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Combustion for Low Emission Applications of Natural Gas

Periodic Reporting for period 2 - CLEAN-Gas (Combustion for Low Emission Applications of Natural Gas)

Reporting period: 2017-01-01 to 2018-12-31

The scientific goal of the CLEAN-Gas project is to develop new experimental and numerical tools for improving natural gas combustion in innovative burners.
It is important to underline that Natural Gas is of great interest to the European Energy policy due to its widespread availability and its environmental and technological benefits. Therefore, a deep understanding and high-level training in the experimental and numerical tools for investigating natural gas combustion in new burners are of upmost importance for future scientific and technological developments. Natural gas is present in all sectors from companies/business to personal/private sector.
Natural gas is a fossil fuel whose energy conversion is mainly achieved by combustion. The combustion process induces two main side effects: the production of greenhouse gases (CO2) and the emission of pollutant species such as nitrogen oxides (NOx) and soot particles. Conventional techniques used to reduce these emissions, already low compared to usual fossil fuels, are often post-combustion treatments and they include CO2 storage, flue gases cleaned up by catalytic and non-catalytic conversions. Another solution is to act directly on the combustion process in order to limit pollutant emissions at the source while maximizing combustion efficiency. New processes are currently using this strategy, for example regenerative burners, flameless (MILD) combustion, combustion of highly diluted mixtures or oxy-combustion.
These processes, despite of the fact that they are already used in different industrial units, are still poorly understood and very difficult to transpose from one industry to another. Therefore, it is extremely important to develop academic and research studies on these new combustion processes to make best use of existing resources while limiting their environmental impact. These new processes are very different from existing technologies and constitute real technological breakthroughs.
The CLEAN-Gas project covered different topics that are relevant for the clean combustion of natural gas, from chemical kinetics of combustion to fluid dynamics and new technologies, both experimentally and theoretically. Considering the complex nature of the controlling pollutant formation and flame stability, the use of both experimental investigations and Computational Fluid Dynamics (CFD) proved to be essential for the development and implementation of such novel combustion technologies. In particular, the appropriate description of the interactions between the combustion process, and the system aerodynamics is crucial in order to develop innovative, clean and safe combustion systems.

Figure 1. CLEAN-Gas partners.
The project involved different academic partners and companies, belonging to different European countries, as described in figure 1. The project contributed to the training of several ERSs, and improved the understanding of the combustion processes involving natural gas. The project partners worked on the development of detailed kinetic mechanism able to describe the combustion of natural gas in conventional and non conventional systems, including the formation of NOx and particulate emissions. Moreover, experimental and modelling activities addressed the combustion of natural gas in laminar and turbulent systems. The whole approach is based on a comprehensive investigation of different turbulent flames at different levels of complexity. The turbulent flames of increasing complexity vary in their nozzle design, fuel, and pressure conditions.
Kinetic model reduction and automatic optimization tools were developed, in order to facilitate the adoption of detailed chemistry inside complex and computationally intensive 3D simulations of turbulent systems. Finally, the numerical tools developed and validated inside the project were adopted for the simulation of industrial-scale combustion devices with the aim to improve the combustion process and reduce the formation of pollutant species.
The project involved the development of a comprehensive kinetic mechanism to model the combustion of natural gas. Special attention has been devoted the chemistry of pollutant species, particularly PAHs, carbonaceous particles (soot) and NOx. Several sooting flames were studied with good agreement with experimental data (fig.2 ad fig. 3).

Figure 2. Sooting flame prediction using CFD simulations.

Figure 3. Comparison between measurements and simulation of soot volume fraction.

The kinetic mechanism has been improved also for promising “unconventional” and environmentally friendly combustion conditions, such as diluted MILD (Moderate or Intense Low oxygen Dilution) and oxy-fuel conditions. The complex interactions between chemistry and turbulence were studied especially in MILD combustion conditions by developing simulation tools able to provide high-fidelity numerical experiments. Particular attention was devoted to the extension and improvement of existing models targeting the simulation of industrial devices, which adopt the combustion technologies which are more environmentally friendly in terms of CO2 and pollutant emissions.
For example, since in many industrial devices lean-premixed flames are stabilized by a swirling flow, during the project several experiments studied flame stabilization and response to flow disturbances (Fig. 4). Thermo-acoustic instabilities, blow-off and flashback limits were experimentally characterized during the project. Project work provided a reliable dataset of turbulent flames experiments for the validation of numerical simulation tools in the pressurized conditions of interest for gas turbine combustion.

Fig. 4. Flame light distribution for different configurations tested with different swirl intensity (S).

Particular attention was also devoted to the role of radiative energy transfer and to the effect of different configurations (oxygen enrichment, CO2 dilution, high pressure) on the emissions from turbulent methane flames (Figure 5).

Fig 5. LES simulation of Instantaneous field of Radiative Power.

Overall, project already resulted in more than 20 papers published on international peer-reviewed journals, and 31 presentations and posters at international conferences.
Main outcomes of the program are skilled people, scientific and transferable knowledge.
All the ESRs completed their training and are finalizing their thesis in view of the PhD defenses. They are now well-trained experts in Natural Gas combustion and its applications in a general context of environmental needs. They master the necessary tools and methodologies from a cross approach perspective (theory, computation and experimentation), and are professionals able to address the main challenges in the combustion of natural gas.
The second outcome is scientific knowledge and expertise. The multi-scale and multi domain approach approach adopted inside the CLEAN-GAS project greatly contributed to the knowledge development in the field of natural gas combustion. Novel modeling approaches were developed both in the chemistry, fluidynamics analysis and in the simulation of the practical applications. Publications, conference participations, and seminars allowed the ESRs to interact with the scientific and technical community. They also interacted with partner companies which hosted the students and advised the CELAN-GAS project partners about the industrial perspective.