G. Skodras

Kinetic studies of elemental mercury adsorption in activated carbon fixed bed reactor

Activated carbons are suitable materials for Hg(0) adsorption in fixed bed operation or in injection process. The fixed bed tests provide good indication of activated carbons effectiveness and service lives, which depend on the rates of Hg(0) adsorption. In order to correlate fixed bed properties and operation conditions, with their adsorptive capacity and saturation time, Hg(0) adsorption tests were realized in a bench-scale unit, consisted of F400 activated carbon fixed bed reactor. Hg(0) adsorption tests were conducted at 50 degrees C, under 0.1 and 0.35 ng/cm(3) Hg(0) initial concentrations and with carbon particle sizes ranging between 75-106 and 150-250 microm. Based on the experimental breakthrough data, kinetic studies were performed to investigate the mechanism of adsorption and the rate controlling steps. Kinetic models evaluated include the Ficks intraparticle diffusion equation, the pseudo-first order model, the pseudo-second order model and Elovich kinetic equation. The obtained experimental results revealed that the increase in particle size resulted in significant decrease of breakthrough time and mercury adsorptive capacity, due to the enhanced internal diffusion limitations and smaller external mass transfer coefficients. Additionally, higher initial mercury concentrations resulted in increased breakthrough time and mercury uptake. From the kinetic studies results it was observed that all the examined models describes efficiently Hg(0) breakthrough curves, from breakpoint up to equilibrium time. The most accurate prediction of the experimental data was achieved by second order model, indicating that the chemisorption rate seems to be the controlling step in the procedure. However, the successful attempt to describe mercury uptake with Ficks diffusion model and the first order kinetic model, reveals that the adsorption mechanism studied was complex and followed both surface adsorption and particle diffusion.

Mineral matter effects in lignite gasification

Greek lignite samples with different mineral contents were gasified after drying, without any other treatment, in H2 and CO2 atmospheres, in order to study mineral matter effects on lignite gasification. Tests were performed in a fixed-bed reactor operating at ambient pressure. The alkali index (AI) was calculated for each lignite sample in an effort to correlate it with the gasification rate. The inorganic constituents in Greek lignite seem to play a controlling role in determining gasification reactivity. A reasonably good correlation exists between the alkali index and the gasification rate, which, in the studied region of alkali indices, varied almost linearly with the alkali index. For both H2 and CO2, gasification rate increased proportionately to Ca concentration, but such correlation was not evident for Na and K, possibly due to the low content and the chemical form of these elements in the organic structure. An initial increase and a subsequent decrease of gasification rate with Mg concentration was found for both H2 and CO2 gasification. As for the Fe, hydrogasification rate seems to be almost unaffected (a slight decrease is actually observed with Fe concentration), while no correlation was evident for CO2 gasification. D 2002 Elsevier Science B.V. All rights reserved.

Production of special activated carbon from lignite for environmental purposes

A treatment technique involving three sequential stages (demineralisation, activation and sulphur dispersion) was developed for the production of suitable activated carbons from Greek lignite. Demineralisation included three steps of acid treatment and samples received were characterised by X-ray diffraction (XRD). A two-stage activation procedure (pyrolysis under nitrogen, followed by activation under carbon dioxide atmosphere) was used for the production of activated samples. Sulphur impregnation of activated carbons was performed by heating with high purity elemental sulphur flakes under nitrogen flow at temperatures up to 600 jC. SEM and line scattering techniques were used to evaluate sulphur distribution in the impregnated activated carbons. Adsorption of N2 at 77 K and CO2 at 298 K was used for the characterisation of products. Sulphur impregnated activated carbon samples were proven unreactive and stable at the flue gases temperature. D 2002 Elsevier Science B.V. All rights reserved.

Production of carbon molecular sieves by plasma treated activated carbon fibers

Carbon molecular sieves (CMS) are valuable materials for the separation and purification of gas mixtures. In this work, plasma deposition was used aiming to the formation of pore constrictions, by narrowing the surface pore system of commercial activated carbon fibers (ACF). For this reason propylene/nitrogen or ethylene/nitrogen discharges of 80 and 120 W were used. The molecular sieving properties of the plasma treated ACF were evaluated by measuring the adsorption of CO2 and CH4. The CO2/CH4 selectivity was significantly improved and depended on plasma treatment conditions (discharge gas and power). The optimum CO2/CH4 selectivity (26) was observed for C2H4/N2 plasma treated ACF at 80 W. Sample scanning electron microscopy (SEM) analysis after plasma treatment revealed an external film formation and X-ray photoelectron spectroscopy (XPS) analysis showed the incorporation of nitrogen functional groups in the film, which probably interact with CO2, thereby altering CO2/CH4 selectivity. q 2003 Elsevier Ltd. All rights reserved.

Toxic emissions during co-combustion of biomass–waste wood–lignite blends in an industrial boiler

The objectives of this work were to study the PCDD/F emissions during the co-combustion of waste wood/coal co-combustion in an industrial boiler and to determine the relation of the toxic emissions to the fuel properties. Co-combustion experiments were performed in a 13.8 MWthermal industrial moving grate combustor. The fuels which were examined in this study included Greek lignite, natural uncontaminated wood, power poles and medium density fibers (MDFs) which were by-products of the plant production process. Fuel blends were prepared by mixing single components in various concentrations. PCDD/F emissions were collected during experimental runs and were analyzed according to standard methods. Low PCDD/F emissions were obtained during the co-combustion tests, lower than the limit value of 0.1 ng TEQ/Nm3. The lowest values were observed during the combustion of fuel blends containing MDF, possibly due to the inhibitory action of some of the N-containing MDF ingredients, such as urea. No direct correlation was found between the PCDD/F and the copper emissions, while examination of the PCDD/F homologue patterns revealed the predominance of the lower chlorinated isomers over the higher ones.

Thermal exploitation of wastes with lignite for energy production

Abstract The thermal exploitation of wastewood with Greek lignite was investigated by performing tests in a laboratory-scale fluidized bed reactor, a 1-MWth semi-industrial circulating fluidized bed combustor, and an industrial boiler. Blends of natural wood, demolition wood, railroad sleepers, medium-density fiberboard residues, and power poles with lignite were used, and the co-combustion efficiency and the effect of wastewood addition on the emitted pollutants were investigated. Carbon monoxide, sulfur dioxide, and oxides of nitrogen emissions were continuously monitored, and, during the industrial-scale tests, the toxic emissions (polychlorinated dibenzodioxins and dibenzofurans and heavy metals) were determined. Ash samples were analyzed for heavy metals in an inductively coupled plasma-atomic emission spectroscopy spectrophotometer. Problems were observed during the preparation of wastewood, because species embedded with different compounds, such as railway sleepers and demolition wood, were not easily treated. All wastewood blends were proven good fuels; co-combustion proceeded smoothly and homogeneous temperature and pressure profiles were obtained. Although some fluctuations were observed, low emissions of gaseous pollutants were obtained for all fuel blends. The metal element emissions (in the flue gases and the solid residues) were lower than the legislative limits. Therefore, wastewood co-combustion with lignite can be realized, provided that the fuel handling and preparation can be practically performed in large-scale installations.

Simulation of a molten bath gasifier by using a CFD code

In this work the simulation of a foaming molten slag gasification reactor was performed with a commercially available Computational Fluid Dynamics code. In the standard body of the code appropriate User-Defined Subroutines were devised and incorporated for the complete description of the process. The flow field and heat transfer equations were solved together with the simulation of the coal gasification phenomena. The model results are in good agreement with the respective experimental data, indicating the validity of the proposed model and thus providing a useful tool for the analysis of the molten slag gasifier operation.

ANALYTICAL AND NUMERICAL SOLUTIONS OF THE MASS CONTINUITY EQUATION IN THE LUMEN SIDE OF A HOLLOW-FIBER MEMBRANE CONTACTOR WITH LINEAR OR NONLINEAR BOUNDARY CONDITIONS

Mass transfer in fully developed laminar flow in hollow-fiber membrane contactors is encountered in a variety of many important applications, such as supported gas and liquid membranes, reverse osmosis, pervaporation, membrane reactors, and biological processes. In this article the complexity of the partial differential equation that describes the concentration profile in the lumen with the associated linear or nonlinear boundary conditions at the fiber wall is simplified by means of analytical and numerical methods using current computational tools. A comparison between the numerical and analytical solution for the linear case reveals the inadequacy of the latter for the evaluation of the lumen Sherwood numbers in the entrance region. For a nonconstant concentration of the diffusing component in the shell side an integro-differential boundary condition at the fiber wall arises, which was approximated by the Gauss–Jacobi orthogonal collocation method.

APPLICATION OF POLYMER MEMBRANE TECHNOLOGY IN COAL COMBUSTION PROCESSES

The energy efficiency and the environmental consequences of typical coal upgrading processes, such as combustion, depend to a large extent on the degree of gas separation, recovery, and recycle. Among the available methods used in chemical industry for a variety of gas separation tasks, the technology of polymer membranes offers several advantages such as low size, simplicity of operation and maintenance, compatibility, and use with a diversity of fuel sources. To examine the impact of membrane separation on coal upgrading processes, the Aspen Plus simulation software was used, in combination with developed membrane mathematical models. Energy analysis in coal combustion processes, where the main scope is CO2 removal, showed that very promising results can be attained. It is estimated that 95% of the emitted CO2 can be captured with a moderately low energy penalty (10%). This penalty can be further decreased if higher selectivity and/or permeability polymers can be developed.

Emissions during the Co-combustion of Lignite and Waste Wood in a Fluidised Bed Reactor

Co-combustion tests were performed in a lab-scale fluidised bed reactor, in order to define (a) the optimum percentage for substituting Greek lignite by waste wood, and (b) the operation conditions ensuring complete burnout of the fuel blends. Tests were performed at the experimental facility of the NTUA’s Steam Boilers and Thermal Plants Laboratory (NTUA-LSB). Pre-dried lignite, from Ptolemais reserve, and various waste wood species, i.e. uncontaminated wood, demolition timber and railway sleepers, were used to prepare the fuel blends. In all tests, the emissions in flue gases - CO, SO2, N2O, NOx, NO, NO2 and CXHY — were continuously monitored.