A way to reduce the CO2 emissions from the transportation sector is by increasing the
use of biofuels in the sector. DME and methanol are two such biofuels, which can be
synthesized from biomass, by use of gasification followed by chemical synthesis. This
method of producing biofuels is shown to be more cost-effective, less energy consuming
and less CO2 emitting, when considering the total well-to-wheel processes, than first
generation biofuels and second generation ethanol produced by biological
fermentation. It is also shown that trustworthy sources in literature (the IPCC and IEA
Bioenergy) estimate the global biomass resource to be sufficiently great to allow the use
of biomass for fuels and chemicals production. IEA Bioenergy even indicate that it might
be more appropriate to use biomass for fuels and chemicals production than for
electricity production because few and expensive renewable alternatives exists for
biomass in the fuels and chemicals sector, but many cost effective renewable
alternatives exists for biomass in the electricity sector.
The objective of this study was to design novel DME and methanol plants based on
gasification of biomass, with a main focus on improving the total energy efficiency of the
synthesis plants, and lowering the plant CO2 emissions - but also try to improve the
DME/methanol yield per unit biomass input, and integrate surplus electricity from
renewables in the production of DME/methanol.
This objective lead to the design of the following plants: 1. Large-scale DME plants based
on gasification of torrefied biomass. 2. Small-scale DME/methanol plants based on
gasification of wood chips. 3. Alternative methanol plants based on electrolysis of water
and gasification of biomass.
The plants were modeled by using the component based thermodynamic modeling and
simulation tools Aspen Plus and DNA.
The large-scale DME plants based on entrained flow gasification of torrefied wood
pellets achieved biomass to DME energy efficiencies of 49% when using once-through
(OT) synthesis, and 66% when using recycle (RC) synthesis. If the net electricity
production was included, the total energy efficiencies became 65% for the OT plant, and
71% for the RC plant (LHV).
By comparing the plants based on the fuels effective efficiency, it was concluded that
the plants were almost equally energy efficient (73% for the RC plant and 72% for the
Because some chemical energy is lost in the biomass torrefaction process, the total
efficiencies based on untreated biomass to DME were 64% for the RC plant and 59% for
the OT plant.
CO2 emissions could be reduced to 3% (RC) or 10% (OT) of the input carbon in the
torrefied biomass, by using CO2 capture and storage together with certain plant design
changes. Accounting for the torrefaction process, which occurs outside the plant, the
emissions became 22% (RC) and 28% (OT) of the carbon in the untreated biomass.
The estimated costs of the produced DME were $11.9/GJLHV for the RC plant, and
$12.9/GJLHV for the OT plant, but if a credit was given for storing the bio-CO2 captured,
the cost became as low as $5.4/GJLHV (RC) and $3.1/GJLHV (OT) (at $100/ton-CO2).
The small-scale DME and methanol plants achieved biomass to DME/methanol
efficiencies of 45-46% when using once-through (OT) synthesis, and 56-58% when using
recycle (RC) synthesis. If the net electricity production was included, the efficiencies
increased to 51-53% for the OT plants (LHV) - the net electricity production was zero in
the RC plants. The total energy efficiencies achieved for the plants were 87-88% by
utilizing plant waste heat for district heating.
The reason why the differences, in biomass to DME/methanol efficiency, between the
small-scale and the large-scale plants, showed not to be greater, was the high cold gas
efficiency of the gasifier used in the small-scale plants (93%).
By integrating water electrolysis in a large-scale methanol plant, an almost complete
conversion of the carbon in the torrefied biomass, to carbon in the produced methanol,
was achieved (97% conversion). The methanol yield per unit biomass input was
therefore increased from 66% (the large-scale DME plant) to 128% (LHV). The total
energy efficiency was however reduced from 71% (the large-scale DME plant) to 63%,
due to the relatively inefficient electrolyser.
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