Flame pattern analysis for flames under conventional air-fired and oxy-fuel conditions for two different types of coal
Introduction
Despite a decline in use of coal for electricity production over the last years, the overall consumption is projected to rise for the coming decades, based on the current energy policies [1]. Scenarios projecting the -release from coal power plants, remain far below the -emission-reduction required to meet the ambitious goals of the Intergovernmental Panel on Climate Change (IPCC) [2], necessary to prevent an increase of global mean temperature by more than by mid-21st century. The sustainable development scenario projected in the World Energy Outlook (WEO) from 2018 [1] requires a decrease of emissions from use of coal by a factor of ten compared to the currently stated world-wide policies. Therefore different techniques to reduce carbon dioxide emissions have to be implemented. While the WEO highlights the need for Carbon Capture Utilization and Storage (CCUS) techniques, it emphasizes the lack of progress in this area and the need of retrofitting or equipping new power plants with CCUS techniques in order to meet the required emission reductions. This does not only require political action, but also scientific development in this area, to make existing techniques like oxy-fuel combustion more mature. Oxy-fuel combustion, proposed by Abraham et al. [3] in 1982 is a method where the flue gas is enriched with and fed as oxidizer to the combustion process. The resulting flue gas consists primarily of , simplifying its sequestration. Therefore the differences in combustion, due to the different oxidizer composition [4], have to be examined, i.e. the influence of changing the oxidizer’s composition from the conventional mixture with respective volume fractions of to an composition in oxy-fuel-combustion with different volume fractions. Differences may for example arise from changes of heat capacity or gas radiation [4]. Thus, for retrofitting existing coal power plants, comprehensive studies and models are needed to account for changes in fuel type or oxidizer composition on flame stability and heat transfer characteristics. Comprehensive reviews on oxy-fuel combustion are given in [4], [5].
Investigations of major species concentration have been carried out by H. Liu et al. [6] in a 20 kW combustor finding a decrease in NOx production when changing from air to oxyfuel combustion, while SOx production was not influenced by changes in oxidizer composition. K. Andersson et al. [7] found a decrease of when changing from air to oxyfuel combustion with flue gas recycling in a test facility. However, they show that the conversion of fuel-N to NO can be even higher during oxyfuel combustion, in agreement with previous studies, where NO mainly reacts to molecular by chemical processes in the reaction zone [8].
The flow field of pulverized coal flames has been mostly investigated by means of Laser Doppler Anemometry (LDA), determining the coal particle velocity. Different configurations have been considered, covering a range from several tens of kilowatt to a half MW size, e.g. Schnell et al. [9], [10] over the MW-range, e.g. [11], [12], [13] to several tens of MW, e.g. [14]. Corrêa da Silva et al. [15] have shown, that the stabilization of oxycoal flames is determined in the burner front and strongly depends on the inflow conditions. Similar observations have been made by Leuckel and Fricker [16], postulating from velocity measurements below the burner that flow stabilization of IFRF Type-2 flames occurs within the burner quarl. In a recent study, Becker et al. [17] employed a burner geometrically similar to the one used in the present study, but with optical access to the quarl (diffuser). The burner was operated under air and oxy-fuel conditions with methane as a fuel. This study shows that the flame dynamics occurring inside the quarl, a location usually not accessible by instrumentation, are important to understand the flame stabilization mechanisms. Therefore, information about the flow fields near the burner is necessary in order to understand flame stabilization behavior.
The radiative heat flux characteristics of lignite-flames with varying -content have been investigated in [18], finding similar radiative heat flux characteristics at -content in the / mixture as compared to conventional air combustion. The influence of coal type on radiative heat flux at different -content in the recycled atmosphere has been evaluated by J.P. Smart et al. [19], concluding that the -content at which heat fluxes of oxy-fuel combustion match those resulting from combustion in conventional air-atmosphere are coal-type dependent. Hjärtstam et al. [20] performed flue gas sampling for lignite fired flames under air- and different oxy-fuel conditions, finding that NO production goes down as flue gas is recycled and that CO-emissions are not influenced by the change in oxidizer even though the spatial distribution of CO production inside the flames changes with oxidizer composition.
In recent publications [21], [22], [23], [24], [25], flow field, local gas composition, particle temperature measurements or narrow-band imaging have been performed on the same setup as used in the present study. The focus in terms of fuel-type has been put on lignite and experiments were dedicated to investigate the influence of oxidizer composition. Various techniques have been employed and improved for detailed flow [24], [26] or temperature measurements [25] and gas sampling [22] was applied to map major product species. It was shown that due to density changes of the oxidizer, when changing its composition, the associated momentum changes at constant inlet velocities are mainly responsible for changes in flow field and flame structure [23]. Therefore the momentum of the inlet streams could be identified to be an important tuning parameter for the flame structure when switching from air- to oxy-fuel-atmosphere.
In [23] it was shown that for the same configuration as in the present study, flames with same momentum have similar flame shape, a principle that can be used for conversion of existing conventionally air fired power plants to oxyfuel combustion as done in [27]. However, the subsequent complementary determination of flow fields using Particle Image Velocitmetry (PIV) and Laser Doppler Anemometry (LDA), [24] of one air and two oxyfuel lignite flames has opened a new question about the source of differences that have been observed for flow field and particle distribution despite the similar inflow conditions. Thus, for the present configuration the question arises if there is a more general mechanism how momentum changes influence velocity field and flame shape that also holds for different fuels? Appending to the first question a second question arises: if a change in momentum influences flame shape, what is the effect on radiative heat transfer characteristics of the flames? Therefore the present study investigates not only the influence of oxidizer and momentum on flame shape but also varies the employed fuel, hence in addition to the already comprehensively investigated lignite flames, two bituminous coal flames are examined in detail. To draw a comprehensive conclusion of the changes in initial conditions on radiative heat transfer, measurements employing a radiative heat transfer probe have been conducted. Therefore the present paper is not only dedicated to give a detailed image of the studied flames but also to derive a better understanding of the influence of tuning parameters (inflow conditions, fuel and oxidizer) on quantities interesting for retrofitting, such as matching heat flux profiles.
Section snippets
Experimental test facility
Experiments were carried out at the test facility of the Institute of Heat and Mass Transfer, RWTH Aachen University, cf. [28]. The facility consists of a cylindrical downfired combustion chamber (cf. Fig. 1a) with an inner diameter of and a total length of . The inside of the combustion chamber is lined with three ceramic layers with embedded heating elements, keeping wall temperatures as high as . This allows self-ignition of coal flames. The inner top section of the
Results
Photos taken through the window of OP2 of all four flames in the near-burner region are shown in Fig. 3. Pictures were taken at constant aperture and ISO-settings; only the exposure time is changed between lignite and bituminous coal flames. The visual inspection of the flame and of these four images shows, that coal-type and oxidizer-composition have a severe impact on flame luminosity in the visible range. Furthermore the images suggest severe differences in flame patterns. While the AIR-L
Discussion
A comprehensive discussion of each flame is given in Sections 4.1–4.4. Differences between the flow fields of the studied flames as well as the impact on radiative heat flux are discussed in Sections 4.5 Discussion of major differences in flow field, 4.6 Discussion of impact on radiative heat transfer characteristics.
Conclusion
The present study is dedicated to investigate the influence of oxidizer composition, fuel-type and inlet conditions on flame-pattern and behavior of pulverized coal flames on flame length, velocity field, minor species concentration in the far field of the burner as well as radiative heat flux characteristics. Not only that experimental studies on bituminous coal flames are in general fewer than on lignite flames, this study furthermore studies bituminous coal flames for the first time for this
CRediT authorship contribution statement
A. Maßmeyer: Conceptualization, Formal analysis, Investigation, Writing - original draft. D. Zabrodiec: Conceptualization, Formal analysis, Investigation, Writing - original draft. J. Hees: Investigation, Writing - review & editing. T. Kreitzberg: Writing - review & editing. O. Hatzfeld: Project administration. R. Kneer: Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research leading to this work has been funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 215035359 – TRR 129 “Oxyflame”. The authors would further like to acknowledge the help of colleagues from the Institute of Heat and Mass Transfer (WSA), RWTH Aachen University: S. Pielsticker, M. Ecker, T. Grooten, B. Thalheim, C. Axt for providing valuable contributions to the experimental data gathering and evaluation process.
References (40)
- et al.
Oxy-fuel combustion of pulverized coal: characterization, fundamentals, stabilization and CFD modeling
Prog Energy Combust Sci
(2012) - et al.
Oxy-fuel coal combustion – a review of the current state-of-the-art
Int J Greenhouse Gas Control
(2011) - et al.
Comparisons of pulverized coal combustion in air and in mixtures of O2/CO2
Fuel
(2005) - et al.
NOx-reduction mechanism in coal combustion with recycled CO2
Energy
(1997) - et al.
Velocity measurements in a swirl driven pulverized coal combustion chamber
Combust Flame
(1988) - et al.
Quarl zone flow field and chemistry of swirling pulverized coal flames: measurements and computation
Symp (Int) Combust
(1992) - et al.
temperatures and stability limits of pulverized oxy-coal combustion
Fuel
(2014) - et al.
Experimental investigation of flame stabilization inside the quarl of an oxyfuel swirl burner
Fuel
(2017) - et al.
Radiation intensity of lignite-fired oxy-fuel flames
Exp Therm Fluid Sci
(2008) - et al.
Combustion characteristics of lignite-fired oxy-fuel flames
Fuel
(2009)
Detailed analyzes of pulverized coal swirl flames in oxy-fuel atmospheres
Combust Flame
Experimental investigation of pulverized coal flames in CO 2/O 2- and N 2/O 2-atmospheres: comparison of solid particle radiative characteristics
Fuel
Pulverized coal swirl flames in oxy-fuel conditions: effects of oxidizer O2 concentration on flow field and local gas composition
Proc Combust Inst
Conversion of existing coal-fired power plants to oxyfuel combustion: case study with experimental results and CFD simulations
Energy Proc
Experimental investigation of NOx emissions in oxycoal combustion
Fuel
Detailed investigation of a pulverized fuel swirl flame in CO2/O2 atmosphere
Combust Flame
Development of an oxycoal swirl burner operating at low concentrations
Fuel
Experimental studies on heat transfer of oxy-coal combustion in a large-scale laboratory furnace
Appl Therm Eng
Flame and radiation characteristics of gas-fired O2/CO2 combustion
Fuel
Simultaneous investigation into the yields of 22 pyrolysis gases from coal and biomass in a small-scale fluidized bed reactor
Fuel
Cited by (7)
Experimental investigation of 40 kW<inf>th</inf> methane-assisted and self-sustained pulverized biomass flames
2023, Proceedings of the Combustion InstituteAdvanced modeling approaches for CFD simulations of coal combustion and gasification
2021, Progress in Energy and Combustion ScienceCitation Excerpt :Thus, they employed conventional models for the solid phase, i.e., single-kinetic rates for devolatilization and a diffusion-kinetic-controlled model for char oxidation. For the Aachen burner, Massmeyer et al. [438] carried out an experimental study examining how the fuel type and oxidizer composition influence the flame structure and combustion behavior. Four pulverized coal flames with different coal types and different atmospheres were studied with a constant thermal output and stoichiometry.
Flow pattern and behavior of 40 kW<inf>th</inf> pulverized torrefied biomass flames under atmospheric and oxy-fuel conditions
2021, Renewable and Sustainable Energy ReviewsCitation Excerpt :To determine radiative heat fluxes (RHF) to the chamber walls, an ellipsoidal radiometer (Manufactured by Green Flames Technology, Livorno, Italy) based on a design of the International Flame Research Foundation (IFRF) [41] was constructed at the tip of a water-cooled probe. The setup is identical to the one employed in Ref. [42]. The probe has an outer diameter of 43 mm, and it is mounted at OP3 (see Fig. 1b), where it can be placed at different radial locations inside the wall tunnel to collect radiation.