Civil Engineering Reference
In-Depth Information
possibility of fuel and ash storage also in
uence on the selection of burning
subsystem.
The air supplied into combustion chamber is divided into:
primary air, when air is introduced with the fuel in a homogeneous mixture
(combustion chambers with burners) or when passing through the fuel layer
(burning on grate);
secondary air,
flame developed inside the combustion
chamber to ensure complete combustion and maintain the combustion
temperature;
introduced into the
tertiary air, introduced into certain areas of the combustion chamber, either to
burn the fuel components incompletely burnt, or to swirl the combustion gases
and ensure uniform temperature in different parts of the combustion chamber
and combustion gas path.
The air is supplied into the combustion chamber either with a fan and/or natural
circulation. The fans used to introduce the air into the combustion chamber and to
extract the combustion gases from the combustion chamber may be radial (cen-
trifugal) and axial. The
flow rates of the air fan and exhaustor are determined by the
calculation of the fuel combustion with air excess, considering an over sizing of
10 % due to the ash deposit on the heat exchange surfaces, which cause the increase
of gas dynamic resistance. The power of the fans is established according to the
fluid to be circulated and the gas dynamic resistances that they must
overcome. Adjustment of air and
flow rate of the
ue gas
flow rates is achieved by changing the
speed of the fan driving engine.
To increase the combustion thermal ef
ciency and stability, the combustion air is
preheated by means of the waste heat contained in
flue gas before being exhausted
in atmosphere.
The characteristics of the fuel supply system depend on the fuel type. This system
is more complex when solid and liquid fuels are used. The solid fuel supply system
consists of the storage and handling facility, the supply facility to the bunker of the
combustion chamber (conveyor belt, scrapers unit, screw feeder, vacuum unit) and
the fuel preparation system (drying and grinding) for pulverized fuel combustion.
Liquid fuel supply systems consist of a tank, circulation pumps,
filters, and preheater,
when high viscosity liquid fuels are used. The simplest system is that using gas fuels.
Except for the case when the direct combustion of fuel is suitable, as is the case
of gaseous fuels, fuels require special preparation for combustion. When solid fuels
are burnt on grate, it is necessary to dry or compact them as bales, pellets, and
briquettes. To burn pulverized solid fuels, they have to be dried and ground in the
dust preparation system. Liquid fuels require preheating before combustion to
reduce viscosity for handling and spraying
fine drops. A gas fuel supply system
includes the supply pipeline on which there are
fitted manual and electrical valves,
filters, pressure gauge, pressure controller, valves, venting pipe, and gas meter.
All combustion facilities should have protection systems which must comply
with the following minimum technical conditions:
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