It look interesting, however I am unconvinced. It turns out that the
seawater acidity is lowered only by concentrating out HCl, in
potentially huge amounts. Some of this may displace existing manufacture
of HCl by chemical industry, but beyond that it's a hazardous waste.
Then there is the problem of MgO discharge: as soon as the ocean is made
alkaline, that provokes precipitation of carbonate, rather than
formation of HCO3- as solute. And then there is the increased energy
use, which even if from solar panels, might be more effectively used to
displace fossil generation.
Oliver.
On 01/02/2015 19:39, Andrew Lockley wrote:
Attached
On 1 Feb 2015 18:25, "Renaud de_Richter" <[email protected]
<mailto:[email protected]>> wrote:
*Thanks to Magnesium, desalination plants could become net
absorbers – rather than net emitters – of carbon dioxide*
http://www.rsc.org/chemistryworld/2015/01/desalination-plant-carbon-dioxide-source-sink
Switching desalination plants from carbon dioxide source to sink
22 January 2015 Katie Lian Hui Lim
<http://www.rsc.org/chemistryworld/more/?author=896>
A UK researcher has proposed a new process to decompose waste
desalination brine <http://xlink.rsc.org/?doi=10.1039/c4ew00058g>
using solar energy that could allow desalination plants to act as
a sink rather than a source of atmospheric carbon dioxide, and
*help to neutralise ocean acidity*.^1
( ^P A Davies, /Environ. Sci.: Water Res. Technol./, 2015, DOI:
10.1039/c4ew00058g <http://xlink.rsc.org/?doi=10.1039/c4ew00058g>
(This paper is free to access.))
Approximately 30 billion m^3 of freshwater is produced by
desalination each year, and this is predicted to double within the
next decade
<http://www.globalwaterintel.com/market-intelligence-reports/> to
meet global demand.^To combat the increased energy consumption and
carbon dioxide emissions associated with this growth in capacity,
research efforts have turned to employing renewable energy.
In the system devised by Philip Davies
<http://www.aston.ac.uk/eas/staff/a-z/dr-philip-davies/> at Aston
University, magnesium chloride in waste brine is hydrolysed by
energy generated by heliostat fields to magnesium oxide, which is
discharged to the ocean. Due to its alkaline nature, this
subsequently neutralises ocean acidity and gradually removes
carbon dioxide through the conversion of magnesium oxide to
bicarbonate, similar to ocean liming, with the advantage that the
neutralising material is sourced from the seawater itself rather
than mined. Hydrochloric acid produced as a byproduct could
potentially be sequestered into silicate rocks.
Although this approach would increase the energy requirement of
the plant by 50%, Davies calculates that this is offset by the
carbon dioxide absorption capacity; each plant would remove 18,200
tonnes of carbon dioxide per year rather than emitting 5300
tonnes. This would result in 0.4% of anthropogenic carbon dioxide
emissions being absorbed given a doubling in the current
desalination capacity.
Davies acknowledges that lowering the energy required to dewater
brine prior to decomposition would be a major benefit. ‘Not much
energy is needed to decompose magnesium chloride in brine to
magnesium oxide, which makes the use of solar energy potentially
very attractive,’ he says. ‘If we could find better ways to
dewater the brine this would become very energy efficient as a
means of avoiding carbon dioxide.’ He also warns that the effects
of magnesium oxide discharge on local marine environments should
be thoroughly assessed, a sentiment echoed by Silvano Mignardi
<http://www.dst.uniroma1.it/Mignardi>, an Earth scientist at the
Sapienza University of Rome in Italy: ‘Environmental issues
involved in the ocean discharge of magnesium oxide and in the
management of hydrochloric acid have to be carefully evaluated.’
Phil Renforth
<http://www.cardiff.ac.uk/earth/academic-staff/dr-phil-renforth/>,
a geo-environmental engineer from Cardiff University, highlights
that a major advantage of Davies’ process is that it can be
appended to existing technology. ‘This approach may allow the
industry to transform itself from a carbon dioxide villain into a
force for good in the climate change debate.
Le mercredi 28 janvier 2015 14:16:16 UTC+1, Schuiling, R.D. (Olaf)
a écrit :
I think that not everybody realizes that some 300 million tons
of CO2 are captured every year by the weathering of basic
silicates, notably the most common one, olivine. To
demonstrate this, the diagram below shows the analytical data
of some 20 spring water samples in olivine rocks in Turkey. It
shows what happens when rain falls on soils on top of olivine
rocks. The rainwater contains essentially only some CO2 and
has a pH in the order of 6. Then it penetrates the soil, which
has much higher CO2 concentrations in the soil atmosphere than
in the atmosphere above. Dead plant material is decaying, the
soil fauna is breathing, both releasing CO2, so the CO2
concentration of the soil atmosphere is often hundred times or
more higher than in the atmosphere. The water equilibrates
with this high CO2 concentration. Then it seeps into the rock,
and reacts with it, releasing magnesium to the solution, and
the pH rises to values around 7.5 to 8.5. This weathering
reaction can be written as
Mg_2 SiO_4 + 4 CO_2 + 4 H_2 O à2 Mg^2+ + 4 HCO_3 ^- + H_4
SiO_4 (so the CO2 is captured as bicarbonate in solution).
At some point this water is emitted again as a spring. This
spring water is very healthy, and we often had to wait in line
for the many people who collect this spring water in
containers and jerrycans to bring home. Most of the water
flows away in small brooks, and finally reaches the sea, where
the calcium and magnesium are used by plankton, corals and
shellfish to form limestones and dolomites, the ultimate
sustainable storage of the CO2.
Just as an afterthought: so if we irrigate semi-arid land on
top of olivine massifs, we have a cheap way to fix CO2 by
increasing the number and the volume of springs in such rocks,
Olaf Schuiling
I attach the paper in which these data were published
\
®
Fig.1: Concentration in meq [Ca^2+ + Mg^2+ ] in spring waters.
Total carbon as mg CO_2 .
® composition of rain water.
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