Page 91 - Wastewater Solids Incineration Systems

60 Wastewater Solids Incineration Systems

4.1.3 Combustion Temperature and Gas Residence Time
For wastewater solids, a combustion temperature of 843°C (1550°F) is typically used,
with a minimum freeboard gas residence time of 6.5 seconds. Although there are no
regulatory temperature and residence time requirements for incinerators in the
United States, the previous figure is conservative compared with European regula-
tory requirement of a freeboard temperature of 850°C (1562°F) for a minimum of
two seconds (European Commission, 2000).

4.2 Combustion Air Temperature versus Solids Content

One of the most important criteria in designing the incineration system is to mini-
mize fuel consumption. Supplementary fuel consumption is calculated from the heat
and mass balances. It depends on two factors: (1) the heat content of the feed mate-
rial (or specifically its solid content, i.e., percent dry solids); and (2) the heat content
of the combustion air (or its temperature).
Solids content of the feed depends on the dewatering equipment and on the
quantity of polymer used as a dewatering aid. Heat content of combustion air
depends on how intense heat recovery is in the heat exchanger. Typically, a heat
exchanger can recover up to 40% of flue gas enthalpy to preheat the combustion air
to approximately 675°C (1250°F).
The theoretical curve of supplementary fuel consumption is presented in
Figure 5.6. The calculation was based on a combustion gas temperature of 843°C
(1550°F) and a throughput capacity of 454 kg (1000 lb) of dry solids per hour. The
feed material is typical of that resulting from WWTP with 75% volatile solids and
a high heating value of 23 260 kJ/kg (10 000 Btu/lb) of volatile solids. Fuel con-
sumption decreases with either percent dry solids or combustion air temperature
increase, or both.
The greater the solids content and the greater the combustion air temperature,
the lower the auxiliary fuel requirement. With a wind box temperature of 648°C
(1200°F), the feed material is burned autogenously, or thermally self-supporting, at a
solid content of 27%.
To avoid emission of nitrogen oxides (NOx emissions increase with dry solids,
according to Dangtran and Holtz, 2001) and to lower polymer consumption, the
system is typically designed based on autogenous combustion at maximum air tem-
perature and minimum dry solids.