Sustainable Energy Mod.1: Fuel Cells & Distributed Generation Systems Dr. Ing. Mario L. Ferrari Thermochemical Power Group (TPG) - DiMSET University of Genoa, Italy
: Gas Turbines
Simple cycle (1/4)
Simple cycle (2/4) Ideal Brayton cycle: efficiency Ideal Brayton cycle: maximum specific work It is possible to demonstrate (first order derivative = 0) that the expression has a maximum for: β
Simple cycle (3/4) Real Brayton cycle: efficiency (1/2) l: limit (real fluid) i: internal (real cycle) r: real
Simple cycle (4/4) Real Brayton cycle: efficiency (2/2) In the case of real cycle efficiency curve shows a maximum value The compression ratio for maximum efficiency is higher than the compression ratio for maximum specific work The designer has to decide if optimising the turbine for maximum efficiency or maximum specific work Usually aeroengines (and aeroderivative turbines) are optimized for η max typically with high β (e.g. 30) and heavy-duty turbines for W max with lower β values (e.g. 17). η β
Recuperated Cycle (1/2) Efficiency increase Effectiveness or regeneration rate
Recuperated cycle (2/2) Efficiency:
Intercooled Cycle Specific work increase Efficiency decrease (if ideal cycle) Efficiency increase possible (if real cycle) More efficiency benefit if recuperator is includes Example: LMS100 GE machine (P=100 MW, η=46%)
Reheated Cycle Specific work increase Efficiency decrease Increase of heat content at turbine outlet (useful aspect for combined cycles or recuperated cycles Essential for recuperator application in high β cycles
Ericsson Cycle 3 4 L H L H Q 2 5 Not real cycle because too many exchangers and machines Efficiency equal to Carnot cycle if cycle with R=1 (recuperated)
Mixed Cycles: STIG (or Cheng) Cycle Efficiency increase (up to 46% in simple cycle), power increase Low cost in comparison with combined cycles Disadvantages: large water consumption, power limitation and surge risks Examples: Allison/General Motors, 4/6 MWe; Kawasaki, 2/4 Mwe, GE LM5000)
Mixed Cycles: RWI Cycle Water inlet Efficiency increase (up to 52%), power increase (lower than STIG) Disadvantages: large water consumption, power limitation and surge risks, large dimension heat exchanger No commercial application because it is no too cheap in comparison with CC
Mixed Cycles: HAT Cycle Efficiency increase (up to 55%), power increase Disadvantages: large water consumption, power limitation and surge risks, saturator is critical No commercial application, but prototype developments
Gas Turbine Emissions Chemical pollution: CO emission are not significant (high air excess): <15-20 ppm HC emission are not significant (high air excess and natural gas no benzene based composites) NO x (to be reduced with apt devices) SO x (not significant if natural gas, low if kerosene): < 2-4 ppm (n.g.) Carbon particulate (not present if natural gas) Thermal pollution: CO 2 emission (low if natural gas) Emission content depends on turbine type, combustor geometry, operating conditions (temperature), maintenance type, fuel composition. Zeldovich reactions (significant if T > 1600 K)
Gas Turbine Emissions Abatement Different approaches are considered: Low temperature operation (not efficient) Steam injection at combustor level Selective catalytic reduction devices Dry low NO x combustors (premixed flames) Steam injection: Steam injection in STIG cycles is useful to have more uniform temperatures in combustors (50% reduction in No x ) Premixed flames (emissions < 15-20 ppm): With premixed flows (upstream of combustor inlet) it is possible to have uniform combustion and no peak temperature (<1600 K) avoiding significant thermal NO x formation.