Investigation into the catalytic cracking/reforming of biomass pyrolysis gas by biochar supported Ni-Ca catalyst

Pyrolysis is widely considered as one of the most effective thermal chemical technologies to convert biomass into high value-added products. However, the biomass tar is inevitably formed during pyrolysis gaseous production. The removal of biomass tar has posed a great challenge to the further commercialization of biomass gasification in recent years. In the present work, a systematic investigation was made to realize the efficient conversion into syngas (H2+CO) using the biochar (BC) supported nickel-calcium catalyst for the biomass tar cracking/reforming. The catalysts were prepared via the one-step pyrolysis of NiCl2, together with CaCl2 pre-loaded biomass at 800℃. Five types of catalysts were also synthesized, including 2Ni-Ca/BC (0.02 mol Ni and 0.01 mol Ca), 2Ni-2Ca/BC (0.02 mol Ni and 0.02 mol Ca), Ni/BC (0.04 mol Ni), Ca/BC (0.04 mol Ca), and BC (without Ni and Ca). Subsequently, the ASAP 2020 Micromeritics instrument was applied to determine the texture structure of catalysts, including specific surface area, the total pore volume, and mean pore diameter. The phase compositions of catalysts were identified by the X-ray Diffractometer (XRD). The morphology and microstructure of catalysts were also characterized using a Scanning Electron Microscope (SEM) and Transmission Electron Microscopy (TEM). A laser Raman spectrometer was used for the surface structure of catalysts. The coke amount of catalyst was determined via a thermal gravimetric analyzer equipped with Mass Spectrometry (MS). The gas composition was finally analyzed using Gas Chromatography (GC). The results indicated that the addition of calcium decreased the crystallite sizes of Ni for the better catalytic performance of catalysts. The graphitization of surface carbon in the 2Ni-Ca/BC catalyst was higher than that in the Ni/BC and Ca/BC catalysts, particularly in the presence of carbon nanotubes. Furthermore, the reaction temperature for all the catalysts greatly contributed to the tar cracking/reforming and syngas production. Alternatively, the catalyst type was another dominant factor during the processing. The catalytic performance was also ranked in the decreased order of 2Ni-Ca/BC, 2Ni-2Ca/BC, Ni/BC, Ca/BC, and BC, in terms of tar cracking and selectivity to the syngas production. Correspondingly, the tar conversion efficiency and syngas yield obtained from 2Ni-Ca/BC catalyst at 700℃ were 91.8% and 607.6 mL/g (H2/CO=1.05), respectively. More importantly, the tar conversion efficiency and syngas yield increased by only 0.7% and 7.6%, respectively, when the cracking/reforming temperature increased from 700 to 800℃, indicating that an excellent catalytic performance occurred at a relatively low temperature. The 2Ni-Ca/BC catalyst performed well in the higher stability after 210 min at 700oC, where the tar conversion efficiency, H2, and CO yields were achieved about 83%, 275 mL/g, and 268 mL/g, respectively. There was no obvious sintering of active, where only a handful of coke (3.6 mmol/g) was produced within 480 min at 700℃, indicating that the 2Ni-Ca/BC catalyst presented an excellent resistance to sintering and coke deposition.