Environmental Engineering Reference
In-Depth Information
plugs. To avoid this problem, the gas industry in Sweden dries compressed
natural gas (CNG) by lowering the temperature to
C at 4 bar,
corresponding to a removal of water down to a level well below 1mg per
Nm
3
. Entrained compressor oil in NGV engines may cause operational
problems with pressure regulators and gas injectors. Another important
aspect is to manage any vehicle incompatibility issues regarding the oil itself
or any of its additives. Oil levels need to be monitored and controlled
through oil removal downstream of the compressor. The developed practice
of maintenance is to have two oil coalescing filters in series, one close to the
compressors and one as far away as possible in order to allow for the
cooling of the gas, leading to precipitation of the dissolved oil.
Another important trace compound that is relevant for both biomethane
and CNG is the content of total sulfur. Lowering the maximum level for
total sulfur content is important because the high content of sulfur dioxide
in exhaust gases has an inhibitory and aging impact on catalysts, with lean-
burn engine applications being the most sensitive. It is clear that most of the
sulfur has its origin in the odorization agent added, with THT
(tetrahydrothiophene) being the most common (typically 15mg is added,
corresponding to 6mg S/Nm
3
; more is added in markets such as in France,
Spain and Italy). More sulfur-poor and even sulfur-free alternatives are
available on the market, although the sulfur-free option is a less likely
candidate, due to its uncharacteristic odor.
The state of standardization is not satisfactory for CNG in general and
renewable compressed biogas (CBG) (biomethane) in particular.
Standardization at the international level is mostly qualitative. Most
standards are at the national level and, with the exception of Sweden,
biomethane is only standardized for injection into the natural gas network.
A mandate (M/475, 2010) from the European Commission to develop CEN
standards for biomethane fed into the grid, or used directly as renewable
CBG, has changed this, and work is ongoing in a new committee (CEN/
TC408 Project Committee - Biogas for use in transportation and injection
into natural gas pipelines). The work started in autumn 2011 and is
scheduled to finish in 2015. A variety of trace elements is under discussion
for inclusion in the new standards; a new development has led to CNG also
being included in the scope, finally leading to quantitative standardization at
international level for all methane-based vehicle fuels.
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