Clean Technol., Vol. 22, No. 4, December 2016, pp. 286-291 청정에너지기술 바이오에너지용억새펠릿의품질및연소특성 문윤호, 이지은, 유경단, 차영록, 송연상 *, 이경보 농촌진흥청국립식량과학원바이오에너지작물연구소 58545 전남무안군청계면무안로 199 (2016 년 8 월 31 일접수 ; 2016 년 9 월 26 일수정본접수 ; 2016 년 9 월 29 일채택 ) Quality and Combustion Characteristics of Miscanthus Pellet for Bioenergy Youn-Ho Moon, Ji-Eun Lee, Gyeong-Dan Yu, Young-Lok Cha, Yeon-Sang Song*, and Kyeong-Bo Lee Bioenergy Crop Research Institute, National Institute of Crop Science, RDA 199 Muan-ro, Cheonggye-myeon, Muan-gun, Jeonnam 58545, Korea (Received for review August 31, 2016; Revision received September 26, 2016; Accepted September 29, 2016) 요 약 본연구는억새바이오매스로성형한연료펠릿의실용화를앞당기기위해소나무톱밥펠릿과비교한성형단계별물리적특성변화, 소요전력그리고성형된펠릿의품질을조사하고, 연소특성개선을위해석회혼합비율별로펠릿을성형하여회분함량등연소특성을조사하였다. 겉보기밀도는억새가원료단계와분쇄후에소나무톱밥에비해낮았으나펠릿성형후에는소나무톱밥과비슷하였다. 수분함량은억새가원료단계에서소나무톱밥에비해높았으나분쇄후에는비슷하였고, 펠릿성형후에는낮아졌다. 억새는소나무톱밥펠릿성형공정에없는밀도증가단계가있지만총소요전력이비슷하였고, 성형된펠릿의내구성과성형율도소나무톱밥과차이가없었다. 억새펠릿은석회혼합비율이증가함에따라회분함량이증가하고고위발열량이다소낮아졌으나, 회분용융점이높아지고 clinker 발생률은감소하는경향이었다. 주제어 : 억새, 펠릿, 품질, 연소 Abstract : In this study we made fuel pellet from miscanthus biomass and investigated changes of physiological characteristics and electricity consumption of pelletizing process in comparison with fuel pellet made of pine sawdust. We also examined combustion characteristics including ash content and clinker forming ratio with fuel pellet made of mixing with micanthus biomass and lime powder. Bulk density of ground-miscanthus and pine sawdust were 158 g L -1 and 187 g L -1, respectively. Bulk density of ground miscanthus was lower than that of pine sawdust, but increased to 653 g L -1 after pelletizing, which was similar to 656 g L -1 of pine sawdust pellet. Moisture content in raw miscanthus and ground miscanthus were 17.0% and 11.8%, respectively. Moisture content in ground miscanthus was similar to that of pine saw dust and decreased to 6.73% after pelletizing, which was 7.7% lower than that of pine sawdust pellet. Although 27 kwh ton -1 were required for compaction press that was an additional process in miscanthus pelleitizing, total required electricity was 193 kwh ton -1 which was similar to 195 kwh ton -1 of pine sawdust pellet pelleitizing. Pellet durability and pelletizing ratio of miscanthus were 98.0% and 99.7%, respectively, which were similar to 98.1% and 99.4% of pine sawdust pellet. When lime mixing ratio increased, ash melting degree and clinker forming ratio of miscanthus pellet increased. While higher heating value and clinker forming ratio of miscanthus pellet decreased. Keywords : Miscanthus, Pellet, Quality, Combustion 1. 서론 최근온실가스배출로인한지구온난화와불안정한국제원유가등범지구적문제의장기적해결을위하여바이오에너 지가주목받고있다 [1]. 연료펠릿은저렴한가격, 취급용이성등으로유럽과북미에서바이오매스보일러용으로생산과이용이증가하고 [2], 우리나라에서도산림지역에서목재펠릿이생산되어온실난방용으로공급되고있다 [3]. 연료펠릿으로 * To whom correspondence should be addressed. E-mail: yssong25@korea.kr; Tel: +82-61-450-0159; Fax: +82-61-453-0085 doi: 10.7464/ksct.2016.22.4.286 pissn 1598-9712 eissn 2288-0690 This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licences/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 286
바이오에너지용억새펠릿의품질및연소특성 287 목재펠릿이가장널리사용되고있지만원료의수집, 운반및건조비용이많이소요되고, 임목이생육속도가느려묘목식재후수년경과하여야만경제적인벌채가가능하여원료수급에어려운면이있다 [4]. 이러한문제점을해결하기위해선진국에서는억새 (Miscanthus species), 스위치그래스 (Panicum virgatum), 갈대 (Phragmites australis) 등다년생초본식물중친환경적이면서바이오매스수량이많은품종을개발하고 [5], 이를활용한펠릿성형연구가진행되고있다 [6]. 이중억새는발열량이높고투입에너지에비해건물수량이많아유망한대체연료자원이다 [7]. 우리나라에는 2007년부터억새등국내자생다년생초본식물유전자원을수집하는한편, 수집유전자원중바이오매스수량이많은물억새 (M. sacchariflorus) 인거대 1호를선발, 보급하였다 [4]. 억새등초본류바이오매스도목재와동일하게펠릿으로성형하면운송및저장비용을절감하고취급시분진발생이적어지는등여러장점이있다 [8]. 저자등은선행연구에서억새등초본류바이오매스펠릿성형기를개발한바있다 [9]. 그러나초본류펠릿은목재펠릿버너에연소시회분용융으로인한 clinker 발생으로공기공급을차단하는등버너에문제를발생시킬수있다 [10]. 따라서본연구는억새바이오매스로성형한연료펠릿의실용화를앞당기기위해소나무톱밥펠릿과비교한성형단계별물리적특성변화와소요전력그리고성형된펠릿의물리적특성을조사하고, 연소특성개선을위해석회혼합비율별로펠릿을성형하여회분함량등연소특성을조사하였다. 정도로건조후바이오매스절단기 (Tomotech Ltd., Seoul, Korea) 로 5 ~ 10 cm 길이로절단하여사용하였다. 소나무톱밥은전남무안군무안읍의제재소에서구입하여유리온실내에서수분함량 15% 로건조하여사용하였다. 펠릿은 억새등초본류바이오매스밀도를증가시키는장치및연료펠릿제조방법 에따라성형하였는데, 억새펠릿은 Figure 1과같이원료 10 kg을절단 분쇄 밀도증가 펠릿성형등의단계를거쳤고 [9], 소나무톱밥펠릿은억새펠릿성형공정중밀도증가단계를생략하고제조하였다. 절단한억새줄기는 20마력 hammer mill과구멍의크기가 3 mm인 screen이장착된분쇄기 (Sunbrand Industrial, Inc., Korea) 로분쇄하였다. 펠릿성형기 (Sunbrand Industrial, Inc., Korea) 는 Figure 2와같이 25마력 3상모터로구동되는원판형금형 (flat die) 과롤러가장착된것을사용하였다. 금형은재질이크롬-니켈강철, 직경과두께가각각 255 mm, 30 mm였고성형구멍 (hole) 의길이와직경이각각 27 mm, 6 mm[ 길이 / 두께비율 (L/D) 4.5 : 1 였다. 금형과쌍을이루는롤러는기어식으로기준피치원지름이 185.5 mm, 기어의높이와폭이각각 2.5 mm, 34 mm였다. 펠릿성형시금형의온도가 100 이상으로높아야만겉보기밀도가높고함수율이낮은펠릿을성형할수있다 [11]. 따라서저자등은미리성형한펠릿을성형기에투입하 2. 실험방법 2.1. 시험재료와펠릿성형공정시험재료로억새는전북익산시용안면석동리소재 거대억새생산단지 (36 14'34.55''N, 126 93'77.95''E) 에서재배한거대1호 (M. sacchariflorus cv. Geodae 1) 를수확하여수분 17% Figure 2. Dies (right) and roller (left) in fuel pelletizer of miscanthus. Figure 1. Schematic of pelletizing process.
288 문윤호ㆍ이지은ㆍ유경단ㆍ차영록ㆍ송연상ㆍ이경보 여예비가동함으로서금형의온도를 100 까지높인후펠릿을성형하였다. 이때금형의온도는비접촉식적외선온도측정기 (FLUKE-566, Fluke Co., USA) 를가동중인금형에조사하여측정하였다. 각단계별소요전력은원료 100 kg 처리하는데소비되는전력을전력량측정기 (WT230, Maxxis Korea, Korea) 로측정하여환산하였는데, 분쇄단계는 Figure 1에서 grinder, screw feeder, cyclone, hopper 가동전력, 밀도증가및펠릿성형단계는각각 compaction press 및 pellet mill 가동전력을측정하였다. 석회혼합비율별억새펠릿은 Figure 1의 grinder를통과한분쇄물에입자의직경이 11 µm 이하인공업용석회분말을무게비율로 0.0, 0.5, 1.0, 1.5, 2.0% 로혼합하여성형하였다. 즉, 분쇄물 10 kg을수분함량 10% 가되도록건조시킨다음, 증류수 1 L에석회분말 50, 100, 150, 200 g을현탁시켜분쇄물과혼합후수분이고루퍼지도록비닐로밀봉하여 15 냉암소에 1개월동안저장후 Figure 1의초본류펠릿공정중절단및분쇄단계를생략하고밀도증가및펠릿성형단계에투입하여성형하였다. 2.2. 품질및연소특성조사겉보기밀도와수분함량, 내구성및펠릿성형율은국립산림과학원의목재펠릿품질규격 [12] 에따라조사하였다. 겉보기밀도는는 1 L PVC 비이커에시료를채우고높이 20 cm 에서가볍게 3회떨어뜨리면서다진다음무게를측정하여용량으로나누었다. 수분함량은시료 20 g을칭량병에넣고 105 건조기 (DE/UF160, Memmert, Korea) 에 48시간동안건조후무게를측정하여백분율로환산하였다. 펠릿의내구성은시료를직경 3.15 mm 체로체거름하여 500 g 칭량후 CEN/TS 15210-1 규정에따라제작한 [12] 내구성시험기 (Sunbrand industrial Inc., Korea) 에넣어 10분동안 500회전후다시직경 3.15 mm 체로사별후잔류한펠릿의무게를측정하여환산하였다. 펠릿성형율은원료 10 kg 투입후최종성형된펠릿의무게를측정하여환산하였다. 고위발열량 (HHV) 은 calorimeter (PARR 6320EF model, Parr instrument, Co., USA) 로측정하였고, 회분용융정도는시료 7 g을도가니에칭량후 800, 850, 900, 950, 1,000 회화로에 4시간동안회화시켜 12시간후꺼내어데시케이터에서 24시간냉각시킨후조사하였다. 용융정도는 Figure 3과같이회분이가루 Figure 4. Combustion of pellet (right), and residual clinker (left). 형태로도가니바닥에흩어진것을 0, 용융되어응집이시작된것 (0.5 mm 이하방울모양관찰 ) 을 1, 용융되어중앙으로응집되었으나냉각후쉽게부서지는것을 3, 용융물이중앙으로응집되어단단해진것을 5로하였다. 회분함량은 1,000 회화후잔류회분무게를측정하여환산하였다. 연소시 clinker 발생률은시료 300 g을도가니식화격자가장착된고기구이용펠릿버너 (BHS-0010, Buheung Safe Inc., Korea) 에 30분동안연소시켜 Figure 4와같이불씨가완전히꺼진후잔류회분과 clinker를긁어내어직경 0.1 mm 체로 1분동안가볍게체거름후잔류한것을칭량하여환산하였다. 3. 결과및고찰 3.1. 펠릿성형단계별겉보기밀도와수분함량변화억새와소나무톱밥의펠릿성형단계별겉보기밀도와수분함량변화는 Figure 5와같다. 원료단계에서억새와소나 Figure 3. Melting degree of ash after cooling. Melting and solid clinker cohered to center was 5 point (right), melting and softly cohered to center was 3 point (middle) and form tiny drops (0.5 mm>) was 1 point (left). Figure 5. Change in bulk density and moisture content by pellet processing stages of pine sawdust and miscanthus.
바이오에너지용억새펠릿의품질및연소특성 289 무톱밥의겉보기밀도는각각 158 g L -1, 247 g L -1 였고분쇄후에는각각 187 g L -1, 255 g L -1 로소나무톱밥이현저하게높았으나펠릿성형후에는각각 653 g L -1, 656 g L -1 로비슷하였다. 이는밀도가낮은초본류도펠릿성형후에는겉보기밀도가목재펠릿과거의동일하다는선행연구 [9,13,14] 와비슷한결과를보여주었다. 수분함량은억새가원료단계에서 17.0% 로소나무톱밥의 15.0% 에비해높았으나분쇄후에는 11.8% 로소나무톱밥의 12.2% 와비슷하였고, 펠릿성형후에는 6.73% 로소나무톱밥의 7.70% 에비해낮았다. 억새는분쇄, 성형단계에서수분함량이크게낮아졌는데, 분쇄단계에서원료와헤머밀의충돌로발생한열로, 성형단계에는원료가금형 (die) 의성형구멍 (hole) 을통과하면서발생한마찰열로원료에함유된수분이증발하기때문인것으로보여진다. Figure 7. Pelletizing ratio and durability of pellets made with pine sawdust and miscanthus. 3.2. 펠릿성형단계별소요전력과품질억새와소나무톱밥의펠릿성형단계별소요전력은 Figure 6과같다. 억새는소나무톱밥펠릿성형공정에는없는밀도증가단계에전력이 27 kwh ton -1 소요되었다. 그러나분쇄및성형단계소요전력이각각 83 kwh ton -1, 85 kwh ton -1 로소나무톱밥의 90 kwh ton -1, 103 kwh ton -1 에비해적어총소요전력은 193 kwh ton -1 로소나무톱밥의 195 kwh ton -1 과차이가없었다. 억새의펠릿성형비용이소나무톱밥에비해많지않다는점에서향후실용생산경제성을판단하기위한중요한지표가될것으로생각된다. 펠릿의내구성은펠릿의강도를나타내는지표로서높을수록운반, 취급시파손율이적어진다 [12]. 소나무톱밥과억새로생산된펠릿의내구성과펠릿성형율을조사한결과는 Figure 7과같다. 소나무톱밥과억새의펠릿내구성은각각 98.1%, 98%, 펠릿성형율이각각 99.4%, 99.7% 로두원료간의차이는거의없어, 목재펠릿이초본류펠릿에비해내구성이높다는 John and John [15] 의결과와일치하지않았다. 본연구에서소나무톱밥펠릿과억새펠릿의내구성차이가없는것은목재펠릿성형공정 ( 분쇄 성형 ) 으로수행한선행연구와는달리억새펠릿성형에밀도증가단계가추가되었기때문인것으로생각된다. 3.3. 석회혼합비율별억새펠릿의연소특성석회혼합비율별억새펠릿의 1,000 연소후회분함량과고위발열량을조사한결과는 Figure 8(a), (b) 와같다. 원료에석회를첨가하지않은소나무톱밥펠릿의회분함량과고위발열량은각각 1.71%, 4,491 kcal kg -1 로억새펠릿에비해연소품질이좋아 Clara et al. [16] 의결과와동일하였다. 억새펠릿의회분함량은석회를혼합하지않은것에서는 2.88% 이었으나혼합비율이높을수록증가하여 2.0% 혼합에서는 3.99% Figure 6. Electricity consumption in pelletizing of pine sawdust and miscanthus. Figure 8. Ash content at 1,000 combustion (a) and higher heating value (b) of miscanthus pellet by mixing ratio of lime. Non-limed sawdust pellet was co-examined for reference.
290 문윤호ㆍ이지은ㆍ유경단ㆍ차영록ㆍ송연상ㆍ이경보 Figure 9. Melting degree of ashes on miscanthus pellet by mixing ratio of lime at different combustion temperature. Nonlimed sawdust pellet was co-examined for reference. 로높아졌다. 고위발열량도석회혼합비율이높을수록낮아져혼합하지않은것에서는 4,300 kcal kg -1 였으나 2.0% 혼합한것에서는 3,894 kcal kg -1 으로낮아졌다. 이는불연성광물인석회혼합비율만큼회분함량이증가하고고위발열량이감소하는것으로판단된다. Clinker는펠릿등고형연료가연소시발생되는회분이용융되어버너에고착되는것으로서발생량이많으면버너의열효율과수명을저하시킨다 [10,17,19]. 석회와혼합하여성형한억새펠릿의회화온도별회분의용융정도를조사한결과는 Figure 9, 펠릿을버너에연소시켜 clinker 발생양상을조사한결과는 Figure 10, 11과같다. 회화온도별회분의용융정도는 석회 0 ~1.5% 혼합펠릿은 800 에서용융되기시작하여 900 ~ 950 에서단단하게응집되었으나 2.0% 혼합펠릿은 950 에서용융이시작되었고, 목재펠릿은 1,000 에서용융이시작되었다. clinker 발생률은석회 0% 혼합펠릿에서 2.56% 로많이발생하였으나혼합비율이높을수록낮아져 2.0% 혼합펠릿은 1.86% 로유의적인차이를보였다. 억새는석회를 2.0% 까지혼합함에따라 clinker 발생률이낮아지는효과가있었다. 바이오매스펠릿의회분용융은재료의화학적조성에따라차이가많아목재와같이 Ca와 Mg 함량이많으면서 K 함량이적으면회분용융점이높아 clinker 발생량이적고, 보리와같은볏과식물은 Ca와 Mg 함량이적으면서 K 함량이많아회분용융점이높아 clinker 발생량이많다 [17]. 억새펠릿이연소시 clinker가많이발생한것은억새가볏과식물로 K 함량이많았기때문인것으로짐작된다. 이러한단점을개선하기위해 Peter et al. [18] 은 K 함량이많은볏과식물을원료로펠릿을성형할때에는석회혼합으로회분용융점을높임으로써 clinker 발생률을낮출수있다고하였다. 본연구에서도억새에석회를혼합함으로서회분용융점을높이고 clinker 발생률을낮출수있었으나석회를혼합하지않은소나무톱밥펠릿의 1,000 와 0.07% 에는미치지못하였다. 현재보급된펠릿연소버너는대부분목재펠릿에적합하도록제작되었다 [15]. 따라서 Lara et al. [19] 이제안한바와같이억새펠릿소비확대를위해서는성형시석회혼합방법개선을통한연소효율향상과더불어가동시 clinker를자동으로제거할수있는버너개발이필요하다고사료된다. 4. 결론억새로성형한연료펠릿의실용화를위해소나무톱밥펠릿과비교한성형단계별물리적특성변화와소요전력및성형된펠릿의품질을조사하고, 연소특성개선을위해석회혼합비율별로펠릿을성형하여회분함량, clinker 발생량등을조사한결과는다음과같다. Figure 10. Clinker ratio of miscanthus pellet by mixing ratio of lime when combusted with roasting burner. Non-limed sawdust pellet was co-examined for reference. Figure 11. Residual clinkers of pellet after combustion with roasting burner. Clinker from non-limed miscanthus pellet (right), 2% limed miscanthus pellet (mid) and non-limed pine sawdust pellet (left). 원료단계의억새와소나무톱밥의겉보기밀도는각각 158 g L -1, 247 g L -1 였고분쇄후에는각각 187 g L -1, 255 g L -1 로소나무톱밥이현저하게높았으나펠릿성형후에는각각 653 g L -1, 656 g L -1 로비슷하였다. 수분함량은억새가원료단계에서 17% 로소나무톱밥의 15% 에비해높았으나분쇄후에는 11.8% 로소나무톱밥의 12.2% 와비슷하였고, 펠릿성형후에는 6.73% 로소나무톱밥의 7.7% 에비해낮았다. 억새는소나무톱밥펠릿성형공정에없는밀도증가단계에전력이 27 kwh ton -1 소요되었으나, 총소요전력은 193 kwh ton -1 로소나무톱밥펠릿의 195 kwh ton -1 과비슷하였다. 소나무톱밥과억새의펠릿내구성은각각 98.1%, 98.0%, 펠릿성형율도각각 99.4%, 99.7% 로차이가없었다. 억새펠릿은석회혼합비율이증가함에따라회분함량이증가하고고위발열량은낮아지는등연소품질은낮아졌다. 억새펠릿은석회혼합비율이증가할수록회분용융점이높아지고 clinker 발생률이감소하여석회 2% 혼합에서는각각 950, 1.86% 였다.
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