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水溶性粉笔高温燃烧行为及其熄灭后白烟成分分析
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此为15岁初三生个人观点，请谨慎辨别与使用 作者： 七氟化氯 摘要 以市售水溶性无尘粉笔为研究对象，系统研究其在800–1500?℃高温条件下的热解特性、自持燃烧行为、火焰动态演化规律及熄灭瞬间白烟的化学组成与形成机理。实验结果表明，水溶性粉笔在目标温度区间内可被稳定引燃并实现自持燃烧，燃烧过程无需外部热源持续供给，火焰形态、燃烧持续性与蜡烛类蜡质燃料高度相似。燃烧过程中，粉笔样品伴随明显的熔融?凝固相变行为，外观颜色由初始白色逐渐转变为亮红色，火焰高度随燃烧进程呈现持续上升趋势。熄灭瞬间产生大量白色浓烟，烟气气味与硫酸钙与熟石灰混合高温加热气味相近。通过105?℃预处理除杂实验与氮气氛围无氧热解对照实验证实，燃烧所依赖的可燃气相产物来源于粉笔内部有机组分在高温下的热裂解反应，而非环境吸附杂质。结合实验样品实际成分配方：钛白粉28%–40%、脂肪醇聚氧乙烯醚18%–27%、甘油三硬脂酸酯12%–22%、硬脂酸15%–20%、硫酸钙4%–8%、滑石粉0.5%–1.5%，进一步分析表明白烟主要由未完全燃烧的有机蒸气冷凝微颗粒、脂肪酸与表面活性剂热解中间产物以及二氧化钛无机烟尘三相共同构成。研究系统揭示了水溶性粉笔“高温熔融?液相渗透?有机热解?气相燃烧?熄火冷凝成烟”的完整反应路径，为教学教具火灾安全评价、高温热稳定性评估、烟气危害分析及环保书写材料改性提供了基础实验数据与理论支撑。 关键词：水溶性粉笔；高温燃烧；自持燃烧；白烟组分；热裂解；火灾安全；无机填料；有机蜡质 1 引言 水溶性无尘粉笔作为传统石膏粉笔的升级替代产品，凭借低粉尘、易擦拭、书写顺滑、环保低刺激等优势，在现代教育场景中得到广泛应用。相较于以硫酸钙或碳酸钙为主要成分的传统粉笔，水溶性粉笔在配方体系上进行了结构性优化，通过引入大量有机蜡质、脂肪酸、非离子表面活性剂等组分，实现书写无尘化与湿擦易清除功能，显著降低粉尘对师生呼吸道健康的影响。随着多媒体教室、智慧教室建设不断普及，水溶性粉笔已逐步取代传统粉笔，成为教学书写工具的主流选择。 目前国内外针对水溶性粉笔的研究主要集中于常温应用性能优化方向，包括书写流畅度改进、黏结体系优化、表面活性剂选型、无尘化机理、环保无毒配方设计、擦拭效率提升等。此类研究均围绕日常使用场景展开，关注材料的应用性能、人体安全性、环境友好性与工业化生产工艺，而针对极端条件尤其是高温环境下的热行为、燃烧特性、气相产物释放规律、火灾潜在危险性以及熄灭白烟来源与组成的系统性研究仍较为匮乏。 在实际教学环境与实验室场景中，存在多种潜在高温热源，例如电器过载发热、实验明火、线路短路高温、阳光聚焦导致局部过热等，均可能使教学教具达到数百摄氏度甚至更高温度区间。800–1500?℃覆盖典型明火高温区、金属熔融温度范围及火灾核心温度区间，研究水溶性粉笔在此温度范围内的燃烧行为，对评估其火灾危险性、热稳定性能、烟气毒性及环境危害具有重要现实意义与工程应用价值。 前期探索性实验观察发现，水溶性粉笔在800–1500?℃高温条件下表现出极为特殊且显著的燃烧现象：样品可被稳定引燃，能够实现自持燃烧，燃烧状态与蜡烛类长链烃燃料高度相似；燃烧过程中火焰高度随时间持续上升，样品表面由白色逐渐转变为亮红色，并伴随熔融、流动与再次凝固的相变行为；燃烧熄灭瞬间产生大量浓密白烟，气味特征接近硫酸钙与熟石灰高温混合加热气味。上述特殊热行为与其配方体系中高比例有机组分密切相关。 根据实验所用市售水溶性粉笔成分分析，其有机组分占比超过50%，主要包括硬脂酸、甘油三硬脂酸酯、脂肪醇聚氧乙烯醚等可燃有机物；无机组分以二氧化钛为主，作为白色颜料提供遮盖性能，同时含有少量硫酸钙与滑石粉作为无机结构填料。高含量有机蜡质与表面活性剂为燃烧提供充足燃料基础，二氧化钛与硫酸钙等无机填料则构成燃烧骨架，影响气相释放、火焰稳定性及最终烟尘组成。 尽管燃烧现象直观明确，但现有研究尚未对其内在机理进行科学解释，仍存在多项关键科学问题亟待阐明：第一，水溶性粉笔在高温下为何能够呈现类似蜡烛的稳定自持燃烧行为；第二，燃烧过程中样品为何出现由白色向亮红色的转变，并伴随熔融?凝固循环；第三，火焰高度为何随燃烧时间持续上升，其动力学控制机制是什么；第四，熄灭白烟的具体来源、化学组成及形成机理如何；第五，燃烧所需可燃气体是否来源于粉笔自身有机热解，而非环境吸附杂质。 针对上述问题，本文通过控制温度燃烧实验、氮气氛围无氧热解实验、预干燥对照实验、燃烧现象动态观测、白烟成分综合分析等方法，系统揭示水溶性粉笔在800–1500?℃下的高温燃烧机理，明确白烟的多相组成特征，完善教学用环保书写材料的高温热行为数据库，为教学环境火灾安全评估、教具热稳定性提升及低燃烧风险新型粉笔开发提供科学依据。 2 实验部分 2.1 实验样品 实验采用市售主流品牌水溶性无尘粉笔，样品外观为圆柱形白色固体，质地均匀，无明显裂纹、气孔与杂质。基于多份专利文献比对与实际样品成分分析，确定其质量分数组成为： 钛白粉（TiO?）28%–40%；脂肪醇聚氧乙烯醚18%–27%；甘油三硬脂酸酯12%–22%；硬脂酸15%–20%；硫酸钙（石膏）4%–8%；滑石粉0.5%–1.5%。 为保证实验重复性与可比性，将粉笔统一加工为规则圆柱试样，直径约10?mm，长度50?mm，单根质量控制在3.0±0.1?g。 2.2 样品预处理 为排除物理吸附水、空气中挥发性有机物等环境杂质对燃烧结果的干扰，所有样品在实验前均置于105?℃鼓风干燥箱中恒温干燥2?h，充分脱除表面吸附水分与小分子挥发性吸附物，确保后续燃烧与热解气体仅来自粉笔自身组分。 2.3 高温燃烧实验 高温燃烧实验在程控高温管式炉中完成，设备温度范围室温至1600?℃，控温精度±5?℃。实验温度设置为800?℃、1000?℃、1200?℃、1500?℃四个典型水平，覆盖研究目标温度区间。将预处理后样品置于刚玉坩埚内，推入管式炉恒温区，使用K型热电偶实时监测样品区域温度，记录引燃时间、火焰形态、燃烧持续时间、火焰高度变化、样品颜色与相变行为。 2.4 氮气氛围无氧热解实验 为验证可燃气体来源，设计惰性气氛对照实验：向炉内持续通入纯度99.999%氮气，吹扫30?min以完全排除空气；在无氧条件下将样品加热至1000?℃，收集热解气体并使用明火测试其可燃性。若气体可被点燃，则证明气相产物来源于粉笔自身有机物热解，而非环境吸附物或氧化反应。 2.5 白烟与燃烧产物分析 燃烧熄灭后迅速采用石英冷阱与纤维滤膜收集白烟固体颗粒，通过感官气味对比、残渣外观观察、无机组分高温行为比对等方式，结合已知配方体系进行白烟组分推断。重点记录白烟生成量、持续时间、颜色、气味及凝聚特性。 2.6 燃烧行为表征指标 实验系统观测以下关键指标：引燃温度区间、自持燃烧稳定性、火焰颜色与高度变化规律、样品熔融与凝固现象、表面颜色由白至红的转变过程、熄灭白烟强度与气味、燃烧总时长、残渣形态与结构。a b b c e e h i i i i n o r s s s t t t u u 3 实验结果与分析 3.1 引燃特性与自持燃烧行为 在800–1500?℃温度区间内，水溶性粉笔均可被稳定引燃。当样品进入预设高温场后，表层有机物迅速受热软化，5–15?s内即可观察到可燃蒸气析出，遇点火源瞬间形成连续火焰。火焰引燃后，移除外部点火源，样品仍可保持稳定燃烧，无需外部热源持续供热，表现出典型自持燃烧特征。 燃烧过程平稳无爆裂、无熔滴飞溅，与蜡烛、硬脂酸制品燃烧行为高度相似。样品质量约3?g时，稳定燃烧时间可达90–150?s，燃烧时长随温度升高略有增加，原因在于高温加速有机相热解，使气相燃料供给更为充足。 温度对引燃效率影响显著：800?℃时引燃延迟时间约10–15?s；1000?℃时缩短至5–8?s；1200?℃以上基本实现瞬时引燃，热滞后效应极弱。表明温度升高可显著促进有机组分熔融、蒸发与热裂解，提升可燃气体释放速率。 3.2 火焰动态演化规律 实验观察到火焰高度随燃烧时间持续上升的独特现象。燃烧初期火焰高度较低，约1–2?cm；中期稳定扩张至3–5?cm；后期可达到6–8?cm，火焰形态由细长变为宽大明亮。其内在原因在于：燃烧持续进行使粉笔内部温度不断升高，无机骨架孔隙通道被熔融有机物充分填充，热解路径更为通畅，单位时间内产生的可燃气量持续增加，从而使火焰高度逐步提升。 火焰颜色以淡黄色至亮蓝色为主，靠近样品根部呈现淡蓝色，代表燃烧较为完全；上部呈现黄色，表明存在部分未完全氧化的碳颗粒与有机蒸气。 3.3 样品相变与颜色变化 燃烧过程中，水溶性粉笔呈现明显的熔融?凝固循环行为。表层有机蜡质受热先熔化形成液相，在毛细作用下渗入无机颗粒骨架间隙；随着温度进一步升高，液相有机物发生热解与气化，参与气相燃烧；未及时分解的有机物在火焰外围温度较低区域重新冷却凝固，形成凹凸不平的固化层。 同时，样品外观颜色由初始白色逐渐转为暗红，最终呈现亮红色高温发光状态。该现象由两方面因素导致：一是高温下固体自身热辐射发光；二是无机填料二氧化钛与硫酸钙在高温下形成致密半透明骨架，增强光辐射与散射效果，使红色更为显著。该颜色变化为典型高温热辐射现象，不代表新物相生成。 3.4 氮气氛围热解实验结果 在氮气无氧氛围下加热至1000?℃时，粉笔仍可释放大量可燃气体，该气体可被明火点燃并形成稳定火焰。结合105?℃预处理除杂实验结果，充分证明：燃烧所依赖的可燃气体并非来自环境吸附杂质，而是粉笔内部有机组分在高温下发生热裂解产生的小分子可燃产物，包括烷烃、烯烃、醛类、CO等。 该实验直接证实水溶性粉笔的燃烧本质为有机热解?气相燃烧模式，而非固相直接燃烧。 3.5 白烟生成特征与成分分析 熄灭瞬间产生大量白烟，烟量大、扩散快，持续时间约3–8?s。气味接近硫酸钙与熟石灰高温混合加热气味，无明显刺激性异味。 结合成分体系分析，白烟由三类物质共同构成： 第一，未完全燃烧的硬脂酸、甘油三硬脂酸酯、脂肪醇聚氧乙烯醚热解蒸气在熄火后快速降温，冷凝形成超细有机微颗粒； 第二，高温下扬起的二氧化钛无机纳米烟尘，作为白烟主要白色载体； 第三，少量硫酸钙与滑石粉高温分解产生的微细粉尘。 其中，有机冷凝颗粒决定白烟浓度与持续时间，二氧化钛决定白烟的白色外观特征。气味主要来源于硫酸钙高温脱除结晶水与表面活性剂热解小分子共同作用。 4 讨论 4.1 自持燃烧机理 水溶性粉笔能够自持燃烧，源于其独特的多相结构与连续热解供气机制。高温下，内部有机蜡质与表面活性剂依次经历熔融、渗流、蒸发、热裂解过程，持续释放可燃气体；气相产物在样品表面与氧气混合燃烧，释放热量进一步加热内部物料，形成自维持的热解?燃烧循环系统。 无机填料TiO?、CaSO?等不参与燃烧，但作为多孔骨架支撑结构，为液相渗透与气相扩散提供通道，使燃烧过程稳定持续。 4.2 火焰高度持续上升的原因 火焰高度随燃烧时间不断上升，主要由三方面因素共同驱动： 第一，燃烧放热使粉笔内部温度持续升高，热解速率加快，可燃气体产量增加； 第二，熔融有机物不断渗入骨架，使热解反应界面持续扩大； 第三，燃烧形成的孔隙结构不断优化，气体扩散阻力降低，供气更为充足。 4.3 白烟形成机制 熄火后，体系温度骤降，未完全反应的有机热解蒸气迅速过饱和并冷凝为细小液滴/固体颗粒，同时裹挟高温扬起的二氧化钛粉尘，形成视觉显著的白烟。其本质为有机冷凝物+无机烟尘的复合气溶胶。 4.4 火灾安全启示 水溶性粉笔在800?℃以上具备自持燃烧能力，表明其在强热源或明火环境下存在一定火灾风险。但其燃烧速度温和、无熔滴、烟气毒性较低，相较于塑料、泡沫等高分子材料危险性更低。在实验室高温设备附近应避免大量堆积，以降低潜在热安全风险。 5 结论 1.?水溶性粉笔在800–1500?℃范围内可被引燃并实现稳定自持燃烧，燃烧行为类似蜡烛，无需外部持续供热。 2.?燃烧过程伴随明显熔融?凝固相变，样品由白色转为亮红色，火焰高度随燃烧时间持续上升。 3.?氮气氛围热解与预干燥实验证实，可燃气体来自粉笔自身有机组分高温热解。 4.?熄灭白烟为复合气溶胶，主要由有机热解蒸气冷凝颗粒与二氧化钛无机烟尘构成。 5.?其燃烧遵循“熔融?渗流?热解?气相燃烧?熄火冷凝成烟”的完整机理。 参考文献 [1] 教学书写材料无尘化技术研究进展. 中国教育学刊, 2020. [2] 硬脂酸系蜡质材料高温热解行为分析. 化工新型材料, 2021. [3] 无机填料对有机复合燃料燃烧特性的影响. 燃烧科学与技术, 2019. [4] 水溶性高分子表面活性剂热解产物研究. 精细化工, 2022. [5] 教育用品火灾危险性评价方法. 消防科学与技术, 2020. High-Temperature Combustion Behavior of Water-Soluble Chalk and Composition Analysis of White Smoke upon Extinguishment Author: Heptachlorine Fluoride Abstract This study takes commercially available water-soluble dust-free chalk as the research object and systematically investigates its pyrolysis characteristics, self-sustaining combustion behavior, dynamic evolution of flame, as well as the chemical composition and formation mechanism of white smoke generated at the instant of extinguishment under high temperatures ranging from 800 to 1500?℃. The experimental results demonstrate that water-soluble chalk can be stably ignited and achieve self-sustaining combustion within the target temperature range, with no need for continuous external heat supply during the combustion process. Its flame morphology and combustion sustainability bear a high degree of similarity to those of wax-based fuels such as candles. During combustion, the chalk sample exhibits an evident melting-solidification phase transition, with its appearance color gradually changing from the initial white to bright red, and the flame height showing a continuous upward trend as combustion proceeds. A large amount of dense white smoke is produced immediately upon extinguishment, whose odor is similar to that of a mixture of calcium sulfate and slaked lime heated at high temperatures. Through 105?℃ pretreatment impurity removal experiments and nitrogen-atmosphere oxygen-free pyrolysis control experiments, it is verified that the combustible gas-phase products supporting combustion originate from the thermal cracking of organic components inside the chalk at high temperatures, rather than environmental adsorbed impurities. Combined with the actual composition formula of the experimental samples (titanium dioxide 28%–40%, fatty alcohol polyoxyethylene ether 18%–27%, glyceryl tristearate 12%–22%, stearic acid 15%–20%, calcium sulfate 4%–8%, and talc powder 0.5%–1.5%), further analysis indicates that the white smoke is mainly composed of three phases: condensed micro-particles of incompletely combusted organic vapor, intermediate products from the pyrolysis of fatty acids and surfactants, and inorganic smoke of titanium dioxide. This research systematically reveals the complete reaction pathway of water-soluble chalk, namely "high-temperature melting-liquid phase penetration-organic pyrolysis-gas phase combustion-smoke condensation upon extinguishment". It provides fundamental experimental data and theoretical support for the fire safety evaluation, high-temperature thermal stability assessment, flue gas hazard analysis of teaching aids, and the modification of environmentally friendly writing materials. Keywords: water-soluble chalk; high-temperature combustion; self-sustaining combustion; white smoke composition; thermal cracking; fire safety; inorganic filler; organic wax 1 Introduction As an upgraded alternative to traditional gypsum chalk, water-soluble dust-free chalk has been widely applied in modern educational scenarios by virtue of its advantages such as low dust generation, easy wiping, smooth writing performance, environmental friendliness and low irritation. Compared with traditional chalk mainly composed of calcium sulfate or calcium carbonate, water-soluble chalk has undergone structural optimization in its formula system. By introducing a large number of organic waxes, fatty acids, nonionic surfactants and other components, it achieves dust-free writing and easy removal by wet wiping, significantly reducing the impact of dust on the respiratory health of teachers and students. With the continuous popularization of multimedia classrooms and smart classrooms, water-soluble chalk has gradually replaced traditional chalk and become the mainstream choice of teaching writing tools. Current domestic and foreign researches on water-soluble chalk mainly focus on the optimization of application performance at room temperature, including the improvement of writing fluency, optimization of bonding system, selection of surfactants, mechanism of dust-free performance, design of environmentally friendly and non-toxic formulas, and improvement of wiping efficiency. Such researches are carried out around daily application scenarios, focusing on the application performance, human safety, environmental friendliness and industrial production technology of materials. However, systematic researches on the thermal behavior, combustion characteristics, release law of gas-phase products, potential fire hazard under extreme conditions, especially high-temperature environments, as well as the source and composition of white smoke after extinguishment are still scarce. In actual teaching environments and laboratory scenarios, there exist various potential high-temperature heat sources, such as overheating caused by electrical appliance overload, open flames in experiments, high temperature due to short circuit of lines, and local overheating caused by sunlight focusing, all of which may heat teaching aids to hundreds of degrees Celsius or even higher. The temperature range of 800–1500?℃ covers the typical open flame high-temperature zone, metal melting temperature range and core fire temperature range. The research on the combustion behavior of water-soluble chalk in this temperature range has important practical significance and engineering application value for evaluating its fire hazard, thermal stability, flue gas toxicity and environmental hazard. Preliminary exploratory experiments have found that water-soluble chalk exhibits extremely special and significant combustion phenomena under high temperatures of 800–1500?℃: the sample can be stably ignited and achieve self-sustaining combustion, and the combustion state is highly similar to that of long-chain hydrocarbon fuels such as candles; during combustion, the flame height rises continuously with time, the sample surface gradually changes from white to bright red, accompanied by phase transition behaviors of melting, flowing and re-solidification; a large amount of dense white smoke is generated at the moment of combustion extinguishment, with an odor characteristic similar to that of calcium sulfate and slaked lime heated at high temperatures. The above special thermal behaviors are closely related to the high proportion of organic components in its formula system. According to the composition analysis of commercially available water-soluble chalk used in the experiments, the proportion of organic components exceeds 50%, mainly including combustible organic matters such as stearic acid, glyceryl tristearate and fatty alcohol polyoxyethylene ether; the inorganic component is mainly titanium dioxide, which serves as a white pigment to provide covering performance, and also contains a small amount of calcium sulfate and talc powder as inorganic structural fillers. High-content organic waxes and surfactants provide sufficient fuel basis for combustion, while inorganic fillers such as titanium dioxide and calcium sulfate form a combustion skeleton, affecting gas-phase release, flame stability and final soot composition. Despite the intuitive combustion phenomena, existing researches have not scientifically explained its internal mechanism, and there are still many key scientific problems to be clarified: first, why water-soluble chalk can present stable self-sustaining combustion behavior similar to candles at high temperatures; second, why the sample changes from white to bright red during combustion, accompanied by melting-solidification cycles; third, why the flame height rises with combustion time, and what its kinetic control mechanism is; fourth, what the specific source, chemical composition and formation mechanism of white smoke after extinguishment are; fifth, whether the combustible gas required for combustion originates from the pyrolysis of organic matters in the chalk itself rather than environmental adsorbed impurities. In response to the above problems, this paper systematically reveals the high-temperature combustion mechanism of water-soluble chalk at 800–1500?℃ through temperature-controlled combustion experiments, nitrogen-atmosphere oxygen-free pyrolysis experiments, pre-drying control experiments, dynamic observation of combustion phenomena, and comprehensive analysis of white smoke components, clarifies the multiphase composition characteristics of white smoke, improves the high-temperature thermal behavior database of environmentally friendly teaching writing materials, and provides a scientific basis for fire safety assessment of teaching environments, improvement of thermal stability of teaching aids and development of new chalk with low combustion risk. 2 Experimental Section 2.1 Experimental Samples Commercially available mainstream brand water-soluble dust-free chalk was adopted as the experimental sample, which is a cylindrical white solid with uniform texture and no obvious cracks, pores or impurities. Based on the comparison of multiple patent documents and actual composition analysis of samples, the mass fraction composition was determined as follows: Titanium dioxide (TiO?): 28%–40%; Fatty alcohol polyoxyethylene ether: 18%–27%; Glyceryl tristearate: 12%–22%; Stearic acid: 15%–20%; Calcium sulfate (gypsum): 4%–8%; Talc powder: 0.5%–1.5%. To ensure the repeatability and comparability of experiments, the chalk was uniformly processed into regular cylindrical specimens with a diameter of about 10?mm, a length of 50?mm, and the mass of a single piece was controlled at 3.0±0.1?g. 2.2 Sample Pretreatment To eliminate the interference of environmental impurities such as physically adsorbed water and volatile organic compounds in the air on combustion results, all samples were dried at a constant temperature of 105?℃ in a forced air drying oven for 2?h before the experiments to fully remove surface adsorbed water and small-molecule volatile adsorbates, ensuring that the subsequent combustion and pyrolysis gases only come from the components of the chalk itself. 2.3 High-Temperature Combustion Experiments High-temperature combustion experiments were completed in a program-controlled high-temperature tube furnace, with an equipment temperature range from room temperature to 1600?℃ and a temperature control accuracy of ±5?℃. The experimental temperatures were set at four typical levels of 800?℃, 1000?℃, 1200?℃ and 1500?℃, covering the target temperature range of the research. The pretreated samples were placed in an alumina crucible and pushed into the constant temperature zone of the tube furnace. A K-type thermocouple was used to monitor the temperature of the sample area in real time, and the ignition time, flame morphology, combustion duration, change of flame height, sample color and phase transition behavior were recorded. 2.4 Oxygen-Free Pyrolysis Experiments in Nitrogen Atmosphere To verify the source of combustible gas, an inert atmosphere control experiment was designed: high-purity nitrogen with a purity of 99.999% was continuously introduced into the furnace and purged for 30?min to completely remove air; the sample was heated to 1000?℃ under oxygen-free conditions, and the pyrolysis gas was collected and tested for flammability with an open flame. If the gas can be ignited, it proves that the gas-phase products originate from the pyrolysis of organic matters in the chalk itself, rather than environmental adsorbates or oxidation reactions. 2.5 Analysis of White Smoke and Combustion Products Immediately after combustion extinguishment, quartz cold traps and fiber filter membranes were used to collect solid particles of white smoke. The composition of white smoke was inferred by means of sensory odor comparison, residue appearance observation, comparison of high-temperature behaviors of inorganic components, combined with the known formula system. The generation amount, duration, color, odor and condensation characteristics of white smoke were recorded emphatically. 2.6 Characterization Indicators of Combustion Behavior The following key indicators were systematically observed in the experiments: ignition temperature range, stability of self-sustaining combustion, change law of flame color and height, melting and solidification phenomena of samples, transition process of sample surface color from white to red, intensity and odor of white smoke after extinguishment, total combustion duration, and morphology and structure of residues. a b b c e e h i i i i n o r s s s t t t u u 3 Experimental Results and Analysis 3.1 Ignition Characteristics and Self-Sustaining Combustion Behavior Within the temperature range of 800–1500?℃, water-soluble chalk can be stably ignited. After the sample enters the preset high-temperature field, the surface organic matters are rapidly heated and softened, combustible vapor can be observed to precipitate within 5–15?s, and a continuous flame is instantly formed when encountering an ignition source. After flame ignition, the sample can still maintain stable combustion without removing the external ignition source, showing typical self-sustaining combustion characteristics without continuous external heat supply. The combustion process is stable without bursting or splashing of molten droplets, which is highly similar to the combustion behavior of candles and stearic acid products. When the sample mass is about 3?g, the stable combustion duration can reach 90–150?s, and the combustion duration increases slightly with the rise of temperature, because high temperature accelerates the pyrolysis of organic phase and makes the supply of gas-phase fuel more sufficient. Temperature has a significant impact on ignition efficiency: the ignition delay time is about 10–15?s at 800?℃; shortened to 5–8?s at 1000?℃; instantaneous ignition is basically achieved above 1200?℃ with extremely weak thermal hysteresis effect. It indicates that the rise of temperature can significantly promote the melting, evaporation and thermal cracking of organic components, and improve the release rate of combustible gas. 3.2 Dynamic Evolution Law of Flame A unique phenomenon that the flame height rises continuously with combustion time was observed in the experiments. At the initial stage of combustion, the flame height is low, about 1–2?cm; it expands stably to 3–5?cm in the middle stage; it can reach 6–8?cm in the later stage, and the flame morphology changes from slender to wide and bright. The internal reason is that the continuous combustion raises the internal temperature of the chalk, the pore channels of the inorganic skeleton are fully filled with molten organic matters, the pyrolysis path is more unobstructed, and the amount of combustible gas generated per unit time increases continuously, so that the flame height rises gradually. The flame color is mainly light yellow to bright blue, with light blue near the root of the sample, representing relatively complete combustion; the upper part is yellow, indicating the existence of incompletely oxidized carbon particles and organic vapor. 3.3 Phase Transition and Color Change of Samples During combustion, water-soluble chalk shows an obvious melting-solidification cycle behavior. The surface organic wax is first heated to melt to form a liquid phase, which penetrates into the gaps of inorganic particle skeleton under capillary action; with the further rise of temperature, the liquid organic matters undergo pyrolysis and gasification to participate in gas-phase combustion; the organic matters that are not decomposed in time re-cool and solidify in the low-temperature area outside the flame, forming an uneven solidified layer. At the same time, the appearance color of the sample gradually changes from the initial white to dark red, and finally presents a bright red high-temperature luminous state. This phenomenon is caused by two factors: first, the thermal radiation luminescence of solids at high temperatures; second, inorganic fillers such as titanium dioxide and calcium sulfate form a dense and translucent skeleton at high temperatures, which enhances the effect of light radiation and scattering and makes the red color more significant. This color change is a typical high-temperature thermal radiation phenomenon and does not represent the generation of new phases. 3.4 Results of Pyrolysis Experiments in Nitrogen Atmosphere When heated to 1000?℃ in an oxygen-free nitrogen atmosphere, the chalk can still release a large amount of combustible gas, which can be ignited by an open flame to form a stable flame. Combined with the results of 105?℃ pretreatment impurity removal experiments, it is fully proved that the combustible gas on which combustion depends does not come from environmental adsorbed impurities, but small-molecule combustible products generated by the thermal cracking of organic components inside the chalk at high temperatures, including alkanes, alkenes, aldehydes, CO and so on. This experiment directly confirms that the combustion essence of water-soluble chalk is an organic pyrolysis-gas phase combustion mode, rather than direct solid-phase combustion. 3.5 Generation Characteristics and Composition Analysis of White Smoke A large amount of white smoke is generated at the moment of extinguishment, with large smoke volume, rapid diffusion and a duration of about 3–8?s. The odor is similar to that of calcium sulfate and slaked lime heated at high temperatures, without obvious pungent odor. Combined with the analysis of the composition system, the white smoke is composed of three types of substances: First, the pyrolysis vapor of incompletely combusted stearic acid, glyceryl tristearate and fatty alcohol polyoxyethylene ether is rapidly cooled after extinguishment to condense into ultrafine organic micro-particles; Second, titanium dioxide inorganic nano-soot raised at high temperatures, serving as the main white carrier of white smoke; Third, a small amount of fine dust generated by the high-temperature decomposition of calcium sulfate and talc powder. Among them, organic condensed particles determine the concentration and duration of white smoke, and titanium dioxide determines the white appearance characteristics of white smoke. The odor mainly comes from the combined effect of calcium sulfate removing crystal water at high temperatures and small molecules generated by the pyrolysis of surfactants. 4 Discussion 4.1 Mechanism of Self-Sustaining Combustion The ability of water-soluble chalk to achieve self-sustaining combustion stems from its unique multiphase structure and continuous pyrolysis gas supply mechanism. At high temperatures, the internal organic waxes and surfactants successively undergo melting, seepage, evaporation and thermal cracking processes to continuously release combustible gas; gas-phase products mix with oxygen on the sample surface for combustion, and the released heat further heats the internal materials, forming a self-sustaining pyrolysis-combustion cycle system. Inorganic fillers such as TiO? and CaSO? do not participate in combustion, but act as a porous skeleton support structure, providing channels for liquid-phase penetration and gas-phase diffusion, so that the combustion process is stable and continuous. 4.2 Reasons for Continuous Rise of Flame Height The continuous rise of flame height with combustion time is mainly driven by three factors: First, the heat released by combustion raises the internal temperature of the chalk, accelerates the pyrolysis rate and increases the output of combustible gas; Second, molten organic matters continuously penetrate into the skeleton, expanding the interface of pyrolysis reaction; Third, the pore structure formed by combustion is continuously optimized, the resistance of gas diffusion is reduced, and the gas supply is more sufficient. 4.3 Formation Mechanism of White Smoke After extinguishment, the temperature of the system drops sharply, the incompletely reacted organic pyrolysis vapor is rapidly supersaturated and condensed into fine droplets/solid particles, and at the same time entrains titanium dioxide dust raised at high temperatures, forming visually significant white smoke. Its essence is a composite aerosol of organic condensates and inorganic soot. 4.4 Enlightenment for Fire Safety Water-soluble chalk has the ability of self-sustaining combustion above 800?℃, indicating that it has a certain fire risk in an environment with strong heat sources or open flames. However, its combustion rate is moderate, without molten droplets, and the flue gas toxicity is low, so the risk is lower than that of polymer materials such as plastics and foams. A large amount of accumulation should be avoided near high-temperature equipment in laboratories to reduce potential thermal safety risks. 5 Conclusions 1.?Water-soluble chalk can be ignited and achieve stable self-sustaining combustion in the range of 800–1500?℃, with combustion behavior similar to candles and no need for continuous external heat supply. 2.?The combustion process is accompanied by obvious melting-solidification phase transition, the sample changes from white to bright red, and the flame height rises continuously with combustion time. 3.?Nitrogen-atmosphere pyrolysis and pre-drying experiments confirm that the combustible gas comes from the high-temperature pyrolysis of organic components in the chalk itself. 4.?The white smoke after extinguishment is a composite aerosol, mainly composed of condensed particles of organic pyrolysis vapor and titanium dioxide inorganic soot. 5.?Its combustion follows the complete mechanism of "melting-seepage-pyrolysis-gas phase combustion-smoke condensation upon extinguishment". References [1] Research Progress on Dust-Free Technology of Teaching Writing Materials. Chinese Journal of Education, 2020. [2] Analysis on High-Temperature Pyrolysis Behavior of Stearic Acid-Based Waxy Materials. New Chemical Materials, 2021. [3] Influence of Inorganic Fillers on Combustion Characteristics of Organic Composite Fuels. Journal of Combustion Science and Technology, 2019. [4] Research on Pyrolysis Products of Water-Soluble Polymer Surfactants. Fine Chemicals, 2022. [5] Evaluation Method for Fire Risk of Educational Supplies. Fire Science and Technology, 2020. 发自小木虫手机客户端 |
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