Ultra-low NOx Research Articles

Comparative analysis on pulverized coal combustion preheated by self-sustained purifying burner with coaxial and centrosymmetric air nozzle structures: Purification, combustion and NOx emission characteristics

To optimize secondary air nozzle structure in purifying burner, this study focused on the comparison of purification, combustion and NOx emission characteristics of pulverized coal preheated by a 30 kW purifying burner with coaxial and centrosymmetric structures. Centrosymmetric structure shifted the position of main burning region down in high-temperature reduction unit (HTRU), and the number of branches differently influenced the temperature in different regions with this structure. For reductive gas components, CO concentration with centrosymmetric structure was higher compared to coaxial structure, while the differences in H2 and CH4 concentrations were smaller. Centrosymmetric structure was more disadvantageous to improve physicochemical properties of pulverized coal compared to coaxial structure, and this structure with four branches further deteriorated its properties compared to two branches. In mild combustion unit (MCU), the temperature at top was lower with centrosymmetric structure, while was higher in the rest. Centrosymmetric structure more effectively reduced NOx emission compared to coaxial structure, but with slight sacrifice of combustion efficiency (η). Moreover, both two-branch and four-branch centrosymmetric structures realized ultra-low NOx emission (<50 mg·m–3) with high η of over 98.50%, and the former was more advantageous. With this optimal structure, η and NOx emission were 99.25% and 40.42 mg·m–3.

Experimental research on wide load operation characteristics of circulating fluidized bed under the action of high-temperature activated gas-solid binary fuels

The circulating fluidized bed (CFB) units, with their robust combustion stability under low load conditions, are poised to play a significant role in future deep peak shaving of power grids. However, issues such as NOx emissions exceedance arise when the CFB unit operates for deep peak shaving. To mitigate these problems, this paper innovatively proposes a CFB coal blended with the high-temperature activated gas-solid binary fuel combustion mode to reduce the NOx emissions of the CFB across a wide load range. The wide load operation characteristics of the CFB under the action of high-temperature activated gas-solid binary fuel were initially examined on a 2 MW experimental platform. A comparison between the conventional combustion mode and the CFB coal blended with high-temperature activated gas-solid binary fuel combustion mode revealed that the latter improved the uniformity of temperature distribution within the CFB across a wide load range, particularly at lower loads. Furthermore, the CFB coal blended with high-temperature activated gas-solid binary fuel significantly reduced the original NOx emissions, decreasing from 136.9 mg/m3, 37.5 mg/m3, and 38.8 mg/m3 to 92.0 mg/m3, 24.7 mg/m3 and 20.6 mg/m3 at 100 %, 50 %, and 30 % load respectively, thereby achieving ultralow original NOx emissions during deep peak shaving operations. However, there is room for improvement in enhancing combustion efficiency using this combustion mode.

Numerical investigation of NO generation reaction pathway with novel embedded flue-gas internal recirculation combustion loop consisting of flue-gas and CH4-air mixture

The novel embedded flue-gas internal recirculation (FIR) combustion technology has garnered increasing attention in gas burners due to its notable advantages in combustion efficiency and ultra-low NOx emissions. In this study, we design FIR channels for a gas burner and investigate the impact of recirculation ratio (R) on temperature, combustion efficiency, concentrations of free radicals (H, O, CH), and NO concentration via Computational Fluid Dynamics (CFD) method. In addition, this paper explores and elucidates the dynamics of four NO production pathways, namely, thermal NO, prompt NO, N2O, and NNH, as well as the contributions of the four pathways to total NO. Observations indicate a constrained range of FIR recirculation ratios, specifically between 23.2 % and 35 %. The combustion efficiency of the FIR burner is 96 % and 99.8 % at R = 23.8 % and R = 35.0 %, which is superior to conventional burners. Moreover, as R increases, both pathways for NO generation from N2O and NNH are intensified as R exceeds 23.8 %. In the embedded FIR combustion system, the contribution of N2O to total NO increases from 0.23 % to 0.78 %, while the percentage of prompt NO decreases to 0.19 %. These findings provide a development direction and technical guidance for the practical implementation of the embedded FIR technology.

Dissecting of Atomic Morphology of Superfine Pulverized Coal Based on X-ray Pair Distribution Function

Resolving the atomic structure information of the aromatic layers in coal plays a crucial role in understanding the generation mechanisms of NOx during coal combustion and further reducing the formation of NOx from the source. This study reveals the distribution of X-ray diffraction bands of superfine pulverized coal using a high-resolution synchrotron radiation X-ray Diffraction (HRXRD) facility, discussing the distribution of atomic distances and atomic density in aromatic layers through pair distribution function (PDF) methods. Furthermore, the influences of mechanochemistry on the evolution of atomic morphology are focused on. The results show that the PDF of coal gradually stabilizes when r &amp;gt; 8 Å, showing the short-range order of graphite-like structure. Additionally, due to the limitations of scanning angle and X-ray energy, atomic distances in aromatic layers for coal are significantly greater than that of pure graphene. Enhanced mechanochemical effects make the peaks 1, 2, and 3 of coal PDF more similar to graphene&amp;apos;s by condensing alkyl side chains into smaller, regular aromatic layers when the particle size decreases. With the enhancement of mechanochemical effects, coals with different metamorphic degrees exhibit different aromatic evolution patterns. The aromaticity of NMG coal first decreases and then increases, while the aromaticity of YQ coal shows the opposite trend. The results can provide deeper insights into the atomic structure of coal macromolecular, which can facilitate the advancement of novel ultra-low NOx combustion methods and support the construction of precise coal macromolecular models.

Novel ultra-low NOx coal combustion technologies based on local microenvironment targeted regulation. Part 1. Selective oxygenation

Developing a novel high-efficiency coal combustion technology with ultra-low NOx emission is urgently needed to sustain the good ecological environment. Here, the targeted regulation of local microenvironment around fuel-N, such as functional groups, radicals, molecular configurations, and reaction atmosphere, is realized by the selective oxidation. The molecular configurations, including pore networks and microcrystalline structure in coal, are well characterized through synchrotron radiation-induced SAXS (small angle X-ray scattering) and WAXS (wide angle X-ray scattering) simultaneously. Furthermore, by combining density functional theory (DFT) and experiments, the effects of the local microenvironment on the nitrogen transformation and NO evolution during the thermal conversion are focused on. The results indicate that for the PPA oxidation, the H radicals attack the adjacent carbon to pyrrole/pyridine nitrogen, promoting the conversion of fuel-N to HCN. On the other hand, for the H2O2 oxidation, disrupting the π bond electron cloud by the CO and C = O on the ortho carbon of pyrrole/pyridine dominates the NH3 generation. Additionally, the increased La (average graphene layer extent), a3 (average interlayer spacing), σ3 (standard deviation of interlayer spacing) and σ1 (standard deviation of the first-neighbor distribution) induce massive smaller pores, promoting the generation of abundant reaction defects inside the particles. Importantly, the intensified adsorption on abundant active sites lead to the decreased HCN and increased NH3 evolution, which is adverse for the interaction between homogeneous and heterogeneous NO reduction. Interestingly, the PAA selective oxidation can reduce NO emission by 31.72 % - 34.30 % during the air combustion, which is far better than the H2O2 oxidation. Overall, the attack of free radicals on nitrogen-containing heterocycles promotes the conversion of fuel-N to HCN, the adsorption of which on char surfaces can further enhance the heterogeneous reduction in a lean oxygen atmosphere. The work here provides a novel route for developing high-efficiency and low-NOx combustion technologies.

Experimental and Simulation Study on Ultra-Low NOx Emissions of Anthracite by Preheated Combustion

ABSTRACT In this study, the ultra-low NOx emission of anthracite was achieved by the preheated combustion technology. Meanwhile, the preheated combustion model was established using Aspen Plus. High-temperature preheating (above 950°C) improved the conversion process from fuel-nitrogen to N2 and precursor of NOx, which involved the cracking of N-5 and N-6. The conversion rate from fuel-nitrogen to N2 was 81.88%. In addition, heterogeneous reduction reaction of NOx was the critical conversion path of fuel-nitrogen for the ultra-low NOx emission during the combustion. The conversion rate from NOx to N2 was 0.43%. The average NOx emission of tail flue gas was 40.77 mg/m3 and 46.15 mg/m3 (@6%O2) for Jincheng anthracite and Yangquan anthracite, respectively. The simulation results showed that the preheated combustion model demonstrated precise prediction capabilities for containing-nitrogen gas emission. The maximum error of NOx emission and N2 concentration were 7.71% and 3.37%, respectively. The simulation and experimental results could be mutually verified.

Catalytic NO&lt;sub&gt;x&lt;/sub&gt; Aftertreatment—Towards Ultra-Low NO&lt;sub&gt;x&lt;/sub&gt; Mobility

Article Catalytic NOx Aftertreatment—Towards Ultra-Low NOx Mobility Navjot Sandhu * , Xiao Yu, and Ming Zheng Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada * Correspondence: sandh12p@uwindsor.ca Received: 26 January 2024 Accepted: 13 March 2024 Published: 20 March 2024 Abstract: The push for environmental protection and sustainability has led to strict emission regulations for automotive manufacturers as evident in EURO VII and EPA2027 requirements. The challenge lies in maintaining fuel efficiency and simultaneously reducing the carbon footprint while meeting future emission regulations. Nitrogen oxides represent one of the major and most regulated components of automotive emissions. The need to meet the stringent requirements regarding NOx emissions in both SI and CI engines has led to the development of a range of in-cylinder strategies and after-treatment techniques. In-cylinder NOx control strategies including charge dilution (fresh air and EGR), low-temperature combustion, and use of alternative fuels (as drop-in replacements or dual fuel operation) have proven to be highly effective in thermal NOx abatement. Aftertreatment methods are required to further reduce NOx emissions. Current catalytic aftertreatment systems for NOx mitigation in SI and CI engines include the three-way catalyst (TWC), selective catalytic reduction (SCR) and lean NOx trap (LNT). This review summarizes various approaches to NOx abatement in IC engines using aftertreatment catalysts. The mechanism, composition, operation parameters and recent advances in each after-treatment system are discussed in detail. The challenges to the current after-treatment scenario, such as cold start light off, catalyst poisoning and the limits of current aftertreatment solutions in relevance to the EURO VII and 2026 EPA requirements are highlighted. Lastly, recommendations are made for future aftertreatment systems to achieve ultra-low NOx emissions.

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

Abstract The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the short helical combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultralow NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In this work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the large eddy simulation method, the shape and liftoff height of the flame are adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected toward the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. This study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications.

New insights into the heterogeneous reduction of NO on coal char based on molecular configuration evolution

No report reveals the relationship between coal char molecular configuration and NO heterogeneous reduction. Here, the molecular configuration of char, including pore structure and microcrystalline morphology, was well characterized through synchrotron radiation-based SAXS and WAXS. Furthermore, the universal relationship between molecular configuration and NO heterogeneous reduction was revealed. The adsorption configurations of NO on different defects were calculated through density functional theory (DFT). The results indicate that smaller pores are affected by both monolayer structure and layer stack characteristics, while larger pores are primarily dominated by the latter. The NO decline rate and final reduction efficiency of large higher-rank chars are driven by NO diffusion in larger pores. On the other hand, the active sites and flow patterns in smaller pores synergistically control the decline rate and reduction efficiency of small higher-rank chars. As for lower-rank chars, the diffusion in larger pores and the flow patterns in smaller pores affect the reduction efficiency and decreasing rate of NO, respectively. It is important to note that edge defects control the initial stage of NO reduction, while the single-vacancy and double-vacancy defects dominate the heterogeneous process in the final NO reduction stage. The findings can provide theoretical support for better predicting the NO heterogeneous behavior and thus promote the development of novel ultra-low NOx combustion technologies.

On the Demonstration of a Humid Combustion System Performing Flexible Fuel-Switch From Pure Hydrogen to Natural Gas With Ultra-Low NOx Emissions

Abstract To fight global warming the European Union formulated the objective of completely decarbonizing the energy sector, stimulating the advent of unconventional fuels and the adaption of the corresponding energy infrastructure. The decarbonization strategy identified hydrogen to play a key role as an energy storage medium, making systems capable of pure hydrogen operation essential. This requirement can be fulfilled with humid power cycles which offer additional advantages such as highly efficient and fuel-flexible operation with low emissions. As an integral part of such a cycle, a humid combustion system was presented previously showing promising results with respect to complete combustion with low emissions for a variety of fuels. The current work introduces an upgraded version of that combustion system. The new Double Swirler system is capable of stable and safe combustion of low calorific value biosyngas surrogate, hydrogen, and natural gas, from dry to steam-rich conditions within the required pressure drops. The inclusion of dry operation of the system can benefit the startup procedure of the humid cycle. The combustor's fuel switching performance is demonstrated by a fast fuel switch at full load from pure hydrogen to pure natural gas and vice versa, while maintaining a stable performance with low NOx-emissions at otherwise constant operation parameters.

Transient and altitude performance analysis of hydrogen fuelled internal combustion engines with different charging concepts

In the internal combustion engine industry, there is a significant shift towards alternative fuels to accomplish zero net carbon dioxide emissions as well as the legislation requirements. According to former studies, in addition to its advantages of being carbon-free and having appropriate combustion characteristics, low nitrogen oxide engine-out emission levels are achieved at lean burn combustion of hydrogen. Lean combustion leads to a turbocharging system requirement. Which is then utilized to generate the expected power output at high altitudes and during transient maneuvers.This study aims to determine the ideal air-charging vessel which can meet the objectives of high altitude. Which is 0 % torque derate and transient response time with respect to 0 %–90 % of maximum torque which is typically ∼3–4 s for diesel engines, while keeping the lean combustion to release ultra-low NOX emissions. Different charging concepts have been investigated by applying 1-D thermodynamic modeling on a H2ICE with a direct injection system which is for heavy-duty vehicles.

Injection strategy of methanol for high loads and low NOx emissions in a neat methanol LTC engine

Methanol is a low carbon and clean burning renewable fuel. Though low temperature combustion (LTC) of neat methanol results in almost no smoke with ultra-low NOx, the extremely low cetane number poses challenges. This experimental study demonstrates LTC of neat methanol at full load by combining direct and port injection of this fuel with controlled intake air temperature (IAT). The effects of important variables including IAT and port (premixed) and direct-injected methanol shares on combustion, performance, regulated and unregulated emissions were investigated. The timing of the direct-injected methanol had to be retarded with an increase in the premixed methanol share and IAT in order to limit the pressure rise rate without affecting the ITE. At high IATs, the premixed methanol burned first, followed by the directly injected methanol, at low IATs, these phases merged, resulting in a high rate of heat release. Employing the lowest possible IAT and the highest possible premixed methanol share led to high ITE. Increase in the MDIP to 450 bar was advantageous due to better fuel evaporation and enhanced combustion. Negligible smoke, very low NOx of 2.7 g/kWh, high ITE of 45% and unburned HC and methanol emissions lesser than 3 g/kWh could be achieved.

CPFD modeling of hydrodynamics, combustion and NOx emissions in an industrial CFB boiler

The ultra-low NOx emission requirement (50 mg/m3) brings great challenge to CFB boilers in China. To further tap the NOx abatement potential, full understanding the fundamentals behind CFB boilers is needed. To achieve this, a comprehensive CPFD model is established and verified; gas-solid flow, combustion, and NOx emission behavior in an industrial CFB boiler are elaborated; influences of primary air volume and coal particle size on furnace performance are evaluated. Simulation results indicate that there exists a typical core-annular flow structure in the boiler furnace. Furnace temperature is highest in the bottom dense-phase zone (about 950 °C) and decreases gradually along the furnace height. Oxygen-deficient combustion results in high CO concentration and strong reducing atmosphere in the lower furnace. NOx concentration gradually increases in the bottom furnace, reaches maximum at the elevation of secondary air inlet, and then decreases slightly in the upper furnace. Appropriate decreasing the primary air volume and coal particle size would increase the CO concentration and intensify the in-furnace reducing atmosphere, which favors for NOx reduction and low NOx emission from CFB boilers.

Evaluation of thermal management technologies for fuel efficient cold-start emissions reduction in diesel engines

The ultra-low NOx emission limits for heavy and mid-duty vehicles, which are expected to be imposed in both Europe and the USA by 2027, together with the enforcement of continuous decrease in CO2 emission levels, make the need for adoption of newer technologies in the automotive industry imperative. To achieve low tailpipe NOx emissions, the accelerated warm-up of the exhaust aftertreatment system is of great importance. On the other hand, the thermal management strategies applied for this purpose result in fuel consumption penalty and consequently higher CO2 as it can be widely found in the literature. Thus, the use and optimization of advanced thermal management technologies is critical to decrease the NOx– fuel penalty trade-off on the basis of complete driving cycles. In the present work an integrated modeled-based approach is implemented by coupling advanced, predictive 1D engine, and aftertreatment simulation models which are extensively validated via dedicated experimental data. The main goal of this approach is to define and investigate multiple engine or aftertreatment-based thermal management technologies, as well as a combination of them, creating a more efficient warm-up mode for the engine. Thermal measures such as cylinder deactivation, retarded start of the main injection, late intake valve closing (Miller cycle), intake throttling, elevated idle speed, and secondary fuel injection upstream the DOC are evaluated for their effect on the exhaust system heat-up time, the fuel consumption penalty and more importantly, for their impact on the tailpipe NOx emissions calculated by this holistic simulation approach.

Combined DFT and Machine Learning Study of the Dissociation and Migration of H in Pyrrole Derivatives.

Systematic DFT calculations of model coal-pyrrole derivatives substituted by different functional groups are carried out. The N-H bond dissociation energies (N-H BDEs) and H-transfer activation energies (H-TAEs) of pyrrole derivatives are fully evaluated to elucidate the effect of the type of substituents and their position on the molecular reactivity. The results indicate that compounds substituted with electron-donating groups (EDGs) are more prone to pyrolysis while those substituted with electron-withdrawing groups (EWGs) are difficult to pyrolyze. Furthermore, quantitative structure-property relationship (QSPR) models for N-H BDEs and H-TAEs about pyrrole derivatives are built with multiple linear regression (MLR) and support vector machine (SVM). The final results show that the SVM-QSPR model has better fitness, prediction, and robustness, while the MLR-QSPR model can express the physical meaning better. The effects of functional groups on pyrolysis are clarified by the models presented in this paper, which will support the optimization of ultra-low NOx combustion.

Effects of hydrogen and EGR on energy efficiency improvement with ultra low emissions in a common rail direct injection compression ignition engine fueled with dimethyl ether (DME) under HCCI mode

This study deals with enhancing the energy efficiency with ultra-low emissions and extending the operating range (maximum load limit) of dimethyl ether (DME) fueled homogeneous charge compression ignition (HCCI) engine using hydrogen and exhaust gas recirculation (EGR). In the neat DME mode, the possible maximum load limit with controlled auto-ignition (CAI) was found as 24% (BMEP of 1.67 bar). Beyond 24% load, the knocking was observed with an advanced combustion phase, and shortened combustion duration resulted in uncontrolled auto-ignition (UAI) combustion. The addition of EGR extended the maximum load limit to 52% (BMEP of 3.62 bar) (60% EGR). Near-zero levels of NOx, zero smoke, and marginal CO and HC emissions reduction were observed with the optimized EGR rate (35%). The hydrogen fuel addition along with DME at an optimized EGR rate resulted in combustion phase retardation with the elimination of the low-temperature reaction (LTR) region. The energy efficiency (EE) and indicated thermal efficiency (ITE) increased along with zero smoke emission, ultra-low NOx, CO, and HC, and lower CO2 emissions till 12% of hydrogen energy share (HES). Thus, the combined strategy of EGR and hydrogen could effectively extend the operating range with ultra-low emissions levels.

Numerical study of the effects of excess air ratio on passive pre-chamber jet performance and ignition mechanism

Pre-chamber (PC) ignition is a promising lean-burn technology to improve the efficiency of spark ignition (SI) engines while achieving ultra-low NOx emissions. However, the significant effects of the excess air ratio (λ) on the PC jet performance and ignition mechanism were still not fully unraveled. The current study numerically investigated the jet characteristics and combustion behaviors of a narrow-throat PC with two rows of orifices on a passive PC engine fueled with methane. The numerical model was validated extensively by the pressure, heat release rate, and natural flame luminosity (NFL) imaging of the PC flame jet. A detailed comparison of the NFL images of the jets and the simulated flame iso-faces presented excellent matching in the flame jet penetration and propagation process. The results reveal that the λ = 0.9 case has the best jet performance, but it doesn’t lead in every jet characteristic. For instance, the jet temperature is highest under λ = 1.0, and OH concentration is most under λ = 1.1. The transition from cold jet to flame jet produces noticeable jet non-uniformity, which is reported for the first time. This observation can be remarked as a more accurate indication of the initiation of pre-chamber flame jet discharge. The asymmetrical mixture inflow from the main chamber produces asymmetrical turbulent kinetic energy (TKE) and temperature distributions; these result in different flame propagation in the PC, jet non-uniformity, and varied stages of jet discharge. The upper jets are quenched in every case because of the much higher TKE, while the lower jets don’t present quenching when the λ ranges from 0.9 to 1.3; especially, the λ = 1.1 case has the best ignition performance for its most OH radicals. The “jet ignition regime”, in which flame quenching takes place, results in highly non-uniform flame propagation and low flame propagation speed. The ultra-rich case shows more signs of jet ignition and thus exhibits worse jet performance than the lean cases. This study uncovers the mechanisms and the optimal λ value for the pre-chamber ignition of the main chamber charge, providing valuable guidance for PC design.

Experimental study on pulverized coal swirl-opposed combustion preheated by a circulating fluidized bed. Part A. Wide-load operation and low-NOx emission characteristics

In this study, the operating characteristics and NOx emission patterns are explored under a wide load range based on a 2MWth pulverized coal preheated combustion test platform. Preheated char with high physical sensible heat and preheated gas with high calorific value greatly improve the ignition and stability of the combustion characteristics of pulverized coal at low loads. Moreover, performing pre-denitrification during the preheating process helps to lower NOx emissions. Stable operation at a wide load range of 26%–106% was achieved, and ultralow loads of 40% and 26% were achieved when the pre-circulating fluidized bed (PCFB) was operated in double-/single-sided operation mode, respectively. Meanwhile, uniform concentration and temperature fields were achieved when both/one side of the PCFB were/was in operation. When operating on the double side of the PCFB, ultralow NOx emissions (@6% O2) of 39.9 mg/m3 and 43.7 mg/m3 were achieved at 40% and 50% loads, respectively. When operating on a single side, the NOx emission amount was 28.3 mg/m3 at 26% load. Furthermore, when the swirling air and the bottom air are equally distributed, the secondary air equivalent ratio is appropriately increased, and the injection position of the burnout air is properly delayed, NOx emissions are reduced.

Dimethyl Ether to Power Next-Generation Road Transportation

Review Dimethyl Ether to Power Next-Generation Road Transportation Simon LeBlanc , Xiao Yu , Linyan Wang , and Ming Zheng , * Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, N9B 3P4 Ontario, Canada * Correspondence: mzheng@uwindsor.ca Received: 20 March 2023 Accepted: 8 June 2023 Published: 19 June 2023 Abstract: The prevailing transportation uses internal combustion engines powered by fossil fuels that bear the reputation of carbon dioxide release among other harmful emissions. As an alternative, dimethyl ether (DME) has shown a high potential to mitigate emission challenges. The properties of DME present a highly reactive and volatile fuel suitable for clean combustion. However, the onboard handling of liquified DME is an ongoing challenge, especially for high-pressure direct injection applications. This paper aims to evaluate the sustainability, fuel handling, and combustion characteristics of DME as a clean and efficient fuel for sustainable on-road transportation. Strategies toward integrating DME fuel for automotive applications are emphasized. An overview of DME production is provided with relevance to current industry practices. Thereafter, the chemical and physical properties of DME are highlighted. The handling challenges of DME are accentuated, and accordingly, recommendations are made for setting up fuel management systems applicable to on-road engines and research laboratories. The DME fueling configurations, e.g., port injection and direct injection, are summarized. Empirical tests studied the engine and emission performance of DME combustion. Ultra-low NOx and smoke emissions, with high combustion efficiency, are achieved.

Experimental and numerical study of MILD combustion characteristics in the long-narrow confined space with different types of flue gas dilution

MILD combustion is extensively applied in wide confined spaces as a new ultralow NOx emission technology but is rarely reported in long-narrow confined spaces. To address this, as exemplified by a coke oven heat flue, a flue gas dual-dilution system in a long-narrow confined space was constructed to achieve MILD combustion, where combustion characteristics such as temperature distribution, flow field, flame morphology and NOx emission were assessed through experiments and numerical simulations. The results indicate that when the dilution rate reaches 25%, single-fuel dilution can achieve unstable MILD combustion, while single-air dilution is still in diffusion combustion, and the corresponding NOx emissions are reduced to 371 ppm and 325 ppm, respectively. When the fuel/air dilution ratio is 10/15 in flue gas dual-dilution, the flame front disappears completely, the flame volume ratio reaches 41.9%, and MILD combustion is established. The corresponding NOx emission is reduced to 58 ppm, which is 95.0% lower than conventional combustion and achieves ultralow NOx emission. An in-depth analysis proves that flue gas dual dilution is the key to achieving MILD combustion in a long-narrow confined space, which is irrelevant to the construction of the flue gas internal recirculation rate.