Full Length ArticleComparative study on the preparation of powdered activated coke for SO2 adsorption: One-step and two-step rapid activation methods
Introduction
SO2 emissions from fossil fuel combustion and solid waste incineration greatly harm the environment and human health [1], [2], [3]. At present, the most commonly used flue gas desulfurization (FGD) technology, namely wet FGD, which utilizes Ca-based absorbents for SO2 removal, has high operating costs, high levels of water consumption, and issues with CO2 leakage and secondary pollution [4], [5], [6]. Alternatively, SO2 adsorption technology using carbon-based adsorbents, such as activated carbon [7], [8], [9], activated coke [10], [11], [12] and activated carbon fibers [13], [14], [15], has been regarded as the most promising choice for the next generation of SO2 removal technology owing to its advantages of water saving, sulfur recovery and multi-pollutant removal.
For the activated coke (AC) FGD technology, the high preparation cost of AC is the main problem, which inhibits its wide spread in industrial applications. In general, the preparation methods of AC include a physical activation method and a chemical activation method [16]. The chemical activation method is limited by its high preparation cost and environmental impact, which is caused by the use of chemical reagents (hydroxides, carbonates, H3PO4, or ZnCl2). As for the physical activation method, it is widely used to prepare AC owing to the fact that it is relatively environmentally friendly and easy to be used in industry [17], [18], [19]. The traditional physical activation method can be divided into one-step and two-step methods [20]. For the two-step method, generally, the materials are first carbonized in an inert atmosphere with a temperature below 800 ℃, and are then activated in an activating atmosphere under a temperature of approximately 700–1000 ℃. As for the one-step method, carbonization and activation are conducted simultaneously [21], [22]. Physical activation methods often use water vapor, CO2, O2 (air), or a mixture of these as the activating agent. Among them, when O2 is involved in activation, the reaction is too intense which makes the activation process difficult to control. Thus, the O2 concentration is ideally kept low at 2–11% [22], [23]. Except for N2, coal-fired flue gas mainly contains O2, CO2, and water vapor, and the concentration of O2 and H2O can be easily adjusted with additional air and water vapor. Therefore, flue gas can be regarded as a good activating agent. Li et al. [12] found that using flue gas to prepare AC for removing SO2 from flue gas is feasible, and that the finer the coal particles, the better the performance of the prepared AC.
At present, the commonly used AC for SO2 adsorption is granular activated coke (GAC) (Φ 5–9 mm) which is usually prepared using the two-step method, and the GAC molding-process is essential [24], [25]. So the preparation of GAC has high cost and complex process. Furthermore, GAC usually requires high hardness to avoid wear and the utilization rate of pore structures and active sites inside it are low [13]. In order to reduce the preparation cost and overcome the limitations of GAC, a process for the rapid preparation of PAC from pulverized coal using a one-step method in a flue gas atmosphere was proposed in this study, as shown in Fig. 1(a), in which part of the pulverized coal burns in the combustion zone to produce high-temperature flue gas. Then the flue gas is adjusted in the ash hopper with air and water, and carries up the other pulverized coals from the bottom of the carbonization and activation zone, completing the rapid preparation of PAC in this zone within approximately 3–5 s. Many efforts have been conducted on PAC preparation through the one-step method by the authors’ research group. Fu et al. and Zhang et al. [26], [27] found that O2 played an important role in the formation of PAC’s pore structure, and appropriate O2 concentration was beneficial to the formation of micropores, and when water vapor was present at the same time, the PAC’s micropore ratio could be further improved, which showed that it was feasible to prepare PAC under flue gas atmosphere. Subsequently a pilot test with PAC production of 30–50 kg/h was conducted, and experimental results further verified the feasibility of this method [28]. On the basis of previous studies, with the help of response surface methodology, Zhou et al. [29] optimized the preparation parameters of the one-step method and obtained the most appropriate parameters combination for the PAC preparation from SL lignite, which provided a reference basis for the optimization of the PAC preparation condition. Besides, An et al. [30], [31], [32] found that the PAC prepared using this method also has a good removal performance on Hg in flue gas, so as to realize the combined control of multiple pollutants.
By drawing a few lessons from the traditional two-step process, based on the one-step method, another process for the rapid preparation of PAC (two-step method) is also proposed in this study, as shown in Fig. 1(b). Pulverized coal is first carbonized in the carbonization zone in the O2-free atmosphere coming from the activation zone to prepare the carbonized semi-coke (CSC). Then, the flue gas coming from the combustion zone is adjusted, and it then carries up the CSC from the bottom of the activation zone, completing the rapid preparation of PAC in this zone within approximately 3–5 s. So far, no research works have been conducted on the properties of the prepared PAC using this method.
In this study, a lignite and a bituminous coal were used as raw materials to prepare PAC using a drop-tube reactor (DTR) through the two methods, respectively. The yields, pore structures and SO2 adsorption capacities of the prepared PACs through the two methods were compared and analyzed, and the activation mechanisms of the two methods were speculated. Furthermore, this study provides a preliminary reference for the selection of PAC preparation methods.
Section snippets
Experimental materials
In this study, Shengli lignite (SL-coal) and Jinjie bituminous coal (JJ-coal) were used as experimental materials for the preparation of PAC, and their coal analyses are given in Table 1. They were dried at 105 ℃ for 8 h and then crushed, ground, and sieved to obtain a particle size of approximately 60–90 μm.
Experimental system
A DTR system was used to prepare the PAC or CSC, as shown in Fig. 2. The system consisted of five parts, namely a gas system, a micro feeder, a DTR, a PAC collector, and a vent treatment
Sample yield and burn-off
Table 3 shows the proximate analyses of all the samples. The carbonization process in the two-step method made the CSC retain some volatiles, while Ad and FCd relatively increased due to the release of volatiles. Then, during the activated process, the further release of volatiles and the ablation of the fixed carbon by the activated components led to the further reduction of the Vd of the PAC2, and to a significant increase in the Ad, whereas its FCd non-significantly changed compared with the
Conclusions
For the two different preparation processes of PAC, corresponding experiments were conducted using a DTR system with SL-coal and JJ-coal as raw materials, and the following conclusions were obtained:
(1) The yield of the prepared PAC from JJ-coal was higher than that prepared from SL-coal under the same preparation conditions. In detail, the yields were 49.24% and 47.61% for the JJ-PAC1 and JJ-PAC2, respectively, which were higher than 48.41% for the SL-PAC1 and 46.36% for the SL-PAC2. Also, the
CRediT authorship contribution statement
Binxuan Zhou: Data curation, Writing - original draft. Tao Wang: Conceptualization, Methodology. Tianming Xu: Validation. Cheng Li: Resources. Jun Li: Data curation, Resources. Jiapeng Fu: Formal analysis. Zhen Zhang: Investigation. Zhanlong Song: Project administration. Chunyuan Ma: Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Key R&D Program of China [grant number 2017YFB0602902].
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