Heat Recovery Steam Generators (HRSGs), Part 3: Predicting Off-Design Performance

Parts 1 and 2 of this series reviewed the basic operating principles of heat-recovery steam generators (HSRGs) working with gas turbine in industrial heating and power-generation schemes. Here, we will examine how to analyze HRSGs based on the operating conditions. Gas-turbine HRSGs operate at different conditions of gas flow, exhaust gas temperature and analysis due to the variations in gas-turbine exhaust parameters. These parameters vary with ambient temperature, fuel composition and load. Hence, predicting HRSG performance under different conditions is a must for consultants, plant engineers and HRSG designers. However, since HRSGs in general are of the convective type, one can easily simulate their off-design performance without actually designing the HRSG in terms of tube sizes, dimensions, surface areas, etc. A little knowledge of basics will help understand the process.

First the design temperature profile as discussed above is obtained at design conditions. We know the duty of each section and the gas/steam temperatures. Then one computes the U.S. values for each section:

(U.S.) = Q/DT, where Q = duty in Btu/h, DT = log-mean temperature difference, °F. U = overall heat transfer coefficient, Btu/ft²h°F and S = surface area, ft²

Then under different gas inlet conditions, the (U.S.) design values are corrected for gas analysis, temperature and gas flow [see my book "Waste Heat Boiler Deskbook" for equations.]

Then the actual duty transferred at each section is obtained using the equation (U.S.) corrected × DT. Gas/steam temperature profiles, duty and steam generation at off-design cases may be then obtained. Note that the pinch and approach points are assumed only for the design case calculations. In off-design mode, the pinch and approach points are arrived at through an iterative calculation, performed by the program using the corrected (US) values.

The author's HRSGs simulation program does these calculations for complex single or multi-pressure, unfired, or fired HRSGs. One can quickly evaluate how a HRSG performs under different gas turbine loads, ambient temperatures, how much supplementary firing is required and steam flows at different modules.

The following table shows the results from a simple simulation calculation using the HRSGs program. It shows how much fuel is required to generate different amounts of steam. Firing temperatures, gas/steam temperatures, oxygen consumed are all obtained in seconds. The first column shows the design conditions of a HRSG in unfired mode using a pinch and approach of 20 °F and the other columns show the off-design performance at higher steam flows.

item	case 1	case 2	case 3
gas flow, lb/h	150,000	150,000	150,000
inlet gas temperature, °F	900	900	900
firing temperature, °F	900	1281	1707
burner duty, million Btu/h	0	16.81	37.52
Steam flow, lb/h	22,780	40,000	60,000
exit gas temperature, °F	326	315	311
boiler duty, million Btu/h	22.66	39.8	59.9
System efficiency, %	68.7	79.2	84.0
[feedwater = 230 F, blowdown = 3%, steam pressure = 200 psig. Gas analysis (vol%): CO2 = 3, H2O = 7, N2 = 75, O2 = 15. Fuel input is on LHV basis.]

Following can be seen from above table:

1. Supplementary firing is an efficient method of generating additional steam in HRSGs. The additional boiler duty is nearly equal to the fuel input, which suggests that supplementary firing is 100% efficient. Compare this to conventional boilers, where fuel efficiency is about 90-93%. Hence if one requires additional steam in cogeneration plants, the first place to look for is the HRSG. While planning cogeneration projects, engineers should see that the HRSG is designed as a fired HRSG, even though it could be more expensive. In the long run, it pays for itself.

2. The exit gas temperature is also reduced at higher steam loads. This is due to the fact that the economizer acts as a bigger heat sink at higher steam loads. Note that the gas flow through the HRSG is nearly the same at 60,000 lb/h as it is at 22,780 lb/h. Hence with the same gas flow, but with higher water low, the economizer is able to pull the gas temperature lower.

3. The oxygen content is reduced due to supplementary firing, because no additional air is required. The oxygen content drops from 15% volume in unfired mode to 10.67 at 60,000 lb/h. This process is opposite to that which happens in conventional boilers, where losses are increased with higher excess air. We reduce the excess air with supplementary firing, which is why the efficiency also improves. The next section shows how the oxygen content varies with amount of fuel fired.

Oxygen Consumption vs. Fuel Input
Gas turbine HRSGs are usually fired with fuels such as natural gas or distillate oils to generate additional steam. Firing temperatures are limited to about 1,650 °F in the case of supplementary fired HRSGs, which have insulated casing design. In these systems, the casing of the HRSG is internally insulated with several layers of ceramic fiber and protected from the hot gases by an alloy steel liner. Duct burners are used for this purpose. No additional air is required as turbine exhaust contains about 13 to 15% oxygen by volume. In case of HRSGs with significant steam injection in gas turbines, the oxygen content reduces to about 11 to 12% and augmenting air may be required if suggested by the burner supplier. Firing above 1,650 °F requires water-cooled furnace design for the HRSG.

Oxygen consumption is often an important factor in determining burner type and process parameters. The following derivation shows the relation between the oxygen consumption and fuel input.

The energy Q in Btu/h on Lower Heating Value basis (LHV) required to raise Wg lb/h of turbine exhaust gases from a temperature of t1 to t2 is given by:

Q = Wg × (h2-h1) where h1 and h2 correspond to the enthalpy of the gas at t1 and t2.

If O is the % volume of oxygen in the exhaust gases entering the burner, the equivalent amount of air Wa in the exhaust is approximately: Wa = 100 × Wg × O × 32/(23 × 100 × 29.5)

In the above equation, we are merely converting the moles of oxygen from a volume to a weight basis.

A molecular weight of 29.5 is used for the exhaust gases. The quantities 32 and 23 are the molecular weights of oxygen and wt% of oxygen in air.

Simplifying the above, we have:

Wa = 0.0417WgO

Now let us relate the air required for combustion with fuel fired. From basic combustion calculations and from articles earlier on combustion, we know that each type of fuel requires a constant amount of air A for combustion for each Million Btu fired. A = 745 for oil and 730 for natural gas. Thus 10⁶/HHV lb of fuel requires A lb of air. Hence Q/LHV lb of fuel requires:

(Q/LHV) × A × HHV/10⁶ lb of air. And this is equal to Wa from above. (Q/LHV) × A × HHV/10⁶ = Wa = 0.0417WgO or Q = 0.0417WgO× 10⁶ × LHV/(HHV × A)

for natural gas and fuel oils it can be shown that LHV/A/HHV = 0.00124. Hence substituting in above,

Q = 58.4WgO

This equation gives the relation between oxygen consumption O and Q.

Example: It is desired to raise the temperature of 150,000 lb/h of turbine exhaust gases from 950 to 1,575 °F in order to double the steam output of a HRSG. If exhaust contains 15% oxygen by volume, determine the oxygen consumed.

Q = 150,000 × 0.31 × (1575-950) = 29 × 10⁶ Btu/h where 0.31 is the average specific heat. The author's burner program or HRSGs program provides accurate computation of the fuel input.

Hence O = 29 × 10⁶/(150,000 × 58.4) = 3.32 or the final oxygen content = 11.68%, which is still high. This tells us that we can raise the gas temperature further if required, provided the HRSG design can be compatible.

By V. Ganapathy