Catalytic combustion without reactant premixing

In the present thesis a method for the catalytic combustion of methane and air that avoids the premixing stage of the two reactants is discussed. Aim of the investigation is to assess the applicability of such reactor concept in a gas turbine combustor. Segregation of the two reactant into two separate chambers is accomplished using a porous ceramic membrane tube. Methane and air diffuse and react inside the wall of the membrane where the active catalyst (Pd or Pt) is deposited. The ratio of methane to air ranges from 0 to infinity throughout the cross section of the porous tube due to the segregation of the reactants. Various experimental apparatuses were prepared and assessed with the aim of achieving a reproducible parametric study of the system. Experiments were carried out on Pd and Pt supported α-Al2O3/SiO2 ceramic membranes in a scale suitable for a laboratory reactor. Ignition of the catalytic reaction on Pd started at 390°C, however CO and H2 were detected at high temperatures and high methane inlet concentrations suggesting that side reactions such as partial oxidation, water gas shift and steam reforming were occurring together with the combustion. Platinum membranes showed lower activity but H2 was not detected and CO production was still low at 580°C. Reaction kinetics were determined for lean and rich methane-air mixtures. In order to reduce the pore size of commercial alpha-alumina membranes silica was deposited inside the pores using a colloidal SiO2 suspension. The catalyst is prepared via wet impregnation and freeze dried in order to obtain a uniform distribution of the active component throughout the cross section. SEM, EDX, Atomic absorption, mercury porosimetry, and permeability tests were used to characterise the catalyst and the support. A 1D mathematical model based on Dusty Gas Model was developed to investigate the behaviour of the reactor under different conditions. This was solved using a custom made FORTRAN programme. According to the results of the modelling, high conversion and outlet temperatures can be achieved with membrane reactors provided that small channels are used. However, care should be taken in designing the reactor and selecting the characteristics of the membrane in order to prevent catalyst overheating. The modelling also showed that the membrane reactor can compete with monolithic reactors in the catalytic combustion of methane applied to gas turbines provided that suitable design parameters are used