Coal-Water Sluny-Fed Gasifiers
The big advantage of coal-water slurry feed gasifiers is the more elegant method of pressurizing the coal. Lock hoppers as used in dry-coal feed gasifiers are costly and bulky equipment with complex valve systems that have to provide a gas-tight block in a dusty atmosphere. Pumping a coal-water slurry is not a simple operation either, but it is definitely less complex than lock-hoppering. In addition, the practical ultimate pressure for dry-pulverized-coal lock hoppers is about 50 bar, whereas for coal-water slurry pumps the pressure could, in principle, be as high as 200 bar.
As there is always a surplus of gasifying agent to be added to the gasifier, there is less benefit in adding a nonslagging second stage. Only when coal is added to the second stage, as is done in the E-Gas process, are part of the disadvantages of having so much water in the feed eliminated.
Single-Stage Gasifiers. In a single-stage coal-water slurry-feed gasifier all the water in the slurry must be evaporated and raised to the full outlet temperature of the slagging operation. This imposes a considerable penalty on the cold gas efficiency of the process to offset against the simplified feed system. This is also reflected in a higher oxygen consumption compared with a dry-feed system. Furthermore space in the reactor is required for the evaporation process.
The high water vapour content of the hot, raw syngas also influences its composition. The CO shift reaction is driven further to the right resulting in a higher H2/CO ratio and higher C02 content than the equivalent dry-feed gasifier. Whether this is important or not will depend on the application under consideration. Additionally the methane content will be even lower although if the potential for higher operating pressures is exploited, this would tend to increase the methane content. However the methane content of all single-stage entrained-flow gasifiers is so small that this is unlikely to be of consequence.
Two-stage gasifiers. The main issues surrounding the staging of coal-water slurry-feed gasifiers are similar to those for dry feed gasifiers. The efficiency of a two-stage gasifier is higher than that of a single-stage process for the same reasons as for the dry feed case. The disadvantages of a higher steam consumption described for dry feed gasifiers are however not applicable, since the amount of steam is dictated by the coal/water ratio in the slurry, which is not affected by the staging arrangement.
