How to balance steam loads efficiently

In brief:

• An understanding of the load vs. efficiency relationship is a guide for better decisions about boiler operation.
• The oxygen content in turbine exhaust gases is sufficient to support the combustion needed for a downstream waste heat boiler.
• The heat recovery steam generator uses about 8% to 10% less fuel input to produce the same amount of steam as a package boiler.

Figure 1. Typical D-type package boiler is ready to be installed.

Many process plants, refineries and chemical plants have package boilers (Figure 1) and supplementary gas-fired heat recovery steam generator (HRSG) (Figure 2). Without a clear understanding of the thermal performance characteristics of these two types of steam generators, plant engineers often don’t assign loads to them judiciously so that the total plant steam is generated most efficiently.

If you know the characteristics of typical natural-gas-fired package boilers and gas-turbine HRSGs used in process or cogeneration plants, you can generate steam optimally and minimize fuel bills. The knowledge also will help when planning which type of HRSGs should be bought, whether unfired, supplementary fired or furnace fired.

Figure 2. A typical furnace-fired HRSG has a water-cooled membrane wall, O-type boiler with furnace, superheater, evaporator and economizer.

Steam generator performance

Table 1. Boiler performance varies as a function of the load.

The first step is to understand the load-versus-efficiency characteristics of a typical package boiler. Table 1 shows performance variation as a function of load. The boiler with an economizer generates 100,000 lb/h of saturated steam at 400 psig using natural gas. At 100% load, the exit gas temperature is 320 °F. Excess air is 15%. The various losses (per ASME PTC 4.1) are shown along with the efficiency.

The efficiency variation isn’t much with load. The casing loss is a function of casing temperature, wind and ambient conditions and doesn’t change with load. Hence, efficiency as a percentage of boiler duty will be smaller at higher loads, and vice versa. The flue gas losses on the other hand will be greater at greater loads as the exit gas temperature is greater at greater loads and decreases as the load decreases.

The combination gives a somewhat flat or parabolic performance curve (Figure 3). The efficiency on a lower heating value (LHV) basis increases slightly from 91.44% at 25% load to about 60% load and then tapers off. To determine burner duty, divide the boiler duty by the efficiency. Using a performance sheet obtained from a boiler supplier, one can get an idea of the fuel consumption at any load.

Figure 3. Boiler performance as a function of load shows a decline at conditions above some load point.

HRSG performance

Heat recovery steam generators behind gas turbines are either unfired or fired. In the fired category, depending on the firing temperature, HRSGs are classified as supplementary fired or furnace fired. Typically, turbine exhaust gases have 14% to 15% by volume of oxygen in the exhaust, which can be used as combustion air. Simply by adding fuel to a burner located in the gas stream, one can increase the exhaust gas temperature from the typical 900 °F to 1050 °F to as high as 2,500 °F to 3,000 °F, depending on boiler design.

When the firing temperature is less than 1,600 °F, use an HRSG with insulated casing. This resembles an unfired unit. However, when firing temperature gets higher, a water-cooled membrane wall design is preferred. The water-cooled enclosure permits you to fire to as high as 3,000 °F. Note that in package boilers, the flame temperature is around 3,200 °F. This type of HRSG is called a furnace-fired unit and is available as shop-assembled units that handle as much as 400,000 lb/h exhaust gas flow. When gas flows are higher, field-erected units might be necessary because of shipping concerns.