Process for melting vitrifiable material in a flame-fired furnace, the process comprising: (a) a high temperature heat recovery stage whereby: i. heat is recovered from hot flue gas evacuated from the furnace in a primary heat exchanger by heat exchange between the hot flue gas and a heat transfer fluid so as to obtain heated heat transfer fluid at a temperature of at least 600°C, preferably of between 600°C and 800°C, and tempered flue gas at a temperature of at least 880°C, preferably of between 900°C and 1000°C, and ii. at least one combustion reactant selected among oxygen-rich oxidant and fuel is preheated in a secondary heat exchanger by heat exchange between the heated heat transfer fluid and the at least one combustion reactant (32, 31) so as to obtain a preheated combustion reactant (5, 6) and a tempered heat transfer fluid; and iii. the at least one preheated combustion (5, 6) reactant is supplied to the furnace; (b) a low temperature heat recovery stage whereby: i. condensibles are removed from the tempered flue gas by precipitation, and ii. heat is recovered from the tempered flue gas (21, 61), process whereby the heat recovered from the tempered flue gas (21, 61) and optionally also heat from the tempered heat transfer fluid is used to heat a working fluid of a Rankine cycle in a tertiary heat exchanger, and whereby mechanical power is generated by the Rankine cycle. (a) a high temperature heat recovery stage whereby: i. heat is recovered from hot flue gas evacuated from the furnace in a primary heat exchanger by heat exchange between the hot flue gas and a heat transfer fluid so as to obtain heated heat transfer fluid at a temperature of at least 600°C, preferably of between 600°C and 800°C, and tempered flue gas at a temperature of at least 880°C, preferably of between 900°C and 1000°C, and ii. at least one combustion reactant selected among oxygen-rich oxidant and fuel is preheated in a secondary heat exchanger by heat exchange between the heated heat transfer fluid and the at least one combustion reactant (32, 31) so as to obtain a preheated combustion reactant (5, 6) and a tempered heat transfer fluid; and iii. the at least one preheated combustion (5, 6) reactant is supplied to the furnace; i. heat is recovered from hot flue gas evacuated from the furnace in a primary heat exchanger by heat exchange between the hot flue gas and a heat transfer fluid so as to obtain heated heat transfer fluid at a temperature of at least 600°C, preferably of between 600°C and 800°C, and tempered flue gas at a temperature of at least 880°C, preferably of between 900°C and 1000°C, and ii. at least one combustion reactant selected among oxygen-rich oxidant and fuel is preheated in a secondary heat exchanger by heat exchange between the heated heat transfer fluid and the at least one combustion reactant (32, 31) so as to obtain a preheated combustion reactant (5, 6) and a tempered heat transfer fluid; and iii. the at least one preheated combustion (5, 6) reactant is supplied to the furnace; (b) a low temperature heat recovery stage whereby: i. condensibles are removed from the tempered flue gas by precipitation, and ii. heat is recovered from the tempered flue gas (21, 61), process whereby the heat recovered from the tempered flue gas (21, 61) and optionally also heat from the tempered heat transfer fluid is used to heat a working fluid of a Rankine cycle in a tertiary heat exchanger, and whereby mechanical power is generated by the Rankine cycle. i. condensibles are removed from the tempered flue gas by precipitation, and ii. heat is recovered from the tempered flue gas (21, 61),