Definition
Waste-heat boiler
A waste-heat boiler recovers heat from hot process gas to generate steam, with fouling, erosion, tubesheet stress and corrosion driving cleaning strategy.
- Subject
- Boilers
- Also known as
- WHB, waste heat recovery boiler
A waste-heat boiler (WHB) recovers energy from hot process gas and generates steam without a separate burner as the main heat source. It may sit downstream of a kiln, incinerator, sulphur recovery unit, FCC regenerator, metallurgical furnace, reformer, gas turbine or chemical reactor.
The term overlaps with heat-recovery steam generator, but WHB is often used for harsher process-gas duties with dust, acid gases, catalyst fines or corrosive condensables.
Fouling characteristics
WHB fouling depends entirely on upstream process chemistry. Cement and metals service may bring abrasive mineral dust. FCC and refinery service may bring catalyst fines. Waste and biomass service may bring sticky alkali chlorides. Sulphur and acid plants may create dew-point corrosion risks if temperatures fall too low.
Deposits reduce heat transfer, increase pressure drop and can produce tube overheating or bypassing. On firetube WHBs, tube-mouth plugging and tubesheet deposits are common concerns. On watertube designs, tube banks, baffles and economiser sections may foul like a conventional boiler pass.
Design and maintenance implications
Important design details include gas velocity, tube pitch, bundle access, refractory protection, expansion allowance, drainability, sootblower lanes, inspection doors and materials at cold-end or acid-risk locations. Upstream trips and start-up conditions matter because sudden temperature swings can crack refractory or stress tube-to-tubesheet joints.
Inspection should connect deposit maps with upstream process events. A WHB that fouls after every feed change or catalyst upset needs process control as much as cleaning hardware.
Sonic-horn duty
Sonic horns are useful where deposits are dry, particulate and reachable by sound: tube banks, inlet chambers, tube mouths, hoppers and gas-distribution zones. They are less effective on fused slag, tarry material, wet sludge or hard scale. In Sylio-style reviews, WHBs are assessed by gas-side deposit type first, because the same boiler geometry can be easy or impossible for acoustic cleaning depending on process chemistry.
Design and operating variables
Waste-heat boiler design starts with the upstream process, not with the boiler alone. Gas temperature, dust loading, corrosive species, pressure, oxygen content, flow variability and upset conditions determine tube layout, materials, cleaning access and protection systems. A boiler behind a sulphur plant, FCC unit, glass furnace, incinerator, steel process or engine exhaust may have a completely different fouling and corrosion profile even if the steam pressure is similar.
Key variables include gas-side velocity, tube pitch, circulation method, steam-drum size, bypass or diverter dampers, approach to dew point, expansion allowance, refractory transitions and whether the boiler must tolerate rapid temperature swings. Dusty services need hoppers, rappers, sootblowers, sonic horns or access for offline cleaning. Clean gas services may be dominated by thermal fatigue, tube vibration or dew-point corrosion instead.
Reliability and acoustic-cleaning relevance
Common problems include tube fouling, ash bridging, acid corrosion at cold ends, erosion at gas inlets, refractory failure, casing leaks, steam-drum level instability and thermal shock during start-up or bypass changes. Operators track steam production, gas outlet temperature, draft loss, tube-metal alarms, feedwater quality and upstream process conditions. Inspection evidence is important because a loss of steam output may be caused by fouling, gas bypass, process load change or water-side scaling.
Acoustic cleaning can be a strong option for waste-heat boilers with dry, friable particulate deposits and difficult access. It is less useful for sticky salt deposits, molten carry-over, wet acid sludge or hard scale. The design should prove that loosened solids will reach hoppers and that horn nozzles do not create casing leaks, thermal bridges or maintenance obstacles.
Related terms
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Related terms
4 terms
- Heat Recovery Steam GeneratorAn HRSG recovers heat from a gas turbine's exhaust to generate steam, the second cycle of a combined-cycle plant. Finned-tube ash deposition and ABS fouling are the main cleaning concerns.
- TubesheetA tubesheet is the thick plate that holds tube ends in a shell-and-tube exchanger, waste-heat boiler or similar heat-transfer vessel.
- Tube foulingTube fouling is deposit build-up on boiler tubes, reducing heat transfer, increasing draft loss and raising corrosion or tube-temperature risk.
- Sonic hornA sonic horn is a pneumatic low-frequency sound emitter used to dislodge particulate fouling from boilers, ESPs, baghouses, ducts and silos while the plant stays online.
References