ENERGY FROM GARBAGE

Garbage - municipal solid wastes, that is - has only about 40% of the BTU's per ton that coal has; also, coal has a much smaller volume for the same weight and is easier to handle, quite apart from the fact that the wastes must usually be sorted first. And these are only some of the reasons why garbage is more expensive than competing fuels. In a nutshell, the utilities pay for regular fuels, but they get paid (at least for the difference) by cities to help take the garbage off their hands.

Disposal of municipal solid wastes is, in fact, an ugly problem that threatens to grow uglier. Most of the wastes are buried in landfills, where moisture can leach out chemicals, and they can find their way into the local water supply; but perhaps an even bigger problem is that large urban areas will, according to the EPA, run out of landfill sites within the next decade. Inflation and transportation to increasingly remote sites is expected to boost the national garbage bill to $10 billion in another two years.

On the other hand, garbage as an energy source is expensive, particularly if the fuel from it is to be good enough to be burned in a power plant without damaging its equipment.

There are at least two ways to accomplish this: pyrolysis and anaerobic digestion, both involving high capital and operating costs.

Pyrolysis is treatment by high temperature without access of oxygen charring, but not burning; similarly, anaerobic digestion is chemical decomposition by bacteria in the absence of free oxygen. Both of these processes liberate methane rich gas (pyrolysis also produces low-sulfur oil). Some of the gas, of course, gets liberated by bacterial decomposition anyway, which is part of the reason why garbage stinks; this comparatively small amount of gas can also be tapped by sinking a gas well into a landfill.

Refuse derived gas burns with no trouble in power plant boilers; but digestors and pyrolytic equipment are expensive in both capital and operating costs. Pyrolytic oil is low in sulfur content, but requires special handling because of higher viscosity (resistance to flow), corrosive activity on conventional steel, and a tendency to degrade at moderate temperatures.

A plant for anaerobic digestion is now under construction in Florida, and another is planned in Delaware. Occidental' s pyrolytic plant at El Cajon near San Diego (AtE Jan.74) is now getting ready to start up with a capacity of 200 tons per day. At present, pyrolytic fuel costs from 25% to 70% more per BTU than low sulfur fuel oil on the market, illustrating the point that refuse derived fuel is more important for waste disposal than as an energy source, which is why municipal authorities usually sell the fuel below cost to the utility. But as disposal sites (and competitive fuels) get more expensive, the economics of the process may change.