Boiler

Figure 1. Thermal power is supplied by a fuel to a boiler.

Boilers are used in power plants in order to produce high pressured steam, so that the plant can generate electricity. The process that does this is known as the Rankine cycle. The boiler takes in energy from some form of fuel such as coal, natural gas, or nuclear fuel to heat water into steam. All but a small fraction of the world's primary energy comes from fuels, and about three-quarters of that fuel ends up going into a boiler (the remainder goes into internal combustion engines, which use the fuel differently).

The design of a boiler is an incredibly important factor in the power plant's efficiency. Three centuries of development have led to the steam-producing boilers today, which produce thousands of tonnes of steam per hour, and have a fuel-to-steam efficiency as high as 90%. Better design means a lower fuel requirement, lower costs and lower emissions of pollutants. The study and innovation of boilers is useful because although they are very efficient, their waste products create some of the world's major pollution problems by emitting greenhouse gases.

Design considerations

The main goal when designing a boiler is to extract as much of the energy from the fuel as possible. To do so, both the fixed carbon and volatile matter must be burned completely. Since one part is solid and the other gas, this task is not easy. The boiler must be at very high temperatures, anywhere near 500^oC, and must burn the fuel continuously at a constant rate.

Another factor in optimizing design is to get the best possible heat transfer from the fuel to the water and steam. Boilers will often have several separate heat exchangers to do so.

A final important design consideration is minimizing undesirable by-products in the form of ash and flue gas which pollute the environment.

Types

Solid fuel

• Grate boiler- Coal or other solid fuels like biomass about a few millimeters in diameter are fed into the boiler from a hopper or conveyor belt. They move across a grate, which has air flowing from underneath into the boiler. The fixed carbon burns on the grate, and the volatile gas burns in the space above. These types are most commonly used for biomass and coal.

• Pulverized fuel boiler- Coal is pulverized into a fine dust (about 0.1 mm in size), and is fed into the boiler in a stream of air. They are by far the most common boiler types for coal. These boilers can reach efficiencies over 90% when they are well run. They can be co-fired with wood or other suitable fuels as well. Since the fuel being burnt is a fine dust, the ash produced is also a fine dust, which, unless filtered properly, can escape with flue gases and pollute the atmosphere.

• Fluidized bed boiler- These offer solutions to some of the pollution problems of coal combustion. There is a thick layer of material - sand or gravel - that lies on a base plate, through which streams of air are blown. As the air gets to higher speeds, the material begins to behave like a liquid, and objects will float on or sink in it. Fuel particles are fed into this "bed" and the fixed carbon and volatile gas will burn quickly and heat the entire bed. The tubes carrying the water and steam are contained in the bed, and because the bed is moving like a liquid this maximizes thermal contact with the pipes, and allows for greater heat transfer. The ash is able to flow out of the bed, separate from the flue gases.

Nuclear

• Light water- The majority of nuclear reactors use ordinary (light) water or steam under high pressure as both the moderator and coolant. There are two main types of light water reactors: Pressurized water reactors (PWR) and Boiling water reactors (BWR).

• Gas-cooled- Reactors like the Magnox reactor use graphite as moderator, and carbon dioxide as coolant. They use natural uranium, meaning that it does not have to be enriched. Advanced gas-cooled reactors (AGRs) also use graphite and CO₂, but the uranium is enriched.

• Heavy water- The CANDU reactors (Canadian-deuterium-uranium) are the only other type of reactor to make some inroad into the dominance of the light water reactors. There are about 20 plants operating in Canada and around the world, and they also use natural uranium like the Magnox. Heavy water absorbs less neutrons than light water, which results in a high neutron economy.

• RBMK- Designed in Russia, the RBMK uses graphite as its moderator and light water as its coolant. They use enriched uranium like most other reactors. The Chernobyl reactors were of this type, and after the disaster in 1986 the plans to make any more were scrapped, and many plants were decommissioned.

Figure 2. A boiling water reactor, the boiler (contained in the Reactor Vessel) produces steam to generate electricity.