Combustion behavior of single iron particles-part I:An experimental study in a drop-tube furnace under high heating rates and high temperatures

Micrometric spherical particles of iron in two narrow size ranges of (38–45) µm and (45–53) µm were injected in a bench scale, transparent drop-tube furnace (DTF), electrically heated to 1400 K. Upon experiencing high heating rates (104–105 K/s) the iron particles ignited and burned. Their combustion behavior was monitored pyrometrically and cinematographically at three different oxygen mole fractions (21%, 50% and 100%) in nitrogen. The results revealed that iron particles ignited readily and exhibited a bright stage of combustion followed by a dimmer stage. There was evidence of formation of envelope micro-flames around iron particles (nanometric particle mantles) during the bright stage of combustion. As the burning iron particles fell by gravity in the DTF, contrails of these fine particles formed in their wakes. Peak temperatures of the envelope flames were in the range of 2500 K in air, climbing to 2800 K in either 50% or 100% O2. Total luminous combustion durations of particles, in the aforesaid size ranges, were in the range of 40–65 ms. Combustion products were bimodal in size distribution, consisting of micrometric black magnetite particles (Fe3O4), of sizes similar to the iron particle precursors, and reddish nanometric iron oxide particles consisting mostly of hematite (Fe2O3).</p

Combustion of Uniformly Sized Glassy Carbon Particles

A method for production of spherical particles of glassy carbon has been developed. The panicles exhibit some differences in surface structure, but are otherwise uniform in size and shape. In drop-tube combustion experiments at wall temperatures ranging from 1200 K to 1600 K, some panicles ignited and burned rapidly, reaching temperatures several hundred degrees higher than the wall temperature, while others burned as much as two orders of magnitude more slowly at temperatures only slightly higher than that of the reactor wall. In spite of these differences in ignition behavior, the particles burned with an intrinsic reaction rate that was in close agreement with the Nagle and Strickland-Constable rate for pyrolytic graphite as well as rates previously measured for carbon black and coal char oxidation. X-ray diffraction measurements show that graphitization occurs rapidly in a hot oxidizing atmosphere. Rapid anneal ing of the carbon structure might account for the similarity of the high temperature oxidation rates of carbons from different sources

Physical Properties of Particulate Matter Emitted from Combustion of Coals of Various Ranks in O-2/N-2 and O-2/CO2 Environments

This work examined the particulate emissions from pulverized coals burning under either conventional or oxyfuel combustion conditions. Oxyfuel combustion is a process that takes place in O-2/CO2 environments, which are achieved by removing nitrogen from the intake gases and recirculating large amounts of flue gases into the boiler; this is done to moderate the high temperatures caused by the elevated oxygen partial pressure therein. In this study, combustion took place in a laboratory laminar-flow drop-tube furnace (DTF) in environments containing various mole fractions of oxygen in either nitrogen or carbon dioxide background gases. A bituminous coal, a sub-bituminous coal, and a lignite were burned at a DTF temperature of 1400 K. Trimodal ash particle size distributions were observed with peaks in the submicrometer region (similar to 0.2 mu m), as well as in the supermicrometer region (similar to 5 mu m and >10 mu m). Both submicrometer and supermicrometer particulate emission yields of all three coals were typically lower in O-2/CO2 than in O-2/N-2 environments. Emission yields typically increased with increasing oxygen concentration in the furnace, with an exception noted at moderate oxygen mole fractions (20%-30%) in CO2, where significant amounts of unburned carbon were detected. Submicrometer particulate yields were found to be comparable in the effluents of all three coals, independently of their ash contents, whereas supermicrometer particulate yields were nearly analogous to the ash contents of the three coals. Scanning electron microscopy (SEM) revealed that submicrometer particles were spherical, whereas supermicrometer particles were often of irregular shapes, fractured spheres, and spheres with small particles attached to their surface

Glassy carbons from poly(furfuryl alcohol) copolymers: structural studies by high-resolution solid-state NMR techniques

The chemical structure of glassy carbon particles produced from poly(furfuryl alcohol) copolymers is studied by ^(13)C cross-polarization/magic-angle spinning (CP-MAS) NMR and high-speed ^1H MAS NMR. In agreement with earlier proposals, ^(13)C NMR spectra confirm the buildup of a highly unsaturated system at the expense of furan rings and aliphatic carbon atoms, and upon heating to 800 K this conversion is essentially complete. Successive carbonization by air oxidation or pyrolysis at temperatures up to 1600 K is reflected in a gradual decrease of the ^(13)C chemical shift from ca. 130 to 115 ppm versus tetramethylsilane. ^1H MAS NMR is used to detect and quantitate the amount of residual C-bonded hydrogen species at various stages of the carbonization process. In addition, these spectra show intense, narrow resonances due to sorbed H_2O molecules, which resonate over a wide range of chemical shifts (between 2.5 and -8 ppm versus tetramethylsilane). In analogy with effects observed by Tabony and co-workers for molecules adsorbed above the basal plane of graphite, the upfield shifts observed for water sorbed in the glassy carbons of the present study are attributed to the large susceptibility anisotropy of submicroscopically ordered, turbostratic, or partially graphitized regions of the samples. The extent of this ordering is inversely correlated with the absolute content of residual C-bonded hydrogen species and depends mainly on the temperature of pyrolysis, whereas the oxygen content of the heating atmosphere and the composition of the initial polymeric material appear to be of secondary importance. The results suggest that sorbed H_2O molecules can function as sensitive NMR chemical shift probes for the initial stages of crystallization processes in glassy carbons

Comparison of Fine Ash Emissions Generated from Biomass and Coal Combustion and Valuation of Predictive Furnace Deposition Indices: A Review

To address important ash-related issues associated with burning solid biomass fuels for power generation, this paper reviews results of studies performed at the Northeastern University (NU) Combustion and Air Pollution laboratory and elsewhere under well-characterized conditions. It compares the physical and chemical characteristics of fine ash emissions generated from the combustion of pulverized biomasses to those from pulverized coals, since biomass is considered as a substitute fuel for coal in power generation, and assesses their furnace surface deposition propensities. Comparisons show that combustion of some biomasses may generate disproportionally higher emissions of submicron ash particles than combustion of coals (0.03-1.1 versus 0.04-0.06 kg/GJ, respectively). The high submicron emissions of biomass are problematic, as conventional particulate control devices have low collection efficiencies for such small particles. Moreover, the chemical composition of submicron particles of biomass typically contain large amounts of alkalis (potassium and sodium), chlorine, sulfur and, often, phosphorous, whereas those collected from combustion of coal contain large amounts of silicon, aluminum, iron, and sulfur. The composition of biomass ashes renders them more amenable to deposition on furnace surfaces, as calculations based on published empirical surface deposition indices show. These calculations, as well as experiences elsewhere, indicate that the slagging and, particularly, the fouling deposition prospects of most biomasses are significantly higher than those of coals. (C) 2015 American Society of Civil Engineers

Chemical Composition of Submicrometer Particulate Matter (PM1) Emitted from Combustion of Coals of Various Ranks in O-2/N-2 and O-2/CO2 Environments

A laboratory-scale investigation has been conducted on the physical and chemical characteristics of particulate matter emissions (ashes) from pulverized coals burning in the air or in simulated oxy-fuel environments. Oxy-fuel combustion is a process that takes place in O-2/CO2 gases, using an air separation unit (ASU) to supply the oxygen and a flue-gas recirculation (FGR) stream to supply the carbon dioxide to the boiler. In order to investigate the effects of the background gas on the particulate matter generated by the combustion of coals of different ranks, a bituminous, a sub-bituminous, and a lignite coal were burned in an electrically heated laminar-flow drop-tube furnace (DTF) in both O-2/N-2 and O-2/CO2 environments (21% < O-2 < 6096). A recent publication by the authors reports on the physical characteristics of the particulate matter; hence, this work focuses on the chemical composition, specifically targeting the difficult-to-capture submicrometer size (PM1) ashes. Particulate matter was collected by a low-pressure multistage cascade impactor and was analyzed for chemical composition by Scanning Electron Microscopy Energy Dispersive X-ray Spectroscopy (SEM-EDS). Selected samples were also examined by Electron Microprobe Analysis (EMA). Results showed that submicrometer (PM1) ashes of the bituminous, the sub-bituminous, and the lignite coals contained mostly Si, Al, Fe, Mg, Ca, K, Na, and S. Prominent components of large submicrometer particle (PM0.56-1) compositions were Si and Al (Ca in sub-bituminous), whereas small submicrometer particles (PM0.1-0.18) were markedly enriched in S. The mass yields of elemental species found in the submicrometer-size particles from all three coals were lower when combustion occurred in CO2, instead of N-2 background gases. The chemical composition of the PM0.56-1 subcategory was not affected by the background gas. To the contrary, the composition of the PM0.1-0.18 subcategory was affected by replacing N-2 with CO2, and mass fractions of Si, Ca, and Al decreased whereas Na, K, and S increased. Furthermore, in PM0.1-0.18, when the O-2 mole fraction increased in either N-2 or CO2, the mass fractions of Si, Ca, and Al increased at the expense mostly of Na, K, and S, but also Fe in the case of the sub-bituminous coal. Experimentally derived partial pressures of the volatile suboxide SiO (P-SiO) at the char surface were compared with the predictions of an ash vaporization model without and with coupling with a particle combustion model; they were found to be in the range of the model predictions

Production of polymer particles in powder form using an atomization technique

A process for producing spherical polymer particles which may be either monodisperse of a predetermined and controlled size, or polydisperse, using a liquid atomization technique. The process includes an aerosol generator to create a stream or multiple streams of liquid droplets sprayed into a thermal reactor. The aerosol generator sprays the feed solution which comprises liquid organic monomers or semi-polymerized monomers, a polymerization catalyst and optionally, a solvent, into the thermal reactor environment. The solvent evaporates allowing polymerization reactions to commence. Polymerization may proceed by a variety of methods. Polymerization is completed during the flight-time of the droplets and the solid polymer particles are collected at the bottom of the reactor. The size of the particles in every batch may be predetermined and controlled by fine tuning the aerosol generator's configuration or operational parameters to adjust the size of the droplets of the feed solution being sprayed into the reactor. In one variation, the feed solution to the aerosol generator may be a polymer dissolved in an appropriate solvent. The aerosol generator then sprays the polymer solution in the thermal reactor to generate particles by evaporating the solvent

Characterization of Particulate Matter Emitted from Combustion of Various Biomasses in O-2/N-2 and O-2/CO2 Environments

This work reports on the physical and chemical characteristics of the ashes of biomass residues burned in air as well as in simulated dry oxy-combustion conditions. Three pulverized biomass residues (olive residue, corn residue, and torrefied pine sawdust) were burned in a laboratory-scale laminar-flow drop tube furnace heated to 1400 K. Olive residue resulted in by far the largest particulate yields both submicrometer (PM1) and supermicrometer (PM1-18)-whereas torrefied pine sawdust resulted in the lowest. The collected particulate yields of these two biomasses were analogous to their ash contents. The collected particulate yields of corn residue, however, were lower than expected in view of its ash content. To investigate the effects of the oxygen mole fraction and of the background gas, the O-2 mole fraction was varied from 20% to 60% in either N-2 or CO2. Submicrometer particulate matter (PM1) emission yields of all three fuels were lower in O-2/CO2 than in O-2/N-2 environments; they typically, but not always, increased with increasing O-2 mole fraction in either background gas. The background gas had little effect on the chemical composition of the PM1 particles. High amounts of alkalis (potassium, calcium, and sodium) as well as of. chlorine were observed in PM1. In addition, phosphorus and sulfur also existed in high amounts in PM1 from combustion of corn residue. Supermicrometer particles (PM1-18) yields exhibited no clear trend when the background gas was changed or when the oxygen mole fraction was increased. The composition of these particles reflected the bulk ash composition of the parent fuels

Oxidation Kinetics of Monodisperse Spherical Carbonaceous Particles of Variable Properties

Synthetic chars of variable physical and chemical properties have been developed to study char oxidation mechanisms and rates. The char particles were spherical and monodisperse, with sizes ranging from a few microns to several tens of microns. The particles were made from a carbon-yielding polymer and pore-forming additives. The surface areas of the chars made from different additives varied by more than two orders of magnitude and the porosities varied by a factor of five. The pore size distributions included both micro and transitional pores. X-ray studies revealed that all chars were amorphous when heat treated to temperatures up to 1600 K in an inert atmosphere. However, upon oxidation at 1600 K, the carbon matrix underwent partial graphitization. This transformation was particularly pronounced for some of the polymer pore-former chars. Combustion experiments showed that the total surface area of the chars increased dramatically with conversion, revealing the existence of a vast network of micropores. Apparent oxidation rates were higher for the chars that contained transitional pores in a microporous matrix. When compared with the rates reported in the literature for coal derived chars, the calculated intrinsic rates were lower at intermediate particle temperatures (800-1600 K) but comparable at elevated temperatures (1800-2300 K). As the temperature was increased further, the intrinsic rates decreased consistent with the Nagle and Strickland-Constable kinetic mechanism [l]