235
Compressed Natural Gas
tanks that may have the tendency in a fire of developing hot spots leading to
serious safety problems and eventual rupture, and spilling out the contents
of the tank with devastating effects. Fuel tanks/cylinders must meet all the
safety regulations in the country of use and be tested at regular intervals.
All fuel tubes, connectors, pressure switches, indicators, regulators, and so
on, and associated equipment must also be of appropriate design and should
comply with safety regulations.
Moreover, NG should be free from impurities like sulfur and water and
so on, or have acceptable levels of purity. Presence of water may cause tank
corrosion problems and reduce the strength of the material. Heating of gas
cylinders would increase the pressure inside the cylinder. If the increase in
pressure is very high, it may cause an explosion of the cylinder if the pressure relief valve is not functioning. Currently, cylinders are made of special materials that at high pressures will not fragment, instead a crack will
develop and release the gas very slowly.
Natural gas has a relatively high flammability limit as compared to the
other fuels. For continuous flame propagation, natural gas requires a minimum of 5% by volume as compared to around merely 2% for propane and 1%
for gasoline vapor. Hence, it is safer than other conventional fuels. Methane,
by virtue of its relatively narrow range of flammability limits, can generate
only a small amount of combustible mixture relative to other fuels. The discharge of one unit volume of methane in air would generate a maximum volume of combustible mixture that is around 40% of that following the release
of an equivalent volume of propane and at 20% the volume of the mixture
formed following the release of a similar amount of gasoline vapor.
The rate of discharge of a gaseous fuel following an accident will depend
on the nature of the leak, and the associated area of discharge. The discharge
from the high-pressure cylinder into the atmosphere will be at rates that will
be proportional to the volume of the area of discharge and inversely proportional to the density of the gas. However, the explosion depends on how the
combustion mixtures formed, and at what rate, and the ignition source availability. The dispersion rate of methane is much higher than gasoline. Hence,
if there is any leakage of CNG from cylinders, gas will be discharged at a
very high velocity jet into the surrounding area, aiding greatly in the rapid
dispersion of fuel (Table 8.2).
CNG has a higher autoignition temperature than gasoline. The minimum
energy required for ignition of a fuel–air mixture within the flammability
range is relatively high for methane gas mixtures. Hence, NG is much more
difficult to ignite/fire than other fuels. Quenching of methane–air flames by
cold surfaces such as through metallic meshes is much easier than in the case
of flames involving LPG or hydrogen. Thus, flame traps are more successful in suppressing methane fires than those involving hydrogen or propane
(Karim and Wierzba 1992).
Astbury (2008) reviewed the safety of vehicles using various alternative
fuels and was summarized as follows. He reported that leakage of NG into
Compressed Natural Gas
tanks that may have the tendency in a fire of developing hot spots leading to
serious safety problems and eventual rupture, and spilling out the contents
of the tank with devastating effects. Fuel tanks/cylinders must meet all the
safety regulations in the country of use and be tested at regular intervals.
All fuel tubes, connectors, pressure switches, indicators, regulators, and so
on, and associated equipment must also be of appropriate design and should
comply with safety regulations.
Moreover, NG should be free from impurities like sulfur and water and
so on, or have acceptable levels of purity. Presence of water may cause tank
corrosion problems and reduce the strength of the material. Heating of gas
cylinders would increase the pressure inside the cylinder. If the increase in
pressure is very high, it may cause an explosion of the cylinder if the pressure relief valve is not functioning. Currently, cylinders are made of special materials that at high pressures will not fragment, instead a crack will
develop and release the gas very slowly.
Natural gas has a relatively high flammability limit as compared to the
other fuels. For continuous flame propagation, natural gas requires a minimum of 5% by volume as compared to around merely 2% for propane and 1%
for gasoline vapor. Hence, it is safer than other conventional fuels. Methane,
by virtue of its relatively narrow range of flammability limits, can generate
only a small amount of combustible mixture relative to other fuels. The discharge of one unit volume of methane in air would generate a maximum volume of combustible mixture that is around 40% of that following the release
of an equivalent volume of propane and at 20% the volume of the mixture
formed following the release of a similar amount of gasoline vapor.
The rate of discharge of a gaseous fuel following an accident will depend
on the nature of the leak, and the associated area of discharge. The discharge
from the high-pressure cylinder into the atmosphere will be at rates that will
be proportional to the volume of the area of discharge and inversely proportional to the density of the gas. However, the explosion depends on how the
combustion mixtures formed, and at what rate, and the ignition source availability. The dispersion rate of methane is much higher than gasoline. Hence,
if there is any leakage of CNG from cylinders, gas will be discharged at a
very high velocity jet into the surrounding area, aiding greatly in the rapid
dispersion of fuel (Table 8.2).
CNG has a higher autoignition temperature than gasoline. The minimum
energy required for ignition of a fuel–air mixture within the flammability
range is relatively high for methane gas mixtures. Hence, NG is much more
difficult to ignite/fire than other fuels. Quenching of methane–air flames by
cold surfaces such as through metallic meshes is much easier than in the case
of flames involving LPG or hydrogen. Thus, flame traps are more successful in suppressing methane fires than those involving hydrogen or propane
(Karim and Wierzba 1992).
Astbury (2008) reviewed the safety of vehicles using various alternative
fuels and was summarized as follows. He reported that leakage of NG into