Page 175 - Advances in bioenergy (2016)

Recently, mixed metal oxide sorbents of Li and Na such as Li ZrO , Li SiO , and Na ZrO 3
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3
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have become subject of great interest. They have a higher CO capacity than HTC and lower
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temperature is needed for regeneration as by the Ca-based sorbents.
In situ CO capture involves suitable reaction conditions under which the whole process can
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be carried out. This requirement limits the available techniques for CO capture, e.g.,
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membrane and cryogenic separation processes require high pressure or low temperature for
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efficient CO capture, adsorption of CO with carbon based adsorbents is limited to low
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temperatures (150–250°C) and hydrotalcite compounds have a significantly diminished
capacity for CO capture beyond 300°C. 50
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Although many metal oxides react with CO to form carbonates, not all metal oxides are
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suitable for CO capture under conditions for biomass gasification. 51
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CaO is capable of scavenging CO to very low concentrations at moderate temperatures (450–
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750°C) and atmospheric pressure. Furthermore, it is low cost and abundant because it can be
derived from naturally occurring precursors including limestone, dolomite, and calcium
hydroxide. The enhanced output from biomass gasification when coupled with CO capture
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using CaO is demonstrated based on thermodynamic equilibrium modeling. 52

The increase of H concentrations corresponding to the capture of CO by CaO is shown in
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Figure 7.9. As can be seen operating window for temperature is 460–600°C. In this
temperature range the highest H yield was achieved. With rising temperature H content is
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decreasing because of deactivation of CaO sorbent. When the sorbent reaches its ultimate
conversion, it can be regenerated to produce CO and CaO by heating to 700–950°C,
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depending on the CO partial pressure. Although the regeneration process requires the input of
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energy, the heat generated by the exothermic absorption reaction partially offsets this demand.

The application of CO capture to enhance H production from biomass coupled with the
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production of a highly concentrated stream of CO represents a suitable and renewable
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pathway for hydrogen production. Furthermore, if the process is combined with CO storage a
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negative balance of CO in the atmosphere can be achieved.
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Demonstration of this process was already done at the biomass CHP Güssing, but the
demonstration plant in Germany was not realized for different reasons.