Understanding fly-ash formation during fluidized-bed gasification of high-silicon-aluminum coal based on its characteristics
Introduction
Coal gasification has attracted increasing interests because it offers one of the cleanest and most versatile methods to convert coal into electricity, synthetic natural gas, hydrogen, liquid fuel and other chemicals [[1], [2], [3]]. In China, high ash-fusion-temperature (AFT, flow temperature, FT > 1400 °C) coal accounts for more than 57% of coal reserves [4]. When these are used in an entrained-flow bed gasifier (EFB), because of the thermal characteristics of the refractory material (the EFB is equipped with refractory materials) or the viscosity–temperature characteristics of the ash/slag (EFB with a water-membrane wall), blocking slag may result in the EFB [5], which may lead to a shutdown of the gasification system. Recently, increasing attentions have been paid to the development of fluidized-bed gasification globally because of its higher feed flexibility to gasify biomass [6,7], deoiled asphalt [8], and solid waste, especially for high-AFT coals [9] and their mixtures [10,11]. In an ash-agglomerate fluidized-bed (AFB) gasification process, coal tends to convert into three parts: most organic matter is transferred into syngas; most inorganic matter changes to bottom ashes (agglomerate, and gangue ashes); small particles, including un-gasified organics and minerals move into the cyclone separator that is entrained by syngas, exits and is referred to as fly ash. The AFB offers a lower carbon-conversion ratio compared with that of EFB because of its low operating temperature. To solve this problem, a multistage-conversion integrated fluidized bed (MFB, see Fig. 1 [12]) has been designed by the Institute of Coal Chemistry (ICC), Chinese Academy of Sciences (CAS). In this system, oxygen is introduced into the upper part of the MFB, which results in an increase in coal-conversion ratio and energy efficiency [12].
The contents of carbon in the fly ashes from fluidized-bed gasifier are generally high [13], some of which are even higher than 50% [14,15]. The utilization of the fly ashes from fluidized-bed gasification is gaining attentions widely in many sections throughout the world. In general, the utilization of fly ashes can be divided into two categories. One is a large-scale utilization, such as, combustion in the pulverized coal boiler directly or in the fluidized-bed boiler after granulation [15], gasification in the fluidized-bed gasifier again through the cyclone separation and return system or in the EFB [13], the manufacture of building materials (e.g., sintering bricks [16,17], fire-resistant materials [18], cement, and glass or ceramics [19]), and as a soil amelioration agent in agriculture [20], etc. The other is refined or value-added utilization, such as, in the formation of mesoporous materials, and application in environmental protection (e.g., the desuphurizer, mercy remover, waste water treatment [21], and sequestration of CO2 [22]), the generations of carbon material (e. g., active carbons and graphite [21]), in the production of zeolites, in the synthesis of geopolymers, for use as catalysts and catalyst supports, and for the metal extraction [19]. The characteristics of fly ashes are fundamental to their high efficient applications. High-AFT coals tend to have a high silicon and aluminum content. Thus, it is necessary to explore the characteristics of fly ashes from the fluidized-bed gasification of high-silicon-aluminum coal.
Fly ashes from fluidized-bed gasifier exhibit higher graphitization degree, higher BET surface area, and richer meso-and macropores than its corresponding chars [13,21,23]. Recently, the composition, ash fusibility, and slgging tendency of fly ashes from AFB have been investigated [14,24]. Buhre et al. [25] reported that coal selection based on char characterization and its AFT could minimize fly-ash formation during combustion. Chuntanapum et al. [26] explored biomass char formation during supercritical water gasification using a compound model, and concluded that the co-presence of other glucose decomposition products resulted in differences in the fly-ash formation mechanism. Zhao et al. [27] examined the surface characteristics and reactivity of residual carbon in the slag from an EFB, and found that inorganic matter in the slag was prone to sphere formation, whereas the residual carbon retained a flocculated morphology.
Fly-ash formation is fundamental to its further usage and to deduce the conversion behaviors of inorganic and organic compositions in coal, to prevent the appearance of ash-related problems (fouling [28], slag [29,30], and corrosion [31]), and for gas purification [32]. However, the investigations on fly-ash formation during AFB gasification are still lacking. Thus, coal-gasification tests were conducted on a pilot-scale pressurized AFB in the Coal Gasification Engineering Center (ICC, CAS) to investigate the formation mechanism of fly ashes during AFB gasification of high-silicon–aluminum coal, and the properties of fly-ash were analyzed in the Coal Chemistry Laboratory (Heze University). This study may provide references for the further usage of fly ashes from AFB gasification and an insight into understanding variation behaviors of coal particle during gasification.
Section snippets
Raw coals and their characteristics
Two air-dried coal samples of Jincheng anthracite and Lu'an bitumite were provided by the ICC, CAS. The raw coal samples were crushed in a mortar to below 0.198 mm, and labeled as RJ and RL, respectively. Table 1 provides the sample proximate analysis (GB/T212–2008), the higher heating value on an air-dried basis, and the ultimate analysis (GB/T31391–2015). The ash compositions and their AFTs (deformation temperature [DT], softening temperature [ST], hemispherical temperature [HT], and FT) in a
Characteristics of fly ashes
The characteristics of the two fly ashes are shown in Table 2. The fixed carbon content (FC) in both samples exceeds 50% (53.18% for FJ, 61.36% for FL), which indicates that it is necessary to investigate the further utilization of fly ashes. The AFTs of the two fly ashes are lower than those of corresponding raw coals (Table 1), which is related to the sinking of some high-density and thermally stable mineral particles with a high melting point (MP, e.g., quartz) in an AFB gasifier [34].
The
Conclusions
The present experimental studies investigated the characteristics of fly ash from AFB gasification of high-silicon–silica coal and its formation were investigated. The conclusions can be summarized as follows:
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The AFTs of the two fly ashes were lower than those of the corresponding raw coal because of differences in mineral type and amount.
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The particle sizes of both fly ashes exhibited a two-peak distribution, and the mean particle size of FJ was smaller than that of FL.
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The carbon content and
Acknowledgments
This work was financially supported by the Natural Science Foundation of Shandong Province, China (ZR2014BM014), the National Natural Science Foundation of China (21506242), the and Youth Natural Science Foundation of Shanxi Province, China (Y5SJ1A1121). We are thankful to all workers in the coal gasification pilot scale center, ICC, CAS.
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