Custom-designed boilers, as the term implies, are designed from scratch. Based on discussions with the burner supplier and the level of NOx and CO desired, one first selects the type of burner to be used and the emission control strategy. A few options could be considered:
Use a large amount of flue gas recirculation (FGR) and a low cost burner, which results in higher operating costs; one may use a large boiler with a wide convection bank to minimize gas pressure drop.
Use an expensive burner, which uses fuel or air staging methods and requires little or no flue gas recirculation. A few burners can guarantee
About 20-30 ppmv NOx (at 3% oxygen dry) on gas firing. Installation and operating costs associated with FGR are minimized.
One can also consider the possibility of using a selective catalytic reduction (SCR) system along with a less expensive burner, which has a low to nil FGR rate.
Steam injection may also be looked into, and the cost of steam versus FGR may be compared.
Depending on the NOx and CO levels desired and the fuel analysis, the solution may vary from case to case, and no obvious solution exists for every situation. Thus one arrives at the best option from an emission control viewpoint and then starts developing the boiler design using the excess air and FGR rates for the fuels in consideration; the furnace dimensions to avoid flame impingement on the furnace walls are then arrived at. Assuming a specific exit gas temperature, the boiler efficiency calculations are done to arrive at the air and flue gas flow rates and the amount of flue gas recirculated. This is followed by an evaluation of furnace performance and design of the heating surfaces. The exit gas temperature from the economizer is arrived at and compared with the assumed value; efficiency is recalculated using the computed exit gas temperature, and revised air and flue gas flows are obtained. (Air and flue gas quantities depend on the amount of fuel fired, which in turn depends on efficiency.) Another iteration starting from the furnace is done to fine-tune the performance. The superheater performance is evaluated at various loads to determine whether the surface areas are adequate.
If different fuels are fired, these calculations are carried out for all the fuels. Efforts are then made to reduce the fuel consumption and also lower the fan power consumption, which are recurring expenses, by fine-tuning the design of the evaporator and economizer. A large economizer may be used to improve the boiler efficiency if the duration of operation warrants it. The designer also has the ability to change the dimensions of the convection section—for example, the number of tubes wide, length, tube spacing, or even tube diameter—to come up with low gas pressure drop and hence low fan operating cost as shown below. Based on partial load performance and gas temperature profiles, bypass dampers may be required if an SCR system is used. Hence it is likely that the steam parameters of several boilers could be the same but the designs different due to the emission control strategy used and degree of custom designing. A computer program is used to perform these tedious calculations.
A 150,000 lb/h boiler firing standard natural gas and generating saturated steam at 285psig with 230°F feedwater uses 15% excess air and 15% flue gas recirculation. The exit gas temperature is 323°F. Compare the performance of a standard boiler with that of a custom-designed unit. The flue gas flow through the boiler is 184,300 lb/h. With 80°F ambient temperature, the efficiency is 83.38% HHV
The results of the calculations are shown in Table 3.1. The following points may be noted from this table:
1. The efficiency is the same in both designs because the exit gas temperature and excess air are the same. Also, the furnace dimensions are the same. Hence the furnace exit gas temperature is the same in both designs.
2. The convection sections are different. In the standard boiler, we used a standard tube spacing of 4 in. In the custom-designed unit, we reduced the surface area significantly by using fewer rows and also made the convection bank tube transverse spacing 5 in. This reduces the gas pressure drop in the convection bank by 4 in. WC. It also reduces the duty of the evaporator section, as can be seen by the higher exit gas Temperature of 683°F versus 550°F.
3. We added a few more rows to the economizer in the custom-designed unit and made its tubes longer to obtain the same exit gas temperature and also to handle the additional duty. Economizer steaming is not a
Table 3.1 Reducing Boiler Gas Pressure Drop Through Custom Designing
Concern in packaged boilers due to the small ratio of flue gas to steam flows (this aspect is discussed later). Hence we can absorb more energy in the economizer, which is a less expensive heating surface than the evaporator. The overall gas pressure drop saving of 4 in. WC results in a saving of 31 kW in fan power consumption (see Example 9.06b for fan power calculation). If energy costs 7cents/kWh, for 8000h of operation per year the annual saving is
31 x 0.07 x 8000 = $17,360.
This is not an insignificant amount. Simply by manipulating the tube spacing of the convection bank, we have dramatically reduced the fan power consumption and the size of the fan. Also the boiler cost for the two designs should be nearly the same because the increase in economizer cost is offset by the smaller number of evaporator tubes, which reduces the material costs as well as labor costs. To improve the energy transfer in evaporators one can also use finned tubes if the boiler is fired with natural gas or distillate fuels. For example, if we desire good efficiency but do not want an economizer because of, say, shorter duration of operation or corrosion concerns, we may consider using extended surfaces in the convection bank to lower the evaporator exit gas temperature by about 40-100°F, which improves the efficiency by 1-2.5% compared to a standard boiler.
4. Another important point is that surface areas should be looked at with caution. One should not purchase boilers based on surface areas, which is still unfortunately being done. It is possible to distribute energy among the furnace, evaporator, and economizer in several ways and come up with the same overall efficiency and fan power consumption and yet have significantly different surface areas as shown in Tables 3.1 and 3.2.