Fresh Air Firing

Fresh Air Firing

Often fresh air firing is resorted to generate steam when the gas turbine is not in operation. However, it should be noted that using ambient air for generating steam in an HRSG is not an efficient process and must be used sparingly or for emergency reasons only and not for continuous operation. It must be clearly understood that in HRSGs, the ratio of gas flow to steam is much larger than in conventional steam generators. Hence, a large amount of energy is wasted by raising the temperature of air from ambient conditions and exhausting the gas at a higher temperature at the stack. One may wonder that we do the same thing in

a steam generator. However, keep in mind that the ratio of flue gas to steam is about 1.1 in a steam generator, while in an HRSG, it will be in the range of 6–8, and so a large amount of exhaust gas is heated from ambient conditions relative to steam generation and then vented to the atmosphere. The sizing of the HRSG should be such that the exhaust gas flow in fresh air mode also should be in the same range as the exhaust gas flow with gas turbine opera- tion or else the gas velocities can be much lower causing nonuniformities in gas flow at the heat transfer surfaces and poor heat transfer. It can be slightly lower than the turbine exhaust gas flow say by 10%–20% but not much lower. The following examples give an idea of the performance in fresh air mode with two different air flows, and one can see the issues here.

Example 5.13

The HRSG in Example 5.1 is to be operated in fresh air mode. Check the performance when 100,000 kg/h of air and 80,000 kg/h of air are used to generate 12,000 kg/h.

The results for both the cases are presented in Figure 5.16a and b. It can be seen that the efficiency is higher when the air flow is lower and vice versa. However, the firing temperature increases with lower air flow. If the HRSG had been designed to accept higher firing temperatures, then we can further lower the air flow and generate the same amount of steam more efficiently. However, we cannot reduce the air flow too much due to gas maldistribution concerns. When sizing the fan, about 80% of the turbine exhaust flow should be adequate. While locating the fan and duct, one should ensure that the flow nonuniformity is minimized as the fan is generally located at right angles to the main duct or at an angle. Grid plates with variable openings may be inserted in the duct,

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

Project—hh Units—Metric case—B Remarks - Amb. temp., °C = 25 Heat loss, % = 1 Gas temp. to HRSG C = 25 Gas flow, kg/h = 80,000

% vol CO 2 =.H 2 O = 1. N 2 = 78. O 2 = 21. SO 2 =. ASME eff., % = 69.85 tot duty, MW = 9.2 Surf.

Gas temp.

Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty

Pres. Flow

°C kcal/h °C Burn 25 551

in/out °C

MW kg/cm 2 a kg/h

Stack gas flow = 80,932 % CO 2 = 2.05 H 2 O = 5.02 N 2 = 76.41 O 2 = 16.49 SO 2 =.

Fuel gas: vol % Methane = 97 ethane = 3

LHV - kcal/cv m = 11 LHV - kcal/kg = 6,623 aug air - kg/h = 0

Sh. Evap. Eco. Project—hh Units—Metric case—B Remarks -

Amb. temp., °C = 25 Heat loss, % = 1 Gas temp. to HRSG C = 25 Gas flow, kg/h = 100,000 % vol CO 2 =.H 2 O = 1. N 2 = 78. O 2 = 21. SO 2 =. ASME eff., % = 62.9 tot duty, MW = 9.

Surf. Gas temp.

Pstm. Pinch Apprch. US Module no. in/out °C

Wat./Stm. Duty

Pres.

Flow

°C kcal/h °C Burn 25 486

in/out °C

MW kg/cm 2 a kg/h

Stack gas flow = 101,016 % CO 2 = 1.79 H 2 O = 4.52 N 2 = 76.61 O 2 = 17.06 SO 2 =.

Fuel gas: vol % Methane = 97 Ethane = 3 LHV - kcal/cv m = 105 LHV - kcal/kg = 11,922 aug air - kg/h = 0

FIGURE 5.16

Fresh air firing cases with 80k and 100k kg/h air flow.

and by measurement of static pressure, one can get an idea of the velocity profile across the cross section at the burner. The profile should also be checked for turbine exhaust gas. The velocity profile cannot be optimized for both turbine exhaust and fresh air as the flow directions are at nearly 90° to each other. Some compromise must be accepted depending on how often fresh air firing is used. More on fresh air firing is discussed in Chapter 4.