Packaged boilers are widely used in cogeneration and even in combined cycle plants as auxiliary boilers providing steam for turbine sealing and steam for other uses when the gas turbine trips and the HRSG is not in operation. These boilers are generally shop-assembled and custom-designed. Typically, boilers of up to 250,000 lb/h capacity can be shop-assembled and larger units are field-erected. Steam parameters vary from 150psig saturated to 1500psig, 1000°F. They typically burn natural gas, distillate fuel oils, and even heavy residual oils. Widely used methods for NOx control are low-NOx burners, flue gas recirculation, and selective catalytic reduction systems (SCRs). Carbon monoxide catalysts are also used if required. Emission control methods are discussed in Chapter 4.

Packaged boilers could be further classified as D, A, or O-type depending on their construction, as shown iN Fig. 3.7. In the A- and O-type boilers, the flue gases exit the furnace and then make a 180° turn, split up into two parallel paths, and flow through the convection section, then recombine to flow through the economizer. Using a convective superheater in this type of boiler is tricky, because it has to be split into two halves. A radiant design may be located at the furnace exit, but it operates in a harsh environment as discussed later.

D-type boilers are widely used in industry. The flue gases generated in the furnace travel though the furnace, make a turn, and go through the convection

Bank and then through the economizer to the stack. The gas flow is not split into two parallel paths as in the A — or O-type designs. If a superheater has to be located in the convection bank, the D-type design is the most convenient, because there is no concern with maldistribution in gas flow between parallel paths as with the O — and A-type boilers, which may lead to thermal performance issues. However, the O — and A-type boilers are more suitable as mobile units, because they have balanced weight distribution; rental boilers, which move from location to location, are generally of A — and O-type designs.

The gas-fired O-type boiler shown in Fig. 3.8 is another variation of packaged boiler design. In this boiler the flue gases do not make a turn at the furnace end; the gases flow straight beyond the furnace to a convection section consisting of bare and finned tubes; the finned tubes make the convection section compact, thus reducing the overall length of the boiler. The advantage of this design is that the width required is not large, because the width of the furnace determines the width of the unit, whereas in a typical O — or A-type boiler the width of the furnace is added to that of the convection bank, making it difficult to ship the boiler to certain areas of the country or the world. Also, a convective type of superheater can be easily located behind a screen section. The advantages of the convective superheater over a radiant design are discussed later.

A recent application for packaged boilers has been in combined cycle plants. These plants require steam for turbine sealing purposes when the HRSG trips, and they need it at short notice, say, within 5-15 min. Packaged boilers with completely water-cooled furnace designs are well suited for fast start-ups, as discussed later.

Very high steam purity as in utility plants can be obtained in packaged boilers through proper design of steam drum internals. Depending on the application, steam purity in the range of 30-100 parts per billion (ppb) can be

Achieved. Packaged boiler designs have evolved over the years and have adapted well to the needs of the industry.