E. Hampartsoumian

Evaluation of the optimal fuel characteristics for efficient NO reduction by coal reburning

Thirteen coals and a char, with volatile matter ranging from 7.9 to 50.4 wt% (daf) and nitrogen content from 1.2 to 2.1 wt% (daf), were evaluated as reburning fuels in an isothermal drop-tube reactor system. The NO reduction efficiency by coal reburning decreased with increasing primary-zone or secondary (reburning)-zone airfuel stoichiometry. The NO reduction efficiency was also nearly proportional to the extent of carbon burnout in the reburning zone and hence to coal reactivity, and generally increased with decreasing coal rank. With a high primary-zone NO concentration (∼650 ppmv) and reburning-zone fuel-rich conditions, the NO reduction efficiency could be improved by increasing the rate of input of reburning-coal volatile nitrogen into the reburning zone. A comparison of the reburning performance of a partly devolatilized char with that of the parent high-volatile bituminous coal showed that the parent coal always achieved a higher NO reduction than the char. A mechanistic model is proposal to determine the relative contributions of volatiles and char to the total NO reduction observed.

Determination of sulfur release and its kinetics in rapid pyrolysis of coal

Abstract Four coals of different rank (one anthracite, one bituminous and two lignites) were flash-pyrolysed at temperatures between 700 and 1200°C in a heated wire mesh reactor at atmospheric pressure. The yields of volatile sulfur-containing products (H2S, COS, CS2 and thiophenic compounds) were determined as a function of pyrolysis temperature by gas chromatography with flame photometric detection. A single reaction model was used to describe the sulfur release and pre-exponential factors and activation energies were determined for each coal. The results showed a relation between coal rank and the kinetic parameters. The activation energy varied between 20 and 100 kJ mol−1, the lowest values corresponding to the lignites.

An investigation of important gas-phase reactions of nitrogenous species from the simulation of experimental measurements in combustion systems

Simulated results from a detailed elementary reaction mechanism for nitrogen-containing species in flames consisting of hydrogen, C1 or C2 fuels are presented, and compared with bulk experimental measurements of nitrogen-containing species in a variety of combustion systems including flow reactors, perfectly stirred reactors, and low pressure laminar flames. Sensitivity analysis has been employed to highlight the important reactions of nitrogenous species in each system. The rate coefficients for these reactions have been compared against the expressions used in three other recent reaction mechanisms: version 3.0 of the GRI mechanism, the mechanism of Glarborg, Miller and co-workers, and that of Dean and Bozzelli. Such comparisons indicate that there are still large discrepancies in the reaction mechanisms used to describe nitrogen chemistry in combustion systems. Reactions for which further measurements and evaluations are required are identified and the differences between the major mechanisms available are clearly demonstrated.

The Reactivity of Coal Chars Gasified in a Carbon Dioxide Environment

Abstract The rates of gasification by CO2 have been determined for three laboratory produced chars derived from U.K. coals with varying ash contents. Data for the temperature range 900 to 1233K were obtained using an isothermal thermogravimetric technique. Reaction rates were determined in atmospheres of either CO2, O2 or CO2/O2 mixtures for which the CO2 partial pressure varied between 10 to 101 kPa. Pre-exponential factors and activation energies for the elementary reaction steps were evaluated using the Langmuir-Hinshelwood expression. High temperature data for the range from 1400 to 1800K were obtained by projecting the char particles through a flat methane-air flame. Particle temperatures in the flame were measured using a two-colour pyrometer system and the rates of char gasification at various CO2 partial pressures were calculated from energy balance equations and then used to provide limited information on the char/CO2 reaction rate above 1400K. The activation energy obtained for the transfer of o...

Variation of char reactivity during simultaneous devolatilization and combustion of coals in a drop-tube reactor

Intrinsic reactivities and activation energies are presented as a function of reaction conditions and carbon conversion for five coal chars produced in situ in the devolatilization zone of a simulated one-dimensional pulverized coal flame environment in an electrically heated drop-tube reactor. Oxygen partial pressures varied from 0.01 to 0.09 atm and gas temperatures from 1123 to 1573 K. Carbon conversion rates were determined by elemental analysis of solid material collected along the axis of propagation. The intrinsic activation energies obtained varied from around 135 kJ mol−1 in the initial char combustion stage to 220 kJ mol−1 towards the end of reactor, where the remaining solid burned with little volatiles evolution. Results are presented indicating that a transition from regime II to regime I conditions takes place over the experimental conditions investigated. The intrinsic reactivities were further compared with the rate of elemental hydrogen release from the particles.

Sulfur release during the devolatilization of large coal particles

Experimental data are reported on sulfur release rates during the flash pyrolysis of coal particles sized up to 2 mm, at heating rates and temperatures found typically in fluidized bed reactors. Predicted sulfur yields are also reported for pyrolysis conditions studied using an existing model for devolatilization of large coal particles, coupled with kinetics from an earlier study. The predictions of the experimental data were found to be sensitive to the value of the thermal diffusion coefficient used in the model, with each coal and pyrolysis temperature requiring a specific value in order to accurately predict the experimental results. The rate of heat transfer is also shown to be an important rate-controlling mechanism and the transition from reaction-controlled to heat-transfer-controlled pyrolysis is determined as a function of thermal diffusivity, particle size and sulfur release kinetics.

The production of ultrafine zirconium oxide powders by spray pyrolysis

Twin-fluid atomisation spray-pyrolysis has been investigated for the production of ZrO2 powders. The atomiser used in this study has a novel internal arrangement that can produce a spray with a mean diameter (SMD) of less than 5 μm. Spray pyrolysis tests with zirconium nitrate as a precursor salt were performed and the formation of ZrO2 powder was studied under substantially different heating rates and initial solution concentrations. A mean particle diameter, d(0.5), of 0.67 μm and 0.77 μm was achieved for 0.05 M and 0.5 M solutions, respectively. It was concluded that the new nozzle design performed well and was successful in producing ultra-fine ZrO2 powder with a principally tetragonal structure when the correct process conditions of heating rate and residence time were applied.

Prediction of lifted, non-premixed turbulent flames using a mixedness-reactedness flamelet model with radiation heat loss

The laminar flamelet approach is frequently applied to model turbulent non-premixed flames based on the assumption of adiabatic combustion. This generally results in the significant overprediction of temperatures for flames where thermal radiation is important. In the present study, an adiabatic, mixedness-reactedness flamelet combustion model has been extended to incorporate the effect of radiation heat transfer using the concept of enthalpy defect. This requires the generation of flamelet data sets using a detailed chemical kinetic mechanism and introduction of enthalpy defect as an additional flamelet parameter. The methodology developed has been applied to simulate lifted, free, turbulent non-premixed natural gas flames for which measurements are reported in the literature. A non-adiabatic flamelet data library for methane-air flames has been generated with the GRI reaction mechanism using the modified CHEMKIN code for the modeling of turbulent radiating flames. The turbulent flame computational results, with and without radiation heat transfer, are compared with experimental data for mean gas temperatures, species concentrations and flame lift-off heights for a number of laboratory- and large-scale lifted turbulent jet flames. Predictions obtained using the non-adiabatic flamelet model are found to be in good agreement with temperature measurements, whereas the original adiabatic model significantly overestimates temperatures in the downstream regions of flames where significant heat loss occurs. Species concentration and lift-off height results show small differences between predictions with and without radiation losses in regions close to the base of the flame, although both methodologies provide satisfactory agreement with the available data.

Evaluation of the chemical properties of coals and their maceral group constituents in relation to combustion reactivity using multi-variate analyses

Abstract The factors affecting the combustion rates of coals have been investigated using multivariate analysis. Coals from Australia, Colombia, Germany, UK and USA and their separated maceral group constituents were subjected to detailed optical and chemical characterisation followed by the measurement of combustion reactivity at low temperatures (up to 1323 K) using TGA, and at high temperatures (1300–1900 K) in an entrained flow reactor. The derived activation energies and pre-exponential factors were used in conjunction with eighty-six detailed chemical and petrographic parameters as the basis for statistical analysis. This demonstrated that the parameters which significantly influence the combustion rate could be used to divide the coals and maceral groups into two classes. One class containing predominantly the northern hemisphere coals with their corresponding maceral groups and the other containing the Australian and Colombian coals with their maceral groups. No significant relationships were found between the reactivity of the coals and the maceral groups within the coal, or between the same maceral group for all the coals. However the chemical composition of the organic matter content was found to be an important parameter in determining coal reactivity and such information could be used to supplement classical petrographical classification and proximate and ultimate analyses in order to predict the reactivity of a given coal. The effect of ash on the burning rate of the individual maceral components was also investigated and the implications for burnout in practical combustion systems discussed.

Fragmentation of large coal particles in a drop-tube furnace

Abstract The fragmentation of large coal particles (1–6 mm) was studied by introducing them singly into an isothermal drop-tube reactor maintained at 1273–1383 K. Fragmentation due to thermal shock and subsequent cracking during the devolatilization and char combustion stages was observed for all particle sizes. The extent of fragmentation increased with increasing initial particle size and temperature. Particle rotation as a result of ejection of volatiles was also observed, with a frequency of 200–1300 Hz depending on particle size.