Pulverized coal firing

from Wikipedia, the free encyclopedia

Coal dust combustion is a dust combustion process for generating thermal energy from ground coal . The process is mainly used in large power plants. Due to rising energy prices, the process is also used in smaller systems (firing of water tube boilers or cement mills, drying ovens).

Procedure

The coal is ground in a grinding plant and dried. It is then either stored in an intermediate bunker ( indirect firing ) or pneumatically conveyed directly from the mill exit (classifier) ​​to the pulverized coal burners ( direct firing ). In modern large power plants (200 MW – 1000 MW), direct dust firing is the norm today. Indirect firing is often found in cement works or in smaller steam boilers .

By grinding the coal, its surface increases so much that it can burn quickly in the furnace. Due to the short reaction times with the combustion air, the stored energy content in the furnace is very low in contrast to grate firing . The main advantage of pulverized coal firing is its better efficiency (high coal burnout, lower excess air ) and the better (faster) controllability of the firing output.

raw material

The grinding and the grain size of the coal particles depend on the coal used. The most important representatives are:

  • Bituminous coal (grinding in vertical bowl mills with an exit grain size of 70 µm and an exit temperature of approx. 100 ° C)
  • Lignite (grinding in beater mills with an exit grain size of 100 µm up to 1000 µm and an exit temperature of approx. 250 ° C)

The coal is conveyed to the burner in direct furnaces with a mass ratio of air to coal of approx. 2. In indirect furnaces this ratio is z. Sometimes much smaller (up to 0.1).

The coal is ground in coal mills . Due to the formation of explosive dusts, explosion protection measures (e.g. inerting of the mill) must be observed. The pulverized coal is pneumatically conveyed to the pulverized coal burner. The burner doses the coal dust and the combustion air into the combustion chamber. The boiler is started up and the lining is heated to the ignition temperature of the coal dust using a pilot burner that is fired with natural gas or heating oil.

burner

There are numerous burner shapes and designs of air supply (so-called air staging) in order to make the combustion as low as possible. The most common goal here is to lower the burner temperature to reduce the formation of nitrogen oxide . A pulverized coal furnace consists of groups of pulverized coal burners (mostly attached to the boiler in levels). As a rule, a mill feeds a group of several burners on one level. A large boiler can reach a height of over 150 m. The pulverized coal furnace is located in the lower part of the boiler (up to approx. 40 m) divided into approx. 3–6 levels, which are each a few meters above the other.

Firing

The pulverized coal firing is mainly used in power plants to generate steam. There are several variants, which are mainly characterized by the arrangement of the pulverized coal burners:

Tangential firing
With this type of firing, the burners are placed around the boiler and aligned in such a way that the coal entered from all burners on one level rises in a spiral path in the boiler. The advantage of this type of firing is the long (spiral) burnout path.
Wall firing
With this type of firing, the burners are attached to a wall (single-sided firing) or on opposite sides (double-walled firing). Sometimes the burners on the opposite sides are also offset from one another (boxer firing). Wall firing has the advantage that you can control the flame closer to the burner. A more variable air flow is thus possible.

Most furnaces nowadays are dry furnaces . This means that the combustion chamber temperature is below the melting point of the slag . Older systems are designed as melting chamber firing. A type of furnace that is still frequently used in the USA is cyclone furnace, which burns the coal in the burner ( cyclone burner ) and is also a melt furnace .

Modern pulverized coal firing systems for coal boilers have approx. 4–10 mills per boiler and 16–40 burners. Such a furnace burns between 100 t and 400 t of coal per hour.

ash

Whereas the furnace resulting ash decreases either downwardly or with the flue gas from the steam generator is discharged. When discharging in dry form, one speaks of dry ash removal . A special design is the melting chamber firing. The furnace temperature is selected so high that the ash can be drawn off in liquid form (as slag). To separate the fly ash from the flue gas, electrostatic precipitators or fabric filters are often used in the downstream flue gas cleaning system.

History and use

The first pulverized coal furnace was built in the USA in the early 20th century. Over a hundred of these large coal firing plants are currently being built each year, most of them in India and China.

In addition to the use of stationary operated systems, pulverized coal firing was also used in steam locomotives . Here it was not able to establish itself because of the constantly changing burner output. Since the combustion of a steam locomotive has to be continuously regulated and the pulverized coal combustion does not store any energy, an optimal combustion performance can never be achieved.

The German cargo ship Nicea was converted to coal dust firing in 1934. However, the technology could not establish itself here either, especially since the steam drive in shipping was subsequently replaced by marine diesel engines .

Economy and use

With the sharp rise in the price of the energy sources oil and gas, dust firing has also established itself in small and medium-sized combustion systems. In addition to municipal and industrial heating and power stations, lignite dust (BKS) is mainly used in the asphalt, cement and lime industries. These decentralized systems usually do not have their own lignite processing, but store finished lignite dust in explosion-proof silos. The delivery of fuel from the coal refining facility to the consumer is carried out by silo trucks or railroad cars. These combustion systems have outputs of approx. 5 to 15 MW and can replace traditional energy sources in industry in certain areas.

developments

Lignite dust firing ( BKS firing ) in combination with industrial boiler systems are also being tested in pilot projects in Europe with low or even CO 2 -free designs. In the immediate vicinity of the Schwarze Pump power plant in Niederlausitz , a 30 MW steam boiler fired with lignite dust was in operation, to which pure oxygen was fed for combustion. After the condensation of the water vapor, almost pure CO 2 is obtained as exhaust gas , which is condensed (liquefied). This then allows the waste gases to be dumped underground .

Web links