A repeat episode of *Dirty Jobs * is just about to air, reminding me of a question I had when I fist saw the program. It relates to the segment about used tire clean-up, reuse and disposal.
The host and his workmates go into the woods to collect old tires that someone has dumped there. They bring them back to the sorting facility where they are grouped into three basic categories: 1. tires good enough to be resold or retreaded, 2. tires good enough to be used as fuel in a cement plant (the ash is also later mixed into the cement, IIRC), and 3. tires unfit for categories 1 and 2 that get shredded and sent to a landfill.
My question concerns category #3 tires. IIRC, the host said that crappy tires that are too big or too small for the cement plant feeder wind up as #3s. But the feeder that they show is little more than a plain old chute that looks like it would hold just about any tire smaller than the ones they put on bulldozers and such. Are they really so picky at these plants and is size really the criteria? Seems to me that there is no need to send any tires to the landfill as long as there is cement to be made.
There’s more to it than just the slot into the incinerator. That plant has an end-to-end automated system for plucking tires out of a heap and transporting them to their destiny. IIRC, they had a vertical conveyor of hooks, and if a tire is smaller than the hooks, it’s not going to go anywhere except to bounce around at the bottom, and if a tire is too large, it could jam the works.
The internal steel belts and sidewall bead wire from big tires might also be a problem at the end of the day or whenever they rake out the unburned leftovers from the incinerator.
The point of automation is to keep costs down by reducing labor, and if they have to pay someone to just babysit a stream of tires, they lose money. It’s easier just to have specs on the incoming materials, and as the truck loading methods are highly manual, it’s trivial to limit the load to passenger car tires at that point in the process.
BTW, that fellow who caught rolling tires in the truck and wove them into an intricate herrigbone pile was amazing.
Two clients of mine in the past have been cement kilns that burned, among other things, tires. I also have several utility clients who burn tires.
The basic issue with oversized tires is that there isn’t usually equipment on-site to chop them down. If they go into the hog/shredder oversized, then they can jam it up, which can take the system down for a good hour or more. Thus oversized tires are just set to the side. I’ve not seen any plants where they had special facilities for cutting the oversized ones into smaller pieces for use in the boiler, as it just wasn’t worth their capital cost and O&M cost to keep such equipment on-site (and it takes a lot of force to cut a tire up, believe me).
I don’t know for certain why undersized tires would be rejected. However, a quick call to a friend at EPRI just now got us speculating that it may be that due to the mechanics of the chopping equipment, smaller tires either slip through whole (due to their small size and flexibility) or else refuse to chop up properly (tires typically need to be cut into a top size of 1x1 (at least in a cement kiln; if you’re burning them in a municipal RDF/TDF furnace, or a stoker, you can go 2x2 or even 3x3 or more).
Tires can make very good fuel for non-pulverized coal furnaces - their heat content is typically equal or greater than many bituminous and anthracite coals. And although they are loaded with iron (steel) and other metals, well, so is the coal ash. And they have some sulfur in them, but less than you get with an Illinois coal, or northern appalachian coal. The EPRI Opportunity Fuels Guidebook, Chapter 5, has much more information on this. As does Google, I reckon. The main reason tires aren’t burned more often is that there are a limited number of furnaces that can accept them, and transportation/collection/sorting/processing costs often (but not always) make them more expensive than coal.
Now aren’t you all glad I’m still hanging around to shovel out useless information that no one really gives a shit about?
