Experimental Study And Numerical Simulation On Super-adiabatic Combustion And Hydrogen Production Based On Pyrolysis Of H₂S In Porous Media

With the rapid development of Chinese economics, the problem of Energy and environment come up to surface. Premixed combustion in porous media has been proved to be an advanced combustion technology over conventional premixed combustion in which the premixed fuel and air burn as a free flame. Premixed combustion in porous media has many advantages such as reduced emissions of pollutants, wider domain of flammability, much higher thermal efficiency and radiant heat outputs, saving energy because of its highly developed inner solid surface and excellent property of heat transfer and heat accumulation. With the support of the National Nature Science Foundation of China (20307007), the characteristics of premixed combustion in porous media were investigated by the methods of experiment and numerical simulation. Hydrogen can be produced from sulfide hydrogen based on advantages of super-adiabatic combustion.First, detailed experimental investigation has been done with wave propagation in porous media of a porous combustion reactor. The Experimental results show that self-propagation reactions of premixed mixture of methane/air combustion are possible at a very low velocity, and the wave velocity is determined by equivalence ratio and size of pores of porous media, larger equivalence ratio has a larger combustion wave velocity, porous media which consist of 6mm diameter spheres have larger combustion wave velocity than which consist of 3mm diameter spheres. Downstream propagation of combustion wave can produce super-adiabatic combustion; otherwise under-adiabatic combustion happens. The emissions of NOx are less than 10ppm under super-adiabatic combustion.Computational fluid dynamics (CFD) combined with detailed chemical kinetics was employed to model the filtration combustion of methane/air in a packed bed of uniform 3 mm diameter alumina spherical particles. The standard k-εturbulence model and a methane oxidation mechanism with 23 species and 39 elemental reactions were used. Various equivalence ratios (1.5, 2.0, 2.2, and 2.5) were studied. The numerical results showed good agreement with the experimental data. For ultra-rich mixtures, the combustion temperature exceeds the adiabatic value by hundreds of centigrade degrees. Syn-gas (hydrogen and carbon monoxide) can be obtained up to a mole fraction of 23%. The numerical results also showed that the combination of CFD with detailed chemical kinetics gives good performance for modeling the pseudo-homogeneous flames of methane in porous media.The experimental study and kinetics investigation on the thermal decomposition of hydrogen sulfide was carried out. The decomposition efficiency of hydrogen sulfide under various temperature and residual time was studied and verified by the experimental data. The simulating results show that the reacting mechanism developed in this work can express the thermal decomposition of hydrogen sulfide accurately. The thermal decomposition of hydrogen sulfide relies on the reacting temperature, only the high temperature conditions lead to the high hydrogen generating efficiency. When the temperature is low, the residual time is the key factor influencing the thermal decomposition. The initial hydrogen sulfide concentration has great influence on the thermal decomposition efficiency, lower hydrogen sulfide concentration results in higher thermal decomposition efficiency. A thermodynamic model was used to study numerically by varying H₂S and oxidizer feed compositions. The results show that the adiabatic temperature is lower in low equivalence ratio, the increase of hydrogen sulfide and oxygen concentrations shifts the super-adiabatic partial oxidation combustion zone to high equivalence ratio; The thermal decomposition of hydrogen sulfide greatly relies on the super-adiabatic combustion temperature, and under higher super-adiabatic combustion temperature the higher hydrogen generating efficiency can be achieved. The results of thermodynamic simulation also suggest that the rage of optimal operation of a super-adiabatic partial oxidation unit is above an equivalence ratio of 6, where the hydrogen output is maximized and SO₂ generation is minimized. Under the same combustion temperature lower hydrogen sulfide concentration results in higher thermal decomposition efficiency.Finally, investigation of hydrogen producing based on super-adiabatic combustion has been done through experiments and numerical simulation. The results show that super-adiabatic combustion could offer the high-temperature environment for the decomposition of hydrogen sulfide to produce hydrogen and sulfur and hydrogen concentration was 2.08% atΦ=2.5. The combination of computational fluid dynamics and chemical kinetics investigation was employed to model the filtration combustion of hydrogen sulfide a 17-species, 57-elemental reaction mechanism. The results of the simulation show good agreement with experimental data.