Firefighters finally put out the blaze at a Georgia sugar refinery Thursday, a week after an explosion set a silo on fire. Sugar dust is thought to have ignited the blast, which claimed at least eight lives. What makes sugar explode?
Surface area. Tiny sugar particles burn up almost instantly because of their high ratio of surface area to volume. Table sugar, or sucrose, is flammable under the right conditions, just like wood (which is made of cellulose, or lots of sugar molecules linked together). Neither a sugar cube nor a wooden log is likely to be ignited by a small spark, but a nano- or micron-sized bit of sugar would be much more susceptible. In that case, a spark could supply enough energy to set off a small explosion, and any place containing sugar dust and lots of oxygen—like a sugar silo—could quickly become a dangerous environment. According to the National Fire Protection Association, a room with at least 5 percent of its surface area covered with anything more than 1/32nd of an inch of organic dust presents an explosion hazard.
No need to fret about your bag of Domino, though. Granulated table sugar won't explode by itself, but it can ignite at high temperatures, depending on the humidity and how quickly it's heated up. (Powdered sugar is marginally more dangerous.) If you heat up sugar on the stovetop or in an oven, it will caramelize before it catches fire; this happens as heat drives out water molecules and the sugar molecules link together in long chains. The resulting sticky material takes on a brown color because the sugar is partially burned in the process. If you keep your stove on a low setting, the sugar will eventually become charcoal, in the same way that logs can smolder without actually catching on fire. At high temperatures, however, the sugar itself can be set ablaze. Extreme heat forces sucrose to decompose and form a volatile chemical called hydroxymethylfurfural, which easily ignites and sets the rest of the sugar on fire. (Some see a potential biofuel in HMF. Table sugar is sometimes an ingredient in homemade firecrackers, and amateur rocket builders like to use a mix of sugar and potassium nitrate * as fuel.)
The especially combustible sugar dust at a refinery comes from the last stage of processing, after the crystals have been poured through a heated dryer to remove the last bits of moisture. As the refined sugar is placed on a conveyer belt and deposited into a silo, dust can float into the air. The factory might also grind sugar to make confectionary powder or just to make sure the grains are all the same size, which also creates more sweet dust. Under the right conditions, an electrical spark from a motor, a pump, from ball bearings rubbing together, or even from a light switch on a wall can set off an explosion. And once a fire gets going in a silo, temperatures can keep rising as long as there's a large fuel source (the sugar, which at this point has no water and is completely flammable) and no easy exit for the heat. The Georgia fire reached 4,000 degrees Fahrenheit at one point.
Dust combustion has also been a problem for flour mills and feed mills. In 1998, a grain elevator in Haysville, Kan., was preparing for the summer wheat harvest when dust ignited and caused a series of blasts (PDF), killing seven people and injuring 10. Today, factories try to minimize the chances of an explosion by sealing off parts of the mills, filling silos gradually, and giving them regular cleanings.
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Explainer thanks John Finley and Ben Legendre of Louisiana State University, Andy Gellman of Carnegie Mellon University, and Zbigniew Witczak of Wilkes University. Explainer also thanks reader Tim Heffernan for asking the question.