Zhan-Guo Zhang

Steam reforming of a clean model biogas over Ni/Al2O3 in fluidized- and fixed-bed reactors

Abstract Simultaneous steam, carbon dioxide reforming of methane was conducted over 11.5xa0wt.% Ni/Al2O3, at 1xa0atm in micro-fluidized- and fixed-bed reactors using a constant molar ratio of CH4/CO2≈1.5 as a model biogas. The performance of a fluidized reactor was compared to that of a fixed-bed reactor under similar conditions (feed gas to steam ratios of 1.5 and 0.75 at a reactor temperature of 750xa0°C, GHSV of 300xa0min−1). Conversions of CH4 and CO2 were 75 and 67%, respectively, in a fixed-bed reactor under the ratio of 1.5. Overall higher conversions (7–15%) were observed in the fluidized-reforming reactor. The initial activity of the fixed-bed reformer decreased rapidly and massive carbon deposition caused reactor blockage at the low steam concentration. Decreased the feed gas to steam ratio to 0.75 reduced carbon deposition and the nature of the carbon was suggested not to be the cause of the catalytic enhancement of the fluidized reforming. A study of the fluidized-bed reformer under decreasing feed gas to steam ratios from 3.0 to 0.3 showed an almost complete CH4 conversion (98% for feed gas to steam ratios below 1.0). With decreasing feed gas to steam ratio the H2/CO ratios increased as expected from 1.5 to 2.7. At the highest feed gas to steam ratio, a poor catalyst fluidization was observed due to massive carbon deposition, which was reduced dramatically when steam was provided in excess. Increasing temperatures from 650 to 850xa0°C enhanced both conversions and lowered H2/CO ratios.

Characterisation of carbon deposits on Ni/SiO2 in the reforming of CH4–CO2 using fixed- and fluidised-bed reactors

Abstract CO2 reforming of methane was investigated with regard to carbon deposition on 4.5 wt% NiO/SiO2 catalyst at 1023 K, 1 atm and a CH4/CO2 ratio of 1.0 employing micro-fluidised- and fixed-bed reactors. A higher catalytic activity (by 20%) was observed in the initial stage (0.5 h) of the fluidised-bed reforming which may be attributed to lesser deactivation of the catalyst compared to fixed-bed operation. Only a limited amount of carbon was deposited in a period of 11 h on stream. In the case of the fixed-bed reactor, a much larger amount of carbon was found on the spent catalyst, particularly, when sampled from the bottom of the bed. TPO results suggest that carbon deposits on the catalyst samples from the fluidised-bed as well as the top of the fixed-bed are rather small and of similar nature. The carbon deposited at the bottom of the fixed-bed reactor contained two distinct species according to XPS results (corresponding to C–O and C–C bonds).

Methane dehydroaromatization over Mo/HZSM-5 in periodic CH4–H2 switching operation mode

Periodically switching flows of CH4 and H2 through beds of 3 wt% Mo/HZSM-5 at 973 K have revealed that coke formed in durations of the CH4 flow over the catalyst is removed in the following H2 flow, and then consequently, the high initial activity of the catalyst for the dehydroaromatization of CH4 to benzene and naphthalene is almost completely recovered and/or maintained. Continuous composition analysis of the effluent streams of the H2 flow has further shown that the coke formed in the CH4 exposures is gasified back by H2 to CH4 and also that duration of the H2 flow required to gasify most of the coke formed on the catalyst depends on the CH4 exposure duration.