Method and device for controlling a process for burning a lime containing mixture to burnt lime

1. A method for controlling a process for supplying a lime containing mixture (CaCO3) to a rotary kiln, burning the lime containing mixture and converting it to calcinated lime (CaO) in the rotary kiln, said rotary kiln having an elongated cavity surrounded by a wall and a burner arranged to heat the cavity, characterised in that the method comprises:

collecting measurement data of the temperature in the wall at a plurality of measuring points along the longitudinal axis of said cavity,

predicting an actual temperature gradient along the longitudinal axis of said cavity based at least on said measurement data of the temperature in the wall, and by means of a thermal model describing the temperature along the longitudinal axis of said cavity of the kiln,

determining a desired temperature gradient along the longitudinal axis of said cavity based on the predicted temperature gradient along the cavity and a predetermined control strategy controlling the temperature in the kiln so that the area of deposition of lime on the wall of the kiln inside said cavity is controlled and any drawbacks of the lime deposition are reduced; and

controlling said burner to adjust the temperature in the kiln to the desired temperature gradient.

2. The method according to claim 1 , characterised in that the method comprises:

collecting measurement data of the supply of the lime containing mixture to the kiln,

collecting measurement data of the supply of fuel to said burner,

wherein said predicting the actual temperature gradient along the longitudinal axis of said cavity is based on the supply of the lime containing mixture and on the supply of the fuel, and

determining set-point values for supply of the lime containing mixture and supply of the fuel to obtain the desired temperature gradient.

3. The method according to claim 2 , characterised in that the method comprises:

determining the rotation speed of said rotary kiln for controlling the temperature in the kiln.

4. The method according to claim 1 , wherein the control strategy controls the temperature in the kiln so that the temperature in said area of deposition is kept essentially at a constant level in order to concentrate the lime deposition to a small area.

5. The method according to claim 1 , wherein the control strategy controls the temperature in the kiln so that the temperature in said area of deposition is varied in order to spread out the lime deposition over a large area.

6. The method according to claim 1 , wherein the control strategy controls the temperature in the kiln so that the temperature in said area of deposition is kept below a predetermined value.

7. The method according to claim 1 , wherein the lime containing mixture is washed in a filter before being supplied to the kiln, and that the method comprises:

determining the alkali content of the lime containing mixture during the washing,

if the alkali content is below a predetermined limit, interrupting the washing and supplying the lime containing mixture to the kiln.

8. The method according to claim 1 , wherein the lime containing mixture is washed in a filter before being supplied to the kiln, and that the method comprises:

determining the alkali content of the a lime containing mixture during the washing,

if the alkali content is below a predetermined limit, the lime containing mixture is supplied to the kiln without interrupting the washing.

9. The method according to claim 7 , wherein the washing produces a filtrate, and the method further comprises measuring the electrical conductivity of the filtrate from the washing and based thereon determining the alkali content of the lime containing mixture.