## 6 Discussion
### Pros & Cons of Yield Methods  
The main pros to using yield methods are that they are less costly than 14C testing methods and appear straightforward to use, although their setting up would take time and involves some 14C testing to define the yield factors. The negatives to the \"type A\" yield method is that the outputs of a refinery may vary significantly, depending on many factors, including process parameters (catalyst age, temperature, etc.) and variations in the composition of the feedstocks. In addition, a new type of feedstock is blended into the system. Therefore, simply ascribing an increase in an output of a certain fraction entirely due to the addition of a bio-feedstock, may miss the effect variation in the aforementioned parameters could have on yields. Ecofys [33] warns that if the input feedstocks change regularly and the relationship between the mass of biogenic input and the mass of biogenic output is not linear, the yield factor may need to be recalculated regularly. This yield method can also be considered more problematicduring FCC co-processing. Hydrogen transfer occurs as shown by [PERSON] et al. [32] where there are interactions between the fossil stream and the biogenic stream. This contradicts the assumption that the fossil yield during such co-processing is unchanged. Regarding the \"type B\" yield method, this appears to be suitable for vegetable oil type feedstocks that can be supplied to the refinery within a strict specification, and to hydrotreaters. In the EU some companies are using this method and it has been specified in the MS legislation where the refinery is based. Looking at the legislation, it appears there is a single fixed yield value for vegetable oil feedstocks. However, a fixed value is only accurate for one vegetable oil type and quality standard. It does not take into account impurities or variations in the vegetable oil quality. For example, [PERSON] et al. [49] co-processed UCO in an oil refinery. The UCO's specifications used were not provided. Their results showed a large variance in yields of HVO from the UCO, between 75.3-88.1 wt%. This implies that it is essential to define strict quality parameters for the vegetable oils. This is easier with virgin vegetable oils such as soybean oil, but virtually impossible with UCO or other oils which are available in relatively small batches from multiple suppliers, unless there is a very strict pre-processing facility to pass the oil through (before sending it for co-processing) which produces a uniform product. In addition, how a hydrotreater is set-up and run affects yields. It is possible to deoxygenate (i.e., turn a vegetable oil into a hydrocarbon) using hydrogen, but the hydrogen comes at a cost. Another option for hydrotreating uses less hydrogen, but some oxygen is removed from the vegetable oil by combining it with bio-carbon, thus producing CO or CO\({}_{2}\) gas. The formation of these waste carbon gases (i.e., CO or CO\({}_{2}\)) reduces the amount of carbon which could be combined with hydrogen to make fuel; thus the yield of bio-fuel in the products is lowered. In summary, in descriptions of yield methods there does not appear to be any requirement for either (a) strict parameters on the vegetable oil (or bio-feedstock) quality, or (b) indications whether a yield factor obtained via this method affects the aforementioned bio-carbon loss or not. Both parameters change the co-processing yield, making the use of a fixed value questionable, unless these other two parameters are defined. In addition (for yield method \"A\"), there is the factor of baseline variability related to petroleum quality and process fluctuations. These points highlight that this method relies on accurate measurements of all inputs and outputs and a consistent baseline, which assumes that the petroleum quality and processing parameters remain constant. The resulting values are valid however only for that batch and the specific combination of feedstocks and process parameters used.