Environmental Engineering Reference
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methane powered Otto engine is higher compared to its petrol counterpart.
In addition, hybridization of the NGV powertrain will make it possible to
decrease the fuel storage size, effectively taking away a large part of the
NGV markup cost and payload penalty.
18.1.2 Gas quality: the effect of trace compounds on NGVs
Technical developments in the form of cryogenic upgrading have made it
possible to clean landfill gas to biomethane quality. Stricter emissions
standards for vehicles have led to a higher level of sophistication in engine
and after-treatment technologies and have made it necessary to better
control the trace elements levels of biomethane. Of particular concern are
siloxanes. Siloxanes are suspected to be accountable for many of the
undiagnosed operational problems in NGVs, since they precipitate as solid
silica during combustion. Problems with lambda sensor clogging have been
attributed to the presence of elementary silicon in petrol. It is unknown
whether similar problems might arise from the presence of siloxanes in
compressed biomethane. There is undoubtedly a risk that the formed silica
will precipitate in after-treatment systems, an accumulative effect that leads
to more rapid deactivation and aging of the catalyst.
With respect to microorganisms, there is much discussion on the potential
health and pipeline integrity hazards of introducing biomethane to the
natural gas system. In fact, the levels of microorganisms are just as high in
natural gas as they are in biomethane. For vehicle gas, it is likely that the
combined effect of heat development during compression, drying of the gas
and filtration with 0.2-1.0micron filters is sufficient to make the compressed
gas quite disease vector free. In addition, Vinnera˚ s et al. (2006) claim that
the amount of biomethane needed to inhale to become infected (even with
severe pathogens, which were not found in the study in question) is so large
that the risk of accidental fire and suffocation is much more pronounced. A
very small amount of gas is released during refueling, approximately 5 cm
3
.
Thus, if pathogens were present, then the absorbed dosage would be too
small to constitute any real risk.
In addition to the biologically derived trace elements it is also evident that
there is a need to regulate more closely the contents of water and entrained
compressor oil. The fact is that most of the operational problems of NGVs
may be attributed to elevated levels of these two substances. These problems
will not lead to engine failure, but do lead to reduced drive performance and
erratic service. All of these problems may easily be controlled by properly
addressing the removal of water and oil in the compressed gas. In winter
climates, if the water level is not kept sufficiently low, the expansion cooling
that occurs when the gas flows from the storage tank through the vehicle's
fuel system may lead to the formation of intermittent hydrocarbon hydrate
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