Effect Of H2 Addition Research Articles

Numerical investigation of soot formation in methane/n-heptane laminar diffusion flame doped with hydrogen at elevated pressure

The addition of hydrogen in a dual-fuel mode based on natural gas and diesel offers great potential for improving engine efficiency and reducing emissions. Therefore, detailed numerical simulations have been used to explore the effects of soot formation characteristics of natural gas-diesel dual-fuel engines doped by hydrogen at elevated pressure. Numerical simulations are compared with available experimental data, the effects of H2 addition on soot formation in laminar CH4 + n-heptane, H2 + CH4 + n-heptane, FH2 + CH4 + n-heptane diffusion flames at 2, 4, 6 and 8 atm are simulated, FH2 is added to separate the H2 chemical effects. The results show that the dilution and thermal effects of H2 inhibit soot formation, while the chemical effects of H2 promote soot formation under different pressures. The addition of H2 increases the concentration of H and OH radicals, which promotes the formation of C2H2 and benzene (A1). By simplifying the reaction path and analyzing the production rate of A1, the mechanism of H2 addition to promote A1 formation is illustrated. The concentrations of polycyclic aromatic hydrocarbon (PAH) pyrene (A4), benzo(ghi)fluoranthene (BGHIF) and benzo(a)pyrene (BAPYR) are inhibited at higher pressures that result in lower inception and condensation rates. Therefore, the increase of hydrogen abstraction acetylene addition (HACA) rate is the main reason for the increase of soot formation under chemical effects. While the O2 oxidation rate decreases and the OH oxidation rate increases, that is due to the decrease of O2 concentration and the increase of OH concentration in the soot formation area under chemical effects.

Effect of H2 addition on the preparation of ZrO2 powder from zircon (ZrSiO4) using a plasma torch

In this study, the plasma torch is utilized for the preparation of highly pure ZrO2 from zircon (ZrSiO4). The incorporation of H2 into the discharge gas Ar improves the plasma physical parameters and serves as a reducing agent to promote the irreversibility of the pyrolysis reaction of ZrSiO4. ZrSiO4 powder is fed internally into a DC arc plasma torch through a powder feeder device. The DC torch generates thermal plasma that can fully heat all the ZrSiO4 within a short period for pyrolysis, extraction, and separation, eventually producing high-purity ZrO2. According to the results, the addition of H2 to the working gas increases the material temperature in the flame by increasing the plasma temperature, thereby promoting the pyrolysis of ZrSiO4. The temperature of the material is 2500–2600 °C, which is higher than the boiling point of the pyrolysis product SiO2 while maintaining the high melting points of ZrSiO4 and ZrO2. Furthermore, H2 plasma acts as a reducing agent in the pyrolysis reaction, where it combines with the O of the product to prevent the reversibility of the pyrolysis reaction of ZrSiO4 and retains only ZrO2.

Effect of H2 addition on laminar burning velocity of NH3/DME blends by experimental and numerical method using a reduced mechanism

Blending dimethyl ether (DME) into ammonia (NH3) can efficiently enhance the combustion of pure NH3, and has attracted increasing attention. Partial dissociation of NH3 can convert NH3/DME to NH3/DME/H2 mixtures. This work measured the Markstein length and laminar burning velocity of NH3/DME/air mixtures for different blend ratios (100/0, 80/20, 40/60, and 0/100) with various H2 additions (0%, 20%, and 40%) at ϕ = 0.7–1.7, 0.1 MPa, and 298 K using a spherical constant-volume combustion method. A reduced kinetics mechanism was developed and optimized based on own prior detailed mechanism. This model gives more accurate predictions for NH3, DME, NH3/DME, and NH3/DME/H2 compared to previous models in terms of laminar burning velocity, ignition delay time, and species concentration. The experimental and modeling results show that the measured Markstein length of NH3/DME/H2 blends as a function of equivalence ratio presents different states for different NH3/DME blend ratios. The existence of H2 can effectively increase the laminar burning velocity of NH3/DME/air mixtures. Relative increase in laminar burning velocity (ESL) shows a non-monotonic tendency with increasing equivalence ratio for different H2 additions. The peak value of ESL for NH3/DME/air mixtures with various H2 additions appears at around ϕ = 1.5. This can be attributed to the dominant effect of C-contain and N-contain reactions rather than HO reactions at the rich burn side. Similar to ESL, the free radicals that dominate laminar burning velocity may transit from H, O, and OH to CH and NH radicals at around ϕ = 1.3 ∼ 1.6. The thermal effect also contributes to the ESL and laminar burning velocity for rich fuel.

Effect of Hydrogen Addition on Soot Formation and Emission in Acetylene Laminar Diffusion Flame.

Hydrogen (H2) has been regarded as a highly competitive alternative fuel that does not produce CO2 and soot during combustion. There are few studies of cofiring H2 with hydrocarbons. Meanwhile, the effect of hydrogen addition on soot formation and emission is questionable. In this study, the effect of H2 addition with varying ratios (between 0 and 50% by molar fraction while the remainder is nitrogen) on soot formation in acetylene (50% by molar fraction) laminar diffusion flames was experimentally and numerically investigated. Results show that with H2 addition, the flame height increases and the temperature decreases. The total soot emission and the primary particle size both increase with H2 addition. The addition of H2 promotes soot formation. In addition, the soot oxidation is weakened due to the lower flame temperature. Chemical kinetic analysis shows that the concentrations of A1, H, and H2O increase with H2 addition, which is consistent with the experimental results. According to the HACA reaction, the increase of the molar fraction of A1 and H radicals could promote PAH growth and soot formation.

Effects of H2 addition on the instability characteristics of CO/air and CH4/air in a horizontal narrow channel

ABSTRACT This paper investigates the instability characteristics of CO/H2/air and CH4/H2/air mixtures in a narrow channel under standard temperature and pressure conditions. The CO/H2 mixture exhibits a greater overpressure growth rate and maximum explosion overpressure as compared to the CH4/H2 mixture at the same hydrogen content. As the hydrogen content increases, the maximum explosion overpressure (P max) value increases, while the corresponding time (θ) value gradually decreases. The maximum explosion overpressure of CH4/H2 and CO/H2 mixtures is 28.27 kPa and 25.09 kPa, respectively. The flame wrinkles to form cells due to the Darius-Landau instability, and the interaction between the vortex tube and premixed flame affects the structure of turbulent premixed combustion. The oscillation frequency of the overpressure increases with increasing hydrogen content, and the oscillation belongs to the Helmholtz oscillation. The CO/H2 mixtures exhibit greater flame thickness than CH4/H2 mixtures, resulting in lower Peclet numbers and therefore greater relative importance of heat losses. As the hydrogen content increases, the sensitivity coefficient of R99 (OH + CO <=> H + CO2) in the CO/H2 mixture gradually decreases, while the sensitivity coefficients of R38 (H + O2 <=> O + OH) and R84 (OH + H2 <=> H + H2O) increase. The elementary reaction R38 plays a leading role in increasing the laminar burning velocity of the CH4/H2 mixture. The trend of R38 variation in the two mixtures is opposite with increasing hydrogen content.

The effects of hydrogen addition on soot formation in counterflow diffusion n-heptane flames

The effects of H2 addition on soot formation are investigated in counterflow diffusion n-heptane flames. Three effects including chemical, thermal, and dilution are fully isolated and characterized by additions of H2, He, and Ar. Soot volume fractions are measured using LE-calibrated LII technique, and flame temperatures are measured using OH-TLAF method along with a thermocouple. Numerical simulations are conducted with a detailed mechanism with soot model. The simulated soot volume fractions and flame temperatures are in good agreement with experimental data. The experimental results show that H2 addition can greatly reduce the soot formation. It is also found that the chemical and dilution effects suppress soot formation, while the thermal effect with increasing flame temperature promotes soot formation. Kinetic analysis suggests that HACA growth rate could be the dominant factor that controls the final soot formation through the three effects due to H2 addition.

Effects of H2 Addition and CO2 Dilution on the Methane–Air Diffusion Flame in a Coflow Burner

Effects of H2 Addition and CO2 Dilution on the Methane–Air Diffusion Flame in a Coflow Burner

Syngas biomethanation: effect of biomass-gas ratio, syngas composition and pH buffer

The concept of syngas biomethanation is attractive, however, it still needs improvement in optimizing the operational conditions. In the present study, syngas fermentations under different carbon monoxide (CO), carbon dioxide (CO2) and hydrogen (H2) compositions were conducted under two different biomass-gas ratio (BGR) systems. The results showed that high BGR enhanced the CO consumption rate, achieving a 60% enhancement with CO as the sole substrate. Stoichiometric H2 addition could successfully convert all the CO and CO2 to pure methane, however, higher H2 partial pressure might decline the CO consumption due to pH inhibition from consumption of bicarbonate. Microbial analysis showed different syngas composition could affect the bacteria community, while, archaea community was only slightly affected with Methanothermobacter as the dominant methanogen. This study provided strategy for efficient syngas biomethanation and deeper insight into effect of H2 addition on CO conversion under different BGR systems.

Impact of hydrogen admixture on interacting premixed flames in domestic boilers

The injection of hydrogen into the natural gas network can contribute to the large-scale integration of renewables, as hydrogen can be easily produced through electrolysis from wind or solar energy. However, the addition of hydrogen to natural gas influences fuel properties, asking for the assessment of the safe and efficient operation of existing end-user equipment, such as domestic burners and boilers. In this work, 3-dimensional resolved numerical simulations based on Computational Fluid Dynamics are carried out to shed light on the effect of H2 addition on the combustion process occurring in condensing boilers equipped with perforated cylindrical burners. To this purpose, multi-hole geometries emulating a portion of a perforated burner are analyzed. Since the burner holes are positioned very close to each other, the interaction of the adjacent laminar premixed flames is observed to occur with influence on the flow and thermo-chemical fields which differ from those of a single premixed flame. The addition of hydrogen was found to lead to an anticipation of the reaction zone, although the general features observed with the G222 gas (23% H2, 77% CH4 ) were alike those of the G20 gas (100% CH4).

Effect of H2 addition on the microbial community structure of a mesophilic anaerobic digestion system

In situ biogas upgrading by H2 addition is a promising technology to increase the calorific value of biogas；however, its effects on microbial community structure remain unclear. Therefore, mesophilic digestion of swine manure was carried out in a reactor with an H2 addition rate of 7.2 mL/min (at a 4:1H2: CO2 ratio) and an organic loading rate of 2.0 g of volatile solids (VS)/(L∙d). After H2 was injected into the reactor, the maximum average methane yield increased from 189 to 245 L/kg VS under intermittent mixing conditions. The acetate concentration increased to a maximum of 1600 mg/L but the methane yield was reduced to 210 L/kg VS, which was attributed to a reduction in acetoclastic methanogen abundance in the system due to continuous mixing. The combination of homoacetogenic Clostridium and the Methanosaeta acetoclastic methanogens instead of hydrogenotrophic methanogens appears to have played a vital role in converting the added H2 into CH4 in the mesophilic reactor. However, hydrogenotrophic methanogens accounted for approximately 10% of the total metabolic activity while having a relative abundance of less than 0.5% among the methanogens, after the acetoclastic were inhibited.

Effects of H2 addition on the formation and emissions of CO/NO2/NOx in the laminar premixed biogas-hydrogen flame undergoing the flame-wall interaction

The overall emissions of CO, NOx and NO2/NOx ratio of laminar premixed biogas-hydrogen flames undergoing the flame-wall interaction (FWI) were measured experimentally while the emission indexes (EI) were calculated. The computational model was employed to calculate the combustion process of the biogas-hydrogen flame, and four major routes to produce the NO were isolated and calculated in the simulations. The effects of H2 addition on the formations and emissions of CO/NO2/NOx of BG75-hydrogen impinging flames were analyzed quantitatively. The results show that, with the H2 addition, the EICO of BG75-hydrogen flame is enhanced steadily at either small separated distance (H) or large H thanks to that the high reactivity of H2 suppresses the CO oxidization efficiently by competing for the OH radical. The H2 addition can improve the NO production via four routes and increase the EINOx of the BG75-hydrogen flame. Although the prompt NO is improved by the H2 addition, its rising trend is decreased considerably as hydrogen addition is larger than 20% owing to the reduced methane proportion and improved premixed combustion. The contribution of prompt NO is decreased with the H2 addition, while that of other routes are increased steadily. The NNH route dominates the increased EINOx at small H while the thermal route dominates the rising trend at large H. EINO2 is enhanced steadily with the H2 addition due to the improved HO2 production in the air mixing region, while the NO2 conversion ratio is improved and suppressed with H2 addition at small and large H, respectively.

Effect of hydrogen enrichment on combustion characteristics of methane swirling and non-swirling inverse diffusion flame

Influence of hydrogen addition on appearance of swirling and non-swirling inverse diffusion flame (IDF) along with emissions characteristics are investigated experimentally. The combustion characteristics including flame length, axial and radial temperature variation, and noise level are analysed for hydrogen addition in methane by mass basis for constant energy input and by volume basis for constant volumetric fuel flow rate. Hydrogen addition in methane IDF produces shorter flame by compressing entrainment zone, mixing zone, reaction zone, and post-combustion zone. Hydrogen addition shift these zones towards fuel and air exit from the burner. Enrichment of methane with hydrogen on a mass basis up to 6% reduces CO emission considerably and increases NOx emission moderately. Effect of H2 addition on combustion and emission characteristics is more prominent in non-swirling IDF. Combustion noise is augmented with the hydrogen addition and the magnitude of sound level depends on the hydrogen concentration.

Effect of H2 addition on process of primary slag formation in cohesive zone

Based on the technology of gas-injection blast furnace (BF), the characteristics of primary slag formation with H2 addition were researched. The results indicate that, compared with traditional BF, the primary melt is formed at a lower temperature, which promotes the deformation of the solid burden particles. With the increase in temperature and H2 content, the quantity of formed melt containing FeO decreases sharply, corresponding to the crystallization of solid 2CaO·SiO2 during reduction. A wider softening range and narrower melting zone could be found in the gas-injection BF with a higher reduction potential. The permeability of burden layer is ameliorated as a result of decreased melt quantity. The influence of H2 on the high-temperature properties of burden is not so conspicuous when the H2 addition is from 10 to 15 vol.% against 5 to 10 vol.%. What is more, the slag shows a better liquidity with the decrease in basicity, owing to the transformation of melt composition from a primary phase field with high melting point to that with low melting point. The process of slag forming in gas-injection BF is characterized by earlier melt formation, less primary slag, higher melting temperature, better permeability and better liquidity, and the phase compositions of primary slag are close to those of final slag.

Effect of hydrogen addition on combustion and heat release characteristics of ammonia flame

As a key parameter of fuel, the heat release characteristics of ammonia (NH3) flames under hydrogen (H2)-enriched conditions constitute the emphasis of this research. The effect of H2 addition on heat release characteristics of premixed laminar NH3 flames are numerically investigated by considering the detailed chemical mechanism. The NH3, O2, H2O, NO, and N2O mole fraction distributions based on the Dagaut-Kéromnès mechanism agree well with experimental results, thereby potentially providing repeatable and accurate outcomes with NH3 flames using this mechanism. The OH × N radical positively affects the net heat release rate at various equivalence ratios and H2 addition ratios conditions. The chemical effect is evidently important at low H2 addition ratios, while the transport effect becomes dominant at high H2 addition ratios because of the high mobility of H2. For the three top endothermic elementary reactions of NH3 flames, the contribution of R236: H + O2=OH + O increases, while R171: H + NO(+M) = HNO(+M) and R240: H2O + O=OH + OH decrease with increasing H2 addition ratios. Diverse key exothermic elementary reactions of NH3 flames occur at various equivalence ratios and different H2 addition ratios. The large contribution of R128: N + NO=N2+O at fuel-rich conditions can be applied to reduce NOx emissions of the NH3 flame under pure and H2 addition conditions.

Effect of H2 addition on combustion and exhaust emissions in a heavy-duty diesel engine with EGR

The effect of the addition of hydrogen (H2) on the combustion process and nitric oxide (NO) formation in a H2-diesel dual fuel engine was numerically investigated. The model developed using AVL FIRE as a platform was validated against the cylinder pressure and heat release rate measured with the addition of up to 6% (vol.) H2 into the intake mixture of a heavy-duty diesel engine with exhaust gas recirculation (EGR). The validated model was applied to further explore the effect of the addition of 6%–18% (vol.) H2 on the combustion process and formation of NO in H2-diesel dual fuel engines. When the engine was at N = 1200 rpm and 70% load, the simulation results showed that the addition of H2 prolonged ignition delay, enhanced premixed combustion, and promoted diffusion combustion of the diesel fuel. The maximum peak cylinder pressure was observed with addition of 12% (vol.) H2. In comparison, the maximum peak heat release rate was observed with the addition of 16% (vol.) H2. The addition of H2 was a crucial factor dominating the increased NO emissions. Meanwhile, the addition of H2 reduced soot emissions substantially, which may be due to the reduced diesel fuel burned each cycle. Furthermore, proper combination of adding H2 with EGR can improve combustion performance and reduce NO emissions.

Numerical Simulation of the Effects of Hydrogen Addition to Fuel on the Structure and Soot Formation of a Laminar Axisymmetric Coflow C2H4/(O2-CO2) Diffusion Flame

ABSTRACTHydrogen is the cleanest energy carrier and has been extensively used as an additive to improve the combustion properties and lower pollutant formation of other hydrocarbons. This numerical study investigated the effects of H2 addition to fuel on the structure and soot formation characteristics of laminar coflow C2H4 diffusion flames burning in an oxidizer stream consisting of 30% O2-70% CO2 at atmospheric pressure using the CoFlame code. Numerical calculations were conducted using a detailed gas-phase reaction mechanism and a soot model comprising the collision of two pyrene molecules as the soot nucleation step, the hydrogen abstraction acetylene addition (HACA) and pyrene condensation as the soot surface growth processes, and soot oxidation by oxygen and OH. Four H2 addition levels of 0, 10, 30, and 50% on volumetric basis were considered. To demonstrate the chemical effects of H2, additional calculations were performed with addition of fictitious hydrogen, FH2. The results confirm the literature finding made in C2H4/air diffusion flames that hydrogen addition to ethylene suppresses soot formation significantly through not only the dilution but also the chemical effects. The effectiveness of H2 on soot formation suppression on per unit percentage addition basis deteriorates as more hydrogen is added to ethylene. The fundamental cause for the reduced soot formation by H2 addition is that the added hydrogen increases the mole fraction of H2, resulting in decreased polycyclic aromatic hydrocarbons (PAHS) concentrations, nucleation rates, and surface growth rates in the soot nucleation and early growth regions. The added H2 to the fuel stream has no chemical interactions with CO2 in the oxidizer stream.

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H2 addition on CH4 decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H2 to CH4 changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H2 flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H2, and when the H2 flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H2 to CH4 and the catalyst deactivation is restrained. Filamentous carbon is obtained when H2 is introduced into CH4 reaction gas compared with the agglomerate carbon without H2 addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.

Numerical Study on the Kinetic Effects of Hydrogen Addition on the Thermal Characteristics of Laminar Methane Diffusion Flames

Abstract The kinetic effects of H2 addition on the thermal characteristics of laminar methane diffusion flames were numerically studied using a detailed chemical kinetics consisting of 53 species and 325 reactions. The variations in the heat release properties and relevant key reactions with H2 addition were analyzed. Results show that the reactions of H + O2 + H2O ⇔ HO2 + H2O (R35), H + HO2 ⇔ OH + OH (R46), H + CH3 (+ M) ⇔ CH4 (+ M) (R52) and OH + H2 ⇔ H + H2O (R84) present important roles in the global heat release and the contributions of these reactions significantly increased as H2 is added to CH4 stream. Moreover, the increase rate of contribution of R84 with H2 addition is much larger than those of the reactions of R35, R46 and R52. The H and OH are the two most important radicals for heat release in the combustion process of CH4-H2 diffusion flame. The reaction of R84 is one of the main contributors for production of H radical and the contribution of R84 significantly increased with H2 addition, while the reaction of H + O2 ⇔ O + OH (R38) dominates the contribution of production of OH, which contributes more than 50 %, no matter whether H2 is added to CH4 stream.

Experimental Study of Hydrogen Addition Effects on a Swirl-Stabilized Methane-Air Flame

The effects of H2 addition on a premixed methane-air flame was studied experimentally with a swirl-stabilized gas turbine model combustor. Experiments with 0%, 25%, and 50% H2 molar fraction in the fuel mixture were conducted under atmospheric pressure. The primary objectives are to study the impacts of H2 addition on flame lean blowout (LBO) limits, flame shapes and anchored locations, flow field characteristics, precessing vortex core (PVC) instability, as well as the CO emission performance. The flame LBO limits were identified by gradually reducing the equivalence ratio until the condition where the flame physically disappeared. The time-averaged CH chemiluminescence was used to reveal the characteristics of flame stabilization, e.g., flame structure and stabilized locations. In addition, the inverse Abel transform was applied to the time-averaged CH results so that the distribution of CH signal on the symmetric plane of the flame was obtained. The particle image velocimetry (PIV) was used to detect the characteristics of the flow field with a frequency of 2 kHz. The snapshot method of POD (proper orthogonal decomposition) and fast Fourier transform (FFT) were adopted to capture the most prominent coherent structures in the turbulent flow field. CO emission was monitored with an exhaust probe that was installed close to the combustor exit. The experimental results indicated that the H2 addition extended the flame LBO limits and the operation range of low CO emission. The influence of H2 addition on the flame shape, location, and flow field was observed. With the assistance of POD and FFT, the combustion suppression impacts on PVC was found.

A numerical investigation of lean operation characteristics of spark ignition gas engine fueled with biogas and added hydrogen under various boost pressures

The lean burn capability of spark ignition engines fueled with biogas was investigated using cycle simulation and Latin hypercube sampling. Dominant variables were CO2 contents, boost pressure, spark timing, relative AF ratio, and H2 contents. Optimum boost pressures were optimized for various biogas compositions and relative AF ratios through maximum brake torque timing (MBT) optimization. Optimized boost pressures were relatively lower under a given turbocharger performance, thus causing a decrease in the mass fraction of fuel burnt. To enhance lean burn capability, effects of H2 addition were also studied under low boost pressure. By adding H2, the mass fraction of fuel burned could be enhanced. Furthermore, H2 could increase the range of spark timing. By advancing spark timing by adding H2, the mass fraction of fuel burned could be remarkably increased, leading to enhancement of lean burn capability.