Production of methane via the anaerobic digestion of biogenic material is an alternative renewable energy source. Among various methods for energy production from biomass (e.g. bioethanol, pyrolysis, biodiesel, etc.), biogas has shown to be a robust, cheap and independent process. It can run on a wide variety of substrates, contributes to carbon and methane capture, does not require sterile conditions, can be very stable under conservative operation, has minimal safety requirements, and the digestate may be used as fertilizer. Still, with the process being carried out by a large symbiotic and constantly changing microbial population (mainly due to mutations, non-sterile feeding, and low dilution rates), and with the sludge making state monitoring costly and maintenance-intensive, there are major challenges in the dynamic analysis and control of the process. This has led to conservative operation of the biogas production process, far from optimal yields and lacking the necessary flexibility to comply with the fluctuations in both, the electricity and feedstock supply grid.
In this work, we study a two-stage reactor design to enable maximization of production by stabilizing the dynamics of the process near the critical dilution rates. In this way, it is possible to achieve: 1) higher yield, 2) higher methane concentration in the output gas, and 3) a more flexible gas production.
The anaerobic digestion process involves four main biochemical steps, namely: 1) hydrolysis, 2) acidogenesis, 3) acetogenesis, and 4) methanogenesis. These steps are carried out by different microbial populations with varying characteristics, growth rates and substrate affinities. Dividing the process in different stages to exploit the characteristics of different populations has already been proposed in the literature . Nevertheless, since optimal performance of the process is near wash-out, increasing the size of the stability region at such operating conditions is of great importance. In order to be able to develop a successful two-reactor biogas production technology, it is necessary to build appropriate models for each stage, as well as advanced control strategies.
A global optimization at steady state biogas production in two reactors shows that an increment of methane flow rate per volume day of 28% is possible against a single reactor concept. Additionally, the concentration of methane in the biogas can also be increased up to 40% (reaching almost 70% methane content). Unfortunately, although stable, these optimal operating points are in the vicinity of washout so that small disturbances in the process may have severe consequences.
The present work addresses problems of feedback stabilization in a two-stage reactor system operating near critical dilution rates, for maximal biogas production. These are tackled through reduced versions of a state-of-the-art anaerobic digestion model , specifically tailored to the operating conditions, as well as concepts of constant-yield control , for the design of stabilizing controllers. Using methane, carbon dioxide, and pH as measurements, the control law is designed to stabilize the system by manipulating the dilution rates in each reactor and the recycling. Lyapunov methods are used to derive estimates of the size of the stability region in open loop and in closed loop. The results show that stable operation of the two-stage reactor process is feasible, leading to increased yield and methane purity.
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