CO2 Dilution Research Articles

Time Series and Spectral Analysis of Thermoacoustic Oscillations for Propane-Oxyfuel Combustion in a Swirl-Stabilized, Nonpremixed Combustor.

In this study, the time series data for sound pressure and heat release fluctuations were analyzed to establish the stability window of the propane-oxyflames under different operating conditions. The CO2 dilution level and the combustor power density were varied at a fixed global equivalence ratio to investigate the thermoacoustic instability of the flames. The phase difference between the fluctuations and the instantaneous Rayleigh index reveals whether the heat release and pressure fluctuations are coupled, which can result in the amplification of the sound pressure fluctuations. Results showed a negative Rayleigh index and uncoupled fluctuations at low (<40%) and high (>60%) CO2 dilution with coupled fluctuations and sound pressure amplification at intermediate CO2 dilution levels. A peak of varying magnitude appeared in the frequency domain at 465 Hz for both the heat release and pressure fluctuations in the coupled mode. The Strouhal number at different CO2 concentrations revealed a range of vortex-shedding frequencies (300-1000 Hz), suggesting that the coupled mode is vortex-induced. Phase space reconstruction for the sound pressure fluctuations was carried out and it is observed that although the pressure fluctuations are amplified in the coupled mode, limit cycle amplitudes have not been reached. The recorded coupling and uncoupling of the oscillations associated with flame-vortex interactions at certain CO2 concentrations provides a valuable insight on the combustor's dynamics and toward the development of nonpremixed-oxy-flames combustors.

Experimental and numerical study of the effect of H2O and CO2 dilution on NH3/CH4 co-oxidation characteristics in a jet-stirred reactor

This work reports an experimental and kinetic modeling study of NH3/CH4 co-oxidation in O2/N2 in a jet-stirred reactor (JSR) covering a reaction temperature of 900–1450 K, equivalence ratio of 0.5–2.0, and dilution degree of 0%–30% with H2O and CO2, respectively. Experimental results show that CO formation is inhibited by H2O dilution while enhanced by CO2 dilution. NO formation is suppressed by either H2O or CO2 dilution under lean condition, however these two diluents generally would accelerate NO formation under stoichiometric and rich conditions. Dong2022 mechanism is selected for further kinetic analysis for NO conversion under stoichiometric condition. It shows that N2H2 plays a more significant role in NO generation in N2/CO2 atmosphere. The reactions of NH + OH ↔ HNO + H and NH2 + O ↔ HNO + H contribute the most for boosting NO formation under both H2O and CO2 dilution conditions.

Comparison of non-reactive flow fields in conventional swirl-air combustion and swirl distributed combustion

Flow field distribution in traditional swirl flow and simulated distributed reaction swirl flow configuration was examined at 3 kHz frequency using high-speed Particle Image Velocimetry (PIV). Non-reactive flow field conditions in distributed reaction zones were fostered from the conventional swirl flow case using the designed inlet mixture preparation with CO2 dilution of the inlet airstream that corresponded to the combustion case at a heat release intensity of 5.72 MW/m3-atm. High-speed imaging of flow fields was performed at a framing rate of 3000 frames/s using a Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser beam at 527 nm wavelength to capture the Mie scattering images of the flow field. Mean flow velocity obtained from the post-processing of raw PIV images showed higher momentum flux injection in the swirl distributed flow zone case as compared to the normal swirl flow case. Enhanced velocity fluctuations were also observed in both the axial and radial direction for the distributed swirl flow case resulting in higher turbulence for the examined condition. The distributed swirl flow case showed a prominent corner recirculation zone along with higher flow entrainment within the recirculation zone. Enhanced interaction of average streamlines with vortices was found in the swirl distributed flow case. Increased turbulent fluctuation in the two different spatial directions was responsible for higher Reynolds stress distribution (u′v′) in the distributed flow case, indicating improved mixing behavior at lower global oxygen concentrations from the carbon dioxide (CO2) dilution, and showed different length scales of turbulent eddies. Furthermore, the spatial local Damkӧhler number values for distributed flow case (for its implication in reacting case) were found to be less than unity. The results showed a shorter flow timescale than the corresponding chemical timescales in distributed reaction zone. These results help support substantiating the difference of reaction regimes on the Borghi diagram (plot of velocity fluctuation/ laminar flame speed versus the integral length scale/ flame thickness) between the distributed reaction zone (that possesses thickened reaction zone) and conventional swirl reaction flow regimes having corrugated flamelet zone.

Küçük Bir Girdaplı Yakıcı ve Fırında CO2 Seyreltmesinin Alev Stabilizasyonu ve NOX Emisyonu Üzerindeki Etkileri

There is a rapid increase in the demand for low NOx hydrogen/methane combustion systems in recent years, and low NOx and low emission burners and furnaces are promising platforms for such applications. Combustion instability is the most important drawback in low NOx burners and uniform thermal field. This paper investigates the influence of CO2 in methane and hydrogen on combustion instability. Numerical simulations were conducted using a small swirl burner and furnace combustion system. The effects of fuel type and CO2 dilution rate, which is a major contributor of hydrogen and methane combustion, on the combustion characteristics and instability are investigated. Combustion instability decreases for higher CO2 dilution rates.

Radiation effects of CO2 and H2O dilution on laminar flame speeds of syngas flames under elevated pressures

The abundant CO2 and H2O in circulating flue gases both chemically affect the combustion characteristics significantly. Meanwhile, the intense radiation effects induced by CO2 and H2O also strongly coupled with chemical reactions by preheat unburned fuels. Thus, the comparison and analysis of dilution effect and interaction mechanism with CO2 and H2O blending is of great significance, especially when considering radiation reabsorption. In this study, laminar flame speeds of premixed syngas flames considering radiation reabsorption were simulated under elevated pressures. It found that radiation reabsorption effect induced by CO2 and H2O dilution both greatly increased the flame speed of syngas flames. Because of the discrepancy in dominant influence mechanisms of radiation reabsorption, different relative enhancements of flame speed as a function of pressure were observed. Moreover, the co-dilution of CO2 and H2O further improved the radiation reabsorption effect by means of the coupling inhibition of OH radical's consumption pathway.

Effect of CO[formula omitted] dilution on structures of premixed syngas/air flames in a gas turbine model combustor

The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.

Role of methyldioxy radical chemistry in high‐pressure methane combustion in CO2

AbstractThe combustion chambers of direct‐fired supercritical CO2 power plants operate at pressures of approximately 300 bar and CO2 dilutions of up to 96%. The rate coefficients used in existing chemical kinetic mechanisms are validated for much lower pressures and much smaller concentrations of CO2. Recently, the UoS sCO2 1.0 and UoS sCO2 2.0 mechanisms have been developed to better predict ignition delay time (IDT) data from shock tube studies at pressures from 1 to 260 bar in various CO2‐containing bath gas compositions. The chemistry of the methyldioxy radical (CH3O2) has been identified as an essential combustion intermediate for methane combustion above 100 bar, where mechanisms missing this species begin to vastly overpredict the IDT. The current literature available on CH3O2 is very limited and often concerned with atmospheric chemistry and low‐pressure, low‐temperature combustion. This means that the rate coefficients used in UoS sCO2 2.0 are commonly determined at sub‐atmospheric pressures and temperatures below 1000 K with some rate coefficients being over 30 years old. In this work, the rate coefficients of new potential CH3O2 reactions are added to the current mechanism to create UoS sCO2 2.1 It is shown that the influence of CH3O2 on the IDT is greatest at high pressures and low temperatures. It has also been demonstrated that CO2 has very little effect on the chemistry of CH3O2 at 300 bar meaning that CH3O2 rate coefficients can be determined in other bath gases, reducing the impact of non‐ideal effects such as bifurcation when studying in a CO2 bath gas. The updated UoS sCO2 2.1 mechanism is then compared to high‐pressure IDT data and the most important reactions which require reinvestigation have been identified as the essential next steps in understanding high‐pressure methane combustion.

Effect of pressure and CO2 dilution on soot formation in laminar inverse coflow flame at conditions close to autothermal reforming

Autothermal reforming (ATR) is an important technology for hydrogen production from natural gas, where soot formation deserves particular attention as it may cause catalyst poisoning. The typical configuration of ATR reactors is that of an inverse diffusion flame (IDF). In this study, soot formation near ATR conditions is investigated experimentally and numerically, focusing on the effects of pressure and CO2 dilution in IDFs with pressure reaching 5 bar. The fuel stream consists of methane diluted with CO2 or N2. The mole fraction of O2 in the oxidant stream is 70%. Polycyclic aromatic hydrocarbons (PAHs) and soot volume fraction are measured by the planar laser-induced fluorescence (PLIF) and planar laser-induced incandesce (PLII) methods. High-fidelity simulations with a detailed soot aerosol model are also performed, and an empirical reactive soot inception model is proposed. A comparison of the results shows that the soot behavior is well predicted by the empirical reactive inception model but not fully captured by the physical inception model based on irreversible dimerization, demonstrating the importance of radical involvement in the soot inception process. Both measurements and predictions show a linear relationship between peak soot volume fraction and pressure, which can be explained by the linear increase of PAH molar concentration with pressure. Simulation analysis indicates that the dimer adsorption and HACA mechanism have a similar quantitative contribution to the soot growth. The CO2 shows stronger suppression effects in soot formation at the investigated pressures compared to N2. The contributions of chemical and thermal effects to soot suppression are numerically analyzed and the results indicate that the chemical effect of CO2 is mainly due to the removal of H radicals through the reaction CO2 + H→CO + OH, providing a soot suppression contribution of two times larger compared to the thermal one.

An Investigation into Burner Configuration Effects on Premixed Flame Characteristics for LPG Diluted with CO&lt;sub&gt;2&lt;/sub&gt;

Meeting stringent emission regulations, the demand for environmentally friendly fuels is increasing by the day. Alternative fuel must be burned alongside conventional fuel to increase the availability of such clean energy sources. The current experimental study investigates the characteristics of the premixed LPG flames with CO2 dilution in tube swirling and non-swirling burners. The study including testing the effects of equivalence ratios, φ, (0.8, 1, 1.2, &amp; 1.4), CO2 dilution ratios (0%, 5%, 7.5%, &amp; 10%), and aspect ratio of the non-swirling burner (2, 4, 6, 8, &amp; 10). Two swirling burners with swirl number was tested, namely 0.78 &amp; 0.48. The dilution of CO2 has been observed lengthens the flame, particularly at higher equivalence ratios and/or flow rates since there is more than one influence, they all agree on a similar influence on flame height. The flame shortens clearly when using a swirling burner. Besides, when increasing the swirl number, the flame height increases slightly. Also, the swirling burner divided the flame's inner core into segments equal to the number of swirl vanes, and a flower-shaped flame was generated at low flow rates. The burner’s aspect ratio affects flame height insignificantly. Flame stability limits increase for a higher equivalence ratio and it enhances due to CO2 addition. The LPG-CO2/air mixture has an improved reply to beat flame flashback. The addition of CO2 expands the flow rate of stable flame by about 40% and 25% for φ = 1 and 1.2 respectively. Utilizing a swirling burner improves flame stability greatly. The limit between flashback and blowout increased by about three times as a result of using a swirling burner.

Effect of N2 and CO2 dilution on syngas/air deflagration characteristics in narrow gap disk burner

ABSTRACT This study explored the typical features of flame from syngas/air premixed flame in a narrow gap disk burner based on diluted N2 and CO2. Four concentrations (i.e. 10%, 20%, 30%, and 40%) of CO2 and five concentrations (i.e. 10%, 20%, 30%, 40%, and 50%) of N2 were adopted in this study. The experiment was conducted in a narrow gap disk burner with a clearance of 2 mm. The results showed that the addition of N2 and CO2 significantly reduced the brightness of flame, the flame propagation velocity, and the pressure resulting from the explosion. When the CO2 concentration reached 40%, the maximum pressure was reduced to 60.78 kPa, which was 41.5% lower than that without diluent. When the N2 concentration was increased to 50%, the maximum pressure was reduced to 67.01 kPa, but at this point, the inhibition effect on the maximum pressure was not as good as 40% of CO2. Additionally, the dilution effect of CO2 was more obvious than of N2. An analysis on the dilution mechanism found that reduced moles of critical radicals H, O, and OH were an important reason for the inhibition effect. However, with the increase of N2 and CO2 concentrations, the unstable flame propagation behavior in a later stage was not inhibited, but enhanced. The unstable propagation behavior was related to the flow field disturbance caused by heat loss.

International Collaborative Cluster-Based Carbon Sequestration Plan

The status quo climate change forecast was analyzed using the ENROADS software and indicates that the net GHG emissions could reach about 90 gigatons of CO2 equivalent by 2100, and global temperatures could rise by 3.6°C. This projection will likely be worse even with moderate estimates. The factors used in the reported status quo forecasts are optimistic. From energy mix to population growth and the assumption that gas use could increase while oil declines, this scenario is limited by available gas reservoirs. There needs to be a more aggressive application of CCS to reach Net-Zero by 2050. Carbon Capture and Storage (CCS) has been identified as the leading technology that could help reduce the amount of CO2 emitted into the atmosphere. At the same time, the search for non-fossil fuel energy sources continues. The amount of CO2 dilution in the air stream complicates the economics of direct air capture. This research proposes two international collaborative clusters for India. Due to India's growth and use of fossil energy sources, this is necessary to manage CO2 emissions from coal-fired plants. The results show that cluster-based CCS facilities possess great potential for the world to develop faster carbon capture and sequestration technology and deploy it faster. These cluster CCS systems will create economies of scale and integration necessary to make the CCS technology succeed to help reduce India's and global CO2 emissions from fossil-fuel-powered electricity generation. If CCS is deployed at the proposed scale, it will reduce atmospheric CO2 significantly, which would capture more than 250 times more than today's effort.

Experimental and modelling study of syngas combustion in CO2 bath gas

Syngas produced from coal and biomass gasification has been proposed as a potential fuel for direct-fired supercritical power cycles. For instance, the Allam-Fetvedt cycle can offer price-competitive electricity production with 100 % inherent carbon capture while utilizing CO2 dilution of about 96 %. In this work, ignition delay times (IDTs) of syngas have been measured in CO2 diluted conditions using a high-pressure shock tube at two pressures (20 and 40 bar) over a temperature range of 1100 – 1300 K. Syngas mixtures in this study were varied in equivalence ratio and H2:CO ratios. The datasets were compared against the predictions of AramcoMech 2.0 and the University of Sheffield supercritical CO2 2.0 (UoS sCO2 2.0) kinetic models. Quantitative comparative analysis showed that the UoS sCO2 2.0 was superior in its ability to predict the experimental IDTs of syngas combustion. We found that the reaction of CO2 and H to form CO and OH caused the separation of H2 and CO ignition in two events, which increased the complexity of determining the IDTs. We investigated this phenomenon and proposed a method to determine simulated IDTs for an effective comparison against the experimental IDTs. The chemical kinetics of syngas combustion in a CO2 and N2 bath gas are contrasted by sensitivity and rate-of-production analyses. By altering the ratio of H2 and CO as well as mixture equivalence ratio, this work provides vital IDT data in CO2 bath gas for further development and validation of relevant kinetics mechanisms.

Sooting tendency of isopropanol-butanol-ethanol (IBE)/diesel surrogate blends in laminar diffusion flames

Recent advancements in metabolic engineering have demonstrated the cost-effective production of isopropanol-butanol-ethanol (IBE) from switchgrass biomass. As such, IBE is seen as a viable future fuel to transition the transportation sector towards renewable biofuels. In this study, an atmospheric co-flow diffusion flame burner was used to examine the sooting tendency of IBE when mixed with a diesel surrogate for blend ratios varying from 0% to 100% by volume. Experimental measurements of soot volume fraction were obtained by implementing a color-ratio pyrometry technique and reporting the variation of sooting tendency in terms of the Yield Sooting Index (YSI). Measurements demonstrated soot formation and hence YSI decreased nonlinearly with increasing IBE content. To gain further insight into the kinetics affecting soot formation, numerical modeling studies were carried out using laminarSMOKE++ and a comprehensive kinetic mechanism with soot chemistry. From a reaction path and rate of production analysis, IBE demonstrated that it can perturb OH and HO2 radical pools and enhance CO and CO2 reactive channels from IBE oxygenated intermediates to suppress the formation of soot precursors. Studies were also extended to examine the role of CO2 dilution on soot formation. It was shown that further soot reduction can be attained by lowering the rate of overall soot formation kinetics, perturbing reaction intermediates leading to soot formation by the depletion of H-atom radical pool, while enhancing the oxidation through enhanced OH-radical production.

Experimental investigations on laminar burning velocity variation of CH4 + air mixtures at elevated temperatures with CO2 and N2 dilution

Experimental investigations on laminar burning velocity variation of CH4 + air mixtures at elevated temperatures with CO2 and N2 dilution

Experimental study on the instability characteristics of CO2-diluted n-butane/air cellular flames on McKenna burner

Present experimental study reports on the effects of CO2 dilution on intrinsic instabilities of non-adiabatic n-butane/air cellular flames on McKenna burner at initial 300 K temperature and 0.1 MPa pressure. The Planar Laser Induced Fluorescence (OH-PLIF and CH2O-PLIF) optical diagnostic technology was utilized to probe the structure of cellular flames. The results show that the cellular flames can be obtained under the conditions with lean, stoichiometric and rich CO2 diluted n-butane/air mixtures. At lean conditions, hydrodynamic instability was dominant and the cellular flames were formed by connected wrinkles. However, at rich conditions, diffusive-thermal instability was dominant and the cellular flames were formed by separated cells. It confirmed that the CO2 dilution can enhance flame instability significantly, as well as the enhanced effect was positive correlation with CO2 dilution ratio. Furthermore, the enhanced effects of flame instability was stronger by CO2 dilution than by N2 dilution. The chemical and thermal effects by CO2 dilution were attributed to enhance flame instability.

Research on the Influence of Valve Sensor Fault Based on Simulation

In the practical application of indoor air quality assurance system, it is difficult to detect the fault of air valves, CO2 sensors and other components, resulting in substandard air quality and increased energy consumption. In this paper, 16 fault conditions and 1 no fault condition were simulated by Simulink simulation software for primary return air conditioning system in summer, and a typical indoor air quality assurance system simulation fault characteristic table was obtained. It is found that PM2.5 sensor fault mainly affects indoor PM2.5 purification speed. The fault of a CO2 sensor affects indoor CO2 dilution and system energy consumption. Small resistance of return air valve increases system energy consumption. Small resistance of supply air valve, exhaust air valve and return air valve decreases the system's ability to control indoor pollutants.

Investigations on Emission Characteristics of Methane in Small Scale a Swirl Flameless Combustor: Using Preheating Air Diluted CO2 and N2 Gas at Various Temperatures

In order to reduce the damage to the environmental, it is desirable to have low emission combustion and high-efficiency system of operation. In achieving this, the multi-stages and exhaust gas recirculation (EGR) were adopted to reduce NOx formation during combustion. In this paper, a numerical study of the impact of diluted preheated oxidizer variables on NOx emissions from methane combustion was performed. This study utilizes Computational Fluid Dynamics (CFD) analysis to determine the influence of preheating fuel and air on the combustion efficiency when asymmetric vortex flameless was applied on different supply tangential air location at different oxygen concentrations. The design parameters utilized are diluted N2 and CO2, oxygen concentrations of 10%, 7% and 5%, and air temperatures that were set at 300 K, 500 K, 700 K and 900 K, respectively. The exhaust gas recirculation (EGR) was simulated using CO2 gas and N2 gas. The results show that by oxygen concentration has direct relationship with NOx emission for all equivalence ratios. Apart from that, CO2 dilution air offers better results than N2 dilution in reducing NOx emissions. In terms of preheating air, higher NOx emission was observed with the increase of air temperature. However, the effect of preheating air on the NOx emission is greater than the effect of preheating of fuel. Based on these results, numerous ports were considered important to achieve a good mix of air and fuel. The analysis outcome shows that more inlet tangential air has a significant influence on combustion temperature and NOx emission.

An experimental investigation of ammonia/landfill/air mixtures' pollutant emissions and temperature distribution under non-preheated moderate or intense low-oxygen dilution combustion.

This paper investigates the non-preheated MILD combustion behavior of landfill/ammonia mixtures. The experiments were carried out in a laboratory-scale furnace. The effects of equivalence ratio variation, CO2 dilution, and ammonia on the furnace temperature distribution and exhaust gas emissions were analyzed attentively. The experiment results indicate that a non-preheated MILD combustion establishment can provide a uniform temperature distribution and lower pollutant emissions while maintaining the same thermal efficiency. According to the results, equivalence ratio changes affect the MILD temperature structure sensibly. Variation of equivalence ratio is also effective on the pollutant emissions. Moreover, the rich burn condition increases the CO emissions by bringing the HCN and H radicals into the related reactions. Whereas in lean-burn states, little amounts of CO are generated, while the NO concentrations are higher due to the NH2 pathways. The results also indicate that CO2 dilution reduces the furnace's peak temperature. However, CO2 dilution may also increase the O and OH radicals, allowing HCN molecules to react and generate NO. Regarding the ammonia addition experiments, it was figured out that ammonia increases NOx emissions, while decreasing CO2 concentrations affect the flame temperature profile.

Heterogeneous and Combined Heterogeneous/Homogeneous Combustion Modeling of SOFC Off-Gases.

The heterogeneous (catalytic) and the hetero-/homogeneous (catalytic and gas-phase) combustion processes of solid oxide fuel cell (SOFC) off-gases with compositions typical of a high cell utilization rate are investigated with high-fidelity 2D simulations in a platinum-coated planar channel using detailed hetero-/homogeneous chemistry. The pressures are 1-8 bar; the reactant streams have volumetric H2 and CO contents 0.7-1.5 and 5.3-9.7%, respectively; H2O and CO2 dilutions are ∼40 and ∼50%, respectively; and the global fuel/air equivalence ratio is 0.90. Water inhibits chemically the catalytic oxidation of H2, as it leads to high H(s) surface coverage that favors the recombinative desorption of H(s) to H2. On the other hand, H2O promotes chemically the catalytic oxidation of CO by creating high OH(s) coverage that in turn accelerates the CO consumption. Strong flames are established at the highest H2 content cases and for pressures p ≥ 3 bar. For all cases with vigorous homogeneous combustion, the catalytic and gas-phase reaction pathways coexist and compete with each other for the consumption of H2 and CO. The large H2O content leads to gas-phase production of H2 via the reaction H2O + H = H2 + OH. However, the gas-phase produced H2 is subsequently consumed by the catalytic pathway, such that nearly complete H2 conversion is attained at the reactor outlet. Gaseous chemistry does not affect the reactor lengths required for complete H2 conversion but substantially reduces the corresponding lengths for CO conversion. The H2 emissions decrease with increasing pressure and are in the range 8-110 ppmv, while the CO emissions increase with rising pressure and span the range 0.3-52 ppmv, thus leading to corrected CO emissions (at 15% O2) of less than 15 ppmv. Finally, the peak wall temperatures are largely acceptable in terms of catalyst thermal stability.

High-Pressure Laminar Flame Speeds and Markstein Lengths of Syngas Flames Diluted in Carbon Dioxide and Helium

Abstract New laminar flame speed and burned-gas Markstein length data for H2–CO–O2–CO2–He mixtures have been measured from spherically expanding flames. Experiments were conducted at 10 atm and room temperature for H2:CO ratios ranging from 2:1 to 1:4 and for overall CO2 mole fractions from 0% to 30%. CO2 dilution had little effect on Markstein length, but CO2 dilutions of 10%, 20%, and 30% caused average reductions in flame speed of 47%, 73%, and 89%, respectively, regardless of H2:CO ratio. The study was designed to isolate the dilution effect of CO2 on flame speed, and a detailed analysis using the FCO2 method was used to show that the chemical-kinetic participation of CO2 was responsible for up to 20% of the reduction in flame speed. Hence, the majority (80% or more) of the reduction in flame speed due to CO2 is from the thermal effect. Accurate flame speed predictions were produced by five different chemical kinetics mechanisms for most conditions, with the slight exception of high-CO, high-CO2 mixtures. A thorough sensitivity analysis highlighted the larger effect of CO2 dilution on the important kinetics reactions than the effect of changing H2:CO. Sensitivity analysis also showed that the chain branching reaction H2O + O ⇌ OH + OH could be modified (albeit beyond its uncertainty) to achieve more accurate flame speed predictions, but also indicated that further improvement of flame speed modeling would require changes to many lesser reactions.