Rapid lightoff of syngas production from methane: A transient product analysis

Abstract

AbstractSteady‐state production of syngas (CO and H2) can be attained within 10 s from room‐temperature mixtures of methane and air fed to a short‐contact‐time reactor by initially operating at combustion stoichiometry (CH4/O2 = 0.5) and then quickly switching to syngas stoichiometry (CH4/O2 = 2.0). The methane/air mixture is first ignited, forming a premixed flame upstream of the catalyst that heats the Rh‐impregnated α‐alumina foam monolith to catalytic lightoff (T > 500°C) in a few seconds. The methane/oxygen ratio is then increased to partial oxidation stoichiometry, which extinguishes the flame and effects immediate autothermal syngas production. Transient species profiles are measured with a rapid‐response mass spectrometer (response time constant ≅ 0.5 s), and catalyst temperature is measured with a thermocouple at the catalyst back face. Because the monolith thermal response time (∼ 1 s) is several orders of magnitude larger than the reaction timescales (∼ 10−12 to 10−3 s), chemistry and flow should be mathematically decoupled from local transient variations in catalyst temperature. Using this assumption, a transient temperature profile is combined with detailed surface chemistry for methane on Rh in a numerical plug‐flow model. This approach accurately reproduces the transient species profiles measured during experimental lightoff for short combustion time experiments and lends insight into how the monolith temperature develops with time. The combined experimental and numerical efforts supply useful information on the transient reactor behavior for various combustion times and identify a combustion time to avoid undershoot or overshoot in catalyst temperature and minimize start‐up time. © 2004 American Institute of Chemical Engineers AIChE J, 51: 247–260, 2005