Waste Heat Recovery in Fired Heaters General Considerations
The fired heaters range from 425 MMBH to 25 MMBH of absorbed duty. The generation of steam in economizers and the use of air preheaters was investigated for each process fired heater. From an economic standpoint, the rankings of the waste heat options are:
1. Generation of 150 PSIG steam
2. Generation of 600 PSIG steam
3. Air Preheaters
The reason that the 150 PSIG generation has an economic advantage over the 600 PSIG is that per dollars invested, more heat and energy are recovered. This is due to its lower temperature and resulting lower investment required to generate 150 PSIG steam.
As mentioned in Section 3150, a steam balance must be developed to support any waste heat recovery project. This is particularly critical for fired heaters, if steam generation, boiler feedwater, or make-up water are involved. As discussed in the boiler section, air preheaters do not have the payouts that economizers do. However, once the steam system is at maximum capacity, the air preheater can be an economical alternative.
From a practical standpoint, several considerations should be reviewed when comparing steam economizers to air preheaters on fired heaters. Some specific guidelines are:
• The use of air preheaters in a multi-box fired heater (such as a rheniformer) is not recommended. This is because of the complexity of the control systems to maintain the correct air/fuel ratio in the fire boxes. Additionally, a rheniformer, with its 1000°F stack temperature can provide steam at the highest pressure that is made in the plant.
• The use of air preheaters with stack temperatures above 900°F to 1000°F presents expansion problems in the rotating elements of regenerative exchangers. On this basis, air preheaters are not normally used in hydrogen plants and rheniformers.
• Steam generation of less than 10,000 pounds/hour is impractical in large process units. The additional auxiliary facilities, maintenance, laboratory testing, and operator attention cannot normally be justified.
• Depending on the cost of fuel, air preheaters for fired heaters would typically have a marginal payout for absorbed duties of 25 to 35 MMBH and less.