ABSTRACT
The necessity to source clean energy and contain CO2 emissions has brought about a global interest in the use of biomass as an environmental friendly renewable energy route. One of the drawbacks in the conversion of biomass to energy via pyrolysis/gasification is the liberation of CO2 from the syngas.This study was carried out in a two- stage catalytic process using rice husk as feed, ZSM-5 catalysts for cracking and reforming of the pyrolysis volatiles and calcineddolomite as CO2 capture material (sorbent).The catalyst include commercial ZSM-5, hierarchical ZSM-5 which was prepared by desilication method using 0.2 mol/dm3 NaOH, and 2 wt.% Zn-wt.%and hierarchical ZSM-5 which was prepared by doping metal on the hierarchical ZSM-5 through wetness impregnation.Calcineddolomite was prepared by size reduction and calcination of the dolomite precursor.Sorbent and catalysts were characterized using XRD, FTIR, BET and TGA techniques. The XRD results of the catalysts indicatedthat the crystalline zeolite structure of HZSM-5 was maintained after alkali treatment and metal modification with slight reduction in intensities of some noticeable peaks as a result of desilication. The structure stability confirmed with XRD was supported with the FTIR resultswhich showed no loss in major functional groups of the HZSM-5 such as the Bands observed at 450 cm-1 represents the T-O-T bending vibration (T=Si or Al) and 550 cm-1 which represents the five membered ring pentasil unit of ZSM-5. At 790 and 1225 cm-1 which represents the external symmetric and asymmetric stretching of the Si-O-Si bond Although on critical examination of the band at 1100 cm-1 which signifies internal tetrahedral bridged Si-OH-Al vibrations in the asymmetric regionit appears that the peak has broadened on alkaline treatment of the catalyst, as compared to the shaper peak of the parent HZSM-5, while the band at 1100 cm-1 signifies internal tetrahedral bridged Si-OH-Al vibrations in the asymmetric region.TGA affirmed the thermal stability of the catalyst with a total percentage weight loss of less than 6% and a notable thermal change at 4000C for parent, hierarchical and Zn-Ni/ hierarchical catalyst. The BETresults showed an increase in surface mesopores from 111 m2 /g in the parent ZSM-5 catalyst to 141 m2 /g and 136 m2 /g in the hierarchical and metal treated hierarchal catalyst respectively.There was also an increase in the volume mesopores and average pore width. These increase was observed after desilication, this is to say there was successful introduction of mesopores into the system hence converting the catalyst to a hierarchical catalyst.Introduction of calcineddolomite into the reformer displayed a reduction of CO2 andCO with a surge in H2 liberated. Upon loading of fractions of hierarchical catalyst to calcineddolomite, it was observed that there was an increase in the generation of non-condensable volatiles with the highest amount of gases produced at 3.0 g hierarchical ZSM-5 catalyst in ratio to 7.0 g calcineddolomite. However, the best in terms of CO2 capture and H2 was noted at 1.5g hierarchical ZSM-5 catalyst in ratio to 8.5g calcineddolomite which gave a high amount of H2 and traces of CO2 with an average of 72 wt. % (vol) and 4wt. %(vol) respectively. Hierarchical catalyst incorporated with metal promoter (2 wt. %Zn- 2 wt. %Ni hierarchical ZSM-5) displayed improved H2yields at the expense of other syngas produced as compared to the hierarchical ZSM-5 and commercial ZSM-5 at similar catalyst to calcineddolomite ratios (1.5g: 8.5g).Increase in loading of hierarchical ZSM5 also led to changes in the composition of the condensates. The FTIR as well as the GC-MS results showed an increase in the aromatic compounds and a decrease in the aliphatic and oxygenated compounds for the catalytic pyrolysis. The GC-MS results confirmed that, hierarchical catalyst showed selectivity towards aconversion of condensates to mostly BTX compounds. Aromatic hydrocarbons composition increased to about 71% of the reformed condensates when Zn/Ni - hierarchical ZSM-5 was used as compared to 61% for hierarchical ZSM-5 and 58% for the parent ZSM-5.