Elsevier

Energy

Volume 214, 1 January 2021, 118991
Energy

Influencing factors and evaluation system for Carbon–Calcium pellet performance in a pyrolysis furnace

https://doi.org/10.1016/j.energy.2020.118991Get rights and content

Highlights

  • A new carbide production process with pre-pyrolysis furnace.

  • The solid block raw materials are ground into powder to make C–Ca pellet.

  • The performance of C–Ca pellet in a pyrolysis furnace are studied experimentally.

  • A comprehensive performance-evaluation system is built to select the best pellet.

Abstract

In the new process of CaC2 production, C–Ca pellets need to be pyrolysed in a prepyrolysis furnace before being placed in a CaC2 furnace. In this paper, the factors influencing the performance of composite pellets in a pyrolysis furnace are studied experimentally. A comprehensive performance-evaluation system is established to calculate and evaluate composite pellets comprising five raw materials. The pyrolytic rate, compressive strength, and dust concentration of the composite pellets are tested through thermogravimetry, universal pressure tester and dust concentration detector, respectively. The effects of C/Ca ratio, temperature, time and raw material composition on these properties are explored. Results show that during the pyrolysis of the composite pellets, the C-containing raw materials have a significant effect on various performance indicators. With increased C/Ca ratio, the pellet pyrolysis rate and compressive strength show a downward trend; with increased pyrolysis temperature and time, the pellet pyrolysis rate, compressive strength and activity show an upward trend; with a change in raw material composition, the pellet activity and dust concentration change significantly. The comprehensive properties of CaO + coke and CaO + anthracite are superior to those of other raw materials in all aspects, and this finding is in line with the industrial-production practice.

Introduction

The China Carbide Industry Survey and Analysis Report and the Market Forecast Report (2015–2020) released by the China Industry Research Network indicated that during the ‘12th Five-Year Plan’ period, China remains in a period of important strategic opportunities for development, and the CaC2 industry is facing rare development opportunities. Coal plays a leading role in the national energy consumption structure, and CaC2 is one of the leading enterprises in the coal chemical industry and a major energy consumer [1]. By the end of 2012, the capacity of the domestic CaC2 industry reached 4000 × 104 t, the capacity under construction reached 2000 × 104 t, and the annual coal consumption of the CaC2 industry alone reached more than 1 × 108 t [2,3]. Therefore, the energy conservation and emission reduction of the CaC2 industry determine the future development of China’s energy structure and ecological environment. Advanced production technology, high-quality products and efficient energy utilisation rate are the goals pursued by the current carbide production industry [4].

The main production process of CaC2 is as follows. A mixture of block semicoke (1–2.5 cm) and block CaO (3–5 cm) is transported to a feed port through a belt and added to the furnace body. The electric furnace is heated to approximately 2000 °C, and CaC2 is generated in accordance with the formula CaO+3C→CaC2+CO. The molten CaC2 produced by arc heating and reaction is discharged from the furnace bottom and naturally cooled, the CO and dust produced by the reaction are discharged from the furnace top, the CO after dust removal and purification is recycled, and the CaC2 dust is generally landfilled after collection [5]. The solid block-feeding reaction process leads to a low reaction rate, which seriously limits the productivity of current production technologies [6,7]. In this paper, a new production process of CaC2 is introduced. The solid block raw materials are ground into lime powder and coal powder (coke powder), which are mixed evenly, pressed into composite pellets (3–5 cm) at a certain pressure, added to a preheating pyrolysis furnace to preheat and pyrolysed at 700 °C-800 °C. Finally, the high-temperature pellets are sequentially fed into an ore-smelting furnace to start reaction. The new process can effectively improve the reverse reaction speed and energy utilisation.

Tagawa [8] crushed CaO and C into a cylinder (20 mm × 30 mm) at a molar ratio of 1:3 and conducted the reaction at a temperature of 1600 °C-1800 °C. The chemical reaction rate is primarily determined by the diffusion of gas through the product layer and solid into the solid and product layers. Wang [9,10] compressed anthracite and lime powder into cubes (5 × 5 × (4.6–5.1) mm) to react at 30 MPa under the conditions of 1700 °C-1850 °C and a C/CaO molar ratio of 2.5, which classify the chemical reactions in accordance with restricted procedures. Brookes [11] et al. crushed CaO and semicoke and mixed them into cylindrical pellets (2.22 cm × 3.18 cm) at a molar ratio of 3:1. A zero-dimensional model of CaC2 reaction was established on the basis of experimental phenomena. Wang Yi [12] systematically studied the traditional carbide production process and the new clean production process of carbide using composite pellets with a diameter of 40–50 mm. Xu Qian [13] proposed a multiphysical reaction transport coupled model based on an implicit finite difference method to address the porous particles of two solids and studied its dynamic characteristics of heat transfer, chemical reaction and phase transitions.

However, the research on the reaction of pellets in a pyrolysis furnace is presently limited, and its effective pyrolysis plays an important role in the formation of CaC2 in the follow-up submerged arc furnace. Therefore, this paper focuses on the reaction changes of composite pellets in the preheating pyrolysis furnace stage, determines the working conditions and selects the composite pellets with high evaluation indices to be applied to CaC2 after the thermolysis in the preheating pyrolysis furnace production. The process accords with the production principles of green, energy saving and high efficiency and is thus significant to the new process of carbide production.

Section snippets

Experimental materials

CaO and CaCO3 are provided by Beijing Tongguang Fine Chemical Corporation; anthracite, soft coal and coke are provided by Henan Zhengzhou Dongda Machinery Corporation. Table 1 shows the specific purity and granularity of the raw materials used in the experiment, and Table 2 presents the industrial analysis of C raw materials.

Experimental system and test instrument

In this study, a box-type muffle furnace is used as the prepyrolysis furnace to study the reaction. Firstly, the heating rate, pyrolysis temperature and time on the control

Effect of C/Ca ratio on pellet performance

Anthracite and CaO are selected as raw materials to explore the effect of C/Ca mole ratio on the performance of the composite pellets. The mass of anthracite is 10 g, and the mass of CaO is 26.8, 20.4, 16.4, 13.7, 11.6 and 10.2 g. That is, 6 mol ratios of C/Ca (1.5, 2, 2.5, 3, 3.5 and 4) are studied. Other experimental conditions are as follows: 30 MPa forming pressure, 3% polyvinyl alcohol as binder and 150 °C drying pellet for later use. The pyrolysis temperature is 750 °C, the heating rate

Comprehensive evaluation system for composite pellet performance

Many performance evaluation indices exist for composite pellets, and the influence degree of each index on the industrial production economic indices of preheating pyrolysis and CaC2 furnaces differs. The evaluation and selection of the composite pellet sent to the pyrolysis furnace are difficult to conduct with only a few indices. Therefore, a comprehensive evaluation system for composite pellet performance is established, and the corresponding weight coefficient is selected for evaluation.

Conclusion

From the pyrolysis experiment of composite pellets; the changes in C/Ca ratio, pyrolysis temperature, time and raw material composition of pellets and the test and analysis of pyrolysis rate, compressive strength, activity and other properties, we obtain the following conclusions:

  • (1)

    As the C/Ca ratio increases, the addition ratio of CaO gradually decreases, and the pyrolysis rate firstly increases and then decreases. When the C/Ca ratio is 1.5 and the CaO addition ratio is 72.8%, the pyrolysis

Credit author statement

Shaowu Yin:Conceptualization, Methodology, Validation. Hongyu Wang:Data Curation , Investigation,Writing-Original draft preparation. Li Wang:Supervision. Chuanping Liu: Visualization,Validation. Lige Tong: Validation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships which have, or could be perceived to have, influenced the work reported in this article.

Acknowledgements

This work is fully supported by the National key research and development plan of China (No.2018YFB0605901). The authors would also like to express their sincere thanks to the anonymous reviewers for their detailed and insightful comments that helped in improving the quality of this paper.

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