Your correspondents seem to be confusing this with «
producer gas», obtained by blowing
air through a thick bed of hot coke, and «
water gas», produced by blowing steam through the same.
Sources of heat include: industrial and process sources such as power stations, industrial processes (such as chemical industries, clinical waste incinerators and food
producers), building cooling systems and refrigeration (such as offices, supermarkets and data centres), sewerage systems and
water treatment works, London Underground tunnels and electricity substations; and environmental sources —
air, ground,
water (that retain solar heat).
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