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Problem Description: Figure 1 Schematics of Cheng Cycle

This problem discusses minimizing NOx emissions from gas turbines by adding steam to the combustor. The student is asked to analyze a modified Cheng cycle with steam injection and calculate the required air temperature for reasonable turbine efficiency. Key aspects include estimating mixture properties, selecting turbine efficiency, and performing energy and mass balances across the mixer and condenser while varying operating parameters like humidity and air-fuel ratio. The consequences of the design on emissions and surrounding environment require in-depth thermodynamic and environmental analysis.

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Muhammad Abdullah
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133 views2 pages

Problem Description: Figure 1 Schematics of Cheng Cycle

This problem discusses minimizing NOx emissions from gas turbines by adding steam to the combustor. The student is asked to analyze a modified Cheng cycle with steam injection and calculate the required air temperature for reasonable turbine efficiency. Key aspects include estimating mixture properties, selecting turbine efficiency, and performing energy and mass balances across the mixer and condenser while varying operating parameters like humidity and air-fuel ratio. The consequences of the design on emissions and surrounding environment require in-depth thermodynamic and environmental analysis.

Uploaded by

Muhammad Abdullah
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOCX, PDF, TXT or read online on Scribd
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Complex Engineering Problem (CEP)

Subject: ME-212 Thermodynamics_II


4TH Semester 2017 session

Problem Description

Currently conventional energy resources (coal, oil and gas) are used to cover about 70% of world
primary energy demand. Energy conversion process with utilization of these resources in thermal power
plants adds huge pollutants (CO2, CO, SOx, NOx, HC, ash etc.) in the environment. Legislation bodies
around the globe (environmental protection agencies like EPA-Pakistan) forces power producers to
adopt policies for minimizing environmental deterioration. This problem is discussing one possible
strategy on minimizing NOx emissions.

Addition of steam to combustors in gas turbines and to internal-combustion engines reduces the peak
temperatures and lowers emission of NOx. Consider a modification to a gas turbine, as shown in Fig.
1 ,where the modified cycle is called the Cheng cycle. In this example, it is used for a cogenerating power
plant. Assume 12 kg/s air with state 2 at 1.25 MPa, unknown temperature, is mixed with 2.5 kg/s water
at 450 ◦C at constant pressure before the inlet to the turbine. The turbine exit temperature is T4 =
500◦C, and the pressure is 125 kPa. For a reasonable turbine efficiency (consult literature according to
operating conditions), estimate the required air temperature at state 2. Compare the result to the case
where no steam is added to the mixing chamber and only air runs through the turbine. Comment on
your results from two stated cases.

Figure 1 Schematics of Cheng cycle


The Cheng cycle, shown in Fig. 1, is powered by the combustion of natural gas (essentially methane)
being burned with 250–300% theoretical air. In the case with a single water-condensing heat exchanger,
where T6 =40◦Cand φ6 =100%, is any makeup water needed at state 8 or is there a surplus? Does the
humidity in the compressed atmospheric air at state 1 make any difference? Study the problem over a
range of air–fuel ratios.

CEP Attributes

Sr. No Attribute Mapping


1 Range of conflicting requirements
2 Depth of analysis required ✔
3 Depth of knowledge required ✔
4 Familiarity of issue
5 Extent of applicable codes
6 Extent of stakeholder involvement and level of conflicting
requirement
7 Consequences ✔
8 Interdependence ✔

Assessment Rubrics

1. Estimation of mixture properties [4]


2. Selection of turbine efficiency with reasonable arguments [2]
3. Calculation of turbine inlet temperature with consideration of mixture properties [3]
4. Consideration of thermal losses in the mixture [2]
5. Energy balance across mixer [2]
6. Mass and energy balance across condensing heat exchanger [4]
7. Effect of variable humidity and air-fuel ratio [3]

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