(532g) Combined Aging Processes for SNG Production By Bio-Syngas Methanation

Combined Aging Processes for SNG Production By Bio-Syngas Methanation

H. Li¹, A. Travert¹, F.Maugé¹, L. Dreibine², Y. Schuurman², C. Mirodatos^*2, F.C. Meunier^1,2

¹ Laboratoire de Catalyse et Spectrochimie, CNRS-ENSICAEN, University of Caen, 14050 Caen, France

² Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France.

Synthesis gas obtained from biomass gasification (biosyngas, a mixture of CO, CO₂, H₂ and impurities) is pertaining to the wider societal agendas not only as a sustainable source of fuels and chemicals, but also as a mean for dealing with global warming challenge. A major barrier in the commercialization of biomass gasification is the presence of bio-impurities such as ammonia, HCl, H₂S and tars in the gas products that are detrimental to downstream processes. Two important downstream processes are the conversion of syngas to (i) methane and light hydrocarbons (Substitute Natural Gas, SNG) and (ii) higher hydrocarbons (Fischer-Tropsch synthesis, FT). The exact role and effects of biomass-derived impurities on the catalysts used for these reactions are poorly known. Furthermore the way that such aging factors can combine with structural aging processes such as metal sintering under operating conditions is not clearly documented either. In the present study, we focused on the SNG process, using Ni based catalysts (alumina supported) in order to evaluate the kinetic sensitivity of various aging factors, combining the effect of model poisons such as ammonia, acetonitrile, heptanes, toluene and benzene together with metal particle re-structuring by sintering under SNG process conditions.

To perform this study, various operando techniques were used, essentially DRIFT spectrometry [1], completed by magnetic measurements and SSITKA analysis.

To sum up the main effects observed with time on stream and added poisons concentrations, the following features can be outlined:

(i) The CO carbonyl IR signal appeared to be correlated with the loss of activity, in the absence of added poisons. This irreversible ageing process was assigned from previous studies [2] to metal sintering and restructuring, as a consequence of metal carbonylation leading to Ni transfer from small to larger particles.

(ii) The deactivating effect of N containing poisons like acetonitrile, NO and ammonia, superimposed with the previous irreversible Ni sintering, was totally or partially reversible. Though acetonitrile and NO were at least partially converted into ammonia under the methanation conditions, the degree of deactivation was ranked as acetonitrile> ammonia> nitric oxide, suggesting that various poisoning modes (e.g. competitive adsorption, coking, electronic effects) might be present.

iii) The effect of adding hydrocarbon compounds representative of tar by-products coming from the bio-mass gasification revealed that ethylene, heptanes and benzene did not show obvious poisoning effects. However, toluene did show somewhat irreversible poisoning effect on accelerating the deactivation rate comparing with the blank reference test.

iv) Accompany with the loss of carbonyl, there were usually an increase of the carboxylate species, which became normally saturated after few hours deactivation during methanation, independant of the increasing concentration of any poison introduction or any continues increasing carbonyl loss. Therefore, the carboxylate can be considered only as in function of time, and can hardly be correlated with carbonyl loss, neither with deactivation process alone.

A tentative rationale of these complex aging processes will be proposed on the basis of a previous kinetic study of the methanation reaction, based the changes in carbonyl and CH_x active intermediates concentrations, directly measured by SSITKA (switch from ¹²CO to ¹³CO and reverse) [3].

For example, by observing that the reversible poisoning effect of N containing species hardly changes the nature and concentration of carbonyl species over the catalyst surface, it is concluded that it affects essentially surface controlling steps such as hydrogen adsorption and/or hydrogenation of CH_x reacting intermediates deriving from carbonyls decomposition. Electronic effects via N interaction with Ni or reactive adspecies combined with Ni surface restructuring might originate the observed decays. For the case the added hydrocarbons, the absence of electronic effects would explain their negligible toxicity, unless surface coking may lead to diffusion limitation for the reacting syngas feedstock.

Strategies for improving the resistance of methanation catalysts to this type of poisoning can be proposed from this mechanistic and kinetic re-investigation of one of the oldest catalytic reaction studied till now.

REFERENCES:

[1] Effective bulk and surface temperatures of the catalyst bed of FT-IR cells used for in situ and operando studies. H. Li, M. Rivallan, F. Thibault-Starzyk, A. Travert, F.C. Meunier, PCCP, 2013, DOI: 10.1039/c3cp50442e

[2] CO hydrogenation on a nickel catalyst. Part I : Kinetics and modeling of a low temperature sintering process.

M. Agnelli, M. Kolb, and C. Mirodatos, J. Catal., 1994, 148, 9-21.

[3] CO Hydrogenation on a Nickel Catalyst. II. A Mechanistic Study by Transient Kinetics and Infrared Spectroscopy. M. Agnelli, H.M. Swaan, C. Marquez-Alvarez, G.A. Martin, C. Mirodatos, J. Catal. 1998, 175, 117.