The use of activated carbon in NOx removal
NOx is a generic term used for the oxides of nitrogen, such as, NO (nitric oxide) and NO2 (nitrogen dioxide). These compounds are produced as a by- product during the combustion of nitrogen at high temperatures. Diatomic nitrogen (N2) is a triply bonded molecule between the 2 N atoms (that is to say that there are 3 bonds between each of the N atoms) and has a bond enthalpy of 945 kJ mol-1 making it one of the strongest bonds in nature (all 3 must be broken simultaneously, in order to separate the two atoms). It forms 70% of our atmosphere and is relatively inert. NOx can be naturally made through lightning strikes and by plants; nitrifying bacteria converts ammonia in the soil to NO2. Most anthropogenic NOx is generated from 3 primary sources:
High temperature oxidation of nitrogen (combustion).
Combustion of nitrogen-rich fuels.
Reaction of atmospheric nitrogen with radicals.
The most significant source of NOx is from vehicle emissions, where large volumes are emitted in urban areas with significant motor vehicle traffic. Although NOx is considered to be an environmental hazard causing large problems in the atmosphere, it can rapidly disperse. Where it does accumulate, NOx forms photochemical smog and in the presence of atmospheric water vapour, forming nitric acid and contributing to acid rain:
4NO2 + 2H2O + O2 → 4HNO3
NOx is involved in the tropospheric production of ozone:
HO2• + NO → •OH + NO2
NO2 + hν→ NO + O
Here, peroxy radicals (HO2•) react with NO to give a hydroxyl radical (•OH) and NO2. NO2 then reacts with light (hν) to produce more NO and a single oxygen atom.
hν is chemistry shorthand for representing light in a mechanism, where h is Planck’s constant and ν is frequency. The full equation is given as:
This relates the energy of a photon (a particle of light), E, to the frequency of the light, ν. This is because light can be thought of as both a particle and a wave, in quantum mechanics this is known as wave-particle duality. The equation defines the relationship between the energy of a photon in terms of the photon’s frequency. Planck’s constant, is a proportionality constant that relates these two properties of light and is fixed at 6.626×10-34 J s-1.
For the reaction above, it means that the reaction is photochemical (in this case, the reaction is known as photodissociation. NO2 is split into two components in the presence of light and photodissociates at 400 nm, which is in the violet band of the visible range of the electromagnetic spectrum).
Whilst ozone is a major component of the stratosphere, (located 20 to 50 km from the Earth’s surface) it performs an important function in that it shields the Earth’s surface from harmful UV and this is known as the ozone layer. Ozone in the troposphere (12 to 20 km) is considered to be a greenhouse gas. Ozone within the troposphere produces toxic oxides due to ozone’s powerful oxidising capabilities. It is also an irritant and is a constituent of smog. Acid rain produced by dissolution of NOx creating weak nitric acid causes serious environmental and structural damage. It is known to cause deforestation and alter the pH of freshwater sources, killing living organisms within them. It is for these reasons that legislation such the Clean Air Act (1993) outline limits for the emission of NOx into the atmosphere.
Most concern for NOx removal lies in the accumulation and the lack of dispersion in modern transport tunnels in cities, especially where the extraction of air is an issue. This causes concentration of NOx levels to be greater than that outlined and recommended in various legal policies such as the Clean Air Act (UK), European Air Quality (EU) etc. Here, the lack of movement of air, means that dispersion is not achieved and NOx is not effectively diluted in large quantities of air. Technologies have to be employed to ensure that guidelines and emission limits are adhered to. Either, facilitating the uptake of NOx from air or converting NOx into a less hazardous form, such as a nitrate salt, would act to minimise the risk of concentrated NOx in this scenario.
There are methods to controlling the amount of NOx that is emitted. This can be achieved in one of two ways. Either the choice of fuel or the combustion conditions can be changed to minimise NOx production, or the NOx can be filtered out or neutralised in situ, using an effective adsorptive medium or reagent, respectively.
NOx from combustion, e.g. flue gas, can be removed using selective catalytic reduction (SCR). Either ammonia (NH3) or urea (CO(NH2)2) reagents combined with a catalyst, crack NOx to give a combination of nitrogen (N2) and water (H2O). CO2 is also produced when urea is used as the reductant:
4NO + 4NH3 + O2 → 4N2 + 6H2O
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
NO + NO2 + 2NH3 → 2N2 + 3H2O
4NO + 2CO(NH2)2 + O2 → 4N2 + 4H2O + 2CO22
Although SCR is highly effective at high temperature, it is impractical to heat large air volumes to remove NOx. Consequently at ambient temperatures, other technologies have been used. As most NOx emissions are associated with vehicle exhaust fumes, roadside air pollution reduction technology is used to combat the removal of NOx. Current technologies have very little effect on the reduction of NOx at the roadside. These include photo-catalytic titanium dioxide (TiO2) painted walls and roadside plantation to utilise micro-organisms in the soil to fix NOx to nitrates (NO3). Activated carbon (AC) fibre units can be employed by the roadside that adsorb NOx and oxidise these compounds to NO3-. AC fibres have a uniformly distributed pore structure on its surface that facilitates this uptake.
The mechanism consists of 2 steps;
- Adsorption of NOx
- Oxidation of NOx into NO3– ions; 2NO + O2 → 2NO2; NO2 + O3 → NO3– + O2
This is part of a regeneration cycle, where steam is passed through the AC fibre units to remove HNO3 on the surface.
From the equations above, it is clear that NO must be oxidised to NO2 before adsorption onto the AC surface. K2CO3, potassium carbonate, impregnated carbon can be used to remove NOx from a gas stream. The dissolved NO2 in water vapour gives HNO3. The presence of K2CO3 neutralises any acid present as demonstrated in the following reactions:
3NO2 + H2O → 2HNO3 + NO
2HNO3 + K2CO3 → 2KNO3 + CO2 + H2O
The production of NO in these reactions can be oxidised in the presence of oxygen using the mechanisms as outlined above. Jacobi Carbons offers the following grade, AddSorb™ VA10 that is specifically designed for the treatment of acid gases. This is a coal based activated carbon that has an elevated impregnation loading of K 2CO3. This is a coal based impregnation, so will have a higher pore volume making it suitable to hold the increased quantity of impregnation compared with other base materials. It is not impregnated with a caustic material, so dissolution of the impregnant is not an issue at elevated humidity levels and therefore capacity for acid gases, or in this case NOx is not diminished. It stands to reason that humidity will facilitate uptake of NO2 in particular, because of the mechanisms already mentioned above, i.e. NO2 dissolution in water gives HNO3, which is then neutralised by the basic impregnation on the carbon in a simple acid-base neutralisation reaction. This gives a salt that is retained on the carbon surface.
|CTC (base)/ %||Ash (base)/ %||Moisture/ % Apparent Density/ kg m3||Ball Pan Hardness/ %|
Table 1: Typical properties of VA10