Peter J.J. Tromp

Gas phase pyrolysis of coal-related aromatic compounds in a coiled tube flow reactor: 2. Heterocyclic compounds, their benzo and dibenzo derivatives

Abstract Selected series of heterocyclic compounds present in coal volatiles have been pyrolysed in a flow reactor to obtain kinetic data on their thermal stability. To investigate the effect of enlargement of the π system, the benzo(b) and dibenzo(b) derivatives were pyrolysed. The ring compounds increase in stability in the sequence furan, cyclopentadiene, pyrrole, pyridine, benzene, thiophene. A general feature of the benzo(b) derivatives is extra stabilization, except for naphthalene which decays like benzene, and benzo(b)thiophene, which is less stable than thiophene but still more temperature resistant than benzene. The extra stabilization is most significant for furan and pyridine. The dibenzo compounds of furan, cyclopentadiene, pyrrole and thiophene exhibit a low temperaure dependence compared with the parent heterocycles and, therefore, are more stable at higher temperatures. Anthracene, as an exception, decomposes more easily than naphthalene and benzene over the complete conversion range.

The influence of potassium carbonate on surface area development and reactivity during gasification of activated carbon by carbon dioxide

In this study it is shown that the uncatalysed gasification of activated peat char can be described by a reaction temperature independent increase of the specific surface area with increasing burn-off and a constant reactivity per unit surface area up to 60% burn off. Unlike nickel catalysis, potassium catalysis causes an increased microporosity due to microchanneling. Above ca. 0.6 atoms per nm2 the increase of the reactivity per unit surface area with increasing burn-off can be described by a first order dependency with respect to the number of potassium atoms per unit surface area. This can be explained by assuming potassium intercalate formation to be an essential step. The development of microporosity appears to be more pronounced at higher reactivities and, consequently, at higher effective catalyst loadings and higher reaction pressures.

Correlation between Raman spectroscopic data and the temperature-programmed oxidation reactivity of coals and carbons

Abstract The reactivity of a number of carbon substrates is determined by temperature-programmed oxidation (TPO). The oxidation temperature is a function of the degree of ordening of the carbon substrate and the presence of catalytically active material. The degree of ordening is determined with Raman spectroscopy. In the absence of catalytic material, the peak position of the “graphitic” Raman band (which is representative for the degree of ordening) is linearly correlated with the oxidation temperature.

Parametric study of N2O formation in coal combustion

Abstract A laboratory scale fluidized bed reactor was used to assess the influence of fuel properties and the major process-related parameters on N 2 O and NO formation during low temperature coal combustion. A set of coals was used, ranging in rank from lignite to anthracite, as well as chars of some of these coals. The results confirm the high N 2 O emissions (100–400 ppm) suggested for combustion facilities burning nitrogen-containing fuels at temperatures between 923 K and 1273 K. The main parameters that determine N 2 O formation from fuel nitrogen are the nitrogen content of the coal, temperature and coal rank. Decreasing temperature results in increasing N 2 O formation and simultaneously decreasing NO formation. Bituminous coals and anthracite coals showed substantially higher N 2 O yields than low rank coals. Formation of N 2 O results from the combustion of the volatiles as well as from char combustion. The air/fuel ratio influences N 2 O yields in particular near stoichiometric combustion conditions, indicating a possible way to N 2 O reduction.

Quantitative heat effects associated with pyrolysis of coals, ranging from anthracite to lignite

Abstract Differential scanning calorimetry (d.s.c.), in combination with thermogravimetry (TG), was performed up to 1050 K to study the heat effects associated with the pyrolysis of 17 different coals. The coals ranged in rank from anthracite to lignite. The net heat effects found were principally endothermic in nature; only in the case of low rank coals in the temperature range 450–750 K were some exothermic heat effects found. No large net heat effects are associated with the principal devolatilization reactions. The main part of the endothermic heat effects occurred above 750 K, and is associated with dehydrogenation of the coal char matrix. For the overall heat of pyrolysis, calculated from the net d.s.c. curves over the temperature range 450–1050 K, a maximum in endothermicity in the order of 450–550 Jg−1 (daf) was found for coals with a volatile matter content of 10–15 wt% (daf). On the other hand, the overall heats of pyrolysis are smaller than the specific enthalpies (∝cp.dT) of the coal chars.

Slow and Rapid Pyrolysis of Coal

Coal is a heterogeneous, mainly organic, material which decomposes upon exposure to high temperatures. This process is called pyrolysis. Coal pyrolysis is the most important aspect of coal behaviour because it occurs in all major coal conversion processes. Upon pyrolysis coal is divided into a hydrogen-rich volatile fraction, consisting of gases, vapors, and tar-components, and a carbon-rich solid residue. Moreover, for certain coals decomposition can result in a temporary softening of the solid material. The pyrolysis process consists of a very complex set of reactions involving the formation of radicals. The overall decomposition of coal is open to manipulation by a variety of experimental conditions. Due to the nature of the coal conversion processes distinction should be made between slow Cheating rate 1000 K/s) pyrolysis of coal. In this chapter the experimental techniques used to study the chemical an the physical changes which occur upon slow and rapid pyrolysis of coal are reviewed. Relevant experimental results are described. The influence of experimental conditions on the yield and composition of the volatile fraction, liberated upon coal pyrolysis, is considered. Some models, which describe the overall process of coal pyrolysis, are given. Finally, this chapter is concluded with a study on the thermal stability of aromatic model compounds, relevant in coal pyrolysis, to elucidate the nature and to describe the extent of primary and secondary reactions.

The thermoplasticity of coal and the effect of K2CO3 addition in relation to the reactivity of the char in gasification

The thermoplastic properties of a medium-volatile and a high-volatile A bituminous coal have been studied by means of high-pressure dilatometry as a function of the heating rate (10 and 65 K min−1), particle size (< 44 μm, < 75 μm, 106–200 μm and 212–400 μm) and gas pressure (1–28 bar). The thermoplastic properties of the coals are significantly different at elevated pressures from those at atmospheric pressure. At atmospheric pressure the volume swelling increases strongly with increasing heating rate and, at 10 K min−1, with increasing particle size. At a pressure of 28 bar however, the swelling is nearly independent of heating rate and particle size. The effect of addition of K2CO3 (20% by weight) was investigated at 65 K min−1 and turned out to depend on the gas pressure and particle size. At atmospheric pressure, K2CO3 reduces the dilatation of the coals almost completely. This reduction decreases with increasing pressure, especially for the larger particle size fraction (212–400 μm). A detailed mechanism for the interaction of alkali metal carbonates with the coal is suggested. The softening and swelling of coal particles has consequences for the available and accessible surface area of the char formed and thus for the reactivity of the char in gasification. Results of reactivity measurements in a CO2 atmosphere in a thermobalance that illustrate this effect are presented and related to the morphology of the char.

An exploratory study of the processing of plastics, by means of pyrolysis, with the emphasis on PVC/aluminum combinations

Abstract The feasibility of an alternative process for the processing of plastic/aluminum combinations by means of pyrolysis has been studied. Below the melting temperature of aluminum, the main fraction of the plastics can be devolatilized. As the plastic fraction often mainly consists of PVC, it is more economical to incinerate the pyrolysis products on-site, in combination with HCl removal and heat recovery. In the case of a high concentration of other plastics, e.g. ABS and PET, it is possible to get a valuable pyrolysis oil. Degradation of PVC mainly occurs through elimination of side groups, i.e. chlorine. 90% or more of this chlorine can be released as HCl. During pyrolysis of PVC, hardly any formation of polychlorinated dibenzodioxins and polychlorinated dibenzofuranes occurs. Depolymerization is the main degradation route of the styrene fraction in ABS (70 wt. %). Reactions of functional groups in the main chain take place during degradation of PET.

High temperature gasification of coal under severely product inhibited conditions: the potential of catalysis

Abstract The CO2-gasification reactivity of three widely differing coals was examined under severely product inhibited conditions. To enhance the reactivity K2CO3 was added as a catalyst. Two types of experiments were performed in a thermobalance: temperature programmed experiments to characterize the gasification behaviour of the different samples relative to each other; and isothermal experiments to determine the overall reactivity of the samples. Relatively low temperature data were extrapolated to predict high temperature reactivity. Extrapolation was carried out according to the Arrhenius equation. The validity of this simple model was experimentally verified for gasification in pure CO2 by using an entrained flow reactor system. The results showed that extrapolation of low temperature reactivity data over a 200–250 K range, according to the Arrhenius equation and using a realistic value for the apparent activation energy of the reaction examined, gives useful information on high temperature reactivity. Under product inhibited conditions complete gasification of coal on a time scale of seconds was found to be possible from 1350 K for K2CO3/lignite. Much higher temperatures were needed for the other coal samples studied, whether catalysed or uncatalysed. The consequences of the results in relation to a new type of iron oxide reduction process are briefly discussed.

Analytical curie-point pyrolysis-gas chromatography as a tool to characterize key parameters relevant to coal reactivity

Abstract Six coal samples, intended to be representative of the entire range of coal ranks and including two thermoplastic coals, were analyzed by Curie-point pyrolysis-gas chromatography to characterize parameters related to coal reactivity such as coal rank, thermoplasticity, and the addition of a catalyst to the coal. It is shown that the number and nature of the pyrolysis products, particularly the degree of oxygen functionality of the one-ring aromatic compounds formed, are characteristics of the rank of a coal sample. The presence of a series of long-chain n -alkanes, which are believed to be part of the so-called mobile phase of bituminous coals, is indicative of a thermoplastic coal. Low-temperature pre-oxidation is an effective method of destroying the thermoplastic properties of bituminous coals. The oxidative treatment affects the volatilization of the aliphatic moieties of the thermoplastic coals and significantly reduces the number and yield of pyrolysis products. The evolution of long-chain n -alkanes is almost completely eliminated upon pre-oxidation. Addition of potassium carbonate, which acts as a catalyst in coal gasification and combustion, results in an interaction of the alkali-metal ion with the carboxyl and hydroxyl groups in the coal. Because of this interaction the yield of phenolic compounds upon Curie-point pyrolysis is reduced. On the other hand, the formation of phenolic compounds, originating from ether linkages in the coal matrix, is not influenced by the addition of potassium carbonate.