Study on coal pyrolysis characteristics by combining different pyrolysis reactors

Abstract

The pyrolysis process of Shendong coal (SD) was first studied by combining the characteristics of thermal gravimetric (TG), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and Gray-King assay (G-K). The results show that the order of coke yields is G-K (76.35% (mass))>TG (73.11% (mass))>Py (70.03% (mass)). G-K coke yield caused by condensation reaction and secondary reaction accounts for 3.08% (mass) and 3.24% (mass), respectively. Compared with slow pyrolysis, fast pyrolysis has stronger fracture ability to coal molecules and can obtain more O-compounds, mono-ring aromatics and aliphatics. Especially, the content of phenolics increases significantly from 15.49% to 35.17%, but the content of multi-ring aromatics decreases from 23.13% to 2.36%. By comparing the compositions of Py primary tar and G-K final tar, it is found that secondary reactions occurred during G-K pyrolysis process include the cleavage of alkane and esters, condensation of mono-ring aromatics with low carbon alkene, ring opening, isomerization of tri-ring aromatics, hydrogenation of aromatics and acids.

1. Introduction

Coal is a preferred energy source in China and will remain a key role in China's energy sources [1]. Coal pyrolysis is the initial phase of coal conversion processes and has an important impact on the subsequent phases [2,3]. Usually, coal pyrolysis is classified according to the final temperature and the heating rate. Low temperature pyrolysis has many advantages, such as mild reaction conditions, simple process, low production cost, less investment and atmospheric pressure production, which can meet with the demand for energy saving and is deemed as an economical coal conversion process [4,5]. Slow pyrolysis is a relatively successful technology compared with fast pyrolysis [6]. But there are still many problems in slow pyrolysis, such as poor quality coal tar, only using lump coal, oil-dust separation difficulty, high cost of tar processing, pipeline blockage, and difficulty in wastewater treatment [7]. None of fast pyrolysis technologies were commercialized up to now. Therefore, understanding coal pyrolysis process and the difference between fast pyrolysis and slow pyrolysis are of great significance for coal conversion and utilization.

A lot of research has been done on coal pyrolysis and it has been found that many factors including coal rank, particle size, temperature, heating rate, atmosphere and reactor have significant effects on coal pyrolysis characteristics [8,9]. In slow pyrolysis process, coal macromolecular structure will gradually condense and become more stable. So for the same coal, as the heating rate increases, the coal conversion rate increases [10]. The presence of a purging gas can greatly avoid secondary reactions and is also dependent on the apparatus of choice [11]. Tian et al. [12] found that coke yields gradually increase with the decrease of heating rate by thermal gravimetric (TG) and Qiao et al. [13] reached the same conclusion. Liu et al. [14] pointed out that temperature increase is the smallest for volatiles generated in small fixed-bed reactors with a purging gas and extending residence time of volatiles at high temperature promotes conversion of tar to coke. Hayashi et al. [15] used Curie-point reactor and fixed-bed reactor to study fast and slow pyrolysis, respectively. He found that secondary reactions of volatiles are sufficiently suppressed in Curie-point reactor and the yield of pyrolysis gas remained unchanged or changed slightly with the increase of heating rate. Xu et al. [16] conducted pyrolysis tests in an improved fixed bed reactor and used Gray-King assay (G-K) as a comparison, found that the tar yield in the fixed bed reactor is higher than that of G-K. Liu et al. [17] pointed out