Gasification of sulphite thick liquor

Abstract

Method for recovering chemicals and energy from sulphite thick liquor containing organic and inorganic compounds obtained when producing pulp by chemical delignification of fibrous raw material using a sulphite pulping process, the method including processing of the organic and inorganic compounds at a global temperature above 800° C. whereby producing partly at least one phase of a liquid material and partly at least one phase of a gaseous material. The processing is carried out by gasification of the sulphite thick liquor in a gasification reactor at sub-stoichiometric conditions and in the presence of an oxidizing medium. The reactor has an opening in its bottom in the form of a chute which opens directly into a quench compartment.

Description

[0001]This application is a Continuation-In-Part of international application No. PCT / SE2011 / 050350, filed 29 Mar. 2011. This application also claims foreign priority to Swedish patent application No. SE 1050299-5, filed 30 Mar. 2010. The complete contents of these applications is incorporated herein by reference.FIELD OF THE INVENTION[0002]The present invention relates to a method for recovering chemicals and energy from sulphite thick liquor, said sulphite thick liquor being obtained when producing paper pulp by chemical delignification of fibrous raw material using a sulphite pulping process, said sulphite thick liquor comprising organic and inorganic compounds; the method comprising processing of said organic and inorganic compounds at a temperature above 800° C. thereby producing partly at least one phase of a liquid material and partly at least one phase of a gaseous material.BACKGROUND INFORMATION[0003]The sulphite pulping process is a chemical pulping process of wood chips w...

AI Technical Summary

Benefits of technology

[0025]Thanks to the invention a more efficient chemical recovery is obtained. Cold gas efficiency obtainable in a commercial scale gasifier is estimated to be 65-75%, leading to high yields of motor fuels produced from the synthesis gas, if this usage of the synthesis gas is selected. Explanations to the high energy efficiency of the process may include the burner design of the gasification reactor that allows operation with high carbon conversion at a relatively low global reactor temperature and also allows gasification without any additional atomizing medium.

[0026]Green liquor sulphidity is significantly lower than for a recovery boiler due to the fact that most of the sulphur may be contained in the raw synthesis gas as hydrogen sulphide. This split of the thick liquor sulphur content is caused by chemical reactions between components of the gas and smelt phases in the reactor, which determine the proportions of liquid sodium sulphide and gaseous hydrogen sulphide. The sulphur in the gas may be returned to cooking liquor preparation in a concentrated gas stream from an acid gas removal unit treating gas from the gasifier, which permits a less complex cooking liquor preparation process. The load on the part of the cooking liquor preparation that converts green liquor sulphide to sulphur dioxide and sulphurous acid is decreased due to the lower sulphur content in the green liquor.

[0027]According to one aspect of the invention, the amount of unburnt charcoal in said green liquor is lower than 5%, preferably lower than 1% and more preferred lower than 0.2%, of the carbon in the sulfite thick liquor, i.e. carbon conversion may be very high, resulting in a good quality green liquor. The high carbon conversion is obtained because of the high flame temperature in the reactor caused by rapid chemical reactions of oxygen or oxygen-containing gas with combustible components and an advantageous recirculating flow pattern in the reactor chamber forced by the limited size of the reactor chamber bottom outlet.

[0028]According to another aspect of the invention, the sulphur found in the green liquor may to an extent of at least 90%, preferably at least 95% and more preferred at least 98%, be in reduced form, i.e. as sulphide. This means that the green liquor produced may have close to 100% sulphur reduction efficiency. High sulphur reduction efficiency increases the efficiency of the pulping process, since it decreases the total amount of sulphur that needs to be circulated by decreasing the so-called dead-load (i.e. inactive sulphur species such as sulphate). The high sulphur reduction efficiency is obtained because of an advantageous recirculating flow pattern in the reactor chamber forced by the limited size of the reactor chamber bottom outlet.

[0029]All these advantages taken together leading to a more efficient and cost effective chemical recovery process with regard to cooking chemicals as well as energy. Said chemical recovery process may no longer be a drawback for sulphite pulping processes compared to Kraft (sulphate) pulping processes.

Problems solved by technology

Certain disadvantages compared to Kraft pulping also exist, such as weaker pulp and difficulties in pulping certain species of wood and more complicated chemical recovery.

The chemical recovery process of the pulping chemicals is dependent on the alkali counter-ion used but is generally more complex than the Kraft process recovery of pulping chemicals.

The recovery boilers used for recovering chemicals and energy from sodium-based sulphite thick liquors are very similar to those used for recovery of black liquor from the Kraft pulping, however burning sulphite thick liquor in such boilers is associated with a number of difficulties as compared to burning Kraft black liquor, which is further discussed below.

The flue gases produced when burning the sulphite thick liquor are more corrosive, which limits the efficiency of liquor energy recovery and causes elevated maintenance costs.

The losses of pulping chemicals, both sodium and sulphur, mostly as fly ash, are significantly higher when burning sodium-based sulphite thick liquor as compared to burning Kraft liquor, which can lead to increased chemical make-up costs in the mill.

When burning sodium-based sulphite thick liquor, the reduction efficiency of sulphur species in the sulphite liquor is relatively low.

The non-reduced sulphur gives disadvantages in the form of dead load in the liquor cycle and a tendency to cause fouling in the process equipment of the liquor cycle.

Tiosulphate decreases pulping efficiency if present in the cooking liquor.

Hence, a large amount of non-active sulphur is present in the liquor cycle, causing a lower efficiency and potential problems with scaling.

In addition, thiosulphate is known to cause corrosion problems in process equipment.

Sulphite thick liquors are known to have a lower reactivity in recovery boilers compared to spent Kraft cooking liquors, which leads to lower capacity when recovery boilers are operated on sulphite thick liquor.

Hence the complex and relatively inefficient chemical and energy recovery from spent sulphite liquors is one reason why the Kraft process has become the dominating pulping process.

Furthermore, persons skilled in the art may have a prejudice against recovery by gasification of sulphite liquors based on earlier experiences.

Further, problems with build-up of smelt layers on the reactor walls were present and the wear on the reactor lining was very severe with the ceramic materials available at that time.

Also the “SCA-Billerud process” was subsequently abandoned due to poor performance.

Pyrolysis at as low temperatures as below 800° C. will however lead to unconverted char in the solid residue from the gasification process and necessitates a second gasification step that is performed in a fluidized bed.

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