Pumping Coal as a Coal-Water Slurry
Pumping coal as a slurry is in principle and in practice a more elegant route to coal pressurization than lock hoppering. In water, coal concentrations of 60-70 wt% can be used. A drawback is that only a small part of the water is required for the gasification, and the majority just constitutes a burden, as it has to be vaporized and heated to 1500°C. This in turn implies that the oxygen consumption is much higher than for dry-coal feed systems and that the CGE is substantially lower. For IGCC applications, this inevitably results in a lower station efficiency (50 and 38%; see Table 6-1).
In order to compensate for virtually all drawbacks of using water as a slurry medium, the merits of substantially preheating the slurry have been investigated. Preheating has the following advantages:
1. The water has to be heated less in the reactor, and the heat of evaporation becomes lower at higher temperatures (see Figure 6-2B).
2. Atomization becomes better with hotter liquid containing feedstocks, increasing the accessibility of the coal for gaseous reactants, especially when the feedstock slurry is preheated such that the carrier flashes upon introduction into the gasification reactor. Also, the reduction in surface tension of the carrier liquid at higher
temperatures enhances the atomization. The risk of rogue water droplets passing practically through the reactor completely is minimized (Bockelie etal. 2002).
3. More reactor space becomes available for the gasification per se, which will increase the carbon conversion.
4. The oxygen consumption will decrease and the cold gas efficiency will increase.
5. The water will expand (see Figure 6-2A), resulting in a lower water requirement to maintain good slurry conditions. To exploit this phenomenon to the fullest, use can be made of a circulating hot-coal slurry at high pressure, in which the relatively cold slurry leaving the slurry pump is blended in.
The water is preheated close to its critical point (see Figure 6-2B), the above effects will be most pronounced and the heat of evaporation is then minimum. Furthermore, not only the water but also the coal is preheated to above 300°C, a feature that is not possible with gasifiers using dry-coal feeding, because there the coal particles will become sticky and will interfere with the fluidization [Holt 2001(a)].
Where higher preheat temperatures can be accomplished in practice, the IGCC efficiency of a slurry feed process is almost equal to that of a dry-coal feed gasifier (49 and 50%, respectively; see Table 5-9).
As shown in Table 5-9, extreme coal-water slurry preheat is far more effective to increase the IGCC efficiency of a gasifier than adding a second stage to a dry-coal feed gasifier. The reason is that the efficiency of the state-of-the-art single-stage slurry-feed gasifier is lower to start with than that of a dry-coal feed gasifier (38 and 50%, respectively; see Table 5-9), and it is much easier to improve the efficiency from a low level of 38% than at a level of 50%. Adding a second stage to a dry-coal feed gasifier increases the efficiency with only 1-51%. The main reason for this low increase is that the steam that is injected into the second stage is for thermodynamic reasons only partly converted, and the remainder just adds burden to the gasifier. Moreover, the syngas cooler duty is decreased as the outlet of the gasifier drops from 1500 to 1100°C. These disadvantages in large part outweigh the advantage of the lower oxygen consumption. By combining the ability of a slurry-feed process to operate at higher pressures of, say, 70 to 100 bar with high-temperature slurry preheating, additional efficiency gains can be made by using a fuel gas expander in the solids free gas.