孔隙结构对焦碳燃烧影响的数值研究

煤粒在热解过程中,由于不同的加热速度和不同的热解最终温度所产生的孔隙结构也不同,本文建立了多组分并存的情况下,综合考虑化学反应,内外传递过程及颗粒孔结构变化在内的单颗粒焦碳着火燃烧模型,计算出３种不同孔隙结构焦碳的转化率,氧化反应率及比面积的变化,比较了它们之间的差异。     

Explosibility parameters for mixtures of pulverized fuel and ash

This paper reports the results of experiments done to examine the explosibility of pulverized fuel dusts (two different coals and petroleum coke) and their waste products (bottom and fly ashes). Tests were conducted for the fuel dusts alone and also blended with other fuels and ashes. The explosion parameters of interest were maximum pressure and rate of pressure rise, and ignition energy and temperature. Petroleum coke was found to present a lesser explosion hazard than the coals. Admixture of either coal or petroleum coke with fly ash resulted in explosible mixtures at volatile contents dependent on the composition of the mixture components and their particle sizes.     

Characterization of Products from the Pyrolysis of Municipal Solid Waste

Pyrolysis of municipal solid waste (MSW) in the form of refuse-derived fuel (RDF) was performed in a fixed bed reactor to recover char, oil/wax and combustible gases. It was established that the yield and composition of the products recovered depended on temperature; the percentage yield of char fell as the pyrolysis temperature was raised from 400°C to 700°C, whereas that of oil/wax and gaseous products rose. The chars recovered were also found to have properties which depended on the size fraction analysed. The gaseous product evolved during the pyrolysis had a high calorific value, making the gases suitable for use as a fuel. Fourier transform infrared spectrometry (FT-ir) of the derived oils, indicated that they were chemically very complex and highly oxygenated, and have the potential of being used either as a petrochemical feedstock or as a substitute liquid fuel.     

Evaluation of beech for production of bio-char,bio-oil and gaseous materials

Evaluation of Oriental beech (Fagus orientalis L.) was investigated with aspect of thermo-chemical conversion to obtain bio-char, bio-oil and gaseous. When the pyrolysis temperature increased, the bio-char yield decreased. A high temperature and smaller particles increase the heating rate resulting in a decreased bio-char yield. The bio-char obtained are carbon rich, with high heating value and relatively pollution-free potential solid biofuel. The liquefaction yield sharply increased with increasing the temperature near critical temperature and after that. In the pyrolysis, increases of liquid yields are considerably sharply for all of the samples with increasing of pyrolysis temperature from 690 K to 720 K. The beechnut oil was converted to biodiesel in supercritical methanol without using the catalyst. Experiments have been carried out in an autoclave at 493, 523 and 593 K, and with molar ratios of 1:6–1:40 of the oil to methanol. The yield of alkyl ester increased with increasing the molar ratio of oil to alcohol.     

Application and effect of negative pressure chambers on pipeline explosion venting

Explosion venting is a frequently-used way to lower explosion pressure and accident loss. Recently, studies of vessel explosion venting have received much attention, while little attention has been paid to pipe explosion venting. This study researched the characteristics of explosion venting for Coal Bed Methane (CBM) transfer pipe, and proposed the way of explosion venting to chamber in order to avoid the influence of explosion venting on external environment, and investigated the effects of explosion venting to atmosphere and chamber. When explosion venting to atmosphere, the average explosion impulse 4.89 kPa s; when explosion venting to 0 MPa (atmospheric pressure) chamber, average explosion impulse is 7.52 kPa s; when explosion venting to −0.01 MPa chamber, explosion flame and pressure obviously drop, and average explosion impulse decreases to 4.08 kPa s; when explosion venting to −0.09 MPa chamber, explosion flame goes out and average explosion impulse is 1.45 kPa s. Thus, the effect of explosion venting to negative chamber is far better than that to atmospheric chamber. Negative chamber can absorb more explosion gas and energy, increase stretch of explosion flame, and eliminate free radical of gas explosion. All these can promote the effect of explosion venting to negative chamber.     

PAN dust explosion inhibition mechanisms of NaHCO3 and Al(OH)3

We investigate the PAN dust explosion inhibition behaviors of NaHCO₃ and Al(OH)₃ in a 20 L spherical explosion system and a transparent pipe explosion propagation test system. The results show that, in the standard 20 L spherical explosion system, the highest PAN dust explosion concentration is 500 g/m³, the maximum explosion pressure is 0.661 MPa, and the maximum explosion pressure increase rate is 31.64 MPa/s; adding 50% NaHCO₃ and 60% Al(OH)₃ can totally inhibit PAN dust explosion. In the DN0.15 m transparent pipe explosion propagation test system, for 500 g/m³ PAN dust, the initial explosion flame velocity is 102 m/s, the initial pressure is 0.46 MPa, and the initial temperature is 967 °C; adding 60% NaHCO₃ and 70% Al(OH)₃ can totally inhibit PAN dust explosion flames. Through FTIR and TG analyses, we obtain the explosion products and pyrolysis patterns of the explosion products of PAN dust, NaHCO₃, and Al(OH)₃. On this basis, we also summarize the PAN dust explosion inhibition mechanisms of NaHCO₃ and Al(OH)₃.     

Biomass to Fuels: Upgrading of Flash Pyrolysis Oil by Reactive Distillation Using a High Boiling Alcohol and Acid Catalysts

We here report our studies on the upgrading of flash pyrolysis oil using an improved alcohol treatment method. The method consists of treating pyrolysis oil with a high boiling alcohol like n-butanol in the presence of a (solid) acid catalyst at 323–353 K under reduced pressure (<10 kPa). Using this approach, the water content of the pyrolysis oil is reduced significantly. Variables like the type of alcohol (n-butanol, ethylene glycol, 2-ethyl-hexyl-alcohol) and liquid and solid acids were explored and the product properties of the resulting upgraded pyrolysis oil (kinematics viscosity, water content, pH and heating value) were determined. On the basis of these screenings studies, n-butanol and the solid acid Nafion SAC13 seem to have the highest potential. The product properties of the upgraded pyrolysis oils, and particularly the heating value and the acidity are considerably improved. These improvements are not only due to blending effects but also the result of the occurrence of chemical reactions (a.o. esterification).     

高温高压下煤孔隙结构的变化对瓦斯吸附特性的影响

为了研究高温高压条件下煤孔隙结构变化对瓦斯吸附特性的影响,选取九里山矿无烟煤,在压力为7 MPa、温度为40～130℃的条件下进行等温吸附实验和压汞实验。研究结果表明:煤样对甲烷的等温吸附曲线在该压力、温度条件下符合Ⅰ型吸附曲线特性,吸附规律符合Langmuir吸附模型;在压力7 MPa和温度130℃条件下,煤样的孔隙结构发生一定的变化,煤的比表面积增大、累计孔体积降低,可见孔及裂隙的数量比例增高,加强了煤样孔隙之间的连通度,导致原本吸附在煤样表面的甲烷分子大量解吸;在压力不变的情况下,随着温度的不断增高,煤的极限吸附量逐渐减小,其主要原因是样品孔隙结构的破坏和分子间作用力的变化。     

Explosion parameters of wood chip-derived syngas in air

The wood gasification process poses serious concerns about the risk of explosion. The design of prevention and mitigation measures requires the knowledge of safety parameters, such as the maximum explosion pressure, the maximum rate of pressure rise and the gas deflagration index, K_G, at standard ambient temperature (25 °C) and pressure (1 bar) conditions. However, the analysis at specific process conditions is strongly recommended, as the explosion behavior of gas mixtures may be completely different.In the work presented in this paper, the explosion behavior of mixtures with composition representative of wood chip-derived syngas (CO/H₂/CH₄/CO₂/N₂ mixtures with and without H₂O) was experimentally studied in a closed combustion chamber. Experiments were run at two temperatures, 300 °C and 10 °C, and at atmospheric pressure. Test conditions were requested by the safety engineering designer of an existing industrial-scale wood gasification plant. In order to identify the specific fuel–air ratios to be analyzed, thus reducing the number of experimental tests, a preliminary thermo-kinetic study was performed.Results have shown that the mixtures investigated can be classified as low-reactivity mixtures, the higher value of K_G found (∼36 bar m/s) being much lower than the K_G value of methane (55 bar m/s @ 25 °C).     

Effect of particle size and dust layer size on ignition characteristics of PMMA dust layer on hot surface

Deposition of combustible dust on a hot surface is a hidden danger of fire. In this work, polymethylmethacrylate (PMMA) dust was selected to analyse the influence of dust layer diameter, dust particle size and dust layer thickness on the ignition characteristics of PMMA dust layer. Critical heating temperatures and ignition time had been measured. The STA-GC/MS-FTIR analysis was used to determine that the main products of PMMA pyrolysis were MMA, CO, CO₂, and C₂H₄, of which CO and C₂H₄ were transported to the ambient to cause gas phase combustion on the surface of the dust layer. For 10 mm thick dust layer, the critical heating temperatures of 5 μm PMMA, 100 nm PMMA, and 30 μm PMMA were 300 °C, 330 °C, and 320 °C. As the thickness of the dust layer increased, the gas transport path became longer, the critical heating temperature and ignition time increased. The characteristic particle size (D [3,2]) was utilized to represent the true particle size, and the ignition time increased with the increase of the characteristic particle size. The increase in the diameter of the dust layer had a slight effect on the temperature history and ignition time of the dust layer.     

Effect of superfine KHCO3 and ABC powder on ignition sensitivity of PMMA dust layer

In order to evaluate the flame-retardant capacity of KHCO₃ and ABC on the ignition of PMMA dust layer accumulation on hot surfaces, the ignition time and critical heating temperature of PMMA/KHCO₃ and PMMA/ABC dust layer were experimentally investigated. The thermal stability of the mixed dust, the condensed phase products and gas phase products of the mixed dust combustion were analyzed to reveal the flame-retardant mechanism. The ignition time of 30 μm PMMA was obviously longer than that of 5 μm PMMA, and the critical heating temperature was close to that of 5 μm PMMA. KHCO₃ and ABC could greatly extend the ignition time of the PMMA dust layer and increase the critical heating temperature of the dust layer. ABC was more effective than KHCO₃. The decomposition of KHCO₃ and ABC absorbed the heat and inhibits the pyrolysis of PMMA. The HPO₃ and P₂O₅ generated by the decomposition of ABC would cover the surface of PMMA aggregates or particles and act as a physical barrier. The main light combustible gas produced by PMMA pyrolysis were CO and C₂H₄. The CO₂ generated during the decomposition of KHCO₃ could dilute the combustible gas in the ambient to inhibit the combustion of PMMA.     

An Experimental Study on Pyrolysis of Biomass

In this study, pyrolysis of sugarcane bagasse was performed in fixed bed tubular reactor under the conditions of nitrogen atmosphere, by varying temperature and different particle sizes. The effect of final pyrolysis temperature from 400 to 500°C and the nitrogen flow rate from 50 to 200 cc min⁻¹ on the pyrolysis product yields from sugarcane bagasse have been investigated. The Maximum bio-oil yield obtained is 24.12 wt% at the final pyrolysis temperature of 450°C, N₂ flow rate of 50 cc min⁻¹ and particle size of mesh number −8 + 12. The yield of bio-oil decreases with increase in temperature from 450 to 550°C and N₂ flow rate from 50 to 200 cc min⁻¹. The various characteristics of pyrolysis oil obtained under these conditions were identified on the basis of standard test methods. The empirical formula of pyrolysis oil with a heating value of 37.01 MJ Kg⁻¹ was established as CH_1.434 O_0.555 N_0.004. The results from the pyrolysis show the potential of sugarcane bagasse as an important source of liquid hydrocarbon fuel.     

Pressure characteristics and dynamic response of coal mine refuge chamber with underground gas explosion

Coal mine refuge chambers are new devices for coal mine safety which can provide basic survival conditions after gas explosion. In order to simulate the propagation of underground methane/air mixture blast wave, and check structural safety of coal mine mobile refuge chamber, an underground tunnel model and a refuge chamber model have been established based on explicit nonlinear dynamic ANSYS/LS-DYNA 970 program. Results show that the reflected wave pressure on the impact surface was about two times higher than that on the incident one. The relationship between the pressure fields of the chamber was analyzed. The maximum pressure of gas explosion reached about 0.71 MPa, and the pulse width was 360 ms. The maximum absolute displacement and stress occurs at the main door center and the connection of stiffeners and the front plate, respectively. The entire coal mine mobile refuge chamber was in elastic state and its strength and stiffness meet the safety requirements. The cabin door, the front plate and the connecting flange at cabin back as well as the stiffeners on each side were the most critical components. Suggestions were put forward for the refuge chamber.     

4种典型易燃烟煤的着火特性分析*

为了研究典型易燃烟煤的着火特性,预防和控制煤尘爆炸,采用粉尘云最低着火温度实验装置和同步热分析仪,分别研究崔木长焰煤、东荣二矿气煤、察哈素不粘煤和丁集焦煤4种烟煤在不同条件下的煤尘云最低着火温度和煤尘热解过程。研究结果表明:当煤尘云浓度从0.90 kg/m3上升到5.99 kg/m3时,4种煤尘的最低着火温度先降后升,在1.50 kg/m3煤尘云浓度时,4种煤尘的最低着火温度均达到最小,分别为450,580,610,620 ℃。随着升温速率的升高,煤的着火温度、峰值温度、燃尽温度和煤样最大放热量整体呈上升趋势,失重率整体呈下降趋势,在5~20 ℃/min的升温速率范围内,崔木长焰煤、东荣二矿气煤、察哈素不粘煤和丁集焦煤热解过程中的最小着火温度分别为354.17,404.37,443.18,484.13 ℃。4种烟煤的最低着火温度和热解过程中的最小着火温度有相对应关系,研究结论可为以上4个煤矿的具体煤样研究和数据分析提供参考依据。     

Determination of heterogeneous reaction mechanisms: A key milestone in dust explosion modelling

Reaction kinetics is fundamental for modelling the thermal oxidation of a solid phase, in processes such as dust explosions, combustion or gasification. The methodology followed in this study consists in i) the experimental identification of the reaction mechanisms involved in the explosion of organic powders, ii) the proposal of simplified mechanisms of pyrolysis and oxidation, iii) the implementation of the model to assess the explosion severity of organic dusts. Flash pyrolysis and combustion experiments were carried out on starch (22 μm) and cellulose (53 μm) at temperatures ranging from 973 K to 1173 K. The gases generated were collected and analyzed by gas chromatography. In this paper, a semi-global pyrolysis model was developed for reactive systems with low Damköhler number. It is in good agreement with the experimental data and shows that both carbon monoxide and hydrogen are mainly generated during the pyrolysis of the solid, the generation of the latter compound being greatly promoted at high temperature. A simplified combustion model was also proposed by adding two oxidation reactions of the pyrolysis products. In parallel, flame propagation tests were performed in a semi open tube in order to assess the burning velocity of such compounds. The laminar burning velocity of cellulose was determined to be 21 cm s⁻¹. Finally, this model will be integrated to a predictive model of dust explosions and its validation will be based on experimental data obtained using the 20 L explosion sphere. The explosion severity of cellulose was determined and will be used to develop and adjust the predictive model.     

秸秆生物质炭的制备及吸附性能研究

将麦秆、向日葵秆和玉米秆低温热解制备生物质炭。采用傅里叶红外光谱、X射线衍射等仪器对其结构进行了表征。研究了热解温度、溶液p H值、初始浓度、吸附时间以及吸附剂用量等因素对各生物质炭吸附亚甲基蓝效果的影响。实验结果表明,3种生物质炭均含有多孔的微晶炭,表面存在大量的活性-OH,-COOH等基团,对亚甲基蓝有较强的吸附作用;在最佳吸附条件下,玉米秆炭、向日葵秆炭以及麦秆炭对亚甲基蓝的吸附容量分别为26.6,26.2,14.7 mg/g。     

Thermal safety assessment for solid organic peroxides

The thermal hazards of dicumyl peroxide (DCP) and benzoyl peroxide (BPO), self-reactive chemicals are identified and characterized using high-pressuredifferential scanning calorimeter, and simultaneous thermogravimetric analyzer, a C80 micro-calorimeter is used. The apparent exothermic onset temperature of DCP is found to be between the range of 112–122 °C for different heating rates in DSC tests. There are two coupled peaks of BPO around 105 °C at both the heating rates of 4.0 and 8.0 °C/min while no endothermic peak showed at lower heating rates. Furthermore, another endothermic peak appears immediately after the exothermic peak at about 211 °C of DCP under high-pressure conditions. For BPO, the endothermic peak before the exothermic peak disappears as the pressure increases to 1.0 and 1.5 MPa. The average values of apparent activation energy calculated by Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods during the conversion rate between 15 and 75% of DCP are 80.69 and 74.05 kJ/mol, and that of BPO are 119.96 and 112.93 kJ/mol, respectively. According to the isothermal tests, the thermal decomposition of DCP behaviors is an n-th order reaction while BPO conforms to the laws of autocatalytic reaction.     

Making hybrid mixture explosions a common case

Explosions of gas-dust hybrid mixtures have long been considered as particular cases encountered in specific industrial contexts. However, it should be reminded that during the explosion of an organic powder, the presence of a hybrid mixture composed of the dust itself and its pyrolysis gases is compulsory. On these premises, an experimental study to determine the role of cellulose pyrolysis products (gaseous, condensable and solid) on the global phenomenon is presented. Hybrid mixture explosion tests were exploited to carry out the investigation. The G-G furnace and the 20 L sphere were employed. Several experimental strategies were chosen to demonstrate the impact of pyrolysis reaction on the explosion of organic powders: i) the fuel equivalence ratio of the reactive mixture (case 1), or ii) the mass of reactants (case 2) were respectively kept constant, iii) the effects of water vapor, char and tar were tested. They were next compared to identify the most suitable one. The two first experimental approaches lead to significantly different results: only case 2 keeps the maximum explosion pressure almost constant, but maximum rate of pressure rises and deflagration index greatly decrease when the pyrolysis gases concentration decreases, which highlights the importance of the pyrolysis reaction on the explosion kinetics. It should also be stressed that the maximum explosion severity is not obtained for the pure gases but when a small dust content is added. The same evolution is observed when a small amount of char is introduced to pyrolysis gases, which underlines the influence of the radiative transfer. Adding small amounts of tar to cellulose tends to increase its explosion severity. However, this impact is less than that generated by the addition of pyrolysis gases.     

An experimental study of an LPG tank at low filling level heated by a remote wall fire

This paper describes an experimental study of 2300 L pressure vessels exposed to remote fire heating by a natural gas fuelled wall fire simulator. The tanks were filled to 15% capacity with commercial liquid propane. The flame intensity and distance were varied to study the effect of different heating levels on the tank and its lading.The fire simulator is first characterized with tests including fire thermocouples, radiative flux meters and thermal imaging. With the appropriate positioning of a target tank it is possible to get very realistic fire heat fluxes at the tank surface.Three tests were conducted with the 2300 L tanks filled to 15% capacity with propane. The tanks were positioned at three different distances from the wall fire resulting in measured average peak heat flux at the tank surfaces ranging between 24 and 43 kW m^?2. The data shows rapid rise in vapour space wall temperatures, significant temperature stratification in the vapour space, and moderate rate of pressure rise. These results provide excellent data for the validation of computer models used to predict the response of pressure vessels exposed to moderate heating from a remote fire.     

Effect of pressure for dust dispersion on minimum ignition temperature

The existing technical standards for determining the minimum ignition temperature (MIT) of dust clouds are inadequate. According to existing technical standards, this study expands the test pressure conditions. The MIT of dust clouds can be determined by a variety of different dispersion pressures, that is, 0.02, 0.04, 0.06, 0.08 and 0.1 MPa. But, whether the MIT of dust clouds tested under different dispersion pressures is consistent is lacking in the corresponding exploration. In this study, a set of measurement system of instantaneous concentration and particle size was established, which can observe the flow of dust clouds and the distribution of the instantaneous particle size and concentration of dust clouds in the heating furnace tube. On this basis, the MIT of dust clouds is determined, and the physical mechanism of dispersion pressure affecting the MIT of dust clouds is revealed. The results show that the MIT of dust clouds is consistent when the dispersion pressure in the range of 0.02–0.06 MPa. When the dispersion pressure is not in this range, the MIT of the dust clouds is very different due to the obvious increase of the instantaneous particle size of the dust clouds.