1. Introduction - Firenze University Press

0.5 – 0.7 kg/kg, resulting typically in 60 – 66% extraction of Mg. Lower temperatures and longer
reaction times give a higher (relative) extraction of iron. Ammonia vapour, NH3, released during the
thermal step is collected and used to give the necessary pH increases for precipitation. It is
thereafter recovered for regeneration of the AS salt downstream, using heat from another process
step. Nonetheless, the recovery of solid ammonium sulphate from the aqueous form incurs a not
insubstantial energy penalty. More detail on the procedure is given by Nduagu et al. [24,25].
3.2.2. Mg(OH) 2 carbonation
The Mg(OH)2 produced as described above is converted into MgCO3 in a pressurised fluidised bed
(PFB) reactor at pressures > 20 bar and temperatures 450 – 600 °C. (Some more detail on the set-up
is given in section 7.2). Results on conversion levels obtained under varying conditions
(temperature, pressure, water content of the gas, time, fluidisation velocity) are reported elsewhere
[16,26,27] for both a synthetic, commercial Mg(OH)2 material and Mg(OH)2 produced from
Finnish or Lithuanian serpentinites. (A few tests were made under supercritical CO2 conditions,
pressure > 74 bar, which showed significantly lower conversion levels and rates, suggesting that
little benefit should be expected from operating at such pressure levels.) It was found that the
Mg(OH)2 materials produced from the serpentinites are much more reactive (as a result of a ~10×
larger specific surface of ~45 m 2 /g vs. ~5 m 2 /g), giving conversion levels of 50% within 15 minutes
for ~300 µm particles.
The product gas from the carbonator is a hot, pressurised mixture of CO2 and H2O, the solids
obtained will be partly recycled for further carbonation conversion. Unfortunately, although the
carbonation reaction is rapid it levels off at carbonation levels up to 65% (the best result obtained so
far) [27], which appears to be the result of calcination of Mg(OH)2 to MgO. However, it is noted
that in order for Mg(OH)2 to carbonate, dehydroxylation (i.e. calcination) has to occur. Apparently,
carbonation at some point becomes slower than dehydroxylation, resulting in a partially calcined
and carbonated product. Thus, below it is assumed that with ~2/3 of the Mg(OH)2 produced also
being carbonated the necessary amount of it is 150% of the stoichiometric amount.
3.2.3. Process energy input requirements
Since CCS is one of the solutions to what is in fact an energy problem, routes that lead to the
production of large amounts of CO2 while producing the power and heat for the CCS process are
obviously not viable. The Meri-Pori plant produces 820 g CO2/kWh electricity, thus CCS with an
electricity consumption of 1/0.82 = 1.22 kWh = 4.39 MJ/kg CO2 would have a zero net output of
both electricity and CO2. The use of electricity in CCS processes should be avoided although some
power consumption will follow from gas compression and crushing/grinding of solid material.
Fortunately, part of the energy input of a CCS processes would be in the form of heat and at ~ 43%
thermal efficiency the Meri-Pori plant produces similar amounts of electricity and (waste) heat.
CCS routes based on CO2 mineralisation appear to be more dependent on heat as energy input than
the “conventional” route that involves underground storage of CO2, while – as done in the ÅA route
– the heat output from the carbonation reaction can be benefitted from. (Therefore the higher
temperature of the carbonation step in the ÅA route, ~500 °C, compared to the earlier suggested
process route from the Albany Research Center (ARC), currently NETL Albany, in the US, results
in a better LCA (life cycle assessment) performance of the ÅA route compared to the ARC route
[28]. The ARC route is based on one-step carbonation in pressurised aqueous solutions at ~150 bar,
~185 °C [29,30].) At the same time, CO2 mineralisation routes that involve electrochemical steps
(electrolysis, fuel cells) are very unlikely to have a net CO2 fixation effect [31].
As presented at ECOS2010 [17] a quick assessment of energy input requirements for the ÅA route
can be made based on the reaction heat QE or HE needed for Mg extraction from rock and the heat
QC or HC released by Mg(OH)2 carbonation. Besides this, crushing/grinding of rock contributes to
only a few % of the energy input requirements while process integration and optimisation will result
in improvements to the energy efficiency [17].
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