Study of the reaction mechanism for preparing powdered activated coke with SO2 adsorption capability via one-step rapid activation method under flue gas atmosphere

Abstract

In this study, the impact of different reaction times on the preparation of powdered activated carbon (PAC) using a one-step rapid activation method under flue gas atmosphere is investigated, and the underlying reaction mechanism is summarized. Results indicate that the reaction process of this method can be divided into three stages: stage I is the rapid release of volatiles and the rapid consumption of O2, primarily occurring within a reaction time range of 0–0.5 s; stage II is mainly the continuous release and diffusion of volatiles, which is the carbonization and activation coupling reaction stage, and the carbonization process is the main in this stage. This stage mainly occurs at the reaction time range of 0.5–2.0 s when SL-coal is used as material, and that is 0.5–3.0 s when JJ-coal is used as material; stage III is mainly the activation stage, during which activated components diffuse to both the surface and interior of particles. This stage mainly involves the reaction stage of CO2 and H2O (g) activation, and it mainly occurs at the reaction time range of 2.0–4.0 s when SL-coal is used as material, and that is 3.0–4.0 s when JJ-coal is used as material. Besides, the main function of the first two stages is to provide more diffusion channels and contact surfaces/activation sites for the diffusion and activation of the activated components in the third stage. Mastering the reaction mechanism would serve as a crucial reference and foundation for designing the structure, size of the reactor, and optimal positioning of the activator nozzle in PAC preparation.

1. Introduction

The combustion of S-containing fuel results in the emission of SO2, which poses a serious threat to both the environment and health [1–3]. At present, the wet flue gas desulfurization (WFGD) is the most popular method for SO2 removal, while this technology is facing challenges due to many problems in its running process, such as high consumption of Ca-based absorbents and water, high operating costs, resulting CO2 leakage and secondary pollution [4–6]. Alternatively, the use of carbon-based adsorbents [7–13] for SO2 adsorption technology has been considered a promising updating technology due to its advantages in sulfur recovery, water conservation and multi-pollutant removal. The aforementioned technology has been employed in Europe and Japan for the purpose of flue gas purification, specifically targeting emissions resulting from the combustion of coal and waste [14].

For the AC-FGD technology, the primary obstacle to its industrial implementation is the exorbitant cost of preparation for the AC adsorbent. In general, there are two methods for the preparation of AC, namely physical and chemical methods. The utilization of the latter is restricted owing to the application of chemical reagents [15,16], while the former is extensively utilized due to its relative environmental friendliness and ease of industrial application [17,18]. Physical activation method generally consists of two steps: carbonization process and activation process. Generally, the carbonization process followed by activation process, and there is an AC forming process between the two processes [19]. At present, the prevailing method for SO2 adsorption involves the use of columnar AC (CAC) with a size of 6 mm × 9 mm, which is typically prepared in this manner. Specifically, the materials are initially carbonized in an inert atmosphere at a temperature below 800 °C, followed by cooling, screening and forming. Finally, they are activated in an activating atmosphere at a temperature of approximately 700–1000 °C. This method is a complex process and its operation cost is high, which leads to the high price of CAC; besides, the activation process is after molding, which is bound to lead to internal and external activation is not uniform, and when the CAC are used for adsorption, the utilization rate of its inner surface is also low. In addition, the CAC products necessitate high levels of hardness to prevent wear and tear.

The activators commonly used in the physical activation method are CO2, H2O (g), O2 or their mixtures, and different activation components have different activation effects. Among them, CO2 can promote the formation of micropores [20,21]; H2O (g) can promote the expansion of micropores, thus increasing the proportion of mesoporous [20,22]; and the mixture of the two has a mutual...