Hydrogen Volumetric Fraction Research Articles

Thermodynamic and emission characteristics of a hydrogen-enriched natural gas-fired boiler integrated with external flue gas recirculation and waste heat recovery

Hydrogen-enriched natural gas (HENG) is a low-carbon fuel and its utilization in combustion devices has been under extensive discussion recently as it is a promising way to reduce the CO2 emission during combustion. The applicability of HENG in the existing natural gas-fired combustion equipment in terms of its efficiency and emissions attracts increasing attention, especially for industrial and domestic small-scale boilers. In this study, the thermal efficiency and pollutant emissions of a 4.2 MWth HENG-fired boiler integrated with external flue gas recirculation (FGR) were respectively evaluated based on the thermodynamic and heat transfer models and the Chemical Reaction Network model. Results suggested that NOx emission rose by ∼19% as the hydrogen volumetric fraction in the fuel increased from 0 to 0.4 at a constant excess air ratio and FGR rate. To suppress the NOx rise, the FGR rate was tuned higher while the system thermal efficiency decreased subsequently. To further improve the overall system thermal efficiency, a cascade flue gas waste heat recovery strategy including sensible heat recovery using an external economizer and latent heat recovery using dual-spray heat exchangers was proposed. The thermodynamic analysis demonstrated that the external economizer improved the overall system thermal efficiency by 0.4% - 0.7% and the dual-spray heat exchangers promoted the overall system thermal efficiency by over 5%. Thus, a comprehensive performance optimization strategy was developed for HENG-fired boilers in terms of their overall system thermal efficiency promotion, NOx emission control, and CO2 emission reduction.

Hydrogen volumetric fraction effects on HCNG refuelling station CAPEX

Greening the transport sector is crucial for the achievement of the ambitious decarbonisation goals set by the Conference of the Parties (COP21) Paris agreement to keep global warming “well below 2 degrees Celsius above preindustrial levels, and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius”. Battery electric vehicles and hydrogen fuel cell electric vehicles will play a key role in the reduction of greenhouse gas emissions in the road mobility. Nonetheless, those technologies still need to face some technological challenges. Hence, hydrogen enriched natural gas (HCNG) can play an important role as bridging technology. This study envisages the upgrade of an existing compressed natural gas (CNG) refuelling station (RS) in HCNG-RS. Particularly, an analysis regarding the impact of different hydrogen volumetric fractions in the blend on the total CAPEX of the HCNG refuelling station is carried out, by evaluating the HCNG fill-up demands for a proper refuelling station sizing. The proposed HCNG refuelling station layout foresee the on-site hydrogen production by means of an electrolyser fed by PV energy. Each component of the station has been adequately sized based on the resulting peak HCNG refuelling demand. The highest CAPEX value is recorded for the 30%vol. HCNG, amounting up to 3.52 M€.

Experimental investigation for combustion performance of hydrogen and natural gas fuel blends

The current study aims to investigate the effect of adding hydrogen into natural gas. Some significant parameters, such as natural gas consumption, heating time and heating value, are studied experimentally to observe the effects of hydrogen addition into natural gas on these parameters. Natural gas combustion in a gas stove is compared with the burning of hydrogen and natural gas blend in three different hydrogen volumetric fractions of 10%, 20%, and 30%. In order to minimize the errors in the experiments, the heating period is prolonged by using 5 kg of tap water heated from the ambient temperature to 60 °C. Two gas stoves are used for natural gas and hydrogen-natural gas blend simultaneously and under the same operating conditions. Tests are repeated and values are checked through an error analysis process. 0.045 m3 of natural gas and 0.011 m3 of hydrogen in the blend (with a hydrogen ratio of 20%) are consumed for the same heating load requirement. The results of this study show that 8% of the natural gas can be saved once hydrogen is added to natural gas in a 20% blend. Meanwhile, there is a 15.9% increase in heating time for the 20% volumetric hydrogen in the blend.

Efficiency and emissions of gas-fired industrial boiler fueled with hydrogen-enriched nature gas: A case study of 108 t/h steam boiler

Hydrogen-enriched natural gas (HENG) is a promising low-carbon fuel in the industry. The fuel flexibility of the existing natural gas-fueled industrial combustion equipment is a crucial issue for the HENG application. This study evaluates the thermal and emission performances of a large-scale industrial steam boiler fueled with HENG. Considering different operation strategies, three boiler operation scenarios were proposed, namely, the unchanged fuel/air valve opening (Scenario 1), the maintained boiler capacity with unchanged valve control logic (Scenario 2), and the maintained boiler capacity with unchanged excess air ratio (Scenario 3). A model considering the thermal equilibrium and heat transfer inside the boiler was built to study the thermal efficiency under the three scenarios, and a chemical reaction network model was established to investigate the NOx emission characteristics. The results showed that thermal efficiency and NOx emission presented a similar variation trend. Both of them decreased in Scenarios 1 and 2 while increased in Scenario 3 as hydrogen volumetric fraction in HENG increased from 0 to 0.9. The differences in decarbonization potentials among the three scenarios were limited. This study implies that the trade-off between thermal efficiency and NOx emission should be carefully concerned when applying HENG to industrial steam boilers.

Effects of hydrogen-enriched methane combustion on latent heat recovery potential and environmental impact of condensing boilers

Decarbonization has received significant interest from the energy sector in recent years. In particular, hydrogen enrichment of fuels has been analyzed by several researchers. A side effect of hydrogen enrichment is that it increases the water vapor content in the resulting flue gas. Although this is an undesirable phenomenon in non-condensing energy systems, the higher water vapor content is an advantage in condensing boilers as more energy can be harnessed through condensation. This study analyzes the carbon emission mitigation and latent heat recovery potential of condensing boilers using a mixture of methane and hydrogen as fuel. Accordingly, analyses were performed considering hydrogen volumetric fractions ranging from 0% (pure methane) to 100% (pure hydrogen). The maximum concentrations of CO2 and CO in the flue gas were calculated using the Ostwald diagram. The characteristics of the hydrogen–methane fuel mixture, namely the partial pressure of the water vapor inside the flue gas and the dew point temperature, were investigated as well. The fuel consumption and overall energy saving potential due to the water vapor inside the flue gas were calculated considering a condensing boiler with a gross heat input of 25 kW. As NOx emissions are correlated to the adiabatic flame temperature, the latter was investigated as well. In addition to carbon emission mitigation, the results demonstrated that the condensing boiler efficiency in terms of the higher heating value increased by 1.3%, owing to the additional latent energy saved as the hydrogen volumetric fraction increased from 0% to 80%.

Effects of hydrogen enhancement on mesoscale burner array flame stability under acoustic perturbations

Partial substitution of hydrocarbon fuel with hydrogen can effectively improve small-scale combustion system stability and performance, potentially opening the way for novel compact power generation and/or propulsion systems in the future. In this study, the effects of hydrogen enhancement between 0% and 40% hydrogen volumetric fractions in methane fuel were experimentally observed in a mesoscale burner array subjected to external acoustic perturbations. The mesoscale burner array utilizes an array of swirl-stabilized burner elements and their interactions with neighboring elements to improve the overall flame stability and simultaneously reduces the combustor length scale. OH∗ chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) were used to image various hydrogen-enriched flames at an equivalence ratio of 0.7, subjected to transverse acoustic perturbations at 320 Hz. Two acoustic modes were imposed by controlling the phase difference between two speakers perturbing the flow. OH∗ chemiluminescence images exhibited flame length scale reduction, leading to a denser flame array. Also, flame arrays with higher hydrogen enrichment were found to be more robust against transverse acoustic perturbations, demonstrated by reduced fluctuations in the global heat release rate. OH-PLIF images showed that flames with higher hydrogen enrichment initiated V- to M-shaped flame shape transition even under fuel lean conditions, thereby improving the combustion stability. OH-PLIF images were also used for flame stability analysis through spectral proper orthogonal decomposition (SPOD). The SPOD analysis showed hydrogen enrichment diminished flame fluctuation structures under fuel lean operation.

Flame structure, turbulent burning velocity and its unified scaling for lean syngas/air turbulent expanding flames

A systematic experimental study of lean premixed syngas/air turbulent expanding flames has been conducted under a wide range of turbulence intensities (0–3.54 m/s), initial pressures (0.5–5 bar), and hydrogen volumetric fractions up to 80% (20%, 50% and 80%). Flame structure and turbulent flame propagation dynamics were investigated. Results show that the flame becomes more refined and wrinkled with the increasing of both turbulence intensity and initial pressure, leading to a larger flame area and the associated turbulent burning velocity (ST). With hydrogen fraction increased, ST is also enhanced significantly, which is mainly due to the promotion of laminar burning velocity (SL) and diffusional-thermal instability. ST/SL is nearly kept constant with hydrogen fraction, which is a trade-off between strengthened diffusional-thermal instability and weakened turbulence stretch. A unified scaling of ST is obtained, indicating that turbulent Reynolds number (ReT) is a practical method to correlate ST when Lewis number is close to unity. Furthermore, at least in the interpretation domain, ST of spherical flames continually increases as the flame expands, which has been referred as flame acceleration phenomenon. It appears that only effective turbulence intensity itself is not able to reflect acceleration phenomenon completely. Turbulent expanding flames follow a self-similar propagation law and the quantitative ST dependence with flame expanding is ST∼R0.5 approximately.

Effect of engine compression ratio on thermodynamic behavior using alternative hydrogen-LPG fuel

ABSTRACT Combustion modeling plays an important role in an internal combustion engine (IC) simulation to predict in-cylinder flow behavior development. Therefore, this work presents a predictive thermodynamic model of an IC engine fueled with gasoline, LPG, and LPG-hydrogen blend, using zero-dimensional modeling. The effects of the compression ratio combining hydrogen enrichment on thermodynamic process are numerically evaluated on gasoline engine fueled alternative hydrogen-LPG fuel. Moreover, hydrogen volumetric fractions of 0%, 5%, 10%, 15%, and 20% were used in the simulation prediction. The main finding in this study was that the compression end temperature in-cylinder pressure, the thermal efficiency, and the supplied power prove that a compression ratio between 9 and 10 is the most reliable. Hence, comparisons with experimental results justify the choice of the compression ratio.

Turbulent burning velocity of stoichiometric syngas flames with different hydrogen volumetric fractions upon constant-volume method with multi-zone model

Syngas has been widely concerned and tested in various thermo-power devices as one promising alternative fuel. However, little is known about the turbulent combustion characteristics, especially on outwardly propagating turbulent syngas/air premixed flames. In this paper, the outwardly propagating turbulent syngas/air premixed flames were experimentally investigated in a constant-volume fan-stirred vessel. Tests were conducted on stoichiometric syngas with different hydrogen volumetric fractions (XH2, 10%–90%) in the ambience with different initial turbulence intensity (u'rms, 0.100 m/s~1.309 m/s). Turbulent burning velocity was taken as the major topic to be studied upon the multi-zone model in constant-volume propagating flame method. The influences of initial turbulent intensity and hydrogen volumetric fraction on the turbulent flame speed were analysed and discussed. An explicit correlation of turbulent flame speed was obtained from the experimental results.

The behavior of methane/hydrogen/air premixed flame in a closed channel with inhibition

Hydrogen enriched natural gas (HNG) is a promising alternative fuel. But the blended fuel will inevitably have different ignition and combustion characteristics as compared to natural gas. The extent of the resulting difference depends on the percentage of hydrogen addition. It may affect the compatibility of combustion systems and have safety implications. The present study was aimed at enhancing the safety of HNG through inhibition by inert gases. Laboratory tests were conducted for methane/hydrogen/air premixed flame propagating in a closed channel with either nitrogen (N2) or carbon dioxide (CO2) as the inhibitor. Mixtures with different hydrogen volumetric fractions in fuel, including 0%, 20%, 50% or 80% were investigated. The flame shape evolution and the overpressure in the channel were recorded by high-speed Schlieren photography and pressure sensor, respectively. The flame shape was found to change in various ways depending on the inhibitor and hydrogen content. The pressure wave had remarkable impacts on flame and pressure dynamics. The effect of buoyancy on the flame deformation was observed and discussed. Both N2 and CO2 were found to have considerable inhibiting effect on the flame speed and overpressure build-up in the channel while the inhibiting effect of CO2 was stronger. The inhibition mechanisms of either N2 or CO2 were revealed from thermal and kinetic aspects.

Onset of cellular instability and self-acceleration propagation of syngas spherically expanding flames at elevated pressures

Systematic study on the onset of cellular instability and self-acceleration propagation for syngas spherically expanding flames is investigated. Three onsets of crack branching, uniform cellularity and transition acceleration are obtained from Schlieren images and Sb-κcurves. Effective Lewis number Leeff and thermal expansion ratio σ are controlled independently by adjusting equivalence ratio and oxygen-nitrogen ratio. Results show that the order of three onsets is crack branching < transition acceleration < uniform cellularity. The critical flame radius is increased with Leeff but decreased with σ. Also, it is decreased with hydrogen volumetric fraction, leading to a much earlier onset at 85% hydrogen fraction. The theoretical result can give a qualitative prediction and closer to uniform cellularity onset quantitatively, although it overestimates Leeff effects a little. The critical Peclet number of uniform cellularity onset is not only increased with Leeff but also increased with σ, mainly due to the variation of flame thickness. Besides, there is a positive correlation between critical Peclet number and Markstein number, indicating a strengthened inhibition of stretch. The onset of similarity acceleration is also obtained through acceleration exponent and it performs a similar variation trend as that of cellular instability. All of these demonstrate that both cellular instability and similarity acceleration are the coupling results of diffusional-thermal instability, hydrodynamic instability and stretch.

Turbulent combustion evolution of stoichiometric H2/CH4/air mixtures within a spherical space

Turbulent combustion evolutions of stoichiometric H2/CH4/air mixtures were experimentally studied within a spherical constant-volume combustion vessel. A series of initial turbulent ambience (with the range of turbulence intensity from 0 to 1.309 m/s) and a series of hydrogen volumetric fraction (with the range from 0.3 to 0.9) were taken as the variables to studied the influences of turbulence intensity and the fuel composition on the turbulent combustion evolutions. The evolutions of explosion overpressure were studied upon the variations of maximal pressure, the influences of turbulence intensity mainly located at heat loss while the influences of fuel composition mainly located at adiabatic explosion. Subsequently, the evolutions of burnt mass were discussed, the competition between pressure rising and temperature rising induced by the heat release during combustion was considered as major influence mechanism. Then, the nexus between burning velocity and the related burnt mass rate were discussed, the variations regulations of maximal burning velocity brought by turbulence intensity and hydrogen volumetric fraction were analysed. Finally, the nexus between maximum burning velocity and heat loss was discussed.

Improving the combustion performance of a gasoline rotary engine by hydrogen enrichment at various conditions

The combustion process within the cylinder directly influences the thermal efficiency and performance of the engines. As for the rotary engine, the long-narrow combustion chamber prevents the mixture from fully burning, which worsens the performance of the rotary engine. As a fuel with excellent properties, hydrogen can improve the combustion of the original engine. In this paper, improvements in combustion of a gasoline rotary engine by hydrogen supplement under different operating conditions were experimentally investigated. The experiment was conducted on a modified hydrogen-gasoline dual-fuel rotary engine equipped with an electronically-controlled fuel injection system. An electronic control module was specially made to command the fuel injection, excess air ratio and hydrogen volumetric fraction. Integral heat release fraction (IHRF) was employed to evaluate the combustion of the tested engine. The tested engine was first run at the idle speed of 2400 rpm and then operated at 4500 rpm to investigate the combustion of the hydrogen-blended gasoline rotary engine under different hydrogen volume fractions, excess air ratios and spark timings. The testing results demonstrated that the combustion of the gasoline rotary engine were all improved when the hydrogen was blended into the chamber under all tested conditions.

Improving the lean performance of an n-butanol rotary engine by hydrogen enrichment

The present paper introduced an experiment survey focusing on exploring the impact of hydrogen enrichment on improving the lean-burn performance of an n-butanol rotary engine. During the test, the engine speed and intake pressure were roughly set at 4000 rpm and 35 kPa, respectively. A constant spark advance of 45 °CA was adopted through the test. Hydrogen volumetric fraction of the total intake was severally kept at 0% and 3%. The testing results manifested that the brake thermal efficiency and peak chamber temperature were heightened with hydrogen addition. Besides, the ignition delay and rapid combustion duration were both reduced with the hydrogen additive. Engine running stability gained an improvement by hydrogen supplement. Moreover, HC and CO emissions from the original n-butanol rotary engine were reduced after hydrogen adding. NOx emissions were increased with hydrogen enrichment while reduced with the increase of excess air ratio. This indicated that NOx emissions from both the n-butanol and hydrogen-blended n-butanol rotary engines could be reduced by lean combustion strategies.

Analysis on hydrogen control system in AP1000 NPP

The three-dimensional computational fluid dynamics code GASFLOW is employed to analyze hydrogen behavior and risk at the accident of a small break of cold leg in 1# steam generation compartment with ADS4 unavailable in AP1000 nuclear power plant. The simulation results show that the ignitors have a great influence on the flow field and the hydrogen volumetric fraction in containment. When the ignitors start, the flow field is enhanced, and the concentration gradient of hydrogen is reduced in the upper space of containment. Hydrogen control system can decrease effectively the hydrogen concentration; eliminate hydrogen risk in containment under severe accident. Flame acceleration and hydrogen detonation can’t occur in the whole containment.

Investigation on combustion and emissions characteristics of a hydrogen-blended n-butanol rotary engine

In this paper, a rotary engine equipped with an n-butanol and hydrogen port-injection system was developed to investigate the combustion and emissions characteristics of a hydrogen-blended n-butanol rotary engine at part load and stoichiometric conditions. A self-developed hybrid electronic control unit was adopted to adjust the injection durations of n-butanol and hydrogen. The rotary engine was run under the conditions of 4000 rpm, a manifold absolute pressure of 35 kPa and a fixed spark timing of 45 °CA before the top dead center during the whole testing operation. The hydrogen volumetric fraction in the total intake was varied from 0% to 6.30%. The test results manifested that the brake thermal efficiency and chamber temperature were simultaneously increased with hydrogen addition. The hydrogen supplement obviously shortened flame development and propagation periods. Both chamber pressure integral heat release fraction versus crank angle were increased when the hydrogen fraction was enhanced. HC emissions were reduced by 54.5% when hydrogen volume fraction was raised from 0% to 6.30%, CO and CO2 emissions were also reduced after increasing hydrogen blending fraction. NOx emissions were mildly elevated due to the improved chamber temperature.

Reducing cyclic variation of a gasoline rotary engine by hydrogen addition under various operating conditions

In the present paper, the cyclic variations of a hydrogen-blended gasoline rotary engine operated under various conditions were experimentally investigated. The experiments were carried out on a modified hydrogen-gasoline dual-fuel rotary engine equipped with an electronically-controlled fuel injection system. An electronic control module was specially made to command the fuel injection, excess air ratio and hydrogen volumetric fraction. The tested engine was first run at idle condition with a speed of 2400 rpm and then operated at 4500 rpm to investigate the cyclic variations of a hydrogen-enriched gasoline rotary engine under different hydrogen volumetric percentages in the total intake, excess air ratios and spark timings. The experimental results demonstrated that the coefficient of variations (in peak pressure, engine speed, flame development period and flame propagation period) of the gasoline rotary engine were distinctly decreased with the increase of hydrogen volume fraction under all the tested conditions. In particular, at idle and stoichiometric conditions, the coefficient of variation in CA0-10 and CA10-90 were reduced from 9.25% to 5.01%, 15.40% to 8.70%, respectively.

Effect of spark timing on performance of a hydrogen-gasoline rotary engine

This paper aimed to study the effect of spark timing on performance of a hydrogen-gasoline dual-fuel rotary engine. For this aim, a modified rotary engine equipped with a dual-fuel port injection system was developed. An electronic management module (ECM) was specially made to command the fuel injection, excess air ratio and hydrogen volumetric fraction. In this study, the engine was operated at 4500rpm with a manifold absolute pressure (MAP) of 35kPa. Hydrogen volumetric percentage of total intake was kept at 0%, 3% and 6%, severally. When the hydrogen volumetric percentage was changed, the gasoline fraction was also adjusted to keep the mixture at the stoichiometric. For a specified hydrogen volumetric fraction, the ignition timing was varied from 24 to 42°CA BTDC (before top dead center) with a fixed interval of 2°CA. Experimental results showed that for a specific hydrogen volumetric percentage, the peak combustion pressure and chamber temperature were increased, brake thermal efficiency was first increased and then decreased with the increase of spark advance. Advancing spark timing caused the increased flame development period, the decreased flame propagation period and exhaust temperature. Cyclic variation was initially weaken and then deteriorated after raising spark advance. HC and NOx emissions were reduced after retarding spark timing. Spark timing had little effect on CO emission.

The Potential of Hydrogen Enriched Natural Gas deriving from Power-to-Gas option in Building Energy Retrofitting

This paper deals with the role of Hydrogen enriched Natural Gas (H2NG) in Hybrid Energy Systems for energy refurbishment purposes. In detail, three different plant layout options were investigated. A photovoltaics (PV) array and two-stage electric heat pump (EHP), a hybrid photovoltaic thermal solar collectors combined to gas heat pump (GHP) fuelled with H2NG, and a CHP fuelled with H2NG connected to a two-stage EHP were compared. The required receiving surfaces of PV array were estimated for three different direct normal irradiation (DNI) values in Italy in order to assess how the available roof surface affects the layout choice. A sensitivity analysis was carried out with varying the building power to heat ratio (PTHR) and the hydrogen volumetric fraction in the H2NG mixtures to assess the primary energy consumption (PEC) and renewable energy fraction. When feasible, the PV+EHP shows the best performance in terms of PEC with a solar energy share equal to 65%. In other cases, the PV/T+GHP fuelled with H2NG @30%vol. can be suitable for PTHR higher than 0.2 approximately. Furthermore, the third layout CHP+EHP is not competitive with the other solutions but, it is the best option where no roof surfaces are available for PV or PV/T installation.

Idle performance of a hydrogen/gasoline rotary engine at lean condition

Because of the limit of properties of gasoline and irregular design of chamber, the pure gasoline rotary engine generally encounters partial burning, increased noxious emissions or even misfire at lean conditions. This situation could be deteriorated at idle because of the high variation in the intake charge and low combustion temperature. Hydrogen addition is proved to remit the deterioration of performance of sparked-ignited (SI) engines at idle and lean conditions. This paper conducted an experiment on a modified rotary engine equipped with gasoline and hydrogen port-injection systems to explore the performance of a hydrogen–gasoline rotary engine (HGRE) at idle and lean conditions. An electronic management unit (EMU) was invented to manage spark and fuel injection. Excess air ratio (λ) and hydrogen volumetric fraction in the total intake (αH2) were also governed through the EMU. For this study, the HGRE was operating at idle and αH2 was kept at 0% and 3%, respectively. For a specific αH2, gasoline flow rate was reduced to make the HGRE run at desired λ. Results indicated that engine fluctuation and fuel energy flow rate were both decreased after hydrogen addition. Combustion duration was cut down and central heat release point was advanced after hydrogen addition. Peak chamber temperature (Tmax), pressure and heat release were enhanced after hydrogen blending. HC, CO and CO2 emissions were simultaneously reduced because of hydrogen enrichment. Specifically, at λ = 1.00, HC, CO and CO2 emissions were respectively reduced from 42,411 to 26,316 ppm, 1.86 to 0.78% and 9.96 to 8.58% when 3% hydrogen was added.