Modification of activated carbon fiber pore structure by coke deposition

The pore structure of activated carbons can be tailored to produce carbon molecular sieves (CMS), by vapour phase thermal decomposition of hydrocarbons. Thermal cracking of propylene at 700°C was used to deposit coke on activated carbon fibers. Pore structure development was studied by determining the adsorption isotherms of N2 at 77 K and CO2 at 298 K, while the molecular sieving properties were investigated by the kinetic uptake curves of CO2 and CH4 at room temperature. Coke deposition resulted in the decrease of surface area and the enhancement of molecular sieving properties during the early stages of hydrocarbon decomposition

CO2 Plasticization of Polyethersulfone/Polyimide Gas-Separation Membranes

This work reports the CO2 plasticization of gas-separation hollow-fiber membranes based on polyimide and polyethersulfone blends. The feed pressure effect on the permeance of pure gases (CO2, N2) and the separation performance of a gaseous mixture (CO2/N2, 55/45%) is examined. Contrary to dense membranes, the permeance of CO2 through ultrathin asymmetric fibers increases immediately with pressure resulting in pronounced apparent plasticization and reduction of the ideal CO2/N2 selectivity. However, no evidence of plasticization was observed when a CO2/N2, 55/45% mixture was fed to the hollow-fiber membranes. In all cases, CO2 permeance decreased with pressure, while that of N2 remained constant. Experimental results were validated by means of mathematical modeling. Membrane-separation performance was overestimated when pressure-independent permeabilities were used in the model, while pressure-dependent permeabilities, due to the overall effect of plasticization and competition phenomena, explained excellently, the obtained stage-cut and permeate purity

Study of Mercury Adsorption by Low-Cost Sorbents Using Kinetic Modeling

In order to make further progress in the field of reducing mercury emissions to the atmosphere, it is necessary to develop efficient and economically viable technologies. Low-cost solid sorbents are a candidate technology for mercury capture. However, kinetic models are required to predict the adsorption mechanism and to optimize the design of the process. In this study, several low-cost materials (biomass chars) were evaluated for the removal of gas-phase elemental mercury and kinetic studies were performed to investigate the mechanism of mercury adsorption. These kinetic studies were also used to predict the behavior of a fixed-bed column. The models applied were pseudo-first-order and pseudo-second-order equations, Fick’s intraparticle diffusion model, and the Yoon–Nelson model. The chars obtained from the gasification of plastic-paper waste demonstrated the best behavior for mercury capture because of their high Brunauer–Emmett–Teller surface area, large total pore volume (mainly micropore volume), and high chlorine content. The Yoon–Nelson model provided a better fitting for the samples with low mercury retention capacities, while in the case of the plastic-paper chars, all of the models provided relatively accurate predictions because their highly microporous structure retarded the internal diffusion process and their increased chlorine content enhanced chemisorption on their surface.The authors thank the project CTM2011-22921, the Energy Research Centre of the Netherlands (ECN) for supplying the chars employed in this study and the Spanish Research Council (CSIC) for awarding Ms. Aida Fuente-Cuesta a pre-doctoral fellowship and for financing her a stay at the Aristotle University of Thessaloniki (Greece).Peer reviewe

A thermochemical conversion study on the combustion of residue-derived fuels

Two different waste-derived by-products were examined and compared. Based on the thermogravimetric tests performed, it was proved that their decomposition occurs in two weight loss steps represented by two shoulders in the derivative thermogravimetric curves. The first shoulder is attributed to the devolatilisation of hemicellulose, cellulose and lignin and the second one to the plastic fraction of the waste. Similarities in the degradation behaviour were observed for both wastes, despite of their different origin. Increased plastic fractions resulted in slightly higher conversions and lower pyrolysis rates. Enhanced lignocellulosic fractions led to higher rates during combustion. The lignocellulosic fraction was increased proportionally to the inorganic residue that remained after combustion. A wide variation of weight losses was attained even in refuse-derived fuel (RDF) samples of the same origin, whilst stronger deviations were observed in the decomposition of the plastic fraction. The independent parallel, first-order, reactions model was elaborated for the kinetic analysis of the pyrolysis results. The thermal degradation of the RDF samples was modelled assuming four parallel reactions corresponding to the devolatilisation of cellulose, hemicellulose, lignin and plastics. Increased activation energies were calculated for the plastics fraction, whilst lignin presented the lowest contribution in the pyrolysis of the samples. Generally, both RDF samples presented similar kinetic constants despite their heterogeneity. © 2008 Springer Science+Business Media B.V

A thermochemical conversion study on the combustion of residue-derived fuels

Two different waste-derived by-products were examined and compared. Based on the thermogravimetric tests performed, it was proved that their decomposition occurs in two weight loss steps represented by two shoulders in the derivative thermogravimetric curves. The first shoulder is attributed to the devolatilisation of hemicellulose, cellulose and lignin and the second one to the plastic fraction of the waste. Similarities in the degradation behaviour were observed for both wastes, despite of their different origin. Increased plastic fractions resulted in slightly higher conversions and lower pyrolysis rates. Enhanced lignocellulosic fractions led to higher rates during combustion. The lignocellulosic fraction was increased proportionally to the inorganic residue that remained after combustion. A wide variation of weight losses was attained even in refuse-derived fuel (RDF) samples of the same origin, whilst stronger deviations were observed in the decomposition of the plastic fraction. The independent parallel, first-order, reactions model was elaborated for the kinetic analysis of the pyrolysis results. The thermal degradation of the RDF samples was modelled assuming four parallel reactions corresponding to the devolatilisation of cellulose, hemicellulose, lignin and plastics. Increased activation energies were calculated for the plastics fraction, whilst lignin presented the lowest contribution in the pyrolysis of the samples. Generally, both RDF samples presented similar kinetic constants despite their heterogeneity. © 2008 Springer Science+Business Media B.V

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 cocombustion in an industrial boiler and to determine the relation of the toxic emissions to the fuel properties. Cocombustion experiments were performed in a 13.8 MWthermal industrial moving grate combustor. The fuels which were examined in this study included Creek 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/Nm(3). 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

Plasma enhanced decomposition of propylene on activated carbon fibers

Activated carbons with uniform size micropores appear to have the highest potential for improvement of their molecular sieving properties by employing chemical vapor deposition techniques. In this work RF plasma was used to enhance propylene cracking and to examine its effect on the pore structure, chemical surface groups and selective CO2 adsorption of a commercial activated carbon fiber. Plasma treatment of carbon fibers was carried out under various experimental conditions. Raman, FTIR and XPS spectroscopy were used to evaluate the modifications on the fibers surface. SEM was also used for the observation of fibers surface before and after plasma treatment