Electrolyser
Oxygen
O
Renewable electricity
H
H S CO
Hydrogen
Gasoline
Syncrude upgrading
FT Synthesis
H /CO Syngas
Diesel
Biomass
Gasi er
Reformer for tars removal
Syngas clean-up
Kerosene
Steam
Figure 4 Hybrid electro-biofuel production for sustainable aviation fuel
raw natural gas from the Bass Strait’s Gippsland Basin with 14% CO 2 . On the other hand, sweet shale gas from the Appalachian Basin contains less than 1% CO 2 . Biomass gasification for liquid fuels : Gasification of residual biomass, such as forestry cuttings and post-harvest waste, is a viable pathway to produce second-generation liquid biofuels. The syngas produced by gasification is very rich in CO, but it does not contain sufficient hydrogen to produce FT synthetic liquid fuels such as kerosene and diesel. In bio-electrofuels production, hydrogen-lean gasifier syngas is blended with electrolytic hydrogen to maximise the carbon yield. During gasification, the ideal chemistry would be for biomass to be converted only to hydrogen and CO. However, the gasifier cannot be controlled to this level of precision. Some heavier molecules break through as tars, and other molecules are combusted to produce CO2 (see Figure 4 ). Use of oxygen instead of air, injection of steam with the oxygen, and gasifier operation at high temperature around 1,400°C can maximise the conversion of carbon to CO and simultaneously minimise CO 2 production. This favours the use of a slagging entrained flow gasifier. For effective downstream conversion of the syngas to FT fuels, CO 2 and other acid gases such as hydrogen sulphide must be removed. This is most suitably achieved using a solvent-
based scrubbing and stripping system with an amine, HPC, or chilled methanol solvent. The captured CO 2 is of high purity and can be liquefied for sale to commercial applications. Waste-to-energy (WtE) : Biomass-fired power plants are premium targets for bioenergy carbon capture and storage (BECCS). They burn wood pellets, wood chips, or straw to create heat and power. Upcoming BECCS projects are responsible for more than 50% of all CDR certificates sold in the VCM to date. WtE facilities incinerate municipal solid waste (MSW) rather than biomass. However, MSW contains between 40 and 60% biogenic material, even after paper has been sorted for recycling and food and garden waste have been removed for conversion to biomethane. Since MSW facilities can only monetise the biogenic fraction of their captured CO2 for CDR in the VCM, the business case for BECCS here is weaker than for biomass-fired plants. However, in cases where there is a meaningful cost avoidance from CO 2 emissions reduction, BECCS from WtE plants can be profitable. Once a WtE facility has jumped the hurdle to invest in CO 2 capture to sequester the biogenic fraction of its emissions for CDR, it will be keen to monetise the fossil fraction to support the business case. This could be the next major low- cost commercial CO 2 source.
Stephen B Harrison sbh@sbh4.de
www.decarbonisationtechnology.com
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