Furnaces – Process – Treating fuel constituent or combustion product
Patent
1997-06-26
2000-05-23
Yeung, James C.
Furnaces
Process
Treating fuel constituent or combustion product
110165A, 431 2, 4232155, 4232391, 423267, F23J 1100, F23J 300
Patent
active
060654090
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to scrubbing flue gases. It is particularly applicable to scrubbing flue gases generated by burning household refuse.
DESCRIPTION OF RELATED ART
Currently, flue gases from household refuse are scrubbed at low temperatures (less than 250.degree. C.) downstream from the recovery boiler of an incineration plant for household refuse. The flue gases are scrubbed by an electrostatic effect, or by means of a filter having socks made of fabric.
Furthermore, flue gases from household refuse are generally treated downstream from the recovery boiler by catalysis or by injecting active coal so as to reduce their furan dioxin content (furan dioxins are organic compounds based on cyclic and chlorinated molecules). Typical furan dioxin emissions in flue gases from household refuse are about 10 ng/Nm.sup.3 at the outlet of the recovery boiler. Furan dioxins are formed most commonly at medium temperatures (240.degree. C. to 450.degree. C.) in the recovery boiler from HCl, from unburnt particles contained in the fly ash of the flue gases, and because of the catalytic effect of certain metals of the copper oxide type.
Hot scrubbing of flue gases has already been considered for coal-fired pressurized-combustion heating systems. Using that technique, the flue gases are scrubbed at a high temperature on the path along which they flow by causing them to pass through a "high-temperature" filter, e.g. of the type including ceramic tubes.
By using a high-temperature filter, it is possible to lighten the structure of the system for treating the flue gases downstream from the recovery boiler, in particular since by hot scrubbing the flue gases it is also possible to reduce the furan dioxin content of said flue gases at the outlet of the recovery boiler if the fly ash is filtered out at a temperature that is high enough, i.e. in the range 450.degree. C. to 650.degree. C. Unfortunately attempts to use such a scrubbing method in waste applications have come up against the problem of removing the filter cake so as to unclog the filter. The fine particles of fly ash have a melting point that is relatively low, which means that they form viscous bridges between them and clog the pores in the ceramic tubes of the filter. The filter cake that forms on the surfaces of the ceramic tubes then becomes extremely difficult to remove and requires frequent stoppages in order to clean or even replace the tubes. This unclogging problem is even more acute when such a method is applied to flue gases from household refuse because the fly ash in such flue gases contains in particular sodium oxide (Na.sub.2 O) which contributes to lowering still further the melting point of the fine particles of fly ash, and therefore to increasing clogging of the high-temperature filter. It is therefore very difficult to reconcile hot scrubbing of flue gases from household refuse with achieving a significant reduction in their furan dioxin content thereby.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method of scrubbing flue gases by causing them to flow through a high-temperature filter placed inside a recovery boiler, which method makes it possible to remove the filter cake.
Another object of the invention is to provide a method of hot scrubbing flue gases from household refuse, which method is capable of reducing the furan dioxin content of the flue gases to a very low value, e.g. to 0.1 ng/Nm.sup.3, so as to satisfy the conditions that will soon be required by regulations or that are already required in certain countries.
To these ends, the invention provides a method of scrubbing flue gases by causing the flue gases to pass through a high-temperature filter element placed on the path along which the flue gases flow inside a recovery boiler, so that the fly ash contained in the flue gases can be filtered out, said method being characterized in that an additional flow of a material is superposed on the flow of fly ash to be filtered, the material having characteristics such that it increases the
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Ciric Ljiljana V.
GEC Alsthom Stein Industrie
Yeung James C.
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