A METHOD AND ARRANGEMENT FOR MANAGING AND CONTROLLING NITROGEN EMISSION FROM A CYCLIC LIQUOR FLOW SYSTEM IN A PULP MILL

Abstract

A method for managing and controlling nitrogen emission from a cyclic liquor flow system in a pulp mill (1), the pulp mill (1) comprising a recovery system (20) for recovering heat and chemicals from a pulping process (30), the recovery system (20) comprising a recovery boiler (8) and a nitrogen oxide scrubber (15), the nitrogen oxide scrubber (15) being arranged to remove nitrogen oxide from flue gas (16) from the recovery boiler (8), the method comprising the steps of: —exposing flue gas (16) from the recovery oiler (8) to an oxidizing agent, thereby oxidising nitrogen oxide in the flue gas (16) to higher nitrogen oxides; —contacting the flue gas (16) with an alkaline aqueous scrubber liquid (17) in the nitrogen oxide scrubber (15), thereby absorbing the nitrogen oxides in the scrubber liquid (17) and producing a nitrogen containing scrubber liquid (17); —introducing all or a part of the nitrogen containing scrubber liquid (17) into the cyclic liquor flow system. An arrangement for managing and controlling nitrogen emission from a cyclic liquor flow system in a pulp mill (1) is also disclosed.

Claims

1. A method for managing and controlling nitrogen emission from a cyclic liquor flow system in a pulp mill, the pulp mill comprising a recovery system for recovering heat and chemicals from a pulping process, the recovery system comprising a recovery boiler and a nitrogen oxide scrubber, the nitrogen oxide scrubber being arranged to remove nitrogen oxide from flue gas from the recovery boiler, the method comprising steps of: exposing flue gas from the recovery boiler to an oxidizing agent, thereby oxidising nitrogen oxide in the flue gas to higher nitrogen oxides; contacting the flue gas with an alkaline aqueous scrubber liquid in the nitrogen oxide scrubber, thereby absorbing nitrogen oxides in the scrubber liquid and producing a nitrogen containing scrubber liquid; and introducing all or a part of the nitrogen containing scrubber liquid into the cyclic liquor flow system.

2. A method according to claim 1, wherein introduction of all or a part of the nitrogen containing scrubber liquid into the cyclic liquor flow system comprises or consists of adding nitrogen containing scrubber liquid in one or more parts of the recovery system.

3. A method according to claim 2, wherein the recovery system comprises a dissolving tank for dissolution of smelt from the recovery boiler and wherein nitrogen containing scrubber liquid is added to the dissolving tank.

4. A method according to claim 2, wherein the recovery system comprises an evaporator for concentrating black liquor from the pulping process before supplying the concentrated black liquor to the recovery boiler, and wherein nitrogen containing scrubber liquid is added to the evaporator.

5. A method according to claim 1, wherein nitrogen containing scrubber liquid is added to a digester in the pulping process.

6. A method according to claim 1, wherein the oxidising agent is an oxygen based oxidizing agent such as hydrogen peroxide or ozone.

7. A method according to claim 1, wherein the alkaline aqueous scrubber liquid comprises sodium hydroxide (NaOH).

8. A method according to claim 1, wherein the pulping process is a Kraft pulping process.

9. An arrangement for managing and controlling nitrogen emission from a cyclic liquor flow system in a pulp mill, the arrangement comprising: a recovery system for recovering heat and chemicals from a pulping process, the recovery system comprising a recovery boiler and a nitrogen oxide scrubber, the nitrogen oxide scrubber being arranged to remove nitrogen oxides from flue gas from the recovery boiler by absorption of nitrogen oxides in the flue gas in an alkaline aqueous scrubber liquid in the nitrogen oxide scrubber, wherein the arrangement for managing and controlling nitrogen emission further comprises an arrangement for introducing nitrogen containing scrubber liquid from the nitrogen oxide scrubber into the cyclic liquor flow system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The methods and arrangements as disclosed herein will be further explained hereinafter with reference to the appended drawings wherein:

[0054] FIG. 1 shows a schematic representation of a pulp mill with a recovery system.

DETAILED DESCRIPTION

[0055] Different aspects of the present disclosure will be described more fully hereinafter with reference to the drawings. The method and the arrangement disclosed herein should not be construed as being limited to the aspects set forth herein but can be varied within the scope of the appended claims.

[0056] The pulp mill 1 which is shown in FIG. 1 in a highly schematic and simplified way is a Kraft pulp mill. Hence, it is to be understood that further equipment and process steps may be included in the pulping process and the recovery system of the pulp mill 1. The liquor flows which are shown in FIG. 1 are only the main liquor flows in the cyclic liquor flow system of the pulp mill 1. Further liquor flows may be part of the cyclic liquor flow system in the pulp mill 1. The pulp mill 1 comprises a digester 2 to which cellulosic raw material 3 such as wood chips is fed. In the digester 2, the cellulosic raw material is digested to free the cellulose fibres and to remove lignin, hemicelluloses and other extractives from the wood. The pulp produced in the digester 2, is washed in a washing arrangement 4 to obtain a refined and optionally bleached paper pulp 5. The cooking liquor from the digester 2, also referred to herein as black liquor 6, is a waste product of the Kraft process and is transferred from the digester 2 to an evaporator 7 where it is concentrated and subsequently introduced into a recovery boiler 8. Black liquor may also be transferred from the washing arrangement 4 to the evaporator 7. The concentrated black liquor 6 leaving the evaporator 7 is burned in the recovery boiler 8 to form a smelt 9 which is drawn off to a dissolving tank 10 where the smelt 9 is dissolved in weak white liquor to form green liquor 11. The green liquor is passed through a causticizer 12 and is clarified and filtered and eventually returned as white liquor 13 to the digester 2. Weak white liquor 14 from the wash process in the causticizer 12 is returned to the dissolving tank 10.

[0057] The pulp mill 1 further comprises a nitrogen oxide scrubber 15. Flue gas 16 from the recovery boiler 8 is first exposed to an oxidizing agent, such as ozone or hydrogen peroxide in order to oxidize NOx in the flue gas to higher nitrogen oxides. Thereafter the oxidized flue gas 16 is contacted with an alkaline aqueous scrubber liquid, such as a scrubber liquid containing sodium hydroxide, NaOH, in a wet section of the nitrogen oxide scrubber 15. The nitrogen oxides in the oxidized flue gas 16 from the recovery boiler 8 are absorbed in the alkaline scrubber liquid and the nitrogen containing scrubber liquid 17 is drawn off and fully or partly introduced into the cyclic liquor flow system of the pulp mill 1.

[0058] As can be seen from FIG. 1 the flow paths in the pulp mill 1 from the digester 2 to the evaporator 7, further to the recovery boiler 8 and the dissolution tank 10 and through the causticizer 12 back to the digester 2 make up the main flow paths of a cyclic liquor flow system in the pulp mill 1.

[0059] The evaporator 7, the recovery boiler 8, the dissolution tank 10, the nitrogen oxide scrubber 15 and the causticizer 12 are parts of a recovery system 20 of the pulp mill 1, the recovery system being arranged for recovery of heat and chemicals from the pulping process 30 which is carried out in the pulp mill 1.

[0060] As disclosed herein, the nitrogen containing scrubber liquid 17 which has absorbed nitrogen oxides, mainly in the form of nitrate NO.sub.3.sup.−, may be introduced in virtually any part of the cyclic liquor flow system in the pulp mill 1.

[0061] It has been found to be particularly advantageous to introduce all or a part of the nitrogen containing scrubber liquid 17 to the dissolving tank 10 as indicated by the unbroken line from the nitrogen oxide scrubber 15 to the dissolving tank 10. Alternatively, or in addition to using the nitrogen containing scrubber liquid 17 in the dissolving tank 10, nitrogen containing scrubber liquid 17 may be supplied to one or more of the evaporator 7 the causticizer 12, the digester 2, etc. The nitrogen containing scrubber liquid 17 may be added directly to one or more of the above-mentioned appliances 2, 7, 10, 12 or may be added to a liquor flow into the appliance 2, 7, 10, 12. Wherever the nitrogen containing scrubber liquid 17 is added in the cyclic liquor flow system in the pulp mill 1, the nitrogen in the scrubber liquid 17 will again pass through the recovery boiler 8 and subsequently be partly removed from the system as ammonia from the dissolving tank 10 and as nitrogen gas from the nitrogen oxide scrubber 15 while approximately 25% of the nitrogen content in the black liquor 6 in the recovery boiler 8 will end up in the nitrogen containing scrubber liquid 17 from the nitrogen scrubber 15 and again be added to the cyclic liquor flow system in the pulp mill 1.

[0062] As disclosed herein, a particular advantage of adding the nitrogen containing scrubber liquid to a dissolving tank is that the scrubber liquid can reduce or replace water addition to the smelt from the recovery boiler such that unwanted dilution of the scrubber liquid is minimized.