Progress in hydrogen enriched hydrocarbons combustion and engine applications

Abstract

The paper summarized the work on hydrogen enriched hydrocarbons combustion and its application in engines. The progress and understanding on laminar burning velocity, flame instability, flame structure flame and chemical kinetics were presented. Based on fundamental combustion, both homogeneous spark-ignition engine and direct-injection spark-ignition engine fueled with natural gas-hydrogen blends were conducted and the technical route of natural gas-hydrogen combined with exhaust gas recirculation was proposed which experimentally demonstrated benefits on both thermal efficiency improvement and emissions reduction.

Keywords

hydrogen enriched hydrocarbon combustion / fundamental study / engine application

Introduction

Clean and efficient combustion is the pursuit in combustion society. Combustion devices like internal combustion engines, boiler and turbines not only consume large amount of primary energy but also produce harmful pollutions and CO_fo the environment [1]. To meet the ever increasing challenges from both energy saving and emission control, a lot of work has been done to combustion devices. In general, these combustion devices use the fossil fuels which primarily consist of hydrocarbons. For example, gasoline and diesel fuels consist of alkanes, alkenes and aromatics. The combustion of these fuels will produce carbon monoxide, unburned hydrocarbons, oxides of nitrogen and soot emissions, meanwhile, the relatively low burning rate of these fuels hinders the further improvement of thermal efficiency of engines [2].

Hydrogen enriched combustion enhancement

Hydrogen is a clean energy with a high burning rate. It can be assumed that hydrogen enriched hydrocarbon combustion can produce high-efficient low-emission combustion. To validate this assumption, a wide range of study was conducted using different hydrogen enriched fuels. A premixed spherically propagating flame was used to measure the flame propagation speeds and laminar burning velocities of natural gas-hydrogen mixtures at various equivalence ratios. The study found that the flame speed was remarkably increased when hydrogen was added into the natural gas, and hydrogen had an exponential enhancement to flame speed. And flame instability was increased with the increase of hydrogen volume fraction in the binary mixture [3]. Hydrogen enriched natural gas combustion in a constant volume bomb showed that the heat release rate was increased with the increase of hydrogen volume fraction in the binary mixture and combustion duration was shortened with the increase of hydrogen volume fraction, i.e., a fast combustion was realized with hydrogen enrichment [4]. Study on laminar burning velocities at varied initial pressures showed that laminar burning velocity was decreased with the increase of initial pressure [5]. Study on laminar stoichiometrically premixed methane/hydrogen/ oxygen/argon flames with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry techniques obtained the mole fraction profiles of major species and intermediates. The results revealed that the major species mole fraction of CO, CO₂ and CH₄ were decreased with the increase of hydrogen mole fraction. The mole fraction of intermediates measured in this experiment decreased remarkably with the increase of hydrogen mole fraction. The study suggested that the increase of H and OH radicals by hydrogen addition and the high diffusivity and activity of H radical promoted the chemical reaction. In addition, the increase of H/C ratio with the increase of hydrogen mole fraction also led to the decrease of the mole fraction of carbon-related intermediates and contributed to the decrease of unburned and incomplete combustion products [6]. The study on dimethyl ether-hydrogen flame also showed a combustion enhancement with the addition of hydrogen [7]. An experimental and numerical study on hydrogen-air flames at elevated pressures and temperatures showed that laminar burning velocities were increased with the increase of initial temperature and decreased with the increase of initial pressure. With the increase of initial pressure, advancement of the onset of cellular instability appeared and Markstein length was decreased, indicating an increase of flame instability with the increase of initial pressure. The study showed insensitivity of flame instability to initial temperature. Laminar burning velocity was dependent on the competition between the main chain branching reactions and the chain termination reaction. The chain branching reactions were the temperature-sensitive reaction, while the termination reaction was the temperature-insensitive reaction. Numerical study also indicated that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) was closely related to the decrease (or increase) of H, O and OH mole fractions in the flames. The study found a strong correlation between laminar burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames, as shown in Fig. 1[8].

Diluted hydrogen and hydrogen enriched hydrocarbon combustion showed that dilution decreased the laminar burning velocity, especially in the case of diluent with high thermal capacity. Dilution increased the flame instability of hydrogen flames or hydrogen enriched hydrocarbon flames with high volumetric hydrogen fraction but increased the flame stability of hydrocarbon flames or hydrogen enriched hydrocarbon flames with low hydrogen volume fraction. Flame images showed that the diffusional-thermal instability was promoted as the mixture became richer, and the hydrodynamic instability was increased with the increase of the initial pressure but decreased with the increase of dilution ratio. The normalized laminar burning velocities showed a linear correlation with respect to the dilution ratio. The effect of nitrogen dilution was more significant at higher pressures [9-13]. Study on the lean methane-hydrogen-air flames showed that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) was closely related to the decrease (or increase) of H, O and OH mole fractions in the flames [14]. Methane-hydrogen study found three regimes depending on hydrogen volume fraction. The three regimes were the methane-dominated combustion regime where hydrogen volume fraction is less than 60%, the transition regime where hydrogen volume fraction is between 60% and 80%, and the methane-inhibited hydrogen combustion regime where hydrogen volume fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increased linearly with the increase of hydrogen volume fraction. However, in the transition regime, the laminar burning velocity increased exponentially with the increase of hydrogen volume fraction in the fuel blends [15]. An experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane-hydrogen-air flames revealed that early onset of cellular instability appeared and the critical radius and Markstein length were decreased with the increase of initial pressure, indicating the increase of hydrodynamic instability with the increase of initial pressure. Flame instability was insensitive to initial temperature compared to initial pressure. With the increase of hydrogen fraction, significant decrease in critical radius and Markstein length was found, indicating the increase in both diffusional-thermal and hydrodynamic instabilities as hydrogen volume fraction was increased [16]. Numerical study on NO formation of the premixed methane-hydrogen-air flames showed that two peaks in the curve of NO versus the equivalence ratio, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with an equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition had little influence on NO formation, while in rich flames, NO concentration was significantly decreased [17]. Hydrogen enrichment could extend the flammabilities of hydrocarbon fuels. Hydrogen addition into the methane extended both upper and lower flammability limits, which suggested that lean burn capability could be enhanced with hydrogen addition [18]. Explosion characteristics of hydrogen combustion showed that a short combustion duration and higher normalized mass burning rate appeared with the increase of equivalence ratio. With the increase of initial temperature, the explosion pressure, the maximum rate of pressure rise and the deflagration index were decreased. Besides, a shorter combustion duration and higher normalized mass burning rate emerged. With the increase of initial pressure, the explosion pressure, the maximum rate of pressure rise and the deflagration index increase, a shorter combustion duration and higher normalized mass burning rate appeared. Dilution could significantly reduce the normalized mass burning rate and the deflagration index and thus the potential of explosion hazards [19]. For hydrogen enriched propane flames, Schlieren images showed that for lean mixture combustion, hydrogen addition would increase the hydrodynamic instability due to the decreased flame thickness and increase the diffusional thermal instability due to the decreased Lewis number. While for rich mixture combustion, the flame front was initially destabilized and later tended to stabilize with the increase of hydrogen volume fraction. This was caused by the competing effects of the hydrodynamic instability and the diffusional-thermal instability [20]. Hydrogen enriched propane study also showed a combustion enhancement with hydrogen addition. An earlier onset of cellular instability and the decrease in the critical radius and the Markstein length appeared with the increase of initial pressure, indicating that the hydrodynamic instability was enhanced with the increase of initial pressure. At the equivalence ratio of 0.8, where the propane-air mixture was thermal-diffusionally stable and the hydrogen-air mixture was thermal diffusionally unstable, the critical radius and the Markstein length decreased significantly with the increase of hydrogen volume fraction, indicating that hydrogen addition would increase the diffusional-thermal and the hydrodynamic instability. At an equivalence ratio of 1.2, where the propane-air mixture and hydrogen-air mixture were both thermal- diffusionally neutral, a moderate decrease in the critical radius and the Markstein length appeared, which indicated the increase of hydrodynamic instability as hydrogen was added [21]. Simulation study on the stoichiometric methane-hydrogen-air freely propagated laminar premixed flames showed that mole fractions of major species CH₄, CO and CO₂ were decreased while their normalized values were increased as hydrogen was added. The rate of production of the dominant reactions contributing to CH₄, CO and CO₂ showed a remarkable increase as hydrogen was added, as illustrated in Fig. 2. The role of H₂ in the flame would change from an intermediate species to a reactant when hydrogen mole fraction in the blends exceeds 20%. Enhancement of combustion with hydrogen addition was ascribed to the significant increase of H, O and OH in the flame as hydrogen was observed. The decrease of the mole fractions of CH₂O and CH₃CHO with hydrogen addition suggested a potential in the reduction of aldehydes emissions of methane combustion as hydrogen was added. The methane oxidation reaction pathways would move toward the lower carbon reaction pathways when hydrogen was available, which had the potential in reducing the soot formation. Chemical kinetics effect of hydrogen addition had a little influence on NO formation for methane combustion with hydrogen addition [22].

The stretch-affected propagation speeds of expanding spherical flames of n-butane-air mixtures with hydrogen addition were measured at atmospheric pressure and subsequently processed by a nonlinear regression analysis to yield the stretch-free laminar flame speeds. Laminar flame speeds were found to increase almost linearly with hydrogen addition parameter and between effective fuel equivalence ratios of 0.6 and 1.4, with the slope of the variation assuming a minimum around stoichiometry. The experimental results also agreed well with computed values using a detailed reaction mechanism. A mechanistic investigation aided by sensitivity analysis identified that kinetic effects caused by the global activation energy, followed by thermal effects resulted from the adiabatic flame temperature, had the most influence on the increase in the flame speeds and the associated linear variation with hydrogen addition parameter due to hydrogen addition, while nonequidiffusion effects due to the high mobility of hydrogen, caused by the global Lewis number, had the least influence. Calculations of methane, ethane, and propane as the fuel showed similar behavior, leading to possible generalization of the phenomena and correlation [23]. Effects of hydrogen addition and turbulence intensity on the natural gas-air turbulent combustion showed that the turbulent combustion rate increased remarkably with the increase of hydrogen volume fraction in fuel blends when hydrogen volume fraction was over 11%. Combustion rate increased remarkably with the introduction of turbulence and decreased with the decrease of turbulence intensity. Lean flammability limit of natural gas-air turbulent combustion was extended with increasing hydrogen volume fraction addition. The sensitivity of natural gas/hydrogen hybrid fuel to the variation of turbulence intensity was decreased with increasing hydrogen addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased with the increase of turbulent intensity at stoichiometric and lean-burn conditions. However, slight influence on combustion characteristics was found with variation of hydrogen volume fraction at the stoichiometric equivalence ratio with and without the turbulence [24]. Study on the effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion showed that the flame kernel was concentrated to the spark position with the increase of premixed ratio and/or hydrogen volume fraction. Flame propagating speed was decreased with the increase of premixed ratio but increased as hydrogen was added to natural gas. Hydrogen addition had little effect on the partially direct-injection natural gas combustion at the stoichiometric fuel-air mixture condition and all premixed ratios. However, hydrogen addition significantly enhanced the combustion rate of natural gas direct-injection combustion at lean mixture condition. Both the initial and main combustion durations were increased with the increase of premixed ratio but decreased as hydrogen was added to natural gas at the lean mixture condition. Partially premixed direct-injection combustion combining with hydrogen addition could achieve the stable spark ignition and fast combustion at the lean mixture condition [25]. Cyclic variations of direct-injection combustion fueled with natural gas-hydrogen fuel blends using a constant volume vessel showed that the cyclic variations were initiated at the early stage of flame development. The flame kernel was closely concentrated to the spark electrode and flame pattern was less irregular with hydrogen addition. Direct-injection natural gas combustion could achieve the stable lean combustion along with low cyclic variations due to mixture stratification. Cyclic variations decreased with the increase of hydrogen addition and this trend was more obvious at ultra-lean-burn condition. Hydrogen addition weakened the effect from turbulent flow on flame propagating process, thus reducing the cyclic variations related to the gas flow. There existed interdependency between the early combustion stage and the subsequent combustion process for direct-injection combustion [26]. Study on ignition delay times of methane/hydrogen/ oxygen/nitrogen mixtures using a shock tube facility showed that ignition was promoted with hydrogen addition. The enhancement of ignition by hydrogen addition became small when ambient temperature was over 1750 K, and the enhancement effect was strong when the temperature was below 1725 K. The ignition delay time of 20%H₂/80%CH₄ was only one-third to that of 100%CH₄ at a temperature of 1500K. Analysis on normalized sensitivity indicated that OH+H₂=H+H₂O was the main reaction for the formation of H radical at a temperature of 1400 K. With the increase of hydrogen volume fraction, chain branching was promoted by H+O₂=O+OH, and the ignition delay times were decreased. At a temperature of 1800 K, CH₃ radical became the key species that influenced the ignition of CH₄/H₂/O₂/N₂ mixtures, and sensitivity coefficients of chain termination reaction 2CH₃(+M)=C₂H₆(+M), and HO₂+CH₃=OH+CH₃O were reduced, lowering the promotion effect from hydrogen addition on ignition of methane/hydrogen mixtures under high temperature [27]. Ignition behavior of the methane-hydrogen mixture depending on pressure resembled that of methane for hydrogen volume fraction less than 40%, with the ignition delays decreasing with increasing pressure. For the hydrogen volume fraction equaling 60%, a negligible promoted effect of pressure on the ignition of the methane-hydrogen mixture was discovered. For hydrogen volume fractions equaling or greater than 80%, however, the ignition response resembled that of hydrogen in that the ignition delay exhibited a complex dependence on pressure and two-step transition in the global activation energy. The rate of production analysis showed that the promoted effect of hydrogen on the oxidation of the methane mainly resulted from the concentrations of the free radicals such as H, O and OH, and this effect was increased with increasing hydrogen volume fraction, and leaded to the total reaction rate is enhanced. Consumption of methane is mainly caused by these reactions in which the active free radicals participate [28]. Hydrogen enriched carbon monoxide combustion showed that a fast burning velocity appeared for the hydrogen-carbon monoxide mixture. Mixture diluents would increase the sensitivity of flame front to flame stretch rate. The normalized laminar burning velocity was only related to dilution ratio and was not influenced by equivalence ratio [29].

Hydrogen enriched natural gas engines

Natural gas engine, a clean engine, has been widely used in buses and taxies in China. Although the natural gas engine does not require fuel evaporation as that in the gasoline engine, it can easily form a homogeneous charge in the cylinder, achieving complete combustion and less emissions. Meanwhile, the natural gas engine also produces less CO₂ emission as it has the smallest C/H ratio. However, natural gas still has its disadvantages of poor lean burn capability and low flame speed, restricting its further improvement on engine thermal efficiency [30]. Hydrogen has the advantages of good lean burn capability and high flame speed. Thus, it is reasonable to think the combination of natural gas and hydrogen can improve engine lean burn capability and realize fast combustion process [31-32]. Table 1 gives the comparison of engine performance and emissions fueled with natural gas and natural gas-hydrogen.

Study on a homogeneous charge natural gas-hydrogen engine showed that engine lean-burn limit was extended by the addition of hydrogen into natural gas which decreased the exhaust hydrocarbon (HC) concentrations. However, addition of hydrogen into natural gas would increase NO_x concentration. Thus, an engine operating on lean-burn natural gas-hydrogen combustion is favorable for getting higher thermal efficiency and lower emissions. In the case of the stoichiometric mixture combustion and the lean mixture combustion, natural gas-hydrogen blended with 10% hydrogen volume fraction gave the highest value of the peak cylinder pressure and the peak heat release rate. The initial combustion duration and the total combustion duration decreased with the increase of hydrogen volume fraction in the natural gas-hydrogen blend. Addition of hydrogen into natural gas decreased the ignition delay. Although the optimum ignition timing was retarded with the increase of the hydrogen volume fraction in the natural gas-hydrogen blends, the heat release process was not postponed [33,34]. HC emissions decreased with the increase of hydrogen volume fraction. This behavior was more apparent under the lean burn operation. NO_x concentration increased with the increase of hydrogen volume fraction, and NO_x got its peak value at an excess air ratio of 1.1. CO₂ emissions decreased with increasing hydrogen volume fraction. Since addition of hydrogen into natural gas could extend the lean burn limit, an engine fueled with natural gas-hydrogen blends operating under lean mixture conditions could get low emissions of HC, CO, CO₂, and NO_x [35]. Using the approach of natural gas-hydrogen blend combined with exhaust gas recirculation (EGR) could further decrease NO_x emission while maintaining high thermal efficiency and low emissions. Because of the presence of hydrogen, the mixture had high tolerance to the exhaust gas recirculation. Study showed that combustion duration increased with the increase of EGR rate and decreased with the increase of hydrogen fraction. Hydrogen addition had a greater influence on flame development duration than that on rapid combustion duration. The engine fueled with natural gas-hydrogen blends combining with proper EGR rate can realize the stable low temperature combustion in gas engine [36]. Large EGR introduction decreased the engine power output. However, hydrogen addition could increase power output at large EGR operation. Effective thermal efficiency increased at small EGR rate and decreased with further increase of EGR rate. NO_x concentration was decreased with the increase of EGR rate and the decrease was more obvious at high hydrogen volume fraction. HC emission was increased with the increase of EGR rate and decreased with the increase of hydrogen volume fraction. CO and CO₂ emissions decreased with the increase of hydrogen volume fraction. The engine fueled with natural gas-hydrogen blend combining with EGR could achieve high-efficiency and low-emission spark-ignition engine [37]. Simulation study on natural gas-hydrogen engine verified the effect of EGR on the reduction of NO_x formation in the cylinder, and quantitative prediction on NO_x could be achieved [38]. Cylinder peak pressure, maximum rate of pressure rise and indicated mean effective pressure decreased and cycle-by-cycle variations increased with the increase of EGR ratio. Combustion stability was promoted and cycle-by-cycle variations were decreased with the increase of hydrogen volume fraction, as depicted in Fig. 3. Slight influence of EGR ratio on indicated mean effective pressure was observed at low EGR ratios while large influence of EGR ratio on indicated mean effective pressure was noticed at high EGR ratios [39].

Study on direct-injection natural gas-hydrogen engine also demonstrated good combustion performance and low emissions. Study showed that heat release rate and combustion duration decreased with the increase of hydrogen volume fraction when hydrogen volume fraction was smaller than a certain volumetric fraction while heat release rate and combustion duration increased with the increase of hydrogen volume fraction when hydrogen volume fraction was greater than a certain value. This phenomenon indicated that only when the hydrogen volume fraction in natural gas reached a certain fraction could a large improvement in combustion be realized. Influence of hydrogen addition on natural gas-hydrogen mixture combustion was larger at low engine speed operation condition than that at high engine speed operation condition [40]. Early injection decreased the excessive-air ratio and made leaner mixtures. Brake mean effective pressure increased with the advancement of fuel-injection timings. An increase in hydrogen volume fraction decreased the brake mean effective pressure when hydrogen volume fraction was smaller than 10%, whereas brake mean effective pressure tended to increase when hydrogen volume fraction was larger than 10%. Combustion durations decreased with the advancement of fuel-injection timing. NO_x and CO₂ increased with advancing fuel-injection timing [41]. Ignition timing had significant influence on engine performance, combustion and emissions. The time intervals between the end of fuel injection and ignition timing were very sensitive to direct-injection natural gas-hydrogen engine combustion. The turbulence in combustion chamber generated by the fuel jet maintained high and relatively strong mixture stratification was discovered when decreasing the time intervals between the end of injection and the ignition timing, giving fast burning rate, high brake mean effective pressure, high thermal efficiency and short combustion durations. Exhaust HC concentration decreased and exhaust NO_x concentration increase with advancing the ignition timing [42]. The phase of the heat release curve advanced with the increase of hydrogen fraction. Rapid combustion duration decreased and heat release rate increased with the increase of hydrogen fraction. This phenomenon was more obvious at low engine speed, suggesting that the effect of hydrogen addition on the enhancement of burning velocity played a more important role at relatively low cylinder air motion. The study suggested that the optimum hydrogen volumetric fraction in natural gas-hydrogen blends was approximately 20% to get the compromise in both engine performance and emissions [43]. Cycle-by-cycle variations decreased with the increase of hydrogen volume fraction at lean mixture operation. The interdependency between combustion parameters and corresponding crank angle tended to be strongly correlated with the increase of hydrogen volume fraction under lean mixture operation. Coefficient of variation of the indicated mean effective pressure was low and was slightly influenced by hydrogen addition under the stoichiometric and relatively rich mixture operation while it decreased remarkably with the increase of hydrogen volume fraction under lean mixture operation. Engine lean operating limit could be extended with hydrogen addition [44]. Natural-gas direct-injection engine at different compression ratios showed that compression ratio had a great influence on engine performance, combustion, and emissions. The penetration distance of the natural-gas jet was decreased and relatively strong mixture stratification was formed as the compression ratio was increased, producing a fast burning rate and a high thermal efficiency, especially at low and medium engine loads. Compression ratio had a significant influence on combustion duration at lean combustion. Experiments showed that a compression ratio of 12 was a reasonable value for a natural gas-hydrogen direct-injection engine to obtain a better thermal efficiency without a large penalty of emissions [45]. Study on a spray guided natural gas-hydrogen direct-injection engine showed that brake thermal efficiency increased with the increase of hydrogen fraction. For later injection timings, the beginning of heat release was advanced with increasing hydrogen fraction, while the beginning of heat release was advanced and then retarded with the increase of hydrogen fraction at earlier injection timings. Brake NO_x emission was increased and then decreased, while brake HC, CO and CO₂ emissions decreased with the increase of hydrogen fraction [46]. Stoichiometric equivalence ratio with exhaust gas recirculation (EGR) in a natural gas-hydrogen direct-injection engine combining with exhaust gas recirculation showed that hydrogen enrichment increased the brake mean effective pressure, which was more obvious at high EGR. NO_x emissions could be decreased greatly with EGR dilution, and hydrogen enrichment could effectively control the combustion variations [47]. Ionic current detection in a natural gas-hydrogen engine suggested that front flame stage and post flame stage in ionization current could be used to analyze the combustion characteristics of natural gas-hydrogen blends. Appearance of front flame stage and post flame stage advanced with the increase of hydrogen fraction. Hydrogen addition could increase the ionization current amplitude. Maximum post flame current showed similar trend to maximum cylinder pressure and correlated between the timing of maximum post flame current and the timing of maximum cylinder pressure. A high correlation coefficient between maximum post flame current and maximum pressure was noticed [48].

Conclusions

(1) Hydrogen enriched combustion can increase the flame speed, shorten the combustion duration and extend the flammability limits.

(2) Hydrogen enriched combustion can decrease carbon-related emissions and is a promising approach to realize clean combustion.

(3) Natural gas-hydrogen engine combined with EGR can realize low-temperature low-emission combustion with low cyclic variations and high thermal efficiency. This technology can meet high stringent emission regulation with fuel modification.

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