PCC is the most commonly used method in coal-fired power plants, and is based on many decades of experience. Units operate at close to atmospheric pressure, simplifying the passage of materials through the plant.
The principal developments involve:
- Increasing plant thermal efficiencies by raising the steam pressure and temperature used at the boiler outlet/steam turbine inlet;
- ensuring that units can load follow satisfactorily; and,
- ensuring that flue gas cleaning units can meet emissions limits and environmental requirements.
The coal is ground (pulverised) to a fine powder, so that less than 2% is +300 µm and 70-75% is below 75 µm, for a bituminous coal. The pulverised coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added.
Combustion takes place at temperatures from 1300-1700°C, depending largely on coal rank. Steam is generated, driving a steam generator and turbine. Particle residence time in the boiler is typically 2-5 seconds, and the particles must be small enough for complete burnout to have taken place during this time.
The technology is well developed, and there are thousands of units around the world, accounting for well over 90% of coal-fired capacity. PCC can be used to fire a wide variety of coals, although it is not always appropriate for those with a high ash content.
Two broadly different boiler designs are used. One is the traditional two-pass layout where there is a furnace chamber, topped by some heat transfer tubing to reduce the FEGT. The flue gases then turn through 180°, and pass downwards through the main heat transfer and economiser sections. The other design is to use a tower boiler, where virtually all the heat transfer sections are mounted vertically above each other, over the combustion chamber.
The relative advantages and disadvantages almost balance each other out. Tower designs have been favoured recently in Europe. They result in taller structures, and this is one reason why they are not used in Japan, which is in an earthquake zone. It is thought to be preferable to reduce the height of structures there wherever possible.
There are variations in the positioning of the burners in the combustion chamber, and designs are offered which use:
- wall-mounted burners on one side,
- opposed-fired wall mounted burners, or
- tangential burners in the corners or on the walls. Some corner burners can be the furnace up or down.
There seem to be few clear cut advantages or disadvantages with the different arrangements, and the choice is based on cost factors, operating experience, environmental considerations and the experience of the various boiler manufacturers.
Boilers with cyclone burners are discussed separately, as a coarser coal feed is used.
Most PCC boilers operate with what is called a dry bottom. Combustion temperatures (with bituminous coal) are held at 1500-1700°C. With lower rank coals the range is 1300-1600°C. Most of the ash passes out with the flue gases as fine solid particles to be collected in ESPs or fabric filters before the stack.
Boilers which use anthracite as the fuel commonly use the downshot burner arrangement to achieve longer residence times and ensure carbon burn-out. Downshot burners send the coal-air mixture down into the cone at the base of the boiler.
Another arrangement used in some boilers is the so-called cell burner. This involves a wall-fired unit where either two or three circular burners are combined into a single vertically orientated assembly that results in a compact intense flame. This would generally not be used in new units, as the higher temperature flame results in more NOx formation which then has to be removed later in the system.
PCC boilers have been built to match steam turbines which have outputs between 50 and 1300 MWe. In order to take advantage of the economies of scale, most new units are rated at over 300 MWe, but there are relatively few really large ones with outputs from a single boiler/turbine combination of over 700 MWe. This is because of the substantial effects such units have on the distribution system if they should ‘trip out’ for any reason, or be unexpectedly shut down.
One of the driving forces which is currently encouraging the use of more efficient power plant is the environmental concern in many countries, and the declared goal of most OECD governments to reduce CO2 emissions to 1990 levels. This is a goal which leaves power generators with many unsolved problems, but increasing the thermal efficiency of converting coal to power is one of the less expensive ways of reducing CO2 emissions. It does, however, involve the construction of new boilers and turbines, as the costs for retrofitting a supercritical steam system to an existing subcritical boiler would be prohibitive.
Increasing thermal efficiency has the potential for reducing other emissions per MWe generated, such as those of SO2 and NOx. Where the coal cost is high, as where traded coals are used, increasing thermal efficiency can result in reduced overall costs in new plants for power generation, as less fuel is needed.
The overall thermal efficiency of some older, smaller units burning, possibly, poor quality coals can be as low as 30%. A commonly used assumption for the average efficiency of larger existing plants with subcritical steam burning somewhat higher quality coals is that it is in the region of 35-36%. New plants, however, with supercritical steam can now achieve overall thermal efficiencies in the 43-45% range.
Various measures can be used to increase the thermal efficiency relative to current design practice, in particular:
- reducing the excess air ratio from 25% to 15% can bring a small increase;
- reducing the stack gas exit temperature by 10°C (while recovering the heat involved) can bring about a similar increase;
- increasing the steam pressure and temperature from 25 MPa/540°C to 30 MPa/600°C can increase efficiency by nearly 2 percentage points;
- using a second reheat stage can add another 1 percentage point;
- decreasing the condenser pressure from 0.0065 MPa to 0.003 MPa can further increase efficiency.
As with all technical options, there is a trade-off between the costs involved (both capital and operating), the risk element in the decision and the amount of additional energy recovered.
Many of the methods for increasing thermal efficiency have been well known for several decades. In some cases they were tried back in the 1950s and 60s, but were abandoned either because of the lack of suitable construction materials or the low energy prices prevailing. This removed much of the incentive for seeking high thermal efficiencies. Small base-load power plants using steam at 35 MPa and 650°C were built in the 1950s. Regenerative preheating of the feed water was introduced as long ago as the 1920s. Steam reheat was introduced in the 1950s and double reheat in the 60s. The more costly options tended to be discounted when oil was cheap, and subsequently as nuclear energy took over base load power generation in many places.
An increase in the steam pressure and temperature involves the use of austenitic material in various parts of the system. Using thin walled austenitic steels for superheater and reheater tubes means that operational flexibility can be largely maintained. In some older plants, thick walled tubes and junctions have been used which means increased start-up times and hence increased start-up losses.
Controlling the excess air is an important function in boiler operation, but requires a careful balance between conflicting requirements. Boilers are normally operated at the minimum practicable excess air amount, but sufficient air is required to burn virtually all the carbon present (99%+), and modern design and practice is to control and stage the addition of air in order to minimise the formation of NOx (air staging).
The maximum efficiencies achievable with lower grade and lower rank coals will be somewhat less in all cases. The maximum efficiencies expected in the brown coal fired plants currently under construction in Germany are around 42% compared with 45% for equivalent new bituminous coal fired units. Net efficiencies of 45-47% are achievable with supercritical steam using bituminous coals and currently developed materials.
New high temperature alloys are under development with the aim of facilitating steam temperatures as high as 700°C. This could make net efficiencies of 50% achievable with PCC. A considerable amount of work on this remains to be done.
Flue gas cleaning/emissions
The various technologies are discussed in separate sections, under particulates control, NOx reduction by primary measures or flue gas treatment, and FGD. Emissions from new PCC units with appropriate flue gas cleaning units can meet all current requirements reliably and economically, and using well-proven technology. The necessary emission control measures can be taken with a relatively small effect on overall thermal efficiency, although the capital cost of these measures can represent about one third of the cost of the unit when meeting the most stringent current standards.
The solid residues consist of 80-90% of fine fly ash with a low level of carbon-in-ash, averaging around 0.5%.