Tri-Reforming of Methane - Operational Studies over a Ni/CeZrO2/MgAl2O4 Catalyst

Methane tri-reforming combines the steam, dry and partial oxidation of methane in a single reactor. The ratio of the syngas (H₂/CO) produced usually ranges from 1.0 to 2.0, and it is the reason for the methane tri-reforming to be considered a process with a great versatility, since syngas quality can be altered by simply changing the feed composition and thus be destined to several applications. An H₂/CO ratio around 2 is usually desired to produce liquid fuels, since it yields hydrocarbons of high chain.

Operational studies related to gas hourly space velocity (GHSV) and feed compositions were carried out in this work aiming to search adequate H₂/CO ratio destined to GTL processes as well as high CO₂ conversion, considering that it competes for the same active sites for water adsorption.

In these studies, Ni (10% wt) supported on MgAl₂O₄ spinel promoted with CeZrO₂ (Ce/Zr molar ratio= 4) was employed and the catalyst was activated in situ under H₂ flow at 750^oC. The catalytic tests were carried out at 750^oC and atmospheric pressure, for 300 minutes. The ratio (n_CO2+n_H2O+ 2.n_O2)/n_CH4 was kept at 1 for any feed composition, where n_i represents the reactant molar flow rate. The CH₄ volumetric flow rate was 73.4 NmL.min^-1 for any feed composition. Water was pre-heated before reaching the reactor. The gases effluent composition was analyzed in a gas chromatograph, which was equipped with two Porapak-N® and a 13X molecular sieve columns. Four space velocities were evaluated: 3.0, 4.0, 5.3 and 10.5 mol.gcat^-1.h^-1.

This study showed that below 4 mol.gcat^-1.h^-1 and under stoichiometric feed (3 CH₄:1 CO₂: 1H₂O: 0.5 O₂), the reactants conversion (CH₄= 77% and CO₂=54%) did not change, while CH₄ and CO₂ conversions decreased up to 58% and 33%, respectively, as GHSV increased up to 10.5 mol.gcat^-1.h^-1. Thus, the feed composition studies were made using a GHSV of 4 mol.gcat^‑1.h^‑1 and varying the O₂/CO₂ and H₂O/CO₂ molar ratios (0, 0.17 and 1.5).

Increasing either O₂/CO₂ or H₂O/CO₂ from 0 to 1.5, the CO₂ conversion decreases from 73% to 30%, despite increasing H₂/CO ratio, once the partial oxidation and steam reforming of methane increase the H₂ production under higher O₂ and H₂O contents, respectively. O₂ consumption was always complete at any reaction condition, due to O₂ affinity to active sites. The O₂/CO₂ ratio of 0.17 in the feed was considered appropriate for operation, once the achieved conversions were 79% for CH₄ and 60% for CO₂, while the H₂/CO ratio was 1.75.