WHRBs are bulky and expensive. The cost of steam and power generated is higher, even with input heat. It makes sense to have a waste heat system only if any one of the following conditions is met:

• The cost of the regular fuels is high and hence the high-cost power and steam from waste heat boilers are acceptable.

• The process demands its inclusion.

In the Middle East, for example, where fuel and interest costs are low, GTs run on open cycle (OC) or simple cycle (SC) with no HRSGs. The power so produced in OC mode is marginally more expensive than the power from closed or combined cycle (CC) but it is still affordable. This is now changing with steadily rising cost of petroleum. In the cement

Industry, a lot of low-grade heat is available in the dusty kiln gases at temperatures —350°C. This makes it very expensive to generate power.

Waste heat recovery boilers are specific to each process. Although the variety is large, the population of WHRBs is rather small, as there can only be one WHRB for each process stream. On the other hand, HRSGs are found only in gas-based power plants and are available only in two designs (vertical and horizontal) and are in use all over the world.

HRSGs are discussed in this chapter; the WHRBs are only of limited interest.

A broad comparison of the WHRBs and the HRSGs is presented in Table 14.1.

Unlike a conventional boiler, the HRSG has upstream equipment in the form of a GT, whose output is used by the HRSG to produce steam power efficiently. Hence the GT has an overriding influence on the size, shape, and dynamics of the HRSG. Although the HRSGs are limited to vertical and horizontal designs (designated by the direction of gas flow), a variety of GT cycles are possible. A good appreciation of GTs and the GT cycles is necessary to understand the HRSGs properly.