Investigation of ammonia and hydrogen as CO2-free fuels for heavy duty engines using a high pressure dual fuel combustion process

This work is a numerical study of the use of ammonia and hydrogen in a high-pressure-dual-fuel (HPDF) combustion. The main fuels (hydrogen and ammonia) are direct injected and ignited by a small amount of direct injected pilot fuel. The fuels are injected using a dual fuel injector from Woodward L’Orange, which can induce two fuels independently at high pressures up to 1800 bar for the pilot fuel and maximum 500 bar for the main. The numerical CFD-model gets validated for of hydrogen-HPDF with experimental data. Due to safety issues at the test rig it was not possible to use ammonia in the experiments, so it is modelled using the numerical model. It is assumed that the CFD-model also gives qualitative correct results for the use of ammonia as main fuel, so a parameter study of ammonia-HPDF is made. The results for the hydrogen-HPDF show, that hydrogen can be used in the engine without any further modifications. The combustion is very stable, and the hydrogen ignites almost immediately when it enters the combustion chamber. The results of the ammonia combustion indicate, that the HPDF combustion mode can handle ammonia effectively. It seems beneficial to inject the ammonia at higher pressures than hydrogen. Also pre-heating the ammonia can increase the combustion efficiency. </jats:p

Numerical Approaches for Modeling Gas–Solid Fluidized Bed Reactors: Comparison of Models and Application to Different Technical Problems

Gas–solid fluidized bed reactors play an important role in many industrial applications. Nevertheless, there is a lack of knowledge of the processes occurring inside the bed, which impedes proper design and upscaling. In this work, numerical approaches in the Eulerian and the Lagrangian framework are compared and applied in order to investigate internal fluidized bed phenomena. The considered system uses steam/air/nitrogen as fluidization gas, entering the three-dimensional geometry through a Tuyere nozzle distributor, and calcium oxide/corundum/calcium carbonate as solid bed material. In the two-fluid model (TFM) and the multifluid model (MFM), both gas and powder are modeled as Eulerian phases. The size distribution of the particles is approximated by one or more granular phases with corresponding mean diameters and a sphericity factor accounting for their nonspherical shape. The solid–solid and fluid–solid interactions are considered by incorporating the kinetic theory of granular flow (KTGF) and a drag model, which is modified by the aforementioned sphericity factor. The dense discrete phase model (DDPM) can be interpreted as a hybrid model, where the interactions are also modeled using the KTGF; however, the particles are clustered to parcels and tracked in a Lagrangian way, resulting in a more accurate and computational affordable resolution of the size distribution. In the computational fluid dynamics–discrete element method (CFD–DEM) approach, particle collisions are calculated using the DEM. Thereby, more detailed interparticulate phenomena (e.g., cohesion) can be assessed. The three approaches (TFM, DDPM, CFD–DEM) are evaluated in terms of grid- and time-independency as well as computational demand. The TFM and CFD–DEM models show qualitative accordance and are therefore applied for further investigations. The MFM (as a variation of the TFM) is applied in order to simulate hydrodynamics and heat transfer to immersed objects in a small-scale experimental test rig because the MFM can handle the required small computational cells. Corundum is used as a nearly monodisperse powder, being more suitable for Eulerian models, and air is used as fluidization gas. Simulation results are compared to experimental data in order to validate the approach. The CFD–DEM model is applied in order to predict mixing behavior and cohesion effects of a polydisperse calcium carbonate powder in a larger scale energy storage reactor.</jats:p

Development of a Continuous Fluidized Bed Reactor for Thermochemical Energy Storage Application

Thermochemical energy storage (TCES) represents one of the most promising energy storage technologies, currently investigated. It uses the heat of reaction of reversible reaction systems and stands out due to the high energy density of its storage materials combined with the possibility of long-term storage with little to no heat losses. Gas–solid reactions, in particular the reaction systems CaCO3/CaO, CaO/Ca(OH)2 and MgO/Mg(OH)2 are of key interest in current research. Until now, fixed bed reactors are the state of the art for TCES systems. However, fluidized bed reactors offer significant advantages for scale-up of the system: the improved heat and mass transfer allows for higher charging/discharging power, whereas the favorable, continuous operation mode enables a decoupling of storage power and capacity. Even though gas–solid fluidized beds are being deployed for wide range of industrial operations, the fluidization of cohesive materials, such as the aforementioned metal oxides/hydroxides, still represents a sparsely investigated field. The consequent lack of knowledge of physical, chemical, and technical parameters of the processes on hand is currently a hindering aspect for a proper design and scale-up of fluidized bed reactors for MW applications of TCES. Therefore, the experimental research at Technical University of Munich (TUM) focuses on a comprehensive approach to address this problem. Preliminary experimental work has been carried out on a fixed bed reactor to cover the topic of chemical cycle stability of storage materials. In order to investigate the fluidization behavior of the bulk material, a fluidized bed cold model containing a heat flux probe and operating at atmospheric conditions has been deployed. The experimental results have identified the heat input and output as the most influential aspect for both the operation and a possible scale-up of such a TCES system. The decisive parameter for the heat input and output is the heat transfer coefficient between immersed heat exchangers and the fluidized bed. This coefficient strongly depends on the quality of fluidization, which in turn is directly related to the geometry of the gas distributor plate. At TUM, a state-of-the-art pilot fluidized bed reactor is being commissioned to further investigate the aforementioned aspects. This reactor possesses an overall volume of 100 L with the expanded bed volume taking up 30 L. Two radiation furnaces (64 kW) are used to heat the reactor. The heat of reaction of the exothermal hydration reaction is removed by water, evaporating in a cooling coil, immersed in the fluidized bed. Fluidization is being achieved with a mixture of steam and nitrogen at operating temperatures of up to 700 °C and operating pressures between −1 and 6 bar(g). The particle size is in the range of d50 = 20 μm. While initial experiments on this reactor focus on optimal operating and material parameters, the long-term goal is to establish correlations for model design and scale-up purposes.</jats:p

Analysis and Comparison of Reactivity and CO2 Capture Capacity of Fresh Calcium-Based Sorbents and Samples From a Lab-Scale Dual Fluidized Bed Calcium Looping Facility

Naturally occurring limestone and samples from a lab scale dual fluidized bed (DFB) calcium looping (CaL) test facility were analysed in a thermo gravimetric analyser (TGA). The reactivity of the samples evaluated at typical carbonation conditions prevailed in the carbonator was compared with raw samples. Carbonations were carried out at 600, 650 &amp;700°C and 5, 10 &amp;15 vol-% CO2 atmosphere using a custom designed sample holder that provided ideal conditions for solid gas contact in a TGA. The rate of carbonation and carbonation capacity of the samples were compared with respect to the following three categories: number of calcination-carbonation cycles, carbonation temperature and CO2 concentration. Notable differences in total conversion (XCaO) and the rates of conversions were observed between the raw and DFB samples in all three cases. It is suspected the much lower activity of the DFB sample is attributed to the differences in experimental conditions: ie., partial carbonation of the DFB particles, fast heating rate in the calciner and thus a rapid calcination reaction, and particle attrition in the CFB calciner riser. These harsh conditions lead sintering and thus a loss of surface area and reactivity. Sintered DFB samples showed low (nearly 1/3 of the raw samples) but stable conversions with increasing number of cycles. The sorbent taken from the DFB facility did not decrease with respect to carbonation rate or maximum conversion over 4 cycles whereas the fresh limestone changed significantly over 4 cycles. Hydration was used as an attempt to regenerate the lost capture capacity of partially carbonated DFB sample. Hydration of the sintered DFB sample was successful in increasing the maximum capture capacity in the fast reaction regime to values almost as high as that of a fresh sample in its first carbonation cycle. Although more investigation is required to investigate the effect of hydration on the CaO particle morphology, a process modification to enhance the CO2 capture efficiency of the carbonator via particle hydration was proposed.</jats:p