Bioethanol, derived from lignocellulosic biomass, has emerged as a sustainable and green alternative to non-renewable gasoline fuel. Bioethanol production is facing the challenge of economical and efficient recovery of sugars from biomass, which is mainly governed by the pretreatment and hydrolysis steps. The biorefinery concept (recovery of multi-product from biomass) may make bioethanol production cost competitive with gasoline. In this study, it was focused to develop a single step sono-assisted sulfuric acid process (SSP) for rice straw biorefinery to recover: sugars for bioethanol, demineralized residue for pure silica and porous residue for bioachar. In the first step, critical factors of sono-assisted sulfuric acid pretreatment (sonication time, temperature and acid concentration) were optimized using response surface methodology. SSP produced 31.78 g of sugar/100g of dry biomass at 80˚C and 10% sulfuric acid concentration after 50 min of sonication. SSP led to > 50% total sugar recovery from RS in one-step. Total sugar recovery reached up to 85.9% upon dilute acid hydrolysis of SSP treated rice straw compared to 85.5% sugar recovery employing enzymatic hydrolysis. Process economics (energy requirement and reagent cost) showed that SSP-dilute acid hydrolysis proved viable technology for commercialization compared to SSP-enzymatic hydrolysis process.
In the second part of this study, SSP treated rice straw was used as a precursor to produce pure mesoporous silica. SSP treatment removed more than 96% of mineral oxides from residual rice straw. This residual waste was combusted at 750˚C for 15 min, which produced ≥ 96% pure mesoporous silica. The hydrolyzed rice straw (without any further processing) was used as biosorbent to remove Methylene blue dye from aqueous solution. The maximum monolayer adsorption capacity obtained was not that high (8.30 mg/g) compared to other biosorbents. Thus, SSP treated rice straw was combusted at 450˚C for one hour to produce rice biochar. The biochar characterization showed high surface area, and oxygen containing functional groups, which led to high adsorption capacity. The adsorption potential of biochar was evaluated using Brilliant Green dye. Adsorption study (equilibrium, kinetic and thermodynamics) showed that BG dye adsorption on RBC was feasible, spontaneous and physical in nature. The maximum monolayer adsorption capacity of BG on biochar was found to be 111.11 mg/g, which was comparable with other commercial adsorbents. These results showed that SSP can be applied to build rice straw biorefinery, where fermentable sugars can be converted into bioethanol, and demineralized rice straw can be used to produce pure silica and commercial biosorbent. Acid can be recycled to improve sugar recovery and process economic. This process intensification can make bioethanol production economical and cleaner process.
목질계바이오매스를이용한바이오에탄올은지속가능하고친환경적으로, 가솔린연료의대체재로부상하고있다. 하지만, 바이오에탄올생산은경제성과함께전처리와당화공정을수반하는바이오매스에서당으로의효과적인전환에어려움을겪고있다. 이에바이오매스로부터다양한부산물을생산하는바이오정제개념은바이오에탄올이가솔린과가격경쟁력을가능하게할수있다. 다음연구에서는초음파가결합된황산처리공법 (SSP)을활용하여볏짚으로부터다양한부산물 (바이오에탄올, 실리카, 바이오숯)로의정제공정에주목하였다.
첫번째단계로, 통계학적방법인 RSM을이용하여초음파가결합된황산처리공법의주요인자(초음파시간, 온도, 산농도)의최적화실험을진행하였다. SSP는 80℃에서 10%의황산을이용하여 50분간초음파를주입한결과, 100g의건조볏짚으로부터 31.78g의당을생산하였다. SSP는한단계만으로 50% 이상의당회수율을보였다. SSP로처리된볏짚을묽은황산으로당화한결과 85.9%의당회수율을보여, 효소당화처리의 85.5%의당회수율과비교할만한결과를나타냈다. 공정의경제성평가는 SSP-황산당화공정이 SSP-효소당화공정에비해상업화에있어더적합한방법임을입증하였다.
두번째연구로 SSP로처리된볏짚은다공성실리카생산의원료로활용되었다. SSP처리는볏짚잔재물로부터 96% 이상의산화무기물을제거하였다. 잔류물은 750℃에서 15분간연소되어 96% 이상의순수다공성실리카를생산하였다. 또한후속공정없이당화된볏짚은수용액상에서Methylene Blue염료의흡착제로도활용되었다. 최대단일흡착능 (8.30mg/g)은다른흡착제에비해그리높지않았다.
따라서, SSP로처리된볏짚은 450℃에서연소시켜바이오숯생산에이용되었다. 바이오숯은성분분석결과, 높은표면적 과높은흡착능을보이는산소를함유한작용기를나타냈다. 바이오숯의흡착능은 Brilliant Green 염료를이용하여분석하였다. 흡착연구 (평형, 동역학, 열역학)는 BG 염료가바이오숯에자연상에서자발적으로잘흡착됨을보였다. BG에대한바이오숯의최대단일흡착능은 111.11 mg/g으로이는다른상업용흡착제성능과견줄만한결과이다. 이러한결과들은 SSP가바이오에탄올위한당생산과순수실리카와상업용흡착제생산을위한광물질제거를위한볏짚의정제공정에활용될수있음을보였다. 또한산은당회수율과경제성향상을위해재이용될수있고, 이는바이오에탄올생산을경제적이고친환경적으로할수있다.
