ON THE POSSIBILITIES OF USING BROWN COAL AS A FUEL FOR POWER BOILERS

1. ON THE POSSIBILITIES OF USING BROWN COAL AS A FUELFOR POWER BOILERS Lyudmila Haponych, NataliyaDunayevska, MaksymYurchenko • The exclusion of natural gas from the fuel base of Ukrainian thermal power plants has led to a coal deficiency of 6 million t f. e./yr (see Table 1). In the nearest future, an increase in the solid-fuel base of power engineering can be reached by the involvement of off-balance fuels, brown coal in the first place.Its balance resources in Ukraine constituteare 2.4 billion t, including those suitable for opencast mining of more than 0.5 billion t. The potential of the annual production of brown coal in Ukraine at the existing and promising enterprises is about 19.2 million t (or 5 to 6 million t f. e.). In the world power engineering, the positive experience of using brown coal was accumulated. However, brown coal practically is not used in Ukraine at present. The existing power plants and boiler houses are not adapted to its burning. The problems of using brown coal for thermal power plants can be solved by applying new, high-efficient technologies. In our opinion, it is reasonable to burn brown coal on the basis of different modifications of fluidized bed (FB). For the development of such technologies, in addition to the well-known results of kinetic investigations of the combustion of brown coal, it is also necessary to study its pyrolysis. We performed an experimental study of the laws of pyrolysis of brown coal for a wide range of parameters characteristic of FB technologies (fast pyrolysis). We present here some results obtainedin these experiments. The technical and elemental composition of brown coal is presented in Table 2. • One of the main problems in studying the pyrolysis of coal is the determination of the composition of gaseous products. Coal-volatile matter, formed as a result of the thermal destruction of coal, consists of components that are condensed (including Н2О) and pyrolysis gas, i. e., Н2, СН4, СО, СО2, and СnНm. Experiments in an inert environment show that the specific yield ofcoal-volatile matter depends on the temperature and correlates with the elemental and petrographic composition of the coal and the degree of its metamorphism. These resultsare summarized in Table 3. • Comparison of the determined elemental composition of pyrolysis gases with the elemental composition of coal-volatile matter according to the standard analysis demonstrates that the main part of elements of the coal-volatile matter, namely, about 56% С, 50% Н, and 96% О, go out in the form of gases Н2, СН4, СО,and СО2. • The pyrolysis of coal in reactors proceeds in the atmosphere of combustion products (up to 20% СО2and 80% N2). Therefore, we carried out experimental investigations of the dynamics of yield of coal-volatile matter in the course of pyrolysis inСО2. The dynamics of gas emission has an extreme character, and, with increase in the gas-carrier velocity, the constants of rates of partial gas emission for each bed temperature grow to their maximal values, independent of the particle sizes. In Table 4, we show the kinetic constants of rates of the yield of Н2 and СН4at different temperatures under a pressure of 0.1 MPa. • Together with kinetic data, the results obtained are sufficient for their use as initial data for engineering calculations of the necessary duration of the residence of brown-coal particles in FB reactors. Table 1– Structure of the resources, consumption, and production of the main organic energy carriers in the world and Ukraine in 2010 f. e. – fuel equivalent is a fuel with a calorifer value of 7,000 kcal/kg Table 2 – Composition of the coal Table 3 – Specific yields of gaseous components Table 4 – Kinetic constants of the pyrolysis of Н2 and СН4 In Ukraine, the main park of small power and hot-water boilers is concentrated in the range from 1.9 to 15.6 MW and presented by DKVR boilers of thermal power 4.0 to 6.5 MW, a significant part of which works on gas and fuel oil. One of the ways of boilers modernization and prolongation of the term of their operation is connected with the mounting of primary furnaces in front of them. • A vortex primary furnace for the burning of brown coal was developed at the Coal Energy Technology Institute. The furnace is intended for use with DKVR-6.5-13 boilers for full or partial replacement of natural gas by brown coal or peat. The primary furnace is joined to the boiler across its frontal wall, which has no water-wall tubes. The design fuel is Dnieperbrown coal with Wtr=25%, Аr=18.8%, andQir=3500 kcal/kg. • The fuel is fed together with tangential air flow, which forms the vortex of combustion products. Brown coal is dividedinto primary and secondary fuels: the first comes to the horizontal vortex chamber, and the second to the horizontal heat-insulated pipe (Scheme). The secondary coal is subjected to drying and fast pyrolysis, which enables one to shorten the time of its firing and combustion. Drying, pyrolysis, and firing are provided by the contact of disperse flow with heated lined walls of the chamber. The combustion products together with residual oxygen through the horizontal pipe come to the boiler furnace. • The flow-rate and thermal characteristics of the primary furnace with vortex stream: • - total air flow rate – 8020 nm3/h, including: - to the vortex chamber – 7200 nm3/h, • - to the pipe – 820 nm3/h; • - consumption of brown coal – to 1.5 t/h, including: - to the vortex chamber – 900 kg/h, • - to the pipe – 600 kg/h; • - temperature at the outlet from the vortex chamber – 1050 оС; • - theoretical temperature in the boiler furnace – 1430 оС ; • - flow rate of combustion products – 8890 nm3 /h; • - thermal power of the primary furnace – 6.1 MW. • The results of tests of the primary furnace have shown the following: • it is possible to organize the stable firing of coal particles in the vortex induced by the radiation of preliminarily heated lining; • the vortex stream of coke particles burns stably in the main part of the primary furnace; • the temperature of the vortex stream is lower than the point of ash softening. This work was carried out within the framework of the Project"Demonstration, dissemination and deployment of CCT(Coal Clean Technology) and CCS (CO2 Capture Storage) in Ukraine“, financed by the European Commission Ukraine Performert: Coal Energy Technologies Institute of the Ukrainian National Academy of Sciences 19, Andriivs'ka str., Kiev 04070, Ukraine Phone: +38 044 255068 Fax: +38 044 5372241