# Adaptation of Temperature Profiles in CO2 Methanation Reactors by an Appropriate Selection of Catalyst and Dilution Agent

## TL;DR

This paper explores how to control temperature in CO2 methanation reactors by using catalysts and dilution agents with different thermal properties.

## Contribution

The study introduces a new method for managing reaction exothermicity through catalyst dilution and axial heat dispersion.

## Key findings

- Al2O3 and ZnO are the best inert materials for heat dispersion in CO2 methanation.
- A trade-off exists between reactor length and hotspot temperature, which can be mitigated by varying catalyst dilution along the reactor.
- Catalyst active phase concentration must be carefully controlled to avoid hotspots and maintain high conversion.

## Abstract

The
control of temperature in CO2 methanation reactors
is a challenging task due to the high exothermicity of the reaction
and the high reaction rate observed when feeding pure reactants. This
study analyses a new concept for moderating the reaction exothermicity
by controlled dilution of a Ni-based catalyst using materials with
different thermal conductivities. The simple decrease in the concentration
of the catalyst active phase is not sufficient to control the temperature
in the reactor because of the parametric sensitivity of the reaction,
which means that a certain threshold of active phase exists, above
which the reaction becomes so fast to cause the formation of a pronounced
reaction hotspot and below which the reaction rate is too low to achieve
high conversion. Therefore, the range of catalyst active phase concentrations
that enables the reactor to remain active while maintaining a temperature
below 550 °C using an externally cooled reactor is too narrow
for practical applications. To achieve reasonable temperature control
and a sufficient CO2 conversion, the catalyst could be
mixed with an inert solid material that can enhance the axial heat
dispersion, so as to induce a good distribution of the heat release
over the axial coordinate. This would reduce the localized load to
the cooling system and spread it over a larger surface. The conductivity
of different standard inert materials was evaluated, showing that
Al2O3 and ZnO are the best options for application
in CO2 methanation. It was then observed that when the
reaction rate decreases due to the approach to thermodynamic equilibrium,
the axial dispersion of heat should be reduced to enhance the reactor
performance, thanks to a fast temperature reduction. This results
in a clear trade-off relationship between the reactor length, to achieve
a given conversion, and the hotspot temperature. To break this trade-off,
it would be necessary to modify the catalyst dilution characteristic
over the axial coordinate, allowing for at least two different catalyst
dilution zones.