Enhanced hydrogen production via catalytic toluene reforming with in situ carbon dioxide capture: Effects of a hybrid iron-calcium composite prepared by impregnation

Calcium looping gasification (CLG) can produce hydrogen-rich syngas with in situ carbon dioxide capture. It is challenging to enhance tar reforming performance and to simultaneously promote cyclic carbonation reactivity and mechanical strength of CaO-based absorbent for biomass gasification. A hybrid iron-calcium absorbent (Ca-Al-Fe) is prepared via a two-step impregnation method and compared with two conventional absorbents (Ca and Ca-Al). Results showed that chemical components of Ca-Al-Fe were CaO, mayenite (Ca12Al14O33) and brownmillerite (Ca2Fe2O5). Ca12Al14O33 was beneficial in hindering sintering and enhancing mechanical strength of the hybrid absorbent. After 10 carbonation-calcination cycles, conversion of CaO in Ca-Al-Fe increased from 43.6% of initial carbonation to 56% of the final cycle. Mechanical strength of Ca-Al-Fe was elevated to 101 N which was much higher than that of conventional CaO (39.4 N). Ca2Fe2O5 was observed to have a catalytic effect on the reforming of biomass tar model component (toluene). Comparing with Ca and Ca-Al, the adoption of Ca-Al-Fe resulted in much higher yield and concentration of hydrogen in reforming gas as well as less residue tar and deposited coke. To further understand performance of the hybrid absorbent at various reaction conditions, influences of mass ratio of iron to CaO (Fe/CaO), reforming temperature (T), mole ratio of steam to carbon in toluene (S/C), absorbent granulation were evaluated on properties of the reforming gas as well as the liquid and solid residue after toluene reforming. For future deployment of CLG, potential reaction conditions were recommended as Fe/CaO 10%, 700℃ and S/C 2.0 in present study. Based on the reforming liquid residue analyses by gas chromatography-mass spectrometer (GC–MS), potential toluene reaction routes were evaluated which revealed that Ca-Al-Fe favored deep dehydrogenation during toluene reforming.