Boiler Performance
Boiler performance parameters, such as efficiency and evaporation ratio decrease with time due to poor combustion, dirty heat exchanger surfaces and poor operation and maintenance. Even for new boilers, reasons such as poor fuel quality and water quality can result in poor boiler performance.
A heat balance can help and identify heat loss that can or cannot be avoided. Boiler efficiency tests can assist in finding boiler efficiency deviations from the best efficiency and target problem areas for corrective action.
Boiler performance evaluation
The heat balance is the balance of the total energy entering the boiler against those leaving the boiler in different forms. Energy losses can be divided into unavoidable losses.
To improve energy efficiency the following losses can be avoided:
1.Loss of flue gas
2. Loss due to unburned fuel.
3.Loss due to blowdown.
4.Loss of condensate
5. Loss due to convection and radiation.
Boiler Efficiency
Boiler efficiency is defined as the percent of incoming energy (heat) used effectively in the resulting system.
There are two methods of assessing boiler efficiency:
1. Direct method, this method is also known as input-output because it only requires output (steam) and heat in (fuel).
This efficiency can be evaluated using the formula:
Boiler efficiency = (Heat out/Heat in) x 100
2. Indirect method, this method is known as heat loss.
The losses that occur in the boiler are heat losses caused by:
a. Dry flue gas
b.Evaporation of water formed due to H2 in the fuel.
c. Evaporation of water content in fuel.
d. The presence of water content in the fuel.
e. Fuel that does not burn fly ash.
f. Unburned fuel in bottom ash.
g. Unaccounted radiation and loss.
Boiler Efficiency Improvement Opportunity
Chimney Temperature Control
The chimney temperature should be as low as possible. However, the temperature should not be so low that water vapor condenses on the chimney wall. This is important for sulfur-containing fuels where at low temperatures will result in sulfur dew point corrosion.
Chimney temperatures greater than 2000C indicate the potential for reuse of waste heat. This also indicates that there has been scale formation on the heat transfer or utilization equipment and it is advisable to shut the blower early for water/chimney side cleaning.
Feedwater Preheat
Usually the flue gas leaving the boiler is 200-3000C. So there is potential to reuse the heat and gases. Preheating the feed water is very necessary so that the heating in the boiler drum does not take too long so that it produces steam faster.
Minimize Incomplete Combustion
Incomplete combustion can arise from a lack of air or excess fuel or poor fuel distribution. This can be seen from the color of the smoke coming out of the chimney. And this should be avoided because in addition to decreasing steam pressure and can affect the environment.
Excess air control
Excess air is required in all combustion practices to ensure complete combustion, to obtain a variety of combustion and to ensure satisfactory stack conditions for some fuels.
Controlling excess air at an optimal level always results in a decrease in exhaust gas loss, for every 1% decrease in excess air there is an increase in efficiency of approximately 0.6%.
Methods for controlling excess air:
1.Portable oxygen analyzers and draft gauges can be used to make periodic readings to determine if the operator is manually adjusting the airflow for optimum operation.
2. Balanced oxygen analyzer with on-site reading, where operator can adjust airflow. A further 10-15% air reduction can be achieved through the former system.
3.The same continuous oxygen analyzer can have a pneumatic damper positioner controlled by a remote control device, where data readings are available in the control room.
Radiation And Convection Heat Loss Avoidance
The outer surface of the boiler shell is hotter than its surroundings. So the surface releases heat to the environment depending on the surface area and the temperature difference between the surface and the surrounding environment. The heat lost from the boiler shell is usually a predetermined energy loss, regardless of the boiler output.
With modern boiler designs, this loss is only 1.5% of the gross calorific value at full speed, but will increase to about 6% if the boiler operates only at 25% output. Repair or expansion of insulation can reduce heat loss of boiler walls and piping.
Automatic Blowdown Control
Continuous uncontrolled blowdown is useless because it will only lead to wasted water. An automatic blowdown control can be installed which is a sensor and responds to the boiler water conductivity and pH. 10% blowdown in a 15 kg/cm2 boiler results in a 3% efficiency loss.
Reduction of scale formation and loss of soot
In oil and coal fired boilers, the soot that builds up in the pipes acts as an insulator to heat transfer, so these deposits must be removed regularly.
An increased chimney temperature can signal excessive soot formation. The same result will also occur due to the formation of scale on the water side. A high exhaust gas temperature in normal excess air indicates poor heat transfer performance.
This condition can result from the gradual formation of deposits on the gas side or the water side. The formation of deposits on the water side requires a review of water treatment methods and pipe cleaning to remove deposits. It is estimated that an efficiency loss of 1% occurs for every 220C increase in flue temperature.
The stack temperature should be checked and recorded regularly as an indicator of soot deposition. When the gas temperature increases by about 200C above the temperature of the newly cleaned boiler, it is time to remove the soot deposits. Therefore, it is recommended to install a dial type thermometer at the base of the chimney to monitor the temperature of the flue gas exit.
Steam Boiler Pressure Reduction
This is an effective way of reducing fuel consumption, if allowed by 1-2%. Lower steam pressures provide lower saturated steam temperatures and no flue heat re-utilization, which results in a decrease in flue gas temperature. Steam is produced at pressures that match the high temperature or pressure demands for a particular process.
In some cases, the process does not operate all the time and there are periods of time during which the boiler pressure must be lowered. However, it must be remembered that a decrease in boiler pressure will decrease the specific volume of steam in the boiler and by aeration, water removal will occur.
Therefore, the energy manager should think about the possible consequences of lowering the pressure carefully, before recommending it. The pressure should be reduced gradually, and should be considered no more than 20% drop.
Variable Speed Control For Fan, Blower And Pump
Variable speed control is an important way of achieving energy savings. Generally, the control of combustion air is influenced by the damper cover valve mounted on the forced and induced draft fan.
Earlier types of dampers were simple, less precise control devices giving poor control characteristics over the upper and lower operating ranges. In general, if the load characteristics of the boiler vary, the possibility of replacing the damper with a VSD should be evaluated.
Boiler Load Control
The maximum efficiency of the boiler does not occur at full load but at about two thirds of the full material. If the load on the boiler decreases continuously then the efficiency also tends to decrease. At zero output and regardless of how much fuel is used only to supply the loss. The factors that affect boiler efficiency are:
1. When the load falls, so does the value of the mass flow rate of the exhaust gases passing through the pipes. A reduction in flow rate for the same heat transfer area reduces the flue gas exit flue temperature by a small amount, reducing sensible heat.
2. Under half the load, most combustion equipment requires more excess air to burn the fuel completely. This increases the sensible heat loss.
In general, boiler efficiency is reduced below 25% load rate and boiler operation below this level should be avoided as far as possible.
Precise Boiler Scheduling
Since optimum boiler efficiency occurs at 65-85% of full load, it is usually more efficient, overall, to operate fewer boilers at higher loads than to operate a large number of boilers at lower loads.
Boiler Replacement
Potency the 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:
1.Old and inefficient
2. Unable to replace cheaper fuel in combustion
3.The size is above or below the existing requirements
4.Not designed for ideal loading conditions.
Boiler Station Inspection
All entrances and plate joints must be kept airtight with effective gaskets. The entire chimney joint system must be effectively closed and isolated where necessary. Boiler walls and parts must be insulated effectively.
At the end of the heating period the boiler must be thoroughly closed, the inner surface exposed during the summer being covered with a sheet containing desiccant.