Search

Dust explosions and fires pose an industrial safety problem, due to the human and material losses caused by them. As many fuel processes and material transport generate powder particles, the effect of granulometry in different flammability properties has been studied to define the relationship between both. Deep knowledge of this relationship reduces the self-ignition and self-combustion processes, and the accidents associated with these processes. In this study, six different samples, including biomass, charcoal, and dog food, are tested in three different particle sizes, so differences in their flammability behavior could be appreciated (not only considering fine particles, but also coarse samples). The transport classification test was carried out, obtaining significant results in two samples, where the same material did not self-ignite when tested at its coarse size, but it did when tested at fine particle size. Similar results were obtained when analyzing initial temperature for related-combustion gasses emission: the finer the particle size, the lower the initial temperature. To understand the heat mechanisms, thermal analyses were performed, such as thermogravimetric analysis and differential scanning calorimetry. Finally, a self-ignition risk was assessed for all samples according to their activation energy and characteristic temperature. All the test results lead to conclude that biomasses easily start self-heating process, but its composition difficult the heat transmission to reach self-ignition, while charcoals are more susceptible to self-ignition. [ABSTRACT FROM AUTHOR]

The self-ignition of a stoichiometric acetone−oxygen mixture diluted with argon was studied both experimentally and numerically. The experiments were performed behind reflected shock waves over a temperature range from 1280 to 1810 K at a total gas concentration of [ M ] 50 ≈ 10−5 mol/cm3. The process was monitored by recording the signals of absorption by methyl radicals (λ = 216.5 nm) and emission from electronically excited OH* radicals (λ = 308 nm) and CO 2 * molecules (λ = 370 nm). Numerical simulations within the framework of various detailed kinetic mechanisms were performed to reproduce our own and published experimental data on the pyrolysis and self-ignition of acetone behind reflected shock waves, including the concentration profiles of acetone and CH 3 in the ground state and electronically excited OH* and CO 2 *, as well as the temperature dependence of the self-ignition delay time. It has been established that various detailed kinetic mechanisms describe the kinetic characteristics of the pyrolysis of acetone in shock waves with varying degrees of accuracy. At the same time, all of them predicted the measured ignition delays within a factor of two. A sensitivity analysis showed that the reactions determining the pyrolysis of acetone produce only a slight effect on the branched-chain ignition process, so that the relevant rate constants only slightly affect the ignition delay time. • Shock tube results of acetone autoignition are presented. • Detailed kinetic mechanisms are capable to describe acetone ignition delay. • The problems of simulation of acetone pyrolysis were revealed. • Kinetics of methyl radicals reactions should be refined. [ABSTRACT FROM AUTHOR]

Simultaneous thermal analysis (TG–DTA) of low-rank coals from different annual periods (2015 and 2018) which originate from Kolubara and Nikola Tesla A (TENT A) coal-fired power plants was studied to identify their combustion characteristics and self-ignition risks. In order to investigate kinetics of thermo-oxidative degradation, model-free models including the Friedman and Kissinger–Akahira–Sunose methods were applied. In accordance with obtained kinetic results, numerical optimization of combustion process was implemented. Homogeneous and heterogeneous types of ignition were identified for the tested coals, and factors that affect their occurrence are the used heating rate and volatile matter content. From thermal susceptibility graph, it was found that the tested coals expand in a wide range of self-ignition risk, depending on their coal rank. The youngest coal (TENT A (2018)) has the largest propensity to self-ignite among high-ranking coals. It was found that less tendency of 2018 coals to show true isokinetic temperature in comparison with 2015 coals is a consequence of intraparticle diffusion limitations, and change of char properties, at higher conversion values. Gamma spectrometry analysis of coals showed that higher concentration of 40K radionuclide (which is in mineral composition) may contribute through its deposition on available sites, decreasing surface area for reaction with oxidizing species. [ABSTRACT FROM AUTHOR]

Hydrogen is efficient and environmentally friendly, but the danger of self-ignition resulted from the leakage of high-pressure hydrogen cannot be ignored. In this work, the self-ignition of high-pressure hydrogen released through different conditions was studied. 700-mm-length tubes with different diameters were adopted in our experiments. It summarized the characteristics of shock waves' attenuation and evolution process of hydrogen jets in tubes. In addition, effects of the boundary layer on the leading shock waves, the contact surface, and expansion waves were discussed. Results indicate that minimum pressure when self-ignition occurs for 15-mm-diameter tube is similar to 10-mm-diameter. And they have closely velocity of shock waves. Simulations show that the greater the release pressure, the more ignition products of hydrogen. Higher release pressure and smaller diameter can create a thicker boundary layer in micro shock tubes, and the boundary layer can lead to a change in the velocity of shock waves' structures. • The minimum pressure when self-ignition occurs is similar in 700-mm-length tubes with different diameters. • Higher release pressure and smaller diameter tubes result in thicker boundary layer. • Boundary layer of micro shock tube affects shock-wave propagation and hydrogen self-ignition. [ABSTRACT FROM AUTHOR]

A series of experiments were conducted to study the pressure and combustion characteristics of the high-pressure hydrogen during the occurrence of spontaneous ignition and the conversion from spontaneous ignition to a jet fire and explosion. Different initial conditions including release pressure (4–10 MPa), tube diameter (10/15 mm), and tube length (0.3/0.7/1.2/1.7/2.2/3 m) were tested. The variation of the pressure and flame signal inside and outside of the tube and the development of the jet flame were recorded. The experimental results revealed that the minimum ignition pressure required for self-ignition of hydrogen at different tube diameters decreased first and then increased with the extension of tubes. The minimum ignition pressure for tubes diameters of 10 mm and 15 mm is no more than 4 MPa and the length of the tubes is L = 1.7 m. The minimum release pressure required for spontaneous ignition of a tube D = 15 mm is always lower than that of a tube D = 10 mm at the same tube length. When the spontaneous ignition occurred, it did not absolutely trigger the jet fire. The transition from spontaneous ignition to a jet fire must go through the specific stages. • Influences of release pressure and tube size are investigated. • The characteristics of self-ignition of high-pressure hydrogen are evaluated. • The transition process from self-ignition to a jet fire is elucidated. • Critical parameters required for spontaneous ignition are determined. [ABSTRACT FROM AUTHOR]

The article presents the results of determining fire and explosion hazard indicators for used lubricants, which are supposed to be used as a dispersion medium in the production of recycling plastic greases. Having determined the flash point of the oils, it is possible to calculate the values of the ignition temperature and self-ignition according to linear regression equations, with an error of up to 2 °C. The flash point can be a criterion when choosing raw materials and layout of the technological scheme for the production of recycling plastic greases. [ABSTRACT FROM AUTHOR]

Standard and alternative methods of assessing the likelihood of coal oxidation and self-ignition in the bed are analyzed. These methods are based on the change in the petrographic, physicomechanical, chemical, and thermal properties of the coal on oxidation. Most of the methods are unsuitable for identification of the initial stage of coal oxidation. The onset of self-heating may be better determined on the basis of the surface properties of the coal: in particular, the hydrophobic–hydrophilic balance and the sorptional properties of internal surfaces. The extraction, transportation, and analysis of the coal sample from the bed must prevent contact with the air. [ABSTRACT FROM AUTHOR]

The article presents the analysis of the correlation between the self-ignition parameters and the ultimate and proximate analysis, as well as the petrographic properties of Polish lignite, sub-bituminous coal, bituminous coal and anthracite. The following coal properties were determined: the moisture, ash content, volatile matter and sulfur content, gross calorific value, net calorific value, C, O, N and H contents, total porosity, ash oxide composition, rates of spontaneous combustion in 237°C and 190°C, and activation energy. During the petrographic analysis, maceral composition and random reflectance were determined. To determine the linear correlation between the self-ignition parameters and the analyzed coal properties, the Pearson correlation coefficient was calculated. The results show that there is no strong linear correlation between the lignite tendency to self-ignition and its petrographic properties. However, a strong negative correlation between the rate of spontaneous combustion and moisture and volatile matter content was observed. In the case of bituminous coal, strong correlations between self-combustion parameters and various coal properties were confirmed. The most noteworthy are the correlations between self-ignition parameters and the maceral composition, that is, between the content of macerals of the inertinite group in coal and content of macerals of the huminite/vitrinite group. The obtained results suggest that the spontaneous combustion tendency of coal increases with the increasing content of semifusinite and liptinite. [ABSTRACT FROM AUTHOR]

To investigate the effects of varying right-angle corner locations inside the L-shaped tube on self-ignition induced by high-pressure hydrogen release, a series of experiments were carried out on L-shaped tubes with different right-angle corner locations and a straight tube was adopted for comparison. It is found that compared with the straight tube, the propagation of shock wave in the L-shaped tubes becomes more complicated due to the existence of reflected shock wave. The pressure profiles detected by pressure transducers before the right-angle corner will undergo secondary rapid rise. The varying right-angle corner locations inside the L-shaped tube have a significant influence on self-ignition of hydrogen. The closer the right-angle corner is to the burst disk, the lower the critical pressure that causes hydrogen self-ignition is. Three possible mechanisms of self-ignition inside the L-shaped tubes are discussed. Nevertheless, different right-angle corner locations have no obvious effects on development process of out-tube jet flame, only the velocity of flame tip and the length and width of jet flame have slight differences. • The effect of varying right-angle corner locations on self-ignition is investigated. • The relationship between the reflected shock wave and burst pressure in L-shaped tube is discussed. • Self-ignition is easy to occur in L-shaped tube of smaller distance from corner to diaphragm. • Three possible mechanisms of self-ignition in L-shaped tube are proposed. [ABSTRACT FROM AUTHOR]

This paper demonstrates experimental investigation on the self-ignition and subsequent flame propagation of high-pressure hydrogen-methane mixture release via a tube. The proportion of methane added to hydrogen is 2.5% (vol.). A transparent rectangular tube (d = 15 mm, L = 400 mm) is used in the experiments. It is shown that the minimum burst pressure required for self-ignition increases 1.57 times for only 2.5% methane addition from 2.89 MPa (pure hydrogen) up to 4.68 MPa (2.5% CH 4 addition). This is mainly caused by the following reasons: on the one hand, methane addition can result in the decease of shock intensity inside the tube, thereby lowering the temperature of the combustible mixture; on the other hand, the hydrogen-methane mixture has the higher minimum ignition energy than that of pure hydrogen. Besides, 2.5% methane addition can increase the initial ignition time, weaken the flame intensity and reduce the flame propagation velocity relative to tube wall inside the tube. Moreover, for cases with 2.5% methane addition, the complete flame throughout the tube is formed closer to the back end of the tube. When the self-sustained flame exits from the tube, the maximum overpressure in a confined space increases with 2.5% methane addition. • The effect of 2.5% methane addition to hydrogen is experimentally investigated. • The shock wave intensity is reduced in the presence of the methane. • The addition of methane can decrease the possibility of hydrogen self-ignition. • Initial ignition occurs in the tube wall and then develops into a complete flame. • The hydrogen-methane mixture brings greater combustion overpressure in chamber. [ABSTRACT FROM AUTHOR]

The effect of mechanically activated coal grinding on the time of their ignition in a vertical tubular reactor is studied experimentally and numerically. The experiments showed that coal, ground by an activator mill (disintegrator mill), ignites faster due to the effect of mechanical activation. According to numerical calculation of the processes of ignition and combustion of mechanically activated pulverized coal fuel, it is clear that the use of non-stationary methods for turbulence simulation and a model of the volatile yield, taking into account the structure of coal matter, allow an estimate of the contribution of mechanically activated grinding to the intensity of devolatilization. [ABSTRACT FROM AUTHOR]

• Using ammonia combustion to reduce drag for the first time. • Ammonia demonstrates superior local drag reduction. • The self-ignition point decides overall anti-drag effects. • The inlet decomposition ratio is not the higher the better. Boundary layer combustion is an effective method to reduce friction in scramjet combustors. In existing research on drag-reducing fuels, hydrocarbons pose challenges related to carbon deposition and emissions. Meanwhile, hydrogen presents significant difficulties in storage and transportation. For the first time, this paper explores ammonia as a drag-reducing alternative. We employed the Reynolds-averaged Navier-Stokes (RANS) method to investigate the characteristics of drag reduction by ammonia combustion in supersonic flow. Numerical results indicate that the local drag reduction by ammonia combustion is better than hydrocarbon and worse than hydrogen compared to existing research. Ammonia exhibits superior potential for drag reduction. However, the overall drag reduction at room ammonia injection temperature is limited due to the significant distance between the ammonia self-ignition point and the injection port. To address this challenge, we proposed two optimization strategies: increasing the ammonia injection temperature and increasing the inlet ammonia decomposition ratio. As the ammonia injection temperature increases, the self-ignition position moves towards the inlet, and the combustion drag reduction region expands upstream. Interestingly, a higher ammonia inlet decomposition ratio does not necessarily lead to a better drag reduction effect. In fact, an excessively high decomposition ratio can inhibit the combustion drag reduction effect. The most effective drag reduction occurs when the ammonia inlet mass fraction is 50%, resulting in a 42.8% reduction in integrated drag compared to the case without NH 3 injection. Overall, this paper offers critical insights into the utilization of ammonia combustion in scramjets to reduce drag. [Display omitted] [ABSTRACT FROM AUTHOR]

The use of lignocellulosic biomass for green energy production is increasing, but the wood storage may lead to ambient odor related issues. This study aims to assess the odor potential and to investigate the mechanisms that promote odorous emissions from wood chip heaps. To achieve this goal, four experimental campaigns were conducted in an Italian thermal power plant. During these campaigns, odor concentration and chemical-physical parameters, that should theoretically affect the odor potential of biomass heaps, were investigated. The first important outcome revealed a strong heterogeneity on the heaps surface, classified as dry or smoking according to the superficial humidity and smoke. Smoking areas proved to be much more emissive and at self-ignition risk than dry regions. Indeed, odor concentration on smoking regions reached values in the order of 105 ou E /m3, significantly higher than dry region ones (102 ou E /m3). In addition, the highest values of odor concentration have been detected in the first days of storage, when fatty acids are oxidized and terpenes are released. Again, after 30 ÷ 80 days, i.e. about 20 days after the temperature peak caused by biological activity, odor concentration increases, suggesting a possible cause-effect mechanism between biological processes and odor release. Unexpectedly, odor emissions decreased during summer, reasonably due to the high ambient temperature which reduces the moisture content limiting biological activity. However, after about 100 days of storage, smoking regions extent and odor emissions can be considered negligible. [Display omitted] • Large amounts of biomass are stored as wood chip heaps in thermal power plants. • Wood chips heaps have a significant odor impact and self-ignition risk. • Heap temperature influences odor emissions. • Odor concentration trend in time shows a peak after 50 ÷ 80 days. • Terpenes and light oxygenates are the main responsible for odor impact. [ABSTRACT FROM AUTHOR]

Abstract—The combustion of hydrogen in its stoichiometric mixtures with oxygen diluted with water vapor, carbon dioxide, and nitrogen at 2.6 MPa was studied. The experiments were performed in a slowly and uniformly heated (1 K/min) tubular stainless steel reactor. The process occurred via conjugate heterogeneous (on the reactor wall) and homogeneous (in the reaction mixture) oxidations. The heterogeneous oxidation of H2 prevailed in N2 and CO2, while the homogeneous oxidation was predominant in H2O. Based on the time dependences of temperature rise caused by the exothermic oxidation of hydrogen, it was found that the self-ignition temperature of the Н2/О2/N2 and Н2/О2/H2O mixtures was ~30 K lower than that of the Н2/О2/СО2 mixture. A small water addition to the H2/O2/N2 and H2/O2/CO2 mixtures led to a decrease in the self-ignition temperature. [ABSTRACT FROM AUTHOR]

This study concerns the numerical simulation of turbulent non-premixed combustion in highly preheated air streams. One of the objectives is to settle an efficient computational procedure to proceed with the numerical simulation of large-scale industrial devices. It is also expected that the availability of such a computational framework may facilitate comprehensive sensitivity analyses as well as the development of mathematical models able to represent turbulence-chemistry interactions (TCI) in such conditions. Based on the salient physical ingredients that characterise scalar mixing, propagation, and self-ignition processes, a turbulent combustion modelling framework is thus introduced and applied to the numerical simulation of well-documented laboratory flames. In the corresponding geometries, the bulk flow velocities of the reactants streams can reach rather large values, which lead the flame to lift from the burner rim. Partially premixed flame edges thus stabilise the whole flame structure and the temperature of the oxidising stream can be increased by vitiation with burned gases so as to promote the corresponding flame-stabilisation processes. For sufficiently large values of the vitiated airstream temperature, self-ignition mechanisms may be triggered thus leading to a competition between mixing, propagation, and ignition processes. In this context, the ratio of the residence time to the self-ignition delay is thought to be a relevant variable to delineate the possible influence of ignition phenomena. Therefore, a modelled transport equation for this normalised residence time is considered. The performance of the corresponding modelling proposal is analysed with special emphasis placed on its ability to reproduce 'memory' or 'lagrangian' effects related to thermal aging processes. In this respect, it is noteworthy that the present set of computations makes use of tabulated quantities associated to (i) steady laminar one-dimensional diffusion flamelets, so as to describe the composition of combustion products, (ii) steady laminar one-dimensional premixed flamelets, to describe the flame brush propagation, and (iii) temporal evolution of zero-dimensional homogeneous mixtures to account for the possible occurrence of self-ignition phenomena. In particular, the tabulated self-ignition time value is used to evaluate the increase in the normalised residence time. Finally, two modelling parameters are put into evidence and studied through a detailed sensitivity analysis. [ABSTRACT FROM AUTHOR]

The time evolution of the chemiluminescence emission signals of CH*, OH*, C 2 *, and CO 2 * during the self-ignition of a number of the simple hydrocarbons is studied. The experiments are performed behind reflected shock waves over a temperature range of 1100–1900 K at a pressure of ∼1 bar. The effects of fuel-to-oxidizer equivalence ratio and the structure of the hydrocarbon molecule on the time profiles of the signals for each of the emitters are examined. A detailed kinetic mechanism for describing the emission signals from CH*, OH*, C 2 *, and CO 2 * recorded during the self-ignition of hydrocarbons is developed. To make the simulations more rigorous and reliable, the NASA thermodynamic polynomials for C 2 *, and CO 2 * were calculated based on the respective rotational and vibrational parameters given in the literature. Numerical simulations satisfactorily reproduce the measured time profiles of the signals from the studied emitters. • Chemiluminescence emission of CH*, OH*, C2*, and CO2* is studied. • Detailed kinetic mechanism for describing the emission signals was developed. • Simulation results well reproduce the measured intensity of emission. • Full width at half maximum of the emission signals are presented. [ABSTRACT FROM AUTHOR]

This paper describes the study of self-ignition delays of stoichiometric binary, triple, and multicomponent mixtures of methane with the addition of up to 10% of normal C2-C6 alkanes in air at temperatures of 523–1000 K and a pressure of 1 atm. It is determined that the additives of all C2-C6 alkanes can reduce the self-ignition delay of methane already at a level of 1%. With the addition of 10%, the ignition delay of this mixture virtually becomes equal to that of the added alkane itself. In binary mixtures with methane, ethane additives have an accelerating effect that is no lower than that of propane and butane additives. In triple and more complex mixtures, the self-ignition delay is determined by the total sum of added alkanes and is virtually independent within the framework of a determination error on their particular ratio. [ABSTRACT FROM AUTHOR]

The self-ignition of hydrogen released from a high-pressure tank using extension tubes (2200 mm) with different diameters was studied. The processes of flame transition at a nozzle and jet flame development were characterized using a high-speed camera. The results indicated that the intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube than that in a 15-mm-diameter tube. The pressure in a 15-mm-diameter tube was weaker than that in the 10-mm-diameter tube at the initial stage; however, it became higher in the later stage. Spontaneous ignition was more likely to happen in a 15-mm-diameter tube. The formation of a stabilized flame at the tube exit and Mach disk were observed during the transition of the flame to a jet fire. The stabilized flame showed a triangular shape because of the influence of a Prandtl–Meyer flow when a hydrogen jet entered a suddenly expanding environment. The formation and separation of a spherical flame were recorded during jet flame development. Large vortexes were formed in front of the flame because of the Kelvin–Helmholtz instability, which resulted in the separation of the spherical flame. The vortexes stopped rotating until the separated flame disappeared. • The intensity of a shockwave and the Mach number decay faster in a 10-mm-diameter tube. • Spontaneous ignition is more likely to happen in a 15-mm-diameter tube. • The stabilized flame at the tube exit is affected by the Prandtl-Meyer flow. • The formed large vortexes have a relationship with the separation of spherical flames. [ABSTRACT FROM AUTHOR]

The following article presents the results of empiric research of the delay time of self-ignition in the compression ignition engine Perkins 1104C-E44T, powered with biofuel – the mixture of fatty acid methyl esters. The research followed the calculations using known correlations that describe the delay of self-ignition in engines powered by petroleum fuels. The comparison of values calculated and achieved in the course of the research is shown in the graphs. • Comparison of methods for determining the combustion ignition delay. • Combustion ignition delay determined for biofuel. • Computational methods provide different results and deviate from empirical results. [ABSTRACT FROM AUTHOR]

The forced cooling of intermediate combustion products of rich oxygen mixtures of methane and its mixtures with propane was studied by computer simulation. The partial oxidation of the hydrocarbon feedstock was performed in the self-ignition mode at a constant pressure at temperatures of 773–973 K and pressures of 0.1–5.0 MPa. The disappearance of acetylene in the intermediate combustion products during the quenching was shown to be more active than when they equilibrated under adiabatic conditions. The forced cooling of the products can accelerate the disappearance of acetylene due to the different orders of the direct and reverse reactions of acetylene with active particles. [ABSTRACT FROM AUTHOR]

The paper presents investigation results on propane combustion in argon, carbon dioxide, and water vapor under the conditions of O2 deficiency and high reagent density when they are uniformly heated up to 620 K. Based on time dependences of reaction mixture temperature, the self-ignition temperature of propane was determined. It has been established that the oxidation of propane in an Ar and H2O medium proceeds according to the mechanism of chain-thermal explosion. The results of mass spectrometric analysis showed that oxidation in a CO2 medium is characterized by the lowest degree of propane conversion. It was also found that at a low density of water vapor, the oxidation of propane is accompanied by a high yield of H2. The mechanisms of CO2 and H2O molecule participation in the oxidation of propane are discussed in the paper. [ABSTRACT FROM AUTHOR]

A mechanism is proposed for the interaction of coal with paramagnetic nitrogen oxides produced in the decomposition of sodium nitrite. The main role in the self-oxidation of coal is played by quinoid structures with high redox potential. Four basic periods are identified in the thermooxidative transformation of coal. These periods differ in the rate of the processes, the quantity of gaseous oxidative products, and their composition. The decomposition products of the coal react most vigorously with the oxidant in the fourth period. That leads rapidly to self-heating of the reaction mixture, culminating in explosive gas liberation. [ABSTRACT FROM AUTHOR]

Highlights • Oxidation of propane, propene, and isopropanol in Ar, CO 2 , and H 2 O media at elevated pressures was studied. • Propane oxidation in Ar and H 2 O media, as well as isopropanol oxidation in Ar and CO 2 media are accompanied by a chain-thermal explosion. • Propane and propene oxidation in CO 2 medium occurs with a low fuel conversion. • Fuels oxidation in water vapor is defined by nearly compete O 2 consumption. • Mechanisms of the influence of CO 2 and H 2 O on the fuels oxidation are considered. Abstract The article discusses research results on oxidation features of high-density fuel-enriched C 3 Н 8 /О 2 , C 3 Н 6 /О 2 , and C 3 Н 7 OH/О 2 mixtures (ρ fuel = 0.22–0.25 mol/dm3, ρ O2 = 0.76–0.88 mol/dm3) diluted with argon, carbon dioxide, or water vapor (from 59 to 72% mol.) at the uniform heating (1 K/min) of tubular reactor to 640 K. Proceeding from the time dependences of the reaction mixtures temperature, it was revealed that the self-ignition temperature of fuels in the Ar medium increases in the following sequence: C 3 H 6 < C 3 H 7 OH < C 3 H 8. The self-ignition temperature of propane and propene is almost independent of the diluent nature, while the self-ignition temperature of isopropanol in the diluents medium increases in the following sequence: Ar < CO 2 < H 2 O. The chain-thermal explosion was observed during the propane oxidation in Ar and H 2 O media, as well as during the isopropanol oxidation in Ar and CO 2 media. Using mass spectrometric analysis, it was shown that the oxidation of propane in CO 2 medium is characterized by a low degree of fuel conversion at almost complete consumption of O 2 , while the oxidation of propene is characterized by a low degree of fuel conversion and O 2 consumption. Oxidation of fuels in the H 2 O medium occurs in several stages and is characterized by almost complete consumption of O 2. In this paper we consider the mechanisms of chemical involvement of CO 2 and H 2 O molecules in fuel oxidation. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Mixtures in a H 2 -fueled HCCI RCEM ignite at lower compression ratio than predicted. • With increasing compression ratio, the stably operable range is extended. • It is more extended if engine performance is considered even with slight knocking. • It should be further improved for the commercialization of H 2 -fueled HCCI engines. Self-ignition and knocking characteristics for hydrogen (H 2)-fueled homogeneous compression charge ignition (HCCI) engines with no carbon emissions are experimentally studied using a rapid compression expansion machine (RCEM) for various compression ratio (ε) and fuel-equivalence ratio (ϕ) conditions without preheating intake air which is generally adopted to HCCI combustion but reduces engine efficiency. Results show that H 2 -air premixtures self-ignite at ϕ = 0.42 and ε = 22.0, while their theoretical self-ignition ε is 27.5, due to different boundary conditions. With increasing ε , the range of ϕ for stable operation with self-ignition and no knocking tends to increase, but it is limited, e.g., ϕ ≈ 0.1–0.2 at ε = 32.0. Depending on ϕ and ε when knocking occurs, various knocking peaks (bands) are observed in the frequency domain by the fast Fourier transform (FFT): the lowest knocking frequencies around 9–13 kHz, exhibiting a tendency of gradually increasing with increasing ϕ , and additional knocking peaks for enhanced ϕ. Compared with the narrow stable operation range in ϕ , the condition where the maximum indicated mean effective pressure (IMEP) is obtained includes moderate knocking and thus the actual operating range is significantly extended, e.g., up to 88% in ϕ at ε = 28.0. [ABSTRACT FROM AUTHOR]

Dust explosions and fires pose an industrial safety problem, due to the human and material losses caused by them. As many fuel processes and material transport generate powder particles, the effect of granulometry in different flammability properties has been studied to define the relationship between both. Deep knowledge of this relationship reduces the self-ignition and self-combustion processes, and the accidents associated with these processes. In this study, six different samples, including biomass, charcoal, and dog food, are tested in three different particle sizes, so differences in their flammability behavior could be appreciated (not only considering fine particles, but also coarse samples). The transport classification test was carried out, obtaining significant results in two samples, where the same material did not self-ignite when tested at its coarse size, but it did when tested at fine particle size. Similar results were obtained when analyzing initial temperature for related-combustion gasses emission: the finer the particle size, the lower the initial temperature. To understand the heat mechanisms, thermal analyses were performed, such as thermogravimetric analysis and differential scanning calorimetry. Finally, a self-ignition risk was assessed for all samples according to their activation energy and characteristic temperature. All the test results lead to conclude that biomasses easily start self-heating process, but its composition difficult the heat transmission to reach self-ignition, while charcoals are more susceptible to self-ignition.

Computer simulation of the initial fire stage dynamics with various growth rate have been carried out with the help of FDS software. In case of a quick fire 1055 kW heat liberation has been reached in accordance with quadratic law within 100 seconds, averagely within 250 seconds and within 500 seconds when the fire rate is slow. Source of fire has been located on the floor and at the height of 2 m. A doorway of 0.8×2.0 m size and two 0.8×0.1 m openings have been used as ventilation holes. One opening has been located at the bottom over the floor and the other one has been positioned under the ceiling. It has been established that in the process of their development fires change their character in the following way: at the initial stage they are controlled by inflammable materials and then the fires are controlled by oxygen. At the first stage heat liberation is changed in accordance with the same law which is used for mass rate of an inflammable material and flame combustion is observed only in the field of the fire source. At the second stage heat liberation dynamics has a form of pulsations which are irregular in amplitude and duration and the dynamics becomes dependable on ventilation conditions. At this stage flame combustion in the source zone and/or self-ignition in the smoke layer provide a corresponding contribution in fire heat liberation.It has been determined that two layers have been formed in case of a fire in the premises with ventilation in the spatial distribution of temperature and air fumigation. These top and bottom layers are practically uniform and they are characterized by irregular pulsations of the parameters which are due to smoke self-ignition occurrence. The paper shows relationship of these distributions with spatial distribution of air pressure changes in the premises during a fire. A significant dependence of heat liberation dynamics on fire source location height is observed at all fire growth rates. An increase in fire source location height above floor level exerts the most significant influence on heat liberation dynamics when ventilation occurs through a doorway and a top opening. Duration of the fire stage when flame combustion occurs in the source zone is reduced and duration of the stage when self-ignition takes place in the smoke layer is increased. In such a case heat liberation pulsation amplitudes, black-out values and air temperature in the bottom smoke zone where the self-ignition occurs are increasing and the rates of smoke layer and heated air sinking are slowing down.

The technological problems occurring in the co-firing of biomass and brown coal (lignite) prompted this research project. During the fuel preparation, accidental self-ignition and explosions were several times reported by power plants operators. The aim of this study was to evaluate brown coal, sunflower husks and sunflower husk pellets as fuels for co-firing in energetic boilers. Sunflower husk had a lower ash content and calorific value than the pellets. The range of the combustion temperatures of the biomass (200–300 °C) was narrower than that of brown coal (200–800 °C). The formation of highly alkaline ash from the biomass resulted in the formation in boiler of agglomerates of ash. The elemental composition, thermogravimetric and biological analyses suggested that the pellets contained synthetic additives difficult to identify. The biological method was proposed for evaluating biomass additives. The use of additional agents in the pelletizing process may influence on the combustion parameters. Mixing biomass with brown coal may occasionally result in self-ignition in the logistic chain. Plastic additives and biological activity may contribute to self-ignition. [ABSTRACT FROM AUTHOR]

Mining in the former Novátor coal and uranium mine ceased in 1957 and a part of the wastes underwent spontaneous ignition in 1959–1970. Since the 1970s, the waste heap has been exposed to continuous weathering. The objective of this study was to assess to what extent the burnt and unburnt parts of the heap contribute to contamination of the local drainage system over a period lasting >40 years after the end of the mining. Analyses of water collected by lysimeters located on the slope of the heap have shown that the burnt part of the pile is still a source of aqueous sulfate and volatile elements (F, Se, and Pb) that have accumulated in this part of the heap during the self-ignition processes. The unburned part of the heap is mainly the source of aqueous carbonate, Mo, and U. The pH values of water flowing through the heap are in the narrow interval of 6.9–8.6. Rare layers of unburnt coal-bearing sediments rich in sulfides, where the pH values are between 2.9 and 3.7 are rather exceptional. These layers are the main source of Zn, Cu, and Co as a result of weathering of pyrite, chalcopyrite and sphalerite. The chemical composition of water draining from the heap ranges between the values determined in both its burnt and unburnt parts. The water collected at the foot of the heap in periods of low drainage contains high levels of aqueous sulfate (up to 400 mg/L), carbonate (up to 208 mg/L), U (up to 585 μg/L), Se (up to 50 μg/L), and Mo (up to 28 μg/L). On the other hand, the concentrations of these elements are lower in periods of high drainage. The pH values in the water drainage are in the near-neutral range, and uranium is mostly present as U(VI) with dominant phosphate and carbonate complexes in the order UO 2 (HPO 4 ) 2 2 −  > UO 2 (CO 3 ) 2 2 −  > UO 2 (CO 3 ) 3 4 −  > UO 2 CO 3 0 . The concentrations of sulfate and trace elements rapidly decrease with increasing distance from the heap as a result of dilution due to influx of water from uncontaminated streams. Stream sediments sampled in the marshy area at the foot of the heap contain high U contents (up to 460 mg/kg), and other elements (Cu, Pb, Zn, Cd, and Se) due to their sorption on Fe- and Mn-hydroxides, organic matter, and co-precipitation with carbonate or sulfate. Similarly, the amount of uranium and other trace elements in sediments decreases with the distance from the heap because of the influx of clastic material from uncontaminated streams. Consequently, potentially harmful elements in water and sediments represent environmental risks only in the close vicinity of the heap. [ABSTRACT FROM AUTHOR]

A numerical analysis of self-ignition of a hydrogen-air-water vapor mixture at different initial pressures is carried out. The results of this analysis are used to make a shortened list of reactions that make the largest contribution to process rate during induction. A simplified analytical description of the system state before self-ignition, which makes it possible to calculate the thermal power and adiabatic heating rate of the system, is presented. A method for estimating the self-ignition limits from the adiabatic heating rate of the mixture is described. [ABSTRACT FROM AUTHOR]

The article discusses novel research results on combustion features of high-density Н 2 /О 2 mixtures (ρ H2 = 0.70–1.89 mol/dm 3 , ρ O2 = 0.32–0.81 mol/dm 3 ) diluted with nitrogen, carbon dioxide, or water vapor (from 46 to 76% mol.) at the uniform heating (1 K/min) of tubular reactor. Based on time dependencies of temperature increment in the reaction mixtures caused by the heat release during oxidation of H 2 , it is found that the self-ignition temperature of Н 2 /О 2 /N 2 and Н 2 /О 2 /H 2 O mixtures is by ≈ 30 K lower than that of the Н 2 /О 2 /СО 2 mixture. Unlike combustion of H 2 in the N 2 medium, in the CO 2 and H 2 O media a chain-thermal explosion is observed at a certain concentration of reagents. The influencing mechanisms of diluents on the H 2 oxidation dynamics, as well as the contribution of homogeneous and heterogeneous reactions in the heat release are revealed. It is established that high heat capacity of H 2 /O 2 /CO 2 mixture, chemical interaction between its components, and presence of CO 2 molecules adsorbed on the reactor inner surface, are the factors determining the H 2 oxidation dynamics in CO 2 medium. At oxidation of H 2 in the H 2 O medium, the process takes place against the background of water evaporation and, as a consequence, is characterized by increased heat capacity and thermal conductivity of the H 2 /O 2 /H 2 O reaction mixture. [ABSTRACT FROM AUTHOR]

This experimental work aims at investigating the potential development of self-heating when torrefied wood chips enter into contact with oxygen. Two kilograms of dried beech chips have been torrefied under nitrogen in a cylindrical reactor at 250 °C or 285 °C. The temperature was then leveled down between 100 °C and 160 °C and nitrogen flow was replaced by oxygen-containing flow. Influences of oxygen volumetric fraction and sweeping gas flow rate were investigated. Temperatures at different bed locations and gaseous product volumetric fractions at the bed output were monitored continuously. In some of the experiments a limited temperature rise was observed while in others a temperature runaway occurred. Thermal considerations indicate a clear enhancement of self-heating propensity with the severity of torrefaction. Gas phase analysis and molecule oxygen balance reveal an adsorption mechanism pathway and a higher rate of oxygen adsorption for the severely torrefied wood. Lowering the oxygen volumetric fraction or increasing the flow rate decrease self-heating behavior and was used to stop self-ignition of the bed. [ABSTRACT FROM AUTHOR]

The detection of supersonic combustion flow field provides valuable reference information for the efficient combustion of hypersonic vehicles. From the perspectives of scramjet operation during flight as well as experiment where the measured information is limited, flow field prediction using deep learning technology is a promising method that can provide future flow field evolution in scramjet combustors. This study proposes a convolutional neural network model of multi-temporal pressure information fusion to predict combustion flow field based on wall pressure. In addition, it analyzes the effect of various wall pressures on the flow field prediction. An experimental dataset of supersonic ground-pulse wind tunnel tests with different injection pressures is built through the combustion chamber wall under self-ignition conditions of a hydrogen-fueled scramjet. The results showed that the predicted flow field of the combustor agrees with the ground test, average correlation coefficient reached more than 90% and when the wall pressure number is 32, the average peak signal to noise ratio reaches about 23. and the main wave structure and turbulent boundary layer region are restored even if there is a shock boundary layer interaction. [ABSTRACT FROM AUTHOR]

Purpose: The aim of the work presented in this paper was the construction of a regression model describing the relationship between the experimentally determined value of the maximum temperature observed during the coal mine waste fire and physicochemical parameters characterizing the coal mine waste. Methods: The model was constructed with the application of the Partial Least Squares method. The experimental data analysed was acquired through the use of a laboratory test stand with a fixed bed reactor and analytical method of gas chromatography. Results: The constructed model was characterized by a good fit of the data used in its construction and the strong predictive ability for the new data. It illustrated the significant impact of the content of H and Fe2O3 and trace elements: Co, Cu, Pb, Sr, V and Zn in a sample on the value of the maximum temperature reached during the fire of coal mine waste. Practical implications: The practical importance of the work presented is clear in the light of the role of coal in the Polish economy and environmental aspects related to coal mining and the coal-based energy sector, in particularly to coal waste disposal and utilization. The model constructed contributes to the development of methods of self-ignition and fire risk assessment on coal waste dumps by determining the relationship between waste dump fire occurrence, the temperature observed during the fire and the physicochemical parameters characterizing the coal mine waste. Originality/value: The novelty of the study presented in the paper consists in both finding the relationships modelled and the data extraction methods applied in the research field concerned.

Purpose: The subject of the research presented in this paper is the analysis of physicochemical parameters characterizing coal mine waste, in terms of their impact on the initialization of the self-ignition phenomenon. Methods: The model was constructed with the application of Hierarchical Cluster Analysis complemented with a colour data map enabling the tracing of similarities between the samples of coal mine waste in the space of parameters and between the examined physicochemical parameters in the space of samples. The data set analysed included parameters characterizing coal mine waste collected from 12 various coal mine waste dumps, either in operation or closed, and where thermal effects either took place or were not reported. Results: The HCA model constructed and complemented with a colour data map revealed that the tendency of coal mine waste to self-ignite is primarily affected by the contents of moisture, ash, volatile matter, C and S, values of heat of combustion, calorific value and contents of SiO2, Al2O3, K2O, SO3, TiO2, Co, Ni and Rb. Practical implications: One of the major environmental hazards associated with the storage of coal mine waste is the possibility of self-ignition. At present, there are no applicable methods of assessment of this risk. The application of Hierarchical Clustering Analysis complemented with a colour data map enabled the analysis of data structures organized in matrix X(m × n) by tracing the similarities between the examined objects in the parameter space and between the measured parameters in the object space, and therefore contributed to the development of procedures of coal mine waste self-ignition risk assessment. Originality/value: The originality of the study presented in this paper comes from finding the parameters affecting the tendency of coal mine waste to self-ignite.

Experiments carried out in a rapid-injection setup with tangential introduction of the mixture into the reactor show that the mixtures self-ignite at temperatures substantially lower than the values reported in the literature. The measured ignition delay times do not exceed 0.1-0.2 s, although thermal conditions in the reactor allow the mixture to ignite with delays of more than 10 s. Videorecording shows that the mixture spontaneously ignites in a small volume at the center of the reactor; i.e., tangential mixture injection produces a vortex with a hotspot having a temperature at its center by more than 200 K higher than that of the reactor walls. An obstacle destroying the vortex eliminates the anomalous behavior of the self-ignition process. Temperatures measured at the center of the reactor with a thermocouple confirm the formation of a hotspot. The flame initiated by the ignition at a hotspot propagates through the mixture at a velocity several times higher than the laminar flame speed characteristic of the mixture. The mechanism of the formation of hotspots in unsteady vortex flows and their possible effect on explosion risk assessment in some practical situations are discussed. [ABSTRACT FROM AUTHOR]

This paper describes the numerical modeling of gas flow in a plane vortex chamber by using the Navier-Stokes equations. The model is based on the laws of conservation of mass, momentum, and energy for nonstationary two-dimensional compressible gas flow in the case of axial symmetry with a tangential component of the gas velocity. The processes of viscosity, thermal conductivity, and turbulence are accounted for. It is shown that the transition of the kinetic energy of gas into thermal energy as a result of transfer processes leads to the formation of hot spots in the boundary layers near the walls of the chamber. The gas temperature at these hot spots can exceed the gas combustion temperature, while the gas remains rather cold in the neighboring regions. This could be the reason for the cold gas self-ignition observed in the experiments. The turbulence of the flow and the processes of mixing and diffusion of the components make a significant contribution to the capacity of gas self-ignition. [ABSTRACT FROM AUTHOR]

Hot plate self-ignition standard tests at constant temperature have been performed on a paddle rice husk of size 75 to 1500 μm and a sieved rice husk of sizes 75 : 106, 106 : 120, and 120 : 150 μm. Modified tests using insulated metal ring have been investigated and compared with the standard test. Gases emissions showed that CO is the main component, and its maximum percentage occurs at the maximum temperature and depends on the dust layer thickness. It was found that as the thickness of dust layer increases, the maximum temperature and its corresponding time increase. Ring with insulation showed lower maximum and ignition temperature and ignition time compared with that without insulation. The flue gases temperature and CO% and CO2% emissions were higher in case of insulated ring. Thermal and physical properties as density, thermal conductivity, heat transfer coefficient, and kinetic parameters as activation energy and frequency factor have been determined. The activation energy of the sieved husk (75 : 106, 106 : 120, 120 : 150 μm) and paddle husk was 174.76, 183.37, 189.1, and 206.1 kJ/mol, respectively. Also, (QA0) factors were 2.696 × 1014, 6.211 × 1016, 6.105 × 1019 and 3.0461 × 1016 (J/kg s), respectively. It has been noted that as the dust particle size increases, the (E) and (QA0) values increase. © 2017 Curtin University of Technology and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In the present study, the self-ignition of high-pressure hydrogen released in atmospheric air through a diaphragm is visualized under various test conditions. The experimental results indicate that the hydrogen that jets through the rupturing diaphragm is mixed with the heated air near the tube wall. The self-ignition event originated from this mixing. The self-ignition was strongly dependent on the strength of an incident shock wave generated at the diaphragm rupture. As a result, a cylindrical flame that formed after the self-ignition shows a tendency to become longer as it propagates in the downstream direction. The head velocities of the hydrogen-air mixture and the cylindrical flame are consistent with that of a contact surface calculated from the measured shock speed. A modified self-ignition mechanism is proposed based on the experimental observations. [ABSTRACT FROM AUTHOR]

• The strength of shock wave is different before and after obstacles. • Obstacles decrease the minimum burst pressure and self-ignition occurs around obstacles. • Mechanism of self-ignition is different before and after obstacles. • Obstacles before the initial ignition location significantly accelerates initial self-ignition. The safe application of hydrogen has attracted increased attention. In particular, fire and explosion disasters may be induced by self-ignition during high-pressure hydrogen leakage. This study experimentally investigates the effect of obstacles on initial self-ignition when high-pressure hydrogen is suddenly released. High-pressure hydrogen release experiments are performed at burst pressures of 2–8 MPa. The hydrogen flame inside the tube is captured by a high-speed video camera and light sensors, and shock wave variations are measured by pressure transducers. The results show that the presence of obstacles inside the tube significantly affects shock wave flow. It is found in this study that obstacles decrease the minimum burst pressure needed for self-ignition and self-ignition is prone to occur in the vicinity of obstacles. The influence of obstacles on the initial ignition process are different at different self-ignition locations. On the one hand, the reflected shock wave originating from obstacles is the main reason for the promotion of self-ignition upstream of obstacles. On the other hand, the formation of a hydrogen/air mixture under the effect of turbulent flow and multi-dimensional shock waves leads to self-ignition downstream of obstacles. Meanwhile, the presence of obstacles before the initial ignition location (L in) shortens the distance between the ignition location and the burst disk and significantly accelerates the occurrence of initial self-ignition. Obstacles participate in the self-ignition process earlier when they are placed closer to the burst disk. However, obstacles after the initial ignition location (L in) show no effect on the initial ignition process. The results can guide the safe use of high-pressure hydrogen. [ABSTRACT FROM AUTHOR]

Most scholars used distilled-water-immersed coal to study the reasons why water-immersed coal was prone to self-ignition, while mine water in field was mostly alkaline. Thus, the influence of the alkalinity of mine water on the self-ignition properties of water-immersed coal was explored. Experimental results showed that during the low-temperature oxidation process, the CO productions of DL, ZX, XT alkaline-water-immersed coals were higher than that of raw coals when the temperature was increased to 392, 382, 370 K respectively. Compared with raw coals, the average apparent-activation-energies (E a) of the distilled-water-immersed DL, ZX, XT coals decreased by 6.89%, 5.94%, 7.75%, respectively. Moreover, that of the alkaline-water-immersed coals decreased by 15.83%, 11.76%, 16.64%, respectively. The reduction in the average apparent E a of the alkaline-water-immersed coal was approximately twice that of the distilled-water-immersed coal. Further investigation revealed that the content of oxygen-containing functional groups in alkaline-water-immersed coals were obviously more than those of the distilled-water-immersed coal. Therefore, the self-ignition ability of the alkaline-water-immersed coal was greater than distilled-water-immersed coal. Compared with the single soaking effect of pure water on coal, many physicochemical variations produced by the reactions of alkali ions with coal may be the main reason for the self-ignition of water-immersed coal. • The CO production of alkaline-water immersed coal increased notably than raw coal. • The average apparent E a of alkaline-water immersed coals reduced more than 10%. • The content of oxygen-containing functional groups increased about 4 times. • The risk of self-ignition of alkaline mine water immersed coal rose remarkably. • The alkali ions have greatly impact on the self-ignition of water-immersed coal. [ABSTRACT FROM AUTHOR]

The influence of addition of methane on the self-ignition of a hydrogen jet was studied experimentally for the pulse discharge into an open channel filled with air. During the pulse discharge from a high pressure chamber a shock wave is formed, which heats the surrounding air and a contact surface. The self-ignition of the jet occurred at the contact surface of the jet with air. A mixture of hydrogen with methane was preliminary prepared in a vessel of 40 L. The initial pressure of the mixture was varied from 3 to 15 MPa. During the discharge of the binary mixture ignition delays were measured relative to the moment of a breaking of the diaphragm. Ignition delays were determined by a photomultiplier tube. The delays were measured for different initial pressures and the methane addition. A molar concentration of the methane addition varied from 5% to 18%. Analysis of the effect of impurities on the thermodynamic parameters of the pulsed jets was carried out. [ABSTRACT FROM AUTHOR]

In the present work, self-ignition of high-pressure hydrogen that is released to air through a ruptured diaphragm is studied experimentally under various test conditions. Diaphragms with several thicknesses and scores are ruptured by high-pressure hydrogen, and the formation of a shock wave in the tube is experimentally observed. The behavior of the ruptured diaphragm and the subsequent self-ignition is photographed using a high-speed camera, while the self-emission intensity from a flame is measured using a photomultiplier. The experimental results show that the diaphragm rupturing condition is one of the main factors affecting the self-ignition phenomena. In addition, the self-ignition causes a ring-shaped flame that travels downstream, along a boundary between the hydrogen jet and the shock-heated air. [ABSTRACT FROM AUTHOR]