Study On Alkali Release And OH Radicals During Coal And Syngas Combustion Using Laser Diagnostic Techniques

The poly-generation technology based on coal pyrolysis and staged-utilization is one of the most important technologies for coal clean utilization, as this system may achieve maximum efficiency while minimize the pollutant emission. In the poly-generation system, coal is pyrolyzed to produce char, tar and gases by heated from coal combustion. After that, syngas can be used as the fuel of gas turbines to produce electric powers.For one thing, the released alkali species during coal combustion, mainly sodium (Na) and potassium (K) compounds, may cause severe problems like fouling and corrosion of the heat transfer surface in industrial furnaces, and also may destroy the gas turbines when these compounds are incorporated into the syngas fuels. For the other thing, the lack of understanding of pyrolysis characteristics of coal restricts the development of the coal poly-generation technology. Moreover, there could be a considerable variation in the composition of syngas dependent on various fuel sources and gasification conditions, which brings many problems to the use of syngas.First, laser-induced breakdown spectroscopy (LIBS) was used in this study to measure quantitatively the Na and K release from burning coal particles. A specially designed laminar premixed burner was employed to provide a post-flame environment. The temperature improved the alkali release during the whole combustion process. For the devolatilization stage, neither O2nor CO2had significant influence on the Na and K release. The release of Na and K during the char stage, however, changed significantly at different O2and CO2concentrations. When the O2concentration increased, the peak concentrations of both Na and K at the char stage increased. When the CO2concentration increased, the release of Na and K was inhibited during the char stage. During the ash stage, the release of Na and K decreased with the O2concentration, whereas it increased with the CO2concentration.For studying the coal pyrolysis characteristics, a single particle experimental system was built in this work. Again, the LIBS technique was employed to measure the species release during the coal pyrolysis. The particle size was found to affect the pyrolysis process significantly. When the particle size decreased, the heat transfer rate was enhanced, resulting in the temperature in particle center increased faster, and the relative mass loss rate also became larger. Accordingly, the release of C, H and O elements occurred earlier. However, the maximum concentrations of these species decreased because of the mass loss rate decreased. Compared with C, H and O elements, the alkali release occurred much earlier. At the experimental condition, the water soluble Na dominated the Na release and the release increased a little when the particle size decreased. Temperature also affected the pyrolysis greatly. At higher temperatures, the particle central temperature increased faster, and the relative mass loss rate also became larger. Meanwhile, the release of C, H, O and Na elements occurred earlier, and the maximum concentrations of these species increased. At lower temperatures, like873K, the non-water soluble Na dominated the Na release. When the temperature increased, the water soluble Na dominated again. With the increase of coal rank, the fuel became less reactive, resulting in the pyrolysis process occurred later and the relative mass loss rate.decreased. Accordingly, the concentrations of C, H and O elements became less and the total release decreased.Then, planar laser-induced fluorescence (PLIF) technique was employed to study the combustion characteristics of syngas, focused on the effects of variations in combustible components. Firstly, laminar flame speeds of typical syngas were studied using laser based Bunsen method and kinetic simulations in CHEMKIN. Both the experimental and simulated results indicated that the flame speed of syngas increased with H2concentration, which, based on the simulation, is attributed to the rapid production of highly reactive radicals and the acceleration of chain-branching reactions by these radicals. When H2was absent, the laminar flame speed increased with the CO content in CO/CH4until80%, then decreased with the CO content increased further. According to the simulation results, CO consumption mainly comes from the oxidation of OH. When OH radical was sufficient, the reaction rate between CO and OH increased with CO content, which contributed to a significant level of heat release rate. When the CO content in CO/CH4exceeded80%, the OH concentration in the flame dropped significantly, which resulted in the reaction rate between CO and OH decreased and the heat release rate became less. The flame speed decreased accordingly. Also, the effects of variations in combustible components on turbulent combustion of syngas were investigated using PLIF technique. The measurements indicated that hydrogen played an important role in the formation of OIl radicals. With the increase of hydrogen content, both the concentration of OH radicals and the burning velocity increased significantly, which facilitated the ignition and combustion of syngas. When the CO content in CO/CH4increased, the OH radical became less in the flame due to the decrease of H in fuel. However, as long as the OH radical was sufficient, the combustion rate would be improved. As the turbulent intensity increases, the flame front becomes more wrinkled, and the concentration of OH radicals also increases, which also facilitated the ignition and combustion of syngas.