Solid fuel combustion

 

In the course of the exit from fossil fuel energy, biogenic fuels are gaining importance, especially the substitution of fossil fuels by renewable raw materials is playing an increasing role. The changed fuel composition compared to coal leads to changed reaction kinetics, hence also to differences in particle combustion and thus influences flame dynamics, temperature, and chemistry.

The research group on solid fuel combustion covers various aspects, starting with the investigation of individual combustion sub-processes – such as the products and kinetics during pyrolysis and coke burnout – over the radiation behavior of different solid fuels to the investigation of dust flames on a pilot plant scale (order of magnitude 10-180 kW_th) and finally to the simulation and investigation of large-scale combustion systems.

A particular focus of the work in recent years has been oxyfuel combustion as a possible technology for CO₂ capture from combustion processes (CCS: Carbon Capture and Storage). With the step towards biomass oxyfuel combustion, the necessary knowledge for BECCUS (Bio-Energy Carbon Capture Usage and Storage, i.e. the capture of CO₂ from biomass combustion), which is urgently needed in the future, is generated in cooperation with the Ruhr-Universität Bochum and the TU Darmstadt within the CRC Oxyflame. Thus, this work contributes to making the goal of the necessary reduction of CO₂ emissions to limit the global mean temperature increase to 1.5°C more achievable.

In this context, the preparation and processing of renewable biogenic fuels were also studied, with the torrefaction of biomass. With this process, the storage and transportability of biomass for firing purposes can be significantly improved. Furthermore, the kinetics and firing behavior of bio-fuels optimized in this way can be investigated using the methods of the other projects.

(Current) Projects

• In the research group solid fuel combustion different areas of thermal utilization of solid fuels are currently investigated:
• Experimental investigation of pulverized biomass combustion for validation of numerical models in air and oxyfuel atmospheres in the power range 40-180 kW_th.(Oxyflame C1)
• Inorganic fine particulate matter formation during turbulent pulverized coal combustion (Joint Sino-German PM Formation)
• Numerical simulation (RANS) of the experimentally investigated flame configurations for a joint deeper investigation (Oxyflame C1, Joint Sino-German PM Formation)
• Experimental investigation of pyrolysis and coke kinetics in well-stirred (fluidized bed) reactor under atmospheric and pressurized conditions in air and oxyfuel atmospheres (Oxyflame A1, Oxyflame T1)
• Modelling of radiative properties of pulverized biomass particles during oxyfuel combustion (Oxyflame C4, Oxyflame T1)

In addition to the investigation of solid fuels, a collaboration has developed to study different plastic fractions for their targeted reuse in a more sustainable economic cycle:

• Experimental investigation of pyrolysis and reaction behavior of different plastic fractions for recycling of higher-grade polymers. (Cluster 4 Plastics Recycling)

Methods and Equipment

For the experimental determination of products and kinetic parameters of the pyrolysis or coke conversion process, a laboratory-scale fluidized bed reactor is used at the WSA. For this purpose, small fuel samples (1-50 mg) are individually fed into the reactor and the reaction products are subsequently determined employing Fourier transform infrared (FTIR) spectroscopy. Concept and optimized design allow the investigation of different boundary conditions: Temperature (300-1200 °C), pressure (1-20 bar), and atmospheric composition (N₂, O₂, CO₂, H₂O). The experimental data obtained form the basis for the development of pyrolysis and coke conversion models of variable complexity.

For efficient calculation of the interaction between thermal radiation and solid fuel particles, numerically efficient models for coupled radiation and flow dynamic simulation is developed. Furthermore, scattering experiments on levitated single particles are performed using an acoustic levitator.

The spectrum of flames investigated includes different pulverized fuels ((torrefied) biomass, lignite, hard coal) in the range 10 - 180 kW_th. For the investigation, two setups – an optically fully accessible combustion chamber to study methane-assisted solid fuel flames and a heated lined combustion chamber primarily for the investigation of self-sustained solid fuel flames – are available.

Several non-invasive measurement techniques are used for flame investigation, such as laser Doppler anemometry to determine flow velocities, a chemiluminescence camera system to identify reaction zones in the flame, or spectrography and 2-color pyrometry to determine particle and wall temperatures in the combustion chamber; in addition, flame investigations are also carried out using probes. Different analyzers – including FTIR spectrometers – are available for the analysis of combustion exhaust gases.

Various submodels have been developed and implemented for the numerical CFD investigation of combustion processes to enable a detailed, scientifically sound simulation of solid combustion.