Coal Diying
Conventional drying processes are thermally based and typically use natural gas as a fuel. For coal gasifiers this is equivalent to using clean gas to dry the coal, and hence a prime product is used for drying the feed. Exergetically this is not attractive, as it lowers the overall energy efficiency. Therefore, for drying the coal it is better to use waste heat of an appropriate low temperature level.
In an IGCC scheme the most logical solution would be to dry it in direct contact with the warm exhaust gas from the heat recovery steam generator (HRSG) of the gas turbine. In this case, low-level heat is used for the drying. However, the exhaust gases from all present gas turbines contain about 15 mol% oxygen, which introduces the danger of spontaneous combustion in the dryer. Recycling flue gas back to the compressor inlet to replace part of the large excess air flow, as discussed in Section 7.3.4, would reduce the oxygen content to 3-5 mol% and would eliminate this problem. The dried coal would then still require a separate coal pressurizing system.
Using coal-water slurries eliminates the problem of drying the coal. By injection of the coal as a coal-water slurry into the hot gas leaving the gasifier, use can be made of waste heat in the fuel gas to evaporate the water from the coal, while still allowing use of the more elegant coal-slurry pump for pressurizing. This principle is used, for example, in the E-Gas process (Section 5.3.5). The problem remains, though, that drying with such hot gases always introduces the risk of some tar formation, and one of the advantages of entrained-flow slagging gasifiers is that no tars were formed in the gasifier proper. Although better than using clean syngas or natural gas for drying, using such high-temperature sensible heat for drying is also exergetically not attractive.