PHOTOBIOREACTOR: A CHALLENGING FUTURE OF REDUCTION CO2 IN THERMAL POWER PLANT Tanay Chattopadhyay Mechanical Operation (Stage-II), Kolaghat Thermal Power Station, WBPDCL, Mecheda, Purbamedinipur, KTPP sub post office, Pin-721137, West Bengal, India. Phone: (+91) 9432075035; E-mail: [email protected]

Abstract: Increasing of green house gas and its effect to global warming, is the major problem today. Many researches have been made to reduce CO 2 emission from different industries. Among them Algae based carbon capture technology have a good future in thermal power plants of India to reduce green house gas emission. In this paper a proposal of CO2 emission reduction with the help of algae culture is proposed and describe. Keywords: Green house gas, Thermal power plant, Photobioreactor, micro-algae. Introduction: Nearly 73% of India s total installed power generation capacity is thermal, of which coal based generation is 90%. About 250 million tones of coal annually burn to generate power [1]. So huge amount of green house gas emits form the thermal power plant. Hence the effect is ‘Global warming’ i.e. average temperature of the Earth's increase, which in turn causes changes in climate. Many gases exhibit such “greenhouse” properties, including those that occur naturally in the atmosphere, such as water vapor, carbon dioxide, methane, and nitrous oxide, sulfur oxides etc. Fossil fuels consist primarily of hydrocarbons, which are made up of hydrogen and carbon. When burned, the carbon combines with oxygen to yield carbon dioxide. Due to the usage of coal, the major types of pollutants emitted into the atmosphere are fly ash, coal dust, soot, and oxides of sulfur and nitrogen. Gaseous emissions discharged through a flue or stacks are called flue gases. Flue gas of thermal power plants contains N 2 (82%), CO2 (14%), CO (80 ppm), O2 (4%), NOx (70 ppm), SOx (it is very small quantity because, Indian coal has low sulfur content) and soot dust (about 50mg/m 3). Seven oxides of nitrogen (NOx) can be grouped as N2O, NO, NO2, NO3, N2O3, N2O4, out of which nitric oxide (NO) and nitrogen dioxide arise from the anthropogenic activities which are classified as major air pollutants Oxidation of sulfur and nitrogen oxides will cause acid rain. These gases also play an important role in the environment through forest damage, effects on vegetation, smog formation, material damage, direct and indirect damage to human health, depletion of the stratospheric ozone layer and the greenhouse effect. Currently, flue gas separation and CO 2 capture are practiced at about a dozen facilities worldwide. The capture process is based on chemical absorption. The captured CO 2 is used for various industrial and commercial processes, e.g., urea production, foam blowing, carbonation of beverages, and dry ice production [2]. Among them Algae capture technology is the resent successful process. Algae can be grown on high concentration of CO2, NOx and at temperature from freezing point to about 80 0C and high pH. Chlorella, Haematococcus, and Dunaliella can be easily harvested at the temperature [3]. Spirulina for example survives and grows well at high pH (9 to 11.5) and because of its spiral shape it is also easy to harvest [4]. Via photosynthesis CO2 and water are basic requirement for algae growth; O 2 and water vapor are the byproduct. Photosynthesis comes from the Greek roots "photo" meaning light and "synthesis” meaning to make something. An example of naturally occurring biological oxidation-reduction reactions is the process of photosynthesis. It is a very complex process carried out by green plants and algae. These organisms are able to harness the energy contained in sunlight, and via a series of oxidation-reduction reactions, produce oxygen and carbohydrates, as well as other compounds, which may be utilized for energy as well as the synthesis of other compounds. Thermal power plant emits nearly 1T of CO 2 for every MWh, by which we can grow nearly 0.5T Algae [5]. Algae culture technique: Flue gas temperature of thermal power station (after induced draft fan or ESP) is about 120 0C. It can be decreased to ~ 60 0C by direct contact of ambient water. In this process heavy ash particle also removed. Then it can be directly fed to the photobioreactor, in which algae can grow. The schematic diagram of this process is shown in Fig-1. V1 valve is used to isolate this process during light up or maintenance condition. At night we can shut down this process because no photosynthesis can take place during night. But if we use green house technique to harvest algae then, we can use artificial light (LED light).

Fig. 1: Design of algae culture form thermal power plant flue gas

This Photobioreactor have different types viz. open pond, closed pond, tubular and plastic bag. The vast bulk of micro-algae cultivated today are grown in open ponds. The concept is to grow algae in artificial ponds, with adding necessary nutrients, and with CO 2 from flue gas. Open ponds can be built and operated very economically and hence offer many advantages as long as the species for cultivation can be maintained. An area is divided into a rectangular grid, with each rectangle containing a channel in the shape of an oval; a paddle wheel is used to drive water flow continuously around the circuit. They usually operate at water depths of 15–20 cm, as at these depths biomass concentrations of 1 g dry weight per liter and productivities of 60–100 mg L−1 day−1 (i.e. 10–25 g m−2 day−1) are possible [6]. The main disadvantage of open systems is that by being open to the atmosphere, they loose water by evaporation at a rate similar to land crops and are also susceptible to contamination by unwanted species. Two types of open pond algae culture is shown in Fig-2 [7].

Fig. 2: Open pond algae culture

Besides this open Photobioreactor, closed reactor is also used because it saves water, energy and chemicals. It require low area for high volume of algae culture i.e. tubular reactors, plate reactors, bubble column reactors ([8-9]; Figs. 3 and 4).

Fig. 3: (a) Tubular reactor, (b) plate reactors, (c) bubble column reactor.

Byproduct: After technical feasibility economics is the critical issue: the algal production systems must have very low capital and operating costs to compete with other crops and alternative energy sources. This CO 2 capture process is more effective because, it reduce green house gas as well as we get bio-diesel, ethanol, agriculture

fertilizer, high protein animal food, biopolymer / bio-plastic etc as a byproduct [2-4, 9]. At present, about 5000 tons of food and feed-grade micro algae biomass is produced annually in large open pond systems [10]. Advantages: The advantages of the CO2 capture through algae culture is as follows [10]: (1) High purity CO2 gas is not required for algae culture. It is possible that flue gas containing 2~5% CO 2 can be fed directly to the Photobioreactor. This will simplify CO 2 separation from flue gas significantly. (2) NOx or SOx can be effectively used as nutrients for micro algae. (3) Micro-algae culturing yields high value commercial products that could offset the capital and the operation costs of the process. (4) This process is a renewable cycle with minimal negative impacts on environment. Concussion: This micro-algae culture for CO2 reduction may more effective near future. Further work is needed before such a system could be considered for practical application. Specifically, issues of light delivery and distribution, enhancing growth rate through increased bicarbonate concentration, flue gas cooling, and harvesting to provide sustained growth, must be addressed for long-term, full-scale functionality. References: [1]S.K.Jain, V.Prakash and S.Kapoor, “Flue gas treatment alternatives for enhancing ESP collection efficiency: NTPC experience”, Workshop on ESP performance: Role of fly ash resistivity, Sept 2324, 2004, IIT-D and NTPC, pp: 1-5. [2]H. Herzog, D. Golomb, “Carbon Capture and Storage from Fossil Fuel Use”, Encyclopedia of Energy, Volume 1. Copyright 2004, Elsevier. All rights reserved, Article Number: NRGY: 00422. [3]S. A. Scott, M.P Davey., J. S Dennis., I. Horst, C.J Howe., D.J Lea-Smith and A.G Smith, “Biodiesel from algae: challenges and prospects”, Current Opinion in Biotechnology , 2010, 21:277– 286. [4]S.A. Khan, Rashmi, M.Z. Hussain, S. Prasad, U.C. Banerjee, “Prospects of biodiesel production from microalgae in India”, Renewable and Sustainable Energy Reviews, 13 (2009) 2361–2372. [5]http://www.powerplantccs.com/ccs/cap/fut/alg/alg.html [6]O. Pulz “Photobioreactors: production systems for phototrophic microorganisms”, Appl Microbiol Biotechnol 57: (2001) 287–293 [7]Y. Chisti, “Biodiesel from microalgae”. Biotechnol Adv; 25: (2007) 294–306. [8]Molina Grima E, Acie´n Ferna´ndez FG, Garcı´a Camacho F, Chisti Y. Photobioreactors: light regime, mass transfer, and scale up”. J Biotechnol;70: (1999) 231–47. [9]P. M. Schenk, S.R. Thomas-Hall, E.Stephens, U. C. Marx, J.H. Mussgnug, C.Posten, O. Kruse, B.Hankamer, “Second Generation Biofuels: High-Efficiency Microalgae for Biodiesel Production”, Bioenerg. Res. (2008) 1:20–43. [10]B.Wang, Y. Li, N.Wu and C. Q. Lan, “CO 2 bio-mitigation using microalgae”, Appl Microbiol Biotechnol (2008) 79:707–718.