Ultrasonic Technology for Accurate Flow Measurement of Biogas

Transcription

1 Ultrasonic Technology for Accurate Flow Measurement of Biogas Accurately Measures Methane Standards Certification Education & Training Publishing Conferences & Exhibits Speaker: Alan Vance E+H Industry Manager 2014 ISA Water / Wastewater and Automatic Controls Symposium August 5-7, 2014 Orlando, Florida, USA

2 Presenter Alan Vance Endress+Hauser Industry Manager W/WW Has worked since 1984 in process control instrumentation: 7 years with Fischer & Porter 22 years with Instrument rep company in FL 1.5 years as Industry Mrkt Mgr with E+H 2

3 Presentation Outline Overview of anaerobic digester (biogas) application Thermal mass Technology & typical problems Ultrasonic flowmeter Technology & benefits Summary 3

4 Energy Consumption - WWTP Sludge treatment 20% Mechanical cleaning and others 10% Biological treatment 70% #1 Aeration Basins #2 Sludge Treatment - Anaerobic digestion 100 MGD WWTP - $3m / year power bill. 4

5 What is Digester Gas or Biogas? Gas produced by the biological breakdown of organic matter in the absence of oxygen. Anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material, and energy crops. Biogas essentially methane (CH 4 ) and carbon dioxide (CO 2 ), small amounts of hydrogen sulphide (H 2 S) and moisture. Component Concentration (Vol%) Methane (CH 4 ) 50 to 75 Carbon dioxide (CO 2 ) 25 to 45 Water (H 2 O) 2 (20 C) 16 (55 C) Nitrogen (N 2 ) <1 Oxygen (O 2 ) <5000 ppm Hydrogen Sulphide (H 2 S) <5000 ppm Hydrogen (H 2 ) <5000 ppm 5

6 Anaerobic Digestion Stable process condition for the bacteria Constant temperature ( F) Nearly neutral ph value (6-7.5) Darkness and absence of oxygen Constant feed Produce 65% methane is ideal 6

7 Why do we need a new technology for biogas? Current Situation Thermal mass meters normally used 1% change in methane causes 0.7% change in volume flow Wet gas causes erratic 30-50% spikes in readings Separate analyzer needed for methane content Thermal meters frequently removed from service for maintenance Low reliability/high maintenance Ultrasonic Technology - New Ultrasonic transit time used for refinery flares since 1982 Variation in gas composition causes no effect Wet gas is normal and creates no problem New ultrasonic meter also computes methane content Ultrasonic transit time meters remain in continuous service High reliability/low maintenance 7

8 Thermal Mass Measuring Principle A heated temperature sensor is kept at a constant differential temperature from the gas reference sensor. The cooling effect generated by mass flow causes an increase in sensor power to maintain the temperature differential. Mass flow is measured directly Mass Flow = Density Velocity The power supplied to the heated sensor is proportional to the mass flow. 8

9 Thermal Sensors Pt 100 ohm RTD 9

10 Measuring Principle Ultrasonic Transit Time Up Flow Down (dn) Pulses of ultrasound are transmitted rapidly in alternate directions With no flow, time upstream (t up ) = time downstream (t dn ) With flow, t up > t dn Transit time difference ( t) is directly proportional to flow velocity (v) Volume flow rate Q = v * A (internal cross-sectional area) 10

11 Speed of Sound The speed of sound is the distance travelled during a unit of time by a sound wave In dry air at 20 C (68 F), the speed of sound is m/s or (1,126 ft/s). The speed of sound in gas is a function of the square root of temperature, but is nearly independent of pressure or density for a given gas. 11

12 Speed of Sound The ultrasonic flowmeter measures the time taken for the ultrasonic pulse to travel through the gas. As the path length is precisely known, the speed of sound in the gas can be accurately determined. As the speed of sound in a gas is dependent on the gas composition, the ultrasonic can use the sound speed to calculate the methane content of the biogas. 12

13 Speed of Sound Because the speed of sound in gas is a function of the square root of temperature, the ultrasonic flowmeter uses an integrated Pt 100 Ω RTD to measure the biogas temperature; this measurement is then used to compensate the effect of temperature on sound speed. Biogas is typically saturated with moisture. The temperature measurement is also used to calculate the moisture content of the gas. 13

14 100% 104 F = m/s 100% 104 F = m/s and. Ideal Condition 14

15 Summary Thermal mass flowmeter works very well in applications where the gas is dry. Ultrasonic flowmeter works well in applications with moisture/dirt Ultrasonic flowmeter can directly measure methane content of biogas which is critical in the health of the digester and if the plant is using the biogas as an energy source. Thank you 15