Integrating propane, gas and steam cycles

The Cascading Closed Loop Cycle (CCLC) system discussed in the previous part of this series has several drawbacks that can be fully overcome by applying a modified Organic Rankine Cycle (ORC) system.

To overcome the drawbacks of the CCLC system, first of all, two separate expanders have to be used to capture all the superheat in the exhaust in the propane of the first expander. Secondly, there is extra piping involved. Thirdly, an extra large and costly heat exchanger has to be applied to transfer the heat from the first expander exhaust to the throttle of the second expander. Another point to consider is the loss in potential heat in the heat transfer process of 50 - 75o F.

It can be argued that the CCLC system might potentially be more thermodynamically efficient, but it is more costly and complicated. However, both systems must have costly hermetically sealed heat exchangers and heaters. There are several interesting observations that can be made in addition to the ones cited above. However, to obtain the exit condition, one must find a way to work backwards, as was done for the CCLC system, and can also be done for the proposed new ORC cycle as well.

Equally efficient Organic Rankine Cycle

The ORC cycle being proposed here overcomes the CCLC objections and provides an equally efficient cycle. The best throttle pressure would be about 2500 psia.  The propane would be heated to the same 500 o F. The exhaust temperature of the HRSG would be about 550 o F to provide a 50 o F approach. Six feed water heaters would be used instead of seven. The first two heaters are combined into one heater operating at a vacuum for the deareator to reflect today's combined cycle technology.

Unexpectedly, the gross ORC efficiency calculates to be 30.0 % and the net efficiency to be 24.5 % when expansion efficiencies are applied and auxiliary losses of fans, pumps, gear and generator losses are considered at an assumed 8 %.  In analyzing the CCLC cycle and the proposed ORC, both cycles have about the same net cycle efficiencies. This is because both cycles have the same heat input, the same work output and the same condenser loss per pound of total working fluid.

However, there are differences in equipment costs, flexibility and low level heat use. The ORC system can accommodate added low level heat of about a 350 o F level as admission flow to the expander. The CCLC cycle requires an expensive intermediate heat exchanger. Both systems would require high 2500 psia propane feed pump. The ORC system would apply only one admission type expander similar to existing partial admission steam turbines. Such an expander would have to be designed.

Possible optimizations

There can be some optimization possible. For the CCLC cycle, the intermediate pressure might be raised from 800 psia to 1000 psia where the approach temperature would be about 20 o F but the cost will be higher. As for the ORC cycle, the second pressure might be lowered from 800 psia to about 700 psia.  In both instances, the amount of superheat could be lowered by about 10 o F to provide extra work. Both cycles will produce more work and a slightly better efficiency at lower ambient temperatures in winter.  

Improving overall cycle efficiency

An additional way to incrementally improve the overall cycle efficiency is possible. Two propane feed pumps are applied. The first one pumps the propane to 850 psia. This propane is then run through regenerative feed water heaters 1 and 2 where the propane is heated to about 190 o F, before being pumped to 2500 psia at constant entropy to exit at 218 o F. Somewhat more feed pump power will be required. A second stream of propane at 100 o F and 850 psia would have to run through the HRPvG before being injected into the expander to maintain the 110 o F HRPvG exit temperature level.

At first, there appears to be a significant ORC gain possible but the added heat input to heat this second stream off sets any ORC gain potential. Only a small gain is obtained from the steam regeneration for the steam cycle and more expander power will occur. This process is similar to feed water heating in a standard steam power plant.    

Raising steam turbine efficiency

In the proposed cycle, the steam turbine efficiency level can be raised from 40.4 % to 42.7 % by adding the four top steam heaters. This increase is without the additional steam regenerative gain provided by the propane feed water heating.  The increase in the steam turbine bottoming efficiency can add another point to obtain a 44 % level. This gain is significant because there is a leverage of some four to one considering the overall bottoming cycle because of the ratio of the power produced by the steam turbine to that produced by the propane expander. 

These last two cycles are truly both examples of combinations and integrations of some six distinct cycles which are: (1)The Brayton, (2)The Rankine, (3)The Combined Steam and Gas, (4) The ORC/CCLC, (5)The  Inverted, and finally (6) The Integrated Steam, Gas and ORC/CCLC cycle. Yes, it is truly ‘The Age of Thermodynamic Cycle Combinations and Integrations.’ Maybe, one of the above combinations will be the winner to achieve and break the 65 % Cycle Target Efficiency set by the author.

Combined cycle power plants offer a way to save a considerable amount of water when compared to the standard steam power plant. In the next part of this series, we shall see how this is possible.

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.