Johannes Janicka’s research while affiliated with Technical University of Darmstadt and other places

Storing electrical energy for long periods and transporting it over long distances is an essential task of the necessary transition to a CO$_2$-free energy economy. An oxidation-reduction cycle based on iron and its oxides represents a very promising technology in this regard. The present work assesses the potential of converting an existing modern coal-fired power plant to operation with iron. For this purpose, a systematic retrofit study is carried out, employing a model that balances all material and energy fluxes in a state-of-the-art coal-fired power plant. Particular attention is given to components of the burner system and the system$'$s heat exchanger. The analysis provides evidence that main components such as the steam generator and steam cycle can be reused with moderate modifications. Major modifications are related to the larger amounts of solids produced during iron combustion, for instance in the particle feeding and removal systems. Since the high particle densities and lower demand for auxiliary systems improve the heat transfer, the net efficiencies of iron operation can be one to two percentage points better than coal operation, depending on operating conditions. This new insight can significantly accelerate the introduction of this innovative technology by guiding future research and the development of the retrofit option.

In this work, a new Large-Eddy Simulation (LES) based approach coupled to a flamelet description of the gas-phase and seamless detailed solid-fuel kinetics is introduced and applied to a self-sustained oxy-fuel coal combustion chamber. LES enables a detailed description of the turbulent flow field, mixing, and heat transfer. The latter also includes thermal radiation for the particles and solving the radiative transport equation using the weighted sum of gray gases for the gas phase. The flamelet description of the gas phase represents an accurate and computationally efficient way to include turbulence-chemistry interactions taking into account the reaction of the complex gas mixture released from the solid fuel particles. For the first time, these two detailed models are coupled with a recently developed solid-fuel kinetic mechanism, allowing of the entire particle conversion process, i.e., devolatilization and char oxidation, to be seamlessly included in the simulation. After the required coupling strategies are discussed, the holistic model is applied to an oxy-fuel combustion chamber. Firstly, the results are extensively validated by the available measurements. Secondly, comparisons to state-of-the-art simplified solid fuel kinetics are carried out to assess the interaction of detailed solid-fuel kinetics with the other models.

As a result of the 2021 United Nations Climate Change Conference (COP26), several countries committed to phasing down coal electricity as soon as possible, deactivating hundreds of power plants in the near future. CO$_2$-free electricity can be generated in these plants by retrofitting them for iron combustion. Iron oxides produced during the process can be collected and reduced back to metallic iron using H$_2$, in a circular process where it becomes an energy carrier. Using clean energy in the recycling process enables storage and distribution of excess generated in periods of abundance. This concept uses and scales up existing dry metal cycle technologies, which are the focus of extensive research worldwide. Retrofitting is evaluated here to determine feasibility of adding these material requirements to markets, in the context of current plans for decarbonization of steel industry, and policies on hydrogen and renewable electricity. Results indicate that not only for a single power plant, but also on larger scales, the retrofitting plan is viable, promoting and supporting advancements in sustainable electricity, steel industry and hydrogen production, converging necessary technological and construction efforts. The maturation and first commercial-scale application of iron combustion technology by 2030, together with developing necessary reduction infrastructure over the next decades, would pave the way for large-scale retrofitting and support the phasing out of coal in many regions. The proposed plan represents a feasible solution that takes advantage of existing assets, creates a long-lasting legacy for the industry and establishes circular energy economies that increase local energy security.

The homogeneous ignition and volatile combustion of pulverized solid fuel in single-particle and particle group configurations were studied numerically in a laminar flat flame burner. Simulations with increasing particle streams were performed to investigate the influence of the interactions in particle groups on homogeneous ignition and combustion. An extensive set of simulations are conducted considering models with different levels of detail for both the gas-phase and solid fuel chemistry. The reference simulations employ the chemical percolation devolatilization model coupled with a detailed chemistry model for gas-phase reactions. The particle-fluid interactions were modeled with a fully coupled Eulerian-Lagrangian framework. Increased ignition delay times for higher particle streams were successfully validated against available experimental measurements. Furthermore, the transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions near the flame base in particle group combustion was observed in both experiments and simulations. The subsequent detailed investigations revealed that the increased heat transfer to particles and, therefore, lower gas temperature for higher particle number densities together with the local oxygen depletion are the primary reasons for this transition. Based on the reference simulation, different simplified model combinations were assessed. The systematic model reduction investigation started with assessing the fixed volatile composition as a required assumption for flamelet models. Finally, the effects of gas-phase chemistry and different simple devolatilization models on ignition and combustion chemistry were studied. Overall, all model combinations provide reasonable predictions of volatile combustion with minor local deficits in the studied conditions.

Large-eddy simulations are conducted for Darmstadt turbulent methane-hydrogen flames (MHFs, 20%H2+80%CH4 in volume fraction). Two different flames are investigated: one without stratification at the fuel inlets (MHF5) and the other with stratification (MHF7). Two different flamelet libraries are generated: one based on the unity Lewis number assumption (Le1) and the other based on the mixture-averaged (MA) approach. Further, two different techniques are adopted to access the flamelet libraries: one solving the transport equations for the six Raman-accessible species mass fractions (M1), and the other solving the transport equations for the three trajectory variables directly (M2). The heat loss effects in the pilot tube are investigated by comparing the adiabatic and non-adiabatic flamelet models. The suitability of the different modeling approaches is evaluated by comparing the simulation results with the available experimental data. The results show that the heat transfer in the pilot tube has significant effects on the variation trends of the gas temperature and the flame position. Overall, M2 performs better than M1 in predicting the thermo-chemical quantities, regardless of diffusion modeling. M2 coupled with different flamelet libraries makes good predictions for both flames in the upstream and inner downstream regions. However, in the outer downstream region only M2 coupled with the unity Lewis number assumption generates good predictions for MHF7, while the other modeling approaches over-predict the gas temperature and CO2 mass fraction for both MHF5 and MHF7. The reason for the different performances of the flamelet models is explored by analyzing the differential diffusion parameter.

Transported probability density function (PDF) methods are widely used to model turbulent flames characterized by strong turbulence-chemistry interactions. Numerical methods directly resolving the PDF are commonly used, such as the Lagrangian particle or the stochastic fields (SF) approach. However, especially for premixed combustion configurations, characterized by high reaction rates and thin reaction zones, a fine PDF resolution is required, both in physical and in composition space, leading to high numerical costs. An alternative approach to solve a PDF is the method of moments, which has shown to be numerically efficient in a wide range of applications. In this work, two Quadrature-based Moment closures are evaluated in the context of turbulent premixed combustion. The Quadrature-based Moment Methods (QMOM) and the recently developed Extended QMOM (EQMOM) are used in combination with a tabulated chemistry approach to approximate the composition PDF. Both closures are first applied to an established benchmark case for PDF methods, a plug-flow reactor with imperfect mixing, and compared to reference results obtained from Lagrangian particle and SF approaches. Second, a set of turbulent premixed methane-air flames are simulated, varying the Karlovitz number and the turbulent length scale. The turbulent flame speeds obtained are compared with SF reference solutions. Further, spatial resolution requirements for simulating these premixed flames using QMOM are investigated and compared with the requirements of SF. The results demonstrate that both QMOM and EQMOM approaches are well suited to reproduce the turbulent flame properties. Additionally, it is shown that moment methods require lower spatial resolution compared to SF method.

In this work, effects of air and oxy-fuel atmospheres on flamelet modeling of NOx and SOx formation in two-dimensional laminar counterflow pulverized coal flames are investigated. The release and combustion of volatile-N, char-N and volatile-S are incorporated in the flamelet model, and a newly developed reaction mechanism for oxy-fuel combustion (129 species and 911 elementary reactions) is employed to describe the chemistry. Two different methods for prediction of pollutant species are evaluated using the flamelet model, in which the pollutant mass fractions are obtained by either extracting the flamelet library directly (“M1”) or solving the corresponding transport equations with the reaction source terms being taken from the flamelet library (“M2”). To evaluate the performance of proposed flamelet models, the flamelet predictions are compared to the reference results of the detailed chemistry solutions, in which the transport equations for the species mass fractions and total enthalpy are directly solved. At first, the atmosphere effects on the NOx and SOx formation are analyzed based on the detailed chemistry solutions, then the effects of atmosphere on flamelet modeling of pollutant formation are evaluated. The results show that M1 overall performs better than M2 at predicting the NOx and SOx species in both air and oxy-fuel atmospheres. The major pollutant species of NO and SO2 are over-predicted by M2 in certain regions for both atmospheres, and the reason for the incorrect prediction is explored by attributing to the interpolation error of the reaction source terms in the middle branch of the S-Shaped curve.

The Eulerian stochastic fields (ESF) method, which is based on the transport equation of the joint subgrid scalar probability density function, is applied to Large Eddy Simulation of a turbulent dilute spray flame. The approach is coupled with a tabulated chemistry approach to represent the subgrid turbulence-chemistry interaction. Following a two-way coupled Eulerian-Lagrangian procedure, the spray is treated as a multitude of computational parcels described in a Lagrangian manner, each representing a heap of real spray droplets. The present contribution has two objectives: First, the predictive capabilities of the modeling framework are evaluated by comparing simulation results using 8, 16, and 32 stochastic fields with available experimental data. At the same time, the results are compared to previous studies, where the artificially thickened flame (ATF) model was applied to the investigated configuration. The results suggest that the ESF method can reproduce the experimental measurements reasonably well. Comparisons with the ATF approach indicate that the ESF results better describe the flame entrainment into the cold spray core of the flame. Secondly, the dynamics of the subgrid scalar contributions are investigated and the reconstructed probability density distributions are compared to common presumed shapes qualitatively and quantitatively in the context of spray combustion. It is demonstrated that the ESF method can be a valuable tool to evaluate approaches relying on a pre-integration of the thermochemical lookup-table.

In this paper novel unified wall function formulations for large eddy simulation of complex turbulent flows and heat transfer are presented. In contrast to existing wall functions, the proposed analytical expressions for velocity and temperature are: (1) valid over the whole range of dimensionless wall distance y + , (2) applicable for complex flow situations that include local non-equilibrium effects, (3) suitable for a wide range of molecular Prandtl numbers, (4) thermo-dynamically consistent with the second law of thermodynamics, and (5) easily extendable to account for contributions of additional source term e.g. gravity, radiation, chemical reactions. Such an universal and consistent wall function formulation is particularly useful for computational fluid dynamics applications in providing appropriate wall boundary conditions for complex high Reynolds number flows. The accuracy and thermodynamic consistency of the proposed wall treatment is first testified by comparison with experimental and direct numerical simulation data. Second, in order to highlight the applicability and performance of the proposed wall functions for computational fluid dynamics, results of large eddy simulations of various complex turbulent flows including heat transfer relevant to internal combustion engine applications are presented and evaluated. It turns out that the suggested wall function approach features a superior physics modeling accuracy in terms of well-known key parameters in near-wall bounded flows in comparison to state-of-the-art wall functions.