Improving Boiler Efficiency

Typical efficiency on lower heating value (LHV) basis for a natural gas–fired boiler is about 93% with 150°C exit gas temperature. In the boiler with elevated steam drum (Figure 3.3), a novel glycol-based closed-loop heat recovery system was incorporated to improve the boiler efficiency by about 2.5% with the exit gas temperature controlled at about 101°C, lower than the feed water temperature! The system also helps increase the air temperature to the forced draft in severe Canadian winter conditions when ambient air temperature ranges from –10°C to –40°C! The scheme is shown in Figure 3.18. There are three glycol heat exchangers in this concept. The boiler capacity is 193 t/h of steam at 3800 kPa with110°C feed water temperature. The package boiler assembly consists of the furnace and convection bank as a single unit built in the shop and was the largest that could be shipped by land in the United States and Canada. Headers were used for the convection bank instead of steam and mud drums. The steam drum was connected to the boiler bank by a system of external downcomers and riser piping. This approach enabled the large steam drum to be shipped separately. Furnace projected area and volume were also liberally sized so that the furnace heat release rates on volumetric and area basis

are low, 587,000 kcal/m 3 h (66,000 Btu/ft 3 h) and 481,000 kcal/m 2 h (177,000 Btu/ft 2 h). The drum holdup time also was large exceeding 5.5 min from normal level to empty as there was no shipping limitation for the drum.

The boiler efficiency was shown to be 86% on HHV basis while generating 193 t/h of steam. The closed-loop glycol system has two air heaters that can be bypassed on glycol side. The first finned tube air heater preheats cold ambient air entering the fan and maintains it at a minimum temperature of about 10°C after mixing with the recirculated flue gas.

Flue gas recirculation

Air heater #2

Air heater #1

To stack Flue gas temperature maintained at 100°C

Natcom

burner

Amient air 40°C–27°C

FD Fan

Glycol flow managed based on boiler load

Scavenger

Glycol pump

Glycol flow through airheater #2

Glycol flow through air

controlled to optimize heat recovery

heater #1 controlled to maintain

the air temperature at fan inlet between 5°C and 40°C

Economizer gas outlet

60% glycol water mixture

FIGURE 3.18

Scheme for improving boiler efficiency using glycol heat recovery system. (Courtesy of Cleaver Brooks Inc., Engineered Boiler Systems, Thomasville, GA.)

Steam Generators 109

TABLE 3.3

Field Data for the Elevated Drum Boiler

Steam flow, t/h

192 Steam pressure, kPa

3751 Feed water in

110 Oxygen in flue gas,%

2.3 2.2 NO x ppmv

46 44 Flue gas recirculation, %

10 11 Ambient temperature, °C

21 22 At air heater exit, °C

66 82 Exit gas temperature, °C

Note: Typical exit gas temperature would have been 150°C–160°C.

The second air heater, located after the fan, preheats the mixture of flue gas recirculated for NO x control and air from the fan. This glycol bypass system maintains the stack gas temperature at about 101°C. If the ambient air temperature is high, as it is in summer, then either air heater may be shut off and the glycol flow bypassed on the tube side of either air heater, as very hot combustion air will increase NO x production. The as-tested performance of the plant is summarized in Table 3.3 [2,3].

Figure 3.19 shows a scheme for improving the efficiency of a gas-fired boiler without using any thermic fluid heating system as discussed earlier. Incoming air from the forced draft fan is heated to about 65°C–75°C by a portion of the feed water. (In case there is no NO x concern, the air temperature can be even further increased and the exit gas temperature from the economizer can be lowered further.) The cooled feed water is heated in another exchanger called scavenger located behind the regular economizer. The increase in air temperature is approximately equal to the drop in flue gas temperature beyond the economizer. If there are no NO x or acid dew point concerns, then the flue gas can be cooled to a lower temperature. This scheme does not require a heat sink such as makeup water to lower the flue gas temperature from the system. Efficiency improvement can be in the range of 1.5%–2% over conventional plants, and the additional cost of the two finned coils and air/gas pressure drop will pay for itself in a short period depending on local fuel cost and the cost of electricity. (This scheme is being patented by Cleaver Brooks, Nebraska, boiler division.)

Improving Boiler Efficiency