Circulating Fluidized Bed Boiler Research Articles

Development of biomass-fired circulating fluidized bed boiler with high steam parameters based on theoretical analysis and industrial practices

Further improvement of steam parameters is essential to developing biomass-fired circulating fluidized bed (CFB) boiler technology, which necessitates solving severe slagging, fouling, agglomeration, and corrosion problems caused by the high content of alkali metals and chlorine in biomass fuels. Studies indicate that by arranging the high-temperature heating surfaces inside the furnace, the slagging and corrosion of the surface can be effectively lightened through the continuous wash and cleaning by solid circulating ash. Meanwhile, the ash fouling on the convective heating surface can be mitigated by applying in-line tube bundles with large intercept, small tube diameter, and parallel flow design. In addition, the significant improvement of circulating loop performance can not only alleviate the bed agglomeration but also help increase the boiler efficiency, decrease slagging on the heating surface, and reduce the NOx emission. Series biomass-fired CFB boilers with high steam parameters have been successfully established based on these technical routes, including one 125 MW super-high pressure boiler with the largest capacity and the highest steam parameters in Asia. Lone-term operational data from four commercial CFB boilers show that they can operate stably within a wide range of load ratios, from 30% to 105%, and continuous operation time can last more than half a year. Moreover, the boiler efficiency can reach up to 90%–93.5%, and the NOx and SO2 emissions can be stably controlled lower than the ultra-low emission standard in China. The variation of some operation parameters with boiler load was also discussed in this work.

Negative CO2 emission from oxy-fuel combustion in CFB boilers

Oxy-combustion by flue-gas recirculation for CO2 capture is applied to an existing, already CO2-neutral, biomass-fired circulating fluidized bed (CFB) boiler, thus resulting in negative CO2 emissions. The required oxygen concentration is determined by a heat balance, but in an existing plant the volume flow is then reduced as well as the fluidization velocity, which affects the heat transfer. Methods to resolve this problem are investigated. The oxygen is usually proposed to be supplied by air separation; a method that consumes a considerable share of the energy produced in the plant. Here, it is instead suggested to use oxygen produced together with hydrogen in electrolysis by excess wind and solar power.

The morphological and mineralogical characteristics and thermal conductivity of ash deposits in a 220 MW CFBB firing Zhundong lignite

Formation of ash deposit on heat transfer surfaces during Zhundong lignite (ZD) combustion in circulating fluidized bed (CFB) boiler has affected boiler operation and its thermal efficiency. This work therefore characterised the morphology, mineralogy and thermal conductivity of the ash deposits in a 220 MW CFB boiler firing ZD for an improved understanding of deposit characteristics. By using XRF, SEM-EDS, XRD, and laser conductometer, results revealed that deposits on superheaters and reheaters were as thick as 3.0–4.5 cm. Moreover, the inner layer of the deposits consisted of fine ash particles enriching in Na, Ca and S in the forms of Na2SO4, CaSO4, and Na2Ca(SO4)2, whereas the outer layer comprising CaSO4, Na6Ca2Al6Si6O24(SO4)2, Fe2O3, SiO2 and Mg2SiO4 with more SiO2 and Al2O3 presented. Besides, thermal conductivity of the inner layer was ca. 1.83 W/(m · K), and that of the middle layer with an increased pore size was ca. 1.50 W/(m · K), both of them were independent of temperature at 550–900 °C and overweighed the out layer being 0.65–0.82 W/(m · K). This would lead to an increase in coal consumption of ca. 12.9%, highlighting the severity of deposit build-up in adversely impacting the heat transfers and deteriorating the fuel economics during ZD CFB combustion.

Investigation on lateral dispersion characteristics of fuel particles in dense phase zone of a large-scale circulating fluidized bed boiler under combustion conditions

The dispersion characteristics of fuel particles in the dense phase zone in circulating fluidized bed (CFB) boilers have an important influence on bed temperature distribution and pollutant emissions. However, previous research in literature was mostly on small-scale apparatus, whose results could not be applied directly to large-scale CFB with multiple dispersion sources. To help solve this problem, we proposed a novel method to estimate the lateral dispersion coefficient (Dx) of fuel particles under partial coal cut-off condition in a 350 MW supercritical CFB boiler based on combustion and dispersion models. Meanwhile, we carried out experiments to obtain the Dx in the range of 0.1218–0.1406 m2/s. Numerical simulations were performed and the influence of operating conditions and furnace structure on fuel dispersion characteristics was investigated, the simulation value of Dx was validated against experimental data. Results revealed that the distribution of bed temperature caused by the fuel dispersion was mainly formed by char combustion. Because of the presence of intermediate water-cooled partition wall, the mixing and dispersion of fuel and bed material particles between the left and right sides of the furnace were hindered, increasing the non-uniformity of the bed temperature near furnace front wall.

Effect of bed material size on gas-solid flow characteristics in a CFB at low solid recirculation rates

The gas–solid flow characteristics greatly influence the heat transfer, combustion and pollutant formation processes in a circulating fluidized bed (CFB) boiler. The effects of particle size on the gas–solid flow characteristics, including flow structure, axial pressure and bed density profiles were studied in a two-dimensional CFB cold apparatus under low solid circulation rates (Gs’s). At the same time, an optical fiber probe was used to assess the particle size effect on the formation of particle clusters and their properties under different operating conditions. Results revealed that when the averaged size of bed materials decreased from ∼200 μm to ∼100 μm, at the same operating condition, the bottom dense bed became remarkably higher, and the upper dilute zone had stronger pressure fluctuation, more even axial and horizontal particle distributions, and higher solid density. Gs was more sensitive to the variation of the superficial gas velocity than the bed inventory. In addition, the standard deviation and power spectrum density of instantaneous pressure decreased in the dense bed and increased in the dilute zone. The particle velocity was slightly lower in the lower section of the riser but noticeably greater in the upper riser section. In addition, the clusters formed more frequently and lasted significantly longer. The study showed that flow characteristics in the bed qualitatively changed from that of Type B particles to that Type A particles in the Geldart’s classification, even at a small Gs, and previous studies with Type B particles could very likely not apply in the CFB boilers operating with fine bed materials.

Model NOx, SO2 Emissions Concentration and Thermal Efficiency of CFBB Based on a Hyper-Parameter Self-Optimized Broad Learning System

At present, establishing a multidimensional characteristic model of a boiler combustion system plays an important role in realizing its dynamic optimization and real-time control, so as to achieve the purpose of reducing environmental pollution and saving coal resources. However, the complexity of the boiler combustion process makes it difficult to model it using traditional mathematical methods. In this paper, a kind of hyper-parameter self-optimized broad learning system by a sparrow search algorithm is proposed to model the NOx, SO2 emissions concentration and thermal efficiency of a circulation fluidized bed boiler (CFBB). A broad learning system (BLS) is a novel neural network algorithm, which shows good performance in multidimensional feature learning. However, the BLS has several hyper-parameters to be set in a wide range, so that the optimal combination between hyper-parameters is difficult to determine. This paper uses a sparrow search algorithm (SSA) to select the optimal hyper-parameters combination of the broad learning system, namely as SSA-BLS. To verify the effectiveness of SSA-BLS, ten benchmark regression datasets are applied. Experimental results show that SSA-BLS obtains good regression accuracy and model stability. Additionally, the proposed SSA-BLS is applied to model the combustion process parameters of a 330 MW circulating fluidized bed boiler. Experimental results reveal that SSA-BLS can establish the accurate prediction models for thermal efficiency, NOx emission concentration and SO2 emission concentration, separately. Altogether, SSA-BLS is an effective modelling method.

Fluidization Characteristics of Circulating Fluidized Bed Boilers

AbstractThe fluidization in the furnace of circulating fluidized bed (CFB) boilers is described. Data have been obtained from a 12 MWth CFB boiler and from literature. The bottom bed is bubbling rather than fast or turbulently fluidized as is often assumed. There is an extended splash zone above this bed, in which particles are thrown up by the bubbles and fall back onto the bottom bed. Other particles are carried further into the transport zone, initially as a saturated gas flow, but some particles move into the wall layers and the density decays towards the exit. The density of the transport zone is low, and the circulation rate is relatively seen small. Therefore, this flow is in a state of dilute‐phase transport and not in the fast fluidization regime, such as is often claimed.

Formation of agglomerates with core–shell structure in a large-scale CFB boiler

Cylindrical agglomerates with cross-sectional diameters over 200 mm are found in the bottom ash discharging chute of a 550 MWe large-scale circulating fluidized bed (CFB) boiler, which results in disfunction of the bottom ash discharging system due to the blockage of the bottom ash discharging chute. The agglomerates with core–shell structure are found to be significant to understand the mechanisms of the agglomeration, and agglomerates with similar structure are also found inside the furnace. The X-ray diffraction analysis on the core–shell structured agglomerate particle shows both the core and the shell comprise a significant amount of mullite; meanwhile, cristobalite is found in the shell, which is supposed to be formed at temperature higher than the bed temperature. The results of the transmission electron microscopy analysis show that mullite crystal in the shell is distributed inside the amorphous SiO2. Sintering experiments show that the formation temperature for mullite and cristobalite is ca. 1050 ℃, while the transformation temperature of quartz to amorphous silica is ca. 1150 ℃, which is much higher than the bed temperature. Thus, the agglomeration is supposed to be resulted from the high-temperature condition caused by the burning char inside the furnace, and the core–shell structure of the agglomerates is supposed to be formed due to the temperature gradient along the particle diameter of the burning char.

Impact of reacting pressure on Na thermal transformation in two full‐scale devices using Zhundong coal at similar temperature

AbstractGasification and combustion are two common methods for Zhundong (ZD) coal utilization. In the present study, it was intended to investigate the thermal transformation of Na in ZD coal at the Lurgi fixed bed gasifier (Lurgi) and circulating fluidized bed boiler (CFBB). To this end, raw coal samples before gasification and gasified slags were gathered from Lurgi, and the raw coal before combustion, burned slag, and fly ash were collected from CFBB. The occurrence form of Na was analyzed by extraction experiments, and the samples were analyzed by inductively coupled plasma‐optical emission spectrometry (ICP‐OES), X‐ray diffraction (XRD), and scanning electron microscopy (SEM). The obtained results revealed that insoluble Na in Lurgi slag exceeded 90% under the high‐pressure gasification process. It was found that the high temperatures and high in Lurgi intensify the conversion of sodium to Na2Ca3Si2O8 and NaAlSiO4. The performed analyses revealed that the insoluble Na content in CFBB slag and fly ash was above 92%, and Na2SiO3 and NaAlSi3O8 were the main sodium minerals in CFBB solid residue. It was found that 76.64% of the initial Na content of raw coal in Lurgi migrated into slag after the gasification process, which was 7.05% higher than the sum of slag and fly ash in CFBB after combustion. During the sodium thermal transformation in Lurgi and CFBB, high pressures significantly reduce the release of gaseous Na from coal ash so more Na was retained in slag.

Feasibility Study of Grinding Circulating Fluidized Bed Ash as Cement Admixture

With the widespread application of circulating fluidized bed (CFB) combustion technology, the popularity of CFB ash (CFBA) has increased dramatically and its production and large-scale utilization have become increasingly important. In the context of carbon neutrality peaking, using CFBA as a cement admixture as an effective method of resource utilization not only reduces the pressures caused by carbon emissions in the cement industry but also solves the environmental problems caused by CFBA depositing. However, the formation conditions of CFBA are worse than those of traditional pulverized coal boilers. CFB ash is the combustion product of coal at 850 °C–950 °C, and the characteristics of CFBA usually include a loose and porous structure with many amorphous substances. Furthermore, it has the disadvantages of large particle size, high water-demand ratio, and low activity index when it is directly used as a cement admixture. In this study, CFBA (including fly ash (CFBFA) and bottom ash (CFBBA)) produced by a CFB boiler without furnace desulfurization with limestone was used as a cement admixture material, and the effect of grinding on the fineness, water-demand ratio, and activity index of CFBA were studied. The experimental results showed that the grinding effect could significantly reduce the fineness and water-demand ratio of CFBA as a cement mixture and improve the activity index. With the increase in the grinding time, the water-demand ratio of CFBA first decreased and then increased. CFBBA ground for 10 min and CFBFA ground for 4 min can reduce the water-demand ratio of CFBA by up to 105% and increase the compressive strength of 28-day-old CFBA cement by 7.05%. The grinding process can ensure that CFBA meets the Chinese standards for a cement admixture and realize the resource utilization of CFBA.

Heat transfer and protection of high-temperature reheater of a 660 MW circulating fluidized bed boiler after black out

• Heat transfer model of buried tube in external heat exchanger was established. • Safety of 660 MW circulating fluidized bed reheater was evaluated when black out. • Correlation between minimum working fluid flow and some key factors was analyzed. With the increase of boiler capacity and steam parameters, compared with other circulating fluidized bed (CFB) boiler, the high-temperature heating exchangers in a 660 MW CFB boiler may face severe potential safety problems after black out, especially the reheater in the external heat exchanger (EHE). In this study, a numerical simulation was conducted to predict the heating transfer process of an EHE after black out in a 660 MW ultra-supercritical CFB boiler. The tube temperature of the reheater inside the EHE was also estimated. It was found that metal tubes of the reheater are probably overtemperature. Therefore, to protect the metal tube from the overtemperature damaged, the additional cold water has to be given inside the reheater. The tube temperature rises first and then falls, and the highest temperature will be reached within tens to hundreds of seconds. The minimum flow rate of cold water to ensure the safety of the pipe was also estimated under different conditions. Some key parameters including the bed temperature of the EHE, thermal diffusivity of the bed materials and thermal deviation of the reheater were also analyzed, which are of great significance to the safety of the EHE and the economics of the emergency water system.

Investigation on the internal and external solid flow rates in large-scale circulating fluidized bed with different layouts of suspended heating surfaces

Suspended Heating Surfaces (SHSs) in large-scale circulating fluidized bed (CFB) boilers have a significant impact on internal ( G s,i ) and external ( G s,o ) circulating solid flow rates that is important for heat transfer. In this paper, the distribution of G s,i and G s,o in a 650 × 680 × 3500 mm 3 CFB apparatus was simulated and verified by experimentations. On this foundation, the effects of SHSs structural layouts on G s,i and G s,o were investigated in a 350 MW supercritical CFB boiler. Results showed that G s,i increased by 65.8% as SHSs number increased from 0 to 20, whereas G s,o decreased by 17.8%, so G s,o / G s,i decreased correspondingly. Meanwhile, increasing SHSs height had a greater improvement on G s,i than width, which would be more conducive to the heat transfer near SHSs. Finally, empirical formulas of G s,o , G s,i and layout parameters of SHSs were developed, and the research results can provide meaningful reference for the operation optimization of large-scale CFB boilers. • Effects of suspended heating surface layout on circulating flow rates are studied. • The ratio variation of internal and external circulating flow rates is explored. • Internal circulating flow rate increases with increasing surface number and area. • Empirical formula of structural parameters on circulating flow rate is achieved.

Thermal and flow calculations of platen superheater in large scale CFB boiler

Circulating Fluidized Bed (CFB) boilers are desirable technology to utilize solid fuels, operate within a wide load range, and promote the high flexibility in burning a wide range of coal and other solid fuels, keeping high efficiency and low gaseous pollution. The paper presents thermal and flow modeling results of steam superheater operated in the large-scale coal-fired CFB boiler, both in steady and transient states. The mathematical model of the superheater wall was developed using Control Volume-based Finite Element Method. The heat transfer coefficient as the sum of radiative and convective heat transfer coefficients was considered in the presented approach to modeling thermal processes in a circulating fluidized bed. The overall heat transfer coefficient was computed for the analyzed type of the boiler at full load and steady-state conditions using available measuring data and results obtained using Computational Fluid Dynamics (CFD) modeling. Depending on the modeling approach adopted in the CFD model, the heat transfer coefficient at full boiler load varies between 159.96 and 189.63 W/(m2·K). In the analyzed period of boiler start-up, results of steam outlet temperature (455 °C–485 °C), heat transfer coefficients on the flue-gas side (100 W/(m2∙K) and 120 W/(m2∙K)), and heat transfer coefficients on the steam-side (925–2008 W/(m2∙K)) vary with time. The methods presented in the paper can be applied to modeling transient flow and thermal processes in superheaters with complicated flow arrangements. The advantage of the proposed method is a short computing time compared to detailed and complex 3D models. The developed model of the superheater with the transfer coefficient determination methods can be used to perform calculations in the full range of boiler load, both during steady and transient boiler operation.

Development and validation of a dynamic flowsheet model for a 350 MWe supercritical circulating fluidized bed boiler

The circulating fluidized bed (CFB) boiler is highly recommended in peak shaving for its wide load regulating range compared with the pulverized coal (PC) boiler. However, the research on dynamic flowsheet model of the CFB boiler is quite limited due to its complex flow regime. In this paper, both steady state and dynamic flowsheet model of a supercritical 350 MWe CFB boiler was developed based on Aspen Plus and Aspen Dynamics software coupled with comprehensively user defined combustion kinetics and heat transfer coefficient with FORTRAN subroutines. This model was then validated by the operating data of the 350 MWe supercritical CFB boiler. Based on this model, the dynamic characteristics of three typical step signal disturbances, coal flow rate, primary flow rate and secondary air flow rate, were further analyzed to provide theoretical references to dynamic operation. Also, the quantitative relation between the stabilization time and step signal amplitude was calculated. The calculated dynamic characteristics demonstrates that the heat distribution does not vary much after adding step signal disturbances of coal flow rate, primary air flow rate and secondary air flow rate. The step signal disturbance of secondary air flow rate has less influence to the CFB boiler system compared with that of primary air flow rate. The superheated steam temperature is more sensitive to step signal disturbances compared with reheated steam temperature. The higher step signal amplitude results in longer stabilization time. Above all, this dynamic flowsheet model can not only be used to conduct dynamic behavior prediction and analysis but also provide transfer functions to the further development of CFB automatic control system.

Dynamic Characteristics Analysis of a 660 MW Ultra-Supercritical Circulating Fluidized Bed Boiler

The 660 MW ultra-supercritical circulating fluidized bed (CFB) boiler, which is the maximum capacity and largest scale boiler in the world has entered construction stage in China. This study established a full-scale dynamic simulation model of the 660 MW ultra-supercritical at 100% boiler maximum continuous rating (BMCR) condition. The model consists of an air-flue gas system, a water-steam system, and an ash circulation system. The “core-annulus” of the gas-solid two-phase flow structure and “six-equation” model of water-steam two-phase flow were applied to simulate the behaviors of the gas-solid phase and water-steam system, respectively. The model was calibrated and verified at 100% BMCR condition, and the steady-state simulation results presented a high accuracy compared with the designed parameters. A dynamic simulation of three typical conditions were carried out as well, including a 5% feed water decrease, 5% air decrease, and 5% coal decrease, respectively. The results showed that the dynamic simulation model established in this study can simulate the dynamic behaviors of the 660 MW ultra-supercritical CFB boiler reasonably.

Effect of bed particle size on the gas-particle hydrodynamics and wall erosion characteristics in a 550 MWe USC CFB boiler using CPFD simulation

To reduce wall erosion in 550 MWe ultra-supercritical (USC) circulating fluidized bed (CFB) boiler, the effect of bed particle size with gas-particle hydrodynamics characteristics is investigated here through computational particle fluid dynamics (CPFD). Most studies on erosion until now have dealt with pilot scale system, but the present study aims to provide valuable information on erosion at full scale in the CFB boiler. The results showed that particle size significantly affected the gas-solid flow in the furnace. Larger particles were difficult to be entrained by the gas stream, which increased the internal circulating rate instead of reducing the external circulation rate. Furthermore, erosion was also influenced by both particle size and circulation rate but did not linearly increase with the particle size. In the cyclone, the erosion rate is highest, which gradually decreases with increasing particle size due to external circulation rate, but the erosion tendency of furnace and other devices are rather highest with the medium-sized particles. As the hydrodynamics and erosion of a USC CFB boiler were studied through CPFD simulation for the first time, the findings of this study may provide considerable insight into and guidance for efficient boiler operation and prevention of shutdown due to erosion.

Study on the Uniformity of Secondary Air of a 660 MW Ultra-Supercritical CFB Boiler

Based on the field test of a 600 MW supercritical circulating-fluidized bed boiler, this paper optimizes the secondary air pipe layout scheme of a 660 MW ultra-supercritical circulating fluidized bed boiler with a similar furnace structure and carried out a numerical simulation on its air distribution uniformity. The secondary air box of the 660 MW ultra-supercritical circulating-fluidized bed boiler adopts a variable section design, and the secondary air branch pipe adopts a separate air inlet mode. The experimental results showed that the oxygen concentration was uniform near the rear wall, but all exhibited a “decrease–increase–decrease” profile along the horizontal line, which indicated that the trajectory of the secondary air jet was first in the shape of bending downward and then upward. To achieve a more uniform secondary air distribution for supercritical CFB boilers, further optimization of the layout of the bellows and branch pipes should be considered. Numerical results showed that the deviation rate of the internal and lower secondary air reached 17%, and there was optimization space for the secondary air branch pipe layout of the boiler. Its uniformity can be increased by adding valves and other measures. The velocity deviation of the secondary air of the 660 MW ultra-supercritical circulating-fluidized bed boiler can be within 3% by means of separate air supply and pipe diameter change, and better uniform air supply can be achieved to reduce the speed deviation by adding valves and adjusting the combustion.

Dynamic prediction of in-situ SO2 emission and operation optimization of combined desulfurization system of 300 MW CFB boiler

As the emission regulation becomes more stringent, in-situ desulfurization and wet flue gas desulfurization have been widely used for circulating fluidized bed (CFB) boilers. However, few studies focus on real-time operation optimization of combined desulfurization system (CDS) in wide-load range based on in-situ SO2 emission prediction. In this work, according to mechanism analysis, a dynamic prediction model of in-situ SO2 emission was developed. Then, closed-loop and step response experiments were conducted to validate the established model, combined with measured data from a 300 MW CFB boiler. Validation results show that the model has satisfactory prediction accuracy in different load conditions and it can capture special dynamic characteristics of in-situ SO2 desulfurization. More importantly, the model can be feasible for building an economical operation strategy of CDS. Thus, a novel control strategy based on dynamic prediction model was designed, which contributes to decrease the SO2 emission fluctuation and operation costs by correcting the limestone-feed rate in advance. The proposed control strategy was applied to actual operation of the 300 MW CFB boiler. In the long-term operation of CDS after optimization, it was easier to reach ultra-low SO2 emission standard and limestone consumption reduces 15.14% compared with before the operation optimization.

Integrated use of coal and coal-containing waste

The current aggravation of such serious challenges as minimization of harmful emissions and suppression of the carbon footprint can be largely resolved through the more efficient and integrated use of fossil fuels with the accompanying production of additional products. Coals, including low-grade, carbon-containing, and vegetable waste, can be recycled into combustible gases, liquid fuels, and special types of coke, or smokeless fuel briquettes. On the other hand, this requires the development of new highly efficient technologies for the preparation, recycling, and use of fuels. The article discusses theoretical and experimental studies, and also presents schemes for thermal contact fuel preparation and recycling aimed at effective integrated use of fuels in standard boilers, and boilers with a circulating fluidized bed (CFB), or in special units with waste heat boilers. In CFB boilers, heat is transferred from the circulating particles to the processed fuel by direct thermal contact. In typical boilers, fuel preparation and recycling are carried out in conveyors with a heating jacket. Recycling of coals into special types of coke is carried out in a thermal-oxidative coking unit.

Experimental study and safety analysis on the heating surfaces in the 660 MW supercritical CFB boiler under sudden electricity failure

AbstractThe necessity of equipping with an emergency water supply system in a supercritical circulating fluidized bed (CFB) boiler has long been a controversial topic in the industry. In this study, the heating surface of a 660 MW supercritical CFB boiler at the Pingshuo Power Station of China Coal Group is taken as the research object. And a boiler electricity failure experiment was accomplished at the power plant, and the variations of thermal parameters during electricity failure were obtained from power plant distributed control system. By analyzing the physical process of the heating surfaces during electricity failure, a mathematical model of heat transfer to the heating surface was established and a calculation program in Fortran language was developed. In this study, the effect of different flow rates and operating times of the supply pumps on the safety of the heating surfaces were then investigated. The numerical simulation results show that an emergency supply pump with a flow rate of 250 t/h, activated within 120 s, ensures the safety of the heating surface. The results of this study have important implications for the selection of emergency supply pumps and the determination of flow rates and pressures for supercritical CFB boilers with external beds.