Coal chemistry

Print edition : November 07, 2008

THE most significant characteristic of Indian coal is its high ash content, which varies from 35 to 45 per cent, compared with that of coal in other parts of the world, which is around 15 per cent.

With legislation requiring, since June 2001, thermal power plants to use coal with less than 35 per cent ash content, and with the quality of Indian coal deteriorating over the years, development of more efficient technologies that can handle high-ash Indian coal, such as Integrated Gasification Combined Cycle, becomes important.

Besides high ash content, another reason for entrained gasifiers (that operate at higher temperatures than fluidised bed gasifiers), commonly used in IGCC plants abroad, being not suitable for Indian coal is its high ash fusion temperature of about 1,500{+0}C.

The ash chemistry of Indian coal is such that it is high in silica and alumina. The ash is also highly abrasive because of high quartz content, which can lead to erosion of the syngas cooling system when it gets fused.

But in a fluidised bed, since the operating temperature is much lower than the fusion temperature, this problem is not serious, according to BHEL.

However, Indian coals sulphur content is low, about 0.5 per cent. So, from a gas clean-up perspective, the flue gas desulphurisation (removal of SOx gases) and NOx removal system is not economically justifiable and, therefore, not important. Also, in the Indian context, this is unnecessary to meet emission norms.

R. Ramachandran

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