Temperature-programmed Combustion Research Articles

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.

TG, TG-MS, and FTIR Characterization of High-Yield Biomass Charcoals

The pyrolysis and combustion characteristics of various high-yield charcoals manufactured in a process development unit were studied by thermogravimetry (TG), thermogravimetry−mass spectrometry (TG-MS), and Fourier transform infrared spectrometry (FTIR). Charcoals resulting from two runs with a macadamia nutshell feedstock and one run with a eucalyptus wood feedstock are compared. The peculiarities of devolatilization, oxygen gasification (temperature-programmed combustion), and the formation of nitrogen oxides are discussed. Small sample sizes (0.2−4 mg) were employed to minimize the effects of heat and mass transfer limitations; nevertheless the results offered a reliable characterization of kilograms of charcoal. Samples produced in different runs or taken from different parts of the reactor behaved similarly with only minor differences that we attribute to reactor inhomogeneities and variations in feedstock composition. In the presence of oxygen, two partial processes were detected. The lower temperat...

Temperature-programmed combustion studies of coal and maceral group concentrates

Temperature-programmed combustion profiles of coals and their maceral group concentrates were obtained by thermogravimetric analysis in air at a heating rate of 15 K min −1. The maceral group concentrates were prepared by density separation of demineralized, micronized coal samples. Particle size had a significant effect on combustion profiles, with increased complexity in terms of the number of peaks observed for micronized coal compared with larger size fractions. Analysis of the combustion profiles of macerals and reconstructed whole coals indicated that in some cases maceral interactions can occur, especially in the presence of liptinite. It was also found that the reactivity of a coal cannot be predicted from the reactivities of its maceral components.

NO Release and Reactivity of Chars during Combustion: The Effect of Devolatilization Temperature and Heating Rate

The study has involved the investigation of the influence of pyrolysis heat treatment temperature (HTT) and heating rate on the reactivity and the release of char-N during temperature-programmed combustion (TPC) of a set of wire mesh reactor chars in a thermogravimetric analyzer−mass spectrometer system. The gas evolution profiles are bimodal and this indicates the presence of species of different reactivity. It was found that increasing pyrolysis temperature and heating rate both produced significant variation in the reactivity of the resultant chars with the former being more influential. This is apparent from the shift of peak positions and the change in the relative intensity of the low- and high-temperature peaks of CO, CO2, and NO evolution profiles. The differences in the CO2 and NO evolution profiles observed between the Gedling entrained flow reactor (EFR) and wire mesh reactor (WMR) chars of similar heat treatment temperatures may be explained by the different extents of pyrolysis experienced by the chars. However, chars produced in the WMR with pyrolysis temperatures up to 1200 °C show little variation in the char-N conversion to NO. This is believed to be due to the highly reactive nature of the chars which give rise to a high extent of reduction of the primary product NO formed during gasification leading to a low NO/char-N ratio. Heat treatment of the chars at lower heating rates and longer soak times to temperatures in the range 1100−1400 °C lead to reduced char reactivity and higher NO/char-N ratios under temperature-programmed combustion conditions. The results are consistent with the reduction of the primary oxidation product NO on the surface and in the pores of the char.

The detection of reactive intermediates in the combustion and pyrolysis of coals, chars and macerals

The temperature-programmed combustion of coal was studied and reactive intermediates were observed by gas sampling directly above the sample in a thermogravimetric analyser-mass spectrometer (t.g.a.-m.s.). Species detected included OCS, OCN, HCN and (CN) 2. NO levels observed at the exit of the t.g.a. differed from those measured directly above the sample, the former usually being higher. This observation can be attributed to the conversion of volatile nitrogen-containing species to NO by homogeneous gas-phase reactions. The temperature-programmed combustion of maceral concentrates was also studied, to assess differences due to heterogeneity in the coal. The differences between the macerals were in species detected and their relative concentrations. Fusinite macerals tended to produce the highest levels of NO measured directly above the sample, compared with vitrinites and semifusinites, and the highest levels of sulfur species. The results are discussed in terms of the release of volatile nitrogen species and the reduction of NO produced in char gasification on the surface and in the pores of the char.

95/06327 The incorporation of nitrogen into isotropic carbons and its release during temperature-programmed combustion

95/06327 The incorporation of nitrogen into isotropic carbons and its release during temperature-programmed combustion

Carbon-13 materials as models for NOx and N2O release during coal char combustion

The influence of structural characteristics, in particular heteroatom content, on the ammoniatreatment of carbon-13 materials has been studied. The resulting carbon-13 materials were investigated for their temperature-programmed combustion characteristics by thermogravimetric analysis-mass spectrometry (TG-MS). The use of ca 99% isotopically pure carbon-13 allowed the analysis of N2O and N2 in the presence of CO2 and CO respectively. Bimodal peaks for N2, NO and N2O were detected in temperature-programmed combustion, suggesting two types of nitrogen functionality in the carbon. The relative yields of the three products were strongly influenced by the heteroatom content of the precursor sample. The sample which had a high heteroatom content gave N2 as the major product rather than N2O. Oxygen, present as an impurity in the carbons, is an important factor in the incorporation of nitrogen by treatment with ammonia.

Detection of reactive intermediate nitrogen and sulfur species in the combustion of carbons that are models for coal chars

The release of nitrogen and sulfur during the combustion of coal chars is a major environmental problem. Model coal chars, prepared by the high-pressure carbonisation of polynuclear aromatic precursors to 873 K and subsequent calcination at atmospheric pressure to 1273 K, were studied using temperature-programmed combustion (TPC) in 20% O 2 He . These carbons, which are derived from pure organic precursors with well-defined nitrogen and sulfur functionality, are highly reproducible and are virtually free of catalytic effects due to the absence of metallic species. The reactions were conducted using a thermogravimetric analyser coupled to a quadrupole mass spectrometer (TG-MS). The evolved gases were analysed throughout the course of the combustion. Sampling of the gases directly above the sample by means of a heated capillary line allowed the detection of reactive species including HCN, C 2N 2, and OCS. Sampling the gases at the exhaust of the TGA allowed the estimation of the gas composition at nearequilibrium conditions. In this case, the reactive species were no longer detected and gas-phase reactions, such as the conversion of HCN and C 2N 2 to NO and the conversion of CO to CO 2, were apparently occurring in the gas phase. The results are discussed in terms of the nature of the surface nitrogen and sulfur species present during combustion.

The incorporation of nitrogen into isotropic carbons and its release during temperature-programmed combustion

The combustion behavior of a suite of isotropic carbons has been investigated using a thermogravimetric analyser mass spectrometer. The carbons were prepared from polyvinylidene chloride (PVDC) and a phenolic resin by carbonisation under argon to 873 K in the pressure range 0.1–7.5 MPa. Nitrogen was introduced into the carbons (HTT 873 K) by both ammonia treatment at the final heat-treatment temperature (1073 K) and during the carbonisation process. Nitrogen was also incorporated into the PVDC and resin carbons by co-carbonisation of the precursor with carbazole. Comparison of the temperature-programmed combustion results obtained for the isotropic carbons with those for the anisotropic acenaphthylene carbons containing nitrogen, introduced by ammonia treatment, showed that there were distinct differences. The reactivities of the carbons were enhanced by the ammonia treatment for the anisotropic acenaphthylene carbons, but reduced for the isotropic phenolic resin and polyvinylidene chloride carbons. The proportions of the two types of nitrogen functionality introduced by ammonia treatment were distinctly different for the isotropic and anisotropic carbons. The results are discussed in terms of the capping of the active sites to form functionalities substituted on the aromatic structures, and nitrogen incorporation into the ring structure of the ‘graphene’ layers. The conversion of char nitrogen to NO during combustion is discussed in relation to char structural characteristics, reactivity, and nitrogen content.

NO x release and reactivity of chars from a wide range of coals during combustion

NO x release from a wide range of chars prepared in an entrained-flow reactor from coals and vitrinite concentrates of different rank and geographic origin was studied. Temperature-programmed combustion of the chars using thermogravimetric analysis and mass spectrometry showed that the fractional conversion of char-N to NO increased with increasing rank of the original coal at first and then reached a plateau above a vitrinite reflectance of ~1.5%. The fractional conversion of char-N to N 2 was roughly constant throughout the suite of chars. The HCN level was significantly lower for the chars than for the parent coals. The HCN is believed to be a devolatilization product in advance of oxidation to N 2, NO and N 2O. The conversion of char-N to NO was correlated with char reactivity. Normalization by surface area halved the range of reactivity values, but the relation between char-N conversion to NO and reactivity was still apparent. The results suggest that as well as the surface area, the intrinsic reactivity of the char is important in determining the reduction of NO on the carbon during combustion.

A comparison of NO release from coals and entrained-flow reactor chars during temperature-programmed combustion

A comparison was made of the temperature-programmed gasification in 20% oxygen in argon of six coals covering a wide range of rank and the chars produced from the coals in an entrained-flow reactor (EFR) at 1000 °C. The temperature-programmed oxidative gasification of the coals has two main regions, representing mainly gasification of the volatiles released and of the char formed during pyrolysis. The NO gas evolution profiles reach a maximum after the corresponding CO and CO 2 profiles for both the coals and EFR chars. The temperature at which these peaks occur increases with increasing rank. In the lower-temperature range where the primary devolatilization of the coal occurs, the NO CO 2 and NO CO ratios decrease with increasing temperature. For the low-rank coals, a peak due to HCN is also detected in this region as well as at the initial char formation stage. In the char gasification region of the coals and for the EFR chars, the NO CO 2 and NO CO ratios increase with increasing temperature and extent of burnoff. The ratio of NO released during gasification to the total nitrogen varies from 0.14 to 0.44 for the EFR chars and from 0.31 to 0.57 for the coals. It is apparent that there are similarities in the release of nitrogen during the combustion of coals of a wide range of rank and of EFR chars derived from these coals.