Numerical simulation of biodiesel fuel combustion and emission characteristics in a direct injection diesel engine

The effect of the physical and chemical properties of biodiesel fuels on the combustion process and pollutants formation in Direct Injection (DI) engine are investigated numerically by using multi-dimensional Computational Fluid Dynamics (CFD) simulation. In the current study, methyl butanoate (MB) and n-heptane are used as the surrogates for the biodiesel fuel and the conventional diesel fuel. Detailed kinetic chemical mechanisms for MB and n-heptane are implemented to simulate the combustion process. It is shown that the differences in the chemical properties between the biodiesel fuel and the diesel fuel affect the whole combustion process more significantly than the differences in the physical properties. While the variations of both the chemical and the physical properties between the biodiesel and diesel fuel influence the soot formation at the equivalent level, the variations in the chemical properties play a crucial role in the NO _x emissions formation.     

Low-pressure-plasma-processed NiFe-MOFs/nickel foam as an efficient electrocatalyst for oxygen evolution reaction

A NiFe bimetallic metal organic framework (MOF) deposited on nickel foam and processed by low-pressure plasmas with 95%Ar+5%H₂, pure Ar, and 95%Ar+5%O₂ gases is used as an electrocatalyst for the oxygen evolution reaction. An alkaline solution (1 M KOH) with 95%Ar+5%H₂ plasma processed NiFe-MOFs/NF exhibits the best electrocatalytic performance with the lowest overpotential of 149 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1. Furthermore, electrical impedance spectroscopy and cyclic voltammetry show that after 95%Ar+5%H₂ plasma treatment, the interfacial impedance greatly reduces, and the electrical double-layer capacitance slightly increased.     

Energy efficiency and the influence of gas burners to the energy related carbon dioxide emissions of electric arc furnaces in steel industry

Determining the complete energy balance of an electric arc furnace (EAF) provides an appropriate method to examine energy efficiency and identify energy saving potentials. However, the EAF energy balance is complex due to the combined input of electrical energy and chemical energy resulting from natural gas (NG) combustion and oxidation reactions in the steel melt. In addition, furnace off-gas measurements and slag analysis are necessary to reliably determine energy sinks. In this paper 70 energy balances and energy efficiencies from multiple EAFs are presented, including data calculated from plant measurements and compiled from the literature. Potential errors that can be incorporated in these calculations are also highlighted. The total energy requirement of these modern EAFs analysed ranged from 510 to 880 kWh/t, with energy efficiency values (η = ΔH_Steel/E_Total) of between 40% and 75%. Furthermore, the focus was placed on the total energy related CO₂ emissions of EAF processes comprising NG combustion and electrical energy input. By assessing multiple EAF energy balances, a significant correlation between the total energy requirement and energy related specific CO₂ emissions was not evident. Whilst the specific consumption of NG in the EAF only had a minor impact on the EAF energy efficiency, it decreased the specific electrical energy requirement and increased EAF productivity where transformer power was restricted. The analysis also demonstrated that complementing and substituting electrical energy with NG was beneficial in reducing the total energy related CO₂ emissions when a certain level of substitution efficiency was achieved. Therefore, the appropriate use of NG burners in modern EAFs can result in an increased EAF energy intensity, whilst the total energy related CO₂ emissions remain constant or are even decreased.     

CH4-CO2 reforming by plasma - challenges and opportunities

CH₄-CO₂ reforming is of rapid growing interest for reasons of the continuous decrease of petroleum resources and the emphasis on the environmental situation for greenhouse gas mitigation. Plasma technology is considered to be one of potential ways for CH₄-CO₂ reforming. This paper presents an overview of CH₄-CO₂ reforming by cold plasmas and thermal plasma. The evaluations for their performances and the key factors in different plasmas are given. In particular, the attention is focused on how to achieve higher conversions at high feed-gas flow rate, so as to lessen the energy consumption in the process by plasma to meet the requirements of industrial application. To obtain the aim, three key factors, electron density, plasma temperature and reactor configuration related to the process are emphasized. Considering the current status of CH₄-CO₂ reforming by plasma, there is an opportunity to improve the energy conversion efficiency and the treatment capacity of the process by optimizing both plasma form and reactor design in future work.     

Dry combustion plasma for open-cycle magnetohydrodynamic electricity generation

Dry combustion plasma (DCP), i.e. plasma produced by seeded combustion of dry fuels such as char, charcoal, petroleum pitch etc. devoid of H₂O, is shown to be a superior medium for open-cycle MHD electricity generation owing to its enhanced electrical conductivity. The calculated conductivity of DCP is about fifty times higher than what has been realized so far in experimental rigs.     

Effects of Fischer-Tropsch diesel fuel on combustion and emissions of direct injection diesel engine

Effects of Fischer-Tropsch (F-T) diesel fuel on the combustion and emission characteristics of a single-cylinder direct injection diesel engine under different fuel delivery advance angles were investigated. The experimental results show that F-T diesel fuel exhibits shorter ignition delay, lower peak values of premixed burning rate, lower combustion pressure and pressure rise rate, and higher peak value of diffusion burning rate than conventional diesel fuel when the engine remains unmodified. In addition, the unmodified engine with F-T diesel fuel has lower brake specific fuel consumption and higher effective thermal efficiency, and presents lower HC, CO, NO _x and smoke emissions than conventional diesel fuel. When fuel delivery advance angle is retarded by 3 crank angle degrees, the combustion duration is obviously shortened; the peak values of premixed burning rate, the combustion pressure and pressure rise rate are further reduced; and the peak value of diffusion burning rate is further increased for F-T diesel fuel operation. Moreover, the retardation of fuel delivery advance angle results in a further significant reduction in NO _x emissions with no penalty on specific fuel consumption and with much less penalty on HC, CO and smoke emissions. __________ Translated from Chinese Internal Combustion Engine Engineering, 2007, 28(2): 19–23 [译自: 内燃机工程]     

Staged combustion properties for pulverized coals at high temperature

Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NO_x emission and carbon in ash. The most important factor for low NO_x combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NO_x emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NO_x emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NO_x emission to 30–60 ppm (6% O₂). The most important NO_x reduction reaction in the fuel-rich zone was the NO_x reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NO_x was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NO_x emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties.     

Intermediate temperature single-chamber methane fed SOFC based on Gd doped ceria electrolyte and La0.5Sr0.5CoO3−δ as cathode

Single-chamber fuel cells with electrodes supported on an electrolyte of gadolinium doped ceria Ce_1−xGd_xO_2−y with x = 0.2 (CGO) 200 μm thickness has been successfully prepared and characterized. The cells were fed directly with a mixture of methane and air. Doped ceria electrolyte supports were prepared from powders obtained by the acetyl-acetonate sol–gel related method. Inks prepared from mixtures of precursor powders of NiO and CGO with different particle sizes and compositions were prepared, analysed and used to obtain optimal porous anodes thick films. Cathodes based on La_0.5Sr_0.5CoO₃ perovskites (LSCO) were also prepared and deposited on the other side of the electrolyte by inks prepared with a mixture of powders of LSCO, CGO and AgO obtained also by sol–gel related techniques. Both electrodes were deposited by dip coating at different thicknesses (20–30 μm) using a commercial resin where the electrode powders were dispersed. Finally, electrical properties were determined in a single-chamber reactor where methane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Temperature, composition and flux rate values of the carrier gas were determinants for the optimization of the electrical properties of the fuel cells.     

A review on the laminar flame speed and ignition delay time of Syngas mixtures

Syngas has shown great success in Integrated Gasification Combined Cycle (IGCC) technology for providing cleaner and higher efficiency energy production with minimal environment impact. Thus, it is promising that Syngas is able to replace the conventional fossil fuel resources, while at the same time minimizing pollutants. The drawback of traditional Gas Turbines that burn Syngas is that they use diffusion flame combustion technology that suffers from low efficiency and high emissions. Recently, Lean premixed combustion technique has emerged as a promising solution, but the variation of hydrogen fraction in Syngas has prohibited its usage. Besides, other gases such as CO₂, N₂, H₂O, NH₃, and CH₄ in Syngas have adverse effects on the combustion characteristics. To address these issues, better understanding of the Syngas's fundamental combustion properties are vital. Hence, recent works published on Syngas combustion at lean-premixed and Gas Turbine relevant conditions are reviewed, classified according to their objectives, and remarks were concluded.     

Floating zone growth of β-Ga2O3: a new window material for optoelectronic device applications

β-Ga₂O₃ is a transparent oxide and intrinsically an insulator. Doping allows the variation of the conductivity for both p- and n-type over a wide range. There are a number of potential applications in optoelectronics such as insulating or conductive window material, or as a substrate. Consequently, the influence of doping on the electrical and optical properties is an issue of crucial importance for pushing this material forward to applications. We report on the successful growth of undoped, Ge- and Ti-doped β-Ga₂O₃ single crystals by the floating zone technique. Both electrical and optical properties were characterized.     

Investigations of seeded combustion products of biogas/air-O2 systems

The composition, temperature and electrical conductivity of the seeded (with K₂CO₃) combustion products of biogas/air-O₂ systems have been analytically evaluated. The effects of preheating and enrichment of air on the combustion temperature and electrical conductivity have been studied. Experimental measurements of the temperature and electrical conductivity of biogas/oxygen systems have been made to validate the theoretical predictions.     

Radiative heat transfer from potassium seeded water gas combustion plasma

Radiative heat transfer calculations from a potassium seeded water gas combustion plasma have been made to estimate the radiative heat losses through the walls of a MHD channel. Both molecular combustion products and seed contribute significantly to the total radiation loss from a plasma. The spectral emission properties of CO₂, H₂O, CO and potassium have been taken into account. It has been shown that the contribution of CO to heat flux is very small and, thus, can be neglected. CO₂ and H₂O are the primary contributors to the radiation from the combustion products. At MHD temperatures, 55–80% of the contribution to heat flux from the combustion products comes from bands lying up to 2.7 μm in the near infrared. It has been shown that accurate knowledge of absorption cross-section data is essential to predict the radiative heat transfer from potassium. It has been estimated that 25–30% of the total radiative heat flux is from the potassium seed.     

DSMC Simulation of Non-Premixed Combustion of H2/O2 in a Y-shaped Microchannel

Combustion at the microscale shows an immeasurable potential in the fields of energy utilization and microelectromechanical systems (MEMS) due to its novel combustion characteristics. Studying the mechanism of this kind of combustion is of great significance for deepening the understanding of microcombustion phenomena and designing related devices. In this article, the non-premixed combustion of H₂/O₂ in a two-dimensional Y-shaped microchannel with a height of 10 μm was numerically studied using the direct simulation Monte Carlo (DSMC) method. The total collision energy (TCE) model and a kinetic mechanism including six species and seven reversible reactions were employed. Predicted distributions of velocity, temperature, heat flux, and components inside the microchannel are presented and analyzed. Influences of the Knudsen number and wall surface conditions on the combustion characteristics are discussed. The results show that the exothermic reaction mainly takes place in the junction area of the branch channels and in the first half of the main channel, and the wall heat flux at the microscale is much higher than that at the conventional scale. This is helpful to effectively heat and ignite the gaseous H₂/O₂ mixture. Moreover, the conversions and distributions of individual components in the flow field are mainly controlled by the chemical kinetics; the Kn number and different wall conditions have a sophisticated influence on the combustion process.     

Experimental study on combustion of torrefied palm kernel shell (PKS) in oxy‐fuel environment

Oxy‐combustion of biomass can be a major candidate to achieve negative emission of CO₂ from a pulverized fuel (pf)‐firing power generation plants. Understanding combustion behavior of biomass fuels in oxy‐firing conditions is a key for design of oxy‐combustion retrofit of pulverized fuel power plant. This study aims to investigate a lab‐scale combustion behavior of torrefied palm kernel shell (PKS) in oxy‐combustion environments in comparison with the reference bituminous coal. A 20 kW_th‐scale, down‐firing furnace was used to conduct the experiments using both air (conventional) and O₂/CO₂ (30 vol% for O₂) as an oxidant. A bituminous coal (Sebuku coal) was also combusted in both air‐ and oxy‐firing condition with the same conditions of oxidizers and thermal heat inputs. Distributions of gas temperature, unburned carbon, and NO_x concentration were measured through sampling of gases and particles along axial directions. Moreover, the concentrations of SO_x and HCl were measured at the exit of the furnace. Experimental results showed that burnout rate was enhanced during oxy‐fuel combustion. The unburnt carbon in the flue gas was reduced considerably (~75%) during combustion of torrefied PKS in oxy‐fuel environment as compared with air‐firing condition. In addition, NO emission was reduced by 16.5% during combustion of PKS in oxy‐fuel environment as compared with air‐firing condition.     

NOx reduction and NO2 emission characteristics in rich-lean combustion of hydrogen

Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NO_x) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NO_x emissions from continuous combustion burners such as gas turbines and boilers, and NO_x reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NO_x reduction effect in previous research. The present study focused on experimental measurements of NO and NO₂ emissions from a coaxial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO₂ were achieved with rich-lean combustion. The NO and NO₂ reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.     

Effect of high temperature and separated combustion on SO2 release characteristics during coal combustion under O2/CO2 atmosphere

This paper studied the effect of high temperature (up to 1873K) and separated combustion mode (volatile combustion and char combustion are separated) on SO₂ release characteristics during pulverized coal combustion under O₂/CO₂ atmosphere. Coal combustion experiments were conducted at different combustion environment temperatures utilizing a high temperature fixed-bed setup. The results show that as temperature rises, the SO₂ release curve is transformed from a single-peak process to a double-peak process. In separated combustion, temperature has little effect on the volatile-SO₂ (SO₂ released during volatile combustion) but brings about a significant effect on char-SO₂ (SO₂ released during char combustion). Char-SO₂ release amount and the ratio of it to fuel-SO₂ release amount (total SO₂ released during coal combustion) increase with temperature rising. The increase of temperature leads to a dramatic decreasing of sulphur mass fixed in the ash and causes SO₂ release amount to rise when temperature is lower than 1573 K. Separated combustion causes a higher SO₂ release amount than coupled combustion (the same as conventional combustion, volatile combustion and char combustion are simultaneous). Thermochemistry equilibrium composition calculation results show that alkali metals and alkaline-earth metals are significant in sulphur retention. CaSO₄ and Na₂SO₄ are the main sulphates at high temperatures.     

The effects of electrical fields upon electron energy exchanges in flame gases

Rates of electron energy gain from applied electrical fields and rates of energy loss in molecular collisions have been computed. To do this, electron mobilities and rate constants for the excitation of the different molecular energy levels have been computed from electron collisional cross-sections. Gases covered in this way are CO, CO₂, H₂O, H₂O, N₂, and O₂ and these results are applied to the products of combustion of methane-air. Rates of ionization by electron collisions have been calculated for different values of electrical field. Comparisons are made of the different rates at which the separate energy levels of the different chemical species gain energy from the field. The influence of electrical fields on the combustion process is discussed on the basis of such data. An increased chemical rate of reaction may arise from a general ohmic heating of the gas and also from preferential excitation of certain molecular energy levels.     

Characterization of crystalline silicon grown by plasma-enhanced CVD for thin-film solar cells

High growth-rate Si epitaxy by plasma-enhanced chemical vapor deposition (PECVD) has been investigated for a thin-film solar cell application. A high growth rate of 50 μm/h was obtained at 1050°C with plasma which is 50% larger than that by the conventional CVD without plasma. The electrical properties are almost the same for epitaxial layers with and without plasma. For undoped n-type layers, the Hall mobility and carrier density were about 600 cm²/V s and low 10¹⁵ cm⁻³, respectively. The electron diffusion length in doped p-type layers was about 20 μm. These electrical properties for the layer with plasma, in spite of higher growth rate, are comparable or better than those without plasma.     

Uncertainty quantification of the premixed combustion characteristics of NH3/H2/N2 fuel blends

Green ammonia is a candidate fuel to decarbonise shipping and other industries. However, ammonia features a lower reactivity compared to conventional fuels and is therefore difficult to burn. To resolve this issue, thermo-catalytic cracking of ammonia using waste heat is often employed to produce NH₃/H₂/N₂ blends as fuel. However, on-site operational variations in this process can become sources of uncertainty in the fuel composition, causing randomness of the flame's physicochemical properties and challenging flame stability. In the present work, a surrogate model is built using the polynomial chaos expansion (PCE) method to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions. Impacts of 1.5% deviation in the fuel composition on the flame properties for different initial pressures (P_i) and unburnt fuel temperatures (T_u) are investigated for a wide range of equivalence ratios covering lean and rich mixtures. The uncertainty effects defined by the coefficient of variation (COV) fluctuate for equivalence ratios greater than 1.1, while no fluctuation is observed in COV for near stoichiometric combustion conditions. It is shown that H₂ variation in the fuel blend has the strongest effect (over 80%) on the uncertainty of all investigated physicochemical properties of the flame. The least affected property is the adiabatic flame temperature with variations of about 2.5% in richer fuel conditions. The results further show that preheating of the reactants can significantly reduce the COV of laminar flame speed. The consequences of these uncertainties upon different combustion technologies are then discussed and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.