Method and a system for quality optimization of green liquor

1. A method for optimizing a reduction and a content of total titratable alkali of a green liquor of a recovery boiler, the method comprising:

producing the green liquor by dissolving a smelt from the recovery boiler in a dissolving tank by conveying a weak white liquor into the dissolving tank,

measuring at least

a content of sodium sulphate (Na 2 SO 4 ) of the green liquor,

a content of sodium hydroxide (NaOH) of the green liquor,

a content of sodium sulphide (Na 2 S) of the green liquor, and

a content of sodium carbonate (Na 2 CO 3 ) of the green liquor,

controlling at least one of a temperature and an oxygen content within the recovery boiler by using the measured content of sodium sulphate (Na 2 SO 4 ) and the measured content of sodium sulphide (Na 2 S) to maximize the reduction in the recovery boiler, and

at a first instance of time, controlling the flow of the weak white liquor into the dissolving tank by using the measured content of sodium sulphate (Na 2 SO 4 ), the measured content of sodium hydroxide (NaOH), the measured content of sodium sulphide (Na 2 S), and the measured content of sodium carbonate (Na 2 CO 3 ) to optimize the content of total titratable alkali of the green liquor.

2. The method of claim 1 , further comprising:

determining a melting temperature for a char bed of the recovery boiler,

measuring a temperature of the char bed, and

controlling the at least one of the temperature and the oxygen content within the recovery boiler in such a way that a flow of the smelt is continuous.

3. The method of claim 2 , further comprising:

measuring at least the content of sodium sulphate (Na 2 SO 4 ) of the weak white liquor, the content of sodium hydroxide (NaOH) of the weak white liquor, the content of sodium sulphide (Na 2 S) of the weak white liquor, and the content of sodium carbonate (Na 2 CO 3 ) of the weak white liquor,

determining the flow rate of a black liquor into a furnace of the recovery boiler,

determining the flow rate of the green liquor and the flow rate of the weak white liquor, and

measuring contents of chlorine (Cl) and potassium (K) from ash produced by the recovery boiler,

wherein the determining of the melting temperature for the char bed of the recovery boiler uses at least

the measured contents of chlorine and potassium, as measured from the ash,

the measured contents of sodium sulphate (Na 2 SO 4 ), sodium hydroxide (NaOH), sodium sulphide (Na 2 S), and sodium carbonate (Na 2 CO 3 ) of the green liquor,

the measured contents of sodium sulphate (Na 2 SO 4 ), sodium hydroxide (NaOH), sodium sulphide (Na 2 S), and sodium carbonate (Na 2 CO 3 ) of the weak white liquor,

the determined flow rate of the black liquor into the furnace of the recovery boiler, and

the determined flow rate of the green liquor and the flow rate of the weak white liquor.

4. The method of claim 3 , further comprising determining, in units of mass per time: (i) the flow rate of the black liquor into the furnace, (ii) the flow rate of the green liquor, and (iii) the flow rate of the weak white liquor, by measuring: (a) the flow rate of the green liquor in units of volume per time, (b) the flow rate of the weak white liquor in units of volume per time, (c) the density of the green liquor, and (d) the density of the weak white liquor.

5. The method of claim 1 , wherein the at least one of the temperature and the oxygen content within the recovery boiler is controlled by controlling at least one process parameter of the recovery boiler selected from the group consisting of:

a flow rate of combustion air into the recovery boiler,

a distribution of the combustion air in the recovery boiler,

a pressure of the combustion air,

a flow velocity of the combustion air,

a pressure of a concentrated black liquor, which is fed to a furnace of the recovery boiler,

a temperature of the concentrated black liquor,

a flow velocity of the concentrated black liquor, and

a size of an orifice of a nozzle used to feed a black liquor into the furnace.

6. The method of claim 1 , further comprising:

measuring a temperature of the green liquor,

determining, using: (i) the measured temperature of the green liquor, (ii) the measured content of sodium carbonate (Na 2 CO 3 ) of the green liquor, (iii) the measured content of sodium sulphate (Na 2 SO 4 ) of the green liquor, (iv) the measured content of sodium hydroxide (NaOH) of the green liquor, and (v) the measured content of sodium sulphide (Na 2 S) of the green liquor:

a maximum allowable value for a quantity indicative of at least an amount of a pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor on the condition that a solubility limit of pirssonite is not exceeded, and

a value of the quantity indicative of at least an amount of the pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor, and

controlling the flow of the weak white liquor into the dissolving tank by using:

the determined maximum allowable value for the quantity indicative of at least an amount of a pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor on the condition that the solubility limit of pirssonite is not exceeded, and

the determined value of the quantity indicative of at least an amount of the pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor, in such a way that

the content of total titratable alkali of the green liquor is maximized on the condition that solid pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) is not formed in the green liquor.

7. The method of claim 6 , further comprising:

determining, using at least the measured temperature of the green liquor, the measured content of sodium carbonate (Na 2 CO 3 ) of the green liquor, and information on the solubility of pirssonite in the green liquor, a theoretical maximum value for the quantity indicative of at least an amount of a component of pirssonite on the condition that solid pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) is not formed in the green liquor,

determining a safety margin for the quantity indicative of at least an amount of a component of pirssonite, and

controlling the flow of the weak white liquor into the dissolving tank by optimizing the value of the quantity indicative of at least an amount of a component of pirssonite towards a target value that is calculated by subtracting the determined safety margin from the determined theoretical maximum value.

8. The method of claim 1 , wherein:

the content of sodium sulphate (Na 2 SO 4 ) of the green liquor, is measured on-line at a second instance of time, and the second instance of time is at most 4 hours earlier than the first instance of time.

9. The method of claim 8 , wherein the second instance of time is at most 1 hour earlier than the first instance of time.

10. The method of claim 1 , further comprising:

measuring, at a second or third instance of time, at least one of:

the content of sodium sulphate (Na 2 SO 4 ) of the green liquor,

the content of sodium hydroxide (NaOH) of the green liquor,

the content of sodium sulphide (Na 2 S) of the green liquor, and

the content of sodium carbonate (Na 2 CO 3 ) of the green liquor,

at a fourth instance of time, which is after the second or third instance of time and before the first instance of time, measuring a density of the green liquor on-line, and

[A]

determining a target value for the density of green liquor using the measured temperature, the measured content of sodium carbonate (Na 2 CO 3 ), the measured content of sodium sulphate (Na 2 SO 4 ), the measured content of sodium hydroxide (NaOH), and the measured content of sodium sulphide (Na 2 S) of the green liquor, and

controlling the flow of the weak white liquor into the dissolving tank by using the measured density of the green liquor and the determined target value for the density of green liquor, or

[B]

determining a value of a quantity indicative of at least an amount of a pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor using the measured density of the green liquor.

11. The method of claim 1 , further comprising:

determining a value indicative of a change of flow rate of the smelt from the recovery boiler into the dissolving tank, and

controlling the flow of the weak white liquor into the dissolving tank by using the determined value indicative of a change of the flow rate of the smelt.

12. The method of claim 11 , further comprising:

determining a target value for at least one feature selected from the group consisting of:

a density of the green liquor,

a content of total titratable alkali of the green liquor, and

a total content of compounds comprising sodium, and

controlling the flow of the weak white liquor into the dissolving tank by using the determined target value of the at least one feature, and a value of the density of the green liquor, the content of total titratable alkali of the green liquor, and a total content of compounds comprising sodium, respectively.

13. The method of claim 11 , further comprising:

conveying the smelt from the recovery boiler to the dissolving tank through a smelt spout,

cooling the smelt spout with a circulation of a heat transfer medium, and

determining a change of a cooling power of the smelt spout to determine a value indicative of a change of flow rate of the smelt.

14. The method of claim 1 , further comprising:

measuring at least one of:

a density of the weak white liquor, and

a temperature of the weak white liquor, and

controlling the flow of the weak white liquor into the dissolving tank by using the measured density and/or temperature of the weak white liquor.

15. The method of claim 1 , wherein at least two contents selected from the group consisting of:

the content of sodium sulphate (Na 2 SO 4 ) of the green liquor,

the content of sodium hydroxide (NaOH) of the green liquor,

the content of sodium sulphide (Na 2 S) of the green liquor, and

the content of sodium carbonate (Na 2 CO 3 ) of the green liquor, are measured by titration.

16. A system for producing a green liquor with optimized reduction and content of total titratable alkali, the system comprising:

a recovery boiler,

a dissolving tank comprising:

a first inlet for receiving a smelt from the recovery boiler,

a second inlet for receiving a weak white liquor, and

an outlet for the green liquor,

a first regulator configured to regulate a flow of the weak white liquor into the dissolving tank,

a second regulator configured to regulate at least one of a temperature and an oxygen content within the recovery boiler,

a first sensor arrangement configured to measure at least

a content of sodium sulphate (Na 2 SO 4 ) of the green liquor,

a content of sodium hydroxide (NaOH) of the green liquor,

a content of sodium sulphide (Na 2 S) of the green liquor, and

a content of sodium carbonate (Na 2 CO 3 ) of the green liquor, the first sensor arrangement being further configured to give at least one signal indicative of the measured contents, and

a processing unit arrangement configured to:

control the at least one of the temperature and the oxygen content within the recovery boiler by using the second regulator and the at least one signal from the first sensor arrangement in such a way that a reduction of the green liquor is maximized, and

control the flow of the weak white liquor into the dissolving tank by using the first regulator and the at least one signal from the first sensor arrangement.

17. The system of claim 16 , wherein the second regulator is configured to regulate the at least one of the temperature and the oxygen content within the recovery boiler by regulating at least one process parameter selected from the group consisting of:

a flow rate of combustion air into the recovery boiler,

a distribution of the combustion air in the recovery boiler,

a pressure of the combustion air,

a flow velocity of the combustion air,

a pressure of a concentrated black liquor, which is fed to a furnace of the recovery boiler,

a temperature of the concentrated black liquor,

a flow velocity of the concentrated black liquor, and

a size of an orifice of a nozzle used to feed a black liquor into the recovery boiler.

18. The system of claim 16 , wherein:

the first sensor arrangement is further configured to measure a temperature of the green liquor, and

the processing unit arrangement is further configured to control the flow of the weak white liquor into the dissolving tank by using the measured temperature of the green liquor, in such a way that a content of total titratable alkali of the green liquor is maximized on the condition that solid pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) is not formed in the green liquor.

19. The system of claim 16 , wherein:

the first sensor arrangement is further configured to measure the content of sodium sulphate (Na 2 SO 4 ) on-line.

20. The system of claim 16 , wherein:

the first sensor arrangement is further configured to measure a density of the green liquor on-line, and

[A]

the processing unit arrangement is further configured to determine a target value for the density of the green liquor using: (i) the content of sodium sulphate (Na 2 SO 4 ), (ii) the content of sodium hydroxide (NaOH), (iii) the content of sodium sulphide (Na 2 S), and (iv) the content of sodium carbonate (Na 2 CO 3 ), or

[B]

the processing unit arrangement is further configured to determine, using the density of green liquor, a target value for a quantity indicative of at least an amount of a pirssonite (Na 2 Ca(CO 3 ) 2 .2H 2 O) forming component in the green liquor.

21. The system of claim 16 , further comprising:

a second sensor arrangement configured to:

measure at least one of

a density of the weak white liquor, and

a temperature of the weak white liquor, and

send at least one signal indicative of the measured density and/or temperature of the weak white liquor to the processing unit arrangement,

wherein:

the processing unit arrangement is further configured to control the flow of the weak white liquor into the dissolving tank by using the first regulator and the at least one signal from the second sensor arrangement.

22. The system of claim 16 , wherein:

the first sensor arrangement comprises a titrimeter configured to measure at least two contents selected from the group consisting of:

the content of sodium sulphate (Na 2 SO 4 ) of the green liquor,

the content of sodium hydroxide (NaOH) of the green liquor,

the content of sodium sulphide (Na 2 S) of the green liquor, and

the content of sodium carbonate (Na 2 CO 3 ) of the green liquor.