Newcastle University

Introduction

As hydrogen becomes more important as an energy vector, the need to find inexpensive, less energy intensive and environmentally friendly ways of producing it increases. Current industrial production methods include steam methane reforming, partial oxidation and auto-thermal reforming. Each of these processes include expensive separation stages to generate hydrogen of a desired purity. Two production techniques which intrinsically require no further purification of hydrogen are chemical looping and membranes. Both methods produce hydrogen by splitting water. In chemical looping this is done over a reduced material, whereas in membranes the oxygen partial pressure gradient is used. So that further purification can be avoided the processes must be carefully designed to achieve an low impurity level (CO <50 ppm) for use in fuel cells. In a simple case of using CO as the reducing/oxygen lean gas the only products are CO₂, but when other gases are utilised (e.g. CH₄), different carbonaceous products can be produced. This increases the number of impurities possible. Results with impurity levels present have been displayed in order to highlight some of the challenges with these techniques: carbon deposition during reduction step and reactive gas break through for chemical looping and membranes respectively.

Introduction

The steam-iron process is a chemical looping cycle which traditionally utilises iron oxide as the oxygen carrier material (OCM). The iron oxide is alternately reduced and oxidised using carbon monoxide as the reducing agent, and steam as the oxidising agent. Water splitting occurs during the oxidation step and can produce pure hydrogen with suitably low CO levels (<50ppm) for use in fuel cells. Air can also be introduced into the cycle to further oxidise the iron oxide (to Fe₂O₃) providing heat to the system.

There is relatively little information on the reaction kinetics of the steam-iron process, however. Most, if not all, of the work on this process up until now has focussed on reactor inlet conditions only. In terms of reactor design and scale up, the reaction kinetics along the whole reactor bed are important. Thus this work investigates the reaction kinetics along the length of a fixed bed reactor. This is achieved within a differentially operated micro-reactor by simulating the changing conditions along the reactor length by changing the inlet gas feed to include quantities of product gas with the reactant gas.

Introduction

Solid oxide fuel cells (SOFCs) have received a great attention owing to their high energy conversion efficiency, fuel flexibility and low emissions. SOFCs require either an external or internal steam reforming step if they are to be used with hydrocarbon fuels. Internal reforming is more efficient but introduction of all of the water in the fuel cell inlet can lead to severe internal temperature gradients within the fuel cell as the reforming reaction is fast and endothermic. Here we intend to introduce water across the SOFC electrolyte while it is operational as shown in the concept. The distributed water introduction will result in controlled reforming, reduce fuel cell temperature gradients and increased membrane humidification. Y - doped BaCeO3 (BCY) was chosen as the electrolyte since it has recently shown proton and oxide ion conductivity simultaneously.

Introduction

‎The study is on the role of sodium addition on electrochemical promotion of catalysis in a Pt/YSZ system. Ethylene oxidation and NO reduction by propene are used as probe reactions. It was found in the first instance that sodium addition at low coverage affect the electrocatalytic activity of the system by lowering the catalyst work function, in analogous to the role of electronic promoter species electrochemically supplied during electrochemical promotion (by modifying the binding strength of adsorbed gases e.g. oxygen adsorbed from the gas phase). At high sodium coverage it appears that sodium increases the catalytic activity possibly by enhancing or creating new sites for, oxygen adsorption, oxygen diffusion and/or oxygen storage. At very high sodium coverage sodium poisoning occurs and the selectivity of NO oxidation to NO₂ is higher than NO reduction to N₂ or to N₂O.

Introduction

Electrochemical Promotion of Catalysis (EPOC) which is due to the catalytically promoting action of ionic species that spillover onto the catalyst surface under the influence of the applied potential can enhance in situ the catalytic activity and selectivity of metal films supported on solid electrolytes. This phenomenon can only take place at the three phase boundary (tpb).

Hypothesis: Available tpb length increases for samples of decreasing feature-length-scale (400µm, 200µm, 40µm, 20µm and 4µm) and it is expected to affect the spillover processes taking place under polarisation.

Objective :To study the effect of varying tpb length on the spillover processes for Pt (platinum) / YSZ (yttria stabilised zirconia) system.

Introduction

As hydrogen becomes more important as an energy vector, the need to find inexpensive, less energy intensive and environmentally friendly ways of producing it increases.  Current industrial production methods include steam methane reforming, partial oxidation and auto-thermal reforming.  Each of these processes include expensive separation stages to generate hydrogen of a desired purity.  One process which intrinsically requires no further purification of hydrogen is the Steam-Iron Process.  This process works by splitting water over a reduced material.  The material that is typically used is iron oxide.  The iron oxide is first reduced in the presence of carbon monoxide and then re-oxidised when exposed to steam.  An additional oxidation step with air is optional after the steam step.  This process takes advantage of the many stable oxidation states of iron, which make it a good oxygen carrier material, namely: metallic iron (Fe), wüstite (FeO), magnetite (Fe₃O₄) and haematite (Fe₂O₃).  Other reducing gases can also be utilised, producing different carbonaceous products . 

Thus in this work methane was used which initially produced CO₂ and H₂O, then CO and H₂.  A thermodynamic model was developed, linking the 3 stages in order to produce an auto-thermal system.  As the syngas produced during the reduction stage can go to synthesise methanol, the inclusion of the reaction enthalpy for methanol production is also investigated.

Introduction

Hydrogen is widely billed as the fuel of the future, but as a buoyant and volatile gas, a storage solution is required to make transport and storage more practical. One attractive solution is hydrogen storage as a liquid organic hydride, which can be dehydrogenated at the point of use to give hydrogen and a carrier which can be recycled by recharging with gaseous hydrogen. This is a convenient solution because it enables long-term storage of hydrogen without significant loss, and convenient transport using the existing liquid fuel infrastructure, with only minor modification. In particular, this PhD project focuses on the Methylcyclohexane-Toluene-Hydrogen (MTH) system, the reactions of which are well understood and documented, and the components of which have particularly favourable physical properties to act as an automotive fuel.