J. C. Diniz da Costa

Ceramic membranes for gas processing in coal gasification

Coal is the most abundant fossil fuel in the world and is likely to outlast gas and oil for centuries. However, with global issues like climate change at the forefront of public attention there is a trend towards the development of a carbon constrained economy. As a result, research has intensified in the last decade on modes of operating coal fired power plants with carbon capture and storage (CCS). In particular, pre-combustion options via coal gasification, especially integrated gasification combined cycle (IGCC) processes, are attracting the attention of governments, industry and the research community as an attractive alternative to conventional power generation. It is possible to build an IGCC plant with CCS with conventional technologies however; these processes are energy intensive and likely to reduce power plant efficiencies. Novel ceramic membrane technologies, in particular molecular sieving silica (MSS) and pervoskite membranes, offer the opportunity to reduce efficiency losses by separating gases at high temperatures and pressures. MSS membranes can be made preferentially selective for H2, enabling both enhanced production, via a water–gas shift membrane reactor, and recovery of H2 from the syngas stream at high temperatures. They also allow CO2 to be concentrated at high pressures, reducing the compression loads for transportation and enabling simple integration with CO2 storage or sequestration operations. Perovskite membranes provide a viable alternative to cryogenic distillation for air separation by delivering the tonnage of oxygen required for coal gasification at a reduced cost. In this review we examine ceramic membrane technologies for high temperature gas separation and discuss the operational, mechanical, design and process considerations necessary for their successful integration into IGCC with CCS systems.

Novel molecular sieve silica (MSS) membranes: characterisation and permeation of single-step and two-step sol-gel membranes

High quality MSS membranes were synthesised by a single-step and two-step catalysed hydrolyses employing tetraethylorthosilicate (TEOS), absolute ethanol (EtOH), I M nitric acid (HNO3) and distilled water (H2O). The Si-29 NMR results showed that the two-step xerogels consistently had more contribution of silanol groups (Q(3) and Q(2)) than the single-step xerogel. According to the fractal theory, high contribution of Q(2) and Q(3) species are responsible for the formation of weakly branched systems leading to low pore volume of microporous dimension. The transport of diffusing gases in these membranes is shown to be activated as the permeance increased with temperature. Albeit the permeance of He for both single-step and two-step membranes are very similar, the two-step membranes permselectivity (ideal separation factor) for He/CO2 (69-319) and He/CH4 (585-958) are one to two orders of magnitude higher than the single-step membranes results of 2-7 and 69, respectively. The two-step membranes have high activation energy for He and H-2 permeance, in excess of 16 kJ mol(-1). The mobility energy for He permeance is three to six-fold higher for the two-step than the single-step membranes. As the mobility energy is higher for small pores than large pores and coupled with the permselectivity results, the two-step catalysed hydrolysis sol-gel process resulted in the formation of pore sizes in the region of 3 Angstrom while the single-step process tended to produce slightly larger pores

Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification

Abstract BSCF hollow fiber membranes possessing an asymmetric layered structure were prepared using a modified phase inversion process followed by subsequent sintering at temperatures from 1100 to 1175 ˚C. The fibers were characterized by SEM, and tested for air separation at ambient pressure and temperatures between 650 and 950 ˚C. Although the prepared hollow fibers resulted in self-supported asymmetric substrate with a very thin densified perovskite layer for mixed conduction, O2 permeation was controlled by surface O2 exchange kinetics rather than bulk diffusion. In order to improve O2 flux, surface modification was carried out by the attachment of Pt particles on the surface of the hollow fiber. The maximum O2 flux measured for pure perovskite hollow fiber was 0.0268 mol m−2 s-1 at 950 ˚C whilst O2 fluxes increased up to 25% after the surface modification using Pt micro-particles.

Combined investigation of bulk diffusion and surface exchange parameters of silver catalyst coated yttrium-doped BSCF membranes

The combined effect of minor yttrium doping and silver catalyst deposition on the surface kinetics (k(chem)) and bulk diffusion (D(chem)) of BSCF (Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ)) perovskite membranes was explored using electrical conductivity relaxation (ECR) and validated using oxygen permeation measurements. Yttrium doping of BSCF to form Ba(0.5)Sr(0.5)Co(0.8)Fe(0.175)Y(0.025)O(3-δ) (BSCFY) improved both the surface exchange kinetics and the bulk diffusion by an average of 44% and 177% respectively, supporting improved oxygen permeation measurements. The deposition of a silver catalyst on BSCFY further improved the surface kinetics by 63-450% at intermediate operating temperatures (600-750 °C), and reduced the activation energy from 163 to 90 kJ mol(-1). Interestingly, these improvements did not translate into enhanced oxygen fluxes for the silver coated thicker 0.5 and 1 mm membranes, indicating that the oxygen ion transport was limited by bulk diffusion. However, oxygen permeation measurements on catalyst-coated 0.3 mm-thick membranes yielded improvements of 20-35% in the range 600-900 °C. The silver catalyst was beneficial in overcoming surface kinetic limitations for the thinner 0.3 mm BSCFY membranes, thus suggesting that the critical thickness of BSCFY membranes lies around ∼0.4 mm and validating the ECR measurements.

Porous ceramic membranes for membrane reactors

This chapter discusses the research and development of porous ceramic membranes and their application as membrane reactors (MRs) for both gas and liquid phase reaction and separation. The most commonly used preparation techniques for the synthesis of porous ceramic membranes are introduced first followed by a discussion of the various techniques used to characterise the membrane microstructure, pore network, permeation and separation behaviour. To further understand the structure-property relationships involved, an overview of the relevant gas transport mechanisms is presented here. Studies involving porous ceramic MRs are then reviewed. Of importance here is that while the general mesoporous nature of these membranes does not allow excellent separation, they are still more than capable of enhancing reaction conversion and selectivity by acting as either a product separator or reactant distributor. The chapter closes by presenting the future research directions and considerations of porous ceramic MRs.

Silica membrane reactors for hydrogen processing

Abstract This paper presents an analysis of membrane reactor operation and design for enhanced hydrogen production. Silica derived membranes were used for gas permeation studies and a membrane reactor for the water gas shift reaction. A model of the equilibrium reaction is developed and analysed with respect to operational factors such as temperature and pressure analysed in consideration for production of a 99% pure H2 stream. These factors influence the optimisation of the reaction and permeation rate as well as the equilibrium conversion. It was found that using H2 permeation membranes, the H2 equilibrium could be shifted towards the products. In turn, this provided better conversion at higher temperatures. The cost of H2 production using membrane reactors is dependent upon several engineering process parameters such as reaction rates, permeation, selectivities, temperature and pressure. Silica membranes assembled in membrane reactors out performed conventional reactor systems. Silica membranes were synthesised showing permeations of 5 × 10−8 mol m−2 s−1 Pa−1 and H2/CO selectivities >10. The silica membrane capital cost per kg H2 produced ranged from US

Characterization and Pervaporation Study on Ethanol Separation Membranes

0·25 to 3&middot00 for 10 to 80%H2 separation respectively.

Oxygen transport membranes: dense ceramic membranes for power plant applications

Inorganic membranes have many advantages for dehydration of azeotropic mixtures of ethanol and water for renewable fuel purposes. In this work, we developed an inorganic membrane from γ-alumina and tested it for its ability to selectively permeate water over ethanol. Ethanol adsorbed both chemically and physically on the surface of γ-alumina, blocking the 47 Å pores sufficiently to enhance water selectivity. Stable flux was observed over 6 h, but after 4–5 h of continuous testing, water selectivity rose above 200 due to this blocking phenomenon.

7 - Perovskite membrane reactors: Fundamentals and applications for oxygen production, syngas production and hydrogen processing

This chapter addresses the latest developments in ceramic materials for use in oxygen transport membranes. Oxygen production is a multibillion dollar business with applications in clean energy, petrochemicals and metallurgical processes. The chapter describes the transport mechanisms involved and possible integration of these membranes in oxyfuel coal combustion and coal gasification. The chapter finishes with a discussion of the most appropriate membrane geometries and considerations for the development of membrane modules for industrial applications.

Modern Environmental Improvement Pathways for the Coal Power Generation Industry in Australia

This chapter addresses research and development of membrane reactors utilizing perovskite materials which can conduct oxygen ions and hydrogen protons at high temperatures; they are therefore finding applications in oxygen and hydrogen separation and reaction processes. This chapter introduces the structure, transport mechanisms and performance of various perovskite membrane materials, followed by an in-depth analysis of employing perovskite membranes for both oxidative and non-oxidative reactions. The membrane function of either selectively removing a reactant to shift the equilibrium or selectively adding a reactant to control the reaction mechanism and associated side reactions is important. We end by discussing future research trends and the major challenges that must be overcome for industrial take-up of this technology.