Characterization of the U.S. Industrial Commercial Boiler Population

Stoker boilers are described by their method of adding and distributing fuel. The most
common type is the spreader stoker, which projects the fuel above the grate, allowing the
fines to combust in suspension while the heavier pieces fall to the grate. Underfeed
stokers push the fuel, usually coal, into the bottom of the bed of fuel and heat causes
volatilization and complete combustion of the fuel by the time it rises to the top of the
bed as ash and is discharged. Chain grate, traveling grate and water-cooled vibrating
grate stokers are less common configurations that use various means to maintain an even,
thin bed of burning fuel on the grate. Stokers burning crushed coal were very common
before 1960 when they gave way to pulverized coal boilers. Sales of stokers have fallen
in recent decades and most new stoker units are fired with wood or wood waste.
Figure 1-5 Cut-Away View of a Traveling Grate Stoker Boiler
Pulverized coal (PC) boilers succeeded stokers as a more efficient method of burning
coal. PC watertube boilers are the predominant technology for power generation coal
boilers but are also used in some large industrial units. In PC boilers, the coal is
pulverized to a dust-like consistency and blown with air into the boiler, where it burns in
suspension in the furnace. PC boilers are characterized by the burner configuration
(tangential, wall, cyclone) and whether the bottom ash exits the boiler in solid or molten
form (wet bottom vs dry bottom).
Fluidized-bed combustors (FBC) are the most recent type of boiler developed for solid
fuel combustion. FBCs burn solid fuel suspended in a bed of inert material at the base of
the furnace. Combustion air is injected from the bottom of the combustor to keep the bed
in a floating or “fluidized” state. Mixing in the fluidized bed provides efficient heat
transfer that allows a more compact design than in conventional watertube designs. The
efficient mixing allows improved combustion at a lower temperature, which reduces the
1-6 Energy and Environmental Analysis, Inc.