Experimental study on cotton stalk combustion in a circulating fluidized bed

This paper summarizes the results of an experimental study on cotton stalk (CS) combustion in a circulating fluidized bed. The mixing and fluidizing characteristics of binary mixture of CS with 10–100 mm in length and alumina bed material with a certain size distribution in a cold test facility were studied. The results show that CS by itself cannot fluidize, and adding inert bed material can improve the fluidization condition. CS can mix well with alumina at fluidization number N = 3–7. As N is more than 7, there will exist a little more segregation. The study concerning combustion characteristics of pure CS was performed on a circulating fluidized bed with a heat input of 0.5 MW. The effects of fluidizing velocity, secondary air flow and gas flow to the loop seal on the bed temperature profiles were investigated. Although there is a little more segregation at N higher than 7 in the cold tests, the hot experimental results indicate that slight segregation has little effect on the steady combustion of the dense region. In this study, the concentrations of major gaseous pollutants (CO, SO₂ and NO) in flue (stack) gas were measured.     

褐煤在鼓泡流化床和循环流化床燃烧的汞迁移试验研究

对褐煤在小型电加热鼓泡流化床和小型电加热循环流化床中燃烧时的汞迁移特性进行了对比试验研究,重点考察了不同燃烧工况对汞迁移特性的影响。试验结果表明,炉膛温度和给煤量增加,鼓泡流化床和循环流化床的烟气总汞HgT均增加,飞灰颗粒汞含量Hg(p)均减少,并且循环流化床的烟气总汞HgT值均低于相同燃烧工况的鼓泡流化床值,循环流化床的飞灰颗粒汞含量Hg(p)值均高于相同燃烧工况的鼓泡流化床的值;流化风速增加,循环流化床的烟气总汞HgT减少,飞灰颗粒汞含量Hg(p)增加,鼓泡流化床烟气总汞HgT增加,飞灰颗粒汞含量Hg(p)减少。     

Experimental study on NOx emissions of pulverized bituminous coal combustion preheated by a circulating fluidized bed

This study investigated nitrogen oxide (NO_x) emissions of pulverized coal combustion preheated by a circulating fluidized bed (CFB). During the test process, high-temperature fuel preheated in a CFB was burned in a down-fired combustor (DFC). The effect of air distribution on NO_x emissions was studied in the DFC, including three types of secondary air nozzle structures, five secondary air ratios, and three tertiary air position arrangements. Under stable conditions, the conversion ratio of fuel-nitrogen to N₂ in the CFB was 41.4%, which resulted in lower NO_x emissions in the platform. In this study, secondary air could be injected into the combustor at the top (annular) or through the side wall (circular) of the DFC, both with high combustion efficiency. This means that the secondary air is completely separated from the burner, and burner structure is greatly simplified. NO_x emissions from secondary air nozzle structures of center, annular, and circular ports were 565.66, 345.45, and 220.38 mg/Nm³ (@6% O₂) respectively. NO_x emissions initially decreased then increased with increases in secondary air ratio with the annular nozzle structure. NO_x emissions could be further inhibited by rationally arranging tertiary air positions.     

Modelling heat transfer in a circulating fluidized bed

A model for the bed-to-wall heat transfer under low temperature condition in a circulating fluidized bed (CFB) was developed based upon a simplified cluster renewal concept. The age of clusters in contact with the wall at different locations along the height of the CFB was estimated as the weighted average age considering their formation and disintegration. One set of experimental data on heat transfer in a 4.5-metre high, 0.15-metre diameter CFB under low temperature condition (67–77°C) was chosen for comparison with prediction of local heat transfer coefficient. The experimental observation and prediction have shown a qualitative agreement.     

Characteristics of co-combustion of anthracite with bituminous coal in a 200-MWe circulating fluidized bed boiler

The effect of co-combustion of Vietnamese anthracite with Australian bituminous coal on the performance of a commercial circulating fluidized bed boiler was observed in the Tonghae thermal power plant.The temperature in the cyclone exit of the boiler increased slightly, which caused a decrease in the desulfurization efficiency as the co-combustion ratio of the bituminous coal increased from 40 to 100%. The unburned carbon fraction also increased. Consequently, the fine particles of the bituminous coal had lower combustion reactivity than those of the anthracite.NO_x emissions decreased as the bituminous coal ratio increased, although the fraction of nitrogen in the bituminous coal was higher than that in the anthracite. However, the emission of dust was found to increase due to an increase in the amount of CaO and MgO in the fly ash, which could lower the efficiency of the electrostatic precipitator.From these results, we concluded that the complete switch from the anthracite to Australian bituminous coal was possible, although the efficiency and the operation stability became lower than before. Additionally, as a future study, it is necessary to monitor the instability of the temperature increase and its effect on the prolonged clinker formation in the boiler.     

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.     

A study of heat transfer in a circulating fluidized bed

An experimental investigation was carried out to study the effects of operating parameters on the local bed-to-wall heat transfer coefficient in a 4.5 m tall, 0.150 m diameter circulating fluidized bed with a bed temperature in the range of 65°C to 80°C, riser flow rate varying from 1400 litres/min to 2000 litres/min, bed inventory in the range of 15 kg to 25 kg of sand, and average sand sizes of 200 μm, 400 μm and 500 μm. A heat flux probe was attached to the riser wall at five different vertical locations for measuring the heat flux from the bed to the wall surface. From the present work, the heat transfer coefficient in the dilute phase was found to be in the range of 62 to 83 W/m²K, 51 to 74 W/m²K, and 50 to 59 W/m² K for sand sizes of 200 μm, 400 μm and 500 μm, respectively. Relevant mathematical correlations were developed to predict local heat transfer coefficient based on the results of the practical work.     

A study of solid circulation rate in a circulating fluidized bed

An experimental investigation under cold condition was made to study the effects of some operating/design parameters and non-mechanical L valve configuration on the solid circulation rate in a 4.5 m tall, 0.15 m diameter circulating fluidized bed with riser flow rate varying from 1400 litres/min to 2000 litres/min and bed inventory from 15 kg to 25 kg of sand of average sizes 200 μm, 400 μm and 500 μm. Solid circulation rate was estimated by measuring velocity of sand particle travelling through a vertical Perspex tube section at the bottom of the return leg. It was found to be in the range of 2.8 to 12.3kg/m²s, 0.07 to 9.1kg/m²s and 0.12 to 2.23kg/m²s for sand sizes of 200 μm, 400 μm and 500 μm, respectively for a horizontal L valve. Two mathematical correlations have been developed from the experimental results to predict solid circulation rate as a function of riser flow rate, aeration flow rate, total bed inventory and particle size used.     

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s⁻¹. Local sand of mean size (d_p) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed. © 1997 John Wiley & Sons, Ltd.     

Studies on the operation of loop-seal in circulating fluidized bed boilers

Loop-seal, considered heart of a circulating fluidized bed (CFB), returns solids captured by cyclone to the base of the riser while preventing direct flow of gas from high pressure riser to the low-pressure cyclone. This non-mechanical valve is used in thousands of CFB systems yet only a limited information is available on its working. Present research studies the flow of solids through a loop-seal and the effect of several design and operating parameters on it. This experimental study was conducted in a loop-seal 110 mm × 448 mm × 400 mm high connected to a riser 152 mm diameter and 5180 mm high. Majority of the experiments was done with 171 μm sand though several other size and type of solids were studied for their flowability. It was found that for the solids to flow through the loop-seal a minimum level of aeration, in excess of that required for minimum fluidization was required. The length of the horizontal passage connecting the supply and recycle chambers of the loop-seal had an important effect on the solids flow. For example, the minimum aeration for the onset of solids flow increases with increase in this length. The pressure drop per unit length across the passage also increased with the passage length. The air fed into the supply chamber is split such that the superficial air velocity in the supply chamber (or the standpipe) remained below the minimum fluidization velocity of the particles while the remaining air conveys solids through the horizontal passage. Present study showed that the solids flowing through the horizontal passage are neither fully fluidized nor moving packed or suspended solids. It moves as a segregated flow of solids driven by hydrostatic pressure and fluid drag.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Discrete injection of oxygen enhances hydrogen production in circulating fast fluidized bed membrane reactors

The coupling of steam and oxidative reforming reactions of methane in a circulating fast fluidized bed membrane reactor (CFFBMR) has been studied. The study is based on rigorous modeling and simulation viewpoints. The concept of oxygen distribution has been investigated by employing the discrete injection approach. Feeding of oxygen through discrete injection points along the length of the reformer is compared with oxygen feeding through dense perovskite membranes (_{La0.4Ca0.6Fe0.8Co0.2O3-δ})$({\mathrm{La}}_{0.4}{\mathrm{Ca}}_{0.6}{\mathrm{Fe}}_{0.8}{\mathrm{Co}}_{0.2}{\mathrm{O}}_{3-\delta })$ and the conventional direct feeding of oxygen (co-feed). The results indicate that the discrete injection of oxygen enables to achieve in situ heat integration at low feed temperature. It is also shown that oxygen distribution along the length of the CFFBMR is beneficial in eliminating the hot spot temperature regions. The hydrogen yield shows an optimal value and is significantly affected by many key parameters such as feed temperature, pressure and number of hydrogen membrane tubes. The result also shows that the CFFBMR has a potential to develop complicated bifurcation behavior.     

Hydrodynamics in a circulating fluidized bed with annular furnace and six parallel cyclones

Systematic measurements were conducted on a cold CFB with annular furnace and six parallel cyclones to study gas-solids flow in the annular furnace and flow non-uniformity among six cyclones.The results show that axial solids holdup in the annular furnace decreases exponentially with height,similar to the conventional rectangular furnace.The uniform transverse distribution of solids holdup suggests a good gas-solids mixing in the annular furnace.The annular furnace presents the core/double-annulus flow structure,and it results in enhanced gas-solids back-mixing than the conventional core/annulus flow structure.The gas-solids flow of the inner wall-layer and the outer wall-layer is very close at most part of the furnace height,and the wall-layer thickness decreases with height.Flow non-uniformity exists among six parallel cyclones in the annular furnace CFB.But non-uniform distribution of solids circulating rates and cyclone pressure drops show no regularity,and the flow non-uniformity is no larger than the CFBs with conventional furnace.Under typical operating conditions,the relative deviation of six solids circulating rates is 8.0％.     

循环流化床锅炉燃烧系统动态特性分析

根据循环流化床锅炉(CFBB)的工作特点以及燃烧系统输入和输出过程变量间的耦合关系,讨论了CFBB的蒸汽压力和床温的动态特性。认为引起蒸汽压力变动的主要原因在于燃料量(内扰)和汽轮机调门的变化(外扰);而影响床温变化的主要因素是给煤量、风量、物料循环量的变动,并从传热和燃烧过程分析了这些因素间的相互耦合关系。这对CFBB燃烧自动控制系统的设计与调试,以及整个控制系统的可靠运行都至关重要。     

Macroscopic modelling of fluid dynamics in large-scale circulating fluidized beds

Macroscopic (semi-empirical) models for fluid dynamics of circulating fluidized bed (CFB) units are presented, with emphasize on applications for conditions relevant to industrial units such as fluidized-bed combustors. In order to make a structured analysis of the models, the CFB unit is divided into 6 fluid dynamical zones, which have been shown to exhibit different fluid-dynamical behaviour (bottom bed, freeboard, exit zone, exit duct, cyclone and downcomer and particle seal). The paper summarizes the main basis and assumptions for each model together with major advantages and drawbacks. In addition, a practical example on how a selected set of these local models can be linked to an overall model of the fluid dynamics of the entire CFB loop is presented. It is shown that it is possible to reach good agreement between the overall model and experimental data from industrial units.     

Experimental investigation of NOx emissions during pulverized char combustion in oxygen-enriched air preheated with a circulating fluidized bed

An experimental investigation on NO_x emissions of pulverized char combustion in oxygen-enriched air was carried out in a new test system consisting of a circulating fluidized bed (CFB) and a down-fired combustor (DFC). In this paper, the pulverized char combustion characteristics and NO_x emission characteristics in the air conditions, the local oxygen-enriched air conditions in the CFB and the overall oxygen-enriched air conditions were studied. The results show that when the primary air oxygen concentration increased from 21.0% to 24.6% and to 28.2%, the ratio of fuel-nitrogen converted to nitrogen in the CFB increased from 39.7% to 45.0% and to 50.8%, respectively. This finding indicates that the preheating process in the CFB was one of the important reasons why the preheating combustion technology could significantly reduce NO_x emissions. Compared with the air combustion conditions, the NO emissions decreased to almost half of the original emissions when only the primary air oxygen concentration increased. On this basis, the NO emissions increased slightly when the air oxygen concentration was also increased in the DFC. Under these conditions, the variations in the char combustion efficiency were consistent with the variations in the temperature distribution. The feasibility and superiority of integrating the preheating combustion technology and oxygen-enriched air combustion technology are verified.     

大型循环流化床电厂运煤系统设计优化及建议

介绍了广东某大型循环流化床电厂运煤系统设计及主要设备运行情况,重点介绍了煤场设置、取料设备、筛碎系统布置、上料管带式输送机的选型及优化,并介绍了运行情况,对以后同类电厂运煤系统的选型设计提出了建议,为今后大型循环流化床电厂运煤系统设计提供参考。     

Characterization of ashes from a 100 kWth pilot-scale circulating fluidized bed with oxy-fuel combustion

Oxy-fuel combustion experiments have been carried out on an oxygen-fired 100 kW_th mini-circulating fluidized bed combustion (CFBC) facility. Coal and petroleum coke were used as fuel together with different limestones (and fixed Ca:S molar ratios) premixed with the fuel, for in situ SO₂ capture. The bed ash (BA) and fly ash (FA) samples produced from this unit were collected and characterized to obtain physical and chemical properties of the ash samples. The characterization methods used included X-ray fluorescence (XRF), X-ray diffraction (XRD), char carbon and free lime analysis, thermogravimetric analysis (TGA), and surface analysis. The main purpose of this work is to characterize the CFBC ashes from oxy-fuel firing to obtain a better understanding of the combustion process, and to identify any significant differences from the ash generated by a conventional air-fired CFBC. The primary difference in the sulfur capture mechanism between atmospheric air-fired and oxy-fuel FBC, at typical FBC temperatures (∼850 °C), is that, in the air-fired case the limestone sorbents calcine, whereas the partial pressure of CO₂ in oxy-fuel FBC is high enough to prevent calcination, and hence the sulfation process should mimic that seen in pressurized FBC (PFBC). Here, the char carbon content in the fly ash was much higher than that in the bed ash, and was also high by comparison with ash obtained from conventional commercial air-firing CFBC units. In addition, measurements of the free lime content in the bed and fly ash showed that the unreacted Ca sorbent was present primarily as CaCO₃, indicating that sulfur capture in the oxy-fuel combustor occurred via direct sulfation. Limestone utilization for oxy-fuel combustion in this unit was generally lower than that in industrial-scale air-firing CFBCs, with better limestone performance found during combustion of petcoke running at relatively higher temperatures. The Brunauer–Emmett–Teller (BET) surface area and also the pore volume in the fly ash were much higher than in the bed ash and smaller size pores predominated in the fly ash samples.     

Prediction of the heat flux profile on the furnace wall of circulating fluidized bed boilers

The purpose of this work is to investigate and develop a correlation for predicting the profile of heat flux on the crest of the membrane water wall tube along the height of circulating fluidized bed furnaces. The heat conduction equation for the tube was solved numerically under various operating conditions and furnace sizes. It was found that the temperature and heat flux profiles decreased rapidly in the region above the secondary air ports and then were found to be nearly constant. In addition, the heat flux increased as the Froude number increased or as the bed particle to the furnace diameter ratio decreased. The results were used to develop a correlation for predicting the heat flux profiles of the boilers having capacities from 12 MW_th to 165 MW_th in terms of dimensionless temperature, the Froude number, the diameter ratio and the height from the ports to the total height ratio.     

Optimization of emissions from fluidized bed combustion of coal,biofuel and waste

Several types of fuel can be burned in a fluidized bed. This paper discusses the impact of low‐ or medium‐volatile fuels (coal) and high‐volatile fuels (biomass and waste) on the emissions of NO, N₂O and other pollutants. It is found that high‐ and low‐volatile fuels behave in different ways, and measures to reduce emissions from coal combustion are not necessarily effective for high‐volatile fuels. This forms a basis for various design options. Copyright © 2002 John Wiley & Sons, Ltd.