Coking in a two-stages catalytic upgrading of biogas

Biogas, essentially a mixture of CH4 and CO2, obtained from the fermentation of waste and a subsequent purification, is exploited selectively separating the methane from the carbon dioxide, through different technologies. The result is the production of highly pure biomethane that can be introduced directly in the natural gas line or used as fuel. CO2 can then be re-used, i.e., in the food industry or in pure gas supply, but the market request could not be such high and alternatives must be investigated. In this thesis, a different technique for the biogas upgrading is proposed: it consists in the application of the Sabatier reaction for an effective upgrading of biogas, where CO2 is converted in CH4, without any preventive separation. The reaction consists in the catalytic heterogeneous hydrogenation of CO2, it is highly exothermic, and it is limited by thermodynamic equilibrium (favored by high pressure and low temperatures). A Ni/Al2O3 industrial catalysts (code name: EX11914) provided by Clariant have been evaluated at atmospheric pressure and at pressure up to 6 bara to understand the behavior of the catalyst under long duration tests and the role of carbon formation on the catalyst’s activity and CO2 conversion. The attention goes toward the second reactor of a two-stage adiabatic biogas methanation process since the high inlet partial pressure of methane is likely to lead to carbon formation by methane cracking. The experimental campaign was divided into two main sections. First, the investigation of the aging of the catalysts at different pressure conditions, by long duration tests (in the order of days). Then, the investigation of the role of addition of water vapor in the feed mixture to a second stage reactor, as an approach to limit carbon formation.