After characterization, the influence of physical factors on the combustion behavior was calculated and the intrinsic reaction rates of the letters were related to their chemical nature. 3.III Properties of materials containing carbon black and PEG 3.IV Chemical composition of glassy carbons.
LIST OF FIGURES
150 5.2 Arrangement of lenses to maximize the efficiency of radiation collection. wavelength curves; selection of wavelengths 152 5.4 Schematic of the pyrometer gain circuits. formed by melting a platinum wire. Arrhenius-type plot of the intrinsic reaction rate coefficient vs. the inverse of the particle temperature. a,b) Temperature-time profile, model: solid line, experiments: dashed line. c,d) Burnout, relative surface oxygen concentration and relative radius vs. time. a,b) Temperature-time profile, model: solid line, case(II) (cenospheres) dotted line; Experiments:
INTRODUCTION
CHAPTER 2
The combustion of the glassy carbon particles was studied for a variety of oxygen partial pressures and wall temperatures in the laminar flow reactor. The cause of the variability between particles is currently unclear.
CHAPTER 3
- Particle Generation and Curing
- Particle Characterization
- Microscopy and Sorption Studies
- X-Ray Diffraction
- Small Angle X-Ray Scattering
- Additional Sorption Measurements
Both solid and hollow spheres can be produced depending on the nature and concentration of the pore former. The pore structure and the chemical properties of the particles can be varied by the addition of pore forming agents.
MATERIALS CONTAINING CARBON BLACK AND PEG
WIDE ANGLE X-RAY DIFFRACTION DATA
SMALL ANGLE PARAMETERS FOR GLASSY CARBONS
AEROSOL GENERATOR
The Chemi'cal Shift
This fractional type of unit has the advantage of being independent of the magnitude of B0, although shielding depends on the strength of the external field. With appropriate calibration of the apparatus and for the case of pure compounds, it may be possible to estimate the absolute number of nuclei responsible for the peak. Because of the low abundance of the 13C isotope, carbon-13 homonuclear interactions are relatively small, and homonuclear decoupling is not necessary.
However, the effect of the magnetic dipole-dipole interactions (as well as the chemical shift anisotropy) can be removed by rapid centrifugation or the sample at the angle of 54.7°. The required spin rates depend on the strength of the dipolar interactions to become lime-averaged; in the case of 1 H :vl AS-.\ ;vi R. speeds of 5-8 kHz are not untypical,. Acetone was added to the polymer in a 2 to 1 volume ratio to lower the viscosity of the polymer and to facilitate mixing and subsequent atomization.
NMR Techniques
The materials used in the present study were particles formed from PF A alone as well as particles formed from mixtures of PFA with different amounts of the following pore formers: tannic acid, polyethylene glycol (PEG), glycerol, and Triton-X 100. Elemental analysis of the materials at different degrees of carbonization and oxidation was performed by oxidizing samples at 1100 K in 02 and monitoring the combustion products using a Perkin Elmer analyzer. Tables I-III give an overview of the samples studied, their carbon and hydrogen content, and (where appropriate) their amount of "burnout", which corresponds to the weight loss on high temperature oxidation.
The cross-polarization pulse program has incorporated spin temperature inversion23 to suppress artifacts and 'flip' the 1 H magnetization in the z direction with an additional 90".'.x pulse at the end of the acquisition period. Serious problems of probe disorder was raised for the case of oxidized samples due to their improved electrical conductivity.The free induction decays were multiplied by an exponential filter function, equivalent to a Lorentzian line broadening of 10-20 Hz, and filled with zeros from 4K to 16K data points, before Fourier transformation.
It is worth noting that the chemical structure of char after this step of the heat treatment is similar to the structure of charred coal, since methylene bridges between aromatic rings are no longer present25-27. 13C CP-MAS NMR studies of the effects of heat treatment and oxidation at temperatures above 800 K proved difficult due to probe tuning problems resulting from the high conductivity of these materials, as mentioned elsewhere for graphites28. The spectra of materials pyrolyzed at 800 K (Figure 6b) are still dominated by this type of hydrogen, although in light of the previously described 13C CP-MAS results, these protons must be chemically different from those of the original starting material.
Its room temperature spectroscopic properties meet all the criteria typical of a species experiencing motion-averaged chemical environments16. The sharpness of the central band, down to very low spin speeds, the weak intensities of the spinning sidebands, as well as the narrow linewidth (about 2 kHz) observed for this component in non-spinning samples, show a significant reduction of the inner core dipole -dipole couplings and chemical shift anisotropies through molecular motion. As in the case of the pyrolyzed samples discussed above, both new peaks can be assigned lo.
Chemical shifts and Ordering States i·n Glassy Carbons
10 and 11 show that the influence of the initial polymer mixture composition on the degree of ordering is on average small. This suggests that the type-Ha peaks are directly associated with the presence of oxygen during high-temperature treatment, possibly leading to some carbon-oxygen bonds on the internal surface of the glassy carbon.41 Thus, the type-Ila resonances can be water represent molecules in the vicinity of such sites. The authors are grateful to Larry HeHling for performing the elemental analysis of the material.
Dependence of 1 H MAS NMR chemical shifts (compared to tetramethylsilane) of sorbed water in all investigated glassy carbons on the residual content (in wt%) of C-bonded hydrogen in the carbon (see text). Dependence of chemical shifts 1 H \fAS :\MR on particle burn-up in all studied glassy carbons. Effect of heat treatment atmosphere on 1H MAS-NMR spectra of PFA chars at constant temperature (1500 K).
LEGEND
BONDED HYDROGEN (wt%)
- Opti"cal System
- Electroni"cs
- Calibration
At the top of the injector was a low-absorbing quartz window press fit with "0-ring". Therefore, the length must be kept to a minimum, consistent with the elimination of the background radiation. The particle feeder is placed in the upper part of the injector below the quartz window.
The small size of the feed pipe did not block the radiation from most events in the combustion chamber. The output signal of the detector at a given wavelength,\ is a function of two independent parameters, the temperature and the area of the particle. 34;Measured temperatures of combustion of pre-pulverized fuel particles and the nature of the primary reaction product.".
OPTICAL FIBER
OPTICAL
DETECTOR (2.4 x2.5mm)
Particle Characterization
Total surface areas were measured by both small-angle scattering (SAXS) and gas adsorption techniques. Apparent densities, porosities, pore volume, and pore area distribution for pore openings greater than 5.5 nm were determined by mercury intrusion using a Quantachrome A utoscan porosimeter. From multilayer adsorption and capillary condensation of nitrogen, we also determined the distribution of pores in the size range from 5 to 30 nm.
The true densities were determined by helium pycnometry at room temperature and, where applicable, by wide-angle X-ray scattering. -angle X-ray diffraction profiles were obtained using a Guinier camera using CuKo: radiation at 40 kV and 25 mA. The camera used monochromator focusing with a bent quartz crystal to record everything without a background. hkl ) reflections on high resolution photographic film.
Combustion Experiments
A water-cooled gas dilution sampling probe was mounted at the bottom of the hot zone to collect partially oxidized particles in experiments where combustion was not complete. The injector was water-cooled and had a wide bore (1 cm i.d.) to facilitate viewing down from the top of the furnace. The furnace wall temperature profile was measured on the outer surface of the alumina tube using permanently installed type S thermocouples and on the inner surfaces with a vanishing optical pyrometer.
These readings have an uncertainty of ± 15 K and were used for the subsequent determination of the combustion environment of the particle. A lens mounted on the top of the injector focused the radiation emitted by the burning particles onto the tip of a forked optical fiber bundle. The viewing was done against the cold background provided by the sample collection probe at the bottom of the oven.
Physi"cal Structure of Parti"cles
The average micropore dimension of the chars was measured by small-angle X-ray scattering and found to be about lOA, depending on the pore former used[12] as shown in Table I. The apparent density of the materials was measured by atmospheric mercury intrusion, and the helium or skeleton density was measured by helium penetration at atmospheric conditions. The results are also shown in Table I together with the total porosity of chars, which includes all pores with a radius of less than 7 µmin.
The total surface areas of the materials were measured by the BET method, using nitrogen adsorption at 77 K. SAXS studies show that the pores expand and take on more regular shapes due to the combined effects of annealing and high-temperature oxidation, and that pore entrances open due to the effect of oxidation alone. The pores become available for external gas penetration and as a result the surface area measured by gas absorption increases with the rate of combustion.
Chemical Properties of the particles
The carbon content of the copolymer also decreases with increasing tannic acid concentration due to the low carbon content of pure tannic acid. The heating value of the charcoal containing 40% glycerol 7% Triton X-100 is the highest, probably because of the higher hydrogen content. The heating value of the charcoal decreases, as expected, during combustion due to hydrogen loss (see Table II).
In partially burned samples, improved two-dimensional ordering is suggested by the appearance of (100) and (110) peaks. The diffraction spectrum of the material formed from 17% tannic acid and PFA is shown in Fig. Therefore, it appears that pore-forming agents change both the physical and chemical structural properties of the letters.
Combustion Behavior
Therefore, the combustion of the synthetic coal particles was not controlled by external mass transfer. An efficiency factor 17 can be defined as the ratio of the apparent velocity (that has been corrected for external diffusion resistance) to the velocity that would be achieved if there were no resistance to diffusion in the pores[4]. The efficiency factor is a function of the Thiele modulus and the effective diffusivity.
The Thiele modulus is given by. where Ac is the specific total surface area of the char. The right-hand side of eq. 9) is a function of the reaction order and known quantities and can therefore be evaluated explicitly. This behavior is further enhanced by the availability of the oxidant, as all high-temperature experiments were performed in pure oxygen.
