All 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.
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 NO 2) 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. Environmental 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, NO x can be formed by three different processes to generate thermal NOx, fuel NO x 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 nitrogen-containing compounds.
Prompt NOx is produced by radical reactions in the flame. In most combustion systems, thermal NOx is the dominant production channel, and NO x 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 particles 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 2.5µm in diameter
- “Coarse fraction” particles are greater than 2.5µm, but less than or equal to 10µm in diameter
- PM10 refers to all particles less than or equal to 10µm in diameter (about one-seventh the diameter of a human hair) n
Major 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
- Clay firing
- Glass melting
- Fluid heating
- Food production
- Ore roasting
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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.