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Technique and treatment of many pollutants

2Nitrogen oxides (NOx)2 NOx include nitrogen monoxide (NO) and nitrogen dioxide (NO2). There are other degrees of oxidation but only NO and NO2 play a significant part in air pollution (over 95% of the emissions) and are therefore regulated.
N2O is not part of NOx but can be regulated as a greenhouse gas. The formation of NOx can be limited at source by limiting the amount of nitrogen contained in reagents (fuels or products used in a process).
NOx emissions can be limited in combustion phenomena by using primary techniques such as low NOx burners or other techniques to adjust the flame temperature (water cooling, air or flue gas recirculation) or the oxygen concentration of the oxidant/fuel mixture (air staging or specific fuel injection).

A wastewater treatment system can be installed (secondary techniques). Nitric acid can be formed by flue gas scrubbing. Ammonia-based treatments can also be envisaged (selective reduction, catalytic or otherwise).
NOx treatment units are often combined with SOx treatment units.

Estimated efficiency

TechniqueLow NOx burnerLow NOx + stagingNat. Gas reburningNat. Gas reburning+ low NOx burnerSNCRNat. Gas reburning. + SNCRSCR
Efficiency 30-55% 35-70 % > 60% 60-70 % 30-50% 90.0% 90,0%

Note: An in-depth study is required to obtain an accurate efficiency value according to the installation. The choice of low NOx burners is most often the most economical solution (depending on the number of burners that need changing) and selective reduction solutions the most costly ones (in particular catalytic solutions).

2Sulphur oxides (Sox)2 SOx include sulphur dioxide (SO2) and sulphur trioxide (SO3). SO3 has a very short lifespan in the atmosphere as it rapidly turns into H2SO4 or other sulphate. This category of pollutants is largely responsible for the acid rain phenomenon.
Fuels with low sulphur content can be used upstream of the process (e.g.: use of fuel with very low sulphur content or VLSC, choice of low-sulphur coal).
Downstream, flue gas desulphurisation processes are scrubbing processes (using water or seawater) forming sulphurous or sulphuric acid, or neutralisation processes using lime or sodium bicarbonate (forming CaSO4, the anhydrous form of gypsum).
These are dry (flue gases/powders), semi-dry (flue gases/liquid aerosol) or wet processes (gas bubbling in a reagent solution). The exchange process is selected according to the pollutant stream to be treated.
SOx treatment units are often combined with NOx treatment units. 2Ozone (O3)2 Ozone is a colourless and odorous gas, made up of three atoms of oxygen. This highly oxidative gas rapidly decomposes when combined with hydrogen, moisture or in contact with solid surfaces. On the ground, its natural level ranges from 0.005 and 0.05 ppm; this level increases with altitude and pollution. For O3, at 25°C and 101 kPa, 1ppm = 2mg/m3. Ground-level ozone (located from the ground up to an altitude of 10 km), considered as a pollutant resulting from primary man-made pollution, should be distinguished from stratospheric ozone (making up the ozone layer at an altitude between 10 and 60 km) considered beneficial and damaged by certain gases emitted by man (freons, halons).
Ozone pollution prevention requires sustainable measures to reduce ozone precursors (NOx and VOC).
Ground-level pollution is very difficult to treat with short-term measures, as ozone is not a primary pollutant.
The measures available are obviously limited, such as the reduction in VOC and NOx emissions (reduction in industrial emissions by limiting production… and, in extreme situations, alternating traffic). At the same time, information is provided to the population to limit risk behaviour (efforts leading to over-ventilation) in sensitive populations.
In confined working atmospheres, ventilation systems should be implemented to disperse the ozone.
In order to limit the damage to the ozone layer, the production of CFCs (chlorofluorocarbons), halons and other substances (listed in appendix to regulation no. 2037/2000/CE) is prohibited, and their use is strictly regulated with a view to future prohibition. 2Volatile organic compounds (VOC)2 Organic compounds comprise all compounds containing at least carbon and one or several of the following elements: hydrogen, halogens, oxygen, sulphur, phosphorus, silicon or nitrogen, with the exception of carbon oxides and inorganic carbonates and bicarbonates.
Volatile organic compounds (VOC) comprise all organic compounds with steam pressure of at least 0.01 kPa at a temperature of 293.15 K or equivalent volatility in specific usage conditions (for example, the fraction of creosote exceeding this vapour pressure value at a temperature of 293.15 K is considered a volatile organic compound).
Methane is distinguished from VOCs called NMVOCs (Non Methane VOCs).
Before envisaging the treatment of VOC emissions, one must assess whether these emissions can be reduced at source by using new products (which should be envisaged systematically for VOCs with risk phrases) or processes. Reduction at source can work on several levels:

  • optimisation of application processes (e.g.: improvement in transfer rate),
  • reduction in these products’ solvent content (e.g.: water-based products),
  • elimination of solvents in products (e.g.: powder products)
    If reduction at source is impossible, VOCs should be captured as far upstream as possible (efficient capture near emission points) and then re-condensed to be reused or destroyed (oxidiser). 2Particles2 Particles are solid elements with very low diameter and weight. Settleable particles, with significant fall velocity and size (>50 mm/s and >45µm), are distinguished from fine suspended particles (<10µm) capable of covering great distances (especially true for those <2µm). They are known as PMxx, “Particulate Matter”, the number xx representing the upper limit of the particle size fraction in µm (traditionally, only total particles, PM10 to PM2.5 and, to a lesser extent, PM1, are studied). Particles dispersed into a gas medium are generally called aerosols. The first type of protection required is the implementation of systems preventing diffuse emissions: flying dust particles associated with movements and wind (grass planting, surface asphalting, hedge planting). Storage of powdered products should be confined or, in case of proven technical impossibility, should provide a flying particle limitation system (sprinkling for example).
    For channelled emissions, several techniques are available, implementing capture and separation processes: mechanical separation using gravimetric force (for example settling by passing through a settling chamber) or centrifugal force (example: centrifugation by passing through a cyclone); wet separation (example: wetting by passing through a wash bottle scrubber, a spray scrubber or venturi scrubber, example illustrated in top drawing); separation by filter (example: passage through porous bag filters, which can be disposed of, regenerated or unclogged, example illustrated in bottom drawing); electrostatic separation (by passing through an intense electrostatic field and captured on a surface).
    Bag filters and electrostatic filters are the most frequently used: bag filters are very efficient for the entire spectrum of particle sizes but require the regular replacement of filter consumables (operating cost on top of significant installation costs); electrofilters are efficient but their operating cost is high (electricity). Finally, the cyclone is economically interesting because of its very low cost but is only efficient for large particles.
    It should be noted that coal ash can be recovered after micronisation to manufacture cement. 2Metals and metal compounds2 Metals are defined as chemical elements capable of easily releasing an electron from their electron cloud (electropositive elements). In the air, metals are mostly found in particulate form (i.e. combined with dust particles) and sometimes in gaseous form (e.g.: mercury). The following emissions are monitored: cadmium (Cd), mercury (Hg), thallium (Tl), arsenic (As), selenium (Se), tellurium (Te), lead (Pb), antimony (Sb), chromium (Cr), cobalt (Co), copper (Cu), tin (Sn), manganese (Mn), nickel (Ni), vanadium (V) and zinc (Zn).
    For gas phases (in particular mercury), they can be captured by scrubbing or sparging (particle filter systems also make it possible to capture part of this gaseous pollution by adsorption or surface condensation of the particles).
    For metals mostly found in particle form, see chapter on “Particles”.