Page 55 - Energize November 2021

TECHNICAL

(Henderson, 2003; Wu, 2006). Contrary This type of burner can process a wider Conventional technology results in a
to NOx, low combustion temperatures range of combustible material than most high temperature flame whereas CFB uses
enhance the formation of N 2O. Reduction conventional combustion systems and it is high temperature mobile hot solids to burn
of N 2O can be achieved by increasing the used for a variety of applications including the fuel in a flameless combustion process.
volatile content of the fuel, air staging, NH 3 waste and biomass combustion, and for
injection and sorbent addition. burning poor quality fossil fuels. Flameless combustion
The fluidised bed comprises inert, non- Normal coal combustion produces a
Uniform temperature distribution combustible material, usually sand, which flame, generated by the combustion of
throughout boiler is fluidised by pumping air from below. volatile material which has been gasified.
The flue gas temperature profile is Combustible material, either in small Flameless combustion (FC) occurs in the
homogenous throughout the furnace, particle or shredded form, is added to the solid material bed only and is a lower
which lowers stress due to reduced fluidised bed through a feed system. The temperature combustion within the body
differential temperature between the gas fuel may be supplemented with additional of the fuel. This form of combustion has
and water side. fuel such as natural gas or liquid fuel such the advantage of uniform heat distribution
as diesel during start-up, or to aid the and controlled temperature. Also known
Fluidised bed combustion complete combustion of poor quality fuels as mild combustion, this process has
Conventional coal fired boilers use a such as municipal waste. Once combustion very low pollutant emissions, especially
pulverised coal (PC) firing system, where has started it will normally continue thermal NOx and CO, which are lowered to
the solid fuel is dried and finely ground and unaided so long as sufficient fuel is added residual values, while maintaining a high
blown into the boiler (using air), where it to the bed. thermal performance of the system. Under
ignites and burns for a few seconds while Fluidised-bed technology offers a the special conditions of the combustion
transiting the boiler before the hot gases good fuel flexibility. Solid fuel and other process, the reactions take place in a
and ash residue exit the boiler. The flame combustible matter make up less than 3% volume sustained by the hot medium
and particles are in contact with the boiler by weight of the hot solids present in the above the self-ignition temperature, and
pipes. This results in very high temperatures, bed. This large source of thermal energy it is not possible to observe any visible
1
in the region of 1300 to 1700°C, with provides an extremely steady combustion flame or luminous effect. The opposite
resultant ash melting and slag formation. environment which is relatively insensitive case occurs in conventional combustion
The fluidised bed combustion system to variations in fuel quality. The fuel systems where reactions are concentrated
exploits the rapid mixing properties of particles fed to the furnace are quickly in a narrow flame front. An important
a fluid, by converting a mass of solid dispersed into the large mass of bed advantage of boilers which operate under
particles into a “fluid” by pumping air solids, which rapidly heat the fuel particles the flameless regime is their ability to burn
through the bed of particles from below. above their ignition temperature without fuels of changing and fluctuating quality, or
This agitation of the bed causes the any significant drop in the temperature LCV fuels.
particles to become “fluidised” so that they of the bed solids. This feature makes it FC offers better mass heat transfer
mix rapidly with one another and with the possible to burn almost any fuel without characteristics than flamed combustion
air injected into the bed, conditions which the use of auxiliary fuels and allows for Problems with flamed combustion are
accelerate reactions between particles several fuels to be burnt without major that the flame is variable in shape and
as well as combustion within the bed. changes in the hardware. position-variable. Fuel also does not melt,
avoiding fouling of radiant and convective
heating tubes.

Heat transfer processes
There are three heat transfer processes
in FBC.
• Radiant: From the burning bed and
circulating hot fuel - mainly to wall
pipes. This differs from PC where radiant
energy from the flame is used. Even heat
distribution allows better control and
lower temperature.
• Convection: From hot gases passing
over various heating stages in the flue
gas path.
• Conduction: From hot returning fuel
Figure 2: Example of a fluidised bed particles in the fluidised heat exchanger.

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