Recovery process and system for a pulp mill

Abstract

A method for burning chlorine-containing liquors in a chemical recovery boiler at a pulp mill, wherein the recovery boiler includes spent liquor sprayers for feeding spent liquor and a number of combustion air levels including: increasing a combustion temperature in the recovery boiler in a burning zone where a chlorine-containing liquor or a chlorine-containing effluent is burned; while burning the liquor or effluent, volatilizing the chlorine in the liquor or effluent to produce chloride-containing salts in flue gases in the boiler, and removing the chloride-containing salts from the flue gases.

Description

RELATED APPLICATION[0001]This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60 / 913,322, filed Apr. 23, 2007, the entirety of which is incorporated by reference.BACKGROUND OF INVENTION[0002]This invention relates to recovery processes for processing natural cellulosic or other fibrous material and, particularly, to the removal of chlorine from such processes.[0003]Chlorine (Cl) is present in wood, in make-up chemicals and pulp bleaching filtrates, especially when using chlorine-containing bleaching chemicals. Chlorides entering with the wood and input chemicals tend to build up in pulping liquors. This may be a particular problem for coastal mills where logs are transported in sea water and become saturated in chloride. Chemicals used in the process may also contain considerable amounts of chlorine. The concentration of chlorine in black liquor varies greatly from one process to another. The chlorine content in black liquor can be as low as 0.1 to 0.8...

AI Technical Summary

Benefits of technology

[0016]A process has been developed and is disclosed herein for treating spent liquors and filtrates or effluents from bleaching using chlorine dioxide at a pulp mill and for removing chlorine (Cl) from the process. This allows high water reuse and effective production of power and heat from spent liquor, such as black liquor, and other energy-containing streams available at the mill, or brought to the mill. The process disclosed herein can also be used for balancing and stabilizing the Cl concentrations in the material circulations of the mill, specially the concentration level in the spent liquor, when chlorine enters the mill in raw materials and chemicals streams. The process disclosed herein preferably relates to sulfate or Kraft pulp mills.

[0017]A process has been developed in which the burning of chlorine-containing liquor and effluents can be controlled in such a way that the operation of the recovery boiler itself is efficient, whereby a high-temperature steam can be produced for power and heat production. The developed process may allow for chlorine to be separated efficiently and for the chlorine level of the pulp mill can be balanced without adversely affecting the pulp quality and the operation of the recovery boiler. Especially corrosion problems in the machinery can be avoided or minimized.

[0018]A method is disclosed herein for burning chlorine-containing liquors in a chemical recovery boiler at a pulp mill, wherein the recovery boiler comprises spent liquor sprayers for feeding spent liquor and a number of combustion air levels. A feature of the disclosed method is that the combustion temperature in the recovery boiler is increased in a zone, where a chlorine-containing liquor or effluent is burned, for improving the volatilization of chlorine from the liquor or effluent into flue gases to produce chloride-containing salts, and that the flue gases are treated to remove the chloride-containing salts. The chlorine-containing stream to be burned is typically a spent liquor, such as black liquor, from pulp production or a chlorine-containing effluent from a bleaching plant of the pulp mill. Also other chlorine-containing streams from the pulp mill can be treated according to the process disclosed herein.

Problems solved by technology

This may be a particular problem for coastal mills where logs are transported in sea water and become saturated in chloride.

Chemicals used in the process may also contain considerable amounts of chlorine.

However, even when decreasing the amount of effluent from the level of 15 m3 to the level of 10 m3, an increase in chemical consumption becomes visible, which thus leads to an ever-increasing amount of organic chlorine compounds from bleaching.

In the recovery boiler, chlorine and potassium are enriched into the fly ash and increase the corrosiveness of the flue gas especially in the superheater.

The above described conventional methods do not address burning chlorine-containing effluents in a recovery boiler such that chlorine could be removed from the chemical recovery loop efficiently.

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