**1. Introduction**

At present, the supply of energy is highly reliant on fossil fuel [1]. The rising demand for energy around the globe is leading to many economic and environmental problems [2]. Ironmaking in China is still dependent on the blast furnace [3]. The direct reducibility and intensity of high-basicity sinter, which is the main raw material for blast furnace ironmaking, markedly influence the production efficiency of ironmaking [4,5]. The combustion characteristic of fuel in sintering mixture materials plays a decisive role in sinter quality [6,7]. Therefore, it is urgent to explore the combustion characteristics of solid fuel in the sintering process in order to provide a theoretical basis for improving the combustion efficiency, improving the quality of sinter and reducing the consumption of solid fuel [8]. Coke is made from natural bituminous coal heated between 950 ◦C and 1050 ◦C in an airless environment. It is the main fuel in the sintering site [9]. Previously, the research on sintering fuel combustion was mostly based on coke breeze [10,11]. However, with the development of the steel industry and the continuous improvement of the grog ratio in the blast furnace, the supply of coke powder for sintering has been short. Therefore, many sintering sites use anthracite wholly or partially to replace coke powder as fuel for sintering [12,13].

In the traditional sense, the experimental samples for studying the combustion characteristics of fuels consist mainly of monomer fuel. However, during the sintering process, the solid fuel in the material layer is usually distributed in a dispersed manner. Hence, the combustion law of sintering fuel should generally be different from the monomer fuel and the fuel layer [14,15]. In recent years, researchers have begun to use a quasi-particle structure to describe the existence of fuel inside the

sintering mixture [16,17]. In order to exclude the interference brought by other reactions, Al2O3 pure powder reagent is often used in many studies on quasi-particles sinter to replace other materials used in production for experimental exploration [18].

Combustion of sintering fuel at low temperature is generally considered as a chemical reaction control process. However, the introduction of a quasi-particle structure will greatly improve the diffusion resistance of internal fuel combustion. Therefore, the extent to which diffusion controls combustion is greatly increased [19,20]. This paper innovatively introduced the double parallel reaction volume model (DVM) and double parallel random pore model (DRPM) to conduct a comparative analysis of the combustion characteristics of monomer blended fuel and quasi-particle fuel and calculated the related kinetic parameters, which are widely used in the kinetic calculation of co-combustion of multiple fuels [21], single-fuel gasification [22] and co-gasification of multiple fuels [23,24]. They can not only obtain the suitable dynamic models for describing the combustion process of two kinds of fuel, but also characterize the effect of quasi-particle structure on the combustion characteristics of sintering fuel. There are many physical and chemical reactions in the sintering process. Due to the influence of sintering temperature and atmosphere, iron oxides undergo different degrees of reduction and oxidation reactions. The occurrence of these reactions is coupled with fuel combustion, which greatly affects the quality of sinter [25]. Therefore, this study systematically explores the influence of factors such as the substitution of anthracite for coke powder, quasi-particle structure and the coupling effect of reduction and oxidation of iron oxide on the combustion characteristics of sintering fuel in order to provide a certain guiding significance for improving fuel efficiency and reducing sintering production cost.

#### **2. Materials and Methods**

#### *2.1. Materials*

The fuel used in the experiment was 25% anthracite blended with 75% coke powder by weight and the particle size was less than 0.105 mm. The specific industrial analysis, elemental analysis and calorific value of the raw materials are shown in Table 1. Because there are many types of reactions of the sintering process of iron ore and the coupling between the reactions is strong, in order to investigate the influence of the quasi-particle structure on the combustion kinetic parameters of the sintering fuel, Fe2O3 and Al2O3 pure powder reagents with a particle size of less than 0.147 mm were used instead of the iron ore and flux applied in an industrial setting. In addition to single anthracite and coke powder, there are also three-blended fuels, namely monomer blended fuel (MBF), quasi-particle fuel (QPF) and sintered mixture (SDM). The schematic diagrams of them are shown in Figure 1, and the ratios of raw materials are shown in Table 2.


**Table 1.** Proximate and ultimate analysis of the fuel (in dry basis).

FC, V and A represent fixed carbon, volatile and ash, respectively; subscript gr means gross calorific value.

**Figure 1.** Schematic diagram of monomer blended fuel (MBF), quasi-granular fuel (QPF) and sintered mixture (SDM).

**Table 2.** Raw materials and proportions used in the experiment, wt%.

