9. Biogas for vehicles
The utilization of biogas in vehicles requires a method of
compact storage to facilitate the independent movement of the vehicle for a
reasonable time. Larger quantities of biogas can only be stored at small volumes
under high pressure, e.g. 200 . . .300 bar, or purified as methane in a liquid
form at cryogenic conditions, i.e. -161 °C and ambient pressure. The
processing, storage and handling of compressed or liquified biogas demand
special and costly efforts.
Compression is done in reciprocating gas compressors after
filtering of H2S. At a medium pressure of about l5 bar the
CO2 content can be "washed out" with water to reduce the final
storage volume. Intermediate cooling and removal of the humidity in molecular
sieve filters are essential as the storage containers should not be subjected to
corrosion from inside. The storage cylinders, similar to oxygen cylinders known
from gas welding units, can be used on the vehicle as "energy tank" and in
larger numbers as refilling store.
One cylinder of 50 l volume can store at a pressure of 200 bar
approximately
- 15 m³ unpurified biogas (CH4 = 65%
Vol) with an energy equivalent of 98 kWh or 101 diesel fuel, or
- 13 m³
purified biogas (CH4 = 95% Vol) with an energy equivalent of 125 kWh
or 12.51 diesel fuel.
The storage volume thus required on the vehicle is still five
times more than is required for diesel fuel. Purification of biogas to
CH4 increases the storage efficiency by 25 . . . 30% but involves an
extra gas washing column in the process.
Purified biogas, i.e. methane, has different combustion features
than biogas because of the lack of the CO2 content. It combusts
faster and at higher temperatures; this requires different adjustments of
ignition timing. Dual fuel methane engines are prone to increased problems with
injector nozzle overheating and have to operate on higher portions of diesel
fuel (about 40%) to effect sufficient cooling of the jets.
Liquification of biogas requires drying and purification to
almost 100% CH4 in one process and an additional cryogenic process to
cool the CH4 down to -161 °C where it condenses into its liquid
form.
Storage is optimal at these conditions as the volume reduction
is remarkable, i.e. 0.6 m³n with an energy content of 6 kWh condense to one
lifer of liquid with an energy equivalent of 0.61 diesel fuel. The required tank
volume is only 1.7 times the volume needed for diesel fuel.
This advantage is opposed by a more sophisticated multistage
process, the handling of the liquid in specially designed cryo-tanks with vacuum
insulation and the fact that for longer storage it has to be kept at its
required low temperature in order to prevent evaporation. This requires
additional energy and equipment. The practicability of such systems is still
being researched with commuter bus traffic in Sao Paulo, Brazil. Data on the
economic viability are not yet available.
The use of biogas as a fuel for tractors on farms has been
elaborately researched. The processing of the gas does not only require about
10% of the energy content of the gas, mainly for compression, but also involves
considerable investment. The tractor itself needs to carry four gas cylinders at
least for a reasonable movement radius. A 40-kW tractor can then operate for
about six to seven hours at mixed/medium load. The modification of the tractor
has to include a three-stage pressure reduction system as the fuel gas is fed to
the mixer at low pressure, i.e. about 50 mbar.
Modification into an Otto gas engine includes the risk of
non-availability of the tractor at biogas shortage. It therefore needs LPG as
spare fuel or another diesel tractor standby. Dual fuel tractor engines, on the
other hand, are difficult to control, especially because of their frequent speed
and load changes during operation in the field.
Biogas for road service has become an issue in Brazil lately. It
must however be seen in connection with the specific situation in this country.
The main issue is to utilize the large natural gas resources for substitution of
diesel fuel which is scarce. Purified biogas is therefore integrated into a
larger "methane program", for which the government may decide to give specific
economic preferences because of energy-political reasons. The biogas will
furthermore be obtained and processed in larger units, e.g. municipal sewage
plants and sugar factories which reduces the cost per m³ considerably.
With the current (political) price of fuel in industrialized
countries the equivalent price for "vehicle biogas" is about two or three times
higher than for diesel fuel. It is therefore presently not economic though
technically feasible to use biogas in vehicles on a larger scale. The
infrastructure for processing and filling however must also be developed
accordingly.
