Steam power plant (SPP) is the most popular power plant and the largest power generator in Indonesia. SPP uses coal as fuel to produce heat energy. The coal that is taken and used by the SPP must be in accordance with the boiler specifications. However, at the end of 2021, Indonesia experienced a coal shortage crisis, forcing SPP to use a lower coal calorific value specification than usual. Therefore, there is a slagging phenomenon in the wall tube which makes the reliability value of the generator worse. The reliability performance of a power plant can be determined from the calculation of the Net Plant Heat Rate (NPHR). NPHR is a value that represents the amount of energy used to produce electrical energy by a generator. The greater the NPHR value indicates that the boiler efficiency worsen. This is because it used more amount of energy to make 1 kW than usual. To overcome the slagging phenomenon that increases the NPHR value, boiler maintenance companies did an injection of coal additive CORINT PC 99 in the combustion chamber and coal silo. Thus the AFT value of coal is getting higher and the slag will collapse from the wall tube. If this happens, it is expected that the heat transfer from the combustion chamber to the walltube will be better and the NPHR value will decreases. After calculation, the injection of coal additives made the NPHR value increase by 4.08% which indicates that the treatment had no significant impact. However, the success parameter of coal additive injection is not only from the NPHR value but also from the amount of slag that falls and the hardness of the fallen slag. According to these 2 parameters, the injection of coal additive has succeeded in collapsing the slag and softening the slag attached to the walltube therefore it is easier to be cleaned by soot blower.