More than half of the world's electricity is produced by burning large amounts of coal in thermal power plants and at the same time producing huge quantities of coal ash. Hydraulic transportation of coal ash from coal fired thermal power plants to ash ponds, using high concentration slurry disposal systems (HCSD) is safe, reliable, economical and attractive method due to its low maintenance costs, round the year availability and being environmentally friendly. Present study reports simulated pressure drop for the flow of high concentration fly ash slurries (in the range of 50 70 % by weight) through sudden and conical pipe contractions at various flow velocities using available experimentally obtained rheological parameters. For calculations the finite volume method is applied, which is the most commonly used and gives very good results in the analysis of problems in fluid mechanics and heat transfer. The method in this paper is adapted for simulation of flow of viscoplastic materials whose behavior is described by viscoplastic (Bingham) model. The simulation results are compared with corresponding analytical solutions and experimental data and good agreement between the results is shown.

This paper presents R&D project of multi fuel concept (MFC) for future coal-based power plants, demonstrated on example of cofiring Middle-Bosnia brown coal with waste woody biomass and natural gas. Pulverised Combustion (PC) lab-scale furnace has been used for the cofiring tests, varying up to 20%w portion of biomass and up to 10%th portion of natural gas in the fuel mix. Tests were purposed to optimize the combustion temperature, air distribution, including Over Fire Air System (OFAS), fuel combination and fuel distribution, including reburning concept, as function of emissions and combustion efficiency estimated through the ash deposits behaviours and unburnt. Considering application of proposed MFC in case of TPP Kakanj unit 6 (118 MWe) set here as a referent power plant, temperature levels and fuel distributions for lowest emissions of CO2 and NOx were found during lab tests, provided that combustion efficiency is at an acceptable level. Derived research results yield input data for calculation sustainability indicators of MFC for the referent power plant, considering 6 fuel options - different combinations of coal, biomass and natural gas. Single criteria analysis and multicriteria sustainability assessment have been done, giving an advantage to the options of cofiring coal with woody biomass and natural gas in the case demonstrated.

This paper describes full lab-scale investigation of Middle-Bosnia coals launched to support selection an appropriate combustion technology and to support opti- mization of the boiler design. Tested mix of Middle-Bosnia brown coals is pro- jected coal for new co-generation power plant Kakanj Unit 8 (300-450 MWe), EP B&H electricity utility. The basic coal blend consisting of the coals Ka- kanj:Breza:Zenica at approximate mass ratio of 70:20:10 is low grade brown coal with very high percentage of ash - over 40%. Testing that coal in circulated fluidized bed combustion technique, performed at Ruhr-University Bohum and Doosan Lentjes GmbH, has shown its inconveniency for fluidized bed combustion technology, primarily due to the agglomeration problems. Tests of these coals in PFC (pulverized fuel combustion) technology have been performed in referent laboratory at Faculty of Mechanical Engineering of Sarajevo University, on a lab-scale PFC furnace, to provide reliable data for further analysis. The PFC tests results are fitted well with previously obtained results of the burning similar Bosnian coal blends in the PFC dry bottom furnace technique. Combination of the coals shares, the process temperature and the air combustion distribution for the lowest NOx and SO2 emissions was found in this work, provided that combus- tion efficiency and CO emissions are within very strict criteria, considering spe- cific settlement of lab-scale furnace. Sustainability assessment based on calcula- tion economic and environmental indicators, in combination with Low Cost Planning method, is used for optimization the power plant design. The results of the full lab-scale investigation will help in selection optimal Boiler design, to achieve sustainable energy system with high-efficient and clean combustion tech- nology applied for given coals.

This paper presents the findings of research into cofiring two Bosnian cola types, brown coal and lignite, with woody biomass, in this case spruce sawdust. The aim of the research was to find the optimal blend of coal and sawdust that may be substituted for 100% coal in large coal-fired power stations in Bosnia and Herzegovina. Two groups of experimental tests were performed in this study: laboratory testing of co-firing and trial runs on a large-scale plant based on the laboratory research results. A laboratory experiment was carried out in an electrically heated and entrained pulverized-fuel flow furnace. Coal-sawdust blends of 93:7% by weight and 80:20% by weight were tested. Co-firing trials were conducted over a range of the following process variables: process temperature, excess air ratio and air distribution. Neither of the two coal-sawdust blends used produced any significant ash-related problems provided the blend volume was 7% by weight sawdust and the process temperature did not exceed 1250oC. It was observed that in addition to the nitrogen content in the co-fired blend, the volatile content and particle size distribution of the mixture also influenced the level of NOx emissions. The brown coal-sawdust blend generated a further reduction of SO2 due to the higher sulphur capture rate than for coal alone. Based on and following the laboratory research findings, a trial run was carried out in a large-scale utility - the Kakanj power station, Unit 5 (110 MWe), using two mixtures; one in which 5%/wt and one in which 7%/wt of brown coal was replaced with sawdust. Compared to a reference firing process with 100% coal, these co-firing trials produced a more intensive redistribution of the alkaline components in the slag in the melting chamber, with a consequential beneficial effect on the deposition of ash on the superheater surfaces of the boiler. The outcome of the tests confirms the feasibility of using 7%wt of sawdust in combination with coal without risk to the efficiency of the unit, its combustion process and with the benefits of emissions reductions. Furthermore, they show that no modification to the existing coal transport system and boiler equipment is necessary to achieve this outcome.

- 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/m n3 . At the same time, the NO x emission is below 320 mg/m n3 during the co-firing of lignite mixtures at 1250 °C, and in the case of brown coal mixtures below 740 mg/m n3 at 1450 °C. In both cases, the net CO 2 emission decreases in proportion to the proportion of biomass in the mixture, while the SO 2 emission is still high, at a level of up to 2500 mg/m n3 for lignite mixtures and up to 6400 mg/m n3 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.