Thermodynamique et modélisation biphasique de la production de l’hydrogène par énergie solaire.

Abstract

Hydrogen appears today as an alternative to the fuels responsible for greenhouse gases, in this context comes this numerical simulation of the phenomenon of methane thermal cracking. In the simulation we took into account the existence of carbon as a homogeneous and nonhomogeneous powder. The mixture is considered poly – phasic, its main phase is the gas phase composed of methane and hydrogen, and the secondary phases are consisting of solid particles of the carbon black powder of the same or of different diameters. The phenomenon of methane cracking into hydrogen and carbon black takes place in a cylindrical cavity 16 cm in diameter and 40 cm in length under the heat of concentrated solar radiation without any catalyst. Three cases have been chosen; in all cases the primary phase contains methane and hydrogen gases. In the first case, we consider two phases; its main phase is formed of CH4 and H2 gases; the secondary phase is a homogeneous carbon black powder with 50 nm of diameter. For the second case we have two mixtures, one of them is formed by a heterogeneous micrometric powder of two diameters (d = 25 and 52 µm) in addition to CH4 and H2. The other mixture is with a heterogeneous nanopowder of two diameters (d = 20 and 80 nm) in addition to CH4 and H2. And finally, a third case of five phases with a powder of four different diameters 20, 40, 60 and 80 nm. The carbon powder particles act as a catalyst for methane cracking due to their significant radiative properties. The low Reynolds k – ε turbulence model was applied. A calculation code ""ANSYS FLUENT"" is to simulate cracking phenomena by choosing the Eulerian - Eulerian model. A simulation by injecting a flow rate of 0.4L / min for the three mixed was performed. After verifying the independence of the results of the first case (homogeneous powder with d = 50 nm) of the mesh and their validation with experimental work, a simulation by injecting a flow rate of 0.4L / min for the two other mixtures (three phases and five phases) has been executed. The first results show that the increase in the diameters of the carbon powder is getting closer and closer to the physical reality of the cracking phenomenon but it requires considerable calculation times. The same results show that the choice of a micrometric powder leads to lower mass fractions of hydrogen compared to a nanometric powder. Thus, a parametric study as a function of the inlet velocity and the intensity of solar radiation is realized. The quantities of hydrogen obtained using a tubular reactor under the heat of two concentrators depend on the input flow and the concentration of the solar concentrator with a carbon black powder formed by particles of 4 different diameters. For a flow rate not exceeding 0.3L / min, the two concentrations give the same amount of hydrogen. These quantities of hydrogen obtained reach maximum values for an inlet flow rate between 0.58 and 0.62 L / min for the two reactors, and then they decrease if we increase the flow rate. Any increase in throughput is therefore unnecessary. All results are then interpreted. The dimensions of the cavity chosen are important and this allows moving from the experimental scale to the industrial scale. Working without a catalyst further facilitates the separation of cracked gases. The use and sequestration of carbon black are proposed as useful solutions for industry and for soil fertility.