Heat flux distribution on circulating fluidized bed boiler water wall

The future of circulating fluidized bed (CFB) combustion technology is in raising the steam parameters to supercritical levels. Understanding the heat flux distribution on the water wall is one of the most important issues in the design and operation of supercritical pressure CFB boilers. In the present paper, the finite element analysis (FEA) method is adopted to predict the heat transfer coefficient as well as the heat flux of the membrane wall and the results are validated by direct measurement of the temperature around the tube. Studies on the horizontal heat flux distribution were conducted in three CFB boilers with different furnace size, tube dimension and water temperature. The results are useful in supercritical pressure CFB boiler design. __________ Translated from Journal of Power Engineering, 2007, 27(3): 336–340 [译自: 动力工程]     

300MW循环流化床锅炉热力特性的研究

为准确把握300 Mw循环流化床(CFB)锅炉的热力特性,针对目前国内3种典型300Mw CFB锅炉炉型,从炉膛、外置床的传热机理和传热计算角度出发,对不同炉型的热力特性进行了分析比较.结果表明:3种炉型CFB锅炉的热力特性均可满足蒸汽参数的要求,且热负荷分配合理.     

循环流化床锅炉中灰循环倍率与炉膛传热系数

循环流化床锅炉中灰循环倍率不仅影响燃烧 ,而且还影响传热。炉膛传热系数是其热力计算的关键数据 ,根据现场测得的数据 ,并参照有关文献 ,提出了一个经验计算公式 ,在几台循环流化床锅炉上亦已证明是正确的     

The heat transfer coefficients of the heating surface of 300 MWe CFB boiler

A study of the heat transfer about the heating surface of three commercial 300 MWe CFB boilers was conducted in this work. The heat transfer coefficients of the platen heating surface, the external heat exchanger (EHE) and cyclone separator were calculated according to the relative operation data at different boiler loads. Moreover, the heat transfer coefficient of the waterwall was calculated by heat balance of the hot circuit of the CFB boiler. With the boiler capacity increasing, the heat transfer coefficients of these heating surface increases, and the heat transfer coefficient of the water wall is higher than that of the platen heating surface. The heat transfer coefficient of the EHE is the highest in high boiler load, the heat transfer coefficient of the cyclone separator is the lowest. Because the fired coal is different from the design coal in No.1 boiler, the ash content of the fired coal is much lower than that of the design coal. The heat transfer coefficients which calculated with the operation data are lower than the previous design value and that is the reason why the bed temperature is rather high during the boiler operation in No.1 boiler.     

循环流化床锅炉水冷壁的传热系数

对循环流化床锅炉内水冷壁的传热机理进行了分析,验证了循环流化床炉内受热面的传热计算经验公式,可供设计、调试和运行时参考。     

Method of calculation of heat transfer coefficient of the heater in a circulating fluidized bed furnace

Knowledge of heat transfer coefficients is important in the design and operation of CFB boilers. It is the key to determining the area and the layout of the heat transfer surfaces in a CFB furnace. Local bulk density has a close relationship to the local heat transfer coefficient. Using a heat flux probe and bulk density sampling probe, the local bed to wall heat transfer coefficient in the furnace of a 75 t/h CFB boiler was measured. According to the experimental results and theoretical analysis of the facts that influence the heat transfer, the heat transfer coefficient calculation method for the CFB furnace was developed. The heat transfer surface configuration, heating condition, and the material density are considered in this method. The calculation method has been used in the design of CFB boilers with a capacity from 130 t/h to 420 t/h. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 540–550, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10056     

船用增压锅炉的炉膛对流传热计算

由于增压锅炉燃烧压力的提高,强化了对流传热,如果仍按常压燃烧锅炉炉膛热力计算,忽略对流传热,将直接影响增压锅炉炉膛热力计算的准确性。文中对增压锅炉与常压燃烧锅炉炉膛特性参数进行了比较,对前苏联增压锅炉的试验数据进行了分析探讨,指出了增压锅炉炉膛对流传热的影响因素。同时提出增压锅炉炉膛传热计算应将辐射与对流传热分开计算,给出了适用于增压锅炉炉膛对流传热的计算公式,并进行了实例计算分析,与前苏联增压锅炉试验的计算数据结果相近。对于完善增压锅炉的炉膛热力计算具有一定的理论和实际指导意义。     

300MW循环流化床锅炉屏式受热面传热系数计算及其变化规律

根据300MW循环流化床（CFB）锅炉现场测试数据并结合以往CFB锅炉传热系数的研究成果,建立了屏式受热面烟气侧的传热模型,包括辐射传热模型和对流传热半经验公式．利用该模型对某300MWCFB锅炉在94％锅炉最大连续蒸发量（BMCR）工况下炉膛内屏式受热面的传热系数进行了计算,分析了屏式受热面管间节距、炉膛温度、工质温度、壁面黑度及烟气速度等因素对传热系数的影响．结果表明：烟气速度、炉膛温度和壁面黑度对传热系数的影响较大,所建立的传热模型能够合理地反映主要因素对CFB锅炉屏式受热面传热的影响．     

海拔高度对锅炉传热影响的理论分析

从现有的锅炉热力计算方法出发,分析推导了海拔高度的增加对锅炉炉内换热,对流放热系数,辐射放热系数和烟气物性的影响以及对锅炉总的换热量的影响,并指出了影响趋势。     

循环流化床锅炉炉膛的传热计算

结合以往对循环流化床(CFB)锅炉炉膛传热系数的研究成果,建立了CFB锅炉炉膛传热系数的计算模型,并对辐射传热系数和对流传热系数的计算分别进行了相应的分析.对某CFB锅炉在100%负荷条件下的炉膛传热系数进行了计算,并对影响该传热系数的主要因素进行了分析,包括受热面结构尺寸、床层温度Tb、工质温度Tf、工质侧传热系数af以及壁面黑度εw等.结果表明:该模型能够合理地反映出以上因素对CFB锅炉炉膛传热系数的影响.     

Heat transfer characteristics in a small-scale fluidized bed boiler

Heat transfer characteristics in a small-scale fluidized bed boiler (2MW_th) were studied using lignite and corn cob as fuels. Depending on air velocity, the heat transfer rates from bed to water membrane wall and from hot flue gas to convective tube bank were in the ranges 75–55% and 25–45% of the total heat absorbed by the boiler, respectively. At designed capacity, the heat transfer flux based on bed cross sectional area and on water membrane wall area were about 0·45 and 0·15 MWm⁻², respectively. Under the conditions studied, it was found that the overall heat transfer coefficient between bed and water membrane wall was 100–300 W m⁻² K⁻¹, whereas that between flue gas and convective tube bank was 10–30 Wm⁻² K⁻¹. The study of heat transfer to a horizontal tube immersed in the bed as well as placed in the freeboard region were also studied. The effective heat transfer coefficients were found to be 300–800 W m⁻² K⁻¹ for in-bed tube and 30–150 W m⁻² K⁻¹ for the freeboard region, depending on air velocity. Comparison of these data with those predicted by both modelling and correlation reported in the literature was also made. For the immersed tube, good agreement was observed for low air velocity, while at high air velocity the experiment produced results twice those estimated from modelling and correlation. For the freeboard region, the model gave a fair prediction.     

Heat transfer effectiveness of three-fluid separated heat pipe exchanger

A heat transfer model for three-fluid separated heat pipe exchanger was analyzed,and the temperature transfer matrix for general three-fluid separated heat exchanger working in parallel-flow or counter-flow mode was obtained.It was found that the forms of temperature transfer matrix are similar for heat pipe rows with equal or different heat transfer surface area.Furthermore,by using the temperature transfer matrix of the heat pipe exchanger,the relationship between heat transfer effectiveness θ 1,θ 2 and M,NTU,U,Δt i were derived for the exchanger operating in parallel-flow or counter-flow mode,and a simple special example was adopted to demonstrate the correctness of these relationships.     

Asymmetric combustion characteristics and NOx emissions of a down-fired 300 MWe utility boiler at different boiler loads

To investigate the aerodynamic field, cold airflow experiments were conducted under different boiler loads in a cold small-scale model of a down-fired pulverized-coal 300 MW_e utility boiler. At 300 MW_e and 250 MW_e loads, a deflected flow field appeared in the lower furnace. In contrast, at a 150 MW_e load, a U-shaped flow field appeared in regions near the left- and right-side walls in the lower furnace. Concurrently, the regions near the two wing walls adjacent to the front arch had received deflected upward airflow emanating from the region near the rear wall. Moreover, a symmetric W-shaped flow field appeared in the central regions below the front and rear arches.Industrial-sized experiments on the full-scale furnace were also performed at different loads with measurements taken of gas temperatures in the burner region and near the right-side wall, as well as heat fluxes and gas components in the near-wall region. Asymmetric combustion appeared at 300 MW_e and 250 MW_e loads, with large differences arising in gas temperatures, gas components, and heat fluxes between zones near the front and rear walls. At 150 MW_e load, gas temperatures, gas components and heat fluxes are, in general, symmetrically distributed throughout the furnace. By decreasing the load, differences in gas temperatures, gas components, and heat fluxes near the front and rear walls decrease, as did NO_x emissions. Meanwhile, the carbon content in fly ash essentially decreased, yielding an increase in boiler efficiency assisted by a drop in exhaust gas temperature.     

Suspension-to-wall heat transfer in a 12-MWth circulating fluidized-bed combustor

An experimental investigation was carried out to study the effects of operating parameters on the local suspension-to-wall heat transfer in the combustor of a 12-MW_th circulating fluidized-bed (CFB) boiler. The heat transfer coefficients were measured with a conduction-type heat flux meter at five different vertical levels. The measurements covered a range of superficial gas velocities from 4 to 6 m/s, a bulk bed temperature from 800 to 850 °C and a suspension density from 6 to 70 kg/m³ for 270-μm silica sand particles. The heat transfer coefficient for the membrane wall in the combustion chamber of the CFB boiler was in the range of 100 to 180 W/m² K for the range of operating conditions employed in this work. The heat transfer coefficient decreased with increasing height and increased with increasing bulk bed temperature, superficial gas velocity and suspension density. Based on the experimental data, a simple correlation is proposed for predicting the suspension-to-membrane wall heat transfer coefficient. The results were analysed and compared with the experimental data of other workers.     

Sensitivity analysis and numerical experiments on transient test of compact heat exchanger surfaces

A single-blow transient testing technique considering the effect of longitudinal heat conduction is suggested for determining the average convection heat transfer coefficient of compact heat exchanger surface. By matching the measured outlet fluid temperature variation with similar theoretical curves, the dimensionless longitudinal conduction parameter λ₁, the time constant of the inlet fluid temperature τ⁺, and the number of heat transfer units N _tu can be determined simultaneously using the Levenberg-Marquardt nonlinear parameter estimation method. Both sensitivity analysis and numerical experiments with simulated measurements containing random errors show that the method in the present investigation provides satisfactory accuracy of the estimated parameter N _tu, which characterizes the heat transfer performance of compact heat exchanger surfaces. __________ Translated from Journal of Power Engineering, 2007, 27(2): 227-232 [译自:动力工程]     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

Combustion and NOx emission characteristics of a retrofitted down-fired 660 MWe utility boiler at different loads

Industrial experiments were performed for a retrofitted 660 MW_e full-scale down-fired boiler. Measurements of ignition of the primary air/fuel mixture flow, the gas temperature distribution of the furnace and the gas components in the furnace were conducted at loads of 660, 550 and 330 MW_e. With decreasing load, the gas temperature decreases and the ignition position of the primary coal/air flow becomes farther along the axis of the fuel-rich pipe in the burner region under the arches. The furnace temperature also decreases with decreasing load, as does the difference between the temperatures in the burning region and the lower position of the burnout region. With decreasing load, the exhaust gas temperature decreases from 129.8 °C to 114.3 °C, while NO_x emissions decrease from 2448 to 1610 mg/m³. All three loads result in low carbon content in fly ash and great boiler thermal efficiency higher than 92%. Compared with the case of 660 MW_e before retrofit, the exhaust gas temperature decreased from 136 to 129.8 °C, the carbon content in the fly ash decreased from 9.55% to 2.43% and the boiler efficiency increased from 84.54% to 93.66%.     

1000MW超超临界锅炉水冷壁壁温计算

采用分区计算简化大容量高参数超超临界锅炉炉内辐射、对流传热模型,研究炉膛水冷壁热负荷及壁温的空间分布情况,并与试验数据进行了对比,计算结果与试验值之间的偏差较小,最大为5.72%.该模型与算法可给出不同锅炉负荷条件下,水冷壁壁面热负荷与壁温沿炉膛宽度方向的分布规律.结果表明,水冷壁热负荷与壁温均呈现出中间高两端低的弧形分布.四角切圆燃烧锅炉火焰位置对炉内传热有很大影响.模拟计算可为超超临界锅炉的运行提供参考,预测了在材料允许温度范围内,火焰中心偏斜最大不超过2 m.     

An advanced model to assess fouling and slagging in coal fired boilers

The assessment of the influence of fouling and slagging on the heat transfer in utility boilers has obtained significant interest both during boiler design and operation. This paper presents a strategy to investigate this influence by introducing heat resistance to represent fouling and slagging on furnace walls. The evaluation of this strategy was performed using the plant heat flux reading data together with a 3‐D computational fluid dynamics code. The model considers the influence of the operating conditions to the incident heat fluxes and these are used to assess the heat transfer resistance from the measured absorbed heat fluxes. Copyright © 2002 John Wiley & Sons, Ltd.     

引进300 MW循环流化床锅炉的技术特点分析

为有效利用我国高硫低质煤资源,加快大型循环流化床(CFB)锅炉在我国的发展,国内采用引进技术制造、投运了多台 300 MW CFB锅炉.在介绍引进的 300 MW CFB锅炉结构及布置特点的基础上,重点分析了引进300 MW CFB锅炉与国内现有的100～ 150 MW级CFB锅炉所采用更新技术:裤衩腿炉膛、高的运行床压和控制系统、长入口中心筒偏置的分离器、管束管径不同的外置式换热器、将多维膨胀简化为一维膨胀的膨胀节、四分仓回转式空气预热器、汽水调节技术、互为备用的给煤线路以及溢流式风水联合冷渣器.