The substantial increase in CO2 emissions from various point sources has become a major contributor to the greenhouse effect, intensifying global warming and exacerbating climate instability. Addressing the rising levels of atmospheric CO2 requires innovative and sustainable mitigation strategies. Integrated carbon capture and conversion (ICCC) is an emerging approach that enables the simultaneous capture and conversion of CO2 into intermediates, such as methane (CH4), carbon monoxide (CO), and syngas, via methanation, reverse water-gas shift (RWGS), and dry reforming of methane (DRM). These intermediates can be further upgraded into value-added liquid fuels via methanol synthesis or the Fischer-Tropsch route, directly supporting the production of renewable methanol and SAF. ICCC could potentially reduce the need for CO2 compression and transport, may lower the energy-intensive requirements for sorbent regeneration, and reduce the reactor systems typically associated with conventional processes. This review critically examines advancements in ICCC technologies, focusing on the design and optimization of dual-function materials (DFMs) and the synergistic interactions between adsorbents and catalysts. It explores optimized reaction pathways, material stability, and the techno-economic feasibility of ICCC systems, while addressing the effects of key operational parameters. Challenges such as thermal sintering, catalyst deactivation, and impurities (e.g., O2, H2O, sulfur species) are addressed. Practical implementation issues, including gas switching between oxidising and reducing atmospheres, purge losses, H2 utilization, product dilution and cyclic reactor heat management, are also discussed as critical determinants of scale-up feasibility. By integrating insights from materials science, process engineering, and economic analysis, this study identifies the material and process conditions under which ICCC may offer advantages over separated capture and conversion routes. The review also highlights that practical deployment remains conditional on long-term DFM stability, realistic gas switching, impurity tolerance, low-carbon H2 availability, heat integration, and reactor-scale validation.
Over the past decade, the pursuit of rapid, scalable, and cost-effective graphene synthesis strategies has intensified, driven primarily by graphene's exceptional structural, electrical, and thermal properties, which underpin its transformative potential in energy storage, electronics, and sensing. However, conventional synthesis methods (e.g., chemical vapor deposition [CVD] and exfoliation) suffer from inherent bottlenecks, including high mass-production costs and limited scalability, making rapid, energy-efficient, and scalable graphene manufacturing an increasingly critical priority to meet the accelerating demand for high-performance carbon materials. This review systematically examines three emerging non-conventional synthesis platforms, direct laser writing (DLW), Joule heating (JH), and atmospheric-pressure plasma (APP) and highlights their distinctive advantages (e.g., rapid processing, feedstock versatility, intrinsic process intensification) and inherent limitations relative to conventional CVD and exfoliation routes. A unified mechanistic analysis is presented, focusing on localized thermofluidic behavior, ultrafast nonequilibrium carbon transformations, defect evolution pathways, and graphitic domain formation across these techniques. Structure–property correlations relevant to energy storage devices, including supercapacitors and various battery systems, are systematically discussed. The review further highlights key challenges in achieving process uniformity, mechanistic predictability, and industrial scalability, while outlining timely opportunities for advancing these methods through integration with emerging reactor concepts, sustainable carbon feedstocks, and hybrid manufacturing strategies. Collectively, these insights position DLW, JH, and APP as promising cornerstones for next-generation rapid graphene synthesis, and this work aims to provide a comprehensive perspective to inspire innovations in process design and mechanistic understanding.
Flammable mixtures of dusts with gases or liquids occur in the process and energy industries. Most research about these so-called ”hybrid mixtures” was, and still is, about coal dust with the admixture of methane because of their occurrence in the mining industry. In the modern industry, hybrid mixture explosions play an increasing role in many existing processes like spray-drying, or in emerging technologies like the direct reduction of iron ore with hydrogen or nuclear/fusion reactors. While some safety characteristics of one of the component substances stay the same or are unaffected by the concentrations that occur in the process, others are severely influenced by only traces of the other substance.This review paper shows in which processes and applications hybrid mixtures pose a risk and gives an overview of the research conducted in the last 150 years. Findings that are reproducible and represent current proven knowledge are stated and compared to each safety characteristic containing only solid particles, gases or liquids as combustible substances. Additionally, fundamental studies on the mechanisms of flame propagation in hybrid mixtures are reviewed. The significance of these studies in enhancing our understanding of explosion behaviors in hybrid mixtures is also discussed. An outlook on what has been missing so far in the literature, is also given comparing the knowledge of single substances with their mixtures, why this might not have been investigated, and where the challenges lie.
Direct air capture (DAC) has emerged as a scalable strategy for atmospheric CO2 removal, with material innovations being central to its advancement. This review explores recent developments in DAC materials, including solid sorbents and liquid solutions, with a focus on their design, performance, and scalability. Key properties such as CO2 sorption capacity, kinetics, and stability under diverse conditions are critically analyzed. Emerging materials, including metal-organic frameworks (MOFs), amine-functionalized adsorbents, and advanced polymeric materials, are examined for their potential to enhance efficiency and reduce energy consumption. Challenges related to material synthesis, regeneration, and long-term durability are discussed alongside strategies for integrating these materials into scalable DAC systems. While previous reviews have addressed DAC scrubbers, a systematic yet comprehensive classification was lacking. This review fills that gap by categorizing DAC materials into a structured quadrant and compiling both foundational studies and recent advancements into detailed tables, providing a clear and organized overview of the field's progression. By synthesizing breakthroughs and identifying future directions, this review underscores the need for innovative materials to drive down costs, improve performance, and facilitate large-scale deployment of DAC in climate mitigation efforts.
Flame aerosol reactors are the preferred industrial method for producing nanostructured materials such as carbon black, fumed silica, and titania pigments. These reactors enable large-scale, reproducible nanopowder synthesis. The field has been revolutionized by the integration of two-phase atomization nozzles in flame spray pyrolysis (FSP) reactors; these reactors enable the processing of virtually any precursor via liquid dissolution and greatly surpasses the limitations of traditional vapor-based approaches. Most importantly, FSP is no longer a "black box"; recent advances in theory and experiments have provided fundamental insights into particle growth dynamics and enabled the precise control over nanoparticle properties with low batch-to-batch variation. This understanding is crucial for biomedical applications, where reproducibility and functional performance are vital. Here, we review the latest developments in flame-made nanoparticles for biomedical applications, with a focus on FSP reactor engineering, surface property control, and direct integration into medical devices. We discuss the theoretical framework behind reactor design and its impact on material performance. While FSP has demonstrated remarkable versatility for medical nanomaterials, addressing challenges such as good manufacturing practice (GMP) compliance, in vivo safety, and clinical translation will be essential for its widespread adoption in biomedicine.
Wet biomass conversion in hot compressed water (HCW) processes operating at temperatures above 250 °C and pressures above 4 MPa offer a promising pathway towards sustainable production of biofuels. This paper provides a multiscale view of HCW, spanning from molecular-level mechanisms to commercialisation and scale-up challenges of related technologies, emphasising the need for scientific and technological innovations, policy support, and market incentives. Key aspects such as sustainability, environmental impact and economic feasibility are critically discussed. Key deployment challenges include catalyst selection, deactivation, reactor design, and process optimisation. Integration with renewable energy systems, such as solar and geothermal, and carbon capture and utilisation technologies is proposed to tackle the high energy requirements and environmental impact of HCW processes. Current developments in data-driven modelling and mechanistic simulations as useful tools that facilitate process analysis and optimisation are also reviewed. Compact, integrated and intensified HCW processes with energy recovery are central to advancing the bioeconomy. This study aims to advance the current state of HCW technologies and outlines a roadmap for future research and technological development integrated with renewable energy systems in more sustainable ways. In summary, this paper is expected to serve as a reference for researchers and industry professionals, providing a guide for addressing real-world deployment issues in HCW integrated technologies and fostering further progress in a field that often focuses more on fundamental chemistry.
For developing next-generation high performance internal combustion engines, the cycle-resolved, real-time in-cylinder combustion information acquisition can be a crucial factor. Ion current detection technology has emerged as a cost-effective diagnostic approach for this purpose. Over the past decades, extensive experimental and numerical studies have been carried out to investigate ion current characteristics. This review highlights recent progress in both the fundamental understanding and practical applications of ion current detection technology. Firstly, the principles of flame ionization, the reaction model development and ion current detection system design are introduced. Then the relationships between ion current signals and combustion behaviors are examined across different setups, ranging from testing combustors to practical engines. Accordingly, the applications of ion current in engine combustion diagnostics and control are then discussed, with emphasis on combustion status identification and optimization. Finally, the key challenges, the potential and the developing tendency of ion current detection technology are also analyzed in this review. The results clearly demonstrate that: Among various in-cylinder combustion diagnostic methods for internal combustion engines, ion current detection technology holds unique advantages in terms of combustion information richness, low cost, and ease of maintenance. Over the past decades, particularly in the last ten years, the shortcomings observed in previous studies—such as the strong influence of near-ion probe flame conditions on the signals, and the weak linear correlation between the signals and specific combustion parameters—have been significantly mitigated through fundamental innovations in detection system design and electronics circuit optimization. In such a context, the high accessibility of ion current signal data, combined with the large-scale data processing capabilities of artificial intelligence models, is expected to make this technology one of the most promising approaches to achieve next-generation intelligence engine combustion control. Overall, by integrating the ion current signal with other information from engine sensing systems, this detection technology is poised to drive profound transformations regarding the engine system design, performance calibration, and cycle-resolved (even intra-cycle) engine combustion control.
Transforming ammonia (NH3) synthesis from the energy-intensive, fossil-fuel-dependent conventional Haber-Bosch (HB) process to a flexible, green hydrogen-based process is pivotal for decarbonization and enabling NH3 utilization in the energy sector. The conventional HB process, operating under high temperature and pressure, is incompatible with green hydrogen systems and economically unviable for downscaled NH3 production integrated with intermittent renewable energies . Therefore, developing alternatives capable of synthesizing NH3 under moderate conditions is crucial for achieving green NH3 production. This necessity has driven the development of a range of emerging technologies, including thermocatalytic, electrocatalytic, photocatalytic, and plasma-assisted processes, amongst which thermocatalysis stands out in terms of production rate, technology readiness, and economic feasibility, demonstrating the greatest potential for NH3 synthesis transformation. This review provides a comprehensive overview of advanced thermocatalytic NH3 synthesis beyond conventional HB process and the system integration with renewable sources. It highlights key limitations and advances in implementing new materials and auxiliary techniques, outlining the challenges and mitigation strategies for achieving high NH3 productivity under mild conditions. Alongside multiscale modeling studies, the review covers catalyst development, reactor intensification, process integration, and system evaluation, examining progress and conducting meta-analysis in reaction mechanisms, emerging separation technologies, and system integration. Scientific obstacles, economic analysis, and environmental impacts are thoroughly discussed, offering state-of-the-art insights into mild NH3 synthesis from fundamental research to practical applications. Additionally, recent industrial projects of green NH3 production are summarized, showcasing scalability and commercial viability. Finally, the remaining challenges and opportunities in advanced thermocatalytic NH3 synthesis are outlined, identifying future research frontiers.
Binary droplet collisions, relevant for various natural phenomena and technological processes, embodies a rich fluid-dynamical platform covering a wide range of physical scales. This review begins with the collision between two identical droplets to reveal the underlying physics of the transition between droplet coalescence, bouncing, and separation. The interplay between the macroscopic droplet motion, the internal flow, and the microscopic interfacial gas film dynamics involving rarefied flow and van der Waals molecular force reflect the essential multi-scale and multi-physics characteristics of the collision dynamics. The review then discusses the collision between unequal-sized droplets, non-Newtonian fluids, dissimilar fluids, and the analogical jet-jet collisions. Fundamental understanding on the basic binary droplet collisions phenomena and its inference on practical processes is emphasized.
Space exploration is a shared human aspiration that presents significant challenges, with fire being a major threat. The unique low-gravity, reduced-buoyancy environments of spacecraft and extraterrestrial habitats profoundly alter fluid dynamics, chemical reactions, and heat-mass transfer, leading to drastic changes in fire behavior. Understanding the solid material combustion under these conditions is vital for spacecraft fire safety and advances fundamental combustion science. This review synthesizes research on flame spread over solid materials under reduced buoyancy/gravity over the past half-century. It uniquely integrates the studies conducted in micro- and partial-gravities with ground-based experiments designed to mimic these environments. The review begins with the theoretical models defining the flame behavior and examines experimental findings from low gravities. These results reveal the important roles of “smothering” effects and radiative heat loss due to the suppressed natural convection, which drive a transition from two-dimensional to three-dimensional flame structures. Ground-based simulation methodologies, including reduced pressure environments and narrow channel apparatus, are critically examined for their ability to replicate low gravities. Stagnation point low-stretch diffusion flames are also included as a ground-based method to simulate the reduced-buoyancy effects on the spreading flame front from a more microscopic perspective. By comparing actual low-gravity data with simulated environments, the review introduces key similarity laws but also discusses the limitations of these methods in fully capturing low-gravity combustion dynamics. As the first integrated review of this topic, this work provides essential insights for ensuring the fire safety of human space exploration in the decades to come.
Organic pollutants are acknowledged as one of the primary environmental hazards in the atmosphere, posing a significant threat to human health and the environment. This work provides a critical review of the recent research on organic pollutants from stationary coal-burning sources, including an overview of the effect of coal composition and coal source, the types of organics material in coal, the generation of organic pollutants during coal combustion, emission of organic pollutants, co-removal by air pollution control devices (APCDs), and the technologies used to remove organic pollutants from coal-fired power plants (CFPPs). Field sampling and analysis showed that the organic pollutants produced from coal combustion processes are mainly composed of volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), and organic components in condensable particulate matter (CPM). The VOCs and CPM are mainly discharged into the atmosphere as part of the gas and particulate phases, respectively, whereas, PAHs are mainly concentrated in fly ash and bottom ash. The generation of organic pollutants during coal combustion is mainly affected by coal grade, temperature, heating rate, residence time, and pressure. Considering that the flue gas temperature and composition can vary from one CFPP location to another, the choice of the sampling methods is based on the specific needs of a CFPP. Because, the selection of a sampling method can significantly affect the final results and should be given special attention. Typical sampling methods for VOCs, PAHs, and CPM are summarized and their advantages and disadvantages are compared. The adsorption tube sampling method is more suitable for the sampling of VOCs because it is not limited by the volume and miscellaneous components of the flue gas. EPA Method 0010 and the dry impactor condensation method (EPA Method 202) are more suitable for sampling PAHs and CPM in flue gas due to their higher accuracy. The APCDs in CFPPs have organic pollutants co-removal abilities besides those for conventional pollutants. As a result, they can remove VOCs, PAHs, and CPM at efficiencies of 55.8–87.6 %, 74.7–89.8 %, and 36.3–81.5 %, respectively, with corresponding emission concentrations of 0.058–16.29 mg/m3, 0.42–43.3 μg/m3, and 5.9–65.1 mg/m3. Based on current publicly reported data, the VOCs, PAHs and CPM contents emitted by global coal-fired power plants are estimated to be 88.1 Gg, 6.76 Gg (2.22 Gg in flue gas and 4.54 Gg in fly ash), and 600 Gg (organic components: 328 Gg, inorganic components: 272 Gg), respectively. Among the many removal technologies reviewed: include improved selective catalytic reduction (SCR) catalysts; and an advanced oxidation process (AOP) combined with an SCR or with a wet flue gas desulphurization (WFGD) unit respectively are two potentially useful technologies for future use in CFPPs. The information collected and presented in this review will help in the formulation of air pollution control policies and the development and application of efficient removal technologies.
In considering ammonia (NH3) as a carbon-free fuel for large-scale power generation, this review examines the current state of knowledge of NH3 as a fuel in terms of its thermophysical properties and burning characteristics compared to conventional hydrocarbon fuels. The proceeding analysis portrays the challenges associated with NH3 combustion in traditional systems and suggests fluidised-bed NH3 combustion as a plausible means to provide reliable ignition, stable combustion, and reduced NOx emission. A fixed-bed is considered as a research tool, as well as a special case of fluidised-bed, to study NH3 oxidation and combustion in the presence of solid bed material to provide foundational information key to understanding the more complicated fluidised-bed NH3 combustion. The thermophysical properties and burning characteristics of NH3, along with an examination of the combustion of other fuels in the presence of solid media, enable expectations for NH3 combustion in fixed- and fluidised-beds. A general fluidised-bed NH3 combustion system design, along with suggested operating conditions, is presented to provide an appreciation for a practical large-scale fluidised-bed NH3 fired power generation system. The NH3 combustion chemistry and associated NOx formation and destruction pathways are also discussed to appreciate the impact of operating conditions on combustion performance. Finally, the review identifies key knowledge gaps and technical challenges which warrant further research to advance fluidised-bed NH3 combustion technology for large-scale electric power generation in a carbon constrained future.
Fixed-bed combustion is the oldest and most flexible method for generating heat from solid fuels with diverse morphological characteristics. This approach consolidates countercurrent fixed-bed burners as the most employed technique for biomass combustion in small- and medium-scale installations. In recent decades, researchers worldwide have generated significant experimental evidence to decipher the complex mechanisms behind reaction front propagation using laboratory-scale burners, which simulate the conditions of fuel in industrial plants. Many experimental results are available in the scientific literature, and the consolidation and discussion of the most relevant data can be useful for better understanding the current state of the art and identifying knowledge gaps on the subject. This paper presents a systematic review of the available literature concerning experimental studies of biomass fixed-bed combustion in laboratory-scale reactors. The central discussion encompasses the definition of the parameters employed to characterize the reaction zone behaviour and their dependence on fuel properties and primary air conditions. The insights gained in this review were addressed to propose criteria for harnessing the abundant availability of agricultural residues as alternative fuels in grate-firing systems.
Internal combustion (IC) engines have contributed to global economic development in industrialized societies. Hydrocarbon fuels used for fueling the IC engines need to be replaced by sustainable and eco-friendly origins that do not adversely impact the environment locally and globally. Electro-fuels (or E-fuels) and biofuels are essential to displace fossil fuels. They are primarily produced using renewable electricity and feedstocks, respectively, and represent an emerging class of carbon-neutral drop-in fuels for the transport sector, which are becoming increasingly important with every passing day globally and have an essential role in the ‘net-zero’ future. This paper comprehensively reviews the advancements in IC engines to become more efficient in taking the fuel property advantages of various E-fuels and biofuels in existing engines. The paper focuses on several fuels, among the most studied ones in the open literature in the last decade, since their adoption might depend on factors such as the local economic considerations, cultural contexts and the application itself, storability, power requirement, and government policies. The paper covers these fuels by briefly introducing their production pathways and properties. It then focuses on their engine use to meet the future tailpipe and greenhouse gas emissions norms. Challenges regarding the modelling of engines powered by E- or bio-fuels are also included. E-fuels offer a straightforward advantage even in engine-out emissions and after-exhaust emission control technologies. It represents an opportunity to limit GHG emissions. Moreover, the E−(or Bio-) fuels powered engines can cover the same operating range (or a larger one) with similar efficiency or greater than hydrocarbon fuels. However, using these fuels still remain challenging. Retrofitting existing engines for using E−(or Bio-) fuels depends on the cost of the fuel injection system for injecting the fuel into the combustion chamber. The transition from fossil fuels can be done using liquid fuels such as methanol, ethanol, etc., by blending them with conventional fuels. In the long run, methanol, hydrogen, and ammonia are expected to significantly decarbonize the transport sector globally.
The toxicity, climate impact, as well as the physical and chemical properties of ultra-fine soot particles emitted from combustion systems are strongly dependent on their size and morphology. Research attention has been paid in the last three decades to developing more accurate and capable methods to model soot particle coagulation in the presence of inception, surface growth, and oxidation, to predict particle size distribution as well as the detailed aggregate morphology of soot. While soot particle concentrations in hydrocarbon flames are primarily governed by soot kinetics, the morphology of soot particles is controlled by both soot kinetics and particle dynamics. Flame-generated soot particles are fractal aggregates formed by polydisperse and nearly spherical primary particles with a certain degree of overlapping. The properties of fractal aggregates, nanoparticle coagulation, and soot formation chemistry all play important roles in soot formation. This article reviews all these aspects but the focus is on recent progress in macro- and meso-scale modeling of soot particle aggregation in laminar sooting flames to avoid the complexities of turbulence. The reviewed macro-scale methods based on the population balance equation include the commonly used sectional methods and methods of moments. The main features of three recently developed state-of-the-art meso-scale methods, namely the event-driven Discrete Element Method, Monte Carlo Aggregation Code, and detailed stochastic population balance model are reviewed. To highlight the complexities of modeling the particle size distribution and detailed particle morphology without and with surface growth, numerical simulations of three test cases were conducted using the event-driven Discrete Element Method, the Monte Carlo Aggregation Code, and the two macro-scale methods. A detailed analysis of the results was presented to understand how different treatments of particle coagulation and surface growth in the two meso-scale methods affect the predicted particle size and morphology. The remaining challenges in modeling detailed soot particle morphology are outlined.
Growing awareness of the environmental and health impacts of unburned and partially pyrolyzed hydrocarbons emitted by flaring establishes a need for instrumentation that can quantify the performance of flares in terms of overall combustion efficiency (CE) as well as the destruction removal efficiency (DRE) of a particular species. Climate modelers and policymakers need CE estimates to calculate the overall contribution of flaring to global methane inventories, so they may understand how flare emissions impact climate change and develop science-informed regulations; regulators need tools for enforcing current and emerging rules governing flare DRE; flare operators need instrumentation to identify problematic operating conditions in real time; and combustion equipment manufacturers need to quantify improvements in CE/DRE realized through new flare tip designs.This paper reviews the current state-of-the-art in instrumentation and techniques used for quantifying CE and DRE, with a focus on flaring in the oil and gas sector. The paper begins with an overview of flaring, followed by a discussion of the aspects of flaring that make this measurement so difficult to carry out. Techniques for measuring flare CE and DRE are then examined. The paper concludes with an outlook of future challenges and opportunities.
The gaseous Inverse Jet Diffusion Flame (IJDF) is a unique nonpremixed flame that can be established in a simple coaxial burner when central air jet surrounded by annular fuel jet is ignited. The active research on laminar IJDF was initiated in the early 1980's, with a primary focus on its sooting characteristics. The soot formation, evolution and morphology in inverse jet diffusion flame differ fundamentally from that of the normal jet diffusion flame (NJDF) due to its distinct reactant delivery mode and fluid dynamics. The unique feature of the IJDF configuration is its reduced soot formation as compared to the NJDF configuration, particularly at higher air-fuel velocity ratio, even for hydrocarbon fuels. The literature has reported six types of laminar IJDF based on visual appearance and air-fuel velocity ratio. Furthermore, laminar IJDF is mainly utilized as a lab-scale flame by various researchers for the fundamental investigation of soot evolution in nonpremixed flames. Unlike normal jet diffusion flame, which is established with the fuel jet enveloped by an oxidizer jet, the post-flame emissions and the flame stability aspects of turbulent IJDF are relatively less understood. From a global perspective, coherence in the research on inverse jet diffusion flame is lacking and there is a need for an extensive investigation to understand this special type of nonpremixed flame. The present review identifies different emerging areas related to IJDF that the combustion researchers can pursue in the future.Various aspects of laminar and turbulent IJDF, such as flame structure, soot formation, flame height, flame stability, thermal and emission characteristics are discussed in this review. This review may serve as a reference that contributes to the research perspectives on laminar and turbulent inverse jet diffusion flames for adapting the favourable aspects of this flame configuration in a wide range of industrial and domestic applications.
Dense gas-solid reacting flow involves multiphase flow, heat and mass transfer, and chemical reactions. The computational fluid dynamics-discrete element method (CFD-DEM) has emerged as a promising tool for investigating and optimizing dense gas-solid reacting systems at the particle scale. Despite the rapid advancement of CFD-DEM and its successful application to various chemical engineering processes, there is still a lack of a comprehensive review of the theory and applications of CFD-DEM modelling of dense gas-solid reacting flow. This article aims to bridge this gap by providing a systematic review of recent progress in the development of CFD-DEM models and their applications to dense gas-solid reacting systems. This article begins by providing a comprehensive review of sub-models used to describe flow dynamics and thermochemical conversion in dense gas-solid reacting systems. The numerical algorithms and implementations, ranging from flow to heat and mass transfer, as well as speed-up methods, are examined in detail. The focus then shifts to the recent advancements of CFD-DEM applications in chemical engineering processes related to dense gas-solid reacting systems. Specific areas of interest include the thermochemical conversion of biomass and coal, blast furnace ironmaking, chemical looping combustion, solid waste incineration, lime shaft kiln calcination, and more. Furthermore, the challenges associated with effectively and efficiently modelling dense gas-solid reacting flow, particularly about the multi-physics and multi-scale characteristics in both time and space, are thoroughly assessed. By addressing these challenges, this review is expected to foster further progress in the field and enhance our understanding and control of dense gas-solid reacting systems in various applications.
The flash point (FP) behavior of binary ignitable mixtures, which are the simplest form of mixtures and fundamental building blocks, is essential for understanding multicomponent mixture behavior. This knowledge plays a vital role in process and chemical safety as well as in fuel design. In the present review, the FP of 245 independent binary ignitable mixtures, composed of 102 individual pure compounds derived from 69 published articles, was investigated. The mixtures based on their chemical class were categorized. Investigations on their ideal or extreme FP behaviors revealed that certain combinations have a higher potential for demonstrating extreme FP behaviors such as alcohol + aromatic hydrocarbon, alcohol + ester, alcohol + alkane, aromatic hydrocarbon + organic acid, alcohol + organic acid, phenol + alcohol, phenol + ketone, and phenol + pyridine. It was found that the occurrence of extreme FP behaviors is not only related to the chemical class but also to the molecular structure, the non-ideality of binary mixture, and the temperature gap between FP values of the pure constituents in each binary blend. These findings can be utilized to enhance the safety level of processes or operations involving these binary mixtures. Furthermore, this information can be valuable in fuel design for specific purposes and improve combustion, thanks to a comprehensive knowledge regarding the FP tendencies of each binary category and the potential for extreme FP behaviors.
End-gas autoignition, especially with detonation development in a confined space, is a complex physical phenomenon, including premixed flame dynamics, fluid dynamics, autoignition chemistry etc., which is generally considered as the origin of knock and super-knock in internal combustion (IC) engines. Furthermore, the mechanism for detonation initiation is also related to fire safety and industrial disasters. Thus, this review focuses on the recent progress made in the fundamental understanding of the mechanisms of end-gas autoignition phenomena along with detonation combustion in confined spaces through theoretical analyses, optical diagnostics, and high-resolution numerical simulations, with emphasis on the effects of crucial physicochemical factors on the two stages of end-gas autoignition, namely autoignition occurrence and autoignition propagation. Firstly, two basic theories, namely Livengood–Wu (L–W) integral and the reactivity gradient theory, which provide theoretical foundations for understanding autoignition occurrence and autoignition propagation, respectively, are demonstrated. Specially, applications and limitations of L-W integral and the extension of Bradley's diagram to multi-dimensional conditions closer to actual circumstances are elaborated. Then, a comprehensive investigation of several pivotal physicochemical factors involved in end-gas autoignition and detonation development in confined spaces, are conducted, including flame propagation, pressure wave, inhomogeneity, turbulence, chemical reactivity and thermodynamic conditions. The results indicate that, three essential elements are included in end-gas autoignition, namely flame, pressure wave, and autoignition. The flame-pressure interaction induced end-gas autoignition and detonation can be divided into three processes: I-reactivity increase, II-critical and sensitive state, and III-coupling and detonation. The first two processes account for autoignition occurrence and the third accounts for autoignition propagation. As to autoignition occurrence, increasing turbulence flame speed can inhibit end-gas autoignition under weak pressure wave conditions, whereas it can promote end-gas autoignition under strong pressure wave conditions. As to autoignition propagation, various combustion modes can originate from a reactivity gradient induced by temperature, composition, additive, as well as a cold spot within negative temperature coefficient (NTC) region, while the existence of low-temperature chemistry (LTC) and multi-stage ignition complicates autoignition propagation. The results further indicate that an inhomogeneous field with a small characteristic length scale, and an inhomogeneous field with a large characteristic length scale but coupled with the turbulence with a small characteristic length scale and a sufficiently large turbulent velocity fluctuation, can both weaken detonation propensity. Furthermore, the fuel type, diluent gas, and thermodynamic conditions can affect both autoignition occurrence and autoignition propagation. However, the effects of fuel type and energy density on autoignition propagation may not be completely explained by Bradley's diagram, and should be considered separately. Lastly, the review discusses the stochasticity of end-gas autoignition and the similarity and difference of detonation in IC engines from deflagration to detonation transition (DDT) in ducts, and provides inspirations obtained from the present fundamental studies for the engineering applications and future prospects.
