Numerical Study of Biomass Ignition, Combustion, and Pollutant Formation in Laminar Jet Flows

Methods

The thermochemical conversion of solid fuels involves complex mass, momentum, and energy exchange between solid particles and the gas phase. This is captured using an Eulerian–Lagrangian approach: the gas phase is treated in an Eulerian framework, while solid particles are modeled as point particles in a Lagrangian formulation. The governing equations are solved with CIAO, an in-house semi implicit finite element solver. The Eulerian equations use second-order discretization in space and time, with scalar transport handled via a fifth-order WENO scheme for convection and second-order central differences for diffusion. Species mass fractions and temperature are integrated using Strang splitting to separate transport and chemistry. Chemical source terms are computed via finite-rate chemistry using the CVODE solver. Solid particles are advanced with a two stage, second-order Runge–Kutta scheme that updates their state, position, and source terms.

Results

This section presents the results of the parametric study to analyze the ${\mathrm{NO}}_x$ formation pathways using DNS. Note that additional simulations with different biomass types have also been performed to validate the particle and gas phase model and to analyze the ${\mathrm{NO}}_x$ formation pathways within this project, but are not shown here for brevity. The presented analysis focuses specifically on solid pulverized Miscanthus biomass flames under varying atmospheric conditions, aiming to assess the impact of these conditions on ${\mathrm{NO}}_x$ production in a drop tube furnace (DTF) configuration. Figure 1 illustrates the correlation between ${\mathrm{NO}}_x$, mixture fraction, and gas-phase temperature in air atmosphere. Distinct high- and low-temperature zones can be identified around the stoichiometric mixture fraction. The peak ${\mathrm{NO}}_x$ production rate aligns with the maximum consumption rate of nitrogen-containing volatiles (NFuel), which are released from the solid particles. This occurs in the high temperature, fuel-rich regions near the particles, emphasizing the critical role of fuel-bound nitrogen in formation during biomass combustion. Figure 2 compares NO_x mole fraction and gas-phase temperature across three atmospheric conditions. In all cases, the global maximum occurs near the flame tip, where lower temperatures and longer residence times promote accumulation. Local maxima also appear near stoichiometric, high-temperature regions. Oxidizer composition strongly affects flame structure and formation, with air yielding the highest NO_x due to enhanced thermal ${\mathrm{NO}}_x$ production. To analyze the ${\mathrm{NO}}_x$ formation mechanisms in detail, Figure 3 presents a rate of production analysis (ROPA) of nitrogen-containing species. The analysis integrates nitrogen element fluxes across the entire computational domain and simulation duration to offer a comprehensive overview. The results reveal that atmospheric conditions significantly alter formation pathways. Under oxy-fuel conditions, the contribution of atomic nitrogen (N) to NO formation decreases, while the role of HNO increases, indicating a shift in dominant reaction pathways. Additionally, a higher oxygen content in the oxidizer leads to increased production of hydroxyl (OH) radicals, which further enhances NO formation. As Fuel-NO_x pathways dominate formation, the impact of volatile composition on prediction accuracy is evaluated. The fixed volatile composition (FVC), required in reduced-order models, is compared with the detailed dynamic volatile composition (DVC) model. While FVC accurately predicts gas- phase temperatures, it fails to capture ${\mathrm{NO}}_x$ distribution (Figure 4). To address this, a modified FVC model (FVC-N) is proposed, which separates nitrogen-containing volatiles due to their distinct release timescales. This improves prediction, as confirmed by closer agreement with DVC results in Figure 4

Discussion

With the computing time on CLAIX, we conducted a parametric study to investigate ${\mathrm{NO}}_x$ formation pathways under different operating conditions in pulverized solid biomass laminar jet flames. The analysis revealed that the surrounding atmosphere plays a significant role in production. Specifically, lower levels were observed under oxy-fuel conditions, where HNO serves as the primary direct precursor to NO. In contrast, in air, the N-radical is the dominant direct precursor for NO formation. Finally, a modification of the fixed volatile composition (FVC) model, which separately accounts for the release of nitrogen-containing volatiles, successfully captures ${\mathrm{NO}}_x$ formation predictions similar to those of the dynamic volatile composition (DVC) model. This modification significantly improves the reduced-order modeling of ${\mathrm{NO}}_x$ formation in solid pulverized biomass flames.

Additional Project Information

DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: CIAO, MPI
Cluster: CLAIX

Publications

P. Farmand, C. Boehme, P. Steffens, H. Nicolai, F. Loffredo, P. Debiagi, S. Girhe,
H. Chu, M. Gauding, C. Hasse, H. Pitsch.
Numerical investigation and modeling of NOx formation in pulverized biomass flames under air and oxyfuel conditions. Combustion and Flame (accepted).

Thesis
Pooria Farmand, Numerical Study on Ignition, Combustion, and Pollutant Formation Processes Using Solid Pulverized Fuels,
PhD Thesis, Aachen (Germany), 2025.

Christian Boehme, Numerical investigation and model assessment of NOx -formation pathways during coal and biomass combustion,
Master Thesis, Aachen (Germany), 2024.