Technology of coal gasification
Nuon Magnum is based on coal gasification technology. In this system, coal is first converted into a combustible gas: syngas. This gas is cleaned and desulphurised to a purity that is comparable with natural gas. The syngas is therefore suitable for use in a steam and gas turbine, where it is converted into electricity. The combination of coal gasification followed by the generation of electricity is known as IGCC, Integrated Gasification Combined Cycle (STEG or KV-STEG in Dutch).
Technology of coal gasification
The production of the coal gasification process is explained in the image below. Click here to enlarge the image.
A
Coal is delivered from the port to the plant, where it is milled and dried to form pulverised coal. Then the pulverised coal is blended with milled biomass. It is stored under nitrogen to prevent the risk of explosion, brought up to pressure with nitrogen and transported to the gasifier.
B
Oxygen is added to the coal, which is converted into syngas under pressure and at high temperature. The non-combustible part of the coal (minerals) solidifies and is removed as slag. The syngas consists primarily of carbon monoxide and hydrogen and at this stage still contains various pollutants. These pollutants are removed step by step. The syngas, which is combustible and poisonous, is then cooled in the syngas cooler. The heat that is released is converted into steam. In the air separation plant, oxygen and nitrogen are drawn from the atmosphere at very low temperatures. As already stated, these gases are used in the process.
C
Then in two steps the fly ash-fine particles that also contain noncombustible materials from the coal- is removed from the gas.
D
Next the gas is washed with water. This removes the soluble compounds present in the coal, such as chlorides and fluoride, extracting them from the gas.
E
During the subsequent sulphur removal process, the toxic H2S (hydrogen sulphide) is extracted from the syngas. In this process over 99% of the sulphur from the coal is bound and converted into pure sulphur. This is reused in the chemical industry. A very small part of the sulphur goes into the air via the flue gas burner as SO2.
F
The purified syngas is thinned with nitrogen and saturated with water vapour in the saturator so as to achieve low NOx emissions; it then goes to the gas turbine. Here the coal gas is combusted, driving the gas turbine. The hot flue gases from the gas turbine are cooled in the flue gas chamber. The heat released is used to create steam, which drives the steam turbine. The ultimate electricity production takes place in the generator, driven by the steam and gas turbines in tandem.
The gasification process shown above is explained in diagrammatical form in the attached below.