Bubbling fluidized bed combustors

Figure 39. Model of 20 MW bubbling fluidized bed combustor showing tube arrangement. (From Jones and Glicksman, 1986.)...

As is the case for bubbling fluidized bed combustors, CFB-combustors are operated with an inert (sand) bed. At start-up the CFB-combustor circulates sand (pp = 2600 kg/m ), which is gradually supplemented and replaced by shale and eombustion ash (Pp - 2500 kg/m ). The bed characteristics should therefore relate to ash, rather than to sand. The average particle size is approximately 300 pm. 

Singh RI, Mohapatra SK, Gangacharyulu D (2008) Studies in an atmospheric bubbling fluidized-bed combustor 10 MW power plant based on rice husk. Energy Convers Manage 49 3086-3103... 

Donsi et al. [21] derived the following equation for carbon attrition in a bubbling fluidized bed combustor ... 

In a very recent study by Farzaneh Kaloorazi [42], it was presumed that the mechanical behavior of dense granular flows is not approximated sufficiently accurately by the soil mechanics theory representing the mechanical behavior of the system as a rate independent plastic regime characterized by a constant friction coefficient. It was claimed that somewhat improved model predictions were obtained for bubbling fluidized bed combustors with a somewhat more comprehensive theory proposed by Jop et al. [77]. This alternative frictional pressure tensor closure was derived based on visco-plastic fluid analysis. 

In the 1970s commercial fluidized-bed combustors were limited to the atmospheric, bubbling-bed system, called the atmospheric fluidized-bed combustor (AFBC). In the late 1970s the circulating fluidized combustor (CFG) was introduced commercially, and in the 1980s the new commercial unit was the pressurized fluidized-bed combustor (PFBC). 

Glicksman and Farrell (1995) constructed a scale model of the Tidd 70 MWe pressurized fluidized bed combustor. The scale model was fluidized with air at atmospheric pressure and temperature. They used the simplified set of scaling relationships to construct a one-quarter length scale model of a section of the Tidd combustor shown in Fig. 34. Based on the results of Glicksman and McAndrews (1985), the bubble characteristics within a bank of horizontal tubes should be independent of wall effects at locations at least three to five bubble diameters away from the wall. Low density polyurethane beads were used to obtain a close fit with the solid-to-gas density ratio for the combustor as well as the particle sphericity and particle size distribution (Table 6). 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

The residual carbon contents at different axial locations of the combustor were measured in the pilot plant tests (Li et al., 1991), as shown in Fig. 18. These data show that axial variations in carbon content with temperature (from 810 °C-923 °C) are as a whole rather slight, but mean carbon content increases with decreasing excess air ratio. Besides, for excess air ratios greater than 1.2, the carbon content at the top of the combustor is somewhat less than that at the bottom, while for excess air ratio less than 1.2, the opposite tendency is evident. In conclusion, for this improved combustor, an excess air ratio of 1.2 is considered enough for carbon burn-out, leading to reduced flue gas and increased heat efficiency as compared to bubbling fluidized bed combustion. That is probably attributable to bubbleless gas-solid contacting for increased mass transfer between gas and solids in the fast fluidized bed, as explained by combustion kinetics. 

Figure 42 shows that the availability of calcium for desulfurization in coal combustion in a bubbling fluidized bed decreases with increasing particle diameter and is less than 20% for particles larger than 1 mm. On the other hand, fly ash collected from flue gas shows a calcium availability of not more than 10%, possibly due to its short residence time in the combustor. 

The dynamic and steady-state characteristics of a shallow fluidized bed combustor have been simulated by using a dynamic model in which the lateral solids and gas dispersion are taken into account. The model is based on the two phase theory of fluidization and takes into consideration the effects of the coal particle size distribution, resistance due to diffusion, and reaction. The results of the simulation indicate that concentration gradients exist in the bed on the other hand, the temperature in the bed is quite uniform at any instant in all the cases studied. The results of the simulation also indicate that there exist a critical bubble size and carbon feed rate above which "concentration runaway" occurs, and the bed can never reach the steady state. 

In the present work, the transient and steady-state characteristics of a fluidized bed combustor are studied by solving numerically a dynamic model in which lateral solids and gas dispersion, lateral temperature distribution and wide size distribution of coal feed are taken into account. The influences of bubble size, excess air rate, specific area of heat exchangers and coal feed rate on the performance of the fluidized combustor are examined by means of simulation with the model. 

Let us consider a shallow fluidized bed combustor with multiple coal feeders which are used to reduce the lateral concentration gradient of coal (11). For simplicity, let us assume that the bed can be divided into N similar cylinders of radius R, each with a single feed point in the center. The assumption allows us to use the symmetrical properties of a cylindrical coordinate system and thus greatly reduce the difficulty of computation. The model proposed is based on the two phase theory of fluidization. Both diffusion and reaction resistances in combustion are considered, and the particle size distribution of coal is taken into account also. The assumptions of the model are (a) The bed consists of two phases, namely, the bubble and emulsion phases. The voidage of emulsion phase remains constant and is equal to that at incipient fluidization, and the flow of gas through the bed in excess of minimum fluidization passes through the bed in the form of bubbles (12). (b) The emulsion phase is well mixed in the axial... 

In Figure 7 the effects of carbon feed rate and bubble size on the steady-state average carbon concentration are shown. The existence of critical bubble size for a fixed carbon feed rate can clearly be observed in this figure. It can also be observed that a critical carbon feed rate exists above which concentration runaway occurs, and a stable or steady-state condition can not be reached for a given bubble size. The value of the critical feed rate increases with a decrease in the bubble size. Under the critical condition, the maximum attainable rate of oxygen transfer from the bubble phase to the emulsion phase is reached, and it becomes the rate determining step for combustion as explained previously. To increase the carbon feed rate to a fluidized bed combustor, either the oxygen concentration in the air (gas) stream or the rate of mass transfer between the bubble and emulsion phase needs to be increased. ... 

A non-isothermal dynamic model has been developed for a shallow fulidized bed combustor, which can be used to predict, at least qualitatively, the transient and steady-state characteristics of such systems. Parametric studies have been conducted to examine the effects of excess air flow rate, bubble size and carbon feed rate. It has been shown that an appreciable carbon concentration gradient does exist in the bed. This explains why it is necessary to use multiple feed points in large fluidized bed combustors. A surprising result obtained is that the temperature iii the bed is essentially uniform under all conditions studied even though the carbon concentration is not uniform laterally. 

Today the circulating fluidized bed (CFB) has become the dominating design for combustors operated at atmospheric pressure. Pressurized circulating fluidized bed combustors are under development for combined power cycle applications, but so far no clear advantages have been revealed yet. For this reason the existing commercial pressurized fluid bed systems are bubbling beds. 

Fig. 10.9. A typical circulating fluidized bed combustor design [63]. The furnace (riser) is normally operated in the fast fluidization regime. The ash which is entrained from the furnace is separated from the flue gas in the cyclone. Most of the ash particles are sent into the siphon. The siphon is a small bubbling fluidized bed acting as a pressure lock. From the siphon the ash flows back into the riser. Reprinted with permission from Elsevier, copyright Elsevier 2007.

Saraiva et al [121] presented an extended model for a circulating atmospheric fluidized bed combustor (CAFBC) which included hydrodynamics for the fast section at the top of the bed as well a bubbling bed section at the bottom of the CAFBC. For the fast section of the bed, one dimensional momentum and energy balances were used to predict the temperature and velocity profiles for gas and particles throughout the reactor. The model contain species mass balances for five gas species including SO2, as well as a model of SO2 retention by limestone particles. A bubbling bed model was considered to simulate the chemical process at the bottom of the combustor. 

Saraiva PC, Azevedo JLT, Carvalho MG (1993) Mathematical Simulation of a Circulating Fluidized Bed Combustor. Combust Sci and Tech 93 223-243 Simonin O, Viollet PL (1989) Numerical study on phase dispersion mechanisms in turbulent bubbly flows. Proc Int Conf on Mechanics of Two-Phase Flows, 12-15 June, Taipei, Taiwan... 

BCGSTAB Bi-orthogonal Conjugate Gradient STABilized BFBC Bubbling Fluidized Bed Combustion CAFBC Circulating Atmospheric Fluidized Bed Combustor CARPT Computer Automated Radioactive Particle Tracer CBC Convection Boundedness Criterion... 

The influence of surface location and orientation on the bed-to-surface heat transfer coefficient in circulating fluidized bed combustors is summarized in Table 13.6. The geometric construction of the combustor and the heat transfer surface is shown in Fig. 13.17. Besides the location and orientation, differences in local heat transfer can also be found on the heat transfer surface/tube. For example, the upper part of the horizontal tube shows the smallest value for the heat transfer coefficient in dense-phase fluidized beds due to less frequent bubble impacts and the presence of relatively low-velocity particles. 

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