'coal-free' originates from Modern English, specifically the words 'coal' and 'free', where 'coal' meant 'a carbon-rich mineral used as fuel' and 'free' meant 'not having or affected by'.

'coal' comes from Old English 'col' (meaning 'coal'), ultimately from Proto-Germanic *kula; 'free' comes from Old English 'frēo' (meaning 'free'). The compound 'coal-free' developed in modern English by combining these elements to describe something without coal.

Initially the components separately referred to 'coal' and 'free'; over time the compound came to be used specifically to describe systems, products, or policies 'without the use of coal' (especially in environmental and energy contexts).