The sensitivity of filtered Two Fluid Models to the underlying resolved simulation setup

Eulerian-Eulerian modelling based on the Kinetic Theory of Granular Flow has proven to be a promising tool for investigating the hydrodynamic and reactive behaviour inside fluidized beds. The primary limitation of this approach is the very fine grid size necessary to fully resolve the transient solid structures that are typical of fluidized bed reactors. It therefore remains impractical to simulate industrial scale fluidized bed reactors using resolved Two Fluid Model (TFM) simulations. For this reason, there is currently widespread interest in developing sub-grid (filtered) models that allow accurate simulations at coarser grids by correcting for the effects of unresolved solid structures. However, little attention has been paid to the importance of the choice of the underlying TFM closures during the derivation of the filtered models. This paper follows a similar approach to an establish filtered TFM (1) to derive sub-grid closures for the interphase momentum exchange , solids viscosity and solids pressure in 2D periodic simulations. These corrections are obtained for different particle-particle restitution coefficients, frictional pressure models and drag models as a function of the particle phase volume fraction and the filter size. This reveals at which values of the markers the individual resolved TFM model choices have significant effects on the final expressions derived for filtered TFMs. Based on these findings suggestions are made regarding the derivation of new filtered TFMs and the use of the existing models. 1. Y. Igci and S. Sundaresan. Constitutive Models for Filtered Two-Fluid Models of Fluidized Gas–Particle Flows. Ind. Eng. Chem. Res., 50: 13190-13201, 2013

Evaluation of the minimum fluidization velocity at elevated temperature and pressure through experiments and modelling

The minimum fluidization velocity is an important measure used in the design and scale-up of fluidized beds. Due to its importance, a large number of experiments over a wide range of operating conditions have focused on this property. Despite this attention, the amount of data where the combined effect of elevated temperature and pressure on the minimum fluidization velocity was investigated is still limited. In this study the minimum fluidization velocity is determined experimentally in a lab-scale fluidized bed reactor designed for use at elevated temperature and pressure. A central composite design (CCD) is used to design experiments where different operating parameters are varied over a wide range. This includes different particle sizes, pressures up to 5bar and temperatures up to 550°C. The collected data provides the basis for existing correlations, such as that given by Bi and Grace (1), to be evaluated at elevated temperature and pressure and allows for detecting any systematic deviations from the experimental data. In addition to the experiments, the minimum fluidization velocity and the voidage at minimum fluidization is calculated numerically over the CCD using computational. Several different drag models are evaluated, allowing their relative performances to be assessed and any weaknesses to be identified. Recommendations are made for drag model selection in pressurized fluidized bed reactors. 1. H.T. Bi and J.R. Grace. Flow regime diagrams for gas-solid fluidization and upward transport. Int. J. Multiphase Flow, 21: 1229-1236, 1995. Please click Additional Files below to see the full abstract

Blue hydrogen and industrial base products: The future of fossil fuel exporters in a net-zero world

Is there a place for today's fossil fuel exporters in a low-carbon future? This study explores trade channels between energy exporters and importers using a novel electricity-hydrogen-steel energy systems model calibrated to Norway, a major natural gas producer, and Germany, a major energy consumer. Under tight emission constraints, Norway can supply Germany with electricity, (blue) hydrogen, or natural gas with re-import of captured CO2. Alternatively, it can use hydrogen to produce steel through direct reduction and supply it to the world market, an export route not available to other energy carriers due to high transport costs. Although results show that natural gas imports with CO2 capture in Germany is the least-cost solution, avoiding local CO2 handling via imports of blue hydrogen (direct or embodied in steel) involves only moderately higher costs. A robust hydrogen demand would allow Norway to profitably export all its natural gas production as blue hydrogen. However, diversification into local steel production, as one example of easy-to-export industrial base products, offers an effective hedge against the possibility of lower European blue hydrogen demand. Looking beyond Europe, the findings of this study are also relevant for the world's largest energy exporters (e.g., OPEC+) and importers (e.g., developing Asia). Thus, it is recommended that large hydrocarbon exporters consider a strategic energy export transition to a diversified mix of blue hydrogen and climate-neutral industrial base products.publishedVersio

Efficient hydrogen production with CO2 capture using gas switching reforming

Hydrogen is a promising carbon-neutral energy carrier for a future decarbonized energy sector. This work presents process simulation studies of the gas switching reforming (GSR) process for hydrogen production with integrated CO2 capture (GSR-H2 process) at a minimal energy penalty. Like the conventional steam methane reforming (SMR) process, GSR combusts the off-gas fuel from the pressure swing adsorption unit to supply heat to the endothermic reforming reactions. However, GSR completes this combustion using the chemical looping combustion mechanism to achieve fuel combustion with CO2 separation. For this reason, the GSR-H2 plant incurred an energy penalty of only 3.8 %-points relative to the conventional SMR process with 96% CO2 capture. Further studies showed that the efficiency penalty is reduced to 0.3 %-points by including additional thermal mass in the reactor to maintain a higher reforming temperature, thereby facilitating a lower steam to carbon ratio. GSR reactors are standalone bubbling fluidized beds that will be relatively easy to scale up and operate under pressurized conditions, and the rest of the process layout uses commercially available technologies. The ability to produce clean hydrogen with no energy penalty combined with this inherent scalability makes the GSR-H2 plant a promising candidate for further research.publishedVersio

Carbon-negative hydrogen from biomass using gas switching integrated gasification: Techno-economic assessment

Ambitious decarbonization pathways to limit the global temperature rise to well below 2 °C will require large-scale CO2 removal from the atmosphere. One promising avenue for achieving this goal is hydrogen production from biomass with CO2 capture. The present study investigates the techno-economic prospects of a novel biomass-to-hydrogen process configuration based on the gas switching integrated gasification (GSIG) concept. GSIG applies the gas switching combustion principle to indirectly combust off-gas fuel from the pressure swing adsorption unit in tubular reactors integrated into the gasifier to improve efficiency and CO2 capture. In this study, these efficiency gains facilitated a 5% reduction in the levelized cost of hydrogen (LCOH) relative to conventional O2-blown fluidized bed gasification with pre-combustion CO2 capture, even though the larger and more complex gasifier cancelled out the capital cost savings from avoiding the air separation and CO2 capture units. The economic assessment also demonstrated that advanced gas treatment using a tar cracker instead of a direct water wash can further reduce the LCOH by 12% and that the CO2 prices in excess of 100 €/ton, consistent with ambitious decarbonization pathways, will make this negative-emission technology economically highly attractive. Based on these results, further research into the GSIG concept to facilitate more efficient utilization of limited biomass resources can be recommended.publishedVersio

The future of fuels: Uncertainty quantification study of mid-century ammonia and methanol production costs

Fuels supplied 76% of global final energy consumption in 2021. Although ongoing electrification efforts can significantly reduce this dominant share, fuels will continue to play a leading role in the global energy system. Hydrogen is the most studied candidate for decarbonizing fuels, but storage and distribution challenges render it impractical and uneconomical for many applications. Hence, the present study focuses on ammonia and methanol that preserve the practical benefits of traditional petroleum fuels. Four low-carbon fuel production pathways are compared using cost and performance projections to the year 2050: solid fuels (coal/biomass blends) with CO2 capture and storage (CCS), natural gas with CCS, renewables (wind & solar), and nuclear. These pathways are assessed in Europe as a typical fuel importer and in various exporting regions where the cheapest primary energy is available. A thorough uncertainty quantification exercise is conducted for all pathways to map out the likely range of future levelized costs. Results show that there are virtually no plausible scenarios where electrolytic fuels (renewables or nuclear) can compete with fuels produced from hydrocarbons equipped with CCS. Ammonia or methanol from solid fuels present a particularly attractive solution for affordable carbon-negative energy security, whereas ammonia from natural gas offers a promising decarbonized alternative to liquified natural gas exports. Based on these findings, a technology-neutral policy framework is recommended instead of targeted support for electrolytic fuels.publishedVersio

Integration of chemical looping combustion for cost-effective CO2 capture from state-of-the-art natural gas combined cycles

Chemical looping combustion (CLC) is a promising method for power production with integrated CO2 capture with almost no direct energy penalty. When integrated into a natural gas combined cycle (NGCC) plant, however, CLC imposes a large indirect energy penalty because the maximum achievable reactor temperature is far below the firing temperature of state-of-the-art gas turbines. This study presents a techno-economic assessment of a CLC plant that circumvents this limitation via an added combustor after the CLC reactors. Without the added combustor, the energy penalty amounts to 11.4%-points, causing a high CO2 avoidance cost of $117.3/ton, which is more expensive than a conventional NGCC plant with post-combustion capture ($93.8/ton) with an energy penalty of 8.1%-points. This conventional CLC plant would also require a custom gas turbine. With an added combustor fired by natural gas, a standard gas turbine can be deployed, and CO2 avoidance costs are reduced to $60.3/ton, mainly due to a reduction in the energy penalty to only 1.4$96.3/ton when a hydrogen cost of $15.5/GJ is assumed. Advanced heat integration could reduce the CO2 avoidance cost to$90.3/ton by lowering the energy penalty to only 0.6%-points. An attractive alternative is, therefore, to construct the plant for added firing with natural gas and retrofit the added combustor for hydrogen firing when CO2 prices reach very high levels

Comparison of particle-resolved direct numerical simulation and 1d modelling of catalytic reactions in a cylindrical particle bed

This work presents a comparative study of reactive flow in a realistically packed array of cylindrical particles on two widely different scales: particle-resolved direct numerical simulation (PR-DNS) and 1D modelling. PR-DNS directly simulates all transfer phenomena in and around the cylindrical particles, while 1D modelling utilizes closure models to predict system behaviour at a computational cost several orders of magnitude lower than PR-DNS. PR-DNS is performed on a geometry of ~100 realistically packed cylindrical particles generated using the discrete element method (DEM). Simulations are performed over a range of Thiele moduli, Prandtl numbers and reaction enthalpies. The geometry with particles of aspect ratio four is meshed with fine polyhedral elements both inside and outside the particles. Hence, we obtain accurate results for combined internal and external heat and mass transfer in the cylindrical particle array. These results are compared with a 1D packed bed reactor model incorporating appropriate models for intra particle diffusion and for external heat and mass transfer (applicable to cylindrical particles). Results document a good comparison for the heterogeneous first order catalytic simple reaction. Therefore, recommendations are made to guide future 1D modelling works involving reactive flows in packed beds of cylindrical particles