Electric Boilers and Waste Heat Recovery Boilers
Not every boiler burns fuel. An electric boiler turns electricity straight into steam, and a waste heat recovery boiler takes its heat from exhaust gas that would otherwise go up a stack. Both avoid a furnace, and both are judged on economics more than engineering.
You'll learn
- Explain how electric and waste heat recovery boilers work
- Calculate the electricity needed per tonne of steam
- Estimate the steam recoverable from an exhaust gas stream
- Judge where each type is worth it
Before this: Enthalpy, What counts as a boiler
Boilers without a burner
An electric boiler heats water with immersed resistance elements, or by passing current through the water itself between electrodes. There is no combustion, no chimney, no fuel store and no ash. Almost all of the electricity becomes heat in the water.
A waste heat recovery boiler, or heat recovery steam generator, sits in the exhaust of something that is already producing hot gas: a diesel or gas engine generator, a gas turbine, a kiln, a furnace, an incinerator. The gas passes over tubes full of water, cools, and makes steam with no extra fuel at all.
Electricity per tonne of steam
- Heat added per kilogram: 2,444.3 kJ/kg. Conversion efficiency about 99%.
- Per tonne:
Electricity is high-grade energy: each kWh is 860 kcal, and it is usually priced far above the same heat in coal, biomass or gas. So electric boilers win on convenience, cleanliness and space, rarely on running cost — except where electricity is cheap or surplus, or the steam demand is small and intermittent.
Steam from waste heat
- 3,000 kg/h of exhaust at 450 °C, cooled to 200 °C.
- Specific heat of the exhaust about 1.1 kJ/(kg·K) — a typical figure; use the engine maker’s exhaust data where available.
- Heat recovered:
- Steam at 10 kg/cm²(g) from 80 °C feed: 8,25,000 ÷ 2,444.3 = 338 kg/h.
What decides whether they pay
For waste heat, the gas exit temperature is the key design choice. Cooling further recovers more, but every degree below the gas’s acid dew point risks condensing sulfuric acid on the tubes when the fuel contains sulfur. Exhaust from furnace oil or high-sulfur diesel is kept well above that point; clean natural gas exhaust can be cooled further, sometimes with an economizer behind the boiler.
A recovery boiler also has a pinch: steam is made at saturation temperature, and the gas cannot be cooled below that temperature in the evaporator. The hotter and the lower-pressure the steam, the more heat the gas can give. Raising steam pressure for convenience costs recovered steam.
Waste heat is only as reliable as its source. If the engine stops, so does the steam, so a recovery boiler usually runs alongside a fired boiler or has a supplementary burner. Electric boilers face a different constraint: the connected load. A 1 TPH electric boiler draws several hundred kW, which may need a larger supply connection than the plant has.
Key takeaways
- Electric boilers convert about 99% of electricity to heat, but electricity is expensive heat.
- About 686 kWh per tonne of steam at 10 kg/cm²(g) from 80 °C feed.
- Waste heat recovery: of the exhaust, limited by acid dew point and pinch.
- Both are boilers under the Act if they exceed the s. 2(c) thresholds.