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Volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass: differences between sugar cane bagasse and cane trash
Bioresource Technology 96 (2005) 1570–1577
Volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass: differences between sugar cane bagasse and cane trash Daniel M. Keown a, George Favas a, Jun-ichiro Hayashi b, Chun-Zhu Li b
a,*
a Department of Chemical Engineering, Monash University, P.O. Box 36, Monash, Vict. 3800, Australia Centre for Advanced Research of Energy Technology, Hokkaido University, N13-W8, Kita-ku, Sapporo 060 8628, Japan
Received 29 June 2004; received in revised form 14 December 2004; accepted 26 December 2004 Available online 22 February 2005
Abstract Sugar cane bagasse and cane trash were pyrolysed in a novel quartz fluidised-bed/fixed-bed reactor. Quantification of the Na, K, Mg and Ca in chars revealed that pyrolysis temperature, heating rate, valence and biomass type were important factors influencing the volatilisation of these alkali and alkaline earth metallic (AAEM) species. Pyrolysis at a slow heating rate (10 K min1) led to minimal (often <20%) volatilisation of AAEM species from these biomass samples. Fast heating rates (>1000 K s1), encouraging volatile–char interactions with the current reactor configuration, resulted in the volatilisation of around 80% of Na, K, Mg and Ca from bagasse during pyrolysis at 900 °C. Similar behaviour was observed for monovalent Na and K with cane trash, but the volatilisation of Mg and Ca from cane trash was always restricted. The difference in Cl content between bagasse and cane trash was not sufficient to fully explain the difference in the volatilisation of Mg and Ca. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass; Pyrolysis; Fluidised-bed; Volatilisation; Chlorine; Potassium
1. Introduction Gasification/reforming of biomass represents a potentially efficient way to use biomass as a renewable energy source. If coupled with CO2 sequestration, gasification/reforming-based power generation using biomass could lead to a net reduction of CO2 in the atmosphere. In particular, the gasification/reforming of biomass constitutes an attractive way of renewable production of H2 (Asadullah et al., 2002; Demirbas, 2002; Garcia et al., 2002) for efficient power generation using fuel cells or gas turbine systems. As an endothermic process, the gasification of biomass also has the potential of acting as a chemical heat pump to convert the thermal *
Corresponding author. Tel.: +61 3 9905 9623; fax: +61 3 9905 5686. E-mail address: [email protected] (C.-Z. Li).
0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.12.014
solar energy into the chemical energy of environmentally friendly and renewable fuels such as H2. Most of the biomass gasification processes proposed or demonstrated so far are largely based on experiences from coal gasification. The special thermochemical properties of biomass are often ignored in these gasification processes. Compared with most bituminous coals, one of the important features of biomass materials is the presence of significant amounts of alkali and alkaline earth metallic (AAEM) species (mainly K, Na, Mg and Ca). The AAEM species tend to volatilise during pyrolysis (and gasification/combustion) and are an important consideration in all aspects of biomass thermochemical conversion processes such as gasification. While the volatilisation of AAEM species may create slagging and/or fouling problems for the operation of gas turbine blades or in the conventional pulverised fuel combustion
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systems using biomass (Nielsen et al., 2000), the AAEM species retained in char during pyrolysis are important catalysts for the gasification/combustion of char (Raveendran and Ganesh, 1998; Zolin et al., 2001), helping to reduce the gasification temperature and thus increasing the overall gasification process efficiency and process economy. The volatilised AAEM species may also act as catalysts for the steam reforming of volatiles in the gas phase (Hosokai et al., 2004). Clearly, the development of a biomass gasification process would require a good understanding of the volatilisation of AAEM species during the pyrolysis of biomass in order to take advantage of the special thermochemical properties of biomass. Annual biomasses often contain much higher concentrations of Cl than coal. Because the Cl and high proportions of AAEM species (particularly K) in biomass are water soluble, simple leaching/washing can remove large fractions of both alkali metals (in most cases >80% of K and Na) and Cl (>90%) from biomass (or char) and therefore has been considered as a means of mitigating the problems caused by the volatilisation of AAEM (particularly K) (Davidsson et al., 2002; Dayton et al., 1999; Gabra et al., 2001; Jenkins et al., 1996; Jensen et al., 2000, 2001; Mojtahedi and Backman, 1989; Olsson et al., 1997; Turn et al., 1998, 2003). While it is often agreed that K contained in biomass is released into the gas phase mainly as KCl during pyrolysis and gasification of biomass (Dayton et al., 1995; Olsson et al., 1997), the detailed mechanisms remain unclear, often contradicting with results such as volatilisation of a portion of Cl preferential to K (Jensen et al., 2000) and volatilisation of Cl mainly as HCl (Bjo¨rkman and Stromberg, 1997). The ratio of K and Cl in biomass appears to affect the volatilisation of K and Cl (Gabra et al., 2001; Jensen et al., 2000, 2001; Mojtahedi and Backman, 1989; Olsson et al., 1997). In agreement with Olsson et al. (1997), Jensen et al. (2000) showed two steps in the volatilisation of Cl: 200–400 °C and 700–900 °C. Significant amounts of K were only released above 700 °C. Previous results (Quyn et al., 2002a) on the pyrolysis of a NaCl-containing Victorian brown coal also showed that the release of Na and Cl took place independently and not as NaCl molecules during pyrolysis. As Cl and K may volatilise from solid fuels [biomass (Jensen et al., 2000; Olsson et al.,
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1997) and brown coal (Quyn et al., 2002a)] independently during pyrolysis, this poses a question as to whether Cl in biomass would indeed facilitate the volatilisation of K or other AAEM species during pyrolysis. With the preferential release of Cl during the pyrolysis of biomass (Jensen et al., 2000; Olsson et al., 1997), K must have been bonded to the char matrix. In fact, as will be shown in this study, there is often not enough Cl to account for the AAEM species, or even just K, in biomass. In other words, a significant proportion of AAEM species exist in biomass in forms other than chlorides. Furthermore, significant amounts of Ca and Mg also exist in biomass as the essential nutrients for the growth of the biomass. Very little is known about the volatilisation of these non-chloride AAEM species during pyrolysis. The purpose of this study is to investigate the volatilisation of AAEM species during the pyrolysis of biomass. Two biomass samples were chosen in this study: a sugar cane bagasse sample and a cane trash sample. Both bagasse and cane trash are considered as important potential renewable energy sources. A novel fluidised-bed/fixed-bed reactor was used to study the pyrolysis of these biomass samples, providing insights into the mechanisms of the volatilisation of AAEM species during the pyrolysis of biomass.
2. Experimental 2.1. Biomass samples Sugar cane bagasse and cane trash samples were provided by the Sugar Research Institute. The properties of these biomass samples are shown below in Tables 1 and 2. The chlorine contents shown in Table 2 were determined using a slightly modified Eschka method (Chakrabarti, 1978; Quyn et al., 2002a). The sugar cane was grown in Queensland, Australia. The sugar cane bagasse has been washed/leached with water at an elevated temperature during the extraction of its sugar. In contrast, the cane trash sample has high contents of AAEM species and Cl. The bagasse and cane trash samples were air-dried and pulverised using a Bauer refiner and sieved repeatedly to a particle size range of 125–250 lm.
Table 1 Proximate and ultimate analyses of the biomass samples used in this study Biomass type
Bagasse Cane trash a
Particle size range (lm)
125–210 125–210
Oxygen calculated by difference.
Ash yield (wt% db) [±0.1]
6.9 7.6
Ultimate analysis (wt% daf) C [±0.3]
H [±0.1]
N [±0.05]
S [±0.03]
Oa
49.7 49.5
6.1 6.1
0.31 0.31
0.04 0.08
43.8 44.0
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Table 2 Contents of AAEM species and Cl in the biomass samples used in this study Biomass type
AAEM content (wt%, db) Na [±0.005]
K [±0.005]
Mg [±0.005]
Ca [±0.005]
Bagasse Cane trash
0.057 0.043
0.174 0.525
0.104 0.185
0.095 0.368
2.2. Pyrolysis Pyrolysis of the biomass samples was carried out using a quartz fluidised-bed/fixed-bed reactor (Fig. 1) heated with an external furnace. This reactor system was originally developed to examine the volatilisation of AAEM species during the pyrolysis of Victorian brown coal (Quyn et al., 2002a). A 160 g bed of acidwashed zircon sand (150–180 lm) was fluidised with ultra high purity (>99.999%) argon. The reactor could be operated in either fast or slow heating modes. In the fast heating rate mode, biomass particles were entrained in a feeder and fed at a nominal rate of 75 mg min1 via a water-cooled injection probe directly into the hot sand bed which was maintained at the desired experimental temperature. Biomass particles were heated at a rate in excess of 103–104 K s1, based on the estimation by Tyler (1979) for a similar reactor. Due to the fibrous nature and elongated shape of the biomass particles, the 1/8 in. diameter of the water injection probe (Quyn et al., 2002a) was increased to 1/4 in. diameter to avoid biomass blockages during feeding. The reactor design in this study (Fig. 1) differed from a normal fluidised-bed reactor in that a quartz frit was Fluidising gas Biomass particles
To absorption bottles
Watercooled probe
Char particles Volatiles Quartz frits
Cl content (wt%, db) [±0.005]
K:Cl atomic ratio [±0.1]
0.027 0.176
5.9 2.7
installed in the freeboard. The frit prevented char elutriation during pyrolysis so that the volatilised AAEM species and the parent char were separated at the reaction temperature. The elutriation of char out of the reactor would have led to the re-condensation of the volatilised AAEM species on the char surface at temperatures lower than the pyrolysis temperature, causing major errors in the quantification of the true volatilisation of AAEM species. The presence of the frit in the freeboard also allowed the elutriated char particles to be held underneath the frit, subsequently forming a thin fixed char bed. Therefore, the reactor had features of a fluidised-bed reactor (e.g. fast heating rates) and of a fixed-bed reactor. When the reactor was operated at the fast heating rate mode, after about 2 g of biomass had been fed into the reactor (actual mass of the fed biomass was accurately determined later), the reactor was lifted out of the furnace and brought to room temperature by natural cooling. The argon gas continuously flowed through the reactor to prevent any char oxidation during quenching. The flow rates of the fluidising and feeding argon gas were controlled by mass flow controllers. The total flow rate was constant at 4.31 l min1 (at room temperature) for all experiments. The light tars and condensable gases that were present in the volatiles were absorbed using two absorption bottles (containing 0.1 M NaOH) connected in series to the exit of the reactor. This arrangement was sufficient for the collection of all Cl (likely as HCl) and light carboxylates/carboxylic acids in the volatiles (Quyn et al., 2002a,b). In contrast, in the slow heating rate mode, the biomass particles were fed into the reactor (with sand) at room temperature and the reactor was heated slowly (nominally 10 K min1) to the desired temperature, which was maintained for 15 min before cooling. In all experiments, the char yield was determined by weighing the reactor and biomass/char before and after the experiment. 2.3. Characterisation of products
Sand bed
Fig. 1. A schematic diagram of the fluidised-bed/fixed-bed reactor (modified from Quyn, 2002).
The AAEM species in biomass and char were quantified using a previously established procedure (Li et al., 2000). Briefly, the biomass/char sample was ashed in a TGA in air. The ash was digested with 1 ml of concentrated hot HF:HNO3 (1:1) for at least 16 h. The acid
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mixture was then evaporated on a hotplate and the residue re-dissolved in 20 mM CH3SO3H (MSA). The resulting MSA solutions were analysed using a Dionex DX-500 ion chromatograph (IC) equipped with a CS12A column and a CSRS cation suppressor to quantify the concentrations of Na, K, Mg and Ca in each solution. The eluent used for the IC was also 20 mM MSA. The sample size used in the TGA was limited by the need to avoid ignition (Quyn, 2002; Sathe, 2001) and this may have led to non-homogeneous samples being analysed. To minimise the effect of non-homogenous samples, each char was analysed in at least two separate ashings. The same Dionex DX-500 IC equipped with an AS10 column and an ASRS suppressor was also used to analyse the solutions for anions such as Cl, formate, acetate, oxalate and benzoate, following the procedures described elsewhere (Quyn et al., 2002a). The eluent used for the IC was 0.1 M NaOH.
3. Results and discussion 3.1. Char yields
Char yield [wt%, db] Char yield [wt%, db]
Fig. 2 shows the char yields from the pyrolysis of cane trash and bagasse samples as a function of temperature and heating rate. The majority of sample weight loss occurred at temperatures below 400 °C for both the bagasse and cane trash samples (Fig. 2) at both heating rates. Increasing the temperature from 400 to 600 °C only resulted in slight decreases in the char yield. Further increases in temperature to 950 °C at the slow heating rate resulted in less than 3 wt% (db) additional weight loss for both biomass samples.
100 80 60
(A) Cane trash Slow heating rate Fast heating rate
40 20 0 80 60
(B) Bagasse Slow heating rate Fast heating rate
40 20 0 300 400 500 600 700 800 9001000 Pyrolysis Temperature [°C]
Fig. 2. Char yields from the pyrolysis of (A) cane trash and (B) bagasse as a function of temperature and heating rate in the fluidisedbed/fixed-bed reactor.
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For both bagasse and cane trash, the fast heating rate experiments at temperatures below 400 °C gave higher char yields than the slow heating rate experiments. These differences are likely attributed to the relatively short holding time in the fast heating experiments (i.e. the reactor was lifted out of the furnace as soon as feeding was completed, so some particles had only a few seconds at temperature). At temperatures above 700 °C, the fast heating rate experiments gave noticeably lower char yields than the corresponding slow heating rate experiments (Fig. 2). A number of factors may have contributed to the observation of the effects of heating rate on char yields shown in Fig. 2. Firstly, for many solid fuels, a fast heating rate tends to give lower char yields than a slow heating rate, due to the fact that the recombination reactions inside a pyrolysing particle are less favoured by high heating rates. Secondly, in a reactor such as used in this study that encourages the interactions between volatiles and char, the ‘‘self-gasification’’ of nascent char by reactive components in the volatiles is favoured at the fast heating rate mode. When the biomass particles were pyrolysed at the fast heating rate mode, volatiles generated from the particles fed into the reactor would have to pass through the bed of char underneath the frit in the freeboard (Fig. 1) that was formed from the biomass particles fed at an earlier stage. The reactive components in the volatiles such as H2O and CO2, produced from the thermal cracking of the light hydrocarbons and tar at elevated temperatures, would react with the nascent char to result in the gasification of the nascent char. Moreover, to avoid changes in biomass structure due to complete drying, the biomass samples in this study were only air-dried prior to pyrolysis: the biomass samples had moisture contents in equilibrium with ambient atmosphere (the moisture contents were determined and considered in the char yield determination). This moisture inherently present in the biomass samples would have been driven out of the biomass as steam, gasifying the nascent char. Indeed, the observed char yields at the fast heating rate decreased with increasing temperature due to the intensification of these ‘‘self-gasification’’ reactions. Thirdly, soot formation and destruction could also have affected the observed char yields, particularly at the fast heating rate. Soot formation could occur as the volatiles passed through the thin char bed held underneath the frit in the freeboard. It is believed that the possible soot formation may have partially offset the effect of heating rate on the char yield, leading to the insensitivity in char yield with heating rate at around 400–600 °C. However, at high temperatures (>600 °C), the thermal cracking of soot would intensify and the in situ reforming of volatiles (Hosokai et al., 2004) by steam would have minimised the soot formation on char particles. The reforming reactions are likely to be catalysed by the AAEM species volatilised into the
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gas phase (see later) or retained on char surface. Data on Victorian brown coal (Li et al., 2004), which produces volatiles that are more aromatic and thus of higher sooting propensity than the volatiles from biomass, did show that in situ soot destruction was enough to eliminate the char yield increases due to soot formation. The data in Fig. 2 indicate that the combined effects of heating rate and H2O/CO2 gasification of char was greater than any possible effects of soot formation on the observed char yields at temperatures higher than 600 °C. In contrast, for slow heating rate experiments, most H2O and CO2 would have been released by the time the reactor reached 450 °C during pyrolysis (Wojtowicz et al., 2003), and the majority of volatiles would have been released at temperatures lower than 500 °C and carried out of the reactor (Fig. 2). Therefore, the selfgasification reactions would have taken place to a minimal extent. With the same reasoning, soot formation would have also been eliminated at the slow heating rate mode. The char yields at the slow heating rate remained almost unchanged with increasing temperature above about 600 °C (Fig. 2); any small decreases in char yield would have been due to the intensified thermal cracking of char with increasing temperature.
100 80 60 40 20 100 80 60 40 20
The retentions of Na, K, Mg and Ca in the chars after pyrolysis of the biomass samples are shown in Figs. 3 and 4. Immediately clear are the significant amounts of AAEM species that were volatilised at temperatures as low as 500 °C for both biomass samples at both heating rates. The volatilisation of AAEM species was also observed by Olsson et al. (1997) during the pyrolysis of biomass at temperatures as low as 180 °C. As is shown in Table 2, there was insufficient Cl in the biomass samples to account for the AAEM species: K/Cl atomic ratios were 2.70 and 5.85 for cane trash and bagasse respectively. Some AAEM species must exist in these biomass samples in forms other than chlorides, such as carboxylates. Quyn et al. (2002b) showed that some carboxylate structures in coal might be detached and volatilised as light carboxylates, becoming an important route of the volatilisation of AAEM species at temperatures as low as 300 °C. Similarly, the thermal breakdown of carboxylates in the biomass from large molecular mass structures and the subsequent release of light carboxylates might also be an important mechanism for the release of AAEM species from the pyrolysis of biomass at low temperatures (e.g. 500 °C in Figs. 3 and 4). The yields of acetate and formate from the pyrolysis of cane trash were determined for fast heating rate experiments. At 500 °C, approximately 25 wt% (db) of the cane trash fed to the reactor was released as acetate and 1 wt% (db) as formate. Subsequent increases in
(B) K
0 100 80 60 40 20
(C) Mg
0 100 80 60 40 20
3.2. AAEM retentions in char
(A) Na
0 Retention of species in char [%]
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(D) Ca
0 500
600
700
800
900
Temperature [°C] Slow heating rate
Fast heating rate
Fig. 3. Retention of (A) Na, (B) K, (C) Mg and (D) Ca in char during the pyrolysis of bagasse as a function of temperature and heating rate in the fluidised-bed/fixed-bed reactor.
reaction temperature resulted in decreasing yields of both acetate and formate [0.5 wt% (db) and 0.01 wt% (db) at 900 °C for acetate and formate respectively]. Diminished yields of acetate and formate with increasing temperature are possibly due to the thermal cracking of carboxylates in the gas phase. These data indicate that structures containing –COOH or –COOX (X = K, Na, Mg or Ca) in biomass can be stable enough to be released as structures containing –COOH and –COOX. In other words, the thermal decomposition of carboxylates to release CO2 is not the only fate of the carboxylates. This is at least partially related to the fact that the release of volatiles from biomass takes place at relatively low temperature (Fig. 2) and is a relatively rapid process favouring the survival of –COOH and –COOX structures in the volatiles. The experimental procedure applied in this study does not allow for a clear distinction between carboxylic acids and carboxylates when released from the biomass. It is likely that the majority of acetate and formate detected may have been released as acetic acid and formic acid from the biomass. However, it is highly probable that at least some were in the carboxylate salt form,
D.M. Keown et al. / Bioresource Technology 96 (2005) 1570–1577 100 80 60 40 20
(A) Na
0 100 Retention of species in char [%]
80 60 40 20
(B) K
0 100 80 60 40 20
(C) Mg
0 100 80 60 40 20
(D) Ca
0 500
600 700 800 Temperature [°C]
Slow heating rate
900
Fast heating rate
Fig. 4. Retention of (A) Na, (B) K, (C) Mg and (D) Ca in char during the pyrolysis of cane trash as a function of temperature and heating rate in the fluidised-bed/fixed-bed reactor.
particularly considering that –COOX (X = K, Na, Mg or Ca) structures tend to be thermally more stable than –COOH structures. Furthermore, some carboxylates, even of relatively low vapour pressure, may have been released from the biomass particles, e.g. due to the entrainment with the volatiles being released. Alternatively, the carboxylates were released as part of high molecular mass volatiles that may further decompose outside the particles to give rise to the light carboxylates. In any case, the release of carboxylate structures would mean the volatilisation of AAEM species, providing a plausible explanation for the release of AAEM species at temperatures as low as 500 °C (Figs. 3 and 4) or even lower (Li et al., 2000; Olsson et al., 1997). The data in Figs. 3 and 4 show that the effects of increasing temperature on the volatilisation of AAEM species depend on the type of biomass substrate, heating rate and valence of the AAEM species. Increasing temperature from 500 to 900 °C at the slow heating rate caused little changes in the volatilisation of all AAEM species investigated (Na, K, Mg and Ca) for both bagasse and cane trash. These results indicate that the
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bonds between AAEM species and char matrix are thermally stable regardless of the original forms (chlorides or carboxylates) of AAEM species in the biomass substrates. In contrast, when temperature was increased from 500 to 900 °C for the fast heating rate experiments, the effects of increasing temperature strongly depended on the biomass substrate. For the pyrolysis of bagasse, increasing temperature from 500 to 900 °C led to significant reductions in the retention of all AAEM species (Na, K, Mg and Ca): less than 40% of these AAEM species present in the bagasse were retained in the char at 900 °C (Fig. 3). This behaviour is contrastingly different to the slow heating rate experiments (Fig. 3). Significant differences in the retentions of AAEM species between the fast and the slow heating rate experiments were found at 700 °C with more notable differences with increasing temperature. This temperature region also corresponds to the significant differences in the char yield found between the fast and slow heating experiments (Fig. 2). Clearly, the large release of AAEM species at higher temperatures at the fast heating rate were at least partly due to the interactions of volatiles and char. It is expected that the tar and light hydrocarbons produced during the pyrolysis of biomass would be very aliphatic in nature and prone to thermal cracking at elevated temperatures, leading to the production of significant amounts of free radicals (especially H radicals). Free radicals interacting with char in the fixed-bed would take part in substitution reactions allowing the volatilisation of AAEM species (Wu et al., 2002), which may be represented symbolically with the following reaction: R þ CM X ! CM R þ X
ð1Þ
where CM denotes the char matrix, X represents the AAEM species and R represents free radicals (including H radicals). These data indicate that, while the bonding between AAEM species and char may be thermally stable in the absence of radicals (as is the case of slow heating rate experiments explained above), the (H) radicals present due to the volatile–char interactions would be energetic enough to activate and break down the bonds between AAEM species and char to result in the volatilisation of AAEM species. Another possible reason for the decreasing retention with increasing reaction temperature for fast heating rate experiments is the reactions of volatile precursors inside particles. During primary pyrolysis, concentrations of hydrogen and other radicals would be much higher than those from the gas phase (from volatiles). Higher fluidised-bed temperature will result in a higher heating rate and therefore accelerated progress of the primary pyrolysis. This will lead to enhancement of reaction (1) during the radical/volatile-precursor formation inside the particle.
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Volatile–char interactions seem to explain the increased volatilisation of K and Na with increasing temperature during the pyrolysis of cane trash at the fast heating rate (Fig. 4). However, the volatile–char interactions were apparently not strong enough to activate and break down the AAEM–char bonds in the cane trash char for the release of Mg and Ca: the retentions of Mg and Ca in the cane trash char were not affected by the heating rate (Fig. 4). The retention behaviour of Mg and Ca in cane trash is similar to that of Mg and Ca in a Victorian brown coal (Li et al., 2004; Wu et al., 2002). The higher retentions of Ca and Mg in cane trash char than those in the bagasse char are surprising considering the higher Cl content in cane trash (0.176 wt% db) than in bagasse (0.027 wt% db) (see Table 2). In other words, the higher Cl content in cane trash compared to bagasse did not lead to more extensive volatilisation of Mg and Ca (or K and Na) compared with those in bagasse (Figs. 3 and 4), contrary to the common belief that Cl in biomass is likely to lead to higher volatilisation of AAEM species (Dayton et al., 1999; Jensen et al., 2000; Olsson et al., 1997). In fact, it is likely that Cl would inhibit the release of Ca and Mg, probably through the formation of MgCl2 and CaCl2, which are of high lattice energies. The extent of Cl volatilisation from cane trash was determined for fast heating rate experiments. Results indicate that approximately 1% of Cl originally present in raw cane trash was retained in the char after a 500 °C fast heating rate experiment. The extensive volatilisation of Cl observed at this low temperature is in qualitative agreement with the Cl release reported in literature for biomass (Bjo¨rkman and Stromberg, 1997) and coal (Quyn et al., 2002a). With increasing reaction temperature, the retention of Cl increased: about 80% of Cl was retained in char at 900 °C. This is a direct result of interactions between Cl-containing volatiles and char held underneath the frit in the freeboard (see Fig. 1). While Cl may be bonded into the char organic matrix, it may also react with Mg and Ca to form refractory MgCl2 or CaCl2. In any case, the data in Figs. 3 and 4 as well as the release of Cl at low temperature indicate that a higher content of Cl in biomass does not mean a high volatilisation level of AAEM species. In fact, as was pointed above, there is not sufficient Cl in bagasse or in cane trash to account for their AAEM species. Two other possible explanations may be proposed for the high retention of Mg and Ca in cane trash char compared with that in the bagasse char. The first one is the structures of bagasse and cane trash may be very different in terms of the forms and dispersion of Mg and Ca. For example, the Mg and Ca species in cane trash may be in close vicinity with SiO2 in the cane trash. On heating, Ca and Mg react with SiO2 to form silicates to inhibit the volatilisation of Mg and Ca. The second
possibility may be due to the difference in the microstructure of bagasse and cane trash. During the process that precedes bagasse production, sugar cane is mechanically crushed in ‘‘roll mills’’ with hot water (65–90 °C) so as to squeeze sugar-containing juice from it. This process may destroy the original cell wall structure. Cane trash undergoes no such process, implying that its cell structure is intact at least prior to pyrolysis. Similar to bagasse, cane trash showed little evidence of melting during pyrolysis: its particles did not stick to the sand particles in the fluidised-bed. It is then likely that its cell structure was largely retained during pyrolysis. It may be imagined that Mg and Ca in cane trash were tightly bound inside the cell wall and organelles and struggle to be released: the cell structure may have had a ‘‘caging effect’’ for the rather large Ca and Mg species and inhibited their volatilisation. In contrast, the ‘‘open’’ cell structure of bagasse would have had little resistance for the volatilisation of AAEM species.
4. Conclusions A sugar cane bagasse sample and a cane trash sample have been pyrolysed in a novel fluidised-bed/fixed-bed reactor. The volatilisation of AAEM species (Na, K, Mg and Ca) during pyrolysis has been quantified. Our results show that the volatilisation of AAEM species depends on many factors such as biomass properties, valence of AAEM species, heating rate and temperature. Even for the bagasse and cane trash grown in the same area (Queensland, Australia), the volatilisation of AAEM species during pyrolysis showed distinct characteristics. The difference in their chlorine contents is not sufficient to account for the difference in the volatilisation of AAEM species between them. In fact, Cl is volatilised independently from K or other AAEM species and does not show any ability to facilitate the volatilisation of AAEM species. Pyrolysis at a slow heating rate with minimised volatile–char interactions would lead to minimal (often <20%) volatilisation of AAEM species from these biomass samples. Under fluidised-bed conditions encouraging fast particle heating rates and volatile–char interactions, over 80% of Na, K, Mg and Ca may be volatilised from bagasse during pyrolysis at 900 °C. While the volatilisation of monovalent Na and K species from cane trash was similar to that from bagasse, the volatilisation of Mg and Ca from cane trash was always limited. In addition to possible differences in the form and dispersion of Mg and Ca in cane trash and in bagasse, the difference in the cell structure between cane trash and bagasse may be partly responsible for their difference in the volatilisation of Ca and Mg: the open cell structure of bagasse may favour the volatilisation of Mg and Ca compared with the intact cell structures in cane trash.
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Acknowledgement The authors gratefully acknowledge the financial support of this study by New Energy and Industrial Technology Development Organisation (NEDO) in Japan. The authors also thank Drs. T.F. Dixon and P. Hobson at Sugar Research Institute (SRI) in Australia for supplying sugar cane bagasse and cane trash. References Asadullah, M., Ito, S.-I., Kunimori, K., Yamada, M., Tomishige, K., 2002. Biomass gasification to hydrogen and syngas at low temperature: novel catalytic system using fluidized-bed reactor. Journal of Catalysis 208, 255–259. Bjo¨rkman, E., Stromberg, B., 1997. Release of chlorine from biomass at pyrolysis and gasification conditions. Energy & Fuels 11, 1026– 1032. Chakrabarti, J.N., 1978. In: Karr, C. Jr. (Ed.), Analytical Methods for Coal and Coal Products. Academic Press, New York, pp. 333–345. Davidsson, K.O., Korsgren, J.G., Pettersson, J.B.C., Jaglid, U., 2002. The effects of fuel washing techniques on alkali release from biomass. Fuel 81 (2), 137–142. Dayton, D.C., French, F.J., Milne, T.A., 1995. Direct observation of alkali vapor release during biomass combustion and gasification. 1. Application of molecular beam/mass spectrometry to switchgrass combustion. Energy & Fuels 9, 855–865. Dayton, D.C., Jenkins, B.M., Turn, S.Q., Bakker, R.R., Williams, R.B., Belle-Oudry, D., Hill, L.M., 1999. Release of inorganic constituents from leached biomass during thermal conversion. Energy & Fuels 13, 860–870. Demirbas, A., 2002. Hydrogen production from biomass by the gasification process. Energy Sources 24, 59–68. Gabra, M., Nordin, A., Ohman, M., Kjellstrom, B., 2001. Alkali retention/separation during bagasse gasification: a comparison between a fluidised bed and a cyclone gasifier. Biomass and Bioenergy 21 (6), 461–476. Garcia, L., Benedicto, A., Romeo, E., Salvador, M.L., Arauzo, J., Bilbao, R., 2002. Hydrogen production by steam gasification of biomass using Ni–Al coprecipitated catalysts promoted with magnesium. Energy & Fuels 16, 1222–1230. Hosokai, S., Hayashi, J.-I., Shimada, T., Kobayashi, Y., Kuramoto, K., Li, C.-Z., 2004. Spontaneous generation of tar decomposition promoter in a biomass steam reformer. Chemical Engineering Research and Design, Submitted for publication. Jenkins, B.M., Bakker, R.R., Wei, J.B., 1996. On the properties of washed straw. Biomass and Bioenergy 10 (4), 177–200. Jensen, P.A., Frandsen, F.J., Dam-Johansen, K., Sander, B., 2000. Experimental investigation of the transformation and release to gas phase of potassium and chlorine during straw pyrolysis. Energy & Fuels 14, 1280–1285. Jensen, P.A., Sander, B., Dam-Johansen, K., 2001. Removal of K and Cl by leaching of straw char. Biomass and Bioenergy 20 (6), 447– 457.
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Li, C.-Z., Sathe, C., Kershaw, J.R., Pang, Y., 2000. Fates and roles of alkali and alkaline earth metals during the pyrolysis of a Victorian brown coal. Fuel 79 (3–4), 427–438. Li, X., Wu, H., Hayashi, J.-I., Li, C.-Z., 2004. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VI. Further investigation into the effects of volatile–char interactions. Fuel 83 (8), 1033–1039. Mojtahedi, W., Backman, R., 1989. The fate of sodium and potassium in the pressurised fluidised-bed combustion and gasification and peat. Journal of the Institute of Energy 62, 189–196. Nielsen, H.P., Frandsen, F.J., Dam-Johansen, K., Baxter, L.L., 2000. The implications of chlorine-associated corrosion on the operation of biomass-fired boilers. Progress in Energy and Combustion Science 26 (3), 283–298. Olsson, J.G., Jaglid, U., Petterson, J.B.C., 1997. Alkali metal emission during pyrolysis of biomass. Energy & Fuels 11, 779–784. Quyn, D.M., 2002. Transformations of alkali and alkaline earth metallic species during gasification of Victorian brown coal. Ph.D. thesis. Department of Chemical Engineering, Monash University, Melbourne. Quyn, D.M., Wu, H., Li, C.-Z., 2002a. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples. Fuel 81 (2), 143–149. Quyn, D.M., Wu, H., Bhattacharya, S.P., Li, C.-Z., 2002b. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part II. Effects of chemical form and valence. Fuel 81 (2), 151–158. Raveendran, K., Ganesh, A., 1998. Adsorption characteristics and pore-development of biomass-pyrolysis char. Fuel 77 (7), 769–781. Sathe, C., 2001. Fate and roles of alkali and alkaline earth metallic species during the pyrolysis of low-rank coals. Ph.D. Thesis. Department of Chemical Engineering, Monash University, Melbourne. Turn, S.Q., Kinoshita, C.M., Ishimura, D.M., Zhou, J., 1998. The fate of inorganic constituents of biomass in fluidized bed gasification. Fuel 77 (3), 135–146. Turn, S.Q., Kinoshita, C.M., Jakeway, L.A., Jenkins, B.M., Baxter, L.L., Wu, B.C., 2003. Fuel characteristics of processed, high-fiber sugarcane. Fuel Processing Technology 81 (1), 35–55. Tyler, R.J., 1979. Flash pyrolysis of coals. 1 Devolatilization of a Victorian brown coal in a small fluidized-bed reactor. Fuel 58 (9), 680–686. Wojtowicz, M.A., Bassilakis, R., Smith, W.W., Chen, Y., Carangelo, R.M., 2003. Modeling the evolution of volatile species during tobacco pyrolysis. Journal of Analytical and Applied Pyrolysis 66 (1–2), 235–261. Wu, H., Quyn, D.M., Li, C.-Z., 2002. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part III. The importance of the interactions between volatiles and char at high temperature. Fuel 81 (10), 1273–1279. Zolin, A., Jensen, A., Jensen, P.A., Frandsen, F., Dam-Johansen, K., 2001. The influence of inorganic materials on the thermal deactivation of fuel chars. Energy & Fuels 15, 1110–1122.
Volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass: differences between sugar cane bagasse and cane trash Daniel M. Keown a, George Favas a, Jun-ichiro Hayashi b, Chun-Zhu Li b
a,*
a Department of Chemical Engineering, Monash University, P.O. Box 36, Monash, Vict. 3800, Australia Centre for Advanced Research of Energy Technology, Hokkaido University, N13-W8, Kita-ku, Sapporo 060 8628, Japan
Received 29 June 2004; received in revised form 14 December 2004; accepted 26 December 2004 Available online 22 February 2005
Abstract Sugar cane bagasse and cane trash were pyrolysed in a novel quartz fluidised-bed/fixed-bed reactor. Quantification of the Na, K, Mg and Ca in chars revealed that pyrolysis temperature, heating rate, valence and biomass type were important factors influencing the volatilisation of these alkali and alkaline earth metallic (AAEM) species. Pyrolysis at a slow heating rate (10 K min1) led to minimal (often <20%) volatilisation of AAEM species from these biomass samples. Fast heating rates (>1000 K s1), encouraging volatile–char interactions with the current reactor configuration, resulted in the volatilisation of around 80% of Na, K, Mg and Ca from bagasse during pyrolysis at 900 °C. Similar behaviour was observed for monovalent Na and K with cane trash, but the volatilisation of Mg and Ca from cane trash was always restricted. The difference in Cl content between bagasse and cane trash was not sufficient to fully explain the difference in the volatilisation of Mg and Ca. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass; Pyrolysis; Fluidised-bed; Volatilisation; Chlorine; Potassium
1. Introduction Gasification/reforming of biomass represents a potentially efficient way to use biomass as a renewable energy source. If coupled with CO2 sequestration, gasification/reforming-based power generation using biomass could lead to a net reduction of CO2 in the atmosphere. In particular, the gasification/reforming of biomass constitutes an attractive way of renewable production of H2 (Asadullah et al., 2002; Demirbas, 2002; Garcia et al., 2002) for efficient power generation using fuel cells or gas turbine systems. As an endothermic process, the gasification of biomass also has the potential of acting as a chemical heat pump to convert the thermal *
Corresponding author. Tel.: +61 3 9905 9623; fax: +61 3 9905 5686. E-mail address: [email protected] (C.-Z. Li).
0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.12.014
solar energy into the chemical energy of environmentally friendly and renewable fuels such as H2. Most of the biomass gasification processes proposed or demonstrated so far are largely based on experiences from coal gasification. The special thermochemical properties of biomass are often ignored in these gasification processes. Compared with most bituminous coals, one of the important features of biomass materials is the presence of significant amounts of alkali and alkaline earth metallic (AAEM) species (mainly K, Na, Mg and Ca). The AAEM species tend to volatilise during pyrolysis (and gasification/combustion) and are an important consideration in all aspects of biomass thermochemical conversion processes such as gasification. While the volatilisation of AAEM species may create slagging and/or fouling problems for the operation of gas turbine blades or in the conventional pulverised fuel combustion
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systems using biomass (Nielsen et al., 2000), the AAEM species retained in char during pyrolysis are important catalysts for the gasification/combustion of char (Raveendran and Ganesh, 1998; Zolin et al., 2001), helping to reduce the gasification temperature and thus increasing the overall gasification process efficiency and process economy. The volatilised AAEM species may also act as catalysts for the steam reforming of volatiles in the gas phase (Hosokai et al., 2004). Clearly, the development of a biomass gasification process would require a good understanding of the volatilisation of AAEM species during the pyrolysis of biomass in order to take advantage of the special thermochemical properties of biomass. Annual biomasses often contain much higher concentrations of Cl than coal. Because the Cl and high proportions of AAEM species (particularly K) in biomass are water soluble, simple leaching/washing can remove large fractions of both alkali metals (in most cases >80% of K and Na) and Cl (>90%) from biomass (or char) and therefore has been considered as a means of mitigating the problems caused by the volatilisation of AAEM (particularly K) (Davidsson et al., 2002; Dayton et al., 1999; Gabra et al., 2001; Jenkins et al., 1996; Jensen et al., 2000, 2001; Mojtahedi and Backman, 1989; Olsson et al., 1997; Turn et al., 1998, 2003). While it is often agreed that K contained in biomass is released into the gas phase mainly as KCl during pyrolysis and gasification of biomass (Dayton et al., 1995; Olsson et al., 1997), the detailed mechanisms remain unclear, often contradicting with results such as volatilisation of a portion of Cl preferential to K (Jensen et al., 2000) and volatilisation of Cl mainly as HCl (Bjo¨rkman and Stromberg, 1997). The ratio of K and Cl in biomass appears to affect the volatilisation of K and Cl (Gabra et al., 2001; Jensen et al., 2000, 2001; Mojtahedi and Backman, 1989; Olsson et al., 1997). In agreement with Olsson et al. (1997), Jensen et al. (2000) showed two steps in the volatilisation of Cl: 200–400 °C and 700–900 °C. Significant amounts of K were only released above 700 °C. Previous results (Quyn et al., 2002a) on the pyrolysis of a NaCl-containing Victorian brown coal also showed that the release of Na and Cl took place independently and not as NaCl molecules during pyrolysis. As Cl and K may volatilise from solid fuels [biomass (Jensen et al., 2000; Olsson et al.,
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1997) and brown coal (Quyn et al., 2002a)] independently during pyrolysis, this poses a question as to whether Cl in biomass would indeed facilitate the volatilisation of K or other AAEM species during pyrolysis. With the preferential release of Cl during the pyrolysis of biomass (Jensen et al., 2000; Olsson et al., 1997), K must have been bonded to the char matrix. In fact, as will be shown in this study, there is often not enough Cl to account for the AAEM species, or even just K, in biomass. In other words, a significant proportion of AAEM species exist in biomass in forms other than chlorides. Furthermore, significant amounts of Ca and Mg also exist in biomass as the essential nutrients for the growth of the biomass. Very little is known about the volatilisation of these non-chloride AAEM species during pyrolysis. The purpose of this study is to investigate the volatilisation of AAEM species during the pyrolysis of biomass. Two biomass samples were chosen in this study: a sugar cane bagasse sample and a cane trash sample. Both bagasse and cane trash are considered as important potential renewable energy sources. A novel fluidised-bed/fixed-bed reactor was used to study the pyrolysis of these biomass samples, providing insights into the mechanisms of the volatilisation of AAEM species during the pyrolysis of biomass.
2. Experimental 2.1. Biomass samples Sugar cane bagasse and cane trash samples were provided by the Sugar Research Institute. The properties of these biomass samples are shown below in Tables 1 and 2. The chlorine contents shown in Table 2 were determined using a slightly modified Eschka method (Chakrabarti, 1978; Quyn et al., 2002a). The sugar cane was grown in Queensland, Australia. The sugar cane bagasse has been washed/leached with water at an elevated temperature during the extraction of its sugar. In contrast, the cane trash sample has high contents of AAEM species and Cl. The bagasse and cane trash samples were air-dried and pulverised using a Bauer refiner and sieved repeatedly to a particle size range of 125–250 lm.
Table 1 Proximate and ultimate analyses of the biomass samples used in this study Biomass type
Bagasse Cane trash a
Particle size range (lm)
125–210 125–210
Oxygen calculated by difference.
Ash yield (wt% db) [±0.1]
6.9 7.6
Ultimate analysis (wt% daf) C [±0.3]
H [±0.1]
N [±0.05]
S [±0.03]
Oa
49.7 49.5
6.1 6.1
0.31 0.31
0.04 0.08
43.8 44.0
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Table 2 Contents of AAEM species and Cl in the biomass samples used in this study Biomass type
AAEM content (wt%, db) Na [±0.005]
K [±0.005]
Mg [±0.005]
Ca [±0.005]
Bagasse Cane trash
0.057 0.043
0.174 0.525
0.104 0.185
0.095 0.368
2.2. Pyrolysis Pyrolysis of the biomass samples was carried out using a quartz fluidised-bed/fixed-bed reactor (Fig. 1) heated with an external furnace. This reactor system was originally developed to examine the volatilisation of AAEM species during the pyrolysis of Victorian brown coal (Quyn et al., 2002a). A 160 g bed of acidwashed zircon sand (150–180 lm) was fluidised with ultra high purity (>99.999%) argon. The reactor could be operated in either fast or slow heating modes. In the fast heating rate mode, biomass particles were entrained in a feeder and fed at a nominal rate of 75 mg min1 via a water-cooled injection probe directly into the hot sand bed which was maintained at the desired experimental temperature. Biomass particles were heated at a rate in excess of 103–104 K s1, based on the estimation by Tyler (1979) for a similar reactor. Due to the fibrous nature and elongated shape of the biomass particles, the 1/8 in. diameter of the water injection probe (Quyn et al., 2002a) was increased to 1/4 in. diameter to avoid biomass blockages during feeding. The reactor design in this study (Fig. 1) differed from a normal fluidised-bed reactor in that a quartz frit was Fluidising gas Biomass particles
To absorption bottles
Watercooled probe
Char particles Volatiles Quartz frits
Cl content (wt%, db) [±0.005]
K:Cl atomic ratio [±0.1]
0.027 0.176
5.9 2.7
installed in the freeboard. The frit prevented char elutriation during pyrolysis so that the volatilised AAEM species and the parent char were separated at the reaction temperature. The elutriation of char out of the reactor would have led to the re-condensation of the volatilised AAEM species on the char surface at temperatures lower than the pyrolysis temperature, causing major errors in the quantification of the true volatilisation of AAEM species. The presence of the frit in the freeboard also allowed the elutriated char particles to be held underneath the frit, subsequently forming a thin fixed char bed. Therefore, the reactor had features of a fluidised-bed reactor (e.g. fast heating rates) and of a fixed-bed reactor. When the reactor was operated at the fast heating rate mode, after about 2 g of biomass had been fed into the reactor (actual mass of the fed biomass was accurately determined later), the reactor was lifted out of the furnace and brought to room temperature by natural cooling. The argon gas continuously flowed through the reactor to prevent any char oxidation during quenching. The flow rates of the fluidising and feeding argon gas were controlled by mass flow controllers. The total flow rate was constant at 4.31 l min1 (at room temperature) for all experiments. The light tars and condensable gases that were present in the volatiles were absorbed using two absorption bottles (containing 0.1 M NaOH) connected in series to the exit of the reactor. This arrangement was sufficient for the collection of all Cl (likely as HCl) and light carboxylates/carboxylic acids in the volatiles (Quyn et al., 2002a,b). In contrast, in the slow heating rate mode, the biomass particles were fed into the reactor (with sand) at room temperature and the reactor was heated slowly (nominally 10 K min1) to the desired temperature, which was maintained for 15 min before cooling. In all experiments, the char yield was determined by weighing the reactor and biomass/char before and after the experiment. 2.3. Characterisation of products
Sand bed
Fig. 1. A schematic diagram of the fluidised-bed/fixed-bed reactor (modified from Quyn, 2002).
The AAEM species in biomass and char were quantified using a previously established procedure (Li et al., 2000). Briefly, the biomass/char sample was ashed in a TGA in air. The ash was digested with 1 ml of concentrated hot HF:HNO3 (1:1) for at least 16 h. The acid
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mixture was then evaporated on a hotplate and the residue re-dissolved in 20 mM CH3SO3H (MSA). The resulting MSA solutions were analysed using a Dionex DX-500 ion chromatograph (IC) equipped with a CS12A column and a CSRS cation suppressor to quantify the concentrations of Na, K, Mg and Ca in each solution. The eluent used for the IC was also 20 mM MSA. The sample size used in the TGA was limited by the need to avoid ignition (Quyn, 2002; Sathe, 2001) and this may have led to non-homogeneous samples being analysed. To minimise the effect of non-homogenous samples, each char was analysed in at least two separate ashings. The same Dionex DX-500 IC equipped with an AS10 column and an ASRS suppressor was also used to analyse the solutions for anions such as Cl, formate, acetate, oxalate and benzoate, following the procedures described elsewhere (Quyn et al., 2002a). The eluent used for the IC was 0.1 M NaOH.
3. Results and discussion 3.1. Char yields
Char yield [wt%, db] Char yield [wt%, db]
Fig. 2 shows the char yields from the pyrolysis of cane trash and bagasse samples as a function of temperature and heating rate. The majority of sample weight loss occurred at temperatures below 400 °C for both the bagasse and cane trash samples (Fig. 2) at both heating rates. Increasing the temperature from 400 to 600 °C only resulted in slight decreases in the char yield. Further increases in temperature to 950 °C at the slow heating rate resulted in less than 3 wt% (db) additional weight loss for both biomass samples.
100 80 60
(A) Cane trash Slow heating rate Fast heating rate
40 20 0 80 60
(B) Bagasse Slow heating rate Fast heating rate
40 20 0 300 400 500 600 700 800 9001000 Pyrolysis Temperature [°C]
Fig. 2. Char yields from the pyrolysis of (A) cane trash and (B) bagasse as a function of temperature and heating rate in the fluidisedbed/fixed-bed reactor.
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For both bagasse and cane trash, the fast heating rate experiments at temperatures below 400 °C gave higher char yields than the slow heating rate experiments. These differences are likely attributed to the relatively short holding time in the fast heating experiments (i.e. the reactor was lifted out of the furnace as soon as feeding was completed, so some particles had only a few seconds at temperature). At temperatures above 700 °C, the fast heating rate experiments gave noticeably lower char yields than the corresponding slow heating rate experiments (Fig. 2). A number of factors may have contributed to the observation of the effects of heating rate on char yields shown in Fig. 2. Firstly, for many solid fuels, a fast heating rate tends to give lower char yields than a slow heating rate, due to the fact that the recombination reactions inside a pyrolysing particle are less favoured by high heating rates. Secondly, in a reactor such as used in this study that encourages the interactions between volatiles and char, the ‘‘self-gasification’’ of nascent char by reactive components in the volatiles is favoured at the fast heating rate mode. When the biomass particles were pyrolysed at the fast heating rate mode, volatiles generated from the particles fed into the reactor would have to pass through the bed of char underneath the frit in the freeboard (Fig. 1) that was formed from the biomass particles fed at an earlier stage. The reactive components in the volatiles such as H2O and CO2, produced from the thermal cracking of the light hydrocarbons and tar at elevated temperatures, would react with the nascent char to result in the gasification of the nascent char. Moreover, to avoid changes in biomass structure due to complete drying, the biomass samples in this study were only air-dried prior to pyrolysis: the biomass samples had moisture contents in equilibrium with ambient atmosphere (the moisture contents were determined and considered in the char yield determination). This moisture inherently present in the biomass samples would have been driven out of the biomass as steam, gasifying the nascent char. Indeed, the observed char yields at the fast heating rate decreased with increasing temperature due to the intensification of these ‘‘self-gasification’’ reactions. Thirdly, soot formation and destruction could also have affected the observed char yields, particularly at the fast heating rate. Soot formation could occur as the volatiles passed through the thin char bed held underneath the frit in the freeboard. It is believed that the possible soot formation may have partially offset the effect of heating rate on the char yield, leading to the insensitivity in char yield with heating rate at around 400–600 °C. However, at high temperatures (>600 °C), the thermal cracking of soot would intensify and the in situ reforming of volatiles (Hosokai et al., 2004) by steam would have minimised the soot formation on char particles. The reforming reactions are likely to be catalysed by the AAEM species volatilised into the
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gas phase (see later) or retained on char surface. Data on Victorian brown coal (Li et al., 2004), which produces volatiles that are more aromatic and thus of higher sooting propensity than the volatiles from biomass, did show that in situ soot destruction was enough to eliminate the char yield increases due to soot formation. The data in Fig. 2 indicate that the combined effects of heating rate and H2O/CO2 gasification of char was greater than any possible effects of soot formation on the observed char yields at temperatures higher than 600 °C. In contrast, for slow heating rate experiments, most H2O and CO2 would have been released by the time the reactor reached 450 °C during pyrolysis (Wojtowicz et al., 2003), and the majority of volatiles would have been released at temperatures lower than 500 °C and carried out of the reactor (Fig. 2). Therefore, the selfgasification reactions would have taken place to a minimal extent. With the same reasoning, soot formation would have also been eliminated at the slow heating rate mode. The char yields at the slow heating rate remained almost unchanged with increasing temperature above about 600 °C (Fig. 2); any small decreases in char yield would have been due to the intensified thermal cracking of char with increasing temperature.
100 80 60 40 20 100 80 60 40 20
The retentions of Na, K, Mg and Ca in the chars after pyrolysis of the biomass samples are shown in Figs. 3 and 4. Immediately clear are the significant amounts of AAEM species that were volatilised at temperatures as low as 500 °C for both biomass samples at both heating rates. The volatilisation of AAEM species was also observed by Olsson et al. (1997) during the pyrolysis of biomass at temperatures as low as 180 °C. As is shown in Table 2, there was insufficient Cl in the biomass samples to account for the AAEM species: K/Cl atomic ratios were 2.70 and 5.85 for cane trash and bagasse respectively. Some AAEM species must exist in these biomass samples in forms other than chlorides, such as carboxylates. Quyn et al. (2002b) showed that some carboxylate structures in coal might be detached and volatilised as light carboxylates, becoming an important route of the volatilisation of AAEM species at temperatures as low as 300 °C. Similarly, the thermal breakdown of carboxylates in the biomass from large molecular mass structures and the subsequent release of light carboxylates might also be an important mechanism for the release of AAEM species from the pyrolysis of biomass at low temperatures (e.g. 500 °C in Figs. 3 and 4). The yields of acetate and formate from the pyrolysis of cane trash were determined for fast heating rate experiments. At 500 °C, approximately 25 wt% (db) of the cane trash fed to the reactor was released as acetate and 1 wt% (db) as formate. Subsequent increases in
(B) K
0 100 80 60 40 20
(C) Mg
0 100 80 60 40 20
3.2. AAEM retentions in char
(A) Na
0 Retention of species in char [%]
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(D) Ca
0 500
600
700
800
900
Temperature [°C] Slow heating rate
Fast heating rate
Fig. 3. Retention of (A) Na, (B) K, (C) Mg and (D) Ca in char during the pyrolysis of bagasse as a function of temperature and heating rate in the fluidised-bed/fixed-bed reactor.
reaction temperature resulted in decreasing yields of both acetate and formate [0.5 wt% (db) and 0.01 wt% (db) at 900 °C for acetate and formate respectively]. Diminished yields of acetate and formate with increasing temperature are possibly due to the thermal cracking of carboxylates in the gas phase. These data indicate that structures containing –COOH or –COOX (X = K, Na, Mg or Ca) in biomass can be stable enough to be released as structures containing –COOH and –COOX. In other words, the thermal decomposition of carboxylates to release CO2 is not the only fate of the carboxylates. This is at least partially related to the fact that the release of volatiles from biomass takes place at relatively low temperature (Fig. 2) and is a relatively rapid process favouring the survival of –COOH and –COOX structures in the volatiles. The experimental procedure applied in this study does not allow for a clear distinction between carboxylic acids and carboxylates when released from the biomass. It is likely that the majority of acetate and formate detected may have been released as acetic acid and formic acid from the biomass. However, it is highly probable that at least some were in the carboxylate salt form,
D.M. Keown et al. / Bioresource Technology 96 (2005) 1570–1577 100 80 60 40 20
(A) Na
0 100 Retention of species in char [%]
80 60 40 20
(B) K
0 100 80 60 40 20
(C) Mg
0 100 80 60 40 20
(D) Ca
0 500
600 700 800 Temperature [°C]
Slow heating rate
900
Fast heating rate
Fig. 4. Retention of (A) Na, (B) K, (C) Mg and (D) Ca in char during the pyrolysis of cane trash as a function of temperature and heating rate in the fluidised-bed/fixed-bed reactor.
particularly considering that –COOX (X = K, Na, Mg or Ca) structures tend to be thermally more stable than –COOH structures. Furthermore, some carboxylates, even of relatively low vapour pressure, may have been released from the biomass particles, e.g. due to the entrainment with the volatiles being released. Alternatively, the carboxylates were released as part of high molecular mass volatiles that may further decompose outside the particles to give rise to the light carboxylates. In any case, the release of carboxylate structures would mean the volatilisation of AAEM species, providing a plausible explanation for the release of AAEM species at temperatures as low as 500 °C (Figs. 3 and 4) or even lower (Li et al., 2000; Olsson et al., 1997). The data in Figs. 3 and 4 show that the effects of increasing temperature on the volatilisation of AAEM species depend on the type of biomass substrate, heating rate and valence of the AAEM species. Increasing temperature from 500 to 900 °C at the slow heating rate caused little changes in the volatilisation of all AAEM species investigated (Na, K, Mg and Ca) for both bagasse and cane trash. These results indicate that the
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bonds between AAEM species and char matrix are thermally stable regardless of the original forms (chlorides or carboxylates) of AAEM species in the biomass substrates. In contrast, when temperature was increased from 500 to 900 °C for the fast heating rate experiments, the effects of increasing temperature strongly depended on the biomass substrate. For the pyrolysis of bagasse, increasing temperature from 500 to 900 °C led to significant reductions in the retention of all AAEM species (Na, K, Mg and Ca): less than 40% of these AAEM species present in the bagasse were retained in the char at 900 °C (Fig. 3). This behaviour is contrastingly different to the slow heating rate experiments (Fig. 3). Significant differences in the retentions of AAEM species between the fast and the slow heating rate experiments were found at 700 °C with more notable differences with increasing temperature. This temperature region also corresponds to the significant differences in the char yield found between the fast and slow heating experiments (Fig. 2). Clearly, the large release of AAEM species at higher temperatures at the fast heating rate were at least partly due to the interactions of volatiles and char. It is expected that the tar and light hydrocarbons produced during the pyrolysis of biomass would be very aliphatic in nature and prone to thermal cracking at elevated temperatures, leading to the production of significant amounts of free radicals (especially H radicals). Free radicals interacting with char in the fixed-bed would take part in substitution reactions allowing the volatilisation of AAEM species (Wu et al., 2002), which may be represented symbolically with the following reaction: R þ CM X ! CM R þ X
ð1Þ
where CM denotes the char matrix, X represents the AAEM species and R represents free radicals (including H radicals). These data indicate that, while the bonding between AAEM species and char may be thermally stable in the absence of radicals (as is the case of slow heating rate experiments explained above), the (H) radicals present due to the volatile–char interactions would be energetic enough to activate and break down the bonds between AAEM species and char to result in the volatilisation of AAEM species. Another possible reason for the decreasing retention with increasing reaction temperature for fast heating rate experiments is the reactions of volatile precursors inside particles. During primary pyrolysis, concentrations of hydrogen and other radicals would be much higher than those from the gas phase (from volatiles). Higher fluidised-bed temperature will result in a higher heating rate and therefore accelerated progress of the primary pyrolysis. This will lead to enhancement of reaction (1) during the radical/volatile-precursor formation inside the particle.
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Volatile–char interactions seem to explain the increased volatilisation of K and Na with increasing temperature during the pyrolysis of cane trash at the fast heating rate (Fig. 4). However, the volatile–char interactions were apparently not strong enough to activate and break down the AAEM–char bonds in the cane trash char for the release of Mg and Ca: the retentions of Mg and Ca in the cane trash char were not affected by the heating rate (Fig. 4). The retention behaviour of Mg and Ca in cane trash is similar to that of Mg and Ca in a Victorian brown coal (Li et al., 2004; Wu et al., 2002). The higher retentions of Ca and Mg in cane trash char than those in the bagasse char are surprising considering the higher Cl content in cane trash (0.176 wt% db) than in bagasse (0.027 wt% db) (see Table 2). In other words, the higher Cl content in cane trash compared to bagasse did not lead to more extensive volatilisation of Mg and Ca (or K and Na) compared with those in bagasse (Figs. 3 and 4), contrary to the common belief that Cl in biomass is likely to lead to higher volatilisation of AAEM species (Dayton et al., 1999; Jensen et al., 2000; Olsson et al., 1997). In fact, it is likely that Cl would inhibit the release of Ca and Mg, probably through the formation of MgCl2 and CaCl2, which are of high lattice energies. The extent of Cl volatilisation from cane trash was determined for fast heating rate experiments. Results indicate that approximately 1% of Cl originally present in raw cane trash was retained in the char after a 500 °C fast heating rate experiment. The extensive volatilisation of Cl observed at this low temperature is in qualitative agreement with the Cl release reported in literature for biomass (Bjo¨rkman and Stromberg, 1997) and coal (Quyn et al., 2002a). With increasing reaction temperature, the retention of Cl increased: about 80% of Cl was retained in char at 900 °C. This is a direct result of interactions between Cl-containing volatiles and char held underneath the frit in the freeboard (see Fig. 1). While Cl may be bonded into the char organic matrix, it may also react with Mg and Ca to form refractory MgCl2 or CaCl2. In any case, the data in Figs. 3 and 4 as well as the release of Cl at low temperature indicate that a higher content of Cl in biomass does not mean a high volatilisation level of AAEM species. In fact, as was pointed above, there is not sufficient Cl in bagasse or in cane trash to account for their AAEM species. Two other possible explanations may be proposed for the high retention of Mg and Ca in cane trash char compared with that in the bagasse char. The first one is the structures of bagasse and cane trash may be very different in terms of the forms and dispersion of Mg and Ca. For example, the Mg and Ca species in cane trash may be in close vicinity with SiO2 in the cane trash. On heating, Ca and Mg react with SiO2 to form silicates to inhibit the volatilisation of Mg and Ca. The second
possibility may be due to the difference in the microstructure of bagasse and cane trash. During the process that precedes bagasse production, sugar cane is mechanically crushed in ‘‘roll mills’’ with hot water (65–90 °C) so as to squeeze sugar-containing juice from it. This process may destroy the original cell wall structure. Cane trash undergoes no such process, implying that its cell structure is intact at least prior to pyrolysis. Similar to bagasse, cane trash showed little evidence of melting during pyrolysis: its particles did not stick to the sand particles in the fluidised-bed. It is then likely that its cell structure was largely retained during pyrolysis. It may be imagined that Mg and Ca in cane trash were tightly bound inside the cell wall and organelles and struggle to be released: the cell structure may have had a ‘‘caging effect’’ for the rather large Ca and Mg species and inhibited their volatilisation. In contrast, the ‘‘open’’ cell structure of bagasse would have had little resistance for the volatilisation of AAEM species.
4. Conclusions A sugar cane bagasse sample and a cane trash sample have been pyrolysed in a novel fluidised-bed/fixed-bed reactor. The volatilisation of AAEM species (Na, K, Mg and Ca) during pyrolysis has been quantified. Our results show that the volatilisation of AAEM species depends on many factors such as biomass properties, valence of AAEM species, heating rate and temperature. Even for the bagasse and cane trash grown in the same area (Queensland, Australia), the volatilisation of AAEM species during pyrolysis showed distinct characteristics. The difference in their chlorine contents is not sufficient to account for the difference in the volatilisation of AAEM species between them. In fact, Cl is volatilised independently from K or other AAEM species and does not show any ability to facilitate the volatilisation of AAEM species. Pyrolysis at a slow heating rate with minimised volatile–char interactions would lead to minimal (often <20%) volatilisation of AAEM species from these biomass samples. Under fluidised-bed conditions encouraging fast particle heating rates and volatile–char interactions, over 80% of Na, K, Mg and Ca may be volatilised from bagasse during pyrolysis at 900 °C. While the volatilisation of monovalent Na and K species from cane trash was similar to that from bagasse, the volatilisation of Mg and Ca from cane trash was always limited. In addition to possible differences in the form and dispersion of Mg and Ca in cane trash and in bagasse, the difference in the cell structure between cane trash and bagasse may be partly responsible for their difference in the volatilisation of Ca and Mg: the open cell structure of bagasse may favour the volatilisation of Mg and Ca compared with the intact cell structures in cane trash.
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