Efficiency Of Steam Boiler Systems

PART 1

A Steam Boiler generates steam which is used to power steam turbines and for usage in various process applications. While the cost of a steam boiler is quite high, depending on the fuel type, the running costs of the boiler can also be quite high. To make the overall boiler operations cost effective, it is important that the system achieves the desired efficiency. With passage of time, due to wear and tear of equipment and due to varied quality of fuel used, the efficiency of steam boiler may decrease. This can have adverse effect on safety and profitability of operations.

What Is Efficiency Of A Steam Boiler?

The boiler efficiency is the ratio of the energy output of the boiler to the energy supplied to the boiler.

Boiler efficiency = Energy output /Energy input x 100

A boiler has many components that can affect the overall boiler efficiency. The efficiency of the boiler is classified as:-

  • Combustion efficiency
  • Thermal Efficiency

Why The Efficiency Of Boilers Is Important?

The steam boiler efficiency (fuel-to-steam efficiency or fuel-to-water efficiency) impacts your business costs. The payback period of the boiler will be governed by the fuel to steam cost and maintenance of the boiler.

  1. Calculating the efficiency of a steam boiler will help businesses to predict fuel costs and manage operations accordingly.
  2. Businesses can also use efficiency calculation to determine the viability of purchasing new boiler vs. additional maintenance costs or modification costs.
  3. Fuel efficiency calculations will help businesses to determine the benefits of fuel conversions, which could be very beneficial for businesses in India.

Combustion Efficiency Of A Steam Boiler

The combustion efficiency of a steam boiler is the measure of how effectively the heat content of the fuel can produce useable heat. The factors determining the efficiency is the amount of unburnt fuels left in the fluegas and the quantity of excess oxygen in the stack gases. Increasing the excess air will decrease the quantity of unburnt fuels in the fluegas but increases enthalpy losses of the boiler. Boiler operators struggle to balance the quantity of unburnt fuel in the fluegas vs. the enthalpy losses of the boiler. Fuel characteristics and GCV also affects combustion efficiency.

Thermal Efficiency Of A Steam Boiler

The thermal efficiency of the boiler measures the efficiency of the heat exchanged in the boiler. Thermal efficiency measures the heat transfer from the combustion process to the water or steam. The effectiveness of heat transfer devices is measured and the thermal efficiency does not calculate radiation losses and convection losses of the boiler components. Maintenance of your boiler can affect the thermal efficiency. Scale formation and soot formation on the heat transfer components lower the thermal efficiency.

To calculate thermal efficiency of a steam boiler, the quantity of steam produced and fuel consumption is measured at hourly intervals.

Boiler efficiency η = Heat output/heat input x 100

Boiler efficiency η = heat in steam output/ heat fuel input x 100

          = Quantity of Steam x (Enthalpy of steam – Enthalpy of feed water) X100

              Quantity of fuel x GCV

The direct efficiency test is easy to conduct and can be done at the plant. The test requires a few measuring devices. The test method does not let plant operators know why efficiency is low and the method does not account for different losses.

The indirect efficiency is measured by adding the losses that occur in the boiler. This method is more cumbersome than direct efficiency but is more accurate.

The fuel quality is important factor in indirect efficiency. The losses measured are:

  • L1 – Loss due to dry flue gas (Sensible heat of the flue gas)
    Loss due to dry flue gas is a major loss in indirect efficiency. If the flue gas temperature leaving the boiler is high it can reduce efficiency.
  • L2 – Loss due to hydrogen –formed water (moisture in fuel)
    The high hydrogen content of the fuel reduces the efficiency of the boiler.
  • L3- Loss due to evaporation of fuel moisture
    If the fuel has high moisture content the ignition heat will be required to evaporate moisture and then burn the fuel.
  • L4 – Loss from moisture in the air
    The humidity in the air is superheated and is lost in the flue gas and is included in heat loss.
  • L5- Loss from unburnt combustion.
    Products left after incomplete combustion could be burnt again to release energy. Carbon monoxide levels help to determine unburnt levels.
  • L6- Radiation Losses
    Radiation loss is the heat lost through boiler casing surfaces, ducts, flues in the boiler. Radiation loss is a function of the temperature difference between the casing and the ambient temperature. The ABMA chart for radiation is referred to learn the approximate value.
  • L7- Unaccounted Losses
    Unaccounted losses are minor losses are based on factors such as manufacturer safety factor and unexpected performance losses.

Total Losses L = L1+L2+L3+L4+L5+L6+L7

Efficiency η = 100 – L

L1, L2, L3, L4, and L5 are heat losses based on flue gases.

Indirect Efficiency calculations require instrumentation to measure flow rate, temperature, and pressure and the water condition. Flue gas is composed of carbon dioxide, moisture, oxygen, and nitrogen. These readings can help calculate the quantity of wet and dry flue gas. We determine excess air after measuring carbon dioxide and oxygen in the flue gas.

Indirect efficiency calculations do not account for standby losses, blowdown losses, soot blower steam, and auxiliary equipment losses. But the method is considered more accurate than the direct efficiency method.

Seasonal Efficiency

When seasons change, the ambient operating conditions such as air temperature, humidity, and density also changes. This can marginally affect the efficiency of the boiler. Boiler operators may need to tune the boiler when there are major seasonal differences.

Factors affecting boiler efficiency

Boiler efficiency depends on the following factors:-

  • Excess Air – Combustion of any fuel requires air is greater than the theoretical requirement to burn the fuel. Excess air increases the amount of oxygen required for the combustion. Low levels of oxygen can result in the formation of carbon monoxide and NOx. The excess air absorbs the heat of combustion and lowers efficiency levels. Boiler operators manage the excess air levels to optimize operations.
  • Flue Gas Temperature – Flue gas exit temperature is also called stack temperature is the temperature measured when the flue gas exits the boiler. A high flue gas temperature may suggest that effective heat transfer is not taking place. Utilizing the high flue gas temperature involves installing additional heat exchange equipment.
  • Convection and radiation Losses are unavoidable. – The losses can be minimized by providing refractory insulation and optimizing the boiler design.
  • Fuel Specification – The fuel specification has a massive effect on boiler efficiency. Fuels with high GCV, high volatile matter, low ash content, and low moisture content are considered ideal.
  • Ambient Temperature- The average temperature of combustion air entering a boiler is called ambient temperature. This air is impelled into the boiler by the forced draft fan or FD Fan. Ambient temperature can affect the net stack temperature (flue gas exit temperature – ambient temperature). It may be tempting to reduce ambient temperature to reduce the stack temperature. Low ambient temperature will reduce efficiency. In India ambient temperature is taken at 35 to 40 C.
  • Turndown – Turndown ratio is the ratio of minimum and maximum output. A boiler with a high turndown ratio shows it can supply steam at lower rates without shutting down or restarting. A high turndown will help the boiler run more efficiently. The ideal turndown ratio for the boiler is 4:1.

Improving Steam Boiler Efficiency

Boiler Drum

A steam boiler can be a bi-drum boiler (with a steam drum and water drum) or a single drum (only steam drum). The steam drum is the heart of the boiler. The drum acts as a phase separator i.e. separating the steam and water. In a bi-drum boiler, the balance water is sent back to the water drum for recirculation. The drum can be affected by scaling, reducing the quality and quantity of steam. The steam separator equipment is more vulnerable to water quality. Boiler manufacturer’s manuals stress the importance of water quality for smooth boiler operation and improved efficiency.

Superheater

The superheater is a heat exchanger that converts saturated or wet steam into superheated or dry steam. The superheater helps to provide dry steam for process and power generation. It helps to improve the efficiency of steam power plants. It helps minimize erosion of turbine blades and eliminates entrained water particles from turbine steam.

Radiant and convection superheater are the two types of superheaters. The superheater is a critical high pressure and high-temperature equipment and the system design engineer needs to account for the distribution difference between the flue gas and steam. They need to provide adequate side spacing for ash cleaning.

The steam temperature in the superheater is affected by factors such as heating surface area, the ratio of radiating to convection absorbing surface. Variables such as excess air, feedwater temperature and quality, fuel characteristics, and ash deposits on the heating surface affect the efficiency of the superheater.

Sootblower is periodically used to remove soot build-up from the superheater tubes. The attemperator between the primary and secondary superheater helps to regulate the temperature of the steam, ensuring safety and improving efficiency.

Furnace Design

The furnace is a large volume surrounded by water-cooled walls for combustion. The shape and volume of the furnace depends on the boiler specification and fuel being burnt. While designing the furnace the clearance of the fuel firing system from furnace walls is important to ensure complete burning. The designer must prevent the flame from interacting with the fuel stream. Boiler furnaces can have a membrane wall to reduce maintenance and lower flue gas temperature. Factors such as the furnace area and furnace height affect efficiency of a steam boiler system.

Economizer

The economizer is a heat exchanger that uses the flue gas as it leaves the boiler to heat the water entering the boiler. The economizer helps in preventing oxygen scavenging and protecting the boiler from the effect of cold water. As the economizer helps to lower the exit flue gas temperature, it improves the steam boiler efficiency, and helps in saving fuel.

Air Preheater

The air preheater is a heat exchanger usually placed after the economizer. The air preheater uses the heat of flue gas to heat the air entering the furnace. The air preheater ensures that hot air enters the furnace, thereby increasing the temperature of the furnace. It also helps to dry the fuel. The air preheater also helps in reducing the flue gas temperature leaving the boiler thus increasing the efficiency of the boiler.

Variable frequency drive

Variable frequency drives are installed in motors such as burners, feeders, draft equipment (ID Fan, SA Fan, and FD Fan), and pumps. There is a massive power saving by controlling the speed of the motors.

Soot blowers

There is a gradual build-up of soot in the fireside of the furnace tube, superheater, boiler bank/evaporator, and economizer. The soot acts like an insulator and brings down the heat transfer rate, thereby increasing the fuel costs. The soot blower prevents the accumulation of soot on the tubes thereby improving the heat transfer efficiency.

Insulation and Refractory

Large boilers have an enormous surface area which is vulnerable to heat losses. Insulation and Refractory help to reduce convection and radiation losses. Insulation protects the boiler from heat losses to the surrounding and cold face temperatures. Insulating valves can help reduce heat losses further.

Deaerator

The deaerator is used to remove oxygen from the feed water before it enters the boiler. The deaerator prevents oxygen pitting and increases the temperature of the water.

Return condensate to the boiler

The condensate forms when steam losses heat. This water should not be wasted. The treated water can be reused easily in the boiler. The hot water requires less fuel to heat to generate steam. Reusing condensate also helps to reduce the quantity of markup water, water treatment costs, and sewer costs.

Boiler Controls

Boiler controls help to reduce operational costs. The control system is used to optimize boiler operations and improve efficiency. Advanced control systems can accommodate boiler operations to adjust for varying boiler load and ambient temperature. These minor operational changes will save energy and fuel.

Boiler maintenance

Periodic maintenance of the boiler will help improve efficiency, reduce operating costs, increase the life of the boiler. During the inspection damaged tubes can be replaced, scaling, and soot can be removed from the tubes that can help decrease heat transfer loss. Along with daily, periodic, and annual maintenance, the operator can use a boiler checklist will help optimize operation.

Residue in form of ash, clinkers

The firing equipment burns the fuel to generate the required heat to burn the fuel. The firing equipment chosen depends on fuel properties and it needs to be checked frequently for clinker formation. Clinker formation reduces the efficiency of the boiler. We need to remove the clinker from the boiler for efficient operations. Clinkering is more common in the high-temperature zone of the boiler. Clinkering is caused by:-

  • Poor fuel quality and fuel sizing,
  • Poorly maintained and operated firing equipment.
  • Poor fuel to air ratio can cause incomplete combustion.

Large clinker pieces can prevent air from passing through the grate, reducing efficiency, and increasing maintenance costs.

Clinkering can be reduced by using good quality fuel. The fuel quantity needs to be monitored. In coal-fired boiler, water is used to flush molten slag and ash from the hopper to prevent deposit.

Slagging and fouling refer to ash deposits in the convection zone. It is caused because of secondary combustion in the upper furnace zone. It is formed when the furnace temperature is very high or the furnace is undersized. An undersized furnace results in incomplete combustion. Incorrect tube pitching can also increase fouling.

Slagging reduces the effectiveness of heat transfer. Slagging can be reduced by using a soot blower. The fuel/air ratio needs to be optimized. We need to uniformly distribute the secondary air over the firing equipment.

Fuel feeding system

The fuel feeding rate needs to be calibrated with the burning rate of the fuel. An incorrect calibrated fuel feeding system can increase slagging and clinkering in the boiler.

Chimney parameters

The stack exists through the boiler system through the chimney. The incorrectly sized chimney can cause the flue gas to backfire. This can cause accidental fires.

Regular maintenance ensures that the steam boiler performs at desired efficiency levels. A business should also consider modification or retrofitting of existing steam boiler to improve its efficiency.

We at Mago Thermal, provides technical solutions for increasing a steam boiler efficiency. Please feel free to contact us for no obligation technical analysis of your system.