CO₂ methanation: Optimal start‐up control of a fixed‐bed reactor for power‐to‐gas applications

Abstract

Utilizing volatile renewable energy sources (e.g., solar, wind) for chemical production systems requires a deeper understanding of their dynamic operation modes. Taking the example of a methanation reactor in the context of power-togas applications, a dynamic optimization approach is used to identify control trajectories for a time optimal reactor start-up avoiding distinct hot spot formation. For the optimization, we develop a dynamic, two-dimensional model of a fixed-bed tube reactor for carbon dioxide methanation which is based on the reaction scheme of the underlying exothermic Sabatier reaction mechanism. While controlling dynamic hot spot formation inside the catalyst bed, we prove the applicability of our methodology and investigate the feasibility of dynamic carbon dioxide methanation.

Transient Flow Rate Ramps for Methanation of Carbon Dioxide in an Adiabatic Fixed‐Bed Recycle Reactor

“…With the aim to reduce the costs of the hydrogen storage by minimizing upstream storage capacities (up to 8% of the total investment costs can be saved), the unsteady‐state operation of the exothermic methanation reaction is demanded. However, the occurring phenomena and operation strategies under dynamic conditions are not exhaustively addressed in scientific literature thus far, but receive more attention in recent years . One promising approach is adiabatically operated fixed‐bed reactors, which show a good partial and excess load behavior, since the chemical equilibrium is reached within the residence time.…”

“…The PS means that “small changes in one or more of the reactor input parameters lead to much larger changes in the output variables.” Multiple steady‐states can be expected in the PS regime known from the ignition and extinction behavior attributed to polytropic reactors. Bremer et al discussed reactor control strategies to operate at the unstable working point for a cooled methanation reactor, requiring very fast changes of the coolant temperature and being possible with a PI controller. A differential flow arises when two or more distinct, interactive species move in a homogeneous flow field with their individual flow velocity, which changes the dynamics of the system drastically allowing DIFIs to occur .…”

“…However, the occurring phenomena and operation strategies under dynamic conditions are not exhaustively addressed in scientific literature thus far, but receive more attention in recent years. [6][7][8][9][10][11][12][13][14][15][16][17] One promising approach is adiabatically operated fixed-bed reactors, which show a good partial and excess load behavior, since the chemical equilibrium is reached within the residence time. The variation of the inlet volumetric flow rate indeed leads to a transition of the axial steady-state profiles, though without changing the outlet temperature and composition.…”

Applying Reaction Kinetics to Pseudohomogeneous Methanation Modeling in Fixed‐Bed Reactors

“…In most studies, the set of three rate equations proposed by Xu and Froment is used for methanation simulations , , . It was initially created to model methane steam reforming, which takes place at higher temperatures than the methanation reactions.…”

“…In pseudohomogeneous reactor modeling, pore diffusion effects can either be considered by a fixed effectiveness factor η (e.g., η = 0.1 , , ) or an analytical solution of the reaction‐diffusion differential equation based on Thiele moduli ϕ (Eq. ) (e.g., reaction order n = 1 ).…”