Economiser in Thermal Power Plant | Economiser

Fuel-fired power plants convert only part of the fuel’s energy into electricity. In a coal-fired thermal power plant, a substantial share leaves with condenser heat and hot flue gas. Recovering useful flue-gas heat can reduce the fuel required for electrical power generation.

For a given electrical output, improving boiler heat recovery lowers fuel use, operating cost and combustion-related emissions from power generation.

Plant efficiency depends on steam conditions, turbine and generator performance, condenser conditions, auxiliary loads, fuel properties and equipment design. It cannot be assigned a single range based only on plant capacity.

An economiser recovers part of the flue-gas energy that would otherwise leave through the stack. The following sections explain how an economizer works and what limits its heat recovery.

What is an Economizer?

An economiser is a heat exchanger that reduces energy use by preheating a process fluid. In a steam boiler, hot flue gas transfers heat to incoming feedwater before the gas leaves the stack. Flue gas from a fuel-fired thermal power plant contains mainly nitrogen, carbon dioxide, water vapour and excess oxygen, with smaller amounts of pollutants and particulate matter that depend on the fuel and controls used at the power plants.

The economiser in thermal power plants reduces the heat that must be supplied to feedwater during electrical power generation. The preheated water then enters the boiler, where it is evaporated and may later become super-heated steam. An air preheater can recover additional flue-gas heat by warming combustion air, but its placement and duty are coordinated with the economiser, emissions controls and stack-temperature limit.

Working Principle of Economizer

Hot flue gas leaving the steam boiler furnace flows across the economiser heat-transfer surface. Feedwater flows through the tubes, commonly in counterflow to the gas, and gains sensible heat before entering the evaporating section. Fins or gills may increase the gas-side surface area where the design must compensate for the gas’s lower heat-transfer coefficient.

Economisers in thermal power plant are sized from flue-gas flow and temperature, feedwater conditions, required heat recovery, fuel composition, allowable gas-side and water-side pressure drop, fouling, corrosion risk and available heat-transfer area.

In a condensing steam cycle, superheated steam expands through the turbine and then enters the steam condenser of turbine. The condensate returns through feedwater heaters and the economiser before re-entering the boiler. The economiser is the final feedwater-heating surface in the lower-temperature flue-gas path.

The economiser is installed in the flue-gas path downstream of hotter boiler surfaces and upstream of the stack or later heat-recovery equipment. A typical water-tube unit uses many small-diameter tubes connected between headers, with flue gas flowing outside the tubes. Counterflow improves the available temperature difference.

Process of Heat Transfer in Economizer, Evaporator and Superheater

Heat transfer to water in a steam generator occurs in three regimes. In the diagram, pressurised liquid water is heated sensibly in the economizer from state 4 to state 5. Its outlet may be subcooled or close to saturation, depending on boiler design and operating condition.
The evaporator then supplies the latent heat that changes the working fluid from liquid to saturated vapour between states 5 and 6.

The superheater raises the temperature of the saturated vapour from state 6 to state 1. For unit mass of working fluid, the heat added in the three heat exchangers equals the corresponding enthalpy rise:
Q_Economizer = h₅ – h₄
Q_Evaporator = h₆ – h₅
Q_Superheater = h₁ – h₆
All three surfaces receive energy from the combustion-gas path. The economiser improves overall heat recovery by using lower-temperature gas after the hotter evaporator and superheater duties.

Types of Economizer

CI Gilled Tube Economizer

Gilled tube economizers use cast-iron elements with integral or mechanically attached extended surfaces. Their characteristics include:

1. The gills increase gas-side surface area and must maintain good thermal contact with the tube.
2. Cast iron can tolerate some corrosive and fouling service, but material selection still depends on flue-gas composition, temperature and cleaning method.

Round Gilled Tube Economizer

A round-gilled unit uses carbon-steel tubes with welded round or square fins. Its main design requirement is:

1. A sound fin-to-tube joint that provides a reliable heat path and withstands thermal cycling.

Coiled Tube Type Economizer

Coiled-tube units are used in some thermal power plants and industrial boilers. They may use seamless carbon-steel tube and offer these features:

1. A long tube path can provide substantial heat-transfer area.
2. The coil can fit a large surface area into a compact enclosure.

Horizontal Finned Tube Economizer

A horizontal finned-tube economiser uses carbon-steel tubes with welded fins assembled across the gas path. Its main features are:

1. The welded fin joint provides a continuous heat path from the fin to the tube wall.
2. The arrangement is widely used where horizontal tube banks suit the boiler and soot-cleaning layout.

Non-Condensing and Condensing Economizers

The main thermal distinction among types of economizers is whether the unit keeps flue gas above its dew point or deliberately condenses water vapour. The selection depends on fuel, return-water temperature, corrosion-resistant materials, condensate treatment and the available use for low-temperature heat.

Non-Condensing Economizer

A non-condensing economizer keeps the heat-transfer surface and outlet gas above the applicable dew-point limit. Finned coils in the boiler-exit duct transfer sensible heat from flue gas to feedwater. A design minimum near 250^oF (120^oC) may be appropriate for some fuels and surfaces, but it is not universal. The required temperature depends on fuel sulphur, excess air, moisture, surface temperature and material.

Cooling flue gas further can recover more heat, but it may also cool metal surfaces below the acid dew point. Sulphur content varies widely among coals and other fuels. When sulphur oxides and water vapour are present, corrosive acid can condense on vulnerable surfaces below the relevant dew point, shown here as 250^oF for the example condition.

Below 250^oF in the example condition, vulnerable surfaces may face unacceptable acid condensation. A non-condensing economizer therefore controls feedwater inlet temperature, heat-transfer area or bypass flow so protected surfaces remain above the design minimum of 250^oF. The efficiency gain cannot be stated as one fixed percentage; it depends on inlet and outlet temperatures, mass flow, excess air, fouling and the boiler’s initial stack loss.

Condensing Economizer

Condensing economizers cool flue gas below its water-vapour dew point and recover both sensible heat and part of the latent heat of condensation. For natural-gas combustion products, the dew point is often about 135^oF (57^oC), although it changes with excess air and moisture. A sufficiently cool heat sink, such as make-up water, is needed. Useful recovery should be calculated for the actual operating conditions.

A condensing unit needs corrosion-resistant heat-transfer surfaces, a compatible stack or liner, drainage and treatment for acidic condensate. Natural gas is a common application because its flue gas has little sulphur, but contaminant-free gas is not an absolute requirement when the system is designed for the expected condensate chemistry. Reported efficiency improvements depend on whether efficiency uses the fuel’s higher or lower heating value.

Applications of Economizer

Boiler economisers are common where flue-gas temperature, feedwater demand and operating hours justify the heat exchanger. They can reduce fuel use for a given steam output and may increase available steam production when other boiler limits allow.

The word economiser also describes related energy-saving arrangements. Common applications of economizer include:

• In steam power plants, a feedwater economiser transfers heat from boiler flue gas to incoming feedwater.
• In HVAC systems, an air-side economiser uses suitable outdoor air to reduce or eliminate mechanical cooling.
• Refrigeration: a refrigerant-side economiser can use vapour injection, liquid subcooling or a separate pressure stage to increase capacity or improve cycle performance under suitable operating conditions.

Advantages and Benefits of Economizer

The advantages of an economizer include:

1. It recovers useful flue-gas heat at a temperature suited to feedwater heating, complementing rather than automatically replacing an air preheater.
2. It can reduce fuel use and combustion-related emissions per unit of steam from suitable power plants.
3. Where a downstream process requires cooler gas, useful heat recovery may reduce separate quenching or cooling duty. The effect on water use depends on the emissions-control design.
4. Feedwater heating from flue gas can reduce the need for another heating source, but the net benefit must include pump and fan power, pressure drop, fouling, cleaning and maintenance.