(716a) Optimal Design of Integrated Electric-Power and Algal Biofuel Generation Systems

Optimal Design of Integrated Electric-Power and Algal
Biofuel Generation Systems

César G. Gutiérrez-Arriaga,¹ Medardo
Serna-González,¹ José M. Ponce-Ortega,¹and Mahmoud M.
El-Halwagi^2,3

¹Chemical Engineering Department, Universidad Michoacana de San Nicolás
de Hidalgo, Morelia, Michoacán, 58060, México.

²Chemical Engineering Department, Texas A&M University, College
Station Texas, 77843, USA.

³ Adjunct Faculty at the Chemical and Materials Engineering
Department, King Abdulaziz University, Jeddah, Saudi Arabia

            With
increasing concerns over global climate change resulting from increased
concentration of greenhouse gases in the atmosphere, large reductions in CO₂
emissions, while satisfying the worldwide increase of energy demands, will be
needed to address this challenging issue. In this regard, fossil-fuel fired power plants play a significant role since they
generate about 65% of the electricity used in the world and 35.8% of all of current anthropogenic emissions. To avoid the worst global warming scenarios,
CO₂ emissions from the electricity sector must be reduce by 50-80%
below today's levels by 2050. Achieving this reduction involves many technical
alternatives such as carbon capture, fuel switching, CO₂ storage,
and process integration. The CO₂ biofixation
by microalgal photosynthesis using flue gases from fossil-fuel fired power
plants is also a suitable technology to reduce the CO₂ emissions
from the electricity sector, since it is a natural and environmentally friendly
alternative. In addition, capture and utilization of CO₂ from power
plants by microalgae followed by algal biofuel production seems to be a good technology
option to achieve sustainable integrated systems composed by a fossil-fuel
fired power plant and an algae-based biorefinery.

            Therefore, this work
presents a multi-objective optimization methodology to determine simultaneously
the optimal design of steam power plants, considering simultaneously economic and
environmental criteria, together with microalgae CO₂ sequestering from
the combustion gases of different fossil fuels (coal, oil and natural gas) and
biofuels (biogas, biomass, softwood and hardwood) selected in the boiler and
the subsequent microalgae biomass conversion to biofuels (bioethanol and
biodiesel). As can be seen in Figure 1, all or part of the algal biofuel
produced can be used as fuel in the boiler. This system allows the sustainable
reduction of the huge CO₂ emissions from the fossil-fuel fired power
plants.

Figure 1. Integrated
scheme for the microalgal CO₂ capture in power plants.

            The proposed methodology is
based on a search procedure through genetic algorithms due to the large number
of non-convex terms present in the model and to avoid get trapped prematurely in
local solutions. The methodology considers the optimal selection of the type of
fuel to be used because this affects drastically and simultaneously the
economic and environmental objective functions. The economic objective function
considers the maximization of the total net profit accounting for the sales
profit and the fuel costs, as well as the capital costs for the turbine,
boiler, condenser, pumps, feed-water heaters and deaerator, as well as the
costs to operate such equipment (fuel, electricity, cooling water, etc.), while
the environmental objective function is based on the eco-indicator-99 constituted by harmful materials for the health
and the ecosystem to measure the global environmental impact of these systems, following the life cycle analysis approach. An approach to determine
a set of optimal solutions that simultaneously compensate both objectives is
proposed. The problem can be easily solved using the proposed methodology in a
relatively small computation time. The application of the proposed methodology
for a case study in Mexico indicates that combination of fossil fuels (coal,
oil and natural gas) and biofuels (biogas, biomass, softwood and hardwood) is
required to compensate the economic and environmental objectives, in addition
to the economic and environmental benefits obtained from the algal biofuel produced
by the integrated system.