Laser-based IR spectroscopy for measuring sulfur trioxide in the exhaust gas of gas power plants

1. A method for determining a sulfur trioxide content in a combustion gas also containing sulfur dioxide and water, comprising:

taking a sample of a gas comprising sulfur dioxide at <50 ppm and reducing a gas pressure of the sample, and

with a wave number tunable monochrome light source, carrying out a wave number resolved transmission measurement on the sample and from the measurement deriving a sulfur trioxide content,

wherein the measurement is a single spectral measurement in a window around 1365.5 cm −1 , and

the sulfur trioxide content is derived by compensating sulfur trioxide absorption at 1365.49 cm −1 for a cross-influence of sulfur dioxide absorption between 1365.52 and 1365.54 cm −1 .

2. The method as claimed in claim 1 ,

wherein the sample is brought to a pressure below 100 hPa.

3. The method as claimed in claim 1 ,

wherein a sample temperature above 200° C. is set.

4. The method as claimed in claim 1 ,

wherein the measurement takes place by wave length modulation spectroscopy (WMS) with detection of the 2nd harmonic or else higher harmonics.

5. The method as claimed in claim 1 ,

wherein the spectral measurement is carried out in a long path cell with a multiply folded beam path.

6. The method as claimed in claim 5 ,

wherein an absorption section is a maximum of 15 m.

7. The method as claimed in claim 1 ,

wherein a measured spectrum is compared in a curve fitting with a model spectrum, and the concentration of sulfur trioxide enters as parameter into the model spectrum.

8. The method as claimed in claim 7 ,

wherein in addition to the concentration of sulfur trioxide, concentrations of interfering gases also enter into the model either as known values which originate from a second independent measuring method, or as fit parameters.

9. A method for operating a power plant with a gas turbine and with a heat recovery steam generator, the method comprising:

determining a sulfur trioxide content in combustion exhaust gas of the gas turbine as claimed in claim 1 , and

adapting an exhaust gas temperature in the heat recovery steam generator on the basis of the sulfur trioxide content so that a sulfuric acid dew point is not fallen below in the heat recovery steam generator.

10. A measuring system for determining a sulfur trioxide content in a combustion gas also containing sulfur dioxide at <50 ppm and water, comprising:

a first gas line from a gas take-off point to a measuring cell,

a pressure regulation device for the measuring cell,

a wave number tunable monochrome light source in the region of a sulfur trioxide absorption band,

a control for carrying out a transmission measurement in the measuring cell, and

an evaluation unit for determining the sulfur trioxide content,

wherein the light source is suited to generate monochromatic light in a window around 1365.5 cm −1 , and

the evaluation unit determines the sulfur trioxide content by compensating sulfur trioxide absorption at 1365.49 cm −1 for a cross-influence of sulfur dioxide absorption between 1365.52 and 1365.54 cm −1 .

11. The measuring system as claimed in claim 10 ,

wherein the pressure regulation device comprises a pressure regulator and a vacuum pump, which are connected to the measuring cell via a second gas line.

12. The measuring system as claimed in claim 10 ,

wherein at least one of the two, first gas line and measuring cell, is heatable.

13. The measuring system as claimed in claim 10 , further comprising

a particle filter connected into the first gas line.

14. The measuring system as claimed in claim 10 , further comprising

a throttle device connected into the first gas line.

15. The measuring system as claimed in claim 10 ,

wherein the measuring cell is a long path cell with a multiply folded beam path, which has an absorption section of 5 to 15 m.

16. A power plant comprising:

a gas turbine,

a heat recovery steam generator, and

a measuring system as claimed in claim 10 .

17. The method as claimed in claim 5 ,

wherein an absorption section is a maximum of 10 m.

18. The method as claimed in claim 5 ,

wherein an absorption section is a maximum of 5 m.

19. The measuring system as claimed in claim 15 ,

wherein the absorption section is 7 to 12 m.

20. The method as claimed in claim 9 , further comprising adapting the exhaust gas temperature in the heat recovery steam generator so that the sulfuric acid dew point is not fallen below by regulating a condensate flow temperature in response to the determined sulfur trioxide content.