The relative price of coal and natural gas determines where Ratcliffe sits in the merit order. Cheap coal, and it come on ahead of CCGT plant.
Its not just that. A few other factors:
* No need for future maintenance, so it is cheaper to run the coal unit(s) than put ‘miles on the clock’ of the gas units that will need maintenance.
* Ratcliffe will be burning stuff that has sat at the bottom of the stockpile for a long time. This will be low grade, but is ‘free’ (bought and paid for a long time ago) and therefore bringing in some new coal to mix / bulk it up helps.
* similarly it is possible (although i really dont know) that there is ‘spare‘ limestone in the Flue Gas Desulphurisation stockpile, and that it is cheaper to burn more coal than cart that away for disposal.