10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Properties of Carbon Particles Generated by Methane Decarbonization in Oxygen Deficient Gas Streams
Mohammad Javad Afroughi, Farjad Falahati, Larry W. Kostiuk, JASON S. OLFERT, University of Alberta
Abstract Number: 593 Working Group: Combustion
Abstract At a sufficiently high temperature, methane (CH4) can be decomposed to solid carbon black (C(s)), a feedstock for many industrial products, and hydrogen (H2), which can be used as a carbon-free fuel. As such, methane decarbonization is viewed as a potential method to produce hydrogen (or heat or electricity if the hydrogen is used as a fuel) without CO2 emissions. Furthermore, the solid carbon can be sold for use in industrial products; however, its value depends on the physical properties (i.e. size, morphology) of the carbon. Several methane decarbonization processes have been proposed including bubbling the methane through molten metal, heating the methane by a plasma, or in the hot products of combustion gases.
This study investigates the physical properties of carbon particles generated during decarbonization of methane in hot products of two different premixed flames. An inverted burner is designed to provide a rich, laminar, premixed flame of propane- and methane-air, with a total constant flow rate of 36.5 SLPM. The burner later injects methane co-flow to be decomposed within a closed quartz tube (20 cm in diameter and 68 cm in length). By placing ceramic blocks with axial concentric holes, a thermally insulated, reaction chamber of 4 cm diameter is created inside the tube. The formed products of decarbonization are discharged through an exhaust with two sampling branches. Measuring gaseous concentrations, a branch of products is dried and sent to a gas chromatographer (GC), while the other branch samples carbon particles with a nitrogen dilution system. Size distribution, effective density, and volatile mass fraction of particles are obtained with sequential arrangements of a differential mobility analyzer (DMA), catalytic denuder, centrifugal particle mass analyzer (CPMA), and condensation particle counter (CPC). Dilution ratios are calculated using simultaneously measured CO2 concentrations in exhaust products and diluted samples. Axial temperature distribution in the reaction chamber is also measured by a K-type thermocouple.
Different flow rates of methane (0.5 to 5 SLPM) are injected in oxygen deficient gas stream of propane- and methane-air premixed flames, with maximum temperature of 1170 and 1135 °C, respectively, to be decarbonized. Results show that properties of generated particles are strongly dependent on type of the premixed fuel, and decarbonized methane flow rate. Generally, decarbonization using propane-air premixed flame produces larger particles with higher number concentrations, for a given injected methane flow rate; e.g. total particle number concentration and median mobility diameter increase from 1.27×108 to 1.69×108 cm-3 and from 21 to 58 nm, respectively, due to change of premixed fuel from methane to propane at a constant decarbonized methane flow rate of 0.5 SLPM. Furthermore, median diameter of size distribution is inversely proportional to decarbonized methane flow rate, and decreases from 58 to 18 nm (21 to 18 nm) following the injection of 0.5 to 5 SLPM of methane to the propane-air (methane-air) premixed flame. It is also notable that effective density and volatile mass fraction of particles are size-dependent and decrease from ~2000 to 500 kg m-3 and from ~0.55 to 0.35, respectively, with the increase in mobility diameter from 14 to 700 nm.