Page 64 - Wastewater Solids Incineration Systems

Chapter 4

Combustion Theory

1.0 THERMODYNAMIC 3.0 EFFECTS OF OPERATING
PROPERTIES OF SOLIDS 33 PARAMETERS 39
2.0 ACTUAL FURNACE 4.0 REFERENCES 41
OPERATING
CONDITIONS 38 5.0 SUGGESTED READINGS 42

1.0 THERMODYNAMIC PROPERTIES OF SOLIDS

Before delving into the combustion of wastewater treatment plant (WWTP) residuals,
it is important to develop a basic understanding of their composition. From a ther-
modynamic perspective, the dewatered feed cake going into the incinerator is com-
posed of three key components: moisture, ash, and volatiles.
In a furnace, the moisture changes from liquid to vapor during the evaporation
process; there is no change in chemical composition. The ash is typically chemically
inert and does not undergo any chemical reactions in a furnace. It is the volatile (com-
bustible) fraction that reacts with the oxygen in the air and in doing so changes com-
position and liberates heat.
An effective study of the combustion process depends on a detailed knowledge of
specific fuel characteristics. The key tool in developing this information is an analysis by
a laboratory that specializes in such moist solids fuels. The American Society of Testing
and Materials (ASTM) and other organizations have published standards that govern
this kind of testing and which describe in detail the lab procedures to be followed to
generate consistent, reproducible test results. Feed cake can benefit from several tests:

• Proximate analysis—moisture, volatile matter, fixed carbon, ash.
• Ultimate analysis—moisture, ash, carbon, hydrogen, nitrogen, oxygen,
chlorine (sometimes), sulfur, heating value.

33