A limestone furnace calcination process involves injecting finely divided limestone particles into a zone in a furnace at which the temperature of the flue gas stream, as it passes through the zone, is above the minimum calcination temperature and below the minimum effective quicklime utilization/sulfation temperature. In conventional furnaces, the minimum calcination temperature, or the calcium carbonate decomposition temperature, ranges from about 1,365 to 1,430° F. The minimum effective quicklime utilization/sulfation temperature refers to the temperature below which the rate of quicklime sulfation of the lime produced by calcination of the limestone is sufficiently slow to result in negligible calcium sulfate formation on the resultant lime, and in conventional furnace applications ranges from 1,600 to 1,800° F. The zone is preferably sized such that limestone particles injected therein will remain at a temperature above the minimum calcination temperature, as the particles are carried downstream, for a period sufficient for substantially complete calcination of the limestone particles to lime while minimizing reactions between the resultant lime particles and sulfur dioxide to form calcium sulfate while the particles are in the furnace and to minimize sintering of the lime and complex calcium compound formation. The resultant lime particles may then be utilized in conventional downstream flue gas desulfurization processes including wet and semi-dry processes.