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Jordanian-German Winter Academy 2006. Combustions Emissions By Eng. Samar Jaber February, 2006.

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Winter Academy 2006

Combustions Emissions


Eng. Samar Jaber

February, 2006

- Combustion of standard fossil fuels results in nine emissions, carbon dioxide, nitrogen, oxygen, water, nitrogen oxide, carbon monoxide , sulfur oxides, volatile organic compounds (VOCs) , and particulate matter.

- We will describe the formation and control of each pollutant

what is no x
What is NOx ?

- A Nitrogen Oxide, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as

a reddish-brown layer

over many urban areas.


Where does NOx come from?

Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process.

In an internal combustion engine, a mixture of air and fuel is burned. When the mixture is tuned so as to consume every molecule ofreactant (in this case fuel and oxygen) it is said to be "running atstoichiometry". With this burns, combustion temperatures reach a high enough level to actually burn some of the nitrogen in the air, yielding variousoxides ofnitrogen, the results of which can be seen over major cities such as Los Angeles , California in the summer in the form of brown clouds of smog.


In general, the contribution of mobile sources to the total NOx level ranges from 60 to 80 percent

For stationary sources, it ranges between 20 and 40 percent.


Thermal NOx: refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primary function of temperature and theresidence time of nitrogen at temperature.

N2 + O → NO + N

N+ O2 → NO + O

N + OH → NO + H

  • Prompt NOx : This is a fast reaction between the N2, O2 and hydrocarbon (CH) fragments:
Fuel NOx : The major source of NOx production from burning fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO.
  • Fuel NOx can contribute as much a s 50% of total emissions when combusting oil and as much as 80% when combusting coal.
The most significant factors affecting NOx formation are flame temperature , the amount of nitrogen in the fuel , excess air level and combustion air temperature.
carbon monoxide emissions
Carbon Monoxide Emissions

Carbon monoxide is a pollutant that is readily absorbed in the body and can impair the oxygen-carrying capacity of the hemoglobin. Impairment of the body's hemoglobin results in less oxygen to the brain, heart, and tissues. Even short-term over exposure to carbon monoxide can be critical, or fatal, to people with heart and lung diseases. It may also cause headaches and dizziness in healthy people.

During combustion, carbon in the fuel oxidizes through a series of reactions to form carbon dioxide (CO2). However, 100 percent conversion of carbon to CO2 is rarely achieved in practice and some carbon only oxidizes to the intermediate step, carbon monoxide.

sulfur compounds so x
Sulfur Compounds (Sox)
  • The primary reason sulfur compounds, are classified as a pollutant is because they react with water vapor (in the flue gas and atmosphere) to form sulfuric acid mist. Airborne sulfuric acid has been found in fog, smog, acid rain, and snow. Sulfuric acid has also been found in lakes, rivers, and soil.
  • The acid is extremely corrosive and harmful to the environment.
The combustion of fuels containing sulfur (primarily oils and coals) results in pollutants occurring in the forms of SO2 (sulfur dioxide) and SO3 (sulfur trioxide), together referred to as SOx (sulfur oxides). The level of SOx emitted depends directly on the sulfur content of the fuel.
  • Typically, about 95% of the sulfur in the fuel will be emitted as SO2, 1-5% as SO3, and 1-3% as sulfate particulate. Sulfate particulate is not considered part of the total SOx emissions.
volatile organic compounds vocs hydrocarbons hc
Volatile Organic Compounds (VOCs)/Hydrocarbons (HC)

VOCs are compounds containing combinations of carbon, hydrogen, and sometimes oxygen. VOCs vaporize easily once emitted into the air and are of concern because of their role in ground level ozone formation.

Formation of VOCs result from poor or incomplete combustion .

VOC's are vapors released from gasoline, paints, solvents, pesticides, and other chemicals.

Formation of VOCs in commercial and industrial boilers primarily result from poor or incomplete combustion due to improper burner set-up and adjustment.
In the combustion chamber the unburned hydrocarbon emissions have several different sources :

1. During compression and combustion, the increasing cylinder pressure forces some of the gas in the cylinder into crevices, or narrow volumes connected to the combustion chamber: The volumes between the piston, rings and cylinder wall are the largest of these.

2. Most of this gas is unburned fuel-air mixture much of it escapes the primary combustion process because the entrance to these crevices is too narrow for the flame to enter.
3. This gas which leaves these crevices in the expansion and exhaust processes, is one of the unburned hydro carbon emissions.
particulate matter pm
Particulate Matter (PM)
  • Emissions of PM from combustion sources consist of many different types of compounds, including nitrates, sulfates, carbons, oxides, and any uncombusted elements in the fuel.
  • Particulate pollutants can be corrosive, toxic to plants and animals, and harmful to humans.
  • PM emissions are primarily dependent on the grade of fuel fired in the boiler. Generally, PM levels from natural gas are significantly lower than those of oils.
PM emissions generally are classified into two categories, PM and PM10. PM10 is a particulate matter with a diameter less than 10 microns. All PM can pose a health problem. However, the greatest concern is with PM10, because of its ability to bypass the body's natural filtering system.
  • When burning heavy oils, particulate levels mainly depend on four fuel constituents: sulfur, ash, carbon residue, and asphalenes. These constituents exist in fuel oils, particularly residual oils, and have a major effect on particulate emissions. By knowing the fuel constituent levels, the particulate emissions for the oil can be estimated.
health and environmental impacts
Health and Environmental Impacts
  • The main reason that NOx is considered an environmental problem is because it initiates reactions that result in the production of ozone and acid rain. Ozone and acid rain can damage fabric, cause rubber to crack, reduce visibility, damage buildings, harm forests and lakes, and cause health problems.
  • By controlling NOx levels, along with the other pollutants, the levels of acid rain and ozone can be reduced.

Smog is a kind of air pollution, originally named for the mixture of smoke and fog in the air. Classic smog results from large amounts of coal burning in an area and is caused by a mixture of smoke and sulfur dioxide.

In the 1950s a new type of smog, known as Photochemical Smog, was first described. This is a noxious mixture of air pollutants including:NitrogenOxides, Tropospheric Ozone and Volatile Organic Compounds (VOCs)

All of these chemicals are usually highly reactive and oxidizing. Due to this fact, photochemical smog is considered to be a problem of modern industrialization.

Photochemical smog is a concern in most major urban centres but, because it travels with the wind, it can affect sparsely populated areas as well.

Smog is caused by a reaction between sunlight and emissions mainly from human activity.

acid rain
Acid Rain

NOx and sulfur dioxide react with other substances in the air to form acids which fall to earth as rain, fog, snow or dry particles. Some may be carried by wind for hundreds of miles.

Acid rain damages; causes deterioration of cars, buildings and historical monuments; and causes lakes and streams to become acidic and unsuitable for many fish. Acid rain also reduces how far and how clearly we can see through the air, an effect called visibility reduction.

NOx reacts with ammonia, moisture, and other compounds to form nitric acid and related particles. Human health concerns include effects on breathing and the respiratory system, damage to lung tissue, and premature death. Small particles penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory disease such as emphysema and bronchitis, and aggravate existing heart disease.

water quality deterioration
Water Quality Deterioration

Increased nitrogen loading in water bodies, particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic plants and animals. Additional nitrogen accelerates "eutrophication," which leads to oxygen depletion and reduces fish and shellfish populations.

global warming
Global Warming

One member of the NOx is a greenhouse gas. It accumulates in the atmosphere with other greenhouse gasses causing a gradual rise in the earth's temperature. This will lead to increased risks to human health, a rise in the sea level, and other adverse changes to plant and animal habitat.

toxic chemicals
Toxic Chemicals

In the air, NOx reacts readily with common organic chemicals and even ozone, to form a wide variety of toxic products, some of which may cause biological mutations.

visibility impairment
Visibility Impairment

Nitrate particles and nitrogen dioxide can block the transmission of light, reducing visibility in urban areas and on a regional scale in our national parks.

no x control technologies
NOx Control Technologies

Post combustion methods address NOx emissions after formation

Combustion control techniques prevent the formation of NOx during the combustion process.

Post combustion methods tend to be more expensive than combustion control techniques

selective non catalytic reduction
Selective Non-catalytic Reduction

Selective non-catalytic reduction involves the injection of a NOx reducing agent, such as ammonia or urea. The ammonia or urea breaks down the NOx in the exhaust gases into water and atmospheric nitrogen. Selective non-catalytic reduction reduces NOx up to 70%.

selective catalytic reduction
Selective Catalytic Reduction

- Selective catalytic reduction involves the injection of ammonia in the boiler exhaust gases in the presence of a catalyst.

- The catalyst allows the ammonia to reduce NOx levels at lower exhaust temperatures than selective non-catalytic reduction. Unlike selective non-catalytic reduction, where the exhaust gases must be approximately 1400-1600 °F, selective catalytic reduction can be utilized where exhaust gasses are between 500 °F and 1200 °F, depending on the catalyst used.

- Selective catalytic reduction can result in NOx reductions up to 90%.

low excess air lea firing
Low Excess Air (LEA) Firing

One of the factors influencing NOx formation is the excess air levels. High excess air levels (>45%) may result in increased NOx formation because the excess nitrogen and oxygen in the combustion air entering the flame will combine to form thermal NOx.

Low excess air firing can be used on most boilers and generally results in overall NOx reductions of 5-10% when firing natural gas.

low nitrogen fuel oil
When firing fuel oils, NOx formed by fuel-bound nitrogen can account for 20-50% of the total NOx level.

One method to reduce NOx levels from boilers firing distillate oils is through

the use of low nitrogen fuel oil

Low Nitrogen Fuel Oil

Reburning is a method of NOx control that uses hydrocarbon radicals to convert nitrogen oxide (NO) to nitrogen (N2) and carbon dioxide (CO2).

Reburning can be applied to boilers that cannot use standard low NOx combustion modification techniques due to the need to maintain high furnace temperatures, such as wet bottom boilers.

In many cases, reburning can be more economical than post combustion NOx controls that would otherwise be used in these instances.


Reburning is accomplished by diverting a portion of a boiler's fuel, typically 10-20%, to a point above the primary combustion zone where it is injected to create a fuel rich "reburn zone." The remaining combustion air is then injected above the reburn zone to provide the necessary burnout air.

water steam injection
Water/Steam Injection

By injecting water or steam into the flame, flame temperatures are reduced, thereby lowering thermal NOx formation and overall NOx levels.

Water or steam injection can reduce NOx up to 80% (when firing natural gas) and can result in lower reductions when firing oils.

Many times water or steam injection is used in conjunction with other NOx control methods such as burner modifications or flue gas recirculation.

flue gas recirculation
Flue Gas Recirculation

FGR entails recalculating a portion of relatively cool exhaust gases back into the combustion process in order to lower the flame temperature and reduce NOx formation. It is currently the most effective and popular low NOx technology for fire tube and water tube boilers.

In many applications, it does not require any additional reduction equipment to comply with regulations.

FGR technology can be classified into two types;
  • External FGR utilizes an external fan to recirculate the flue gases back into the flame. External piping routes the exhaust gases from the stack to the burner. A valve controls the recirculation rate, based on boiler input.
  • Induced FGR utilizes the combustion air fan to recirculate the flue gases back into the flame. A portion of the flue gases are routed by duct work or internally to the combustion air fan, where they are premixed with the combustion air and introduced into the flame through the burner.

Consider the Nitric Oxide formation in the post flame gases of a stoichiometric propane-air mixture at atmospheric pressure. Assuming adiabatic conditions, how does the initial rate of NO formation (ppm/s) from the Zoldivich mechanism compare for no dilution and 25% dilution by N2 (moles of N2 added equals 0.25 the number of moles of air). The reactants and N2 diluents are initially at 298 K.

Each method results in a different

degree of NOx control.

For example, when firing natural gas, low excess air

firing typically reduces NOx by 10%, FGR by 75%,

and selective catalytic reduction by 90%.

Selecting the best low NOx control package should be made with total boiler performance in mind

Some NOx control technologies used to reduce NOx levels by lowering flame temperatures by modifying air/fuel mixing patterns. The lower flame temperature and decreased mixing intensity can result in higher CO levels.
  • An induced FGR package can lower NOx levels by reducing flame temperature without increasing CO levels. CO levels remain constant or are lowered because the flue gas is introduced into the flame in early stages of combustion and the air fuel mixing is intensified. Intensified mixing offsets the decrease in flame temperature and results in CO levels that are lower than achieved without FGR.
co control technologies
CO Control Technologies
  • High flame temperatures and intimate air/fuel mixing are essential for low CO emissions.
  • Some NOx control technologies reduce NOx levels by lowering flame temperatures by modifying air/fuel mixing patterns. The lower flame temperature and decreased mixing intensity can result in higher CO levels.
An induced flue gas recirculation package can lower NOx levels by reducing flame temperature without increasing CO levels. CO levels remain constant or are lowered because the flue gas is introduced into the flame in early stages of combustion and the air fuel mixing is intensified.
  • Intensified mixing offsets the decrease in flame temperature and results in CO levels that are lower than achieved without FGR. But, the level of CO depends on the burner design. Not all flue gas recirculation applications result in lower CO levels.
so x control technologies
SOx Control Technologies
  • Methods of SOx reduction include switching to low sulfur fuel, desulfurizing the fuel, and utilizing a flue gas desulfurization (FGD) system.
  • Fuel desulfurization, which primarily applies to coal, involves removing sulfur from the fuel prior to burning. Flue gas desulfurization involves the utilization of scrubbers to remove SOx emissions from the flue gases.
For users of industrial boilers, utilizing low sulfur fuels is the most cost effective method of SOx reduction. Because SOx emissions primarily depend on the sulfur content of the fuel, burning fuels containing a minimal amount of sulfur (distillate oil) can achieve SOx reductions, without the need to install and maintain expensive equipment.
vocs control technologies
VOCs Control Technologies

To control VOC emissions from commercial and industrial boilers, no auxiliary equipment is needed; properly maintaining the burner/boiler package will keep VOC emissions at a minimum. Proper maintenance includes keeping the air/fuel ratio at the manufacturer's specified setting, having the proper air and fuel pressures at the burner, and maintaining the atomizing air pressure on oil burners at the correct levels. An improperly maintained boiler/burner package can result in VOC levels over 100 times the normal levels

pm control technologies
PM Control Technologies

The emission levels of particulate matter can be lowered by switching from a residual to a distillate oil or by switching from a distillate oil to a natural gas. Additionally, through proper burner set-up, adjustment and maintenance, particulate emissions can be minimized, but not to the extent accomplished by switching fuels