An optimizing start-up strategy for a bio-methanator |
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Authors: | Mihaela Sbarciog Mia Loccufier Alain Vande Wouwer |
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Institution: | (1) Automatic Control Laboratory, University of Mons, 31 Boulevard Dolez, 7000 Mons, Belgium;(2) Department of Electrical Energy, Systems and Automation, Ghent University, 914 Technologiepark, 9052 Ghent, Belgium |
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Abstract: | This paper presents an optimizing start-up strategy for a bio-methanator. The goal of the control strategy is to maximize
the outflow rate of methane in anaerobic digestion processes, which can be described by a two-population model. The methodology
relies on a thorough analysis of the system dynamics and involves the solution of two optimization problems: steady-state
optimization for determining the optimal operating point and transient optimization. The latter is a classical optimal control
problem, which can be solved using the maximum principle of Pontryagin. The proposed control law is of the bang–bang type.
The process is driven from an initial state to a small neighborhood of the optimal steady state by switching the manipulated
variable (dilution rate) from the minimum to the maximum value at a certain time instant. Then the dilution rate is set to
the optimal value and the system settles down in the optimal steady state. This control law ensures the convergence of the
system to the optimal steady state and substantially increases its stability region. The region of attraction of the steady
state corresponding to maximum production of methane is considerably enlarged. In some cases, which are related to the possibility
of selecting the minimum dilution rate below a certain level, the stability region of the optimal steady state equals the
interior of the state space. Aside its efficiency, which is evaluated not only in terms of biogas production but also from
the perspective of treatment of the organic load, the strategy is also characterized by simplicity, being thus appropriate
for implementation in real-life systems. Another important advantage is its generality: this technique may be applied to any
anaerobic digestion process, for which the acidogenesis and methanogenesis are, respectively, characterized by Monod and Haldane
kinetics. |
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