Is car­bon-neg­a­tive the an­swer?

Car­bon-neg­a­tive tech­nolo­gies se­quester more car­bon than they re­lease. But ex­perts say they can­not be a sub­sti­tute for re­duc­tion of green­house gases

Down to Earth - - CONTENTS - DOWN TO EARTH INDU MATHI S

Ex­perts ques­tion the viability of car­bon-neg­a­tive tech­nolo­gies

AMONG THE var­i­ous strate­gies sci­en­tists are work­ing on to tackle cli­mate change, car­bon-neg­a­tive tech­nolo­gies have gained a lot of at­ten­tion in the past cou­ple of decades.As the name sug­gests, the tech­nolo­gies help re­duce car­bon in the at­mos­phere by cap­tur­ing and se­ques­ter­ing it. But are they re­ally ef­fec­tive, or even safe?

Ac­cord­ing to Pete Smith, pro­fes­sor at the Uni­ver­sity of Aberdeen, Scot­land, neg­a­tive-emis­sion tech­nolo­gies have been proved only the­o­ret­i­cally in as­sess­ment mod­els and the tech­nolo­gies are con­tro­ver­sial. Re­ly­ing heav­ily on such tech­nolo­gies is risky, says Smith. He gives the ex­am­ple of bioen­ergy with car­bon cap­ture and stor­age (beccs), a rel­a­tively new en­trant in the field of car­bon-neg­a­tive tech­nolo­gies.

In beccs, a large quan­tity of biomass (high-en­ergy crops such as poplar, wil­low or tem­per­ate grasses) is burnt to pro­duce elec­tric­ity and the car­bon emis­sions are cap­tured and se­questered in de­pleted un­der­ground reser­voirs or in­jected into por­ous rocks. This re­sults in net re­duc­tion in the at­mo­spheric car­bon diox­ide (see ‘Off­set­ting dam­age’). The tech­nol­ogy can be em­ployed in power plants or other in­dus­tries that make fuel.

Ac­cord­ing to a study by re­searchers at the Uni­ver­sity of Cal­i­for­nia, Berke­ley (ucb), usa, use of beccs could make power gen­er­a­tors store more car­bon than they emit. Their study, based on a com­puter model of the west­ern North Amer­i­can elec­tri­cal power grid, says a com­bi­na­tion of beccs, re­new­able en­ergy and fos­sil emis­sions re­duc­tions can cre­ate a car­bon-neg­a­tive power sys­tem in west­ern North Amer­ica by 2050, with up to 145 per cent emis­sions re­duc­tion from 1990 lev­els. The study was pub­lished in Na­ture Cli­mate Change on Fe­bru­ary 9.

Opin­ions, how­ever, re­main di­vided over beccs’ prac­ti­ca­bil­ity. “beccs may sound ideal, but it has many com­mer­cial un­cer­tain­ties,” says Daniel Sanchez, a grad­u­ate stu­dent in ucb’s En­ergy and Re­sources Group, who was part of the study. “Nev­er­the­less, we’re tak­ing this tech­nol­ogy and show­ing that in the West­ern United States 35 years from now, beccs does not merely let you re­duce emis­sions by 80 per cent but gets the power sys­tem to neg­a­tive car­bon emis­sions.”

Doubts have also been raised about the safety of un­der­ground se­ques­tra­tion of car­bon. Ac­cord­ing to a 2014 re­port by the In­ter­gov­ern­men­tal Panel on Cli­mate Change, it can cause con­tam­i­na­tion of wa­ter aquifers or leak into the at­mos­phere. The re­port also said that in­jec­tion of car­bon diox­ide in un­der­ground por­ous rocks can trig­ger tiny earth­quakes which, in turn, can re­lease the car­bon diox­ide.

Daniel Kam­men, pro­fes­sor at ucb and direc­tor of the Re­new­able and Ap­pro­pri­ate En­ergy Lab­o­ra­tory, where the re­search was con­ducted, has a dif­fer­ent view. He says beccs could be one of the few cost­ef­fec­tive car­bon-neg­a­tive op­por­tu­ni­ties avail­able to mit­i­gate the worst ef­fects of an­thro­pogenic cli­mate change. This strat­egy will be par­tic­u­larly im­por­tant should cli­mate change be worse than an­tic­i­pated, or emis­sions re­duc­tions in other por­tions of the econ­omy prove par­tic­u­larly dif­fi­cult to achieve, he says.

Sim­i­lar tech­nolo­gies

Apart from beccs, there are a few other car­bon-neg­a­tive tech­nolo­gies which are be­ing stud­ied. Ac­cel­er­ated weath­er­ing of mag­ne­sium ox­ide-bear­ing rocks is one such tech­nol­ogy in which ig­neous rocks con­tain­ing cal­cium and mag­ne­sium sil­i­cates are made to re­act with at­mo­spheric car­bon diox­ide. This re­sults in the pro­duc­tion of car­bon­ates that store car­bon. The tech­nol­ogy, how­ever, is quite ex­pen­sive, cost­ing US $1,000 for ev­ery tonne of car­bon diox­ide re­moved, says Jen Wil­cox, pro­fes­sor of en­ergy re­sources en­gi­neer­ing at Stan­ford Uni­ver­sity, usa.

Di­rect air cap­ture of car­bon diox­ide is an­other ex­am­ple.It in­volves us­ing ma­te­ri­als such as amines, min­er­als and ze­o­lites, sodium hy­drox­ide and ac­ti­vated car­bon to di­rectly cap­ture car­bon diox­ide from ex­haust gases from in­dus­tries and other sources.

Biochar, a char­coal made by slowly heat­ing veg­e­ta­tion in the ab­sence of oxy­gen, is yet an­other ex­am­ple. Biochar stores car­bon, pre­vent­ing its es­cape into the at­mos­phere. It can be used as a fer­tiliser but is harm­ful to earth­worms (see ‘Car­bon curbs’).

Re­duc­ing GHG a must

Car­bon neg­a­tive tech­nolo­gies can be used along with mea­sures to re­duce car­bon emis­sions.

“Plan A must be to re­duce green­house gas emis­sions. A fail­ure of beccs or other neg­a­tive-emis­sions tech­nolo­gies to de­liver ex­pected mit­i­ga­tion in the fu­ture leaves us with no plan B,” warns Smith. He also says that over-re­liance on beccs and other neg­a­tive emis­sions sys­tem in the fu­ture, along with con­tin­ued use of fos­sil fuel, may be ex­tremely risky be­cause our abil­ity to sta­bilise the cli­mate de­creases with the in­crease in emis­sions.

His view is mir­rored by the Na­tional Academy of Sciences which re­cently re­leased a two-vol­ume re­port on car­bon re­duc­tion. “Strate­gies to re­move car­bon diox­ide from the at­mos­phere are limited by cost and tech­no­log­i­cal im­ma­tu­rity, but they could con­trib­ute to a broader port­fo­lio of cli­mate change re­sponses with fur­ther re­search and devel­op­ment,” says the re­port.

Off­set­ting dam­age

Source: Na­ture

Bioen­ergy with car­bon cap­ture and stor­age (BECCS) tech­nol­ogy pro­duces elec­tric­ity by burning biomass. The car­bon emis­sions are cap­tured and se­questered un­der­ground

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