Current Aspects of Carbon Dioxide Fixation by Microalgae in Korea

The carbon dioxide fixation by microalgae has several merits such as the COz recovery and the production of useful chemicals. Since the flue gas from a industry has some unfavorable conditions such a s high CQ concentration and toxic chemicals like SO., NO, contained in flue gas for the cultivation of m s t microalgae, it is important to apply a suitable strain of microalgae for the fixation of carbon dioxide emitted from various industrial sources. The current status for carbon dioxide emission from thermal power plants, which are major in Korea, is going to be described. Some research work has been done in KIER to get a suitable microalgae. Among the various microalgae tested in our laboratory, Chlorella sp., HA-1 and GMoroaxcum littode showed the satisfactory performance The experimental results for culturing of the two microalgae will be presented. INTRODUCTION Global warming due to increased carbon dioxide concentration in the atmosphere is a threat causing widespread concern. CQ is a green house gas, as are methane, chlorofluorocarbons and water vapor. The CQ concentration in the atmosphere is reported to be risen by about 25% since industrial revolution(1,Z). In the same time, the temperature of the northern hemisphere has also increased by 0.5'c. A prudent response is to reduce emissions of gmnhouse gases while the science underlying global warming is resolved. Various physicochemical methods such as wet and dry absorption, adsorption, and membrane separation technologies have been developed and the absorption method has been put into practice for the elimination of carbon dioxide(3) but further d i s ~ s a l of the trapped CQ is a costly m e s s . Biological methods, in particular using microalgal photosynthesis, have several merits such as no requirements for the pretreatment of flue gas and the further disposal of trapped CQ and mild conditions for COz fixation. Carbon fixed by microalgae is incorporated intn carbohydrates, lipids, so energy , chemicals, or fmds can be produced from algal biomass. The flue gas from power plants and steel mills contain about 10-20% of COZ. If the CQ gas is once condensed by chemical or physical means to remove toxic chemicals, the gas fed for microalgal cultivation may contain higher concentration of CQ. I t has been reported, however, that high concentrations of CQ inhibits the rate of algal photosynthesis. Silva and Pirt reported that Chlorella vulgaris 211/8k strain, which is tolerant to high CQ concentration, showed a maximum growth rate at 7% C G concentration(4). Practical growth studies with high concentrations of COz have been initiated(5,6). Negoro et al. examined several strains of marine microalgae on their tolerance to 5 or 15% CQ and reported that some of them could grow without pH adjustment(7). Strains prefening low PH values can cut operation costs considerably by elminating pH control. If the microalgae can be grown at high densities, the size of culhuing facilities may be reduced. Desirable properties of micmalgae, for the direct biological utilization of CQ from flue gas, are summarized in Table 1. Many works have been cam& out to find some microalgae to fit the requirements described above(8-10). Various microalgae, which may be used for COz futation work, have been obtained fmm several culture collections and tested in our laboratory for their feasibility for C G fixation in Korea. At fusf in this paper, m n t aspects for the CQ emission in Korea is going to be described, and subsequently some preliminary works for the development of the biological fixation process of COz fmm flue gas in KlER will be presented. CURRENT ASPECTS OF CARBON DIOXIDE EMISSION IN KOREA As Korea is getting d e v e l d , the amount of CQ emitted has been increased rapidly. Recently global warming by CQ have been emerged as a major environmental problem threatening global ecosystem and a concern over it is increasing gradually in Korea like in many other nations. On account of this, Korean government assisted national R&D program for COZ by energy conservation and renewable energy utilization, C Q fixation by various methods and conversion or utilization of COz for various uses. Among these projects, the biological fixation of is considered to be the most environmentally friendly technology. and hence it is started to be investigated in several universities, government institutes such as Korea Research Institute of Chemical Technology(KR1CT) as well as Korea Institute of Energy Research(KIER). Total CCk emission in the world was reported to be 5,696 million tons of carbon in 1988(11). Korea is responsible to a 0.88% of the total emission. It corresponds to 49.89 million tons of 1391 carbon. The power generation shares 15.9% of total Korean COz emission. Indushy shares 2.1%, transportation 14.9%, others 35.1%. Among the major COL emiaas, only the indushy and power generation sectors can take the measures to reduce or recover the CCk from exhausted gases. The capacity of fossil-fuel burning power plants in Korea today is about 14,300 megawatts(m), about 50% of it is driven by heavy oil burning plants. Coal burning power plants shares 31%, LNG Mng power plants 18%. As shown in Figure 2, the contirbution of several fuels for elebicity generation and hence for CCh emission is predicted to be changed significantly in the future. According to the national plan, the number of coal-burning power plants will be remarkably increased; the capacity of coal-burning power plants is planned to be 67% of total thermal power capacity 2010 (Figure 2). The flue gas composition is greatly dependent on the type of fuel used as shown in Table 2. The flue gas from coal-burning power plants generally has the highest concentrations of SO,, NO. which are toxic compounds against the microalgal growth. Unless some pretreatments to reduce the SO.. NO. concentrations is made, the gas can not be directly purged into microdeal culture. On the other hand, flue gas from LNG and diesel oil buming power plants is believed to have acceptable concentrations of SO., NO, concerning literature. Carbon dioxide in the flue gas can be directly fixed by microalgae. The growth rate of microalgae is greatly dependent on the cultivation temperatue and most microalgaes exhibit the maximum growth rate between 25°C and S C . Since CCk fixation pmcess by microalgae requires a large area for light absorption, it is believed to be installed outdoors for large scale application. Thus the control of cultivation temperature become normally very difficult In case of power plant application of the process, cooling water discharge temperature might determine the cultivation iemperahe of microalgae. Average temperature of cooling water from typical fossil-fuel buming power plant in Korea is shown in Table 3. The table shows that the minimum temperature worded in February is 13.4C. The lowest temperature is apparently too low for the proper cultivation of normal microalgae. However, the problem is believed to be overcome by thermal insulation or solar energy technologies such as a green house in winter. BIOLOGICAL CARBON DIOXIDE FIXATION WORK W KIER Recently several applied studies aimed at the direct biological fixation of COz out of the flue gases from thermal power plants have been carried out(l1-13). Although the direct use of 7 discharged gases reduces the cost of pretreatment, it imposes extreme conditions on microalgae such as high concentrations of COZ and the presence of toxic chemicals like SO,, NO, to microalgae. The best suited microalgal strain for this purpose should be selected in terms of tolerance to environmental stresses. /I