Combustion Characteristics Of Coal Particles Research Articles

Effect of coal particle density on coal properties and combustion characteristics

In the present work, the combustion characteristics of coal particles with varying average specific gravity (ASG) have been reported. Proximate analysis, ultimate analysis, GCV, petrographic analysis and FTIR analysis were carried out to identify variation in coal properties with varying ASG of coal from1.24 to 1.9. Burning profile parameters and combustion indices are evaluated from TGA analysis. Experimental results inferred that with the increase in coal ASG from 1.24 to 1.9 GCV decreased from 7266 kcal/kg to 3741 kcal/kg, heat release rate varied between 1386 kcal/min to 120 kcal/min, burning time increased from 9 to 30 min and vitrinite varied from 58% to 2.5%, liptinite varied from 13.5% to 1.5%, inertinite varied from 19.2% to 65.7%. Differences in the combustion properties were correlated with hydrocarbon functional groups obtained from FTIR analysis and macerals composition further exploration in details with varying ASG of coal.

Ignition and combustion characteristics of coal particles under high-temperature and low-oxygen environments mimicking MILD oxy-coal combustion conditions

The experiments of coal particles combustion were conducted to mimic moderate and intense low-oxygen dilution (MILD) oxy-coal combustion on a diffusion-flamelet-based Hencken burner under different O2/CO2 and O2/N2 environments with the coflow temperatures of 1400–1800 K and oxygen concentrations of 5%–20%. The images of coal particles combustion flame were captured by a digital camera to determine the ignition standoff distances of coal particles. The radiation spectrums of burning coal particles at different coflow temperatures and oxygen concentrations were measured by a fiber optic spectrometer and the temperatures of burning coal particles were deduced with the two-color method. It is found that either increasing the coflow temperature or the oxygen concentration leads to brighter flame luminosity, shorter ignition standoff distance and higher burning particles temperature. The ignition standoff distance of coal particles is increased by 0.07–0.49 cm when carbon dioxide is used as diluent gas instead of nitrogen. The temperatures of the burning coal particles under O2/CO2 environments are lower by 51–185 K than those under O2/N2 environments at the same conditions due to the combined physicochemical properties of CO2 with higher heat capacity and the endothermic reaction of char gasification. Moreover, a method based on the coal particles temperature uniformity coefficient is proposed to quantitatively identify the combustion modes of MILD coal combustion and MILD oxy-coal combustion. All coal combustion flames are visible under both O2/CO2 and O2/N2 environments, and the MILD coal combustion flames under high-temperature and low-oxygen concentration conditions are weaker and more uniform with dark red color.

Influence of particle sizes on combustion characteristics of coal particles in oxygen-deficient atmosphere

The combustion characteristics of coal samples with different particle size were investigated using a thermogravimetric analyzer. Three different heating rates were set in the oxygen-deficient atmosphere of 12 vol % oxygen concentration. Experiments were conducted on four particle sizes with a diameter of 0.6, 1.0, 2.0, and 4.0 mm. It was found that ignition and burnout temperatures of coals rose approximately linearly with the increasing particle size. The ignition temperature of the coal with a diameter of 4.0 mm (482 °C–520 °C) varied greatly at different heating rates compared with that of 0.6 mm (473 °C–481 °C). The effect of particle size on the ignition temperature decreased with the increase in heating rate, while the effect on the burnout temperature was promoted. The calculation of the combustion index and the results of the kinetic analysis indicated that the reactivity decreased as the coal particle size increased. The results of this paper can provide guidelines for the development of a staged fuel-feeding horizontal circulating fluidized bed boiler.

Gas-Particle Flow and Combustion Characteristics of Pulverized Coal Injection in Blast Furnace Raceway

Abstract The two-dimensional steady-state discrete phase mathematical model is developed to analyze gas-particle flow and combustion characteristics of coal particles, as well as components concentration and temperature distribution of coal gas in the process of pulverized coal injection of blast furnace raceway. The results show that a great deal of coal gas discharges on the top of raceway away from the tuyere, and the residence time of coal particles in the region of blowpipe and tuyere is 20 ms or so and 50 ms when it reaches raceway boundary. The pressure is the highest at the bottom of raceway and the maximal temperature is about 2 423 K. The char combustion is mainly carried out in the raceway and the maximum of char burn-out rate attains 3×10 −4 kg/s.

Numerical Studies on Combustion Characteristics of Interacting Pulverized Coal Particles at Various Oxygen Concentration

The two-dimensional laminar combustion characteristics of coal particles at various oxygen concentration levels of a surrounding gas have been numerically investigated. The numerical simulations, which use the two-step global reaction model to account for the surrounding gas effect, show the detailed interaction among the inter-spaced particles, undergoing devolatilization and subsequent char burning. Several parametric studies, which include the effects of gas temperature (1700 K), oxygen concentration, and variation in geometrical arrangement of the particles on the volatile release rate and the char burning rate, have been carried out. To address the change in the geometrical arrangement effect, multiple particles are located at various inter-spacings of 4–20 particle radii in both streamwise and spanwise directions. The results for the case of multiple particles are compared with those for the case of a single particle. The comparison indicates that the shift to the multiple particle arrangement resulted in the substantial change of the combustion characteristics and that the volatile release rate of the interacting coal particles exhibits a strong dependency on the particle spacing. The char combustion rate is controlled by the level of oxygen concentration and gas composition near particles during combustion. The char combustion rate is highly dependent on the particle spacing at all oxygen levels. The correlations of the volatile release rate and the change in total mass of particles are also found.

The impact of soot on the combustion characteristics of coal particles of various types

The products of coal devolatilization are radically transformed by secondary pyrolysis after they are expelled into hot gases until only H 2, C 2H 2, CO, CO 2, H 2O and soot remain. The coal combustion model developed in this article represents the limiting scenario of infinitely fast secondary pyrolysis followed by combustion of gaseous fuels and soot in flame sheets, either on or around individual coal particles. Soot and gases from different coal types are distinguished by different evolution rates, elemental compositions, and transport properties. Thermophoresis and radiation are accounted for in the transport analysis. This study also develops separate limiting behavior for instantaneous soot oxidation in envelope flames and for frozen soot oxidation chemistry. Comparisons among predicted and observed flame lifetimes and maximum flame standoffs select the most realistic modeling scenarios. Radiation from soot into the surroundings dictates the overall impact of secondary pyrolysis on macroscopic combustion characteristics. Soot is almost 1000 times more efficient than its host particle in radiating energy, and dissipates up to 90% of the radiation during the initial stages of combustion. Soot radiation cools flame temperatures by up to 300 K, reducing the differences among flame temperatures for diverse coal types in 8% O 2 to only 100 K around 2200 K. At such temperatures, water/gas shift equilibrium determines the distribution of combustion products, and the energy carried away by intermediates becomes negligible. For envelope flames around 100 μm particles of all coal types, about 60% of the heat of combustion is fedback to the particle, and one-third is conducted or radiated into the surroundings. But for attached flames on smaller particles more than 90% is retained by the particle.