June 24 2012 - TI Staff

*After discussing heat losses in combined cycles and the three basic principles for GT CC cycles to achieve maximum efficiency, Ivan Rice talks about a specific strategy to inch aeroderivative combined cycle efficiencies up to 60%.*

Fuel gas heating is independent of any particular CC arrangement and is being used by some of the latest combined cycles. The cycle efficiency of the Combo 5 LM 6000s is increased right at one percentage point. An average specific heat value of .640 BTU/Lb/oF obtained from the methane (natural gas) Mollier diagram between 100 and 600 o F at 500 psia has been used in the calculations.

**(An LMS100 aeroderivative gas turbine)**

The boiler supplementary firing temperatures have been calculated to be 1290 o F for 120 MW of steam turbine power and 1347 o F for 125 MW, both for 1050 o F steam. A rather high specific heat value of .275 was used for the exhaust flow due to the extra water from Sprint and the water of combustion of the fuel.

The decrease in steam flow can be readily calculated by applying the various ratios of the TSRs involved. The effect of topping power at 3413 BTU/KWH can likewise be easily calculated which equates to the fuel added. These calculations are based on the efficiency equation of:

CC Efficiency = Work Output/Heat Input

with the heat (fuel) input known of the 5 LM 6000s and the output work (GT plus ST) known of the referenced cycle being used as a starting point. Incremental additions of input and output have been introduced to obtain individual results.

As of now, computer runs have not been done on this. Computer programs for combined cycles are most useful and helpful to obtain accurate and exact data but they cannot think through a process and come up with new combined cycle approaches. The thermodynamic reasoning presented above regarding steam turbine topping power approaching 3413 BTU/KWH and the lower TSRs for the Combo 5 LM 6000 power plant with supplementary fired steam super heating only in the central unit should be verified by computer runs for the two cycles. The ThermoFlow and the Gate computer programs have to be manipulated for the split steam cycle as they were designed primarily for the conventional combined cycle.

Another example of steam splitting is the application of 2 uprated LMS 100 125 MW units as anticipated by using the 80E1 gas generator and modifying the first compressor and power turbine and using 1 uprated LM 6000 unit whereby only the HRSG of the LM 6000 is supplementary fired for heating all the initial superheat and reheat steam for 1250 psia 1100/1100 o F steam conditions. This Combo 3 system, using the current 80C2 100 MW LMS 100 and applying 1250 psia and 1050/1050 o F steam conditions, yields a CC efficiency of about 56 % with about 85 MW of steam turbine power produced for a total of around 335 MW.

A low pressure inter cooler boiler with 2 to 4 psig steam pressure and 300 o F output can be used to produce admission steam to the steam turbine. Considering 40,000 LB/Hr per LMS 100, the steam turbine will generate about 4 MW where the TSR is 18.5 Lb/KWH for 2" Hg condenser pressure for the two LMS 100s. The CC cycle efficiency is raised about one percentage point.

The CC efficiency increases to about 59 % with added steam turbine output and decreased fuel input when

(1) steam conditions are raised to 1100/1100 o F,

(2) fuel gas heating is incorporated and

(3) the IC low pressure admission steam is included.

The intercooler heat loss, if not incorporated in the cycle some way, will degrade the overall combined cycle efficiency by about 2 percentage points. Therefore, a workable solution has to be incorporated. Perhaps the ORC propane cycle or a low steam pressure admission to the LPT section of the steam turbine can be applied to use up an optimum portion of the intercooler heat rejection and inch up closer to a 60 % cycle efficiency level. The projected new 80E1 LMS 100 Combo 3 should top 60 % efficiency at an increase of about 70 MW to exceed 400 MW total output. Additional refinements in each example can be made but are not discussed herein.

In the concluding part of this series, the author talks about the significance of flexible features of a proposed multi-cycle unit.

*(This article is the third part of a series by the author)*

*Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.*

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