Research paperNumerical study on the heat recovery and cooling effect by built-in pipes in a graphitization furnace
Introduction
Graphitization furnaces are mainly used in high temperature processing for purification of graphite powder, and can manufacture electrode material, carbon fiber, cemented carbide, precision ceramic materials and other particular components. Thus they are an important part of carbon industrial manufacturing. Generally speaking, the raw material takes 1–2 days to heat, 1 day for thermal insulation and 8–10 days for naturally cooling down before being taken out. However, it takes about 4000–4500 kWh to produce each ton of carbon electrode, and each furnace consumes 300–400 MWh in one production cycle [1]. Even the advanced lengthwise graphitization furnace also consumes 2800–3200 kWh per ton of carbon electrode [2]. In order to accelerate cooling, the top insulation layer of the furnace is removed in the cooling period, but no mechanical method is adopted to reduce the cooling time or recover heat.
Many researches are being conducted to improve the performance of graphitization furnaces. There are two main approaches. Some focus on the relationship between the electric charge curve and temperature field variation, then optimize the heating curve to make good use of the power. Kutuzov et al. [3] constructed a mathematical model of a graphitization furnace and performed calculations to determine the temperature fields in the furnace when a more efficient heat-insulating charge is used. Lin et al. [4] proposed a mathematical model of heat transfer in a graphitization furnace based on the analysis of its thermal, electric and physical characteristics, and reasonable plans for supplying electricity are made according to the model. Chen et al. [5] studied the production process of a six-electrode continuous graphitization furnace numerically to understand its electric parameters and the change of temperature distribution, and the feasibility and security of the proposed furnace are verified. In addition, some studies try to adjust the treatment of the furnace to suit different materials. Hamada et al. [6] analyzed the technology to produce lithium battery anodes using an Acheson graphitization furnace, and the heat-treated process was well studied. Boudard et al. [7] characterized the physico-chemical features and the in vitro impact on the biological activity of five SiC powders through different treatments by an Acheson graphitization furnace. Takaku et al. [8] studied the structural development on the heat treatment of carbon fibers by a graphitization furnace, and the correlation independence of heating condition and carbon fiber types was found. It can be seen that the present researches mainly focus on the heating period of the graphitization furnace, and pay less attention to heat recovery and the cycle-reducing potential. No waste heat recovery or mechanical cooling system has been put forward in this area.
Nevertheless, numerous approaches have been proposed for optimizing industrial energy systems [9] and cooling systems, which may provide a reference to develop a heat recovery and fast cooling system for graphitization furnaces. Power stations, the steel industry and cement industry are common heat recovery fields. Mokkapati et al. investigated exhaust heat recovery systems of generators with twisted tape inserts [10], [11], [12], [13], [14], [15], showing great energy conservation potential. Barati et al. developed waste heat recovery technologies towards molten slag, offering good opportunities to save energy in the steel industry [16], [17], [18], [19], [20], [21]. Karellas et al. evaluated waste heat recovery systems in the cement industry by energy and exergy analysis, identifying the most efficient and economical solutions [22], [23], [24], [25], [26]. As for industrial cooling, there are also many applications. Desideri et al. tried to use solar power to meet the cooling load in multiple types of factories [27], [28], [29]. Wang et al. studied the methods for fast cooling in the steel industry [30], [31]. Zhang et al. [32], [33], [34] presented a micro-structure heat exchanger for a special cooling requirement in industrial manufacturing. These researches are all expected to find sustainable and efficient solutions in the basic materials industries [35].
As concerns the graphitization furnace, its core is usually heated over 2800 °C, and its chamber is filled with solid instead of fumes. Thus, common heat recovery and cooling systems cannot be adapted to it immediately although the ideas can be imitated. In the meantime, the safety of the system is another important issue due to the high temperature.
To overcome these problems, an idea for a graphitization furnace based on heat recovery and fast cooling is presented in this article. The heating, insulation and cooling periods of the improved and original furnaces are studied numerically. Comparative analysis testifies to the energy conservation and production promotion potential. The key design parameters of the improved furnace are also obtained and the safety of the system is checked.
Section snippets
Description of the heat recovery and fast cooling system
Fig. 1 sketches the principle of the heat recovery and mechanical system for the graphitization furnace. Pipes are buried in the furnace and connected with the mechanical system for heat recovery or mechanical cooling. Air or inert gas is circulated in the system as a heat medium in the cooling period, transferring heat from the hot furnace to the heat recovery system to generate electricity, steam etc. In this period, valves V3, V4, V5, and V6 are open while V1 and V2 are closed. With the
Geometric model
As shown in Fig. 3, a typical graphitization furnace is investigated in the study. The dimensions of the furnace are 17 m (length, Z) × 4.8 m(width, X) × 4.4 m (height, Y). Thirty-two pipes are arranged in the two sides at intervals of 180 mm and twenty pipes are arranged in the bottom at intervals of 150 mm. The diameter of each pipe is 100 mm. The core and insulation sections are further divided into the inner core, outer core, inner insulation layer and outer insulation layer (different from
Results and discussion
The simulation of the original furnace in the model validation section indicates that the location of the built-in pipes can be heated to over 1350 °C in the heating period, which may cause damage to the pipes. Thus it is necessary to ventilate low speed gas in the later heating period. In addition, the thermal conductivity of the insulation layer is small, so appropriate heat extraction at the insulation layer in the heating period may not have any impact on the core but will help the cooling
Conclusions
As a basic equipment in the carbon industry, the traditional graphitization furnace consumes a large amount of energy for heating and takes a long time for natural cooling. A heat recovery and fast cooling system is presented for graphitization furnaces, in which cooling pipes are arranged into the insulation layer and gas is ventilated through the pipes. A numerical method is adopted to simulate the heating, insulation and cooling period of the improved furnace and the original furnace
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