## 6 Discussion
### Pros & Cons of Energy & Mass Balance Methods  
The energy and mass balance method estimates the energy input using the mass and lower heating value (LHV) of the biogenic and fossil feedstocks. This method has the advantage of being relatively quick and easy to implement. Although the LHV could be a value from literature or found by conducting a direct measurement on the feedstock (using a bomb calorimeter), the method has no requirement to define the quality of the feedstock. For example, if Used Cooking Oils (UCO) are co-processed, the moisture, impurities and FFA content can vary between batches. Thus, if a literature value for the LHV of UCO is used to calculate the bio-energy of this feedstock and the actual batch of UCO contains impurities (dirt, water, etc.), the actual UCO LHV will be lower than in the literature.  
This method also assumes that the share of biogenic content in all the outputs from a co-processing step in an oil refinery is equal to the share of biogenic content in the refinery's inputs. This is termed \"equal-allocation\". However, this is an over-simplification, and in practice, different shares of bio-component can be expected in different fuels. In Europe, vegetable oils are co-processed in hydrotreaters. For these types of feedstocks when hydrotreated, most of the bio-component will end up in the diesel or kerosene fraction, and a small fraction will be propane and fuel gas (which can be burned on-site in the oil refinery for energy, or used as a component in LPG gas). Thus the distribution of the bio-carbon is not equal across hydrotreater products. As an alternative, one solution is to use unequal allocation, based on the actual shares of the bio-component that ends-up in each fuel fraction (gasoline, kerosene, diesel, heavy fuel oil, etc.) produced in the process. This requires however, conducting radiocarbon measurements of all inputs and outputs to establish the bio-content in each fuel fraction produced. The resulting values are valid only for that batch and the specific combination of feedstocks type and composition and process parameters. A challenge remains the accuracy of measurements, in particular at low levels of bio-content.  
However, if the vegetable oil is co-processed in an FCC, the bio-component in the products is less easy to predict, as the FCC \"cracks\" or splits hydrocarbon chains into smaller chains. If the vegetable oil is cracked into smaller components, these will be lighter so more bio-component could be expected in the gasoline fraction or lighter gases than the heavier diesel fraction. In an FCC, hydrodeoxygenation is the dominant reaction mechanism followed by decarboxylation and decarbonylation. The latter two reactions cause losses of bio-carbon (as bio-CO\({}_{2}\) and bio-CO). This loss in carbon occurs in the vegetable oil only. Thus a simple energy-balance will ignore such bio-losses (and overestimate the bio-component in the final fuels).  
Biomas oils tend to be relatively more complex and contain carbon chain lengths which are less defined or regular. So it is difficult to predict into which fuels the bio-component would finally end up and at what share. Additionally, in doing so, one would have to assume the biomass oil remains uniform in nature throughout a period of co-processing, which is unlikely to be the case.