Standard boilers, which are pre-engineered packages, are inexpensive and are used in applications that are not very demanding in terms of process or emission limits. Decades ago, various manufacturers had developed so-called standard designs for boilers of 40,000-200,000 lb/h capacity with fixed dimensions of furnace, tubes, tube spacing, lengths, and surface areas. If someone wanted a boiler for a particular capacity that was not listed, the next or closest standard model would be offered. Standard models are less expensive than custom designs because no engineering is required to design and build them. It must be borne in mind that these designs were developed 30-50 years ago when the concept of flue gas recirculation and low-NOx burners were unheard of. They also had a lot of refractory in their design—on the floor, front walls, and rear walls—because completely water-cooled furnace designs had not yet been developed. The concerns with refractory-lined boilers are discussed later. However, emission regulations are forcing suppliers to custom design the boilers.
As discussed in Chapter 4, the effect of flue gas recirculation and changes in excess air levels have to be reviewed on a case-to-case basis depending on the NOx and CO levels desired. Hence standard furnace dimensions may or may not be suitable for a given heat input, because the flame shape varies according to the NOx control method used. Flame lengths with low-NOx burners can be wider or even longer than with regular burners. Hence the use of low-NOx burners makes it difficult to select a standard boiler that meets the same need and is also an economical option. The furnace size could be compromised, which may result in flame impingement concerns with the burners used, or the gas pressure drop across the convection surfaces could be very large due to the flue gas recirculation rates used; the efficiency also could be lower due to the higher exit gas temperature associated with the larger flue gas flow. The operating cost due to a higher gas pressure drop is discussed below and in Chapter 4.
Often gaseous and oil fuels are fired at excess air ranging from 10% to 20%; flue gas recirculation could be in the range of 10-35%, depending on the NOx level desired. In a few boilers, 9ppmv NOx has been achieved with the burner operating at 15% excess air and 35% flue gas recirculation rate on natural gas firing. Thus it is possible to have a ‘‘standard’’ steam generator handling nearly 30-40% more flue gases than it was designed for in the good old days when 5-10% excess air was used without gas recirculation: A 100,000 lb/h standard boiler could be operating at gas flow conditions equivalent to those of a
140,0 lb/h boiler if it is not custom-designed. Of course, one could select a larger standard boiler, but it may or may not meet all the requirements of furnace dimensions, because developers of standard boilers generally increase furnace lengths for higher capacity but not the width or height, due to shipping constraints, particularly when the capacity is large. However, standard boilers are useful where one is not concerned about optimizing all the parameters such as efficiency, gas pressure drop, and emission levels and low initial cost is a primary objective.
Packaged steam generators of today are custom-designed with an eye on operating costs and emissions. The furnace design also has undergone major design innovations, the completely water-cooled furnace (Fig. 3.2) being one of them. This design offers several advantages over the refractory-lined boilers designed decades ago.