Page 129 - Sustainable On-Site CHP Systems Design, Construction, and Operations

CHAPTER 7

Carbon Footprint—

Environmental Benefits and

Emission Controls

Dharam V. Punwani

ven though the primary products of all fossil fuel combustion are water vapor
and carbon dioxide (CO ), there are several products of combustion that are con-
2
Esidered undesirable for a healthy environment. These include nitrogen oxides
(NO ), carbon monoxide (CO), unburned hydrocarbons (HC), sulfur oxides (SO ), and
x x
particulate matter (PM).
The emissions of undesirable products or pollutants depend on the composition of
the fuel used and the type of combustion system deployed and its mode of operation.
All the sulfur in the combusted fuel appears in the emissions as SO . Total emissions of
x
NO consist of the following two components:
x
1. Fuel NO
x
2. Thermal NO
x
Fuel NO correspond to the products of combustion involving the oxidation of
x
nitrogen content of the fuel, while thermal NO refer to the products of combustion
x
involving the oxidation of nitrogen in the combustion air. The emissions of thermal NO
x
products are independent of the nitrogen content of the fuel but are directly propor-
tional to the temperature of combustion. Higher combustion temperatures result in
higher production of thermal NO .
x
The emissions of CO, unburned HC, and PM depend on the air-fuel ratio and the
tuning of the combustion system. The emissions of PM depend primarily on the type of
fuel used.
Most applications of CHP systems use natural gas or a biogas (produced by bio-
logical conversion of waste materials). Fuel oil/diesel, biodiesel are also used for some
applications. Since gaseous fuels are used most often in CHP systems and the sulfur
content of most gaseous fuels is low, SO emissions are generally not a concern for CHP
x
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