Manufacturing cement is a process of producing very fine particles. Most cement particles range from about 3 to 30 µm. The manufacturing process effectively captures all those fine particles using a variety of dust collection techniques. In essence, that ‘dust’ is our product. Then there’s the ‘dust’ that we see: dust suspended in the air by wind or by a variety of manmade activities. Even simple things like driving down a dirt road can produce dust. This dust is termed fugitive dust. Cement manufacturers capture product and control fugitive dust quite effectively; we’ve done it for years. But there’s a third type of ‘dust’ that requires much more attention: combustible dust.
Combustible dust has the potential to cause serious injury and even death. It can cause explosions and fires. Properly controlled, it safely fuels kilns and calciner vessels. It’s that dichotomy that deserves our attention.
What makes a dust combustible?
Most of us are familiar with the chemistry behind combustion. Simply put, fuel and oxygen are combined with an ignition source and achieve combustion. That’s the combustion triangle: fuel, oxygen, and an ignition source. Remove any one of the three components and combustion cannot be initiated or maintained. Fuels at a cement plant include coal, petcoke, and a wide variety of alternative fuels. The chemical component that makes these materials a fuel is the element carbon. The fuels are fired using the air that we breathe; the same air that contains 21% oxygen and 78% nitrogen. And the ignition source can be as simple as a spark.
That combustion triangle forms the basis for how we process fuel. It also forms the basis for actions to take when trying to stop that combustion. The fuel supply can be shut down and inerting a vessel, literally flooding it with carbon dioxide or nitrogen, removes oxygen.
Combustible dust explosions require two additional elements: suspension and confinement. A pile of combustible dust might burn but it won’t explode. Take that same pile of dust and turn it into a dust cloud by suspending it in air and you’re half way to a disaster. A suspended pile of dust – a dust cloud – can still ignite but it won’t explode until the final element is added: confinement. Entrain a pile of coal dust into the atmosphere outside and you may get a sizeable fireball if you were to ignite it. Take that same pile of coal dust and entrain it in a room or a dust collector or a pressure vessel and ignite it and now it explodes. The suspension of dust means that the dust can combust much more quickly while the confinement of the dust allows the combustion to build up the explosive pressures.
Years ago, chemistry instructors would demonstrate these principles using flour dust. First, they would try to ignite about a half a cup of flour dust to no avail. Next, they would place the flour dust in a funnel connected to tubing. They would blow through the tubing to ignite the cloud of flour dust as it was entrained. Flame heights of several metres were not uncommon, which is perhaps why these demonstrations no longer take place.
By adding the suspension and confinement, the combustion triangle becomes the explosion pentagon. And it doesn’t take much dust. Decades of experiments at the experimental mine in Bruceton, Pennsylvania, demonstrated that, ‘[a] minimum 5/1000-inch (0.12-mm) thick layer (about the thickness of a sheet of paper) of pulverised float coal dust deposited on top of a 3/8-inch-thick uniform concentration of 80% rock dust and 20% float coal dust would propagate an explosion. The thicker the float coal floor layer, the more violent the explosion.’1 Experts theorise that a recent pharmaceutical plant explosion was caused by dust accumulations of less than 0.25 in. (0.6 cm) in depth.2
But still, what makes a dust combustible? In general, any organic material that’s fine enough has the potential to be classified as a combustible dust. It’s a lengthy list that includes familiar organic materials like coal and coke, as well as other more exotic organics like coffee dust, cocoa bean dust, soy dust and tofu dust. Some inorganic dusts can also be problematic. Titanium, aluminium, powder coatings and rubber coatings are just some examples of inorganic materials that can be transformed into combustible dusts.3
1. CDC NIOSH Technology News, No. 515, Milestones in Mining Safety and Health Technology, Float Coal Dust Explosion Hazards, April 2006.
2. US Chemical Safety and Hazard Investigation Board, Investigation Report Combustible Dust Hazard Study, Report No. 2006-H-1, November 2006.
3. US Department of Labor Occupational Safety and Health Administration, Directorate of Standards and Guidance, Combustible Dust Expert Forum, Washington, D.C. May 13, 2011, Meeting Summary Report prepared by Eastern Research Group, Inc.
This is part one of a three-part article written by Rick Bohan for World Cement’s April 2015 issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Check back tomorrow for part two of the article on www.worldcement.com.
Read the article online at: https://www.worldcement.com/the-americas/24122015/the-dangers-of-dust-part-one-3/