Greek Lignite Research Articles

Thermodynamic simulations of lignite-fired IGCC with in situ desulfurization and CO 2 capture

IGCC cycles running on Greek lignite with in situ desulfurization and CO 2 capture were simulated using the ASPEN PLUS process simulator and compared with other power cycles. Dolomite was chosen as the material for in situ desulfurization. Considering the power produced and residual sulfur content of the off-gas, an oxygen-blown gasifier operated at 1.8 MPa and 950°C, having a thermal efficiency of 38.9%, gives the best performance. The efficiency loss due to CO 2 capture can be largely compensated using IGCC cycles.

96/04855 The effect of mineral matter and pyrolysis conditions on the gasification of Greek lignite by carbon dioxide

96/04855 The effect of mineral matter and pyrolysis conditions on the gasification of Greek lignite by carbon dioxide

The effect of mineral matter and pyrolysis conditions on the gasification of Greek lignite by carbon dioxide

Coal specimens with different mineral matter contents were produced from Greek lignite using various acid treatment conditions. Ash content and chemical composition of mineral matter depended on the type of acid used and the sequence of treatment stages. Gasification rates of coals were investigated by thermogravimetric analysis in a carbon dioxide atmosphere in the temperature range 700–900°C. The combined effects of inorganic constituents and carbonization conditions such as heating rate and final temperature were determined. Gasification rates of chars with high ash content were higher than those of similarly prepared char with low ash content, due to the presence of catalytically active inorganic constituents. An almost proportional increase of gasification rate with Mg concentration was found, but such correlation was not evident for Ca, Na and K, possibly due to the chemical form of these elements in the organic structure. Slow carbonization led to the production of chars with higher reaction rates than those of chars prepared by rapid carbonization. The gasification rate increased with the concentration of CO 2 in the reaction gases. The effects of heating rate and CO 2 concentration on char gasification rate were more pronounced for samples from untreated lignite than for those from acid-washed lignite.

Pollutant emissions from circulating atmospheric fluidised beds burning low-grade coal

More than 70% of the total electricity production in Greece is based on the utilisation of soft brown coal of medium ash content ( 15%), high moisture ( 58%) and low calorific value (4.5-5 MJ kg −1 ). The increasing demand (twice the EC average) for electrical power imposes the need for the best exploitation of the existing brown-coal surface mines, together with new ones producing solid fuel of the same or even lower quality. This fact, combined with the high cost of the DeNO x /DeSO 2 facilities required when the conventional pulverised-fuel combustion technology is applied, implies a need for investigating the applicability to the particular coal quality of alternative coal-combustion technologies such as fluidised-bed combustion. This paper presents the results of combustion tests on Greek lignite in two CFBC (circulating fluidised bed combustor) facilities. The first, a laboratory-scale facility of approximately 100 kW thermal power, is at NTUA (the National Technical University of Athens); the second, which is of semi-industrial scale (1.2 MW thermal power), is at RWE's power station at Niederaussem in Germany. The two installations are of different size and different design, and the concepts represent the major CFBC construction trends: the Niederaussem facility operates with a thermally insulated («hot») cyclone, whereas the CFBC in Athens features a non-insulated («cold») cyclone. The results reported in this paper include determination of the operating conditions to achieve proper fuel burnout, examination of the influence of air-staging on the temperature distribution inside the reactor, and investigation of the combustion behaviour of the particular fuel type in relation to the size of fuel particles. As regards pollutant emissions, the CO, SO 2 , NO x and N 2 O emissions are recorded during various test cases for both facilities. Particular interest applies to the examination of additives of various types in order to obtain the maximum SO 2 retention inside the reactor, because the fuel that was used has a high sulphur content (1.5%). Several conclusions are drawn concerning the necessary modifications and requirements of the plant layout when a large-scale CFBC installation is designed to burn low-grade brown coal. The air-staging, furnace cooling, reactor height and cyclone type are considered the critical parameters for proper ignition and burnout conditions. Note, however, that the results in this paper are not exclusively of industrial interest: they also include documented experimental data from various test cases, which can be used for the evaluation of computational models aiming to simulate the combustion of coal in CFBCs

Coals of Greece: distribution, quality and reserves

Abstract Greek coals occur in a number of sedimentary basins and range in age from Eocene to Quaternary. The petrographic data indicate a wide variation in petrographic and chemical composition. The rank ranges from the transition zone peat-lignite to subbituminous. There are no deposits of hard coal in the country. Lignite constitutes the most abundant type of coal in Greece and the most important of the Greek lignite deposits formed during the Pliocene and Pleistocene in shallow lakes and marshes of closed intramontaine basins (Ptolemais, Florina, Drama in Macedonia, northern Greece and Megalopolis in Peloponnesus, southern Greece). The proved lignite reserves are currently estimated at 6750 Mt, excluding the 4300 Mm 3 of ‘Phillipi’ peat in Macedonia. There, 58% (about 3900 Mt) is considered to be economically recoverable. The probable and possible reserves are estimated to be of the order of 4000 Mt. The Kozane-Ptolemais-Amynteo-Florina basins in Macedonia contain most (about 64%) of the nation’s coal resources. These lignites, which are all already being exploited, have a very low calorific value (at Ptolemais-Amynteo, 1400 kcal kg −1 ; at Megalopolis, 900 kcal kg −1 ) and high ash and low sulphur contents. The lignite production for 1992 was over 54 Mt. the greatest centres of lignite production are in Macedonia, at the opencast mines of Ptolemais and Amynteo, and in Peloponnesus, at the opencast mine of Megalopolis. The vast majority (98%) of the extracted lignite is used for electricity generation and feeds power plants which have a total capacity of 4533 MW. The lignite-based power plants account for more than 720f the total electricity generation of the country. Today, through detailed geological exploration and evaluation, efforts are being made to locate and develop other lignite deposits throughout Greece.

Acid treatment of lignite and its effect on activation

Coal specimens with different contents of inorganic constituents were produced from Greek lignite under various acid treatment conditions. Activated carbons were produced with heat treatment of the raw and acid-treated lignites at 900°C. Characterization of the activated carbons was accomplished by adsorption of N 2 at 77 K and CO 2 at 298 K. Adsorption isotherms were analysed by the Dubinin-Radushkevich method to give an assessment of the micropore volumes. The ash content of the lignites depended upon the type of the acid used and the treatment stages. Removal of the catalytically active inorganic constituents brought about a lowering in the reactivity of the acid-treated coals. Complete demineralization resulted in activated carbons with a highly developed microporosity and very large pore volumes. However, activated carbons from lignites with medium ash content presented values of pore volume lower than the respective values of the above samples, yet exceeded the corresponding volumes of carbons from untreated lignite.

The prediction of pulverized greek lignite combustion in axisymmetric furnaces

A general numerical method for calculating pulverized coal combustion in axisymmetric geometries is utilized, aiming at the optimization of combustion model constants for Greek lignite. The two-equation, “ k-ϵ” turbulence model is used for the gas phase, whereas a stochastic approach, based on the lagrangian technique, is used for the particulate phase. The method employs a “constant devolatilization rate” model for both the volatiles and the moisture of the fuel, a char combustion model, a relation of the “eddy-dissipation” family for the reaction rate of the gaseous mixture, and the “nonequilibrium diffusion” radiation model. The maximum permissible rate of devolatilization and the preexponential factor of the char oxidation reaction rate are optimized in order to match available experimental data for Greek lignite; these parameters were found to influence considerably, albeit in a different fashion, the combustion process in cylindrical furnaces operating on pulverized lignite. Finally, the liftoff characteristics of the lignite flame were also predicted using a temperature criterions, as well as a flame stability model.

The Effect of Activated Carbon Properties on the Adsorption of Toxic Substances

The objectives of this work were to relate the activated carbon properties to its adsorptive capacity. The activated carbon needed was produced in the lab from Greek lignite coal. Subsequently, adsorption studies were performed in order to evaluate the efficiency of the various activated carbons to remove toxic substances from water. Two organic substances were used. These were phenol and fulvic acid. Additionally, the adsorption of arsenic (V) was, also, investigated. It was found that the adsorptive capacity of the activated carbons depended primarily on the ash content and the compound. The capacity of the carbon to remove phenol, expressed as mg of phenol removed per g of activated carbon (carbon loading), decreased linearly as the amount of ash in the activated carbon increased. Ash-free activated carbons could adsorb 4 times as much phenol as the activated carbons with a high ash content. On the other hand, fulvic acid and arsenic adsorbed poorly on the ash-free activated carbons. Even for the high surface area activated carbons (over 1000 m2/g), the quantity of fulvic acid or arsenic adsorbed was significantly less than that exhibited by the high ash activated carbons (maximum surface area measured hardly exceeded 300 m2/g). As the amount of ash in the carbon increased, the carbon loading increased as well, up to a certain level, beyond which the amount of ash played no significant role. The beneficial role of ash was explained by the ability of the fulvic acid and arsenic to interact with metal oxides and metal ions, which constitute a significant fraction of the ash.

Flash pyrolysis of Greek lignite

A study of flash pyrolysis of Greek lignite was carried out using the temperature range from 550 to 950°C in a captive sample reactor in order to determine the yield distribution of the various products. Residence times were in the range of 0.1–1.0 s while heating rates were estimated at about 300°C/s, for lignite particle diameters of 90–125, 125–150 and 150–300 μm. Experiments for the determination of the total weight loss were also carried out in the temperature range from 100 to 1000°C. Tar yields decrease with temperature. Liquid hydrocarbon yields also decrease slowly with temperature while total gas yields increase with temperature. The gases produced from flash pyrolysis of Greek lignite are dominated by oxygen compounds (CO, CO 2). The kinetics of total weight loss was modeled with the Anthony and Howard model.

Stability of pulverized coal burners

Stability limits of pulverized fuels, burned with the support of natural gas in quarl burners with swirling secondary air, are quantified. The potential heat release of the solid fuels was up to about 14 kW while the gas support was reduced, typically to less than about 12 kW, to find the extinction limits as a function of the swirl number, quarl length, secondary average axial velocity, downstream confinement and for Welsh anthracite, English bituminous coal and Greek lignite (the latter with 12% moisture). In unconfined operation, attached and lifted flames were produced at swirl numbers between 0.3 and 0.6, depending on the quarl length and the air velocity. The bituminous coal provided the broadest stability limits: dried lignite gave comparable lean stability limits but also introduced rich stability limits as the solid feed rate was increased. In confined operation, common rich stability limits were found for attached flames of all coal types: the lean limits were also common for swirl numbers less than than about 0.7. As lean extinction of confined bituminous-fired flames was approached, attached flames were followed by extensive lifted regions. As burner confinement increased, attached flames were observed, for all coal types, at lower values of swirl numbers.

Product distribution and kinetic predictions of Greek lignite pyrolysis

This paper aims to determine the distribution of all species produced from Greek lignite under fast pyrolysis and to derive an appropriate kinetic model. A fluidized bed reactor system was used with sand as the heat carrier. For the product distribution determination, experiments were carried out over a temperature range of 500–850 °C. The yield distribution was determined for the following main volatile products: CO, CO 2, CH 4, H 2, C 2H 4 and HC (all other hydrocarbons). The kinetic model of Anthony and Howard is used, as a basis, to predict the experimental evolution curves.

Evaluation and kinetic modelling of Greek lignites

Two different types of Greek lignite were evaluated using the fast pyrolysis process in a fluidized bed reactor. The innovation in this experimental system is that the reactor effluent is connected on line with a flame ionization detector (FID) and a thermal conductivity (TC) detector for the continuous monitoring of total hydrocarbons and total gases, respectively. The total yield distribution of the various products was obtained as a function of pyrolysis temperature and lignite particle size. Two kinetic models for coal decomposition were also tested for the determination of appropriate kinetic parameters. From this work it can be concluded that: the gases produced from pyrolysis of Greek lignites are dominated by oxygen compounds (CO 2, CO); a number of hydrocarbons are also produced (C 1-C 6); the experimental results correlate well with the competing reaction model.

Effect of catalyst impregnation conditions on Greek lignite hydrogasification

Hydrogasification of Greek lignite impregnated with potassium salts and metal chlorides was studied isothermally, between 750 and 900 °C, in a fixed bed reactor, in correlation with the impregnation conditions (pH, molarity, temperature). Potassium carbonate favours light hydrocarbon formation. Lowering of pH of impregnation significantly decreases potassium chemisorption on the lignite matrix and is followed by a drastic decrease in methane production and carbon conversion during hydrogasification. Initial porosity and surface area decrease with catalyst impregnation and their development during hydrogasification is suppressed.

Effects of drying upon lignite macro-pore structure

The effect of drying upon the macro- and partly meso-pore structure of Greek lignite was investigated. Drying of lignite lumps (∼d p : +3 – 15 mm) placed on a fixed bed arrangement, was carried out under vacuum (∼0.2 Torr) in a constant-temperature (100 – 250 °C) environment, for varying drying times (5 – 180 min). Drying caused a considerable particle contraction (by ∼1/3 of its original size), a minor decrease in the macro- and partly meso-pore volume and a marked increase in the relevant surface area, particularly in the range of high weight losses (35 – 40%). Residual moisture contents ranged from ∼0% to 42%, specific pore volume fell in the range 0.20 – 0.25 cm 3/g (dry sample), and specific surface area in the range 9 – 17 m 2/g (dry sample). Pore volume and surface area frequency distributions, determined from mercury porosimetry (MP) measurements done on raw or partially dried samples, clearly indicate the formation of pores in the size range D p : + 75 – 1500 Å at the expense of pores in the range D p : +1500 – 10 4 Å. This observation might be the result of pore shrinkage and pore emptying due to the counteraction of particle contraction and moisture removal respectively. Mercury penetration — retraction hysteresis was evident in every MP run. Mercury entrapment drop (from 85% to 38%) varied with residual moisture content, an indication that drying favours the formation of a more regular and less complex physical pore structure.

Megalopolis lignite drying kinetics based on TGA experimental results

Lignite drying kinetic data are of critical importance for the design of drying and occasionally gasification process equipment because of the particularly high (55–60%) moisture content held in the lignite particles. A lumped kinetic model, R d( t) = η k d( T)( W * - W( t)) n d( T) , describing the dependence of the drying rate upon the residual moisture content (dry basis) of lignite single-particle systems is proposed in this paper. The mathematical analysis is based on data obtained from TGA experimental studies carried out on lumps or on samples of pulverized Greek (Megalopolis) lignite in a nitrogen atmosphere under transient temperature conditions. (A rate of temperature rise 20°C/min, up to 100°C, was applied.) The temperature dependence of both the relevant rate constant and the kinetic order was determined by the best fit of the theoretical model to the relevant experimental data. Megalopolis lignite drying can be described satisfactorily by the kinetic equation ▪ η = f( d p) and T = f′( t) are described graphically in the text.