Elsevier

Fuel

Volume 189, 1 February 2017, Pages 312-321
Fuel

Full Length Article
Impact of calcium on the synergistic effect for the reactivity of coal char gasification in H2O/CO2 mixtures

https://doi.org/10.1016/j.fuel.2016.10.100Get rights and content

Highlights

  • Mechanism of synergistic effect during co-gasification was revealed.

  • Catalytic effect of CaO facilitates the synergistic effect.

  • H2O can make CaO smaller in size and higher in dispersity.

  • Interaction of H2O and CO2 with Ca participation promotes the pore development.

  • Sintering of CaO at higher temperature causes the weak synergistic effect.

Abstract

In this paper, the effect of calcium on coal gasification in mixture agents containing both H2O and CO2 was investigated by comparing the gasification behaviors of the demineralized coal and calcium-loaded coal using Wucaiwan (WCW), a calcium-enriched low rank coal, as the raw material. TG analysis of pure CaCO3 was conducted at different conditions to reveal the catalytic mechanism of calcium. The properties of the chars as well as partially-gasified chars were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moreover, pore structures of partially-gasified chars were obtained by BET analysis. It was found that under calcium catalysis, the normalized gasification rate K increased more than 20 times in H2O/CO2 mixtures, and this value was much higher than that in pure H2O and pure CO2. The results suggest that under the influence of calcium, there is a synergistic effect between H2O and CO2 during co-gasification when temperature is lower than 900 °C. The reason for this enhanced gasification is the fact that CaO, the main form of calcium mineral during coal catalytic gasification, can catalyze coal gasification in H2O/CO2 mixtures much more effectively and create a strong synergistic effect. H2O can decrease the size of the CaO particles and increase their dispersity, thus facilitating CO2 adsorption on CaO. Furthermore, with CaO participation, interaction between H2O and CO2 can allow both of them to enter the interstices or pores of the coal char more rapidly, which accelerates the reaction rate and results in the synergistic effect. The synergistic effect becomes weaker as the temperature rises, which can be attributed to reduced adsorption of CO2 on CaO and the sintering of CaO at higher temperature.

Introduction

Coal gasification is one of the most important technologies for the development of modern coal chemical industry and it is considered as a clean and environment-friendly approach for coal utilization [1]. With the demands of saving energy and reducing emission of pollutants, recycling of CO2 during chemical processes is considered to be an effective way to utilize CO2 [2], [3]. Especially in the fixed bed dry bottom gasifier, addition of CO2 to replace partial H2O can not only reduce the amount of phenolic wastewater but also limit the emission of CO2 [4]. Moreover, this method can make full use of the carbon source. Nevertheless, introducing CO2 into the gasifier may reduce the gasification reaction rate of coal with H2O. However, our previous work demonstrated that the reactivity of coal char gasification with H2O/CO2 mixtures was better than that with H2O or CO2 alone, which can be attributed to the synergistic effect between H2O and CO2 on the gasification reactivity [5]. Wang et al. [6] found that the synergistic effect disappeared after the coal was demineralized by HCl, and it was further revealed that the synergistic effect depends on the catalytic action of calcite inherent in coal. Therefore, catalytic coal gasification in mixtures of H2O and CO2 may be a reasonable and available way to ensure gasification efficiency.

Catalytic gasification is an attractive option due to its low operating temperature, high gasification reaction rate, and selective reaction pathways towards production of desired gases [7], [8], [9], [10], [11]. Calcium, an important alkaline earth metal, can show superior catalytic activity, but it reacts to a much lesser degree with clay minerals in coal and hardly volatilizes during the heating process [12], [13]. Owing to the superiority of calcium among other potential catalysts including potassium and sodium [14], [15], [16], it has been favored by more and more researchers and is a better choice for catalytic gasification.

Hengel and Walker [17] studied the gasification reactivity of calcium-loaded coal in air, CO2 and steam separately. The coal with added calcium showed outstanding activity in any gasifying agents. Murakami et al. [18] investigated the catalytic performance of CaCO3 in the steam gasification of Indonesian sub-bituminous coal. The results showed that catalytic activity of CaCO3 was as high as that of Ca(OH)2, and CaCO3 continued to display excellent performance even at low catalyst loading. Shuai et al. [19] prepared calcium-loaded coal by wet-mixing method and found that H2 concentration was greatly improved due to the catalytic effect of Ca(OH)2 on the water gas shift reaction.

Up to now, most of the studies have mainly focused on the catalytic action of calcium on the gasification of coal or char with single agents. There is limited research on the catalytic effect of calcium on the gasification of char with H2O and CO2, and it is still unclear how calcium catalyzes coal char gasification in mixture of H2O and CO2. Thus, the purpose of this study is to examine the catalytic effect of calcium on coal gasification with H2O/CO2 mixtures and attempt to reveal the catalytic mechanism of calcium for co-gasification. Wucaiwan (WCW), a calcium-enriched low rank coal, was used as the raw material. The effect of calcium on coal char gasification was evaluated by comparing the gasification behaviors of the demineralized coal and calcium-loaded coal in different gasifying agents including pure H2O, pure CO2 and H2O/CO2 mixtures. The chemical forms and dispersion of calcium on the surface of coal char were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Moreover, the pore structures of coal chars were analyzed by N2 adsorption and the dissociative-chemisorption behavior of CO2 on CaO was studied by thermal gravimetric analysis (TGA).

Section snippets

Coal sample

A sub-bituminous coal from Wucaiwan (WCW), Xinjiang province in China was used in this study. The sample was ground and sieved to a particle size less than 0.178 mm. Proximate and ultimate analyses of WCW are listed in Table 1 and the chemical composition of the coal ash is given in Table 2.

Preparation of demineralized coal sample and calcium-loaded coal sample

HCl and HF were used to wash WCW in order to remove the minerals inherent in coal. The specific demineralization process has been described in our previous work [5]. The final demineralized coal obtained

Reactivity of Ca-WCWD and WCWD in H2O/CO2 mixtures

Fig. 1, Fig. 2 show the carbon conversion and normalized gasification rate of Ca-WCWD at different gasification conditions, respectively. It can be seen that the gasification behaviors in different gasifying agents followed the order of 66.7% H2O + 33.3% CO2 > 100% H2O > 100% CO2 at 800 °C and 850 °C (Fig. 1(a) and (b)), except for the initial stage where the reactivity of H2O was lower than that of CO2 due to the slower adsorption rate of H2O compared to that of CO2 [23]. While at 900 °C, the

Conclusions

The catalytic effect of calcium on coal gasification in mixtures of H2O and CO2 was investigated, and the following are the main conclusions that were drawn from the results:

  • (1)

    The main active form, CaO, plays a remarkable catalytic role during coal char gasification with H2O/CO2 mixtures and promotes the synergistic effect. This effect becomes weaker with increasing temperature and no longer exists after 900 °C.

  • (2)

    H2O can decrease the size of CaO particles and increase their dispersity on char

Acknowledgments

This work was supported by the National High Technology Research and Development Program of China (863 Program) (No. 2015AA050503), National Natural Science Foundation of China (Grant No. 21506142) and the Shanxi Coal Based Key Scientific and Technological Project (No. MH2014-02).

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