Parasitic Power
Parasitic power is usually defined as the electrical energy required for drivers of auxiliary machines in an IGCC. In connection with an oxygen-blown gasifier, this includes the air compressor (if any) for the ASU, the oxygen compressor, and the nitrogen compressor (again, if any). For an air-blown gasifier, the parasitic power for the provision of oxidant is the air booster compressor (see Figure 6-14). Unfortunately, this definition very quickly creates misunderstandings or confusion, because it does not include the power used by the air compressor of the gas turbine. Any air integration included in the cycle removes mechanical energy in the shaft from the generator, where it can produce electric power. Since the compression energy internal to the gas turbine is not included in the normal definition of parasitic power, one needs to be very careful about drawing any conclusions from a single number provided for any particular project. In practice, finding an optimum integration between the gas — turbine air compressor and the provision of oxidant is highly dependant on the characteristics of the air compressor itself, and it should come as no surprise to find that different optima can be presented by different gas turbine suppliers for the same project.
Klosek, Sorensen, and Wong (1993) point out that the parasitic power demand for an oxygen-blown IGCC without nitrogen integration can be significantly lower than that of an air-blown unit. This initially surprising result can be attributed to two facts. First, the air-blown unit must handle much more air, and second, the air must all be compressed to the gasifier inlet pressure, which is in the current designs significantly higher than the turbine combustor pressure. Reducing the pressure drop over the gasifier and treating could well change some of the above conclusions.
|
FEEDSTOCK Figure 6-14. Typical Air-Blown IGCC |
6.10.1 Deductions
Essentially, the reasons why most real IGCCs have been built using oxygen as oxidant are economic, and the technical background to these economics has been described in the preceding sections. These plants, whether demonstration (e. g., Buggenum, Puertollano, Wabash, and Tampa at 250-300 MW) or commercial (e. g., Sarlux at 500 MW), are, however, all large projects. There is no disguising the fact that the favorable economics of oxygen-blown technology has an entry price, which is the investment cost of the ASU. At this scale, the entry price is well worth paying.
For small plants, smaller than say 50 MW, which is largely the realm of biomass and waste fuels, the initial investment in an ASU is less attractive. Savings on the much smaller equipment and the improved efficiency of oxygen operation is no longer able to pay for this investment. This is why many gasification projects in this size range use air-blown gasifiers. It is, however, not possible to provide any hard-and-fast guidelines for determining the break-even point between the two technologies. The number of variables, which include feedstock pricing and supply, gas turbine characteristics as discussed, and the potential for synergies in oxygen supply (see Section 8.1), make a project-specific evaluation inevitable.