赵文涛，张 毅，于光鑫，FRANK Behrendt,2，何 芳

炭棒与生物质棒稳态阴燃特性对比

赵文涛1，张 毅1，于光鑫1，FRANK Behrendt1,2，何 芳1※

（1. 山东理工大学交通与车辆工程学院，淄博 255049；2. Institute of Energy Engineering，Technische Universität Berlin，Berlin 10623，Germany）

为研究炭与生物质稳态阴燃的特性差异，对不同直径（2～8 mm）的炭棒与绝干、空干生物质棒竖直向下的阴燃进行了试验，并编写程序计算了棒状燃料阴燃过程的耗氧速率。结果表明：1）所制炭棒与生物质棒均能自行调节反应区形状以维持稳态阴燃。2）炭棒的阴燃传播速度约为生物质棒的4.2倍，最高温度比生物质棒高约50 ℃，反应区长度约为相应生物质棒的3.8倍，燃料消耗速率约为生物质棒的2.4倍。3）计算和试验烟气轮廓吻合较好，炭棒耗氧速率约为生物质棒的3.4倍。研究结果可为稳态阴燃机理的深入研究及应用中燃料选择提供参考。

炭；生物质；稳态阴燃；传播速度；耗氧速率

0 引 言

中国农林生物质废弃物总量约为1.2×109t/a[1]，资源量巨大，将其在低氧条件下高温热裂解可制成生物质炭[2-4]。炭及农林生物质常见的利用方式为燃烧，这种方式易造成设备腐蚀和环境污染等问题[5]。阴燃是一种缓慢、低温、无焰的燃烧过程[6]，具有燃料适应性强、固相低温可避免钾逸出造成设备粘污[7]、灰分可做肥料[8]等优势，发展潜力巨大。炭及农林生物质的阴燃在中国传统上常用于冬季农村住宅[9]或蔬菜大棚[10]供暖等场景（如炕、燃池等）。其阴燃特性（如阴燃最高温度[11]、传播速度[12]、燃料消耗速率[13]等）的进一步研究，有助于对应用中的阴燃过程有更为普遍的理解，包括确定燃料配比[11,14]、通风条件[15-16]、烟气处理策略[17]及提高能源效率[18]等，并可为阴燃装置的设计及开发连续运行工艺提供重要参考。

炭及生物质阴燃的研究已有几十年的历史，主要涉及煤炭[19-20]、森林[21]的火灾，香烟的燃烧[22]，近期有机固废的处理利用[23-24]和建筑的供暖[25-26]等。从应用的角度来说，一般需要阴燃过程的稳态和可控，便于烟气处理，并满足不同热负荷的供暖需求。稳态阴燃研究多采用棒状燃料[27]。早在1967年，KINBARA等[28]在不同温度的燃烧室中对熏香阴燃传播速度进行了研究，发现其阴燃传播速度范围在1～8 mm/min，并给出了阴燃传播速度（）与点火温度（i）及环境温度（a）之间的关系（2∝1/(i-a)）。近年来，对棒状燃料稳态阴燃的研究主要集中在不同含水率、空气流速、氧含量及氧分压等因素下的阴燃传播过程。高振强等[29]研究了含水率对佛香阴燃传播速度的影响，发现不同含水率（0～35%）的佛香，其传播速度在初始阶段（< 20 min）有显著差异。MUKUNDA等[30]研究了不同空气流速（0～7 m/s）及氧含量（23%～44%）对佛香阴燃过程的影响，发现正向阴燃时，其传播速度随气流速度及氧含量的增加而增加，而在逆向阴燃时其传播速度存在峰值。LIN等[31]对不同空气流速下的佛香阴燃进行了研究，发现随空气流速增大，佛香阴燃会依次进入氧气控制、热控制及化学控制阶段。KADOWAKI等[32]对不同氧气质量分数（0.1～0.5）下艾柱的阴燃过程进行了研究，发现传播速度及最高温度随氧气质量分数的增加而增加。YAMAZAKI等[33]研究了佛香阴燃的火焰转捩，发现氧分压低于0.3时不会出现明火。YAN等[34]研究了不同直径、不同灰分含量炭棒的阴燃特性，发现氧气在其阴燃过程中起主导作用。这些研究为稳态阴燃技术开发提供了依据。

然而，同一种生物质和其制备的炭在稳态阴燃特性方面的异同，目前并不明确，也未见报道。本文拟采用试验的方法解决这一问题，并对过程中起关键作用的氧传输进行理论分析。试验中应用中国传统佛香制备方法制作生物质棒，并对其热解制备炭棒。以期为深入研究稳态阴燃机理和改进阴燃应用提供参考。

1 材料与方法

1.1 炭棒与生物质棒的制备

购买产自四川省、颗粒直径小于0.15 mm的纯榆树皮粉和纯柏木粉为原料，制得空干、绝干生物质棒和炭棒。具体制作方法如下：将榆树皮粉、柏木粉和水按质量比例约1∶1∶4混合均匀，挤压成直径3、5、8 mm的生物质棒，在空气中自然风干（>48 h），得到空干生物质棒；将空干生物质棒置于101型电热鼓风干燥箱中，在105 ℃下保温24 h制得绝干生物质棒，绝干生物质棒制作完成后放置于干燥皿中储存；炭棒由空干生物质棒热解制成：将空干生物质棒放在石英试管中，并用石棉将试管口封堵，放置在马弗炉中，以10 ℃/min的加热速率将其从室温加热至500 ℃，保温2 h后，关闭电源即制得炭棒，待其在炉中冷却后（>12 h），放入密封袋中储存。

对所制炭棒与生物质棒进行测量，并参照GB/T 28731—2012（固体生物质燃料工业分析方法）进行分析，分析结果如表1所示。需要注意的是，在制作过程中，因水分蒸发或挥发分逸出导致的收缩，使炭棒与生物质棒的实际直径和制作直径（3、5、8 mm）略有差别。

表1 样品参数及工业分析

注：表中“-”表示成分含量可忽略；*为在炭棒工业分析测定挥发分含量时，为防止炭粉的氧化，在其表面覆盖约4 mm厚的珍珠岩粉。

Note: The “-” in the table indicates that the component content can be ignored. *is in the proximate analysis of char rods to determine volatile content, to prevent the oxidation of char powder, about 4 mm thick perlite powder was covered on its surface.

1.2 试验方法

在大气环境中（25±3）℃，对棒状燃料竖直向下的稳态阴燃过程进行试验。具体试验方法如图1所示。首先使用点火器将长度约15 cm的棒状燃料顶部点燃。待阴燃稳定后（约4 min），采用刻度尺和计时器记录反应前锋移动距离（Δ）和时间（Δ），可得阴燃传播速度（sm=Δ/Δ）。棒状燃料稳态阴燃过程中采用红外热成像仪（ThermoProTM TP8）记录反应区温度分布，并利用自制纹影系统记录棒状燃料的烟气层轮廓。随后将棒状燃料插入生物质灰中，隔绝氧气使其熄灭，并用徕卡显微镜（M135C）记录反应区形状。需要注意的是，红外图像分析时取各棒状燃料发射率为0.93。

注：Δx、Δt、usm分别为移动距离、移动时间及阴燃传播速度。

2 结果与分析

试验发现，各棒状燃料在竖直状态下均能稳定阴燃。炭棒与生物质棒的反应区形状及红外温度图像如图2所示，图中“+”为最高温度的位置标记。可以看出，各反应区（炭锥）形状明显不同，这表明不同的炭棒与生物质棒可通过自行调节其反应区形状以维持稳态阴燃。另外，棒状燃料阴燃高温范围也随反应区形状有明显不同，最高温度范围均在620～770℃之间。

3种物料阴燃特性参数随直径变化的试验结果如图3所示。由图3可知，炭棒与生物质棒在阴燃传播速度、最高温度、反应区长度和燃料消耗速率方面均有差异。

注：图中d、Tmax分别为炭棒与生物质棒的直径及最高温度；各图中左图与右图分别为炭棒与生物质棒的反应区形状及红外温度图像。

图3 炭棒与生物质棒阴燃特性对比

2.1 炭棒与生物质棒阴燃传播速度对比

由图3a可知，炭棒与生物质棒的阴燃传播速度分别在6.7～11.2及1.3～3.1 mm/min之间。且直径2.27 mm炭棒的阴燃传播速度最高，约为1.1 cm/min；生物质棒阴燃传播速度与文献[27,29,31]中佛香的阴燃传播速度（3.2～6.5 mm/min）在同一数量级。若将各炭棒的阴燃传播速度分别与绝干、空干生物质棒作比并平均，可知炭棒的阴燃传播速度显著大于生物质棒，整体上约为生物质棒的4.2倍，分别约为绝干、空干生物质棒的3.9和4.5倍。而绝干生物质棒的阴燃传播速度仅约为空干生物质棒的1.1倍。

2.2 炭棒与生物质棒阴燃最高温度对比

由图3b可知，炭棒与生物质棒的阴燃最高温度分别在660～760和620～720 ℃之间，二者的温度区间分别与文献[27]和文献[32]中的温度值相近。在相同时间内，炭棒的传播速度较大带来了更多的热量；且本试验中生物质棒阴燃与相关文献[27,33]类似，未有明显的挥发分燃烧，热解过程仍表现为吸热，导致炭棒的阴燃最高温度整体上比生物质棒高约50 ℃，分别比绝干、空干生物质棒高约43和55 ℃。含水率不同对生物质棒的阴燃最高温度无显著影响[12,35]，绝干生物质棒的最高温度仅比空干生物质棒高约7～20 ℃。

2.3 炭棒与生物质棒反应区长度对比

2.4 炭棒与生物质棒燃料消耗速率对比

3 阴燃烟气及耗氧速率分析

氧气传输控制着棒状燃料的阴燃过程[31-32]，影响其稳态阴燃特性。下述将对棒状燃料阴燃过程中的耗氧速率进行计算。

3.1 氧传输模型

炭棒及生物质棒阴燃过程的氧传输模型及单元划分如图4所示。在图4a中，氧传输模型可分为固体区和气体区。固体区从上到下又可分为灰分区、碳氧化区、炭锥区、热解区（炭棒无此区）、干燥区和原物料区。气体区从中轴线到外侧分为烟气区和空气区。烟气区由碳氧化产生的烟气和干燥及热解产生的烟气（炭棒无热解烟气）构成。阴燃过程中氧气需穿过烟气区和灰分区才能到达碳氧化区进行氧化反应。具体的单元划分如图4b所示。

注：图4b中，r为单元的半径；z为单元距炭锥底面的高度；下标1、i、n分别表示第1、i、n个单元，zp为炭锥单层高度。

3.2 数学描述

计算棒状燃料碳氧化区耗氧速率的主要思路：首先需确定烟气层半径，然后确定烟气层及灰分区中的氧气传质阻力，进而确定其表达式。

3.2.1 棒状燃料烟气层半径计算

式中sc为标准状况下，1 mol气体所占体积，0.022 4 m3；为烟气温度313 K时，对sc的修正系数313K/273K≈1.15；c为固定碳的密度，kg/m3；a、s分别为灰分区及碳氧化区的半径，m；sm为棒状燃料的阴燃传播速度，m/s；c为碳的摩尔质量，kg/mol。

式中f,c为碳氧化产生的烟气半径（m）；f为烟气流速，m/s，本文取0.01 m/s。

由于c=c,1，可得因碳氧化产生的烟气半径：

碳氧化产生的烟气周围还环绕着干燥及热解产生的烟气，其满足下述表达式：

式中rod为棒状燃料的半径，m；w、v分别为水分及挥发分的密度，kg/m3；w、v分别为水及热解烟气的摩尔质量，kg/mol，本文取v=31.22×10-3[36]；tf为烟气层半径，m。

由式（4）可得棒状燃料阴燃产生的烟气层半径：

需要注意的是，绝干生物质棒及炭棒的烟气层半径表达式中分别不含w/w及v/v项。

3.2.2 烟气层及灰分区氧气传质阻力计算

氧气在烟气层中的扩散量由菲克定律确定：

式中f为氧气在烟气层中的扩散系数，m2/s，本文取4.53×10-5m2/s[37]；f为氧气在烟气层进行扩散的单元面积，m2；为氧气浓度，kg/m3；为半径，m。

对上式移项，并在积分区间（从c→a，从tf→a）内积分，可得：

式中c、a分别为外界环境及灰分区表面的氧气浓度，kg/m3。由式（7）可确定烟气层内单位面积的氧气传质阻力为f

同样地，可利用菲克定律确定灰分区单位面积的氧气传质阻力为a

式中a为氧气在灰分区的扩散系数，m2/s，本文取其计算式为a= 0.677g1.18[a/ 273]1.75[38]；为灰分区孔隙，%；s为棒状燃料碳氧化区的氧浓度，kg/m3。

3.2.3 耗氧速率表达式

式中0为棒状燃料碳氧化区的面积，m2。

3.3 计算流程图

耗氧速率的计算流程如图5所示。需根据试验获得的炭锥，采用图像数据处理软件，沿轴方向进行单元划分，取各单元的半径值。多次试算发现，炭棒与生物质棒划分的单元数分别为55和35个，是满足读图误差和离散误差两者均较小的较优组合。自=0向上，计算每个单元的反应面积、烟气层半径、氧气传质阻力，最终计算出耗氧速率。计算采用自编程的Matlab程序进行。

图5 计算流程图

3.4 烟气层计算结果及验证

由式（8）及式（10）可知，棒状燃料的烟气层半径对耗氧速率存在影响。因此以制作直径5 mm的炭棒与生物质棒为例，验证烟气层半径计算值的合理性。炭棒与生物质棒的烟气层轮廓如图6所示。从试验直拍图片中可以看出，灰色背景下直接观察并不能清晰捕捉到烟气层轮廓。而从数值计算结果及纹影试验图片中可以看出，计算出的烟气层轮廓（红色及白色虚线所示）及纹影试验烟气层轮廓在阴燃前锋处都有弧形且增长较快，炭锥处增长缓慢，炭锥以上几乎没有增长；整个烟气层外轮廓的计算与试验尺寸相差并不大，两者的烟气层轮廓吻合较好。

注：各图中从左至右依次为数值计算结果、试验直拍图片和纹影试验图片。

3.5 炭棒与生物质棒耗氧速率计算结果

炭棒与生物质棒阴燃过程的耗氧速率如图7所示。可以看出，炭棒与生物质棒的耗氧速率分别在70～470和19～100 mg/min之间。若将各炭棒的耗氧速率分别与绝干、空干生物质棒作比并平均，可知炭棒的耗氧速率显著大于生物质棒，约为2种生物质棒的3.4倍，分别约为绝干、空干生物质棒的3.1和3.6倍；绝干生物质棒的耗氧速率约为空干生物质棒的1.2倍。需要注意的是，炭锥轮廓取点常存在误差，可能使耗氧速率结果存在0～15%的误差。

炭棒与绝干、空干生物质棒耗氧速率的比值、传播速度的比值、反应区长度的比值三者大致相近。可以预见，相同来源的炭棒与生物质棒稳定阴燃时，当外界空气条件一定的情况下，反应区尺寸增大，其阴燃传播速度增加。在应用设计中，需根据所选物料及阴燃强度，合理设计反应区尺寸。

图7 炭棒与生物质棒耗氧速率对比

4 结 论

1）所制炭棒与绝干、空干生物质棒在竖直状态下，均能自行调节反应区形状以维持稳态阴燃；阴燃最高温度范围均在620～770 ℃之间。

2）炭棒的阴燃传播速度约为生物质棒的4.2倍，最高温度比生物质棒高约50 ℃，反应区长度约为相应生物质棒的3.8倍，燃料消耗速率约为生物质棒的2.4倍。

3）计算和试验烟气轮廓吻合较好，炭棒阴燃耗氧速率约为生物质棒的3.4倍。相同来源的炭棒与生物质棒稳定阴燃时，当外界空气条件一定的情况下，反应区尺寸增大，其阴燃传播速度增加。在应用设计中，需根据所选物料及阴燃强度，合理设计反应区尺寸。

[1] 王芳，刘晓风，陈伦刚，等. 生物质资源能源化与高值利用研究现状及发展前景[J]. 农业工程学报，2021，37(18)：219-231. WANG Fang, LIU Xiaofeng, CHEN Lungang, et al. Research status and development prospect of energy and high value utilization of biomass resources[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(18): 219-231. (in Chinese with English abstract)

[2] 肖鹏飞，安璐，吴德东. 基于文献计量学分析的全球生物质炭研究进展[J]. 农业工程学报，2020，36(18)：292-300. XIAO Pengfei, AN Lu, WU Dedong. Research progress of biochar in the world based on bibliometrics analysis[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2020, 36(18): 292-300. (in Chinese with English abstract)

[3] CHEN W, MENG J, HAN X. Past, present, and future of biochar[J]. Biochar, 2019, 1: 75-87.

[4] 霍丽丽，姚宗路，赵立欣，等. 典型农业生物炭理化特性及产品质量评价[J]. 农业工程学报，2019，35(16)：249-257. HUO Lili, YAO Zonglu, ZHAO Lixin, et al. Physical and chemical properties and product quality evaluation of biochar from typical agricultural residues[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2019, 35(16): 249-257. (in Chinese with English abstract)

[5] 柳善建，王帅超，易维明，等. 多层二次风配风对玉米秸秆颗粒燃烧降低NOx产率及结渣的影响[J]. 农业工程学报，2021，37(15)：222-231. LIU Shanjian, WANG Shuaichao, YI Weiming, et al. Influences of multi-layer secondary air distribution on NOx reduction rate and slagging tendency of maize stover particle combustion[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(15): 222-231. (in Chinese with English abstract)

[6] TORERO J L, GERHARD J I, MARTINS M F, et al. Processes defining smouldering combustion: Integrated review and synthesis[J]. Progress in Energy and Combustion Science, 2020, 81: 100869.

[7] 苏现强，何芳，李秀华，等. 秸秆燃烧中钾逸出的几种模型的对比[J]. 太阳能学报，2022，43(5)：391-398. SU Xianqiang, HE Fang, LI Xiuhua, et al. Comparison of models for release of potassium during agro-stalks combustion[J]. Acta Energiae Solaris Sinica, 2022, 43(5): 391-398. (in Chinese with English abstract)

[8] 张毅，张政清，高振强，等. 不同燃烧方式生物质灰土壤循环特性对比[J]. 可再生能源，2016，34(4)：615-620. ZHANG Yi, ZHANG Zhengqing, GAO Zhenqiang, et al. Comparison on recycling characteristics in soil of biomass ashes from different burning methods[J]. Renewable Energy Resources, 2016, 34(4): 615-620. (in Chinese with English abstract)

[9] ZHUANG Z, LI Y, CHEN B, et al. Chinese kang as a domestic heating system in rural northern China: A review[J]. Energy and Buildings, 2009, 41(1): 111-119.

[10] 王铁良，白义奎，刘文合. 燃池在日光温室加热的应用试验[J]. 农业工程学报，2002，18(4)：98-100. WANG Tieliang, BAI Yikui, LIU Wenhe. Experimental research on using fire pit to increase temperature in solar greenhouse[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2002, 18(4): 98-100. (in Chinese with English abstract)

[11] GIANFELICE G, DELLA ZASSA M, BIASIN A, et al. Onset and propagation of smouldering in pine bark controlled by addition of inert solids[J]. Renewable Energy, 2019, 132: 596-614.

[12] HE F, YI W, LI Y, et al. Effects of fuel properties on the natural downward smoldering of piled biomass powder: Experimental investigation[J]. Biomass and Bioenergy, 2014, 67: 288-296.

[13] HUANG X, REIN G. Thermochemical conversion of biomass in smouldering combustion across scales: The roles of heterogeneous kinetics, oxygen and transport phenomena[J]. Bioresour Technol, 2016, 207: 409-421.

[14] YERMáN L, HADDEN R M, CARRASCAL J, et al. Smouldering combustion as a treatment technology for faeces: Exploring the parameter space[J]. Fuel, 2015, 147: 108-116.

[15] 李劲松，李君，杨轶楠. 不同通风条件下的煤颗粒堆积床阴燃传播特性[J]. 燃烧科学与技术，2021，27(5)：545-552. LI Jinsong, LI Jun, YANG Yinan. Smouldering propagation characteristics of coal piles under different ventilation conditions[J]. Journal of Combustion Science and Technology, 2021, 27(5): 545-552. (in Chinese with English abstract)

[16] 王学宝，郑冰涛，曹大郅. 纵向通风风速对锯末正向阴燃过程的影响[J]. 消防科学与技术，2020，39(7)：896-899. WANG Xuebao, ZHENG Bingtao, CAO Dazhi. The influence of longitudinal ventilation velocity on the forwards moldering process of sawdust[J]. Fire Science and Technology, 2020, 39(7): 896-899. (in Chinese with English abstract)

[17] CHEN Y, LIN S, LIANG Z, et al. Smouldering organic waste removal technology with smoke emissions cleaned by self-sustained flame[J]. Journal of Cleaner Production, 2022, 362: 132363.

[18] RASHWAN T L, TORERO J L, GERHARD J I. The improved energy efficiency of applied smouldering systems with increasing scale[J]. International Journal of Heat and Mass Transfer, 2021, 177: 121548.

[19] YUAN H, RESTUCCIA F, REIN G. Computational study on self-heating ignition and smouldering spread of coal layers in flat and wedge hot plate configurations[J]. Combustion and Flame, 2020, 214: 346-357.

[20] SONG Z. Modelling oxygen-limited and self-sustained smoldering propagation: Underground coal fires driven by thermal buoyancy[J]. Combustion and Flame, 2022, 245: 112382.

[21] 黄鑫炎，林少润，刘乃安. 林火中的阴燃现象：研究前沿与展望[J]. 工程热物理学报，2021，42(2)：512-528. HUANG Xinyan, LIN Shaorun, LIU Naian. A review of smoldering wildfire: Research advances and prospects[J]. Journal of Engineering Thermophysics, 2021, 42(2): 512-528. (in Chinese with English abstract)

[22] SAIDI M S, HAJALIGOL M R, MHAISEKAR A, et al. A 3D modeling of static and forward smoldering combustion in a packed bed of materials[J]. Applied Mathematical Modelling, 2007, 31(9): 1970-1996.

[23] RASHWAN T L, FOURNIE T, TORERO J L, et al. Scaling up self-sustained smouldering of sewage sludge for waste-to-energy[J]. Waste Management, 2021, 135: 298-308.

[24] SONG Z, HE T, LI M, et al. Self-sustaining smoldering as a novel disposal approach for food waste with high moisture content[J]. Fuel Processing Technology, 2022, 228: 107144.

[25] ZHANG X, CHEN B, ZHAO J R, et al. Optimization of thermal performance in a Chinese traditional heating system-Burning cave[J]. Energy and Buildings, 2014, 68: 423-431.

[26] YU K, BAI L, ZHANG T, et al. Improving the thermal performance of the traditional Chinese Kang system by employing smoldering combustion and mechanical ventilation: An experimental study[J]. Energy and Buildings, 2022, 256: 111736.

[27] YAMAZAKI T, MATSUOKA T, LI Y, et al. Applicability of a low-pressure environment to investigate smoldering behavior under microgravity[J]. Fire Technology, 2019, 56(1): 209-228.

[28] KINBARA T, ENDO H, SEGA S. Downward propagation of smoldering combustion through solid materials[J]. Symposium (International) on Combustion, 1967, 11(1): 524-531.

[29] 高振强，何芳，尤彦彦，等. 含水率对佛香阴燃速率影响的实验研究[J]. 消防科学与技术，2015，34(5)：563-566. GAO Zhenqiang, HE Fang, YOU Yanyan, et al. Experiments study on the influence of moisture content on the smoldering speed of buddha incense[J]. Fire Science and Technology, 2015, 34(5): 563-566. (in Chinese with English abstract)

[30] MUKUNDA H S, BASANI J, SHRAVAN H M, et al. Smoldering combustion of “incense” sticks-Experiments and modeling[J]. Combustion Science and Technology, 2007, 179(6): 1113-1129.

[31] LIN S, CHOW T H, HUANG X. Smoldering propagation and blow-off on consolidated fuel under external airflow[J]. Combustion and Flame, 2021, 234: 111685.

[32] KADOWAKI O, SUZUKI M, KUWANA K, et al. Limit conditions of smoldering spread in counterflow configuration: Extinction and smoldering-to-flaming transition[J]. Proceedings of the Combustion Institute, 2021, 38(3): 5005-5013.

[33] YAMAZAKI T, MATSUOKA T, KUWANA K, et al. Study on the flaming-transition behavior of a downwardly smoldering biomass stick utilizing low pressure[J]. Proceedings of the Combustion Institute, 2021, 38(3): 5073-5080.

[34] YAN S, HE F, CAI J, et al. Characteristics of char cone covered by ash in steady smoldering of a char rod[EB/OL]. Combustion Science and Technology, 2022: 1-10. https://doi.org/10.1080/00102202.2022.2150079

[35] 辛颖，历美岑. 含水率对森林腐殖质阴燃传播的影响[J]. 消防科学与技术，2017，36(4)：448-450. XIN Ying, LI Meicen. The influence of moisture content on smoldering combustion of forest humus[J]. Fire Science and Technology, 2017, 36(4): 448-450. (in Chinese with English abstract)

[36] 刘慧慧，邹俊，邓勇，等. 改性生物质炭对棉秆热解挥发分析出特性的影响[J]. 农业工程学报，2016，32(22)：239-243. LIU Huihui, ZOU Jun, DENG Yong, et al. Influence of modified biomass char on releases characteristics of volatiles during pyrolysis of cotton stalk[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2016, 32(22): 239-243. (in Chinese with English abstract)

[37] LU Z. Structure and extinction of reverse smolder waves in the presence of heat losses: A premixed-flame perspective[J]. Combustion and Flame, 2022, 242: 112201.

[38] CHEN H, REIN G, LIU N. Numerical investigation of downward smoldering combustion in an organic soil column[J]. International Journal of Heat and Mass Transfer, 2015, 84: 253-261.

Comparison on the steady smoldering characteristics of char and biomass rods

ZHAO Wentao1, ZHANG Yi1, YU Guangxin1, FRANK Behrendt1,2, HE Fang1※

(1.,,255049,;2.,,10623,)

To investigate the different characteristics of char and biomass rods in steady smoldering, experiments were carried out on the vertical downward smoldering of char, absolute dried biomass and air-dried biomass rods with different diameters (2-8 mm) made from elm-bark powder and cypress powder. The detailed production methods of three rod fuels are as follows: The elm bark powder, cypress powder and water were mixed uniformly at a mass ratio of about 1:1:4, and extruded into biomass rods with diameters of 3, 5, 8 mm. The rods were dried in the air (>48 h) to obtain air-dried biomass rod. The air-dried biomass rod was placed in electric drying oven and kept at 105 ℃ for 24 h to obtain absolute dried biomass rod. After absolute dried biomass rod was cooled, it was placed in drying basin for storage. The char rod was made by pyrolysis of air-dried biomass rod: The air-dried biomass rod was placed in quartz tube, and the nozzle was sealed with asbestos. It was placed in muffle furnace and heated from room temperature to 500 ℃ at a heating rate of 10 ℃·min-1. After holding for 2 h, the power supply was turned off to obtain a char rod. After it was cooled (> 12 h), it is stored in a sealed bag. The char and biomass rods were characterized by proximate analysis following GB/T 28731-2012. In the smoldering experiment, the tops of the respective char and biomass rods (about 15 cm in length) were ignited. After the smoldering got stabilized (about 4 minutes), smoldering propagation velocity was obtained by using the scale and timer to record the moving distance and time of reaction front, respectively. During the steady smoldering process of the rod, the temperature distribution of the reaction zone was taken using infrared thermal imager (ThermoProTM TP8), and the contours of flue gas layer around the rod were recorded by a self-made schlieren device. The rod fuel was then inserted into biomass ash to isolate oxygen and get extinguished, and the shapes of reaction zone were recorded by Leica microscope (M135C). The emissivity of each rod was measured as 0.93 in infrared image analysis. Moreover, the oxygen consumption rate of char and biomass rods was calculated by a self-written code. The results showed that 1) the char and biomass rods can self-adjust the shape of reaction zone to maintain steady smoldering, and the maximum temperature range of each rod fuel is between 620 and 770 ℃. 2) The smoldering propagation velocity of char rods is about 3.9 and 4.5 times that of absolute dried and air-dried biomass rods, respectively. Not endothermic pyrolysis in the smoldering process of char rods is observed, and the larger smoldering propagation velocity produces more heat during the same time, thus the maximum temperature of char rods is about 43 and 55 ℃ higher than that of absolute dried and air-dried biomass rods, respectively. The length of reaction zone of char rods is increased due to the greater propagation velocity and higher maximum temperature provide a larger reaction area to maintain steady smoldering. The length of the reaction zone of char rod is about 3.5 and 4.1 times that of absolute dried and air-dried biomass rods, respectively. The fuel consumption rate of char rods is about 2.4 times that of biomass rods. 3) The calculated and experimental contours of flue gas layer are in good agreement, and the oxygen consumption rate of char rods is about 3.1 and 3.6 times that of absolute dried and air-dried biomass rods, respectively. In the process of steady smoldering of char and biomass rods from the same source, when the external air conditions are certain, the size of reaction zone increases, so that the smoldering propagation velocity increases. In the application design, the size of reaction zone should be reasonably designed according to the selected materials and smoldering intensity. The finding can provide a theoretical reference to study further the steady smoldering mechanism and fuel options in the application.

char; biomass; steady smoldering; propagation velocity; oxygen consumption rate

2022-12-01

2023-04-10

中德合作交流互访项目（M-0183）；山东省自然科学基金项目（ZR2022ME038）；科技型中小企业创新能力提升工程项目（2021TSGC1114）

赵文涛，研究方向为生物质能利用。Email：zwtaow@163.com

何芳，博士，教授，研究方向为生物质燃烧基础理论和燃烧设备开发研究。Email：hf@sdut.edu.cn

10.11975/j.issn.1002-6819.202212009

S21; TK6

A

1002-6819(2023)-08-0215-07

赵文涛，张毅，于光鑫，等. 炭棒与生物质棒稳态阴燃特性对比[J]. 农业工程学报，2023，39(8)：215-221. doi：10.11975/j.issn.1002-6819.202212009 http://www.tcsae.org

ZHAO Wentao, ZHANG Yi, YU Guangxin, et al. Comparison on the steady smoldering characteristics of char and biomass rods[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2023, 39(8): 215-221. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.202212009 http://www.tcsae.org