Analysis of coal suspensions evolution during combustion process

Agnieszka Kijo-Kleczkowska

摘要

Purpose - The paper aims to undertake coal-water suspension combustion, in air and in fluidised bed conditions. Fluidised bed conditions are the best to efficiently and ecologically use fuel. Combustion technologies using coal-water fuels create a number of new possibilities for organising combustion processes so that they fulfil contemporary requirements. The aim of the process was to show how the specificity of combustion of coal -water suspensions in the fluidised bed changes the kinetics of the process, compared to combustion in the air stream. Changes of the surface and the centre temperature and mass of the coal suspension during combustion, and evolution of fuels during process are presented in the paper. Design/methodology/approach - Experimental character of the research required the research stand preparation, as well as working out of the measurements methodology (Kijo-Kleczkowska, 2010). The research stand (Figure la) was made of ceramic blocks in which the quartz pipes were put. The heating element of the stand comprised three heating coils of 2.0 kW. Each heater was placed in small quartz tubes. These tubes were built into the quartz tube which was thermally insulated by fibre material Al_2O_3 and which was covered with steel sheet. Combustion chamber constituted the quartz pipe, which was additionally insulated thermally, to keep the necessary temperature of the entering gas and to reduce the heat loss. The compressed air was transported to the quartz tube through the electro-valve, the control valve and the rotameter. This study stand allowed for the comparison of the combustion process of coal-water suspensions, in air and in fluidised bed conditions. To study in the fluidised bed, quartz sand was used. Depending on the velocity of air inflowing from the bottom of the bed, different bed characteristics were obtained - from bubble - to circulating-beds. The fumes were removed outside by means of a fan fume cupboard. To regulate the temperature inside the combustion chamber, the Lumel microprocessor thermoregulator was applied. The regulator controlled the work of tri-phase Lumel power controller supplying the main heating elements (gas heater) allowing to measure the actual temperature with accuracy of measurements to 20°C. The temperature measurements in the combustion chamber were carried out by means of the thermocouple NiCr-NiAl. To establish the centre and surface temperature and mass of the fuel, a special instrument stalk was constructed (Figure 1b). It had two thermocouples PtRh10-Pt, placed in two thin quartz tubes connected to the scale. One of the thermocouples was located inside the fuel, while the other served as a basket which was to support the fuel. It also touched the surface of the fuel. The thermocouples were connected to the computer to record the experimental results. The essential stage of the preliminary work was to make out a suspension, which was a mixture of fuel dust (hard coal dust or dried coal-sludge dust) and water. To produce the suspension it was necessary' to prepare fuel dust after grinding and sifting it, and then adding water, to obtain a suspension moisture of 20, 35 or 50 per cent. The hard coal was applied in the research. The analysis of fuel dust (in air-dry state) is shown in Table I. The testing of the porosity of fuel was made with mercury porosimetry, carried out in the Pascal 440 apparatus, applying pressure from 0.1 to 200 MPa. This method involves the injection of mercury into the pores of the fuel, using high pressures (Kijo-Kleczkowska, 2010). Findings- 1. Under experimental conditions, during combustion in the fluidised bed, intensive heating of the suspension is observed in the initial stage of the process, followed by the removal of heat from the suspension by the contacting quartz material, leading to lowering of the average fuel temperature and extension of the combustion time, compared to the process carried out in air. 2. Measurements using mercury porosimetry enable the identification of the change of suspension porosity. 3. Devolatilisation and combustion of volatiles lead to an increase in the pores' size in the fuel and their coalescence. 4. Combustion of fuel leads to the development of cracks in the suspension, and its structure changes under the influence of temperature. Cracks are caused by the formation of thermal stresses inside the fuel. 5. Under experimental conditions, suspension combustion in the fluidised bed causes an increase in volume participation of pores, with larger sizes of pores (3,500-5,000 nm), compared to combustion in the air. Originality/value The paper undertakes the evolution of suspension fuel, made of a hard coal and a coal-sludge, during combustion in air and in the fluidised bed.

机译：目的 - 本文旨在在空气和流化床条件下进行煤水悬浮燃烧。流化床条件是最好和生态使用的燃料。使用煤水燃料的燃烧技术为组织燃烧过程创造了许多新的可能性，以便他们满足现代要求。该方法的目的是展示流化床中煤水悬浮液的燃烧的特异性如何改变该方法的动力学，与空气流中的燃烧相比。纸质期间，燃烧过程中煤悬浮液的表面和中心温度和质量的变化，并在工艺中燃料的演化。设计/方法/方法 - 研究的实验性质需要研究支架准备，以及解决测量方法（Kijo-Kleczkowska，2010）。研究站（图1a）由陶瓷块制成，其中陶瓷块被放置了石英管。支架的加热元件包括3.0 kW的三个加热线圈。每个加热器都放在小石英管中。将这些管内置在石英管中，该石英管由纤维材料进行热绝热，纤维材料AL_2O_3并用钢板覆盖。燃烧室构成了石英管，其另外绝缘，以保持进入气体的必要温度并降低热量损失。压缩空气通过电阀，控制阀和轮毂输送到石英管。本研究允许比较煤水悬浮液，空气和流化床条件的燃烧过程。为了在流化床中学习，使用石英砂。根据从床的底部流入的空气速度，获得不同的床特性 - 从泡泡到循环床。烟丝通过风扇烟囱除去外面。为了调节燃烧室内的温度，施加腔微处理器热调节器。该调节器控制了提供主加热元件（气体加热器）的三相舷外功率控制器的工作，允许测量到20°C的精度测量实际温度。燃烧室中的温度测量通过热电偶NiCr-Nial进行。为了建立燃料的中心和表面温度和质量，构建了一种特殊的仪器茎（图1B）。它有两个热电偶PTRH10-PT，放置在连接到刻度的两个薄石英管中。其中一个热电偶位于燃料内部，而另一个热电偶是用作支撑燃料的篮子。它还触及了燃料的表面。热电偶连接到计算机以记录实验结果。初步工作的基本阶段是制造悬浮液，这是一种燃料粉尘（硬煤尘或干煤污泥粉尘）和水的混合物。为了产生悬浮液，必须'在研磨后制备燃料粉尘和筛分，然后加入水，得到20,35或50％的悬浮液。在研究中应用了硬煤。燃料粉尘（空气干燥状态）的分析如表I所示。用汞孔隙测定法进行燃料孔隙率的测试，在帕斯卡440装置中进行，施加0.1至200MPa的压力。这种方法涉及使用高压（Kijo-Kleczkowska，2010）注入燃料的毛孔中的汞。调查结果 - 1.在实验条件下，在流化床中的燃烧过程中，在该过程的初始阶段观察到悬浮液的强烈加热，然后通过接触石英材料从悬浮液中除去热量，导致降低与空气中进行的过程相比，平均燃料温度和燃烧时间的延伸。 2.使用汞孔隙测量法测量能够识别悬浮孔隙率的变化。 3.挥发物和挥发物的燃烧导致燃料中孔径的增加和它们的聚结。 4.燃料燃烧导致悬浮液中的裂缝的发展，其结构在温度的影响下变化。裂缝是由燃料内部形成热应力引起的。在实验条件下，流化床中的悬浮燃烧导致孔隙的容量参与，与空气中的燃烧相比，孔隙尺寸的尺寸（3,500-5,000nm）。原创性/价值本文在空气和流化床中燃烧过程中，悬浮燃料的演变，由硬煤和煤污泥制成。

Analysis of coal suspensions evolution during combustion process

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