laser-based standoff methane sensors for enhancing coal miner safety

Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

Physical

Sciences Inc.

Laser-Based Standoff Methane Sensors for

Enhancing Coal Miner Safety

Mickey Frish1, Clinton Smith1, Richard Wainner1 , James Rutherford2, Steve Chancey2, Paul Wehnert2

1Physical Sciences Inc., 20 New England Business Center, Andover MA2Heath Consultants 9030 Monroe Rd., Houston TX

Pittcon 2015New Orleans, LA

March 8, 2015

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This material is based upon work supported by The Centers for Disease Control and Prevention. Any opinions, findings and conclusions

or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the CDC.

Physical Sciences Inc.

Overview

Motivation

– Enhance miner safety by supplementing current methane measurement

practices with remote methane detection

Technology: Backscatter Laser Sensor

– Hand-held sensor locates areas of increased methane

• Demonstrated in research and working mines.

• Standoff range up to > 150 ft.

• Measures path integrated concentration.

– Sensor pair maps methane concentrations via tomography

• Map actual concentration.

• Demonstrated in coal mine face simulation lab.

Summary

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During mining operations, methane emanates from the sidewalls and

ceilings.

– Can create pockets that are explosion or breathing hazards

Point sensors utilized for methane measurement might not detect

pockets trapped in areas that are difficult to access

Remote methane detection can provide additional knowledge

regarding the location, volume and extent of methane pockets,

enhancing miner safety.

Benefit of Remote Methane Detection

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Technology Platform:

The Remote Methane Leak Detector (RMLD)

Like a flashlight, laser beam

Illuminates a surface up to

150 feet distant

Senses target gas between

surveyor and illuminated surface

>3000 in use for natural gas (CH4) distribution pipeline leak surveying– leak sensitivity comparable to commonly-used flame ionization detectors

Being adopted for CO2 pipeline monitoring at sequestration sites

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Handheld Use Scenarios

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Portable tool warns before entering a hazardous area

− Including through doors and windows

Searches for methane pockets in inaccessible areas such

as high roof

Manually scans mine face for abnormally high methane

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Tests in Research Coal Mine

Hung a transparent 1 m2 Tedlar bag against a representative coal

mine wall

– Filled w/ 2.1% methane, nominal 12.5 cm path ~2600 ppm-m.

– Directed laser beam at selected wall areas around and through the bag at

ranges of 50 ft., 75 ft., 100 ft., and 150 ft.

– Achieved acceptable laser return signal and detected the increased

methane from all ranges

– Established good return signals for different wall compositions

Performed a controlled detection test using 2.1% methane released

from pressurized cylinder

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Targeting a Representative Coal Mine Wall

1) Brown

stone

2) Grey

stone

3) Brown/grey

stone

4) 2.1%

methane

bag5) coal

8) coal

7) coal6)

Brown

stone

Laser beam was directed at each of the 8 locations

Surface of coal mine face was wet

Measurements made from distance of 50, 75, 100, 150 ft.

Backscatter signal strengths are sufficient to detect methane from

distances > 200 ft.

1) 2)

3)4)

5)

6) 7)

8)

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Detected Methane from Remote Distance

2.1% methane concentration in bag against mine wall

– Bag thickness (i.e., methane path) is ~5 inches

Able to detect the methane at all distances measured

2.1% methane pocket (5” deep) detectable from >200 ft.

– Power law extrapolation based on return signal

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4000

3500

3000

2500

2000

1500

1000

500

050 70 90 110 130 150

Distance From Wall (ft.)

0

10

20

30

40

50

60

70

80

90

2

1

r

1f at Methane Bag Vs. DistancePath Integrated Concentration ofMethane Bag Vs. Distance

50 70 90 110 130 150

Distance From Wall (ft.) L-3129

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Rock Types Have Little Impact on Signals

Aimed RMLD away from the methane bag

Brown stone, grey stone, and coal all return similar laser power

– Path integrated concentration increases with distance due to ambient methane

• Positions #5, #6, #8 exhibit anomalies at 150 ft. range due to laser beam sampling

only a portion of the methane bag

– Beam diameter @ 150’ is ~75 cm

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1200

1000

800

600

400

200

050 90 110 130 150


70 50 90 110 130 150


700

20

4060

80

100

120140

160

Methane Concentration for Different Surfaces Vs. Distance

L-3130

1. Brown stone 2. Grey stone 3. Brown/grey stone 5. Coal

6. Brown stone 7. Coal 8. Coal

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Controlled 2.1% Methane Release:

Search for Source

Search wall area away

from sourceSearch wall area away

from source

Methane Pocket

Concentration increases

near methane source

Manually scanned from 50 ft. range

Methane pocket created by release from bottle was readily detected

Methane released in back corner

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Dual-Sensor Tomography

Operator scans two tripod-mounted laser beams across the mine face

– Employs two sensors on rotational stages to make in-plane horizontal scans over the region of

interest.

– Pathlength retrieval with rangefinders.

Log signals to rugged laptop for software processing

A Graphical User Interface computer displays methane spatial concentration

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Tomography: Coal Mine Face Simulation Lab Tests

• Performed tomographic scans with varying

flow rates and leak sources (2.1% methane)

− 20 ft. distance from wall

− Single source (0.5 L/min)

− Single source (>2 L/min)

− Three sources (> 2 L/min)

• 2.1% methane flow at 2 L/min is equivalent

to 0.08 scfh pure methane

− Less than a pilot light

X2X1

X=RMLD + laser rangefinder

mounted on tripod (~4ft. High)

40

ft.

ro

om

de

pth

16.5 ft. room width

Control cables

Simulated coal face with leak(s)

single source

Three sources

X1

X2

leak sources

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Tomographic Scan of Single Leak Source – 20 ft. away

Single source high flow (>2L/min.)Single source low flow (0.5 L/min.)

Leak source is located in the middle of the face; Background methane is less than 15 ppmv

– Methane collects near far wall (conforms with historical observation)

Tomographic low flow peak is ~25 ppmv; high flow peak is ~60 ppmv

– Note: These values are 2-3 orders-of-magnitude below concentrations of concern at coal

mine faces

Electrochemical sensors deployed for comparison

– Detected high flow (~2% methane) when placed at leak source

• Could not detect methane several inches from leak source

– Could not detect low (0.5L/min.) flow

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Three sources high flow (sources near scan plane)

Tomographic Scan of Three Leak Sources – 20 ft. away

X2X1



40

ft.

ro

om

de

pth

16.5 ft. room width

Control cables

Simulated coal face with leak(s)

Three sources

2.1% methane; Flow rate > 2 L/min.

Peak of 0.01% (100 ppm) detected from stronger (upstream) source

– All three sources are distinguishable

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Coal Mine Face Simulation Lab Tests – 40 ft. away

X2X1



40

ft.

ro

om

de

pth

16.5 ft. room width

Control cables

2.1% methane flow rate of > 2 L/min.

Peak of 0.08% (800 ppm) detected from stronger

(upstream) source

Two sources are distinguishable

Three sources high flow (sources near scan plane)

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Summary

1. Successful remote methane detection inside an experimental mine

– Single laser backscatter sensor readily identified areas of increased

methane from distances > 150 ft.

– Successfully located the source of a controlled 2.1% methane release from a

distance of 50 ft.

• Tests were done against wet coal faces and a fresh coal mine face

2. Tomographic scans located and quantified methane near

simulated mine faces from distances of 20 ft. to 40 ft.

– Identified releases of 2.1% methane at 0.5 L/min and 2 L/min

– Sensitivity > 100x better than concentrations of concern in mines

– New application for backscatter TDLAS laser sensor systems.

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Acknowledgments

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Research funded by CDC/NIOSH Contract #200-2011-40565

and Heath Consultants Inc.

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Questions?

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Machine Mounted Mapping Tool Scenario

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Automated tool installed on mining machine that continuously

maps methane concentrations at mine face

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Absorption Spectroscopy Fundamentals

• Gas molecules absorb light at

specific colors (“absorption lines”)

Beer-Lambert law

Iν = Iνo exp [S(T) g( - o) N]

where:

= optical frequency (= c/l)

o = line center frequency

g() = lineshape function

= path length

N = absorbing species number density

S(T) = temperature dependent linestrength

Iνo = unattenuated laser intensity

I = laser intensity with absorption

DI = change in intensity (= I0-I)

c = speed of light

l = wavelength of light

Absorbance = - ℓn (I /Io)

≈ DI/I0 ( with small DI)

CH4 Spectrum

Mid-IR fundamental

Near-IR overtone

200x linestrength advantage is

the mid-IR appeal

(when deployed judiciously)

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laser-based standoff methane sensors for enhancing coal miner safety

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