The thermal energy sector is very important at this time of global energy transition. From the aspect of stable energy independence, this is especially important in countries whose energy system is based on fossil fuels, especially coal. This is why decarbonisation of the energy sector is necessary, with a partial and gradual substitution of coal with renewable fuels. However, the use of these fuels is not always possible in existing plants. This refers to the establishment of energy, economic and environmentally acceptable proportion of the co-firing of these fuels, i.e. a mixture of coal and renewable fuels. The success of the establishment of this process is a function of several variables, the dominant of which are the aggregate properties of the fuel, basic and their mixtures, including the ash properties of those fuels. With the motive of contributing to a more successful implementation of the energy transition in the thermal energy sector, and with the aim of obtaining new scientific knowledge about the characteristics of the combustion of lignite and brown coal with different renewable fuels, laboratory research was carried out. In particular, different mixtures of lignite and brown coal, waste woody biomass and Miscanthus as a fast-growing energy crop were subjected to co-firing with variable process conditions. In addition to changing the composition of fuel mixtures, the test regimes included a significant change in process temperature (1250-1450 °C) and the primary measure of staged air supply to the furnace. In these combustion conditions, the emission of undesirable and harmful components into the environment, the efficiency of combustion, and the tendency of the ash from the fuel mixture to possible soiling of the heating surfaces in the furnace were evaluated - an analysis of the characteristics of the ash samples from the reaction zone and samples of the slag and ash to the furnace. Here, in both cases of co-firing, lignite and brown coal with different types of biomass, it was shown that in real conditions it is possible to establish a sustainable primary energy conversion process from fuel with a low unburnt carbon content in the slag (unburnt carbon content, UBC<1%) as well as low CO emission, below 350 mg/mn 3. At the same time, the NOx emission is below 320 mg/mn 3 during the co-firing of lignite mixtures at 1250 °C, and in the case of brown coal mixtures below 740 mg/mn 3 at 1450 °C. In both cases, the net CO2 emission decreases in proportion to the proportion of biomass in the mixture, while the SO2 emission is still high, at a level of up to 2500 mg/mn 3 for lignite mixtures and up to 6400 mg/mn 3 for brown coal mixtures. None of the treated types of biomass, up to the level of applicable content in the mixture with lignite and brown coal, does not worsen the progress of the process from the aspect of possible slagging/fouling heating surfaces in the boiler.

The energy transition basically implies a phased and significant reduction in the use of fossil fuels, until the final cessation of use in the near future. Renewable energy sources and alternative fuels are an increasingly important part of the energy transition. At the same time, it is necessary that energy systems increase energy efficiency and environmental acceptability. In this regard, great efforts are being made to expand the portfolio of primary fuels in existing large energy systems, e.g. thermal power plants. Therefore, today in large boilers, in order to reduce NOx emission, staged air supply for combustion is inevitably used as the primary measure in the furnace (air staging). In addition to the previous measure, there are many examples of staged fuel supply to the combustion zone (fuel staging), and still a small number of examples of the use of third or additional fuel, e.g. application of biogas or natural gas in combustion of pulverized solid fuels (reburning technology). General, these measures simultaneously increase the efficiency of primary energy conversion from fuel and reduce the emission of undesirable components into the environment. The results of this paper show the extent to which the effects of the use of natural gas in the co-firing of Bosnian coal and waste wood biomass are expressed. In this process there is an additional reduction of NOx in proportion to the share of gas - at a process temperature of 1350 °C and at 10% of the energy content of gas in combustion with coal, compared to emissions without additional fuel, recorded reduction of NOx emissions by more than 250 mg/mn 3.

Abstract The paper describes the function and importance of safety devices on pressure equipment, legal regulations for the necessary scope and periods of functional inspections of safety valves in thermal power plants. The mode of operation of the apparatus for in-site testing and adjustment of safety valves on pressure vessels is described, as well as a comparative presentation of the obtained test results compared to the results obtained by the conventional method that uses hydraulic pressure of the medium for valve spring load. The on-site method uses a motor drive to load the valve spring, and uses sensitive displacement, force and sound sensors to generate signals, which are processed in a suitable software application to obtain test results. Unlike the conventional method, the on-site method allows testing of safety valves without their disassembly from the installation site. A comparison of the test results obtained by the modern method with the test results by the conventional method was made on several safety valves. It has been shown that in addition to better precision, which is important for valves with lower operating pressures, the application of the on-site method shortens the test time several times.

In general, the mixing the reactants is very important to the process and the rate of combustion of each fuel, especially low-ranked coals with low reactivity. The appropriate excess air coefficient for combustion and the position and mode of supply of individual portions (air sta- ging) of the total air in the furnace are one of the key influ- encing factors for the mixing, respectively the efficiency of the combustion process, and the level of emissions. Today, the air staging (primary, secondary, tertiary and over fire air-OFA) is an indispensable process in the design of new industrial and energy boilers. Also, this measures are used for the reconstru- ction of existing furnace of large boilers. The implementation of this primary measure in the furnace does not require signi- ficant financial resources - the costs of investing in the intro- duction of air staging supply for combustion in the furnace are very low in relation to the multiple positive effects and bene- fits of such a system, especially a lower NOx emissions. However, the first condition for the correct and reliable design of the air staging system in the furnace is the choice of the optimal position for the supply OFA air. Determination of this position on the furnace depends on several factors, primarily the type and characteristics of the fuel, grinding quality, co- mbustion technology, furnace structure, type, number and arra- ngement of burners. On this topic, the Faculty of Mechanical Engineering of the University of Sarajevo, in the Laboratory for Coal and Biomass Combustion, conducted extensive rese- arch on the pulverized combustion of coal blends of the Ce- ntral Bosnia Mining Basin, including co-firing of these coals with waste wood biomass (beech and spruce sawdust). Here is presented some of the results of those NOx emission studies for different fuels and combustion temperatures, depending on the mode of supply and the positions of the combustion air supply in the boiler furnace.