Relating Oxygen and Energy Input in Turbine Exhaust Gases

Relating Oxygen and Energy Input in Turbine Exhaust Gases

Gas turbine exhaust gases typically contain 13%–16% oxygen by volume compared to 21% in atmospheric air. If steam is injected in the gas turbine for NO x control, the oxygen con- tent will be further reduced. Still there is enough oxygen to raise the exhaust gases to about 1600°C (see Figure 1.6). Sometimes, augmenting air is introduced at the burner to ensure a stable combustion process.

The energy Q in GJ/h required to raise the temperature of exhaust gases from t 1 to t 2 °C is given by an energy balance around the burner, but approximately it is Q = 10 –6 ×W g × (h 2 –h 1 )

FIGURE 1.4

Burner for turbine exhaust. (Courtesy of Natcom Burner, Division of Cleaver Brooks, Thomasville, GA.)

18 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Modular HRSG

Integral deaerator

CFD velocity contours

HP evaporator

LP evaporator

CFD velocity vectors

Super heater

HP economizer

LP economizer

CB-Natcom duct burner

Modular HRSG

FIGURE 1.5

Duct burner in a HRSG. (Courtesy of Natcom Burner, Division of Cleaver Brooks, Thomasville, GA.) 19

11 100 , MM kcal/h 2 and % H 9

%O 60

7 Burner duty

40 Burner duty 5

500 600 700 800 900 1000 10 1200 1300 1400 1500 1600 Firing temperature, °C

FIGURE 1.6

HRSG firing temperature, burner duty, and exhaust gas analysis.

where h 1 ,h 2 are the enthalpies of the gas before and after combustion in kJ/kg, and W g is the exhaust flow in kg/h. The fuel quantity is small and can be neglected when compared to the exhaust gas flow. A more accurate expression will be (W g +W f )h 2 –W g h 1 = 10 6 × Q where W f = fuel consumption in kg/h. W f = 10 6 × Q/LHV where LHV is the lower heating value in kJ/kg. If O% volume of oxy- gen is available in the exhaust gas, the equivalent amount of air W a in W g kg/h of exhaust gases may be shown to be

Combustion Calculations

W a = (100/23) × W g × O × 32/100/28.4 = 0.049 W g × O kg/h air where the oxygen in % volume is converted to mass basis by multiplying with its MW of 32 and dividing by the exhaust gas MW of 28.4 (typical gas turbine exhaust analysis is used. % volume CO 2 = 3,

H 2 O = 7, N 2 = 75, and O 2 = 15). The (100/23) is the conversion from oxygen to air on mass basis. The energy input on HHV basis = 10 6 × HHV × (Q/LHV). Now 1 GJ of energy input requires A kg of air for combustion as shown in Table 1.5. Hence, 10 6 × HHV × (Q/LHV) requires 10 6 × HHV × (Q/LHV) × A kg/h air = W a = 0.049 × W g × O. Simplifying, Q = 0.049 ×

10 –6 ×W g × O × LHV/(A × HHV). For typical natural gas and fuel oils, (LHV/A/HHV) may

be approximated as 0.00287. Hence within ± 3% margin, Q = 140 × 10 –6 ×W g × O = 0.000140 W g × O; Q is in GJ/h, W g in kg/h

(1.14a) Q = 60 W g O in British units. Q in Btu/h, W g in lb/h

(1.14b) For example, with a fuel input of 30 GJ/h (28.44 MM Btu/h), the % volume of oxygen consumed

with 70,000 kg/h (154,000 lb/h) of exhaust gases will be O = 30/(0.000140 × 70,000) = 3%. This can raise the temperature of 70,000 kg/h gas by about 360°C.