Heat Recovery Steam Generators and Waste Heat Recovery Boilers

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

Comparison of Waste Heat Recovery Boilers (WHRBs) versus Heat Recovery Steam Generators (HRSGs)

Parameter

WHRB

HRSG

Type of plant

Process

Power and cogen

Steam use

Power and process

Power and process

Power plant cycle

Rankine, cogen, or process

Combined or cogen

Upstream

Process plant

Gas turbine

Gases

Dusty

Clean

Fired/unfired

Both

Both

Design types

Various

Limited

Steam pressure

Mostly <70 bar

Up to 150 bar

Pressure levels

Single

Mostly multiple

Steam temperature

<500°C

Up to 570°C

Tubes in boiler

Plain mostly

Finned mostly

Erosion issues

In many cases

No issue

Corrosion issues

In many cases

Only occasionally

Gas cleaning upstream

Many times required

No

Gas cleaning

Many times required

No

Downstream

Draft

Balanced

Pressurized

Fans

Required

Only with fresh air-firing mode

Refractory usage in

Plenty

Negligible

Boiler

Number of boilers

Limited

Many

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.

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