Encyclopedia of Environmental Science and Engineering, Volume I and II

402

F

FLUIDIZED BED COMBUSTION

INTRODUCTION

The technology for reacting suspended coal particles with

a gas fl owing through them dates back to the 1920s when

the Winkler gas generator was developed in Germany. The

petroleum industry was responsible for the commercial

expansion of fl uidization techniques in the U.S. (1940s),

particularly in the use of solids which catalytically crack

vaporized heavy oils to produce gasoline and other petro-

leum fuels. The application of fl uidized bed combustion

(FBC) technology (to various solid fuels) is widespread in

the U.S. and in other countries for all types of industrial

processes. More than 350 atmospheric fl uidized bed units

are operating in North America, Europe and Asia. FBC is

part of the answer to the question—how do we control our

major emissions from coal sources? Briefl y an FBC boiler is

a fi nely divided bed of solid fuel particles in admixture with

limestone particles which are suspended or conveyed by

primary combustion air moving in the vertical upward direc-

tion. The limestone reacts with sulfur dioxide to remove it

from the fl ue gas. The low uniform temperature (ca 1550
F)

has a benefi cial effect on nitrogen oxide suppression. The

emission from coal combustion schemes of nitrogen oxides

(NO x ) and sulfur dioxide (SO 2 ), together with carbon oxides

(CO and CO 2 ), particulate matter and solid wastes must

always be compared when evaluating various alternative

schemes. The potential consequences of gaseous emissions,

include the greenhouse effect and acid rain, which have

received much publicity in recent years. The practical FBC

limit of SO 2 removal is currently about 95%. Nitrogen oxide

formation is lower than with conventional pulverized coal

(PC) boiler NO x control.

TYPES OF FLUIDIZED BED COMBUSTORS (FBCS)

FBCs are generally referred to as either circulating (CFB) or

bubbling beds. However, the bubbling type may be classifi ed

according to whether reaction takes place at atmospheric

(AFB) or under pressurized conditions (PFB).

A. Circulating Fluidized Bed

Combustors

In the basic CFB combustor, coal or some other type of fossil

fuel, e.g., natural gas or petroleum, is injected into the com-

bustor together with a calcium based material such as lime-

stone or dolomite to be used as a sorbent for SO 2. The bed

material is entrained by fl uidizing air usually in the velocity

range of 12–30 ft/sec. The entrained material is forced into

a refractory-lined cyclone located between the combustor

and the convective pass. The separated larger particles are

reintroduced at the bottom of the combustion chamber or,

as in some designs, to an external heat exchanger. The mean

bed particle size is usually between 50 and 300 microns.

Combustion temperature will vary but generally is kept

between 1550
F and 1650
F.^1 In this temperature range SO 2

sorption is optimized and the formation of nitrogen oxides

is minimized.

The heavier solids fall to the bottom of the cyclone and

are recirculated at a ratio of between 15:1 and 100:1 (solids to

feed). The carbon content of the bed is usually about 3–4%.

Calcium sulfate, ash, and calcined limestone make up the

bulk of the recirculated material. The fl ue gas exits the top of

the cyclone, travels through the convective pass and typically

goes into an economizer (heat exchanger—superheated steam

produced) and into a tubular air preheater. From there the gas

may enter an electrostatic precipitator or a bag house dust

collector (for removal of fi ne particulate matter from the gas).

An induced draft fan is fi nally employed to force the gas up a

stack and into the atmosphere.

Combustion air is provided at two levels of the combus-

tor. Primary air enters through the bottom of the combustor

and is evenly distributed by a gas distributor plate. Secondary

air enters through a number of ports in the sidewalls of the

combustor. Hence, there are two staged areas of combustion

within the combustor. In the lower combustor, combustion

takes place under reducing conditions. In the upper com-

bustor nitrogen oxides are further reduced as is particulate

matter. The admission of secondary air is also benefi cial

in controlling the temperature of the combustor as well as in

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