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Boiler heat recovery: Engineering and integration

Compact add-ons, as long as the acid stays away

What this tab covers Engineering and operating integration: how tightly it is coupled to the host process, how steady the waste stream is, and how hard it is to retrofit.

These are mature, compact add-ons to the boiler house. The main engineering care points are acid corrosion at the cold end and protecting the boiler from contaminated condensate.

Process interdependence (coupling risk)

Favourable

An economiser sits on the feedwater line, so a tube leak can force a boiler outage unless gas-side and water-side bypasses are fitted, which is standard practice. Direct-contact condensers can only heat makeup water up to about 43 to 60 C, so they depend on a steady cold-water demand. Returned condensate can carry oil or process contamination into the boiler; plants monitor quality and dump bad condensate rather than risk the boiler. Failure of these devices usually lowers efficiency rather than stopping production.2

Direct-contact makeup water limit about 43 to 60 C (110 to 140 F)2 Minimum exit gas, condensing designs about 24 C (75 F)2

Load factor and intermittency

Favourable

DOE suggests economisers for boilers above about 100 boiler horsepower (about 1 MW), operating above about 5 bar (75 psig) and at significant load all year. Its worked examples assume 8,000 to 8,400 hours a year. Condensing gains depend on having cold water to heat; if return water is warm, little vapour condenses and the extra gain disappears.15

Economiser candidate boiler above about 100 boiler hp, above 75 psig, year-round load1 Operating hours in DOE examples 8,000 to 8,400 h/yr15

Footprint and retrofit complexity

Favourable

Economisers fit in the flue duct and can often be installed in a planned boiler shutdown. Condensing units need a drain for acidic condensate and a stack able to handle cool, wet gas. Condensate return can be the hardest retrofit because it needs piping and pumps back from scattered users across the site. An automatic blowdown control system costs only about USD 2,500 to 6,000.4

Automatic blowdown control system cost USD 2,500 to 6,0004 Blowdown control payback often 1 to 3 years4

Sources

  1. Use Feedwater Economizers for Waste Heat Recovery, Steam Tip Sheet #3 (DOE/GO-102012-3412) US DOE Advanced Manufacturing Office, 2012
  2. Considerations When Selecting a Condensing Economizer, Steam Tip Sheet #26B (DOE/GO-102012-3397) US DOE Advanced Manufacturing Office, 2012
  3. Install an Automatic Blowdown-Control System, Steam Tip Sheet #23 (DOE/GO-102012-3402) US DOE Advanced Manufacturing Office, 2012
  4. Return Condensate to the Boiler, Steam Tip Sheet #8 (DOE/GO-102012-3410) US DOE Advanced Manufacturing Office, 2012