Page 315 - Wastewater Solids Incineration Systems

276 Wastewater Solids Incineration Systems

1.0 INTRODUCTION

Combustion is the controlled release of heat caused by the chemical reaction between
a fuel and an oxygen source. The incineration of sludge cake is a combustion process.
The volatile fraction of the total solids in sludge cake is the fuel that ultimately reacts
with oxygen and releases heat. High moisture content distinguishes combustion of
sludge cake from that of more common fuels because the amount of heat required to
convert water from liquid to vapor is high. A significant portion of the heat released
when sludge cake is combusted must be used to evaporate cake moisture. Thus, only
the smaller remaining fraction is available to raise the temperature of the products of
combustion to proper levels. When the thermal properties of the sludge cake and
design of the furnace combine to allow sludge incineration to proceed without the
need for an auxiliary fuel, such as natural gas or oil, the combustion process is con-
sidered autogenous.
Each wastewater treatment plant (WWTP) incinerator operator strives to achieve
autogenous combustion amidst ever-changing biological treatment processes and
increasingly stringent environmental regulations.
This appendix is broken down into four major sections. The first section reviews
some engineering fundamentals associated with the combustion process. The second
section provides an introduction to the basic science of sludge combustion and exam-
ines typical compositions of auxiliary fuels and sludges. The third section examines
the combustion of the most common auxiliary fuels, natural gas and fuel oil. The
fourth section describes the combustion of wastewater treatment plant sludge.

2.0 ENGINEERING FUNDAMENTALS ASSOCIATED
WITH COMBUSTION

Because wastewater treatment and combustion each has its own basic terminology
and units of measure, it is sometimes difficult to switch between the disciplines. This
section will illustrate some important concepts and relationships.
2.1 Describing Physical Quantities in a Mechanical System
Four basic quantities are typically used in a mechanical system:

• Length feet (ft),
• Time seconds (sec),
• Force pounds of force (lbf), and
• Mass pounds (lb).