The original chain grate boiler had low thermal efficiency and high NOx emissions, failing to meet current environmental emission standards. The boiler was retrofitted by retaining the drum, removing the chain grate, furnace, and corresponding heating surfaces at the tail, and adding a circulating fluidized bed furnace, cyclone separator, and tail flue. This upgrade improves boiler efficiency while significantly reducing NOx emissions, with initial emissions below 150 mg/Nm³.