粉煤在不同温度下热解行为
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Keywords: pf combustion; Pyrolysis chars; Pyrolysis temperature; Char volatiles; Reactivity; Char morphology; Optical texture
1. Introduction The combustion of pulverised coal particles under the environmental conditions prevailing in an industrial boiler is known to occur as a two-stage process [1]. The first one is the thermal decomposition (pyrolysis) of coal, which consists of the sudden release of volatiles accompanied by drastic changes in the morphology and molecular structure of the remaining solid, commonly referred to as char. While the second one is the chemical reaction between the oxygen and both the volatiles (homogeneous combustion) and the solid char (heterogeneous combustion). The generally accepted and well-supported fact that pyrolysis and heterogeneous combustion are consecutive rather than simultaneous and that little, if any, interaction occurs between them, led to the assumption that char particles were true intermediate products in the combustion process. Different experimental devices have been designed with the aim to submit coal particles to the same time–
Abstract This study reports the chemical and petrographic characteristics of chars obtained at 1000 and 1300ЊC (within the range of temperatures reached by coal particles in the near burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are low and high volatile bituminous vitrinite-rich coals, and the third one has similar rank to the high volatile bituminous coal but a very different maceral composition (97% inertinite). The residual volatiles remaining after pyrolysis at the two temperature tested were determined by heating the chars in a thermobalance at 1100ЊC under nitrogen. Also, the morphology and optical texture of the chars were studied by optical and scanning electron microscopy. The increase in preparation temperature enhanced the plastic behaviour of the low volatile bituminous coal, resulting in an extensive development of secondary porosity. The drop in reactivity from low to high temperature char might be due to the enhanced fluidity of the metaplast, which increases the likelihood for polymerisation reactions to take place, with a parallel reduction in the number of active sites. Incomplete devolatilisation and poor plastic properties were achieved during low temperature pyrolysis of the inertinite-rich coal, whereas at 1300ЊC most of the material passed through a fairly plastic stage and was very efficiently pyrolysed. No significant differences were observed between the low and the high temperature chars from the high bituminous vitrinite-rich coal. This behaviour could be due to either extensive crosslinking reactions involving hydroxylic oxygen in an early stage of the pyrolysis process, or, which seems more likely, to the sheltering effect of soot particles formed in the vicinity of pyrolysing coal particles, which could minimise the effect of the increased flame temperature. ᭧ 1999 Elsevier Science Ltd. All rights reserved.
´ n, CSIC, La Corredoria s/n Ap. 73, 33080 Oviedo, Spain Instituto Nacional del Carbo Received 5 February 1999; received in revised form 20 April 1999; accepted 17 May 1999
Fuel 78 (1999) 1501–1513 /locate/fuel
Pyrolysis behaviour of pulverised coals at different temperatures
´ ndez* M.J.G. Alonso, A.G. Borreg
1. Introduction The combustion of pulverised coal particles under the environmental conditions prevailing in an industrial boiler is known to occur as a two-stage process [1]. The first one is the thermal decomposition (pyrolysis) of coal, which consists of the sudden release of volatiles accompanied by drastic changes in the morphology and molecular structure of the remaining solid, commonly referred to as char. While the second one is the chemical reaction between the oxygen and both the volatiles (homogeneous combustion) and the solid char (heterogeneous combustion). The generally accepted and well-supported fact that pyrolysis and heterogeneous combustion are consecutive rather than simultaneous and that little, if any, interaction occurs between them, led to the assumption that char particles were true intermediate products in the combustion process. Different experimental devices have been designed with the aim to submit coal particles to the same time–
Abstract This study reports the chemical and petrographic characteristics of chars obtained at 1000 and 1300ЊC (within the range of temperatures reached by coal particles in the near burner zone of pulverised fuel boilers) from three different coals. The coals were selected according to petrographic criteria: two of them are low and high volatile bituminous vitrinite-rich coals, and the third one has similar rank to the high volatile bituminous coal but a very different maceral composition (97% inertinite). The residual volatiles remaining after pyrolysis at the two temperature tested were determined by heating the chars in a thermobalance at 1100ЊC under nitrogen. Also, the morphology and optical texture of the chars were studied by optical and scanning electron microscopy. The increase in preparation temperature enhanced the plastic behaviour of the low volatile bituminous coal, resulting in an extensive development of secondary porosity. The drop in reactivity from low to high temperature char might be due to the enhanced fluidity of the metaplast, which increases the likelihood for polymerisation reactions to take place, with a parallel reduction in the number of active sites. Incomplete devolatilisation and poor plastic properties were achieved during low temperature pyrolysis of the inertinite-rich coal, whereas at 1300ЊC most of the material passed through a fairly plastic stage and was very efficiently pyrolysed. No significant differences were observed between the low and the high temperature chars from the high bituminous vitrinite-rich coal. This behaviour could be due to either extensive crosslinking reactions involving hydroxylic oxygen in an early stage of the pyrolysis process, or, which seems more likely, to the sheltering effect of soot particles formed in the vicinity of pyrolysing coal particles, which could minimise the effect of the increased flame temperature. ᭧ 1999 Elsevier Science Ltd. All rights reserved.
´ n, CSIC, La Corredoria s/n Ap. 73, 33080 Oviedo, Spain Instituto Nacional del Carbo Received 5 February 1999; received in revised form 20 April 1999; accepted 17 May 1999
Fuel 78 (1999) 1501–1513 /locate/fuel
Pyrolysis behaviour of pulverised coals at different temperatures
´ ndez* M.J.G. Alonso, A.G. Borreg
