The Econmist - USA (2021-11-06)

70 Science&technology TheEconomistNovember6th 2021

originalemissionthusdealtwith,lessce­

mentisneededfora givenjob,lowering

overallemissionsstillfurther.McKinsey,a

consultancy,reckonsreverse calcination

could,atpresent,sequesterupto5%ofce­

ment’semissions.Asthetechnologyim­

provesit expectsthatmightriseto30%.

Several companiesare starting down

thisroute.CarbonCure,aCanadianfirm,

hasfittedequipmentwhichinjectsCO 2 in­

toready­mixedconcretetomorethan 400

plantsaroundtheworld.Itssystemhas

beenusedtoconstructbuildingsthatin­

cludea newcampusinArlington,Virginia,

forAmazon,anonlineretailer(andalsoa

shareholder inCarbonCure), andan as­

semblyplantforelectricvehicles,forGen­

eralMotors,inSpringHill,Tennessee.

AtpresenttheCO 2 usedbyCarbonCure

hasbeencapturedbyindustrial­gascom­

panies. Butfirmsare developing equip­

mentintendedtocollectthegasdirectly

fromcementkilns. And Calix,basedin

Sydney,Australia,isworkingonanelectri­

cally powered system which heats the

limestoneindirectly,fromtheoutsideof

thekilnratherthantheinside.Thaten­

ablespureCO 2 tobecapturedwithouthav­

ingtocleanupcombustiongasesfromfuel

burntinsidethekiln—so,iftheelectricity

itselfcamefromgreensources,theresult­

ingcementwouldbecompletelygreen.

Apilotplantusingthistechnologyhas

runsuccessfullyaspartofa EuropeanUn­

ionresearchprojectona siteinBelgium

operatedbyHeidelbergCement,a German

firmthatisoneofworld’sbiggestcement­

makers.A largerdemonstration plantis

duetoopenin2023,inHanover,tohelp

scaleupthetechnology.

Energisingrubbish

Another approach—less green, but still

betterthanusingfossilfuels—istosubsti­

tutesomeofthecoalburntinkilnswith

municipaland industrial waste. Several

firmsarealreadydoingthis.Cemex,a Mex­

icanbuilding­materialsgiant,forexample,

makesa kilnfuelcalledClimafueloutof

municipalwastethathasbeendenudedof

itsrecyclablesubstances.Thisisrich,in

theformofplantmaterial(“biomass”),in

carbonthathasrecentlybeenintheatmo­

sphere,andissimplyreturningthere,rath­

erthanhavingbeendugupasfossilfuel.

Upto60%ofthecoalusedbysomeofCe­

mex’sBritishcementplantshasbeenre­

placedwithClimafuel.

Companiesarealsolookingatwaysto

substitutesomeofthecementinconcrete

withothermaterials.Manyaddflyash,a

by­productofcoal­firedpowerplants,or

crushedslagfromtheblastfurnacesused

to make iron. But neither of these ap­

proachesissustainableinthelongrun.As

PeterHarrop,bossofidTechEx,a firmof

analystsinCambridge,England,andthe

co­authorofa newreportonthefutureof

concreteandcement,observes,coaluseis

dwindlingandsteelproductionaspiresto

movetonewer,cleanertechnologies.

ForDrHarrop,animportantpartofthe

answeristo “tech­up”concreteinways

whichmeanthatlessofit willbeneededto

do particular jobs. This means adding

thingslikesyntheticandnaturalfibres—or

evengraphene,a substancestrongerthan

steelthatconsistsofsingle­layersheetsof

carbon atoms. Only small amounts are

neededtoproducebeneficialresults.

Grapheneandotherreinforcementwill

leadtonew,ultra­high­performancecon­

cretes,whichDrHarropthinkswillbepar­

ticularly suitable for 3 d printing. This

The ins and outs of making cement

Manufacturing processes per one tonne of cement

Source:McKinsey

CO2 emissions

Kilograms

Energy used

Megajoules

Total: 3,895 MJ 95 kg

Cooler; cement mill;

logistics

Kiln and calcinator

Calcination

479

Fossil fuels

319

Quarry; crusher;

transport; raw mill

↑

↑

185

3,150

(^56099)

8
M
ostpeoplewhothinkaboutsuch
things agree that replacing fossil
fuels with renewable electricity, either
directly or indirectly, is the best way to
decarbonise industry, transport and the
heating and cooling of buildings. But
there are some holdout areas where this
is hard. Cement (as described in the
adjacent piece) is one. Aviation is anoth­
er, because batteries are too heavy and
hydrogen (which could be made using
renewable electricity) too bulky to do the
job easily. Hydrocarbon aviation fuels are
thus likely to be around for a while.
But such fuels need not be fossil. They
might be synthesised from the CO 2 ex­
haust of various industrial processes.
And a study just published in Nature, by
Aldo Steinfeld of ethZurich, a techno­
logical university in Switzerland, and his
colleagues, shows how they might liter­
ally be plucked from thin air.
Dr Steinfeld and his team devised and
tested a system that, in essence, reima­
gines the natural process of photosyn­
thesis. Plants take in atmospheric CO 2
and water and, with sunlight providing
the energy, turn those raw materials into
organic molecules. And that is exactly
what Dr Steinfeld has done.
The process has three stages. The first
absorbs CO 2 and water from the atmo­
sphere using a so­called direct­air­cap­
ture device made by Climeworks, a spin­
off of ethfounded by two of Dr Stein­
feld’s students that made the news re­
cently by opening a demonstration car­
bon­capture­and­storage system in
Iceland. There, however, the CO 2 is re­
acted with basalt rock to dispose of it. Dr
Steinfeld’s system makes use of it.
The second stage is the clever bit. It
employsconcentratedsunlighttoheat a
material called cerium oxide which,
when so heated, reacts with both CO 2 and
water. The reaction with CO 2 creates
carbon monoxide. The one with water
creates hydrogen. In both cases the by­
product is oxygen, which is vented into
the atmosphere. But a mixture of carbon
monoxide and hydrogen is a familiar one
to industrial chemists. It is called syngas,
and is widely used as a raw material to
make other things.
The third part of the process is there­
fore to turn the syngas into organic
molecules. For the hydrocarbons that
make up aviation fuel an industrial
chemist would normally turn to what is
known as the Fischer­Tropsch process.
For their demonstrator, the team chose
another route, which led to methanol
rather than hydrocarbons. But the gener­
al idea is the same.
The team’s demonstration rig, which
they installed on the roof of eth’s Mach­
ine Laboratory Building, had a typical
yield of 32ml of pure methanol per sev­
en­hour day—tiny, but a clear proof of
principle. A back­of­the­envelope calcu­
lation suggests that substituting the
world’s aviation­fuel market entirely in
this way would need 45,000km^2 of suit­
ably insolated land. That sounds a lot,
but is equivalent to about 0.5% of the
area of the Sahara Desert. 
Air­captured aviation fuel would
certainly need its path to market
smoothed by appropriate carbon taxes
on the fossil variety, and possibly other
measures. But Dr Steinfeld’s rig does
seem to have demonstrated a credible
and potentially scalable way to go about
making the stuff.
Greenaviation
Liquid sunshine
Away of combining atmospheric CO 2 and water to make aircraft fuel