Effect Of Hydrogen Fraction Research Articles

Retrofitting Natural Gas–Fired Boiler for Hydrogen Combustion: Operational Performance and NOx Emissions

Abstract The effects of hydrogen fraction (HF: volumetric fraction of H2 in the fuel mixture of CH4 + H2) from 0% to 100% by volume, on the thermal and environmental performance of a 207-MW industrial water tube boiler, are investigated numerically at a fixed excess air factor, λ = 1.15. This study aims to determine the hardware modifications required for boilers to be retrofitted for pure hydrogen operation and investigates how NOx emissions are affected by hydrogen enrichment. The results showed insignificant increases in maximum combustion temperature with increasing the HF, though the distributions of temperature profiles are distinct. In reference to the basic methane combustion, H2 flames resulted in a positive temperature rise in the vicinity of the burner. Increasing the HF from 0% to 2% resulted in higher average thermal NOx emissions at the boiler exit section from 37 up to 1284 ppm, then it decreased to 1136 ppm at HF = 30%, and later it leveled up to 1474 ppm at HF = 100%. The spots for higher differences in NO formation compared to the reference case are shifted downstream at higher HFs. The effect of hydrogen enrichment on CO2 and H2O as radiation sources, as well as the volumetric absorption radiation of the furnace wall and the heat flux at furnace surfaces, has all been presented in relation to the effect of hydrogen addition on boiler performance.

Effect of hydrogen fraction and initial pressure on the inhibition of methane/hydrogen/air explosions by NaHCO3

To explore the effects of the hydrogen fraction and initial pressure on the inhibition of methane/hydrogen/air explosions, the inhibition of stoichiometric methane/hydrogen/air explosions (hydrogen fraction ranging from 0 to 0.8) by NaHCO3 at various initial pressures (0.8 atm, 1.0 atm, 1.4 atm) was investigated in a 36 L spherical vessel. The experimental findings indicate that the effectiveness of NaHCO3 in suppressing methane/hydrogen/air explosions is significantly influenced by the hydrogen fraction and initial pressure. For a given hydrogen fraction, higher initial pressures weaken the inhibitory effect of a given NaHCO3 concentration on explosions. The drop ratios in the maximum explosion pressure for initial pressures of 0.8 atm, 1.0 atm, and 1.4 atm are 41.2 %, 19.7 %, and 15 %, respectively, at a hydrogen fraction of 0.8 and a NaHCO3 concentration of 400 g/m3. Additionally, the inhibitory impact of a given NaHCO3 concentration diminishes as the hydrogen fraction increases at the given initial pressure. An increasing hydrogen fraction decreases the drop ratio of the maximum explosion pressure while simultaneously increasing the maximum pressure rise rate and deflagration index. A thermodynamic equilibrium model for the endothermic decomposition of NaHCO3 particles was built to explore the impact of the hydrogen fraction and initial pressure on the decomposition characteristics of NaHCO3 particles of different sizes. The laminar burning velocity was derived using a theoretical model through the experimental pressure history and was calculated through detailed chemical kinetic modeling. The effect of the hydrogen fraction on the suppression mechanisms of NaHCO3 was evaluated by using the normalized laminar burning velocity. The dominant inhibition mechanism of NaHCO3 on the laminar burning velocity changes from physical inhibition to chemical inhibition with increasing hydrogen fraction.

Effects of hydrogen fraction in co-injection gas on combustion characteristics of the raceway in low carbon emission blast furnace

Co-injection of coal and hydrogen-rich gas with oxygen blast at tuyere is a promising technology for low carbon emission blast furnace ironmaking, which favors the development of hydrogen-based reduction and affordable carbon capture. Effects of hydrogen fraction in the co-injection gas on the combustion characteristics in raceway are numerically studied. Results show that increase of hydrogen fraction in the co-injection gas accelerates the preferential combustion of the injected gas, which promotes the preheating and pyrolysis of pulverized coal. This compensates the oxygen-grabbing effect and improves the burnout of coal from 83.2% to 86.8% with the hydrogen fraction in co-injection gas increase from 20 vol% to 80 vol%. With the CO/H2 molar ratios in the co-injection gas of 8:2, 6:4, 4:6 and 2:8, the outlet CO/H2 molar ratios are 54:46, 42:52, 32:68 and 24:76 respectively. Combustion, gasification and water-gas shift reactions play conjoint roles in the redistribution of reducing gas species.

Effects of compression and mixing ratio on NH3/H2 fueled Si engine performance, combustion stability, and emission

• NH 3 /H 2 are attractive alternative fuels for the worldwide decarbonization movement. • H 2 is an excellent ammonia additive because it eliminates the cold start problem. • The combustion of ammonia/hydrogen reduces fuel waste while increasing BP. • There are no greenhouse gas emissions from ammonia/hydrogen combustion. • As a result, it appears that fuel NOx predominates over thermal NOx. Carbon-free fuels for the worldwide decarbonization movement are ammonia and hydrogen. The experiment is conducted under WOT conditions with a constant ignition timing of 24°CA BTDC to evaluate performance, combustion stability, and emissions with varying CR (12 to 15), hydrogen energy fractions (5 to 21%), and engine speeds between 1500 and 1700 RPM. BP increased by 31.2% at 1700RPM and BTE increased by 39.0% at 1500RPM, despite a 9% decrease in volumetric efficiency at 1700RPM, from 5% hydrogen fraction at CR12 to 21% hydrogen fraction at CR15.The combustion process is sped up by the effect of hydrogen fraction and CR, causing the flame development and propagation period to shorten. NOx emission was increased significantly with hydrogen and CR, with an increase of 42.34% from 5% hydrogen at CR12 to 21% hydrogen at CR15 at 1700 RPM. Excessive NOx emissions are a drawback that can be successfully controlled by installing after treatment or exhaust gas recirculation technologies. Ammonia is another important key element used to reduce NOx emissions from vehicles because it is used in SCR.

Effects of Hydrogen Fraction on Explosion Characteristics of Hydrogen/Methane/ Air in a Vented Chamber

Effects of Hydrogen Fraction on Explosion Characteristics of Hydrogen/Methane/ Air in a Vented Chamber

Flame characteristics of premixed H2-air mixtures explosion venting in a spherical container through a duct

The explosion venting duct can effectively reduce the hazard degree of a gas explosion and conduct the venting energy to the safe area. To investigate the flame quantitative propagation law of explosion venting with a duct, the effects of hydrogen fraction and explosion venting duct length on jet flame propagation characteristics of premixed H2-air mixtures were analyzed through experiment and simulation. The experiment results under initial conditions of room temperature and 1 atm show that when hydrogen fraction was high enough, part of the unburned hydrogen was mixed with air again to reach an ignitable concentration, resulting in the secondary combustion was easier produced and the duration of the secondary flame increased. With the increase of venting duct length, the flame front distance and propagation velocity increased. Meanwhile, the spatial distribution of pressure field and temperature field, and the propagation process and mechanism of the flame venting with a duct were analyzed using FLUENT software. The variation of the pressure wave and the pressure reflection oscillation law in the explosion venting duct was captured. Therefore, in the industrial explosion venting design with a duct, the hazard caused by the coupling of venting pressure and venting flame under different fractions should be considered comprehensively.

Numerical and experimental study of swirl premixed CH4/H2/O2/CO2 flames for controlled-emissions gas turbines

Stoichiometric hydrogen-enriched oxy-methane (CH4/H2/O2/CO2) flames are investigated numerically by large eddy simulations (LES) and experimentally in a premixed dry-low-emission (DLE) swirl combustor to study the combustion behavior as well as stability characteristics. The study focuses on the effects of hydrogen fraction (HF), oxygen fraction (OF), and inlet bulk velocity (Uin) of the combustible mixture on the stability and combustion characteristics of the flames. Some distinctive features of the flames are reported in this study for possible application in controlled-emissions gas turbines. The results show that the reaction rates are increased with increasing percentages of hydrogen and oxygen in the reactant mixture, and the flame structures are observed to be more stable and compact. It is noted that the reaction rates are more sensitive to OF than they are to HF. The reaction rates are reduced with increasing inlet flow velocity, owing to the associated shorter residence time. The outer recirculation zone plays a significant role in flame stabilization at smaller values of HF and OF; however, its role diminishes at higher values. The CO concentration at the combustor exit increases with increasing HF and OF, due to higher CO2 decomposition at elevated temperatures. The effluent CO concentration, on the other hand, decreases with increasing inlet velocity. Higher HF and OF are characterized by higher Damköhler number (Da) and reduced flame thickness. The effect of velocity on flame thickness, however, was found to be negligible, indicating that the flame microstructure is governed by the overall stoichiometry. The Da showed declining behavior with increasing the inlet velocity because of smaller residence time. The reaction rates thus dominate the flame behavior, while turbulence affects only the macro-structure of the flame.

Study on the effect of hydrogen fraction on the premixed combustion characteristics of syngas/air mixtures

Effect of hydrogen fraction on the combustion characteristics of syngas/air mixtures was experimentally investigated at various hydrogen fractions and equivalence ratios in a turbulent combustion bomb under atmospheric temperature and pressure. The laminar burning velocity and turbulent burning velocity at various hydrogen fractions were measured, and the promotion of turbulence on the burning velocity at various hydrogen fractions was systematically investigated. The results show that the laminar burning velocity of the H2/CO/air mixture exhibits a non-linear increasing trend with increasing hydrogen fraction. With increasing equivalence ratio, the laminar burning velocity increases remarkably. Reaction CO + OH = CO2 + H is identified as the most dominate chain branching reactions and reaction H + OH + M = H2O + M is the most dominant chain termination reaction. The laminar burning velocity exhibits linear and non-linear growth with the maximum concentration of radical H and OH, respectively. With increasing the hydrogen fraction, the value of uT/uL shows a non-linear decreasing trend as a whole and the growth rate gradually decreases. As the equivalence ratio increases, the uT/uL value decrease. When the hydrogen fraction is more than 50%, the promoting effect of turbulence on the burning velocity decreases gradually.

Research of performance and emission indicators of the compression-ignition engine powered by hydrogen - Diesel mixtures

The purpose of this study is to use the hydrogen – diesel mixture in Audi/VW 1.9 TDI turbocharged CI engine equipped with dynamometer and examine the performance and emission indicators by comparing it with sole diesel mode. The recent diesel emission scandals because of manufacturers cheating the laboratory tests, have initiated the discussions about the sustainable and environmentally friendly diesel engines. The CI engine without major engine modifications was set to operate at two speeds of 1900 rpm and 2500 rpm. At each of speed, the experiment was conducted at three BMEP: 0.4 MPa, 0.6 MPa, and 0.8 MPa. The test engine was operated using diesel fuel with amounts of 10 l/min, 20 l/min, and 30 l/min of hydrogen gas, supplied with air into intake manifold before the turbocharger. Relatively low hydrogen fraction (max. 15.74%) has effect on diesel combustion process and performance indicators at the all range of BMEP. The in-cylinder peak pressure at both speeds of 1900 rpm and 2500 rpm was lower than that with pure diesel fuel, as the small amount of hydrogen shortens the CI engine ignition delay period and decreases the rate of pressure rise. The decrease of BTE noticed, and increase of BSFC was registered with low hydrogen fraction (hydrogen amounts of 10 l/min, 20 l/min). However, with increase of hydrogen amount to 30 l/min, the BTE increased and BSFC decreased to the level, which was lower than that at the pure diesel test. The supply of hydrogen positively effects on engine emissions: the smokiness, NOx, CO2, CO decreased, the only hydrocarbon increased. The effect of hydrogen fraction on the combustion and emission characteristics of the diesel - hydrogen mixture was validated by AVL (Anstalt für Verbrennungskraftmaschinen List) BOOST and analysed with presentations of the main limitations and perspectives.

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H2/CO/air mixtures. The laminar burning velocity of H2/CO/air mixtures diluted with CO2 and N2 was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H2/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H2 fraction in H2/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H2/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H2/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.

Effect of dilution on laminar burning characteristics of H2/CO/CO2/air premixed flames with various hydrogen fractions

The laminar burning characteristics of syngas-premixed (H2/CO/CO2/air) laminar spherical flames have been systematically investigated through an experiment using Schlieren system. In the experiment, the volume fraction of H2 in the H2/CO part of the H2/CO/CO2/air mixtures was varied from 30% to 100%, the volume fraction of CO2 in the H2/CO/CO2 mixtures was varied from 0% to 60%, and the equivalence ratio was varied from 0.4 to 1.0. The effects of hydrogen fraction, carbon dioxide fraction and equivalence ratio on flame propagation, Markstein length, and laminar burning velocities have been studied. The obtained laminar burning velocities were compared with existing data from the literature and excellent agreement was obtained. The flames become more unstable by decreasing the equivalence ratio and increasing dilution fraction. The value of laminar burning velocity decreases apparently with the increase of carbon dioxide dilution fraction due to combined effect of decrease in adiabatic temperature, thermal diffusivity and the CO2 participation in the chemical reaction of CO+OH↔CO2+H. With the increase of hydrogen fraction, the laminar burning velocity increases significantly due to the chemical effect and diffusion effect rather than thermal effect.

Laminar burning velocities and flame characteristics of CO–H2–CO2–O2 mixtures

Laminar burning velocities of CO–H 2 –CO 2 –O 2 flames were measured by using the outwardly spherical propagating flame method. The effect of large fraction of hydrogen and CO 2 on flame radiation, chemical reaction, and intrinsic flame instability were investigated. Results show that the laminar burning velocities of CO–H 2 –CO 2 –O 2 mixtures increase with the increase of hydrogen fraction and decrease with the increase of CO 2 fraction. The effect of hydrogen fraction on laminar burning velocity is weakened with the increase of CO 2 fraction. The Davis et al. syngas mechanism can be used to calculate the syngas oxyfuel combustion at low hydrogen and CO 2 fraction but needs to be revised and validated by additional experimental data for the high hydrogen and CO 2 fraction. The radiation of syngas oxyfuel flame is much stronger than that of syngas–air and hydrocarbons–air flame due to the existence of large amount of CO 2 in the flame. The CO 2 acts as an inhibitor in the reaction process of syngas oxyfuel combustion due to the competition of the reactions of H + O 2 = O + OH, CO + OH = CO 2 + H and H + O 2 (+M) = HO 2 (+M) on H radical. Flame cellular structure is promoted with the increase of hydrogen fraction and is suppressed with the increase of CO 2 fraction due to the combination effect of hydrodynamic and thermal-diffusive instability. ► A novel type of combustion mixtures was studied. ► Laminar burning velocities of CO–H 2 –CO 2 –O 2 were measured and calculated. ► Radiation characteristics of various mixtures were compared. ► Effect of large fraction of H 2 and CO 2 on the mixtures was discussed. ► Flame intrinsic instability of the novel mixtures was studied.

Effect of Hydrogen Addition on Combustion Characteristics of a Spark Ignition Engine Fueled With Low Heat Value Gas

An experimental investigation on the effect of hydrogen fraction on the combustion characteristics of a spark ignition engine fueled with low heat value gas-hydrogen blends was studied. The results show that engine indicated thermal efficiency, indicated mean effective pressure and maximum combustion pressure are increased with the increase of hydrogen fraction in the blends. And hydrogen addition shows remarkable influence on engine power and emissions. At the same excess air ratio, HC emissions decrease, CO and NOx emissions increase with the increase of hydrogen fraction in the blends. And engine power is influenced by both hydrogen fraction and heat value in low heat value gas-hydrogen blends combining. Hydrogen significant extends the lean burn limit of combustion of low heat value gas.

Experimental study on combustion characteristics of a spark-ignition engine fueled with natural gas–hydrogen blends combining with EGR

An experimental study on the effect of hydrogen fraction and EGR rate on the combustion characteristics of a spark-ignition engine fueled with natural gas–hydrogen blends was investigated. The results show that flame development duration, rapid combustion duration and total combustion duration are increased with the increase of EGR rate and decreased with the increase of hydrogen fraction in the blends. Hydrogen addition shows larger influence on flame development duration than that on rapid combustion duration. The coefficient of variation of the indicated mean effective pressure increases with the increase of EGR rate. And hydrogen addition into natural gas decreases the coefficient of variation of the indicated mean effective pressure, and this effectiveness becomes more obviously at high EGR rate. Engine fueled with natural gas–hydrogen blends combining with proper EGR rate can realize the stable low temperature combustion in gas engine.

Hydrogen addition effect on laminar burning velocity, flame temperature and flame stability of a planar and a curved CH 4–H 2–air premixed flame

The effects of hydrogen fraction on laminar burning velocity, flame stability (Markstein number) and flame temperature of methane–hydrogen–air flame at global equivalence ratios of 0.7, 1.0 and 1.2 have been investigated numerically based on the full chemistry and the detailed molecular species transport. The effect of stretch rate on combustion characteristics is examined using an opposed-flow planar flame model, while the effect of flame curvature is identified by comparing a tubular flame to the opposed-flow planar flame. The difference in response on hydrogen fraction between the planar and curved flames has been observed. The results show when hydrogen fraction increases, the flame temperature and laminar burning velocity increases, and this effect is more significant at a large stretch rate; while Markstein length decreases. At a fixed stretch rate of 400 s −1, under which the flame approaches extinction limit, the flame temperature of the tubular flame is considerably higher than that of the planar opposed flow flame, which results most likely from the contribution of the positive flame curvature to the first Damkohler number.