Energy Conservation Method in power plant - ASKPOWERPLANT

Energy Conservation Method in power plant

Energy Conservation methods in power plant
BOILER OPERATION
4

Energy Conservation Method in power plant

In this article we discussed about various energy conservation method which should be adopted in power plant for getting maximum efficiency during operation
Here various energy efficiency techniques adopted in boiler system which is related to combustion, heat transfer, avoidable losses, high auxiliary power consumption, water quality and blow down. Some important energy conservation method discussed below which is very effective to used for energy conserve .

1. Stack Temperature low as possible

When the fuel gas is exhausted from the boiler after combustion through I.D fan.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 significant sulphur as low temperature can lead to sulphur dew point corrosion. Stack temperatures greater than 200°C indicates potential for recovery of waste heat. It also indicates the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning.

2. Preheating Feed Water using Economizer

Preheating Feed Water using Economizer is another energy conservation method which is adopted in power plant. in this method the flue gases leaving a modern 3-pass shell boiler are at temperatures of 200 to 300 oC. Thus, there is a highly potential to recover heat from these gases which is coming from the outside of the boiler as a waist flue gases .it should be possible the flue gas exit temperature from a boiler is usually maintained at a minimum of 200 oC, so that the sulfur oxides in the flue gas do not condense and cause corrosion in heat transfer surfaces which is also called the dew point corrosion . 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 below 200oC.
The energy saving in the power plant 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 260oC, an economizer could be used to reduce it to 200oC, increasing the feed water temperature by 15oC. Increase in overall thermal efficiency would be in the order of 3%. For a modern 3-pass shell boiler firing natural gas with a flue gas exit temperature of 140oC a condensing economizer would reduce the exit temperature to 65oC increasing thermal efficiency by 5%.

3. Preheat Combustion Air

In power plant boilers Combustion air is a preheating source of feedwater heating. In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 oC. Most gas and oil burners used in a boiler plant are not designed for high air preheat temperatures. Modern burners which is used in the boiler for tengsile firing 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 of fuel

Incomplete combustion of fuel in the boiler during operation is also a important factor which effect the energy conservation .low excess air and low grade fuel is also causes of incomplete combustion. It is usually dedicated 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. Improper 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.

5. Excess Air Control should be controlled

Excess air is required for the complete combustion in the boiler, 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 of flue gases varies with furnace design, type of burner, fuel and process variables. It can be determined by conducting tests with different air fuel ratios.

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.
Here various methods are available to control the excess air:
• Portable oxygen analyzers 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 usually method is adopted 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.
• Oxygen analyzer contains 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.

6. Radiation and Convection Heat Loss

The outermost area of boiler is 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 energy loss from the boiler outermost shell is generally 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 of damage insulation can reduce heat loss through boiler walls and piping.

7. Installed automatic Blow down Control

Whenever boiler steam drum water chemistry disturbed like Ph of boiler steam drum water increase then we take the action for giving the blow down through IBD ( intermittent blow down ), so IBD is also a energy loss from the boiler so we can’t understand what amount of IBD is given which correct the boiler steam drum pH up to desired level. Uncontrolled IBD ( intermittent blow down ) is very wasteful. Automatic blow down controls can be installed over come from this problem so that it 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 on boiler tubes

In modern power plant oil and coal-fired boilers which is used for process and power generation, soot deposition on tubes acts as an insulator against heat transfer. This type of deposits should be removed on a regular basis. Increases in stack temperatures may indicate excessive soot buildup. Same happening is occurred inside the tube when the deposited scaling increase inside the tube the heat transfer rate reduce ftom 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 in the D.M plant side and tube cleaning to remove deposits. It estimated that 1% efficiency loss occurs with every 22oC increase in stack temperature. Stack temperature should be checked and recorded regularly as an indicator of soot deposits. When the temperature of flue gas rises about 20oC 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 deposit on boiler tubes can cause an increase in fuel consumption by 2.5% due to increased flue gas temperatures. Periodic shut down 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

Reduction of Boiler Steam Pressure 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.
In process plant 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 at same temperature and pressure, and there are periods when the boiler pressure could be reduced. The energy manager should consider pressure reduction carefully, before recommending it. It show Adverse effects , such as an increase in water carryover from the boiler owing to pressure reduction,. 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

In modern power plant VFD (variable speed drive controller) is an important means of achieving energy savings. During boiler operation 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 with new one

If boiler running long period of time then boiler started running under faulty condition like super heater tube damage soon, not take load according the demand and also not burn the cheap fuel properly .when the boiler is not erected according the demand and these type boiler need to be replace The potential savings from replacing a boiler depend on the anticipated change in overall efficiency. If any change in a boiler take place it should 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 possible study should be examining 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.

 

 

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