Development of intensified catalytic reactors for energy conversion

Abstract

In this work, two intensified catalytic reactor systems applicable for energy conversion have been studied. The first is an intensified membrane reactor combining oxygen separation from air using dense oxygen selective composite perovskite type membranes with a chemical reaction at the permeate side. In this part of the work, a membrane reactor made of stainless steel was designed, constructed and a fabricated planar oxygen ceramic membrane of the L₀.₆S₀.₄C₀.₂F₀.₈O₃-δ (LSCF6428) type tested in it. A challenge in this work was developing a procedure to hermetically seal the ceramic membrane to the stainless steel holder. An inexpensive soft glass composition was used and with some surface treatment of the stainless steel substrate was found to bond well with both the ceramic membrane and the stainless steel casing. Oxygen permeation experiments were conducted using the membrane reactor at a temperature of 650°C, under inert (helium) and reactive (CH₄or CO) conditions. Results obtained have shown that oxygen permeation increased with a chemical reaction by 1-2 orders of magnitude compared to permeation under permeate inert conditions. At operating temperature of 650°C, oxygen flux of about 0.025mLmin⁻¹cm⁻² and 0.40mlmin⁻¹cm⁻² under air/helium gradient and air/(He+CO) or air/(He+CH₄) gradient respectively were obtained. This result shows that from an oxygen permeation point of view, performance is superior in a combined separation and combustion mode than in a separation first to produce oxygen and combustion in a separate chamber. Further studies can explore the possibility of depositing catalytic nanoparticles on the permeate membrane surface to prevent complete oxidation and rather promote partial oxidation of methane to CO and H₂. Post operation examination of membrane showed permeate side surface changes which show some chemical stability issues of the membrane material. The second is a DBD plasma activated reforming of methane to hydrogen or syngas. In this study, the effects of different parameters such as applied plasma power, feed gas mixture flowrate, molar composition ratio of CO₂ and CH₄and inclusion of catalyst particles in plasma volume on the conversion of reactants and selectivity of products were experimentally investigated. Results obtained have shown the potential of plasma activated reforming of methane with carbon dioxide in one step to produce hydrogen or syngas and Higher Hydrocarbons (HCs) and oxygenates. The results from the experimental investigations in parametric effects can be used to optimise the process for the desired conversions and product selectivity. An important finding of this work was that the main products of this reforming process is not syngas as widely reported in literature, but a mixture of HCs and oxygenates, which may actually be very valuable products. Only about 10-20% of the carbon in the converted methane forms CO while the rest forms HCs, oxygenates, and, depending on conditions, carbon black. Experimental results have shown that carbon black forms in low concentration of CO₂in the feed gas. CO₂in higher quantities inhibits formation of carbon deposits in the plasma volume. The reported energy inefficiency (in literature) of plasma based reforming process may have been concluded from a syngas yield point of view while other products were considered as worthless by-products. An analysis of the “by-products” has shown that this might not be the case.