Environmental Engineering Reference
In-Depth Information
11.6.2
Chemical Recycling
Before PVC containing waste can be converted into fuels by liquefaction or gas-
ification, dechlorination has to be conducted. Dechlorination can be carried out
thermally by heating plastic waste at temperatures at about 300 °C when most of
the chlorine is removed. If more efficient dechlorination is required, hydrogenation
can be carried out. In the presence of hydrogen at elevated pressures and tempera-
tures, heteroatoms such as halogens, oxygen, and nitrogen are removed as HCl,
HBr, water, and ammonia. High quality paraffinic oil is obtained which is sold as
syncrude.
Another possibility is wet dechlorination in aqueous or organic alkaline solu-
tions. In aqueous solutions, high pressures are required, while the reaction can be
carried out at atmospheric pressure in ethylene glycol at 190 °C. The residual de-
chlorinated PVC is a black rigid material that can be treated with other plastics in
liquefaction or gasification plants. Chlorine is recovered as NaCl.
11.7
Energy Recovery
Today's energy supply relies on the fossil fuels natural gas, oil, and coal. It can be
seen from Fig.
11.8
that natural gas provides the highest amount of energy during
combustion expressed as heating value, followed by propane, petroleum products,
and coal. This order can be explained by the elemental composition of these fuels:
the combustion of hydrogen provides more energy than the combustion of carbon.
As a consequence natural gas consisting of more than 90 % methane (CH
4
) and
an H/C ratio of 4 has the highest heating value. Lower is that of petroleum with
an H/C ratio of 2. Coal consisting mainly of carbon has an H/C ratio of about 0.5.
Vice versa, the highest emissions of the greenhouse gas CO
2
are generated by coal,
since mainly carbon contributes to the combustion. CO
2
emissions of petroleum and
especially natural gas are much lower, since the combustion of hydrogen makes a
large contribution.
Most plastics are petrochemical products. Therefore, they also have high heating
values. Polyolefins (PE, PP) and polystyrene are hydrocarbons with chemical struc-
tures also present in petroleum; these plastics can be seen as solid fuels. The heating
values of heteroatom containing plastics such as PET and PVC are significantly
lower. Because of the oxygen content, PET can be seen as a partially oxidized mate-
rial. Since oxygen bonds do not contribute to the combustion, the energy released is
reduced accordingly. PVC contains 56 % (wt.) chlorine as a non-burnable portion.
It can be seen from this that the energy that can be recovered depends strongly on
the elemental composition of the material.
Similar effects are observed in the combustion of bio-waste. Char coal which is
derived from heating wood in the absence of oxygen at temperatures below 400 °C,
exhibits heating values in the range of fossil coal. Heteroatoms especially oxygen
