Catalysts based on cellulose-derived carbon aerogels for the cleaning and upgrading of gasification product gas

FERNÁNDEZ, C.; GÓMEZ, O.; SEGURA, C.; JIMÉNEZ, R.; ARTEAGA, L.:
Proceedings of the 5th Latin American Congress on Biorefineries – From laboratory to industrial practice, January 2019 pp162.

Abstract

The use of gasification product gases for energy applications or as platform for chemical synthesis is very limited because of their tar content, which exceeds the limits allowed for energy systems (<50 mg/Nm3) and synthesis applications (<10 ppm). The catalytic decomposition of tars is an effective and energy efficient alternative to clean the syngas, provided that an inexpensive and stable catalyst can be used to selectively convert tars into simpler molecules, which also increases the heating value of the gas. Novel biomass-derived carbon aerogels (CAG), having promising properties as catalyst supports, are studied in this research project for the cleaning and further upgrading of gasification product gases. The objectives are (i) to synthesize CAG-based catalysts with suitable properties for tars decomposition and COx hydrogenation into methane, (ii) to propose kinetic models that properly describe the reactions studied with model gases, and (iii) to test the long-term use of catalysts in a bench-scale reactor, under real gas conditions. An optimized method to synthesize cellulose-derived carbon aerogels was developed, resulting in a predominantly mesoporous turbostratic carbon with high surface area, low mass density and improved thermal stability. Monometallic (Ni and Co) and bimetallic (Ni-Co) catalysts were prepared on CAG and SiO2 supports. Highly dispersed metal particles with narrow size distribution were formed. These materials were tested for tar conversion via steam reforming (using benzene and naphthalene as tar model compounds) and methane production via CO hydrogenation, at different temperatures and residence times. High and similar tar conversions (95–99%) were obtained for Ni and Co catalysts at high temperatures (>800 °C), and kinetic expressions were proposed for the steam reforming of benzene and naphthalene, using a quasi-first order approach. In the case of CO methanation at 250–350 oC, Co resulted more active than Ni, while the highest activity was obtained for bimetallic Ni-Co catalysts. For Co catalysts, the intrinsic rate of methane production increased with the size of Co clusters. A hydrogen-assisted CO dissociation model properly represents the kinetic data. An integrated system of gasification and syngas cleaning/upgrading has been implemented and the catalytic tests will be soon performed.

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