Coal maceral separation scale up

Conference

Abstracts

conversion in coal concentrates containing larger proportions of vitrinites and liptinites. It is thus possible to envisage a coal cleaning programme which produces coal fractions to give high liquefaction yields with the remaining fractions being used for combustion or gasification on the same site. Present work being carried out at Nottingham is investigating the application ofcoal beneficiation to improve liquefaction yields, together with improvements in associated analytical techniques, especially for the more rapid petrographic analysis of coal. In this paper work on the density separation of different size fractions of two British coals will be described. In order to achieve any separation of maceral groups, the individual macerals need to be liberated. This is a function of the maceral associations in the coal and the degree of grinding. In these experiments it is not intended to carry out very fine grinding, but to evaluate the potential separation which can be achieved with moderate grinding and the use of standard beneficiation techniques. The coals are obtained uncrushed and, following air-drying, three separate samples are ground to -3.3 mm, - 1.0 mm and - 500 /cm. Each sample is then screened to produce up to eight similar size fractions, depending on the starting material, with a linal fraction of - 38 to + 20 urn in each case. Densitv seoarations are then carried out on selected fractions for each starting sample. The density separations are carried out using a dense-medium of aqueous sodium polytungstate

solution. On each sample a petrographic

and proximate analysis is

carried out. Liquefaction behaviour is assessed using tubing bomb experiments with both phenanthrene and hydrogenated anthracene oil as solvent. The phenanthrene is used because it is not a hydrogendonating solvent and will highlight significant differences in the relative digestions of the different fractions produced. To date, the separations for one coal are complete and the analytical results are being processed. REFERENCES

I 2 3

Moore, S. A. EEC Conference, Palermo, October 1990 Cronauer, D. C. and Swanson, A. J. Am. Chrm. So<. Dia. Furl Clwn~. Prrpr. 1991. 36, 61 Comolli, A. G. rr al. Am. Chem. SK. Die. Fuel Chem. Prrpr. 1991. 36, 75

Emission of toxic and fire hazardous gases from atmospheric stockpiled coal Samuel L. Grossman*t, Shoshana Davidi* and Haim Cohen*1 * R. Bloch Coal Research Center and Chemistry Department, Ben Gurion University, Beer Sheva, Israel t National Coal Supply Corp., PO Box 21253, Tel Aviv, Israel $ NRCN Chemistry Department, PO Box 9001, Beer Sheva, Israel When fresh coal is exposed to air it undergoes exothermic chemisorption of oxygen which is followed by formation of surface oxides and to some extent oxidation of the coal resulting also in the emission of various gases (the most prominent of which being CO,, H,O and CO). Large stockpiled bituminous coals that are stored for long periods may develop hot spots due to autogenous heating. In extreme cases spontaneous fires have been reported. The self heating process occurs if the heat produced by the exothermic processes (namely the chemisorption of oxygen and the oxidation of the coal to carbon dioxide) is not dissipated efficiently by the heat conducting processes within the coal pile. These processes may raise environmental and economical problems to coal consumers who transport and store large coal piles. Israel started to fire bituminous coals in utility plants 11 years ago. At present more than 5 million tons are consumed annually in two utility plants producing more than 2500MWh and the amount of consumed coal is expected to reach at least 9 million tons in the next decade. As the country has no coal resources of its own, the coal is imported in large ships. Upon arrival to the port the coal is unloaded and transferred via a conveyer to large stockpiles stored for long periods under open air. The delayed use of the atmospheric stored coal increases the costs and may contribute to environmental problems. Furthermore the initial temperature of the coal piles in Israel is relatively high > 25°C. In order to assess the storage behaviour of the piles a portable sampling unit which can sample gases and temperatures in a coal pile up to 6 m deep, without interfering with the coal conveying to the utility plant. The stockpiled coal was monitored for more than 12 months. The maximum temperature measured in a hot spot was 172’C at a depth of 1.2m. Carbon monoxide, low molecular weight

708

Fuel 1993

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hydrocarbons (up to C,) as well as molecular hydrogen have been detected. The results of the monitoring studies and the conclusions on how the different toxic and fire hazardous gases are distributed within the coal pile will be discussed in detail. The behaviour of different potential coals (to be used in the utility plants in Israel) and their oxidation resistance (in the temperature range 50-l 50°C) was studied in simulation experiments in the laboratory using small quantities of coal (l-100 g). The activation energies and the kinetics of reaction will be reported. It has been observed that emission of molecular hydrogen carbon monoxide. low molecular weight hydrocarbons and olefins accompany the oxidation process. The implications of coal storage in confined spaces (ships’ holds, coal mines, silos, etc.) upon the dangers of explosion (due to molecular hydrogen emissions) will also be discussed.

Mineral effects on the high- and low-temperature Beulah Zap lignite char

reactivity of

R. F. Cope, 7: H. Fletcher and W C. Hecker Department

Combustion University, Mineral

of Chemical Engineering Provo,

UT

Engineering and Advanced Research Center, Brigham Young 84602,

USA

catalysis

can have a significant impact on the low-temperature rates of low rank coal chars, but its significance on high-temperature (2 1400 K) oxidation rates is less certain. This study examines the significance of mineral catalysis, at both high and low particle temperatures, in the oxidation of a lignite char with and without potentially catalytic metal ions. The 63-75 pm size fraction of a pulverized Beulah Zap lignite was devolatilized in a CH, flat-flame (50ms residence time; -4% post flame oxygen). After devolatilization (65% mass loss), a portion of the char was washed with HCI to remove much of the mineral fraction including the alkali and alkaline earth metals. Some of the HCl-washed char was subsequently loaded with Ca by ion-exchange with calcium acetate. The untreated, washed and Ca-loaded chars were then oxidized to various burn-out levels (lo-90%) and particle temperatures (1500~1800 K) in a heated-wall drop-tube reactor (3% 0,). An optical pyrometer measured the in situ temperature, velocity and diameter of the reacting char particles as they passed through the drop-tube reactor. A water-cooled, N,-quenched probe collected partially reacted char samples at the various particle burn-out levels. Particle mass loss and residence time were used to determine high-temperature char oxidation rates in the drop-tube reactor. Early oxidation results show that the untreated char experienced some densification, and under some conditions used in the study, burned near the film-diffusion limit. Intrinsic kinetic rates and rate parameters (A, and E,) were obtained by removing diffusion effects from those high-temperature results that were not film-diffusion limited. Particle samples collected in the drop-tube reactor were also analysed in a thermogravimetric analyser (10% O,, 80 K min-’ heating rate) to determine low-temperature (65&750 K) intrinsic oxidation rates and rate parameters (A, and E,). Changes in these parameters, as a function of oxidation burn-out level, were also investigated. The significance of mineral catalysis at both temperature levels was examined by comparing the intrinsic kinetics obtained from TGA measurements with those obtained from high-temperature rate measurements in the drop-tube reactor.

(< 750 K) oxidation

Coal maceral separation scale up J. C. Crelling, E. J. Hippo*, D. Tandon* and M. Blankenship* Departments of Geology and * Mechanical Engineering and Energy Processes, Southern Illinois University, Carbondale, IL 62901. USA Coal is a heterogeneous material and every coal utilization process is strongly affected by this heterogeneity. Mineral subcomponent of coal not only corrode/erode and foul combustion equipment, trace elements and sulfur also pollute the environment. Combustion, pyrolysis and coking processes are affected by the organic subcomponents (macerals) composition of the coal. Inertinite macerals lower the liquefaction yields. Because of the heterogeneity (presence of various minerals and maceral subcomponents) coal cannot be used in the diesel engines,

Conference

Abstracts

MHD systems, and direct fired gas turbines. Reduced heterogeneity should improve operations, increase process efficiencies, and expand application of more environmentally acceptable coal feedstocks. Although quite a few sulfur removal and separation processes have been developed, the cleaned product has a low value added to it because the only product considered is combustion feedstock, which makes the economics of most processes unfavourable. In 1990 researchers at SIU at Carbondale began exploring a new concept in coal processing termed ‘Coal Refining’. Coal refining is the separation of coal into macerals each of which may be used as a high quality fuel or a premium chemical feedstock. Until recently availability of pure maceral fractions were very limited. Pure macerals were available at l-10 g level and required 2 man-years of technician time to produce. Macerals have been found to inhibit desulfurization under both pyrolysis and supercritical methanol extraction. Maceral interactions influence combustion profiles, char yields and elemental analysis. This paper will discuss technique developed and results obtained from large scale maceral separation.

changes and revisions in the design. These were incorporated in a modified test facility that allowed ‘routine’ testing. Results of a small number of tests, conducted on South African coal, are reported here. Due to the preliminary nature of the results they may only be considered as an indication of what may be expected. Much work must still be done before quantitative conclusions may be drawn.

Techniques were developed to demineralize coal on a large scale using hydrochloric and hydrofluoric acids. A semi-continuous centrifuge (40OOOrevmin~‘) was employed to concentrate macerals. Cesium chloride was used to make coal slurry density. Large samples of

A reaction scheme consisting of three independent parallel reactions has been used to describe the pyrolysis reactions of volatile matter of coal in a fixed bed reactor, in the temperature range 700-900°C. A simple first-order kinetic model has been applied to the evolution of liquid and gases based on the distribution of carbon in the various products. The model predicts the experimental data within reasonable accuracy. Activation energies of 43.75, 55.55 and 25.00 kcal mol-’ for the non-catalytic coal and 33.33, 37.50 and 18.75 kcal mol-’ for the catalytic coal have been estimated for the evolution of gases, liquid and char, respectively.

demineralized coal were processed at five specific gravities (1.22, 1.25, 1.31, 1.35 and 1.65). Recoveries on the mass balances were over 90%. Based on the petrography, 100% of inertinite and 99% of vitrinite was recovered. Liptinite recovery was low and the loss appeared preferential. This may be due to liptinite maceral sticking to the walls of the filter funnel during filtration and sticking to the walls of the oven during drying. The sink at 1.65 specific gravity was nearly 100% pyrite with a small amount of unliberated organics in some particles. Although these fractions are not pure, these maceral sub-groups can be purified by DGC processing. Using this process it is possible to obtain lOtX2000 g of pure maceral with only 3 man-months of effort. This reduces time requirements by 87% and increases yield by 200 times. Thus, cost per gram of pure maceral has been reduced from -$5000 to $100.

Liquid fuels from coal in conjunction with power plant operation J. Dayan, A. Shauit and C. Gutjinger Department of Mechanical Engineering, Technion Institute of Technology, Haifa 32000, Israel

- Israel

The feasibility of incorporating liquid fuel production with a coal burning power plant is considered. Coal-fired power plants could produce liquid fuels at rates of - l&15% of their coal consumption by low temperature pre-pyrolysis of the coal, without compromising the operation of the power plant. This may become an important new source for liquid fuel and chemical feedstock. The larger consumption of coal may be offset by the higher economic and strategic value of the liquids. The amount of liquids and their quality as a source for fuels and chemical feedstock as well as the combustion properties of the char are important parameters of such a scheme. The process also affects the distribution of the sulfur compounds between the various streams, It may provide a way to reduce emission of sulfur compounds to the environment. The use of a fluidized bed combustor in a power plant reduces also the emission of NO, as compared to other types of combustion systems. The main goal of this research was to evaluate this parameter as a function of the operating conditions. The research was carried out in two stages. First, a long residence time batch partial pyrolysis was used. Two types of coals, commonly used in Israeli power plants, South African and Australian coals, were pyrolysed at temperatures ranging between 400°C and 600°C. The yield and the properties of the liquids and gases under long residence time were studied. Since long residence times may cause cracking of the liquids and reduce its yield, the results, therefore, reflect a lower limit to the liquid yield. Preliminary results of the laboratory bench scale study on partial pyrolysis of coal are presented. They show that, in general, the yield increases with pyrolysis temperature in most of the temperature range considered. The yield from the Australian coal dropped only slightly for pyrolysis temperature above 550°C indicating that secondary decomposition or cracking of the products at the relatively low pyrolysis temperatures, although present, was not very pronounced. Second, a continuous process fluidized bed pyrolysis pilot plant was designed and constructed. Monitoring and control of the system is accomplished with the aid of a commercial computer software (Paragon). Preliminary tests were carried out to determine the feasibility of partial pyrolysis of coal in a single-stage fluidized bed. Several problems were encountered in the operation of the plant that required

Kinetics of coal and catalytic coal pyrolysis reactions in a fixed bed reactor P. K. Dey

Department of Chemical Engineering, Centre of Energy Studies, Indian Institute of Technology, Haus Khas, New Delhi-l 10016. India

Kinetics of coal/catalyst-treated coal pyrolysis reactions in a fixed bed reactor under sweep gas flow conditions P. K. Dey

Department

Engineering,

Centre

Studies, Indian Institute of Technology, New Delhi-l 10016, India

of Chemical

Haus

for Energy Khas,

A kinetic reaction model based on a reaction mechanism consisting of three independent parallel reactions has been developed to describe the ovrolvsis reactions of volatile matter of coal in a fixed bed reactor in ;he temperature range 70@9OOC under sweep gas flow conditions. A zero-order kinetic model has been used for the determination of appropriate kinetic parameters based on the carbon content of the products. The mode1 was found to be sufficiently accurate in interpreting the yield data. Activation energies for the evolution of gases, liquid and char were estimated and found to be 27.77, 28.57 and 9.09 kcal mol 1 for non-catalytic coal and 27.27, 27.77 and 14.28 kcal mol-’ for calcium-catalysed coal, respectively.

Kinetics of calcium-catalysed carbon dioxide and steam

coal gasification in a mixture of

P. K. Dey

Department of Chemical Engineering, Centre for Energy Studies, Indian Institute of Technology, Haus Khas, New Delhi- I 10016, India The kinetics of reactions involved in the gasification of calciumcatalysed coal with the carbon dioxide-steam mixed gasifying agent in a small fixed bed reactor has been studied in the temperature range 75&9OO”C. A reaction scheme consisting of three-step gasification reactions has been used to describe the gasification reaction processes based on the distribution of carbon in the various products. In the first step, coal material pyrolyses to yield volatiles (liquid and gases). In the second step, the liquid products of the volatile components undergo reactions with steam and carbon dioxide simultaneously. Then the reaction of char with the mixed gasifying agent takes place in the third step. The gasification rates of coal material with the steamcarbon dioxide mixture can be predicted using the rate constants obtained from separate experiments, A first-order kinetic model for the coal-steam reaction and a zero-order kinetic mode1 for coal-carbon dioxide reactions have been applied to obtain the kinetic parameters. The mode1 has been proved to be adequate for the predictions of experimental results. The desired ratio of CO/H, ~0.66 in the product

Fuel 1993

Volume

72 Number

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