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Combustion of thermochemically torrefied sugar cane bagasse
Accepted Manuscript Combustion of Thermochemically Torrefied Sugar Cane Bagasse M. Valix, S. Katyal, W.H. Cheung PII: DOI: Reference:
S0960-8524(16)31459-6 http://dx.doi.org/10.1016/j.biortech.2016.10.053 BITE 17208
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
16 September 2016 18 October 2016 19 October 2016
Please cite this article as: Valix, M., Katyal, S., Cheung, W.H., Combustion of Thermochemically Torrefied Sugar Cane Bagasse, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/j.biortech.2016.10.053
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COMBUSTION OF THERMOCHEMICALLY TORREFIED SUGAR CANE BAGASSE M. Valix∗, S. Katyal and W.H. Cheung The University of Sydney, Sydney, NSW 2006, Australia *Corresponding author. Tel: + 61 2 9351 4995, Fax: + 61 2 9351 2854, E-mail: [email protected]
Keywords: bagasse, dry torrefaction, thermochemical torrefaction, biological stability, combustion
Abstract This study compared the upgrading of sugar bagasse by thermochemical and dry torrefaction methods and their corresponding combustion behavior relative to raw bagasse. The combustion reactivities were examined by non-isothermal thermogravimetric analysis. Thermochemical torrefaction was carried out by chemical pre-treatment of bagasse with acid followed by heating at 160-300°C in nitrogen environment, while dry torrefaction followed the same heating treatment without the chemical pretreatment. The results showed thermochemical torrefaction generated chars with combustion properties that are closer to various ranks of coal, thus making it more suitable for co-firing applications. Thermochemical torrefaction also induced greater densification of bagasse with a 335% rise in bulk density to 340 kg/m3, increased HHVmass and HHVvolume, greater charring and aromatization and storage stability. These features demonstrate the potential of thermochemical torrefaction in addressing the practical challenges in using biomass such as bagasse as fuel.
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1. Introduction Bioenergy from biomass combustion or its co-firing with coal represents an important renewable energy and it currently represents 10% of the global primary energy production. The decreasing reserves of fossil fuels and the resulting adverse effects of their use in terms of generation of greenhouse gases, acid rain and changes in climate conditions are driving the interest in utilisation of renewable energy such as biomass materials as fuel (Uris et al., 2014). The potential bioenergy resources worldwide are large and diverse. Unused biomass residues and wastes are significant under-exploited resources and these have the potential to assist in various government commitment globally to provide energy through renewable sources (2009/28/EC, 2009; Schmidt, 1998). Bagasse the fibrous by-product from sugar cane milling offers a cheap, plentiful and low polluting fuel that can have an important role in the renewable energy mix. In Australia approximately 11.4 million tons of wet bagasse and equivalent quantities of green waste are generated annually(Purchase et al., 2014). It is currently used in generating steam for the sugar mills and in cogeneration processes where it is used to supply both steam and electricity for the mill and the excess is sold to the grid (Lowry et al., 2001). Various technologies are available for converting bagasse to energy including thermochemical (involving pyrolysis, gasification and combustion) (Gigler et al., 1999; vandenBroek et al., 1996), biological (Limayem & Ricke, 2012; Luciano Silveira et al., 2015) and chemical (Climent et al., 2014) methods. But among these the key technologies that dominate energy production from bagasse are either direct combustion and/or co-firing of bagasse with coal (Akram et al., 2015; Barnes, 2015; Cardozo et al., 2014; Liao et al., 2015; Ndibe et al., 2015a; Ndibe et al., 2015b). The use of bagasse as fuel is challenged by its inherent properties including its residual sugar content and hydrophilic nature that makes it prone to chemical biodegradation and spontaneous combustion (Breccia et al., 1997; Macaskill et al., 2001). Bagasse can only be stored for a maximum of 10 weeks limiting its further use during the non-milling period. In addition, the high and variable moisture content of bagasse, which can be as high as 45-55%, can suppress its combustion efficiency (Macaskill et al., 2001; Ndibe et al., 2015b; Orang & Honghi, 2015). Whilst its low volumetric energy density, elongated particle shape, poor grinding properties and low particle density present transportation and handling difficulties (Orang & Honghi, 2015). Pre-treatment by torrefaction can transform bagasse to overcome these practical challenges (Bach & Skreiberg, 2016). Torrefaction is a mild pyrolytic process that converts biomass to chars thereby inducing characteristics such as lower moisture content, higher energy density and handling properties that are similar to that of coal (Liu et al., 2016). Current torrefaction can be achieved by dry and wet methods. Dry torrefaction involves heating of biomass at 200-300°C for periods from 2-3 hours typically under inert conditions (Deng et al., 2009; Liu et al., 2016; Madanayake et al., 2016). Although oxygen and carbon dioxide atmospheres, from waste flue gas, have also been used with nitrogen gas to achieve torrefaction (Eseltine et al., 2013). Wet torrefaction involves pre-treatment of biomass in hydrothermal media or hot compressed water at temperatures within 180-260°C (Bach & Skreiberg, 2016; Makela et al., 2016; Quang-Vu et al., 2015). Despite these benefits, the current torrefaction methods are not without their technical challenges. The bulk density 2
of dry torrefied biomass is still considerably lower compared to coal. Integration of wet torrefaction to downstream pyrolysis or gasification processes is difficult and the need to dispose liquid waste poses an additional challenge (Chen et al., 2015). To address these issues, this study considered an alternative wet torrefaction method involving chemical reagents. The combination of reagents such ZnCl2, H3PO4, H2SO4, CaCl2, and Ca(OH)2 in thermal pre-treatment of bagasse are known to increase the particle density of the biomass whilst leading to improved surface area (Syna & Valix, 2003; Valix et al., 2004; Valix et al., 2008). There appears to be very little known on the effect of chemical torrefaction to the fuel properties and long term stability of bagasse. The aims of this study were to examine and compare the stabilisation and upgrading of bagasse to higher energy dense products by chemical and dry torrefaction and their corresponding combustion characteristics. 2. Materials and methods 2.1 Bagasse material Sugar cane bagasse was obtained from Wilmar Sugar Mills (formerly CSR Sugar Mills) in Queensland, Australia. As bagasse is very heterogeneous, the sample was dried, ground and sieved. The bagasse particle size is from 1.0 to 5 mm. On the average, bagasse contained about 45-50% moisture, 43-52% fiber, and 2-6% soluble solids. 2.2 Torrefaction Thermal torrefaction tests were performed by heating raw and chemically pre-treated bagasse in a vycor reactor tube heated by a horizontal tube furnace. Dry torrefaction of the raw bagasse was carried out at temperatures from 160-300°C for two hours in the presence of N2 metered at 350 ml/min. Chemical pre-treatment was performed by mixing bagasse with 98% H2SO4 in a 0.75 (w/w) bagasse to acid ratio followed by similar thermal treatment to dry torrefaction. Chars generated were collected, washed with water and dried at 105°C for two hours. The char yield was estimated using the following equation:
ℎ = ( ) 100
(1)
The higher heating values (HHVmass) of bagasse and torrefied bagasse were calculated by Equation 2 (Sheng & Azevedo, 2005), where C, H, S, O are the mass percentages of these respective elements on a dry ash free basis.
%$!!" #$& = 0.3517 + 1.1626 ! + 0.1047 / − 0.111 1
(2)
To determine the amount of energy that is retained in the char as a result of torrefaction, the energy densification factor (DE) was multiplied by the char yield (see equation 1) to give the energy yield (YE): 6
23 = 4ℎ 53 = 4ℎ 6
(3)
and the densification factor (DE) was estimated using the following equation (Bach & Skreiberg, 2016): 3
7789:;<=>
53 = 778
(4)
9:?@A9=BB
where HHVm-char and HHVm-biomass are the heating values of the char and raw bagasse respectively. 2.3 Characterisation The bulk density of the bagasse samples was determined using ASTM Standard method E 873-82(ASTM, 1986a). Proximate analysis were performed according to ASTM method D 3172-73 through D3177-82(ASTM, 1986b). Ultimate analysis was performed by CHNS analyser and O was determined by difference. 2.4 Testing combustion characteristics The combustion characteristics of the bagasse fuels were performed in a TA Instrument Thermogravimetric Analyser (TGA 2950). Approximately 5.0 mg of samples were distributed in the sample pan in a single layer to minimise inter-particle diffusional effects and heat and mass transfer restrictions within the particles. Samples were heated from 30 to 1000°C with a heating rate of 20°C/min with air metered at 40ml/min. Tests were repeated in triplicates. Weight loss of the samples was recorded continuously using TA Instruments software and analysed by Microsoft Excel. 3. Results and discussion 3.1 Bagasse Fuel Properties The enhanced upgrading of bagasse achieved with thermochemical torrefaction over dry torrefaction is demonstrated by the results in Table 1. The density, char yield, proximate and ultimate compositions of raw bagasse, dry and chemically torrefied bagasse are compared with the properties of lignite, sub-bituminous, bituminous and anthracite coals in Table 1. As shown the method and temperature of torrefaction have significant effect on char yields. Over the temperatures of 160 to 300°C, dry torrefaction resulted in char yields of 55 to 95%. As shown the yield declined with increasing torrefaction temperature. While the char yields resulting from thermochemical torrefaction are lower but varied over a narrower range (45 to 57%). This shows the majority of the degradation by thermochemical torrefaction is achieved at the lower temperature (160°C) and very little additional degradation is achieved by further heating to higher temperatures. This acid induced low temperature degradation of bagasse has been proposed to result from the intercalation reactions of acid within the bagasse fibers (Valix et al., 2004). In this process acid was proposed to act as fillers that forces bagasse structures apart modifying the thermal degradation process of the major components of bagasse (lignin, cellulose, and hemicellulose). Therefore, thermochemical torrefaction is able to deliver a lower temperature of thermal upgrading of bagasse. Thermochemical torrefaction is also shown to induce significant densification of bagasse. The bulk density of bagasse samples varied between 95 and 100 kg/m3 and these are consistent with bagasse densities reported in literature (Bernhardt, 1999). Thermochemical torrefaction 4
increased the bulk density of bagasse by 335% to 340 kg/m3 whereas dry thermal torrefaction had little effect on bagasse density. The inability of dry torrefaction to induce densification in bagasse is also evident in other biomass (Chiou et al., 2016; Rodrigues & Rousset, 2009). Thus briquetting and pressing of the torrefied biomass is often necessary to achieve the desired fuel density (Araujo et al., 2016; Hu et al., 2016). The merits of thermochemical torrefaction is also evident in its effect on the proximate and ultimate properties of the chars. Biomass have higher volatile matter (VM), in particular combustible volatile matter and lower fixed carbon to volatile matter ratio in comparison to coal. Thus biomass have lower ignition temperature, rapid ignition and faster combustion (Vassilev et al., 2015). This higher reactivity poses storage issues and instability during combustion. Lower ignition and its higher reactivity gives bagasse a greater propensity for spontaneous combustion, which becomes an issue in storing bagasse (Breccia et al., 1997; Macaskill et al., 2001). In a practical sense, the combustion of biomass with high volatile matter is also more difficult to control as it creates segregated combustion when co-fired with coal. Thus biomass fuels with VM/FC ratios closer to the ratios of coal are considered more beneficial. Table 1 shows torrefaction reduced the volatile matter (VM) contents of the chars consequently resulting in a lower volatile matter to fixed carbon (VM/FC) ratio. Bagasse has volatile matter of 83.1% and VM/FC ratio of 6.5. Dry torrefaction reduced the volatile matter by 4-32% at 200-300°C, while very little change was observed at 160 °C. This resulted in chars with VM/FC ratios of 1.7- 6.7. While thermochemical torrefaction induced 37 to 50% reduction in volatile matter resulting in chars with VM/FC ratios of 1.1-1.2. Although these VM/FC ratios are still incomparable compared to VM/FC ratios of the various coal ranks of 0.12 - 1.3 as reported in Table 1 (Ahn et al., 2014; Idris et al., 2012; Toptas et al., 2015; Wang et al., 2014), nonetheless the thermochemical method bring the bagasse fuel closer to the coal properties than the dry torrefaction method. The O/C and H/C ratios of biomass fuel can have a significant impact on its calorific value and stability. Fig. 1 compared the O/C and H/C ratios of the bagasse fuels to coals using the absolute elemental values reported in Table 1. Raw bagasse has O/C and H/C ratios of 0.8 and 1.2 respectively. While the various ranks of coal exhibited O/C and H/C ratios of 0.4−0.08 and 1.02−0.078. Fig. 1 shows that increasing the temperature of torrefaction, by dry and chemical method, resulted in lowering O/C ratios, which is consistent with greater charring of the chars (Enders et al., 2012; Spokas, 2010). While the corresponding reduction in H/C ratio as a result of torrefaction reflects the formation of greater aromaticity in the chars (Kuhlbusch, 1995). Dry torrefaction reduced O/C ratio down to 0.46−0.74 and H/C to 0.461.12, whilst thermochemical torrefaction reduced the O/C ratio 0.34-0.49 and H/C to 0.3-0.74. These results show that thermochemical torrefaction is able to achieve greater charring and aromaticity and is consistent with their greater densification. The greater shift in the elemental ratios resulting from thermochemical torrefaction indicates the improvements in the fuel properties of bagasse and that its properties are closer to coal. The S contents of the thermochemically torrefied bagasse are from 0.2-0.9 wt.%. These are comparable or lower 5
compared to the S contents of the various coal ranks and these also reflects the efficiency of washing off the residual acids from the char. The corresponding effect of chemical changes induced by torrefaction on the heating values, energy density (DE) and energy yield (YE) of bagasse fuel are compared with the reported heating values of various coal ranks in Table 2. The higher heating values of raw, thermally and thermochemically torrefied bagasse in Table 2 show that torrefaction increased the energy density of the chars. The energy density (HHVmass) of raw bagasse is 15.9 MJ/kg on a dry basis. Dry torrefaction increased the energy density with torrefaction temperature raising HHVmass by up to 27% to 20.2 MJ/kg at 300°C. Thermochemical torrefaction also increased the HHVmass of the char with temperature raising the energy density by 39% to 22.1 MJ/kg at 250°C, but was reduced at 300°C. The rise in heating values is consistent with the observed loss of volatile matter and lowering of O/C and H/C ratios with torrefaction (see Table 1 and Fig. 1). The higher increase in the calorific values of the thermochemically torrefied bagasse could be attributed to the catalysed decomposition of lower energy density components of bagasse; hemicelluloses (13.6 MJ/kg) and cellulose (18.6 MJ/kg)(Sheng & Azevedo, 2005). Thermochemical torrefaction is able to volatilise these lower calorific components leaving behind the higher calorific lignin fractions (27 MJ/Kg). This is consistent with the higher fixed carbon (FC) and lower volatile matter (VM) contents of the thermochemically torrefied bagasse. However at higher temperatures, above 250°C, it appears further decomposition of the FC occurs resulting in loss of the calorific value of the fuel. The greater loss of the lower calorific components must be taken into account in the evaluation of the torrefaction process by incorporating the mass yield. The energy yield (YE) for dry torrefied chars are from 70-97.5%, whilst the energy yields of thermochemically torrefied chars are lower at 53 to 74%. The benefit of torrefaction, however, on upgrading the fuel of bagasse is more evident when the energy yields are compared with the char yield. Tables 1 and 2 shows that all torrefied bagasse have energy densities that were either equal or greater than the char yields, however the enhancement achieved by thermochemical torrefaction is greater. For dry torrefied char, the ratio of the energy yield to char yield is 1.0 -1.26, while thermochemically torrefied chars resulted in ratios of 1.3-1.4. These higher ratios suggest the enhancement in the energy density of the chars achieved with thermochemical torrefaction more than outweighs the mass loss resulting from the thermal process. The volume based heating values (HHVvolume) determined by multiplying HHVmass with the bulk density of the chars, further reflects the benefits achieved by thermochemical torrefaction. The impact of this greater densification is shown by the increase in HHVvolume of the chars. The HHVvolume of raw bagasse is 1.59 GJ/m3. The HHVvolume was raised by up to 25% thermally to 1.98 GJ/m3. Whilst thermochemical torrefaction raised the energy density by 365% at 250°C to 7.39 GJ/m3. Given that bagasse will need to be transported to boilers where they are used as fuel, the greater energy density offered by chemical conversion, and as reflected by the higher HHVvolume, has a significant potential in overcoming the greater economic cost of transporting the low energy density raw bagasse feedstock. 6
3.2 Stability Characteristics of Torrefied Bagasse A number of methods for comparing the stability of chars exposed to various environment have emerged (Antal & Gronli, 2003; Crombie et al., 2013; Enders et al., 2012; Kuhlbusch, 1995; Spokas, 2010). In this study, the stability of the thermochemically torrefied bagasse (160°C) was tested by storing the chars in temperature controlled chamber at 30°C in air for 6 to 18 months. The extent of degradation during storage was monitored from the proximate O/C and H/C molar ratio analyses of the chars (see Fig. 2). Proximate analysis has been used to assess the quality of coals. As shown in Fig 2a there is relatively little difference in the proximate analysis of the chars over the 18 months storage. There is only a slight decline in the higher heating values of the chars over time. The HHVmass of the chars declined from its initial value of 20.6 MJ/kg to 18.6 MJ/kg in 6 months and 18.1 MJ/kg by 18 months. Approximately 12% of the calorific value was lost in 18 months. Fig 2b shows the loss in calorific values is attributed to the rise in the O content and reduction in the H content of the chars. The rise in the O/C ratio infers the systematic oxidation of the char with storage (Enders et al., 2012; Spokas, 2010). The O/C ratio rose from 0.48 to 0.52 in 6 months and was relatively stable thereafter. Spokas (2010) suggested that chars characterised by O/C molar ratio >0.6 would have properties closer to the biomass and would have a residence time of 100 years. Whilst lower O/C ratio of 0.2−0.6 would be expected to have a residence time of 100-1000 years. The lowering of the H/C content of the chars suggests the rise in aromaticity and thus it would have greater resistance to biological degradation (Kuhlbusch, 1995). Although the chars are unlikely to be stored for more than a year, these results demonstrate that thermochemically torrefied bagasse have sufficient stability to be stored during the non-milling period. 3.3 Combustion Characteristics of Bagasse Fuels Fig. 3 shows the TG and DTG curves obtained from the thermogravimetric analysis of bagasse at a heating rate of 20 oC/minute. As shown the combustion process is characterised by three stages. The first stage corresponded to drying of the chars up to the temperature of 180 oC. The second stage at 180– 450 oC corresponded to combustion of pyrolytic volatiles and the last stage is attributed to the combustion of the fixed carbon (Moon et al., 2013). As shown, the char oxidation appears as a minor thermal event shown as a shoulder on the oxidative pyrolysis peak. The reactivity of the fuel under the oxidation reaction was characterised by the ignition temperature (Ti), peak temperature (Tm) and the maximum rate of reaction (DTGmax) at the given thermal event and burnout temperature (Tb). The ignition temperature was taken at the point when the rate of weight loss first becomes 1.0 wt.%/minute directly after the horizontal period of moisture loss (Cumming & McLaughlin, 1982). The burnout temperature, which represented the temperature where sample oxidation was completed, was taken as the point immediately before reaction ceased, when the rate of weight loss was 1.0 wt. %/min. The reactivity of a fuel is directly proportional to its maximum weight loss velocity (DTGmax) and inversely proportional to Tm (Ghetti et al., 7
1996). Both parameters were used to give an average value of the fuel reactivity (Rm)(Zheng & Kozinski, 2000): C =
DEF9=G E9
100
(5)
These indicators were used in comparing the reactivity behaviour of the bagasse fuels in the present work. Fig. 4 shows the combustion characteristics of dry thermally torrefied bagasse in comparison to raw bagasse. TG is on the left and DTG on the right. Fig. 5 shows the combustion behavior of thermochemically torrefied bagasse. The corresponding reactivity indicators are summarised in Table 3. Most biomass like bagasse typically consists of hemicellulose, cellulose and lignin. The reactivity of these components during torrefaction determines the final composition of the char. The amorphous structure and low degree of polymerization of the hemicellulose makes is highly susceptible to thermal degradation, whilst the greater crystallinity and aromaticity of cellulose and lignin makes them more thermally stable (Bach & Skreiberg, 2016). Typically hemicellulose has a characteristic DTG peak at 300°C and forms a shoulder to the degradation of cellulose around 280- 380 °C (Liu et al., 2016). Whilst lignin decomposes more slowly and over a wider temperature range of 200-500°C (Brebu & Vasile, 2010). The DTG of raw bagasse in Fig 4 shows the typical degradation of hemicellulose and cellulose with a peak temperature of 335 °C. The lack of change in the DTG peak of bagasse thermally torrefied at 160°C suggest this temperature was insufficient to thermally degrade the bagasse constituents. This is also consistent with lack of change in the volatile matter of the resulting char at160°C (see Table 1). Higher torrefaction temperatures resulted in greater degradation. This is reflected in the reduction of the first DTG peak attributed to hemicellulose and cellulose combustion, and the rise in the second peak at 417438°C reflecting the combustion of lignin. The effect of thermochemical torrefaction, as shown in Fig 5, was more pronounced. The cellulose and hemicellulose peaks in bagasse torrefied at 160 and 200 °C are absent and only the second peak at 428°C was present suggesting only the lignin survived the process. Further rise in torrefaction temperature above 200°C led to shifts in the lignin peak to lower temperatures (365-378°C) inferring further degradation of lignin. These results are consistent with the lowering of HHV values resulting from thermochemical torrefaction of bagasse at 300°C (see Table 2). These results suggest that thermochemical torrefaction at temperatures of 160 -200°C offer optimal energy densification of bagasse. Table 3 shows torrefaction is able to increase char ignition temperatures (Ti). Dry torrefaction increased Ti from 245°C to 246-281°C when torrefied at 160-300°C. While thermochemical torrefaction increased Ti to 265-281°C. The rise in Ti is consistent with the observed reduction in volatile matter with torrefaction (see Table 1). These slightly higher ignition temperatures may partially reduce the risk of self-ignition of the chars providing safer storage. 8
Torrefaction reduced the rate of combustion of pyrolytic products and shifted the peak and burnout to higher temperatures. Increasing the temperature of torrefaction either thermally or thermochemically resulted in lower reactivity. The reduction in reactivity as a result of thermochemical torrefaction, however, is greater and resulted in reactivities closer to that of coal of 6.2-6.9%/min. The DTGmax of dry torrefied bagasse under oxidative pyrolysis were 107-145%/min, whilst thermochemically torrefied bagasse showed rates of 40-75%/min. These are consistent with the characteristic lower volatile matter and higher fixed carbon with torrefaction. Torrefaction also increased the burnout temperature. Fig. 6 compares the temperature range (Ti and Tb) when combustion occurs. The combustion region for bagasse, 245-362°C, does not coincide with the combustion regions for any of the coal ranks. The coals have Ti values of 359-611°C and Tb values of 601-714°C. This demonstrates one of the key challenges in cofiring biomass with coal. The higher reactivity of biomass results in segregated combustion where the more reactive biomass burns first and in a shorter period (Haykiri-Acma et al., 2015; Park et al., 2012). Segregated volatile matter emission and combustion along the combustor introduces instability that are of concern in regards to the safe and efficient operation of industrial scale reactors. Torrefaction was able to overcome this issue to different extents. The burnout temperatures of dry torrefied bagasse were shifted to 363-485°C while the burnout temperatures of thermochemically torrefied bagasse were shifted to higher temperatures of 419-535°C. Chars dry torrefied above 200°C and all the thermochemically torrefied bagasse have burnout temperatures that exceeded the ignition temperatures of lignite to bituminous coals of 359-411°C. This commonality in combustion profiles of these chars with the various coal ranks allows them to achieve greater combustion stability and attain synergy when used in co-firing with coal (Liu et al., 2016). 4. Conclusions Thermochemical torrefaction is able to address many of the practical challenges in using bagasse as fuel. It induced greater densification, higher HHVmass, HHVvolume, greater charring and aromatisation resulting in char storage stability. It shifted the oxidative pyrolysis of the chars to temperatures allowing the combustion of the chars to coincide with that of various ranks of coal. The commonality of the combustion profiles will permit the co-firing of thermochemically torrefied bagasse and coal to achieve greater combustion stability and interaction between the biomass and coal fuels. Thermochemical torrefaction at 160°C offered the best method for stabilising and upgrading bagasse.
Acknowledgements This research was supported by Sugar Research and Development Corporation and Material and Mineral Processing Research Unit.
References
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22. Ghetti, P., Ricca, L., Angelini, L. 1996. Thermal analysis of biomass and corresponding pyrolysis products. Fuel, 75(5), 565-573. 23. Gigler, J.K., Meerdink, G., Hendrix, E.M.T. 1999. Willow supply strategies to energy plants. Biomass & Bioenergy, 17(3), 185-198. 24. Haykiri-Acma, H., Yaman, S., Kucukbayrak, S. 2015. Does carbonization avoid segregation of biomass and lignite during co-firing? Thermal analysis study. Fuel Processing Technology, 137, 312-319. 25. Hu, Q., Yang, H.P., Yao, D.D., Zhu, D.C., Wang, X.H., Shao, J.G., Chen, H.P. 2016. The densification of bio-char: Effect of pyrolysis temperature on the qualities of pellets. Bioresource Technology, 200, 521-527. 26. Idris, S.S., Rahman, N.A., Ismail, K. 2012. Combustion characteristics of Malaysian oil palm biomass, sub-bituminous coal and their respective blends via thermogravimetric analysis (TGA). Bioresource Technology, 123, 581-591. 27. Kuhlbusch, T.A.J. 1995. Method for determining black carbon in residues of vegetation fires. Environmental Science & Technology, 29(10), 2695-2702. 28. Liao, Y., Cao, Y., Chen, T., Ma, X. 2015. Experiment and simulation study on alkalis transfer characteristic during direct combustion utilization of bagasse. Bioresource Technology, 194, 196-204. 29. Limayem, A., Ricke, S.C. 2012. Lignocellulosic biomass for bioethanol production: Current perspectives, potential issues and future prospects. Progress in Energy and Combustion Science, 38(4), 449-467. 30. Liu, Z., Hu, W., Jiang, Z., Mi, B., Fei, B. 2016. Investigating combustion behaviors of bamboo, torrefied bamboo, coal and their respective blends by thermogravimetric analysis. Renewable Energy, 87, 346-352. 31. Lowry, G., Compagnoni, F., Keevers, P., Cranfield, P. 2001. A modern cogeneration power plant in an established sugar mill. 32. Luciano Silveira, M.H., Morais, A.R.C., da Costa Lopes, A.M., Olekszyszen, D.N., Bogel-Lukasik, R., Andreaus, J., Ramos, L.P. 2015. Current Pretreatment Technologies for the Development of Cellulosic Ethanol and Biorefineries. Chemsuschem, 8(20), 3366-3390. 33. Macaskill, C., Sexton, M.J., Gray, B.F. 2001. A reaction-diffusion model of stored bagasse. Anziam Journal, 43, 13-34. 34. Madanayake, B.N., Gan, S., Eastwick, C., Ng, H.K. 2016. Thermochemical and structural changes in Jatropha curcas seed cake during torrefaction for its use as coal co-firing feedstock. Energy, 100, 262-272. 35. Makela, M., Benavente, V., Fullana, A. 2016. Hydrothermal carbonization of industrial mixed sludge from a pulp and paper mill. Bioresource Technology, 200, 444-450. 36. Moon, C., Sung, Y., Ahn, S., Kim, T., Choi, G., Kim, D. 2013. Thermochemical and combustion behaviors of coals of different ranks and their blends for pulverized-coal combustion. Applied Thermal Engineering, 54(1), 111-119. 37. Ndibe, C., Grathwohl, S., Paneru, M., Maier, J., Scheffknecht, G. 2015a. Emissions reduction and deposits characteristics during cofiring of high shares of torrefied biomass in a 500 kW pulverized coal furnace. Fuel, 156, 177-189. 38. Ndibe, C., Maier, J., Scheffknecht, G. 2015b. Combustion, cofiring and emissions characteristics of torrefied biomass in a drop tube reactor. Biomass & Bioenergy, 79, 105-115. 39. Orang, N., Honghi, T. 2015. Effect of feedstock moisture content on biomass boiler operation. Tappi Journal, 14(10), 629-637. 11
40. Park, S.-W., Jang, C.-H., Baek, K.-R., Yang, J.-K. 2012. Torrefaction and lowtemperature carbonization of woody biomass: Evaluation of fuel characteristics of the products. Energy, 45(1), 676-685. 41. Purchase, B.S., Rosettenstein, S., Bezuidenhoudt, D.V. 2014. Challenges and potential solutions for storage of large quantities of bagasse for power generation. International Sugar Journal, 116(1388), 592-602. 42. Quang-Vu, B., Khanh-Quang, T., Skreiberg, O. 2015. Accelerating wet torrefaction rate and ash removal by carbon dioxide addition. Fuel Processing Technology, 140, 297-303. 43. Rodrigues, T.O., Rousset, P.L.A. 2009. Effects of torrefaction on energy properties of eucalyptus grandis wood. Cerne, 15(4), 446-452. 44. Schmidt, M. 1998. Innovative cements quick-setting cement, spray cement, ultrafine cement, and cements with high resistance to sulphates or acids. Zkg International, 51(8), 444-+. 45. Sheng, C.D., Azevedo, J.L.T. 2005. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass & Bioenergy, 28(5), 499-507. 46. Spokas, K.A. 2010. Review of the stability of biochar in soils: predictability of O:C molar ratios. Carbon Management, 1(2), 289-303. 47. Syna, N., Valix, M. 2003. Assessing the potential of activated bagasse as gold adsorbent for gold-thiourea. Minerals Engineering, 16(6), 511-518. 48. Toptas, A., Yildirim, Y., Duman, G., Yanik, J. 2015. Combustion behavior of different kinds of torrefied biomass and their blends with lignite. Bioresource Technology, 177, 328-336. 49. Uris, M., Ignacio Linares, J., Arenas, E. 2014. Techno-economic feasibility assessment of a biomass cogeneration plant based on an Organic Rankine Cycle. Renewable Energy, 66, 707-713. 50. Valix, M., Cheung, W.H., McKay, G. 2004. Preparation of activated carbon using low temperature carbonisation and physical activation of high ash raw bagasse for acid dye adsorption. Chemosphere, 56(5), 493-501. 51. Valix, M., Cheung, W.H., Zhang, K. 2008. Role of chemical pre-treatment in the development of super-high surface areas and heteroatom fixation in activated carbons prepared from bagasse. Microporous and Mesoporous Materials, 116(1-3), 513-523. 52. vandenBroek, R., Faaij, A., vanWijk, A. 1996. Biomass combustion for power generation. Biomass & Bioenergy, 11(4), 271-281. 53. Vassilev, S.V., Vassileva, C.G., Vassilev, V.S. 2015. Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview. Fuel, 158, 330-350. 54. Wang, S.J., Wu, F., Zhang, G., Zhu, P., Wang, Z.Y., Huang, C.J., Chen, S.T. 2014. Research on the combustion characteristics of anthracite and blended coal with composite catalysts. Journal of the Energy Institute, 87(2), 96-101. 55. Zheng, G., Kozinski, J.A. 2000. Thermal events occurring during the combustion of biomass residue. Fuel, 79(2), 181-192.
12
List of Figures
Fig. 1. Elemental O/C and H/C molar ratios of bagasse and coal fuels. Fig. 2. Effect of storage period on the ultimate and proximate analyses of bagasse thermochemically torrefied at 160°C. Fig. 3. Combustion characteristic of bagasse thermochemically torrefied at 160°C. Fig. 4. Combustion characteristics of dry bagasse and dry-torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG). Fig. 5 Combustion characteristics of dry bagasse and thermochemically torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG). Fig. 6. Comparison of ignition and burnout temperatures of fuels.
13
Elemental ratios (mole/mole)
1.6 1.4 1.2
Dry Chemically treated
Coal
1 O/C
0.8
H/C
0.6 0.4 0.2 0
Fuels
Fig. 1. Elemental O/C and H/C molar ratios of bagasse and coal fuels.
14
elemental molar ratios (mole/mole)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
(b)
O/C H/C
0
6 Time (months)
18
Fig. 2. Effect of storage period on the ultimate and proximate analyses of bagasse thermochemically torrefied at 160°C.
15
Fig. 3. Combustion characteristic of bagasse thermochemically torrefied at 160°C.
16
Fig. 4. Combustion characteristics of dry bagasse and dry-torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG).
17
Fig. 5 Combustion characteristics of dry bagasse and thermochemically torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG).
18
Fuels
Anthracite Bituminous coal Sub-bituminous coal Lignite Chemical 300°C Chemical 250°C Chemical 200°C Chemical 160°C Dry 300°C Dry 250°C Dry 200°C Dry 160°C Bagasse
Ti Tb
0
200 400 600 Temperature (°C)
800
Fig. 6. Comparison of ignition and burnout temperatures of fuels.
19
Table 1 Proximate and ultimate compositions of raw, dry and thermochemically torrefied bagasse
Fuel
T (o C)
Char Yiel d (w/w %)
Bagass e Dry torrefie d
Proximate Analysis Den (%, dry sity basis) kg/ 3 m V A F M sh C 100 83 4. 12 .1 2 .7
16 0
94
102
85 .1
2. 3
12 .6
20 0
90
100
79 .8
4. 3
16 .0
25 0
75
100
62 .9
27 .9
30 0
55
91
56 .4
Thermo 16 0 chemic ally 20 torrefie 0 d 25 0
57
290
52 .5
1 0. 0 1 0. 4 4. 4
53
335
47 .0
9. 0
44 .0
49
335
48 .5
38 .6
30 0
45
340
41 .9
1 3. 0 1 9. 9 1 1. 5 3. 4 5
53 .5
Lignite
49 .9
Subbitumin ous
42 .6
20
33 .2 43 .1
38 .2 38 .6
Ultimate Analysis (%, dry ash free basis) C
H N S
O
4 5. 5 4 6. 8 4 9. 3 6 0. 4 5 9. 4 5 8. 0 6 1. 6 6 6. 5 5 8. 8 -
4 . 6 4 . 4 3 . 1 2 . 3 2 . 9 3 . 6 2 . 7 1 . 7 2 . 0 -
0 . 4 0 . 2 0 . 3 0 . 4 0 . 8 0 . 4 0 . 2 0 . 5 0 . 7 -
0 . 1 0 . 0 0 . 3 0 . 1 0 . 0 0 . 7 0 . 9 0 . 6 0 . 2 1 . 1
49 .5
6 0. 0
5 . 1
1 . 9
0 32 . .6 3
References
46 .0
This work
47 .1
This work
36 .8
This work
37 .0
This work
37 .2
This work
34 .6
This work
30 .7
This work
38 .3
This work
-
(Toptas et al., 2015) (Idris et al., 2012)
Bitumi nous
34 .4
7. 7
57 .9
Anthrac ite
8. 4
2 3. 4
68 .1
21
7 5. 9 8 8. 9 8
4 . 8 0 . 5 8
1 . 4 0 . 4 5
0 10 . .2 0 0 9. . 38 6 1
(Ahn et al., 2014) (Wang et al., 2014)
Table 2 Fuel properties of raw, dry and thermochemically torrefied bagasse
Fuel
Bagass e Dry torrefie d
Thermo chemic ally torrefie d
* Estimated equation (1) ultimate
T (o C)
16 0 20 0 25 0 30 0 16 0 20 0 25 0 30 0
HHV
HHVVol
mass ume
(MJ/ kg)
GJ/m3
15.9
1.59
16.5
1.68
15.7
1.57
19.8
1.98
20.2
1.84
20.6
5.97
21.5
7.20
22.1
7.39
18.8
6.38
DE YE ( ( % % ) )
References
This work 1. 0 1. 0 1. 2 1. 3 1. 3 1. 4 1. 4 1. 2
97 .5 88 .9 93 .6 69 .8 73 .9 71 .7 68 .1 53 .2
This work This work This work This work This work This work This work This work
Lignite
25.1
(Toptas et al., 2015) (Idris et al., 2012)
Subbitumin ous Bitumi nous
23.6 * 31.3 *
(Ahn et al., 2014)
Anthrac ite
31.0 *
(Wang et al., 2014)
22
using using the fuel analysis
Table 3 Combustion characteristics of bagasse, dry and thermochemically torrefied bagasse and various coal rank.
T (°C)
Ti (°C)
160 200 250 300 160 200 250 300 -
245 246 247 265 281 275 281 265 273 359
Oxidation Pyrolysis Tm DTGmax (°C) (%/min) 335 145 335 145 425 75.6 417 128 438 107 428 40 428 42 365 75 378 58 484 1.3
Subbituminous Subbituminous Bituminous
-
398
471
6.9
700
-
343
426
-
601
-
-
402
510
-
654
-
Anthracite
-
611
667
6.2
714
Fuel Bagasse Dry
Thermochemical
Lignite
Tb (°C)
Rm (%/min.K )
Residue (wt.%)
References
362 363 428 479 485 506 535 419 486 644
23.8 23.8 10.8 18.6 15.0 5.7 6.0 11.8 8.9 0.2
2.2 4.2 1.7 10.7 12.9 5 9 13 20 -
0.9
-
This work This work This work This work This work This work This work This work This work (Toptas et al., 2015) (Idris et al., 2012) (Ahn et al., 2014) (Ahn et al., 2014) (Wang et al., 2014)
0.7
-
T is the torrefaction temperature, Ti is the ignition temperature, Tm is the peak temperature at this regime, DTGmax is the peak rate at this regime, Tb is the burnout temperature.
23
Highlights Chemical based thermochemical torrefaction (TT) offers new method to upgrade bagasse TT increased the bulk density of bagasse by 335% to 340 kg/m3 TT achieved greater charring, aromatisation and storage stability TT increased HHVmass by 27% to 22.1 MJ/kg and HHVvolume by 335% to 7.39 GJ/m3. TT bagasse exhibited combustion features that are in common with various ranks of coal.
24
S0960-8524(16)31459-6 http://dx.doi.org/10.1016/j.biortech.2016.10.053 BITE 17208
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
16 September 2016 18 October 2016 19 October 2016
Please cite this article as: Valix, M., Katyal, S., Cheung, W.H., Combustion of Thermochemically Torrefied Sugar Cane Bagasse, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/j.biortech.2016.10.053
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COMBUSTION OF THERMOCHEMICALLY TORREFIED SUGAR CANE BAGASSE M. Valix∗, S. Katyal and W.H. Cheung The University of Sydney, Sydney, NSW 2006, Australia *Corresponding author. Tel: + 61 2 9351 4995, Fax: + 61 2 9351 2854, E-mail: [email protected]
Keywords: bagasse, dry torrefaction, thermochemical torrefaction, biological stability, combustion
Abstract This study compared the upgrading of sugar bagasse by thermochemical and dry torrefaction methods and their corresponding combustion behavior relative to raw bagasse. The combustion reactivities were examined by non-isothermal thermogravimetric analysis. Thermochemical torrefaction was carried out by chemical pre-treatment of bagasse with acid followed by heating at 160-300°C in nitrogen environment, while dry torrefaction followed the same heating treatment without the chemical pretreatment. The results showed thermochemical torrefaction generated chars with combustion properties that are closer to various ranks of coal, thus making it more suitable for co-firing applications. Thermochemical torrefaction also induced greater densification of bagasse with a 335% rise in bulk density to 340 kg/m3, increased HHVmass and HHVvolume, greater charring and aromatization and storage stability. These features demonstrate the potential of thermochemical torrefaction in addressing the practical challenges in using biomass such as bagasse as fuel.
1
1. Introduction Bioenergy from biomass combustion or its co-firing with coal represents an important renewable energy and it currently represents 10% of the global primary energy production. The decreasing reserves of fossil fuels and the resulting adverse effects of their use in terms of generation of greenhouse gases, acid rain and changes in climate conditions are driving the interest in utilisation of renewable energy such as biomass materials as fuel (Uris et al., 2014). The potential bioenergy resources worldwide are large and diverse. Unused biomass residues and wastes are significant under-exploited resources and these have the potential to assist in various government commitment globally to provide energy through renewable sources (2009/28/EC, 2009; Schmidt, 1998). Bagasse the fibrous by-product from sugar cane milling offers a cheap, plentiful and low polluting fuel that can have an important role in the renewable energy mix. In Australia approximately 11.4 million tons of wet bagasse and equivalent quantities of green waste are generated annually(Purchase et al., 2014). It is currently used in generating steam for the sugar mills and in cogeneration processes where it is used to supply both steam and electricity for the mill and the excess is sold to the grid (Lowry et al., 2001). Various technologies are available for converting bagasse to energy including thermochemical (involving pyrolysis, gasification and combustion) (Gigler et al., 1999; vandenBroek et al., 1996), biological (Limayem & Ricke, 2012; Luciano Silveira et al., 2015) and chemical (Climent et al., 2014) methods. But among these the key technologies that dominate energy production from bagasse are either direct combustion and/or co-firing of bagasse with coal (Akram et al., 2015; Barnes, 2015; Cardozo et al., 2014; Liao et al., 2015; Ndibe et al., 2015a; Ndibe et al., 2015b). The use of bagasse as fuel is challenged by its inherent properties including its residual sugar content and hydrophilic nature that makes it prone to chemical biodegradation and spontaneous combustion (Breccia et al., 1997; Macaskill et al., 2001). Bagasse can only be stored for a maximum of 10 weeks limiting its further use during the non-milling period. In addition, the high and variable moisture content of bagasse, which can be as high as 45-55%, can suppress its combustion efficiency (Macaskill et al., 2001; Ndibe et al., 2015b; Orang & Honghi, 2015). Whilst its low volumetric energy density, elongated particle shape, poor grinding properties and low particle density present transportation and handling difficulties (Orang & Honghi, 2015). Pre-treatment by torrefaction can transform bagasse to overcome these practical challenges (Bach & Skreiberg, 2016). Torrefaction is a mild pyrolytic process that converts biomass to chars thereby inducing characteristics such as lower moisture content, higher energy density and handling properties that are similar to that of coal (Liu et al., 2016). Current torrefaction can be achieved by dry and wet methods. Dry torrefaction involves heating of biomass at 200-300°C for periods from 2-3 hours typically under inert conditions (Deng et al., 2009; Liu et al., 2016; Madanayake et al., 2016). Although oxygen and carbon dioxide atmospheres, from waste flue gas, have also been used with nitrogen gas to achieve torrefaction (Eseltine et al., 2013). Wet torrefaction involves pre-treatment of biomass in hydrothermal media or hot compressed water at temperatures within 180-260°C (Bach & Skreiberg, 2016; Makela et al., 2016; Quang-Vu et al., 2015). Despite these benefits, the current torrefaction methods are not without their technical challenges. The bulk density 2
of dry torrefied biomass is still considerably lower compared to coal. Integration of wet torrefaction to downstream pyrolysis or gasification processes is difficult and the need to dispose liquid waste poses an additional challenge (Chen et al., 2015). To address these issues, this study considered an alternative wet torrefaction method involving chemical reagents. The combination of reagents such ZnCl2, H3PO4, H2SO4, CaCl2, and Ca(OH)2 in thermal pre-treatment of bagasse are known to increase the particle density of the biomass whilst leading to improved surface area (Syna & Valix, 2003; Valix et al., 2004; Valix et al., 2008). There appears to be very little known on the effect of chemical torrefaction to the fuel properties and long term stability of bagasse. The aims of this study were to examine and compare the stabilisation and upgrading of bagasse to higher energy dense products by chemical and dry torrefaction and their corresponding combustion characteristics. 2. Materials and methods 2.1 Bagasse material Sugar cane bagasse was obtained from Wilmar Sugar Mills (formerly CSR Sugar Mills) in Queensland, Australia. As bagasse is very heterogeneous, the sample was dried, ground and sieved. The bagasse particle size is from 1.0 to 5 mm. On the average, bagasse contained about 45-50% moisture, 43-52% fiber, and 2-6% soluble solids. 2.2 Torrefaction Thermal torrefaction tests were performed by heating raw and chemically pre-treated bagasse in a vycor reactor tube heated by a horizontal tube furnace. Dry torrefaction of the raw bagasse was carried out at temperatures from 160-300°C for two hours in the presence of N2 metered at 350 ml/min. Chemical pre-treatment was performed by mixing bagasse with 98% H2SO4 in a 0.75 (w/w) bagasse to acid ratio followed by similar thermal treatment to dry torrefaction. Chars generated were collected, washed with water and dried at 105°C for two hours. The char yield was estimated using the following equation:
ℎ = ( ) 100
(1)
The higher heating values (HHVmass) of bagasse and torrefied bagasse were calculated by Equation 2 (Sheng & Azevedo, 2005), where C, H, S, O are the mass percentages of these respective elements on a dry ash free basis.
%$!!" #$& = 0.3517 + 1.1626 ! + 0.1047 / − 0.111 1
(2)
To determine the amount of energy that is retained in the char as a result of torrefaction, the energy densification factor (DE) was multiplied by the char yield (see equation 1) to give the energy yield (YE): 6
23 = 4ℎ 53 = 4ℎ 6
(3)
and the densification factor (DE) was estimated using the following equation (Bach & Skreiberg, 2016): 3
7789:;<=>
53 = 778
(4)
9:?@A9=BB
where HHVm-char and HHVm-biomass are the heating values of the char and raw bagasse respectively. 2.3 Characterisation The bulk density of the bagasse samples was determined using ASTM Standard method E 873-82(ASTM, 1986a). Proximate analysis were performed according to ASTM method D 3172-73 through D3177-82(ASTM, 1986b). Ultimate analysis was performed by CHNS analyser and O was determined by difference. 2.4 Testing combustion characteristics The combustion characteristics of the bagasse fuels were performed in a TA Instrument Thermogravimetric Analyser (TGA 2950). Approximately 5.0 mg of samples were distributed in the sample pan in a single layer to minimise inter-particle diffusional effects and heat and mass transfer restrictions within the particles. Samples were heated from 30 to 1000°C with a heating rate of 20°C/min with air metered at 40ml/min. Tests were repeated in triplicates. Weight loss of the samples was recorded continuously using TA Instruments software and analysed by Microsoft Excel. 3. Results and discussion 3.1 Bagasse Fuel Properties The enhanced upgrading of bagasse achieved with thermochemical torrefaction over dry torrefaction is demonstrated by the results in Table 1. The density, char yield, proximate and ultimate compositions of raw bagasse, dry and chemically torrefied bagasse are compared with the properties of lignite, sub-bituminous, bituminous and anthracite coals in Table 1. As shown the method and temperature of torrefaction have significant effect on char yields. Over the temperatures of 160 to 300°C, dry torrefaction resulted in char yields of 55 to 95%. As shown the yield declined with increasing torrefaction temperature. While the char yields resulting from thermochemical torrefaction are lower but varied over a narrower range (45 to 57%). This shows the majority of the degradation by thermochemical torrefaction is achieved at the lower temperature (160°C) and very little additional degradation is achieved by further heating to higher temperatures. This acid induced low temperature degradation of bagasse has been proposed to result from the intercalation reactions of acid within the bagasse fibers (Valix et al., 2004). In this process acid was proposed to act as fillers that forces bagasse structures apart modifying the thermal degradation process of the major components of bagasse (lignin, cellulose, and hemicellulose). Therefore, thermochemical torrefaction is able to deliver a lower temperature of thermal upgrading of bagasse. Thermochemical torrefaction is also shown to induce significant densification of bagasse. The bulk density of bagasse samples varied between 95 and 100 kg/m3 and these are consistent with bagasse densities reported in literature (Bernhardt, 1999). Thermochemical torrefaction 4
increased the bulk density of bagasse by 335% to 340 kg/m3 whereas dry thermal torrefaction had little effect on bagasse density. The inability of dry torrefaction to induce densification in bagasse is also evident in other biomass (Chiou et al., 2016; Rodrigues & Rousset, 2009). Thus briquetting and pressing of the torrefied biomass is often necessary to achieve the desired fuel density (Araujo et al., 2016; Hu et al., 2016). The merits of thermochemical torrefaction is also evident in its effect on the proximate and ultimate properties of the chars. Biomass have higher volatile matter (VM), in particular combustible volatile matter and lower fixed carbon to volatile matter ratio in comparison to coal. Thus biomass have lower ignition temperature, rapid ignition and faster combustion (Vassilev et al., 2015). This higher reactivity poses storage issues and instability during combustion. Lower ignition and its higher reactivity gives bagasse a greater propensity for spontaneous combustion, which becomes an issue in storing bagasse (Breccia et al., 1997; Macaskill et al., 2001). In a practical sense, the combustion of biomass with high volatile matter is also more difficult to control as it creates segregated combustion when co-fired with coal. Thus biomass fuels with VM/FC ratios closer to the ratios of coal are considered more beneficial. Table 1 shows torrefaction reduced the volatile matter (VM) contents of the chars consequently resulting in a lower volatile matter to fixed carbon (VM/FC) ratio. Bagasse has volatile matter of 83.1% and VM/FC ratio of 6.5. Dry torrefaction reduced the volatile matter by 4-32% at 200-300°C, while very little change was observed at 160 °C. This resulted in chars with VM/FC ratios of 1.7- 6.7. While thermochemical torrefaction induced 37 to 50% reduction in volatile matter resulting in chars with VM/FC ratios of 1.1-1.2. Although these VM/FC ratios are still incomparable compared to VM/FC ratios of the various coal ranks of 0.12 - 1.3 as reported in Table 1 (Ahn et al., 2014; Idris et al., 2012; Toptas et al., 2015; Wang et al., 2014), nonetheless the thermochemical method bring the bagasse fuel closer to the coal properties than the dry torrefaction method. The O/C and H/C ratios of biomass fuel can have a significant impact on its calorific value and stability. Fig. 1 compared the O/C and H/C ratios of the bagasse fuels to coals using the absolute elemental values reported in Table 1. Raw bagasse has O/C and H/C ratios of 0.8 and 1.2 respectively. While the various ranks of coal exhibited O/C and H/C ratios of 0.4−0.08 and 1.02−0.078. Fig. 1 shows that increasing the temperature of torrefaction, by dry and chemical method, resulted in lowering O/C ratios, which is consistent with greater charring of the chars (Enders et al., 2012; Spokas, 2010). While the corresponding reduction in H/C ratio as a result of torrefaction reflects the formation of greater aromaticity in the chars (Kuhlbusch, 1995). Dry torrefaction reduced O/C ratio down to 0.46−0.74 and H/C to 0.461.12, whilst thermochemical torrefaction reduced the O/C ratio 0.34-0.49 and H/C to 0.3-0.74. These results show that thermochemical torrefaction is able to achieve greater charring and aromaticity and is consistent with their greater densification. The greater shift in the elemental ratios resulting from thermochemical torrefaction indicates the improvements in the fuel properties of bagasse and that its properties are closer to coal. The S contents of the thermochemically torrefied bagasse are from 0.2-0.9 wt.%. These are comparable or lower 5
compared to the S contents of the various coal ranks and these also reflects the efficiency of washing off the residual acids from the char. The corresponding effect of chemical changes induced by torrefaction on the heating values, energy density (DE) and energy yield (YE) of bagasse fuel are compared with the reported heating values of various coal ranks in Table 2. The higher heating values of raw, thermally and thermochemically torrefied bagasse in Table 2 show that torrefaction increased the energy density of the chars. The energy density (HHVmass) of raw bagasse is 15.9 MJ/kg on a dry basis. Dry torrefaction increased the energy density with torrefaction temperature raising HHVmass by up to 27% to 20.2 MJ/kg at 300°C. Thermochemical torrefaction also increased the HHVmass of the char with temperature raising the energy density by 39% to 22.1 MJ/kg at 250°C, but was reduced at 300°C. The rise in heating values is consistent with the observed loss of volatile matter and lowering of O/C and H/C ratios with torrefaction (see Table 1 and Fig. 1). The higher increase in the calorific values of the thermochemically torrefied bagasse could be attributed to the catalysed decomposition of lower energy density components of bagasse; hemicelluloses (13.6 MJ/kg) and cellulose (18.6 MJ/kg)(Sheng & Azevedo, 2005). Thermochemical torrefaction is able to volatilise these lower calorific components leaving behind the higher calorific lignin fractions (27 MJ/Kg). This is consistent with the higher fixed carbon (FC) and lower volatile matter (VM) contents of the thermochemically torrefied bagasse. However at higher temperatures, above 250°C, it appears further decomposition of the FC occurs resulting in loss of the calorific value of the fuel. The greater loss of the lower calorific components must be taken into account in the evaluation of the torrefaction process by incorporating the mass yield. The energy yield (YE) for dry torrefied chars are from 70-97.5%, whilst the energy yields of thermochemically torrefied chars are lower at 53 to 74%. The benefit of torrefaction, however, on upgrading the fuel of bagasse is more evident when the energy yields are compared with the char yield. Tables 1 and 2 shows that all torrefied bagasse have energy densities that were either equal or greater than the char yields, however the enhancement achieved by thermochemical torrefaction is greater. For dry torrefied char, the ratio of the energy yield to char yield is 1.0 -1.26, while thermochemically torrefied chars resulted in ratios of 1.3-1.4. These higher ratios suggest the enhancement in the energy density of the chars achieved with thermochemical torrefaction more than outweighs the mass loss resulting from the thermal process. The volume based heating values (HHVvolume) determined by multiplying HHVmass with the bulk density of the chars, further reflects the benefits achieved by thermochemical torrefaction. The impact of this greater densification is shown by the increase in HHVvolume of the chars. The HHVvolume of raw bagasse is 1.59 GJ/m3. The HHVvolume was raised by up to 25% thermally to 1.98 GJ/m3. Whilst thermochemical torrefaction raised the energy density by 365% at 250°C to 7.39 GJ/m3. Given that bagasse will need to be transported to boilers where they are used as fuel, the greater energy density offered by chemical conversion, and as reflected by the higher HHVvolume, has a significant potential in overcoming the greater economic cost of transporting the low energy density raw bagasse feedstock. 6
3.2 Stability Characteristics of Torrefied Bagasse A number of methods for comparing the stability of chars exposed to various environment have emerged (Antal & Gronli, 2003; Crombie et al., 2013; Enders et al., 2012; Kuhlbusch, 1995; Spokas, 2010). In this study, the stability of the thermochemically torrefied bagasse (160°C) was tested by storing the chars in temperature controlled chamber at 30°C in air for 6 to 18 months. The extent of degradation during storage was monitored from the proximate O/C and H/C molar ratio analyses of the chars (see Fig. 2). Proximate analysis has been used to assess the quality of coals. As shown in Fig 2a there is relatively little difference in the proximate analysis of the chars over the 18 months storage. There is only a slight decline in the higher heating values of the chars over time. The HHVmass of the chars declined from its initial value of 20.6 MJ/kg to 18.6 MJ/kg in 6 months and 18.1 MJ/kg by 18 months. Approximately 12% of the calorific value was lost in 18 months. Fig 2b shows the loss in calorific values is attributed to the rise in the O content and reduction in the H content of the chars. The rise in the O/C ratio infers the systematic oxidation of the char with storage (Enders et al., 2012; Spokas, 2010). The O/C ratio rose from 0.48 to 0.52 in 6 months and was relatively stable thereafter. Spokas (2010) suggested that chars characterised by O/C molar ratio >0.6 would have properties closer to the biomass and would have a residence time of 100 years. Whilst lower O/C ratio of 0.2−0.6 would be expected to have a residence time of 100-1000 years. The lowering of the H/C content of the chars suggests the rise in aromaticity and thus it would have greater resistance to biological degradation (Kuhlbusch, 1995). Although the chars are unlikely to be stored for more than a year, these results demonstrate that thermochemically torrefied bagasse have sufficient stability to be stored during the non-milling period. 3.3 Combustion Characteristics of Bagasse Fuels Fig. 3 shows the TG and DTG curves obtained from the thermogravimetric analysis of bagasse at a heating rate of 20 oC/minute. As shown the combustion process is characterised by three stages. The first stage corresponded to drying of the chars up to the temperature of 180 oC. The second stage at 180– 450 oC corresponded to combustion of pyrolytic volatiles and the last stage is attributed to the combustion of the fixed carbon (Moon et al., 2013). As shown, the char oxidation appears as a minor thermal event shown as a shoulder on the oxidative pyrolysis peak. The reactivity of the fuel under the oxidation reaction was characterised by the ignition temperature (Ti), peak temperature (Tm) and the maximum rate of reaction (DTGmax) at the given thermal event and burnout temperature (Tb). The ignition temperature was taken at the point when the rate of weight loss first becomes 1.0 wt.%/minute directly after the horizontal period of moisture loss (Cumming & McLaughlin, 1982). The burnout temperature, which represented the temperature where sample oxidation was completed, was taken as the point immediately before reaction ceased, when the rate of weight loss was 1.0 wt. %/min. The reactivity of a fuel is directly proportional to its maximum weight loss velocity (DTGmax) and inversely proportional to Tm (Ghetti et al., 7
1996). Both parameters were used to give an average value of the fuel reactivity (Rm)(Zheng & Kozinski, 2000): C =
DEF9=G E9
100
(5)
These indicators were used in comparing the reactivity behaviour of the bagasse fuels in the present work. Fig. 4 shows the combustion characteristics of dry thermally torrefied bagasse in comparison to raw bagasse. TG is on the left and DTG on the right. Fig. 5 shows the combustion behavior of thermochemically torrefied bagasse. The corresponding reactivity indicators are summarised in Table 3. Most biomass like bagasse typically consists of hemicellulose, cellulose and lignin. The reactivity of these components during torrefaction determines the final composition of the char. The amorphous structure and low degree of polymerization of the hemicellulose makes is highly susceptible to thermal degradation, whilst the greater crystallinity and aromaticity of cellulose and lignin makes them more thermally stable (Bach & Skreiberg, 2016). Typically hemicellulose has a characteristic DTG peak at 300°C and forms a shoulder to the degradation of cellulose around 280- 380 °C (Liu et al., 2016). Whilst lignin decomposes more slowly and over a wider temperature range of 200-500°C (Brebu & Vasile, 2010). The DTG of raw bagasse in Fig 4 shows the typical degradation of hemicellulose and cellulose with a peak temperature of 335 °C. The lack of change in the DTG peak of bagasse thermally torrefied at 160°C suggest this temperature was insufficient to thermally degrade the bagasse constituents. This is also consistent with lack of change in the volatile matter of the resulting char at160°C (see Table 1). Higher torrefaction temperatures resulted in greater degradation. This is reflected in the reduction of the first DTG peak attributed to hemicellulose and cellulose combustion, and the rise in the second peak at 417438°C reflecting the combustion of lignin. The effect of thermochemical torrefaction, as shown in Fig 5, was more pronounced. The cellulose and hemicellulose peaks in bagasse torrefied at 160 and 200 °C are absent and only the second peak at 428°C was present suggesting only the lignin survived the process. Further rise in torrefaction temperature above 200°C led to shifts in the lignin peak to lower temperatures (365-378°C) inferring further degradation of lignin. These results are consistent with the lowering of HHV values resulting from thermochemical torrefaction of bagasse at 300°C (see Table 2). These results suggest that thermochemical torrefaction at temperatures of 160 -200°C offer optimal energy densification of bagasse. Table 3 shows torrefaction is able to increase char ignition temperatures (Ti). Dry torrefaction increased Ti from 245°C to 246-281°C when torrefied at 160-300°C. While thermochemical torrefaction increased Ti to 265-281°C. The rise in Ti is consistent with the observed reduction in volatile matter with torrefaction (see Table 1). These slightly higher ignition temperatures may partially reduce the risk of self-ignition of the chars providing safer storage. 8
Torrefaction reduced the rate of combustion of pyrolytic products and shifted the peak and burnout to higher temperatures. Increasing the temperature of torrefaction either thermally or thermochemically resulted in lower reactivity. The reduction in reactivity as a result of thermochemical torrefaction, however, is greater and resulted in reactivities closer to that of coal of 6.2-6.9%/min. The DTGmax of dry torrefied bagasse under oxidative pyrolysis were 107-145%/min, whilst thermochemically torrefied bagasse showed rates of 40-75%/min. These are consistent with the characteristic lower volatile matter and higher fixed carbon with torrefaction. Torrefaction also increased the burnout temperature. Fig. 6 compares the temperature range (Ti and Tb) when combustion occurs. The combustion region for bagasse, 245-362°C, does not coincide with the combustion regions for any of the coal ranks. The coals have Ti values of 359-611°C and Tb values of 601-714°C. This demonstrates one of the key challenges in cofiring biomass with coal. The higher reactivity of biomass results in segregated combustion where the more reactive biomass burns first and in a shorter period (Haykiri-Acma et al., 2015; Park et al., 2012). Segregated volatile matter emission and combustion along the combustor introduces instability that are of concern in regards to the safe and efficient operation of industrial scale reactors. Torrefaction was able to overcome this issue to different extents. The burnout temperatures of dry torrefied bagasse were shifted to 363-485°C while the burnout temperatures of thermochemically torrefied bagasse were shifted to higher temperatures of 419-535°C. Chars dry torrefied above 200°C and all the thermochemically torrefied bagasse have burnout temperatures that exceeded the ignition temperatures of lignite to bituminous coals of 359-411°C. This commonality in combustion profiles of these chars with the various coal ranks allows them to achieve greater combustion stability and attain synergy when used in co-firing with coal (Liu et al., 2016). 4. Conclusions Thermochemical torrefaction is able to address many of the practical challenges in using bagasse as fuel. It induced greater densification, higher HHVmass, HHVvolume, greater charring and aromatisation resulting in char storage stability. It shifted the oxidative pyrolysis of the chars to temperatures allowing the combustion of the chars to coincide with that of various ranks of coal. The commonality of the combustion profiles will permit the co-firing of thermochemically torrefied bagasse and coal to achieve greater combustion stability and interaction between the biomass and coal fuels. Thermochemical torrefaction at 160°C offered the best method for stabilising and upgrading bagasse.
Acknowledgements This research was supported by Sugar Research and Development Corporation and Material and Mineral Processing Research Unit.
References
9
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12
List of Figures
Fig. 1. Elemental O/C and H/C molar ratios of bagasse and coal fuels. Fig. 2. Effect of storage period on the ultimate and proximate analyses of bagasse thermochemically torrefied at 160°C. Fig. 3. Combustion characteristic of bagasse thermochemically torrefied at 160°C. Fig. 4. Combustion characteristics of dry bagasse and dry-torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG). Fig. 5 Combustion characteristics of dry bagasse and thermochemically torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG). Fig. 6. Comparison of ignition and burnout temperatures of fuels.
13
Elemental ratios (mole/mole)
1.6 1.4 1.2
Dry Chemically treated
Coal
1 O/C
0.8
H/C
0.6 0.4 0.2 0
Fuels
Fig. 1. Elemental O/C and H/C molar ratios of bagasse and coal fuels.
14
elemental molar ratios (mole/mole)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
(b)
O/C H/C
0
6 Time (months)
18
Fig. 2. Effect of storage period on the ultimate and proximate analyses of bagasse thermochemically torrefied at 160°C.
15
Fig. 3. Combustion characteristic of bagasse thermochemically torrefied at 160°C.
16
Fig. 4. Combustion characteristics of dry bagasse and dry-torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG).
17
Fig. 5 Combustion characteristics of dry bagasse and thermochemically torrefied bagasse in 20 ml/min air heated 20°C/min (left graph: TG, right graph: DTG).
18
Fuels
Anthracite Bituminous coal Sub-bituminous coal Lignite Chemical 300°C Chemical 250°C Chemical 200°C Chemical 160°C Dry 300°C Dry 250°C Dry 200°C Dry 160°C Bagasse
Ti Tb
0
200 400 600 Temperature (°C)
800
Fig. 6. Comparison of ignition and burnout temperatures of fuels.
19
Table 1 Proximate and ultimate compositions of raw, dry and thermochemically torrefied bagasse
Fuel
T (o C)
Char Yiel d (w/w %)
Bagass e Dry torrefie d
Proximate Analysis Den (%, dry sity basis) kg/ 3 m V A F M sh C 100 83 4. 12 .1 2 .7
16 0
94
102
85 .1
2. 3
12 .6
20 0
90
100
79 .8
4. 3
16 .0
25 0
75
100
62 .9
27 .9
30 0
55
91
56 .4
Thermo 16 0 chemic ally 20 torrefie 0 d 25 0
57
290
52 .5
1 0. 0 1 0. 4 4. 4
53
335
47 .0
9. 0
44 .0
49
335
48 .5
38 .6
30 0
45
340
41 .9
1 3. 0 1 9. 9 1 1. 5 3. 4 5
53 .5
Lignite
49 .9
Subbitumin ous
42 .6
20
33 .2 43 .1
38 .2 38 .6
Ultimate Analysis (%, dry ash free basis) C
H N S
O
4 5. 5 4 6. 8 4 9. 3 6 0. 4 5 9. 4 5 8. 0 6 1. 6 6 6. 5 5 8. 8 -
4 . 6 4 . 4 3 . 1 2 . 3 2 . 9 3 . 6 2 . 7 1 . 7 2 . 0 -
0 . 4 0 . 2 0 . 3 0 . 4 0 . 8 0 . 4 0 . 2 0 . 5 0 . 7 -
0 . 1 0 . 0 0 . 3 0 . 1 0 . 0 0 . 7 0 . 9 0 . 6 0 . 2 1 . 1
49 .5
6 0. 0
5 . 1
1 . 9
0 32 . .6 3
References
46 .0
This work
47 .1
This work
36 .8
This work
37 .0
This work
37 .2
This work
34 .6
This work
30 .7
This work
38 .3
This work
-
(Toptas et al., 2015) (Idris et al., 2012)
Bitumi nous
34 .4
7. 7
57 .9
Anthrac ite
8. 4
2 3. 4
68 .1
21
7 5. 9 8 8. 9 8
4 . 8 0 . 5 8
1 . 4 0 . 4 5
0 10 . .2 0 0 9. . 38 6 1
(Ahn et al., 2014) (Wang et al., 2014)
Table 2 Fuel properties of raw, dry and thermochemically torrefied bagasse
Fuel
Bagass e Dry torrefie d
Thermo chemic ally torrefie d
* Estimated equation (1) ultimate
T (o C)
16 0 20 0 25 0 30 0 16 0 20 0 25 0 30 0
HHV
HHVVol
mass ume
(MJ/ kg)
GJ/m3
15.9
1.59
16.5
1.68
15.7
1.57
19.8
1.98
20.2
1.84
20.6
5.97
21.5
7.20
22.1
7.39
18.8
6.38
DE YE ( ( % % ) )
References
This work 1. 0 1. 0 1. 2 1. 3 1. 3 1. 4 1. 4 1. 2
97 .5 88 .9 93 .6 69 .8 73 .9 71 .7 68 .1 53 .2
This work This work This work This work This work This work This work This work
Lignite
25.1
(Toptas et al., 2015) (Idris et al., 2012)
Subbitumin ous Bitumi nous
23.6 * 31.3 *
(Ahn et al., 2014)
Anthrac ite
31.0 *
(Wang et al., 2014)
22
using using the fuel analysis
Table 3 Combustion characteristics of bagasse, dry and thermochemically torrefied bagasse and various coal rank.
T (°C)
Ti (°C)
160 200 250 300 160 200 250 300 -
245 246 247 265 281 275 281 265 273 359
Oxidation Pyrolysis Tm DTGmax (°C) (%/min) 335 145 335 145 425 75.6 417 128 438 107 428 40 428 42 365 75 378 58 484 1.3
Subbituminous Subbituminous Bituminous
-
398
471
6.9
700
-
343
426
-
601
-
-
402
510
-
654
-
Anthracite
-
611
667
6.2
714
Fuel Bagasse Dry
Thermochemical
Lignite
Tb (°C)
Rm (%/min.K )
Residue (wt.%)
References
362 363 428 479 485 506 535 419 486 644
23.8 23.8 10.8 18.6 15.0 5.7 6.0 11.8 8.9 0.2
2.2 4.2 1.7 10.7 12.9 5 9 13 20 -
0.9
-
This work This work This work This work This work This work This work This work This work (Toptas et al., 2015) (Idris et al., 2012) (Ahn et al., 2014) (Ahn et al., 2014) (Wang et al., 2014)
0.7
-
T is the torrefaction temperature, Ti is the ignition temperature, Tm is the peak temperature at this regime, DTGmax is the peak rate at this regime, Tb is the burnout temperature.
23
Highlights Chemical based thermochemical torrefaction (TT) offers new method to upgrade bagasse TT increased the bulk density of bagasse by 335% to 340 kg/m3 TT achieved greater charring, aromatisation and storage stability TT increased HHVmass by 27% to 22.1 MJ/kg and HHVvolume by 335% to 7.39 GJ/m3. TT bagasse exhibited combustion features that are in common with various ranks of coal.
24