Supplementary Firing

Supplementary Firing

Supplementary firing is an excellent way to generate additional steam in cogeneration plants. Steam generators firing oil or gas have a lower heating value (LHV) efficiency of about 93%, while it is nearly 100% when the HRSG operates in the fired mode. That is, if

8 MW of additional energy is to be added to steam, a maximum of 8 MW of fuel on LHV basis alone need be fired in an HRSG. With a steam generator, one would fire about 6% more fuel or about 8.5 MW fuel unless it has a condensing economizer, which is a special case. To illustrate this point, an example is provided.

Example 5.7

Saturated steam is generated at 40 kg/cm 2 g in a boiler. In the unfired mode with an

exhaust gas of 100,000 kg/h at 500°C, the HRSG makes about 15.4 t/h. Determine the fuel required to generate 25 t/h and 40 t/h of steam.

Solution

The HRSG was simulated using a pinch point of 8°C and an approach point of 7°C. Results are summarized in Tables 5.3 and 5.4, and Figures 5.4 and 5.5 show the off- design performance runs.

We see that the additional fuel requirement is less than the boiler duty in the fired modes. To generate 25,000 kg/h, we are using only 5.84 MW of fuel energy while the additional duty is 6.3 MW! Similarly, to generate 40 t/h, we are using only 15.53 MW

274 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

TABLE 5.3

HRSG Performance—Design Case

Gas Wat./Stm. US, Temp. in/

Appr. kcal/h Module Surf.

In/Out,

out, °C °C

MW

kg/cm 2 a kg/h

°C

ch., °C °C No.

65,692 1 Note: Project—Example 5.7 Units—METRIC Case—Example 5.7 Remarks—Amb. temp., C = 20 heat loss,

% = 1 gas temp. to HRSG C = 500 gas flow, kg/h = 100,000 % vol CO 2 = 3. H 2 O = 7. N 2 = 75. O 2 = 15. SO 2 =. ASME eff, % = 68.41, tot. duty, MW = 10.1

TABLE 5.4

Off-Design Fired Performance

40,000 Boiler duty, MW

10.1 16.4 26.3 Burner duty, MW

0 5.84 15.53 Additional boiler duty, MW

0 6.3 16.2 Exit gas temperature, °C

139 Pinch point, °C

8 12 18 Approach point, °C

additional fuel while the additional duty is 16.2 MW! That means we are generating the additional steam at more than 100% efficiency in these cases! Hence, supplementary firing is more than 100% efficient. This was also seen in Chapter 4. The exit gas tempera- ture is lower in the fired mode for reasons given in Chapter 4. Again, the economizer is acting as a bigger heat sink when we increase the steam flow. Hence, the HRSG is more efficient in the fired mode. In a steam generator, the exit gas temperature increases with increase in steam generation as discussed in Chapter 4, while in an HRSG, it is the other way around. By firing fuel in an HRSG, we are in effect reducing excess air as we do not add air for this fuel but only utilize the excess oxygen in the exhaust gas.

Note that we assume the pinch and approach points only in the design mode. In fired cases, the pinch and approach points fall in place as the surface area has been selected by assuming pinch and approach points. We see that the pinch is 18°C and approach is 62°C in the 40 t/h case. How will one know what pinch and approach points to use if we had started the design in the fired mode? Yet another reason that we should always start the design in the unfired mode!