Encyclopedia of Environmental Science and Engineering, Volume I and II

660 MANAGEMENT OF SOLID WASTE

molecules. The process is carried out in the absence of added

oxygen (or with very limited oxygen), and with the addition

of heat, at temperatures in the range of 900–1700°F. The pro-

cess will produce a fuel gas, oil and carbon. A study in San

Diego indicated that about 50% of the refuse is susceptible to

pyrolysis.^34 This study yielded low BTU gas, char and oxy-

genated hydrocarbons. Cities Services has evaluated a similar

process and believed it to be potentially economic in plants

with capacities of 5000 tons per day which are subsidized at

the rate of $2 to $8/ton.^35 Pilot plant studies by the Bureau of

Mines indicate that both industrial and municipal refuse yield

large amounts of gas and solid, as shown in Table 23.

A process using pyrolysis has been piloted for tires,

which present some particularly difficult disposal problems

in incineration and landfill.^37 Similar processes can be effec-

tive in recovering chemicals from plastics but have not been

developed because of separation problems.^51

The US Bureau of Mines has also piloted a high pressure

process where refuse free of glass and metal is reacted in the

presence of water and carbon monoxide (hydro-oxynation)

at 1400 psig and 500 to 700°F to yield oil, gas and carbon.^37

This process appears to have very favorable oil yields.

Process variable studies have shown that conversions as high

as 90% can be obtained with a 40% yield of oil; typical yield

is shown in Table 24. These yields indicate a potential oil

production rate of 2  10 8 tons annually as compared to US

crude production of 5  10 8 tons per year. Similar studies

using hydrogen showed lower yields and conversions. The

use of CO does present some significant operating problems

as well as economic debits.

Several systems using pyrolysis were ready for commer-

cialization. Hercules in Delaware planned a unit to pyrolyze

industrial waste. Monsanto Environchem built a pyrolysis

unit using the LANDGARD process (Figure 7). This pro-

cess emphasized waste reduction (with recovery of ferrous

metal), rather than recovery of variable byproducts; it has

been piloted in a 35 T/day semiworks facility. The process

reduces the solid waste, typically, by 90%; a typical stack gas

analysis is given in Table 25 and indicates the very low par-

ticulate matter in the effluent. Unfortunately the unit did not

operate successfully at full scale because particulate removal

did not meet expectations and costs became prohibitive.

In contrast to the Landgard system, a pyrolysis process

emphasizing recovery of valuable products was developed

by Garrett Research and Development Co. This process was

piloted at a 4 T/day level. It consisted of shredding, air clas-

sification, pyrolysis and pyrolysis product separation steps.

(A full scale unit was built in San Diego, but never operated a

full capacity because of mechanical problems). Product recov-

ery was similar to that obtained by the Bureau of Mines.

One may ask why solid refuse should be subjected to

complex processes such as hydropulping or pyrolysis.

Where actual wastes such as paper or cellulose fiber can be

recovered, hydropulping is certainly attractive; on the other

NON-MAGNETIC

TRASH

SUPER

CONDUCTING

IRON MAGNET

&

STEEL

ALUMINIUM AIR

CLASSIFIER

GLASS

MAGNETIC

SEPARATOR

NOTE:

ALL DUST & FINES

TO COLLECTOR

METAL, GLASS

HEAVY PLASTIC

TERTIARY

CLASSIFIER

PLASTIC

GLASS

PLASTIC

DUST*

DUST*

DUST*

DUST*

AIR

AIR

AIR

AIR

AIR

AIR

CARDBOARD

PAPER

CONVEYOR

SECONDARY

CLASSIFIER

LEAVES

PLASTIC

RAGS

CYCLONE

CYCLONE

PRIMARY

CLASSIFIER

PRIMARY

SHREDDER

FINE

SHREDDER

MUNICIPAL

REFUSE

START

BALLISTIC

REJECT

LIGHT

FINES

GRIT

FIGURE 4 Proposed waste separation system, using SRI Air Classifier.

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