On-line Moisture Measurement in Biomethane for Injection to Natural Gas Network. by Andy Benton

Transcription

1 On-line Moisture Measurement in Biomethane for Injection to Natural Gas Network by Andy Benton

2 Typical schematic for biomethane On-line analysis: O 2, CO 2, H 2 S, COS, CH 4, H 2 O On-line analysis: CV, Wobbe, Density O 2, CO 2, H 2 S, COS, THT, H 2 O

3 Gas quality standards for network injection biomethane National specifications currently apply in Europe: France (GrDF SBLI001) and Italy (UNI/TR 11537): -5 C dew point at process pressure. Germany (DVGW G 260): Moisture content: 200mg/m 3 (MOP 10 bar) 50mg/m 3 (MOP >10 bar) European Standard In development by CEN/TC: pren and pren :2014

4 DVGW G 260: Moisture content, mg/m 3 Gas Infastructure to pren :2014 : Dew point -8 o C at 70 bar 10 bar *European Association for Streamlining Energy Exchange >10 bar EASEE-gas * -8 Cdp@70bar

5 Dehydration processes Refrigeration drying: - Separation of condensed water vapour at +2 C and 10 bar over-pressure. Desiccant drying. - Water vapour adsorption. Molecular sieve or silica gel.

6 Moisture measurement for biomethane Biomethane production and network injection: Metal-oxide capacitance dew-point transmitters. Key points within grid networks receiving biomethane: TDLAS (Tuneable Diode Laser Absorption Spectroscopy) Moisture Analyser.

7 Metal-oxide Capacitance Dew-point Transmitters Economical solution for integration to biomethane plant

8 Sensor Impedance, K ohms Ceramic Metal-oxide Sensor Technology Typical calibration characterisation Dewpoint, deg. C Capacitance sensor with hygroscopic metal-oxide dielectric. Porous to adsorb/de-sorb polarised moisture molecules. Responds to water vapour pressure which relates directly to dew point temperature

9 Dew-Point Transmitter installations Dew-Point temperature at process pressure or converted to moisture content

10 TDLAS Moisture Analysis High accuracy, fast response moisture analysis in natural gas and biomethane

11 Measurement Principle: Photometric gas absorption IO Absorbent concentration (c) Ix Beer Lambert law*: A =εlc A = Absorption (I 0 - I x ) c = concentration of gas (moisture) l = pathlength (gas cell length) = extinction coefficient (absorption strength) of gas (moisture) Path length (l) * Here simplified for trace moisture measurement For known pathlength and molar extinction coefficient, concentration (of moisture) is proportionate to absorption.

12 Measurement principle: TDLAS Typical broadband source bandwidth Scan range Peak of interest sawtooth laser scan Laser line width = nm Conventional IR source gives very broad transmission Potential interferant gases Gas absorption Bandwidth typically 0.05nm Potential interferant gases

13 Why TDLAS for moisture measurement? Non-contact measurement technique Fast response to process moisture changes Highly selective and sensitive gas measurement, - Accuracy: +/-1ppmV or 1% of reading Wavelength modulation spectroscopy (WMS): - Maximum signal to noise Background gas composition compensation: - Consistent measurement performance with changing gas.

14 Natural gas networks carry gases from multiple and changing sources: Conventional: Natural gas (often from multiple sources) LNG and storage gas during demand peaks Unconventional: Biomethane Other SNG SynGas including shale gas, coalbed methane Hydrogen injection Conclusion: Analysis of moisture must be unaffected by rapid changes in background composition.

15 Examples of moisture absorption peaks within the NIR region (HITRAN database). Unfortunately, other natural gas components absorb NIR.

16 Methane also absorbs NIR Requirement: High sensitivity to moisture content and low sensitivity to methane (and other gas components).

17 Illustration of spectral interference: Detail of nm water peak with moisture content reduced to 30 ppmv Defined fixed composition: May be possible to calibrate out background offset. Variable composition: Requires active compensation.

18 TDLAS: The Michell Approach Active compensation requires measurement of composition variations For example, the concentration of methane can be measured directly: - Tuning the laser to a nearby methane absorption peak (with no water interference) as separate secondary measurement. In simplest form, the absorption due to the variable methane subtracted from the moisture giving a fundamental clean detection

19 D-Met Dynamic compensation

20 Independent testing: National Physical Laboratory

21 Network gas downstream of biomethane injection in periods of low demand. Tested against the NPL multi-gas multi-pressure humidity generator.

22 Rich natural gas Tested against the NPL multi-gas multi-pressure humidity generator.

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24 TDLAS Moisture Analyser Installations

25 On-line Moisture Measurement in Biomethane for Injection to Natural Gas Network Thank you Any Questions?