By membrane separation will not give any strict
separation, but partial
enrichment. This means that
the content of methane in the produced gas may not be suitable for use as
vehicle fuel directly. By applying cryogenic techniques, a very clean CH4
can be achieved. This is favorable if the product should be used for vehicles.
At the same time, also CO2 can be separated, which may be of
high interest in possible future carbon capture and storage (CCS) plants. A
drawback of cryogenic techniques is the equipment costs, which limits the use
to large scale plants. The combination of gasification and CHP enables cleaning
of the gas from problematic substances that are evaporated and passing with the
gaseous phase such as chlorides and metal salts,
such as alkali salts.
It could facilitate the
introduction of biomass and waste in today´s existing CHP plants using fossil
fuel-based feed-stocks. Co-combustion with biomass and waste is generally not possible
in these plants, since the introduction would cause severe materials problems
in form of high temperature corrosion on heat exchanger surfaces, where the
chlorides and alkali metals are the major concerns. Since rebuilding of existing
plants is generally too expensive and therefore not an option, a gasification
process is a feasible step to extend the use of existing infrastructure and
simultaneously enable the introduction of renewable feedstock as well as future
CCS.
The economic conditions
combining CHP with biomass gasification are not yet verified, but rough
estimates appear to give reasonable figures. In case of black liquor
gasification, experimental investigations it is observed that the catalytic effect of the
alkali salts reduces the tar formation to almost zero in the product gas, while
the reaction temperature can be reduced significantly for the same gasifier
capacity compared to solid biomass.
By keeping a relatively low
temperature it may therefore be possible to get a high CH4 production with
respect to heating value. This could be interesting form applications, related
to smaller pulp and paper mills, as a complement, where today no recovery is made
at all of the spent liquors. The residual CO + H2, could also, in this case, be
used either in a CHP or only for steam production, where there is a significant
demand in all pulp and paper industries internally. For methane production, in
this particular case, cost effective separation technologies need to be
developed.
CHP IN COMBINATION WITH BIOMASS GASIFICATION
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