Low Temperature Activation of Carbon Dioxide by Ammonia in Methane Dry Reforming—A Thermodynamic Study

Anand Kumar

doi:10.3390/catal8100481

Abstract

Methane dry reforming (MDR) is an attractive alternative to methane steam reforming for hydrogen production with low harmful environmental emissions on account of utilizing carbon dioxide in the feed. However, carbon formation in the product stream has been the most challenging aspect of MDR, as it leads to catalyst deactivation by coking, prevalent in hydrocarbon reforming reactions. Common strategies to limit coking have mainly targeted catalyst modifications, such as by doping with rare earth metals, supporting on refractory oxides, adding oxygen/steam in the feed, or operating at reaction conditions (e.g., higher temperature), where carbon formation is thermodynamically restrained. These methods do help in suppressing carbon formation; nonetheless, to a large extent, catalyst activity and product selectivity are also adversely affected. In this study, the effect of ammonia addition in MDR feed on carbon suppression is presented. Based on a thermodynamic equilibrium analysis, the most significant observation of ammonia addition is towards low temperature carbon dioxide activation to methane, along with carbon removal. Results indicate that ammonia not only helps in removing carbon formation, but also greatly enriches hydrogen production.

Highlights

The reaction CH4 + CO2 → CO + H2 or methane dry reforming (MDR) for producing hydrogen from methane provides an alternative way to the industrially used methane steam reforming (MSR) [1,2,3,4,5]
Looking at the distribution of various compounds (Figure 1a), it is clear that elemental carbon and water are the two most stable products at low temperature and both start to decrease as temperature increases
There is no CO formation until 600 K, whereas hydrogen starts to appear as early as 400 K itself and both CO and H2 show an exponential increase in concentration with temperature, indicating that higher temperature is more favorable for syngas formation and low temperatures are suitable for coking

Summary

Introduction

The reaction CH4 + CO2 → CO + H2 or methane dry reforming (MDR) for producing hydrogen from methane provides an alternative way to the industrially used methane steam reforming (MSR) [1,2,3,4,5]. The advantage of MDR reaction is that it uses CO2 as a feed, ameliorating the environmental impact by decreasing the concentration of this ubiquitous greenhouse gas, and generating a ratio of H2 and CO that can be adjusted to obtain the required synthesis gas (syngas) composition. Starting from syngas, using Fischer–Tropsch chemistry, long chain hydrocarbons and liquid fuels can be produced on transition metal oxide catalysts, as described in Equation (1): nCO + (2n + 1) H2 → Cn H2(n+1) + nH2 O (1). Supported noble metals suffer less deactivation, but are costly, and the use of alkali promoters increases time on stream (TOS), but decreases activity and selectivity [3,9,12,13,14,15]. The direct dissociation is anticipated to take place at a high

Methods

Results

Conclusion