This research work presents the results of studies on the effect of modifiers comprising the salts CuSO 4 ·2H 2 O, NaCl, NH 4 Cl, MgSO 4 ·7H 2 O, CaCl 2 , and urea at various concentrations on the combustion of fine coal. The tests were carried out in a 12-kW laboratory boiler equipped with a rotary-grate retort furnace. The emission levels and concentrations of CO, CO 2 , SO 2 , NO x , and TOC in the flue gas were measured using analyzers. A modifier composed of 350 ppm Cu, Na, Mg, NH 4 + , Ca, and urea showed particularly high activity in the combustion of fine coal. The flue-gas levels of CO, NO x , and SO 2 were reduced by approximately 9%, 12%, and 10%, respectively, in comparison with the modifier-free sample. In this case, the boiler efficiency also increased from 65% in the tests with no modifier to 76% in the tests with the modifier. The proprietary application system, enabling the modifier to be added in exact amounts to the variable flow of fine coal, is also described. It was found that the use of the modifier in coal combustion tests results in lower emissions of harmful fuel components, a higher amount of heat obtained from the fuel unit mass, lower corrosive impact of the fuels, lower boiler maintenance costs, and extended service life.Energies 2019, 12, 4572 2 of 15In Poland, even as much as 80% of electric energy is still based on coal, including brown coal [10]. Looking at the latest investments in the power engineering industry in Poland, such as new power units in power plants, as well as the national energy policy, it seems that coal will continue to be the essential raw material in the generation of electric energy in Poland [10,11].The growing mechanization of coal mining leads to an increased efficiency of the production of fine coal (grain size 0-20 mm). Small coal has a high content of sulfur; therefore, its direct combustion is a serious source of environmental pollution. Reasonable utilization of fine coal is important for sustained development in power engineering [12].Direct coal combustion is a commonly used technology in the production of electric and thermal energy. There are essentially two coal combustion techniques: Bed combustion and suspension combustion. According to the former technique, coal is fed onto a fire grate and the combustion type depends on the directions of flow of the fuel and of the air. This type of combustion takes place in a stoker. The latter type, taking place in a fluidized bed, is a process in which suitably prepared solid particles are suspended in a uniformly distributed upward flow of the fluid. These are high efficiency devices, capable of attaining heating capacities as high as 135 tons of steam per hour [13].Stoker furnaces are the oldest and the most commonly used devices for industrial coal bed combustion. In these devices, crushed coal (typically, 95% particles below 32 mm and 20% to 60% below 6 mm) is fed onto the stoker where primary air flows upward through the coal bed. The crushed coal pieces are heated, dried, degassed, and combusted, lea...