The effect of fire retardants on combustion and pyrolysis of sugar-cane bagasse
Highlights
► I study the effect of fire retardants on the pyrolysis and combustion of bagasse. ► I find these increase the char yield during pyrolysis. ► I find these reduce energy release during combustion. ► I find combining the chemicals shows no synergistic effects.
Introduction
Sugar-cane bagasse constitutes the fibrous residue of sugar-cane after undergoing conventional milling and the extraction of the sugar-cane juice. It contains 27–54% cellulose, 22–39% hemicellulose, 14–24% lignin and 3–5% inorganic matter (Aiman and Stubington, 1993). Sugar mills typically utilise this waste material to generate heat and power to run the sugar milling process (Edwards, 1991, Islam et al., 2010), but a significant proportion of the bagasse must be stockpiled. The stockpiled bagasse is of low economic value and constitutes an environmental problem to sugar mills and surrounding districts, especially if stockpiled for extended periods, due to the risk of spontaneous combustion occurring within the pile (Dawson et al., 1990). As a means of minimising this hazard, many mill furnaces are operated highly inefficiently so as to burn large portions of the excess bagasse – this, however, can have other environmentally detrimental effects to the surrounding community. Globally, 240 Mt of bagasse are produced annually from 800 Mt of processed cane (Kalderis et al., 2008) therefore by adopting more energy efficient modes of operation a substantial source of biomass suitable for alternative uses can be made available.
The utilisation of excess bagasse for energy generation (beyond that required by the sugar mills) has received increased attention as a possible method of partially replacing fossil fuels and ameliorating the negative environmental effects of carbon dioxide emissions. Furthermore, pyrolysis of biomass has the potential to provide extra environmental benefits – the gases or liquids generated by pyrolysis may be used as fuel and the residue solids may be utilised as biochar; a stable, carbon rich material that decomposes extremely slowly and can significantly improve the quality of soil when added (Sohi et al., 2009, Kameyama et al., 2010). A life cycle analysis by Gaunt and Lehmann (2008) investigating the utilisation of biomass by slow pyrolysis showed that maximising biochar production is the optimum strategy for mitigating greenhouse gas emissions.
Alternatively, bagasse fibres may be incorporated into building materials or composites (John and Thomas, 2008); Bagasse-filled polymer composites have been shown to exhibit similar performance to wood-filled composites (De Sousa et al., 2004, Stael et al., 2001). However, the use of bagasse as a building material raises the concern of its fire behaviour and the addition of chemical retardants that may be useful in reducing unfavourable flammability characteristics.
Although there have been a number of studies conducted on the pyrolysis and oxidation of bagasse (Aiman and Stubington, 1993, Stubington and Aiman, 1994, Bilba and Ouensanga, 1996, Islam et al., 2010, Munir et al., 2009) the effect of chemical addition on the reaction kinetics and char yield is much less comprehensive. Varhegyi et al., 1988, Varhegyi et al., 1989 investigated the effect of dilute inorganic salts (NaCl, FeSO4 and ZnCl2) on bagasse degradation during pyrolysis and found charcoal production was enhanced. There has been considerable research on the effect of chemical treatments when bagasse is pyrolysed to produce activated carbon (e.g. Valix et al., 2004), but this research focused on the effect the chemical additives have on the surface characteristics of the char and optimising these parameters. This research is directed towards studying the effect of chemical additives on the degradation kinetics and char yields from bagasse. Furthermore, the effect of chemical additives on the fire characteristics of bagasse is not evident in the literature and therefore formed part of this study.
This paper reports the affect of three commonly used fire retardants – sodium borate (borax – Na2B4O7·5H2O), boric acid (H3BO3) and ammonium sulphate ((NH4)2SO4) – on the combustion and pyrolysis behaviour of bagasse. TGA and cone calorimetry techniques were used to study ignition, rates of degradation, and heat release of untreated and treated bagasse samples. The compounds chosen are known to affect the condensed phase degradation rate of lignocellulosic materials and, therefore, their incorporation into bagasse could provide a method by which the production of biochar may be increased and/or the heat release by combustion decreased. These compounds can be applied as aqueous solutions so that they may be impregnated into the material where they deposit on microfibre surfaces (Kandola et al., 1996). Impregnated Borax (BX) and Boric acid (BA) act by a physical mechanism whereby a glassy coating or protective layer forms over the fibres at elevated temperatures thus acting as a barrier excluding oxygen and entrapping volatile components (Kandola et al., 1996). Moreover, BA has been shown (Wang et al., 2004) to catalyse the dehydration of wood components thus promoting the charring of this material and the production of diluent H2O into the gas phase. BX and BA are commonly used together as fire retardants for wood (Baysal et al., 2007) and other cellulosic materials as BX tends to reduce flame spread but can promote smouldering whereas BA suppresses smouldering but has less affect on flame spread (Wang et al., 2004).
Ammonium sulphate (AS) is a salt that has a dual fire retardant action. The salt decomposes at about 284 °C to produce NH3 – a diluent that inhibits gas phase combustion, and H2SO4 – a catalyst for the dehydration reaction and promotion of charring. The char may act as a thermal barrier, reducing heat transfer to the interior of the material and, thereby, inhibiting the rate of decomposition (Statheropoulos and Kyriakou, 2000). Ammonium sulphate is commonly used as a fire retardant in lignocellulosic boards and panels (Kozlowski and Wladyka-Przybylak, 2000) and for suppression of forest fires (Pappa et al., 1995).
Section snippets
Preparation of bagasse
Fresh bagasse, retrieved from Invicta Mill, Giru, Queensland was washed, dried and sieved to produce samples with particle size between 3 and 5 mm. Proximate analysis of the bagasse was 83.7% volatile matter, 14.3% fixed carbon and 2.0% ash. Using the correlation reported by Shen et al. (2010), the ultimate analysis of the bagasse is 48.7% carbon, 6.0% hydrogen and 44.8% oxygen. Dried bagasse was immersed in aqueous solutions of 0.1–0.5 M borax (Na2B4O7·5H2O, Sigma–Aldrich, purity > 98%), boric
Mass-loss kinetics of bagasse in nitrogen environment
Fig. 1 shows differential thermal gravimetry (DTG) plots for untreated and chemically treated bagasse when heated at 10 K/min under a nitrogen environment. The DTG plot for the untreated bagasse clearly shows that the bagasse begins to degrade at 200 °C and exhibits two major peaks with maxima at 308 and 358 °C. Previous studies (Manya and Arauzo, 2008, Varhegyi et al., 1988, Bilba and Ouensanga, 1996) on the thermal decomposition of bagasse under inert atmospheres indicated that the initial
Conclusions
Experiments conducted by TGA and cone calorimetry show that treatment of bagasse using ammonium sulphate, boric acid and borax affected the solid-phase degradation of the bagasse so as to increase the amount of char produced during pyrolysis and decrease their respective PHRR and THR during combustion. Combination of the chemicals did not show enhanced results. Treatment of bagasse by these chemicals could be useful to enhance biochar yields in pyrolysis plants or to reduce flammability risk in
Acknowledgements
The author wishes to acknowledge Mr. Paul Britton and Mr. Joseph Lai for supply of bagasse and some chemically treated bagasse. Further, the author acknowledges Mr. Ashley Bicknell for performing the cone calorimetry experiments.
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