PYROLYTIC PROCESSING OF ORGANIC WASTES
Kenneth A. Zeltner, Product Engineer
Energy Systems
Surface Division
Midland-Ross Corporation
Toledo, Ohio 43691
The United States used 74.4 quads (1015 Btu) of energy in 1981 [1]. This staggering
magnitude converts to a per-capita consumption of 15 hp. In the last decade, increasing
attention has focused on waste conversion to energy technologies to assist in supplying this
demand. Direct combustion of fossil fuels is giving way to advanced thermal processes like
pyrolysis and gasification. These processes can accept as a feedstock virtually any organic
"waste" material generated by industry, agriculture and municipality. Conventional waste
disposal methods, like landfilling and ocean dumping, may not be legal or practical in the
foreseeable future. The fossil fuel outlook provides additional incentives because of escalating prices, questionable foreign suppliers and inevitable resource depletion.
SRI International found in an exhaustive 1975 survey that the United States generated
more than 500 million tons of organic solid waste, equal to 5 quads of energy [2]. Presently,
nonfossil fuels contribute 1.8 quad/yr (equal to the power generated by nuclear reactors)
and have a 5% annual growth rate [3].
Midland-Ross has developed a versatile waste-to-energy conversion system based on pyrolysis followed by incineration. The Surface Division of Midland-Ross has been in the
thermal processing business since 1915 and has a strong background in technologies involving furnace design, combustion, heat-treating and waste heat recovery. Because of the
variable physical and chemical properties of different waste streams, each waste-to-energy
system must be custom-designed. The bench- and pilot-scale test facilities housed at the
Thermal Systems Technical Center are ready to research and develop challenging systems
that require a high degree of sophistication. To date, successful commercial applications
include the thermal processing of pharmaceutical sludge, municipal sewage sludge, painting-
line fixtures, oily machine shavings, wax molds for precision castings, chemical warfare
agents and various liquid wastes [4-6].
PYROLYSIS FUNDAMENTALS
Pyrolysis is the chemical change of a substance by means of heat alone. As temperature
increases, bond energies decrease to the point where molecules break apart. True pyrolysis
takes place in an inert atmosphere without catalysts; if oxygen is present at elevated temperatures, combustion reactions will occur. Heat can be supplied with indirect radiation, by
electrical heating elements or with direct-fired burners operating stoichiometrically.
Inside a pyrolyzer, organic gases are driven off the feedstock, leaving behind granular
residue; offgases are combusted in a separate incinerator. The terminology is confusing in
gasification and "starved-air" processes where combustion and pyrolysis takes place in
different zones of the same reactor. Oxygen combines with fixed carbon in the combustion
zone and the heat evolved pyrolyzes nearby material, assuming all oxygen has been consumed.
Parameters governing the pyrolytic transformation include temperature, residence time,
hydrocarbon partial pressure, total pressure and chemical composition of the feedstock.
Offgas production mechanisms involve liquid volatilization, solid sublimation and pyrolytic
reformations.
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