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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 NO x 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 NO x and SO 2 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. [Display omitted] • Proposing technical routes for biomass-fired CFB boilers with high steam parameter. • Circulating loop performance is significant to the boiler operating performance. • Data from four high/super-high steam pressure CFB boilers were first disclosed. • Efficiency of biomass-fired CFB boiler can reach up to 90%–93.5%. • Ultra-low NO x /SO 2 emissions can be maintained for biomass-fired CFB boiler. [ABSTRACT FROM AUTHOR]

Graphical abstract The effects of the secondary air on the gas–solid flow are studied systematically by experiment. With the increasing of the secondary air ratio (SAR), the particles preferred to accumulate at the bottom of the riser, as a result, the particle concentration in the dense phase zone increased. Correspondingly, the particle concentration in the dilute phase zone decreased gradually and the circulating rate will also decrease. Highlights • The effects of the secondary air on the gas–solid flow are studied by experiment. • The solids carrying capability of secondary air is weaker than the primary air. • The solids concentration in the dense bed mainly depends on the primary air rate. • One transitional state was found in the transition zone. • The circulating rate with the secondary air can be predicted by a modified model. Abstract The design and operation of the secondary air (SA) have a great influence on the axial distribution of particles, the structure of the redox atmosphere, and the gas–solid mixture diffusion in the circulating fluidized bed (CFB) boiler. In this paper, the effects of the SA on the gas–solid flow in the CFB boiler, such as the axial particle distribution and the circulating rate were studied systematically through experimental analysis. All experiments were carried out under the ambient temperature and atmospheric pressure, and quartz sand was selected as the bed material in the 4.5 m high CFB test rig made of Plexiglas. The results showed that the SA whose capability to carry particle is affected by the particle concentration at the SA injection port, has a weaker effect on the entrainment of particles than the primary air (PA). The particle concentration in the dense phase zone mainly depends on the PA rate. The increasing of the secondary air ratio (SAR) may lead to the accumulation of particles in the dense phase zone. In addition, one transitional state was found in the transition zone. The variation of particle concentration in the transition zone with the increasing of the SAR is determined by the magnitude of the particle concentration at the SA injection port. The circulating rate decreased with the increasing of SAR or the SA injection height. The effect of the SA injection on the circulating rate in a circulating fluidized bed can be predicted by a modified model with the dimensionless parameters. [ABSTRACT FROM AUTHOR]

Graphical abstract The fluctuations of pressure drop across the standpipes. Highlights • Severe pressure fluctuations in parallel external loops were experimentally studied. • Two different mechanisms contributing to the pressure fluctuation were found. • Aeration air distribution would cause periodical fluctuation in external loops. • Parallel external loops would be more unstable at high solid circulation rate. Abstract Severe pressure fluctuations in parallel external loops were found in the circulating fluidized bed (CFB) boilers recently, so a CFB test rig with two external loops was established to investigate the unstable performance of parallel external loops. A long-length horizontal section was used in the loopseal to intensify the impact of gas flow inside horizontal section on solid circulation rate, and CO 2 tracer method was used to detect the variation of gas flow rate in the horizontal section. The results showed that the solid circulation rate would significantly influence the performance of parallel external loops. The pressure drop in the parallel external loops at low solid circulation rate was relatively stable if aeration air distributed evenly between the two parallel external loops, and the high-frequency pressure fluctuation in the riser was ascribed to the bubble behavior at turbulent fluidization. However, the fluctuation of gas flow rate inside the horizontal section aggravated at high solid circulation rate, leading to the unstable solid recycling rate, as well as the low-frequency and high-amplitude fluctuation of pressure drop in the parallel external loops. For this reason, a single blower is not recommended to use to supply the aeration rate among parallel external loops. [ABSTRACT FROM AUTHOR]

In order to improve the consumption of renewable energies, certain amounts of circulating fluidized bed (CFB) boiler units should assume the task of deep peak regulation. However, the effect of CFB boiler load on pollutant emissions such as NO x still needs to be clarified. In this paper, the NO x emission characteristics of two industrial-scale CFB boilers within a wide load range (35%–100%) were further analyzed by using a comprehensive one-dimensional, two-phase CFB mathematical model. Simulation results reveal that, when the load ratio decreases, the NO x emission decreases first and then increases. The non-monotonic variation trend is also confirmed by the operational data collected from the SC-350 boiler. However, for different boilers, the load ratio corresponding to the turning point of NO x emission may be different, e.g., for the 135 MW e super high steam pressure boiler, it is about 40%, while for the 350 MW e supercritical boiler is 50%. On the one hand, the decrease in boiler load leads to a decline in the furnace temperature, which contributes to reducing NO x emission due to the decrease of volatile yields, the lower conversion rate of Vol-N to NO x , and the enhancement of the overall NO reduction on chars. On the other hand, at low loads, the excess air coefficient is generally set to high values, and air staging is weakened, resulting in adverse effects on the NO x emission control. In addition, when the CFB boiler operates at low loads, the solid circulation loop performance usually worsens, and the heat loss caused by incomplete combustion may increase. This study points out that high NO x emission is an unavoidable issue in the process of deep peak regulation for CFB boilers. [Display omitted] • NO x emissions are lowest at 40%–50% load and increase at lower and higher load. • A comprehensive CFB mathematical model is applied to further analyze the mechanism. • Performance of the solid circulation loop usually becomes worse at low loads. • Heat loss due to the incomplete combustion of fuel increases as load decreases. • Operational data from two commercial CFB boilers support the modeling results. [ABSTRACT FROM AUTHOR]

The NO x emission of circulating fluidized bed (CFB) boilers in China is more and more stringently controlled due to the updated national emission regulation (NER). Based upon the theory of CFB combustion and previous research results related to NO x formation mechanism, a technical route was proposed by means of re-specifying the fluidization state in CFB boilers, to achieve the ultra-low NO x emission without installing the deNO x equipment additionally. The main method is to increase the solid circulating rate and decrease the bed inventory as well as the mean size of bed material, which would influence the bed temperature and oxidizing/reducing atmosphere distribution, and thereby benefit for the reduction of NO x emission. Besides, the reasonable control of bed temperature and air supply should be considered. This technique has been demonstrated to be effective in three CFB units with different capacities (150 t·h − 1 , 260 t·h − 1 and 560 t·h − 1 ) and burning different types of coal. No obvious decrease of combustion efficiency was found in the field tests. The technical route provides a feasible approach to further control the NO x emission for CFB boilers. [ABSTRACT FROM AUTHOR]

In this paper, the erosion characteristics in a circulating fluidized bed (CFB) were investigated on time scale with Euler-Lagrange model based computational particle fluid dynamics (CPFD). By analyzing average erosion rate data, two different erosion characteristics were found. In addition to the common linear erosion caused by downflow particles, another type of erosion is characterized by step growth and is caused by upwelling particles. Linear erosion at the height of 0.1 m is close to that at 0.2 m from the air distributor, but the value of stepped erosion at 0.1 m is much greater than that at 0.2 m. The simulation results show that stepped erosion is the main cause of erosion in the simulated object, accounting for about 70% of the total erosion. The present numerical study implies that the special gas-solid flow of CFB will bring different characteristics of erosion, and the anti-wear method should be targeted at the causes of erosion. [Display omitted] • Two erosion mechanisms were identified: linear erosion and stepped erosion. • The distribution of stepped erosion and linear erosion was evaluated. • The boundary effects of linear erosion and stepped erosion were analyzed. • The consideration of the erosion mechanism for heavily worn areas is recommended. [ABSTRACT FROM AUTHOR]

• The solid concentration was measured in a CFB test rig with Geldart B particles. • Soong's criterion and wavelet analysis were used to identify clusters. • A comparison of the two cluster identification criteria was made. • The effect of fluidization state on the dynamic properties of clusters was studied. • A cluster solid holdup model with higher prediction accuracy was proposed. Particle clusters have a significant influence on the gas–solid flow behavior in circulating fluidized beds (CFB). To further investigate the parametric effects of fluidization state on the characteristics of clusters for Geldart group B particles, measurements of the transient solid concentration at different axial elevations and transverse positions were conducted in a 10.3-m high CFB test rig with quartz sand. Soong's criterion and wavelet analysis were both used to specify the cluster identification threshold, and a following comparison of the two established thresholds showed that Soong's criterion was a better choice for identifying clusters. By the selected threshold, the dynamic properties of clusters at different fluidization states were determined. The results revealed a weak dependence of the properties on the fluidization states. Lastly, based on the abundant experimental data provided, a modified cluster solid holdup model with higher prediction accuracy at ε s < 0.01 was proposed. [ABSTRACT FROM AUTHOR]

There are two peaks in the axial distribution of total released volatiles: coal inlet area and bottom dense bed, which is mainly attributed to the wide size range of feeding fuel. • A model is developed aiming at the axial distributions of volatiles in a CFB boiler. • There are two peaks in the axial distribution of total volatiles. • Quantitatively the release profiles of individual volatile species are different. • More volatile-N will be released above dense bed if the feeding fuel size decreases. • The share of volatile-N in dense bed only changes a little if temperature increases. The proper understanding and description of the spatial distributions of volatile species is important to the circulating fluidized bed (CFB) reactor design and modeling. Aiming at this issue, a mathematical model has been developed in the present work, which constitutes of two main modules that are validated by experiments, one-dimensional fluid dynamic model and single fuel particle devolatilization model. The simulation of a 135 MWe CFB boiler was conducted. Results show that all volatile species exhibit in general a vertical release profile of bimodal shape (coal inlet area and bottom dense bed) while with quantitative differences, which is mainly attributed to the wide size range of feeding fuel. Compared to the influence of bed temperature, the effect of feeding coal size on the volatile nitrogen release is more obvious. This model can be applied as the devolatilization sub-model for the integral combustion simulation of a CFB boiler. [ABSTRACT FROM AUTHOR]

• Improvement of cyclone performance contributes to reducing the NO x emission. • Reducing coal particle size can decrease the NO x emission. • NO x emission may be further reduced by reducing the height of the coal inlet. • Effects of cyclone efficiency and coal size on combustion efficiency are discussed. • A comprehensive CFB mathematical model is applied to further analyze the mechanism. The NO x emission of circulating fluidized bed (CFB) boilers can be evidently reduced via optimization of the cyclone performance and the coal particle size, which has been proved in industrial practices, while there are few reasonable interpretations for these phenomena so far. In this paper, a comprehensive 1-dimensional/1.5-dimensional CFB mathematical model was applied to investigate the effect of these two important factors on NO x emission. Simulation results on two commercial CFB boilers indicate that increasing the cyclone efficiency and reducing the coal particle size can significantly decrease the average size of bed materials and increase the solid suspension density in the freeboard zone; namely, the bed quality is improved. The local gas–solid flow behaviors inside the CFB furnace will be accordingly changed, such as the increasing volume fraction of bubble phase in dense bed, shortening of secondary air penetration depth, enrichment of particles in the downward-flow annulus or clusters, etc. Consequently, the reducing atmosphere will be formed or enhanced in local areas, leading to the effective inhibition of NO x formation. Meanwhile, the increase in the bed inventory of fine char particles also facilitates the reduction of NO x. Moreover, if appropriately reducing the height of the coal inlet or feeding fine coal particles from the solid recycle system, the NO x emission may be further reduced due to the adjustment of the volatiles release pattern. In addition, the reduction of coal particle size can contribute to the increase in fuel combustion efficiency, while the unburned carbon content in fly/bottom ash may increase or decrease with the improvement of cyclone efficiency. This study further analyzes the influence of gas–solid flow characteristics on the chemical reaction process and provides a new solution to the low-cost pollutant emission control for CFB boilers. [ABSTRACT FROM AUTHOR]

The effect of particle properties on gas solid flow in the whole circulating fluidized bed (CFB) system was investigated. Two Geldart-B particles with different sizes were used and the differences in pressure balance and material balance of the whole CFB system with a loop seal were described. The results show that coarse particles have a larger transport velocity U tr to fall into the fast bed regime, which means accumulation in the riser is easier for the coarse particles at the same fluidization velocity U r . The coarse particles also have a smaller solid saturation carrying rate G s , max and a lower height of the acceleration region in the fast fluidization regime. Considering the pressure balance of the system, a higher pressure head should be supplied by the standpipe at the aeration tap of the loop seal under the same G s when coarse particles are used. The pressure drop through the cyclone is lower for coarse particles under the same solid concentration at the entrance of it. In the loop seal, coarse particles need more aeration gas to reach the same G s because of its higher minimum fluidization velocity. Coarse particles show a higher pressure drop through the weir part and a lower pressure drop through the horizontal part of the loop seal. In the standpipe, coarse particles have a lower drag coefficient and need a higher gas solid slip velocity to build up the same pressure gradient, so fine particles suffer less bypassing gas into the cyclone and have higher material seal capability. However, because of the same density of the two particles, the maximum pressure gradient built up in the standpipe is similar. [ABSTRACT FROM AUTHOR]

In a circulating fluidized bed (CFB) boiler, loop seal is one of the most commonly used non-mechanical valves. When the solids are conveyed horizontally from the supply chamber to the recycle chamber through the horizontal section, the length of the horizontal section will cause much flow resistance, which not only influences the fan burden but also the pressure and mass balance between the furnace and the external loop. In this paper, a new structure loop seal with a slope passage connecting the supply and recycle chambers (named N-type) is described. The flow characteristic of quartz sand (Geldart-B particles) in the new loop seal was studied by comparison with the traditional loop seal (named U-type). The experimental data show that the passage design connecting the two chambers of a loop seal has an obvious effect on the gas solid flow not only in the loop seal, but also in other components in the whole loop. For the N-type loop seal, the minimum aeration rate required for the onset of solid flow is equal to the minimum fluidization velocity of the bed materials, much less than with the U-type loop seal. The solid recycling capability of the N-type loop seal is better than of U-type one at low aeration rate, but this status will be reversed at high aeration rate. A small amount of vertical aeration into the bottom of the standpipe can further improve the solid recycling capability of the N-type loop seal, by reducing the inter-particle friction and build up a higher pressure drop gradient in the standpipe. At the same time, less air bypassing upward into the cyclone through the standpipe is found for the N-type loop seal at high level of aeration rate, while, at the same condition the U-type suffers severe gas bypassing with a fluidized state in the standpipe. At last, the pressure drop of the new type loop seal is much less than the pressure drop of the traditional one with the same solid flow rate, which provides an advantage for reducing fan power and sustaining more solids in the boiler furnace. [ABSTRACT FROM AUTHOR]

Schematics of the CFB combustor (left) and 1-D/1.5-D hybrid CFB model (right). [Display omitted] • A comprehensive 1-D/1.5-D hybrid CFB combustion model is developed. • Gas mixing processes in splash zone are considered in 1-D/1.5-D CFB modeling. • Homogeneous reaction system is described by a detailed chemical kinetic mechanism. • The model is validated against field test data from three commercial CFB boilers. • Gas-solid fluidization state has significant impacts on NO x emission. A comprehensive 1-dimensional/1.5-dimensional hybrid mathematical model is developed for predicting NO x emission of a circulating fluidized bed (CFB) combustor under broader operating parameters. In this model, the local gas–solid fluidization state and gas/heat transfer conditions in different regions of a CFB combustor are specifically considered. Some two- or three-dimensional problems, such as bubble breakage over dense bed surface, secondary air injection, core-annular flow structure, and particle clusters in freeboard, are also taken into account in 1-D/1.5-D modeling. The detailed chemical kinetic mechanism is creatively used to describe the homogeneous reaction system towards CFB combustion simulation. In addition to operating parameters and fuel-specific inputs, no other model parameters can be trimmed from case to case. This integral CFB model is validated against the field test data obtained from three commercial CFB boilers with different capacities, some of which are first disclosed. Favorable comparisons are obtained between the predicted and measured results, involving particle size distributions, temperature and pressure profiles, and NO x /SO 2 emissions. The final NO emission, as well as gas profiles, are somewhat different among the cases, which may be attributed to the discrepancy in boiler structure, fuel properties, and operating conditions. Further sensitivity analysis indicates that the proportion of volatile-N in total fuel-N, char combustion reactivity, and char-NO reactivity significantly impact the NO x emission for CFB combustion. Meanwhile, the gas–solid fluidization state also plays an essential role in the NO x emission and the in-furnace combustion efficiency, such as the gas flow distribution between phases, bubble size, secondary air penetration depth, etc. However, the NO x emission seems insensitive to the particle external gas mass transfer coefficients. [ABSTRACT FROM AUTHOR]

Co -firing coal slime in circulating fluidized bed (CFB) boilers is an effective method for utilizing coal slime, as industrial solid waste, with high water content, high viscosity, high ash content and low calorific value. In order to study the morphology and drying characteristics of coal slime dough injected into the furnace, this paper studied in detail the influence of compression degree and water content on the tensile force of coal slime dough. The results show that the degree of compression is positively correlated with the tensile force of coal slime dough, while, the water content is negatively correlated. A one-dimensional quadratic mathematical relationship between the tensile force of coal slime dough and the water content is established. At different feeding locations, i.e., at the top or middle of the boiler furnace, the breaking length of the coal slime dough was predicted. With top feeding, the length is 0.17 m, and that value for the middle feeding is about 0.15 m. Using the coupling model of the movement and drying of coal slime dough inside the furnace of the CFB boiler, the temperature change of the slime dough was calculated. The results showed that the surface temperature of coal slime dough rose rapidly during the drying process, then decreased slowly before it dived into the dense bed. When the furnace temperature is 900 °C and furnace height is about 38 m, the temperature of the slime surface can reach 200 °C at the highest. • Complete the morphology and drying characteristics of coal slime dough ejected into the furnace. • Propose a one-dimensional quadratic mathematical relationship between the tensile force of coal slime dough and the water content. • Study detailed the influence of compression degree and water content on the tensile force of coal slime dough. • Calculate the surface temperature of coal slime dough change of the slime dough. [ABSTRACT FROM AUTHOR]

A 1-dimensional CFB boiler model has been developed to simulate the mass balance and SO 2 emission of the world first 550 MW e ultra-supercritical circulating fluidized bed boiler. • A 1-dimensional CFB boiler model has been developed. • Mass balance and SO 2 emission of the world first USC CFB boiler were simulated. • The modelling results agree well with the field test data. • Improving cyclone efficiency can improve in-situ desulphurization performance. • Properly reducing limestone size can obtain higher desulphurization efficiency. In the present work, the in-situ desulphurization performance of the world first 550 MWe ultra-supercritical (USC) circulating fluidized bed (CFB) boiler was investigated mainly through simulation method. A pseudo steady-state 1-dimensional CFB model has been developed that integrates and simultaneously predicts the particle balance, combustion, lime sulphation and some other aspects. The specially designed integrated recycle heat exchanger (INTREX) and bottom ash self-circulation system of this boiler are also considered. The predicted median diameters of fly ash, bottom ash and circulating ash are 16 μm, 238 μm and 142 μm, respectively, which agree well with the field test data. Both test and modelling results reveal that the when the fine limestone with a median diameter of 50 μm is applied, the in-situ desulphurization efficiency can reach up to 95% and the final SO 2 emission is only around 20 ppm, while no any back-end FGD system is applied. A sensitivity analysis is carried out using the model to examine the effects of cyclone efficiency and limestone particle size on the in-situ desulphurization performance. It indicates that by improving the cyclone efficiency, the SO 2 emission in the outlet of furnace significantly decreases. Besides, appropriately reducing the limestone size can obtain higher desulphurization efficiency. The model presented in this paper can be used to further study the operation characteristics of USC CFB boilers, also including in-situ desulphurization, which is important for accurate prediction and cost-effective control of SO 2 emission. [ABSTRACT FROM AUTHOR]