Energy Efficiency and Clean Coal

Technology

Overview

Introduction on Efficiency Key Areas in a Coal-Fired Power Plant for

Improving Efficiency Flow Diagram of a Thermal Power Plant Thermal Areas Auxiliaries Other Areas Within the System Clean Coal Technology Integrated Gasification Combined Cycle Pulverized Coal Technology Conclusion

Introduction on Efficiency

The energy efficiency of a thermal power station, considered as salable energy(in MW) produced at the plant as a percent of the heating value of the fuel consumed, is typically 33% to 48% efficient.

Efficiency losses are essentially the result of heat and mechanical friction losses within the system.

Flow Diagram of a Thermal Power Plant

Key Areas in a Coal-Fired Power Plant for Improving Efficiency

A)Thermal Areas: Boilers Turbines Condensers Economizers Air Pre-Heaters

Key Areas in a Coal-Fired Power Plant for Improving Efficiency

B) Auxiliaries: Soot-Blowers Boiler Feed and Circulating Water

Pumps Boiler and Furnace Fans

Key Areas in a Coal-Fired Power Plant for Improving Efficiency

C) Other Areas Within the System: Combustion Coal Quality Deviation w.r.t. Design Insulation to Reduce Heat Losses Feed Water Impurities and Corrosion

Thermal Areas

Boilers

To secure high thermal efficiencies, boilers are operated with the use of high pressure steam cycles, high temperatures and high feed water temperatures.

To ensure a high feed water temperature to the boiler, a series of HP heaters can be used.

Steam that has been bled from the main turbine after it has done some useful work can be used to pre-heat the boiler feed water. This is known as Regenerative Heating and it greatly increases the combined overall thermal efficiency of the boiler.

To provide for variations in fuel quality, an automatic control system is important in obtaining the final steam temperature.

Thermal Areas

Turbines

To maximize efficiencies, the heat rate can be improved by control:

Main steam pressure Main steam temperature Reheat steam temperature

Cylinder Efficiency can be improved with regular inspections of HP/IP/LP Turbines to prevent nozzle block and turbine blade erosion.

Thermal Areas

Condensers

It condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and the efficiency of the cycle increases.

For best efficiency , the temperature in the condenser must be kept as low as possible in order to achieve the lowest pressure in the condensing steam. Since the condenser temperature can almost always be kept below 100⁰C, where the vapour pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum.

Thermal Areas

Economizers

The economizer is a heat recovery appliance which is placed in the path of the flue gases to abstract heat that would otherwise be rejected to the chimney.

It can be considered that for a fixed evaporation the economizer provides a portion of the total heat added to the feed water to convert it into steam; since this heat is obtained at virtually no cost it effects an appreciable saving in fuel.

Thermal Areas

Air Pre-Heaters

The air pre-heaters also extracts heat from the flue gases but transfers that heat to the air fed into the furnace.

The effects claimed from air pre-heating are as follows:

The recovery of heat from the flue gases reduces the heat loss from this source.

The flame temperature in the combustion space is raised with greater rate of heat transfer by radiation, thus increasing the effective heat transfer and increasing the amount of steam made per boiler.

Lower grades of fuel can be burned successfully with pre-heated air, that would not be effectively burned with cold air.

Auxiliaries

Soot-Blowers

Soot-blowers are used to keep the boiler surfaces free from deposits of dust and grit which would otherwise reduce the rate of heat transmission in the boiler, lowering the boiler efficiency.

It has been well established that regular soot-blowing is essential if a high thermal efficiency is to be maintained in a water-tube boiler over any appreciable length of time.

Boiler Feed and Circulation Water Pumps

Pump efficiency can be improved by applying a polymer coating on the pump internals.

For energy savings, it is more economical to install electrically driven pumps than turbine driven pumps at power stations where electricity is generated in very efficient sets.

Auxiliaries

Boiler and Furnace Fans

The use of a fan draught allows the maximum amount of heat to be abstracted from the products of combustion before they are discharged into the chimney.

It makes possible enhanced rates of combustion of low grade fuels together with higher furnace temperatures and higher rates of heat transfer.

Furnace fans assists in the maintenance of correct air-fuel ratio. To ensure they are operating efficiently, regular lubrication and cleaning is necessary.

The pressure and volume can be controlled by varying the fan speed which can be effected by using a variable speed driver. Variable speed drivers are most economical to run.

Other Areas Within the SystemCombustion

Theoretically, perfect combustion is attained when fuel is burned with the exact quantity of air required.

But in practice, complete combustion is not achieved unless a substantial excess of air is provided.

The fuel and air must be brought into contact under the right conditions, namely:

An adequate proportion of air at the right places throughout the fuel;

Adequate time and turbulence must be arranged at a temperature sufficiently high to ensure that the chemical reactions involved are completed within the available combustion space.(These 3 factors are referred to as the ‘the three Ts’.)

Other Areas Within the SystemCoal Quality Deviation w.r.t. Design

Boiler design is highly dependent on the coal characteristics.

Performance of the boiler and ultimately performance of the entire unit can change if the coal being used has a different heating value from the desired design heating value.

It starts from the combustion calculation. The C-H ratio decides the heating value of the coal.

This decides the quantity of air for combustion and the flame temperature.

This affects the furnace behavior.

Other Areas Within the SystemCoal Quality Deviation w.r.t. Design

A higher heating value leads to a higher temperature• Equipment failure• Decrease in Metal Strength • Increase Nox emissions

A lower heating value leads to a lower temperature • Incomplete combustion• Wasted fuel• Unburned hydrocarbons • Increase soot emissions

Therefore its essential that the coal quality complies with the furnace design for optimum efficiency.

Other Areas Within the SystemInsulation to reduce Heat Losses

To lower heat losses it is preferable to insulate furnaces and foundations particularly around flues.

The furnace should be separated from the foundations by an air gap otherwise the soakage of heat from large furnaces into the foundations may result in concrete cracking.

A composite wall of firebrick and insulating brick may be used for insulation for the furnace.

Other Areas Within the SystemInsulation to reduce heat losses

The lagging of pipes, chimneys, boilers and all hot surfaces can greatly reduce heat loss within the system.

The magnitude of losses that can take place from steam pipes is explained by an example:

If a 1 inch steam pipe, having a surface temperature of 200⁰C is unlagged for a length of 200ft, it may waste 35 tons of coal per year.

As the temperature the surface rises, so the heat loss increases; consequently the efficiency of the insulation must be stepped up correspondingly.

Therefore even if a pipe is lagged, it cannot be assumed to be efficiently lagged.

Other Areas Within the SystemFeed water impurities and corrosion

The purpose of the boiler plant is to make steam that will not cause corrosion or deposits in the superheaters, turbines or process plant.

Therefore the water used in the boiler should be treated to remove impurities.

At boiler temperatures, water reacts with steel to form magnetite.

Other Areas Within the SystemFeed water impurities and corrosion

However in boilers, it seems that the reaction is brought to a stop because the magnetite forms a layer on the metal surface.

Dissolved oxygen would tend to break down this layer and promote corrosion.

The use of caustic soda and sodium phosphate should assist in repair at points of breakdown.

Corrosion is also caused by CO2 in steam. Oxygen stimulates this attack and may localise to form deep pits. To solve this problem, the feed should be deoxygenated, and bicarbonates should be reduced.

Clean Coal Technology

Clean coal technologies which have positive effects on energy efficiency:

Integrated Gasification Combined Cycle

Pulverized Coal Technology

Integrated Gasification Combined Cycle (IGCC)

Integrated gasification combined cycle (IGCC) power generation is a technology in which coal is gasified with either oxygen or air, and the resulting synthesis gas (or syngas, consisting of hydrogen and carbon monoxide), is cooled, cleaned and fired in a gas turbine.

The hot exhaust from the gas turbine passes through a heat recovery steam generator (HRSG) where it produces steam that drives a steam turbine, therefore power is produced from both the gas and steam turbine-generators.

IGCC has the potential to improve coal’s fuel efficiency rate to 50 percent:

As it avoids the need for heat exchange before the turbine, thereby reducing capital costs and heat exchange losses.

A CO2 fed gasifier preserves coal heating values and the latent heat losses related to steam fed gasifiers are avoided.

IGCC Process

Pulverized Coal Technology

Pulverized coal technology is the most widely used coal-firing option worldwide. The steam conditions (pressure and temperature) at the inlet of the steam turbine define different types of pulverized coal plants and determines the plant efficiency:

A subcritical pulverized coal plant has a steam outlet pressure below 22.1 MPa. The typical steam outlet temperature is around538⁰C and produces a net plant efficiency between 35-38 % .

A supercritical pulverized coal plant has a steam outlet pressure above 22.1 MPa. Typically, the pressure is around 24.7 MPa and has a steam outlet temperature of 565⁰C, resulting in a net plant efficiency between 38-40 %

An ultra-supercritical pulverized coal (USC) plant has a steam outlet pressure, typically around 27 MPa, with a maximum steam outlet temperature of 625⁰C . The net plant efficiency is between 40.0-42.5%.

Ultra Supercritical Process

Conclusion

Energy efficiency offers significant potential for reduction of fossil fuel consumption and costs, apart from the environmental benefits and climate change impacts.

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