Modelling study of two chemical looping reforming reactor configurations: Looping vs. switching

Autothermal Chemical Looping Reforming (CLR) is a promising technology for hydrogen production with integrated CO2 separation. Conventional CLR employs two fluidized beds (fuel and air reactors) with an oxygen carrier circulating between them. In this way, CLR supplies heat to the endothermic reforming reaction while avoiding fuel/nitrogen mixing. This configuration can achieve steady operation and low gas leakages between reactors, but has some drawbacks. The complex interconnected configuration is challenging to scale up, especially under the pressurized conditions required for high process efficiency. Moreover, the external circulation of particles through cyclones and loop seals increases reactor costs and imposes a narrow operating window. These challenges can be circumvented by carrying out the reduction/oxidation reactions in a single bubbling/turbulent fluidized bed alternatively fed with fuel and air. This gas switching (GS) concept has been demonstrated experimentally (1) and thermodynamically (2) for chemical looping combustion (CLC) and can be extended to CLR. The primary drawbacks of the GS concept are the undesired mixing between fuel and nitrogen after the gas feed switch and the need for high temperature valves at the reactor outlet. The objective of this paper is to compare the conventional CLR configuration against the GS configuration using a generic phenomenological model. This model is based on the probabilistic approach (3) which makes it applicable to the fluidization regimes used in both concepts. Steady state (looping) and transient (switching) simulations are completed and results are compared in terms of important variables such as methane conversion and CO2 separation efficiency. REFERENCES 1. Zaabout, A., Cloete, S., Johansen, S. T., Sint Annaland, M. van, Experimental Demonstration of a Novel Gas Switching Combustion Reactor for Power Production with Integrated CO2 Capture. Industrial & Engineering Chemistry Research, 2013. 52(39): p. 14241-14250. 2. Cloete, S., Romano, M. C., Chiesa, P., Lozza, G., Amini, S., Integration of a Gas Switching Combustion (GSC) system in integrated gasification combined cycles. International Journal of Greenhouse Gas Control, 2015. 42: p. 340-356. 3. Abba, I. A., Grace, J. R., Bi, H. T., Spanning the flow regimes: Generic fluidized-bed reactor model. AIChE Journal, 2003. 49(7): p. 1838-1848

High-purity H2 production with CO2 capture based on coal gasification

A novel hybrid concept is proposed, combining Pd-alloy membrane and low temperature separation technology, to produce pure H2 from gasified coal and capture the main part of the generated CO2. 75% of the H2 produced from gasification and water-gas shift is separated from the shifted syngas through H2-selective Pd-alloy membranes. After water removal, the H2-depleted, CO2-rich retentate stream is compressed and cooled, after which CO2 is condensed out at a purity level of ∼99%. The “waste” volatiles from the low-temperature CO2 separation constitute a low heating value syngas that is burnt in a gas turbine. The gas turbine with a steam bottoming cycle generates a surplus of electricity that could be employed for H2 liquefaction. Altogether, the concept has the potential to be developed into a stand-alone high-purity H2 production unit with CO2 capture, suitable e.g. for remote areas from where H2 and possibly also CO2 must be transported by ship. However, the investigations of three different process alternatives, as well as three membrane separator parameters, illustrate that there are many degrees of freedom in the proposed concept that require further analysis, both individually and how they interact, in order to establish an optimized and purposeful stand-alone H2 production concept.acceptedVersio

A Novel Swing Adsorption Reactor Cluster (SARC) for cost effective post-combustion CO2 capture: A thermodynamic assessment

The SARC consists of a number of standalone reactors where a solid sorbent is carbonated by a flue gas and regenerated by a combination of vacuum and temperature swing. Efficiency is maximized through heat integration between carbonation and regeneration using a heat pump. Initial power plant simulations showed 9.4%-points energy penalty when integrated into a pulverized coal plant. This is in-line with reported energy penalty for MEA and VPSA technologies, but great potential for further efficiency improvements exists. Future studies will investigate the effect of SARC process parameters and sorbent material selection on the energy penalty

The effect of chordae tendineae on systolic flow

When using Computational Fluid Dynamics to simulate ventricular blood flow in the heart, it has been common practice to neglect the effect of the sub-valvular apparatus and the trabeculae on the flow conditions. In this study, we analyze the effect of neglecting the chordae tendineae on the fluid flow and pressure drop. To test the assumption we use a previously developed dynamic 3D model of the left ventricle, aorta and valves that is based on 3D echocardiographic recordings. To this model we add the chordae tendineae as a sub-grid model. The previously developed 3D model for the left ventricle during systole is based on real-time three-dimensional echocardiography (RT3DE) recordings of a 30 years old female volunteer. The segmented ventricular wall does not include details of the aorta and the mitral valve, so these were reconstructed. The subgrid model for the flow across the chordae tendineae is based on the Actuator Line Method, which means that they are represented by drag coefficients. The analysis shows that the effect of the chordae tendineae on the pressure drop and work efficiency of the normal heart during systole is minor, and it seems that for simulating ventricular fluid flow and pressure drop during systole, one can follow the current practice and ignore the chordae. However, there can be local effects such as small vortices behind the chordae. Whether such effects are important for a particular application must be evaluated for the given case.publishedVersio

Mass transfer between bubbles and seawater

Mass transfer between bubbles and seawater is an important mechanism when determining how much gas reaches the atmosphere from gas sources at the seabed. The mass transfer coefficient is a governing parameter for the phenomenon. Experiments on small bubbles in seawater have been performed where the bubble size has been monitored. The observed evolution of the bubble size has been compared with theoretical predictions of the bubble size. Based on this comparison, it is shown that mass transfer correlations for contaminated conditions is more consistent with experiments than correlations for clean conditions. It is also learned that simultaneous desorption of gases dissolved in the liquid must be accounted for.acceptedVersio

Simulation of gas-liquid flows in separators. A Lagrangian approach

In order to simulate the separation efficiency of gas scrubbers, we have formulated and implemented a version of the Single-Particle Method. The method is suitable for CFD simulations of gas-droplet flows, and is based on using Lagrangian tracking of droplets. An implementation of the method has been made in a commercial CFD tool. The methodology and the CFD implementation have been validated against analytical results in the literature.publishedVersio

Simulation of gas-liquid flows in separators. A Lagrangian approach

In order to simulate the separation efficiency of gas scrubbers, we have formulated and implemented a version of the Single-Particle Method. The method is suitable for CFD simulations of gas-droplet flows, and is based on using Lagrangian tracking of droplets. An implementation of the method has been made in a commercial CFD tool. The methodology and the CFD implementation have been validated against analytical results in the literature

Development of an algorithm to detect hydraulic jacking in high pressure rock mass grouting and introduction of the PF index

Pre-excavation rock mass grouting is a common procedure for reducing water ingress into tunnels during construction in Norwegian tunnelling. The level of grouting pressure is a disputed subject and the knowledge of how the rock mass responds to the high pressure and how this inflicts on the grouting results is sparse and little investigated. For this reason, it is of interest to use data from high pressure grouting performed in Norwegian projects to investigate these matters. This paper presents the development of a method for identifying hydraulic jacking during rock mass grouting and the making of an algorithm to perform computerized detection of hydraulic jacking in grouting logs. The algorithm is the base for a larger study, where screening for hydraulic jacking is performed in over hundred grouting rounds, distributed on several Norwegian tunnels excavated in rock mass. The relation between grout flow and grouting pressure has shown to be vital for the understanding of the grouting progress and events occurring during the grouting. Interpretation of pressure and flow as two separate variables, which are affected by aliasing, caused by low and irregular sampling frequency is a challenging task, and it was found to be helpful to create a parameter to represent this relationship, named the PF index (Pressure Flow index). This parameter has also shown to be useful in other applications such as monitoring the grouting progress on site

Energy assessments of onboard CO2 capture from ship engines by MEA-based post combustion capture system with flue gas heat integration

An early phase feasibility study was carried out for offshore CO2 capture from ship engines of a CO2 transport ship. A flexible in-house process simulator was applied in the assessments. Parametric studies of the overall onboard process were enabled by a fast data-driven capture plant model derived from supervised machine learning by PLS regression of a large dataset of rigorous simulations. The results show, based on the given models and assumptions, that the thermal energy coming from the ship engine exhaust gas is not sufficient alone to cover the thermal energy demand of an absorption-based CO2 capture unit operating above 50% capture rate using 30 wt% MEA (mono-ethanolamine) as solvent. The thermal energy demand can be met using a fuel afterburner as heat source. The added fuel consumption is estimated to increase the fuel consumption by 6–9% when operating with liquefied natural gas (LNG) as fuel source, while an increase of 8–12% is expected with diesel as fuel source. The effect of absorber height on energy consumption at a given CO2 capture rate is limited, especially for lower capture rates, and may be an important degree of freedom for optimizing the CAPEX/OPEX trade-offs. Use of state-of-the art solvents with lower specific energy consumptions will shift the results towards higher capture rates before a fuel afterburner is required to meet the thermal energy demands.publishedVersio

Simplified model description of a CLOP reactor for system simulation and analysis

In order to perform overall system simulations and optimization at the flowsheet level, simplified models of process units are required. We present a simplified model of the CLOP reactor (chemical looping for oxygen production) and compare it against a rigorous dynamic fixed bed model, which uses a 1D phenomenological approach. The model is validated towards the detailed model to verify that the performance is captured correctly. In this way, after model validation, system simulations can be performed and optimized both based on process flow configuration, and temperature/pressure ranges. When combined in a process simulation, the model can give an understanding of the potential of a given oxygen carrier material (OCM) for usage in power plants utilizing the novel COMPOSITE concept, which is a concept for energy production with CO2 capture. Both the rigorous and the simplified models are based on using fuel burning to maintain the desired operating reactor temperature. The model can be used for finding equilibrium points in the air and fuel reactors, and thus identify what is the limiting factor for the reactor performance.publishedVersio