Self-cleaning processes

Many appliances for gas sensors in households are still under development. For example, several attempts have been made in the past to include gas sensors in kitchen ovens to control cooking and frying processes as well as to control the self-cleaning process (pyrolysis). 

The crucial meanings of photosensitization for environmental self-cleaning processes and conversion of solar energy into a useful one are discussed in Chapters 9 (section 9.4) and 21 (section 21.4) [113-124]. 

Ciesla P, Mytych P, Kocot P, Stasicka Z. Role of iron and chromium complexes in environmental self-cleaning processes. Separation Sci Technol 2007 42 1651-66. 

The role of photochemistry in nature is difficult to overestimate especially when we consider conversion of solar energy into the chemical energy in natural photosynthesis or ozone production, which protects the Earth from hazardous UV radiation. Much less is known about the role of sunlight in environmental self-cleaning processes. 

The most common sacrificial electron acceptor in the environment is molecular oxygen, whereas the main sacrificial donors are organic compounds. In consequence, the self-cleaning processes consist in oxidation of organic pollutants by molecular oxygen in its triplet ground state the reactions are driven by energy from solar radiation. In nature, many different photoinitiators or photosensitizers are reactive, but the most common environmental photosensitizers include hiunic substances (HS), whereas the best photoinitiators are transition metal complexes. 

A very significant peculiarity of the atmosphere is that so-called self-cleaning processes take place within the air. Due to these processes even those trace constituents which have no other chemical or biological sinks are relatively quickly removed from the air. Hence these removal mechanisms are of great importance for the control of the atmospheric pathways of some trace gases and aerosol particles (see Chapter 5). 

Some oil products are biologically quite easily metabolized, such as alkanes, alkenes, cycloalkanes and aromates. Products of this activity are differently oxidized oxygen derivatives (peroxides, alcohols, phenols, aldehydes, acids, etc.). In this way microorganisms enable self-cleaning processes in soil and rivers. The rate of these degradation reactions depends on sufficient access of oxygen from the air. 

Oil spills are especially dangerous for biota during the warm season, with the maximum of solar radiation. In this case oil becomes more toxic, which is extremely harmful in the Arctic environment, where self-cleaning processes are slow [Arctic, p. 39]. 

Air cleaning systems are often used to remove dust or vapors from plant or process exhaust streams. Dust collecting systems such as filters or electrostatic precipitators that handle heavy loads of dust are usually designed to be self-cleaning, but it is stiU. necessary to enter the air cleaner periodically for inspection or repair. Dust deposits inside the equipment are likely to be stirred up and inhaled by unprotected workers. Baghouses are particularly likely to cause exposure because large amounts of dust may be retained in the cloth and released when the bags are handled. 

The oxidative degradation of organic pollutants in water and air streams is considered as one of the so-called advanced oxidation processes. Photocatalytic decomposition of organics found widespread industrial interest for air purification (e.g., decomposition of aldehydes, removal of NO , ), deodorization, sterilization, and disinfection. Domestic applications based on Ti02 photocatalysts such as window self-cleaning, bathroom paints that work under illumination with room light, or filters for air conditioners operating under UV lamp illumination have already been commercialized. Literature-based information on the multidisciplinary field of photocatalytic anti-pollutant systems can be found in a number of publications, such as Bahnemann s [237, 238] (and references therein). 

The control of the self-cleaning procedure (pyrolysis) of especially equipped kitchen ovens is another focus of development. The underlying idea is to burn the organic residues at elevated temperatures (around 400 °C) and to detect the emerging volatile compounds. In order to minimize energy consumption, the process time should be kept as short as possible. During this process considerable amounts of CO and C02 are released. A decrease in concentration of these compounds can thus be taken is an indicator of the end of process. The most direct method would be the detection of C02 in the flue gas. The most common C02-... 

On the other hand, economically advantageous routes by continuous baking process have also been described in recent years. The processes are carried out in self-cleaning mixing apparatus such as double screw extruders. More recent patent literature claims yields as high as 80%. 

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