State-of-the-Art IGCC
The integrated gasification combined cycle (IGCC) is in its present form basically a combination of gasification and a CC plant. The advantage of incorporating gasification into these plants is to convert solid and residual liquid fuels into a form that gas turbines can accept. Furthermore, gasification provides a means of desulfurization of the fuel before combusting the gas in the CC plant to levels not achievable at reasonable cost with current flue gas desulfurization (FGD) technologies. Thus, with a gasifier and the proper gas treating train behind it, one obtains a fuel that can be combusted as simply and as environmentally friendly as natural gas. IGCC plants with coal as feedstock can be found in power stations, whereas IGCC plants with heavy residual fractions as feedstock are found in refineries.
Although the two component parts of an IGCC (gasification and gas turbines) are both well developed technologies, the combination is nonetheless relatively new. As is typical for a technology in such a stage of development, there is considerable variation in the optimization of flowsheets for IGCC. This is particularly true in the matter of effective integration of the two core technologies involved (Holt 2002).
The most important integration that is applied in almost all IGCC plants is the integration of the steam system. The steam cycle of a standard CC has an efficiency of about 38%. The pinch problem caused by the evaporation of the water is the main reason why it is so low. On the other hand, a steam system that derives its heat only from the syngas cooler of a gasifier has also a low efficiency of perhaps 38% because superheating is difficult in a syngas cooler. Combining both steam sources alleviates the restrictions, as the pinch in the HRSG is eased by the large evaporating duty in the syngas cooler and more saturated steam can be superheated in the HRSG, albeit at a lower superheat temperature. It is therefore not surprising that when combining the heat sources from HRSG and syngas cooler the efficiency becomes about 40%. This is represented by the “conventional integration” case shown in Figure 7-18.
Two other possibilities for integration that are applied in some IGCCs both involve the air separation unit. One possibility is air integration, which involves
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Figure 7-18. Block Flow Diagram of an Integrated IGCC Power Plant |
using extraction air from the combustion turbine compressor as feed air for the ASU. This results in an overall somewhat lower compression duty. The ASU main column then operates at a higher than the normal atmospheric pressure (about 10 bar instead of 5 bar) that results in the production of oxygen and nitrogen at an elevated pressure of 5 bar instead of 1 bar. Hence the oxygen for the gasification requires less compression. Nitrogen integration uses the nitrogen stream from the ASU as diluent for the clean fuel gas to reduce the flame temperatures and hence NOx emissions. Also, here it is advantageous that the nitrogen already has an elevated pressure.
Although the efficiency of the IGCC increases with a high degree of integration, there is a risk of the availability suffering. A compromise that is applied in new plants is one where part of the air for the ASU is supplied as extraction air from the compressor of the gas turbine and part from a dedicated compressor in the ASU. This adds an additional piece of rotating equipment to the plant, but it facilitates the start-up and improves the overall availability of the plant.
The extraction air leaving the adiabatic gas turbine compressor has first to be cooled before it can be used in the ASU. This is a further cycling of the temperature of a main gas stream, which is one of the reasons why the efficiencies of IGCC plants are still relatively low. In the next section some ideas are discussed to improve this situation.