Method for monitoring a cleaning of a process gas

1. A method for cleaning and for monitoring a cleaning of a process gas passing through a wet scrubber, the wet scrubber comprising an inlet zone, a contact zone and an outlet zone, the contact zone divided into at least two sections, the process gas flowing through the inlet zone and into the contact zone for mixing with an absorption liquid within the wet scrubber, the absorption liquid absorbing at least part of a sulphur dioxide content of the process gas, the process gas then flowing out of the wet scrubber via the outlet zone, the method comprising:

measuring a concentration of sulphur dioxide in at least a first measuring point corresponding to a first section of the at least two sections of the contact zone to obtain measurement information; and

comparing the obtained measurement information to a reference value for determining a local sulphur dioxide removal performance of the first section.

2. The method according to claim 1 further comprising utilizing the local sulphur dioxide removal performance to determine a need for an action to regulate a local operating condition of the first section.

3. The method according to claim 1 further comprising utilizing the local sulphur dioxide removal performance to determine a need for an action to regulate a local operating condition of the first section, wherein the action to regulate the local operating condition of the first section is at least one of:

increasing or decreasing the amount of absorption liquid supplied to the first section of the contact zone;

increasing or decreasing a concentration of a reagent in the absorption liquid of the first section of the contact zone;

increasing or decreasing an amount of process gas supplied to the first section of the contact zone; and

cleaning, replacing, or rebuilding equipment of the wet scrubber corresponding to the first section.

4. Method according to claim 1 further comprising controlling absorption of sulphur dioxide in the first section of the contact zone based on local sulphur dioxide removal performance.

5. The method according to claim 1 further comprising controlling absorption of sulphur dioxide in the first section of the contact zone based on local sulphur dioxide removal performance wherein the controlling absorption is through at least one of:

increasing or decreasing the amount of absorption liquid in the first section of the contact zone;

increasing or decreasing the concentration of a reagent in the absorption liquid in the first section of the contact zone; and

increasing or decreasing an amount of process gas flowing to the first section of the contact zone.

6. The method according to claim 1 , wherein a concentration of sulphur dioxide is measured downstream of the contact zone.

7. The method according to claim 1 , wherein a reference value is selected from the group consisting of:

a concentration of sulphur dioxide in the process gas upstream of the wet scrubber;

a concentration of sulphur dioxide in the process gas downstream of the wet scrubber;

a concentration of sulphur dioxide in the contact zone measured in a measuring point corresponding to a second section of the at least two sections; and

a predetermined value.

8. The method according to claim 1 further comprising measuring the concentration of sulphur dioxide in at least a first, a second and a third measuring point corresponding to a respective first, second and third section of the contact zone to obtain measurement information.

9. The method according to claim 1 further comprising measuring the concentration of sulphur dioxide in at least a first, a second and a third measuring point corresponding to a respective first, second and third section of the contact zone to obtain measurement information and comparing the measurement information to reference values to determine a need for action to modify a local operating condition in any of the first, second and third sections.

10. The method according to claim 1 , wherein the action to modify the local operation condition is through controlling by at least one of:

increasing or decreasing the amount of absorption liquid in the first section of the contact zone;

increasing or decreasing the concentration of a reagent in the absorption liquid in the first section of the contact zone; and

increasing or decreasing an amount of process gas flowing to the first section of the contact zone.

11. The method according to claim 1 , wherein the wet scrubber is a wet scrubber of the open spray tower type and wherein each of the at least two sections comprises at least one nozzle adapted for spraying absorption liquid.

12. The method according to claim 1 , wherein the wet scrubber further comprises at least one demister positioned downstream of the contact zone and concentration of the sulphur dioxide is measured downstream of the demister.

13. The method according to claim 1 , wherein the wet scrubber comprises a first demister and a second demister arranged in series and the concentration of the sulphur dioxide is measured downstream of the second demister.

14. The method according to claim 1 , wherein the wet scrubber is of a bubbling bed type of scrubber wherein the contact zone comprises:

a plate with a plurality of apertures for passage of process gas from the inlet zone to the outlet zone through the apertures;

sections comprising at least one of the apertures; and

a layer of absorption liquid flowing over an upper side of the plate.

15. The method according to claim 1 , wherein the wet scrubber further comprises at least one sulphur dioxide detecting sensor positioned in measuring points for measuring the concentration of sulphur dioxide.

16. The method according to claim 1 , wherein the wet scrubber further comprises at least one sulphur dioxide detecting sensor, and at least one collector, the method further comprising the step of

collecting gas from the at least one measuring point by means of the at least one collector and transporting the gas to the at least one sulphur dioxide detecting sensor.

17. A gas cleaning system for cleaning a process gas, comprising:

a wet scrubber for separating sulphur dioxide from the process gas passing through the wet scrubber by contact with an absorption liquid within a contact zone within the wet scrubber;

at least one sulphur dioxide detecting sensor; and

at least a first measuring point; wherein

the first measuring point corresponds to a first section of at least two sections of the contact zone, and the at least one sulphur dioxide detecting sensor is operable to measure a concentration of sulphur dioxide in the first measuring point to obtain measurement information to determine a local sulphur dioxide removal performance of the first section of the contact zone.

18. The gas cleaning system according to claim 17 , further comprising a controller to utilize measurement information to determine a need to modify the local operating condition in one of the at least two sections of the contact zone to which the at least one measuring point corresponds.

19. The gas cleaning system according to claim 17 , further comprising a controller wherein the controller is adapted to determine a need to modify at least one local operating parameter of the wet scrubber in the first section.

20. The gas cleaning system according to claim 17 further comprising a controller and a control device used for at least one of:

increasing or decreasing the amount of absorption liquid in the first section of the contact zone;

increasing or decreasing the concentration of a reagent in the absorption liquid in the first section of the contact zone;

increasing or decreasing an amount of process gas in the first section of the contact zone.

21. Use of the system according to claim 17 for determining a malfunction in the operation of one section of at least two sections of a wet scrubber.