Ask any boiler operator what the boiler consumes and you will hear about husk — lorries of it, 29 tonnes a day at the practice plant. Nobody mentions the other raw material, though the boiler takes nearly seven times more of it by weight: air. Roughly 190 tonnes of it every day walks in through the forced-draught fan, reacts, picks up the fuel's heat, and leaves by the chimney — and not one kilogram of it appears on a purchase order or crosses the weighbridge. This lesson is the arithmetic of that invisible flow: what burning actually is, how much air a kilogram of fuel is owed by chemistry, and why the answer sets the size of your fan, the reading on your flue analyser and the ceiling on your firing rate. Lesson B4 turns the flue reading into money; this lesson explains where that reading comes from.
Burning is bookkeeping
Strip away the flame and combustion is three tidy reactions. Carbon takes oxygen and becomes CO₂ — and the mass balance is fixed by atomic weights: every kilogram of carbon demands 2.67 kg of oxygen (32 ÷ 12). Hydrogen burns to water vapour and is greedier: 8 kg of oxygen per kilogram of hydrogen. Sulphur, where a fuel has any, takes its own weight in oxygen. Nothing vanishes and nothing is created: the flue gas weighs exactly what the fuel and the air weighed together, minus the ash left on the grate. A boiler is a chemical ledger with a chimney.
Two refinements finish the sum. First, some fuels — biomass especially — carry oxygen of their own, chemically bound in the fuel, and that oxygen pays part of the bill: the hydrogen entry is really H − O/8, the "net hydrogen" left after the fuel's own oxygen has claimed its share. Second, air is not oxygen — it is only 23.2% oxygen by mass; the rest is nitrogen that walks through the fire unburnt, gets heated for nothing, and leaves by the stack. Every kilogram of oxygen the chemistry demands drags 3.3 kg of freeloading nitrogen with it. That nitrogen is why excess air costs money — but that is B4's story.
The air a kilogram of husk is owed
Take the practice plant's rice husk, with a typical as-fired analysis (teaching values — your fuel contract's lab sheet is the real source): carbon 38%, hydrogen 5%, oxygen 32%, ash 14.5%, moisture 10%, nitrogen traces, sulphur nil.
So every kilogram of husk that goes up the chute is followed, invisibly, by six and a half kilograms of air — four and a seven-tenths owed to chemistry, the rest the deliberate margin lesson B4 tunes with the damper.
Every fuel, one rule
Run the same three-line sum on the other fuels of lesson B2's basket and a pattern appears:
| Fuel (typical analysis) | Theoretical air, kg/kg fuel | GCV, kcal/kg | Air per 1,000 kcal |
|---|---|---|---|
| Rice husk | ≈ 4.7 | ≈ 3,200 | ≈ 1.5 kg |
| Indian coal (high-ash) | ≈ 5.3 | ≈ 4,000 | ≈ 1.3 kg |
| Furnace oil | ≈ 13.9 | ≈ 10,200 | ≈ 1.4 kg |
| Natural gas | ≈ 17.2 | ≈ 13,300 | ≈ 1.3 kg |
Per kilogram the fuels look wildly different — oil is owed three times husk's air — but per thousand kilocalories released, every industrial fuel is owed roughly 1.3–1.5 kg of air. The band is approximate and the direction is not: air demand follows heat release, not tonnage. It is why a fuel switch changes the fan arithmetic less than the tonnage suggests, and why a boiler is best imagined as a machine for processing air, with fuel as the additive that makes the air worth heating.
Why the fire gets more than it is owed
Feed a real furnace exactly its stoichiometric air and it smokes. The arithmetic assumes every oxygen molecule finds its carbon atom; a real grate has thick spots, thin spots and drafts, and mixing is never perfect. Combustion people call the fix the three T's — temperature, turbulence, time: keep the fire hot, mix it hard, give the gases long enough in the furnace. Whatever mixing quality your firing system achieves, the shortfall is bought with excess air — and the better the mixing, the less excess it takes. That single idea ranks the firing systems of lesson B1: a gas burner premixes almost perfectly and runs at 5–10% excess; an oil burner atomises and manages 10–20%; a fluidised bed churns its fuel in a sandstorm and holds ~20–25%; a husk grate, where lumps of fuel wait for air to find them, needs 30–60% — which is why B4's fair target for the practice plant sits at O₂ 6%, about 40% excess.
Starve the fire below its owed air and the ledger goes ugly fast: carbon stops at CO instead of CO₂ — leaving two-thirds of that carbon's heat unreleased — the stack smokes, unburnt carbon blackens the ash, and unburnt gas accumulating in a hot furnace is the explosion risk lesson B8's purge interlocks exist for. Too little air always costs more than slightly too much: the damper's safe side is rich in air, lean in fuel.
The plant scale — your fan is the appetite
Two hundred-odd tonnes of gas moved, every day, by a fan most plants have never nameplated against the arithmetic. The fan's capacity is the ceiling on the firing rate: a boiler that cannot breathe cannot burn, whatever the grate can hold.
- Get the ultimate analysis (C, H, O, S, ash, moisture) onto every fuel contract's lab sheet — proximate analysis alone cannot feed this arithmetic.
- Nameplate the FD fan: its m³/h × 1.15 kg/m³ is the air it can deliver; divide by 6.6 and you have the husk rate it can honestly support. Compare with the boiler's rating before blaming the grate for low steaming.
- Switching fuel — husk to briquette, coal to bagasse? Redo the three-line sum first. Same grate, different appetite: smoke after a fuel change is usually the old damper setting serving the new chemistry.
- Weigh one shift's ash against fuel × ash%. Ash notably heavier than the sum says means unburnt carbon in the pit — B4's loss ledger, caught with a spring balance.
- Near the air minimum, trust CO and smoke before O₂ alone — B4's rule; the dial and the money live in that lesson.
- Carbon is owed 2.67 kg of O₂ per kg; hydrogen 8 kg — less what the fuel's own oxygen pays (H − O/8). Air is only 23.2% oxygen by mass.
- Husk: ~4.7 kg air owed per kg; fed ~6.6 at the 40% husk-grate margin.
- Universal band: ~1.3–1.5 kg of air per 1,000 kcal released, whatever the fuel.
- Excess air buys imperfect mixing: gas 5–10% · oil 10–20% · FBC 20–25% · grate 30–60%. Better mixing, less margin.
- 29 t of husk brings ~191 t of air and sends ~216 t up the stack — the boiler house's biggest flow is the invisible one.
Until the 1770s the best minds in chemistry believed burning released a substance — phlogiston — because flames so obviously give something off. Antoine Lavoisier settled it with a balance and a sealed flask: burn something in closed glass and the total weight does not change; open the flask and air rushes in to replace the oxygen the fire consumed. Burning takes from the air, it does not give to it. Every combustion calculation since — including the three lines this lesson runs on a kilogram of husk — is Lavoisier's ledger with newer handwriting: weigh everything in, weigh everything out, and let the difference confess.
FAQ
How much air does my boiler actually need?
Chemistry's minimum is about 1.3–1.5 kg of air per 1,000 kcal your fuel releases — for husk that is ~4.7 kg per kg of fuel — plus the excess-air margin your firing system needs for mixing (gas burners 5–10%, husk grates 30–60%). At plant scale it is tonnes per hour: the practice plant's 29 t/day of husk needs roughly 191 t/day of air, all of it through the FD fan.
If the fire needs air, why is there oxygen left in my flue gas?
Because you deliberately feed more air than the chemistry minimum, and the unused share leaves unreacted. That leftover is exactly what the flue analyser reads: O₂ 6% in the stack means roughly 40% more air went in than the fuel was owed. The reading is not a fault — it is the measurement of your margin, and lesson B4 shows how to turn it into the excess-air number and the money it costs.
Does more air ever mean better combustion?
Up to the point where mixing stops being the limit, yes — that is what the excess-air margin is for. Beyond it, every extra kilogram is cold nitrogen heated to stack temperature and thrown away, plus a cooler furnace working against you. The honest optimum is the lowest excess your firing system can hold without CO climbing or smoke appearing — better mixing (the three T's) moves that optimum down; more damper never does.
Why does husk need so much less air per kilogram than furnace oil?
Two reasons. Husk is one-third oxygen by weight, and that built-in oxygen pays most of the hydrogen's bill before any air arrives. And husk simply carries less heat per kilogram — 3,200 kcal against oil's ~10,200 — so there is less combustion to feed. Per 1,000 kcal released the two fuels are owed nearly the same air, which is the number that actually sizes fans and ducts.
What happens if the damper closes the air below the chemistry minimum?
Carbon stops half-burnt at CO, giving up only a third of its heat; the stack smokes; unburnt carbon blackens the ash; and unburnt gas in a hot furnace is an explosion waiting for a spark — the reason lesson B8's purge and interlock sequences exist. Short of air is always the expensive side of the setpoint: when in doubt, err lean on fuel, rich on air, and fix the mixing.
Check yourself
Five quick questions on this lesson. No marks, no records — the score is for you.