The emission factors used are shown in Table A3. Included in this category is an emission from the combustion of petroleum coke. This emission arises from the operation of fluidized bed catalytic crackers. During the cracking processes coke is deposited on the catalyst degrading its performance. The catalyst must be continuously regenerated by burning off the coke. The hot flue gases from the regeneration stage are used as a source of heat for the process. Since the combustion provides useful energy and the estimated amount of coke consumed is reported (DTI, 1999), the emissions are reported as energy emissions rather than as a fugitive emissions. Emission factors are either based on operators’ data (Environment Agency, 1999) or IPCC (1997) defaults for oil. The NAEI definition of refinery aims to include all combustion sources and includes refinery fuels, electricity generation in refineries and fuel oils burnt in the petroleum industry.