Only Connect! Just a few days ago I was writing about Temple Grandin making the killing floor in abattoirs more efficient and humane. It takes insight and a team of process engineers. Then news came in from Taiwan over the weekend of a bad case of killing flour. In the immediate aftermath, they were able to say nobody died but one of the 500 horribly injured young students has now succumbed. As I expressed my treatises on road traffic accidents and end of life issues, there may be worse things than dying. It's probable that among the injured in many hospitals near Taipei are kids with no eyelids or no skin on one side of their bodies who wish they were dead.
Combustion is all about oxidation: lots of chemicals, when they combine with oxygen, do so as an exothermic (energy generating) process. It's happening in your brain right now as you try to get my drift - glucose and oxygen are being converted to carbon dioxide and water yielding about 16kJ of energy per gram. That's enough to keep a 100W light-bulb going for 2.5 minutes. And 100W is about what is being dissipated through the top of your head every waking moment. If oxygen is in short supply, this exothermic reaction needs help to get started. A candle just sits until a flaring match supplies the energy of activation to get things going. In pure oxygen, the reaction is easier to start. My investigations of explosives have tried to show that the more oxygen incorporated into the chemical, the more energy can be generated: TNT C7H5N3O6 has about 1/3 the explosive power of nitroglycerine C3H5N3O9.
I make a lot of bread, so a lot of flour goes through our kitchen, but it sits there inert in a 2kg bag until mixed with yeast and water. The dough sits like a damp blob [safe] and even after heating to 200oC for 40 minutes to create a loaf it has no likelihood of exploding.The energy is released later in my head - a blob is converted miraculously into The Blob. But if you trick about with the proportions, you can create a neat whoomph at the kitchen table. You can scale that up into a devastating dust explosion in which finely powdered flammable material is mixed in the right/wrong proportions with the 20% oxygen in the air. Coal dust in mines, and flour or grain dust in silos are known to be potentially hazardous. An ignition source is necessary but this can be a lit cigarette or even a static electricity spark from a nylon jump-suit. These are enclosed spaces, so it looks like the dust has to be concentrated w.r.t. the available air.
When I was in school, I had a Summer job working in a flour mill as a general dogsbody. One of my tasks was to make the 19thC mill-buildings air-tight in preparation for the annual fumigation against flour beetle Tribolium castaneum, grain weevil Sitophilus granarius, or just plain mice Mus musculus. It was a soul-destroying task. I was given a large roll of paper tape and some flour and water paste and told to go round all the windows. That was fiddly but the hoist-towers were four or five stories high and made of clapper-board that whistled in the wind. They'd never had an explosion in 100 years because any dust leaked out through the holes in the building's structure.
In was like that at Imperial Sugar in Port Wentworth, Georgia. Granulated sugar was stored in enormous silos and brought to the packing and processing floors by a system of conveyors, lifts, augurs and hoppers all designed by process engineers. Sugar dust was everywhere: in heaps on the floor, on top of rarely-cleaned light fittings, underneath the conveyor belts, in the hair of all the operatives. You may be sure that the workers bristled when they were welcomed home by "Hello, sweetie". Like my flour mill, that worked fine for many years until Health and Safety applied their own agenda to the process and enclosed the main conveyor running beneath the silos: to prevent contamination. That built up the concentration of dust to hazardous levels and, on 7th February 2008, the sugar-dust and oxygen mix met a red-hot faulty bearing and blew up. The fireball travelled up through the silos and set them off which triggered a chain reaction through the factory floor. The blast blew more sugar dust into the air in a frighteningly sustainable fashion. 14 dead and 40 injured. Explanatory 4min video.
The lesson from this is that you have to work quite hard to cause an explosion using flour-dust. Last weekend on 27th June 2015, impressarios at Formosa Fun Coast in Taiwan thought it would be kool to host a huge end of term party reflecting the Hindu Spring festival of Holi where everyone celebrates by throwing coloured dust and water on each other. Accordingly the fun-park ordered 3 tons of coloured corn-starch, called the event Color Play Asia and started to sell tickets. With 1,000 people in the arena each with 3kg to disperse, the dust/air ratio crossed the critical threshold and someone lit a cigarette. Whoomph! It was all over in 20 seconds. The young people, 180 of whom are in intensive care, will be paying for their tickets for some time to come.