Background. KM-75-40 boilers have been widely used at pulp and paper mills in Russia for the utilization of bark and wood waste (BWW). Their design with inclined grates and chain stoker furnaces, when firing fuel with moisture content above 50 %, leads to reduced steam-rated capacity, the need for constant oil support firing, intensive slagging of heating surfaces, and frequent outages. Under the current transition to carbon-neutral technologies, there is a demand for higher efficiency, and environmental performance of biomass combustion equipment. Thus, the vital task of the study is to develop and justify a retrofit option for KM-75-40 boiler by replacing the grate furnace with a bubbling fluidized bed (BFB) furnace to ensure stable combustion of high-moisture BWW with simultaneous increase in steam-rated capacity and reduction of emissions.
Materials and methods. The study has been carried out using the normative method of boiler heat balance, the TRAKT computer program, and field test data. The analysis has covered design solutions for replacing the chain stoker with a BFB furnace, including the arrangement of air supply, the air-distribution grate, and equipment for sand and large particle removal. The evaluation has accounted for fuel moisture, ash chemistry, gas-path temperature profiles, and steam generation rates under various operating modes.
Results. A modernization option for KM-75-40 boiler is proposed. Thermal engineering calculations of the basic operating parameters of the boiler under various modes have been carried out. It has been established that the retrofit has increased boiler efficiency from 58,6 % to 86,3 %, achieved a steam-rated capacity of 90 t/h for BWW moisture levels of 30–55 %, and reduced NOₓ and SOₓ emissions by 2,1 and 3,8, respectively. Stable operation has been achieved at up to 70 % fuel moisture. Furnace temperature has decreased by more than 200 °C, improving component lifetime and reducing NOₓ formation.
Conclusions. The modernization of KM-75-40 boilers has significantly improved efficiency, reliability, and environmental safety of the equipment. Stable combustion of BWW with up to 70 % moisture has been demonstrated with minimal use of auxiliary fuel. The proposed solution is recommended for retrofitting similar boilers and other biomass-fired plants. Future work should focus on design optimization and automation of BFB control systems.