Coal gasification which converts elements C and H from coal into effective syngas (CO + H2) is the key technology for clean and efficient utilization of coal and production of coal-based chemicals (Chang et al., 2016). Industrial coal gasifiers are usually categorized into the fixed bed gasifier, the fluidized bed gasifier, and the entrained-flow gasifier (Wu et al., 2010; Dogru and Erdem, 2019; Pan et al., 2016; Ayola, Yurdakos and Gurgen, 2019; Li et al., 2018; Tamošiūnas et al., 2019; Zeng et al., 2014). Fluidized bed gasification develops rapidly in China's coal chemical industry and occupies a large market share due to its advantages of high heat and mass transfer efficiency, mature process, high annual operation rate, and low production cost (Matsuoka et al., 2013). Unfortunately, the operating temperature of the fluidized bed gasifier is low at about 900–1000 ℃, so the char particles formed by coal devolatilization only undergo partial combustion and gasification reactions, and eventually form fly ash with higher carbon content. A large amount of fly ash was contained in the generated raw syngas, which reduces the quality of syngas and even affects the stable operation of the gasifier system (Chen, Namioka and Yoshikawa, 2011; Xu et al., 2010).
To mitigate the above problems, post-treatment systems for the removal of fly ash are typically installed at the gasifier outlet. These systems successively include cyclones, bag dust collectors, and various wet removal processes. In the case of a cyclone separator, only the larger particles (> 5 µm) are separated and returned to the gasifier chamber through the return conduit, while the remaining fly ash is carried into a bag dust collector by the flow of the raw syngas. Much of the fly ash is then captured by the filter cake on top of the bag filter material, such that the relatively small fly ash particles (0.1-5 µm) are separated. The remaining material with particle sizes less than 0.1 µm has to be removed using a wet process, such as a spray tower, wash tower, or Venturi tube scrubber (Yoshida, Ono and Fukui, 2005; Wang et al., 2013), employing a liquid as the dust collector to capture particles from the gas stream. This type of post-treatment system suffers from several challenges. First, the high rate of water consumption required makes such processes unsuitable for use in regions where water is scarce. Second, the sewage generated by the wet removal has to be treated, which significantly increases costs. Finally, these physical removal processes cannot reduce the carbon content in the fly ash.
The fly ash contained in the raw syngas from a fluidized bed gasifier tends to have a high carbon content (20–40%) (Kelebopile, Sun and Liao, 2011; Ouyang et al., 2018). The fly ash particles are internally porous, have loose structures, and are readily crushed. In addition, the pores in these particles can absorb significant amounts of water and high-temperature sintering loss. These characteristics together tend to restrict the potential utilization of fly ash as a raw material. As such, the dry ash separated from the bag dust collector cannot be used to make cement or concrete admixtures, or to produce foamed glass or ceramics. In addition, the wet ash filtered from the sewage is mud-like and can contain numerous pollutants, such as heavy metals and small amounts of tar and phenol. This material is also inconvenient to transport and cannot be used directly. Therefore, fly ash recovered from post-treatment systems is often simply disposed of via landfill (Hurt et al., 1998), even though this is not a sustainable option as it uses valuable land and can lead to environmental degradation (Ahmaruzzaman, 2010). Discarding fly ash also represents a monetary loss because this material contains a significant quantity of unburned carbon and thus could serve as fuel in a fluidized bed combustor. Ouyang et al. (Ouyang et al., 2018) investigated the combustion characteristics and NOx emissions associated with the coal gasification of fly ash in a 0.4 MW preheated combustion test rig, and found a maximum fly ash combustion efficiency of 98.6% with NOx emissions of 155 mg/Nm3. Some commercial processes (Blissett and Rowson, 2012) also use fly ash as a secondary fuel, which removes most of the unburned carbon in fluidized bed combustors. However, there have been few reports focused on the gasification of fly ash using an entrained-flow gasifier.
A novel technology for the purification of raw syngas has been developed in which fly ash is treated in an entrained-flow gasifier after coming directly came from a fluidized bed. This process is referred to as fly ash entrained-flow gasification, and the first 80,000 Nm3/h fly ash entrained-flow gasifier was constructed in Liaoning, China in 2015 (Fang et al., 2020). As this unit can be considered to be dilute-phase entrained-flow reactors, it does not directly gasify pulverized coal, but have the same advantages as coal entrained-flow gasifiers. In addition, because the fine ash is rapidly heated in this system to generate molten particles, the unburned carbon is almost completely exhausted, while the remaining ash forms a liquid slag that is discharged from the gasifier. In this manner, a fly ash entrained-flow gasifier effectively converts the small particles into raw syngas. Consequently, this kind of gasifier can replace the wet removal equipment in a typical fly ash post-treatment system while removing the generation of sewage and lowering equipment costs. This process can also reduce the carbon content in the fly ash to less than 3% while recycling the carbon and energy from the fly ash.
In view of the extensive application of numerical simulation to visually provide detailed information on the complex process of reactions in gasifiers (Wu et al., 2010; Hurt et al., 1998; Myöhänen, Palonen and Hyppänen, 2018; Li et al., 2020; Fang et al., 2019; Chen, Hung and Chen, 2012), a comprehensive three-dimensional numerical simulation was developed to get a full understanding of the particle flow and gasification characteristics in industrial-scale fly ash entrained-flow gasifier. The operating conditions can affect the performance of a gasifier. The structural parameters also have a significant impact on the operation performance, although these effects have only rarely been researched. Adjusting the burner inclination angle helps to optimize the design of the gasifier. Therefore, the present work assessed the effects of different burner inclination angles (0°, 8.5°, 17°, and 25.5°) on the flow and reaction behaviors in a gasifier.