Full Length ArticleCo-gasification reactivity of petcoke and coal at high temperature
Introduction
With the increasing demand of petroleum and the installation of more petroleum deep conversion process units, the output of petcoke has increased rapidly [1]. Petcoke has a high carbon content and calorific value which usually be used as fuel to combust, inevitably with low efficiency and high pollution especially high-sulfur petcoke. Fortunately, it is increasingly considered as an attractive feedstock for gasification to produce hydrogen, power, and steam in the petroleum industry [2], [3]. However, the low reactivity and ash content of petcoke has greatly restricted its applicability for gasification [4], [5], [6].
Increasing the gasification temperature is one of the most effective methods to improve the gasification reactivity of petcoke. In our previous work [7], it was found that the CO2 gasification rate of petcoke increases 25 times as the temperature increases from 1200 °C to 1600 °C. However, the gasification rate of petcoke was still several times lower than that of coals even at 1600 °C in both CO2 and steam atmosphere. In order to achieve a high carbon conversion, a higher temperature or a larger residence time is necessary. But this will reduce the gasification efficiency and economics. Moreover, the low ash content of petcoke is not suitable for entrained-flow slagging gasifiers with a cooled membrane wall.
On the other hand, it is known that coal is the most common feedstock used for gasification. The reactivity of coal often varies with its rank, which decreases from low rank coal to high rank coal. The low rank coal usually has a high reactivity as its high content of volatile and mineral matter [8]. However, it is not often used as feedstock in most entrained-flow gasifiers, which is because of its high moisture, low calorific value and low concentration in coal-water slurry [9], [10]. How to utilize the large deposits of low rank coal remains a serious problem, especially in China.
Considering the characteristics of petcoke and low rank coal, co-gasification will be a better method to improve the low reactivity of petcoke and also offer a suitable way to utilize the large amount of low rank coal. Understanding the co-gasification behavior and reaction mechanism of petcoke and coal is essential for the development of co-gasification technology. Up to now, there are a number of studies concerning on co-gasification of coal and petcoke. It is important to note that some synergetic effects may occur during co-gasification of petcoke with coal or other materials [11], [12], [13]. Due to the synergy effect, the gasification reactivity, carbon conversion, syngas heating value and cold gas efficiency of petcoke can be improved by co-gasifying with coal or other materials [14], [15], [16].
However, it should be noted that although the studies in the literatures are valuable for understanding the co-gasification behavior of petcoke with coal, there is still some insufficiency especially for co-gasification reaction mechanism at high temperature. As the complicated properties of petcoke and coal, the detail reasons of synergetic effect during co-gasification is not very clear. It is worth to verify which components in coal has positive effect on co-gasification reaction at high temperature. Moreover, it is necessary to investigate whether the synergetic effect is existent at temperatures higher than the coal ash fusion temperature.
In this work, an attempt was made to understand the co-gasification behavior of petcoke with coal with respect to CO2 in the temperature range of 1200–1400 °C. A drop-in-fixed-bed reactor coupled with a mass spectrum (MS) was used to analyze the co-gasification reactivity. Four different rank coals, including two low rank coal, one middle rank coal and one high rank coal were chosen to seek the key factor that influence the co-gasification reactivity. The effect of coal ash on gasification of petcoke was investigated. XRD method was used to analysis the major mineral matters in the residual char.
Section snippets
Samples
Four coals from different rank (XLT and SL, low rank coal; XCG, middle rank coal and GP, high rank coal), and two petcokes (YS and JS) were selected. The samples were pulverized to pass through a 100-mesh screen (0.149 mm) and dried in vacuum oven at 110 °C for 24 h prior to use as the raw sample. The results of the proximate and ultimate analyses of the raw samples are listed in Table 1.
The powder petcoke and coal or ash sample were thoroughly mixed with a certain ratio in a mortar for 5 min.
The influence of blending ratio
In order to investigate the effect of blending ratio on the gasification process, XLT coal was chosen to mix with YS or JS petcoke. The blending ratio of XLT coal to petcoke is 100:0, 75:25, 50:50, 25:75, and 0:100, respectively. The blends were compressed into pellets (Section 2.1). The gasification reactivity of these pellets is presented in Fig. 3. In the figure, the solid line was drawn from the experimental data, while the dot line was drawn from calculated data (the weighted average
Conclusions
In this work, experimental study on co-gasification reactivity of petcoke with coal under CO2 atmosphere were investigated in a drop-in-fixed-bed reactor in the temperature range of 1200–1400 °C. The effect of coal ashes on gasification of petcoke at high temperature were also studied. The results reveal that the gasification reactivity of petcoke can be improved greatly by co-gasifying with some coals or by loading their ashes, even at temperatures higher than the ash fusion temperature. It is
Acknowledgements
The authors gratefully acknowledge the financial support from the Hebei Natural Science Foundation in China (B2015109004), the National Basic Research Program of China (2010CB227003) and the Major Scientific Research Program of Handan University (14103). Prof. Bijiang Zhang, Prof. Jianli Yang, Dr. Muxin Liu and Dr. Shijie Qu of Institute of Coal Chemistry, Chinese Academy of Sciences, are acknowledged for their valuable suggestions and help with experiments.
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