MINE GAS DITECTORS

INTRODUCTION

At present the mine gas concentration is detected by fixed gas sensor which mounted

at fixed locations in China coal mine, and then connect to the working station through the

underground cable ,at last connect to the monitoring center. With the extension of the mining

face, The distance between the main roadway and the mining face can stretch to several

hundred meters or several kilometers, a large number of gas emission will cause gas

overrunning and abdominal mass near the mining face in the process of mining, The gas

concentration can not be detected effectively in the conditions of movement and the on-site

maintenance of big mechanism equipments in the temporary working location, including

laying the communication lines out of time, sensor can not meet the requirements of dynamic

detection, real-time transmission and rapid deployment

A gas detector is a device that detects the presence of gases in an area, often as part of

a safety system. This type of equipment is used to detect a gas leak and interface with a

control system so a process can be automatically shut down. A gas detector can sound an

alarm to operators in the area where the leak is occurring, giving them the opportunity to

leave. This type of device is important because there are many gases that can be harmful to

organic life, such as humans or animals.

Gas detectors can be used to detect combustible, flammable and toxic gases,

and oxygen depletion. This type of device is used widely in industry and can be found in

locations, such as on oil rigs, to monitor manufacture processes and emerging technologies

such as photovoltaic. They may be used in firefighting.

Gas leak detection is the process of identifying potentially hazardous gas leaks by

sensors. These sensors usually employ an audible alarm to alert people when a dangerous gas

has been detected. Common sensors include infrared point sensors, ultrasonic

sensors, electrochemical gas sensors, and semiconductor sensors. More recently, infrared

imaging sensors have come into use.

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Methane gas detection is very important because this gas is not only an explosive but

also acts as asphyxiant. It can therefore cause serious harm to you and your family's health

and home. Methane gas detection was pioneered in the twentieth century by coal miners who

used canary birds that would pass out if there were any spike in the amount of methane in the

atmosphere. Thankfully, methods of methane gas detection have moved on from this

primitive means of highlighting its presence in the air. There are now a number of methods

and devices you can use for methane gas detection. Before discussing these methods of

methane gas detection, let us look at why the gas poses such a threat to human health.

Methane can lead to explosions in coal shafts - the gas trapped in the rock can be released as

a result of mining - and also fires in landfill sites as a result of a process known as

methanogenesis. In the home, methane is found in the natural gas used for cooking. There is

also a risk of methane leaking into your home from sewer pipes. Methane in the atmosphere

is dangerous because our lungs only function normally when the atmospheric concentration

of oxygen is more than twenty per cent. If the level falls below this, including as a result of it

being replaced by methane, it can trigger asphyxiation and if undetected will ultimately lead

to death. Let us now turn to the various methods of methane gas detection. As methane is

odorless and colorless, it can be very difficult to detect. A methane gas detector is by far the

best method of alerting yourself to the presence of this gas in the air around you. Methane gas

detectors are normally portable so that you can carry them around in your kitchen or near

sewers to pick up the presence of methane gas. If you use a fixed methane gas detector it

should be installed high up near the ceiling because methane being lighter than oxygen rises

to the top of whatever space it is in. Methane (CH4) is the principal component (97%) of

natural gas. Methane is a colorless, odorless gas, which is lighter than air. It is formed by the

decomposition of organic carbons under anaerobic conditions. Methane is abundant in nature

and thus a desirable fuel. However, since it is a gas at normal temperature and pressure, it is

difficult to transport from its source. It is generally transported in bulk by pipeline or

liquefied natural gas (LNG) carriers. Natural gas is found with other fossil fuels, in coal beds,

as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and

landfills. It is an important fuel source, a major feedstock for fertilizers, and unfortunately a

potent greenhouse gas. Methane gas is flammable and therefore should be monitored in

enclosed or underground spaces such as mines or power plant. It also poses the danger of

asphyxiation, as it displaces oxygen. Also carbon monoxide is a byproduct of methane, so

proper ventilation is critical.

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MINE GASES The Earth’s atmosphere is made up of a mixture of gases. On the surface of the Earth,

the ratio of these gases stays the same. Surface events, such as fires, explosions, volcanic

eruptions and even animals and plants breathing, can have an effect on the proportions of

these gases in the air. These effects are usually short-lived and will only affect a small area,

because unusual gas concentrations are quickly dispersed by wind and air currents.

Coal mines are confined spaces, and abnormal gases do not always have the chance to

disperse in the same way. Some gases may build up in the mine, and can cause danger to

people working underground.

BLACKDAMP AND CARBON DIOXIDE Blackdamp is a mining term for a build-up of carbon dioxide. This gas is not

poisonous, but because it replaces oxygen in the air, it means that people have nothing to

breathe and can suffocate.

Carbon dioxide can form due to oxidation, where coal that is in contact with the air

uses up the oxygen to produce carbon dioxide. It is not usually a problem when mine

workings are well ventilated. It is heavier than air, so will lie in lower areas of a mine or in

old, disused roadways.

FIREDAMP AND METHANE

Firedamp is flammable gas found in coal mines. It is the name given to a number of

flammable gases, especially methane. It is particularly found in areas where the coal

isbituminous. The gas accumulates in pockets in the coal and adjacent strata, and when they

are penetrated, the release can trigger explosions. Historically, if such a pocket was highly

pressurized, it was termed a "bag of foulness".

Methane gas is known as firedamp to miners. As this name suggests, methane can

burn and in certain conditions can cause explosions. This gas is formed with coal over

millions of years. It can be released as coal is mined, when pockets of gas will seep into the

pit.

Explosive mixtures of methane can form, and a naked flame, overheated machine, or

a spark might then cause an explosion. There are very strict rules about what can be taken

underground and about care of working machines.

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Firedamp is explosive at concentrations between 4% and 16%, with most explosions occurring at around 10%. It caused much loss of life in coal mines before the invention of theGeordie lamp and Davy lamp. The invention was prompted by the Felling mine disaster near Newcastle upon Tyne claiming 92 lives on 25 May 1812. Davy experimented withiron gauze, determining the maximum size of the gaps and the optimum wire thickness to prevent a flame passing through the gauze. If a naked flame was thus enclosed totally by such a gauze, then methane could pass into the lamp and burn safely above the flame. He did not patent his invention.

COALBED METHANE (CBM OR COAL-BED METHANE)

Coalbed gas, coal seam gas (CSG), or coal-mine methane (CMM) is a form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, Australia, and other countries.

The term refers to methane adsorbed into the solid matrix of the coal. It is called 'sweet gas' because of its lack of hydrogen sulfide. The presence of this gas is well known fromits occurrence in underground coal mining, where it presents a serious safety risk. Coalbed methane is distinct from a typical sandstone or other conventional gas reservoir, as the methane is stored within the coal by a process called adsorption. The methane is in a near-liquid state, lining the inside of pores within the coal (called the matrix).

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The open fractures in the coal (called the cleats) can also contain free gas or can be saturated with water.Unlike much natural gas from conventional reservoirs, coalbed methane contains very little heavier hydrocarbons such as propane or butane, and no natural-gas condensate. It often contains up to a few percent carbon dioxide.

COAL MINE METHANE (CMM)Coal mine methane (CMM) is a type of gas present in active, working mine sites. This

gas is extracted from the air in the coal mine helping improve safety and preventing

uncontrolled release of methane to atmosphere. CMM is a mixture of methane & air released

during the process of coal mining and must be vented for safety reasons. Methane has

significant effects as a greenhouse gas being 21 times higher than that of carbon dioxide,

therefore its capture and use in gas engines has significant environmental benefits. CMM

typically has an oxygen content of 5-12%. The methane content ranges from 25-60%.

However, the methane/air proportion can change suddenly, thus complicating its use in gas

engines

ABANDONED MINE METHANE (AMM)

Even after coal mines are shut down, coal mine gas continues to be released. Coal

mine gas from abandoned mines typically contains no oxygen, and its composition changes

slowly. The methane content ranges from 60-80%.

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UNDERGROUND COAL GASIFICATION (UCG)

Underground Coal Gasification is an industrial process by which coal is gasified in

situ. This process converts the physical coal to a product gas (a type of synthetic/syngas).

Synthetic gases are covered more on this site on their dedicated syngas page found here.

GAS ENGINES AND COAL GAS

The composition of CBM and AMM presents no technical difficulties for combustion

in gas engines. Although the sudden changes in the composition of coal mine gas from active

mining (CMM) put greater demands on engine design. GE offers specially modified gas

engines that make efficient use of this gas for power generation. The electrical energy

generated can be used in the coal mine to meet electricity requirements or fed into the public

power grid. The thermal energy can be used for heating purposes on site or fed into a district

heating scheme.

ADSORPTION CAPACITY Adsorption capacity of coal is defined as the volume of gas adsorbed per unit mass

of coal usually expressed in SCF (standard cubic feet, the volume at standard pressure and

temperature conditions) gas/ton of coal. The capacity to adsorb depends on the rank and

quality of coal. The range is usually between 100 to 800 SCF/ton for most coal seams found

in the US. Most of the gas in coal beds is in the adsorbed form. When the reservoir is put into

production, water in the fracture spaces is pumped off first. This leads to a reduction of

pressure enhancing desorption of gas from the matrix

TYPES OF GAS DETECTORS

Gas detectors can be classified according to the operation mechanism (semiconductors, oxidation, catalytic, infrared, etc.). Gas detectors come packaged into two main form factors: portable devices and fixed gas detectors.

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Portable detectors are used to monitor the atmosphere around personnel and are worn on clothing or on a belt/harness. These gas detectors are usually battery operated. They transmit warnings via audible and visible signals, such as alarms and flashing lights, when dangerous levels of gas vapors are detected.

Fixed type gas detectors may be used for detection of one or more gas types. Fixed type detectors are generally mounted near the process area of a plant or control room, or an area to be protected, such as a residential bedroom. Generally, industrial sensors are installed on fixed type mild steel structures and a cable connects the detectors to a SCADAsystem for continuous monitoring. A tripping interlock can be activated for an emergency situation.

ELECTROCHEMICAL GAS SENSOR

Electrochemical gas detectors work by allowing gases to diffuse through a porous

membrane to an electrode where it is either chemically oxidized or reduced. The amount of

current produced is determined by how much of the gas is oxidized at the

electrode, indicating the concentration of the gas. Manufactures can customize

electrochemical gas detectors by changing the porous barrier to allow for the detection of a

certain gas concentration range. Also, since the diffusion barrier is a physical/mechanical

barrier, the detector tended to be more stable and reliable over the sensor's duration and thus

required less maintenance than other early detector technologies.

last only 1–2 years before a replacement is required. Electrochemical gas detectors are

used in a wide variety of environments such as refineries, gas turbines, chemical plants,

underground gas storage facilities, and more.

The sensors contain two or three electrodes, occasionally four, in contact with

an electrolyte. The electrodes are typically fabricated by fixing a high surface area precious

metal on to the porous hydrophobic membrane. The working electrode contacts both the

electrolyte and the ambient air to be monitored usually via a porous membrane.

The electrolyte most commonly used is a mineral acid, but organic electrolytes are

also used for some sensors. The electrodes and housing are usually in a plastic housing which

contains a gas entry hole for the gas and electrical contacts.

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INFRARED POINTInfrared (IR) point sensors use radiation passing through a known volume of gas; energy from the

sensor beam

is

absorbed at certain wavelengths, depending on the properties of the specific gas. For example, carbon

monoxide absorbs wavelengths of about 4.2-4.5 μm. The energy in this wavelength is compared to a

wavelength outside of the absorption range; the difference in energy between these two wavelengths is

proportional to the concentration of gas present.

This type of sensor is advantageous because it does not have to be placed into the gas to detect it

and can be used for remote sensing. Infrared point sensors can be used to detect hydrocarbons[6] and other

infrared active gases such as water vapor and carbon dioxide. IR sensors are commonly found in waste-

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water treatment facilities, refineries, gas turbines, chemical plants, and other facilities where flammable

gases are present and the possibility of an explosion exists. The remote sensing capability allows large

volumes of space to be monitored.

Engine emissions are another area where IR sensors are being researched. The sensor would detect

high levels of carbon monoxide or other abnormal gases in vehicle exhaust and even be integrated with

vehicle electronic systems to notify drivers

INFRARED IMAGING

Infrared imaging sensors include active and passive systems. For active sensing, IR

imaging sensors typically scan a laser across the field of view of a scene and look for

backscattered light at the absorption line wavelength of a specific target gas. Passive IR

imaging sensors measure spectral changes at each pixel in an image and look for

specific spectral signatures that indicate the presence of target gases. The types of compounds

that can be imaged are the same as those that can be detected with infrared point detectors,

but the images may be helpful in identifying the source of a gas.

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Semiconductor sensors detect gases by a chemical reaction that takes place when the

gas comes in direct contact with the sensor. Tin dioxide is the most common material used in

semiconductor sensors, and the electrical resistance in the sensor is decreased when it comes

in contact with the monitored gas. The resistance of the tin dioxide is typically around 50 kΩ

in air but can drop to around 3.5 kΩ in the presence of 1% methane. This change in resistance

is used to calculate the gas concentration. Semiconductor sensors are commonly used to

detect hydrogen, oxygen, alcohol vapor, and harmful gases such as carbon monoxide. One of

the most common uses for semiconductor sensors is in carbon monoxide sensors. They are

also used in breathalyzers Because the sensor must come in contact with the gas to detect it,

semiconductor sensors work over a smaller distance than infrared point or ultrasonic

detectors.

ULTRASONIC

Ultrasonic gas detectors use acoustic sensors to detect changes in the background

noise of its environment. Since most high-pressure gas leaks generate sound in the ultrasonic

range of 25 kHz to 10 MHz, the sensors are able to easily distinguish these frequencies from

background acoustic noise which occurs in the audible range of 20 Hz to 20 kHz. The

ultrasonic gas leak detector then produces an alarm when there is an ultrasonic deviation from

the normal condition of background noise.

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Ultrasonic gas leak detectors cannot measure gas concentration, but the device is able

to determine the leak rate of an escaping gas because the ultrasonic sound level depends on

the gas pressure and size of the leak.

Ultrasonic gas detectors are mainly used for remote sensing in outdoor environments

where weather conditions can easily dissipate escaping gas before allowing it to reach leak

detectors that require contact with the gas to detect it and sound an alarm. These detectors are

commonly found on offshore and onshore oil/gas platforms, gas compressor and metering

stations, gas turbine power plants, and other facilities that house a lot of outdoor pipeline.

HOLOGRAPHIC

Holographic gas sensors use light reflection to detect changes in a polymer film

matrix containing a hologram. Since holograms reflect light at certain wavelengths, a change

in their composition can generate a colorful reflection indicating the presence of a gas

molecule. However, holographic sensors require illumination sources such as white light

orlasers, and an observer or CCD detector.

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A holographic sensor aims to integrate the sensor component, the transducer and the display in one device for fast reading of molecular concentrations based in colorful reflections or wavelengths

Certain molecules that mimic biomolecule active sites or binding sites can be incorporated into the polymer that forms the holographic film in order to make the holographic sensors selective and/or sensitive to certain medical important molecules like glucose, etc.

The holographic sensors can be read from a fair distance  because the transducer element is light that has been refracted and reflected by the holographic grating embedded in the sensor. Therefore they can be used in industrial applications where non-contact with the sensor is required. Other applications for holographic sensors are anti counterfeiting

CALIBRATIONAll gas detectors must be calibrated on a schedule. Of the two form factors of gas

detectors, portables must be calibrated more frequently due to the regular changes in

environment they experience. A typical calibration schedule for a fixed system may be

quarterly, bi-annually or even annually with more robust units. A typical calibration schedule

for a portable gas detector is a daily "bump test" accompanied by a monthly calibration.

Almost every portable gas detector requires a specific calibration gas which is available from

the manufacturer. In the US, the Occupational Safety and Health Administration (OSHA)

may set minimum standards for periodic recalibration.

CHALLENGE (BUMP) TEST

Because a gas detector is used for employee/worker safety, it is very important to make sure it is operating to manufacturer's specifications. Australian standards specify that a person operating any gas detector is strongly advised to check the gas detector's performance each day and that it is maintained and used in accordance with the manufacturers instructions and warnings.

A challenge test should consist of exposing the gas detector to a known concentration of gas to ensure that the gas detector will respond and that the audible and visual alarms activate. It is also important inspect the gas detector for any accidental or deliberate damage by checking that the housing and screws are intact to prevent any liquid ingress and that the filter is clean, all of which can affect the functionality of the gas detector. The basic calibration or challenge test kit will consist of calibration gas/regulator/calibration cap and hose (generally supplied with the gas detector) and a case for storage and transport. Because 1 in every 2,500 untested instruments will fail to respond to a dangerous concentration of gas, many large businesses use an automated test/calibration station for bump tests and calibrate their gas detectors daily.

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METHODS OF METHANE GAS DETECTION

A PORTABLE REMOTE METHANE DETECTOR

Gas distribution companies use portable instruments for detecting natural gas leaks, in

support of their emergency response and surveillance services. The conventional method used

to detect gas leaks involves positioning instruments in close proximity to the area to be

checked. However, this can be a difficult operation and often entails lengthy inspection

periods particularly in elevated or narrow locations. To overcome these difficulties, optical

methods, particularly laser based methods, were studied by lots of groups in the gas industry.

The laser based methods provides us with remote detection of methane leaks and

thereby improves the operational efficiency and safety levels of the natural gas distribution

facilities. In particular, Tunable Diode Laser Absorption Spectroscopy (TDLAS) is a

promising method to lead a compact and costeffective remote methane detector. In previous

works, the author and his research group presented a portable remote methane detector based

on TDLAS. To the best of the author’s knowledge, this detector was world’s first product that

is person-portable and capable of remote detection of methane leaks. In the present work,

Tokyo Gas Co., Ltd. and Anritsu Corporation jointly developed a new version of the detector.

The author and his research group improved the userfriendliness and cost-effectiveness

dramatically from the old version.

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CONCEPT OF REMOTE DETECTION

The device transmits an infrared (IR) laser beam with the wavelength set at one of the

absorption wavelength (absorption line) of methane. It then receives a fraction of the

backscatter reflected from the target. In this configuration, the received power can be

expressed by the Lambert-Beer law as use as strong an absorption line as possible. Methane

has two strong absorption bands, or groups of absorption lines, centered at 3.3 μm (ν3 band)

and 7.6 μm (ν4 band). However since a near infrared diode laser is used for cost

effectiveness, the available laser wavelength is limited lower than 2.2 μm. Below 2.2 μm, the

strongest absorption band of methane is located at 1.64 to 1.70 μm (2ν3 band).

WAVELENGTH MODULATION SPECTROSCOPY

Laser based Methane has to measure very little power since it collects limited diffused

reflections from a target. In a typical case, for example, it will receive as little as 100 nW

from an initial laser power of 10 mW. In addition, it has to detect very weak absorptions. For

example, 100 ppmm methane corresponds to an optical depth of less than 10-4. These are

significant technical challenges of remote methane detection using a near infrared diode laser.

To overcome them, Laser based Methane employs the second-harmonic detection of

wavelength modulation spectroscopy (WMS).

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METHANE GAS DETECTION USING DIFFERENTIAL ABSORPTION

The Institute of Optoelectronics Military University of Technology developed the

prototype of laser system for gas detection using differential absorption of radiation

backscattered from topographic targets. Both gas lasers were excited by RF discharge. We

have obtained the output power of about 10 mW for He - Ne laser for lengths of 50 cm. The

presence of methane on the distance up to 50 m can be measured by using receiver optics

with the diameter of 7 cm and thermocooled HgCdTe detector. The new solution is under

construction. In order to increase the range of measurement Casseigrain optics with diameter

of 30 cm is being prepared. Using the special construction of gas lasers with the output power

of 30 mW, the measurement distance of 200 m is expected. In case of remote monitoring,

taking into account the possibility of long-range sensing, system should operate in so called

atmosphere absorption windows. Comparing the typical characteristic of atmosphere

transmittance with the absorption bands of methane, it can be easily found, that its

fundamental absorption band of n4 = 1306 cm-1 (7.66 mm) coincide with the band of strong

water vapour absorption as well as that only the n3 band (3020cm-1 or 3.31 mm) lies in the

transmission window. Other absorption bands of methane, representing combinations or

overtons of fundamental bands, for example n2 + 2n3 (1.3 mm), 2n3 (1.6 mm) are very weak,

with absorption strength of single percents of fundamental ones. On the basis of complex

analysis of methane absorption bands, atmosphere transmission data, available laser sources

and detection systems as well as Polish technological potentials, He-Ne laser generating

radiation in the 3.39 mm band was selected as measurement source (lon). This line of

generation coincides very exactly with oscillationrotational absorption line P(7) of methane.

Moreover, after practical analysis of He-Ne laser generating characteristics we propose

further utilisation its two single lines, namely 3.3922 mm and 3.3912 mm.

The aim is to substantially increase the measurement range of methane

concentrations. The line of 3.3922 mm, having a very high absorption coefficient, allows

very sensitive methane concentration detection, but only up to around 2600 ppm (referring to

measurement cell of 15 cm). The additional line 3.3912 mm, with several times lower

absorption coefficient allows to extend the measurement range to 2x105 ppp (much higher

than LEL - lower explosive limit). As the reference source (loff), the proposal is to develop a

gas laser generating wavelength of 3.5 mm. This wavelength lies so near to the measurement

one (3.39 mm), that we avoid the additional influence of different atmosphere transmittance.

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Our experiments have shown possibility of use of RF discharge to obtain 7 mW of output

power without optimisation of gas mixture and pressure, for reasonable dimensions of laser

head. The idea of proposed measurement system is shown in fig.3. Transmitted laser beams

are AM modulated due to pulse RF excitation of both lasers (TTL level modulation of power

sources). The small amount of lasers output powers, through beamsplitters go to detectors D2

and D3 for on-line control of the level of output power and as reference signals for data

acquisition circuits. The Cassegrain optical transmitting/receiving subassembly aims laser

beams to the target (area) and collects, through D1 detector backscattered radiation. After

amplification, detector electronic signals are directed to the electronic part of system. The

result of methane concentration measurement is calculated on the base earlier developed

algorithm.

The only one cause of the different signals from two wavelengths is the presence of

methane. It should be emphasised, that utilisation of modern, RF excited lasers allows not

only to increase the output powers and extend their lifetimes, but also to simplify and

increase the stability of electronics (TTL level AM modulation instead of mechanical

chopping). Moreover, it increases also signal energy, measurement range and accuracy.

Several methods applied for methane concentration measurements (also other gases) can be

divided to groups using different criteria, among others "in-situ" - place of sampling or

"stand-off" - place of gas real presence. The first group utilises mainly chemical,

physicochemical and spectral methods. A big selection of portable or stationary systems is

offered. The second group offers distant measurements, with typical example - lidar. This

relatively low price, mobile measurement system (on the board of helicopter or van), which

links DIAL and DOAS methods and capable to detect methane up to 200 m, appears to be a

good supplement to existing methods

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REMOTE DETECTION OF METHANE BY INFRARED

SPECTROMETRY

LMLD(LED Methane Leak Detector) is an Optical Gas Leak Detection instrument for

measuring the presence of methane and ethane gas. It has been developed since 2006 in our

institute. The instrument relies on measuring the absorption of light by methane in the mid

infrared region. A light source made by LED (Light Emitting Diode) mounted on one end,

facing an optical detector located on the opposite end.

The light source produces a wide range of wavelengths, including wavelength bands

absorbed by methane. LEDs of the mid-infrared band, which are made from In AsSbP,

provide narrow bandwidth illumination that is projected through the sample volume and then

detected by a photo detector. A photo detector made by a photodiode(PD) contains a glass

lens and a mirror that concentrates ray reached from a LED light source. LEDs and PDs are

equipped with the function of electric thermo cooling. The power is used to supply 12V from

the battery. The figure of a whole system of methane detection system. Then concentration is

computed through a procedure series: emitting LEDs, amplifying, AD converting, calculation

and displaying.

The schematic figure of CH4 detector and the structure of the LED transmitters and

a PD receiver along with the modulator-controller. Thermoelectric elements, controlling

operating temperature, are mounted upon LEDs and PDs.

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The characterics of these InAsSbP LEDs(IBSG, Russia) are follows : The current

threshold at 1kHz rate is maximum 1A. FWHM is 0.7μm. Optical power at 1A is 400μW.

Quantum yield is 0.5%.Switch time is 50ns. Operating temperature is from 77 to 320K.The

receiver consists of a focusing lens system and a InAsSbP pin photo diode(IBSG, Russia).

The characteristics are as follow: Cut-of wavelength at 10% is 3.8 μm. Peak wavelength at >

90% is from 2.8 to 3.4μm. Responsibility at _p is 1.0-1.2 A/W. Detection sensitivity is about

3*109 cm·Hz1/2/W. Fig show appearances of LEDs and PDs

FLAME IONIZATION DETECTOR

Flame Ionization Detector (FID) is one of the most used detectors for gas chromatography

(GC). The application area is wide. For example, petrol for air planes, kerosines, are carefully

analyzed with GC-FID as a routine control. The composition of the kerosines is of great

importance for the energy conversion. A completely different area is packaging of food. Your

take-away hamburger is wrapped in an insulating polystyrene box. During the processing of

polystyrene different hydrocarbons are added to create the end-product. When polystyrene is

used within food industry, it is crucial that the product is analyzed for any residues of the

hydrocarbons, since they can harm the quality of the food and your health. The GC-FID is

well suited for analysis of hydrocarbons such as methane, ethane, acetylene etc., but also for

organic substances containing hydrocarbons and for volatile organic compounds (VOCs). In

an FID the sample undergoes a combustion in a hydrogen/synthetic air fl ame. Ions and free

electrons are formed in the fl ame. The charged particles produce a measurable current flow

in the gap between two electrodes in the detector. The resulting current flow is of greater

strength than the signal produced by the pure carrier gas and the fuel gas flame alone. This

signal differential provides information about the sample. The current is proportional to the

information which depends on the composition of the separated sample. The FID is a general

detector which, with extra configurations, can be used for more specific components. For

example, with placing a methanizer ahead of the FID, components containing carbon can

undergo a formation to methane and thereby be suited for further FID analysis. CO and CO2

are commonly analyzed this way. For determination of organic nitrogen/phosphorus

compounds a different FID configuration is needed. The sample passes a heated alkali source

where charged particles are formed in contact with the alkali source. This method is normally

named alkali flame ionization, also named thermionic detector and belongs to the group of

detectors in which thermal energy is used as source for ionization.

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The abbreviation of this method is often NPD. Flame ionization detectors are

extremely sensitive and have a wide range of linearity. The only disadvantage is that it

consumes the sample.

An important facet of the GC-FID is the use of a carrier gas to transfer the sample

from the injector, through the column and into the FID detector. The carrier gas must be inert

and may not be adsorbed by the column material. Helium or nitrogen are normally used as

carrier gas with GC-FID, and sometimes hydrogen. The detector gases, hydrogen and

synthetic air, serve respectively as fuel gas and oxidizing gas during the combustion process.

Since hydrocarbon impurities, moisture and oxygen produce a greater baseline noise which

has an adverse effect on the detection limit, these impurities in the detector gases should be

kept as low as possible. Like all chromatographic analytical processes, gas chromatography is

a relative method, i.e. calibration with a standard mixture is required, both to check linearity

and as calibration for the sample .

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STATIONARY METHANE SENSORS

Many stationary methane gas sensors are placed in coal mines. Some signals from the

sensors are sent to a monitoring room on the ground, while automatic shutdown mechanisms

of electric devices will be activated upon detection of a high concentration of methane gas. In

any case, high precision measurement is necessary to prevent the disasters. Operators in the

monitoring room especially concentrate on trends of the gas concentration and swiftly react

when suspicious changes are observed.

 

HANDY METHANE SENSORS

Beside the stationary gas detectors, individual workers often carry a handy type

methane sensor. A kind of interferometer equipped with a filter for removing CO2 is often

used for the purpose. Concentration rage of 0.25 to 10% is measurable with this device.It is

needless to say that any work in underground coal mines should be serious one regardless of

its purpose. 

Even though the production scale of the coal mine is much smaller than before, the

distance to the test site is still about 2000m. As a train we got on board accelerating in a dark

and small tunnel, I felt the speed much faster. It must be a great photo opportunity to express

the feeling in the dark; it was not allowed to take digital cameras because the use of electric

devices is strictly restricted in coal mines.

DEPARTMENT OF MINING Page 20

METHANOMETERS IN COAL MINES

No regular monitoring of methane is required in areas of gassy coal mines outby the

mining face. Methane ignitions that have occurred in mine outby areas indicate the need to

provide better protection to workers. Handheld methane monitors are now used by some

miners to make periodic measurements of methane at the working face. The IYONI II gas

detector which is incorporated into a miner’s cap lamp and worn on a miner’s helmet can

continuously provide an alarm signal whenever methane levels exceed a set level.

Tests were conducted to evaluate the performance characteristics of this methane

detector by measuring response times with methane gas supplied through a calibration fixture

or adaptor. Other response time tests were performed with the detector in an environmental

test box. Performance was also evaluated in a full scale test gallery where face methane

emission and underground ventilation were simulated. Procedures for calibration by response

time measurement of the IYONI II detector have been developed. In limited testing, the

IYONI II detector was found to reliably detect the presence of 1 percent by volume methane.

DEPARTMENT OF MINING Page 21

CARBON MONOXIDE Carbon monoxide is a poisonous gas caused by fires. The gas is colourless and does

not smell, so cannot be easily detected by people.

Carbon monoxide affects small animals more quickly than people. Caged canaries

were used as a simple way of detecting carbon monoxide. The birds would fall from their

perch before the miners were affected, and they could move quickly to an area of fresh air.

Canaries affected in this way could often be revived. Modern equipment can detect carbon

monoxide and tell miners exactly how much of the gas is present.

CARBON MONOXIDE DETECTOR 

A carbon monoxide detector or CO detector is a device that detects the presence of the carbon

monoxide (CO) gas in order to preventcarbon monoxide poisoning. In the late 1990s Underwriters

Laboratories (UL) changed their definition of a single station CO detector with a sound device in it to a

carbon monoxide (CO) alarm. This applies to all CO safety alarms that meet UL 2034; however for

passive indicators and system devices that meet UL 2075, UL refers to these as carbon monoxide

detectors. CO is a colorless, tasteless and odorless compound produced by incomplete combustion of

carbon containing materials. It is often referred to as the "silent killer" because it is virtually undetectable

without using detection technology and, in a study by Underwriters Laboratories, "Sixty percent of

Americans could not identify any potential signs of a CO leak in the home". [1] Elevated levels of CO can be

dangerous to humans depending on the amount present and length of exposure. Smaller concentrations can

be harmful over longer periods of time while increasing concentrations require diminishing exposure times

to be harmful.

CO detectors are designed to measure CO levels over time and sound an alarm before dangerous

levels of CO accumulate in an environment, giving people adequate warning to safely ventilate the area or

evacuate. Some system-connected detectors also alert a monitoring service that can dispatch emergency

services if necessary.

While CO detectors do not serve as smoke detectors and vice versa, dual smoke/CO detectors are

also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO

detectors detect and warn people about dangerous CO buildup caused, for example, by a malfunctioning

fuel-burning device. In the home, some common sources of CO include open flames, space heaters, water

heaters, blocked chimneys or running a car inside a garage.

DEPARTMENT OF MINING Page 22

SENSORSEarly designs were basically a white pad which would fade to a brownish or blackish

color if carbon monoxide was present. Such chemical detectors were cheap and were widely

available, but only give a visual warning of a problem. As carbon monoxide related deaths

increased during the 1990s, audible alarms became standard.

The alarm points on carbon monoxide detectors are not a simple alarm level (as in

smoke detectors) but are a concentration-time function. At lower concentrations (e.g. 100

parts per million) the detector will not sound an alarm for many tens of minutes. At 400 parts

per million (PPM), the alarm will sound within a few minutes. This concentration-time

function is intended to mimic the uptake of carbon monoxide in the body while also

preventing false alarms due to relatively common sources of carbon monoxide such as

cigarette smoke.

There are four types of sensors available and they vary in cost, accuracy and speed of

response, listed below. The latter three types include sensor elements that typically last up to

10 years. At least one CO detector is available which includes a battery and sensor in a

replaceable module. Most CO detectors do not have replaceable sensors.

DEPARTMENT OF MINING Page 23

BIOMIMETIC

A biomimetic sensor works in a fashion similar to hemoglobin which darkens in the

presence of CO proportional to the amount of carbon monoxide in the surrounding

environment. It uses cyclodextrins, a chromophore, and a number of metal salts. This can

either be seen directly or connected to an infrared source of photons such as an IRLED and

then monitored using a photodiode. Battery lifespan usually lasts 2–3 years with conventional

alkaline, but a lithium battery will last the life of the product. The biotechnology based

sensors have a useful operational life of 6 years. These products were the first to enter the

mass market, but because they cost more than other sensors they are mostly used in higher-

end areas and RVs.

The technology has been improved and is the most reliable technology, according to a

report from Lawrence Berkeley National Laboratory.] The technology is the only one tested

false alarm free and is preferred by those with larger facilities like hospitals, hotels and

apartments that use air fresheners, alcohols and other disinfectants where the cost of one false

alarm is very high. This technology was invented in the United States and is manufactured in

California.

ELECTROCHEMICAL

This is a type of fuel cell that instead of being designed to produce power, is designed

to produce a signal current that is precisely related to the amount of the target gas (in this case

carbon monoxide) in the atmosphere. Measurement of the current gives a measure of the

concentration of carbon monoxide in the atmosphere. Essentially the electrochemical cell

consists of a container, two electrodes, connection wires and an electrolyte - typically sulfuric

acid. Carbon monoxide is oxidized at one electrode to carbon dioxide while oxygen is

consumed at the other electrode. For carbon monoxide detection, the electrochemical cell has

advantages over other technologies in that it has a highly accurate and linear output to carbon

monoxide concentration, requires minimal power as it is operated at room temperature, and

has a long lifetime (typically commercial available cells now have lifetimes of five years or

greater). Until recently, the cost of these cells and concerns about their long term reliability

had limited uptake of this technology in the marketplace, although these concerns are now

largely overcome. This technology is now the dominant technology in USA and Europe.

DEPARTMENT OF MINING Page 24

SEMICONDUCTOR

Thin wires of the semiconductor tin dioxide on an insulating ceramic base provide a

sensor monitored by an integrated circuit. This sensing element needs to be heated to

approximately 400 °C in order to operate. Oxygen increases resistance of the tin dioxide

while carbon monoxide reduces resistance. The integrated circuit monitors the resistance of

the sensing element. Lifespans are approximately five to 10 years.

The large power demand of this sensor means that it is usually powered from the

mains. A battery-powered, pulsed sensor is available with a lifetime in months.

This technology has traditionally found high utility in Japan and the far east with some

market penetration in USA. However the superior performance of electrochemical cell

technology is beginning to displace this technology.

DIGITALAlthough all home detectors use an audible alarm signal as the primary indicator,

some versions also offer a digital readout of the CO concentration, in parts per million.

Typically, they can display both the current reading and a peak reading from memory of the

highest level measured over a period of time. These advanced models cost somewhat more

but are otherwise similar to the basic models.

The digital models offer the advantage of being able to observe levels that are below

the alarm threshold, learn about levels that may have occurred during an absence, and assess

the degree of hazard if the alarm sounds. They may also aid emergency responders in

evaluating the level of past or ongoing exposure or danger.

DEPARTMENT OF MINING Page 25

PORTABLE

Portable CO detectors are also available; these are typically used for professional

applications or in some cases by consumers such as property managers for maintenance and

diagnosis issues (i.e. sourcing a CO leak). Most offer real time measurements of CO down to

a few ppm (usually shown on a digital display), and are more expensive than home safety CO

detectors (e.g. ~$250 vs $25). There are two types of portable detectors, one that is designed

for aircraft, cars and trucks. They will warn the driver and passenger if there is a CO hazard.

Another type is used by industrial hygienists and first reponders. Digital, fast responsing

portable type CO detectors are usually a better choice for real time "on the go" applications as

they respond to low levels of CO in seconds rather than minutes or hours (which is the case

for UL2034 listed residential alarm).

SENSOR NODEMine gas sensor consists of electrical bridge, signal conditioning circuit, Alarm circuit. Block

diagram of mine gas sensor hardware architecture.

DEPARTMENT OF MINING Page 26

CONCLUSIONThe underground mine gas concentration detection based on the dectector can realize

the wireless data transmission and greatly improve the intrinsic safety of the mine gas

detection system with the advantage of the low cost and flexibility. It will play a great role in

the Coal Mine Safety Monitoring systems as a supplement to the present wire transmission.

Because of these considerations, the conclusion seems natural that, unless the action of the air and the

general weathering processes introduce some other factors here overlooked, the newly made coal dust

at the working faces of the mine should, on a chemical basis alone, other things being equal, possess

greater explosive potentialities than the old dust along the main haulage ways. One of the uncertain

operative factors is the occluded oxygen, which may be supposed to facilitate an explosion of dust.

DEPARTMENT OF MINING Page 27

REFERENCES

1.  Wali, Russeen (2012). "An electronic nose to differentiate aromatic flowers using a

real-time information-rich piezoelectric resonance measurement". Procedia

Chemistry 6: 194–202. doi:10.1016/j.proche.2012.10.146.

2.  Detcon, http://www.detcon.com/electrochemical01.htm

3.  United States Patent 4141800: Electrochemical gas detector and method of using

same,http://www.freepatentsonline.com/4141800.html

4. ^ Jump up to:a b c Muda, R., 2009

5.  International Society of Automation, http://www.isa.org/Template.cfm?

Section=Communities&template=/TaggedPage/DetailDisplay.cfm&ContentID=23377

6. Edward Naranjo and Shankar Baliga and Philippe Bernascolle, "IR gas imaging in an

DEPARTMENT OF MINING Page 28