CO2 Mitigation With Microalgae Systems (1)

8/12/2019 CO2 Mitigation With Microalgae Systems (1)

1/5

~ Pergamon Energy Convers Mgmt Vol. 38, Suppl., pp. 475- 479, 1997 1997 Elsevier Science Ltd. All rights reservedPrinted in Great Britaine l l : S0196-8904 96)00313-5 0196-8904/97 17.00 + 0.00

C O 2 M I T I G A T I O N W I T H M I C R O A L G A E S Y S T E M SJ o h n R . B e n e m a n n343 Carave l le Drive , W alnut Creek, CA 94598, U .S .A.

SUMMARYM icroalgae present one of the few technologies for the capture and ut i lizat ion of CO 2 emi t ted bypow er p lan t s. These m ic roscop ic p lan t s wou ld be g rown in l a rge open ponds , in to wh ich pow erp lan t f lue gas o r pu re CO 2 (captured from pow er p lants) i s sparged, and, af ter harvest ing , thebiomass wo uld be conve rted to a fossil fu el replacement , preferably a h igh value l iquid fuel such asbiodiesel . Th e requi reme nts for large areas of land, favorable c l imate , and ample w ater suppl ieswi l l res t r ic t the potent ia l of th is technology. Also , even wi th ra ther favorable technical assump-t ions, the cu rrent ly projected costs of microalgae-fuels are h igh, s imi lar to m ost pow er p lant CO 2capture and disposal options. How ever, i f the technology of microalgae could achiev e very h ighproduct iv i t ies , equivalent to 10% so lar energy conversion, and i f projected low- cost cul t ivat ion ,harvest ing and processing techniques could be developed, microalgae technology could beco me alow-cos t CO2 mi t igat ion opt ion, particularly i f pr ices for fossi l fuels increase in the fu ture . In thenearer- term m icroalgae CO 2 ut il iza tion can be integrated wi th wastewater t reatment and reclamat ion,providing an ea rly appl ication of th is technology. Long-term basic and appl ied R&D are requi redto develop th is technology, as one of the man y options that may be requi red in the fu ture to helppres erve o ur plan etary a tmos pher e and biosph ere. 1997 ElsevierScienceLtd

KEYWORDS Microa lgae , C 02M i t iga t ion , Greenhouse Gases , G loba lWarming .I N T R O D U C T I O N

Red ucing the bui ld-up o f a tmospheric CO2, the major driv ing force in projected g lobal warm ing,can be accom pl ished by three conceptual ly di fferent methods:1 . reduc ing the use of fossi l fuels ;2 . rem oving CO 2 from the a tmosphere; and3. capturing and sequestering or u ti liz ing the CO 2 emit ted by fossil fuel com bust ion before i tenters the a tmosphere .

Th e f i rs t can be accom pl ished, for examples , by increasing the eff ic iency of pow er generat ion ,decreasing d em and through m ore effect ive energy ut il iza tion, or by subst itu ting fossi l fuels wi thn o n - C O 2 gener at ing energy sources (nuclear , b iofuels , o ther renewables) . Th e sec ond opt ion,rem oving CO 2 from th e a tmosphe re , is current ly only achievable wi th h igh er p lants , e .g . t rees , tobui ld-up the carbon content of the b iosphere , mainly in s tanding forests a nd soil s. Thes e indi rectmi t igat ion opt ions are the focus o f most o f the current technological developm ent and CO 2mitigation activit ies. Ho we ver, i t is l ikely, even certain, that these will not be eno ug h to stabilizea tmospher ic CO 2 levels suff ic ient ly to avoid a fu ture greenhouse w orld .Thus , the th i rd opt ion, the capture sequestra tion or u t i l iza t ion of CO 2 from pow er p lants and otherfossi l fuel comb ust ion systems, the focus of This Con ference, wi l l a l so need to be con sidered inECM 38/SUPI Q 475


2/5


3/5

B E N E M A N N : C O _, M I T I G A T I O N B Y M I C R O A L G A E 4 77product iv it ies projected for m icroalgae systems ov ercom e at least som e of the l imi ta t ions of h igherplant systems - principal ly the need for very large land areas . Indeed, microalgae systems can useland, such as hard-pan a nd high c lay soi ls , and w ater resources, suc h as wa ste or brackish wa ters ,not su i table for convent ional agricul ture or forestry , m inimizing the compet i t ion wi th food and f iberproduct ion.Th i s de fense o f mic roa lgae R&D fo r CO 2 mi tigat ion i s not only requi red to coun ter the argum entsma de by det ractors of th is technology, but , perhaps most cri tica l , to neut ra l ize the ove rblownrhetoric and unsupportable c la ims by m any of i ts som et imes overenthusiast ic proponents . M icro-algae wiU not l ikely be a m ajor fu ture fuel source , e i ther in the U.S . or g lobal ly , despi te som eclaims to the cont rary . No r can the development of h ighly com plex and c learly exorbi tantlyexpensive ph otobioreactors be just i f ied in the context of CO 2 mi t igation and fuel produ ct ion.He re I presen t the conclusions of a recent , updated , conceptual econom ic analysis of microalga ep o w e r p l a n t C O 2 ut i liza t ion and conversion to h igh value l iquid fuels (Be nem ann and Osw ald ,1996), and discuss a potent ia l pathway fo r the fu ture develop men t of th is technology . F i rs t , how -ever, I address one of the cr i t ica l i ssues in m icroalgae production: the achievable product iv i t ies ofmicro algal systems.

M I C R O L G E P R O D U C T I V I T IE SOn e o f the c la ims fo r microalgae cul tures i s that they have the potential for ach ieving very h ighproduct iv i t ies . Ho wev er, such projections are often based on se lect ive ext rapola tions of limi teddata from sho rt - term and smal l -scale pond operat ions, ev en of laboratory experiments . Notw i th-s tanding som e assert ions to the cont rary , microalgae have no part icular or inherent c la ims tounusu al ly h igh product iv i ties. Microalgae conta in the sam e photosynthet ic mac hinery present inhighe r p lants . Indeed, the est imated ( in the absence of re l iable publ ished data) that the product iv i tyof m icroalgae in comm ercia l operations and from wastew ater t reatment ponds i s about 20 to 30mt/ha/yr , not rem arkable wh en com pared to i rrigated crops u nder s imi lar c limat ic condi tions.Ho we ver, m icroalgae cul tures exhibi t some features that argue for potent ia lly h igher product iv i ties :

con tinuou s prod uction avoids the establishment periods of conv ention al plants; abi l i ty to prov ide opt imal nut r ient levels (e .g ., CO 2, N, P , e tc . ) a t a ll t imes; abse nce of non-ph otosynthet ic support ing s t ructures (roots, s tems, fru i t s, e tc . ) . abil i ty to adjust harvest rates to keep culture densit ies at optimal levels at al l t imes; an d cont ro l over cel l com posi t ion (for h igh o i l contents , for example) , wi thout decreasingprodu ctivit ies - m axim izing potential fuel prod uction an d C O 2 util iTation.

St i ll , these argum ents are no t s t rong - and maxim al product iv it ies for microalgae are unl ikely toexce ed the max imal levels observed wi th h igher p lants, such as sugar cane, grow n under opt ima lcondi t ions o f sunl ight , temperature , wa ter supply and fert il izers - about 100 mt /ha/yr of dr y b io-mass , corresp onding to about 3 .5 - 4 to ta l (7 - 8 v is ib le) so lar l ight energy ) conve rsioneff ic iency. Ev en achieving such levels wi l l requi re considerable R& D. For exam ple , techniquesm ust be d eve lope d to ma intain tmialgal cultures of specific, desirable algal strains, prev entinginvasion by other a lgae , zooplankton grazers and bio t ic infect ions general ly . An other i ssue is howto m inimize the respi ra t ion by a lgal ce l l s a t the h igh ox ygen tensions present in mass c ul ture ponds.Ho we ver, in th e laboratory , photosynthet ic v is ib le l ight conversion eff ic iencies ( light energ yt ransform ed in to b iomass h igher heat ing value) are rout inely me asured a t 20 - 24 . But th is i sonly a chie ved at low light intensities: at high l ight levels the photosynthetic apparatus is unable toprocess a l l the photons absorbe d by the chlorophyl l and other p igments . This resul t s in the a lgalcel l s wast ing from 50 to 80 of the incident l ight, and i s one the major reasons that a lgal cul tureproduct iv i ties a t the h igh l ight in tensit ies typical of sunl ight are muc h low er than in laboratoryexperiments .This proble m has been recogn ized for over forty years 1950 's (Vandeva r Bush , 1953), and asolut ion proposed ne arly as long ago: reduce the so-cal led antenna p igments of the photosynthet icapparatus , to prevent th is wastage of l ight energy. Al though th is a pproach i s yet to bedem onst ra ted in the laboratory , le t a lone in pract ice , i t provides a c lear experimental approach to


4/5

478 B E N E M A N N : C O2 M I T I G A T I O N B Y M I C R O A L G A Eachieving the h igh solar conversion ef f ic iencies requi red for microalgal CO 2 fixation and fuelproduct ion systems. The a l ternat ive approach, a lso recognized for many decades , i s to usedevices such as opt ical fibers, to d isperse l ight even ly throughout an a lgal culture . This can b edem ons trated in the laboratory to increase culture productivit ies, but is impractical in any scale-up.

M I C R O A L G A E P R O D U C T I O N S Y S T E M SM icroalgal b iomass, l ike o the r p lant b iomass, i s potent ia l ly sui table for conversion to l iquid (gas-ol ine , b iodiesel , e thanol) and gaseous (methane, and hydrogen) fuels. Ho we ver , even laboratorydata on su ch conversions i s l imi ted and scale-up has not been a t tempted. Fur thermore, the pro-duc t ion o f t he a lga l b iomass fo r ene rgy p roduc tion has no t been a t tempted beyond a f ew smal l-scale outdoo r s tudies , of l imi ted durat ion, wi th the largest s tudy using tw o 0 .1 ha pon ds operatedfo r a two seasons in N ew M exico (Wei ssman and Ti l le t t, 1990 , unpub li shed). Thus , any d i s -cuss ion and ana lysi s o f m ic roa lgae t echno logy fo r pu rposes o f CO 2 mit igat ion and fuel product ionmu st be bas ed o n ext rapolations f rom other systems, in par ticular the com me rcia l product ion ofmicroa lgae for hum an foods and , perhaps most applicable, microalgae wastew ater t reatment ponds.The s t andard com merc i a l t echno logy fo r mic roa lgae b iomass p roduc t ion uses r aceway- type , padd lewh eel mixe d ponds. Such system s are f lexible , re la t ively easi ly scalable , and of low cost . Thre ecom me rcia l p lants based on th is design, ranging f rom about 5 to 15 ha in s ize , are operat ing in theU.S. , producin g high-v alue food supplem ents (beta-carotene, Spi rul ina b iomass) . N o al ternat ivedesign i s apparent , despi te scores of photobioreactor concep ts proposed. For CO 2 mi t igat ion andenergy p roduc t ion r aceway ponds wou ld be used , excep t tha t ind iv idual g rowth ponds w ou ld beover 5 h a in s i ze (vs . 0 .5 ha n ow) , and they w ou ld be un l ined (o r use a s imple c l ay l i ne r, no texpens ive p l ast i c l ine r s a s used in comm erc i a l sys tems) . Harves t ing wou ld a l so need to be o f l owcost , whic h restr ic ts considerat ion to s imple se t tl ing or s tra in ing processes. This cou ld be fo l low edby a cent r i fugat ion or chem ical flocculat ion step, to increase the densi ty of the b iom ass to a l lowextract ion of the o i l s or fermen tat ion of carbohydrates to e thanol . Th e residues would be ferm entedto m ethan e, to recove r as mu ch fuel values as possible , and w ater , nut rients , CO 2, and wa stesrecycled.Ini t ia l proposals o f th is con cept of m icroalgae for CO 2 ut i lizat ion and conversion to fuels dates bac kover f o r ty yea r s and i t has been deve loped exper imen ta l ly and concep tua l ly by Osw ald andGo lueke (1960). Th ey concep tual ized large earthen growth ponds located near the pow er p lant ,wi th the b iomas s conver ted to me thane by anaerobic d igestion. M ost of the water and residues( con ta in ing the N , P and o the r nu t ri en t e l emen t s ) were to be r ecyc led to t he pon ds , a long wi th anyC O 2 f rom the conve rsion process. Al thou gh th is in i t ia l assessment was super ficia l, m ore deta i leddev elopm ent of th is conce pt car r ied out in the 1970 's and ear ly 1980 's , in response to the energ ycr i s is ( r ev i ewed in Be nem ann an d O swald , 1996), a l so conc luded that , w i th f avorab le a ssump-t ions, such system s could potent ia l ly provide a comp et i t ive fuel source. S ince the 1980 's work inthis f ie ld has e mp hasized the product ion of h igh value l iquid fuels , in p ar t icular b iodiesel , theme thyl or e thyl esters of veg etable o il s.A rec ent ly updated analysis (Benem ann and Osw ald, 1996) of several a l ternat ive cases (d i rect useo f f l ue gas and cap tu r ed CO 2, a t product iv it ies equivalent to 5% and 10% so lar conversionef f ic iencies , and cru de oi l pr ices of $25 and $35/bar re l) conclu ded that CO 2 mit igat ion costs w ouldbe $10 0/mt C O 2 for the least favorable case and less than $10/mt for the most favorable. The ma jorconc lusion wa s that no insurmou ntable show-stoppers we re apparent, but that considerablelong- term R& D, in par t icular in the areas of b iomass product iv ity and cul ture s tabil ity w i l l berequired to ach ieve such cost -goals .

C O N C L U S I O N SD e s p i te o v e r fo r ty y e a rs o f R & D , w i th s o m e $ 5 0 m i l li o n i n s u pp o rt b y t h e U .S . D e p a ~ e n t o fEnergy over t he pas t two decades , m ic roa lgae t echno log ies f o r CO 2 f ixation an d fue l product ionhave ba r e ly advan ced beyond the concep tua l s tage. An even l a rge r, bu t more r ecen t , Japaneseprog ram QOsui and Ike nouc hi , in The se Proceedings) has conce ntra ted on c losed photobioreactors .How ever , desp i t e f if t y yea r s o f deve lopm en t , c losed pho tob io reac to r s have no t ye t been even


5/5

BENEMANN: CO_, MITIGATION BY MICROALGAE 479applied in the com mercia l production of h igh value (> $100/kg biomass) microalgae products . Andthe current com mercia l microalgae production systems, using o pen pond s designs, produce only afew hu ndre d tons of biomas s annually, at costs approaching $10,000/rot. The g oals of m icroalga eproduction for CO 2 capture and fuel production require alm ost one o rder of ma gnitu de higherproduc tivities (mt/ha/yr), two orders of magnitud e low er production costs ($/mt), and three ordersof m agn itude higher production levels (mt/yr) than current systems - dau nting goals indee d.Co upled with th e climatic restrictions, lan d requirements, a nd limited numb ers of sites w here suchsystem s could be realistically established in the U.S., the prospects for deve lopin g micro algae-b a s e d CO2 mit igat ion processes might appear doubtful . How ever , past R& D programs d id not , inm ost part, ad dress central issues of this technology , resulting in lim ited progress. A nd resourceand tech nolog y limitations are apparent for most of other CO 2 capture and disp osal/utilizationoptions. Inde ed, eve n a dozen such facilities, comp rising a few thousand hectares, w ould result inover a m i l l ion tons o f CO 2 mitigated, a significant im pact.Finally, and perhaps m ost important, som e nearer-term approaches to micro algae CO 2 utilization/recyc ling are possible by com binin g such systems w ith wastewater treatment. Indeed , the earlierw ork in this field clearly identified he potential of such approaches both conceptually (Osw ald andGolueke , 19 60) and experim ental ly (Benem ann et a l . , 1980). Wastew ater treatment provides bothresource s (water, nutrients) and eco nom ic incentives for such systems, allow ing smaller-scale andmore econom ic sys tems fo r CO 2 utilization-providing a pathway for the d evelo pm ent and eventu alcom me rcialization of this technology.

R E F E R E N E SBe nem ann, J .R. , Uti l iza t ion of Carbon Dioxide from Fossi l Fuel-Burning Power Plants w ithBiolog ical Systems, Energ y Conserv. Mgm t. , 34 :99 9 - 1004 (1993).Be nem ann , J .R. , and W .J. Osw ald, Svstems and Economic Analysis of Microalgae Ponds forCon version of CO , to Biom ass. F inal R eport to the Pi ttsburgh E nergy Tecfmology Center .U.S. Dep t . of Energy (March 1996).Be nem ann , J .R. , in Algal Biotechnologv, in R.C. Cresswell , T .A.V. R ees, and N. Shah, eds . ,Lon gm an, Londo n pp. 317 (1990).Be nem ann , J .R. , B.L. Koo pma n, J .C. W eissman, D.E. Eisenbe rg and R.P. Goeb el , in G.She lef, and C .J. So eder, eds., Algal Biomass, Elsevier, Amster., p. 457 (1980).Bu sh, V ., qu ote d in J. B urlew , Alga e Culture: From Laboratorv to Pilot Plant, Carn egie Institute,W ashin gton D.C. (1953).Osw ald, W.J . , and C.G. Golueke, Solar Energy Conversion with Microalgae System s, Ad v.Appl. Microbiol . . 11:2 23 (1960).

Documents

CO2 Mitigation With Microalgae Systems (1)