Single Drum Dryer Heat Source: Coal, Gas, or Waste Heat?

A single drum dryer is a heat exchanger with a burner bolted to one end, and buyers spend most of the evaluation on the drum. The heat is where the money is. Coal, natural gas and waste heat all reach the 700–800 °C inlet that the published sheet allows, and on the same duty they can differ by a factor of two or more in running cost and by a much larger factor in what has to be built around the drum.

The comparison is only meaningful per tonne of water evaporated, because that is the unit the physics fixes. A dryer removing 2.8 t of water an hour and one removing 8 t are not comparable on fuel per hour. Start from the gigajoules, convert to whichever fuel is on site, then add the equipment each source drags in with it.

What Each Source Brings to the Drum

The three options do not differ only in price per gigajoule. Each arrives with a different temperature, a different equipment list and a different relationship with the product inside the drum.

How to read this table: the temperature bands are typical of each source rather than published limits of this machine. The drum itself publishes a maximum inlet of 700–800 °C, which all three can satisfy — but they satisfy it in different ways, and the waste-heat route satisfies it only after the gas has been diluted with air, which costs volume rather than temperature.

Coal: The Cheapest Heat, the Largest Plant

Coal at 23 MJ/kg and 85% combustion efficiency is about $3.90 per gigajoule at $90 a tonne, or $4.60 once the unburned fraction is counted. Natural gas at 35.8 MJ/Nm³ and 98% is about $11.20 per gigajoule at $0.40 a cubic metre, or $11.40 once combustion is counted. The gap is roughly two and a half to one, and that gap is the entire argument for coal.

What the gap buys is equipment. A coal-fired drum needs a hot gas generator with a combustion chamber, a fuel yard and handling, ash removal, and a dust collector sized for both the extra gas and the ash load. It needs a fuel analysis, because 5,500 kcal is an assumption and the delivered coal may be 4,800 with 12% ash and 1% sulphur. And it puts combustion products in direct contact with the material, which is fine for slag, sand, clay for brick and fly ash, and wrong for anything where brightness or whiteness is the specification — a point that matters on kaolin, where the temperature recipe is already tight.

Gas: Fewer Parts, Pricier Heat

Gas removes the yard, the ash and the ash-related collector load. The burner train is compact, turndown is wide, and the flame is clean enough to dry a white mineral without a colour complaint. The cost is per gigajoule, and it is volatile: the same burner that looks reasonable at $0.25 a cubic metre is expensive at $0.55, and a dryer sized on a cheap contract can be stranded when the contract ends.

The compensation is that gas heat is easy to modulate, which matters on a drum that has to hold a discharge temperature rather than a flame. On a product with a 550–600 °C structural limit, the ability to cut heat quickly is worth something in scrap product avoided, and this is where the fuel choice stops being only an energy decision.

Waste Heat: Free Fuel, Three Times the Gas Volume

Waste heat is the cheapest heat in the world once the duct exists, and it is the reason so many dryers are bolted onto plants that already have a hot stack. What it is not is free of consequence. Heat delivered at 300 °C carries far less energy per kilogram than heat at 700 °C, so the same gigajoules require far more gas — and the drum, duct, fan and bag filter are all sized on gas volume.

How to read this table: all five columns are calculated on 4.25 GJ per tonne of water, 1.05 kJ/(kg·K) for the gas, a 110 °C outlet and a 2.5 kPa system pressure drop with a 65% fan efficiency. The last column is the honest cost of free fuel: at 300 °C the fan uses roughly three times the electricity per tonne of water that it does at 700 °C, and the bag filter handles three times the volume. A waste-heat drying system is a fuel saving and a fan bill, not a saving with no bill.

There is a second constraint that never appears in the fuel comparison: availability. Kiln preheater gas exists while the kiln runs. If the dryer must run 24 hours through a kiln shutdown, it needs a backup burner — and then two heat sources have been bought, with two sets of safety interlocks and two sets of operating procedures.

The Bill per Tonne of Water

Put the three on one line, at example prices, and the ranking is clear — as long as the prices are replaced with local ones before any decision is taken.

How to read this table: electricity is priced at $0.10/kWh and the fan figures come from the volume column above; the coal and gas prices are illustrative only and vary widely by market. What does not vary is the shape: coal is the cheapest purchased heat per gigajoule, gas is roughly two and a half times dearer, and waste heat converts the fuel line into a fan line. On a line evaporating 4 t of water an hour, running 8,000 hours a year, that is about 32,000 t of water and a difference of roughly $925,000 a year — against which the gas installation saves the fuel yard, the ash handling and the collector load.

Five Numbers for the Enquiry

Delivered gas temperature and volume at the mixing point, not at the burner. Fuel analysis — calorific value, ash, sulphur, moisture. Outlet gas temperature and the dew point margin behind it. Turndown ratio, because a dryer spends most of its life part-loaded. Annual availability of the source in hours, which decides whether a backup burner is in the scope. Add the dust loading of any waste gas, since that decides the collector. The sizing arithmetic on the drum side is set out in what one ton of water costs to evaporate, and the drum itself is described on the Single Cylinder Dryer page; where the product has a temperature ceiling, the setpoints in the kaolin temperature guide apply before any fuel is chosen.

Frequently Asked Questions

Q1: Which heat source is cheapest to run?
A: At coal $90/t and gas $0.40/m³, coal costs about $20 per tonne of water and gas about $49, before the fuel yard and ash handling that coal requires. Waste heat is fuel-free but pays about $1.30 in fan power at 300 °C. Local prices can reverse the coal and gas ranking; they rarely touch the waste-heat case where the gas already exists.

Q2: Can waste heat reach the 700–800 °C the drum allows?
A: Rarely. Kiln preheater gas is commonly 300–350 °C. The drum does not need 700 °C to work — it needs the heat — but low-temperature gas means roughly three times the mass flow and volume, so the fan, ducts and bag filter all grow. That is the capital cost of free fuel.

Q3: Does a single drum accept all three sources?
A: Yes. The published maximum inlet is 700–800 °C, which a coal or gas hot gas generator reaches after blending with tempering air, and waste heat sits well below it. What changes is the burner or duct arrangement, not the drum.

Q4: Do I need a bigger dust collector if I burn coal?
A: On two counts. Combustion adds ash to the gas stream, and blending a hot gas generator down to the drum inlet adds air volume. Both show up as collector area and fan power, and both belong in the coal-versus-gas comparison rather than in a footnote.

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