Pulverised coal combustion (PCC) is the most commonly used method in coal-fired power stations. Also known as pulverised coal-fired (PCF) or pulverised fuel (PF) systems.
PCC process consists of stages:
• Stockpiling, Reclaiming and Blending
• Pulverising
• Combustion
• Heat Transfer and Steam Generation
• Disposal of Waste Products
Let us look at each stage of the process.
• Stockpiling, Reclaiming and Blending
Coal is received from sources that can be imported coals or domestic coals depending on country. Domestic coals can be mine mouth operations where the power station is located adjacent to mine.
Power stations retain a minimum level of stockpiled coal to ensure continuity and security of supply.
The stockpiling process depends on plant location, coal source/s and number of types of coal fired at the plant.
Good stockpile management is important to prevent spontaneous combustion and degradation of coal.
Remember that the feed must be consistent quality. Excessive variations in quality cause problems with coal firing.
If blending coals then must be properly blended. Partial blending leads to uneven feed quality.
• Pulverising
The coal feed is pulverised to a fine powder in a stream of air, typically 70 to 75 % passing 75 micron and less than 2 % plus 300 micron.
The most common forms of pulverisers are vertical spindle (ball or roller) units and horizontal tube (ball) units.
The tube ball mills are generally more expensive, consume more power and noisier. However the horizontal mill produces a more consistent size product and requires less maintenance.
The product size from a vertical mill changes with the number of hours used due to metal wear.
The pulverised coal is conveyed to the burners pneumatically. The air that transports the coal is called primary air and usually represents 25% of total combustion air. The air is heated to dry the coal. The air temperature depends on coal moisture content, usually about 300ºC.
The Hardgrove Grindability Index (HGI) result provides an indication of the ease or difficulty of pulverising. The Abrasion Index (AI) result provides a measure of the abrasiveness of the coal.
Pulverising the coal is a significant part of the operating and maintenance costs of a power station.
• Combustion
The pulverised coal is blown with part of the combustion air into boiler through a series of burner nozzles. Secondary and tertiary air may also be added.
There is a stoichiometric amount of air required for combustion (to ensure complete combustion). This can be calculated as:
Approximate theoretical air (l/kg fuel)
= 35.8 (2.67 C + 8.00 H + 2.29 S + S –O)
Ultimate results on dry mineral matter free basis.
In practice an amount of excess air is used to ensure complete combustion.
Combustion takes place at temperatures from 1300 to 1700ºC. Particle residence time in the boiler is typically 2-5 seconds so the particles must be small enough for complete combustion to occur during this time.
Two different boiler designs are commonly used. One is the traditional two-pass layout where there is a furnace chamber, topped by heat transfer tubing. The flue gases pass through a 180º bend and downwards through the main heat transfer and economiser sections.
The other design is a tower boiler where the heat transfer sections are mounted vertically above each other, over the combustion chamber.
The geometry and dimensions of the boiler furnace are related to the combustion system and fuel to be used. The furnace cross-section must accommodate the flame shape plus burner arrangements/spacing. The flame must not impinge on the furnace walls.
The height of the furnace is designed to minimise convection pass slagging and fouling. A number of geometries are reported as being investigated but do not indicate any clear advantage over the common rectangular furnace.
The boiler atmosphere has reducing zone (mainly flame area) and oxidizing (convective areas).
Removal of bottom ash formed during combustion can be done two ways.
- Dry bottom
The ash is removed in a dry state from bottom of furnace.
- Wet bottom
The ash is removed in a molten (slag) state from bottom of furnace.
The dry bottom furnace is the most common type these days. Wet bottom furnace can handle larger size coal but the AFT temperatures must be lower so the ash remains in a molten state.
Burner technology plays a large role in the combustion efficiency and production of pollutant gases. Burners are designed to achieve stable ignition and flame conditions.
There are variations in the positioning of burners in the combustion chamber:
- Wall mounted burners on one side
- Opposed fired wall mounted burners
- Tangential burners in the corners or on the walls
Low NOx burners are used to minimise the formation of NOx gases. These burners achieve a cooler flame so combustion is longer under reducing conditions. NOx formation increases at higher temperatures and excess combustion air.
The choice of burners is based on cost, operating experience, environmental considerations and manufacturer preference.
Generally the higher the volatile matter content the more stable the combustion process. To fore low volatile coals (20% daf) special designs must be used.
• Slagging and Fouling
Slagging is the accumulation of deposits on the radiant heat exchange surfaces. Molten ash may stick to the surfaces on the water tubes and gradually build up over time.
Fouling is the accumulation of deposits on the convection heat exchange surfaces. High temperature deposits occur on the superheater tubes and low temperature deposits occur on the economiser and air heater.
Both slagging and fouling reduce heat transfer efficiency. In severe cases the boiler may have to be shut down and deposits removed from the surfaces.
The ash content, ash chemistry, chlorine, sulphur, process temperature and design affect the tendency of a coal to foul or slag.
Table on following page details some of the relationships used to estimate the potential slagging or fouling factor of a coal.
Note the relationships are estimations and predictive ability varies from coal to coal.
Recent research shows that a better prediction of coal slagging and fouling is made from looking at solid, liquid and gaseous phases of the Al-Ca-Mg-Fe-Si oxides at high and low temperatures. This is done by sophisticated testing and computer modeling.
High moisture and pyritic sulphur can affect the pulveriser performance.
• Heat Transfer and Steam Generation
The heat of combustion is used to produce superheated (supercritical) steam. Typical properties of superheated steam are 2500 psi and 550ºC.
The furnace chamber is divided into two zones.
- Combustion
- Convection
Heat transfer takes place at the combustion zone by radiation from the flame to water tubes producing saturated steam and to a radiant superheater.
In the convection zone heat is transferred from hot flue gases to superheater tubes. The temperature of the flue gases must be decreased before convective transfer. This typically below 1050ºC. This means the radiant heat transfer surface must be sufficient to achieve temperature drop.
Heat transfer also takes place by convection in the economiser (water heater) and air heater.
PCC steam turbines range in size from 50 to 1300 MWe. Most are over 300 MWe to take advantages of economies of scale.
• Boiler Efficiency
Boiler efficiency is measured as the energy produced (e.g. MWh) divided by the coal feed energy (e.g. GJ/t).
Factors are:
- Energy of feed coal
Directly related to efficiency, higher energy higher output
- Coal moisture
Higher moisture means lower efficiency
- Coal hydrogen content
Higher hydrogen means loss of heat in flue gas from higher moisture
- Boiler design
Design related to feed coal/s
Corrosion problems associated with higher temperatures.
- Excess air ratio
Decreasing excess air ratio increases efficiency
- Steam pressure and temperature
Higher steam pressure and temperature (up to 30 MPa and 600ºC) means better heat transfer. Restriction is material failure at higher temperatures. Research is looking at advanced alloy material for superheater and furnace wall surfaces.
- Heat transfer efficiency
Using thicker materials to improve strength and wear at higher temperature means loss in heat transfer efficiency. Stronger and thinner alloys hope to resolve this problem.