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2.2 Process modeling and results
The system is modeled with two software tools: AspenPlus is used to model methanol fuel heating and decomposition, as well as syngas-fueled gas turbine; GTPro is used to model steam bottoming cycle (including an HRSG, steam turbine and cooling system) and the CO2 capture and compression process.
An F-class gas turbine is selected with a turbine rotor inlet temperature (TIT) of 1,327°C. A simplified turbine cooling model is adopted for simulating the gas turbine. ISO ambient conditions are used. The major assumed parameters are listed in Table 1.
The performance results of the methanol indirect combustion combined cycle are summarized in Case 2 of Table 2. The results indicate that net plant efficiency reaches as high as 54.73 percent HHV while the net output is 233.3 MW.
Figure 2 shows the flow schematic of a conventional combined cycle based on methanol direct combustion (by solid lines), which is the same as a natural gas-fired combined cycle except that liquid methanol fuel is used. It is modeled based on the same assumed parameters and methodology as for Case 2, and the results are presented in Case 1 of Table 2. It has a net plant efficiency and net output of 49.97 percent HHV and 271.7 MW respectively.
In comparison, combined cycle efficiency is improved by 4.76 percentage points through methanol indirect combustion, which will be discussed in detail in Section 2.3.
2.3 Advanced performance by methanol indirect combustion
2.3.1 Exergy analysis
Section 2.2 indicates that efficiency of Case 2 is 4.76 percent higher than Case 1. To examine performance difference between the two cases, an exergy analysis is carried out.
Table 3 shows a comparison of distribution of exergy destruction of Case 1 and Case 2.
As can be seen, the largest process takes place in the GT combustor because fuel combustion degrades available energy significantly. Because of methanol indirect combustion, the exergy destruction in the GT combustor for Case 2 is much lower than Case 1 (24.6 percent vs. 32.6 percent). This will be further analyzed in Section 2.3.2.
2.3.2 Cascade utilization of chemical exergy
Exergy analysis in Section 2.3.1 shows the exergy destruction of the GT combustor in Case 2 is lower than Case 1 by 8 percentage points. This can be reasonably explained by a principle of cascade utilization with a combination of chemical exergy and physical exergy [7-9].
In Figure 3, the A-t coordinates represent energy level and temperature respectively. The area above the Carnot efficiency curve (ηC) illustrates chemical exergy, while the area below shows physical exergy. The cascade utilization of physical exergy is achieved by integrating the Brayton and Rankine cycles based on the thermal energy levels. As for the chemical exergy of hydrocarbon fuels in combustion, their energy levels (Af) could be as high as about 1.0, while the energy level of hydrogen-rich syngas fuel (Asyn) has a value between 0.83 and 0.9 . As a result, it is possible to effectively utilize the chemical exergy of fuels with different energy levels, similar to the cascade utilization of physical energy in a power cycle.