Direct Conversion of Biogas into Synthetic Natural Gas (SNG) over the 0.1-0.5wt% Ru/Al2O3 Catalyst

Introduction

Biogas and landfill gas are the two main sources of greenhouse gases (GHG) emissions which are rich in both carbon dioxide (CO₂) and methane (CH₄). These streams could be potentially utilized as synthetic natural gas (SNG), sometimes also referred to as renewable natural gas (RNG). However, the CO₂ content (up to 50 vol%) has to be eliminated prior to the use. Current commercial solutions rely on CO₂ removal via either membrane separation or pressure swing adsorption (PSA) with the removed CO₂ being vented to the atmosphere. As an alternative way, the thermocatalytic Sabatier reaction can be used to convert the CO₂ contained in biogas and landfill gas into synthetic CH₄, upgrading these waste streams to a pipeline quality SNG. A highly active, selective and stable catalyst is a pre-requisite for implementation of this conversion pathway. In this study, we use the 0.1-0.5wt% Ru/Al₂O₃ catalyst to convert a simulated biogas mixture directly without any upstream CO₂ separation. Our results show a promising new technological avenue for biogas upgrading into SNG.

Methodology

A series of Ru/Al₂O₃ catalysts were prepared using a wet impregnation method. Commercial alumina support was crushed and sieved prior to impregnation. Appropriate amounts of ruthenium chloride were dissolved in acetone and the sieved alumina particles were added to the solutions with different Ru concentrations. The resulted slurries were sonicated and dried. In the flow system setup, mass flow controllers, a back pressure regulator, and a furnace were used to control feed flow rate, pressure, and temperature. Infrared analyzers were used to measure outlet gas concentrations. Catalytic performance was investigated as a function of temperature and space velocity for a range of Ru loadings to identify the optimal active phase content. The catalyst stability was specifically examined. Characterization techniques included ICP-OES, TGA-MS-FTIR and temperature programmed reactions (TPA, TPR, and TPO).

Results

The results of direct conversion of the simulated biogas (1:1 CO₂:CH₄) over the 0.5wt% Ru/g-Al₂O₃ are shown on the left figure. At 350-450 °C, CH₄ selectivity is nearly 100% followed by a gradual decrease at higher temperature. The optimal temperature for this catalyst is around 450 °C with 80% CO₂ conversion and 97% CH₄ selectivity achieved. From the figure on the right it is clearly seen that both CO₂ conversion and CH₄ selectivity were stable over the 65 hour time on stream. The temperature programmed oxidation (TPO) test of the spent catalyst suggested that the extent of coking is negligible. The effect of Ru loading was also investigated. Lowering Ru loading caused the CH₄ selectivity to drop significantly, and optimal operating temperature shifted to higher temperatures as the Ru loading was reduced. The lower limit for maintaining decent methanation performance was identified as 0.1wt%.

Significance

The results of this study indicate that it is possible to directly convert a mixture of CO₂ and CH₄ (representing biogas) to an upgraded mixture predominantly containing CH₄ with some unreacted H₂, achieving CO₂ conversions above 90%. The proposed technological solution could eliminate the need for the costly CO₂ separation increasing two-fold the rate of RNG production from biogas and landfill gas.