The effect of fuel injection on NOx emissions and undesirable combustion for hydrogen-fuelled piston engines

In order to determine the potential of direct cylinder injection for hydrogen-fuelled engines, an experimental study was performed with an ASTM-CFR engine. Both the standard Otto head and the standard diesel head were used. Measurements were made of power output, thermal efficiency, and oxides of nitrogen emissions. The feasibility was investigated of a scheme in which injection of gaseous hydrogen starts late in the compression stroke, ignition occurs as soon as possible thereafter, and combustion rate is determined by injection rate. This scheme prevents undesirable combustion phenomena such as pre-ignition, high rates of cylinder pressure rise, and high amplitude pressure waves in the cylinder. Furthermore, it obviates flashback into the carburetor. The potential of hydrogen as a low pollution fuel was investigated by operating the Otto head engine on both hydrogen and indolene, and by comparing the resulting NO_x, emissions. Hydrogen yielded very low NO_x emissions provided the fuel-air equivalence ratio was less than 0.5, and provided the hydrogen and air were well mixed. For equivalence ratios greater than 0.5, hydrogen yielded NO_x emissions that were higher than those obtained with indolene. The timing of hydrogen injection was found to have a significant effect on NO_x emissions. With an unthrottled air intake and hydrogen injection at equivalence ratios between 0.3 and 0.8, indicated mean effective pressures ranged from 0.3 to 0.78 MPa. Corresponding indicated thermal efficiencies ranged from 43 to 31%. By decreasing the equivalence ratio to 0.1, the IMEP could be reduced to 0.07 MPa, thus providing an indicated load range of more than a factor of 10.     

Gas/particle flow and combustion characteristics and NOx emissions of a new swirl coal burner

Due to the limits of reserves and price for the high rank coal, the low rank coal has been employed as fuel for power generation in China and will be eventually employed in the world. To burn low rank coal, centrally fuel-rich swirl coal combustion burner has been studied in Harbin Institute of Technology. This paper reviews and analyzes the major research results. The work has included both experiments and numerical simulation. The experiments were conducted using small-scale single-phase experimental equipment, a gas/particle two-phase test facility and 200- and 300-MW_e wall-fired utility boilers. For the burner, the primary air and glass beads partially penetrate the central recirculation zone and are then deflected radially. At the center of the central recirculation zone, there is high particle volume flux and large particle size. For the burners the local mean CO concentrations, gas temperatures and temperature gradient are higher, and the mean concentrations of O₂ and NO_x in the jet flow direction in the burner region are lower. Moreover, the mean O₂ concentration is higher and the gas temperature and mean CO concentration are lower in the side wall region. Centrally fuel-rich burners have been successfully used in 200- and 300-MWe wall-fired pulverized coal utility boilers.     

Application of the Miller cycle to reduce NOx emissions from petrol engines

A conceptual analysis of the mechanism of the Miller cycle for reducing NO_x emissions is presented. Two versions of selected Miller cycle (1 and 2) were designed and realized on a Rover “K” series 16-valve twin-camshaft petrol engine. The test results showed that the application of the Miller cycle could reduce the NO_x emissions from the petrol engine. For Miller cycle 1, the least reduction rate of NO_x emission was 8% with an engine-power-loss of 1% at the engine’s full-load, compared with that of standard Otto cycle. For Miller cycle 2, the least reduction rate of NO_x emission was 46% with an engine-power-loss of 13% at the engine’s full-load, compared with that of standard Otto cycle.     

Combustion and NOx emissions characteristics of a down-fired 660-MWe utility boiler retro-fitted with air-surrounding-fuel concept

Air-surrounding-fuel is a well-known concept used within tangential and wall-fired boilers. Here, we report for the first time on industrial experiments performed to study the effects of this concept on a 660 MW_e full-scale down-fired boiler. Data are reported for the gas temperature distributions along the primary air and coal-mixed flows, furnace temperatures, gas compositions, for example O₂, CO and NO_x, and gas temperatures in the near-wall region. The influence of concentration control valve (CCV) opening on combustion and NO_x emission in the furnace were determined. The results show that the flame stability, temperature distribution, unburnt carbon are influenced by both concentration ratios and fuel-rich flow velocities. As CCV opening increases, NO_x emissions decrease from 2594 mg/m³ to 1895 mg/m³. Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening.     

Investigation of NOx conversion characteristics in a porous medium

The conversion of nitric oxide (using CNG/air as fuel/oxidizer) inside a porous medium is investigated in this study. Unlike freely propagating flames, porous burners provide a solid medium that facilitates heat exchange with the gaseous phase. The heat exchange allows the stabilization of a variety of fuel mixtures from lean to rich and with a variety of calorific values. In addition, it allows the control of the reaction zone temperature and thus the control of pollutant formation while maintaining flame stability. An experimental porous burner was designed and manufactured for this purpose. The effects of equivalence ratio and flow velocity on the flame stabilization, NO_x and TFN (total fixed nitrogen) conversion ratios, and temperature profiles along the burner are investigated. In addition, numerical calculations using the PLUG flow simulator model and the GRI 3.0 kinetic mechanism reveals the key reactions which control the conversion efficiency. It was found that under slightly fuel-rich conditions (φ?1.3) NO_x mostly converts to N₂ with a maximum conversion ratio of 65%, while for higher equivalence ratios (φ>1.3) a large proportion of NO_x converts to NH₃. Results from experiments and numerical modeling showed that the temperature profile along the burner has significant effects on the NO_x and TFN conversion ratios. It was also found that temperatures between 1000 and 1500 K are most desirable for NO_x and TFN conversion in the porous burner. Analysis of the chemical paths for the low- and high-equivalence-ratio cases showed that the formation of nitrogen-containing species under very rich conditions (φ>1.3) is due to the increased importance of the HCNO path as compared to the HNO path. The latter is the dominant path at low equivalence ratios (φ?1.3) and leads to the formation of N₂. The NO concentration in the initial mixture was found to improve the conversion by up to 20% at low equivalence ratios (φ?1.3) and to have negligible effect at higher equivalence ratios.     

Sensitizing effects of NOx on CH4 oxidation at high pressure

The CH₄/O₂/NO_x system is investigated in a laboratory-scale high pressure laminar flow reactor with the purpose of elucidating the sensitizing effects of NO_x on CH₄ oxidation at high pressures and medium temperatures. Experiments are conducted at 100, 50, and 20 bar, 600-900 K, and stoichiometric ratios ranging from highly reducing to oxidizing conditions. The experimental results are interpreted in terms of a detailed kinetic model drawn from previous work of the authors, including an updated reaction subset for the direct interactions of NO_x and C_1-2 hydrocarbon species relevant to the investigated conditions. The results reveal a significant decrease in the initiation temperature upon addition of NO_x. A similar effect is observed with increasing pressure. The sensitizing effect of NO_x is related to the hydrocarbon chain-propagating NO/NO₂ cycle operated by NO₂+CH₃?NO+CH₃O and NO+CH₃OO?NO₂+CH₃O as well as the formation of chain-initiating OH radicals from interactions between NO/NO₂ and the H/O radical pool. At low temperatures, reactions between NO/NO₂ and CH₃O/CH₂O also gain importance. The results indicate a considerable intermediate formation of nitromethane (CH₃NO₂) as a characteristic high-pressure phenomenon. The formation of CH₃NO₂ represents an inactivation of NO_x, which may result in a temporary reduction of the overall hydrocarbon conversion rate.     

Predicting the performance and NOx emissions of a turbocharged spark-ignition engine generator fueled with biogases and hydrogen addition under down-boosting condition

The characteristics of an engine generator fueled with methane-based biogas and its blend with hydrogen (H₂) were numerically investigated using a one-dimensional cycle simulation and the fractional factorial design of experiment method. Based on the experimental results, the numerical model was validated and calibrated under various excess air ratio (EAR), boost pressure, spark timing, and biogas composition conditions. The conventional and current optimization methods, maximum brake torque (MBT), and minimum input fuel (MIF) conditions, were compared in terms of engine performance, NO_x emission, and generating efficiency. Under MBT conditions, although higher torque and mass fraction burned (MFB) could be achieved, boosting degradation was evident at the optimum MBT timing. On the contrary, under MIF conditions with down-boosting, reduced NO_x formation and improved generating efficiency could be achieved, but was accompanied by an MFB decrease. Moreover, the reduced MFB could be enhanced by H₂ addition and spark-timing control. Compared to MBT, MIF optimization had advantages in terms of generating efficiency, NO_x emission reduction, and spark-timing control, and could avoid boosting degradation over the entire operating range for various fuel compositions.     

Global observation of EKC hypothesis for CO2, SOx and NOx emission: A policy understanding for climate change mitigation in Bangladesh

Environmental Kuznets Curve (EKC) hypothesis is critical to understanding the developmental path of a nation in relation to its environment. How the effects of economic development processes dictate environmental changes can be found through the study of EKC. To understand the EKC phenomena for climate change, this study was undertaken by reviewing the available literature. As CO₂, SO_x and NO_x are the significant greenhouse gases (GHG) responsible for global warming, thus leading to climate change, the study focused on those GHGs for EKC consideration. With an understanding of the different EKC trajectories, an attempt was made to determine the implications for the economic development of Bangladesh in regards to the EKC. It was shown that EKC for CO₂ was following a monotonous straight line in most cases. SO_x were shown to follow the full trajectory of the EKC in most situations and NO_x was shown the hope only for the developed countries getting the low-income turning point. The type of economic policy that Bangladesh should follow in regards to the discussed pollutants and sources is also revealed. From these discussions, contributions to policy stimulation in Bangladesh are likely to be made.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

A detailed numerical study of NOx kinetics in low calorific value H2/CO syngas flames

The present study provides an extensive and detailed numerical analysis of NO_x chemical kinetics in low calorific value H₂/CO syngas flames utilizing predictions by five chemical kinetic mechanisms available out of which four deal with H₂/CO while the fifth mechanism (GRI 3.0) additionally accounts for hydrocarbon chemistry. Comparison of predicted axial NO profiles in premixed flat flames with measurements at 1 bar, 3.05 bar and 9.15 bar shows considerably large quantitative differences among the various mechanisms. However, at each pressure, the quantitative reaction path diagrams show similar NO formation pathways for most of the mechanisms. Interestingly, in counterflow diffusion flames, the quantitative reaction path diagrams and sensitivity analyses using the various mechanisms reveal major differences in the NO formation pathways and reaction rates of important reactions. The NNH and N₂O intermediate pathways are found to be the major contributors for NO formation in all the reaction mechanisms except GRI 3.0 in syngas diffusion flames. The GRI 3.0 mechanism is observed to predict prompt NO pathway as the major contributing pathway to NO formation. This is attributed to prediction of a large concentration of CH radical by the GRI 3.0 as opposed to a relatively negligible value predicted by all other mechanisms. Also, the back-conversion of NNH into N₂O at lower pressures (2–4 bar) was uniquely observed for one of the five mechanisms. The net reaction rates and peak flame temperatures are used to correlate and explain the differences observed in the peak [NO] at different pressures. This study identifies key reactions needing assessment and also highlights the need for experimental data in syngas diffusion flames in order to assess and optimize H₂/CO and nitrogen chemistry.     

Post combustion CO2 capture by carbon fibre monolithic adsorbents

As generation of carbon dioxide (CO₂) greenhouse gas is inherent in the combustion of fossil fuels, effective capture of CO₂ from industrial and commercial operations is viewed as an important strategy which has the potential to achieve a significant reduction in atmospheric CO₂ levels. At present, there are three basic capture methods, i.e. post combustion capture, pre-combustion capture and oxy-fuel combustion. In pre-combustion, the fossil fuel is reacted with air or oxygen and is partially oxidized to form CO and H₂. Then it is reacted with steam to produce a mixture of CO₂ and more H₂. The H₂ can be used as fuel and the carbon dioxide is removed before combustion takes place. Oxy-combustion is when oxygen is used for combustion instead of air, which results in a flue gas that consists mainly of pure CO₂ and is potentially suitable for storage. In post combustion capture, CO₂ is captured from the flue gas obtained after the combustion of fossil fuel. The post combustion capture (PCC) method eliminates the need for substantial modifications to existing combustion processes and facilities; hence, it provides a means for near-term CO₂ capture for new and existing stationary fossil fuel-fired power plants.This paper briefly reviews CO₂ capture methods, classifies existing and emerging post combustion CO₂ capture technologies and compares their features. The paper goes on to investigate relevant studies on carbon fibre composite adsorbents for CO₂ capture, and discusses fabrication parameters of the adsorbents and their CO₂ adsorption performance in detail. The paper then addresses possible future system configurations of this process for commercial applications.Finally, while there are many inherent attractive features of flow-through channelled carbon fibre monolithic adsorbents with very high CO₂ adsorption capabilities, further work is required for them to be fully evaluated for their potential for large scale CO₂ capture from fossil fuel-fired power stations.     

Effect of ignition timing retard strategy on NOx reduction in hydrogen-compressed natural gas blend engine with increased compression ratio

Hydrogen-compressed natural gas blend (HCNG) engines can extend the lean burn limit because of the wide flammability range of hydrogen. Lean combustion helps facilitate high efficiency and fundamentally reduces NO_x emission. Increasing the compression ratio (CR) of an HCNG engine was reported to improve its thermal efficiency. However, the high risk of knock occurrence and the increase in NO_x emission can hinder CR increase.     

An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C₁-C₆, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NO_x and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NO_x and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N₂, O₂, n-C₇H₁₆, CO₂, CO, H₂), and reasonably good agreement for intermediate (CH₄, C₂H₄, C₂H₂, C₃H_x) and higher hydrocarbon species (C₄H₈, C₄H₆, C₄H₄, C₄H₂, C₅H₁₀, C₆H₁₂) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ?<5.0, characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ?>5.0. The NO_x and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NO_x emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.     

Structural and magnetic properties of Lix(MnyFe1−y)PO4 electrode materials for Li-ion batteries

A series of LiMn_yFe_1−yPO₄ samples have been prepared in the whole range 0 ≤ y ≤ 1. Chemical delithiation could be achieved to obtain Mn_yFe_1−yPO₄ in the range 0 ≤ y ≤ 0.8, keeping the same crystal phase (olivine structure, space group Pnma). The composition y = 0.8 is the limit where the delithiated phase is still crystallized, but abruptly suffers strains at the molecular scale evidenced by both optical spectroscopy and X-ray diffraction. The analysis of the magnetic properties shows that in all the samples the concentration of impurities is negligible. The concentration of polarons, either holes associated to Li vacancies in LiMn_yFe_1−yPO₄ or electrons associated to the existence of Li left in the matrix of Mn_yFe_1−yPO₄, is found to be small (≤1%) in all the samples. For y ≤ 0.6, all the Mn³⁺ ions Mn_yFe_1−yPO₄ are in the high-spin state (S = 2). At larger manganese concentration, however, the Mn³⁺ ions in excess of the critical concentration y_c = 0.6 undergo a transition to the low-spin state (S = 1). As a consequence, and in contrast with prior works, we find that Mn_0.8Fe_0.2PO₄ has magnetic interactions that are much smaller, and no antiferromagnetic ordering in this compound is detected, at least above 20 K. Antiferromagnetic ordering that had been reported so far for Mn_yFe_1−yPO₄ at large y-composition might come from incomplete delithiation. The spin-transition of Mn³⁺ in concentration (y–y_c) to the low-spin state is at the origin of the strain fields at the molecular scale that increase with y for y > 0.6, and ultimately prevents the full delithiation for y > 0.8. This result sheds light on the reason for the degradation of cathode properties in Mn-rich compounds of the heterosite–purpurite series, while the electrochemical properties are good in the range y ≤ 0.6 but only at slow rates, due to the very small hopping mobility of the small polaron.     

Suitability analysis of advanced diesel combustion concepts for emissions and noise control

During the last years, the preservation of the atmospheric environment has played an increasingly important role in society. The Diesel engine can be considered an environmentally friendly engine because of its low consumption and the subsequent carbon dioxide (CO₂) emissions reduction. However, in the near future it will face strong restrictive emission standards, which demand that the current nitrogen oxides (NO_x) and soot emissions are halved. To comply with these restrictions new combustion concepts are emerging, such as PCCI (premixed charge compression ignition), in which the fuel burns in premixed conditions. Combustion noise is thus deteriorated and consequently end-users could be reluctant to drive vehicles powered with Diesel engines and their potential for environment preservation could be missed. In this paper, Diesel combustion is addressed through the analysis of performance, emissions and combustion noise in order to evaluate the suitability of PCCI engines for automotive applications. The results show that PCCI combustion offers great possibilities to fulfill future emission restrictions, but the engine noise is strongly deteriorated. The great sensitivity of users to this factor requires vehicle manufacturers to focus their efforts on the optimization of passive solutions for implementing the PCCI concept in passenger car and light-duty engines, even with the subsequent increase in the cost of vehicle. This aspect is less restrictive in heavy-duty engines, since the great benefits in emissions reduction compensate the deterioration of engine noise.     

Experimental investigation of Cu(In1−x,Gax)Se2/Zn(O1−z,Sz) solar cell performance

In this study we investigate the performance of Cu(In_1−x,Ga_x)Se₂/Zn(O_1−z,S_z) solar cells by changing the gallium content of the absorber layer in steps from CuInSe₂ to CuGaSe₂ and at each step vary the sulfur content of the Zn(O,S) buffer layer. By incorporating more or less sulfur into the Zn(O,S) buffer layer it is possible to change its morphology and band gap energy. Surprisingly, the best solar cells with Zn(O,S) buffer layers in this study are found for close to or the same Zn(O,S) buffer layer composition for all absorber Ga compositions. In comparison to their CdS references the best solar cells with Zn(O,S) buffer layers have slightly lower open circuit voltage, V_oc, lower fill factor, FF, and higher short circuit current density, J_sc, which result in comparable or slightly lower conversion efficiencies. The exception to this trend is the CuGaSe₂ solar cells, where the best devices with Zn(O,S) have substantially lowered efficiency compared with the CdS reference, because of lower V_oc, FF and J_sc. X-ray photon spectroscopy and X-ray diffraction measurements show that the best Zn(O,S) buffer layers have similar properties independent of the Ga content. In addition, energy dispersive spectroscopy scans in a transmission electron microscope show evidence of lateral variations in the Zn(O,S) buffer layer composition at the absorber/buffer layer interface. Finally, a hypothesis based on the results of the buffer layer analysis is suggested in order to explain the solar cell parameters.