ABSTRACT
Boiler and HRSG designs have undergone significant changes during the last few
decades with the enforcement of emission regulations in various parts of the
world. Decades ago boiler and HRSG users were concerned about two issues
only: the initial cost of the boiler or HRSG and the cost of operation. Low boiler
efficiency, for example, meant higher fuel cost, and a large pressure drop across
boiler heating surfaces resulted in increased fan power consumption. Each
additional 1 in. WC pressure drop in a boiler of 100,000 lb=h capacity results in about 5 kW of additional fan power consumption. In a gas turbine HRSG, an
additional 4 in. WC of gas pressure drop decreases the gas turbine power output
by about 1.0%. At 320F stack gas temperature, the difference in efficiency between 5% and 15% excess air operation on natural gas is about 0.4%.
Therefore, steam generators were operated at the lowest possible excess air,
about 5% or so, to maintain good efficiency. With strict emission regulations in
vogue throughout the world, present-day steam generators or HRSGs, in addition
to having low operating costs, must limit the emissions of CO2;CO;NOx; SOx, and particulates. The expression ‘‘low NOx, no SOx, and no rocks’’ aptly
describes the direction in which we are headed. However, several of the
techniques used for emission control increase the cost of owning and operating
the boilers and HRSGs. For example, in order to meet the stringent levels of NOx
and CO, today’s boilers have to operate at higher excess air and use some flue gas
recirculation (FGR), which affects their efficiency as well as their operating costs
significantly, as we discuss later.
