一维热爆炸试验传热的数值模拟

针对加热过程所涉及的主要传热学问题,分别建立空气夹层复合传热模型和炸药受热分解放热数值模型;计算分析了473K下油浴和空气浴加热时系统辐射率、空气对流换热系数对RDX、HMX、TATB 3种炸药热点火延滞时间的影响。结果表明,473K油浴加热和空气浴加热时RDX分别在151.7s和3 372.6s时发生热点火,表明相同温度和炸药,油浴加热时热点火延滞时间远小于空气浴加热;空气浴加热时,同一种炸药在相同温度下,随系统辐射率的降低,热点火延滞时间增加;RDX、HMX和TATB炸药辐射率由0.9降至0.1时,热点火延滞时间分别增加了180.1%、168.9%和169.3%;相同温度、相同系统辐射率条件下,对流换热系数减小,热点火延滞时间增加。     

Time‐resolved measurements of pyrolysis and combustion products of PMMA

We studied transient chemical speciation during high‐temperature solid material pyrolysis and combustion in air. Our objective was to develop a database of chemical burn signatures. The material tested was the thermoplastic PMMA. Material samples were heated in an infrared furnace until they pyrolyzed, ignited, and combusted in air. Time‐resolved quantitative measurements of the exhaust species CO₂, O₂, hydrocarbons, and CO along with exhaust gas temperature were obtained. Two categories of experiments were conducted: (1) pyrolysis tests in which there was no combustion; (2) combustion tests with chemical reaction and heat release. During heating, the sample underwent numerous processes that appear as diagnostic sequences. In the pyrolysis tests, as the furnace temperature was raised, the CO and hydrocarbon (HC) signals underwent transition from one peak to two peaks. In the combustion tests, spontaneous ignition occurred at higher test temperatures as evidenced by an exothermic reaction reported by the thermocouples, leading to three‐peak CO and HC profiles. The measured O₂/CO₂ ratio of 1.3 ± 0.1 agreed with stoichiometric methyl‐methacrylate monomer decomposition. Calculations of the power output using two independent methods supported (1) that combustion was experimentally observed in the furnace, and (2) the accuracy of the combustion gas analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

Feedback-Controlled Firing of Reaction-Bonded Aluminum Oxide

The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1^°C/min to 1100^°C. Heat-treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30^°C/mm) and compositional gradients (>5000 mol·(m³·mm)⁻¹), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h.     

Experiment study of the altitude effects on spontaneous ignition characteristics of wood

This paper has studied the influence of ambient pressure and oxygen content on spontaneous ignition of wood by conducting contrastive experiments with wood slab exposed to high temperature radiation at two different altitudes. The measurement of mass loss, time to ignition, and surface temperature of wood are carried out. Results show that mass loss rate of wood at high altitudes (3650 m) is higher than the one at low altitudes (50 m), while ignition delay time of the sample at high altitude is shorter. The surface temperatures at the time of ignition in the two different places are both close to each other, which indicates that the pressure did not affects the ignition temperature. The theoretical analysis on the phenomenon of different ignition behavior of wood in these two altitude environments has been presented.     

Thermal decomposition and ignition of coal

A study of the thermal decomposition and ignition of coal as functions of pelletizing pressure and dwell time has revealed that: (1) ignition and thermal behaviour are related to the apparent density of the pelletized coal; (2) for a given apparent density of pelletized coal, the ignition temperature is related to the rate constants of thermal decomposition. Isothermal decomposition in air at 550 °C has been shown to fit the Avrami-Erofeev equation for three-dimensional growth of nuclei.     

分解炉空气分级燃烧及NOx排放特性研究

随着我国经济的飞速发展,作为重要基础材料的水泥产品需求量极大且趋于稳定。水泥生产过程中的NOx排放与燃煤火电厂和汽车尾气产生的NOx排放已成为空气污染的主要来源,而分解炉是降低水泥生产工艺中NOx排放的有效设备。笔者在引入高温烟气的模拟分解炉内进行空气分级燃烧试验,研究配风位置、配风比例以及石灰石/煤比例对分解炉内燃烧和NOx排放特性的影响规律。试验稳定过程中,高温烟气发生装置的给煤量和配风量保持不变。此时,高温烟气发生装置的时间平均温度为911℃,其产生的高温烟气温度稳定在750℃左右,高温烟气中NOx主要以NO和N2O的形式存在,其浓度分别为261.49×10^-6和12.96×10^-6。该股高温烟气将模拟实际回转窑产生的烟气进入分解炉内。在分解炉的上部区域(距离顶部0~2 000 mm区域)的温度为800~1 000℃,与实际分解炉运行温度一致,排放烟气中NOx主要以NO和N2O形式存在。随着中间配风位置的下移,煤粉燃烧放热区域下移,而顶部区域的石灰石吸热量变化较小,则原有热量平衡被打破且原有吸热量高于现有放热量,导致顶部区域内燃烧温度降低。此时,还原气氛中煤粉燃烧和石灰石分解反应时间均变长,导致NOx的还原反应更加充分。但石灰石分解产生的氧化钙(CaO)作为中间产物会促进NO的生成反应,其反应时间增加也促进了NO的生成;另一方面,石灰石作为催化剂参与焦炭和挥发分还原NO的反应过程,分解炉顶部区域的温度下降使得该还原反应变弱。综上,NO的最终排放浓度是以上反应的综合结果。随着配风位置的下移,该变化对NO的生成作用更加明显,故NO的排放浓度逐渐升高。当一级风量与二级风量的配风比例降低时,分解炉上部区域的煤粉燃烧份额减少和石灰石分解量降低,而分解炉下部区域的煤粉燃烧份额增加和未分解的石灰石份额增加,但石灰石的吸热增加量高于燃烧增加份额的放热量,因此分解炉内整体温度均降低。分解炉内NO浓度是由石灰石催化的氧化过程和还原过程综合决定的。一级风量变小时,尾部CO浓度随之增加,烟气中NO浓度呈现降低的趋势。当石灰石/煤比例增加时,分解炉内沿程温度逐渐下降。随着石灰石给粉量增加,分解炉内石灰石受热分解产生的CaO浓度增加,CaO催化NO还原反应更剧烈,从而NO浓度逐渐降低。而石灰石给粉量增加和分解炉温度降低的过程导致尾部的CO浓度升高。     

煤粉富氧燃烧着火温度预测的优化随机森林(GA-RF)模型

二氧化碳排放是造成温室效应的主要原因之一,富氧燃烧作为一种有效的碳减排与封存技术具有广泛的研究前景。在燃煤电厂中煤粉富氧燃烧的着火温度是燃烧器设计和运行安全的重要指标,并且与煤粉组成成分、煤粉粒径以及燃烧氛围都有复杂的相关性。因此,对煤粉富氧燃烧着火温度的预测模型研究意义重大。采用滴管炉分别测量了5种煤粉在O2体积分数为30%、35%、40%、50%、60%、70%、80%、90%、100%富氧条件下的着火温度,分析了氧气体积分数和煤粉的组成成分与着火温度之间的关系。研究发现,随着氧气体积分数分数的增加,5种煤样的着火温度均显著下降,且挥发分越高的煤,下降幅度越大。将45组试验着火温度数据与其他研究者采用同样方法测得的69组着火温度数据组成机器学习样品库,以煤粉的元素分析、工业分析、煤粉粒径及氧气体积分数为输入条件,以着火温度T为目标输出,构建了遗传算法优化的随机森林模型(GA-RF模型),准确预报了煤粉富氧燃烧的着火温度,其预报精度为:R2>0.99,RMSE<16,MAE<8。通过模型参数重要性分析发现,氢组分超过5%后,着火温度出现阶跃式上升,现有煤粉着火数据也证实了该现象。     

INVESTIGATION OF IGNITION OF THERMOPLASTICS THROUGH THE HOT WIRE IGNITION TEST

The purpose of this study was to investigate the ignition phenomena of selected polymeric materials—Plexiglas® (PMMA), polyethylene, Lexan® (polycarbonate), and nylon using the Hot Wire Ignition Test. Underwriters' Laboratories (UL) prescribe this test for polymeric materials used in enclosures or insulation systems of electrical equipment. The main objective of this study was to identify the effect of specimen thickness on the ignition time. In addition, temperature changes at the surface of selected polymeric materials during the different phases of the ignition process were also investigated.

The apparatus used for this test was specified by ASTM D 3874–90a—Standard Test Method for Ignition of Materials by Hot Wire Sources. It consisted of a supply circuit capable of maintaining a continuous 60-Hz power density of 0.26 W/mm over 12 in. (304.8 mm) of No. 24 AWG, Nichrome V wire and a variable transformer for adjustment of the voltage to achieve the desired current.

Temperature measurements were also performed using a Cole-Parmer Digi Source® 12-channel scanning thermometer and 30 AWG chromel–alumel (type K) thermocouples.

A linear relationship between time to ignition and thickness was verified for the four polymers of thicknesses 1/16 in., 1/8 in., 3/16 in., and 1/4 in. The different polymers evaluated in terms of ease of ignition from most ignitable to least ignitable were in the order: Plexiglas, polyethylene, Lexan, and nylon. From the temperature measurements performed in tests with Plexiglas, it appears that the major factor affecting the time to ignition is the rate of temperature rise during the decomposition process. One important recommendation is that temperature measurements similar to those performed with four thicknesses of Plexiglas should be performed with Lexan (polycarbonate), nylon, and polyethylene. These data would confirm the principal role played by the rate of temperature rise during the decomposition process on the time to ignition of thermoplastic materials.     

高炉喷吹煤燃烧性能的实验研究

介绍了利用差热分析天平,采用程序升温法对高炉喷吹煤在不同升温速率和空气流量下的燃烧性能进行的实验,分析了升温速率和空气流量对失重率和燃烧特征温度的影响。实验结果表明:失重率随升温速率的增大而降低,随空气流量的增大而升高。燃烧特征温度随着升温速率的增大而升高,随着空气流量的增大而降低。     

Kinetic study on thermal decomposition of woods in oxidative environment

The purpose of this work is to gain knowledge on kinetics of biomass decomposition under oxidative atmospheres, mainly examining effect of heating rate on different biomass species. Two sets of experiments are carried out: the first set of experiments is thermal decomposition of four different wood particles, namely aspens, birch, oak and pine under an oxidative atmosphere and analysis with TGA; and the second set is to use large size samples of wood under different heat fluxes in a purpose-built furnace, where the temperature distribution, mass loss and ignition characteristics are recorded and analyzed by a data post-processing system. The experimental data is then used to develop a two-step reactions kinetic scheme with low and high temperature regions while the activation energy for the reactions of the species under different heating rates is calculated. It is found that the activation energy of the second stage reaction for the species with similar constituent fractions tends to converge to a similar value under the high heating rate.     

Factors influencing the ignition of flames from air-fired swirl pf burners retrofitted to oxy-fuel

Combustion tests were undertaken in a vertical pilot-scale furnace (1.2 MWt) at the IHI test facility in Aioi, Japan, to compare the performance of an air fired swirl burner retrofitted to oxy fired pf coal combustion with the oxy fired feed conditions established to match the furnace heat transfer for the air fired case. A turn down test at a reduced load was also conducted to study the impact on flame stability and furnace performance.Experimental results include gas temperature measurements using pyrometry to infer the ignition location of the flames, flue gas composition analysis, and residence time and carbon burnout. Theoretical computational fluid dynamics (CFD) modelling studies using the Fluent 6.2 code were made to infer mechanisms for flame ignition changes.Previous research has identified that differences in the gas compositions of air and oxy systems increase particle ignition times and reduce flame propagation velocity in laminar systems. The current study also suggests changes in jet aerodynamics, due to burner primary and secondary velocity differences (and hence the momentum flux ratio of the flows) also influence flame shape and type.For the oxy fuel retrofit considered, the higher momentum flux of the primary stream of the oxy-fuel burner causes the predicted ignition to be delayed and occur further distant from the burner nozzle, with the difference being accentuated at low load. However, the study was limited to experimental flames being all Type-0 (low swirl with no internal recirculation), and therefore future work consider higher swirl flames (with internal recirculation) more common in industry.     

硼烷合乙二胺的点火、燃烧及热解特性

为探索高氢含量硼烷合乙二胺储氢材料在空气中的燃烧性能,使用气相合成方法制备了硼烷合乙二胺(EDAB)微米颗粒。通过连续激光点火实验,测试了其点火和燃烧参数;通过在氮气和空气中的热解实验,研究了其燃烧过程机理。结果表明,硼烷合乙二胺的燃烧具有点火延迟时间短、点火能量低的特点,在常温常压静止空气流中,点火功率密度为10~9 W/m~2量级时,微米级硼烷合乙二胺的点火延迟时间为0.000 2~0.000 9s,最小点火能量仅0.000 1J;其连续激光点火燃烧过程分为两个阶段,分别产生亮蓝色与黄色火焰。结合材料在氮气及空气中的热解行为,推测该材料燃烧第1阶段蓝色火焰对应其热解释放氢气的燃烧,第2阶段的黄色火焰对应其骨架高温裂解所生成挥发物的燃烧。     

Correlation between the hot plate ignition test and other laboratory-scale flammability tests on rubber conveyor belts with fabric skeletons

The hot plate ignition test was conducted on ten different conveyor rubber belts with fabric skeletons. In this test, a 25 × 25 mm² belt sample was placed on a stainless steel plate in an electric furnace and heated. The ‘ignition temperature’ was determined on each sample in almost the same way as proposed in Canada. The test results were compared with those from other laboratory-scale flammability tests (i.e. the small-scale flame tests, the Oxygen Index test and the laboratory-scale gallery test). The calculated results of both sample and rank correlation coefficients imply that the ‘10 signition temperature’ in this hot plate ignition test has a correlation with the results from both the Oxygen Index test and the laboratory-scale gallery test, but that it has very little or no correlation with those from the small-scale flame tests. On the other hand, it is found that the ‘60 s ignition temperature’ in this test has a far better correlation both with the results from the small-scale flame tests and with the time to ignition in the laboratory-scale gallery test than the ‘10 s ignition temperature’.     

Study of Ignition of High-Energy Materials with Boron and Aluminum and Titanium Diborides

This paper describes the ignition of high-energy materials (HEMs) on the basis of ammonium perchlorate and ammonium nitrate and an energetic binder, containing the powders of Al (base composition), B, AlB₂, AlB₁₂, and TiB₂, upon initiation of the process by a CO2 laser in the heat flux density range of 90–200 W/cm². The ignition delay time and surface temperature of the reaction layer during the heating and ignition of HEMs in air are determined. It is obtained that the complete replacement of a micron-sized aluminum powder by amorphous boron in the composition of HEMs significantly reduces the ignition delay time of the sample (by 2.2–2.8 times) with the same heat flux density, and this occurs due to the high chemical activity of and difference between the mechanisms of oxidation of boron particles. The use of aluminum diboride in HEMs reduces the ignition delay time by 1.7–2.2 times in comparison with the base composition. The ignition delay time of the HEM sample with titanium diboride decreases slightly (by 10–25%) relative to the ignition delay time of the base composition.     

煤粒着火及燃烧稳定性

用热天平研究了几种煤焦的化学着火时间，发现与环境温度有较明显的联系，并综合煤粒加热物理着火时间，提出了一个衡量煤在实际锅炉中的燃烧稳定性的影响。     

Influence of the adjustable vane position on the flow and combustion characteristics of a down-fired pulverized-coal 300 MWe utility boiler

Experiments with a small-scale cyclone burner used for burner enrichment in a down-fired pulverized-coal 300 MW_e utility have been conducted on an air/particle test facility. Particle separating efficiency was obtained with different positions of an adjustable vane. Industrial experiments were performed on a full-scale boiler. The gas temperature distribution along the primary air and coal mixture flow, gas temperature distribution of the furnace, and gas components such as O₂, CO, CO₂ and NO_X in the near-wall region were measured for the first time. The influence of the adjustable vane position on coal combustion in the furnace was determined. With the adjustable vanes at the nozzle, ignition of the primary air and pulverized-coal mixture was delayed and the gas temperature peak was above the burner arch, with high NO_X emission. Raising the vanes can bring forward the ignition point but results in the fuel-rich flow being up ahead of time, leading to a rise in carbon content in fly ash and NO_X emission.     

不同干燥程度胜利褐煤燃烧与自燃特性

为了探究高水分褐煤干燥后的燃烧与自燃特性变化,采用一维火焰炉、煤粉着火炉以及自燃试验台对不同干燥程度的胜利褐煤进行了试验研究。试验结果表明,试验样品的着火温度随着干燥程度的增加而降低,随着风煤比的增加而增加,煤粉细度同样会对着火温度产生影响。在燃用干燥褐煤过程中宜采用较高一次风率以提高制粉系统的安全性。胜利褐煤及其干燥褐煤均属于极易燃尽煤种,燃尽率均在99.4%以上,水分的变化对燃尽率影响不大。建议胜利褐煤与20%水分干燥褐煤的运行氧含量控制在3.5%左右。随着干燥程度的加深和粒径的减小,褐煤越容易自燃。胜利褐煤干燥到20%以下可能有自燃的风险。     

Transient group combustion of the pulverized coal particles in spherical cloud

A transient group combustion model for the pulverized coal particles in a spherical cloud is developed to predict the transient group combustion characteristics. The submodels, which account for the detailed combustion process of both homogeneous and heterogeneous reactions, are also applied for the pulverized coal particles. The numerical simulation of the collective behaviors of ignition and its subsequent burning were carried out. The ignition mechanism is characterized by the heterogeneous process of the dilute cloud and the homogeneous process of the dense cloud. Two dominant flame structures are observed: one flame penetrating inside the cloud and the other moving outside the cloud. The effects of various parameters (i.e., radiation heat transfer, group combustion number, air temperature, air oxygen concentration, particle size distribution, and particle number density) on the particle mass burning rate and on the overall performance of the group combustion are examined. The results are in good agreement with existing experimental data.     

Effect of Secondary Mixing on Starting Characteristics of a Cold Diesel Engine

Results of experiments performed in a constant-volume bomb are presented. The conditions in the bomb were typical of starting regimes of diesel engines. Cooled (down to 286 K) or noncooled inserts were placed in the bomb volume on the way of the fuel–air jet. These inserts were plates simulating a sector of the combustor wall. Leaf deflectors of the jet were mounted on the plates to obtain the effect of secondary mixing. In the case of an elevated pressure in the medium (2.5 MPa and higher), the presence of the inserts, including those equipped by deflectors, independent of their thermal state, affects ignition only at extremely low temperatures of the medium, at which the primary ignition of fuel vapors and products of its thermal decomposition occurs in an immediate vicinity of the insert. If the initial pressure in the medium is rather low (0.9 MPa), the influence of the insert is significant: a noncooled surface reduces both the induction period (by a factor of 1.5 as compared to injection into a free volume) and the limiting temperature of ignition. If the insert surface is cooled, however, the ignition delay increases by 20%, and the limiting temperature of ignition increases by 100 K. Under these conditions, a leaf deflector of the jet mounted on the wall-simulator surface significantly improves the ignition characteristics of the mixture: the ignition delay and the limiting temperature decrease down to values that do not exceed those upon injection into a free volume. It is assumed that the secondary injection of the mixture from the edge of the leaf deflector into the free space of the working volume leads to the formation of a closed circular vortex containing both the liquid phase (atomized fuel drops) and the gaseous phase (fuel vapors, products of its partial thermal decomposition, and air). Key words: diesel engine, mixing.     

工业萘管式炉节能及自动化升级改造

针对山西豪仑科化工有限公司36万t/a焦油加工装置工业萘精馏单元存在的煤气单耗高、安全系数低等问题,通过更换燃烧器、在管式炉顶部增加空气预热器、自动化升级方面增加自动点火装置和快速切断阀等措施,使管式炉排烟温度由450℃降低到小于140℃,助燃空气温度由30℃提高到280℃,管式炉热效率由原来的68.5%提高到81.4%,加工吨焦油的煤气单耗降低约0.61 m³,加热炉操作便捷性和安全稳定性得到提高。