Pressurized Circulating Fluidized Bed Research Articles

Investigation on the radiation heat transfer in dilution zone of a 100 kWth oxy-fuel pressurized circulating fluidized bed

Due to high pressure, complicated flue gas components and high particle concentration in dilute phase region, radiation heat transfer in the oxy-fuel combustion pressurized circulating fluidized bed (PCFB) is complex. To indicate the radiation heat transfer, both experiments and numerical simulations were carried out, where the Computational Fluid Dynamics (CFD) modelling was based on the full spectrum correlated k-distribution model (FSCK) combined with Mie theory. The multiphase flow behaviors, combustion reactions and radiation heat transfer of the PCFB under oxy-fuel combustion at 0.6 MPa were revealed with good agreement between the simulated result and the experimental data. The predictions of radiation heat transfer were accomplished under different oxygen concentration and operating pressure. The results illustrated that the existence and accumulation of particles lead to the increase of flue gas optical thickness, which hinders more radiation energy transmitted to the wall. Various oxygen concentration leads to the different distribution of temperature and radiative medium concentration, which all have an impact on total radiation intensity. Compared with temperature, particle radiation and gas radiation have less effect on radiation heat transfer under different atmosphere. In addition, the operating pressure affected the flue gas temperature as well as particle concentration and radiative gas concentration in the dilution zone, and the combustion atmosphere also has an influence on the above characteristics. Complicated thermodynamic states lead to dissimilar thermal radiation characteristics under different operating condition.

Radiative heat transfer in splash and dilution zones of the pressurized oxy-fuel combustion CFB considering particle-dependent scattering

• The radiative heat transfer in the oxy-fuel combustion PCFB is investigated. • The radiative heat transfer considering particle-dependent scattering is studied. • The importance of particle concentration on radiative heat transfer is summarized. • The mechanism of radiative heat transfer action in different regions is revealed. Radiative heat transfer is dominant in splash and dilution zones of the oxy-fuel combustion pressurized circulating fluidized bed (PCFB). However, the particle radiation fields interact with each other due to the high particle concentration. Moreover, the particle concentration distribution, the flue gas components and pressure change drastically due to pressurized oxy-fuel combustion technology. In this paper, the radiative heat transfer in splash and dilution zones of the oxy-fuel combustion PCFB is investigated when particle-dependent scattering is considered. The results show that the maximum error of the particle radiation model is less than 1% in predicting the incident heat flux at the wall when particle-dependent scattering is considered. The particle concentration distribution has a significant impact on the radiative heat transfer in splash and dilution zones of oxy-fuel combustion PCFB. The simplified particle concentration distribution model cannot capture the zero-source term phenomenon in the core region and has a large error in the annular region. In addition, the mechanism and importance of the flue gas composition and pressure on the radiative heat transfer in splash and dilution zones of oxy-fuel combustion PCFB are analyzed.

Transient hydrodynamics and heat transfer behaviour in a pressurized circulating fluidized bed during abrupt changes in operating pressure

Pressurized circulating fluidized beds (PCFB) are subjected to various fluctuating conditions due to fluctuations in load demands owing to its vast utilization as a combination of heat and power generating unit. As such, analysis of transient heat transfers and hydrodynamics during these fluctuations in PCFB is very important. The present work includes experimental investigation on the transient behaviour of a PCFB unit during abrupt changes in operating pressure. The experiments are performed on a laboratory scale PCFB set up consisting of a stainless-steel riser of height 2 m and an inner diameter of 54 mm. The operating pressure is regulated by a pressure regulator and is measured using a piezo resistive pressure sensor attached to the inlet of the riser. Static pressures and temperatures are also measured along the riser column at various heights. Experiments are performed with 500 g sand as bed inventory at two operating pressure of 2 and 3 bar. Drastic changes in voidage values were observed mostly in the upper region of the riser column for transient conditions in the operating parameters. The experiments showed that with a sudden drop in operating pressure, the heat transfer coefficient momentarily increase and subsequently decrease steadily. The reverse is observed for a sudden increase in operating pressure.

Experimental study of the solid circulation rate in a pressurized circulating fluidized bed

The solid circulation rate is essential for design of pressurized circulating fluidized beds (PCFBs). With increasing pressure from atmospheric pressure to a few bars, the gas density linearly increases with the pressure, which affects the gas–solid flow characteristics. In this work, experiments were performed at room temperature in a cold PCFB apparatus with a riser of 3.3m in height and 0.05m in diameter. The solid circulation rate was studied from 20 to 80kg/(m2·s) under various conditions with increasing pressure from 0.1 to 0.6MPa and fluidizing gas velocity from around 1.5 to 8.0m/s for different Geldart B group particles. Most of the conditions were in the flow regimes of core-annulus flow (CAF) only and CAF with a turbulent fluidized bed at the bottom. The trend of the apparent slip factor with the dimensionless slip velocity was similar at different pressures and for different average particle sizes, and it converged to an exponential function. An empirical equation was obtained by fitting the solid circulation rate with the operating parameters (particle transport velocity, particle volume fraction, Archimedes number, and Froude number), which is helpful for design and operation of PCFBs.

Heat recovery from the Downcomer of a pressurized circulating fluidized bed during various transient conditions

An enormous amount of heat energy is available in the cyclone and downcomer of a circulating fluidized bed (CFB). The downcomer section can thus be a major source of heat recovery. There is scope for investigation on prospects of recovering heat along the downcomer. In the present work, an attempt is made to utilize the heat from the downcomer of a laboratory-scale Pressurized circulating fluidized bed (PCFB) set up by incorporating a spiral type of heat exchanger inside it. Transient behaviour of the heat recovery for various transient operating conditions such as superficial air velocity and power input which are operated under either step and ramp increase or decrease are reported. The PCFB unit in the present study consists of a riser of 2000 mm height and 54 mm inner diameter. The length of the downcomer is 1465 mm with an inner diameter of 54 mm. The results indicated that a maximum amount of around 350 W of heat could be recovered in the downcomer. It is also observed in some cases that this heat recovery fluctuates in phase with the transients in various operating conditions while it fluctuates with delays and phase shifts in others.

Effects of pressure on flow characteristics in a pressurized circulating fluidized bed

The effects of operating pressure on the gas–solid flow characteristics in a pressurized circulating fluidized bed (CFB) gasification system were studied by experiment and simulation. Elevated operating pressure was found to increase the particle uniformity in the axial and radial directions. Experiments and simulations both showed that as the operating pressure increased, the pressure drops along the riser height decreased. The experimental pressure drops at the same locations decreased when the operating pressure increased, but the opposite trend was observed in the simulation results. A proper drag model must be developed for the accurate simulation of pressurized CFB operation, and the accuracy of electrical capacitance tomography (ECT) measurement and cross-correlation analysis should be improved for better prediction of the particle circulation flux.

Gasification studies of low-grade Indian coal and biomass in a lab-scale pressurized circulating fluidized bed

The present research is focused on the gasification study of a lab-scale pressurized circulating fluidized bed (PCFB) gasifier considering low-grade Indian coal and biomass such as sawdust and rice husk with a mean particle size of 600 μm as feed materials. Syngas composition, lower heating value (LHV), dry gas yield (Y), carbon conversion efficiency (CCE), and cold gas efficiency (CGE) are evaluated based on the experiments in the range of pressure 1–4 bar. It is observed that the concentration of CH4 increases with pressure for all the three feed materials. The concentration of CH4 is found to be increasing from (3.65–4.86 vol%) and (2.98–3.46 vol%) for sawdust and rice husk, respectively. Similarly, the LHV is found to be increasing with pressure for coal and sawdust. There is a significant increase of 12% in LHV with an increase in pressure from 1 to 4 bar for both coal and sawdust. The CGE is found to increase by 34, 51, and 61% for rice husk, coal and sawdust, respectively with the increase in pressure from 1 to 4 bar.

Hydrodynamic study of low-grade Indian coal and sawdust as bed inventory in a pressurized circulating fluidized bed

Low-grade coal and biomass waste such as bamboo dust, sawdust, rice husk, etc. are abundantly available in India. Utilization of such resources to generate electricity is a challenging task with conventional methods. Atmospheric circulating fluidized bed (ACFB) is considered to be one of the potential devices to utilize coal or biomass or both for power production through gasification or combustion routes. In this paper, the effect of various operating parameters such as bed inventory, particle size, superficial velocity, blends of coal with sawdust (percentage by weight) and operating pressure on bed hydrodynamics, i.e., bed voidage, suspension density, and solid circulation rate were studied experimentally in a pressurized circulating fluidized bed (PCFB). The bed voidage decreases with an increase in operating pressure and increases with an increase in particle size. Maximum of 8.2% decrement in bed voidage is observed as operating pressure varies from atmospheric to 4 bar. Further, the suspension density is found to increase with an increase in inventory weight. Peak increment of 28.3% in suspension density is perceived with an increase in inventory weights at a pressure of 4 bar.

Hydrodynamic and gasification studies of sawdust in a pressurized circulating fluidized bed

The present research is focused on the hydrodynamics and gasification studies of a pressurized circulating fluidized bed (PCFB) gasifier considering saw dust as a bed inventory. The operating pressure range of the PCFB gasifier is maintained in between 1 to 4 bar and the suspension density, syngas composition, lower heating value (LHV), gas yield, carbon conversion efficiency (CCE) and cold gas efficiency (CGE) were calculated and presented in this paper. The suspension density increases with rise in operating pressure and decreases with rise in superficial velocity. The concentration of CH4 in the syngas was found to be increasing with rise in bed pressure and is found to be 4.78 % at 4 bar. However, no significant effect of operating pressure on H2 concentration is observed and is found to vary between 8.3 to 7.95 %. The CO concentration increases up to 2 bar pressure and decreases gradually afterwards. The LHV, gas yield, CCE and CGE has a positive response to the rise in operating pressure and found maximum at 4 bar. The CCE and CGE of the PCFB gasifier are increased by 57 % and 58 %, respectively when the operating pressure is changed from 1 to 4 bar.

Оцінка ефективності впровадження парогазових і ЦКШ-технологій на ТЕС України

At present, various technologies for the production of electric and heat energy are being developed and actively implemented in the world in accordance with international requirements for efficiency, economy and ecological performance. At the same time, the main attention in construction, reconstruction and modernization of power plants is given to the use of high-efficiency energy technologies for coal combustion and gasification. This paper analyzes world experience in the implementation of combined-cycle and circulating fluidized bed (CFB) technologies: combined-cycle plants (CCP) with coal gasification (with flue gas desulphurization and denitrification), combustion in atmospheric and pressurized CFB, CCP with combustion in bubbling fluidized bed under pressure, and natural gas-fired CCP in comparison with pulverized coal combustion with flue gas desulphurization and denitrification. For the new construction and reconstruction of TPP in Ukraine that burn Ukrainian coal and ensure the achievement of modern emission standards for the main pollutants, a software-and-information complex has been developed at the Institute of General Energy of the NAS of Ukraine, and a feasibility study has been carried out with comparing the average prime cost of electricity for the life cycle for advanced technologies. Our study shows that the lowest prime costs are in the case of coal combustion by the atmospheric or pressurized CFB technologies. At present, due to the high prime cost of electricity for the life cycle, the considered energy technologies are not yet competitive for the energy sector of Ukraine, although the country has a full cycle of development and production of industrial gas turbines. Combined-cycle technology is still not widely used in Ukraine. Ukrainian industry has not yet formed the necessary infrastructure for the production of CCP of high capacity, but about 80–85% of CCP equipment can be manufactured at domestic enterprises. Taking into account the high ecological performance and problems of CO2 utilization, which must be solved in the next 10-15 years, CCP and CFB technologies can be competitive for the energy sector of Ukraine in the future, and it is advisable to introduce them.

Study of bed-to-wall heat transfer with twisted tape at the upper splash region of a pressurized circulating fluidized bed unit

In the present investigation, bed-to-wall heat transfer coefficient was studied at the upper splash region of a pressurized circulating fluidized bed (PCFB) riser of internal diameter 54mm and height of 2000mm. The experiments were carried out at three different temperatures. At each temperature, experiments were performed at three different operating pressures viz. 1, 3 and 5bar to investigate the influence of operating pressure. At each pressure condition, bed temperature distributions along the height of the riser were investigated. The superficial velocity was maintained constant throughout the experiment at 7m/s. The effect of twisted tape inserts on heat transfer at the upper splash region of the riser was studied at three different solid inventories of 400, 600 and 800g and the results obtained are compared without twisted tape inserts. The average particle size of sand used in the experiment was of 307μm. The overall uncertainty in calculating the heat transfer coefficient was found to be ±12.3%.

Some Studies on Wall-to-bed Heat Transfer in a Pressurized Circulating Fluidized Bed Unit

Abstract In the present work, a pressurized circulating fluidized bed (PCFB) unit of 54 mm inner diameter and riser height of 2000 mm has been fabricated to investigate the effect of pressure on suspension density and heat transfer. The effects of blending of biomass in sand, and superficial velocity on bed hydrodynamics and heat transfer has also been studied. Experiments have been conducted at four different percentage blending of biomass such as 2.5%, 7.5%, 15% and 20% in sand with two different weight composition ratios and at a superficial velocity of 5 m/s. Operating pressure is varied from 1 to 5 bar in a step of 2 bar. Results show that, the axial heat transfer coefficient increases from the bottom to the top of heat transfer probe with the increase in operating pressure. The radial variation of heat transfer coefficient decreases from the wall to the core of the heat transfer probe. The heat transfer coefficient is also found to be higher in between the 7.5 to 15% biomass blending in sand. The overall uncertainty in calculating heat transfer coefficient is found to be 3.90%.

Effect of pressure on thermal aspects in the riser column of a pressurized circulating fluidized bed

In the present paper the effect of pressure on bed-to-wall heat transfer in the riser column of a pressurized circulating fluidized bed (PCFB) unit is estimated through a modified mechanistic model. Gas–solid flow structure and average cross-sectional solids concentration play a dominant role in better understanding of bed-to-wall heat transfer mechanism in the riser column of a PCFB. The effect of pressure on average solids concentration fraction ‘c’ in the riser column is analysed from the experimental investigations. The basic cluster renewal model of an atmospheric circulating fluidized bed has been modified to consider the effect of pressure on different model parameters such as cluster properties, gas layer thickness, cluster, particle, gas phase, radiation and bed-to-wall heat transfer coefficients, respectively. The cluster thermal conductivity increases with system pressure as well as with bed temperature due to higher cluster thermal properties. The increased operating pressure enhances the particle and dispersed phase heat transfer components. The bed-to-wall heat transfer coefficient increases with operating pressure, because of increased particle concentration. The predicted results from the model are compared with the experimentally measured values as well as with the published literature, and a good agreement has been observed. The bed-to-wall heat transfer coefficient variation along the riser height is also reported for different operating pressures. Copyright © 2005 John Wiley & Sons, Ltd.

Parametric analysis of a coal based combined cycle power plant

In the present paper thermodynamic analyses, i.e. both energy and exergy analyses have been conducted for a coal based combined cycle power plant, which consists of pressurized circulating fluidized bed (PCFB) partial gasification unit and an atmospheric circulating fluidized bed (ACFB) char combustion unit. Dual pressure steam cycle is considered for the bottoming cycle to reduce irreversibilities during heat transfer from gas to water/steam. The effect of operating variables such as pressure ratio, gas turbine inlet temperature on the performance of combined cycle power plant has been investigated. The pressure ratio and maximum temperature (gas turbine inlet temperature) are identified as the dominant parameters having impact on the combined cycle plant performance. The work output of the topping cycle is found to increase with pressure ratio, while for the bottoming cycle it decreases. However, for the same gas turbine inlet temperature the overall work output of the combined cycle plant increases up to a certain pressure ratio, and thereafter not much increase is observed. The entropy generation, the irreversibilities in each component of the combined cycle and the exergy destruction/losses are also estimated. Copyright © 2005 John Wiley & Sons, Ltd.

Estimation of the Effect of System Pressure and CO 2 Concentration on Radiation Heat Transfer in a Pressurized Circulating Fluidized Bed Combustor

The present work reports the effect of CO 2 concentration and system pressure on radiation heat transfer in a pressurized circulating fluidized combustor. The basic heat transfer mechanism in a pressurized circulating fluidized bed combustor is presented. For a given CO 2 concentration in a pressurized circulating fluidized bed combustor, radiation heat transfer increases slightly with system pressure due to increased gas partial pressure and gas emissivity. The results demonstrate that for the given CO 2 concentration, the effect of system pressure on radiation heat transfer is minimal in a pressurized circulating fluidized bed (PCFB) combustor. The effect of variation in CO 2 concentration levels on bed to wall radiation heat transfer during combustion in a pressurized circulating fluidized combustor is not significant for a narrow combuster.

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.

Effect of operating parameters on sulphur capture in a pressurized circulating fluidized bed combustor

Experiments are conducted to investigate the effect of system pressure, Ca/S ratio and primary air velocity on sulphur capture in a pressurized circulating fluidized bed (PCFB) combustor. The pressure inside the PCFB combustor is varied from 200 to 700 kPa. The Ca/S ratio is varied from 1.6 to 3.0. The primary air velocity ranges from 3 to 7 ms−1. The bed temperature is maintained at 750°C. The sulphur capture increases with system pressure in the present range of experimental investigations. The sulphur capture also increases with Ca/S ratio up to a certain ratio and then shows a decreasing trend for the given operating conditions. A semi-empirical model is developed for explaining the sulphur capture mechanism in the pressurized circulating fluidized bed combustor under batch combustion conditions. The experimental data are validated with the model predictions and a reasonable agreement has been observed. Copyright © 2002 John Wiley & Sons, Ltd.

A model for heat transfer in a pressurized circulating fluidized bed furnace

A mechanistic model based on cluster renewal approach is proposed to predict bed to wall heat transfer coefficient in a pressurized circulating fluidized bed (PCFB). The model takes into account the effect of pressure on cluster density, cluster thermal conductivity and particle convection heat transfer coefficient. The effect of pressure and bed temperature on bed to wall heat transfer coefficient is investigated. Published information on cluster velocity and bed hydrodynamics are used in the formulation of the model. The effect of cross-sectional average volumetric solids concentration on bed to wall heat transfer coefficient is also reported. The model predictions are validated against the experimental data obtained from a PCFB riser 52.4 mm in diameter and 2020 mm high. The unit is heated by electrical resistance heaters. The experimental results are reported for pressures up to 600 kPa and bed temperatures up to 650°C. The experimental data and model predictions are in reasonable agreement with each other. The model predictions are also validated with the data from the published literature, and a fair agreement is observed.

Effect of pressure and carbon dioxide concentration on heat transfer at high temperature in a Pressurized Circulating Fluidized Bed (PCFB) combustor

Experiments are performed in an electrically heated 52.5 mm diameter and 2020 mm high pressurized circulating fluidized bed to investigate the bed-to-wall heat transfer in it. The bed-to-wall heat transfer is determined from the radial temperature distribution in the wall. The CO 2 content of the fluidizing gas is changed by either adding varying amounts of CO 2 to the air or by burning coal and coke with and without an addition of limestone. The heat transfer coefficient increases with both system pressure and bed temperature due to increased contribution of gas convection and radiation. The heat transfer coefficient also increases with CO 2 concentration due to increased non-luminous radiation from CO 2.

Effect of pressure and temperature on cluster and particle heat transfer in a pressurized circulating fluidized bed

The present work reports the influence of pressure and bed temperature on particle-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB). The particle convection heat transfer plays a dominant role in determining the bed-to-wall heat transfer coefficient. So far, no information is reported on the effect of pressure and bed temperature on particle-to-wall heat transfer in a PCFB in the published literature. The present investigation reports some information in this direction. The effect of system pressure and bed temperature are investigated to study their influence on cluster and particle heat transfer. The particle convection heat transfer coefficient increases with system pressure and bed temperature due to higher cluster thermal conductivity. The increase in particle concentration (suspension density) results in greater cluster solid fraction and also the particle concentration near the wall is enhanced. This results in higher cluster and particle convection heat transfer between the bed and the wall. Higher particle convection heat transfer coefficient results in enhanced heat transfer between the bed and the wall. The results will also help to understand the bed-to-wall heat transfer mechanism in a better way in a PCFB. Copyright © 2001 John Wiley & Sons, Ltd.