Flowerpot Power Plant

Needs must when the temperature gets to -3c . My greenhouse cacti and succulents might turn to mush. B&Q have stopped selling paraffin greenhouse heaters due to environmental issues. Charcoal is renewable so I've started putting 3 or 4 blocks into a clay pot for overnight. Who knows if it's a wise move...I may look out one morning and have nothing left but puddles of aluminium and glass!