Jordanian-German Winter Academy 2006. Combustions Emissions By Eng. Samar Jaber February, 2006.
Winter Academy 2006
Eng. Samar Jaber
- We will describe the formation and control of each pollutant
- 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.
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.
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
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.
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.
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.
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.
Smog is caused by a reaction between sunlight and emissions mainly from human activity.
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.
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.
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.
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.
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.
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.
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 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 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%.
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.
One method to reduce NOx levels from boilers firing distillate oils is through
the use of low nitrogen fuel oilLow 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.
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.
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.
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.
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
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
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