Re: Please give more details on Activated Carbon.

Date: Mon Mar 29 10:19:11 1999
Posted By: Anna Steding, Grad student, Ecological Engineering, University of California/Department of Civil and Environmental Engineering
Area of science: Chemistry
ID: 919823304.Ch

Message:

The charring process involves three steps:

Drying at 150 to 200 deg. C;
Pyrolysis (or carbonization - used to drive off hydrocarbons but with an insufficient supply of air to sustain combustion) at 400 to 600 deg. C; and finally..
Activation by exposure to a mildly oxidizing atmosphere (steam, carbon dioxide, air) at 600 to 900 deg. C.

It is this final step that develops the pore structure in the precursor material. This precursor material, as you probably know, can be bituminous or lignite coal, peat, wood, coconut hulls, almond hulls, or walnut hulls. It is the second step - carbonization - which can produce harmful byproducts of incomplete combustion, including carbon monoxide, nitrogen oxides, sulfur oxides, particulate matter, and volatile organic compounds (including benzene, styrene, trihalomthane), as well as other hydrocarbons (polycyclic aromatic hydrocarbons, for example).

Carbon monoxide, nitrogen oxide, and hydrocarbons could be removed from the exhaust gas by a catalytic converter (like the one found in cars), which uses a solid catalyst (usually platinum) to lower the activation energy of the reactions necessary to convert CO, NO, and hydrocarbons to less hazardous forms. With a lower activation energy, complete combustion can occur - at a lower temperature. Specifically, NO is reduced to nitrogen gas, CO is oxidized to carbon dioxide, and hydrocarbons are oxidized to carbon dioxide and water. Nitrogen oxides can also be removed using what is called a thermal deNOx process: ammonia is added to the exhaust gas and subsequently undergoes thermal decomposition to form an amine (NH2), which in turn reacts with NO to form water and nitrogen gas.

Sulfur oxides can be removed from the exhaust with a scrubber that removes the pollutant from the gas phase by dissolving it in a liquid phase. The liquid phase of choice for sulfur oxide removal is a slurry composed of 90% water and 10% slaked lime or limestone. Volatile organic compounds can be removed from the exhaust gas by incineration, condensation (causing the gas to undergo a phase change to liquid by cooling or increasing pressure), and through membrane recovery. Ironically, adsoprtion (i.e. with activated carbon) can also be used to remove VOCs! Finally, particulate matter can be removed in a settling chamber, a cyclone (which causes air to flow in a vortex, thereby separating the particles by inertia), a filter, or an electrostatic precipitator (which creates strong electrostatic charge on particles and then uses electric field to separate particles from gas stream).

You are correct that some toxins can be left behind in the charring process, notably metals. Consider that the activated carbon is used to remove hazardous materials from solution, and therefore becomes classified as hazardous waste itself and must be treated as such when being regenerated or disposed.

For more information on air pollution control, you might want to try a text on air pollution control. A very recent one is by R.J. Heinsohn (1999): "Sources and control of air pollution", Prentice Hall publishers. Some information on activated carbon is available in "Wastewater Engineering: Treatment, Disposal, and Reuse" (1991) by Tchobanoglous and Burton, McGraw-Hill publishers.

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