Comparative studies on the pyrolysis of cellulose,hemicellulose, and lignin based on combined kinetics

The thermal degradation behavior and pyrolytic mechanism of cellulose, hemicellulose, and lignin are investigated at different heating rates from 10 Kmin^?1 to 100 Kmin^?1 with a step-size of 10 Kmin^?1 using thermogravimetric analysis (TGA) equipment. It is observed that there are one, two, and three stages of pyrolytic reactions takes place in cellulose, hemicellulose, and lignin respectively. Isoconversional method is not suitable to analyse pyrolysis of hemicellulose and lignin as it involves multi-step reactions. The activation energies of the main decomposition stage for cellulose, hemicellulose, and lignin are 199.66, 95.39, and 174.40 kJ mol^?1 respectively. It is deduced that the pyrolysis reaction of cellulose corresponds to random scission mechanism while the pyrolysis reaction of hemicellulose and lignin follows the order based reaction mechanisms.     

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (${\mathrm{X}}_{{\mathrm{H}}_2}$) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for ${\mathrm{X}}_{{\mathrm{H}}_2}$ less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for ${\mathrm{X}}_{{\mathrm{H}}_2}$ = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.     

Experimental study on ignition and combustion of coal-rice husk blends pellets in air and oxy-fuel conditions

The ignition and combustion characteristics of anthracite-rice husk (AC-RH) and bituminous coal-rice husk (BC-RH) pellets were investigated in a vertical heating tube furnace under different experimental condition, for gas temperature (873 K–1073 K) and under air and different oxygen concentration (21–70%) in CO₂/O₂ atmosphere. The investigation of the ignition and combustion characteristics focused on ignition mechanism, ignition delay, ignition temperature and combustion process. AC-RH pellets had two ignition mechanism in CO2/O2 atmosphere: homogeneous ignition of volatile and heterogeneous ignition of char. Heterogeneous ignition region decreased while homogeneous ignition increased as rice husk blending ratio increased in oxygen concentration-gas temperature plane. Only homogeneous ignition was observed when rice husk blending ratio was 30%. As for BC-RH pellets, only homogeneous ignition occurred in all experimental conditions. The effect of the rice husk blending on the anthracite was more pronounced than the bituminous coal for ignition mechanism. As oxygen concentration increased, a significant reduction in ignition delay and ignition temperature was observed at low rice husk blending ratio and low gas temperature. but at 1073 K, high oxidizer temperature weakened the effect of biomass blending and oxygen concentration on ignition delay and ignition temperature. Meanwhile, at 20% and 30% rice husk blending ratio, it also weakened the effect of oxygen concentration and oxidizer temperature on ignition delay and ignition temperature. In contrast, blending ratio had a more significant effect on ignition behavior. The replacement of N₂ by CO₂ at the same oxygen concentration contributed to an increase in ignition delay time and internal ignition temperature, which suppressed the ignition behavior. Different ignition mechanisms corresponded to different combustion processes.     

Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures

Experimental and numerical study on the effect of pressure and equivalence ratio on the ignition delay times of the DME/H₂/O₂ mixtures diluted in argon were conducted using a shock tube and CHEMKIN II package at equivalence ratios of 0.5–2.0, pressures of 1.2–10 atm and hydrogen fractions of 0–100%. It was found that the measured ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$ ≤80%, the ignition is controlled by the DME chemistry and ignition delay times present a typical Arrhenius pressure dependence and weak equivalence ratio dependence. For the DME/H₂ mixture at 80% < XH₂$X_{{\text{H}}_2}$ < 98%, the ignition is controlled by the combined chemistries of DME and hydrogen, and the ignition delay times give higher ignition activation energy at higher pressures and a typical Arrhenius equivalence ratio dependence. However, for the DME/H₂ mixture at XH₂$X_{{\text{H}}_2}$≥98%, the ignition is controlled by the hydrogen chemistry and ignition delay time shows complex pressure dependence and weak equivalence ratio dependence. Comparison of the measurements of neat DME and neat hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco Mech 1.3 [1], LLNL DME Mech 2, 3 and 4 and Princeton-Zhao Mech [5], shows that NUIG Aramco Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio dependence at various hydrogen fractions. The sensitivity and normalized H-radicals consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions that control the ignition characteristics of DME/H₂ mixtures were revealed. Further chemical kinetic analysis was made to interpret the ignition delay time dependence on pressure and equivalence ratio at varied hydrogen fractions.     

Isothermal torrefaction kinetics of hemicellulose, cellulose, lignin and xylan using thermogravimetric analysis

In recent years, torrefaction, a mild pyrolysis process carried out at the temperature range of 200-300 °C, has been considered as an effective route for improving the properties of biomass. Hemicellulose, cellulose, lignin and xylan are the basic constituents in biomass and their thermal behavior is highly related to biomass degradation in a high-temperature environment. In order to provide a useful insight into biomass torrefaction, this study develops the isothermal kinetics to predict the thermal decompositions of hemicellulose, cellulose, lignin and xylan. A thermogravimetry is used to perform torrefaction and five torrefaction temperatures of 200, 225, 250, 275 and 300 °C with 1 h heating duration are taken into account. From the analyses, the recommended values of the order of reaction of hemicellulose, cellulose, lignin and xylan are 3, 1, 1 and 9, respectively, whereas their activation energies are 187.06, 124.42, 37.58 and 67.83 kJ mol⁻¹, respectively. A comparison between the predictions and the experiments suggests that the developed model can provide a good evaluation on the thermal degradations of the constituents, expect for cellulose at 300 °C and hemicellulose at 275 °C. Eventually, co-torrefaction of hemicellulose, cellulose and lignin based on the model is predicted and compared to the thermogravimetric analysis.     

Characteristics and conditions for ignition of bio-coal mixtures based on coal and forest combustible material

The results of the experimental studies of the ignition process of a pulverized fuel mixture based on coal and biomass — forest combustible material (FCM) have been given. As the second component of the fuel the wastes of various deciduous or coniferous species of trees have been used.The experiments have been carried out on the equipment that provides a fairly low level of error when registering the main characteristics (ignition delay time of the fuel particles _tign, ambient temperature T_g) of heat and mass transfer processes occurring together during ignition of bio-coal fuel during thermal preparation. It has been established that the addition of biomass to coal leads to a significant reduction (up to 30%) of the entire ignition period of the fuel mixture. The video recording of the ignition processes has allowed to identify the main stages of thermal preparation and ignition of the bio-coal fuel particles. It has been established that the particles of biomass (leaf or fir needles) ignite faster than coal.A mathematical model has been developed based on the results of a detailed analysis of the videograms of the ignition process of the bio-coal mixtures describing the joint flow of the main processes of heat and mass transfer under conditions of the intense phase and thermochemical transformations. A numerical simulation of the ignition process has been carried out and the ignition delay times have been established. A comparative analysis of the theoretical and experimental values of t_ign has shown their good conformance.     

Experimental study on spark ignition of non-premixed hydrogen jets

The leaks of pressurized hydrogen can be ignited if an ignition source is within a certain distance from the source of the leaks, and jet fires or explosions may take place. In this paper, a high speed camera was used to investigate the ignition kernel development, ignition probability and flame propagation along the axis of hydrogen jets, which leaked from a 3-mm-internal-diameter nozzle and were ignited by an electric spark. Experimental results indicate that for successful ignition events, the ignition delay time increases with an increase of the distance between the nozzle and the electrode. Ignitable zone of the hydrogen jets is underestimated if using the predicted hydrogen concentration along the jets centerline. The average rate of downstream flame decreases but that of the upstream flame increases with the electrode going far from the nozzle.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

Experimental study on explosion characteristics of syngas with different ignition positions and hydrogen fraction

The influence of different ignition positions and hydrogen volume fractions on the explosion characteristics of syngas is studied in a rectangular half-open tube. Three ignition positions were set at the axis of the tube, which are 0 mm, 600 mm and 1100 mm away from the closed end, respectively. A range of hydrogen volume fraction (φ) from 10% to 90% were concerned. Experimental results show that different ignition positions and hydrogen volume fraction have important influence on flame propagation structure. When ignited at 600 mm from the closed end on the tube axis, distorted tulip flame forms when flame propagates to the closed end. The formations of the tulip flame and the distorted tulip flame are accompanied by a change in the direction of the flame front propagation. The flame propagation structure and pressure are largely affected by the ignition position and the hydrogen volume fraction. At the same ignition position, flame propagation speed increases with the growing of hydrogen volume fraction. And the pressure oscillates more severe as the ignition location is closer to the open end. And pressure oscillations bring two different forms. The first form is that the pressure has a periodic oscillation. The amplitude of the pressure oscillation gradually increases. It takes several cycles from the start of the oscillation to the peak. For the second form, the pressure reaches the peak of the oscillation in the first cycle of the start to the oscillation.     

Experimental methodology and heat transfer model for identification of ignition kinetics of powdered fuels

A methodology for investigating and quantifying the thermal processes leading to ignition of rapidly heated metal powders was developed. The simple experiment involves observing ignition of a powder coated on the surface of an electrically heated filament and is well suited for a variety of powdered fuels. In an experimental case study, the ignition temperature of spherical Mg powder was detected optically at different heating rates. To interpret the results, a heat transfer model was developed for a multilayer powder coating on the heated cylindrical filament. The thermal contact resistance between particles was determined from the measured bulk thermal diffusivity of the powder considering the experimental particle size distribution. An Arrhenius type expression was used to describe the exothermic chemical processes leading to ignition with the pre-exponent as an adjustable parameter. For Mg, a pre-exponent value identified by matching the calculations with the experimental data was found to be 10¹⁰ kg/m² s. The match between the experimental and predicted temperatures and times of ignition was good for different heating rates, which validated the proposed heat transfer model and indicated that the developed methodology is practically useful.     

Effects of hydrogen peroxide addition on two-stage ignition characteristics of n-heptane/air mixtures

Rising fuel cost and environmental concerns of greenhouse emissions have driven the development of advanced engine technology with optimal fuel strategy that can simultaneously yield high thermal efficiency and low emissions. Due to its strong reactivity and extra oxygen atom serving as an oxidizer, hydrogen peroxide (H₂O₂) has been used along with other hydrocarbons to promote overall combustion process. To explore the potential benefits of H₂O₂ in clean combustion technology, a numerical study with detailed chemistry is conducted to investigate the effects of H₂O₂ addition on the two-stage ignition characteristics of n-heptane/air mixtures at low-to-intermediate temperatures (below 1000 K), with due emphasis on how the negative temperature coefficient (NTC) behavior is affected. The results show that H₂O₂ addition shortens both the first-stage and total ignition delay times of n-heptane/air mixtures and suppresses the NTC behavior by reducing the upper turnover temperature. With increasing H₂O₂ addition, the lower turnover temperature, corresponding to the first-stage ignition delay minimum, is found to increase first and then decrease. Chemical kinetic analyses show that the addition of H₂O₂ promotes both first- and second-stage ignition reactivity by enhancing OH production through H₂O₂ decomposition. Furthermore, low-temperature chemistry controls the first-stage ignition, while H₂O₂ chemistry dominates the second-stage ignition.     

Further study on the ignition delay times of propane–hydrogen–oxygen–argon mixtures: Effect of equivalence ratio

Using a shock tube facility, measurements on ignition delay times of propane/hydrogen mixtures (hydrogen fraction X_H2 is from 0% to 100%) were conducted at equivalence ratios of 0.5, 1.0 and 2.0. Results show that when X_H2 is less than 70%, ignition delay time shows a strong Arrhenius temperature dependence, and the ignition delay time increases with the increase of equivalence ratio. When X_H2 is larger than 95%, the ignition delay times do not retain an Arrhenius-like temperature dependence, and the effect of equivalence ratio is very weak when the hydrogen fraction is further increased. Numerical studies were made using two selected kinetic mechanisms and the results show that the predicted ignition delay times give a reasonable agreement with the measurements under all test conditions. Both measurements and predictions show that for mixtures with X_H2 less than 70%, the ignition delay time is only moderately decreased with the increase of X_H2, indicating that hydrogen addition has a weak effect on the ignition enhancement. Sensitivity analysis reveals the key reactions that control the simulation of ignition delay time. Further investigation of the H-atom consumption is made to interpret the ignition delay time dependence on equivalence ratio and X_H2.     

Experimental investigation on ignition and burnout characteristics of semi-coke and bituminous coal blends

Ignition and burnout characteristics of semi-coke and bituminous coal blends were investigated by thermogravimetric analyzer and drop tube furnace. The results showed that the ignitability index and the comprehensive combustion characteristic index of the blends decrease as the blending proportion of semi-coke increases, but the average activation energy of the blends increases gradually. Ignition mode of bituminous coal is changed from homogeneous to hetero-homogeneous ignition with the increasing of semi-coke content in the blends. When the mixing proportion of semi-coke is lower than 45%, the burnout rate is lower than the weighted value in the early stage of combustion and gradually higher than the weighted value with the development of combustion process. However, the burnout is always lower than the weighted value to mix with 67% semi-coke. Increasing furnace temperature from 850 °C to 1050 °C can improve the mid-term reaction process, alleviate the negative effects of semi-coke on the co-combustion process and increase the burnout rate. So less than 45% semi-coke blending ratio and increasing furnace temperature are recommended for semi-coke and bituminous coal co-combustion.     

Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass

Torrefaction is a thermal pretreatment process for biomass where raw biomass is heated in the temperatures of 200-300 °C under an inert or nitrogen atmosphere. The main constituents contained in biomass include hemicellulose, cellulose and lignin; therefore, the thermal decomposition characteristics of these constituents play a crucial role in determining the performance of torrefaction of lignocellulosic materials. To gain a fundamental insight into biomass torrefaction, five basic constituents, including hemicellulose, cellulose, lignin, xylan and dextran, were individually torrefied in a thermogravimetry. Two pure materials, xylose and glucose, were torrefied as well for comparison. Three torrefaction temperatures of 230, 260 and 290 °C, corresponding to light, mild and severe torrefactions, were taken into account. The experiments suggested the weight losses of the tested samples could be classified into three groups; they consisted of a weakly active reaction, a moderately active reaction and a strongly active reaction, depending on the natures of the tested materials. Co-torrefactions of the blend of hemicellulose, cellulose and lignin at the three torrefaction temperatures were also examined. The weight losses of the blend were very close to those from the linear superposition of the individual samples, suggesting that no synergistic effect from the co-torrefactions was exhibited.     

Effect of cellulose and lignin content on pyrolysis and combustion characteristics for several types of biomass

Fundamental pyrolysis and combustion behaviors for several types of biomass are tested by a thermo-gravimetric analyzer. The main compositions of cellulose and lignin contents for several types of biomass are analyzed chemically. Based on the main composition results obtained, the experimental results for the actual biomass samples are compared with those for the simulated biomass, which is made of the mixture of the cellulose with lignin chemical. The morphological changes before and after the reactions are also observed by a scanning electron microscope. The main compositions in the biomass consisted of cellulose and lignin. The cellulose content was more than lignin for the biomass samples selected in this study. The reaction for the actual biomass samples proceeded with the two stages. The first and second stage corresponded to devolatilization and char combustion during combustion, respectively. The first stage showed rapid mass decrease caused by cellulose decomposition. At the second stage, lignin decomposed for pyrolysis and its char burned for combustion. For the biomass with higher cellulose content, the pyrolysis rate became faster. While, the biomass with higher lignin content gave slower pyrolysis rate. The cellulose and lignin content in the biomasses was one of the important parameters to evaluate the pyrolysis characteristics. The combustion characteristics for the actual biomass depends on the char morphology produced.     

A review on the laminar flame speed and ignition delay time of Syngas mixtures

Syngas has shown great success in Integrated Gasification Combined Cycle (IGCC) technology for providing cleaner and higher efficiency energy production with minimal environment impact. Thus, it is promising that Syngas is able to replace the conventional fossil fuel resources, while at the same time minimizing pollutants. The drawback of traditional Gas Turbines that burn Syngas is that they use diffusion flame combustion technology that suffers from low efficiency and high emissions. Recently, Lean premixed combustion technique has emerged as a promising solution, but the variation of hydrogen fraction in Syngas has prohibited its usage. Besides, other gases such as CO₂, N₂, H₂O, NH₃, and CH₄ in Syngas have adverse effects on the combustion characteristics. To address these issues, better understanding of the Syngas's fundamental combustion properties are vital. Hence, recent works published on Syngas combustion at lean-premixed and Gas Turbine relevant conditions are reviewed, classified according to their objectives, and remarks were concluded.     

The ignition,oxidation, and combustion of kerosene: A review of experimental and kinetic modeling

For modeling the combustion of aviation fuels, consisting of very complex hydrocarbon mixtures, it is often necessary to use less complex surrogate mixtures. The various surrogates used to represent kerosene and the available kinetic data for the ignition, oxidation, and combustion of kerosene and surrogate mixtures are reviewed. Recent achievements in chemical kinetic modeling of kerosene combustion using model-fuels of variable complexity are also presented.     

气化炉渣着火特性试验研究

煤炭是我国的主题能源,我国正处于工业化快速推进阶段,未来较长时期,能源需求仍将快速增长.以煤气化为核心技术的煤制油、煤制气等产业在我国得到了大力发展,高残炭的气化炉渣作为燃料是一种重要的利用途径.本文依托哈尔滨锅炉厂有限责任公司一维火焰燃烧炉对气化炉渣的着火特性进行试验研究,研究表明:该种气化炉渣的着火温度为716℃,...     

Measurements and kinetic study on ignition delay times of propane/hydrogen in argon diluted oxygen

Measurements on ignition delay times of propane/hydrogen mixtures in argon diluted oxygen were conducted for hydrogen fractions in the fuel mixtures (XH₂)$\left(X_{{\text{H}}_2}\right)$ from 0 to 100%, pressures of 1.2, 4.0 and 10 atm, and temperatures from 1000 to 1600 K using the shock-tube. Results show that for XH₂$X_{{\text{H}}_2}$ less than 70%, ignition delay time shows a strong Arrhenius temperature dependence and it decreases with the increase of pressure, while for XH₂$X_{{\text{H}}_2}$ larger than 90%, there is a crossover pressure dependence of the ignition delay time with increasing temperature. Numerical studies were made using the selected kinetic mechanisms and results show that the predicted ignition delay time gives a reasonable agreement with the measurements. Both measurements and predictions show that for XH₂$X_{{\text{H}}_2}$ less than 70%, the ignition delay time is only moderately decreased with the increase of XH₂$X_{{\text{H}}_2}$, indicating that hydrogen addition has weak effect on ignition enhancement. Sensitivity analysis reveals the key reactions that control the simulation of ignition delay time. Kinetic study is made to interpret the ignition delay time dependence on pressure and XH₂$X_{{\text{H}}_2}$.     

The effect of ignition delay time on the explosion behavior in non-uniform hydrogen-air mixtures

The purpose of this study is to examine the explosion characteristics of non-uniform hydrogen-air mixtures with turbulent mixing. In the experiment, hydrogen is first filled into a 20 L spherical chamber to a desired initial pressure, then air is introduced into the same chamber through a fast response solenoid valve, by adjusting the ignition delay time (t_d), i.e., the time period between the end of air injection and the action of ignition, the turbulent mixing strengthen (or called uniformity of hydrogen-air mixture) is then changed. The experimental results show that the explosions are overall enhanced as t_d decreases, which indicates that turbulence plays a leading role in enhancing the explosion behaviors. In addition, it is found that the effect of turbulence on p_max is more prominent in end-wall ignition than that in center ignition. This is because the heat loss per unit time is higher in end-wall ignition due to the flame front continuously contacts with inner wall of the chamber throughout the explosion process, although the explosion duration time t_e for both ignition cases is reduced when turbulence is introduced, heat loss reduction for end-wall ignition is generally larger than that in center ignition. Lately, a systematical analysis of the turbulent effect associated with various equivalence ratios on the explosion characteristics is conducted in end-wall ignition. Those experimental results illustrate that the turbulence-enhancing influence is more noticeable when hydrogen-air mixtures move toward the lower explosion limit. However, no significant influence of turbulence on explosion process can be found as combustible mixtures tend to the fuel-rich side. This is mainly because that when hydrogen-air mixtures tend to fuel-rich side, τ_e reduction caused by the presence of turbulence is relatively weak as compared with that under quiescent condition, resulting in heat loss during explosion process changes slightly, hence there is no significant impact on explosion parameters.