Sulfur distribution in coke and sulfur removal during pyrolysis
Introduction
Sulfur species in coal exist in the forms of inorganic compounds as sulfides and sulfates, and of organic compounds with aliphatic, aromatic or heterocyclic sulfur structures. The inorganic sulfurs can be removed or reduced by industrial methods such as coal washing, flotation and oil agglomeration. The organic sulfur compounds of aliphatic types are thermally less stable. They tend to form H2S when heated or pyrolyzed and sometimes are converted partly to the more stable compounds of heterocyclic structures (usually being thiophenic compounds) [1], [2]. Despite of a great deal of efforts were devoted to the removal of major organic sulfur compounds from coals, no economically feasible process is available as yet.
The species of organic and inorganic sulfurs from coals change significantly during pyrolysis. Pyrites in the coals tend to be transformed into sulfides and elemental sulfur at the temperatures above 350 °C [3]. During the coal pyrolysis, the nascent elemental sulfur tends to react with hydrogen to form H2S, or be captured by organic matrix to form organic sulfurs residing in semi-coke or tar [4], [5], [6], [7], or be fixed by minerals in the coal to form sulfide remaining in semi-coke [8], [9], [10], [11], [12]. Generally speaking, about 70% of total sulfur in coal is distributed in coke [13], [14] after coal pyrolysis, and part of sulfurs from the coal are volatilized in the coke oven gas (COG). The sulfur content in coke affects remarkably the energy consumption of iron and steel making processes and the quality of molten iron. The previous studies concerning coal desulfurization indicate that the energy consumption will increase 1.5% and the productivity will decrease about 5% if the sulfur content in coke increases 0.1% [15]. A process based on mixing some materials, such as CaO and CaCl2, with coking coal to form stable CaS has been proven to be able to reduce the amount of sulfur that enters the iron [16]. However, this process has not been applied in industry yet due to the fact that the strength of coke may decrease and the amount of coke ash may increase during the process. Other studies indicated that some of the gaseous sulfur in pyrolysis gas would go back to the semi-coke when gases were released from semi-coke column [17]. This is one of the reasons causing high sulfur content in coke. Furthermore, it is always assumed that the sulfur in coke might be distributed uniformly in spatial in view of the presumed releasing way of pyrolysis gas. Based on this assumption, one would further infer that some of sulfur in pyrolysis gas would remain in coke during the release of sulfur-containing gas from semi-coke column. Besides, if the residence time of sulfur-containing pyrolysis gas was decreased by mixing a large quantity of desulfurization gas with pyrolysis gas, the opportunity of the releasing sulfur during the pyrolysis reacting with organic matrix or mineral matters in coal might be reduced.
Our previous studies [18] disclosed that if the semi-coke bed was fed with desulfurization gas such as H2, the sulfurs in iron sulphide and thiophene in the semi-coke could be reduced to H2S and thus removed as part of COG in the later coking stage. This process may be used to reduce sulfur content in coke, and the sulfur thus removed might be further recovered by desulfurization treatment of COG. Thus the primary aim of this work is to study the distribution behavior of sulfur in coke and investigate the feasibility of coke desulfurization during pyrolysis process. Some mechanisms regarding sulfur transformation and distribution in pyrolysis stage may be essential to the effective and economic removal of sulfur, and consequently to the development of desulfurization technology.
Section snippets
Apparatus and coal sample
A vertical tube furnace with 230 mm in diameter and 1700 mm in height was used in the experiments to investigate the distribution of sulfur in coke. There was a temperature-uniform zone in the furnace with approximately 1500 mm in height and the temperature deviation with time was within the range of ± 10 °C. The furnace was operated under a pressure of 1 atm and the heating rate was controlled by a programming temperature controller. The experimental apparatus, heating rate curve and sampling
Distribution of sulfur in different forms
Fig. 3 presents the sulfur contents at the radial positions of (I)–(IV) from axes and at vertical positions of (1)–(5) from the top of the coke column. At the radial positions, both organic and inorganic sulfur contents increase gradually from the center to brim. The average increment in the contents of organic and inorganic sulfur are approximately 0.04% and 0.08%, respectively. Obviously, the increase of organic sulfur is larger than that of inorganic sulfur, and both of the organic and
Conclusions
Sulfur was mainly released from coke during the pyrolysis at the range of 300–600 °C. Under this temperature range, the sulfur content in coke can be reduced by 0.05–0.06% by blowing N2, CO or CH4, and by 0.14% by blowing H2 at a space velocity of 1.2 mm/s. The total, organic and inorganic sulfur contents in coke increase with increasing the diameter of coking chamber under identical pyrolysis conditions. Both organic and inorganic sulfur contents in coke increase gradually from the center to
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 50474044) and the National Science Fund for Distinguished Young Scholars (No. 50225415) in China.
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