To ensure the sustainable and effective use of coal, an abundant fossil fuel, the environmental pollution issues associated with coal consumption are required to be adequately addressed [
1,
2,
3,
4], especially in China. Coal gasification technology which converts coal into electricity, hydrogen, chemical products, and other valuable energy is versatile and clean [
5,
6,
7]. The deployment of CO
2 capture technology is more feasible in coal gasification systems due to coal decarbonization, producing clean syngas. The clean syngas can be used for power generation and production of chemicals and hydrogen [
5,
6,
7]. Thus, coal gasification is an important sustainable clean coal technology.
Coal gasification technology is a multiphase reaction technology involving both gaseous phase and solid phase reactions [
8,
9,
10,
11]. This implies complex interaction of pyrolysis, heterogeneous reactions of carbon with O
2, CO
2, and H
2O, and homogeneous reactions [
12]. The entrained-flow approach has received significant attention in comparison to the fixed-bed and fluidized-bed for coal gasification due to its improved multiphase reaction rates, which is more promising for large-scale application. Compared with the fixed-bed and fluidized-bed technologies, the entrained-flow gasification technology accomplishes gasification in several seconds with a high conversion rate of coal. The entrained-flow gasification technology accomplishes a high gasification rate of coal through the following means [
9,
13]: (1) the raw coal is pulverized to increase contact surface area for multiphase reactions; (2) the pure oxygen is used as gasifying agent to promote the reaction rate; (3) the gasification temperature is further increased.
As an endothermic reaction, the reaction rate of coal gasification increases with temperature. However, high temperature is expected to bring about an increase in oxygen consumption, which in turn means lower yield of syngas in the gasification process. This implies higher cost, due to the need for larger down-stream dehumidifier. However, the gasification temperature of most entrained-flow gasification technologies is approximately 1500 °C, because they use slag-tapping technique which requires that gasification temperature be at least 100 °C higher than ash melting temperature. The two-stage entrained-flow gasification technology was developed to address the need for high temperature zone needed for slag-tapping technique, and to create optimum temperature zone for effective gasification reaction. The temperature for reaction area of the first stage is approximately 1500 °C, which meets the need for slag-withdrawal, and the temperature for the reaction area of the second stage declines to 1100–1200 °C, a temperature that is the most efficient for gasification reactions. Therefore, the gasification efficiency is improved, and the specific oxygen consumption is lowered. In the reaction region of the second stage, the pulverized coal is injected without oxygen injection at the same time. The Huaneng Clean Energy Research Institute (HNCERI) of China has also developed a two-stage entrained-flow gasification technology, which has a very high conversion rate of carbon [
9].
Many studies have been reported in literature on two-stage coal gasification. Albal et al. [
8] focused the influential factors on an advanced two-stage gasification system, using an experimental approach. Chen et al. [
14,
15,
16,
17] extensively studied the gasification reactions and reactant mixing process with a simulation method. Watanabe and Otaka [
18] simulated a 2 t/day gasifier using CFX software. Shi et al. [
19] performed a simulation of a water–coal–slurry gasifier. Wang et al. [
20] also simulated the coal gasification process. Liu et al. [
21] and Yang et al. [
22] simulated the gasification behaviors of coal in a similar way. Peralta and Wang et al. [
23,
24] investigated the gasification characteristics of various Chinese coals under conditions related to the entrained-flow gasification process. As for the HNCERI gasification process, Li et al. [
25] experimentally studied the contact between slag viscosity and temperature. Ren and co-workers [
13] evaluated the application of a two-stage pressurized dry pulverized-coal gasifier in the Tianjin demonstration power plant of integrated gasification combined cycle (IGCC) and compared the performance of the first stage and the second stage gasification. In addition, a novel two-stage entrained-bed gasification system has been proposed by Gao et al. [
26], and they simulated the coal pyrolysis and gasification characteristics using Aspen Plus software as well. Watanabe et al. [
27] assessed the influences of CO
2 recirculation on gasification performance of a two-stage coal gasifier in an oxy-fuel IGCC plant using a numerical method. Wang et al. [
28] numerically studied the coal gasification performance in a two-stage entrained flow gasifier using computational fluid dynamics (CFD) software. The soot formation during coal gasification in two-stage entrained-flow gasifiers was rarely specifically paid attention to [
29]. During the past few years, extensive investigations on biomass gasification via the two-stage gasification process have been conducted, owing to the increasing attention on biomass energy. For example, Jeong et al. [
30] studied the two-stage gasification of biomass via active carbon, for the sake of enhancing the hydrogen production and lowering tar production using lab-scale and pilot-scale experimental systems. They also evaluated the co-gasification performance of coal and dried sewage sludge [
10]. Jahromi et al. [
31] developed a CFD model for biomass gasification and syngas formation. Niu et al. [
32] proposed a new two-stage gasifier to gasify biomass, which consists of a swirl-melting furnace and a fluidized bed gasifier. A new concept of two-stage gasification for biomass was proposed by Pei et al. [
33] to overcome the disadvantages of biomass gasification. A 1.5 MW
th demonstration plant was also designed, and operated, to evaluate the technical and economic feasibility. The gasification behaviors of coal differ greatly from those of biomass, due to their considerably different inherent properties. However, further experiments on coal gasification characteristics in the second stage (also known as reductor) are still needed. Most previous investigations have been conducted only under lab-scale and pilot-scale conditions. There are few reported experimental investigations to clarify the key operation parameters that affect coal gasification and reaction between the gasification products. Moreover, there are insufficient experiments to test coal gasification characteristics in the reductor under high load and long period of the operation’s condition. Therefore, the gasification features of the reductor are still unclear.
In the present work, the effects of different concentrations of gasifying agents and coal input rates on coal gasification characteristics in the reductor were experimentally investigated in a full-scale plant, and the gasifier was also tested for a long period under high load condition of the second stage. The effects of steam input to the gasifier and the amount of coal fed into the reductor on syngas production (CO2, H2, CO, and CH4) were studied as well. In addition, the effect of steam addition on the reaction between the gasification products was further analyzed, based on the element balance. This is the first publication of the performance of the reductor under a range of conditions. The test results were compared with existing data to determine the two-stage reaction characteristics. The present study will not only elucidate coal gasification characteristics in the reductor, but also provide the basis for the design and operation of a two-stage entrain-flow gasifier.