Abstract: Nanofilamentous carbon was grown on a carbon foam by catalytic chemical vapour deposition (CVD) using the decomposition of ethylene/hydrogen mixtures over Ni. The carbon foam was obtained from a coal by a two-stage thermal process, with the first stage taking place at a temperature within the plastic region of the precursor coal. The extent of porosity and the pore size of the foam were mainly influenced by the pressure reached in the reactor during the first stage. In the CVD process, 700 degrees C was the optimum temperature for obtaining good yields of nanofilaments. A low ethylene/hydrogen ratio (1/4) in the reactive gas gave rise to almost only short and thin carbon nanostructures. A higher proportion of C2H4 (4/1, C2H4/H-2) gave better yields of nanofilaments, with good proportions of higher-length and higher-diameter. (up to around 0.5 mu m) structures. Among the carbon forms produced, transmission electron microscopy revealed the predominance of fishbone-type nanofibres, with some bamboo-like nanotubes being also observed. (c) 2008 Elsevier Ltd. All rights reserved.
Abstract: The pyrolysis of glycerol was carried out over carbonaceous catalysts to produce synthesis gas (up to 81 vol.%). The catalytic effect of two activated carbons was tested and compared with quartz glass chips as packing materials in a fixed-bed reactor. The main advantage of using a carbonaceous catalyst seems to be its higher selectivity toward hydrogen, resulting in a synthesis gas with a greater H-2/CO ratio. The influence of the heating method (electrical furnace and microwave oven) and temperature (from 400 to 900 degrees C) on the pyrolysis of glycerol over a selected activated carbon was also studied. As a result, an elevated gas fraction with a higher H-2 + CO Composition was obtained under microwave heating, even at low temperatures. This proves that microwave heating could be used to upgrade the industrial surplus generated from bio-diesel production. (c) 2009 Elsevier B.V. All rights reserved.
Abstract: The effect of nickel distribution and content in Ni-doped carbon nanospheres on hydrogen storage capacity under conditions of moderate temperature and pressure was studied. It was found that the nickel distribution, obtained by using different doping techniques and conditions, has a noticeable influence on hydrogen storage capacity. The samples with the most homogeneous nickel distribution, obtained by pre-oxidising the carbon nanospheres, displayed the highest storage capacity. In addition, storage capacity is influenced by the amount of nickel. it was found a higher storage capacity in samples containing 5 wt.% of Ni. This is due to the greater interactions between the nickel and the support that produce a higher activation of the solid through a spillover effect. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Abstract: The possibility to improve the electrochemical behaviour of AB(5)-alloy commercial electrodes was studied using different carbons as support, such as carbon blacks and a selection of commercial and in-lab synthesised carbon nanotubes. The carbons selected for this work present different morphologies (i.e., spherical and tubular). Furthermore, they also present a variation of the porous structure and subsequent surface area, which are also going to influence their further electrochemical behaviour. Carbon samples were texturally characterised by the adsorption-desorption of N-2 and CO2 at 77 and 273 K, respectively. The carbon structure was analysed by XRD, Raman, TEM, and the chemistry of the samples was also characterised by elemental analysis. The charge and discharge techniques, cyclic voltammetry, rate capability and linear polarisation were used for the electrochemical characterisation of the electrodes studied. The electrochemical behaviour of all the samples was related to their morphological, textural and chemical properties. The results show that there is a clear influence of the nature of the carbon support on the hydriding/dehydriding reaction of the alloy, and in the case of active carbons the kinetics decreases with the increase of the surface area of the carbon support. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: In classic carbon supports is very difficult to control pore size, pore size distribution, and surface chemical properties at the same time. In this work microporous carbons derived from furfuryl alcohol are used as support to prepare Ni-doped carbon materials. The N-2 flow rate used during the carbonisation process of the precursor influences on the size of the nanospheres obtained but not in their textural properties. Microporous carbon nanospheres have been synthesised with a narrow pore size distribution centred in 5.5 angstrom. The surface chemistry of these materials can be easily modified by different treatments without detriment of the pore structure of the doped carbon nanospheres. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: In this work a series of solid sorbents were synthesized by immobilizing liquid amines on the surface of a mesoporous alumina. The samples were chemically characterized and BET surface areas calculated from the N-2 adsorption isotherms at 77 K. The CO2 capture performance of the sorbents and their thermal stability was studied by thermogravimetric methods. The effect of amine loading on the CO2 capture performance of the prepared sorbents was also evaluated. Analysis of TG-DTG curves showed that thermal stabilization of the amines is significantly improved by immobilizing them on an inorganic support. Temperature-programmed CO2 adsorption tests from 298 K up to 373 K at atmospheric pressure, proved to be a useful technique for assessing the capacity of sorbents for CO2 capture. Alumina impregnated with diethylenetriamine presented the highest CO2 adsorption capacities throughout the tested temperature range.
Abstract: The hydrophobic-hydrophilic character of a series of microporous activated carbons was explored as a key factor in competitive adsorption of a non-polar compound from liquid phase. The selectivity of the carbon surface towards naphthalene was explored by performing the adsorption isotherms in water, cyclohexane and heptane. Solvent polarity and adsorbent hydrophobic character were found to strongly influence the adsorption capacity of naphthalene. In aqueous media, despite the non-polar character of the adsorbate, surface acidity lowered adsorption capacity. This is attributed to the competition of water from the adsorption sites, via H-bonding with surface functionalities and the formation of hydration clusters that reduce the accessibility and affinity of naphthalene to the inner pore structure. In organic media the uptake decreased due to competition of the hydrophobic solvent for the active sites of the carbon and to solvation effects. This competitive effect of the solvent is minimized in oxidized carbons as opposed to the trend obtained in aqueous solutions. The results confirmed that although adsorption of naphthalene strongly depends on the narrow microporosity of the adsorbent, competitive adsorption of the solvent for the active sites becomes important.
Abstract: Textural properties of carbon gels can be controlled by varying the synthesis and drying process conditions. In this work, the influence of the initial pH and the drying method on the final properties of carbon gels, synthesized using methanol as a solvent, was evaluated. Furthermore, the use of microwaves as a drying method for the synthesis of carbon xerogels was assessed. In the light of the results obtained, in order to synthesize monolithic and microporous carbon gels in a short period of time, the use of a multimode microwave oven is proposed. The use of pH 7 also leads to shorter gelation times and more consistent monoliths. Furthermore, the multimode microwave drying can produce homogeneous microporosity and surface areas of up to 1341 M-2 g(-1), in a very short time (i.e., only 6 min is required for the drying step). (C) 2007 Elsevier B.V. All rights reserved.
Abstract: In this work a series of commercial carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. The highest H-2 storage capacity at 298 K and 90 bar was 0.5 wt%, while at 77 K and atmospheric pressure it was 2.9 wt%. It is also showed that, in order to predict the hydrogen storage capacity of carbon material both at cryogenic and ambient temperature, the only use of BET surface area or total micropore volume obtained from N-2 adsorption isotherm may be insufficient, the characterization of the narrow microporosity is needed due to its high contribution to hydrogen adsorption capacity. The process involved in hydrogen storage in pure carbon materials seems to be physisorption. Morphological or structural characteristics have no influence, at least on gravimetric storage capacity.
Abstract: The aim of this work was to study the growth of nanofilaments on two carbon-based materials (i.e. an activated carbon and carbon fibres) from microwave-assisted decomposition of CH4. It was found that the nature of grown nanofilaments differed depending on the carbon support, probably due to the presence of diverse metals which catalyze the nanofilaments growth. Besides, it was also studied the influence of the composition of the in-flow gases on the growth of nanofilaments. Thus, tests with pure CH4 and different CH4/N-2 mixtures were carried out, the yield in nanofilaments being higher when mixtures were fed in. No nanofilaments were found after analogous tests carried out under conventional heating, therefore it can be stated that microwave heating is a promising technology for growing nanofilaments. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Microwave drying was used to prepare resorcinol-formaldehyde aqueous gels, without performing any pretreatment, and to see whether it was possible to use this drying option to obtain porous carbon xerogels with controlled textural properties. By using microwave drying, the process for obtaining carbon gels is greatly simplified, textural properties are controlled likewise with other drying methods, but the time involved in the process is significantly reduced and no pretreatment is necessary. Therefore, microwave drying could help to simplify the carbon xerogels synthesis and reduce the associated costs. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: Two approaches, thermal swing desorption over a range of temperatures and time in an atmosphere of CO2 and using nitrogen as a stripping gas at elevated temperatures, are explored for the regeneration of the polyethylenimine based adsorbents. Thermal swing regeneration was demonstrated to give good cyclic regeneration capacities (2 mmol g(-1)). However, further reaction of the regenerable carbamate ion to form urea linkages, significantly reduces cyclic capacity and therefore the lifetime of the adsorbent. Regeneration of this secondary reaction product at elevated temperatures was attempted in a nitrogen atmosphere, and whilst recovering some of the original capacity did not fully regenerate the adsorbent. Adsorbent regeneration with nitrogen as a stripping gas was used as an alternative regeneration method, the results of which suggest that steam stripping may be a potential method for adsorbent regeneration. Crown Copyright (C) 2008 Published by Elsevier Inc. All rights reserved.
Abstract: Resorcinol-formaldehyde xerogels synthesised with different resorcinol/sodium carbonate molar ratios were chemically activated either after drying or after drying and pyrolysis, using potassium hydroxide. It was found that organic (i.e. dried) and carbon (i.e. pyrolysed) xerogels behave differently when subjected to chemical activation. In the case of carbon xerogels, the increase in the microporosity takes place without any significant modification to the meso/macroporosity formed during the synthesis step, leading to micromesoporous or micro-macroporous materials with a larger micropore volume. Furthermore, control of the microporosity is possible because its development depends on the amount of KOH used. However, when organic xerogels are activated, mainly microporous materials with BET specific surface areas of up to 2000 m(2) g(-1) are obtained, there hardly remaining any of the meso/macroporosity formed during the gel synthesis. Thus, the combination of different synthesis conditions and chemical activation with potassium hydroxide allows the textural properties of carbon xerogels to be tailored at both micropore and meso/macropore levels. (c) 2008 Elsevier Inc. All rights reserved.
Abstract: Ignition tests were carried out on blends of three coals of different rank - subbituminous, high volatile and low volatile bituminous in two entrained flow reactors. The ignition temperatures were determined from the gas evolution profiles (CO, CO2, NO, 0,), while the mechanism of ignition was elucidated from these profiles and corroborated by high-speed video recording. Under the experimental conditions of high carbon loading, clear interactive effects were observed for all the blends. Ignition of the lower rank coals (subbituminous, high volatile bituminous) enhanced the ignition of the higher rank coal (low volatile bituminous) in the blends. The ignition temperatures of the blends of the low rank coals (subbituminous-high volatile bituminous) were additive. However, for the rest of the blends the ignition temperatures were always closer to the lower rank coal in the blend. (c) 2007 Elsevier Ltd. All rights reserved.
Abstract: In the present work, the mechanisms involved in NO-char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 degrees C) where NO chemisorption takes place and high temperature (T > 250 degrees C) where NO-C reaction occurs. Step response experiments combining consecutive reaction stages with NO and (NO)-N-15 were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N-2 and CO2 to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N-2 emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: Adsorption is considered to be one of the more promising technologies for the capture of CO2 from flue gases. In general, nitrogen enrichment is reported to be effective in enhancing the specific adsorbent-adscirbate interaction for CO2. Nitrogen enriched carbons were produced from urea-formaldehyde and melamine-formaldehyde resins polymerised in the presence of K2CO3 as a chemical activation agent, with activation undertaken over a range of temperatures. CO2 adsorption capacity was determined to be dependent upon both textural properties and more importantly nitrogen functionality. Adsorbents capable of capturing above 8 wt-% CO2 at 25 degrees C were produced from the chemical activation of urea-formaldehyde resin at 500 degrees C. Chemical activation seems to produce more effective adsorbents than CO2 activation. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: The aim of this work was to investigate the role of porous and chemical heterogeneities of activated carbons in the adsorption of naphthalene from aqueous media. A commercially available activated carbon was used as the adsorbent, and its surface heterogeneity was systematically altered by heat treatment at 450 and 850 degrees C, obtaining a series of carbons with various oxygen contents and similar surface functionalities. The results confirmed that the adsorption of naphthalene depends strongly on the pore size distribution of the adsorbent, particularly narrow microporosity. Moreover, oxygen functionalities reduced the accessibility and affinity of naphthalene to the inner pore structure via formation of hydration clusters. Consequently, the hydrophobic/hydrophilic character of the adsorbent is important, since it dominates the competitive adsorption of water. Adsorbents with a high non-polar character (i.e., low oxygen content) have proven to be more efficient for naphthalene adsorption. (c) 2007 Elsevier B.V All rights reserved.
Abstract: The aim of this work was to correlate the textural and chemical features of carbonaceous adsorbents with the adsorption capacity of naphthalene from aqueous phase, at the concentration in which this compound is usually found in wastewater from coke ovens. The study reveals that the adsorption capacity in different carbon materials depends not only on the textural characteristics of the material but also on the functionalities of the activated carbons. The micropores of the adsorbents, particularly those of narrower diameter, were found to be active sites for the retention of naphthalene. In contrast, the modification of the surface chemistry of the carbon materials led to a decrease in the adsorption capacities. Dispersive forces play an important role, and adsorbents with a higher non-polar character have proven to be more efficient for the naphthalene adsorption. This behaviour has been linked to the presence of specific interactions between the basal planes and the polyaromatic structure of the naphthalene molecule. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: The success of CO2 capture with solid sorbents is dependent on the development of a low cost sorbent with high CO2 selectivity and adsorption capacity. Immobilised amines are expected to offer the benefits of liquid amines in the typical absorption process, with the added advantages that solids are easy to handle and that they do not give rise to corrosion problems. In this work, different alkylamines were evaluated as a potential source of basic sites for CO2 capture, and a commercial activated carbon was used as a preliminary support in order to study the effect of the impregnation. The amine coating increased the basicity and nitrogen content of the carbon. However, it drastically reduced the microporous volume of the activated carbon, which is chiefly responsible for CO2 physisorption, thus decreasing the capacity of raw carbon at room temperature. CO2 2007 Elsevier Ltd. All rights reserved.
Abstract: The mechanism of reactive adsorption of dibenzothiophene (DBT) on a series of modified carbons derived from the recycled PET was investigated. The influence of the oxygen functionalities of the adsorbent on the DBT adsorption capacity was explored. The results revealed that adsorption of DBT on activated carbons is governed by two types of contributions: physisorption on the microporous network of the carbons and chemisorption. Introduction of surface acidic groups enhanced the performance of the carbons as a result of their specific interactions with DBT. The nature of the acidic groups is a decisive factor in the selectivity of the reactive adsorption process. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: For ecological reasons, there is an increasing demand for recycling polyethylene terephthalate (PET) wastes in developed countries. Although one potential application might be its utilisation for the production of activated carbons, the behaviour of these wastes when subjected to different heat treatments and activation processes is still not very well known. In the present work, samples with different degrees of burn-off were prepared by pyrolysis in an inert atmosphere and subsequent CO, activation at high temperatures. The derived changes in the textural and structural properties of the residual solids were studied by helium picnometry, N-2 and CO2 adsorption isotherms, powder XRD, Raman spectroscopy and XPS. The study reveals that CO2 activation of PET wastes develops a carbonaceous matrix with micropores. Helium measurements showed that the mass density of the activated samples increased as the degree of burn-off increased. Characterisation studies revealed that the structural changes derived from pyrolysis and further CO2 activation mostly involved a progressive decrease in the number of structural units.
Abstract: The present study was aimed to investigate different methods of activation of carbon nanofibres, CNF, in order to determine the beneficial effect on the hydrogen sorption capacities of increasing the surface area. Two activation systems were used: physical activation with CO2 and chemical activation with KOH. A range of potential adsorbents were thus prepared by varying the temperature and time of activation. The structure of the CNF proved more suitable to activation by KOH than by CO2, with the former yielding higher surface area carbons (up to 1000 m(2) g(-1)). The increased surface area, however, did not correspond directly with a proportional increase in hydrogen adsorption capacity. Although high surface areas are important for hydrogen storage by adsorption on solids, it would appear that it is essential that not only the physical, but also the chemical, properties of the adsorbents have to be considered in the quest for carbon based materials, with high hydrogen storage capacities. (C) 2005 Elsevier Ltd. All rights reserved.
Abstract: Carbon nanofibres, obtained at industrial scale by chemical vapour deposition, were subjected to oxidative treatments under CO2 and air at different temperatures. The experiments were performed in electric and microwave furnaces, both at bench scale. The effect of the different treatments on the fibre content (i.e., depuration degree), surface chemistry, porous structure and graphitic properties, of the product obtained in the industrial reactor, were evaluated. The results obtained show that microwave treatments were less selective regarding the depuration process, but this process presents advantages like the low time consuming, besides the increase of the oxidation degree and the graphitic properties of the final sample.
Abstract: Model compounds, with a controlled heteroatoms content and well-defined functionalities, were used to study the release of nitrogen compounds from char combustion. In the present work, the mechanisms involved in NO-char heterogeneous reduction were studied with a synthetic coal (SC) char as carbon source. Another synthetic char (SN) without any nitrogen in its composition was also employed in these studies. Temperature programmed reduction (TPR) tests with a gas mixture of 400 ppm NO in argon and with isotopically labelled nitric oxide, (NO)-N-15 (500 ppm (NO)-N-15 in argon), were carried out. The gases produced were quantitatively determined by means of MS and FTIR analysers. Under the conditions of this work the main products of the NO-C reaction were found to be N-2 and CO2. The main path of reaction involves the formation of surface nitrogen compounds that afterwards react with nitrogen from the reactive gas to form N-2. It was observed that fuel-N also participates in the overall heterogeneous reduction reaction, although to a lesser extent. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Coal blends are now widely used by the power generation industry and the general characteristics are well known. Attention is still directed to the emission of NOx, which is subject to more stringent regulation, and to the amount of carbon in ash. The latter is increased when low NOx burners are employed, which is the norm now. It is also increased as a result of additional air staging when over-fire air is added in furnaces, especially tangential fired systems. Such a furnace is studied here. Two approaches can be employed for prediction of NOx and unburned carbon. The first approach uses global models such as the 'slice' model which requires the combustor reaction conditions as an input but which has a detailed coal combustion mechanism. The second involves a computational fluid dynamic model that in principle can give detailed information about all aspects of combustion, but usually is restricted in the detail of the combustion model because of the heavy computational demands. The slice model approach can be seen to be complimentary to the CFD approach since the NOx and carbon burnout is computed using the slice model as a post-processor to the CFD model computation. The slice model that has been used previously by our group is applied to a commercial tangentially fired combustor operated in Spain and using a range of Spanish coals and imported coals, some of which are fired as blends. The computed results are compared with experimental measurements, and the accuracy of the approach assessed. The CFD model applied to this case is one of the commercial codes modified to use a number of coal combustion sub-models developed by our group. In particular it can use two independent streams of coal and as such it can be used for the combustion of coal blends. The results show that both model approaches can give good predictions of the NOx and carbon in ash despite the fact that certain parts of the coal combustion models are not exactly the same. However, if a detailed insight into the combustor behaviour is required then the CFD model must be used. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Carbon foams were obtained from a bituminous coal with good plasticity properties by a two-stage thermal process under different pressure and temperature conditions. The first stage was a controlled carbonisation treatment under pressure at 450 and 500 degrees C. In the second stage the carbonisation product was baked at 1100 degrees C. The foams produced display a macroporous texture with pressure and temperature determining the mean pore size and the amount of pores. The pressure increase reduces the pore size, while the increasing temperature increases the pore volume. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The influence of polymerization with formaldehyde on the thermal reactivity of a low-temperature coal tar pitch has been investigated. The mechanism and extent of the polymerization depends on the catalyst used, the greatest extent of polymerization being achieved under basic catalytic conditions. After the polymerization treatment, samples were carbonized at 420 degrees C and the products were characterized by optical microscopy. According to the results, polymerization with formaldehyde increases the reactivity of the pitch, giving rise to increased carbonization yields and leading to the formation of the mesophase with milder conditions. The polymerization process also affects the morphology of the resultant anisotropic material, giving rise to the formation of irregularly shaped mesophase particles and reducing the optical texture size of the anisotropic domains, giving mosaic texture, especially when basic catalysis is used.
Abstract: Perhydrous coals are characterized by a high hydrogen content and exhibit a modified composition and physico-chemical properties in comparison with normal coals. These modifications affect the behaviour of perhydrous coals during pyrolysis and, therefore, may have an influence on the subsequent combustion process. In this work the combustibility behaviour of a series of perhydrous coals was evaluated in order to study the effect of hydrogen enrichment during the thermal treatment of the coals in an oxidant atmosphere. To this end temperature programmed combustion tests for the coals, and air isothermal (500 degrees C) reactivity tests for their chars, were carried out in a thermogravimetric analyser. A clear relationship between the combustion behaviour of the perhydrous coals, and the aromatic to aliphatic hydrogen ratio was found.
Abstract: Adsorption is considered to be one of the more promising technologies for capturing CO2 from flue gases. For post-combustion capture, the success of such an approach is however dependent on the development of an adsorbent that can operate competitively at relatively high temperatures. In this work, low cost carbon materials derived from fly ash, are presented as effective CO2 sorbents through impregnation these with organic bases, for example, polyethylenimine aided by polyethylene glycol. The results show that for samples derived from a fly ash carbon concentrate, the CO2 adsorption capacities were relatively high (up to 4.5 wt%) especially at high temperatures (75 degrees C), where commercial active carbons relying on physi-sorption have low capacities. The addition of PEG improves the adsorption capacity and reduces the time taken for the sample to reach the equilibrium. No CO2 seems to remain after desorption, suggesting that the process is fully reversible. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: In this work, the CO2 capture capacity of a series of activated carbons derived froth recycled polyethylene terephtalate (PET) was tested, facing two problems at the same time: minimising plastic waste and developing an adsorbent for CO2 capture. The PET raw material, obtained from post-consumer soft-drink bottles, was chemically activated with KOH. In addition, a series of nitrogen-enriched activated carbons was obtained by mixing the raw material with different nitrogen compounds (i.e., acridine, carbazole and urea). The influence of temperature on the CO2 capture capacity of the adsorbents was evaluated in a thermogravimetric system. The CO2 uptake was also related to the chemical and textural charactefistics of the samples. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: The regeneration of activated carbons used for the decontamination of water and wastewater polluted with organic compounds related to the pharmaceutical industry, has been studied in this work. To this end, the pyrolysis of exhausted activated carbons was performed in a quartz reactor. The influence of the pyrolysis of saturated activated carbons on their adsorptive capacities and porous structure was evaluated. Textural characterisation of the samples was carried out by helium density, and N-2 and CO2 adsorption isotherms, at -196 and 0 degrees C, respectively. Consecutive cycles of pyrolysis of activated carbons exhausted with phenol and salicylic acid, generated a widening of the microporosity, and a decrease in the apparent BET surface areas. The adsorptive capacities for phenol retention after a series of adsorption/ pyrolysis cycles were found to decrease drastically. In contrast, consecutive cycles gave higher salicylic acid adsorptive capacities, as compared to phenol. The DTG pyrolysis profiles of the samples saturated with salicylic acid appeared at lower temperatures than those of phenol, which showed two peaks, while those exhausted with salicylic acid only showed one. These results provide evidence of the different adsorption mechanisms of these two pollutants. (C) 2005 Elsevier B.V. All rights reserved.
Abstract: The nitrogen enrichment of active carbons is reported to be effective in enhancing the specific adsorbate-adsorbent interactions for CO2. In this work, nitrogen-enriched carbons were prepared by co-pyrolysis of sugar and a series of nitrogen compounds with different nitrogen functionalities. The results show that although the amount of nitrogen incorporated to the final adsorbent is important, the N-functionality seems to be more relevant for increasing CO2 Uptake. Thus, the adsorbent obtained from urea co-pyrolysis presents the highest nitrogen content but the lowest CO2 adsorption capacity. However, the adsorbent obtained from carbazole co-pyrolysis, despite the lower amount of N incorporated, shows high CO2 uptake, up to 9 wt.%, probably because the presence of more basic functionalities as determined by XPS analysis. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: NOX emissions from sewage sludge combustion are a concern, because of the usually high nitrogen content of this fuel. The interactions during co-combustion in a fluidized-bed reactor of sewage sludge and a bituminous coal were evaluated, in relation to the nitrogen evolution during the combustion process. The nitrogen stable isotope measurements provide novel results regarding the tracing of nitrogen during combustion. Our preliminary results show that the co-combustion chars retain more nitrogen than expected, with the additional nitrogen being mainly derived from the sludge. Additional measurements are planned on the resultant co-combustion gases, to aid source apportionment of the NOX arising from coal/sewage interactions.
Abstract: An experimental study to determine the temperature and mechanism of coal ignition was carried out by using an entrained flow reactor (EFR) at relatively high coal feed rates (0.5 g min(-1)). Seven coals ranging in rank from subbituminous to semianthracite, were tested and the evolved gases (O-2, CO, CO2, NO) were measured continuously. The ignition temperature was evaluated from the gas evolution profiles, and it was found to be inversely correlated to the reactivity of the coal, as reflected by the increasing values of the ignition temperature in the sequence: subbituminous, high volatile bituminous, low volatile bituminous and semianthracite coals. The mechanism of ignition varied from a heterogeneous mechanism for subbituminous, low volatile bituminous and sentianthracite coals, to a homogeneous mechanism for high volatile bituminous coals. A thermogravimetric analyser (TGA) was also used to evaluate coal ignition behaviour. Both methods, TGA and EFR, were in agreement as regards the mechanism of coal ignition. From the SEM micrographs of the coal particles retrieved from the cyclone, it was possible to observe the external appearance of the particles before, during and after ignition. The micrographs confirmed the mechanism deduced from the gas profiles. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: Vapour grown carbon nanofibres (VGCNFs) produced at industrial scale were subjected to different treatments in N-2, air and CO2 atmospheres. The effect of these treatments on the homogeneity of the industrial product was investigated by means of TG analysis. Temperature programmed oxidation (TPO) experiments were performed to obtain DTG profiles. The DTG curves were deconvoluted into a mixture of Gaussian-Lorentzian curves, so that the contribution of each peak could be evaluated and assigned to phases of different reactivity. It was found that treatments in air are more selective in removing the most reactive phase (which has a higher content of amorphous carbon) at low burn-off, while at an elevated burn-off degree CO2 treatments result in a greater enrichment of the product in the less reactive phase (which has a higher content of fibre). This behaviour is attributed to the presence of a certain amount of iron in the industrially produced materials and its catalytic effect on the oxidative reactions. The effect of the treatments on the porous texture, surface chemistry and graphite-like character of the samples was also investigated. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: This paper studies the chemical composition of several supercritical gas (SCG) extracts and its influence on the thermal behaviour under carbonisation conditions. The extracts were obtained from a Spanish lignite (Mequinenza), a low-quality coal from the point of view of energy applications. The lignite was treated with toluene, ethanol (EtOH) and tetrahydrofuran (THF) as solvents under different supercritical temperature and pressure conditions. The extracts display high aliphatic nature and enhanced concentrations of oxygen functional groups, aided by the contribution of hydrogenation and oxygen incorporation reactions occurring in the SCG extraction with EtOH and THF Thiophenic compounds are also present in great concentrations derived from the exceptionally high organic sulphur content of the parent coal. The carbonisation of the extracts renders anisotropic material with fine mosaic texture, as a consequence of the significant thermal reactivity inferred by the aliphatic and oxygenated groups. The size of the mosaic increases with the temperature of the SCG extraction and varies with the supercritical solvent in the order: toluene < EtOH < THF (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Polyethyleneterephthalate (PET) has become one of the major post-consumer plastic wastes. PET products present a problem of considerable concern due to the huge amount of solid waste produced. The disposal of this waste, together with its low bio- and photo-degradability represents a serious challenge for industrial countries all over the world. Pyrolysis could provide an alternative and economically viable route for processing PET waste due to the potential uses of different by-products: energy from the pyrolysis gases (58% yield in this work), recovery of terephthalic acid and other subproducts (20%), and a solid residue (22%), which has shown a high textural development after activation. The pyrolysis of PET waste was performed in a quartz reactor (i.d. 35 mm) under an inert atmosphere. Further activation was carried out at a temperature of 925degreesC, with a flow rate of 10 ml min(-1) of CO2. A series of carbon materials with different burn-off degrees was obtained. Textural characterisation of the samples was carried out by performing N-2 adsorption isotherms at -196 degreesC. Changes in the morphological and structural properties of chars were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The carbons obtained were isotropic and highly microporous materials with apparent BET surface areas of up to 2500 m(2) g(-1). The suitability of the samples for hydrogen storage was studied by performing H-2 adsorption measurements in the 0-1 bar pressure range. Adsorption-desorption experiments showed that reversible physisorption takes place in all the samples. The hydrogen adsorption capacities of the activated PET waste compare favourably well with those attained with high-value carbon materials. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Poly(ethylene) terephthalate (PET), has become one of the major post-consumer plastic waste. In this work special attention was paid to minimising PET residues and to obtain a high value carbon material. Pyrolysis and subsequent activation of PET from post-consumer soft-drink bottles was performed. Activation was carried out at 925 degreesC under CO2 atmosphere to different burn-off degrees. Textural characterisation of the samples was carried out by performing N-2 adsorption isotherms at - 196 degreesC. The obtained carbons materials were mainly microporous, presenting low meso and macroporosity, and apparent BET surface areas of upto 2500 m(2) g(-1). The capacity of these materials for phenol adsorption and PAHs removal from aqueous solutions was measured and compared with that attained with commercial active carbons. Preliminary tests also showed high hydrogen uptake values, as good as the results obtained with high-tech carbon materials. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: Poly(ethyleneterephthalate), PET, is nowadays one of the polymers more widely used. However, due to its big production, it accounts for a large part of the wastes generated and it becomes necessary to minimise them. The PET incineration with energy recovery is the most common way to eliminate these residues. Further research is needed in order to find alternative processes to recycling PET. In this work, it is shown that the post-consumer PET is in interesting source of carbonaceous materials that develop a well controlled microporosity, giving the possibility of application in several fields.
Abstract: Knowledge of surface properties is essential for understanding the reaction mechanisms involved in several coal conversion processes. However, due to the complexity and heterogeneity of coal this is rather difficult and the use of known model compounds could be a valuable tool. Single model compounds have been widely used, but they give a quite simplified picture. In this work a mixture of model compounds in a phenol-formaldehyde matrix was cured in order to create cross-linked structures. The obtained synthetic coal was pyrolysed in a fixed bed reactor, under helium atmosphere. The surface composition of the chars was evaluated by XPS, adsorption gravimetry of water vapour, temperature-programmed desorption and potentiometric titration. Texture was characterised by N-2 and CO2 adsorption isotherms at 77 and 273 K, respectively, and immersion calorimetry in benzene. The results obtained from the different techniques were contrasted in order to give an overview of the surface properties (chemical and physical) of the samples studied. Chars obtained under the same operating conditions from a high volatile bituminous coal were used as a reference. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Knowledge of the amount and distribution of active sites in carbons is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas solid reactions. In this work a commercial active carbon, CM, was treated at several temperatures in order to obtain a series of samples with different textural and structural properties. The results showed that the loss of reactivity of the samples, determined by thermogravimetric analysis, is related not only to the lower surface area but also to the decrease in the amount of active sites due to a higher structural ordering.
Abstract: Understanding the ignition behaviour of coal is of utmost importance for the design of boilers and control of the combustion process. In recent years there has been an increasing utilisation of coal blends for combustion, but information on the possible interactive effects during ignition of the individual components is scarce. In this work the ignition behaviour of a series of coal blends, made up from three coals of different rank, sub-bituminous, high volatile and low volatile bituminous, was studied. To this end a thermogravimetric analyser linked to a mass spectrometer for evolved gas analysis was used. Different ignition behaviour was observed for the coals studied; the sub-bituminous and low volatile bituminous coals ignited heterogeneously, while homogeneous ignition occurred for the high volatile bituminous coal. In the case of blends of the low and high volatile bituminous coals, different mechanisms of ignition were observed depending on the blends composition.
Abstract: Coal pyrolysis is the initial, accompanying reaction of a number of coal conversion processes such as hydrogenation, combustion and gasification. However, because of the inherent complexity of coal composition, it is difficult to describe coal pyrolysis clearly. Single model compounds have been used before in order to provide additional insight into the complex processes that occur in the pyrolysis of coal. Yet the picture obtained is a simplified one and certain important aspects such as coal structure, interactions between different surface groups and cross-links are omitted. The approach used in this work involves the preparation of a synthetic coal, SC, with a known structure by curing a mixture of single, well-defined model compounds. By means of chemical characterisation, the SC was shown to contain the macroscopic features of a high volatile coal (proximate and ultimate analyses). FTIR characterisation revealed the presence of functional groups similar to those of coal in the structure of the SC. Temperature-programmed pyrolysis tests were performed in a thermobalance linked to a mass spectrometer and a Fourier transform infrared analyser (TG/MS/FTIR). The thermal behaviour of the synthetic coal (i.e., rate of mass loss and the evolution profiles of gaseous compounds during pyrolysis tests) is very similar to that of the high volatile bituminous coal which was used as a reference material. The great advantage of using SC lies in the fact that its composition and structure can be accurately determined and employed in subsequent applications in basic and mechanistic studies. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: Perhydrous coals are characterised by high H/C atomic ratios and so their chemical structure is substantially modified with respect to that of conventional coals. As a result, perhydrous coals show different physico-chemical properties to common coals (i.e. higher volatile matter content, enhancement of oil/tar potential, relatively lower porosity and higher fluidity during carbonisation). However, there is little information about thermal behaviour during the pyrolysis of this type of coal. In this work, six perhydrous coals (H/C ratio between 0.83 and 1.07) were pyrolysed and analysed by simultaneous thermogravimetry/mass spectrometry. The results of this work have revealed the influence of high H/C values on the thermal behaviour of the coals studied. During pyrolysis the perhydrous coals exhibit very well defined, symmetrical peaks in the mass loss rate profiles, while normal coals usually show a broader peak. The shape of such curves suggests that in perhydrous coals fragmentation processes prevailed over condensation reactions. The high hydrogen content of perhydrous coals may stabilise the free radicals formed during heat treatment, increasing the production of light components. (C) 2003 Elsevier Science B.V. All rights reserved.
Abstract: The effect of curing temperature on smokeless fuel briquettes has been studied by Fourier transform infrared spectroscopy (FT-IR), mass spectrometry (MS), and temperature programmed decomposition (TPD). These techniques help to predict the final properties of these briquettes which were prepared with a low-rank coal, sawdust, and olive stone as biomasses and humates as binder. The best mechanical properties are reached with both the mildest thermal curing at 95 degreesC and the cocarbonized at 600 degreesC of Maria coal (M2) and sawdust (S) due to the fibrous texture of sawdust. The temperature of curing causes the release of a certain amount of oxygenate structures and the decrease of the mechanical resistance.
Abstract: The increasing environmental concern caused by the use of fossil fuels and the concomitant need for improved combustion efficiency is leading to the development of new coal cleaning and utilisation processes. However, the benefits achieved by the removal of most mineral matter from coal either by physical or chemical methods can be annulled if poor coal combustibility characteristics are attained. In this work a high volatile bituminous coal with 6% ash content was subjected to chemical demineralisation via hydrofluoric and nitric acid leaching, the ash content of the clean coal was reduced to 0.3%. The original and treated coals were devolatilised in a drop tube furnace and the structure and morphology of the resultant chars was analysed by optical and scanning electron microscopies. The reactivity characteristics of the chars were studied by isothermal combustion tests in air at different temperatures in a thermogravimetric system. Comparison of the combustion behaviour and pollutant emissions of both coals was conducted in a drop tube furnace operating at 1000 degreesC. The results of this work indicate that the char obtained from the chemically treated coal presents very different structure, morphology and reactivity behaviour than the char from the original coal. The changes induced by the chemical treatment increased the combustion efficiency determined in the drop tube furnace, in fact higher burnout levels were obtained for the demineralised coal. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: A mixture of carbon compounds was pyrolysed under an inert atmosphere at different temperatures in a fixed bed reactor. The resultant chars were characterised in terms of texture and thermal behaviour. Textural characterisation of the chars was carried out by N-2 and CO2 adsorption isotherms at -196 and 0 degreesC, respectively. Char isothermal reactivity in air at 500 degreesC, and in CO2 at 1000 degreesC, was performed in a thermogravimetric analyser (TGA). Temperature-programmed combustion tests under 20% oxygen in argon were also performed in the TGA linked to a mass spectrometer (TGA/MS). The results showed that char textural properties do not always relate well to their reactivity. Not only do physical properties (e.g. surface area, porosity) but also chemical properties (e.g. active sites concentration and distribution) play an important role in the reaction of carbonaceous material and oxidant. On the other hand, in terms of chemical composition the chars obtained from the mixture of carbon compounds were very similar to the chars produced under the same experimental conditions by a high volatile bituminous coal. The fact that carbon compounds are well known makes it easier to obtain knowledge about the functional groups present in synthetic char, and to study the mechanisms of heterogeneous reactions such as the reduction of NO with carbon. (C) 2003 Elsevier Ltd. All rights reserved.
Abstract: Nitrogen oxides are one of the major environmental problems arising from fossil fuel combustion. Coal char is relatively rich in nitrogen, and so this is an important source of nitrogen oxides during coal combustion. However, due to its carbonaceous nature, char can also reduce NO through heterogeneous reduction. The objectives of this work were on one hand to compare NO emissions from coal combustion in two different types of equipment and on the other hand to study the influence of char surface chemistry on NO reduction. A series of combustion tests were carried out in two different scale devices : a thermogravimetric analyzer coupled to a mass spectrometer and an FIR (TG-MS-FTIR) and a fluidized bed reactor with an on line battery of analyzers. The TG-MS-FTIR system was also used to perform a specific study on, NO, heterogeneous reduction reactions using chars with different surface chemistry. According to the results obtained, it can be said that the TG-MS-FTIR system provides valuable information about NO heterogeneous reduction and it can give good trends of the behavior in other combustion equipments (i.e., fluidized bed combustors). It has been also pointed out that NO-char interaction depends to a large extent on temperature. In the low-temperature range (<800 degreesC), NO heterogeneous reduction seems to be controlled by the evolution of surface complexes. In the high-temperature range (>800 degreesC), a different mechanism is involved in NO heterogeneous reduction, the nature of the carbon matrix being a key factor.
Abstract: The purpose of this work was to study the formation of mesophase spherules from a low-temperature coal tar pitch under carbonization conditions. For comparison, the carbonization of a high-temperature coal tar pitch and a petroleum pitch were also considered. Different operating conditions during the carbonization process were used in an attempt to cover different degrees of mesophase formation and development for each pitch. The parent pitches and the semicokes thus obtained were analyzed by elemental analysis, optical microscopy, and Fourier transform infrared spectroscopy (FT-IR). More significantly, the samples were subjected to thermal decomposition under well controlled operating conditions from room temperature to 850 degreesC in a thermogravimetric analyzer (TG). The use of a mass spectrometer linked to the TG (TG-MS) provides additional data about the devolatilization process, yielding information about the evolution,of different volatile products and about possible chemical reactions occurring during thermal decomposition. Thus an-insight into the process of mesophase formation is obtained. The results from FT-IR, elemental analysis, and the TG-MS tests were compared with the different extents of mesophase formation, checked by optical microscopy. According to the results, several stages can be distinguished as temperature increases in the carbonization process of the pitches. In them low-temperature coal tar pitch, the devolatilization of light components, especially phenols, accounts for the most significant weight loss. Moreover, cross-linking contributes greatly to the formation and development of mesophase, resulting in the predominance of bulk mesophase in A relatively short time in the case of the low-temperature coal tar pitch.
Abstract: Polymer waste nowadays accounts for a large part of municipal solid waste and so poses a problem of considerable concern for the environment. In this study, it is reported that poly(ethylene terephthalate) pyrolysis is a valuable process with no residual solids, as all the products obtained are of potential use, i.e., energy from the gases produced (58%), terephthalic acid (approximately 20%), and a solid black residue (22%) which, when activated, shows interesting properties, making it useful as an adsorbent. Pyrolysis of raw PET was performed under nitrogen atmosphere, and ulterior activation was achieved at 925degreesC, with a flow rate of 10 mL min(-1) Of CO2, up to different burn-off degrees. Textural characterisation of the samples was carried out by measuring apparent density (mercury at 0.1 MPa), mercury porosimetry, and N-2 and CO2 adsorption isotherms, at -196 and 0degreesC, respectively. The active carbons obtained were mainly microporous, with BET apparent surface areas of up to 2468 m(2)g(-1).
Abstract: The aim of this study was to investigate the influence of coal rank and operating conditions during coal pyrolysis on the resultant char texture properties, morphology and reactivity. A range of bituminous coals were pyrolysed in a fixed bed reactor at different heating rates, It was found that the higher the heating rate and the lower the coal rank, the more microporous chars were obtained. Isothermal (500 degreesC) gasification in 20% oxygen in argon of the chars was carried out using a differential thermogravimetric system (DTG), The results of this work indicated that the increase in the availability of char-active surface sites led to an increase in char reactivity, not only for oxygen but also for other reactive gases, in particular NO, diminishing emissions during the combustion process. (C) 2002 Elsevier Science B.V All rights reserved.
Abstract: Emissions of nitrogen oxides during coal combustion are a major environmental problem. The chemically bound nitrogen in fuel accounts for up to 80% of total NOx emissions. In this respect, fundamental studies are needed to clarify the mechanisms and to identify the different species that are precursors in the formation of the NOx. Mass spectrometry (MS) has been used for decades as a successful technique in evolved gas analysis. However, MS is normally used to identify typical volatile compounds formed during coal pyrolysis (i.e. H-2, CH4, CO, CO2 and H2O) but very few works on the detection by MS of nitrogen compounds during coal devolatilisation can be found. In this work, the possibility of detecting different nitrogen compounds by means of thermogravimetric-MS during the temperature-programmed pyrolysis of coal was evaluated. Interferences in the N-compounds MS signals were determined. The use of model compounds provided additional information on the MS response factors of the volatile compounds produced. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Combustion profiles determined by TGA and experiments in a laminar entrained flow reactor (EFR) were used in this work to assess the relative combustion reactivities of different rank coals and their binary coal blends. The combustion behaviour of coal blends in TGA was greatly influenced by coal rank and the proportion of each component in the blend, Higher volatile coals exerted more influence in the low-temperature region and less reactive coals in the char combustion zone. The results in the EFR indicated that coal blends burnout and NO emissions show additivity in the case of similar nature coals. When one of the components was a high-rank coal, the burnout of the blend exhibited, in some cases. positive synergistic effects. while a clear deviation from linearity was found in NO emissions. (C) 2002 Elsevier Science B.V All rights reserved.
Abstract: In carbon materials, active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas-solid reactions. However, although ASA reveals interesting information about the sample, there is a need for a reliable and standardised method for its estimation. The aim of this work is to compare ASA determination by the usual methods (i.e., gravimetry, TPD) with another method, which is based on pressure measurements in order to perform an oxygen chemisorption isotherm (OCI). The results showed that the OCI method seems to be a valuable and alternative method for ASA determination, as it avoids the main potential source of errors inherent in the usual methods.
Abstract: Coal blending is becoming of increasing importance in power stations firing pulverised coal as a result of increasing competition, stricter emission legislation and is an attractive way of improving plant economic and combustion performance. Presently, the two general methods used by power station operators to assess or predict the performance of an unknown coal blend to be fired in power station boilers are by the use of experimental large scale rig tests or correlation indices derived from experience of firing other coal blends in the power station environment. The first is expensive and the second is of doubtful accuracy in some cases. This paper evaluates the application of mathematical modelling of the combustion of a series of binary coal blends in the test situation of a drop tube reactor to predict the NO emissions and degree of char burnout. Its applicability to low NOx burners used in power stations is discussed and it is concluded that present mathematical coal combustion models are not developed sufficiently to enable an adequate description of the binary blends and the physical and chemical processes, which may include interactions, during combustion of the blend. This means that accurate predictions cannot be made. (C) 2001 Elsevier Science Ltd. All rights reserved.
Abstract: The aim of this work was to determine the influence that an advanced demineralisation procedure has on the combustion characteristics of coal. A high-volatile bituminous coal with 6.2% ash content was treated in a mixture of hydrofluoric and fluorosilicic acids (HF/H2SiF6). Nitric acid was used either as a pretreatment, or as a washing stage after HF/H2SiF6 demineralisation, with an ash content as low as 0.3% being attained in the latter case. The structural changes produced by the chemical treatment were evaluated by comparison of the FTIR spectra of the raw and treated coal samples. The devolatilisation and combustibility behaviour of the samples was studied by using a thermobalance coupled to a mass spectrometer (TGA-MS) for evolved gas analysis. The combustibility characteristics of the cleaned samples were clearly improved, there being a decrease in SO2 emissions. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Knowledge of the coal devolatilisation rate is of great importance because it exerts a marked effect on the overall combustion behaviour. Different approaches can be used to obtain the kinetics of the complex devolatilisation process. The simplest are empirical and employ global kinetics, where the Arrhenius expression is used to correlate rates of mass loss with temperature. In this study a high volatile bituminous coal was devolatilised at four different heating rates in a thermogravimetric analyser (TG) linked to a mass spectrometer (MS). As a first approach, the Arrhenius kinetic parameters (k and A) were calculated from the experimental results, assuming a single step process. Another approach is the distributed-activation energy model, which is more complex due to the assumption that devolatilisation occurs through several first-order reactions, which occur simultaneously. Recent advances in the understanding of coal structure have led to more fundamental approaches for modelling devolatilisation behaviour, such as network models. These are based on a physico-chemical description of coal structure. In the present study the FG-DVC (Functional Group-Depolymerisation, Vaporisation and Crosslinking) computer code was used as the network model and the FG-DVC predicted evolution of volatile compounds was compared with the experimental results. In addition, the predicted rate of mass loss from the FG-DVC model was used to obtain a third devolatilisation kinetic approach. The three methods were compared and discussed, with the experimental results as a reference. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: The results presented in this work are part of a more extensive research programme aimed at assessing the impact of coal porous structure on density-based process evaluation and modelling. The coal samples used were obtained from two different density-based cleaning processes, a Vorsyl dense medium separator for treating an anthracite (TW) with a size fraction of 0.5-8.0 mm and a spiral concentrator for heating a bituminous coal (DH) with a size of less than 2 mm. Textural characterisation of the samples was carried out by measuring hue (helium) and apparent (mercury) densities and mercury porosimetry up to a maximum pressure of 200 MPa. Adsorption isotherms in CO2 at 273 K were also determined for both coal series. In the case of the bituminous coal series a linear relationship between porosity and ash level was found. This may have important implications if coal porosity and/or textural parameters need to be incorporated into new density-based simulation models. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: In this work the combustion behaviour of four coals of varying rank from high volatile bituminous coal to anthracite and their binary blends was studied by means of temperature programmed combustion tests (TPC), using a thermogravimetric analyser. Isothermal gasification in air at 500 degrees C was also carried out. The individual coals were mixed to generate blends containing 25%, 50% and 75% by weight of the lower rank coal. The characteristic parameters, extracted from TPC profiles, of the coal blends could not be predicted from the weight percentage of the individual coals in the blend. This was more accentuated in the case of mixing very different rank coals, i.e. anthracite and high volatile bituminous coals. In addition, the burnout temperature increased as the percentage of anthracite in the blend increased but its value was always lower than that of the anthracite. Similar results were obtained for the other blends. This indicates that interaction between the samples occurred giving, as a result, an improvement in the combustibility of the less reactive coal in the blend.
Abstract: In the present work a study was carried out in order to assess the NO, emissions when increasing the fraction of high-volatile coals in blends with low-volatile coals -anthracitic, semianthracitic type-. To this end four coals of different rank were ground below 150 mu m and mixed in different proportions. The samples were subjected to temperature programmed combustion (TPC) tests in a thermogravimetric analyser coupled to a mass spectrometer (TGMS). This allowed the combustion behaviour of the samples and the nitrogen compounds evolved during TPC tests to be studied. In addition, coal fractions of 75-150 mu m in size were sieved, blended and introduced in a drop tube reactor (2 m length, 4 cm internal diameter) which can simulate the conditions encountered in industrial furnaces in terms of temperature, heating rate, particle size and gas composition. Burnout and NO emissions were determined for the individual coals and their blends. A non-additive behaviour was attained for one blend with regard to both burnout and NO emissions.
Abstract: The emission of sulfur oxides during the combustion of coal is one of the causes, among other air pollution problems, of acid rain. The contribution of coal as the mainstay of power production will be determined by whether its environmental performance is equal or superior to other supply options. In this context, desulfurization of coal before combustion by biological methods was studied. Four Spanish high-sulfur content coats of different rank were inoculated with bacteria isolated from mine-drainage waters and with naturally occurring bacteria inherent in the coals to be treated. Higher levels of desulfurization were obtained in the case of the samples treated with their own accompanying bacteria and when aeration was increased. All the samples were amenable to the biodepyritization processes. However, it is of little Value to achieve large sulfur reductions if a decrease in coal combustion performance is obtained in the process. For this reason, a comparison was made between the combustibility characteristics of the original coals and those of the biodesulfurized samples. Results indicated that combustibility was not substantially modified by the overall biological treatment. The benefits of reduced sulfur emissions into the atmosphere ought to be taken into account as part of the general evaluation of the processes.
Abstract: The combustion of char nitrogen is the main contributor to NOX emissions in both pulverised fuel combustion in low-NOX burners and in fluidised bed combustion. In the latter case char textural properties seem to play an important role in NO heterogeneous reduction on the char surface. Active surface area (ASA) is one of the main topics of concern in the study of heterogeneous reactions (e.g. gasification, NOX reduction, etc.). In this work a low volatile bituminous coal was pyrolysed at several heating rates to produce chars with different textural properties. The results show that there is a linear relationship between char reactivity and active surface area (ASA). The effect of coal rank on char textural properties was also studied by using a range of bituminous coals (low, medium and high volatile matter content). The results are discussed in terms of the influence of both textural properties and reactivity on NO emissions and on the heterogeneous reduction of NO.
Abstract: Grinding of a high volatile bituminous coal was performed in three comminution devices: Raymond Mill (RM), Rolls Crusher (RC) and Ball Mill (BM). The pulverised samples were sieved to obtain four particle size fractions, and temperature-programmed combustion (TPC) was used for the evaluation of their combustion behaviour. In addition, three coals of different hardness and rank were mixed in various proportions in order to compare the combustibility characteristics of the binary coal blends with those of the individual coals. The effect of coal blending on grindability was also studied. It was found that grindability was non-additive especially when coals of very different Hardgrove Grindability Index (HGI) were blended. The combustion studies also suggested that there exists an interaction between individual coals when they are burnt as a blend.
Abstract: Simultaneous thermogravimetry-mass spectrometry was used to study the pyrolysis behaviour of an anthracite and three bituminous coals of different volatile matter content. This system was optimised by using calcium oxalate as a reference for calibration. A normalisation method that permitted a semiquantitative comparison between the volatile species of the coals was also developed. The instantaneous evolution of the volatile compounds was studied by means of temperature-programmed pyrolysis experiments. The peaks varied in shape, temperature and size, and showed a marked dependence on coal rank. This can be attributed to the varying amounts of the different functional groups in the coals studied. Special attention was paid to the nitric oxide released during pyrolysis, together with its precursor species. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: NO is the primary product of the oxidation of char nitrogen, and in some combustion processes the NO can be reduced on the char surface to give N2O and/or N-2 In this study a range of bituminous coals (low, medium and high volatile matter content) were pyrolysed in a fixed bed reactor at various heating rates. Textural characterisation was carried out by measuring true (He) and apparent (Hg) densities and N-2 (-196 degrees C) and CO2 (0 degrees C) adsorption isotherms. Pore volume distributions and surface areas were obtained for the chars studied. A thermogravimetric analyser coupled to a quadrupole mass spectrometer (TG-MS) was used to study the combustion behaviour of the samples and the nitrogen compounds evolved during temperature-programmed combustion. Results are discussed in terms of the influence of both textural properties and reactivity on NO emissions and on the heterogeneous reduction of NO. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: Coal combustion computational fluid dynamic (CFD) models are a powerful predictive tool in combustion research. In existing coal combustion CFD models, the process is described by three kinetic rates: coal devolatilisation, volatile combustion and char combustion. A general, representative devolatilisation rate for coal is a matter of some contention. and measured rates depend upon the type of experimental system employed in their determination. Thus the reported rates vary considerably, causing difficulties in the choice of rate expression for CFD modelling applications. In this investigation, a laminar flow CFD model of a drop-tube furnace was used to assess the influence of global devolatilisation rates on overall combustion behaviour, and in particular, NOx emissions. The rates chosen include some of the common expressions employed by researchers in the field. Analysis, and comparison of the modelling results with those of the experimental, indicated that a single-step devolatilisation rate can give satisfactory profiles. This rate can be calculated from the tar release rate using a network model such as FG-DVC (functional group, depolymerisation, vaporisation and cross-linking), together with the nitrogen partitioning between gas and char during pyrolysis. The use of these single-step models result in good predictions of NOx, and the inclusion of soot/NOx interactions can improve the model significantly to give an excellent agreement with experimental results. (C) 1999 Elsevier Science Ltd. All rights reserved.
