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Cheng Cycle Heat Recovery System V.Ganapathy

For plants requiring power and process steam,a simple cycle consisting of a gas turbine and HRSG is adequete when the thermal load or process steam demand is constant.However when the process steam demand drops off,the exhaust gas may have to be bypassed,which is a waste of energy or the gas turbine load may have to be decreased,which is also not a good solution. An extraction steam turbine added to the system can help handle process steam variations;however the resulting combined cycle is expensive and has more components for a small plant.The Cheng cycle is a good solution to these type of situations.
The Cheng cycle,see figure below,helps plants generate varying amounts of electrical power and process steam depending upon the demand of each without having to compromise on either.It offers a few advantages over combined cycle plants.

The Cheng cycle consists of a gas turbine,which is capable of being injected with large amount of superheated steam(15 to 20 % of the exhaust flow) and a HRSG which can generate both saturated as well as superheated steam with firing capability.The ratio between amount of superheated and saturated steam can be varied by using control valves in the line to the superheater and at the process steam outlet.Electrical power increases as more of  high temperature superheated steam is injected into the gas turbine and vice versa(due to the increased gas mass flow and higher specific heat).Process steam needs can be handled by decreasing the amount of superheated steam generated. Hence we are able to achieve a wide ranging electrical and process steam demand variations using this concept,which is also simpler with fewer compnents compared to a combined cycle.Note that the duct burner is located between the superheater and the evaporator.This enables the superheater to run dry at times as discussed below.
There are basically 4 modes of operation with widely varying combinations within these boundary points.
Unfired mode with all superheated steam injected into the gas turbine; maximum electrical power is generated,nearly 50 to 70% more than that without injection.No process steam is provided.The gas analysis to the HRSG will have to consider the large amount of steam injected. Large % volume of water vapor and mass flow  increases the gas specific heat and thermal conductivity and hence heat transfer ability,heat flux,gas pressure drop etc.
Unfired mode with no injection of superheated steam. Only saturated steam for process is generated in this mode.The superheater runs dry.Normal electrical power is generated by the gas turbine.
Full injection of steam with full firing of the duct burner.Maximum electrical power is generated with significant amount of process steam.Both superheated and saturated steam are generated.
Fully fired with no steam injection. Maximum process steam is generated. Superheater runs dry.
As mentioned earlier,one can vary the amount of injection steam or amount of firing to have several operating points within these 4 major boundary points.
With a modified Allison 501 KB gas turbine,the electrical power output varies from about 3.5 to 5.5 MW,while process steam can vary from nil to 50,000 lb/h at about 200-400 psig.
The following points should be noted with respect to the design of the HRSG.
The superheater can run dry and wet on and off,depending upon the power and process steam demand and electrical/thermal load scheduling.(Some plants use the Cheng cycle to maximize on the sale of power  when electricity costs are expensive and optimize on a ratio between electrical power,process steam based on cost of electricity and process steam.) Hence there is a possibility of exfoliation (dislodging and carrying away with steam) of iron oxide deposits from the inside of superheater tubes into the gas turbine. This concern is minimized by using alonized T11 tubes.Chromizing may also be considered,however this may be more expensive.
The turbine exhaust gas contains nearly 30% water vapor and 11 % oxygen (versus 7 % water vapor,14 % oxygen in dry mode) when full injection occurs.Hence in order to ensure stability of the duct burner flame,an augmenting air fan is added to the burner system.
NOx is reduced due to the injection of steam in the gas turbine,which is an additional advantage of the system.
In order to ensure excellent steam purity of steam for injection into the gas turbine(less than 10 ppb sodium),an external steam separator is added in the line between the drum and superheater.
In order to improve the efficiency of the system and reduce the amount of deaeration steam,a feed water preheater is included. This exchanger preheats the makeup water going into the deaerator,while cooling the feed water to the economizer.
More information is available in the author's Waste Heat Boiler Deskbook.

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