Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Patent
1998-06-04
2000-03-07
Griffin, Steven P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
423240R, 423240S, 42324308, 42324407, 4232155, 423DIG18, B01D 5334
Patent
active
060336390
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for cleaning process gases, such as flue gases formed during combustion in a boiler, especially during combustion of waste, in which method a sorbent, such as an absorbent and/or an adsorbent, is supplied to the process gases for reaction with gaseous pollutants that are present therein, such as acidifying components and/or heavy metals and/or organic compounds, whereby particulate pollutants are formed, which are separated in a subsequent dust separator.
BACKGROUND OF THE INVENTION
For sorption of gaseous pollutants, two different methods are usually applied, the so-called dry method and the so-called wet-dry method.
As a rule, the dry method is applied in the combustion of waste and is distinguished in that a sorbent in the form of a dry, finely divided powder is supplied to the flue gases in a contact reactor. The sorbent can be, for instance, an absorbent, such as lime, which thus absorbs the acidifying components that are present in the flue gases, whereby particulate pollutants are formed, which together with the remaining so-called residual products are separated in a subsequent dust separator, such as a bag filter. To obtain an operating temperature that is suitable for the absorption, the flue gases are usually cooled by injection of water which is evaporated therein in a cooling tower arranged before the contact reactor. The wet-dry method is usually applied in the combustion of waste or coal and is distinguished in that a sorbent solution or sorbent suspension is injected into the flue gases in a contact reactor. Also in this case, the separation of the particulate pollutants formed takes place in e.g. a bag filter. An operating temperature that is suitable for the sorption is obtained by the water in the solution or suspension being evaporated in the flue gases. Both methods result in a completely dry residual product.
It is known that the sorption capacity and, thus, the degree of separation depends to a great extent on the temperature and moisture content at which the sorption takes place. The ratio of the temperature of the flue gases to the moisture content thereof can be expressed in their relative humidity or so-called condition.
It may be mentioned that relative humidity is, by definition, the ratio of the current partial pressure of the water vapour to the corresponding saturation pressure at the same temperature. The moisture content of the flue gases derives on the one hand from the inlet moisture content after combustion and, on the other hand, from the amount of water supplied for cooling the flue gases. With an increased moisture content, the sorption capacity increases, i.e. the tendency of the sorbent to react with the gaseous pollutants of the flue gases, which, for a satisfactory sorbent utilisation, makes it necessary to decrease the temperature as much as possible. There is, however, an upper limit for the moisture content in respect of the separated residual products. At too high a moisture content, the residual products will be moist and, thus, sticky, which results in clogging of the filter bags of the bag filter. The residual products will also be practically impossible to handle. The hygroscopic calcium chloride formed by the reaction between lime as absorbent and the hydrogen chloride of the flue gases is very important to the moisture content of the separated residual products. On the other hand, too low a moisture content reduces the sorption capacity, which makes the sorbent consumption increase for a given degree of separation. Thus, it is difficult to control the condition of the flue gases to a level which is optimum for the process concerned.
The temperature and moisture content of the flue gases as well as the amount of acidifying components often fluctuate widely, among other things owing to great variations in the composition of the fuel that is being burnt, especially in the combustion of waste. These fluctuations may also be very rapid and thus result in emission peaks, i.e. temporarily hig
REFERENCES:
patent: 4208381 (1980-06-01), Isahaya et al.
patent: 4208383 (1980-06-01), Kisters et al.
patent: 4229411 (1980-10-01), Kisters et al.
patent: 5084256 (1992-01-01), McElroy et al.
ABB Flakt AB
Griffin Steven P.
Vanoy Timothy C
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