Practical Issues
6.1 EFFECT OF PRESSURE
The effect of gasification pressure and temperature on gas composition, yield, and cold gas efficiency was discussed in Section 2.3.1. There are other aspects to consider, however, when deciding on the values of these parameters in a process, and these are discussed here.
The pressure in a gasifier is generally based on the requirements of processes downstream of the gasifier. This requirement is easily met when the downstream process is a combined cycle (CC) that typically requires a pressure in the gasifier of 20-40 bar. Other processes such as methanol or ammonia synthesis require much higher pressures of 50-200 bar and thus compression of the synthesis gas.
In principle, it looks more attractive to pressurize the feed to a gasifier than to pressurize the gas. However, it should be realized that most of the advantages in terms of equipment compactness and lower compression energy are already obtained when gasifying at a pressure of 15-25 bar. Moreover, where the feedstock is a solid such as coal or biomass, pressurizing becomes more and more complicated at higher pressures. In the case of air gasification, there is in principle less reason to prefer pressurization of the blast, since the savings on syngas compression are much less due to the large percentage of inerts in both the blast and the product gas.
For high-temperature entrained-flow gasifiers, this theoretical argument of pressurizing the blast components remains valid for quite high pressures of 100-150 bar because of the low methane content in the gas. For fluid-bed gasifiers that operate at much lower temperatures, the higher methane contents in the gas at such high pressures would be unfavorable for nonfuel gas applications. In moving-bed gasifiers, the methane content is already high owing to the pyrolysis reactions. High pressures raise the methane content further, even to the extent of almost doubling it as was demonstrated in the Ruhr 100 plant (see Section 5.1.3). This may not be desirable for syngas applications, but for SNG production it reduces the load on the downstream plant considerably.
There are also a number of practical aspects to be reviewed when considering gasification at very high pressures, which sometimes reduce the attractiveness of such a solution.
Compression of Reactants
Large oxygen compressors are available for pressures up to 70 bar. Above this pressure oxygen is mostly pressurized by pumping liquid oxygen. This facilitates the pressurizing and reduces the energy for syngas compression. However, in the ASU more energy is required for compression as the cold from the evaporation of the liquid oxygen now comes available at a somewhat higher temperature. Overall, there may be still an advantage to gasify at a pressure of, say, 100 bar.
Raising the pressure of heavy oil residues for gasification at 80 bar with plunger pumps is normal commercial practice in Texaco plants, and pilot plants have operated at 100 bar. Coal-water slurries are also relatively easy to pump, although more difficult than a pure liquid. Gravity feed of lump coal through lock hoppers to a moving-bed gasifier has been demonstrated at 100 bar (Lohmann and Langhoff 1982). The situation is different for dry-coal feed systems relying on pneumatic conveying, as in entrained — flow systems or screw conveyors. For such systems the maximum practical pressure is about 50 bar (see also Section 6.2.1).
Compression of the moderator, which in virtually all cases is steam, is not a problem, as pumping water requires relatively little energy.
Equipment
All gasification reactors require some form of protection between the high- temperature reaction space and the outer pressure shell, which must be maintained at moderate temperatures of 200°C to 300°C. This protection either takes the form of a thick (50-70 cm) insulating refractory wall, or a membrane wall that in current designs is at least as thick. One of the potential advantages of gasifying at higher pressures is that the reactor volume and thus cost required for a given throughput decreases, particularly in fluid-bed and entrained-flow reactors, where the volume is determined by the gas phase. Since the volume taken up by the pressure shell protection system is virtually independent of the pressure, the economics of designs much above 30 to 40 bar tends to be confronted with diminishing returns in IGCC applications using coal as a feedstock. When the downstream application of the gas requires very high pressures, as has been the case in most heavy oil gasifiers where the gas is mostly used for ammonia or methanol synthesis, the combination of the savings in compression cost and the fact that oil is easy to pressurize outweigh the disadvantages of a somewhat higher cost reactor.
Side Reactions
When looking at the possibilities of high-pressure gasification, one should not forget the effect of pressure on side reactions. When the feedstock contains iron or nickel (the latter being typical for refinery residues), the formation of carbonyls is favored by higher pressures and becomes significant at pressures over about 30 bar (see Section 6.9). Although this is not an argument against higher pressure per se, it will cause additional expense in the gas clean up.
Similarly, formation of formic acid in the liquid phase is favored by higher partial pressures of carbon monoxide. This will tend to lower the pH in water washes or process condensate and at high pressures will need to be considered in the material selection.