• Energy Research

Abstract The ignition of pulverized fuel particles in air under laser irradiation with an optical fibre was studied. Six coals of different rank were used, in three size fractions. An Nd-YAG laser (continuous wave) with radiation wavelength 1.06 μm and variable output power was used as the energy source to heat single particles on the end of the optical fibre (100 μm core diameter). Two laser power levels were defined: 1. (1) the critical laser power passing through the fibre, as measured; 2. (2) the minimum laser power absorbed by the particles, estimated using the optical properties of particles. The experimental results indicate that there is a minimum laser power required to ignite a fuel particle, dependent on coal type and particle size. The measured critical (or minimum) laser power ( P f , c ) for ignition to occur, which comes from the optical fibre end, decreases with increasing particle size, whereas the minimum laser power ( P ab ) predicted to be absorbed by the particle increases. Three different ignition mechanisms under laser heating, depending on the volatile matter of the particle, were observed. Supporting theoretical analysis of the experimental data, based on a heterogeneous ignition assumption, is also presented and brief comparisons are made with other ignition techniques.

Abstract It is shown that laser irradiation of a fine coal or char particle in a combustible gas environment can result in ignition of the particle before explosion occurs in the gas. The relation between critical power required for ignition and size of particle is examined.

Coal blends are commonly used in pulverised fuel fired power plants. Past experience has shown that some coals exhibit synergistic effects when co-fired with other coals. Despite the recent progress in the field, there is still a general lack of understanding about how and why such synergistic effects take place. It is, therefore, imperative to develop reliable techniques to predict the combustion characteristics of coal blends. In the present paper, a commercially available computational fluid dynamics (CFD) software package, known as Fluent, is applied to simulate the combustion of binary coal blends of Australian black coals in a pilot-scale furnace. The modelling was mainly concerned with employing the two-mixture-fraction-pdf approach to separately track the individual components of each blend. Additionally, another approach, which takes the blend as a single coal with the weighted average properties of the components, was also used and compared. The main combustion characteristics of coal blends, such as ignition, burnout and NOx emissions were studied and corresponding predictions were validated against measurements in a pilot-scale furnace. The comparisons indicated that the CFD model with the two mixture fraction approach can successfully predict the non-additive (synergistic) combustion behaviour of coal blends, providing an effective tool for full-scale applications. The single coal approach presents nearly additive prediction of coal bends, and is only applicable to the blends of similar coals.

Microwave pyrolysis of an Indonesian lignite is investigated in this study. The effects of experimental parameters, such as microwave receptor/coal ratio, residence time, temperature, microwave pow...

Deposits formation on heat transfer surfaces is one of the main problems associated to biomass co-combustion. It reduces plant efficiency and availability and increases maintenance costs. It is obvious that an increasing amount of low-temperature melting components in fuel ash accelerates and aggravates this process. Research is done to evaluate the validity of thermal analysis methods to characterise fusion of biomass and waste ashes. Laboratory ashes from a set of biomass and waste fuels are leached in successive steps. The original and the leached ashes are analysed by Thermo-Mechanical Analysis (TMA). Traces obtained from TMA show to be promising ash fingerprints to classify deposition tendencies. Additionally Simultaneous Thermal Analysis (STA) is performed on selected samples. Furthermore, improved chemical equilibrium calculations are proposed to predict the proportion of melted species resulting from combustion of biomass fuels. The model takes into account the reactivity of the inorganic matter in the fuel as issued from ash leaching.

Abstract The Callide Oxy-fuel Project is the world's largest operating oxy-fuel plant. This work details an experimental test campaign at the Callide Oxy-fuel Project monitoring mercury and SO3 levels exiting the fabric filter during transitions between air and oxy firing conditions. The measurements were taken using two custom built probes; the first allowing combined collection of SO3 and mercury over short time intervals; the second allowing on-line measurements of Hgtotal and Hg0 with SOx removal. Total mercury emissions in oxy-firing measured a maximum of 6–7 μg/m3 of which 89% was in oxidised form (Hg2+). The use of low NOx burners had an overriding influence on the mercury measurements reducing the total mercury levels to 0.13 and 0.15 μg/m3 (air, oxy respectively) with no Hg2+ being measured. The SO3 concentrations were also lower than expected, estimated at ∼0.5–0.8 ppm (based on a practical estimate of 1% conversion of SO2). Overall mercury capture in either operating mode was estimated at 92–93% for the existing burners and 98–99% with the low NOx burners used (being 2 of the 4 burners operating). Total SOx captured from the flue gas was 16% in oxy-mode and 19% in air firing. These findings suggest that operational conditions have a primary impact on capture of Hg and SOx during transitions with a secondary impact of firing mode (i.e. air or oxy).

Abstract Literature data on the gasification of coal chars with CO2 at moderate temperatures and high pressures has been reviewed, the focus being the factors affecting the reactivity. A model has been developed to extrapolate this reactivity data to high-temperature conditions. The intrinsic reactivity for various chars were predicted from the moderate temperature apparent reactivity data, assuming that reaction is in regime I, and the effects of pressure, char type and temperature on intrinsic reactivity were obtained. The apparent char reactivity was predicted at high temperature by incorporating the intrinsic reactivity with an effectiveness factor. It is shown that both apparent and intrinsic reaction rates at 1123 K increase continually with CO2 partial pressure. The char type has a more significant effect on the intrinsic reaction rate than the pressure. The char surface area is an important factor in obtaining char apparent reaction rate. Gasification temperature has the greatest influence on reactivity. The large variation in predicted intrinsic reaction rate for various chars at high temperatures is observed due to different activation energies. The activation energy for char–CO2 gasification generally decreases as coal char rank decreases. For char–CO2 gasification of 100 μm particles in an entrained flow coal gasification system where the gasification temperature exceeds 1700 K, the char apparent gasification rate is limited by mass transfer in porous structure of char particles. The predicted apparent reactivity showed a reasonable agreement with experimental measurements that were obtained under high-temperature low-pressure gasification conditions. The extrapolated high-temperature gasification kinetics can be used in modeling the performance of an entrained flow gasifier.

Abstract A char morphology system is outlined, based on the physical and optical characteristics known to influence the burning of pulverized coal. Application of the system to high temperature char has established links between coal microlithotype predecessors and char types for subbituminous and bituminous coals. Quantities of high density and thick-walled char produced by pyrolysis at 1500 °C correlate with quantities of high density char in combustion residue, and with unburnt carbon content in fly ash for seven Australian energy coals. The proportions of different pyrolysis char types produced by any coal are shown to depend on coal rank, petrographic composition, maceral fusibility, and possibly ash content and composition, Quantities of high density chars in combustion residue can also be related to petrographic properties, including infusible inertinite content and percentage of microlithotypes of high inertinite content. Vitrinite reflectance is found to be a good parameter to differentiate burnout performance of coals with significant differences in rank. By comparing the proportions of high density, thick-walled pyrolysis char that form at 1500 °C, an estimate of burnout behaviour based on the combustion characteristics of char is obtained. Study of char types remaining after combustion also indicates potential for improvement of burnout performance, based on knowledge of the nature and origin of the unburnt carbon particles.