Application of exhaust gas fuel reforming in diesel and homogeneous charge compression ignition (HCCI) engines fuelled with biofuels

This paper documents the application of exhaust gas fuel reforming of two alternative fuels, biodiesel and bioethanol, in internal combustion engines. The exhaust gas fuel reforming process is a method of on-board production of hydrogen-rich gas by catalytic reaction of fuel and engine exhaust gas. The benefits of exhaust gas fuel reforming have been demonstrated by adding simulated reformed gas to a diesel engine fuelled by a mixture of 50% ultra low sulphur diesel (ULSD) and 50% rapeseed methyl ester (RME) as well as to a homogeneous charge compression ignition (HCCI) engine fuelled by bioethanol. In the case of the biodiesel fuelled engine, a reduction of NO_x emissions was achieved without considerable smoke increase. In the case of the bioethanol fuelled HCCI engine, the engine tolerance to exhaust gas recirculation (EGR) was extended and hence the typically high pressure rise rates of HCCI engines, associated with intense combustion noise, were reduced.     

Simulation of HCCI combustion in air-cooled off-road engines fuelled with diesel and biodiesel

The present work describes the elaboration of a predictive tool consisting on a phenomenological multi-zone model, applicable to the simulation of HCCI combustion of both diesel and biodiesel fuels. The mentioned predictive tool is created with the aim to be applied in the future to perform engine characterization during both pre-design and post-design stages. The methodology applied to obtain the proposed predictive model is based on the generation of an analytical mechanism that, given a set of regression variables representing the engine operative conditions, provides the user with the optimal figures for the scaling coefficients needed to particularize both the ignition delay and the heat release rate functional laws, which rule the combustion development in the proposed multi-zone model for HCCI engines. The validation of the proposed predictive multi-zone model consists on the comparison between chamber pressure curve derived from the simulations and experimental data based on a DEUTZ FL1 906 unit modified in order to allow HCCI combustion operation mode using diesel EN590 and rapeseed biodiesel. Finally, evidences of the capabilities of the proposed model to be used as a predictive tool applicable to the analysis of off-road engines under HCCI conditions are provided, consisting in the characterization and optimization of the operational maps related to both Brake Specific Fuel Consumption and NOx emissions.     

Improving the environmental performance of biofuels with industrial symbiosis

In the production of biofuels for transport many critics have argued about the poor energy efficiency and environmental performance of the production industries. Optimism is thus set on the production of second generation biofuels, while first generation biofuels continue to dominate worldwide. Therefore it is interesting to consider how the environmental performance of first generation biofuel industries can be improved. The field of industrial symbiosis offers many possibilities for potential improvements in the biofuel industry and theories from this research field are used in this paper to highlight how environmental performance improvements can be accomplished. This comes in the form of by-product synergies and utility synergies which can improve material and energy handling. Furthermore, the processes and products can gain increased environmental performance improvements by the adaption of a renewable energy system which will act as a utility provider for many industries in a symbiotic network. By-products may thereafter be upcycled through biogas production processes to generate both energy and a bio-fertilizer. A case study of an actual biofuel industrial symbiosis is also reviewed to provide support for these theories.     

Numerical study of effects of reformed exhaust gas recirculation (REGR) on dimethyl ether HCCI combustion

The effects of reformed exhaust gas recirculation (REGR) on combustion and emissions of dimethyl ether (DME) homogeneous charge compression ignition (HCCI) engines are studied by multi-dimensional CFD coupled with chemical kinetic model. The results show that REGR combing EGR and DME reformed gases (DRG) improves combustion and emissions. REGR can delay ignition time by both EGR and DRG, and makes main combustion closer to top dead center (TDC), which is beneficial to reducing compression negative work and broadening load range of HCCI engines. The interaction of DRG and EGR helps avoid too high pressure rise rate or low power performance when being applied independent of each other. HC, CO and NO_x emissions can be controlled simultaneously by REGR. Both advantages of DRG and EGR are used to decrease the emissions of HCCI engines by REGR, while the disadvantages of high emissions are alleviated when one of them is applied.     

One-dimensional turbulence simulations of hydrogen-fueled HCCI combustion

Homogeneous charge compression ignition (HCCI) engines fueled by hydrogen have the potential to provide cost-effective power with high efficiencies and very low emissions. This paper investigates the ability of two of the most commonly used injection methods, port fuel injection (PFI) and single-pulse direct injection (DI), to prepare an ideal in-cylinder hydrogen-air mixture and control the autoignition process. Computations are performed using the one-dimensional turbulence (ODT) model formulated for engine simulations. It is found that direct injection is able to prepare a more uniformly lean mixture and control the autoignition more effectively than port fuel injection. A combination of ignition modes are found to be operating when PFI is used as compared to mainly volumetric autoignition in the case of DI. Also, DI is able to maintain comparatively lower temperatures than PFI.     

Numerical study of EGR effects on reducing the pressure rise rate of HCCI engine combustion

The effects of the inert components of exhaust gas recirculation (EGR) gas on reducing the pressure rise rate of homogeneous charge compression ignition engine combustion were investigated numerically by utilizing the CHEMKIN II package and its SENKIN code, as well as Curran’s dimethyl ether reaction scheme. Calculations were conducted under constant volume combustion and engine combustion (one compression and one expansion only, respectively) conditions. Results show that with constant fuel amount and initial temperature and pressure, as EGR ratio increases, combustion timings are retarded and the duration of thermal ignition preparation extends non-linearly; peak values of pressure, pressure rising rate (PRR), and temperature decrease; and peak values of heat release rate in both low temperature heat release (LTHR) and high temperature heat release decrease. Moreover, maximum PRR decreases as CA50 is retarded. With constant fuel amount, mixtures with different EGR ratios can obtain the same CA50 by adjusting the initial temperature. Under the same CA50, as EGR ratio increases, the LTHR timing is advanced and the duration of thermal ignition preparation is extended. Maximum PRR is almost constant with the fixed CA50 despite the change in EGR ratio, indicating that the influence of EGR dilution on chemical reaction rate is offset by other factors. Further investigation on the mechanism of this phenomenon is needed.     

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO_x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO_x emissions while reducing the tendency to knock     

The effect of hydrogen addition on combustion and emission characteristics of an n-heptane fuelled HCCI engine

The mechanisms of the influence of hydrogen enrichment on the combustion and emission characteristics of an n-heptane fuelled homogeneous charge compression ignition (HCCI) engine was numerically investigated using a multi-zone model. The model calculation successfully captured the most available experimental data. The results show that hydrogen addition retards combustion phasing of an n-heptane fuelled HCCI engine due to the dilution and chemical effects, with the dilution effect being more significant. It is because of the chemical effect that combustion duration is reduced at a constant compression ratio if an appropriate amount of hydrogen is added. As a result of retarded combustion phasing and reduced combustion duration, hydrogen addition increases indicated thermal efficiency at a constant combustion phasing. Hydrogen addition reduces indicated specific unburned hydrocarbon emissions, but slightly increases normalized unburned hydrocarbon emissions that are defined as the emissions per unit burned n-heptane mass. The increase in normalized unburned hydrocarbon emissions is caused by the presence of more remaining hydrocarbons that compete with hydrogen for some key radicals during high temperature combustion stage. At a given hydrogen addition level, N₂O emissions increases with overly retarding combustion phasing, but hydrogen addition moderates this increase in N₂O emissions.     

Exergy analysis of diesel/biodiesel combustion in a homogenous charge compression ignition (HCCI) engine using three-dimensional model

Exergy analysis gives the presentation of a system relative to its best performance. In addition, the exergy destructed can react with its surrounding and harm environment processes. This study investigated the effect of biodiesel fuel blended with diesel fuel (i.e. 0%, 20%, and 50% blending of biodiesel fuel with conventional diesel fuel) on various exergy terms in an HCCI engine. To model the energy balance a 3-D CFD code was utilized. Using energy and combustion analyses results, the researchers calculated various exergy terms by developing a FORTRAN based code. To ensure the integrity of modeling, the results of the in-cylinder pressure and heat release rate were compared with the experimental results for pure diesel fuel. This comparison indicated a good agreement between the two. With crank position at three fuel compositions, different rates of exergy and cumulative exergy terms were identified and calculated separately. With the increase in the biodiesel volume percentage from 0% to 20% and 50%, exergy efficiency increased by 4.9% and 5.7%. Also, the cumulative heat loss exergy decreased by 4.4% and 9.7%, respectively.     

Modeling HCCI combustion: Modification of a multi-zone model and comparison to experimental results at varying boost pressure

The present study presents a comparison of the results obtained from a modified HCCI multi-zone model to experimental measurements, at different load and boost pressure conditions. The multi-zone model includes a modified sub-model for the wall heat transfer and accounts for the heat transfer between zones. Gas mixing between cold and hot regions of the combustion chamber, which is of major importance for the emissions formation, is also accounted for throughout compression, combustion and expansion. Combustion is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver. A refined zone configuration near the combustion chamber wall was used, in order to obtain a high resolution at the emissions formation regions. The pressure traces and emissions of nine experimental cases were compared to the multi-zone model results. In these cases the equivalence ratio and the boost pressure were varied, while maintaining constant engine speed. The results show adequate agreement with the pressure traces. The emissions trends are also adequately captured, with the absolute values presenting some deviation from the experimental cases especially for the HC and CO emissions at the relatively low air-fuel equivalence ratios.     

An experimental study on the effect of hydrogen enrichment on diesel fueled HCCI combustion

This paper experimentally investigates the influence of hydrogen enrichment on the combustion and emission characteristics of a diesel HCCI engine using a modified Cooperative Fuel Research (CFR) engine. Three fuels, n-heptane and two middle distillates with cetane numbers of 46.6 and 36.6, are studied.The results show that hydrogen enrichment retards the combustion phasing and reduces the combustion duration of a diesel HCCI engine. Besides, hydrogen enrichment increases the power output and fuel conversion efficiency, and improves the combustion stability. However, hydrogen enrichment may narrow the operational compression ratio range and increase the knocking tendency. Both the overall indicated specific CO emissions (isCO) and CO emissions per unit burned diesel fuel mass are reduced by hydrogen enrichment. Although hydrogen enrichment decreases the overall indicated specific unburned hydrocarbon emissions (isHC), it does not significantly affect the HC emissions per unit burned diesel fuel mass.     

Natural resource demand of global biofuels in the Anthropocene: A review

The Anthropocene is the later part of the Holocene where human activity has become a major driver for global ecosystem development. The demand of natural resources, renewable and non-renewable, is a crucial aspect of environmental (un-) sustainability. When considering a societal transition scheme towards sustainability, bio-based options come to the fore. The article develops a global framework for the analysis of natural resource demand of global biofuels. The framework defines the biofuel system in terms of exergy at four levels, i.e., the foreground system, the supply chain, the anthroposphere, and the ecosphere. Various measures of resource demand, such as cumulative exergy demand, global and anthropogenic exergy budgets are incorporated into the framework. Based on reviews of global biofuel production and natural resource demand of the anthroposphere, the study finds that the production of conventional biofuels, i.e., first generation of biodiesel and bioethanol by key producer countries in 2008 consumed 9.32 E+11 MJ of exergy from non-renewable resources and accounted for 0.23% of the total anthropogenic non-renewable resource demand. In addition, it shows that the contribution to climate change due to the heat emission of the global biofuel production was 5.79 E−05 W/m², which would reach up to 0.002% of global greenhouse warming if anthropogenic heat flux is treated as a climate forcing.     

Experimental study of combustion and emission characteristics of ethanol fuelled port injected homogeneous charge compression ignition (HCCI) combustion engine

The homogeneous charge compression ignition (HCCI) is an alternative combustion concept for in reciprocating engines. The HCCI combustion engine offers significant benefits in terms of its high efficiency and ultra low emissions. In this investigation, port injection technique is used for preparing homogeneous charge. The combustion and emission characteristics of a HCCI engine fuelled with ethanol were investigated on a modified two-cylinder, four-stroke engine. The experiment is conducted with varying intake air temperature (120–150 °C) and at different air–fuel ratios, for which stable HCCI combustion is achieved. In-cylinder pressure, heat release analysis and exhaust emission measurements were employed for combustion diagnostics. In this study, effect of intake air temperature on combustion parameters, thermal efficiency, combustion efficiency and emissions in HCCI combustion engine is analyzed and discussed in detail. The experimental results indicate that the air–fuel ratio and intake air temperature have significant effect on the maximum in-cylinder pressure and its position, gas exchange efficiency, thermal efficiency, combustion efficiency, maximum rate of pressure rise and the heat release rate. Results show that for all stable operation points, NO_x emissions are lower than 10 ppm however HC and CO emissions are higher.     

Ozone-assisted combustion of hydrogen: A comparison with isooctane

Homogeneous Charge Compression Ignition (HCCI) engines have the potential to achieve low emissions of nitrogen oxides and particulate matter but the use of carbon-based fuels leads to significant emissions of carbon monoxide and unburned hydrocarbon which hinder the widespread development of HCCI. Nonetheless, HCCI engines have also the potential to use various types of fuel such as hydrogen, a carbon free fuel. The present study examined hydrogen combustion in an HCCI engine comparing hydrogen with isooctane. The sensitivities of CA05, CA50 and ringing intensity to intake pressure, intake temperature and ozone were investigated for both fuels. Results highlighted that hydrogen autoignites more easily than isooctane despite its higher-octane number. Ozone seeding was therefore used to shift the combustion region of isooctane in order to perform a viable comparison between the two fuels while starting with the same initial conditions. From this point, all of the three parameters were found to promote the HCCI combustion of both fuels but greater sensitivities were observed for hydrogen than for isooctane. Ozone was observed to have the strongest influence, making it a good strategy for a future control approach. The noise level of hydrogen was very high due its high reactivity. Finally, the use of hydrogen together with ozone should enable a very clean engine cycle to be achieved cycle with only water and nitrogen as engine-out emissions.     

A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine

A model for the Homogeneous Charge Compression Ignition (HCCI) of Primary Reference Fuels (PRFs) in a Rapid Compression Machine (RCM) has been developed. A reduced chemical kinetic model that included 32 species and 55 reactions was used and the affect of wall heat transfer on the temperature of the adiabatic core gas was taken into account by adding the displacement volume of the laminar boundary layer to the cylinder volume. A simple interaction between n-heptane and iso-octane was also included. The results showed the well-known two-stage ignition characteristics of heavy hydrocarbons, which involve low and high temperature cycles followed by a branched chain explosion. The first stage energy release decreases and the ignition delay increases nonlinearly with increasing octane number and decreasing the initial pressure. The energy release rate and total energy released were determined primarily by the rate of CO oxidation during the explosive phase following the ignition delay. The model reproduced the pressure curves obtained in the RCM experiments over a wide range of conditions remarkably well and was very sensitive to the fuel structure, the mixture composition and the initial temperature and pressure. Thus, the model can be easily adapted for predicting “knock” in spark-ignition engines and ignition-delays and burning rates in HCCI engines.     

Assessing the sustainability of biofuels: A logic-based model

Over the last decade, the production and consumption of biofuels increased rapidly worldwide, in an attempt to reduce GHG (greenhouse gas) emissions, diversify transportation fuels, promote renewable energy, and create or maintain employment, especially in rural areas and developing countries. Although policy instruments being currently implemented in industrialized regions focus on sustainable biofuels, the definition and assessment of sustainability remains a highly debated issue. Several countries have adopted compulsory targets or financial incentives for promoting biofuels, and only a few countries have accounted for sustainability certification schemes for those biofuels within their policy framework. In this paper, a logic-based model for assessing the sustainability of biofuels is presented. The model uses a hierarchical structure to link multiple factors from the more specific variables to the most general one, sustainability performance. The strengths and limitations of the model are discussed and the anticipated improvements are provided.     

High hydrogen content syngas for biofuels production from biomass air gasification: Experimental evaluation of a char-catalyzed steam reforming unit

This work investigates the performance of a reformer reactor for the upgrading of syngas and char derived from a pilot-scale air gasifier. The proposed setup represents a circular approach for the production of hydrogen-rich syngas from air gasification. Specifically, the reforming-unit was operated under a range of temperatures (from 700 °C to 850 °C) and steam flow rates and for each the improvement in producer gas composition and reducing species yield is evaluated. The results highlight that an increase in hydrogen concentration is obtained at higher temperature, moving from 16.2% to 21.3%, without using steam, and to 45.6%, with steam injection on the char-bed, while CO concentration did not follow a monotonic behavior. Moreover, the gas quality index, defined as a ratio between reducing species and inert species, delivered the highest values at the highest temperatures and steam flow rates. These results provide a guide for future gas quality optimization studies.     

A parametric study on the emissions from an HCCI alternative combustion engine resulting from the auto-ignition of primary reference fuels

The homogeneous charge compression ignition is an alternative combustion technology that can reduce automobile pollution, provided that the exhaust emission can be controlled. A parametric study can be useful in order to gain more understanding in the emission reduction possibilities via this new combustion technology. For this purpose, the inlet temperature, the equivalence ratio and the compression ratio are changed, respectively, from 30 to 70 °C, 0.28 to 0.41 and 6 to 14. Also the diluting, thermal and chemical effects of exhaust gas recirculation were studied. The emission of CO, CO₂, O₂ and hydrocarbons has been measured using primary reference fuels. It appears that an increase in the inlet temperature, the EGR temperature, the equivalence ratio and the compression ratio results into a decrease of the emissions of CO and the hydrocarbons of up to 75%. The emission of CO₂ increased, however, by 50%. The chemical parameters showed more complicated effects, resulting into a decrease or increase of the emissions, depending on whether the overall reactivity increased or not. If the reactivity increased, generally, the emissions of CO and hydrocarbons increased, while that of CO₂ increased. The increase of CO₂ emissions could be compensated by altering the compression ratio and the EGR parameters, making it possible to control the emission of the HCCI engine.     

Production of dimethyl ether and hydrogen by methanol reforming for an HCCI engine system with waste heat recovery – Continuous control of fuel ignitability and utilization of exhaust gas heat

Homogeneous charge compression ignition (HCCI) is a promising technique to achieve high thermal efficiency and clean exhaust with internal combustion engines. However, the difficulty in ensuring optimal ignition timing control prevents its practical application. Previous research has shown that adjusting the proportion of dimethyl ether (DME) and hydrogen-containing methanol-reformed gas (MRG) can control the ignition timing in an HCCI combustion engine fueled with the two fuels. As both DME and MRG can be produced in endothermic methanol reforming reactions, onboard reforming utilizing the exhaust gas heat can recover the waste heat from the engine. A very high overall thermal efficiency can be achieved by combining the high engine efficiency with HCCI and the waste heat recovery. This research investigates the basic characteristics of methanol reforming in a reactor tube with different catalysts with the aim to produce fuels for the HCCI combustion system.     

Microbial hydrogen production by bioconversion of crude glycerol: A review

Hydrogen is a clean source of energy with no harmful byproducts produced during its combustion. Bioconversion of different organic waste materials to hydrogen is a sustainable technology for hydrogen production and it has been investigated by several researchers. Crude glycerol generated during biodiesel manufacturing process can also be used as a feedstock for hydrogen production using microbial processes. The possibility of using crude glycerol as a feedstock for biohydrogen production has been reviewed in this article. A review of recent global biodiesel and crude glycerol production and their future market potential has also been carried out. Similarly, different technical constraints of crude glycerol bioconversion have been elaborately discussed and some strategies for improved hydrogen yield have also been proposed. It has been underlined that use of crude glycerol from biodiesel processing plants for hydrogen production has many advantages over the use of other organic wastes as substrate. Most importantly, it will give direct economic benefit to biodiesel manufacturing industries, which in turn will help in increasing biofuel production and it will partially replace harmful fossil fuels with biofuels. However, different impurities present in crude glycerol are known to inhibit microbial growth. Hence, suitable pretreatment of crude glycerol is recommended for maximum hydrogen yield. Similarly, by using suitable bioreactor system and adopting continuous mode of operation, further investigation of hydrogen production using crude glycerol as a substrate should be undertaken. Furthermore, isolation of more productive strains as well as development of engineered microorganism with enhanced hydrogen production potential is recommended. Strategies for application of co-culture of suitable microorganisms as inoculum for crude glycerol bioconversion and improved hydrogen production have also been proposed.