## 2 Biomass Feedstocks Processing with Fossil Fuels in a Common Process
### The Role of Co-Processing in Decarbonising Transport  
Most of the drop in biofuels are produced nowadays at commercial scale by hydrotreating of lipid-based material, such as vegetable oils or Crude Tall Oil (CTO) to produce is Hydrotreated Vegetable Oil (HVO) often referred to as renewable diesel. Renewable diesel is been produced in standalone plants using vegetable oil or crude tall oil such as Neste Oil and UPM [3,4].Technical issues and high initial investment costs have been challenging for reaching economic viability of thermochemical processes (such as gasification combined with Fischer-Tropsch synthesis or pyrolysis/hydrothermal liquefaction--HTL) from lignocellulosic feedstock, despite of their potential to provide drop-in biofuels. Upgrading of pyrolysis/hydrothermal liquefaction bio-oils/biocrudes/into finished fuels is challenging technically and this option has not reached yet commercial operation. Unlike vegetable oils that have a relatively uniform nature, the composition of biomass-derived oils varies considerably and requires further upgrading by removing oxygen as well as cracking to reduce the chain length. This variation can be a result of the type of process used to convert biomass into an oil (e.g., via fast or slow pyrolysis, catalytic or hydro-pyrolysis, or via HTL) as well as the type of biomass feedstock used (for, e.g., wood, agricultural residues, sludge, etc.). The complex composition of bio-oils and their physical-chemical properties together with their instability prevent the direct blending of bio-oils with fossil fuels and pose large challenges for further upgrading into biofuels [5,6]. An alternative to standalone plants is the possibility to process bio and fossil feedstocks together in an oil refinery; this produces fuels (such as gasoline and jet fuel or diesel) which are chemically uniform with the biofuel portion indistinguishable from the fossil part. A schematic is provided to give an overview of the various co-processing pathways (see Figure 1). Oil refineries already have the infrastructure to produce fuels and base chemicals and, therefore they do not require additional major investments for processing additional alternative feedstocks. Therefore, co-processing vegetable oil, used cooking oil, or animal fat as well as other biomass-derived oils (biobased intermediates) with fossil fuels in a common processes in oil refineries has the potential to play an important role in the near future [2,7,8]. Co-processing has the advantage that small amounts of biomass-derived oils can be blended with petroleum feeds in order to mitigate the problems that come with pyrolysis oil processing [2]. Oil companies around the world are increasingly interested in  
Figure 1: Overview of possible co-processing pathways. blending bio or other renewable feedstocks into their refineries, as it is seen as a method of reducing the carbon intensity of the resulting fuel products. Co-processing allows biofuels to be produced without requiring the need for the construction of a dedicated facility and uses existing oil refinery assets. This brings significant economic advantages due to the avoidance of large capital investments generally associated with the production of advanced biofuels, although some modifications or equipment upgrading may still be required. Co-processing of biomass feedstocks has started and trials have been carried out in refineries [2; 5], with a focus on the use of bio-oils from fast pyrolysis as well as bio-crudes from HydroThermal Liquefaction (HTL) [9; 10; 11; 12; 13]. An oil refinery produces a range of final fuels, including diesel fuel, gasoline, kerosene, Heavy Fuel Oil (HFO), and other less valuable products such as Liquefied Petroleum Gas (LPG), coke, fuel gas, etc.). Crude oil refining in refineries is highly complex that entails various processes and processing units, having multiple outputs. Depending on the market demand, process parameters are changed to enable the production of the different outputs.