In the first part of this series, the author wrote about a new aero combined cycle innovation to overcome aero's relatively low exhaust temperatures compared to the latest HD gas turbines. In this article, he talks about the heat losses in combined cycles.
(The aeroderivative Trent gas turbine)
There are three heat losses in all combined cycles. The first one is the unavoidable parasitic loss that amounts to about 2 % of the fuel input: the radiation and convection heat loss of the outside surfaces of the two turbines, the ducts, the piping, the HRSGs, the lube oil cooling loss, the electric generator loss, and air leakage loss. Some units externally cool the GT cooling air which accounts for another 2 to 3 % loss such as the Alstom GT 24 and 26s and some of the MHI/W older units. The IC GE LMS 100 also has a significant intercooler heat loss that is higher than the stack loss. Ways to reduce this loss must be addressed if competitive CC efficiencies are to be realized.
Stack and latent heat loss
The two largest and significant heat losses are the ones cycle manipulators try to reduce to an absolute minimum. There is the stack heat loss with its sensible heat and latent heat loss of the fuel burned. The 200 o F stack reduces this sensible and some of the latent heat loss to a low value. The other and largest heat loss is the steams condenser loss. This loss deserves attention to minimize the steam flow to the condenser. Every pound of steam requires 1000 BTU to condense. Having only one feed water heater adds a little to the condenser loss but allows a lower stack loss with cooler water entering the economizer, a better trade off.
Another way to gain on the condenser loss relative to the steam power generated is to increase the superheat and reheat temperatures to lower the theoretical steam rates (TSRs) and to increase steam turbine output. The latest HD GTs with higher exhaust temperatures have allowed higher steam temperatures without supplementary duct firing. Siemens, MHI, Alstom and GE have all gone from 1000 to 1100 o F in their latest HRSG and steam turbine designs. Another way for gaining on the condenser has just been discussed in our previous example by steam superheat splitting without adding stack loss and by lowering the condenser flow (loss).
There are three basic principles to apply for all GT CC cycles for maximum efficiency:
1. The cycle should be configured to provide the lowest steam flow for the highest steam pressure and temperature to give the maximum steam power and the lowest condenser heat loss. A minimum boiler pinch point is required. No extra steam by boiler firing can be tolerated.
2. The cycle should be configured to obtain a low stack temperature of about 200 o F by applying a single feed water heater for a vacuum deaerator. Also, the HRSG should be of the multi-pressure design (three drum or a three pressure once through design). The lowest stack heat loss should be realized.
3. The cycle should be configured by applying the steam turbine with the highest expansion efficiency. Gearing the HP section of the steam turbine to the IP section can be considered to obtain a high HP expansion efficiency for the lower steam flow units.
Supplementary firing each HRSG in our split steam example generates far too much steam and does not satisfy item 1. The excess steam generates too much steam turbine power at the 40 % bottoming efficiency level and degrades the overall combined cycle efficiency. In our example, when only one HRSG is supplementary fired, no extra steam is generated by the system. More cycle topping power is produced. Condenser and stack losses are kept to a minimum.
When there is no additional steam flow or less flow and no resulting increase in system condenser heat loss and when there is likewise no increase in stack heat loss of the system, then the total sensible heat added by supplementary firing into the single superheater is transformed into pure topping incremental steam turbine power approaching 3413 BTU/KWH (100 % efficiency) degraded only by the small amount of aforementioned parasitic heat loss and about 1 % weight increase of the added fuel at a slightly higher specific heat.
The moisture content at the condenser remains at the 7 % level for 2 ” Hg and 100 o F. Increasing the steam temperature to 1100/ 1100 o F and lowering the TSRs adds considerably more steam turbine power at greater than topping efficiency. It is therefore understandable why the combined cycle efficiency level is increased significantly. The first law of thermodynamics dictates and is supported by the second law dealing with entropy.
Topping power and higher steam conditions
The two factors previously mentioned which increase the combined cycle efficiency from a standard reference point of 54 % gross to about 58 percent are explained as follows: The first is the pure topping power produced by the supplementary firing of fuel added at constant condenser steam flow of the referenced unit's 200 Lb/sec. The second is the ability to go to higher steam conditions of 1800 psia and 1050/1050 o F and then on to 1100/1100 o F. The first (topping power) increases the efficiency by about 2 points and the second adds another 2 points by reducing the condenser steam flow and increasing steam power because of the lower steam rates.
Advancing to 1100/1100 o F steam conditions further increases topping power and also reduces the steam rates further, which further reduces the condenser flow and increases the steam turbine power. Heating the fuel gas to about 600 o F by the stack gas and by saturated steam from the second and third drums reduces the fuel gas input directly (100 % efficiency).
The next article focuses on the calculations of the combined cycle efficiency, throws light on thermodynamic reasoning, and highlights some additional refinements that can be considered to improve efficiency.
(This article is the second 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.