Carbon burnout from the char of a single cylindrical pellet

Q3 Earth and Planetary Sciences
Mykola M. Zhovmir
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引用次数: 0

Abstract

In countries with limited wood fuel resources, the use of straw for energy should be increased to comply with environmental commitments. Boiler houses burning whole straw bales have limited application because of unfavourable straw logistics and obstacles to their construction in most densely built-up settlements. Straw pellets are more convenient for transportation and safer for storage. Boilers with pellet burning can be operated in fully automatic mode. However, burning straw pellets instead of wood pellets is complicated with increased ash content and lower ash melting temperature. Approaches for low-temperature burning of straw pellets at temperatures below the initial deformation temperature of their ash are of practical interest. At low temperatures, burning is slowing and incomplete burnout with energy loss is possible. The rate of burnout also depends on the form and dimensions of the particle. It was common to accept the rate of carbon burnout as uniform on the reacting surface of the particle. This work aimed to study the low-temperature carbon burnout from the char of the cylindrical pellets of various lengths and to estimate the energy loss with unburned carbon. The aim was achieved by the mathematical description of pellet char burning, in which the actual cylindrical char was regarded as the infinite cylinder intersecting with the endless plate. The burnout of fixed carbon was accepted as layered, the carbon burning fronts as infinitely thin, and different rates of carbon burning in radial and axial directions were accepted. An experimental study of the low-temperature burning of fixed carbon from the char of wood and straw pellets was conducted at free air access at 700°C in the furnace, i.e., the temperature around a single pellet was certainly lower than the possible initial deformation temperature of the pellet ash. The duration of carbon burnout from the char of single cylindrical pellets depending on their lengths was studied. The research findings are as follows. Equations in a dimensionless form, describing changes in the remaining share of unburned fixed carbon in pellet char in time, were deduced analytically. At the experimental burning of single straw pellets, loose particles of ash with no signs of melting formed, but they contained unburned carbon. The share of unburned fixed carbon in ash was 0.016–0.020. The coefficient of determination of calculated and experimental duration of complete burnout of fixed carbon from wood pellet char was R2 = 0.96, and R2 = 0.87–0.91 at incomplete burnout from straw pellet char. The most significant scientific result is that for long pellets, the fixed carbon burnout is controlled mainly by its slower burnout rate in the radial direction, and for the shortest pellets by more intensive burnout rate in the axial direction. The practical value of the results obtained is that the use of shorter pellets, which are characterised by faster burnout, may become purposeful for intensive combustion. Conversely, for slow combustion, and especially with the aim of arranging low-temperature burning of straw pellets, it may be feasible to use longer pellets with extended burnout. In the given conditions of straw pellet burning, the unburned carbon presents in ash, but losses of the pellet energy with unburned carbon were estimated at 0.61–0.72%, which is acceptable for boiler burners.
由单个圆柱形颗粒的炭化而产生的碳燃尽
在木材燃料资源有限的国家,应增加使用秸秆作为能源,以遵守环境承诺。在人口密集的居民点,由于秸秆运输不便和建设障碍,燃烧全秸秆的锅炉房应用有限。秸秆颗粒运输更方便,储存更安全。具有颗粒燃烧的锅炉可以在全自动模式下运行。然而,用秸秆颗粒代替木屑颗粒燃烧会增加灰分,降低灰分熔化温度。在低于其灰分初始变形温度的温度下低温燃烧秸秆球团的方法具有实际意义。在低温下,燃烧是缓慢的,不完全燃尽与能量损失是可能的。烧坏的速度还取决于颗粒的形状和尺寸。通常认为碳的燃尽速率在颗粒的反应表面是均匀的。本研究旨在研究不同长度柱状球团的低温碳燃尽,并估算未燃碳的能量损失。通过对球团炭燃烧过程的数学描述,将实际的柱状炭视为与无底板相交的无限大圆柱体,从而达到了这一目的。固定碳的燃尽是分层的,碳燃烧前沿是无限薄的,在径向和轴向上可以接受不同的碳燃烧速率。在炉膛内700℃自由通风条件下,即单个球团周围的温度肯定低于球团灰分可能的初始变形温度,对木质和秸秆球团中的固定碳进行了低温燃烧实验研究。研究了单圆筒形球团的焦化时间随球团长度的变化规律。研究结果如下:以无量纲形式推导了描述颗粒炭中未燃烧固定碳剩余份额随时间变化的方程。在单个秸秆颗粒的实验燃烧中,形成了没有融化迹象的松散灰烬颗粒,但它们含有未燃烧的碳。未燃烧固定碳在灰分中所占比例为0.016 ~ 0.020。木屑颗粒炭固定碳完全燃尽的计算时间和实验时间的决定系数R2 = 0.96,秸秆颗粒炭完全燃尽的决定系数R2 = 0.87-0.91。最显著的科学结果是,对于长颗粒,固定碳的燃尽主要受其在径向上较慢的燃尽速率控制,而对于短颗粒,其在轴向上的燃尽速率更强。所获得的结果的实用价值是,使用较短的颗粒,其特点是更快的燃烬,可能成为有目的的密集燃烧。相反,对于缓慢燃烧,特别是为了安排秸秆颗粒的低温燃烧,使用延长燃尽的较长颗粒可能是可行的。在给定的秸秆颗粒燃烧条件下,未燃烧的碳以灰的形式存在,但未燃烧碳的颗粒能量损失估计在0.61-0.72%之间,对于锅炉燃烧器来说是可以接受的。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Energetika
Energetika Energy-Energy Engineering and Power Technology
CiteScore
2.10
自引率
0.00%
发文量
0
期刊介绍: The journal publishes original scientific, review and problem papers in the following fields: power engineering economics, modelling of energy systems, their management and optimi­zation, target systems, environmental impacts of power engi­neering objects, nuclear energetics, its safety, radioactive waste disposal, renewable power sources, power engineering metro­logy, thermal physics, aerohydrodynamics, plasma technologies, combustion processes, hydrogen energetics, material studies and technologies, hydrology, hydroenergetics. All papers are re­viewed. Information is presented on the defended theses, vari­ous conferences, reviews, etc.
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