Course: BFM 202 Biodiesel Production

Lecture

Exhaust Emissions
Credits
BFM 202 Biodiesel Production Unit 2 Lecture has been adapted from "ENGY 131 Unit 2 Commentary: The Diesel Engine" by Southeastern Illinois College.

ENGY 131 Biodiesel Production course content by Southeastern Illinois College is licensed under a Creative Commons Attribution 3.0 Unported License. Except where indicated otherwise.

Exhaust Emissions

As the fuel-air mixture in the cylinder combusts, soot particulates and
oxides of nitrogen (NOx) form.
The highest temperatures are found in the combustion chamber where the
fuel has a higher air-fuel ratio and particulates are formed. Carbon,
hydrogen, and inorganic compounds form as high-molecular-weight
hydrocarbon particulates. These hydrocarbons adsorb, gathering on the
condensed liquid, and are emitted through the exhaust pipe. Biodiesel
reduces soot production, possibly by binding with oxygen in the fuel.
Biodiesel's low volatility causes a small portion of the fuel to survive
the combustion process and coat the cylinder walls, where it is then
released during the exhaust process.

NOx

When the fuel-air mixture in the cylinder combusts, the results
are the formation of soot particles and oxides of nitrogen (NOx).
High gas temperatures and lean fuel conditions create NOx emissions,
resulting in smog. If diesel engines are modified to control NOx,
particulate emissions increase as does fuel consumption.
It is possible that the carbon reduction from biodiesel fuel also
reduces the cylinder's heat loss thus increasing the cylinder's
temperature. Since NOx is created in this high temperature area of the
cylinder, higher NOx can be produced when biodiesel fuels are used.

Sulfur

In most engines, diesel fuel's carbon burns to carbon dioxide with the
hydrogen converted to water vapor. Sulfur that exists in the fuel oxidizes
to sulfur dioxide and sulfur trioxide. The sulfur oxides combine with water
vapor to form sulfuric acid and sulfate compounds. Sulfates then become
exhaust particulates and increase the particulate emissions.
In 1993, the EPA set significantly reduced diesel fuel exhaust limits
for sulfur at 500 ppm.  In 2006, the EPA further reduced sulfur exhaust
limits to 15 ppm. This obviously eliminated sulfur as an exhaust
particulate and introduced the need for catalytic after-treatments for
diesel engine exhaust. There are limited options for controlling these
emissions since sulfur is a catalyst poison.
Comparatively, biodiesel from soybean oil is very low in sulfur while
biodiesel from animal fat contains higher amounts of sulfur.