A new rheometer for direct measurement of the flow properties of coal ash at high temperatures

Development of more effective technologies of utilising low rank coals for power generation has been driven by a demand for higher efficiency, low capital costs and minimal environmental impacts. Fluidised bed systems are regarded as one of the more promising alternative technologies for power generation to overcome the disadvantage of the existing pulverised coal burning power generation plants for low rank coals. However, ash deposition and bed agglomeration are potential problems in fluidised bed processing of coals with high alkali and sulphur contents. In order to gain a better understanding of the mechanism of agglomeration in fluidised beds, a good knowledge of the rheological behaviour of coal ash deposits at high temperatures and under the processing conditions is necessary. Rheological characterisation of materials at high temperatures is difficult due to lack of standard instruments and reliable measurement techniques. We have recently developed a rheometer that has the capability of measuring the rheological properties of coal ash slag over a wide range of temperatures from 600 to 1300°C and under different processing atmospheres. In this paper the features of this unique instrument are described and the experimental technique developed for flow property measurement is outlined. Some typical measured rheological properties of coal ashes from different Australian low-rank coals are presented and discussed to illustrate the potential applicability of the rheometer for high-temperature rheological characterisation. Furthermore, by means of the experimental results obtained it is demonstrated that the alkali content of the coal ash plays a significant role in controlling the rheological characteristics of the ash deposit, which in turn has an important implication on agglomeration in fluidised bed combustion processes.     

Time- and temperature-resolved field ionization mass spectrometry of coals and their extracts

Field ionization mass spectra of three coals (85, 78 and 72 wt% C, daf) and their pyridine extracts were obtained between 50 and 500 °C using the direct introduction system with a linear heating rate of 1.0 °Cs⁻¹. Scan numbers and temperatures were plotted versus the total ion current (thermograms) and selected ion intensities (profiles). All spectra of coals and extracts showed that the volatile material gives almost exclusively molecular ions with f.i.m.s. and mainly comprises substances in the range 200 to 800 daltons. The spectra of a coal and its respective extract show striking similarity. The yields of volatiles from the coals ranged from 11 to 30 wt%, daf. The thermograms of extracts were clearly shifted towards lower temperatures compared with those of the coals. The profiles of selected ions show at low and high mass resolution that the same substance may be volatilized over the whole heating range from 50 to 500 °C, indicating that the volatilized material had been clathrated in the coals and that thermally induced diffusion preceded its evaporation. The diffusion of molecules occluded in the pore structure or chemical species generated by thermal fragmentation is assumed to occur at the initial step of all technical processes such as liquefaction, carbonization, combustion and gasification. In general, the behaviour of a coal is expected to depend strongly upon the amount of material capable of diffusion, which is characteristic for the processes of coal utilization at a given heating rate. Time-/temperature-resolved field ionization mass spectrometry of coal offers a unique means of fast and reliable monitoring of such characteristics.     

Hot-compaction behaviour of coals

Hot-compaction behaviour of semi-anthracite, low-, medium- and high-volatile caking, oxidized medium-volatile, sub-bituminous and lignite coals was studied under a constant load of 3.45 MPa. Generally, total compaction decreased and initial compaction temperatures increased with increasing coal rank. Initial compaction of the caking coals was noted at temperatures below their Ruhr dilatometer softening temperatures. For the high-volatile sample, this softening temperature was low enough, at about 250 °C, to imply possible thermal breakdown of such coals in preheating processes used before coke-making. The visco-elastic behaviour of coals of different rank over a range of temperature was examined. Densification mechanisms are suggested and the rate-controlling steps are discussed. Devolatilization is an important factor in particle-flow and shrinkage/dilatation mechanisms, as inferred from the similarities of the activation energies associated with these mechanisms and the reported values for devolatilization processes. Non-caking coals exhibit visco-elasticity at temperatures where they are expected to be semi-chars, in contrast with the caking coals.     

Minimization of Moisture Readsorption in Dried Coal Samples

Low-rank coals csonstitute a major energy source for the future as reserves of such high-moisture coals around the world are vast. Currently they are considered undesirable since high moisture content entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Furthermore, low-moisture-content coal is needed for the various coal pyrolysis, gasification developed. Hence, various upgrading processes have been developed to reduce the moisture content. Moisture readsorption and spontaneous combustion are important issues in coal upgrading processes. This article discusses results of laboratory experiments conducted to study the options for minimization of readsorption of moisture after drying of selected coal samples. Results suggest that there is little benefit in drying low-rank coal at high temperatures. It was found that the higher the amount of bitumen used for coating, the lower is the readsorption of moisture by dried coal. Also, mixing high-temperature-dried coal with wet coal in appropriate proportion can yield reduced moisture content as the sensible heat in the hot coal is utilized for evaporation.     

Liquefaction characteristics of selected vitrinite- and liptinite-rich coals from British Columbia,Canada

Four coals from British Columbia, Canada are described micropetrographically, and their liquefaction potential was tested in a rocking autoclave system. Rank was determined by measuring vitrinite reflectances and was found to range from subbituminous C to high volatile A bituminous. Composition was determined by maceral analysis. All four coals were found to be extremely high in the reactive maceral groups vitrinite and liptinite (> 97%) and were thus considered to be susceptible for liquefaction.Results of the liquefaction experiments indicate that in fact all four coals were converted easily into liquid and gaseous products. Overall conversion was found to be >90% and liquid yields (THF solubles) were beyond 80%. Alginite-rich coals from southeastern British Columbia were found to produce greater amounts of gases, indicating that temperatures used in the experiments might have been excessive for liquefaction of alginite-rich coals. Higher liquid yields and less gaseous products were obtained from the vitrinite-rich coals from northeastern British Columbia, indicating that these coals might have the optimum blend of vitrinite/liptinite macerals.     

Laboratory-scale coal reactivity testing for entrained gasification

A relatively simple and rapid micro-gasification test has been developed for measuring gasification reactivities of carbonaceous materials under conditions which are more or less representative of an entrained gasification process, such as the Shell coal gasification process. Coal particles of < 100 μm are heated within a few seconds to a predetermined temperature level of 1000–2000 °C, which is subsequently maintained. Gasification is carried out with either CO₂ or H₂O. It is shown that gasification reactivity increases with decreasing coal rank. The CO₂ and H₂O gasification reactions of lignite, bituminous coal and fluid petroleum coke are probably controlled by diffusion at temperatures 1300–1400 °C. Below these temperatures, the CO₂ gasification reaction has an activation energy of about 100 kJ mol^?1 for lignite and 220–230 kJ mol^?1 for bituminous coals and fluid petroleum coke. The activation energies for H₂O gasification are about 100 kJ mol^?1 for lignite, 290–360 kJ mol^?1 for bituminous coals and about 200 kJ mol^?1 for fluid petroleum coke. Relative ranking of feedstocks with the micro-gasification test is in general agreement with 6 t/d plant results.     

Principal component analysis of upgraded Western Canadian coals

Three Western Canadian coals with different ranks have been upgraded by evaporative drying in several atmospheres at temperatures up to 500°C. Data from proximate and ultimate analyses, along with equilibrium moistures and heating values, have been evaluated by principal component analysis. The results indicate that significant changes in the products occur above 400°C for lignite and above 300°C for subbituminous B and high-volatile C bituminous coals. No significant differences are observed when the data for the upgraded coals are compared with published data for naturally occurring Western Canadian and US coals of the same rank.     

Diffusion models for gas production from coal: Determination of diffusion parameters

A method for the laboratory measurement of transient diffusivity parameters for methane in coal has been developed. Previously described Chromatographic techniques for measurement of single diffusivity parameters have been modified for the bi-disperse pore diffusion model. This model, which is necessary to describe methane diffusion and desorption in coal for both short and long times, requires the determination of three parameters to characterize a particular coal sample. The experimental technique uses a pulse of methane gas which is introduced into a helium stream flowing through a long tube containing the coal sample. The three desired parameters are determined by matching the experimental and theoretical elution curves. Further refinement of the technique accounts for tracer pulse duration thus allowing operation over a greater flow rate and sample size range. This technique has been applied to anthracite, bituminous and subbituminous coals. For all three coal types, the macropore diffusivity (≈10^?5 s) was about two orders of magnitude higher than the micropore diffusivity. The anthracite adsorbs more methane in the micropores than the other coals. These results showed the expected trends taking the known change of pore structure with rank into account.     

Flow adsorption calorimetry of coals before and after heat and oxidation treatment

Flow microcalorimetry has been used to determine the heats of preferential adsorption of n-butanol and ammonium hydroxide on a number of coals immersed in n-heptane and water respectively. The determinations have also been carried out on the coals subjected to heating in air to temperatures ranging from 100 °C to 200 °C, to illustrate how the calorimetric technique can detect changes in the surface properties of the coals subjected to various treatments. The heats of adsorption have provided information on the relative surface acidities and hydrophobicities of the coals.     

Free-radical involvement in the drying and oxidation of victorian brown coal

The technique of electron spin resonance spectroscopy has been used to study the changes in free-radical nature and content of brown coals as a result of vacuum drying the coal at various temperatures, and exposing the dried coal to either air at room temperature or oxygen at 105 °C. Free-radical concentrations increased during the vacuum-drying process and this is thought to be a result of organic functional group breakdown. Free-radical concentrations in dry coals following air or oxygen exposure have been found to vary with exposure time. The observations can be explained in terms of free-radical involvement in the oxidation process.     

A reactivity study of chars obtained at different temperatures in relation to their petrographic characteristics

This study reports on the reactivity of chars obtained at 1000°C and 1300°C (within the range of temperatures reached by coal particles in the near-burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are high volatile bituminous coals differing in maceral composition and the third one is a vitrinite-rich low volatile bituminous coal.The morphology and optical texture of the chars were studied by optical microscopy. The kinetic parameters for the combustion of the high temperature chars under Regime I (combustion controlled by chemical kinetics) have been obtained and related to the optical texture and reflectance of the chars. The intrinsic reactivity of the high temperature chars was found to be lower than that of the low temperature chars, whereas the enhanced porosity observed in the high temperature chars had a positive effect on their combustion reactivity under Regime II (combustion controlled by oxygen pore diffusion). The intrinsic reactivities of the chars decreased following the sequence: vitrinite-rich low rank char>inertinite-rich char>vitrinite-rich high rank char. As the combustion temperature increases, the reactivity of the inertinite-rich char approaches that of the low rank vitrinite-rich char, which justifies the good performance observed for high volatile bituminous inertinite-rich coals in power plants.     

Thermal and chemical desulfurization of low sulfur coals

This paper describes desulfurization characteristics of low sulfur coals prior to combustion and optimum conditions of three different desulfurization processes. These processes include two thermal treatment processes (mild pyrolysis and air oxidation) and an H₂O₂ leaching process. Dual processes composed of thermal and leaching processes were also evaluated. Low sulfur coals employed were two imported bituminous coals and two domestic anthracite coals. The optimum reaction temperatures and times of the thermal processes were 500–550 °C and 15–20 minutes, respectively. The optimum condition for the leaching process was obtained when the experiment carried out for 60 min at 90 °C using 30% H₂O₂. The dual process showed the best sulfur removal efficiency as expected among the evaluated processes. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Kinetic parameters and activation criteria of the oxidative mechanical degradation of coals

The kinetic parameters of the oxidative mechanical degradation of various coals on treatment in various dispersers were determined. The rate constants of these reactions are the criteria of the mechanical activation of coals. Based on the experimental results, the effective temperatures of these processes were calculated.     

Diffusion of Argon in a Potassium-Lime-Silica Glass

Diffusion coefficients for bulk diffusion of argon in a potassium-lime-silica glass have been determined at temperatures in the range 308° to 470°C. by a radioactive counting technique. Argon diffusion was found to follow classical bulk diffusion theory within the limits of experimental accuracy. From a plot of log D versus 1000/ T , an activation energy of 42 kcal. per mole was obtained for the argon diffusion process. The activation energy and diffusion coefficients obtained for argon by the radioactive counting technique are compared with values for helium and argon obtained by other investigators by use of the permeation technique.     

Some aspects of the role of coal thermoplasticity and coke structure in coal gasification: 3. The effect of rank,pitch and sodium carbonate on Brabender plastometry parameters

The thermoplastic properties of coal are a significant factor in determining gasification behaviour in most gasification processes. This investigation has involved the use of constant shear rate (Brabender) plastometry to study the plastic properties of a wide range of coals varying from 101 to 902 in the NCB Classification. The results obtained show more complex behaviour during the carbonization process than reported previously and are compared with ASTM Gieseler plastometry and BS dilatometry results. Some coals have two peaks in the caking region of the plastometry curve. This has been attributed to an initial peak corresponding to softening of the coal followed by caking due to active decomposition. The phenomenon was observed for coals with ranks 301a to 602, the temperature difference between the peaks being largest for the prime coking coals. It has been shown that this technique produces much additional information about the initial phase of the carbonization process. The effect of pitch and sodium carbonate addition on the plastometry parameters has also been studied. The results show that small amounts of these additives can radically change the plastometry curves. Pitch increases the plastic range whereas sodium carbonate reduces it. The addition of sodium carbonate tends to modify the plastometry curves so that they are similar to coals of lower rank whereas pitch addition, in general, tends to increase fluidity and, in the case of low-rank coals, produces plastometry curves similar but not identical to higher-rank coals.     

Reactivity of chars from New Zealand coals

Equipment for measuring the reactivity of chars from highly reactive coals, based on recent Norwegian developments, has been assembled. Measuring techniques are described, and the results of measurement with a range of New Zealand coals are presented. The measured reactivities are compared with results obtained by using the German standard method for reactivity, and the comparison confirms the advantages of the Norwegian method for use with coals of high reactivity. By overseas standards, New Zealand's subbituminous coals have phenomenally high reactivities.     

In-Situ Evaluation of the Diffusion of Epoxy and Amine in Thermoplastic Polymers

The current study investigates the diffusion of a bisphenol A type epoxy resin and a diamine curing agent into polysulfone (PSU) using a Fourier Transform Infrared Spectroscopy-Attenuated Total Reflectance (FTIR-ATR) technique. The diffusion of epoxy or amine through the film was monitored in real time by measuring the changing intensity of selected characteristic absorbance bands in each of the polymers. The diffusion coefficients for the epoxy and amine were determined independently using the appropriate solution to the diffusion equation and the experimental data. Diffusivities were evaluated at several temperatures, and an Arrhenius relationship was observed. Results show that the diffusivity of amine is an order of magnitude greater than the diffusivity of epoxy at each temperature tested. Furthermore, the epoxy diffusion appears to be Fickian, while the diffusion of amine appears to be non-Fickian.     

水泥回转窑煤粉燃烧过程的数值模拟

为了研究水泥回转窑NOx浓度与温度的变化规律,对其内煤粉燃烧过程进行数值模拟,模拟结果与监测数据相对吻合.研究结果表明:当煤粉喷入量为一次风量的0.5～1.0倍时,水泥回转窑冷却段、煅烧段、过渡段、出口NOx浓度分别为0、100～1300 mg/m3、200～1300 mg/m3、100～1000 mg/m3,温度分别为1100～1300 K、1400～1900 K、1600～1750 K、1300～1700 K;煤粉喷射量为一次风量的0.6～0.8倍时,水泥回转窑内NOx浓度较低、温度较高,较为适合实际水泥生产工艺.     

Direct determination of organic sulphur in raw and chemically desulphurized coals

An electron beam technique has been developed for the direct determination of organic sulphur in coal, using a scanning electron microscope coupled with an energy-dispersive X-ray analysis system with quantitative data reduction capabilities. The direct measurement of organic sulphur is performed on a number of particles of the maceral vitrinite, previously located within the mounted coal sample by reflected light microscopy. The particles are first examined by SEM at ≈5000 × magnification to allow proper identification of most mineral inclusions and to avoid these areas during analysis. The X-rays generated from an area 3 μm in diameter are monitored quantitatively by EDX for sulphur and iron, as well as for calcium, aluminium and silicon. After ensuring that no other elements, indicating mineral matter, are present at the point of analysis, the sulphur value obtained is taken as a measure of the organic sulphur in coal. The results obtained by this technique also allow estimation of the intra- and inter-particle variation in the organic sulphur level. Statistical methods can be used to construct confidence bands about the mean sulphur levels and estimate standard deviations within and between particles. This permits evaluation of the precision of the analysis on a sample-by-sample basis and provides a measure of the heterogeneity of the coal. The SEM-EDX method has been applied to a large number of raw coals and coals chemically desulphurized by the Ames, PETC and Ledgemont oxydesulphurization processes, as well as the molten caustic (Gravimelt) process of TRW Systems Inc. The results of this direct technique agree fairly well with those of the ASTM procedure, for both raw and chemically processed coals, although for the latter the SEM values often show less desulphurization than the ASTM values.     

THE DIFFUSION COEFFlCIENT OF HEHEHP IN HEPTANE MEASURED BY THE MODIFIED STOKES CELL TECHNIQUE AT TEMPERATURES BETWEEN 25 AND 55°C.

The diffusion coefficient of HEHEHP in heptane has been measured by the modified Stokes cell technique at temperatures between 25 and 55^°C. The Stokes cell was calibrated by measuring acetic acid diffusion with the pH-stat technique. The average diffusion coefficient of HEHEHP in heptane was determined to be 0.97 × 10 ^?9 m²/s at 25 ^°C for HEHEHP concentrations of 0 - 0.1 kmol/m³. The diffusion coefficient did not vary significantly with HEHEHP concentration. The Wilke-Chane relationship was used to predict D values of 1.53 × lO^?9m²/s and 1.01 × 10 ^?9m²/s for the monomelic and dimeric species respectively. Comparison of the measured and predicted diffusion coefficients led to the conclusion that the HEHEHP was substantially dimenzed over the concentration range studied

An activation energy of 6.90 kJ/mol was calculated from the diffusion coefficient measurements at elevated temperature.