biogas. Material is regenerated through reducing pressure and
applying vacuum to the adsorbent material [3].
Trace compounds such as siloxanes are of special interest when
biogas is utilized as a fuel in gas engines and in traffic vehicles
[4–6]. During combustion siloxanes form abrasive silicon dioxide
which form deposit layers on vital engine parts thereby reducing
heat conduction, lubrication and changing combustion chamber
geometry. Furthermore silicon dioxide can deactivate catalytic
converters which will lead to higher exhaust emissions [6]. This
leads to two choices; considerably more frequent engine maintenance
or removal of siloxanes from biogas. The concentration of
siloxanes is an important factor when determining the best course
of action. Acceptable siloxane concentrations in electricity generating
gas engines are now quoted in the range of 2.8 mg[Si]/Nm3 as
gas engine manufacturers have made the terms of engine warranty
significantly more stringent in recent years [7]. A commonly used
technique for siloxane abatement is adsorption to activated carbon.
Also silica gel and alumina are promising since these materials
can also be used for gas drying and easily regenerated back to almost
full capacity by heating. Activated carbon has only limited
regeneration ability [6].
Physical and chemical absorption can be used to remove siloxanes.
The most promising physical absorbent is organic solvent
Selexol™ (polyethylene glycol or dimethyl ethers) [6]. Water is
also a physical absorbent but in past studies it has not been very
effective media for siloxane removal [8]. Since many organic silicon
compounds are at least partially water soluble, more comprehensive
study in this field is warranted [6,9].
Fresh water use can be a major expenditure when using water
absorption in biogas upgrading especially if potable water is the
only source of water. For this reason water recycling is an economically
attractive choice. However recycling water in upgrading system
requires that it is equipped with gas desorption and water
cooling which means greater investment and maintenance costs.
Even when recycling absorption water, it is common practice to dilute
part of the system water with fresh, otherwise the water pH
becomes too low [10]. It could be argued that with good desorption
and cooling systems almost completely closed water circulation
could be achieved. The most commonplace method for CO2 desorption
is air stripping in a desorption vessel which can lead to H2S
oxidation to elemental sulphur by chemotrophic sulphide oxidizing
bacteria causing deposits and abrasion damage in the system
[10–12] This same principle is used for example in biological desulphurisation
of biogas in presence of O2 [1]. By using under pressure
in the desorption vessel, however, instead of air stripping
these difficulties could be avoided.
Water absorbs different gases in different amounts and according
to Henry’s Law this depends on the gas pressure and temperature
of the water [12]. At lower temperature the absorption
capacity of water is higher but associated costs in water cooling
are also higher since water is warmed by the process itself. It could
be more economical to use higher temperature if the absorption
capacity is still sufficiently high and, if water recirculation is used,
the water flow speed doesn’t have a dominate effect on economics
regarding to the cost of pure water. However higher temperature
could also mean that reduced absorption capacity must be compensated
for using higher pressure or higher water recycling and
flow speed and this of course increases electricity consumption
per upgraded gas unit. Therefore temperature, pressure, water
recycling and flow speed have a certain tradeoffs and most suitable
(economical) parameters have to be found for each individual
upgrading system.
The objective of this study was to study landfill gas upgrading
with a pilot-scale counter current water absorption system with
a special 3:1 height to width ratio in the absorption column. The
study focused on the effect of water recycling, water temperature
and absorption pressure on the product gas quality and trace compounds.
The pilot system has been used in a previous study Rasi
et al. [8] and the findings in that study were used to set up the research
objectives of this study. Contrary to that study, in the present
study focus was in water recycling, water quality, product gas
trace compounds and exhaust gas as well as longer term trials on
previously established process parameters.