Effect of Alkali Vapor Exposure on Ni-MgO/γ-Al2O3/Cordierite Monolithic Catalyst for Biomass Fuel Gas Reforming

Abstract

Fly ash compounds, such as alkali salts, in the raw biomass fuel gas can contaminate and deposit on traditional granular Ni-based catalysts, which resulted in catalyst deactivation and pressure increase of the downstream reformer. The impact of alkali salt exposure (KCl, K2SO4, K2CO3, by evaporation at about 7.8 mg/L for 6 h) on dry CH4/CO2 reforming of model biomass fuel gas (H2/CO/C2H4/CH4/CO2/N2 = 15.8/12.1/2.51/ 15.0/22.1/32.6 vol %) over Ni-MgO/γ-Al2O3/cordierite monolithic catalyst (MC) was investigated and studied. The results showed that CH4 and CO2 conversions and CO and H2 yields increased at 700−850 °C for undeposited and deposited MC. Compared with undeposited MC, the deposited catalysts show lower CH4 conversion but higher CO2 conversion and CO yield at 750−830 °C. The stability tests also show that CH4 conversion and H2 content in the tail gas decreased dramatically from 87.2% to 32.0% and from 35.1% to 26.7%, respectively, after 17 h time on stream (TOS) for the deposited MC, while CH4 conversion kept steady of above 90% after 60 h TOS for undeposited MC at 750 °C. Characterization by N2-physisorption, XRD, ICP-AES, SEM-EDS, and XPS of MC indicate that alkali salt aerosol covering the catalyst surface or blocking mesopore channels was the main reason for the decreased reforming performance and MC deactivation, which occurred mainly at the top part of monolithic catalyst (K = 1.39 wt % by EDS), vicinal to the alkali source. The reforming of real biomass fuel gas (H2/CO/C2H4/CH4/CO2/N2 = 10.2/16.8/0.5/6.4/15.2/51.0 vol %) from air gasification of pine sawdust in the pilot plant (200−250 kg/h) by the reformer packed with MCP, larger in size than MC, exhibits pressure drop of less than 700 Pa, CH4 conversion of about 84%, and tar content from 4.8−5.3 g/m3 to 0.12−0.14 g/m3 during 60 h TOS at 600 °C. The porosity structure of MCP catalytic bed and relatively low alkali (K, Na = 0.03−0.07 wt %) deposition by fly ash from real biomass fuel gas were the main reasons for the excellent reformer performance.