Effect of hydrogen addition on the performance of a biogas fuelled spark ignition engine

Hydrogen was added in small amounts (5%, 10% and 15% on the energy basis) to biogas and tested in a spark ignition engine at constant speed at different equivalence ratios to study the effects on performance, emissions and combustion. Hydrogen significantly enhances the combustion rate and extends the lean limit of combustion of biogas. There is an improvement in brake thermal efficiency and brake power. However, beyond 15% hydrogen the need to retard the ignition timing to control knock does not lead to improvements at high equivalence ratios. Significant reductions in hydrocarbon levels were seen. There was no increase in nitric oxide emissions due to the use of retarded ignition timing and the presence of carbon dioxide. Peak pressures and heat release rates are lower with hydrogen addition as the ignition timing is to be retarded to avoid knock. There is a reduction in cycle-by-cycle variations in combustion with lean mixtures. On the whole 10% hydrogen addition was found to be the most suitable.     

The effect of hydrogen addition on biogas non-premixed jet flame stability in a co-flowing air stream

An investigation of the stability limits of biogas jet non-premixed (diffusion) flames in a co-flowing air stream was conducted. The stability limits were determined experimentally for two different methane–carbon dioxide mixtures that represent the typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel, the stability limits were very low and the flames can only be stabilized over a very small range of co-flowing air velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region for stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen into the fuel. Results obtained in the current experimental setup indicate that an increase in the stability limits by approximately four-fold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of the addition of hydrogen on the enhancement of biogas stability is most significant with a 10% initial addition. The degree of enhancement diminishes with further increases in hydrogen addition from 10% to 30%.     

Effects of hydrogen addition on the characteristics of a biogas diffusion flame

This work describes an experimental study of the effect of hydrogen addition on the stability and impingement heat transfer behaviors of a biogas diffusion flame. The amount of hydrogen added was varied from 5% to 10% of the biogas by volume. The results show that upon hydrogen addition in the biogas flame, there is a corresponding change in the appearance, stability and heat transfer characteristics of the flame.     

Effect of hydrogen addition on the combustion and emission of a diesel free-piston engine

The free-piston engine (FPE) is a new crankless engine, which operates with variable compression ratio, flexible fuel applicability and low pollution potential. A numerical model which couples with dynamic, combustion and gas exchange was established and verified by experiment to simulate the effects of different hydrogen addition on the combustion and emission of a diesel FPE. Results indicate that a small amount of hydrogen addition has a little effect on the combustion process of the FPE. However, when the ratio of hydrogen addition (R_H2) is more than 0.1, the R_H2 gives a positive effect on the peak in-cylinder gas pressure, temperature, and nitric oxide emission of the FPE, while soot emission decreases with the increase of hydrogen addition. Moreover, the larger R_H2 induces a longer ignition delay, shorter rapid combustion period, weaker post-combustion effect, greater heat release rate, and earlier peak heat release rate for the FPE. Nevertheless, the released heat in rapid combustion period is significantly enhanced by the increase of R_H2.     

Effects of hydrogen addition on the structure and pollutant emissions of a turbulent unconfined swirling flame

This article aims at investigating the effect of hydrogen addition on the temperature and pollutant emissions of turbulent unconfined swirling methane/air flame. A computational approach utilizing the steady laminar flamelet and the realizable k–ε combustion and turbulence models, respectively, has been used. The turbulence–combustion interaction has been modeled by a β-shaped presumed probability density function. The percentage of hydrogen in the fuel stream is modeled at a wide range from 0% to 50% of the fuel volume flow rate. Results show that with the increase of volumetric hydrogen percentage in the fuel stream the flame structure changes considerably. The size of maximum temperature region decreases significantly to a small region at flame tip and peak temperature rises which leads to increase in NO emission levels. The flame with 10% hydrogen is observed to be slightly of the general trend. This is deemed to be due to the change in flow field as a result of change in fuel density, while the amount of hydrogen is not effective enough to change the combustion characteristics of the flame.     

Counterflow diffusion flame of hydrogen-enriched biogas under MILD oxy-fuel condition

Biogases are commonly found renewable fuels. Meanwhile they are difficult to be economically utilized because their low calorific values are very small and the induced costs of upgrading are expensive. To overcome the above deficiencies, in this paper we discuss the feasibility to utilize biogases under the MILD oxy-fuel operation recently proposed by the present authors. A popularly used counterflow configuration is adopted as the research prototype in this work. The effects of (1) the preheated temperature of the oxidizer mixtures, (2) the oxygen concentration in the oxidizer flow and (3) the hydrogen concentration in the fuel mixtures on the reaction structure of biogas under the new combustion condition are investigated with the aid of the lattice Boltzmann method (LBM). Through numerical simulation, it is found that the MILD oxy-fuel combustion fueled by biogas can be sustained even with relatively low preheated temperature of the oxidizer, extremely highly diluted oxygen concentration in the oxidizer flow and little hydrogen addition in the fuel mixtures, which provide a solid theoretical basis to develop a novel scheme to respond to the challenge caused by CO₂ emissions. Moreover, our discoveries imply the breakdown of the popularly used flamelet approach and emphasize the urgency to develop new turbulent combustion models for this novel combustion strategy.     

Effect of addition of hydrogen and exhaust gas recirculation on characteristics of hydrogen gasoline engine

The effects of exhaust gas recirculation (EGR) on combustion and emissions under different hydrogen ratios were studied based on an engine with a gasoline intake port injection and hydrogen direct injection. The peak cylinder pressure increases by 9.8% in the presence of a small amount of hydrogen. The heat release from combustion is more concentrated, and the engine torque can increase by 11% with a small amount of hydrogen addition. Nitrogen oxide (NOx) emissions can be reduced by EGR dilution. Hydrogen addition offsets the blocking effect of EGR on combustion partially, therefore, hydrogen addition permits a higher original engine EGR rate, and yields a larger throttle opening, which improves the mechanical efficiency and decreases NOx emissions by 54.8% compared with the original engine. The effects of EGR on carbon monoxide (CO) and hydrocarbon (HC) emissions are not obvious and CO and HC emissions can be reduced sharply with hydrogen addition. CO, HC, and NOx emissions can be controlled at a lower level, engine output torque can be increased, and fuel consumption can be reduced significantly with the co-control of hydrogen addition and EGR in a hydrogen gasoline engine.     

Effects of ammonia substitution on hydrogen/air flame propagation and emissions

In order to evaluate the potential of partial ammonia substitution to improve the safety of hydrogen use and the effects on the performance of internal combustion engines, the propagation, development of surface cellular instability and nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions of spark-ignited spherical laminar premixed ammonia/hydrogen/air flames were studied experimentally and computationally. With ammonia being the substituent, the fundamental unstretched laminar burning velocities and Markstein numbers, the propensity of cell formation and the associated flame structure were determined. Results show substantial reduction of laminar burning velocities with ammonia substitution in hydrogen/air flames, similar to hydrocarbon (e.g., methane with a similar molecular weight to ammonia) substitution. In all cases, ammonia substitution enhances the NO_x and N₂O formation. At fuel-rich conditions, however, the amount of NO_x emissions increases and then decreases with ammonia substitution and the increased amount of NO_x and N₂O emissions with ammonia substitution is much lower than that under fuel-lean conditions. These observations support the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use with low NO_x and N₂O emissions in fuel-rich hydrogen/air flames. The potential of ammonia as a suppressant of both preferential-diffusional and hydrodynamic cellular instabilities in hydrogen/air flames was also found particularly for fuel-lean conditions, different from methane substitution. However, it should be noted that the use of ammonia also imposes considerable technological challenges and public concerns, particularly those associated with toxicity and the specific properties such as high reactivity with container materials and water, which should be completely resolved.     

Effect of hydrogen addition on the structure and stabilization of a micro-jet methane diffusion flame

The micro-jet diffusion flame can act as the heat source for the micro power generation systems due to some advantages. The present work investigates the effect of hydrogen addition on the structure and stabilization of micro-jet methane diffusion flame by numerical simulation. The results show that the oval flame becomes more and more circular with the increase of hydrogen addition fraction. The addition of hydrogen remarkably suppresses the increase of the flame height with the inlet velocity. The methane sharply decreases around the outlet of the micro-jet tube due to the high fresh fuel temperature. The intermediate species (e.g., H₂ and CO) increase sharply before the flame front, and they are consumed sharply within the flame front. With the increase of hydrogen addition fraction, the concentration gradients of reactive species increase before the flame front, while the flame temperature decreases. In addition, with the increase of hydrogen addition fraction, the micro-jet flame root shifts toward the tube-wall and downstream direction at the radial and axial directions, respectively, and the addition of hydrogen decreases the anchoring temperature of the micro-jet flame root, which is conductive to improve the flame stabilization. Meanwhile, a large hydrogen addition fraction is detrimental for the flame stabilization in terms of the thermal interaction between the micro-jet flame and tube-wall. However, the positive effects brought by a large hydrogen addition fraction are noticeably larger than the adjunctive negative effects. This study not only provides the guideline for further expanding the operating range of the micro-jet methane diffusion flame but also helps us to gain insights into the mechanism of hydrogen addition on improving the flame stabilization.     

SI engine assessment using biogas,natural gas and syngas with different content of hydrogen for application in Brazilian rice industries: Efficiency and pollutant emissions

After Asia, Brazil is the world's largest rice producer. During the processing of the grain, large amounts of husk are generated, corresponding to 22% of its weight. On the other hand, in the process of parboiling, in turn, the final result is considerable volumes of effluents rich in organic matter, generating large amounts of methane gas through anaerobic treatment. Therefore, the SI engine can operate with mixtures of biogas and syngas, generating electricity and heat in the Brazilian rice industries. In addition, it reduces the emissions of polluting gases that are generated with a direct burning of the husks instead of their gasification, as well as the use of methane gas. Accordingly, in this work, it was used the spark-ignition engine operating with one of the typical biogas and syngas compositions generated in the rice industries, named Bio65 (containing 65% of CH₄ by vol.), syngas1 (containing 18,3% of H₂ by vol.), and syngas2 (containing 13,5% of H₂ by vol.), respectively. Additionally, the tests with natural gas as a reference fuel have been performed. It was evaluated the emissions of polluting gases such as CO, NO_x and HC, as well as the thermal and electrical efficiency of all tested fuels. An important result that could be observed was that for both natural gas and biogas fuel, the increase in excess ratio (λ) value from 1 to 1.5 led to lower NO_x and CO emissions, even if with increased HC emissions. On the other hand, the Indicated Specific Energy Consumption increased to all the fuels tested in lean conditions in almost all ignition advances angles. The research tried to show that biogas and syngas can be used in parboiling rice industries, taking the advantage of the generated gases for energy self-sufficiency as well as reducing emissions.     

On the stability of a turbulent non-premixed biogas flame: Effect of low swirl strength

Biogas like other low calorific value fuels has a very narrow stable region when operating in diffusion flame mode owing to their low burning velocity in conjunction with the unburned flow high velocity. This paper presents an experimental study on the effect of the burner geometry on the stability limits of a turbulent non-premixed biogas flame. The main focus of the study is on the role of the low swirl strength of the co-airflow, and the fuel nozzle diameter. The results revealed that the swirl plays a dominant role on the flame mode (attached or lifted) as well as on its operating/stability limits. However, the results revealed that the swirl effect prevails only at relatively moderate to high co-airflow velocity. That is, the swirl does not have an apparent effect at weak co-airflow when the flame is attached. Whereas, it becomes dominant at relatively high co-airflow velocity where the attached flame lifts off and stabilizes at a distance above the burner. Correlations were proposed to describe the lifted biogas flame blowout limits.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

Exergy analysis of hydrogen production via biogas dry reforming

Among the alternative pathways for hydrogen production, the use of biogas from organic waste via dry reforming of methane (DRM), water gas shift reaction and pressure swing adsorption (PSA) is often seen as an interesting option. In this work, the thermodynamic performance of this type of biohydrogen energy system –additionally including a combined-cycle scheme that satisfies the electricity and steam requirements of the process– is evaluated through exergy analysis. The main data needed for the analysis are acquired from a predictive simulation model implemented in Aspen Plus^®. The system shows an exergetic efficiency of 55%, with the DRM and the power generation subsystems arising as the main sources of irreversibility. Furthermore, given the significant influence found for the PSA off-gas on the thermodynamic performance of the system, two alternative process configurations based on the use of this stream are evaluated. In this regard, full recirculation of the PSA off-gas to the DRM reactor is found to improve the system's exergetic performance.     

Overview of hydrogen production technologies from biogas and the applications in fuel cells

Traditionally, H₂ is a large-scale production by the reforming process of light hydrocarbons, mainly natural gas, used by the chemical industry. However, the reforming technologies currently used encounter numerous technical/scientific challenges, which depend on the quality of raw materials, the conversion efficiency and security needs for the integration of H₂ production, purification and use, among others. Biogas is a high-potential versatile raw material for reforming processes, which can be used as an alternative CH₄ source. The production of H₂ from renewable sources, such as biogas, helps to largely reduce greenhouse gas emissions. Within this context, the integration of biogas reforming processes and the activation of fuel cell using H₂ represent an important route for generating clean energy, with added high-energy efficiency. This work expounds a literature review of the biogas reforming technologies, emphasizing the types of fuel cells available, the advantages offered by each route and the main problems faced.     

Estimation of hydrogen output from a full-scale plant for production of hydrogen from biogas

An experiment was conducted to produce hydrogen from biogas using dairy cow waste as the main material, and the results were used as the basis for simulation of a full-scale plant (referred to here as a BTH-plant) combining a biogas plant and a hydrogen production facility. In this study, BTH-plant operation method to minimize greenhouse gas emissions was identified using linear programming, and the available hydrogen supply capacity was estimated.     

Theoretical investigation of the combustion performance of ammonia/hydrogen mixtures on a marine diesel engine

Ammonia is a good hydrogen carrier and can be well combined with hydrogen for combustion. The combustion performance of the mixtures of ammonia and hydrogen in a medium-speed marine diesel engine was investigated theoretically. The HCCI combustion mode was selected for reducing thermal-NO_x production. The start fire characteristic of the NH₃–H₂ mixtures was studied under different equivalence ratio, hydrogen-doped ratio, and intake air temperature and pressure. Then, the combustion performance of the NH₃–H₂ mixtures (doping 30% hydrogen) was analyzed at a typical operation condition of engine. The addition hydrogen improved the laminar flame velocity of ammonia, and affected the NO_x emission. For the medium-speed marine engine fueled with NH₃–H₂, reducing combustion temperature, introducing EGR and combining with post-treatment technology would be a feasible scheme to reduce NO_x emission.     

Emissions of a diesel engine using B20 and effects of hydrogen addition

The blended biodiesel with up to 20% biodiesel in petroleum diesel (B20) is considered nowadays as available in production. Previous studies investigating the effect of B20 on engine emissions led to some contradictory results. The present study continued the investigation on B20, 20% biodiesel (rapeseed methyl esters) blend effects and was also extended on B20 enriched with hydrogen. It was conducted on a conventional tractor diesel engine running alternatively with B20 and petroleum diesel at various speeds and full load and then, with the same fuels enriched with hydrogen, at 60% load and two speeds.     

Investigation of biogas and hydrogen production from waste water of milk-processing industry in Turkey

This study is conducted to determine the potential for producing both biogas and hydrogen from a milk-processing waste water in Turkey. The results of this study indicate that a maximum of 54.2 million m³ biogas/yr and 12,670 ton H₂/yr can be produced from milk-processing waste water. A total of $15.1 million worth of energy may be supplied every year from the produced biogas. Some Reference calculations for the production of biogas and the economic evaluation are carried out using actual data taken from the plant. Overall hydrogen production energy efficiency for different types of reforming and for different ambient temperatures ranges between 19 and 70% whereas the overall exergy efficiency for 900 °C reforming and different ambient temperatures changes between 8 and 48%, respectively.     

Exergy analysis of the demonstration plant for co-production of hydrogen and benzene from biogas

The dehydro-aromatization reaction of methane is one of the methods to utilize biogas and co-produces hydrogen and benzene. To demonstrate the industrial co-production of hydrogen and benzene from biogas, we constructed a demonstration plant. The purpose of this study is to evaluate the feasibility of the demonstration plant, which can co-produce 134 N m³/day of hydrogen and 8.6 L/day of benzene from 200 N m³/day of biogas, by means of clearing energy requirement, exergy loss and CO₂ emission. Energy requirement was 3783 MJ/day. Only 21% of the input exergy was converted to hydrogen and benzene, and 79% was consumed. The sub-system of the dehydro-aromatization reformer showed the largest exergy loss among the eight sub-systems. Assuming electricity was supplied by thermal power generation, CO₂ emission was 634 kg/day. On the other hand, assuming electricity was supplied by biogas engine, fuel biogas requirement was 555 N m³/day. However, CO₂ emission was not counted in this case because of the carbon-neutral principle.