What Is HRSG Simulation?

Knowing exhaust gas flow, temperature, gas analysis, and steam parameters, one can establish gas–steam temperature profiles and duty of each section such as superheater, evaporator, and economizer in the design mode. Then, in the off-design mode, determine how the HRSG performs when exhaust gas conditions or any of the steam parameters change. In other words, one can thermally design an HRSG and evaluate its performance without knowing about its geometry per se [1]. HRSG suppliers do what is called physical design. With physical design, one should know the HRSG configuration and the tube and fin geometry details; then, the overall heat transfer coefficient U is estimated for each component based on gas velocity, tube geometry, fin configuration, tube spacing, tube length,

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

and then, the required surface area for each component is estimated using the equation

A = Q/(UΔT). Once the design mode is established, the number of transfer units (NTU) method is used to arrive at the off-design performance of the complete HRSG or any of its components as discussed in Chapter 4 and in Appendix A. With simulation process, we do not compute U but compute the term UA = Q/ΔT for each heating surface, which is possible from gas–steam temperature profiles alone. Then, in the off-design mode, we correct this term (UA) for each surface such as superheater, evaporator, or economizer for variations in gas flow, temperature, and analysis and use the NTU method to predict its off-design performance. Thus, the geometry of the HRSG need not be known to obtain performance information. The results obtained from simulation will be close to that obtained using physical design as will be shown later.

Note that simulation will not give results such as surface area per se. The value (UA) is a proxy for surface area. The product (UA) is obtained in design and off-design cases. Since U varies with gas flow, analysis, and temperatures, (UA) will vary depending on these parameters. Also gas pressure drop or tube wall temperature is not evaluated as physical design data are not used in simulation calculations. One can get a good idea of the overall thermal performance as shown in the following examples.

Since simulation calculations are tedious particularly if multimodule HRSGs are involved, a program has been developed by the author. Figure 5.1 shows how one may arrive at complex HRSG configurations by combining basic modules such as a superheater, an evaporator, and an economizer. These modules may be arranged in any order to opti- mize the HRSG performance. The concept of common economizer and superheater is used, which allows an economizer to feed an evaporator located anywhere in the gas path or a superheater to be fed by an evaporator located anywhere in the gas path. This enables one to simulate complex multimodule HRSGs such as those shown in the figure.

Example 3 HRSG configuration

Sh. Evap. Eco. Sh. Evap.

Evap.

Evap. Eco.

Eco.

Sh.

Sh. Burner Sh. Sh. Evap. Example 1 HRSG configuration

Example 2 HRSG configuration

1 2 3 Example 4 HRSG configuration

Evap. Sh. Evap.

Eco.

Evap.

Sh. Eco. Evap.

1 2 3 4 Burner

Evap. Sh. Evap. Eco. Eco. Example 5 HRSG configuration

1 2 3 4 5 Example 7 HRSG configuration By combining the six basic

modules shown above complex HRSG configurations can be

Sh. Evap. Sh.

Evap. Eco. Evap. Eco.

developed. The HRSGS

1 Burner 2 3 4 5 6 7 1 software contains these 2 3 examples.

Sh. Sh. Evap. Eco.

FIGURE 5.1

By combining various modules shown earlier, complex HRSG configurations can be simulated.

HRSG Simulation 265

Applications of HRSG Simulation

• Given the exhaust gas flow and temperature, the first question on any process

engineer’s mind is how much steam can be generated at a given pressure and temperature and what the exit gas temperature is. In the design mode, using the simulation concept, one can determine the gas–steam temperature profiles, duty of each component, and steam generation. Design condition is typically the unfired mode of HRSG operation at the design or guarantee point. This is done by simply selecting pinch and approach points at each evaporator as explained later. Hence, by simply assuming pinch and approach points, we get a lot of information about the HRSG.

• We can obtain the off-design performance and see what happens to the HRSG

gas–steam temperature profiles when exhaust gas flow or temperature conditions or any of the steam parameters change. This can in part load or fired mode of operation. This situation arises very often in any plant. There is only one design case, but there can be numerous off-design cases, both unfired and fired.

• One can estimate fuel required for generating a given quantity of steam in the

fired mode without approaching the HRSG suppliers! Thus this is also a great planning tool.

• Check if the economizer is likely to generate steam at low loads. One can change

approach temperature and redesign the HRSG if necessary (increase approach point in design mode or decrease the pinch point and ensure that steaming does not occur at low loads).

• Plant engineers can evaluate different gas turbines to see which one matches the

steam needs of the plant better. There can be many gas turbines that can deliver

a certain amount of power that the plant needs, but their exhaust gas conditions may be different. The exhaust gas flow can be higher or lower and so also the exhaust gas temperature. By doing a simulation study, plant engineers can get an idea of the steam generation, fuel required in fired mode, and efficiency of HRSG in various modes of operation for each gas turbine in consideration. Using this information, they can zero in on any particular gas turbine that meets their power as well as steam needs better.

• Plant or process engineers can also manipulate the HRSG configuration or

arrangement to maximize the energy recovery. They can relocate the heating surfaces such as superheater, evaporator, or economizer and see if the energy recovery can be improved; this manipulation is necessary when there are multi- modules in the HRSG. The HRSG arrangement can be optimized, and then, con- sultants can develop specifications based on this configuration. One should not expect HRSG suppliers to spend time on this type of analysis as they may not have the time and also they know less about the plant than the plant engineers concerned. For example, process steam may be taken out of the low-pressure (LP) evaporator for deaeration or heating purposes, or import steam from other boilers can be superheated in the HRSG. Plant engineers can quickly find out how this affects the HRSG performance without waiting for a reply from the HRSG supplier.

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

• Field performance of an existing HRSG may also be evaluated and correlated

with guarantee values to see if the performance is acceptable. This is illustrated later. One can also get an idea which surface is underperforming or overperforming.

All these points are illustrated with the following examples.

What Is HRSG Simulation?