Strumentazione analitica per la misurazione di composizione di biogas e biometano Lorenzo Cocola, Fabio Melison et al. CNR Istituto di Fotonica e Nanotecnologie Padova Verso un’economia circolare: tecnologie al servizio delle rinnovabili in agricoltura: principi guida e casi studio. DEI- Aula Magna A. LEPSCHY via Gradenigo, 6b 35100 Padova 15 settembre 2020 Gaseous fuel mixtures

• Mainly hydrocarbons (HC)

• LPG (Liquified Petroleum Gas)

Propane (C3H8), N-butane, I-butane (C4H10), liquid at few bars (RT). Wildly variable composition. • NG (Natural Gas)

Methane (CH4), small amounts of ethane (C2H6) and heavier HCs up to pentane (C5H12), H2, N2, CO2 and contaminants (depending on source type). Usually distributed in gaseous phase. • Town Gas (obsolete)

Hydrogen (H2), methane (CH4), carbon monoxide (CO), ethylene (C2H4). • SNG (Substitute Natural Gas) Other mixtures (such as propane-air) mimicking the fuel properties of NG.

• Odorizing gas added for safety, detectable by nose at ppm levels.

• Common contaminants: water vapour (H2O), hydrogen sulfide (H2S)

L. Cocola, F. Melison et al. - Strumentazione analitica per la 2 misurazione di composizione di biogas e biometano Higher Heating Value

An indicator of the energy value of a fuel gas is the Higher Heating Value:

• The quantity known as higher heating value (HHV) (or gross energy or upper heating value or gross calorific value (GCV) or higher calorific value (HCV)) is determined by bringing all the products of combustion back to the original pre-combustion temperature, and in particular condensing any vapor produced.

• Main gas families, typical HHV in MJ/Nm3: 1 Town gas: 22.5 to 30.0 2 Natural gas: 39.0 to 45.0 3 LPG: 72.0 to 87

L. Cocola, F. Melison et al. - Strumentazione analitica per la 3 misurazione di composizione di biogas e biometano Contaminants of natural gas

Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:

• Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons. When processed and purified into finished by-products, all of these are collectively referred to as Natural Gas Liquids or NGL.

• Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).

• Other gases: nitrogen (N2) and helium (He). • Water: water vapor and liquid water. Also dissolved salts and dissolved gases (acids). • Liquid hydrocarbons: perhaps some natural-gas condensate (also referred to as casinghead gasoline or natural gasoline) and/or crude oil. • Mercury: very small amounts of mercury primarily in elemental form, but chlorides and other species are possibly present. • Naturally occurring radioactive material (NORM).

Hydrogen is being added in the gas distribution network in significant and increasing amounts as a way to store excess energy production from photovoltaic farms.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 4 misurazione di composizione di biogas e biometano Typical composition of natural gases by region

Region CH4 % C2H6 % Other HC % CO2 % N2 % Other gas % HHV [MJ/m3] Italy 99.53 0.06 0.03 0.03 0.35 - 37.7 Russia 97.50 1.06 0.45 0.15 0.83 0.01 38.0 Northern EU 90.32 4.62 1.33 1.37 2.33 0.03 38.6 Algeria 87.92 6.95 1.77 0.97 2.29 0.10 39.7 Libya 85.98 6.80 2.95 1.44 3.42 0.11 39.9 Regassified 90.05 7.82 1.24 - 0.84 0.05 40.4 Pure methane 100.00 - - - - - 37.7

L. Cocola, F. Melison et al. - Strumentazione analitica per la 5 misurazione di composizione di biogas e biometano Biogas

Compound Vol. % (typ.) A mixture of different gases produced by the breakdown of organic matter in the absence of oxygen Methane 50 to 75 Produced from: • agricultural waste Carbon dioxide 25 to 50 • manure • municipal waste Nitrogen 0 to 10 • plant material • sewage Hydrogen 0 to 1 • green waste • food waste Hydrogen sulfide 0.1 to 0.5 • Biogas is a renewable energy source Oxygen 0 to 0.5 • Different from NG

L. Cocola, F. Melison et al. - Strumentazione analitica per la 6 misurazione di composizione di biogas e biometano Biogas upgrading

• H2S is corrosive and contamination of pipeline infrastructure should be avoided. • Methane in biogas has to be concentrated via a biogas upgrader to the same standards as fossil natural gas • Before grid injection carbon dioxide, water, hydrogen sulfide, and particulates must be removed if present.

• There are several main methods of upgrading: water washing, pressure swing absorption, selexol absorption, amine gas treating and the use of membrane separation technology. Biogas gas-grid injection into the methane grid (natural gas grid). • Until the breakthrough of micro combined heat and power two-thirds of all the energy produced by biogas power plants was lost (as heat). • Using the grid to transport the gas to customers, the energy can be used for on-site generation, resulting in a reduction of losses in the transportation of energy. Typical energy losses in natural gas transmission systems range from 1% to 2%; in electricity transmission they range from 5% to 8%.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 7 misurazione di composizione di biogas e biometano Current gas testing protocols

 Calorimetric: measurement of small variations over a predefined gas composition Combustion based; unable to resolve contaminants; for clean NG.

 Measurement of slight Wobbe Index variations, suitable for energy billing purposes.

 Electrochemical/NDIR/sensor fusion: measurement of single components through individual sensors Some of the used techniques are sensitive to contamination or require frequent recalibration; unable to sense nitrogen; good for trace-level contaminant detection in biogas.

 Measurement of ammonia and hydrogen sulfide levels for process control.

 Gaschromatographic: complete gas analysis Resolution of ppm-range contaminants; requires periodic maintenance and carrier gas (e.g. He) supply.

 Measurement of complete gas composition and contaminant detection, more expensive, often done remotely after sampling

L. Cocola, F. Melison et al. - Strumentazione analitica per la 8 misurazione di composizione di biogas e biometano Raman spectroscopy

• A candidate for composition analysis of an arbitrary gas mixture is Raman spectroscopy • Non-contact (only optical access to gas sample required)

• Sensitive to diatomic gases (H2, N2) • Weak signals (need for efficient and powerful laser sources) 3D printed nylon – alpha cell prototype

L. Cocola, F. Melison et al. - Strumentazione analitica per la 9 misurazione di composizione di biogas e biometano Preliminary study setup

• Experimental setup • 2W 532nm (2f DPSS Nd:YAG, Lasever); • Custom gas interaction cell; • Custom high-throughput lens-based spectrometer, coupled to CMOS industrial camera; • Normalization photodiode.

Beam dump

Edge filter

Gas in Slit

Gas out

Focusing lens Laser in L. Cocola, F. Melison et al. - Strumentazione analitica per la 10 misurazione di composizione di biogas e biometano Spectrometer design

• An optimized high brightness miniature spectrometer was custom made for the application • f/2.4 • dispersion ab. 10nm/mm • Lens-based, Czerny-Turner design • Imaging of a 6mm high slit • Compatible with different detectors • Astronomical-grade cameras • C-mount industrial cameras

L. Cocola, F. Melison et al. - Strumentazione analitica per la 11 misurazione di composizione di biogas e biometano Preliminary calibration

• The instrument was calibrated:

Molecul CH4 C2H6 C3H8 i-C4H10 n-C4H10 CO2 O2 N2 H2O CO H2 e Ref. 99.5% 99,5% 50% 7,81% 7.99% 500ppm 21% 78% 2% 5% 4% value ± 2% ± 2% ± 2% ± 2% ± 2% ± ± 0.5% ± 2% ± 2% rel. rel. rel. rel. rel. 50ppm rel. rel. Source sample sample sample sample sample measur avg. val. avg. val. measur sample sample in N2 in N2 in N2 ed in air in air in air ed in air in N2 in N2

L. Cocola, F. Melison et al. - Strumentazione analitica per la 12 misurazione di composizione di biogas e biometano Calibration data

• • Methane Calibration data for HCs obtained with test mixtures cylinders; • Ethane • Propane • Operation at ambient pressure; • n-butane • MATLAB fitting routine was developed • i-butane for improved performance on mixtures

mainly containing CH4

L. Cocola, F. Melison et al. - Strumentazione analitica per la 13 misurazione di composizione di biogas e biometano Tests on sample gas mixtures • Bottled mixtures simulating the NG mixtures from different wells were sampled and evaluated with the instrument

• The measurement routine works by scaling and (small) shifting calibration spectra to fit experimental data.

• The instrument proved to perform higher heating value estimation (according to ISO 6976) accurate within 0.5% when compared with a certified gas-chromatograph output.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 14 misurazione di composizione di biogas e biometano Tests on sample gas mixtures

• Measurements taken at 532nm, 1W optical power.

• CMOS camera, uncooled, 4s integration time.

• 1bar abs.

Mix HHV nominal (kJ/ HHV avg. 4s measurement Error (%) m3) detected (kJ/m3) std. dev. (kJ/m3)

P17 37712 37690 20 0.058 P18 38356 38322 21 0.089 P19 38800 38810 20 -0.026 P20 40567 40559 27 0.020

L. Cocola, F. Melison et al. - Strumentazione analitica per la 15 misurazione di composizione di biogas e biometano Using a diode pump

High power diode lasers are available up to several watts output, featuring:

• High wallplug efficiency (~30%) • No active cooling needed • Wavelengths in the blue region • High Raman efficiency (goes with λ-4) • Very low cost (~40€)

• No single mode operation • Wavelength and power drifting with temperature • Low beam quality

L. Cocola, F. Melison et al. - Strumentazione analitica per la 16 misurazione di composizione di biogas e biometano Using a diode pump

• Collection geometry is similar to the one used with the DPSSL; • Max 3W optical power at 450nm; • Same custom spectrometer; • Same CMOS detector; • Normalization photodiode on the pump output.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 17 misurazione di composizione di biogas e biometano Tests with a diode laser

A 450nm 3W laser diode (Osram) was used as a pump

• Lab-grade Peltier mount allowed an evaluation of the temperature effects on Raman spectra. • The pump linewidth is already narrow enough to resolve hydrocarbon lines.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 18 misurazione di composizione di biogas e biometano Temperature compensation

Temperature drifts can be compensated via calibration and resampling of the image data

• Stray light artifacts due to the fixed wavelength pump rejection filter must be taken into account. • Proper design of the gas interaction cell keeps stray light down.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 19 misurazione di composizione di biogas e biometano Performance with diode laser (NG)

HHV [MJ/m3] Test performed with ambient pressure P24 mix LD Nominal (P24 mix): 37.83 (°C) • Detection of • Methane 30°C Estimated: 37.84 • Ethane Deviation [%]: 0.026 • Propane • i-Butane 40°C Estimated [%]: 37.79 • N-butane Deviation [%]: -0.105 • Carbon dioxide • Nitrogen 50°C Estimated [%]: 37.85 Deviation [%]: 0.053 • 4s integration time • About 2W diode output power 60°C Estimated [%]: 37.88 Deviation [%]: 0.130

70°C Estimated [%]: 37.87 L. Cocola, F. Melison et al. - Strumentazione analitica per la Deviation [%]: 0.105 20 misurazione di composizione di biogas e biometano Performance with diode laser (biogas)

Biogas sample from organic fraction of municipal waste Test performed with ambient pressure, Composition CH CO O N BAL PCS RMSE 4 2 2 2 50°C source temperature Ref. 56,200 40,700 0,200 0,000 2,900 21,256 Measurement 59,082 40,918 0,000 0,000 0,000 22,349 • Detection of: Deviation 2,882 0,218 -0,200 0,000 -2,900 5,143 2,049 • Biogas sample from agicultural waste and plant material Methane • Oxygen Composition CH4 CO2 O2 N2 BAL PCS RMSE Ref. 51,400 45,400 0,000 0,000 3,200 19,440 • Carbon dioxide Measurement 54,661 44,904 0,000 0,435 0,000 20,680 • Nitrogen Deviation 3,261 -0,496 0,000 0,435 -3,200 6,377 1,664 Calibrated bottle • 4s integration time Composition CH CO O N BAL PCS RMSE 4 2 2 2 • About 2W diode output power Ref. 64,800 35,200 0,000 0,000 0,000 24,510 Measurement 65,092 34,908 0,000 0,000 0,000 24,618 • Reference methods: Deviation 0,292 -0,292 0,000 0,000 0,000 0,439 0,207 • Geotech 5000 (field measurement) for sampled gases • Bottle certification

L. Cocola, F. Melison et al. - Strumentazione analitica per la 21 misurazione di composizione di biogas e biometano Results and further steps

With a proper calibration, the pump source can work at least over a 30°C to 70°C temperature range while being well within the 0.5% HHV accuracy specification for NG measurement. The instrument provides actual nitrogen and water vapour measurement which is not provided by electrochemical reference methods used on biogas. Water vapour should be measured online (biogas) due to the high value, close to saturation levels. Biogas sampling with bags proved to be tricky; careful evacuation of the bag is important and sample storage time is limited to <24h

• A simplified calibration technique is under development to reduce the amount of calibration data required.

• A deconvolution of the image data may help improving resolution for a more generic (analytic) instrument application and to simplify calibration.

L. Cocola, F. Melison et al. - Strumentazione analitica per la 22 misurazione di composizione di biogas e biometano Spectrometer engineering

A simplified optical layout is under test towards a low power, portable industrial prototype.

• An integrated cell-spectrometer assembly has been realized to reduce cost and complexity while improving mechanical stability; • Spectral dispersion exceeded useful resolution; a smaller spectrometer has been developed and is now under test for portable operation. • Free running, passively cooled laser source

L. Cocola, F. Melison et al. - Strumentazione analitica per la 23 misurazione di composizione di biogas e biometano Performance with integrated spectrometer Natural Gas sample

Composition N2 CH4 C2H6 C3H8 n-C4H10 i-C4H10 CO2 O2 PCS RMSE Ref. 4,030 84,427 5,030 1,990 0,000 0,513 4,010 0,000 37,740 First tests were performed Measurement 4,120 83,948 4,619 1,312 0,268 0,993 4,301 0,440 37,560

Deviation 0,090 -0,479 -0,411 -0,678 0,268 0,480 0,291 0,440 -0,477 0,401 on NG, methane-hydrogen Methane + Hydrogen (20%) mixtures and biogas from

Composition N2 CH4 C2H6 C3H8 n-C4H1 i-C4H10 CO2 O2 H2 PCS RMSE Ref. 0,000 80,190 0,000 0,000 0,000 0,000 0,000 0,000 19,810 32,670 municipal waste. Measurement 0,000 80,570 0,000 0,000 0,000 0,000 0,000 0,000 19,430 32,767 Deviation 0,000 0,380 0,000 0,000 0,000 0,000 0,000 0,000 -0,380 0,298 0,170 Methane + Hydrogen (10%)

Composition N CH C H C H n-C H i-C H CO O H PCS RMSE 2 4 2 6 3 8 4 1 4 10 2 2 2 Reference measurements Ref. 0,000 89,890 0,000 0,000 0,000 0,000 0,000 0,000 10,110 35,169 Measurement 0,000 90,023 0,000 0,000 0,000 0,000 0,000 0,000 9,977 35,204 provided by bottle Deviation 0,000 0,133 0,000 0,000 0,000 0,000 0,000 0,000 -0,133 0,099 0,060 certification or GC sample Biogas from organic fraction of municipal waste

Composition CH CO O N PCS RMSE 4 2 2 2 measurement Ref. 58,900 35,830 1,010 4,260 22.270 Measurement 56,714 38,729 0,842 3,715 21.449 Deviation -2,186 2,900 -0,168 -0,545 -3.687 1.838

L. Cocola, F. Melison et al. - Strumentazione analitica per la 24 misurazione di composizione di biogas e biometano Thanks for your attention!

L. Cocola, F. Melison et al. - Strumentazione analitica per la 25 misurazione di composizione di biogas e biometano