28-12-2012, 02:15 PM
Air Pollution
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INTRODUCTION
Air pollution is the introduction into the atmosphere of chemicals, particulate matter, or biological materials that cause discomfort, disease, or death to humans, damage other living organisms such as food crops, or damage the natural environment or built environment.
The atmosphere is a complex dynamic natural gaseous system that is essential to support life on planet Earth. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well as to the Earth's ecosystems.
Nitrogen Oxides (NOx)
Sources
Nitrogen oxides are produced in combustion processes, partly from nitrogen compounds in the fuel, but mostly by direct combination of atmospheric oxygen and nitrogen in flames. Nitrogen oxides are produced naturally by lightning, and also, to a small extent, by microbial processes in soils. Man-made emissions of nitrogen oxides dominate total emissions in Europe, with the UK emitting about 2.2 million tonnes of NO2 each year. Of this, about one-quarter is from power stations, one-half from motor vehicles, and the rest from other industrial and domestic combustion processes. Unlike emissions of sulphur dioxide, emissions of nitrogen oxides are only falling slowly in the UK, as emission control strategies for stationary and mobile sources are offset by increasing numbers of road vehicles.
Atmospheric chemistry and transport
The primary pollutant, directly emitted, is nitric oxide (NO), together with a small proportion of nitrogen dioxide (NO2). NO is oxidised by ozone in the atmosphere, on a time scale of tens of minutes, to give NO2. In rural air, away from sources of NO, most of the nitrogen oxides in the atmosphere are in the form of NO2. NO and NO2 are collectively known as NOX because they are rapidly inter-converted during the day. NO2 is split up by UV light to give NO and an O atom, which combines with molecular oxygen (O2) to give ozone. Therefore, during the day NO, NO2 and ozone exist in a quasi-equilibrium which depends on the amount of sunlight. Eventually, NO2 is oxidised to nitric acid (HNO3, vapour) which is absorbed directly at the ground, is converted into nitrate-containing particles, or dissolves in cloud droplets. At night, different oxidation processes convert NO2 to nitrates.
Although nitric acid is rapidly absorbed on contact with surfaces (cloud droplets, soil or vegetation), the other nitrogen oxides are removed only rather slowly, and may travel many hundreds of km before their eventual conversion to nitric acid or nitrates. Consequently, emissions in one country will be deposited in others. The UK exports about three-quarters of its emissions of NOX.
Removal
Branch Environmental can provide several different technologies for removing oxides of nitrogen (NOx) from air or flue gas. The best treatment method for your application will depend on the conditions of operation.
Selective Catalytic Reduction
SCR systems use our catalytic design to react ammonia injected into the air stream with the NOx present. This approach requires an operating temperature of 320-700º F (160-320º C) depending on the catalyst used.
For flue gas that is already in this range, the SCR system will be a good choice. Systems use about .4 wt. NH3/wt. NOx .
Because there are so many variables, please contact us for specific information.
Wet Phase Catalyst
To avoid the problem of high chemical cost, a special surface catalyst media was developed to be used at ambient temperatures. This unit uses conventional scrubber chemistry with a caustic solution, operates at ambient temperature, and can handle variable loads.
This surface catalyst is relatively expensive initially, so the best choice will depend on concentration of NOx, temperature, required efficiency...
Wet Scrubbing
The lowest initial cost system will be a wet scrubber. However, as NOx absorbs in water, it reacts to form NO (insoluble form) which will slowly reoxidize to NO2. In a conventional scrubber, 60-70% removal occurs. To improve this, several possible chemicals can be introduced into the towers combined with a very long contact time.
The disadvantage of this approach is the cost of the special chemicals required. Additionally, there will be a waste water treatment requirement.