Properties of Carbon Black (cas 1333-86-4) produced by the thermal decomposition of methane in the products of premixed flames

Add time:09/02/2019    Source:sciencedirect.com

The thermal decomposition of methane is a technique used to manufacture hydrogen gas and Carbon Black (cas 1333-86-4). The physical properties of carbon black produced by the thermal decomposition of methane (TDM) in the O2-deficient gas products of two premixed flames (propane- or methane-air) were investigated under different flow rates of decomposing methane injection (0.5–5 SLPM). An inverted burner was designed to provide a fuel-rich, laminar premixed flame, to produce hot gas into which methane was injected to thermally decompose inside a reactor. Particles from TDM were extracted by a nitrogen dilution system at the immediate exit of the reaction chamber, where another branch of the exhaust was dried and directed to a gas chromatograph. The carbon black particles were characterized by size resolved number concentration, mass concentration, effective density, volatility, and internal mixing state using different arrangements of a differential mobility analyzer, catalytic denuder, centrifugal particle mass analyzer, and condensation particle counter, as well as by morphology and primary particle size using transmission electron microscopy. A bimodal number-size distribution was observed at all conditions with count median diameters (CMDs) less than 58  nm and 21  nm when using propane- or methane-air premixed flames as the heat source, respectively. Higher number concentrations and mass concentrations with larger CMDs were achieved under lower flow rates of decomposing methane injection. For a given flow rate of decomposition methane, mass concentration and CMD increased significantly when using propane as the fuel, compared to the methane fuel. The size segregated mass fraction of internally mixed volatile content in particles was similar for both heat sources, showing a roughly constant fraction of volatile material in particles produced by the decomposition of 0.5 and 5 SLPM of methane (10%–30%) but a measurably larger fraction (55%–30%) with a decreasing trend as a function of particle size from the decomposition of 1 SLPM of methane. The effective density of denuded particles was similar, but slightly higher, than the effective density of soot from a wide range of internal combustion engines. A higher denuded effective density was observed in the particles with higher volatile contents (particles from decomposition of 1 SLPM of methane), suggesting the restructuring of carbon black into more compact clusters due to excessive volatile condensation. TEM analysis revealed some similarity between the produced carbon black and engine soot in terms of morphology and primary particle diameter (both below 40  nm). Beside the carbon black properties, the efficiency of TDM in this configuration was also investigated by evaluation of methane destruction efficiency, as well as carbon black and hydrogen production efficiencies, based on the product gas composition and the particle mass concentration. It was found that longer residence times corresponding to lower flow rates resulted in higher conversion efficiencies in terms of methane destruction efficiency (maximum of ∼95%), hydrogen production efficiency (maximum of ∼80%), and carbon black production efficiency (maximum of ∼1%). It was concluded that the significantly low efficiency of carbon black production is due to the high amount of CO formation, likely through the gasification process.

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