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

Abstract

A method for determining a sulfur trioxide content in a gas. A sample of the gas is taken, and a gas pressure of the sample is reduced. A wave number-resolved transmission measurement is carried out on the sample using a wave number-tunable monochrome light source, and a sulfur trioxide content is derived from the measurement. The measurement is carried out in the sulfur trioxide absorption band between 1360 and 1410 cm.sup.1, in particular in a window around the sulfur trioxide absorption at 1365.49 cm.sup.1. A method for operating a power plant, a measuring system for determining a sulfur trioxide content in a gas, and a power plant are adapted to determine a sulfur trioxide content in a gas.

Claims

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in further detail by way of example with the aid of the drawings. There are shown diagrammatically and not to scale:

(2) FIG. 1 a gas and steam turbine plant,

(3) FIG. 2 a measuring system according to the invention,

(4) FIG. 3 the method for determining a sulfur trioxide content in a gas according to the invention and

(5) FIG. 4 a WMS spectrum (2.sup.nd harmonic) of sulfur trioxide and sulfur dioxide around 1365.5 cm.sup.1.

DETAILED DESCRIPTION OF INVENTION

(6) FIG. 1 shows diagrammatically and by way of example a power plant 7, in particular a gas and steam turbine plant. Steam for the operation of the steam turbine 22 is generated by the exhaust gas 1 of the gas turbine 8 in the heat recovery steam generator 9 downstream of the gas turbine 8. Steam which is expanded in the steam turbine 22 is condensed in the condenser 23 and is delivered again to the heat recovery steam generator 9.

(7) FIG. 2 shows a measuring system 10 according to the invention. The gas take-off takes place at the heat recovery steam generator 9 at a suitable gas take-off point 12 (at the point at which the lowest temperature is to be expected). In order to keep particles away from the measuring system 10, the sample 2 (i.e. the measurement gas) passes firstly through a particle filter 20 with adapted pore size, which withstands the operating conditions at the place of use. A possibility would be a PTFE filter, which is temperature-stable up to 260 C. and is supplied with various pore sizes. Following the filter 20 is a throttle device 21, for example a simple throttle or alternatively a mass flow controller 24, which provides the desired throughput at underpressure operation. A first gas line 11 from the gas take-off point 12 to the measuring cell 13 should be flexible, owing to the thermal expansion of the heat recovery steam generator 9, able to be heated to 200 C. and embodied to be as short as possible, in order to keep the dwell time of the sample 2 small. The greater the ratio of surface to volume of the first gas line 11, the greater is the risk that interaction between sample 2 and surface such as adsorption or catalytic conversion processes take place to an interfering extent. Inert surfaces further reduce the interactions. Teflon pipes are approved for the transport of measurement gases, if concentrations in the sub-ppm range are to be measured. Alternatively, high-grade steel surfaces can be provided with a silicon dioxide coating, which reduces the interaction with the surface. As the spectroscopy does not require a drying of the sample 2, a gas conditioning is dispensed with. Thereby, a further source for a falsification of the gas composition is eliminated. From the heated first gas line 11, the sample 2 is directed into the likewise heated measuring cell 13. The required absorption section 6 (approximately 10 m) is provided by a long path cell 4 for a multiply folded beam path 5. After the measuring cell 13, the sample 2 is cooled in a gas cooler 25, which is connected into a second gas line 19, to temperatures which are compatible for the pressure regulator 17 of a pressure regulation device 14. The pressure regulator 17 keeps the pressure in the measuring cell 13 constant at low pressure (e.g. 100 hPa), the pressure at which the spectral line widths are so small that the individual absorption lines can be separated. A vacuum pump 18 generates the underpressure required for the operation of the pressure regulator 17. For the examination of the sample 2 in the measuring cell 13, a wave number tunable monochrome light source 3 (laser), a photodiode 26, and in addition a control 15 for carrying out a transmission measurement and an evaluation unit 16 are required.

(8) FIG. 3 shows the method for determining a sulfur trioxide content in a gas, in which in a first step 31 a sample 2 of the gas 1 is taken and in a second step 32 a gas pressure of the sample 2 is reduced, and in a third step 33 with a wave number tunable monochrome light source 3 on the sample 2 a wave number resolved transmission measurement is carried out in the sulfur trioxide absorption band between 1360 and 1410 cm.sup.1, in particular in a window around the sulfur trioxide absorption at 1365.49 cm.sup.1 and in a fourth step 34 a sulfur trioxide content is derived from the measurement.

(9) FIG. 4 shows a wavelength modulation spectrum (2.sup.nd harmonic) of sulfur trioxide and sulfur dioxide around 1365.5 cm.sup.1. The wavelength modulation spectroscopy method practically completely eliminates the influence of the water absorption in the region of the sulfur trioxide absorption.