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2*
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1
Chemical Engineering Department, Texas A&M University, College Station Texas, 77843, USA.
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Chemical Engineering Department, Universidad Michoacana de San Nicolás de Hidalgo, Morelia,
Michoacán, 58060, México.
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Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz
University, P. O. Box 80204, Jeddah, 21589, Saudi Arabia.
* Author for correspondence: J.M. Ponce-Ortega
E-mail: jmponce@umich.mx
Tel. +52 443 322 3500 ext. 1277
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The proposed methodology considers a detailed thermodynamic modelling of the power plant is carried out using the IAPWS-IF 97 formulation (see Wagner et al. 2000) for prediction of steam and water thermodynamic properties. Also according to Figure 2 , the steam power plant considers the sustainable generation of electricity incorporating an advanced regenerative-reheat steam power plant employing different energy sources (i.e. fossil fuels, biofuels, and residual biomass). This power plant consists of one stage of steam reheat, and includes a deaerator and six closed feedwater heaters (shelland-tube heat exchangers) receiving steam bled from the turbine and feedwater or high-pressure subcooled liquid water coming from the condenser. It should be noted that a wet-cooling tower connected with the condenser removes the waste heat from the turbine exhaust steam through direct contact with air; consequently, subcooled liquid water or condensate is obtained. The cooling tower is followed by a pump, which returns the cooled water to the condenser and the condensate is pressurized by a pump to get a pressure sufficient to pass through the three low-pressure closed feedwater heaters.
Later, it enters the deaerator to improve the water quality by removing dissolved oxygen and other noncondensable gases. This unit is placed at the middle of the feedwater heater system owing to the thermodynamic conditions are better to remove the dissolved gases from the water (see El-Wakil
1984). A second pump is installed after the deaerator with the purpose of deliver water through three high-pressure closed feedwater heaters to the boiler. The boiler generates high-pressure superheated steam firing different types of fuel as well as the residual algae biomass coming from the oil extraction plant of the algae cultivation system. Finally, the superheated high-pressure steam produced in the boiler is used to generate electric power in high pressure (HP), medium or intermediate pressure (IP) and low pressure (LP) turbines. It is worth noting that the steam leaving the high-pressure turbine at
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some intermediate pressure goes back through the boiler before sending it to the next turbine stage with the purpose of the turbine exhaust steam does not contain excessive moisture.
On the other hand, the algae cultivation system transforms the algal biomass into biodiesel with microalgae biofixation of CO
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in the flue gas from the power plant. This process starts with the biomass-cultivation stage that requires culture inoculum ( Chlorella vulgaris ), water, flue gas CO
2
, sunlight and inorganic nutrients (i.e., nitrates and phosphates). The Chlorella vulgaris is cultivated in open-ponds mixed with paddle wheels over a twelve-month growing season. Several methodologies have reported different optimal values for some design parameters for the algae cultivation system such as production rate, the microalgal lipid content, the ratios between the biodiesel generated/the amount of nutrients required and the CO
2
provided/algal biomass, the overall CO
2
consumption efficiency, a standard design for each raceway pond, etc. (Brune et al. 2009; Campbell et al. 2011; Benemann and
Oswald 1996; Yang et al. 2011; Lardon et al. 2009). The following step in the algae cultivation system is the algae biomass harvesting, which consists of biomass recovery from the culture medium. Algal biomass is first recovered using a coagulation-flocculation-settling system operating with the recovery yield of 97% and it has a concentration of 30 g/L (see Benemann and Tillet 1987). Additional information for the design of the algae biomass harvesting can be found in the literature (Ventura et al.
2013; Molina-Grima et al. 2003; Heasman et al. 2000). The next step is the oil extraction process. It is worth mentioning that the algae cultivation system can be located in a sunny area near the Cuitzeo
Lake, in the central part of Mexico, due to the large amounts of water and land area available to support the algal culture facility. The proposed mathematical formulation for the algae cultivation system is presented in Appendix A. Furthermore, there is an adjacent industrial plant demanding external utilities where the thermal pinch analysis is carried out to establish utility targets for heat integration among the process streams and to obtain the minimum hot and cold utility required; thus the hot utility
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requirements are replaced by the residual energy from the steam power plant (avoiding the burning of fossil fuels to satisfy the heating utilities).
Benemann JR, Oswald WJ (1996) System and economic analysis of microalgae ponds for conversion of CO
2
to biomass. Quarterly Technical Progress Report to the Department of Energy Pittsburgh
Energy Technology Center. Grant No. DEFG22-93PC93204
Benemann JR, Tillet DM (1987) Effect of fluctuating environments on the selection of high yielding microalgae. Final Report, Subcontract XK-4-04136-06, Solar Energy Institute, Golden,
Colorado, USA
Brune DE, Lundquist TJ, Benemann JR (2009) Microalgal biomass for greenhouse gas reductions:
Potential for replacement of fossil fuels and animal feeds. Journal of Environmental
Engineering 135:1136-1144
Campbell PK, Beer T, Batten D (2011) Life cycle assessment of biodiesel production from microalgae in ponds. Bioresource and Technology 102:50-56
Heasman DJ, O’Connor W, Sushanes T, Foulkes L (2000) Development of extended shelf-life microalgae concentrate diets harvested by centrifugation for bivalve molluscs – a summary.
Aquaculture Research 31:637-659
Lardon L, Hélias A, Sialve B, Steyer JP, Bernard O (2009) Life-cycle assessment of biodiesel production from microalgae. Environmental Science and Technology 43:6475-6481
Molina-Grima E, Belarbi EH, Acién-Fernández FG, Medina AR, Chisti Y (2003) Recovery of microalgal biomass and metabolite: process options and economics. Biotechnology Advances
20:491-515
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Ventura JRS, Yang B, Lee YW, Lee K, Jahng D (2013) Life cycle analyses of CO
2
, energy and cost for four different routes of microalgal bioenergy conversion. Bioresource Technology 137:302-310
Yang J, Xu M, Zhang, XZ, Hu QA, Sommerfeld M, Chen YS (2011) Life-cycle analysis on biodiesel production from microalgae: water footprint and nutrients balance. Bioresource and Technology
102:159-165
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