Trans. of the Korean Hydrogen and New Energy Society(2015. 6), Vol. 26, No. 3, pp. 247~259 DOI: http://dx.doi.org/10.7316/khnes.2015.26.3.247 ISSN 1738-7264 eissn 2288-7407 혐기성수소발효를결합한생물학적 2단공정의유기성폐자원처리및바이오에너지생산 이채영 1 ㆍ유규선 2 ㆍ한선기 3 1 수원대학교토목공학과 하천환경기술연구소, 2 전주대학교토목환경공학과, 3 한국방송통신대학교환경보건학과 Two-stage Bioprocesses Combining Dark H 2 Fermentation: Organic Waste Treatment and Bioenergy Production CHAE-YOUNG LEE 1, KYU-SEON YOO 2, SUN-KEE HAN 3 1 Dept. of Civil Engineering Institute of River Environment Technology, The University of Suwon, 17 Wauan-gil, Bongdam-eup, Hwaseong-si, Gyeonggi-do 445-743, Korea 2 Dept. of Civil & Environmental Engineering, Jeonju University, 303 Cheonjam-ro, Wansan-gu, Jeonju-si, Jeollabuk-do 560-759, Korea 3 Dept. of Environmental Health, Korea National Open University, 169 Dongsung-dong, Jongno-gu, Seoul 110-791, Korea Abstract >> This study was performed to investigate the application of dark H 2 fermentation to two-stage bioprocesses for organic waste treatment and energy production. We reviewed information about the two-stage bioprocesses combining dark H 2 fermentation with CH 4 fermentation, photo H 2 fermentation, microbial fuel cells (MFCs), or microbial electrolysis cells (MECs) by using academic information databases and university libraries. Dark fermentative bacteria use organic waste as the sole source of electrons and energy, converting it into H 2. The reactions related to dark H 2 fermentation are rapid and do not require sunlight, making them useful for treating organic waste. However, the degradation is not complete and organic acids remain. Thus, dark H 2 fermentation should be combined with a post-treatment process, such as CH 4 fermentation, photo H 2 fermentation, MFCs, or MECs. So far, dark H 2 fermentation followed by CH 4 fermentation is a promising two-stage bioprocess among them. However, if the problems of manufacturing expenses, operational cost, scale-up, and practical applications will be solved, the two-stage bioprocesses combining dark H 2 fermentation with photo H 2 fermentation, MFCs, or MECs have also infinite potential in organic waste treatment and energy production. This paper demonstrated the feasibility of two-stage bioprocesses combining dark H 2 fermentation as a novel system for organic waste treatment and energy production. Key words : Two-stage bioprocess( 생물학적 2 단공정 ), Dark H 2 fermentation( 혐기성수소발효 ), CH 4 fermentation ( 메탄발효 ), Photo H 2 fermentation( 광합성수소발효 ), Microbial fuel cells( 미생물연료전지 ), Microbial electrolysis cells( 미생물전해전지 ) Corresponding author : skhan003@knou.ac.kr Received : 2015.5.13 in revised form : 2015.6.5 Accepted : 2015.6.30 Copyright c 2015 KHNES 247
248 혐기성수소발효를결합한생물학적 2 단공정의유기성폐자원처리및바이오에너지생산 1. 서론 현재전지구적에너지수요의대부분은석유, 석탄, 천연가스와같은화석연료를이용하여충당하고있다. 그러나화석연료의부존량은충분하지않으며또한화석연료의사용으로부터많은종류의오염물질이발생하고있다 1,2). 이로인해수소는미래의청정에너지원으로서전세계적으로큰각광을받고있다. 수소는연소시순수한물만을배출하기때문에, 대기오염뿐만아니라지구온난화문제를해결할수있다. 그리고수소의에너지함량 (122 kj/g H 2) 은메탄보다약 2.1배그리고휘발유보다약 2.8배정도더큰장점이있다 1-3). 수소를생산하는방법에는다양한물리화학적및생물학적방법이있다. 물리화학적방법에는광화학적, 전기화학적, 광전기화학적, 열화학적수소생산등이있으나, 화석연료의연소로부터얻어지는전기를이용해야하는단점이있다 4,5). 따라서최근에는지속가능하며친환경적인생물학적방법이각광을받고있다. 생물학적방법에는녹조류 (green algae) 를이용한직접물분해 (direct biophotolysis of water), 시아노박테리아 (cyanobacteria) 를이용한간접물분해 (indirect biophotolysis of water), 자색비황세균 (purple non-sulfur bacteria) 을이용하는광합성수소발효 (photo H 2 fermentation), 그리고혐기성발효균 (dark fermentative bacteria) 을이용하는혐기성수소발효 (dark H 2 fermentation) 가있다 6-8). 이중에서도혐기성수소발효는다른생물학적방법에비해반응속도가빠르고, 기술적으로단순하며, 빛이필요하지않고, 또한유기성폐자원으로부터수소를생산할수있는장점이있어약 15~20년전부터지금까지수많은연구가이루어지고있다 9-13). 하지만혐기성수소발효는수소수율이높지않을뿐아니라수소와함께발생하는유기산의후처리가필요한단점이있어실제적인적용을위해서는생물학 적 2단공정의구성이반드시필요하다 14,15). 그러므로본리뷰논문에서는혐기성수소발효와결합할수있는다양한후단공정들을알아보고, 이를바탕으로향후적절한생물학적 2단공정의대안을제시하고자한다. 2. 혐기성수소발효 (dark H 2 fermentation) Fig. 1에서보는바와같이복합유기물질의혐기성분해는가수분해 (hydrolysis), 산생성 (acidogenesis), 초산생성 (acetogenesis) 및메탄생성 (methanogenesis) 의단계를거쳐서이루어진다. 이때, 유기물의 COD 는약 72% 가초산을통하여그리고약 28% 가수소를통하여메탄으로전환이되는데, 여기에서메탄으로전환되는마지막단계를억제한다면메탄대신수소를얻을수있다는것이혐기성수소발효의기본원리이다 15). 따라서수소를이용하여메탄을생성하는미생물, 즉수소소비메탄생성균 (H 2-consuming methanogens) 의활성을저해시켜서수소를얻고자하는다양한방법이시도되었다. 공기 ( 산소 ) 를공급하는방법, 산또는염기로전처리하는방법, 동력학적으로조절하는방법, 아세틸렌이나 2-bromoethanesulfonate (BES) 와같은저해물질을주입하는방법등이사용되었는데, 현재가장많이이용하고있는방법은열처리 Fig. 1 Anaerobic degradation of complex organic compounds to methane 16) >> 한국수소및신에너지학회논문집
이채영ㆍ유규선ㆍ한선기 249 (heat-shock treatment) 방법이다 15,17,18). 이것은미생물배양액에높은열을가해주면대부분의미생물은사멸되지만, 수소생성능력이있는포자형성균 (spore-forming bacteria) 은포자를형성하여살아남을수있다는원리에근거한것이다. 약 90~100 C에서 10분이상의가열을통해얻어진포자는이후적절한조건을만나게되면다시활발하게성장한다 19,20). Clostridium sp. 는포자형성및수소생성의능력을다같이가지고있는대표적인미생물로서가장널리연구되고있다. 이들은절대혐기성균으로화학종속영양미생물에해당하는데, 최적 ph는약 5.5이며최적온도는약 35 C이다 10). 많은종류의 Clostridium sp. 중에약 22종이유기산과더불어수소를발생하는데, 그중에서도 Clostridium butyricum이가장널리알려져있다. 이러한 Clostridium sp. 는탄수화물을분해하는 saccharolytic acidogen에해당되기때문에, 탄수화물을많이포함한유기성액상폐자원이수소발효에많이이용되고있다 21). 이론적으로 1 mol의포도당 (glucose) 이완전히산화되면식 (1) 과같이 12 mol의수소가생성된다. 그러나이러한반응은혐기성수소발효에서열역학적으로발생하지않는다 22). 혐기성수소발효에서는식 (2) 와같이 1 mol의포도당에서이론적으로 2 mol의초산 (acetate) 과함께 4 mol의수소가발생된다. 이경우수소수율 (H 2 yield) 은 4 mol H 2/mol glucose이며수소전환율 (H 2 conversion efficiency) 은 33%(=4/12) 에지나지않는다 10). 등이변화되면, 식 (3)~(4) 와같이초산대신에뷰틸산 (butyrate) 이나프로피온산 (propionate) 을생성하는것으로대사경로를바꾸어안정성을향상시킨다. 만약에 1 mol의포도당에서 1 mol의뷰틸산이발생하게되면 2 mol의수소만이발생하게되어수소전환율은 16.7% (=2/12) 로감소한다. 그리고 1 mol의포도당에서 2 mol의프로피온산이발생하게되면수소의생산대신오히려 2 mol 의수소를소비하게된다 23). (3) (4) 혐기성수소발효는상대적으로반응조가작고간단하며, 빠른반응으로높은수소생산속도를나타내고, 태양광을필요로하지않으며, 유기성폐자원을처리함과동시에청정에너지를생산할수있는장점이있다. 하지만식 (2)~(3) 에서보는바와같이수소가생성되는경우에도, 수소수율이높지않으며또한수소와함께발생하는유기산의추가적처리가필요한단점이있다 15). 따라서혐기성수소발효를적용하는경우, 그후단에별도의반응조를연결하여유기산을처리함과동시에여분의에너지 ( 메탄, 수소, 전기등 ) 를생산하는것이바람직하다. 3. 혐기성수소발효를결합한생물학적 2 단공정 (1) (2) 그러나충격부하 (shock loading) 가있거나또는과다한수소발생으로인해 ph, 수소분압 (H 2 partial pressure), 산화환원전위 (oxidation-reduction potential) 3.1 혐기성수소발효와메탄발효를결합한생물학적 2 단공정 (dark H 2 fermentation + CH 4 fermentation) 메탄발효 ( 혐기성소화 ) 는혐기성수소발효의부산물인유기산을처리함과동시에여분의메탄을생산할수있는공정이다. 대표적인메탄발효균은 Methanosaeta, Methanosarcina 등으로서유기산을이용하여메탄을 제 26 권제 3 호 2015 년 6 월
250 혐기성수소발효를결합한생물학적 2 단공정의유기성폐자원처리및바이오에너지생산 론적으로 4 mol의수소와더불어 2 mol의메탄이생성된다 26). (6) Fig. 2 Schematic diagram of two-stage bioprocess (dark H 2 fermentation + CH 4 fermentation) 생산할수있다. 이들은절대혐기성균으로화학종속영양미생물에해당하는데, 최적 ph는약 7.0이며최적온도는약 35 C이다 24). 오래전부터하수처리장에서잉여슬러지를처리하기위해사용된메탄발효는안정적인거동을보여줄뿐만아니라, 그동안축적된많은경험을바탕으로이미상용화가이루어진공정이다. Fig. 2는혐기성수소발효와메탄발효를결합한생물학적 2단공정을보여주고있다 25,26). 혐기성수소발효의기본반응식은앞서살펴본식 (2) 와같은데, 식 (2) 의부산물인 2 mol의초산을메탄발효의기질로이용하면다음의식 (5) 와같이여분의메탄을더얻을수있다. (5) 그리고식 (2) 와식 (5) 를합치면식 (6) 과같은혐기성수소발효와메탄발효를결합한생물학적 2단공정의식을얻을수있다. 즉, 1 mol의포도당에서이 포도당, 수소및메탄의저위발열량을각각 2,888, 242, 801 kj/mol로가정하여에너지전환율을계산하면생물학적 2단공정의값은약 89% (=(4 242+2 801)/2,888) 이며, 이것은메탄발효공정 (Glucose 3CH 4 + 3CO 2 ) 의값인약 83% (=(3 801)/2,888) 보다더높다. 따라서이론적인측면에서생물학적 2단공정이메탄발효공정보다약 6% 정도더효율적임을알수있다 27). Table 1은혐기성수소발효와메탄발효를결합한생물학적 2단공정의결과들을보여주고있다. 음식물쓰레기를이용한실험에서는수소수율이약 0.07~ 0.31 m 3 H 2 /kg VS로나타났으며메탄수율이약 0.21~ 0.55 m 3 CH 4 /kg VS로나타났다. 그리고도시고형폐기물중유기성분 (organic fraction of municipal solid waste, OFMSW) 을이용한실험에서는수소수율이약 0.21 m 3 H 2 /kg VS, 메탄수율이약 0.46 m 3 CH 4 /kg VS로나타나음식물쓰레기를이용한결과와유사하였다. 한편, 음폐수를처리하는파일롯플랜트실험에서는생산단가에서별차이가없었음에도불구하고생 Table 1 Hydrogen and methane production by a two-stage bioprocess (dark H 2 fermentation + CH 4 fermentation) Substrate Food waste Food waste Food waste OFMSW b) OLR a) (H 2 reactor) 11.9 kg VS/m 3 d (mesophilic) 22.7 kg VS/m 3 d (mesophilic) 39 kg COD/m 3 d (thermophilic) 38.4 kg VS/m3 d (thermophilic) a) OLR: organic loading rate b) OFMSW: organic fraction of municipal solid waste H 2 yield (m 3 H 2/kg VS) 0.31 0.07 0.11 0.21 OLR a) (CH 4 reactor) 5.4 kg COD/m 3 d (mesophilic) 4.6 kg VS/m 3 d (mesophilic) 4.2 kg COD/m 3 d (thermophilic) 6.6 kg VS/m 3 (mesophilic) CH 4 yield (m 3 CH 4/kg VS) Ref. 0.21 10 0.55 28 0.45 29 0.46 30 >> 한국수소및신에너지학회논문집
이채영ㆍ유규선ㆍ한선기 251 물학적 2단공정이메탄발효에비해약 25% 의전기를더많이생산하여이론적인값보다훨씬더효율적임을보여주었다 31). 또한 OFMSW 를이용하여전기및열을동시에생산하는 co-generation 연구에서는생물학적 2단공정이메탄발효에비해서약 26% 의전기와약 23% 의열을더많이생산하였다 32). 혐기성수소발효의기본반응식은앞서살펴본식 (2) 와같은데, 식 (2) 의부산물인 2 mol의초산을광합성수소발효의기질로이용하면다음의식 (7) 과같이여분의수소를더얻을수있다 21). (7) 3.2 혐기성수소발효와광합성수소발효를결합한생물학적 2 단공정 (dark H 2 fermentation + photo H 2 fermentation) 광합성수소발효는혐기성수소발효의부산물인유기산을처리함과동시에추가적으로여분의수소를생산할수있는공정이다. 대표적광합성수소발효균은 Rhodobacter, Rhodopseudomonas와같은자색비황세균 (purple non-sulfur bacteria) 으로서유기산을이용하여수소를생성할수있다. 이들은일반적으로혐기성조건에서광종속영양적으로성장하는것을선호하는데, 최적 ph는약 7.0이며최적온도는약 35 C이다 21). Fig. 3은혐기성수소발효와광합성수소발효를결합한생물학적 2단공정을보여주고있다 22,26). Fig. 3 Schematic diagram of two-stage bioprocess (dark H 2 fermentation + photo H 2 fermentation) 그리고식 (2) 와식 (7) 을합치면식 (8) 과같은혐기성수소발효와광합성수소발효를결합한생물학적 2단공정의식을얻을수있다. 1 mol의포도당에서 12 mol의수소를얻을수있기때문에, 이론적으로 100%(=(4+8)/12) 의수소전환율을얻을수있다 21). (8) 그러나실제의경우, 혐기성수소발효에서초산뿐만아니라다양한유기산이생성되고또한미생물의성장과유지에기질이사용되기때문에생성되는수소의몰수는감소한다. Table 2는혐기성수소발효와광합성수소발효를결합한생물학적 2단공정의결과들을보여주고있다. 다양한기질에대하여혐기성수소발효의수소수율은약 0.7~2.5 mol H 2 /mol hexose로나타났으며광합성수소발효의수소수율은약 1.5~4.9 mol H 2 /mol hexose로나타났다. 그리고전체생물학적 2단공정에대한총수소수율은약 3.3~6.0 mol H 2 /mol hexose로 나타나수소전환율은약 27.5~50% (=3.3/12~6/12) 에 Table 2 Hydrogen and methane production by a two-stage bioprocess (dark H 2 fermentation + photo H 2 fermentation) Substrate Dark H 2 fermentation H 2 yield (mol H 2/mol hexose) Photo H 2 fermentation H 2 yield (mol H 2/mol hexose) Sucrose Mixed culture 1.8 Rhodobacter sphaeroides SH2C 1.5 34 Food waste Mixed culture 1.8 Rhodobacter sphaeroides ZX-5 3.6 35 Potato starch Mixed culture 0.7 Rhodobacter capsulatus B10 4.9 36 Cassava starch Mixed culture (mainly Clostridium sp.) 2.5 Mixed culture (mainly Rhodobacter palustris) Ref. 3.5 37 제 26 권제 3 호 2015 년 6 월
252 혐기성수소발효를결합한생물학적 2 단공정의유기성폐자원처리및바이오에너지생산 해당하였다. 하지만상업화가이루어지기위해서는포도당 1 mol로부터적어도 8 mol의수소가생성되어야한다. 즉, 수소전환율이약 67% (=8/12) 이상을유지해야한다 33). 3.3 혐기성수소발효와미생물연료전지를결합한생물학적 2 단공정 (dark H 2 fermentation + microbial fuel cell (MFC)) 미생물연료전지 (microbial fuel cell, MFC) 는혐기성수소발효의부산물인유기산을처리함과동시에추가적으로전기를생산할수있는공정이다. 미생물연료전지는전기화학적활성을지닌혐기성미생물의촉매작용을이용하여유기물에함유된화학에너지를직접전기에너지로변환시키는것이다 38). 일반적인미생물연료전지의형태는 Fig. 4에서보는바와같이이실 (double-chamber) 미생물연료전지의형태이다. 산화전극 (anode) 이포함된반응조, 환원전극 (cathode) 이포함된반응조, 분리막 (proton(cation) exchange membrane) 및양쪽의전극을도선으로연결한외부전기회로로구성된다. 단, 단실 (single-chamber) 미생물연료전지에서는분리막이제외된다. 미생물연료전지에서는산화전극표면에생물막 (biofilm) 형태로존재하는 Shewanella, Geobacter 와같은전자방출균 (exoelectrogen) 에의해서기질의분해가이루어진다. 이들은절대 혐기성균으로화학종속영양미생물에해당하는데, 최적 ph는중성영역이며최적온도는중온영역이다 40). 기질의분해로부터전자 (electron) 와양성자 (proton), 이산화탄소 (CO 2) 가생성되는데, 이때전자는전자방출균의다양한전자전달메커니즘에의해서산화전극으로전달된후, 외부저항 (resistor) 이유발한전위차에의해서도선을통해환원전극으로이동한다. 그리고양성자는용액과분리막을통해환원전극으로이동한다. 최종적으로이러한전자와양성자는외부에서환원전극으로공급된산소 (O 2) 와함께반응하여물 (H 2O) 을생성한다 41). Fig. 5는혐기성수소발효와미생물연료전지를결합한생물학적 2단공정을보여주고있다 14,26). 혐기성수소발효의기본반응식은앞서살펴본식 (2) 와같은데, 식 (2) 의부산물인 2 mol의초산을미생물연료전지의기질로이용하면다음의식 (9)~(10) 과같은산화전극과환원전극의반응에의해서전력을생산할수있다 42). Anode: (9) Cathode: (10) 하지만실제운전의경우에는옴저항 (ohmic resistance) 이존재하고전극에서의전압손실 (overpotential) 이발생하기때문에, 초산으로부터회수되는전력은 Fig. 5 Schematic diagram of two-stage bioprocess (dark H 2 Fig. 4 Schematic diagram of microbial fuel cell (MFC) 39) fermentation + microbial fuel cell (MFC)) >> 한국수소및신에너지학회논문집
이채영ㆍ유규선ㆍ한선기 253 Table 3 Electricity production from acetate by microbial fuel cell (MFC) MFC type Anode material Separator material Cathode material Max. Power density (W/m 3 ) SCMFC a) Carbon cloth Membrane-free Carbon cloth/platinum 16.0 44 Tubular SCMFC a) Graphite brush 4 Stacked MFC Titanium plates CEM AEM b) Membrane-free c) 21.2 45 /CoTMPP d) Titanium plates e) 144.0 46 /MMO Double f) Carbon cloth None-woven cloth Carbon cloth/platinum 2,080 47 CEA MFC a) Single-chamber microbial fuel cell b) Anion exchange membrane c) Cobalt tetramethoxyphenylporphyrin d) Cation exchange membrane e) Mixed metal oxide f) Cloth electrode assembly microbial fuel cell Ref. 그값이크지않다. 그리고시스템의재료및구조에따라그리고운전조건 (ph, HRT, 기질농도, 버퍼등 ) 에따라그값은큰편차를갖는다 43). Table 3은혐기성수소발효의부산물인초산으로부터전력을생산한미생물연료전지의결과들을보여주고있다. Liu 등 44) 은탄소천 ( 산화전극 ) 과백금이코팅된탄소천 ( 환원전극 ) 을장착한단실미생물연료전지를이용하여초산으로부터약 16.0 W/m 3 의전력밀도를나타내었다. Zuo 등 45) 은흑연브러쉬 ( 산화전극 ) 와 CoTMPP (cobalt tetramethoxyphenylporphyrin) 가코팅된음이온교환막 ( 환원전극 ) 을장착한관 ( 튜브 ) 으로된단실미생물연료전지를이용하여초산으로부터약 21.2 W/m 3 의전력밀도를나타내었다. 또한 Dekker 등 46) 은티타늄판 ( 산화전극 ) 과 MMO (mixed metal oxide) 가코팅된티타늄판 ( 환원전극 ) 그리고 CEM( 분리기 ) 을장착한 4개의스택미생물연료전지를이용하여초산으로부터약 144.0 W/m 3 의전력밀도를나타내었다. 그리고그이후에 Fan 등 47) 은탄소천 ( 산화전극 ) 과백금이코팅된탄소천 ( 환원전극 ) 그리고 none-woven cloth( 분리기 ) 를장착한이중 (double) CEA (cloth electrode assembly) 미생물연료전지를이용하여초산으로부터약 2,080 W/m 3 의전력밀도를나타내었다. 지금까지알려진최대 (maximum) 전력밀도 (power density) 중가장높은값은이중 CEA 미생물연료전지를이용해서얻은 2,080 W/m 3 이며, 이상적인운전이이루어질경우에 2.87 kw/m 3 까지도가능한것으로보고되었다 47). 이것은기존메탄발효조를이용해서얻을수있는최대전력밀도 (1.1 kw/m 3 ) 보다각각 1.9배및 2.6배높은값이다 48). 그리고이중 CEA 미생물연료전지의전류밀도 (current density) 는 16.4 A/m 2 (10.9 ka/m 3 ) 로서 93.5 kg/m 3 d의 COD 제거율로환산될수있는데, 이는혐기성소화조의값 (25 kg/m 3 d) 보다약 3.7배높은값으로유기물제거효율도매우높음을보여준다 47). Fig. 6 Schematic diagram of microbial electrolysis cell (MEC) 39) 제 26 권제 3 호 2015 년 6 월
254 혐기성수소발효를결합한생물학적 2 단공정의유기성폐자원처리및바이오에너지생산 3.4 혐기성수소발효와미생물전해전지를결합한생물학적 2 단공정 (dark H 2 fermentation + microbial electrolysis cell (MEC)) 미생물전해전지 (microbial electrolysis cell, MEC) 는혐기성수소발효의부산물인유기산을처리함과동시에추가적으로수소를생산할수있는공정이다. Fig. 6과같이미생물연료전지의구조및운전방법을간단하게변형함으로써전기대신높은수율의수소를얻을수있다 39). 즉, 환원전극에대한산소공급을중단하고전기회로에약간의전압을공급하면환원전극으로부터수소가생산된다. 미생물전해전지에서는환원전극에대한산소공급이없기때문에, 산화전극으로산소가누출되지않아효율을높게유지할수있다. Fig. 7은혐기성수소발효와미생물전해전지를결합한생물학적 2단공정을보여주고있다 26,49). 혐기성수소발효의기본반응식은앞서살펴본식 (2) 와같은데, 식 (2) 의부산물인 2 mol의초산을미생물전해전지의기질로이용하면다음의식 (11)~ (12) 와같은산화전극과환원전극의반응에 Fig. 7 Schematic diagram of two-stage bioprocess (dark H 2 fermentation + microbial electrolysis cell (MEC)) 의해서전력을생산할수있다 50). 이때환원전극에이론적으로약 0.11 V의외부전압을공급하면되는데, 실제적으로는옴저항과전극에서의전압손실로인해 0.11 V 보다좀더큰전압의공급이필요하다 39). Anode: (11) Cathode: (12) 따라서식 (2) 에서얻은 4 mol의수소와식 (11) 에서얻은 8 mol의수소를합치면 1 mol의포도당으로부터총 12 mol의수소를얻을수있기때문에, 이론적으로 100% (=(4+8)/12)) 의수소전환율을얻을수있다. Table 4는혐기성수소발효의부산물인 2 mol의초산으로부터수소를생산하는미생물전해전지의결과들을보여주고있다. Rozendal 등 51) 은흑연펠트 ( 산화전극 ) 와백금이코팅된티타늄메쉬 ( 환원전극 ) 그리고양이온교환막 ( 분리기 ) 을장착한미생물전해전지에 0.5 V를공급하여초산으로부터 4.2 mol H 2/2 mol acetate의수소를생산하였다. Hu 등 52) 은탄소천 ( 산화전극 ) 과백금이코팅된탄소천 ( 환원전극 ) 을장착한미생물전해전지에 0.6 V를공급하여초산으로부터 5.0 mol H 2/2 mol acetate의수소를생산하였다. Table 4 Hydrogen production from acetate by microbial electrolysis cell (MEC) Applied Voltage (V) Anode material Separator material Cathode material H 2 yield (mol H 2/2 mol acetate) 0.5 Graphite felt CEM a) Titanium mesh/platinum 4.2 51 0.6 Carbon cloth Membrane-Free Carbon cloth/platinum 5.0 52 0.85 Carbon cloth CEM a) Carbon cloth/platinum 2.2 53 0.9 Graphite brush AEM b) Stainless steel mesh/platinum 6.6 54 a) Cation exchange membrane b) Anion exchange membrane Ref. >> 한국수소및신에너지학회논문집
이채영ㆍ유규선ㆍ한선기 255 Table 5 Comparison of various two-stage bioprocesses combining dark H 2 fermentation Dark H 2 fermentation + methane fermentation Dark H 2 fermentation + photo H 2 fermentation Dark H 2 fermentation + microbial fuel cell (MFC) Dark H 2 fermentation + microbial electrolysis cell (MEC) Advantages 25-26% increase in electricity compared to single CH 4 fermentation Stable operation Light independent process Improved H 2 yield (theoretical yield: 12 mol H 2/mol hexose) Direct conversion of organic matter into electricity by MFC Efficient MFC operation at ambient and even low temperatures Light independent process Improved H 2 yield (theoretical yield: 12 mol H 2/mol hexose) Light independent process Disadvantages CO 2 emissions from methane combustion Light supply Low light penetration efficiency High cost of photo-bioreactors Scale-up problem Limited practical applications O 2 supply (only in double MFC) Increased system complexity High cost of MFC Scale-up problem Limited practical applications Power supply Increased system complexity High cost of MEC Scale-up problem Limited practical applications 또한 Kyaze 등 53) 은탄소천 ( 산화전극 ) 과백금이코팅된탄소천 ( 환원전극 ) 그리고양이온교환막 ( 분리기 ) 을장착한미생물전해전지에 0.85 V를공급하여초산으로부터 2.2 mol H 2/2 mol acetate의수소를생산하였다. 그리고그이후에 Nam 등 54) 은흑연브러쉬 ( 산화전극 ) 와백금이코팅된스테인리스강메쉬 ( 환원전극 ) 그리고음이온교환막 ( 분리기 ) 을장착한미생물전해전지에 0.9 V를공급하여초산으로부터 6.6 mol H 2/2 mol acetate의수소를생산하였다. 4. 혐기성수소발효를결합한생물학적 2 단공정의비교 다양한생물학적 2단공정의특성이 Table 5에나타나있다. 첫째, 혐기성수소발효와메탄발효를결합한생물학적 2단공정은기존메탄발효조와비교했을때약 25~26% 의전기생산이증가하며, 운전이안정적이고, 빛이필요없어하루종일운전이가능하다. 하지만생산된메탄가스의연소는 CO 2 의배출로지구온난 화를가속화시킨다. 둘째, 혐기성수소발효와광합성수소발효를결합한생물학적 2단공정은매우높은수소수율이가능하다. 하지만빛이필요하며, 기질에대한빛의투과율이낮고, 광반응조의비용이고가이며, 규모확대가어렵고, 실용화실적이거의없다. 셋째, 혐기성수소발효와미생물연료전지를결합한생물학적 2단공정은유기물을처리하면서직접전기를생산할수있으며, 상온이나저온에서도미생물연료전지의운전이가능하고, 빛이필요없어밤에도운전이가능하다. 하지만이실미생물연료전지의경우는산소의공급이필요하며, 시스템이복잡하고, 미생물연료전지의비용이고가이며, 규모확대가어렵고, 실용화실적이거의없다. 그리고넷째, 혐기성수소발효와미생물전해전지를결합한생물학적 2단공정은매우높은수소수율이가능하고, 빛이필요없어하루종일운전이가능하다. 하지만전력의공급이필요하며, 시스템이복잡하고, 미생물전해전지의비용이고가이며, 규모확대가어렵고, 실용화실적이거의없다. 제 26 권제 3 호 2015 년 6 월
256 혐기성수소발효를결합한생물학적 2 단공정의유기성폐자원처리및바이오에너지생산 그러므로아직까지는혐기성수소발효와메탄발효를결합한생물학적 2단공정이가장유망하다고할수있다. 그러나광합성수소발효, 미생물연료전지, 미생물전해전지의반응조제작비용과운전비용이감소하고, 스케일업기술이향상되며, 실용화관련자료가축적된다면, 이들을결합한생물학적 2단공정들의향후응용가능성과전망도매우크다고할수있다. 5. 결론 화석연료의고갈과지구온난화로인해수소는미래의청정에너지로서각광을받고있다. 수소를생산하는방법에는여러가지가있지만, 그중에서도혐기성수소발효는유기성폐자원의처리및에너지의생산이라는일석이조의장점으로인해최근많은연구가이루어지고있다. 하지만혐기성수소발효는수소수율이높지않고또한수소와함께발생하는유기산의추가적처리가필요한단점이있기때문에, 실제적인적용을위해서는생물학적 2단공정의구성이반드시필요하다. 생물학적 2단공정은크게 4가지로나눌수있는데, (1) 혐기성수소발효와메탄발효를결합한것, (2) 혐기성수소발효와광합성수소발효를결합한것, (3) 혐기성수소발효와미생물연료전지를결합한것, (4) 혐기성수소발효와미생물전해전지를결합한것이이에해당한다. 아직까지는기술적인그리고경제적인측면에서혐기성수소발효와메탄발효를결합한생물학적 2단공정이가장유망하다고할수있다. 그러나광합성수소발효, 미생물연료전지, 미생물전해전지의반응조제작비용과운전비용이감소하고, 스케일업기술이향상되며, 실용화관련자료가축적된다면, 이들을결합한생물학적 2단공정들의향후응용가능성과전망도매우크다고할수있다. References 1. A. Ghimire, L. Frunzo, F. Pirozzi, E. Trably, R. Escudie, P. N. Lens, and G. Esposito, A review on dark fermentative biohydrogen production from organic biomass: Process parameters and use of by-products, Appl. Energ., Vol. 144, 2015, pp. 73-95. 2. C. Sambusiti, M. Bellucci, A. Zabaniotou, L. Beneduce, and F. Monlau, Algae as promising feedstocks for fermentative biohydrogen production according to a biorefinery approach: A comprehensive review, Renew. Sust. Energ. Rev., Vol. 44, 2015, pp. 20-36. 3. S. K. Khanal, Anaerobic biotechnology for bioenergy production: Principles and Applications, 1st ed., John Wiley & Sons, New York, USA, 2008, pp. 189-192. 4. M. Momirlan, and T. Veziroglu, Current status of hydrogen energy, Renew. Sust. Energ. Rev., Vol. 6, No. 1, 2002, pp. 141-179. 5. J. D. Holladay, J. Hu, D. L. King, and Y. Wang, An overview of hydrogen production technologies, Catal. Today, Vol. 139, No. 4, 2009, pp. 244-260. 6. J. Benemann, Hydrogen biotechnology: progress and prospects, Nat. Biotechnol., Vol. 14, No. 9, 1996, pp. 1101-1103. 7. R. Kothari, D. P. Singh, V. V. Tyagi, and S. K. Tyagi, Fermentative hydrogen production An alternative clean energy source, Renew. Sust. Energ. Rev., Vol. 16, No. 4, 2012, pp. 2337-2346. 8. K. Y. Show, D. J. Lee, J. H. Tay, C. Y. Lin, and J. S. Chang, Biohydrogen production: Current perspectives and the way forward, Int. J. Hydrogen Energ., Vol. 37, No. 20, 2012, pp. 15616-15631. 9. R. Nandi, and S, Sengupta, Microbial production of hydrogen: an overview, Crit. Rev. Microbiol., Vol. 24, No. 1, 1998, pp. 61-84. 10. S. K. Han, and H. S. Shin, Biohydrogen production by anaerobic fermentation of food waste, Int. J. Hydrogen Energ., Vol. 29, No. 6, 2004, pp. >> 한국수소및신에너지학회논문집
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