Fuels, Combustion and Gas-Turbine Cycles
Comprehensive university textbook unit covering fundamental principles, governing relationships, and engineering applications of Fuels, Combustion and Gas-Turbine Cycles.
§3.1 Fuel characterization and stoichiometry
**Core topic:** air requirement and excess air. **Representative relation:** `AFR_st = m_air_st / m_fuel`. **Case context:** boiler burner with varying fuel composition.
§3.2 Steam-generator heat balance
**Core topic:** boiler efficiency and steam production. **Representative relation:** `Q_to_steam = m_dot_s*(h_out-h_in)`. **Case context:** feedwater-to-superheated steam balance.
§3.3 Ideal Brayton pressure-ratio trends
**Core topic:** pressure ratio under constant-property ideal assumptions. **Representative relation:** `eta_B = 1-r_p**(-(gamma-1)/gamma)`. **Case context:** gas-turbine performance thought experiment.
§3.4 Compressor and turbine specific work
**Core topic:** gas-path shaft balance. **Representative relation:** `w_net = cp*(T3-T4)-cp*(T2-T1)`. **Case context:** simple open Brayton cycle.
§3.5 Combustion emissions and operational trade-offs
**Core topic:** emissions intensity and denominators. **Representative relation:** `e_CO2 = m_CO2 / E_net`. **Case context:** plant output versus fuel-carbon accounting.
Step-by-Step Power Plant Engineering Solutions
Three original questions connect calculations, definitions and interpretation. Open each solution to follow the reasoning.
Problem Statement: For ideal pure methane, CH4 + 2 O2 → CO2 + 2 H2O. One mole of methane has mass about 16 g and needs 64 g oxygen; hence **4 kg O2/kg CH4**. If dry air has 23.2% oxygen by mass, the stoichiometric air requirement is 4/0.232 = **17.24 kg air/kg methane**. This ignores real fuel mixtures and moisture.
Worked Engineering Derivation:
**Verification and limitations.** Using the wrong oxygen mass fraction or an LHV/HHV basis changes a combustion calculation materially. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: Feedwater at 850 kJ/kg is heated to steam at 3200 kJ/kg at a mass flow of 20 kg/s. Gross steam-side duty is 20(3200 − 850) = **47 MW(th)**. This is not identical to fuel input, because the boiler has losses, blowdown and possibly attemperation or other mass-flow branches.
Worked Engineering Derivation:
**Verification and limitations.** A measured flue-gas stack temperature is not enough to compute complete boiler efficiency without a fuel and mass balance. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: Assume an ideal reversible Brayton cycle with a constant heat-capacity ratio γ = 1.4 and compressor pressure ratio 10. The ideal expression 1 − 10^(−0.4/1.4) gives **48.21%**. Real simple-cycle efficiency will generally differ because of compressor/turbine isentropic losses, combustor pressure drop and turbine inlet temperature limits.
Worked Engineering Derivation:
**Verification and limitations.** Do not interpret the simple air-standard value as a vendor map or predicted guaranteed turbine performance. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: With cp = 1.05 kJ/(kg K), compressor inlet/outlet temperatures 300 and 610 K, and turbine inlet/outlet temperatures 1400 and 900 K, net gas-path specific work is 1.05[(1400 − 900) − (610 − 300)] = **199.5 kJ/kg**. Turbine work alone would be 525 kJ/kg and would badly overstate shaft output.
Worked Engineering Derivation:
**Verification and limitations.** The example assumes identical gas and air mass flows and constant cp solely for introductory comparison. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: An illustrative emissions inventory gives 16 tonnes CO2 for 80 MWh(e) net export in the same hour. The intensity is (16,000 kg)/(80 MWh) = **200 kg CO2/MWh**. This is a synthetic arithmetic case, not an emissions factor for a fuel or technology. Boundary choice must state whether the metric covers direct combustion, upstream supply or full lifecycle.
Worked Engineering Derivation:
**Verification and limitations.** Emissions accounting must not conflate greenhouse-gas intensity with NOx or air-quality compliance. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.