Can Flour Really Explode?
– Powder That Barely Burns in the Bag Can Flash-Burn in Mid-Air
It can. Hold a flame to a heap of flour and the surface just scorches. But when the same powder is thrown into the air as a cloud, flame races from grain to grain in an instant, and it becomes an explosion. What makes the difference is not the amount of powder. It is how much surface touches the air, and the air between the grains.
You are about to bake. You tear open a bag of flour, and a white puff rises into the air. Right next to you, the gas stove is lit.
We don't usually think of flour as a cause of fires. Hold a match to a heap of it and it hardly catches.
Yet flour mills and coal mines have suffered big dust explosions many times over the years. It is the same powder. So what is different?
Just two reasons
Things burn at the surface that touches the air. Crush a lump into bits and the faces that were hidden inside come out into the open. As a powder, the same amount has thousands of times more surface.
In a heap, there is little air between the grains, and a fire cannot travel deep. Thrown into the air, every grain is surrounded by air, and flame jumps from one grain to the next.
The heat released by burning is the same for a lump and for a powder. What changes is the time it takes. Heat that would normally come out over minutes is packed into a single instant. That is what an explosion is.
How much more surface does crushing give?
Picture a sugar cube, 1 cm on each side. Its outer surface, all six faces together, is 6 square centimetres. Cut it into little cubes 1 mm on a side, and the total surface becomes 10 times larger: 60 square centimetres.
Grind it as fine as flour grains and the total becomes 1000 times larger: 6000 square centimetres, about 40 postcards' worth. The weight is still that of one sugar cube. Try changing the grain size with the slider in Figure 1.
Roughly speaking, burning speed is thought to be proportional to the surface touching the air. With 1000 times the surface, the same amount burns up in far less time.
Why is it dangerous when it "floats up"?
If reason 1 were the whole story, a heap of powder would be dangerous too. In practice, the powder in a heap only scorches on top and the fire hardly spreads. The grains are packed together, and the air (oxygen) in the gaps runs out quickly.
Once the powder is in the air, things change. Every grain is wrapped in air. When one grain burns, its heat warms the grains nearby, and they catch one after another. Flame spreads through the whole cloud at once, and the suddenly expanding hot air shoves everything around it aside.
Even scarier is the "second explosion" in Figure 2. The blast from a small first explosion throws up the dust lying on floors and shelves. When flame reaches it, an even bigger cloud burns all at once. In many serious factory accidents, the second explosion is said to have done most of the damage.
Fine coal dust also explodes when thrown into the air. In 1963, a coal dust explosion at the Mitsui Miike Mikawa mine in Fukuoka Prefecture killed 458 people. Dust lying in the tunnels was thrown up, and flame is said to have raced a long way.
Flour is not the only danger. Sugar, starch, powdered milk, wood shavings and metal aluminium powder are also said to explode when finely dispersed. In 2015 at an outdoor event in Taiwan, sprayed coloured starch powder caught fire, and 15 people died.
So what should we do?
- Don't throw powder into the air near a flameDon't rip open a bag or slap flour off your hands near the stove.
- Never pour flour on burning oilThe powder can fly up and spread the fire in a flash. Cover the pan with a lid that fits, or use a fire extinguisher.
- Clean up settled dust; don't blow it awayBlowing dust off a work area makes a cloud. Use a vacuum cleaner or a damp cloth.
First turn off the gas flame or switch off the machine to cut the heat source. If the flames look likely to spread, don't try to put them out. Get away from the spot at once and evacuate. Once outside, call 119 (the emergency number in Japan). A dust cloud can burn in an instant, so don't go closer to check.
Summary
Whether a powder explodes depends less on what it is than on how fine it is and how well it is mixed with air. Fine grains have a lot of burning surface, and airborne grains are wrapped in air. When both are true, heat that normally takes minutes to come out is released in an instant.
There is no such thing as a powder that won't burn.
There are only powders that haven't been thrown into the air yet.
For how heat can build up and start a fire on its own, see "Why do oily rags catch fire with no flame nearby?". For why you must never put water on an oil fire, see "Why does water on a burning tempura-oil pan send flames up to the ceiling?".
- Get one sugar cube and the same weight of granulated sugar.
- Fill two glasses with the same amount of water, drop each in at the same moment, and watch without stirring.
- The granulated sugar should dissolve and vanish much sooner. The more surface touches the water, the faster the change goes.
Burning works the same way: the reaction proceeds from the surface that is in contact. Never try setting powder alight as an experiment, including copying videos you find online.
For those who want more – terms, formulas and links to textbooksEach part is labelled by level, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school chemistry
- HS+Advanced high-school material, or a textbook sidebar
- UniversityUniversity specialist courses (combustion and safety engineering), not taught in high school
- ResearchNot yet settled even at university; researchers are still studying it
Junior highTerms: this phenomenon has a name
- Dust explosion: when combustible powder suspended in air is ignited and flame spreads all at once, causing an explosion.
- Surface area: the total size of an object's surface. For the same amount, it grows the more finely the object is divided.
- Lower explosive limit: the borderline concentration below which flame cannot pass from grain to grain in airborne dust.
Junior highHigh schoolChecking with a formula: how much surface does 1 cm³ of powder have?
Cut a lump of a given volume into small cubes of equal side length, and work out the total surface area.
| In symbols | S ＝ 6 × V ÷ d |
| In words | Total surface area ＝ 6 × total volume ÷ size of one grain |
| Where it comes from | One grain has 6 faces, so "6 × d²"; the number of grains is "V ÷ d³". Multiply them and one d cancels, so the smaller the grain, the more surface. |
| S (symbol) | Total surface area. Unit: square centimetres |
| V (symbol) | Total volume. Here 1 cubic centimetre (about one sugar cube) |
| d (symbol) | Size of one grain (one side). 1 cm for the lump, 0.001 cm (0.01 mm) for fine powder |
| Area of one postcard | 10 cm × 14.8 cm = 148 square centimetres |
| Lower explosive limit (many powders) | Said to be around a few tens of grams per cubic metre |
| 6 × V (V＝1) | 6 × 1 ＝ 6 |
| As a lump (d＝1 cm) | 6 ÷ 1 ＝ 6 square centimetres |
| As a powder (d＝0.001 cm) | 6 ÷ 0.001 ＝ 6000 square centimetres |
| How many times larger | 6000 ÷ 6 ＝ 1000 times |
| How many postcards | 6000 ÷ 148 ≒ 40.5 postcards |
| Grains along one side | 1 ÷ 0.001 ＝ 1000 grains |
| Total grains (length × width) | 1000 × 1000 ＝ 1000000 grains |
| Total grains (× height as well) | 1000000 × 1000 ＝ 1000000000 grains |
A sugar cube's worth of powder splits into about a billion grains, with about 40 postcards' worth of "doorway to burning". Each time you make the grains 10 times finer, that area grows another 10-fold.
| Floor area of a 6-tatami room | About 9.9 square metres |
| Room volume (ceiling 2.4 m) | 9.9 × 2.4 ≒ 23.8 cubic metres |
| 1 kg (1000 g) of flour spread evenly through the room | 1000 ÷ 23.8 ≒ 42 grams (per cubic metre) |
Even under the extreme assumption that it spreads evenly through the whole room, it barely reaches the concentration regarded as the lower limit. So it is hard to imagine a whole room exploding from ordinary kitchen use. Real dust clouds, however, are not even. Near the mouth of a bag or beside a stove, they can be far denser locally. The danger is the "small dense cloud" that forms near a flame.
High schoolHS+Reaction speed and surface area
High schoolIn the "reaction rate" unit of chemistry, you learn that reactions involving a solid go faster the larger its surface area. A dust explosion is an extreme case. The total heat released by burning (the heat of combustion) does not change; only the speed at which the heat comes out rises by orders of magnitude.
HS+For ignition, the dust concentration has to fall within a certain range. Too thin and flame cannot reach the next grain; too thick and oxygen runs short. This range is called the explosive limits. A common way to sum it up is that an explosion needs five things together: combustible dust, oxygen and an ignition source, plus "dispersion in the air" and "a confined space".
UniversityMeasures of dust explosion severity
In combustion and safety engineering, the danger of a dust is compared by its minimum ignition energy, lower explosive limit, maximum explosion pressure and so on. The violence of an explosion is expressed by the "explosion index (Kst value)": the maximum rate of pressure rise in a vessel multiplied by the cube root of the vessel's volume. The finding that this stays nearly constant whatever the vessel size is called the "cube-root law", and it is used to design explosion vents in factories. Whether the gas released by heating the grain surface burns, or the grain surface itself burns, is said to differ from one powder to another.
📖 Derivations and further reading: Dust explosion (Wikipedia, in Japanese) ／ Explosive limit (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- How fine can it get before the danger levels off? If grains are too small, they stick together into clumps, and the danger is said not to keep rising. What happens with nano-sized powders is still being studied.
- The effect of turbulence. Even for the same powder, flame moves faster when the air is stirred up. Predicting accurately how flame accelerates in the complex airflow of a real factory is said to be difficult.
- Can flame outrun sound? In long tunnels or pipes, flame can accelerate and turn into a "detonation" with a shock wave. The conditions for this are less well understood than for gaseous fuels.
In other words, this article too is "an explanation of what is known so far." Still, the advice to "not throw powder into the air near a flame" and "not let settled dust build up" holds up whatever the research shows.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: "Combustion (changes that combine with oxygen)" | Burning needs oxygen / why a heap of powder is hard to burn |
| High school | Chemistry: "Reaction rate" | The larger the surface area, the faster the reaction |
| HS+ | Chemistry (advanced): "Heat of combustion, explosive limits" | Total heat is the same; only the speed of release changes |
| University | Combustion engineering, safety engineering | Explosion index, cube-root law, minimum ignition energy |
| Research | Powder technology, combustion science | Behaviour of nano powders, flame acceleration in turbulence |
| – | Links to daily life | Handling powder in the kitchen, cleaning workshops, events that use powder |
- Dust explosion – Wikipedia (in Japanese)
- Explosive limit – Wikipedia (in Japanese)
- Fire and Disaster Management Agency (総務省消防庁), Ministry of Internal Affairs and Communications
- National Institute of Occupational Safety and Health, Japan (労働安全衛生総合研究所), "Technical materials on preventing dust explosions and fires" (粉じん爆発・火災の防止に関する技術資料)
- NFPA 652, Standard on the Fundamentals of Combustible Dust (National Fire Protection Association, USA)
* This article is a popular-science explanation. The figures given are rough guides to help you understand the mechanisms. In a fire or accident, follow the instructions of the fire service and local authorities.