High-ash Char Research Articles

Experimental study on the contribution of included/excluded minerals to particulate formation during coal char combustion

The contributions of included and excluded minerals to particulate matter formation are quantified in this work. By devolatilization of three typical Chinese coals, three coal chars with low (LA), moderate (MA), and high ash (HA) content (with both included and excluded minerals) were prepared, from which three float coal chars with low (LA−), moderate (MA−), and high ash (HA−) content (without excluded minerals) were separated by density differentiation method. The coal chars before and after separation were burned in a drop-tube furnace at 1600 K, the results of three float coal chars can directly reveal the contribution of included minerals, and the comparison of two kinds of chars with and without excluded minerals would reveal the excluded mineral contribution to ash distribution. The size distributions of the combustion-generated ash particle were obtained from the combination of ELPI+ and laser particle-size analyzer. The fractions of included/excluded minerals were obtained from SEM images. The results from the combustion of the float chars indicate the physical transformation of included minerals, while the results from original coal chars involve the transformation of both included and excluded minerals. Results indicate that the distribution of ash particles from included minerals for HA− char possesses a single peak, while the ash distributions of MA− and LA− char show a bi-modal shape. The excluded minerals in LA and MA chars mainly contribute to the ash emissions with sizes larger than 80 μm. The contribution from excluded minerals reach 86% to the ash particles of 205 μm for HA char, 54% for MA char and 51% for LA char. The number fractions of included minerals continuously decrease with a transformation into excluded minerals during char dynamic combustion.

Gasification reactivity of high ash Indian coals in varying concentrations of CO<SUB align="right">2

Optimisation of coal gasification processes requires a good understanding of coal reactivity of the CO2 and steam gasification reactions. Presently there are very few studies on gasification behaviour of high ash coals. Therefore, present paper investigates the gasification behaviour of char samples in 100, 70 and 30% concentrations of CO2 in the temperature range of 900–1050°C using thermogravimetric analyser. Char samples were prepared from high ash Indian coals in inert atmosphere at 800, 900 and 1,000°C. Effects of coal/char properties, char preparation temperatures, char gasification temperatures and varying concentrations of CO2 on gasification reactivity of high ash char samples have been reported. Higher pyrolysis temperature shows adverse effect on CO2 gasification, whereas higher gasification temperature, CO2 concentration, alkali index, surface area as well as porosity favour CO2 gasification reactions. [Received: February 27, 2017; Accepted: December 1, 2017]

Properties of high ash coal-char particles derived from inertinite-rich coal: II. Gasification kinetics with carbon dioxide

The reaction rate of carbon dioxide-nitrogen gas mixtures with a well-characterised high ash char derived from an inertinite-rich coal discard was investigated by experimentation and reaction rate modelling. Experimentation with a thermogravimetric analyzer at 87.5 kPa and 287.5 kPa between 850 °C and 900 °C and with 1mm diameter particles, similar to operating conditions used for bubbling fluidised bed gasification, was carried out. The char consisted of a large proportion of dense char formed from the inertinite in the parent coal, fine pores from the low concentration of reactive macerals and cracks formed as a result of thermal deflagration. The effects of carbon dioxide concentration, temperature and pressure on the carbon conversion with time were found to follow expected trends with long reaction times. The random pore model, which accounts for intraparticle structural changes, was examined to predict the overall reaction rate. For this evaluation, a new procedure was developed to determine the structural parameter which could not be calculated directly from initial characterisation results. This procedure consists of defining a reduced time parameter, which conveniently eliminates the effect of the intrinsic kinetics when conversion versus the reduced time results is used. Thus, the structural parameter, which characterises the pore growth and coalescence, can be evaluated by a regression procedure using experimental results obtained at all temperatures and pressures. Intrinsic reaction parameters based on the power rate law were also calculated from carbon conversion versus real time results using a stepwise regression procedure. It was found that the random pore model predictions for the carbon conversion with time using the determined parameters correlated very well with experimental results, thus confirming that the reaction rate is chemical-reaction controlled.

Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical, structural and petrographic characteristics

An investigation was undertaken to determine the properties of high ash coal–chars derived from South African discards rich in inertinites, for the development of suitable overall reaction rate models at low temperatures (<900 °C). Detailed characterisation results of the parent coal and chars prepared at 700 °C and 900 °C obtained from standard coal analytical methods, petrographic techniques, CCSEM image analysis and a surface adsorption method are presented. The parent coal consisted of 32% by volume of inertite (“pure” inertinite), 7% of vitrite (pure “vitrinite”), and 13% of bi- and tri-macerite, 30% of maceral/mineral mixtures (carbominerite) with 18% of mineral-rich material. Reflectances obtained from measurements taken on vitrinites and total maceral reflectance scans increased dramatically on charring at 900 °C and were accompanied by an extension of vitrinite reflectance class distributions indicating higher molecular ordering. Volatiles were liberated essentially from the original parent vitrinites, creating fine gas pores. Inertinites increased in reflectance but not in porosity and were characterised as dense char fractions in the final charred product, according to a coal form analysis. Structural change due to low temperature thermal stress fracturing (passive deflagration) occurred early on in the temperature regimes, creating increased surface areas and porosity. The chars consist of a high proportion (52%) of extraneous rock fragments together with minerals mainly as fine inclusions in carbon rich particles (13%). The chars had very low porosities and surface areas. These were created by the devolatisation of reactive maceral associations and deflagration. Such materials could introduce intra-particle diffusional effects during gasification and combustion of millimetre size particles at low temperatures.

The removal of reactive dyes using high-ash char

The thermodynamics and kinetics of adsorption of reactive dyes on high-ash char was studied. Equilibrium data were obtained using the static method with controlled agitation at temperatures in the range of 30 to 60ºC. The Langmuir isotherm model was used to describe the equilibrium of adsorption, and the equilibrium parameters, R L, in the range of 0 to 1 indicate favorable adsorption. The amount of dye adsorbed increased as temperature increased from 30 to 40ºC, but above 40ºC the increase in temperature resulted in a decrease in the amount of dye adsorbed. The kinetic data presented are for controlled agitation at 50 rpm and constant temperature with dye concentrations in the range of 10 ppm to50 ppm. The film mass transfer coefficient, Kf, and the effective diffusivity inside the particle, De, were fitted to the experimental data. The results indicate that internal diffusion governs the adsorption rate.

An Integrated Approach to the Utilization of Low Rank Coals and Biofuel

While suggesting an integrated approach for utilization of inferior low rank coals for power in India, the importance of low temperature carbonization followed by retrieval of all value-based products has been stressed. It is further suggested that tar, obtained in the process, could be hydrogenated and fractionated in a central plant for conversion to hydrocarbons. High ash char, the principal product of pyrolysis, has been experimentally found to be amenable to beneficiation, yielding suitable fractions for power generation, briquetting, or blending. Experimental studies have shown that forest litters and agricultural wastes, containing significant proportions of spore, cuticle, and exine considered as precursors of hydrocarbon-generating coal macerals also yield large quantities of tar, ammonical liquor, and the principal product, char, which can be respectively utilized for the production of petroleum substitutes, value-based chemicals, and source material for blending, briquette making, and char-water slurries, opening up new avenues for fuel utilization and conservation.

Evolution of pore structure of high-ash char during activation

Experimental results are presented for pore structure evolution in high-ash char during activation with CO 2, H 2O and air. Surface areas and pore size distributions of the char at various activation times are reported. It is shown that activation with H 2O results in the most surface area and micropore volume, and air the least.

Comparison of structural models for high-ash char gasification

Structural models for the gasification of high-ash char are developed from different approaches to the depiction of the char pore structure. The description of the pore structure of a high-ash char particle is based on the structural characteristics of high-ash containing coal reject char. A finite overlapping grain model (FOG), modified random pore model (MRP) and a model based on the percolation theory on Bethe lattice (PBL) are formulated. These models take into account the effect of ash content by considering it as an inactive inclusion. Comparisons of these models are presented based on the parametric simulation results. It has been shown that both the MRP and PBL models can predict similar behavior of high-ash char gasification. The effect of ash content on the surface area and reaction rate evolution is shown to be significant. The percolation model has an advantage in that it can describe the closed pore evolution and predict particle fragmentation.

Steam gasification of German hard coal using alkaline catalysts: Effects of carbon burn-off and ash content

Laboratory-scale experiments were performed on chars from German hard coals with potassium carbonate addition. The steam gasification rate at 4 MPa and 700 °C as a function of the amount of catalyst added is described for a low-and high-ash char. From the burn-off behaviour the reaction order relative to carbon was determined. For the low-ash char a uniform reaction order was found but the high-ash char indicated a complex interaction of catalytic gasification, catalyst deactivation, and the development of the reacting surface.