Chemical Engineering March 2018 - 32

Facts At Your Fingertips
Industrial Combustion Products
Department Editor: Scott Jenkins
ll chemical process industries
(CPI) operations involving combustion
need to be concerned
with the emission of combustion
products, due to their potential environmental
and health impacts. This
one-page reference provides information
about several of the most common
combustion byproducts from
industrial heating processes.
Combustion provides heat for many
industrial processes and accounts
for over 80% of the power used by
turbines to generate electricity. The
listing in the box shows several operations
within the CPI where combustion
is used. Modern industrial
combustion - both for process heat
and power generation - is characterized
by an increasingly diverse fuel
supply and a greater need to reduce
potentially polluting emissions.
A
ronmental Protection Agency (EPA)
requires control of NOx in combustion,
because NOx reacts with organic
vapors in the presence of sunlight
to produce ground-level ozone. The
low-cost way to limit NOx is to do
so upstream in the combustion process,
for instance, via low-NOx burners,
and low-oxygen firing systems.
In a flame, NOx can be formed by
three different processes to generate
thermal NOx, fuel NOx and prompt
NOx. Thermal NOx is produced at
high temperatures by elementary reactions
of the Zeldovich mechanism.
Fuel NOx is produced from combustion
of nitrogen-containing fuel
components. Fuel-generated NOx
can be avoided by using fuels without
significant quantities of nitrogencontaining
compounds.
Prompt NOx is produced by radical
Combustion products
The following sections discuss some
of the main potential pollutants that
result from the combustion of fossil
fuels in an industrial setting.
Carbon dioxide. CO2 is the principal
product from the combustion of
fossil fuels. It is a colorless, odorless
gas with a density greater than air.
For each pound of carbon burned in
a CPI process, 3.67 lb of CO2 is released.
CO2 is a greenhouse gas, and
is present in the earth's atmosphere
at concentrations around 400 parts
per million (ppm). Its IDLH (immediately
dangerous to life and health)
concentration is 40,000 ppm
Carbon monoxide. CO is a colorless,
odorless gas that is formed when carbon
in fuel is not burned completely.
Adverse health effects can occur at
CO concentrations as low as 10 ppm
with prolonged exposure. In the atmosphere,
CO can be a component
of smog.
Oxides of nitrogen (NOx). Oxides of
nitrogen (primarily NO and NO2) form
when fuel is burned at high temperatures.
The primary sources of
NOx include motor vehicles, power
generation facilities and other industrial
sites, as well as commercial and
residential sources. The U.S. Envi30
reactions
in the flame. In most combustion
systems, thermal NOx is the
dominant production channel, and
NOx emissions can be controlled by
limiting the flame temperature.
Sulfur byproducts. Sulfur dioxide
(SO2), belongs to the family of sulfur
oxide gases (SOx). These gases dissolve
easily in water. Sulfur is prevalent
in all raw materials, including
crude oil, coal, and ores that contain
common metals, such as aluminum,
copper, zinc, lead and iron.
SOx gases are formed when fuel
containing sulfur, such as coal and
oil, is burned, and when gasoline is
extracted from oil, or metals are extracted
from ore. SO2 dissolves in
water vapor to form acid and interacts
with other gases and particles
in the air to form sulfates and other
products that can be harmful to humans
and the environment.
Particulate matter.
Particulate matter
(PM) is the general term used to
describe a mixture of solid particles
and liquid droplets found in the air.
PM can either be emitted directly or
formed in the atmosphere. Different
sources of particles include:
* " Primary " particles are formed
from combustion sources and
are emitted directly into the atmosphere.
Examples of primary parMAJOR
PROCESSES INVOLVING
COMBUSTION
Combustion heating temperatures range
from 400-500K for bread baking and paint
drying, to 2,000K for cement and steel
making.
*
*
*
*
*
*
*
*
*
*
*
*
Metal melting
Metal heat treating
Curing and forming
Drying
Calcining
Clay firing
Agglomeration
Glass melting
Fluid heating
Distillation
Food production
Ore roasting
ticles are dust from roads or black
carbon (soot).
* " Secondary " particles are formed
in the atmosphere from primary
gaseous emissions. Examples of
secondary particles are sulfates
formed from SO2 emissions from
power plants and industrial facilities;
nitrates formed from NOx
emissions from power plants, automobiles,
and other combustion
sources; and carbon formed from
organic gas emissions from vehicles
and industrial facilities.
Some particles are large enough to
be seen as dust or dirt. Others are so
small they can be detected only with
an electron microscope.
* PM2.5 describes the " fine " particles
that are less than or equal to
2.5 µm in diameter
* " Coarse fraction " particles are
greater than 2.5 µm, but less than
or equal to 10 µm in diameter
* PM10 refers to all particles less
than or equal to 10 µm in diameter
(about one-seventh the diameter
of a human hair)
n
References
1. Baukal, C., " Industrial Combustion Pollution and Control, "
Taylor and Francis, 2003.
2. McGowan, T., Air-Pollution Control: Assessing the Options,
Chem. Eng., August 2011, pp. 62-70.
3. Dutton, J., Energy Conservation and Environmental
Protection, Lesson 4, Online course materials, Penn
State University, accessed from: www.e-education.psu.
edu/egee102.
4. Littlejohn, D. et al., Burner Design for Fuel Flexibility and
Efficiency, Chem. Eng., May 2011, pp. 44-47.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2018
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Chemical Engineering March 2018

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