2.3 Effect of operating parameters on heat transfer
2.3.6 Effect of other operating parameters on heat transfer
Winaya and Basu (2001) studied the effect of CO2 addition in PCFB. They found that the heat transfer coefficient increases with increase in both system pressure and bed temperature due to increased contribution of gas convection and radiation and volumetric concentration of CO2 in the PCFB riser. They also observed that the moderate increase of heat transfer coefficient with limestone addition due to increase in CO2 concentration through the calcinations of CaCO3. Reddy and Basu (2002) investigated the effect of CO2 concentration and system pressure on radiation heat transfer through a bed to wall heat transfer mechanistic model. As observed, the bed to wall radiation heat transfer increases slightly with system pressure and the variation in CO2 concentration during combustion in the PCFB combustor does not influence the bed to wall radiation heat transfer coefficient significantly. Tsukada et al. (1994) studied the effect of pressure on transport velocity in a CFB and found that the core diameter at transport velocity is 80 % of the bed diameter and thickness of the annulus was slightly decreasing with the operating pressure. It is also reported that the fluidizing velocities are appreciably lower and heat transfer coefficient is significantly higher in PFBC than in atmospheric units. As the PCFB technology is new and the cost involved in conducting the experiments is high at high pressure hence the published literature related to experimental study is very less. Most of the work reported in literature is on the investigation heat transfer by using mathematical model. Some of the reported work in this direction in discussed in Table-2.3.
Although much work has been done on the investigation of bed voidage profile, solid circulation rate and heat transfer at varied system pressure both in bubbling and circulating fluidized bed, there is no specific information about the investigation of hydrodynamics and heat transfer characteristics at varied percentage mixing of biomass in sand has been reported.
Table-2.3 Effect of various operating parameters on heat transfer
Investigator (s) Experimental variables Observations
Basu and Nag (1987)
• Heat transfer coefficient was measured for different superficial velocities and solid circulation rates.
• Effect of suspension density, superficial velocity, circulation rate, particle size, temperature on heat transfer coefficient was studied.
• Model was validated experimentally.
• Heat transfer in a circulating fluidized bed combustor can be predicted by cluster renewal model.
• Bed to wall heat transfer coefficient increases with increasing suspension density; but it decreases as fluidization velocity increases while keeping the recycle rate constant.
• Within the present range of experimental conditions finer particles resulted in a higher heat transfer coefficient for a given suspension density.
• A higher heat transfer at higher bed temperature is predicted by the cluster renewal model due to the effect of radiation as well as increased thermal conductivity of the gas film at elevated temperature.
Basu (1990) • Heat transfer to the wall of a fast fluidized bed has been measured for 4 different particle sizes, two sizes of heat transfer probes and several temperatures from 30-900 °C.
• The local heat transfer coefficient is higher at the wall than at the centre of the bed.
Basu and Chang (1996)
• Studied the effect of pressure on heat transfer, bed suspension density, particle size and superficial velocity.
• Pressure variation studied from 100 kPa to 600 kPa.
• Result of cluster renewal model was validated by experiment.
• The entire test rig was maintained in a temperature controlled electric furnace.
• Two measuring devices were used to measure the pressure drop along the bed (i) a Dura block:~ Solid Plastic Stationary Gage; (ii) a Model 700D5"24V4 Pressure Transducer.
• Maximum fall velocity of cluster was found to be 1.26 m/s.
• Most particles were separated in the primary cyclone and recycled to the bed through the standpipe and the loopseal.
• The bed-to-wall convective heat transfer coefficient increases with increasing system pressures and bed suspension density but not with particle size.
• The effect of superficial velocity on heat transfer coefficient was reported to be negligible.
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Investigator (s) Experimental variables Observations
Yates (1996) • Reviewed the effect of temperature and pressure on gas solid fluidization especially on Geldart A and B particles.
• Heat transfer in both bubbling and circulating fluidized bed and scaling was also studied.
• With the increase in pressure, the enhanced bed-to-surface heat transfer coefficients in beds of Group A powders is due to the suppression of bubbles while in beds of Group B materials the enhancement is through an increase in the gas convective component of the heat transfer coefficient.
Basu and Nag (1996)
• Studied the heat transfer to the wall of CFB at atmospheric pressure.
• Reviewed the hydrodynamics condition in furnace.
• Studied the lab and commercial scale experimental observations.
• Effect of operating parameters such as suspension density, bed temperature, fluidization velocity, particle size, vertical length of heat transfer surface was reviewed.
• Mechanistic models of heat transfer were reviewed.
• Suspension density is the most significant factor influencing heat transfer in a CFB furnace.
• Direct effect of particle size is evident with short heat transfer surfaces, but is not significant with long surfaces similar to those used in commercial boilers.
• Heat transfer coefficient 100-200 W/m2-K varies with suspension density.
• Mechanistic models can be used prediction of heat transfer coefficient.
Winaya and Basu (2001)
• Effect of pressure and carbon dioxide concentration on bed to wall heat transfer in PCFB combustor.
• Effect of superficial velocity (1, 3, 5 m/s) and Ca/S ratio (1.6, 2, 2.5, 3) was investigated.
• Pressure and temperature was varied from 100 kPa to 600 kPa.
• Heating of the setup was done by electrical means and maintained at 925-1125 K.
• Heat transfer coefficient increases with both system pressure and bed temperature due to increased contribution of gas convection and radiation.
• Heat transfer coefficient increases with volumetric concentration of CO2 in the PCFB riser.
• Average bed densities at constant inventory are observed to be 11.1 kg/m3 at 2 bar, 16.19 kg/m3 at 4 bar and 19.21 kg/m3 at 6 bar.
Reddy and Basu (2001)
• The model takes into account the effect of pressure on cluster density, cluster thermal conductivity and particle convection heat transfer coefficient.
• Effect of pressure, temperature and hydrodynamic parameters on bed-to-wall heat transfer coefficient was investigated.
• Pressure and temperature was varied from 100 kPa to 600 kPa and 350-650 °C, respectively.
• Heat transfer coefficient increases with system pressure.
• The increase in suspension density results in increase in heat transfer coefficient.
• Heat transfer coefficient increase with bed temperature.
Investigator (s) Experimental variables Observations
Gupta and Nag (2002)
• Effect of pressure and other relevant operating parameters were investigated experimentally.
• Superficial velocities were varied from: 0.25-1.25 m/s.
• Bed inventories considered are 1, 1.25 and 1.5 kg.
• Pressure variation studied from 2 to 6 bar.
• The axial bed voidage along the height of the bed is observed to be less in the bottom zone and is high in the top zone. It is also observed that the bed voidage increases in the bottom zone and decreases in the top zone with increase in operating pressure.
• The heat transfer coefficient is found to be increasing with the increase in operating pressure as well as increase in gas superficial velocity. Also increases monotonically with the increase in bed temperature
Kolar and Sundaresan (2002)
• Studied the heat transfer characteristics at an axial copper tube of 6.9 mm OD and 0.6 m height in a CFB riser.
• Tube was located axially at distances of 0.97 m, 1.62 m, 3.0 m and 4.0 m from the distributor plate.
• Fluidizing air velocity varied from 4.5 – 7.3 m/s.
• Solid circulation flux varied from 21 to 72 kg.m-2 s-1.
• Heat transfer coefficient varied from 58-101 W/m2-K and showed a decrease trend from the riser bottom to the riser exit.
• Heat transfer coefficient decreases with increase of fluidizing velocity and increases with suspension density.
• Suspension density is not an independent parameter but a derived one depend upon the combination of particle size, fluidizing velocity and the solids recycle rate for a given bed material, bed geometry and the bed holdup.
Reddy and Basu (2002)
• Effect of CO2 concentration and system pressure on radiation heat transfer was investigated by using a mechanistic model.
• Pressure variation studied from 100 kPa to 600 kPa.
• The entire test rig was maintained at a constant temperature of 1123 K.
• For the CO2 released during combustion in the PCFB combustor, the gas partial pressure increases.
• The bed to wall radiation heat transfer increases slightly with system pressure.
• The variation in CO2 concentration during combustion in the PCFB combustor does not influence the bed to wall radiation heat transfer coefficient significantly.