Ash Cooler Research Articles

Mathematical model for a single screw ash cooler of a circulating fluidized bed boiler

The paper presents the mathematical model of the heat transfer processes for a single screw ash cooler (SSAC) working with a circulating fluidized bed (CFB) boiler. The validation of the model is based on experimental measurements made on a 230 MWe power unit. The analysis of measurement data from two bottom ash coolers showed that the correct prediction of the outlet temperatures of ash and cooling water requires the determination of their individual ash filling characteristics (i.e. coolers). The numerical results confirm that the mixing model proposed by Schlünder correctly predicts the heat transfer coefficient for ash.The described mathematical model can monitor the operating conditions of the single screw ash cooler. It enables the determination of the optimal volumetric flow rates of the cooling water in order to get its highest possible outlet temperature, and then for this water to be used, e.g. for sanitary purposes.

Design and operation of the double-cylinder rotary ash cooler for large CFB boilers

The bottom ash cooler is an important auxiliary equipment of a circulating fluidized bed (CFB) boiler. As the CFB boiler technology develops, the rotary ash coolers have partially substituted the fluidized bed bottom ash coolers in the industrial practice due to its excellent reliability. To meet the increased output requirement, a double-cylinder rotary ash cooler, rather than the conventional single-cylinder one, was proposed in the present study. This paper provided the design of the double-cylinder rotary ash cooler and presented its application in a 300 MWe CFB boiler. The design parameters, structural features, heat transfer performance, as well as the field trial results were discussed in the present study. In addition, a semi-empirical model was proposed to provide theoretical support for the design principle of the double-cylinder rotary ash cooler. The field trial results showed that the actual output of the installed double-cylinder rotary ash cooler was up to 32 t/h with the ash temperature at the outlet being the designed value, much greater than the value (~23 t/h) of the single-cylinder one at approximately the same size. Particularly, this novel bottom ash cooler had run continuously for more than three years without any maintenances due to its reasonable design.

Thermodynamic analysis of a 660 MW ultra-supercritical CFB boiler unit

While the power plant units are the developing towards larger capacity, the relative shortage in thermal efficiency of the circulating fluidized bed (CFB) boiler unit, which has obtained significant development in recent years, is becoming more and more apparent. However, the thermal efficiency computational model applied for different CFB boiler units are not derived yet. Based on Generalized Energy Efficiency Distribution Matrix (GEEDM) model which is designed for normal pulverized coal boiler units, this paper innovatively built a computational model of cycle efficiency for CFB boiler units. Moreover, the mathematical influences of main local influencing factors like terminal difference, extraction steam pressure loss, heat loss, drained water cooling insufficient, steam desuperheater, shaft seal leakage of steam turbine and connection schemes of ash cooler to this model were further proposed to strengthen its industrial applications. In the meanwhile, the detailed thermodynamic analysis of a 660 MW ultra-supercritical unit in China was studied to provide some optimized design and operation suggestions. Particularly, for this unit, an optimized connection scheme of the ash cooler, a core component in the CFB boiler to reclaim physical heat in the bottom ash, was selected from 20 different connection schemes.

Exergy-based analysis combined with LCA for waste heat recovery in coal-fired CHP plants

Coal-fired power generation brings about severe environmental impacts, such as air emissions, water pollution, and solid wastes. An exergy-based (exergetic, exergoeconomic, and exergoenvironmental) study of a coal-fired CHP system with waste heat recovery has been conducted. Firstly, waste heat profile is plotted with the extracted data to evaluate the recovery potential and feasibility. Models of rolling-cylinder ash cooler (RAC) and absorption heat pump (AHP) are developed and integrated in the CHP system. Then, an analysis method that combines thermodynamic with economic analysis and life cycle assessment is proposed for thermal-economic-environmental considerations. Finally, the method is applied in three cases, named base case (without heat recovery), R1 case (applying RAC and AHP), and R2 case (applying RAC, AHP, and a new steam turbine). The results show that the overall efficiency increases from 32.94% (base) to 34.14% (R1) and 34.89% (R2). Solid waste and global warming are dominant impacts caused by coal burning. The unit cost and environmental impact of hot water have a sharp decrease (28.3% and 29.1%, respectively) in R2 case, which shows a better economic and environmental performance.

A hybrid simulation of a 600 MW supercritical circulating fluidized bed boiler system

A hybrid dynamic model for a 600 MW supercritical circulating fluidized bed (CFB) boiler is presented. The combustion system includes combustion chamber, cyclone separator, standpipe, and ash cooler. The steam-water system mainly contains water wall, external heat exchanger, superheater, reheater, and economizer. The ‘core-annulus cell model’ is employed to simulate complex physical and chemical processes in the boiler. A series of classical empirical models have been chosen to model main physical and chemical processes. The water-steam system is modelled by means of modular modelling method, a new user-defined model for external heat exchanger is established and validated. The combustion system is coupled with the water-steam system via heat exchange in water wall tubes. The static state calculation is taken to compare with the design values indicating the model has reached a rather high precision. Dynamic simulations under three typical transient conditions are conducted which are 5% fuel decrease, 5% secondary air decrease and 5% supply water decrease, respectively. The results demonstrate that the hybrid model is capable of performing good characteristics of CFB. The modelling method has important theoretical and engineering values for researchers, and the hyrbid model can be very helpful to optimize operating and design parameters to get a better performance of large-scale commercial CFB.

Beneficial effects of dry bottom ash extraction and recycling in modern PCF power plants

The article describes the operational experience of the first industrial application of the Magaldi Ash Recycling (MAR®) system. That installation was commissioned in 2007 at a 2 × 320 MW pulverized coal-fired (PCF) power plant in Italy.The MAR® system is a recycling process that transforms bottom ash into fly ash by returning the “dry” bottom ash to the boiler combustion chamber.The dry bottom ash is removed from beneath the boiler using the MAC® Magaldi Ash Cooler. This technology, with more than 200 installations worldwide on boilers generating more than 70,000 MW, extracts and cools down the bottom ash by means of a small amount of ambient air. Then, the MAR® system conveys the ash back to the coal feeders where it is mixed with coal, milled and reintroduced into the furnace.The effects of the bottom ash reinjection, including the results of a 2-year site investigation, will be discussed in the present paper. The benefits of the MAR® system for PCF power plants are remarkable and can be summarized by the following achievements:1.Conversion of all bottom ash into saleable fly ash.2.Complete elimination of costs associated with bottom ash disposal or dedicated slurry system to a bottom ash pond.3.LOI content reduction in fly ash due to the dilution effect of dry bottom ash (∼8% reduction on average).4.Fly ash in compliance with international standards: EN 197-1 (cement) and EN 450-1 (concrete) in the EU.5.Environment respect: mixing fly ash with cement, CO2 emissions decrease. Around 0.9 tons of CO2 are generated from the production of a ton of Portland cement.

Study of heat exchange processes in a screw ash cooler installation

The paper presents an example of the thermal-flow analysis of a bottom ash cooler cooperating with a circulating fluidized bed (CFB) boiler. The authors present the methodology of the conducted studies and examples of thermal balance calculations basing on measurement results. The comparison of two different ash coolers indicates a significant relationship between the outlet temperatures of the ash and cooling water depending on the construction and arrangement of the analysed ash coolers. The analysis presented in this paper enables determining the optimal rotation speed of the screw in order to achieve the required temperature of the slag depending on the actual working parameters of the CFB boiler. It provides an opportunity to optimize the heat exchange conditions in order to obtain the highest acceptable water temperature at the outlet of the ash cooler while adhering to safety regulations. The hot water could further be used for internal sanitary purposes or to support heating systems operating in the power plant.

Numerical modelling of series-parallel cooling systems in power plant

The paper presents a mathematical model allowing one to study series-parallel hydraulic systems like, e.g., the cooling system of a power boiler's auxiliary devices or a closed cooling system including condensers and cooling towers. The analytical approach is based on a set of non-linear algebraic equations solved using numerical techniques. As a result of the iterative process, a set of volumetric flow rates of water through all the branches of the investigated hydraulic system is obtained. The calculations indicate the influence of changes in the pipeline's geometrical parameters on the total cooling water flow rate in the analysed installation. Such an approach makes it possible to analyse different variants of the modernization of the studied systems, as well as allowing for the indication of its critical elements. Basing on these results, an investor can choose the optimal variant of the reconstruction of the installation from the economic point of view. As examples of such a calculation, two hydraulic installations are described. One is a boiler auxiliary cooling installation including two screw ash coolers. The other is a closed cooling system consisting of cooling towers and condensers.

Design, development, experimental and CFD analysis of a prototype fluidized bed stripper ash cooler

In this study, a novel stripper ash cooler for Circulating Fluidized Bed (CFB) boiler has been designed and a cold test rig has been fabricated. Pressure drop characteristics and the flow pattern of ash have been captured using a cold test rig. Numerical analysis using the Eulerian Eulerian multiphase model coupled with closures from the kinetic theory of granular flow has been used to simulate the flow pattern and heat transfer characteristics. The model has been validated using the experimental data. The influence of particle size and the fluidization velocity of ash on the flow pattern and heat transfer performance have been studied. The results show that there is a reduction in bubble diameter on increasing the particle size. The motion of the solid particles around the immersed heat exchanger has been predicted. The variations in the instantaneous heat transfer coefficient with particle size and fluidization velocity has been presented. The effect of Ranz- Marshall and the Gunn model on the wall-to bed heat transfer coefficient has also been analyzed. It has been observed that the heat transfer coefficient varies in the range of 250–400W/m2K and the heat transfer coefficient is highest for a fluidization velocity of 4m/s and a particle size of 600μm among all cases considered.

CFD modeling of stripper ash cooler of circulating fluidized bed boiler

Summary The stable operation of a bottom ash cooler is vital for the operation of the circulating fluidized bed boiler. To assess, the stability of the ash cooler, it is important to have a thorough understanding of the flow behaviour. Although, many experimental results been reported in literature, CFD modelling of the ash cooler has not been carried out. In this paper, the transient computational analysis of a novel stripper ash cooler has been carried out using the Eulerian–Eulerian multiphase approach. The phase coupled SIMPLE algorithm has been used to solve the multiphase equations and the Gidaspow drag model has been employed to model the interaction between the fluidized air and ash. Two cases have been analysed in this paper. In the first case, the filling of the ash in the cooler has been analysed and in the second case, the phenomenon of fluidized bed bubbling in the ash cooler has been simulated. The study the of flow characteristics of hot ash has been studied. The contours of temperature, phase volume and bubbling have been analyzed in this paper.

Agglomeration in Stripper Ash Coolers and Its Possible Remedial Solutions: a Case Study

The bottom ash of circulating fluidized bed (CFB) boiler contains large amounts of physical heat. When low quality coals are used in these types of boilers, the ash content is normally more than 40 % and the physical heat loss is approximately 3 % if the bottom ash is discharged without cooling. Bottom ash cooler (BAC) is often used to treat the high temperature bottom ash to reclaim heat, and to facilitate the easily handling and transportation of ash. The CFB boiler at BLA Power, Newari, MP (India) is facing problems of clinker formation in strip ash coolers of plant since the installation of unit. These clinkers are basically agglomerates, which leads to defluidization of stripper ash cooler (BAC) units. There are two strip ash coolers in unit. Each strip ash cooler is capable of working independently. The proper functioning of both strip coolers is very important as it is going to increase the combustion efficiency of boiler by stripping of fine unburnt coal particles from ash, which are injected into the furnace. In this paper causes, characterization of agglomerates, thermo gravimetric analysis of fuel used, particular size distribution of coal and sand and possible remedial solution to overcome these agglomerates in strip ash coolers has also been presented. High temperature in compact separators, non uniform supply of coal and not removing small agglomerates from stripper ash cooler are among main causes of agglomeration in stripper ash cooler. Control of compact separator temperature, replacing 10–12 % of bed material and cleaning stripper ash cooler periodically will decrease agglomeration in stripper ash cooler of unit.

Study of thermal-flow processes in ash cooler cooperating with CFB boiler

Summary The article presents an example of thermal-flow analysis of the bottom ash cooler cooperating with the circulating fluidized bed boiler. There is presented a mathematical model of series-parallel hydraulic system supplying the ash cooler in cooling water. The numerical calculations indicate an influence of changes of the pipeline geometrical parameters on the cooling water flow rate in the system. Paper discusses the methodology of the studies and presents examples of the results of thermal balance calculations based on the results of measurements. The numerical results of the thermal-flow analysis in comparison with the measurements on the object indicate that the presented approach could be used as a diagnostic tool investigating the technical state of the bottom ash cooler.

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity umf and critical velocity for full fluidization umff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size dp on the umf and umff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the umf and umff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the umf and umff increase with the increase of taper angle α, static bed height H and particle size dp, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of umf and umff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the umf of the modified bed.

Experimental study on lateral mixing of particles in a quasi-slot-rectangular spouted bed

A novel fluidized bed ash cooler called Bi-spouted-bed fluidized bed ash cooler was developed for circulating fluidized bed (CFB) boilers. It consists of chambers that have some characteristics of slot-rectangular spouted bed (SRSB), and the flow structure is similar to a combination bubbling bed and SRSB, thus we call it quasi-slot-rectangular spouted bed (QSRSB). A cold model of quasi-slot-rectangular spouted bed (QSRSB) with a cross-section of 900 mm × 200 mm was built. And a binary mixture of different sized particles was adopted, and the jetsam (bigger) and flotsam (smaller) component were used as tracers and bed material respectively. Experimental investigation on lateral mixing of tracer particles has been conducted. The lateral mixing process of tracer particles was studied. In addition, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. Meanwhile, a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. It is found that the tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration. Furthermore, the effective lateral dispersion coefficient of tracers increase with the increase of superficial gas velocity u, static bed height h and air inlet section width δ, but decrease with the increase in size of bed material dp, respectively. Additionally, the effective lateral dispersion coefficient of bubbling fluidized bed appears to be larger than that of QSRSBs when gas velocity at the upper section of bed, U, is greater than 0.3 m/s. However, the minimum U that ensuring the normal operation of QSRSBs is less than that of bubbling fluidized bed in the present work, which means that QSRSB has a better adaptability to the tracer granularity with the same gas flowrate.

Development of a dry bottom ash extraction/afterburning system from pulverized fuel co-fired utility boilers

Co-firing of coal with waste/biomass in PF boilers is frequently associated with coarse unconverted waste/biomass particles reporting to the bottom ash. Accordingly, ash postcombustion may still be active during dry bottom ash extraction from utility boilers. This phenomenon has been frequently observed in plants equipped with dry ash belt extraction/conveying systems, such as the MAC (Magaldi Ash Cooler) proprietary technology.In the present work a model is presented that describes the fate of ash particles from a PC boiler co-fired with coal and RDF during their fall-out from the boiler and during the subsequent extraction on the belt-conveyor. The physico-chemical properties and the reactivity of ashes have been characterised experimentally. The model deals with the broad distribution of ash particles falling from the furnace on the belt and the establishment of a polydisperse granular bed on the conveyor. Modelling of heat transfer and combustion in the granular bed of ash is accomplished considering two distinct length- and time-scales: at the macroscale the deposited solids undergo transient heat transfer within the granular bed and with the environment; at the microscale ash particles of different size and temperature exchange heat among themselves, equalising their temperatures. Model results confirm that extensive carbon conversion can be achieved on the belt and that the MAC system can be effectively implemented to operate as a bottom ash afterburner.

Heat transfer in the rotary ash cooler with residual char combustion considered

In circulating fluidized-bed (CFB) boilers, some residual char is left in the ash when the ash is drained into the rotary ash cooler (RAC). The residue char will combust in the RAC given oxygen is present. It is important to understand and predict the influence of residual char combustion on the heat transfer process in the RAC in order to avoid slagging and fouling inside the RAC. In this paper, an improved heat transfer model of the RAC was developed in which the residual char combustion process was considered. Based on the analysis, the shrinking sphere model was selected to describe the residual char combustion. The improved model was validated by the final ash temperature and cooling water temperature data measured from a few RACs equipped with the coal fired CFB boilers with different steam output. The predictions of the improved model agree well with the field measurement data, within an error range of ± 10%. The simulation results also show that the heat released from residual char combustion counts for about 60–80% of the total heat released by ash in the inlet section of the RAC, and the influence of residual char combustion decreases rapidly along the ash flow. The final ash temperature increases accordingly with the increase of inlet carbon content, and decreases with the increases of residual char size. As particle size is larger than 3 mm, the final ash temperature changes little at a given carbon content due to a low mass burning rate. It is suggested that combustion of residual char should be considered when the inlet carbon content is over 2.5%; otherwise, the combustion of residual char in the RAC could be neglected.

Characteristics of clinker formation in a circulating fluidized bed boiler firing Korean anthracite

To shed light on the causes of clinker formation during the initial operation period of the Tonghae circulating fluidized bed (CFB) boiler, which uses Korean anthracite as fuel, the properties of ash, sand and limestone particles and the mixtures of each gradient have been characterized. The clinkers formed in the loopseals and the fluidized bed ash cooler (FBAC) of the CFB boiler were also characterized by analyzing the composition, the surface phenomena and the crystal structure of the clinkers. As a result, the black clinker was found to come from the sand particles and the composition of the white clinker was found to be similar to that of ash particles. The cause of the clinker formation in the FBAC proved that ash was sticking to molten or sintered phases in the high temperature regions in the boiler. On the other hand, the composition of the ash changed with the particle size, showing an enrichment of Fe2O3 as the particle size decreased. Also, the ash particles between 75–100 μm contained more than 11% CaO which resulted in low initial deformation temperature of the particles. So it is possible to explain that the amount of Fe and Ca in the fine particles of the ash plays a crucial role in the formation of agglomerates in the CFB boiler.

Development of a novel fluidized bed ash cooler for circulating fluidized bed boilers: Experimental study and application

A novel bottom ash cooler (BAC) called compound fluidized bed ash cooler (CFBAC) was developed in this paper. The CFBAC combined the major technical features of spouted bed and bubbling bed, and could achieve the selective discharge on the bottom ash. Experiments about the gas–solid flow characteristics of the CFBAC were conducted in a visible cold test rig. The experimental results indicated that the separation chamber working in the spouted bed state had a good particle separation effect on the boiler bottom ash. A small quantity of fluidizing air was needed for the cooling chambers to work in the bubbling bed state. The particle separation effect could be controlled by the fluidizing air flow and physical dimensions such as the height of separation partition. The CFBAC had also been industrially applied in a 300 MW circulating fluidized bed unit. The application showed that the CFBAC had a well separation effect, an excellent adaptability on the bottom ash, a good cooling effect and a large ash discharge capacity over 30 t/h. Compared with the water-cooled BAC, the CFBAC had a better energy conservation performance.

Influence of CFB (circulating fluidized bed) boiler bottom ash heat recovery mode on thermal economy of units

CFB (circulating fluidized bed) boiler bottom ash contains large amounts of physical heat. A BAC (bottom ash cooler) is often used to treat high temperature bottom ash to reclaim heat, and to have the ash easily transported. The unit thermal economic indicators of three CFB power plants in China were derived based on heat balance calculation and analysis on the principled thermal system in turbine heat acceptance condition, taking the influence of two different bottom ash heat recovery modes into account. One of the two bottom ash heat recovery modes was the FBAC (fluidized bed ash cooler) mode, and the other was the RAC (rolling-cylinder ash cooler) mode. The results indicated that two modes both improved the thermal economy of units. Compared with the RAC mode, the FBAC mode obtained higher plant thermal efficiency, lower plant heat rate and less standard coal consumption. The standard coal consumption rate of the FBAC mode was less nearly 2 g/(kW h) than the RAC mode in the three CFB power plants, when the net calorific power of standard coal was 29.27 MJ/kg.

Heat-Transfer Model of the Rotary Ash Cooler Used in Circulating Fluidized-Bed Boilers

Rotary ash coolers are widely used in circulating fluidized-bed (CFB) boilers for bottom ash cooling, attributed to their high efficiency and excellent reliability. However, they are mostly still designed empirically, and one of the most important reasons is due to the lack of an appropriate heat-transfer model. In this paper, a comprehensive heat-transfer model was developed and the model parameters were determined by experiments. Against the literature findings, the experimental results showed that the dimensionless thickness of gas film used in the thermal contact resistance calculation is different for different ash particle sizes. The new model was further applied to predict the heat-transfer characteristics for a rotary ash cooler used in a commercial 300 MWe CFB boiler. It was found that the heat-transfer coefficients of ash−air and air−water are close but much lower than that of ash−water, indicating that the heat transfer of ash−water dominates in the total heat transfer. In addition, the ash tem...