Despite receiving increasing attention during the last few decades, the production of microalgal biofu-els is not yet sufficiently cost-effective to compete with that of petroleum-based conventional fuels. Among the steps required for the production of microalgal biofuels, the harvest of the microalgal biomass and the extraction of lipids from microalgae are two of the most expensive. In this study, two electrochemical har-vesting processes were developed and tested. A conductive filter called ‘electro-membrane’ was synthesized and tested both in cross-flow electro-filtration (CFEF) process and modified electro-floatation (MEF) process. At continuous mode of CFEF, the applied electric field caused to enhance harvesting performance by 150% comparing to using commercial membrane, demonstrating anti-fouling property of the synthesized electro-membrane. At discrete mode, membrane surface, in which microalgae cells were clogged, was almost com-pletely cleansed by periodic powerful repulsive force, and it resulted in flux recovery to the initial high level. In MEF process, iron plate was used as a sacrificing anode to donate ferric ion (Fe3+). At the time that com-plete harvest of algae was obtained, the final pH was about 9.2 when the current intensities were 2.0-5.0 mA/cm2, and pH was about 8.3 with the current intensities of 0.5-1.0 mA/cm2. The optimal condition was found to be 1 mA/cm2 in term of high harvesting rate of 306.0 g/m2/h, as well as low energy consumption of 0.62 kWh/m3. The pH of solutions increased from 6.8 to about 8-9 after ECF treatment, which could be at-tributed to the continuous production of hydroxyl ions (OH-) at the cathode with the generation of Fe3+ at anode
In the second part of this study, two cell disruption methods were newly developed. An optimal ex-traction condition of chemical method, via the response surface methodology (RSM), was identified using response with respect to FeCl3 concentration (1-3 mM), reaction time (30-90 mins) and temperature (60-100 °C). At a condition of FeCl3 of 2 mM, 90 mins, and 87 °C, a maximum extraction yield of 81.1% and a FAME conversion rate of more than 80% were achieved. Differently from chemical process, developed elec-trochemical method provided direct H2O2 production form the surface of cathode, resulting in the production of strongly oxidative hydroxyl radical (oOH) via electro-Fenton (E-Fenton) process. Extraction yields via elec-trochemical way were proportional to the increase in temperature, but not proportional to the increase in amount of applied current density. The highest extraction yield of 84.6% was obtained at the condition of 80°C and 10 mA/cm2. Energy consumption required for two developed methods were calculated as 4.9 and 3.8 kWh/m3 for chemical and electrochemical methods, respectively.
In order to make our developed processes more economically feasible, we combined harvesting step with lipid extraction step for the purpose of reducing both energy and chemical consumption. In this study In this study, mature solution of chlorella sp. KR-1 which has the initial concentration of 1.7 g/L was concen-trated by coagulation to 20 g/L, then crude oil was directly extracted from harvested wet sample simultane-ously using FeCl3 and Fe2(SO4)3 both as coagulant and cell disrupting catalyst. Optimal extraction conditions, via the Response Surface Method (RSM), were identified using response with respect to H2O2 concentration (1-3 %), reaction time (30-90 mins) and temperature (80-120 °C). Even though it has been found that there are synergistic effects between one variable and the others, the effect of temperature has the greatest influ-ence than other two variables, and high lipid extraction yields more than 90% were achieved at 120 °C.
Finally, two methods that reduce consumption of major two chemical of FeCl3 and H2O2 in combined process of harvesting and lipid extraction were suggested and confirmed. By controlling pH, reutilization of Fe3+ was successfully obtained. In addition, in-situ production of H2O2, which can be produced using H2 and O2, thus possibly produced from electrochemical harvesting or cell disruption process, was carried out using Pd-based catalyst. In light of iron salt being a reference coagulant, even for microalgae harvesting, its use for continuous lipid extraction can be a promising option to conceivably reduce the overall cost of microalgae-derived biodiesel production.
최근 미세조류는 바이오디젤 생산을 위한 가장 유망한 원료 중 하나로 각광받고 있다. 미세조류 기반의 바이오디젤 생산공정은 배양-수확-추출-전환의 세부 공정들로 나눌 수 있는데, 미세조류의 낮은 배양농도 (Open pond 기준 0.5 g/L 이하)와 두꺼운 세포벽으로 인해 수확과 추출공정에 많은 에너지가 소모되는 실정이다. 본 연구에서는 수확 및 추출 공정에 대하여 에너지 및 화학약품 소모 측면에서 기존공정 대비 더 효율이 좋은 공정들을 화학적, 전기화학적인 방법을 적용하여 새롭게 개발하였고, 나아가서 두 공정을 통합하여 경제성을 더욱 높이는 연구를 진행하였다. 추가로, 수확/추출 통합공정에 사용되는 염화제2철 (FeCl3)와 과산화수소 (H2O2)의 사용에 소모되는 비용을 줄이기 위해서 염화제2철을 재이용 할 수 있는 방법을 고안하였고 개발된 전기화학적 기술들로부터 직접 과산화수소를 저렴하게 생산할 수 있는 방법 또한 개발하여 연구를 진행하였다.
