Comparison of co-refining of fast pyrolysis oil from Salix via catalytic cracking and hydroprocessing

Lignocellulosic biomass from energy crops, i.e., short rotation coppice willows such as Salix spp., can be used as feedstock to produce transportation biofuels. An attractive thermochemical route for the introduction of large-scale production of biofuels in the transport sector in the near future is to convert the biomass to a liquid (pyrolysis oil) via fast pyrolysis followed by co-processing with fossil oil in existing refinery infrastructures. In this study, Salix was first liquefied using ablative fast pyrolysis in a pilot scale unit. The resulting pyrolysis oil, rich in oxygenates, was co-refined in 20 wt.% ratio with fossil feedstocks using two different technologies, a fluidized catalytic cracking (FCC) laboratory unit, and a continuous slurry hydroprocessing pilot plant. In the FCC route, the pyrolysis oil was cracked at 525 °C using a commercial FCC catalyst at atmospheric pressures and resulted in significant deoxygenation (97 wt.%). To determine the biogenic carbon in the co-refined liquid product, C14 analysis was conducted. The whole conversion route resulted in 11 wt.% of the biogenic carbon from Salix ended up in liquid products, the rest being gaseous products, bio-char from the pyrolysis, and coke (16 wt.%) from the FCC upgrading. The hydroprocessing was conducted at 420 °C, at a hydrogen pressure of 150 bars at a liquid hourly space velocity of 0.67 h-1, in the presence of an unsupported molybdenum sulfide catalyst. The hydroprocessing resulted also in significant deoxygenation of the liquid product (93 wt.%). The whole conversion route resulted in 26 wt.% of the biogenic carbon from Salix ending up as liquid hydrocarbons, the rest was mainly gaseous hydrocarbons, carbon oxides, and bio-char. The study showed that it is feasible to co-refine pyrolysis oil using both methods, the main difference being that hydroprocessing results in a significantly higher yield of hydrocarbon products but also would require an input of H2 while in the cracking route a significant part ends up as gas and as coke on the catalyst. The choice of routes is foremost dependent on the available amount of bio-oil and refining infrastructures. Besides liquid biofuel, it is also vital to utilize the biogenic carbon material in the bio-char and in the gaseous products.