Gas cycles

Question 1

Common data: gas turbines

Answer 1

TIT:1000 -1300ºC
•material restriction (metallurgical limit)
•cooling of turbine blades
•temperature up to 1600ºC are anticipated in the future

Question 2

Compressor ratio vs. Turbine inlet temp

Answer 2

describe diagram

Question 3

Intercooling

Answer 3

• Compression is divided into 2 or more stages with cooling between
• Reduces the work of the compressor
• Ideal intercooling:
• same inlet-T in both stages
• same pressure ratio βin the compressionsteps
• “Bulky” installations
• Only installed in large plants

Question 4

Reheat cycle

Answer 4

• Heat addition between expansion steps → increased work output, decreasedefficiency
• Supplied heat
• Effect of reheat cycle related to the gas properties:vertical distance between constant pressure lines increases with increasing entropy (figure a)
• Optimum reheat conditions = equal temperature drops (or pressure ratios) in LP and HP turbine stages
• Efficiency decreases compared to simple cycle (without reheat)-Reduced temperature range of the cycle

Question 5

Gas turbine combustion systems-design criteria

Answer 5

• Emission formation
• Combustion/exhaust temperature
• Turbine inlet temperature distribution
• Minimize pressure losses
• Flame stability

Question 6

Emission formation depends on

Answer 6

, 
air/fuel ratio
-Stoichiometry
-Temperature
-Residence time

Question 7

NOx formation in gas turbines

Answer 7

describe diagram

Question 8

Combustion principles

Answer 8

• Diffusion (A)-fuel and air mixes in the combustion chamber-high peak temperatures => high NOx
• Premix (B)-mixture of fuel and air enters combustion chamber-Lean mixture is combusted (high air to fuel ratio)-Low temperatures => low NOx

Question 9

Staging principle

Answer 9

• Fuel/air ratio varied
• Hydrocarbon rich zones generated (reducing conditions)
• Reburning mechanism used to reduce Nox
• Double annular combustors
• Parallel/Axial staging

Question 10

Burner systems

Answer 10

Can-type combustors

• small to medium size units
• Separated flow
• Combustion chambers situated around gas turbine axis

Annular combustors

• medium size to large units
• compact design
• Low NOx emissions

Silo

• type combustors
• large units
• Reduced pressure losses
• High fuel flexibility
• complete burnout of fuel
• Low load operation
• NOx problems

Question 11

Blade cooling

Answer 11

• necessary to reduce thermal stress
• Reduces efficiceny
• pressurized air bled off

Question 12

Pressure loss factor

Answer 12

look at equation

Question 13

Flame stability

Answer 13

Stability loop limitsLimit = flame blows outWeak: unsufficientfuel flowRich: toomuchfuel

Question 14

Steam and Gas (STAG) combined cycles

Answer 14

• High efficiency through implementation of the rule: high temperature of heat addition and low temperature of heat rejection
• Current efficiencies up to 60%
• Typical configuration–Gas turbine = topping cycle (in most cases)–HRSG = Heat Recovery Steam Generator–Steam cycle = bottoming cycle