c) On all burners where oil will be the only fuel, it is
recommended that the flame sensing elements be restricted to the frequency-
response (lead sulfide) type or the ultraviolet type.
d) All oil burners should have two guns, one of which may be
removed for cleaning while the other maintains the load.
e) An economic analysis should determine if oil burners should be
steam or air atomizing unless steam is required for greater turndown. When
air is used for atomizing, a steam connection should be provided for emergency
All burners should have automatic recycling controls.
Wood Firing. Combustion systems for wood are usually designed
specifically for the material and mixture of fuels to be burned. When the
moisture content is high, over 60 to 65 percent, supplemental firing of coal,
oil, or gas can be used or the wood must be mixed with low-moisture fuels so
that enough energy enters the boiler to support combustion. Dry wood may have
a heating value of 8,750 Btu/lb (20 388 kJ/kg); but at 80 percent moisture, a
pound of wet wood has a heating value of only 1750 Btu/lb (4078 kJ/kg). Table
11 shows the moisture-energy relationship.
The usual practice when burning wood is to propel the wood particles
into the furnace through injectors along with preheated air with the purpose
of inducing high turbulence in the boiler. The wood is injected high enough
in the combustion chamber so that it is dried and all but the largest
particles are burned before they reach the grate at the bottom of the furnace.
Spreader stokers and cyclone burners work well for this application. Before
choosing wood as a fuel to produce steam or HTW, methods should be researched
thoroughly. Success of similar operations, adequacy of fuel source, and
economics should be evaluated.
Suspension Burning of Wood. Small wood chips or saw dust may be
blown into the furnace chamber and burned in suspension. The ash or unburned
particles are collected on a traveling-grate and transported to an ash pit.
In wood burning applications, heat releases have been as high as 1,000,000
Btu/square foot/hr (3155 kJ/square meter per s) of active grate area.
Wood Stokers. Wood chips and waste products such as bark and hog
fuel are successfully burned in stokers. Spreader stokers and vibrating-grate
stokers have been primarily used for this fuel.
Solid Waste Firing. Solid waste, like wood, derives most of its
heating value from its cellulose content. The firing methods are therefore
very similar for the two fuels.
Mass Burning. In mass burning, water-wall incinerator type boilers
use vibrating grates, underfire and overfire air systems, induced draft and