Energy is an indispensable resource that is required to run the economy
and sustain the well being of people. Every day new discoveries are made
to improve efficiency of the methods of power generation and
transmission. After all, energy conservation must be our social
responsibility.
Social responsibilities for exploring, harnessing, adopting the given
energy sources to obtain effective means to consume those are no less
important. The rapidly developing economies of Third World countries
like India, China need to generate increasing amounts of energy as they
grow. Energy shortages with rising cost confront every nation,
especially those aspiring for industrial development. A time has come
when no resource of this planet, including energy, should be used
wastefully. To achieve this ideal, the society as a whole would require
collective efforts of all groups, especially between the organizations
harnessing energy and the consumers of energy. And both must understand
the underlying principles of energy generation and energy consumption.
Every day new discoveries are made to improve efficiency of the methods
of power generation and transmission. Some of the discoveries would fail
to attract the users while some may be rejected or replaced by new
claims. Such a circumstance would demand repeated explorations of energy
opportunities and the way social responses can facilitate the use of
new discoveries.
Energy Conservation Opportunities
The various energy efficiency opportunities in boiler system can be
related to combustion, heat transfer, avoidable losses, high auxiliary
power consumption, water quality and blowdown. Flue gases are the single
most important cause of energy loss. As much as 18 to 22 percent of
available energy goes up the chimney. Heat radiation and convection from
boiler walls raise heat loss another 1 to 4 percent. Examining the
following factors can indicate if a boiler is being run to maximize its
efficiency:
1. Stack Temperature
The stack temperature should be as low as possible. However, it should
not be so low that water vapor in the exhaust condenses on the stack
walls. This is important in fuels containing signficant sulphur as low
temperature can lead to sulphur dew point corrosion. Stack o
temperatures greater than 200 C indicates potential for recovery of
waste heat. It also indicate the scaling of heat transfer/recovery
equipment and hence the urgency of taking an early shut down for water /
flue side cleaning.
2. Feed Water Preheating using Economiser
Typically, the flue gases leaving a modern 3-pass shell o boiler are at
temperatures of 200 to 300 C. Thus, there is a potential to recover heat
from these gases. The flue gas exit temperature from a boiler is
usually maintained at a o minimum of 200 C, so that the sulphur oxides
in the flue gas do not condense and cause corrosion in heat transfer
surfaces. When a clean fuel such as natural gas, LPG or gas oil is used,
the economy of heat recovery must be worked out, as the flue gas
temperature may be well o below 200 C.
The potential for energy saving depends on the type of boiler installed
and the fuel used. For a typically older model shell boiler, with a flue
gas exit temperature of o 260 C, an economizer could be used to reduce
it to o o 200 C, increasing the feed water temperature by 15 C. Increase
in overall thermal efficiency would be in the order of 3%. For a modern
3-pass shell boiler firing o natural gas with a flue gas exit
temperature of 140 C a condensing economizer would reduce the exit o
temperature to 65 C increasing thermal efficiency by 5%.
3. Combustion Air Preheat
Combustion air preheating is an alternative to feed water heating. In
order to improve thermal efficiency by 1%, o the combustion air
temperature must be raised by 20 C. Most gas and oil burners used in a
boiler plant are not designed for high air preheat temperatures. Modern
burners can withstand much higher combustion air preheat, so it is
possible to consider such units as heat exchangers in the exit flue as
an alternative to an economizer, when either space or a high feed water
return temperature make it viable.
4. Incomplete Combustion
Incomplete combustion can arise from a shortage of air or surplus of
fuel or poor distribution of fuel. It is usually obvious from the colour
or smoke, and must be corrected immediately.
In the case of oil and gas fired systems, CO or smoke (for oil fired
systems only) with normal or high excess air indicates burner system
problems. A more frequent cause of incomplete combustion is the poor
mixing of fuel and air at the burner. Poor oil fires can result from
improper viscosity, worn tips, carbonization on tips and deterioration
of diffusers or spinner plates.
With coal firing, unburned carbon can comprise a big loss. It occurs as
grit carry-over or carbon-in-ash and may amount to more than 2% of the
heat supplied to the boiler. Non uniform fuel size could be one of the
reasons for incomplete combustion. In chain grate stokers, large lumps
will not burn out completely, while small pieces and fines may block the
air passage, thus causing poor air distribution. In sprinkler stokers,
stoker grate condition, fuel distributors, wind box air regulation and
over-fire systems can affect carbon loss. Increase in the fines in
pulverized coal also increases carbon loss.
5. Excess Air Control
Excess air is required in all practical cases to ensure complete
combustion, to allow for the normal variations in combustion and to
ensure satisfactory stack conditions for some fuels. The optimum excess
air level for maximum boiler efficiency occurs when the sum of the
losses due to incomplete combustion and loss due to heat in flue gases
is minimum. This level varies with furnace design, type of burner, fuel
and process variables. It can be determined by conducting tests with
different air fuel ratios.
Typical values of excess air supplied for various fuels are given in
Table. The Table gives the theoretical amount of air required for
combustion of various types of fuel. Controlling excess air to an
optimum level always results in reduction in flue gas losses; for every
1% reduction in excess air there is approximately 0.6% rise in
efficiency.
* Portable oxygen analysers and draft gauges can be used to make
periodic readings to guide the operator to manually adjust the flow of
air for optimum operation. Excess air reduction up to 20% is feasible.
* The most common method is the continuous oxygen analyzer with a local
readout mounted draft gauge, by which the operator can adjust air flow. A
further reduction of 10-15% can be achieved over the previous system.
* The same continuous oxygen analyzer can have a remote controlled
pneumatic damper positioner, by which the readouts are available in a
control room. This enables an operator to remotely control a number of
firing systems simultaneously.
The most sophisticated system is the automatic stack damper control, whose cost is really justified only for large systems.
6. Radiation and Convection Heat Loss
The external surfaces of a shell boiler are hotter than the
surroundings. The surfaces thus lose heat to the surroundings depending
on the surface area and the difference in temperature between the
surface and the surroundings.
The heat loss from the boiler shell is normally a fixed energy loss,
irrespective of the boiler output. With modern boiler designs, this may
represent only 1.5% on the gross calorific value at full rating, but
will increase to around 6%, if the boiler operates at only 25 percent
output. Repairing or augmenting insulation can reduce heat loss through
boiler walls and piping.
7. Automatic Blowdown Control
Uncontrolled continuous blowdown is very wasteful. Automatic blowdown
controls can be installed that sense and respond to boiler water
conductivity and pH. A 10% blow down in a 15 kg/cm2 boiler results in 3%
efficiency loss.
8. Reduction of Scaling and Soot Losses
In oil and coal-fired boilers, soot buildup on tubes acts as an
insulator against heat transfer. Any such deposits should be removed on a
regular basis. Elevated stack temperatures may indicate excessive soot
buildup. Also same result will occur due to scaling on the water side.
High exit gas temperatures at normal excess air indicate poor heat
transfer performance. This condition can result from a gradual build-up
of gas-side or waterside deposits. Waterside deposits require a review
of water treatment procedures and tube cleaning to remove deposits. An
estimated o 1% efficiency loss occurs with every 22 C increase in stack
temperature.
Stack temperature should be checked and recorded regularly as an
indicator of soot deposits. When the flue o gas temperature rises about
20 C above the temperature for a newly cleaned boiler, it is time to
remove the soot deposits. It is, therefore, recommended to install a
dial type thermometer at the base of the stack to monitor the exhaust
flue gas temperature. It is estimated that 3 mm of soot can cause an
increase in fuel consumption by 2.5% due to increased flue gas
temperatures. Periodic off-line cleaning of radiant furnace surfaces,
boiler tube banks, economizers and air heaters may be necessary to
remove stubborn deposits.
9. Reduction of Boiler Steam Pressure
This is an effective means of reducing fuel consumption, if permissible,
by as much as 1 to 2%. Lower steam pressure gives a lower saturated
steam temperature and without stack heat recovery, a similar reduction
in the temperature of the flue gas temperature results. Steam is
generated at pressures normally dictated by the highest pressure /
temperature requirements for a particular process. In some cases, the
process does not operate all the time, and there are periods when the
boiler pressure could be reduced. The energy manager should consider
pressure reduction carefully, before recommending it. Adverse effects,
such as an increase in water carryover from the boiler owing to pressure
reduction, may negate any potential saving. Pressure should be reduced
in stages, and no more than a 20 percent reduction should be considered.
10. Variable Speed Control for Fans, Blowers and Pumps
Variable speed control (VSD) is an important means of achieving energy
savings. Generally, combustion air control is affected by throttling
dampers fitted at forced and induced draft fans. Though dampers are
simple means of control, they lack accuracy, giving poor control
characteristics at the top and bottom of the operating range. In
general, if the load characteristic of the boiler is variable, the
possibility of replacing the dampers by a VSD should be evaluated.
11. Effect of Boiler Loading on Efficiency
The maximum efficiency of the boiler does not occur at full load, but at
about two-thirds of the full load. If the load on the boiler decreases
further, efficiency also tends to decrease. At zero output, the
efficiency of the boiler is zero, and any fuel fired is used only to
supply the losses.
The factors affecting boiler efficiency are:
* As the load falls, so does the value of the mass flow rate of the flue
gases through the tubes. This reduction in flow rate for the same heat
transfer area, reduced the exit flue gas temperatures by a small extent,
reducing the sensible heat loss. * Below half load, most combustion
appliances need more excess air to burn the fuel completely. This
increases the sensible heat loss.
In general, efficiency of the boiler reduces significantly below 25% of
the rated load and as far as possible; operation of boilers below this
level should be avoided.
12. Proper Boiler Scheduling
Since, the optimum efficiency of boilers occurs at 65- 85% of full load,
it is usually more efficient, on the whole, to operate a fewer number
of boilers at higher loads, than to operate a large number at low loads.
13. Boiler Replacement
The potential savings from replacing a boiler depend on the anticipated
change in overall efficiency. A change in a boiler can be financially
attractive if the existing boiler is:
* old and inefficient
* not capable of firing cheaper substitution fuel
* over or under-sized for present requirements
* not designed for ideal loading conditions
The feasibility study should examine all implications of long-term fuel
availability and company growth plans. All financial and engineering
factors should be considered. Since boiler plants traditionally have a
useful life of well over 25 years, replacement must be carefully
studied.
Energy Conservation Boilers
Burning of unprocessed coal can release enormous quantities of obnoxious
gases. Combustion of coal, like any other fossil fuel produces carbon
dioxide (CO ), 2 nitrogen oxide (NOx) along with varying amounts of
sulphur dioxide (SO ). Sulphur dioxide reacts with 2 oxygen to form
sulphur trioxide (SO ), which with water 3 forms sulphuric acid.
Sulphuric acid passed into the atmosphere is returned to thearth as acid
rain. Many other pollutants are present in coal power station
emissions, as solid coal is more difficult to clean than petroleum.
After observation of the above different factors the following steps to
be considered for the energy conservation boilers.
* Ensure proper selection fuel-firing equipment, viz. Burners, mechanical stokers etc.
* Ensure correct temperature and pressure of fuel oil at the burner tip as per manufacturers specifications.
* Reduce radiation losses from boilers, furnaces and auxiliary equip-
meant by improved thermal insulation. This will also improve thermal
insulation. This will also improve the working conditions within the
building and eliminate, unnecessary ventilation.
* Employ blow down and water process-heat to preheat the boiler to feed water.
* Use of steam and power within the boiler house should be subject to
critical scrutiny. For instance, loss of steam from relief valves and
other fittings should be minimized.
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