Structure and combustion characteristics of semi-cokes from a pilot-scale entrained flow gasifier using oxygen-enriched air

Highlights

• Semi-coke was prepared from an entrained flow gasifier with oxygen-enriched air.

• The pore structure, morphology, and crystalline of semi-cokes was determined.

• Combustion performances of semi-coke got worse with rising temperature and O₂.

• The volatile content and surface area affect the combustion kinetic of semi-coke.

Abstract

To explore the structural characteristics and further utilize the semi-cokes obtained from a self-designed pilot-scale entrained flow gasifier at the temperatures of 800–1200 °C with oxygen-enriched air as gasifying agents (21, 50, and 100% O₂ in N₂), the gasified semi-cokes were characterized by N₂ adsorption, scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods, and their combustion performances and kinetic analysis were explored using a thermogravimetric analyzer. Results indicated that the volatile release was significantly enhanced with rising gasification temperature, which would contribute to the rapid development of pore structure in semi-cokes. The increasing oxygen concentration mainly worked on promoting both the volatile release and the carbon conversion through enhancing the oxidation reactions with carbon matrix, which would result in the dramatic decrease of both surface area and micropore volume. Thermal analysis showed that the ignition and comprehensive combustion performances of semi-cokes got worse as the gasification temperature and oxygen concentration increased, indicating that the volatile matter is vitally important to promote the ignition as well as maintain the combustion stability. While their combustion intensity and burnout index were more dependent on the pore structure. The activation energy of semi-cokes calculated by both FWO and KAS methods increased with the temperature and oxygen concentration, and different order reaction models (ranged from 0.8 to 2.1) were responsible for the combustion of semi-cokes. It is notable that the volatile content and pore structure are the key factors affecting the variation of activation energy with conversion degree and the magnitude of the reaction kinetic model.

Introduction

The clean and efficient utilization of coal still has a sense of urgency and necessity for China considering its energy reserves characteristics of rich coal, meager oil, and little gas [1]. The advanced coal gasification technology has attracted extensive attention and researches owing to the fact that it provides the opportunity to produce high-quality syngas (CO + H₂), which can be used as Fischer-Tropsch fuels or converted to high-value chemical and petrochemical products [2]. Furthermore, the current state of the art for entrained flow gasification with high throughput and fuel flexibility can achieve high carbon conversion as well as low pollution emission, representing the development direction of coal gasification technology [3].

In order to achieve the sophisticated design and optimal efficiency of this technology, quantities experimental and modeling studies have been conducted to evaluate the effects of key operating parameters, such as temperature, agent, particle size, and residence time. Sankar et al. [[4], [5], [6]] from Monash University, Australia, did a lot of researches on a high temperature, electrically heated entrained flow gasifier and found that both the CO content and carbon conversion would be improved with the increase of temperature (1000–1300 °C), input CO₂ concentration (10–40% in N₂), and residence time (5–7 s). The studies regarding gasifying agents on gasification performance have been carried out by Meng et al. [7], and it was found that the gasifying agents largely influenced the producer gas properties, and the use of oxygen-enriched air would favor the syngas quality and raise the lower heating value compared with air owing to reducing N₂ dilution. Moreover, it was reported by Wang et al. [8] that the oxygen levels in the gasifying agent had a noticeable impact on the content of H₂ + CO in syngas, and the highest carbon conversion efficiency of 80% and the syngas with over 70 v% of H₂ + CO were obtained when using 99.5 v% oxygen as the gasifying agent. In parallel, gasification models using the thermodynamic equilibrium model [9,10] and CFD calculation [11] have been developed and validated with experimental results. In conclusion, the oxygen-enriched gasification technology provides a simple, easy to implement, and feasible approach for improving syngas quality, which is also favorable for the renovation of small industrial boilers.

Semi-coke is the solid residues from coal gasification with significantly high carbon and ash content, and its environmental and efficient utilization is the key to promoting the industrialization and commercialization of pulverized coal gasification technology. The main utilization way of semi-coke is burning, which depends intensively on its physic-chemical characteristics. Deep and thorough studies have been conducted on the structure and reactivity of pyrolysis char derived from the inert atmosphere [[12], [13], [14]], while the relevant researches about the semi-cokes obtained under gasification conditions, especially the oxygen-enriched air gasification process were rather scarce. Billaud et al. [15] conducted the gasification experiments in a drop tube furnace and simulation to evaluate the influence of temperature and gasifying agents on gasification products. Results showed that char gasification was largely enhanced when temperature attained 1200 °C and above. Furthermore, the O₂ addition would reduce the char and soot yields when the equivalence ratio rose from 0 to 0.61, which is attributed to the more combustion reactions. Zhang et al. [16] indicated that the combustion reactivity of semi-coke lowered with the increase of pyrolysis temperature and residence time due to the incremental release amount of volatile matter. Moreover, the addition of 5% O₂ in the pyrolysis atmosphere would significantly reduce the burnout ratio while beneficial to the combustion behavior of semi-coke due to the promotion of pore development. What calls for special attention is that the heating rate and thermal history also have a great influence on the structure and reactivity of semi-cokes. Gu et al. [17] compared the physical and chemical properties of two fly ash samples collected from Texaco gasifiers and a coal char prepared at rapid pyrolysis in a drop tube furnace, and found that the fly ashes (carbon content of 31–37%) exhibited higher specific surface area, more disordered carbon crystalline structure, and better CO₂ gasification reactivity than coal char (carbon content of 58–59%). Tremel et al. [18] also found that the volatile yield and char reactivity acquired from a pressurized high temperature entrained flow reactor were quite different from those properties of reference char produced with a low heating rate at 900 °C (standard DIN 51720). It should be noted that the commercial entrained flow gasifiers operate a single-step process where coals are gasified directly, and part of the coals was burned to supply enough energy for the autothermal continuous movement of endothermic gasification reactions. Thus, it is highly imperative to address the shortage of fundamental data of semi-cokes obtained from the pilot-scale entrained-flow gasifier under oxygen-enriched air conditions to facilitate the development of commercial application of gasified semi-cokes or optimize the design of a future integrated gasification-combustion system.

In this study, semi-cokes were prepared by a self-designed pilot-scale high-temperature entrained flow gasifier under different temperatures and oxygen concentrations. The main objectives of this research are to explore how the physical properties of the obtained semi-cokes changed with gasification conditions and correlate these intrinsic properties of semi-cokes to their subsequent combustion performances as well as reaction kinetics. The physical characterization of the gasified semi-cokes was carried out employing N₂ adsorption, scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods. Meanwhile, their combustion reactivities and kinetic parameters were estimated by thermogravimetric analysis. It was hoping that this research would provide basic data and instructive suggestions for further utilization of semi-cokes and improving the comprehensive utilization efficiency of coal gasification technology.