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SWEETCORN TECH
An air preheater (APH) is a general term used to describe any device
designed to heat air before another process (for example, combustion in a
boiler) with the primary objective of increasing the thermal efficiency
of the process. They may be used alone or to replace a recuperative
heat system or to replace a steam coil.
In particular, this article describes the combustion air preheaters used
in large boilers found in thermal power stations producing electric
power from e.g. fossil fuels, biomass or waste.
The purpose of the air preheater is to recover the heat from the boiler
flue gas which increases the thermal efficiency of the boiler by
reducing the useful heat lost in the flue gas. As a consequence, the
flue gases are also conveyed to the flue gas stack (or chimney) at a
lower temperature, allowing simplified design of the conveyance system
and the flue gas stack. It also allows control over the temperature of
gases leaving the stack (to meet emissions regulations, for example).
Types
There are two types of air preheaters for use in steam generators in
thermal power stations: One is a tubular type built into the boiler flue
gas ducting, and the other is a regenerative air preheater. These may
be arranged so the gas flows horizontally or vertically across the axis
of rotation.
Another type of air preheater is the regenerator used in iron or glass manufacture.
Tubular type
Construction features
Tubular preheaters consist of straight tube bundles which pass through
the outlet ducting of the boiler and open at each end outside of the
ducting. Inside the ducting, the hot furnace gases pass around the
preheater tubes, transferring heat from the exhaust gas to the air
inside the preheater. Ambient air is forced by a fan through ducting at
one end of the preheater tubes and at other end the heated air from
inside of the tubes emerges into another set of ducting, which carries
it to the boiler furnace for combustion.
Problems
The tubular preheater ductings for cold and hot air require more space
and structural supports than a rotating preheater design. Further, due
to dust-laden abrasive flue gases, the tubes outside the ducting wear
out faster on the side facing the gas current. Many advances have been
made to eliminate this problem such as the use of ceramic and hardened
steel.
Many new circulating fluidized bed (CFB) and bubbling fluidized bed
(BFB) steam generators are currently incorporating tubular air heaters
offering an advantage with regards to the moving parts of a rotary type.
Dew point corrosion
Dew point corrosion occurs for a variety of reasons. The type of fuel
used, its sulfur content and moisture content are contributing factors.
However, by far the most significant cause of dew point corrosion is the
metal temperature of the tubes. If the metal temperature within the
tubes drops below the acid saturation temperature, usually at between
190 °F (88 °C)and 230 °F (110 °C), but sometimes at temperatures as high
as 260 °F (127 °C), then the risk of dew point corrosion damage becomes
considerable.
Regenerative air preheaters
There are two types of regenerative air preheaters: the rotating-plate
regenerative air preheaters (RAPH) and the stationary-plate regenerative
air preheaters (Rothemuhle).
Rotating-plate regenerative air preheater
The rotating-plate design (RAPH) consists of a central rotating-plate
element installed within a casing that is divided into two (bi-sector
type), three (tri-sector type) or four (quad-sector type) sectors
containing seals around the element. The seals allow the element to
rotate through all the sectors, but keep gas leakage between sectors to a
minimum while providing separate gas air and flue gas paths through
each sector.
Tri-sector types are the most common in modern power generation
facilities.[11] In the tri-sector design, the largest sector (usually
spanning about half the cross-section of the casing) is connected to the
boiler hot gas outlet. The hot exhaust gas flows over the central
element, transferring some of its heat to the element, and is then
ducted away for further treatment in dust collectors and other equipment
before being expelled from the flue gas stack. The second, smaller
sector, is fed with ambient air by a fan, which passes over the heated
element as it rotates into the sector, and is heated before being
carried to the boiler furnace for combustion. The third sector is the
smallest one and it heats air which is routed into the pulverizers and
used to carry the coal-air mixture to coal boiler burners. Thus, the
total air heated in the RAPH provides: heating air to remove the
moisture from the pulverised coal dust, carrier air for transporting the
pulverised coal to the boiler burners and the primary air for
combustion.
The rotor itself is the medium of heat transfer in this system, and is
usually composed of some form of steel and/or ceramic structure. It
rotates quite slowly (around 3-5 RPM) to allow optimum heat transfer
first from the hot exhaust gases to the element, then as it rotates,
from the element to the cooler air in the other sectors.
Construction features
In this design the whole air preheater casing is supported on the boiler
supporting structure itself with necessary expansion joints in the
ducting.
The vertical rotor is supported on thrust bearings at the lower end and
has an oil bath lubrication, cooled by water circulating in coils inside
the oil bath. This arrangement is for cooling the lower end of the
shaft, as this end of the vertical rotor is on the hot end of the
ducting. The top end of the rotor has a simple roller bearing to hold
the shaft in a vertical position.
The rotor is built up on the vertical shaft with radial supports and
cages for holding the baskets in position. Radial and circumferential
seal plates are also provided to avoid leakages of gases or air between
the sectors or between the duct and the casing while in rotation.
For on line cleaning of the deposits from the baskets steam jets are
provided such that the blown out dust and ash are collected at the
bottom ash hopper of the air preheater. This dust hopper is connected
for emptying along with the main dust hoppers of the dust collectors.
The rotor is turned by an air driven motor and gearing, and is required
to be started before starting the boiler and also to be kept in rotation
for some time after the boiler is stopped, to avoid uneven expansion
and contraction resulting in warping or cracking of the rotor. The
station air is generally totally dry (dry air is required for the
instrumentation), so the air used to drive the rotor is injected with
oil to lubricate the air motor.
Safety protected inspection windows are provided for viewing the preheater's internal operation under all operating conditions.
The baskets are in the sector housings provided on the rotor and are
renewable. The life of the baskets depend on the ash abrasiveness and
corrosiveness of the boiler outlet gases.
Problems
The boiler flue gas contains many dust particles (due to high ash
content) not contributing towards combustion, such as silica, which
cause abrasive wear of the baskets, and may also contain corrosive gases
depending on the composition of the fuel. For example, Indian coals
generally result in high levels of ash, sulfur and silica in the flue
gas. The wear of the baskets therefore is generally more than other,
cleaner-burning fuels.
In this RAPH, the dust laden, corrosive boiler gases have to pass
between the elements of air preheater baskets. The elements are made up
of zig zag corrugated plates pressed into a steel basket giving
sufficient annular space in between for the gas to pass through. These
plates are corrugated to give more surface area for the heat to be
absorbed and also to give it the rigidity for stacking them into the
baskets. Hence frequent replacements are called for and new baskets are
always kept ready. In the early days, Cor-ten steel was being used for
the elements. Today due to technological advance many manufacturers may
use their own patents. Some manufacturers supply different materials for
the use of the elements to lengthen the life of the baskets.
In certain cases the unburnt deposits may occur on the air preheater
elements causing it to catch fire during normal operations of the
boiler, giving rise to explosions inside the air preheater. Sometimes
mild explosions may be detected in the control room by variations in the
inlet and outlet temperatures of the combustion air.
Stationary-plate regenerative air preheater
The heating plate elements in this type of regenerative air preheater
are also installed in a casing, but the heating plate elements are
stationary rather than rotating. Instead the air ducts in the preheater
are rotated so as to alternatively expose sections of the heating plate
elements to the upflowing cool air.
As indicated in the adjacent drawing, there are rotating inlet air ducts
at the bottom of the stationary plates similar to the rotating outlet
air ducts at the top of the stationary plates.
Stationary-plate regenerative air preheaters are also known as
Rothemuhle preheaters, manufactured for over 25 years by Balke-Dürr GmbH
of Ratingen, Germany.
Regenerator
A regenerator consists of a brick checkerwork: bricks laid with spaces
equivalent to a brick's width between them, so that air can flow
relatively easily through the checkerwork. The idea is that as hot
exhaust gases flow through the checkerwork, they give up heat to the
bricks. The airflow is then reversed, so that the hot bricks heat up the
incoming combustion air and fuel. For a glass-melting furnace, a
regenerator sits on either side of the furnace, often forming an
integral whole. For a blast furnace, the regenerators (commonly called
Cowper stoves) sit separate to the furnace. A furnace needs no less than
two stoves, but may have three. One of the stoves is 'on gas',
receiving hot gases from the furnace top and heating the checkerwork
inside, whilst the other is 'on blast', receiving cold air from the
blowers, heating it and passing it to the blast furnace.
