Article pubs.acs.org/EF
Evaluation of CO2 Gasification Reactivity of Different Coal Rank Chars by Physicochemical Properties Xuliang Jing,†,‡ Zhiqing Wang,† Qian Zhang,†,‡ Zhongliang Yu,†,‡ Chunyu Li,† Jiejie Huang,† and Yitian Fang*,† †
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: The isothermal and non-isothermal CO2 gasification of eight different coal rank chars was conducted by a thermogravimetric analyzer (TGA), and the reactivity index and peak temperature were obtained to express the gasification reactivity. In addition, the physicochemical properties of these chars, such as the alkali index, Brunauer−Emmett−Teller surface area (SBET), micropore area (Smic), and carbon crystalline structure, were examined by an inductively coupled plasma (ICP) spectrometer, N2 adsorption, and X-ray diffraction (XRD). The CO2 chemisorption was also measured by a TGA, and the total, strong, and weak chemisorbed volumes (Ctotal, Cstr, and Cwea) were obtained. All of these properties were used to evaluate the CO2 gasification reactivity of these chars. The results show that the gasification reactivity of chars decreases with an increasing coal rank. In comparison to the other properties, the Ctotal and Cstr are the best parameters for predicting the gasification reactivity, and the linear correlation coefficients between Ctotal/Cstr and gasification reactivity can reach up to 0.90 or even 0.98. It suggests that the CO2 chemisorption can be used to evaluate the gasification reactivity of different coal rank chars.
1. INTRODUCTION As a clean coal utilization technology, gasification is widely used in many industrial areas. Gasification reaction in a gasifier can be divided into two procedures: coal pyrolysis and char gasification, of which, the rate of char gasification is much lower than that of coal pyrolysis. At the same time, the gasification reactivity of chars with CO2 is lower than that of O2 or H2O, and therefore, it is regarded as the rate-determining step.1 Thus, a clear understanding of the char CO2 gasification reactivity is of great significance for the design and operation of a gasifier. For a given parent coal and a specific gasification condition, the CO2 gasification reactivity of char mainly depends upon pyrolysis conditions.2 In general, the rapid pyrolysis coal chars have a larger specific surface area and higher gasification reactivity than those of slow pyrolysis.3,4 In addtion, a decrease in the pyrolysis temperature and pressure increases the gasification reactivity of coal chars.5−7 However, for the chars of different coal ranks, the CO2 gasification reactivity is difficult to be predicted because of the significant difference in their physicochemical properties.8 It has been noted that the CO2 gasification reactivity of low-rank coal chars is controlled by the catalytic effects of alkali and alkaline earth metals (AAEMs, including K, Na, Ca, and Fe), whereas the gasification reactivity of high-rank anthracite chars is proportional to the alkali index.9,10 Other researchers have indicated that the CO2 gasification reactivity of different coal rank chars is also associated with their pore and carbon crystalline structure.11,12 Kim et al.13 found out that, for the different coal rank chars, the influence extent of individual coal property on CO2 gasification reactivity decreases in the order of alkali ratio, volatile content, mesopore volume, alkali index, and specific surface area. Nevertheless, even the highest nonlinear correlation coefficient (R2, alkali ratio) was lower than 0.79. Thus, an acceptable © XXXX American Chemical Society
gasification-determining factor, which can be used to predict the CO2 gasification reactivity of different coal rank chars, should be investigated. The oxygen-exchange mechanism for the CO2 gasification, which is proposed by Ergun,14 has been widely accepted and describes the CO2 gasification as follows: Cf + CO2 ⇌ C(O) + CO
(1)
C(O) + C → CO + Cf
(2)
where Cf represents the free carbon active site and C(O) is the carbon−oxygen surface complex. On the basis of this mechanism, CO2 first dissociates at a free carbon active site with which a carbon−oxygen surface complex and a molecule of CO are formed. Then, the carbon−oxygen complex produces a new free active site and another molecule of CO.15 This mechanism indicates that the gasification reactivity of chars can be evaluated by the number of active sites. However, there is still no universal measurement method for the number of active sites. Zhu et al.16 measured the CO2 chemisorbed volume in a self-made apparatus and regarded it as an indicator for the number of active sites. This concept has also been accepted by Molina et al.,17 and a conventional volumetric chemisorption unit was used in their experiment. However, the problem is that the CO2 chemisorbed volume of coal char is extremely small, which means more effort must be paid to obtain a more accurate result, and an apparatus with a high accuracy should be employed. In this case, the thermogravimetric analyzer (TGA) is an ideal instrument for Received: August 16, 2013 Revised: November 13, 2013
A
dx.doi.org/10.1021/ef401639v | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
proposed by Takarada et al.,24 was used to compare the isothermal gasification reactivity of different samples. It was defined as
testing the CO2 chemisorption of chars because of its high sensitivity to weight variation and ease of operation.18−20 The related results also show that the TGA performs well on CO2 chemisorption and the CO2 chemisorbed volumes correlate well with the CO2 gasification reactivity. However, only one type coal was used by past works, and few studies concentrated on the chemisorption of different coal rank chars. Gasification reactivity of different coal rank chars plays a pivotal part in our industrial utilization of a new coal in a certain gasifier. Thus, the development of a universal and various chars-suitable evaluation method for gasification reactivity is essential, and this method can be universally used for every rank coal. In this paper, the CO2 gasification reactivity of eight different coal rank chars was measured by the TGA. The alkali index, pore structure, carbon crystalline structure, and CO2 chemisorbed volumes were correlated with the gasification reactivity. The aim is to find the most appropriate property that can estimate the CO2 gasification reactivity of different coal rank chars more accurately. Meanwhile, this development can help us obtain a further understanding of the gasification mechanism.
R s = 0.5/τ0.5
2. EXPERIMENTAL SECTION 2.1. Preparation of Samples. The eight different coal rank chars were prepared in a fixed-bed reactor.21 In brief, the crucible with 10 g of coal sample (
