Effect of hydrothermal dewatering on the slurryability of brown coals

https://doi.org/10.1016/j.enconman.2011.11.016Get rights and content

Abstract

Two brown coals from China were dewatered under hydrothermal dewatering (HTD) conditions at 250–320 °C for 1 h in a 2 L autoclave. The hydrothermally dewatered products were used to prepare coal water slurry (CWS) with a lower viscosity than brown raw coal slurry. Moreover, the coal rank and heat value of the brown coal increased as the inherent moisture and oxygen content decreased during the HTD process. The maximum solid concentration of CWS prepared from XiMeng coal increased from 45.7% to 59.3%, whereas that of CWS prepared from BaoTou coal increased from 53.7% to 62.1%, after being dewatered at 320 °C. The improvement in the slurryability of brown coal significantly depended on the final temperature of the HTD process, the mechanism of which can be explained by the chemical analysis of oxygen functional groups, zeta potential, and the contact angle of the surface between coal and water. The oxygen functional groups, the oxygen/carbon ratio and hydrogen/carbon ratio in brown coal decreased, indicating that the coal rank was upgraded during the HTD process. As a result, both the point of zero charge and the contact angle increased, implying that the HTD products were highly hydrophobic.

Highlights

Brown coals are upgraded by hydrothermal dewatering. ► The moisture content and oxygen functional groups decrease during the process. ► The point of zero charge and the contact angle rise as the temperature increases. ► The products were highly hydrophobic. ► The improvement on slurryability of solid products were examined.

Introduction

Low-rank coals (brown coal and lignite) are abundant worldwide, and they constitute a significant resource for both energy and chemical feedstock. However, despite their abundance and relatively low market price, these low-rank coals have not been utilized as much as higher rank coals [1]. Preparing coal water slurry (CWS), a clean liquid fuel, as an oil substitute is an economic choice for the utilization of brown coals. However, the presence of significant inherent moisture and oxygen functional group contents in brown coals [2], [3] results in strong tendencies for self-ignition during both transportation and storage and presents great difficulty in preparing these materials for CWS [4]. Therefore, dewatering is the first essential step in almost all brown coal utilization processes.

Brown coals dried using conventional evaporative methods will reabsorb water over time. Hydrothermal dewatering (HTD) is a non-evaporative method by which water is removed as a liquid so that the latent heat of vaporization is saved and a large amount of the water-soluble inorganic material is leached out [5], [6]. Peat [7], [8], biomass [9] and other low quality coals [10] could be also upgraded, dewatered, desulfurized or de-ashed by hydrothermal treatment at temperatures ranging from 150 to 380 °C. A study reported that when Australian brown coal was treated with water at 350 °C under 18 MPa, the coal was separated into four fractions, namely, gaseous product by 8% yield, water-soluble extract at room temperature (soluble) by 23% yield, extract precipitates as solid at room temperature (deposit) by 23% yield, and residual coal (upgraded coal) by 46% yield [11]. The upgraded coal by the HTD process has lower moisture and oxygen contents compared with raw coal. JGC Corporation has developed the technology to improve the quality of low rank coal on CWS at a temperature of 300 °C and pressure of 12 MPa. And it is constructed a pilot plant in Indonesia by utilizing hydrothermal treatment [12], [13]. Besides, the factors have been studied that affect the HTD of low rank coal, including temperature, residence time, and dry coal/water ratio and so on, and the modification of coal properties, such as intra-particle porosity [6], [14], [15].

Previous researches mainly focused on the influence factors and composition of HTD products. Studies of the effect of coal rank and wettability on CWS properties were relatively few. In the present paper, two Chinese brown coals were hydrothermally treated for dewatering and upgrading at 250–320 °C. The effects of HTD temperature on the slurryability of brown coals were studied by analyzing the coal rank and oxygen functional groups as well as zeta potential, and contact angle of the treated coals.

Section snippets

Procedure for HTD

Two Chinese brown coals, XiMeng (XM) and BaoTou (BT) were used as raw samples, which were collected from Inner Mongolia, the largest brown coal producer in China. HTD treatment was performed using a hydrothermal reaction system (Fig. 1), comprising a 2 L cylindrical autoclave (WeiBa WHFS-2) that was equipped with an automatic temperature controller and had a maximum pressure of 25 MPa and a maximum temperature of 350 °C, an electrically heated furnace, a magnetic stirrer, and a controller. In the

Coal property analysis

The proximate and ultimate analyses of brown coals modified by HTD are shown in Table 1. The equilibrium moisture content of the two brown raw coals is greater than 15%, which then decreased to the minimum of 5.17% after the hydrothermal treatment process. Moreover, the moisture can be more efficiently removed at a high treatment temperature, indicating that the coal capacity for holding water weakened after HTD. Thus, it is beneficial to improve CWS concentration because the free water, as the

Conclusion

HTD is a process of simulating the growth process of coal in nature. Coal rank can be upgraded by the HTD process deduced from the decreasing of O/C and H/C. And the treated products had a higher CV than raw coals, as indicated that the two raw coals were 18624 kJ/kg and 18395 kJ/kg and the products was between 21,600 kJ/kg and 25,000 kJ/kg. At the same time, both equilibrium moisture and oxygen contents are simultaneously decreased. The hydrophilic groups are removed during the HTD process,

Acknowledgment

The authors gratefully acknowledge the support of the Special Funds for Major State Basic Research Projects of China (2010CB227001).

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