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• 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]

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 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]

[Display omitted] • The influence of initial temperature and equivalence ratio on the exergy loss of iso-octane self-ignition was researched. • The exergy loss of chemical reactions under unit heat release decreases with combustion temperature increasing. • Under constant equivalence ratio condition, the equivalence ratio ≤ 1.0 is beneficial to control exergy loss. • Under the condition of variable equivalence ratio, shifting equivalence ratio from the rich to the lean is beneficial for controlling exergy loss and maintaining heat release. With the severe low-carbon policies, energy utilization from the maximum useful-work exergy becomes one of the breakthrough points towards engine high efficiency. Hence, for constraining the combustion which is the main exergy loss source, the energy analysis from second law is imperative for the further efficiency improving. In this paper, the influence of boundary conditions on the heat release and exergy loss of chemical reaction during the constant-volume combustion of iso-octane at engine-like conditions was studied. It was found that the boundary conditions: initial temperature of 700 K, 900 K and 1100 K, equivalence ratio of 0.5, 1.0 and 1.5, have the same trend effect on the heat release and exergy loss rates of the same chemical reaction. It is impossible to increase the heat release and reduce the exergy loss at the same time, but different equivalent ratios have different degree effects on the reaction. Hence, a parameter α, the ratio of exergy loss on heat release, which stands for the exergy loss of unit heat release was promoted to consider the cost feature of chemical reactions. The results show that enhancing the medium and high temperature reaction during the combustion process is beneficial to reduce the parameter α, and the lean mixture is beneficial to obtain a small α. From the perspective of reaction type, it showed that the α of Fuel Fragment series reaction is the largest and its α is controlled by both temperature and equivalent ratio, the rich mixture should be conducive to reduce its α. The α of H2O2 Loop, H-O and CO-CO2 series reaction are small, and they are mainly controlled by the equivalence ratio, and lean mixture is favourable for reducing their α. Therefore, under the constant equivalence ratio, the best way to optimize the heat release and exergy loss is to control the equivalence ratio as small as 1.0. Under the strategy of the variable equivalence ratio, it is the best to change the mixture from rich to lean in the combustion process, and the reactions would sequentially go through the best α region of Fuel Fragment, H-O, CO-CO2, H2O2 Loop reactions, which would keep the α of whole combustion process maintain at the lowest level. [ABSTRACT FROM AUTHOR]

• Devolatilisation and combustion reactions was key reactions to stage II. • Consumption of functional groups agreed mainly with free radical reaction theory. • Proposed two-step reaction mechanism made 2.06% reduction in results deviation. • Reaction model of coal was f (α) = (1– α)0.5957 + (1– α)2.0020 by model reconstruction. • Differential evolution algorithm improved time–cost (16.7%) and accuracy (3.5%). Self-ignition of coal is a world-class problem leading to resource wastes and pollutions, whose solution required further study about coal reaction. The application of multiple reaction mechanisms on coal oxidation was analysed. The two-step reactions were determined for multi-step reaction mechanism by analysing mass loss rate characteristics, gaseous products concentration, and kinetic parameters. The key temperatures with standard deviations (maximum mass temperature, ignition temperature, and burnout temperature) under 10 °C/min were 315.7 ± 0.3, 471.2 ± 0.2, and 631.2 ± 1.1 °C. The equivalent chemical expression of coal was C 64.2 H 43 O 7.1 based upon proximate and elemental analyses. According to TG-FTIR and TG-GC/TCD results, the CO 2 (2337 cm−1) and CO (2097 cm−1) can be generated by carboxyl and aldehyde group respectively. Specially, the generation amount of CO 2 and CO increased rapidly after 417 °C and the maximum values appeared at 526 °C at 10 °C/min. The activation energy calculated by two isoconversional methods lied in 63.4 ± 0.1 to 133.37 ± 0.1 kJ/mol. The deviation equation was proposed in view of same effect (γ = 0.5) on simulation results for mass and mass loss rate. Kinetic triplets of coal reaction were computed using differential evolution algorithm. The average deviation was decreased by 2.06% by utilising two-step reaction mechanisms. [ABSTRACT FROM AUTHOR]

Livestock manure is an animal waste that is generated worldwide, and is one of the most abundant waste materials generated in Japan. Its excess application as a fertilizer can cause negative environmental problems; i.e., groundwater pollution and greenhouse gas emissions. When livestock manure is used as a solid fuel, a significant amount of energy is required to remove the high water content by conventional heating, and the resulting dry material has a self-ignition risk that makes it difficult to treat and store. This study focused on an energy-saving dewatering technology using liquefied dimethyl ether (DME) at room temperature, and investigated the dewatering properties of DME and the resulting mass balance of water in cattle manure. The fuel characteristics and the self-ignition risk of the dewatered cattle manure were also investigated. Under optimum conditions, over 98% of the water and some of the crude fat content in cattle manure could be removed after 70 min (seven batches) at a total DME/initial water ratio of 28.6. The manure was more efficiently dewatered than some other wet biomass such as sewage sludge and vegetable biomass. Moreover, a similarity in the kinetics between thermal drying and DME dewatering also emerged. The lower heating value (LHV) of DME dewatered manure under these conditions increased to 13.8 MJ/kg, which was 18.1 times that of original cattle manure. The cattle manure dewatered by DME had a lower self-ignition risk than thermally dried cattle manure because of the extraction of crude fat by DME. Therefore, it could be more efficiently produced and safely used as a biomass fuel following DME dewatering. [ABSTRACT FROM AUTHOR]

Storage of various types of biomass can result in spontaneous combustion processes, as they are capable of absorbing oxygen to produce exothermic oxidation reactions. Self-combustion is frequent in the storage and handling of biomass products and there are several factors related to their physical and chemical properties that influence their thermal susceptibility, that is, their tendency to oxidise and the subsequent ignition of the substance. Biomass from agriculture, forestry and waste origins with different chemical compositions are studied to analyse their behaviour and to determine the main factors that have an influence on the self-ignition risk. Among those factors, chemical composition, physical treatments and flammability characteristics are studied. Also thermal analysis methods are included in this study. Based on classical techniques of thermogravimetry and differential scanning calorimetry, particular experimental applications are used to assess the self-ignition risk of biomass products. Results are statistically analysed looking for correlations and grouping of samples. Statistical analysis concludes that the chemical composition of the biomass has an overriding role in characterising the self-ignition tendency of these biomasses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Examined self-ignition in high-pressure hydrogen jet owing to diaphragm rupture. • Visualization of flame growth behavior in a tube and out of a tube. • The longer a tube, the more flames develop in the tube. • The flame trapped in the vortex ring is the ignition source of the continuously released hydrogen jet. • The longer the tube length, the lower the burst pressure, and the jet flame is observed on the outside of the tube. This study investigates the behavior of self-ignited flames caused by high-pressure hydrogen release in seamless acrylic tubes. Such flames develop in and around vortex rings generated by the release of a preceding shock wave. The combination of hydrogen, which mixes with air as it flows through the vortex rings, and a sufficiently developed self-ignited flame yields a jet flame. The experiments performed in this study investigate flame retention outside circular acrylic tubes by varying their length and the burst pressure. As observed, for tube lengths below 1000 mm, the burst pressure decreases with increase in the tube length, and a jet flame is observed outside the tube. For tubes exceeding 1000 mm in length, jet flames are observed at burst pressure exceeding 4.8 MPa. Compared to extant research, a low critical pressure is observed in this study owing to high flame temperature and low thermal conductivity of the acrylic tube. [ABSTRACT FROM AUTHOR]

• The minimum release pressure for self-ignition is significantly higher. • The decrease of binary mixture self-ignition enhances with fuel molecular weight. • Nitrogen addition effectively inhibits flame development inside the tube. • Nitrogen-weaken ignited flame spouting from tube exit tends to blow out. • When self-ignition occurs, a partially premixed flame evolves first. This paper reports on the nitrogen addition effects on the self-ignition of high-pressure hydrogen leakage. It reveals that the intensity of produced incident shock decreases with more nitrogen addition, unbeneficial for self-ignition occurrence. Besides, nitrogen addition in hydrogen significantly reduces self-ignition possibilities inside the tube and self-sustained jet flame formation outside the tube. There exists a certain critical threshold of shock overpressure of about 1 MPa for self-ignition occurrence at different nitrogen additions. In our experiments, the impurity gases (N 2 , CO, CH 4) reduce the self-ignition possibilities in the same order as it reduces the shock intensity, namely, the effect of N 2 is similar to that of CO, larger than that of the same volume of CH 4. It suggests that the decrease of binary mixture self-ignition can be mostly explained by the reduction of shock intensity, and the decrease effect enhances with fuel molecular weight. Furthermore, nitrogen addition inhibits the flame development and propagation inside the tube. Nitrogen-weaken ignited flame barely survives during the expansion outside the tube or is soon extinguished inside the tube with more nitrogen addition. Moreover, the flame has a rapid flame length growth rate of 1–2 mm/μs in the initial stage, but the expanding velocity becomes lower to 0–1 mm/μs after a period of spread. Accordingly, a mechanism of the self-ignition process for high-pressure hydrogen release is proposed for cases with 0%-7.5% nitrogen addition. [ABSTRACT FROM AUTHOR]

• The shock wave and self-ignition of hydrogen in the arc-shaped tube are studied. • Reflected shock waves generated in the bend will spread both upstream and downstream. • Changing the position of the bend can promote or inhibit self-ignition. • Two possible processes of self-ignition are summarized. The self-ignition is a potential risk during the use of hydrogen, and arc-shaped bends exist in industrial hydrogen pipelines. So the experimental and numerical study were conducted to investigate the shock wave and self-ignition induced by high-pressure hydrogen release via three 90° arc-shaped tubes. The pressure profiles, photoelectric signals and jet flame were recorded. It was found that reflected shock waves generated in the bend would spread both upstream and downstream. And the reflected shock wave propagating downstream can enhance the intensity of the leading shock wave. But the velocity of leading shock wave in the bend decreased faster than that in the straight part. In addition, based on numerous experiments, two processes of self-ignition in the 90° arc-shaped tube were summarized. One was only affected by the strong leading shock wave, the other was affected by the combination of reflected shock wave and leading shock wave. Moreover, the distance from the bend to the burst disc had an effect on the self-ignition. When the distance from the bend to the burst disc was the shortest, it was least likely to cause self-ignition. Besides, the phenomena of flame quenching cannot occur in the bend of arc-shaped tube. The development characteristics of the external flame was studied, including the flame morphology near the Mach disk and the variation in the size of the jet flame. [ABSTRACT FROM AUTHOR]

[Display omitted] • A mechanistic model yielded accurate predictions of biomass self-ignition. • Modeling results were most sensitive to reaction kinetic parameters. • No parameter fitting was necessary. • The model is designed for loosely packed biomass dust beds. Loosely packed biomass dusts may self-heat and spontaneously ignite, especially when kept under elevated temperatures as in power plant mills. In this work, a mechanistic model was developed to predict self-ignition of biomass under such conditions. The model takes temperature- and gas phase species gradients into consideration. Reaction kinetic parameters were taken from a previous work, while material properties of biomass where found in the open literature. The model therefore did not require any parameter fitting. Model equations were discretized in one spatial dimension (here: for a cylinder). A series of lab scale experiments (10–40 g biomass dust) with beech, pine, sunflower husk pellets and wheat straw were used to validate the model. Predicted ignition temperatures were in good agreement (≈5% error) with experimental data. A sensitivity analysis showed the model to be most sensitive to reaction kinetic parameters, and to a lesser degree towards parameters influencing heat transfer. Mass transfer limitations (oxygen diffusion) did not appear to have a significant effect on the predicted onset of ignition. A scaling study showed sample size to have a larger influence on ignition temperatures than bulk density of the sample or oxygen availability. This study demonstrates that models based on chemical kinetics, heat- and mass transfer phenomena (as opposed to numerical correlations based on the Frank-Kamenetskii method) can yield accurate predictions of self-ignition temperatures. It also underlines the importance of finding realistic reaction kinetic parameters for low temperatures. [ABSTRACT FROM AUTHOR]

• The moisture absorption ratios of the pretreated products were all lower than 3.8%. • The moisture absorption proneness of pretreated products was obviously inhibited. • The cumulative released heat of extract below 300 °C was 887 kJ/kg lower than RS. • The released heat of biochar and hydrochar below 300 °C was 1000 kJ/kg higher than RS. • The influence mechanism of the three pretreatment methods was elucidated in depth. Three main pretreatment methods (torrefaction, hydrothermal carbonization and degradative solvent extraction) have been developed to deoxygenate and upgrade biomass into biochar, hydrochar and extract, respectively, which have a variety of high-value applications. However, the influence of the three pretreatment methods on the moisture absorption and self-ignition characteristics of biomass still remains unclear, which greatly affects the subsequent storage, transportation and utilization process of the biochar, hydrochar and extract. Therefore, the effects of torrefaction, hydrothermal carbonization and degradative solvent extraction on the moisture absorption and self-ignition characteristics of biomass were investigated and compared in this study. The results showed that the moisture absorption ratios of the pretreated products (biochar, hydrochar and extract) were as low as 3.8%, 1% and 0.8%, respectively, which was obviously lower than that of RS (8%), indicating that the moisture absorption propensities of biochar, hydrochar and extract were all obviously inhibited compared to RS. These results were caused by the dramatic removal of oxygen functional groups during the three pretreatment processes. The heat release of biochar (6950 kJ/kg) and hydrochar (7134 kJ/kg) below 300 °C were obviously higher than that of RS (5950 kJ/kg), while the heat release of extract (5063 kJ/kg) below 300 °C was much lower than that of RS. Compared with RS, biochar and hydrochar had higher specific surface areas and higher contents of inorganic metallic elements, which contributed to their higher self-ignition susceptibilities than those of RS. In contrast, the extract had a dense structure and extremely low oxygen functional groups contents as well as low inorganic metallic elements contents, which jointly led to its lower self-ignition susceptibility than that of RS. This research is expected to provide a comprehensive understanding of the influence mechanism of pretreatment on the moisture absorption and self-ignition characteristics of biomass. [ABSTRACT FROM AUTHOR]

• The critical pressure for ignition increases with methane addition concentration. • Methane addition can effectively inhibit flame development in the tube. • Three mechanisms of self-ignition for hydrogen with methane addition is proposed. • Hydrogen-methane flame spouting from tube exit tends to blow out. • The hydrogen-methane mixture brings greater combustion overpressure in chamber. In this paper, experimental investigations are carried out to study the effects of methane addition (2.5%, 5% and 7.5% vol.) on self-ignition and subsequent flame propagation of pressurized hydrogen release via a tube. The visualization method utilizing the high-speed direct photography and the measurements of pressure and light sensors are applied in this study. The result shows that the existence of methane can reduce the shock wave overpressure and the temperature of shock-heated air. Therefore, it has great effects on the self-ignition possibility. The minimum burst pressure required for self-ignition increases from 2.89 MPa (0% CH 4) up to 6.05 MPa (7.5% CH 4). Besides, methane addition can extremely reduce flame intensity and inhibit flame development inside the tube. Three possible self-ignition mechanisms for the hydrogen with methane addition are discussed. Under the effects of methane addition, hydrogen flame spouting from tube exit tends to blow out. In all cases with 7.5% methane addition, no self-sustained jet flame is formed outside the tube for burst pressure up to 10.0 MPa. When self-sustained flame is formed in a confined space, a great rise in overpressure is generated due to the intense combustion. It is suggested that more methane addition can reduce the self-ignition probability and the formation of self-sustained flame. However, with the increasing of CH 4 additions, once the jet fire is formed in a confined space, it may cause greater overpressure and thus bring greater casualties and property losses. [ABSTRACT FROM AUTHOR]