR内N₂O释放。