Exhaust gas cleaning
As emission removal is of air pollutants from exhaust gases , respectively. For removing solid components in the exhaust gas are the methods of the dedusting applied. For gaseous and liquid or drop-shaped substances, depending on the chemical and physical properties of the substances, the methods described below can essentially be used.
Exhaust gas cleaning and exhaust air cleaning
Exhaust gas or exhaust air cleaning means the removal of harmful components from the gas phase . The terms exhaust gas and exhaust air are used inconsistently. While in the TA Luft, for example, exhaust air is mentioned when it comes to the subject of animal husbandry, the standardization often refers to gas composition and origin: gases contaminated with pollutants that come from rooms where people stay permanently are referred to as exhaust air. The concentration of nitrogen and oxygen must be similar to that of the air. Otherwise it is an exhaust gas.
Exhaust gas cleaning process
As absorbent , water is preferred for several reasons. It is usually inexpensive and widely available in many areas of application. If the absorption capacity of water is insufficient, the absorption must be supplemented by chemical conversion with additives ( chemisorption ). In particular, water-soluble impurities are very well removed from the gas . The dissolved substances dissociate completely or partially into ions . The scrubbing liquid can be contaminated by the absorption of substances from the gas. Often the substances absorbed are acidic or basic chemicals, such as B. hydrogen chloride , nitrogen oxides or ammonia . The absorbent can be kept at a desired pH value through a regulated supply of chemicals . Other absorbents are oils (oil washing) for organic substances.
Absorbers can be constructed as spray washers , vortex washers , jet scrubbers or venturi scrubbers and achieve high degrees of absorption. Disadvantages of the absorption process are the resulting waste water or landfill problems . In addition, aerosols can arise during absorption , which must be separated by means of an aerosol separator so that they are not carried over to the subsequent cleaning stages or even discharged from the exhaust gas cleaning system.
Flue gas desulphurisation has a special position . The processes used under this name are used to remove sulfur dioxide from exhaust gases. This can create FGD gypsum , which can be used in the construction industry.
Adsorption processes are used both for cleaning industrial exhaust gases and in building services, for example in extractor hoods. Air pollutants, in particular hydrocarbons, are removed from the exhaust gas to be cleaned by attaching them to the inner surface of porous adsorbents . A distinction can be made between fixed bed, moving bed, rotor, fluidized bed and entrained flow adsorbers. In multi-stage exhaust gas cleaning systems, adsorbers are used as the final cleaning stage, as so-called police filters.
Large variations in concentration of air pollutants can be problematic if, due to low input concentration in the already loaded adsorbent a desorption occurs and air pollutants in an undesirable concentration of the adsorber leave ( breakthrough ). If such cases are to be expected, the size of the adsorber should be larger or the adsorbent should be replaced more frequently. Used adsorbents must be regenerated or disposed of. Adsorbents can be impregnated for certain separation tasks . The impregnating agent either serves as a reactant ( chemisorption ) or as a catalyst for heterogeneous catalysis .
The catalytic emission control is in any modern car, with both a gasoline engine or diesel engine and also in large industrial facilities are used. The advantage is the comparatively low energy consumption , which is necessary for the chemical reaction of the exhaust gas cleaning. The z. T. higher investment and maintenance costs of the catalysts (compared to other processes) and the sensitivity of the catalysts to impurities and so-called catalyst poisons . Therefore a pre-separation of particles may be necessary.
The three-way catalytic converter in passenger cars with gasoline engines is an example of catalytic exhaust gas cleaning . It consists of a ceramic body, which is filled with precious metals such as platinum , rhodium and the like. a. is coated. On the surface, the chemical reactions of the exhaust gas cleaning process are greatly accelerated and usually take place without any energy supply. The carbon monoxide (CO) contained in the exhaust gas is oxidized to carbon dioxide (CO 2 ), the nitrogen oxides (NO x ) are reduced to nitrogen (N 2 ), but only when the catalytic converter has reached a certain temperature.
Another example is the use of activated carbon in large-scale exhaust gas cleaning systems, for example for the separation of dioxins and furans , heavy metals , dusts and halogens after waste incineration plants or for desulphurisation and denitrification after sintering belts. Depending on the size of the plant and the area of application, the loaded activated carbon can be regenerated or burned in coal-fired power plants. One method is the CSCR procedure.
The exhaust gas cleaning by condensation is used in particular for the separation and recovery of solvents. The range of applications ranges from laboratory scale with a cold trap to large industrial plants.
Depending on the operating temperature, a distinction is made between different types of condensation. The operating temperatures range from solvent condensation with temperatures around 25 ° C to cryogenic condensation with temperatures as low as −120 ° C. The cooling can take place directly or indirectly, with indirect cooling via heat exchangers being preferred, since there is no subsequent phase separation.
Non-catalytic chemical processes
The non-catalytic chemical processes include those processes in which harmful exhaust gas components through chemical reactions with specially added chemicals lead to the pollutants being converted into a less harmful form. A process that is often used in industry is the so-called SNCR process ( selective non-catalytic reduction ). With this form of denitrification of exhaust gases, all nitrogen oxides (NO x ) are reduced to elemental nitrogen (N 2 ) by ammonia (NH 3 ) . The ammonia is injected directly into the exhaust pipe at a temperature of 900 to 1000 ° C. The amount of ammonia used must, however, be matched precisely to the amount of nitrogen oxides, as otherwise ammonia residues can be found in the exhaust gas that would also have to be removed.
Dust reduction process
A wide variety of methods are available for removing particles ( dedusting ) from an exhaust gas, which are used depending on the exhaust gas composition and the cleaning task. Mass force separators , such as gravity or centrifugal separators , are characterized by low investment and operating costs and great reliability. Since the forces effective for the separation are proportional to the particle mass, inertial force separators are preferably used for coarse dedusting. As a further development of Inertia can scrubbers be understood by the resulting particulate mass is significantly increased by the addition of water droplets. Gas scrubbers are able to remove dusty and gaseous pollutants from an exhaust gas in one process step. Another advantage is that they can be used in an explosive atmosphere. The formation of sludge and aerosol formation from the washing liquid can be disadvantageous.
Filtering separators are divided into surface filters and depth filters . However, other distinguishing features, such as the type of filter medium and packaging features, can also be used. In the case of surface filters, also known as cleaning filters, a filter cake is created during the filtration process , which has a significant share in the cleaning performance of the filter and which has to be cleaned at regular intervals. In contrast to surface filters, depth filters (storage filters) without the desired cake formation are used to separate out particulate contaminants from the air intake, exhaust and recirculation of ventilation and air conditioning systems as well as for cleaning process air.
In electrostatic precipitators, gas ions are generated which cause the particles to be separated to be electrically charged. The charged particles are deposited on what is known as a precipitation electrode. The collecting electrode must be cleaned at regular intervals and the dust removed. Because they are independent of particle mass and diameter, they do not have a characteristic separation minimum.
In general, higher degrees of separation can be achieved with surface filters and electrostatic precipitators . Lower degrees of separation are achieved with gas scrubbers and inertial force separators.
Non-thermal plasma (NTP) is used to eliminate odors and break down organic hydrocarbons in the exhaust air. The exhaust air cleaning with the NTP process is carried out by excitation in an electric field between the electrodes and the dielectric barrier. NTP processes are used both as a direct process and as an injection process. In the direct process, the exhaust gas passes through the electric field and the pollutant molecules are directly excited in order to increase their reactivity. In the injection process, an excited air stream that has passed through the electric field is introduced into the exhaust gas stream. Ideally, the pollutant molecules are converted into water and carbon dioxide.
It should be noted that the electrical excitation causes secondary emissions, such as B. ozone and nitrogen oxides can arise. NTP systems are often designed in combination with a further process stage (e.g. adsorption, absorption, catalyst).
The main purpose of the afterburning of exhaust gases is to reduce their hydrocarbon content . To do this, the exhaust gas is heated to such an extent that, ideally, hydrocarbons are oxidized to carbon dioxide and water. In the case of post-combustion systems, there is a
distinguished. The processes listed differ, among other things, in the process control (continuous or discontinuous operation) and the temperatures reached. As a rule, the use of additional fuels is necessary. If the concentration of hydrocarbons in the exhaust gas is large enough, additional fuels can be dispensed with.
In catalytic as well as regenerative afterburning, the need for additional fuel is lower because of the lower process temperatures and the type of process management, but both processes require more space and are more susceptible to particulate contamination. When the catalytic converter is in operation, substances that act as catalytic converter poisons can also appear in the exhaust gas.
Biological exhaust gas cleaning
In biological waste gas purification , organic waste gas components are metabolized by microorganisms and used as an energy source or to build up cellular substance. Usually it is aerobic bacteria , such as B. Pseudomonas , Streptomyces or Xanthobacter , which ideally convert the exhaust gas constituents into carbon dioxide and water. But fungi such as Aspergillus or Penicillium are also among the microorganisms that occur in biological exhaust gas cleaning.
All biological processes have in common that the exhaust gas constituent to be separated must first be dissolved in order to be microbiologically degraded afterwards . Two requirements must therefore be met in order to use this exhaust gas cleaning process:
- The exhaust gas constituent must be water-soluble (the presence of water is absolutely necessary for biological exhaust gas cleaning).
- The exhaust gas constituent must be microbiologically degradable.
The microorganisms are fixed on a surface or suspended in a solution, depending on the process . While their carbon requirements - possibly also the need for sulfur and nitrogen - are covered by the substances in the exhaust gas, other substances, such as B. Phosphorus and trace elements, the microorganisms are supplied in another way. Biological processes are used when
- the exhaust gas constituent is not to be recovered,
- no major changes in the exhaust gas composition are expected,
- the exhaust gas temperatures are in ranges that are compatible with the microorganisms, and
- no exhaust gas components that are toxic to the microorganisms are to be expected.
Process of biological exhaust gas cleaning
The following processes for biological exhaust gas cleaning are used in industry:
- The first biofilters were patented and used in the middle of the last century. An organic carrier material such as. B. Bark mulch or wood chips are filled into a filter bed with a bed height of approx. 1-3 m. The exhaust air flows through the filter bed from bottom to top, whereby the pollutants are biodegraded. The biofilters can be built at ground level or stacked on top of each other in modular construction. Due to its relatively simple design, the biofilter is a very cost-effective exhaust air purification process, but it is only suitable for applications in continuous operation with low levels of solvent or odor.
- Bio washer
- The bio-washer is an absorber with microorganisms suspended in the washing liquid. The pollutants are physically washed out of the exhaust gas and then biodegraded in the aqueous phase. The main breakdown of the pollutants takes place in the absorber sump or in an external regeneration reactor. The absorber sump is sufficient if the pollutants are present in low concentrations and are easily degradable. If, on the other hand, the absorbed air pollutants are difficult to break down, the necessary longer residence time of these substances in the aqueous phase requires a regeneration reactor. Most of the scrubber water regenerated through biological degradation can be circulated. The nutrients necessary for biological degradation must be added to the washing liquid. Bio-scrubbers can treat solvent concentrations in the exhaust gas up to about 1000 mg / m³.
- Bioriesel bed reactor
- The bio-diesel bed reactor differs from the bio-scrubber in that the microorganisms that break down the pollutants are fixed on internals. The difference to the biofilter is that the internals colonized by the microorganisms do not serve as a nutrient substrate and the water flow in the reactor serves to contain and equalize the biofilm and not to prevent it from drying out. The laden exhaust air flows through the inert carrier material in a cross, countercurrent or cocurrent flow in relation to the direction of flow of the sprinkling liquid. The pollutants it contains are absorbed by the sprinkling liquid and serve as nutrients for the microorganisms. Additionally necessary nutrients are supplied via the sprinkling liquid. The bioresel bed reactor is suitable for cleaning solvent and odor-laden exhaust air with concentrations of up to 1.5 g / m 3 .
Descriptions and standards for the aforementioned processes are in the from the Commission on Air Pollution Prevention issued pollution VDI / DIN manual Air included.
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