Volatilisation and catalytic effects of alkali and alkaline earth metallic
species during the pyrolysis and gasi®cation of Victorian brown coal.
Part II. Effects of chemical form and valence
Dimple M. Quyn
a
,Hongwei Wu
a
,Sankar P. Bhattacharya
b
,Chun-Zhu Li
a,
*
a
CRC for Clean Power from Lignite Department of Chemical Engineering, PO Box 36, Monash University, Victoria 3800, Australia
b
CRC for Clean Power from Lignite 8/677 Springvale Road, Mulgrave, Victoria 3171, Australia
Received 6 May 2001; revised 30 July 2001; accepted 1 August 2001
Abstract
Alkali and alkaline earth metallic (AAEM) species (Na,Mg and Ca) exist in Victorian brown coal mainly as carboxylates forming a part of
the coal organic matter or as dissolved salt (NaCl) in the coal moisture. The experimental results in this paper show that the chemical and/or
physical form of sodium in the brown coal is an important factor in¯uencing the volatilisation of sodium during pyrolysis. Signi®cant
amounts of light species containing carboxyl or carboxylate groups such as formate,acetate and oxalate were found in the volatiles from the
pyrolysis of the brown coal. It is believed that the release of AAEM carboxylates is an important mechanism for the volatilisation of AAEM
species,particularly at low temperatures (,6008C). The carrier gas ¯ow rate passing through the coal bed can greatly affect the volatilisation
of AAEM species through this mechanism. Another mechanism for the volatilisation of AAEM species is the breakage of bonds between
AAEM species and char matrix at high temperatures. Under our experimental conditions,the sodium in the form of NaCl in the coal substrate
seems to volatilise more easily than the sodium in the form of carboxylate in the coal substrate. The monovalent species (Na) is volatilised
much more easily that the divalent species (Mg and Ca) during pyrolysis. q 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Volatilisation; Sodium; Magnesium; Calcium; Brown coal; Pyrolysis
1. Introduction
The main alkali and alkaline earth metallic (AAEM)
species in Victorian brown coal are sodium,magnesium
and calcium. They exist in two main forms: as carboxylates
and as soluble salts (NaCl). While the former form of
AAEM species can be considered as a part of the coal
organic substance [1±4] and distribute uniformly at
atomic/molecular scale in the brown coal,the latter form
of AAEM species (mainly NaCl) exist as ionic species
associated with the water [5] in the as-mined coal. Upon
drying,NaCl would tend to be more concentrated in
the abundant pore structures of the brown coal,weakly
associated with the internal pore surface. Some Na and Cl
may also be `included' in the coal matrix. Even for the
AAEM species in the form of carboxylates,the difference
in their valence,for example sodium being monovalent
and magnesium being divalent,implies that they are
bonded differently to the three-dimensional macromolecular
structure in the brown coal. While Na
1
can be bonded to a
single carboxylate group as ZCOONa,Mg
11
(and Ca
11
)
would necessarily be bonded to two carboxylate groups as
ZCOOMgOOCZ. Therefore,the AAEM species exist in
the Victorian brown coal in different chemical and physical
forms and also at different valence status. The purpose of
this part of the study is to investigate the effects of chemical/
physical form and valence of AAEM species on their
volatilisation during pyrolysis.
The AAEM species in brown coal undergo a series of
chemical transformations during pyrolysis. The transforma-
tion of the ion-exchangeable AAEM species (carboxylates)
during pyrolysis has very important implications on the
formation of volatiles [2,6±9]. The physico±chemical
transformation of the AAEM species during pyrolysis is
also likely to be responsible for the volatilisation of the
AAEM species themselves. In Part I of this series [10],
we showed that NaCl in the brown coal could be volatilised
as chlorine and sodium separately; with the volatilisation
of chlorine,sodium is bonded to the char. To understand
the volatilisation of the AAEM species bonded as carboxyl-
ates in the brown coal,we have measured the release of
Fuel 81 (2002) 151±158
0016-2361/02/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(01)00128-4
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* Corresponding author. Tel.: 161-3-9905-9623; fax: 161-3-9905-5686.
E-mail address: Chun-Zhu.Li@eng.monash.edu.au (C.-Z. Li).