A LOT OF EMPTY SPACE

Recycling? The demand has gone down sharply, mainly because recapped tires no longer represent a big saving over new ones. There are some tire disposal methods such as using them as fillers for asphalt and concrete, but by and large, the millions of scrap tires are an ugly nuisance.

The obvious solution is to shred the rubber and burn it as fuel, for the heating value of shredded rubber is some 25% higher than that of coal. That is what General Motors tried in an experiment in 1970. They burned 30 tons of shredded tires in a 10% mix with 90% coal, but the cost of shredding and the material handling problems made the project so expensive compared to pure coal that the project was abandoned.

Three years later, a solution to the bad comparative economics was found by a band of Arab sheiks, who so increased fuel prices (indirectly coal, too) that GM re-evaluated the project and resumed its large-scale tests at its Fisher Body-Pontiac powerhouse.

Tires are shredded, fed into a storage silo, and from there automatically added as a 10% additive to the coal as it moves along conveyers from the coal cars to the bunkers.

The results of a six-month experiment were reported at a conference on Rubber Processing Operations in Akron, Ohio, last March. Initially, the rubber tended to jam and clog the stokers, but these problems have now been overcome. Pollution is increased, mainly by zinc oxide emissions, but these are well below permissible levels. The dollar saving in fuel costs amounts to about 6% compared with burning coal only, not counting depreciation of the required capital equipment; beyond that, there is the saving in energy, and the intangible benefit of disposing of scrap tires.

The full report, Shredded tires as an auxilliary fuel, is available to AtE subscribers from the author, R. Taggart, Environmental Activities Staff, GM Technical Center, Warren, MI 48090.