(best examples and good practices) on household organic waste ...

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produced during this process. For example, high temperature combustion
(>1400°C) increases the emission of thermal NOx from atmospheric nitrogen
(Bontoux.,1999). The presence of chlorine or sulphur in the waste will cause the
emission of HCl <strong>and</strong> SOx, but in cement plants equipped with a cyclone preheater
kiln, they will be to a large extent neutralized by the basic raw materials.
NOx, SOx <strong>and</strong> HCl contribute to the acidification of rain. (Bontoux.,1999)
In view of the environmental <strong>and</strong> health concerns raised by all the emissions
from combustion installations, European <strong>and</strong> national environmental
regulations have set emission limit st<strong>and</strong>ards. The immediate response of the
operators of combustion installations was to apply end-of-pipe flue gas
treatment systems to reduce specific emissions: dust, SO2, etc. The array of
technologies implemented is large: post-combustion chambers, dry <strong>and</strong> wet
scrubbers, electrostatic precipitators, cyclones, activated carbon filters, bag
filters, etc. These processes use energy to transfer the airborne pollution to a
solid phase. The wet scrubbers transfer the contaminants to a water phase that
needs further treatment. All flue gas treatments impact negatively on the
energy balance of the systems that burn waste. (Bontoux.,1999)
3.2.5. Pyrolysis
Pyrolysis is a process which transforms waste into a medium calorific gas, liquid
<strong>and</strong> a char fraction in the absence of oxygen, through the combination of
thermo-cracking <strong>and</strong> condensation reactions. Typically, a pyrolysis process will
produce char, oil, steam <strong>and</strong> syngas. A look at the SilvaGas process for RDF
shows that it is really a pyrolysis process. The chemical equation for the Silva
Gas (Battelle) process is:
C20H32O10 + 11.6H2O (l) + 5.7H20 (g) →7.7C + 1.5CO2 + 5.7CO + 7.3H2
+12.3CH4 + 18.6H20 +1349.3 kJ (1)
There are more water molecules on the right h<strong>and</strong> side of the equation than on
the left h<strong>and</strong> side. Hence, the steam input was not essential to the mass <strong>and</strong>
energy balance <strong>and</strong> can be subtracted from both sides. The result defines the
pyrolysis reaction as
C20H32O10 → 7.7C + 1.5CO2 + 5.7CO + 7.3H2 + 12.3CH4 + 1.3H2O + 1.4C2H4
+1349.3kJ (2)
Pyrolytic processes are always endothermic (they absorb thermal energy). They
are typically performed in packed beds, fluidized beds or rotary kilns. Where a
fluidized bed is used, a fluidizing gas is necessary. Many experimental studies
use nitrogen, but this would not be economic for a commercial plant. Practical