COAL prices are to rise by up to 6 per cent - more than twice the rate of inflation. The Coal Advisory Service (CAS) said coal stocks had been "greatly depleted" over the long winter. It said "small increases in supply prices had been absorbed last year" but that prices would rise this year by 5 to 6 per cent on average. "Hardening prices in the world market for domestic coal is the main reason for the increase," CAS said, adding that prices hadn't increased last year.