Paper Mills work with mostily wet products and geralily dry with steam, which is only 212 F. or slightly higher depending on pressure. Pellet processing is very different, the dryer is not a dryer like in paper opperations pellet operations use a very high temp kiln that can reach well over 1000 F. ( The one in strong was warming up and was at about 900 F. when it exploded.) This particular kiln was heated with a large hot air fernace (useing no steam) and even though all of the plants metal devices and pipeing were grounded somehow a spark or flame ignited the ever growing amount of very dry wood dust, which will explode, espically if it is in a closed space ( like the 50 foot long 97,000 lb kiln.) this created a situation not unlike whats going on in your cars engine on a massive scale. (hmm, maybe we should run our cars on wood dust?)

Mills like the one in strong should take their que from large scale flour/wheat processing plants which deal with large amounts of dust everyday. though it still happens, they have reduced the chances of this happening by installing grounded metal probes in their transport pipes to make actual contact with the static on the dust particals flowing in the pipe. often the dust is moveing so fast it doesn't come in full contact with the metal pipe, thus leaveing a growing amount of static IN the dust which can potentially cause a spark. I wonder if this mill had something like that installed???

Combustible dust related fires and explosions are a regular occurrence globally in wood pellet plants http://dustexplosions.blogspot.com/2008/08/wood-pellet-dust-fire-non-issue.html

I've provided numerous Spark Detection and Extinguishment Systems for this industry. These systems have been used for years in the wood processing plants. We use infrared detection with plant water extinguishment. The water is pressurized to 60 psi and is activated at 300 ms or less. The sparks are typically extinguished on the fly while the process keeps operating.

Dryers are a must to have these systems. Other area's are the hammermills. I typically use two zones in these area's due to the amount of risk. Both processes can produce high amounts of sparks. Double protection is a must.

Other area's in pellet plants that are critical is detection from the pellet mill discharge or the cooler infeed and the cooler air exhaust. If the bearings in the rollers fail, and they do frequently, hot embers are now fed into the cooler. This is typically a detect and deluge application. The duct is a detect and extinguish.

It's not a bad idea to protect bucket elevators and any ducts going to dust collectors.

R. Wann / GreCon, Inc.