Fig.6.38 Comparison of heat transfer coefficient with bed inventory at P = 1 bar 108 Fig.6.39 Comparison of heat transfer coefficient with bed inventory at P = 3 bar 109 Fig.6.40 Comparison of heat transfer coefficient with bed inventory at P = 5 bar 109 Fig.6.41 Comparison of heat transfer coefficient with surface velocity at P = 1. Fig.7.21 Comparison of the heat transfer coefficient in the upper splash region of the riser at W = 400 g.
Motivation
The heat transfer coefficient is found to be more in case of smaller particles ie. the heat transfer coefficient decreases from the wall-to-core of the riser at both pressures. The variation of heat transfer coefficient with system pressure at Usup = 6 m/s in the case of 2.5.
Figure 7.22 Comparison of the heat transfer coefficient in the upper spray zone of the riser at W = 600 g. Figure 7.23 Comparison of the heat transfer coefficient in the upper splash area of the riser at W = 800 g.
Background of fluidized bed technology
Objective of the present work
Outline of the thesis
This chapter also discusses the gas analysis of the gasification product and the efficiency of the biomass feed system. Design of orifice plate, distribution plate, cyclone separator, particle size measurement procedure, thermocouple calibration, design calculation of CFB gasification, uncertainty analysis of heat transfer coefficient and solid circulation rate measurement, experimental data of various biomass mixing, Heisler diagram and reaction values kinetic parameters for five different biomasses are presented in the appendices (Appendices I to XII).
Introduction
Bed hydrodynamics and heat transfer phenomena in CFB
Bed hydrodynamics
In a typical CFB boiler or gasifier, the solids are in the turbulent fluidized bed regime in the lower furnace, the rapid regime in the upper furnace, swirling flow in the cyclone, moving bed in the standpipe, bubbling bed in the loop seal, and pneumatic transport in the back pass. The rate of descent of strands into the wall layer, the duration of their residence at the wall and the time fraction of wall coverage are all important hydrodynamic parameters that influence the heat transfer between the suspension of solid gas particles and the wall.
Effect of operating parameters on bed hydrodynamics
- Bed voidage
- Suspension density
- Minimum fluidization velocity
The suspension density of a single CFB boiler varies exponentially as it does in the free zone of a bubble bed [Li and Kwauk (1980), and. It is claimed that the density of the suspension increases with lowering. 2005) suggested that by changing the bed inventory, the suspension density can be affected.. changes exponentially as it does in the freeboard region of a bubble), and Kunii and Levenspiel (1991)].
Effect of operating parameters on heat transfer
- Effect of suspension density on heat transfer
- Effect of particle size on heat transfer
- Effect of superficial velocity on heat transfer
- Effect of pressure on heat transfer
- Effect of bed temperature on heat transfer
- Effect of other operating parameters on heat transfer
- Mechanistic model for prediction of heat transfer coefficient …
- Enhancement of heat transfer coefficient
Reddy and Basu (2001) considered two particle sizes (234 µm and 489 µm) and investigated the effect of pressure on heat transfer. Gupta and Nag (2002) found that the heat transfer coefficient increases with the increase in superficial velocity.
Review on biomass feeding system
Review on reaction kinetics and thermal analysis
Niksa and Lau (1993) had reported a significant influence of the pre-exponential factors with variation in heating rates. Kalita (2009) studied the thermogravimetric (TG) analysis followed by the study of the DTG curves of different lingo-cellulose materials.
Review on various parameters influencing the performance of biomass
- Gasification processes
- Gasification types
- Gasification in circulating fluidized beds
- Effect of operating parameters on gasification
- Type of biomass and feeding
- Ash content
- Effect of gasifying agent
- Equivalence ratio
- Effect of bed materials
- Effect of temperature
The effects of bed temperature and catalyst on the composition and heating value of the producer gas were investigated by Li et al. Gasification of coal with biomass reduces problems associated with high ash and sulfur contents in the coal (Skoulou et al., 2008).
Summary of the literature review
The temperature profile inside the direct-heated co-gasification reactor is also affected by the percentage of biomass in the coal-biomass mixtures. 2000) observed an average temperature drop of 60 °C (approximately) in the fluidized bed gasifier when the percentage of pine chips was increased from 20 to 100 % in the blends. Both circular and rectangular CFB risers have been reported in laboratory-scale studies. Further, a thorough study on the effect of various operating parameters on the hydrodynamic characteristics, heat transfer behavior and reaction kinetics is very essential to bring further improvements to the current reactor designs and their application in future power plants. .
Scope for research
Information on gas production quality at operating pressures has been scarce in the open literature. Similarly, studies on mixing biomass with bed material and its effect on gas composition in a pressurized CFB are also scarce. Thus, there is a need to conduct further studies to understand the complex phenomena occurring in a pressurized CFB gasification unit.
Introduction
Description of cold bed unit
- Setup description
- Heat transfer probe
- Experimental procedure
- Working formula
The detailed design of the orifice plate and distributor plate are presented in Annexures-I and II respectively. High precision pressure gauge (Swagelok make) is used for measuring the compressor delivery pressure. The suspension density of the bed (ρsus) can be evaluated by the relation as suggested by Kunni and Levenspiel (1991).
Description of hot bed unit
Setup description
A KANTHAL heating element with a capacity of 3500 W (resistance 14 Ohms) is installed in the lower part of the riser. A total of 22 K-type thermocouples are used to measure the temperature at different locations of the CFB unit. Dial gauges (4 nos.) are used to measure the pressure drop across the CFB riser height.
Heat transfer probes
Assuming a constant specific heat capacity of water and steady state conditions, the heat transfer (Q) from the bed to the tubes can be determined from the total heat gained by the water as;. From the energy balance between the total heat gained from the water and the heat transfer from the bed to the tube, the average heat transfer coefficient (h) can be determined using the following expression;.
Experimental procedure
In each experiment, the controlled amount of air is supplied by the fan at a required surface velocity to the CFB loop. The experiments were performed at a surface velocity of 7 m/s and at three different temperatures as well as 450 °C. The heat transfer coefficient with and without twist strip insertion in the upper spray region of CFB in three different sand inventories such as 400, 600 and 800 g are also studied and compared.
Biomass feeding system and its experimental procedure
Experimental procedure for gasification study
The gas coming out of the cyclone separator consists of fine dust particles, which are separated by a dust filter. The product gas composition at biomass to sand mixing ratios of 12.5% and 20.0% was also investigated. All the experiments were performed at three different operating pressures of 1, 3 and 5 bar and at an equivalence ratio (ER) of 0.27.
Summary of the chapter
The cleaned hot gas is then cooled using a condenser and finally the gas sample was collected for gas chromatographic analysis.
Introduction
Evaluation of reaction kinetics
TG data were used to estimate kinetic parameters such as activation energy, pre-exponential factor and reaction order. Determination of kinetic parameters from TG data was based on the following Arrhenius rate expression (Ergudenler and Ghaly, 1992). The simplified form of the rate expression (Eq.4.1) for the fourth-order Range-Kutta is as follows.
Model formulation and description …
The last term of the equation represents the rate of heat generation or consumption resulting from decomposition. The decay rate is given by an nth-order kinetic rate equation of the form. Assuming no expansion of the solid material, eqn (4.6) also gives the rate of change of density in eqn (4.5).
Summary of the chapter
This is expected because the rate of energy consumption or addition due to the decomposition is proportional to ∂ρ ∂t. In addition, the thermal and transport properties of the materials are the functions of the stage of decomposition. The combination of these effects can significantly change the predicted thermal response of the material.
Introduction
Experiments
Characterization and thermal analysis of biomass
The wall-to-bed heat transfer coefficient along the riser height is investigated in the upper spray zone of the riser. The heat transfer coefficient is observed to be greater in the case of smaller particles, thus similar variations of the heat transfer coefficient are observed at all operating conditions in the case of 2.5.
It was found that as the pressure increases, the heat transfer coefficient increases. From these figures, it was found that the heat transfer coefficient decreases from the wall to the core of the riser.
Predictions of TG curves
Transient thermal model and its prediction
The sketch of the measuring arrangement of temperature profile along the length of the rice husk sample is presented in the Fig.5.12. Figures 5.13 and 5.14 show the transient temperature distribution during pyrolysis and gasification along the length of the sample at heat transfer coefficient 20 W/m2-K without considering heat of formation. Figures 5.16-5.19 show the transient temperature distribution during pyrolysis and gasification along the length of the sample at heat transfer coefficient 20 W/m2-K by considering the heat of formation.
Characteristics of other biomass
Summary of the chapter
Introduction
Experiments on cold CFB unit
Investigation of bed hydrodynamics
Variation of bed voidage profile with pressure, inventory and particle
Similar variation of bed space along the height of the rise was found as in the case of 2.5 % sawdust mixture in sand. Fig.6.13 Comparison of variation of bed space along the height of the rise at P = 1 bar. Fig.6.14 Comparison of variation of bed space along the height of the rise at P = 5 bar.
Effect of operating parameters on suspension density
The suspension density plays a key role in the quality of the heat transfer along the riser of a CFB. The comparison of variation of suspension density along the height of the riser at three different particle sizes and at two different operating pressures of 3 and 5 bar is shown in Fig. respectively. 6.27 and 6.28. It has been observed that the suspension density increases with the decrease in particle size.
Variation of solid circulation rate
Investigation of heat transfer
Effects of operating parameters on wall-to-bed heat transfer without
This may be the reason why the heat transfer coefficient decreases in the upper bed when the particle size increases. At these points, the heat transfer coefficient is found to be greater with an average particle diameter of 469 µm. At higher bed inventories, the heat transfer coefficient is found to be higher at all operating pressures.
Effects of operating parameters on wall-to-bed heat transfer with
We can see that as the surface velocity increases, the heat transfer coefficient increases. The variation of the heat transfer coefficient with pressure at four different percentages of biomass mixture in sand and at Usup = 6 m/s is shown in Figure 6.50. It was found that the heat transfer coefficient increases with increasing pressure at all % mixing of biomass in sand.
Variation of wall-to-bed heat transfer coefficient with suspension
Variation of wall-to-bed heat transfer coefficient with weight
A similar change in heat transfer coefficient without biomass mixing was shown by Gupta and Nag (2002). Figures 6.61 to 6.63 show the variation of the heat transfer coefficient along the heat transfer probe at three different operating pressures of 1, 3 and 5 bar. By changing the operating pressure, the heat transfer coefficient related to the percentage of the biomass mixture in the sand and the weight composition also changes.
Variation of heat transfer coefficient along the radial direction
Variation of heat transfer coefficient along the radial direction with
The comparison of the radial variation of the heat transfer coefficient at two different surface velocities is shown in Fig. 6.70 to 6.72. This may be due to a decrease in the concentration of particles from the wall to the core of the riser. This is probably due to the diffusion of particles from the core to the riser wall.
Effect of solid circulation rate on heat transfer
Comparison of experimental results
Uncertainty analysis
Summary of the chapter
Introduction
Experiments on hot bed study
Study of temperature profile and heat transfer
- Variation of bed temperature along the riser height
- Comparison of bed temperature with solid inventory along the riser
- Comparison of bed temperature at different biomass blends
- Comparison of bed temperature at different weight composition
- Effect of twisted tape inserts
- Comparison of bed temperature without and with twisted
- Comparison of heat transfer coefficient without and with
Higher bed temperature has been observed at higher operating pressure along the riser height. A similar variation of bed temperature along the riser height was observed for both biomass mixing percentage. The comparison of the bed temperature along the height of the riser without (probe-2) and with twisted tape (probe-3) for the fixed stock of 600 g at operating pressures of 1, 3 and 5 bar is shown in Figures and 7.20. respectively.
Gas composition analysis
The gas samples are analyzed at different operating pressures and at two different biomass mixing ratios. Figures 7.24 and 7.25 present the result of GC analysis at the mixing ratios of 12.5% and at 1 and 5 bar operating pressure respectively. These two are the representative curves, similar results were found at the other mixing ratios and operating pressures.
Performance of biomass feeding system
The comparison of variation of biomass discharge time with blower exit velocity at four different stocks of and 1000 g is presented in Fig.7.27. The snapshot of the bridge formation, formation of rat holes and shocking is shown in Fig.7.28. Fig.7.30 Flow of biomass for the particle Fig.7.31 Flow of biomass for the particle size.
Uncertainty analysis
Summary of the chapter
Summary of the investigation
- Results of characterization and thermal analysis of biomass
- Results of cold bed studies
- Results of hot bed studies
- Performance of biomass feeding system
We also see that as the surface velocity and system pressure increase, the heat transfer coefficient increases along the height of the heat transfer probe. The effect of coiled tape inserts on heat transfer in the upper spray region of the riser was studied in three different operating modes. The heat transfer coefficient in the upper injection zone increases with increasing operating pressures as well as with the use of a twisted tape with a twist ratio of 4.
Scope for future work
Basu P., and Ngo T., Effect of some operating parameters on heat transfer in vertical fins in a circulating fluidized bed furnace, Powder Technology. Chinsuwan A., and Dutta A., An Investigation of the Heat Transfer Behavior of Ended Longitudinal Membrane Waterwall Tubes in Circulating Fluidized Bed Boilers. Kolar K.A., and Sundaresan R., Heat transfer characteristics in an axial tube in a circulating fluidized bed riser.
![Figure 6.74 presents the variation of heat transfer coefficient with suspension density of the present investigation along with the published data [Basu and Nag (1987), Nag and Moral (1990), Gupta and Nag (2002)]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10495312.0/150.918.222.698.106.341/figure-presents-variation-transfer-coefficient-suspension-investigation-published.webp)
