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Experimental studies on premixed flame propagation characteristics in a sub-millimeter-scale closed chamber are performed. Propane/air mixtures are ignited in the center of the visualized chamber a...

Flame propagation pictures of a propane/air mixture are captured by a high-speed camera in a visualized constant volume combustion chamber with a diameter of 150 mm, a parallel plate height of H = ...

Based on the optimization of the recently proposed gas phase mechanism for n-heptane pyrolysis, mechanisms for polycyclic aromatic hydrocarbon (PAH) formation and a detailed wall surface reaction, a basic gaseous pyrolysis mechanism with 1127 steps and a simplified surface reaction mechanism with 34 steps were developed, respectively. After performing the detailed estimation and modelling procedures for the surface site density (SDEN) related to the particle diameter, particle density, C/H ratio and other parameters determined by the experimental tests and observations, numerical studies were integrated with the developed coke model to simulate the pyrolysis process and the coking properties in a millimetre-sized tube reactor on the Chemkin Pro software platform using a two-dimensional cylindrical shear flow (CSF) module. The prediction values of the pyrolysis product concentration and coking rate at different temperatures were compared with the experimental data, and agreement was obtained. Under pyrolysis conditions of an n-heptane inlet flow rate of 0.5 ml/min, an operating pressure of 1.0 MPa and a furnace temperature of 973–1073 K, the research results showed that the hydrogen abstraction reaction, as the first and key step of coke formation, is easily affected by gaseous diffusion. As the temperature and particle diameter increase, the inhibition of diffusion becomes stronger. Among all the considered radicals, the abstraction ability of the H radical is the strongest, and it is easily restricted by diffusion. The presence of abundant radicals such as CH3, C2H3, and C3H5 at low temperatures can compensate for this limitation. For the unsaturated species addition reactions, the calculation results showed that the addition of olefins, alkynes, diolefins and aromatic hydrocarbons occurs in sequence with an increase in residence time. With increasing temperature, the addition contributions of olefins such as C2H4 and C3H6 gradually weaken, while those of small molecule aromatic hydrocarbons such as benzene (A1) and styrene (A1C2H3) remarkably increase. Alkynes and diolefins, such as C2H2 and C3H4, have low concentrations under the experimental conditions, and their addition contributions are very low and can be ignored. Due to the addition of unsaturated species, the concentration of H2 in the gas phase is significantly improved. The modeling work can predict the coke formation characteristics and quantitatively evaluate the change in the coke formation rate, which is helpful for explaining the intrinsic thermal pyrolysis coking mechanism in a round tube, such as the regenerative cooling system of supersonic engines, among others.

Coke deposition is considered a challenging problem during hydrocarbon pyrolysis. To get more insights into pyrolysis coking characteristics in the regenerative cooling channels, the unsteady-state experiments of n-heptane were performed in a 2.0 mm (internal diameter) passivated STS304 tube at 873–1073 K and 1.0 MPa with a feeding flow rate of 1.0 mL/min during pyrolysis for 5–60 min. The unsteady-state and spatial coke distributions were obtained. Results showed three distinct stages of coking, namely, catalytic coking, the transition stage, and pyrolytic coking. Coke samples obtained under different temperatures were characterized by element analyzer, scanning electron microscope (SEM), thermogravimeter coupled with a Fourier transform infrared spectroscopy (TG-FTIR), X-ray diffraction (XRD), and Raman spectra apparatus. The coke could be classified into two or three types according to its oxidative activity, and was composed of aromatic carbon and aliphatic hydrocarbons. The morphology of the coke transitions from filamentous to spherical with temperature and pyrolysis time. The C/H ratio of coke increased with the temperature in the catalytic coking stage, but decreased in the pyrolytic coking stage. High H contents and extensive defects of the coke observed at high temperatures could attributed to the generation of large amounts of pyrolytic coke and high coking rate, which resulting a lower degree of dehydrogenation. The presence of metallic substances in the coke verifies the catalytic effect at the beginning of coking and indicates the occurrence of carburization corrosion.

A thermal cracking experimental equipment of hydrocarbon fuels was built to study n-heptane pyrolysis and the effect of reaction conditions on this reaction process. The main species were measured and the change rules were analyzed on the range of temperature 873–1073 K and pressure 0.1–3.5 MPa. The total content of alkenes products was more than alkanes on this pyrolysis process. Compared to alkenes with same number carbons, the alkanes were more easy to decompose with temperature but more conducive to formation with pressure increasing. The content of ethylene is usually the most on above reaction conditions, but its descent is also the fastest with pressure increasing. A mechanism model of n-heptane pyrolysis (44 species and 166 reactions) was constructed and validated by experiments on different conditions. Compared with n-heptane oxidation detailed model of Version 3.1 from Lawrence Livermore National Laboratory (LLNL), the pyrolysis model present a better accordant with experiment results on a range of temperature and pressure. The kinetic reaction of n-heptane pyrolysis was analyzed with present pyrolysis model, and the pyrolysis reaction pathway for the main products was obtained. The formation of alkenes are mainly through C C bond dissociation reaction, especiallyβ-C dissociation, and small alkanes are formed mainly by radical metathetical or synthesis reaction, the former are endothermic reactions, but the latter are mostly exothermal reactions. The properties of some main reactions have a critical role for the change of product content with temperature and pressure, which is the main reason for the variety of products selectivity under different conditions. Pressure increased the pyrolysis residence time and mass density but it does not significantly affect the reaction energy, so its contribution to conversion rate of fuels thermal cracking is limited, although it changes the reaction pathway greatly. However, the temperature can increase obviously the reaction activation energy, even though the residence time and concentration is decreased, the conversion rate of n-heptane pyrolysis still increased.

The pyrolysis experiments of n-heptane in the CO2/H2O atmosphere were carried out in a millimeter-sized tube reactor. Mass yields and molar concentrations of gaseous products, including C2H4, C3H6, H2, CO, CH4, and C2H6, were obtained at a temperature range of 1073−1373 K, and atmospheric pressure. The pyrolysis characteristics of n-heptane under different reaction atmospheres and residence times were summarized. The results showed that the mass yields of ethylene and propylene in the mixed atmosphere of CO2 and H2O were 7.9 %–36.8 % higher than those in the single H2O atmosphere under low residence times. The yield and selectivity of olefin increased with increasing the temperature within narrow residence time range (less than 0.2 s). At 1173 K, the total mass yield of ethylene and propylene was more than 60 %, and the olefin selectivity was about 40 %, which was the peak value among all experimental temperature conditions. With a rise in temperature, there also occurred an interaction between pyrolysis and gasification processes as reported in the literature, and the gasification reaction rate increased, which led to the high production of CO and H2. The overall studies in this paper show that reasonable and accurate control of reaction residence time in the CO2&H2O pyrolysis process is important to obtain higher olefin yield and selectivity, especially when the operating temperature is higher than 1173 K.

A model was established to describe the effects of inorganic matter on the char-CO2 gasification reactivity. The effects of seven inorganic elements on the char-CO2 reactivity were considered: K, Na, Ca, Mg and Fe, which play a catalytic role, and Si and Al, which play an inhibitory role. A catalytic/inhibitory index factor model and a saturated catalytic K ratio model were established and then integrated into a char-CO2 reactivity model. In the modelling process, active site adsorption theory was also considered to calculate the surface area of catalytic elements. The char-CO2 reactivity was calculated by combining the catalytic/inhibitory sub-models of inorganic elements and the non-catalytic char-CO2 reactivity model. In the experimental part, inorganic elements were loaded on de-ashed tobacco stem char, and the char-CO2 reactivity was then tested. The catalytic/inhibitory index factors of seven inorganic elements were quantitatively expressed, and the standardized results showed that the catalytic index factors of K, Na, Ca, Mg and Fe were 1.000, 0.510, 0.580, 0.053 and 0.387, respectively. The inhibitory index factors of Si and Al were 0.695 and 0.453, respectively. Finally, eight typical biomass samples were pyrolysed in a drop tube furnace to produce char and then analysed by TGA to test the char-CO2 reactivity validation model. A comparison of the predicted results with the experimental data shows good agreement, proving the applicability of the developed model.