Fixed-Bed Combustors

The first type of furnace was the spreader stoker, in which a mechanical shovel moved between a fuel hopper and the fixed grate, spreading the coal onto the burning zone (Chapter 14). Later improvements in ash handling and coal feeding, however, have made this early stoker design obsolete. 

The stoker system, however, did have several advantages (1) the coal does not have to be pulverized, (2) only a low level of particulate emissions occurs, simplifying flue gas cleanup, and (3) the stoker is easy to operate and can be built in sizes varying from small to large. 

As in coal gasification (Chapters 20 and 21), reaction zones can be established in the fixed bed of coal as it sits on the grate. If the coal is ignited at the bottom, air flow helps the combustion front move upward through the bed, but volatiles released above the flame front are entrained by the flue gases without passing through a hot zone. 

On the other hand, burning from the top down suffers from incomplete combustion, which sometimes necessitates introduction of secondary air above the bed, if there is not adequate bypassing of air through the bed. Combustion above the bed is promoted by excess air, turbulence, and high temperature. Preheating of the incoming coal also assists complete combustion. 

An improvement over the spreader stoker is the sprinkler stoker, where the coal is gravity-fed through a drop tube above the fuel bed. Other modifications that have been tried involve pushing burning coke out of the way with fresh coal. 

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Bryden, K. M. Ragland, R. W. (1966) Numerical Modeling of a Deep, Fixed Bed Combustor, Energy and Fuels, American Chemical Society, 10, 269-275. 

There are three main designs of combustion systems in which coal and air can be reacted as the start to power generation and they are (1) the fixed-bed combustor, (2) an entrained-bed combustor, also called a suspended-bed combustor, and (3) a fluidized-bed combustor. 

In a fixed-bed combustor, the air passes upward through the pulverized coal at a low velocity. The coal is held on a grate, and the bed of hot coal may be several inches thick. The coal remains in a fixed bed since the air velocity is not sufficient to lift the coal particles upward. Ash removal is continuous or semicontinuous by mechanical means. This type of bed, however, does not afford very efficient gas-solid contact. High combustion rates are not possible with this system. 

In the entrained-bed combustor, the feed coal must be introduced to the combustor as small. The particles (generally <200 mesh) are carried by the gas into the furnace and travel in a suspended state through a hot zone where they are consumed. After combustion, approximately 20% of the ash falls to the bottom of the furnace and is removed there. Combustion gases, which contain about 80% of the ash, pass out of the furnace and are treated to remove the remaining particulates (usually in an electrostatic precipitator) and sulfur compounds (usually in a stack gas scrubber). This type of combustion system (PCC) is in common use for large-scale utility boilers. Higher combustion rates can be attained in pulverized fuel combustors than in fixed-bed combustors. 

Once an undesirable material is created, the most widely used approach to exhaust emission control is the appHcation of add-on control devices (6). Eor organic vapors, these devices can be one of two types, combustion or capture. AppHcable combustion devices include thermal iaciaerators (qv), ie, rotary kilns, Hquid injection combusters, fixed hearths, and uidi2ed-bed combustors catalytic oxidi2ation devices flares or boilers/process heaters. Primary appHcable capture devices include condensers, adsorbers, and absorbers, although such techniques as precipitation and membrane filtration ate finding increased appHcation. A comparison of the primary control alternatives is shown in Table 1 (see also Absorption Adsorption Membrane technology). 

Somewhat related is a process proposed and demonstrated on labscale by the University of Siegen (Germany). The process is called the (Herhof)-Integrierte Pyrolyse und Verbren-nung (IPV) process and is decribed in detail by Hamel et al.60 In this process, biomass is converted with high-temperature steam to pyrolysis gas in a fixed-bed reactor. The generated carbon from this reactor is led to a stationary FB combustor from which the hot ash is returned to the first-mentioned reactor. The ash works catalytically to reduce the tar content of the gas produced. The gas is further cleaned and conditioned using a scrubber and electrostatic filter from which the catch is returned to the FB combustor. 

The combustion of coal in a fixed bed (e.g., stokers) is the oldest and most common method of coal combustion. In recent decades, however, the fixed beds have lost some of their popularity due to the increased use of fluidized bed and suspended bed combustors [2,7],... 

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. ... 

Catalysts in thin-wall honeycomb form offer the advantages of low pressure drop, high geometric surface area, and short diffusion distance as compared to conventional pellets and beads in fixed bed reactors (1). Active zeolite catalysts may be extruded in the form of a honeycomb structure or they may be washcoated on ceramic honeycomb substrates. The latter technique has been widely used in automotive emissions control (2), woodstove combustors (3), control of volatile organic emissions from organic solvents (4), ozone abatement in jet aircraft passenger cabins (5), and N0x abatement... 

Coal-burning systems are usually referred to as layer and chambered the former refers to fixed beds while the latter refers to systans designed for pulverized coal. The feed systems applicable to the combustor are (a) overload, (b) front feed, and (c) underfeed. 

For example, coal combustion may be achieved using pulverized coal in entrained systems or as sized particles in fixed or slowly moving beds larger pieces may, in certain instances, also be used. In the case of the fixed- or slowly moving bed combustor, it is usual to employ a mechanical stoker to feed the coal and a grate to support the coal particles as well as to admit air for the combustion process. With regard to the pulverized systems, coal that has been crushed to ca. 200 mesh is carried into the system entrained by the air. 

Figure 7.7 Cross-flow arrangement (left) of the plate-fin methanol steam reformer coupled with a catalytic combustor as developed by Pan and Wang [382] the diagram on the right side shows details of the gas distribution system, which was introduced into the combustor fixed-bed.

Park et ol. [507] increased the size of their methanol steam reformer described above to an electric power equivalent of 28 W and combined steam reforming with catalytic combustion The reactor was sealed by brazing. While the same steam reforming catalyst as described above was coated onto etched channels of200-pm depth and 300-pm width, the catalytic combustor was a small scale fixed-bed of platinum/alumina catalyst spheres... 

The mathematical model for char combustion described in the previous two sections is applicable to a bed of constant volume, i.e., to a fluidized bed of fixed height, Hq, and having a constant cross-sectional area, Aq. The constant bed height is maintained by an overflow pipe. For this type of combustor operating for a given feed rate of char and limestone particles of known size distributions, the model presented here can predict the following ... 

The only design of the pulse combustion fluid bed dryer appears to be the one patented by Lockwood (1983). To avoid attenuation of the pulsating gas stream by a perforated gas distributor, the flue gases from a pulse combustor enter the bed of particulate material just above the solid floor, which rotates under a plurality of ducting blades adjacent to the floor. The radially spaced blades are fixed to the central hub at one end and to the inner annular baffle at the other one (Figure 14.11). The space between the baffle and dryer wall forms a gas manifold connected to the tailpipe of a pulse combustor. Each... 

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