K.M. Thomas

EVOLUTION OF NITROGEN FUNCTIONALITIES IN CARBONACEOUS MATERIALS DURING PYROLYSIS

X-ray photoelectron spectroscopy (XPS) was used to investigate the fate of nitrogen functional forms present in a lignite and its chars, chars derived from the model compounds acridine, carbazole and polyacrylonitrile (PAN). Four different peaks have been found in the XPS patterns, corresponding to at least five different nitrogen functional forms, all being aromatic moieties. The XPS patterns of the synthetic chars were recorded for identification purposes. The distribution of nitrogen functional forms changes with increasing severity of the pyrolysis conditions. Under mild pyrolysis conditions, firstly unstable functionalities like pyridones, protonated pyridinic-N and N-oxides of pyridinic-N are converted to pyridinic-N and secondly pyrrolic-N is converted to pyridinic-N during condensation of the carbon matrix. During the condensation process, nitrogen atoms are incorporated in the graphene layers replacing carbon atoms. After severe pyrolysis all nitrogen is eventually present in 6-membered rings located at the edges of the graphene layers as pyridinic-N or in the interior as quaternary-N. Upon exposure to the ambient, N-oxides of pyridinic-N can be formed. During pyrolysis, differences in nitrogen distribution of the char precursors have diminished. It is presumed that the remaining small differences in the nitrogen distribution of the chars cannot significantly influence the formation of nitrogen oxides during combustion of the chars.

Iron catalysed graphitisation in the blast furnace

Abstract The paper describes the study of iron catalysed graphitisation in a set of coke samples extracted from a blast furnace, by means of optical microscopy, X-ray diffraction (XRD) and Raman microprobe techniques. The direct link obtained between optical microscopy and Raman spectroscopy in the Raman microprobe proved useful in linking morphology to molecular structure. The results show that catalytic graphitisation takes place at temperatures well below the temperature where graphitisation would normally be expected to occur. The mechanism of graphitisation probably involves dissolution and precipitation processes. The most noticeable spectral difference between the graphite produced by the catalytic process and the coke is the resolution of the second order Raman G ′ band of the former into an overlapping doublet, whereas the latter only has a single symmetric peak. These spectral changes are associated with the decrease in the d 002 values and the development of graphitic structure. The results suggest that temperature, amount of iron in contact with the coke and residence time of the coke in the hottest regions of the furnace are important factors in the catalytic graphitisation process. The use of XRD and Raman spectroscopy for the characterisation of the thermal history of blast furnace coke samples is discussed.

The influence of matrix microstructure on the mechanical properties of CFRC composites

Abstract A suite of carbon fibre reinforced carbon (CFRC) composites and carbon matrices with a range of matrix microstructures have been fabricated using a multiple liquid vacuum impregnation process and high-pressure carbonization. The carbon matrices were derived from a range of petroleum and coal tar pitches in addition to pitch/phenolic resin mixtures. The efficiency with which the pitches densified their respective CFRC composites was dependent initially on their carbon yield, but after repeated densification on their rheological properties. The densification efficiency of the pitch/phenolic resin matrix precursors was dependent solely on their carbon yields. The variation of phenolic resin/ pitch composition of the precursor for the matrix carbon had a profound influence on the structural and mechanical properties of the resultant carbon. The intrinsic mechanical strength (microstrength) of a matrix carbon cannot always be translated to its corresponding CFRC composite because of fibre matrix interfacial phenomena. Studies of the systematic variation of matrix microstructure have shown that in the absence of transverse shrinkage phenomena, maximum interlaminar shear strength (ILSS) corresponds to a matrix of mosaic optical texture. Maximum compressive strength corresponds to a matrix with an axial preferred anisotropic optical texture. Consequently, it is possible to fabricate CFRC composites with specific properties by modifying their matrix microstructures.

The anode deposit formed during the carbon-arc evaporation of graphite for the synthesis of fullerenes and carbon nanotubes

A number of different products are formed in the synthesis of fullerenes using the DC carbon arc evaporation of graphite. Fullerenes are found in the soot that condenses from the vapour phase, while nanotubes are located in the material that deposits on the cathode. This paper reports the discovery of a deposit on the anode, which is similar in appearance and composition to that formed on the cathode; i.e. it is a highly oriented graphitic material that contains nanotubes. Carbon isotope distribution studies indicate that the anode deposit, like the cathode deposit, is depleted in 13C, demonstrating that it is derived from small C, (n < 3) species. Materials formed under He/N, atmospheres contain small amounts of nitrogen. This incorporation of nitrogen into carbon products provide further evidence for small precursor species to these materials. The combustion properties of the nitrogen-doped materials were quite different to those of the undoped materials. The results are discussed in terms of mechanisms for the formation of the products of the carbon arc evaporation process. Keywords-Fullerene, nanotube, isotope ratios, anode, cathode, deposit, Raman, SEM, TEM, graphite, mechanism

Temperature programmed combustion studies of the co-processing of coal and waste materials

The disposal of waste materials using incineration methods is becoming increasingly important from the view point of the need to minimise the environmental impact of sewage sludge disposal. This investigation has involved the use of temperature programmed combustion (TPC) to study the interaction between coal, polyethylene, and dried sewage sludge which are possible components in coal/waste materials co-processing combustion systems. The TPC studies were carried out on the raw materials and 1:1 mixtures of sludge/coal and sludge/polyethylene. Comparison of the gas evolution profiles of the mixtures with corresponding profiles obtained by the summation of the profiles of the component materials indicate that there were interactions between the mixtures during the combustion. Significant interactions may occur when the combustion temperature ranges of the materials overlap.

The identification of oxygen functional groups in carbonaceous materials by oxygen K-edge XANES

Abstract The aim of this paper is to assess the potential of oxygen K-edge X-ray absorption near-edge spectroscopy (XANES) for the identification of oxygen functional groups in carbonaceous materials, with a particular view to the identification of functional groups formed during combustion. A suite of aromatic reference compounds was used to provide “fingerprints” for a range of oxygen functional groups. The results indicate that oxygen K-edge XANES is sufficiently sensitive to functional group type for this application. A set of partially gasified chars was prepared from samples of pure acenaphthylene, Wearmouth coal and Cwmbargoed coal. The oxygen K-edge XANES spectra of these chars clearly show that one can follow the simultaneous loss of oxygen species originating in the carbon and formation of new surface complexes during combustion. It is of particular note that anhydrides and lactones can be readily distinguished using this technique. The mechanistic implications of the results for carbon combustion are discussed.

Metal carbonyl decomposition and carbon deposition in the advanced gas-cooled nuclear reactor

Abstract Carbon deposition in advanced gas-cooled nuclear reactors has been found to occur preferentially in those regions of the reactor core which receive gas directly from the coolant reprocessing plant. This discovery has led to the hypothesis that such deposition may be catalysed by iron or nickel carbonyls being transported into the reactor core. This article describes experiments involving the decomposition of metal carbonyls in simulated AGR gas mixtures under conditions of temperature and pressure similar to those found within the reactor cores. Nickel carbonyl has been shown to promote significant carbon deposition from the simulated AGR, gas mixtures. The carbons deposited in the laboratory have been compared and contrasted with those found within the AGR. Those gases present which are most susceptible to decomposition under the influence of the decomposition products of nickel carbonyl have been identified, and the influence of AGR fuel cladding material examined. Taken together the results provide a mechanism accounting for the nature and distribution of the carbon deposits occurring in these areas of AGR cores.

The effect of Fe catalyst on the release of NO during the combustion of anisotropic and isotropic carbons

The release of nitrogen oxides during the combustion of coal chars is a major environmental problem. In this study, the effect of iron catalysts on the release of NO during the combustion of a suite of carbons was investigated using a thermogravimetric analyser-mass spectrometer. The anisotropic carbons were prepared from acenaphthylene while the isotropic carbons were prepared from polyvinylidene chloride and a phenolic resin. Nitrogen was incorporated in the carbon structure by treatment with ammonia. A range of catalyst impregnation and sample treatment procedures were used and the temperature programmed combustion characteristics of the resulting carbon samples were studied. The results were compared with the corresponding combustion characteristics of the pure carbons. Distinct differences were observed in the gas evolution profiles during temperature programmed combustion for the pure carbons, and the various methods of catalyst preparation and dispersion. The results are discussed in terms of the structure of the carbons, incorporation of nitrogen into the carbon structure by ammonia treatment and the catalyst impregnation methods.

Isotope fractionation in the synthesis of fullerenes

Abstract The synthesis of fullerenes by the direct current (DC) carbon arc evaporation method produces a number of different carbon materials. This investigation has involved the structural characterisation of the materials and the study of their isotopic composition. The cathode deposit was a highly ordered graphite with the basal planes oriented in the axial direction of the graphite rod. Raman microprobe spectroscopy, X-ray powder diffraction, and temperature programmed combustion measurements also showed that the material was heterogeneous. The structural characterisation results were consistent with the deposit being formed at progressively lower heat-treatment temperatures with increasing deposition. The fullerene soot was a very heterogeneous material with a large surface area. Isotope composition studies showed that the raw fullerene soot, fullerene depleted soot (toluene extract), C60, and C70 were enriched in 13C relative to the graphite anode, whereas the cathode deposit was depleted in 13C. The measurements show that C60 is richer in 13C compared with both C70 and the toluene-extracted soot. The implications regarding the mechanisms for the formation of fullerenes are discussed.

Assessment of kinetics, selectivity, and capacity in carbon molecular sieves by flow microcalorimetry

Abstract A novel application of the technique of flow microcalorimetry has been used to assess the equilibrium adsorption capacity and kinetic gas separation capability of several commercial carbon molecular sieves and two active carbons. A gravimetric gas uptake technique has been similarly used to study the carbons and the two techniques have been compared and contrasted. This article provides a detailed critique of the use of gas flow microcalorimetry and concludes that the technique is useful for rapid analyses of molecular sieving capabilities.