233
Compressed Natural Gas
foreign matter, and hydrogen sulfide from NG before it is compressed. The
water vapor can condense in the vehicle fuel system, causing corrosion, especially if hydrogen sulfide is present. The dispenser shape is similar to that of a
gasoline/diesel filling pump but the internal parts are different and suitable
for gasoline-fuel applications. CNG dispensers are made from aluminum
and stainless steel. CNG dispensing nozzles have a finite lifetime typically
characterized by the total number of refuelings completed. Nozzles should
be replaced at the end of the lifetime, before failure. The CNG dispensing
hoses are usually stainless steel wire wrap. The inner tube can be Teflon or
nonporous. Like nozzles, the hoses also have finite lifetimes and should be
replaced before its failure. CNG gas refueling systems can be divided into
two categories: slow fill and fast fill.
Fast-fill stations allow vehicles to pull up and refuel in a short period of
time. In order to fuel vehicles quickly, gas must be drawn from prepressurized storage vessels. Cascade type storage vessels are used to store the
high-pressure gas. Cascade storage divided into several compartments can
be independently connected to the refueling pump. The pressure of cascade
is higher than the maximum storage pressure of the cylinders on the vehicle.
As a general rule of thumb, only 40% of the stored gas in a three bank cascade arrangement is available for refueling. This means that a 30,000 cubic
foot storage cascade will deliver about 12,000 cubic feet of NG quickly. This
equates to about 96 equivalent gallons of gasoline. A priority sequential
panel is used to direct compressor discharge to the high, then medium, then
low, bank. When filling, a vehicle will draw first from the low bank, then the
medium bank, and top off from the high bank. The compressor will replenish the cascade by filling the high bank first, then the medium bank, and
finally the low bank (http://www.mckenziecorp.com/cng_storage.htm).
Fast-fill systems dispense the fuel at a faster rate and the gas cylinders have
no time to lose heat to the environment. At the end of fill, gas in the cylinders becomes slightly warmer than the atmosphere. Hence, it is difficult to
complete refueling by the fast-fill method. Moreover, as the cylinder cools,
atmosphere pressure drop will occur. Fast-fill CNG car dispensers require
three minutes to fill a car and one minute to fill an auto rickshaw with a flow
range of 0–15 kg/min. Bus CNG dispensers, with a flow range of 0–75 kg/
min, require 6 minutes to fill.
Slow-fill systems are designed to refuel the vehicles even when the
cylinders are hot. Slow-fill systems do not require cascade storage. Cylinders
are connected in parallel to the compressor. The working pressure of CNG
cylinders is about 200 bar (CONCAWE 1995). Figure 8.2 shows the schematic
of a refueling system.
CNG systems essentially incorporate two kinds of refueling principles
based on
i. Mother-daughter concept
ii. On-line station concept
Compressed Natural Gas
foreign matter, and hydrogen sulfide from NG before it is compressed. The
water vapor can condense in the vehicle fuel system, causing corrosion, especially if hydrogen sulfide is present. The dispenser shape is similar to that of a
gasoline/diesel filling pump but the internal parts are different and suitable
for gasoline-fuel applications. CNG dispensers are made from aluminum
and stainless steel. CNG dispensing nozzles have a finite lifetime typically
characterized by the total number of refuelings completed. Nozzles should
be replaced at the end of the lifetime, before failure. The CNG dispensing
hoses are usually stainless steel wire wrap. The inner tube can be Teflon or
nonporous. Like nozzles, the hoses also have finite lifetimes and should be
replaced before its failure. CNG gas refueling systems can be divided into
two categories: slow fill and fast fill.
Fast-fill stations allow vehicles to pull up and refuel in a short period of
time. In order to fuel vehicles quickly, gas must be drawn from prepressurized storage vessels. Cascade type storage vessels are used to store the
high-pressure gas. Cascade storage divided into several compartments can
be independently connected to the refueling pump. The pressure of cascade
is higher than the maximum storage pressure of the cylinders on the vehicle.
As a general rule of thumb, only 40% of the stored gas in a three bank cascade arrangement is available for refueling. This means that a 30,000 cubic
foot storage cascade will deliver about 12,000 cubic feet of NG quickly. This
equates to about 96 equivalent gallons of gasoline. A priority sequential
panel is used to direct compressor discharge to the high, then medium, then
low, bank. When filling, a vehicle will draw first from the low bank, then the
medium bank, and top off from the high bank. The compressor will replenish the cascade by filling the high bank first, then the medium bank, and
finally the low bank (http://www.mckenziecorp.com/cng_storage.htm).
Fast-fill systems dispense the fuel at a faster rate and the gas cylinders have
no time to lose heat to the environment. At the end of fill, gas in the cylinders becomes slightly warmer than the atmosphere. Hence, it is difficult to
complete refueling by the fast-fill method. Moreover, as the cylinder cools,
atmosphere pressure drop will occur. Fast-fill CNG car dispensers require
three minutes to fill a car and one minute to fill an auto rickshaw with a flow
range of 0–15 kg/min. Bus CNG dispensers, with a flow range of 0–75 kg/
min, require 6 minutes to fill.
Slow-fill systems are designed to refuel the vehicles even when the
cylinders are hot. Slow-fill systems do not require cascade storage. Cylinders
are connected in parallel to the compressor. The working pressure of CNG
cylinders is about 200 bar (CONCAWE 1995). Figure 8.2 shows the schematic
of a refueling system.
CNG systems essentially incorporate two kinds of refueling principles
based on
i. Mother-daughter concept
ii. On-line station concept