Frans Snijkers

Experimental investigation of CaMnO3-δ based oxygen carriers used in continuous Chemical-Looping Combustion

Three materials of perovskite structure, (M = Mg or Mg and Ti), have been examined as oxygen carriers in continuous operation of chemical-looping combustion (CLC) in a circulating fluidized bed system with the designed fuel power 300 W. Natural gas was used as fuel. All three materials were capable of completely converting the fuel to carbon dioxide and water at 900°C. All materials also showed the ability to release gas phase oxygen when fluidized by inert gas at elevated temperature (700–950°C); that is, they were suitable for chemical looping with oxygen uncoupling (CLOU). Both fuel conversion and oxygen release improved with temperature. All three materials also showed good mechanical integrity, as the fraction of fines collected during experiments was small. These results indicate that the materials are promising oxygen carriers for chemical-looping combustion.

Ceramic Foams Coated with Zeolite Crystals

Ceramic foams can be used as filters, dust collectors, light weight components and catalyst carriers. They can be produced by a variety of techniques. The performance of ceramic foams will be strongly improved when their mechanical properties are improved. For this reason, we produced ceramic foams both by a modified reaction bonded (RB) replica technique and by gel casting. With both methods, reticulated foam structures with enhanced mechanical strength were obtained. Zeolites are a special type of materials that are characterized by high catalytic properties. They can be brought on a structured carrier by dip and slurry coating. Nevertheless, in situ coating has as main advantage that the support is used as the base for nucleation. This results in the formation of a chemical bond between the zeolite crystals and the support. The goal of this contribution is twofold: at first we demonstrate how Al2O3 foams with improved mechanical strength can be produced both by the modified RB-alumina replica technique and by gel casting. Secondly, it is shown that these ceramic foams can be coated with (silicalite) zeolite crystals by insitu crystallization from a precursor sol. The two-layer material combinations have been characterized with FESEM, XRD, CT (computer assisted tomography), IA (Image Analysis) and by mechanical tests.

Catalyst design with porous functional structures

The chemical industry is experiencing important changes. The driving force for these changes is a growing need to improve competitiveness and consolidate market positions while complying with the regulations for safeguarding human health, and the environment. Currently, what is known as “green chemistry” or “sustainable technology” is at the heart of the changes the chemical industry is undergoing. Catalysis and materials science will play an important role in this new approach. One domain with specific relevance is porous ceramics and metals, substrates with pores sizes ranging from vacancies at the atomic level to macro pores with sizes of millimeters. There are plenty of emerging applications for porous functional components. Each application will specify the window of properties of the porous material. A technology assessment of the developed porous materials and powder processing techniques for their use as catalyst or membrane system will be presented, limited to inorganic porous materials which can be synthesized by dry and wet powder processing methods. The applications overview for porous materials is focused on macroporous components with a designed functional coating.

Chemical Looping Combustion: an Emerging Carbon Capture Technology

Chemical looping combustion (CLC) is a promising technology for energy production with inherent capture of carbon dioxide at minimal energy penalty. In CLC, oxygen is transferred from an air reactor to a fuel reactor by means of a solid oxygen carrier. Direct contact between air and fuel is avoided, resulting in an undiluted CO2 exhaust stream. As such, CLC was picked up recently as a high potential carbon capture and storage (CCS) technology. While initial focus was on storage projects, CO2 is more and more considered as a valuable chemical substance for enhanced oil/gas recovery projects as well as for the production of chemicals, polymers or building materials. A critical aspect of the CLC technology is the oxygen carrier performance which has a very strong impact on the economic viability. Parameters such as particle size, density, porosity, strength, attrition resistance, reactivity, environmental aspects and cost, define the performance of the oxygen carrier. The first generation oxygen carriers was Ni-based. However, due to cost of nickel and toxicity, a search for Ni-free oxygen carriers was conducted with similar or superior performance in CLC. This lead to the development of Cu-, Fe and Mn-based oxygen carriers, that demonstrate the beneficial oxygen uncoupling effect, with complete fuel conversion as a result. In this contribution it is shown that the industrial spray-drying technique is a very versatile and scalable technique for the fabrication of oxygen carriers. New and promising oxygen carriers with varying compositions, good fluidisability, high sphericity, high attrition resistance, and homogeneity on the micro-scale have been synthesized. Different materials such as perovskite type materials based on calcium-manganate, magnesium manganates, copper based materials, and iron manganates have been investigated for their performance with promising results towards complete combustion and high attrition resistance.

Mixed Conducting Ceramic Capillary Membranes for Catalytic Membrane Reactors: Performance of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Capillaries

Oxygen-permeable perovskite ceramics with mixed ionic-electronic conducting properties can play an important role in the high temperature separation of oxygen from air. Such membranes are envisaged for application in catalytic membranes reactors and in oxy-fuel and pre-combustion technologies for fossil fuel power plants enabling CO2 capture. Since large-scale gas separation applications demand high membrane surface/volume ratios, membranes with capillary or hollow fiber geometry have a distinct advantage over tubular and flat sheet membranes. The fabrication and performance of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) capillary membranes is presented. The capillaries were made by a spinning technique based on phase inversion using a sulfur or non-sulfur containing polymer binder. Attention is given to the polymer solution and ceramic spinning suspension in order to avoid the formation of macrovoids and achieve gastight membranes. The comparison of the performance of sulfur-free and sulfur-containing BSCF capillaries with similar dimensions revealed a profound impact of the sulfur contamination on both the oxygen flux and the activation energy of the overall oxygen transport mechanism. In addition the effect of activation layers on oxygen permeation is studied.

Near-Net-Shape Ceramics by Reactive Processing

Since conventional production of high-temperature materials involves high investments and costly consumption of both energy and time, reaction engineering methodology combined with near-net shaping is often the answer to problems associated with the fabrication of advanced materials. Over the last decades, the number of different reaction–based processing methods for near-net-shaped ceramics has gradually increased. In this review, different reactive processing techniques and their potential for near-netshaping are treated, e.g. SHTS (self-supporting high temperature synthesis), the Lanxide method DIMEX®, reaction bonding (RB), reactive processing of Alumina-Aluminide Alloys (3A) and Al2O3-Al alloyed metal composites (3AMC). In addition to their potential for near-net shaping, other advantages to reactive processing routes are recognized to be reduced processing temperatures, reduced glassy phase formation at the grain boundaries, fine grained microstructures and improved mechanical strength. Since the exothermic reactions constitute the base for reactive processing of high quality materials in an economic way, control of these reactions is essential. The process flows are described together with characteristic features of process and materials. In addition, specific aspects of reaction-based synthesis will be illustrated with examples from own work in the area of reaction bonding of silicon nitride and alumina.

Reaction-Based Processing Methods

Reaction–based processing methods constitute a large collection of different fabrication routes. The aim of this contribution is to give an overview of reaction based processing methods (Reaction Bonded, SHTS, DIMOX, reactive sintering, CVI) and to discuss recent developments and future of these alternative ways to produce ceramic materials. Reaction engineering methodology is uniquely suitable to solve many synthesis, analytical and processing problems associated with the fabrication of advanced ceramics. We discuss in the present paper the principles of the manufacturing routes, their advantages and disadvantages and the possible applications of these special materials.