Energy Fuels 2010, 24, 4638–4645 Published on Web 02/15/2010
: DOI:10.1021/ef901168f
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Torrefaction of Agriculture Residue To Enhance Combustible Properties† Anuphon Pimchuai,‡ Animesh Dutta,*,§ and Prabir Basu )
‡ Department of Mechanical Engineering, Burapha University, Tambon Saensook, Muang, Chonburi 20131, Thailand, §School of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada, and Department of Mechanical Engineering, Dalhousie University, Post Office Box 1000, Halifax, Nova Scotia B3J 1Z1, Canada
Received October 13, 2009. Revised Manuscript Received January 29, 2010
Torrefaction improves the thermochemical properties of biomass that are relevant to combustion, cocombustion with coal, or gasification. This study examines torrefaction of rice husks and four other agriculture residues (sawdust, peanut husks, bagasse, and water hyacinth) in nitrogen. Two main operating parameters of torrefaction, temperature and residence time for the process, were varied in the range of 250-300 °C and 1-2 h, respectively. Product evolution and mass and energy losses during torrefaction were measured. Similar to other work, the torrefied products in the present work were characterized by a more brownish color, reduced moisture content and volatile matter, and increased ash, fixed carbon content, and energy density. The difference between the mass and energy yield was shown to improve for the higher torrefaction temperatures investigated. For the biomass studied, the torrefied bagasse at 300 °C and 1.5 h resulted in the highest higher heating value (HHV) of 25.68 MJ/kg of product, which was comparable to the HHV of lignite. Dependent upon the severity of the torrefaction conditions, the torrefied fuel can contain up to 98% of the original energy content on a mass basis. The combustion behavior of both raw and torrefied rice husks was studied in a spout-fluid bed combustor by measuring its temperature history at different zones. It is observed that torrefied husks ignite faster and raise the bed temperature to a higher level because of its low moisture content.
A process called torrefaction can help overcome some of the above limitations of biomass.3-6 Torrefaction is a thermal treatment in an inert atmosphere, which improves the thermochemical properties of biomass, making it more suitable for energy generation. This process removes moisture and carbon dioxide and depolymerizes the long polysaccharide chains, producing a hydrophobic solid product with an increased energy density (on a mass basis). Torrefaction typically involves slow heating in the temperature range of 230-300 °C. This process releases both moisture and carbon dioxide, both of which remove oxygen from the biomass, resulting in a fuel with a lower O/C ratio. This results in high gasification efficiency.2 Torrefaction also partially decomposes the hemicellulose in the biomass fiber. As a result, much less energy is required to grind the torrefied biomass when co-fired with coal in existing pulverized coal-fired power stations.7-9 The torrefaction process may also be called mild pyrolysis, with the removal of smoke producing compounds and the formation of a solid product.5
1. Introduction The carbon dioxide emissions from the consumption of fossil fuels around the world grow at an average rate of 2.1% per year, and the rate continues to increase.1 To arrest this growth, the world should move toward more sustainable energy sources, such as biomass (agriculture residues), because of its carbon neutrality. Several major shortcomings of biomass, however, limit its wider application in power generation. These include low heating value, high moisture content, hygroscopic nature, smoke during combustion, low energy density, and low combustion efficiency. Biomass, being a high moisture fuel, consumes a considerable amount of energy during drying. Furthermore, if stored for a period of time, the dried biomass again picks up moisture, owing to its hygroscopic (water-absorbing) nature. Therefore, for industrial use, the biomass needs to be hydrophobic (waterrepellant) instead of hygroscopic in nature. Also, a high oxygen content of biomass does not make it an ideal fuel for gasification.2 † This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed. Telephone: þ1-5198244120 ext. 53023. Fax: þ1-5198360227. E-mail: adutta@ uoguelph.ca. (1) Energy Information Administration (EIA). World Oil Markets. Official Energy Statistics from the U.S. Government, 2006 (http://www. eia.doe.gov/oiaf/ieo/pdf/emissions.pdf). (2) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. More efficient biomass gasification via torrefaction. Energy 2006, 31, 3458–3470. (3) Pentananunt, R.; Rahman, A. N. M. M.; Bhattacharya, S. C. Upgrading of biomass by means of torrefaction. Energy 1990, 15 (12), 1175–1179. (4) Bergman, P. C. A. Combined Torrefaction and Pelletisation: The TOP Process; Energy Research Centre of the Netherlands (ECN): Petten, The Netherlands, 2005.
r 2010 American Chemical Society
(5) Arcate, J. R. New process for torrefied wood manufacturing. Bioenergy update, 2000; Vol. 2 (4) (http://www.techtp.com/Bioenergy% 20Update%20.pdf). (6) Zwart, R. W. J.; Boerrigter, H.; Drift, A. V. D. The impact of biomass pretreatment on the feasibility of overseas biomass conversion to Fischer-Tropsch products. Energy Fuels 2006, 20, 2192–2197. (7) Abdullah, H.; Wu, H. Biochar as a Fuel: 1. Properties and grindability of biochars produced from the pyrolysis of mallee wood under slow-heating conditions. Energy Fuels 2009, 23, 4174–4181. (8) Arias, B.; Pedida, C.; Fermoso, J.; Plaza, M. G.; Rubiera, F.; Pis, J. J. Influence of torrefaction on the grindability and reactivity of woody biomass. Fuel Process. Technol. 2008, 89, 169–175. (9) Bergman, P. C. A.; Boersma, A. R.; Zwart, R. W. R.; Kiel, J. H. A. Torrefaction for biomass co-firing in existing coal-fired power stations. Biocoal, Energy Research Centre of the Netherlands (ECN), 2005, ENCC-05-013.
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Although it is a relatively new area, several good works have been performed on this. Sridhar et al10 presented the research work on the torrefaction of bamboo, where a batchtype kiln was used. Nimlos et al.11 studied the sawdust torrefaction and reported that the torrefaction temperature has a more profound effect on the weight loss than residence time. Pach et al.12 studied the effects of the torrefaction temperature and residence time on the product distribution and properties of the product for birch, pine, and bagasse. It has been found that during torrefaction the wood sample gave more solid than the agricultural residue sample. Arias et al.7 studied the effect of the temperature and residence time on the grindability and kinetic parameters of pyrolysis of eucalyptus wood in a temperature range of 220-500 °C. Sadaka and Negi13 torrified straw (rice and wheat) and cotton-gin waste and found that the temperature has a greater effect on the chemical and thermophysical properties than the residence time of torrefaction. Kargbo et al.14 reviewed the pretreatment of rice husks using a number of processes, including torrefaction, but did not provide any data on it. Previous researchers have focused their studies examining the mass loss, carbon content, and energy distribution with changing time and temperature of torrefaction for the woody materials only. A few studies paid attention to the grindability, gasification, co-firing, and reactivity of torrefied wood. Less attention has been paid to the torrefaction of the agricultural field residues13 and their combustion performances. Although rice husks are one of the most widely available biomasses in the world and a good amount of it is burnt for energy production, very little work on the torrefaction of rice husk is available in published literature. Data on combustion properties of torrified biomass are also limited. A small amount of information is based on thermogravimetric analysis (TGA) and a muffle furnace. For a light biomass, such as husk, a spout-fluid bed is ideal for combustion. It drags the light fuel through the dense bed before being thrown in the freeboard. Such an operation ensures lower combustible losses. At the time of writing this paper, no results on the combustibility of torrified biomass in a spout-fluid bed were available. Also, a previous torrefaction study was performed for small sample sizes. The present study reports preliminary results on the torrefaction of rice husks and four other agricultural residues, sawdust, peanut husks, bagasse, and water hyacinth. To provide the base of design of appropriate combustors for torrefied biomass, it also presents the combustibility of torrefied rice husks in a spout-fluid bed combustor.
Figure 1. Schematic diagram of the torrefaction setup.
Figure 2. Photograph of the spout-fluid bed combustor used for combustion tests.
operating conditions for the torrefaction temperature and residence time were set as 250, 270, and 300 °C and 1, 1.5, and 2 h, respectively. A set of nine experiments for each raw material were conducted. The unit consisted of a programmable muffle furnace (Fisher Scientific Isotemp model 10-650-126N, 35.7 L capacity with 10-550P Port Kit to supply N2), a nitrogen cylinder with a pressure regulator valve, a water seal valve, an auto-transformer (variac), and necessary fittings and pipes. The heating rate was maintained at 15 °C/min using the variac. A continuous N2 flow of 15.0 L/min to the reactor was maintained to create inert conditions inside the reactor. Then, the residence time was set by a timer, and the required torrefaction temperature was set by adjusting the digital temperature-controller button. A T-type thermocouple placed inside the reactor was connected with the temperature controller. After the set time, the timer switch cut out the electricity and the sample was moved from the furnace for cooling. The sample was again weighted (for study of the torrefaction yields). Thereafter, the proximate analysis and bomb calorimetry were carried out using American Society for Testing and Materials (ASTM) methods. A sample size of approximately 1 kg was used in each torrefaction experiment. 2.2. Combustion Tests. The combustibility tests for both raw and torrefied biomass were performed in a modified spout-fluid bed combustor. Figure 2 shows a photograph of the combustor, and Figure 3 shows the schematic diagram with all of the measuring points.15 The combustor was an air-blown, spout-fluid bed combustor (flat base), was autothermal, and operated at atmospheric pressure.
2. Experimental Procedure 2.1. Torrefaction. A bench-scale torrefied unit was developed for this research. Figure 1 shows the schematic diagram of the unit. The (10) Sridhar, G.; Subbukrishna, D. N.; Sridhar, H. V.; Dasappa, S.; Paul, P. J.; Mukunda, H. S. Torrefaction of bamboo. Porceedings of the 15th European Biomass Conference and Exhibition, May 7-11, 2007. (11) Nimlos, N. M.; Emily, B.; Michael, J. L.; Robert, J. E. Biomass torrefaction studies with a molecular beam mass spectrophotometer, National Bioenergy Center. Prepr. Symp.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48, 590–591. (12) Pach, M.; Zanzi, R.; Bjornbom, E. Torrefied biomass a substitute for wood and charcoal. Proceedings of the 6th Asia-Pacific International Symposium on Combustion and Energy Utilization, Kuala Lumpur, Malaysia, May 20-22, 2002. (13) Sadaka, S; Negi, S. Improvements of biomass physical and thermochemical characteristics via torrefaction process. Environ. Prog. Sustainable Energy 2009, 28 (3), 427–434. (14) Kargbo, F. B.; Xing, J.; Zhang, Y. Pretreatment for energy use of rice straw: A review. Afr. J. Agric. Res. 2009, 4 (13), 1560–1565.
(15) Thamavithya, M.; Dutta, A. An investigation of MSW gasification in a spout-fluid bed reactor. Fuel Process. Technol. 2008, 89 (10), 949–957.
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Pimchuai et al. Table 1. Height of the Temperature-Sensor Location from the Distributor Plate temperature sensor
height from distributor plate (mm)
T1 T2 T3 T4 T5 T6 T7
110 210 300 1130 1360 1800 2900
Figure 4. Raw and torrefied biomass at 250 °C and 1 h residence time. Figure 3. Working diagram for the gasification with primary air supply and the temperature-sensor location of the combustor.6
The spouting and fluidizing air flow rates were kept constant at 5 and 80 m3/h (89.25 kg/h), respectively. The fuel feed rate of the two fuel types was constant at 100 g/min. The heights of the temperature-sensor locations from the distributor plate are shown in Table 1 and Figure 3 and were measured every 1 min until they reached the steady point. Results are compared for both fuel types.
The combustor was 3000 mm high measured from the distributor plate to the top of the enlargement zone. The distributor plate of the combustor was 450 450 mm in cross-section. It had a 50.8 mm diameter spouting nozzle in the center and 56 tuyere-type nozzles equally distributed on the remaining area. The combustor was divided into three different zones. The first zone was the 500 mm high bed with an inner cross-sectional area of 450 450 mm2. The second zone was the oxidation zone, where secondary air could be injected. It was located 330 mm above the bed zone. It was 620 mm high, with an inside diameter of 150 mm. The third zone was the enlargement zone located 130 mm above the oxidation zone. It had an inside diameter of 300 mm. To reduce heat loss through the wall and protect against deformation because of thermal stress, thermal insulation was provided on the wall of the combustor using firebricks and castable materials for high-temperature application. A cylindrical cyclone was attached at the gas outlet of the enlargement zone to capture ash and unburned particles carried away by the flue gas. The detachable return leg was optional and was attached when required for experimentation. Seven K-type (Chromel-Alumel) thermocouples were installed for continuous measurement of temperatures at different locations along the height of the combustor. Six of them were installed at 110, 210, 300, 1130, 1360, and 1800 mm above the distributor. To measure the gas temperature leaving the combustor, a thermocouple was installed in the exhaust pipe located between the cyclone and the enlargement zone of the combustor. An air blower with a capacity of 200 m3 h-1 was used to supply air through the air distributor. A small wire net was provided on the spouted nozzle to prevent the bed material from falling down into the air-supplying pipes. The optional secondary air-supply port was placed at 1100 mm above the primary air distributor. The combustion test was performed with the primary air supply alone without secondary air injection. The bed was preheated by burning charcoal in it. Once charcoal combustion raised the temperature of the bed to 550 °C and the charcoal almost entirely burned out, the test fuels were fed into the combustor. The amount of preburning of charcoal for such tests was found by trial and error.
3. Results and Discussion 3.1. Agriculture Residue and Torrefied Product. Figure 4 shows the color of biomass before and after the torrefaction experiments. The torrefied products are more brownish in color than the raw biomass. This is similar to the color change noted when coffee beans are roasted. As the severity of roasting increases, the coffee beans changes color from green to gray, to brown, and finally, to black. 3.2. Proximate Analysis. Figure 5 shows the influence of the torrefaction temperature and time on proximate analysis of rice husks. It can be seen from Figure 5 that the moisture content and volatile matter content decrease with the increase in the torrefaction temperature and residence time. However, the ash content and fixed carbon content increase with the increase in the temperature but majorly decrease with the increase in the residence time. Table A1 of the Appendix presents the proximate analysis of all torrefied products at each experimental condition for the five raw or green biomasses studied. The first column in Table A1 presents the proximate analysis of the raw biomass. From Table A1, it can be seen that percentage moisture losses in bagasse (81-89%) and water hyacinth (83-88%) were higher than rice husks (70-84%), sawdust (71-82%), and peanut husks (71-79%) for all of the tests conducted. For volatile matters, the change was not substantial (2-3%) for rice husks, sawdust, peanut husks, and bagasse when the temperature and residence time was maintained at 250 °C and 1 h. However, for water hycinth, the change was 41%. When the torrefaction temperature and time increase to 300 °C and 2 h, the change was substantial. Overall, the 4640
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Figure 5. Influence of the temperature and residence time on the (a) moisture content, (b) volatile content, and (c) ash content of rice husks.
volatile matter reduction was varied between 3 and 47% for rice husks, between 3 and 45% for sawdust, between 3 and 39% for peanut husks, between 3 and 40% for bagasse, and between 41 and 56% for water hyacinth. The variation of mositure and volatile matter losses among the five biomass studies is explained in the following paragraph. In general, biomass consists of cellulose, hemicelluloses, and lignin. Hemicellulose is the most reactive part of biomass and is subjected to limited devolatilization and carbonization below 250 °C; cellulose decomposes at 305-375 °C; and lignin gradually decomposes over the temperature range of 250-500 °C.3,9 This may be one of the reasons that moisture
reductions in bagasse and water hyacinth are higher compared to rice husks, sawdust, and peanut husks. Because devolatilization is limited below 250 °C, the reduction in volatile matters of rice husks, sawdust, peanut husks, and bagasse were insignificant for the torrefaction temperature of 250 °C. These results are in agreement with Aries et al.,8 Sadaka and Negi,13 and Prins et al.16 3.3. Mass and Energy Yield. Figure 6 presents the influence of the temperature and residence time on mass and energy (16) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. Torrefaction of wood. Part 1. Weight loss kinetics. J. Anal. Appl. Pyrolysis 2006, 77, 28– 34.
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Figure 6. Influence of the temperature and residence time on (a) mass yield and (b) energy yield of rice husks.
yields of rice husks. Table A2 of the Appendix shows mass and energy yields from the raw biomass after each experiment for all of the raw biomass studied here. It can be seen from Figure 6 that the percentage of mass and energy yields continually decrease with the increase in the torrefaction temperature and residence time. It can also be noticed that the temperature has more effect on torrefaction than the residence time. This could be explained by the decrease in the moisture content and volatile matter content of the biomass. Because of the decomposition of some reactive components of the hemicelluloses, there is a significant mass loss at the beginning. At higher residence times, the mass loss can be attributed to the decomposition of the less reactive components of the hemicelluloses. Similar results were reported by Prins et al.16 and Arias et al.8 However, the yields of the solid product are significantly higher than that of pyrolysis products. Mass and energy yields were in the range of approximately 41-78% of the initial weight and 55-98% of the original energy content. The mass and energy yields were lowest at the highest torrefaction temperature. Similar results were reported by Aries et al.,8 Sadaka and Negi,13 and Prins et al.16 3.4. Energy Density. As can be seen from Table 2, the energy density continues to increase with the increase in the temperature. The increase in energy density because of greater residence times, however, was insignificant. It can also be seen that the increase in energy density varied with the type of fuels investigated. The largest increment ratio of
Table 2. Influence of the Temperature and Residence Time on the Energy Density of the Torrefied Agriculture Residuea energy density temperature (°C) 250 270 300 a
time (h)
rice husks
sawdust
peanut husks
bagasse
water hyacinth
1 1.5 2 1 1.5 2 1 1.5 2
1.11 1.12 1.16 1.12 1.14 1.2 1.23 1.24 1.24
1.08 1.11 1.06 1.13 1.21 1.22 1.31 1.35 1.37
1.12 1.14 1.23 1.27 1.27 1.28 1.3 1.31 1.32
1.36 1.38 1.42 1.42 1.43 1.45 1.58 1.66 1.62
1.22 1.24 1.21 1.28 1.31 1.35 1.36 1.38 1.43
yield ð%Þ energy density ¼ energy mass yield ð%Þ
energy density is found to be 1.66 times for bagasse at 300 °C and 1.5 h residence time. Similarly, a minimum increment ratio is 1.08 times for sawdust at 250 °C and 1 h residence time. The comparison of mass and energy yields for bagasse after torrefaction at various conditions is shown in Figure 7. Mainly, moisture is lost during the torrefaction process, which results in a significant mass loss of raw feedstock without compromising much of its energy content. 3.5. Heating Value. Table 3 presents the influence of the temperature and residence time on the higher heating value (HHV) of the torrefied products. The HHV of torrefied products increases between 9 and 16.6% for all of the feed4642
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to the increase in the heating value. An increase in the carbon concentration lead to an increase in the heating value.11,17 This clearly shows the importance of torrefaction if one wishes to increase the heating value of an agriculture residue and reduce its weight. 3.6. Hydrophobic Characteristics. One of the major problems of raw biomass is its hydrophobic character. A biomass, even after drying, easily absorbs moisture from the atmosphere. This special property of biomass makes extended storage very expensive in terms of the energy spent in evaporating the moisture during combustion or gasification. Torrefaction greatly improves this property of the biomass by reducing its hydrophobicity. Experiments were conducted to measure the reduction in hydrophobicity by immersing the fuel in water for 2 h, and then, its moisture content was measured. It is observed that a specimen torrefied at a higher operating temperature results in a lower moisture absorption. The total moisture absorption is also found to be biomass-specific. For example, as seen in Table 5, the rise in the moisture content of the torrefied sawdust was 2.16% compared to 150.33% for raw sawdust. The maximum rise in the moisture content is 17.71% for the torrefied water hyacinth, while it is 197.54% for raw water hyacinth. This further demonstrates that torrefaction is a method that can solve the classical problem of moisture absorption by biomass during storage. 3.7. Combustibility of Torrefied Biomass. The objective of this test was to examine how the combustion behavior of biomass is affected by torrefaction. Both raw and torrefied rice husks were used for the experiments. This is performed by observing the temperature distribution along the height of a spout-fluid bed combustor during startup. 3.7.1. Spout-Fluid Bed Combustor Temperature. Figure 8 plots the measured temperature against time at different zones of the combustor for both raw and torrefied husks as the fuel. The values are averaged over every minute stating from startup, until it reaches the steady-state condition. There are noticeable differences observed in temperature distributions at different zones of the combustor. When these are compared to the fuel types used for the test, torrefied husk combustion results in higher temperatures at all of the zones. The torrefied husks have higher fixed carbon and much lower moisture content than the raw rice husks. An increase in carbon and low moisture concentrations in torrefied husks may be the main factors contributing to the increase in the heating value and results in a higher temperature during combustion. The following observations can be made from Figure 8: Because no secondary air is injected in the narrow oxidation zone (Figure 3), the temperature at this zone did not rise because of secondary combustion. (i) The temperature in the bed is found to be higher than that measured in other zones. Because the reactor was an insulated one and no heatabsorbing surfaces were projected in the combustor, this confirmed that the bulk if not the entire combustion took place in the bed. (ii) The bed temperature for both raw and torrefied rice husk combustion increases at a similar rate of 41 °C/min in the bed until about 550 °C at the eighth minute.
Figure 7. Comparison of the mass and energy yields of bagasse at various conditions.
stock when the operating temperature increases from 250 to 300 °C. With the increase in the residence time from 1 to 2 h, it increases only 2.5-4.7%. This is yet another obvious confirmation that the temperature has more effect on the torrefaction process than that of the residence time. It is also noted that the rise in HHV contents (Table 4) is different for various feedstocks. The maximum increase is 39.89% for bagasse at 300 °C and 1.5 h residence time, and the minimum is 7.21% for sawdust at 250 °C and 1 h residence time. Moisture reduction may be the main factor contributing
(17) Bergman, P. C. A.; Boersma, A. R.; Kiel, J. H. A.; Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. Torrefaction for entrained-flow gasification of biomass; Van Swaaij, W. P. M., Fjallstrom, T., Helm, P., Grassi, A., Eds.; Second World Biomass Conference, Rome, Italy, May 10-14, 2004.
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Table 3. Influence of the Temperature and Residence Time on the HHV of the Torrefied Agriculture Residue HHV (MJ/kg of product) temperature (°C) 250 270 300
time (h)
rice husks
sawdust
peanut husks
bagasse
water hyacinth
1 1.5 2 1 1.5 2 1 1.5 2
15.89 16.07 16.55 16.07 16.3 17.13 17.59 17.77 17.81 14.32
19.55 20.09 20.29 20.47 21.96 22.16 23.8 23.94 25.09 18.14
16.35 16.64 17.98 18.6 18.64 18.74 18.96 19.1 19.36 14.63
21.02 21.35 21.99 21.86 22.06 22.39 24.39 25.68 25.03 15.44
12.68 12.92 13.14 13.25 13.61 14 14.06 14.33 14.85 10.36
raw biomass
Table 4. Influence of the Temperature and Residence Time on HHV Rise (%) of the Agriculture Residuea HHV rise (%) temperature (°C)
time (h)
rice husks
sawdust
peanut husks
bagasse
water hyacinth
250
1 1.5 2 1 1.5 2 1 1.5 2
9.9 10.89 13.48 10.89 12.19 16.44 18.60 19.42 19.61 14.32
7.21 9.72 10.59 11.39 17.41 18.13 23.80 24.24 27.72 18.14
10.55 12.07 18.63 21.35 21.54 21.95 22.85 23.42 24.43 14.63
26.56 27.68 28.81 29.39 30.02 31.05 36.69 39.89 38.33 15.44
18.30 19.86 21.22 21.84 23.89 26.04 26.35 27.73 30.25 10.36
270 300 raw biomass a
HHV rise ð%Þ ¼
HHVproduct -HHVraw HHVraw
100%
Table 5. Rise in the Moisture Content after the Hydrophobic Test (Submerging the Specimen for 2 h into Water)a
principle, one can understand that the volatile matter in the fuel combusts first before the combustion of char/fixed carbon. Although the volatile content of raw rice husks (56.71%) was a little higher than the torrefied rice husks (54.76%), a higher moisture content with the raw rice husks (13.12%) compared to the torrefied husks (3.95%) might contribute to the similar rate of temperature rise until the eighth minute of the tests. (iii) The temperature in the bed reached a steady condition after 8 min at 550 °C after startup for raw rice husks and 15-16 min at 905 °C for torrefied rice husks. For other zones, it varies between 12 and 13 min for raw rice husks and between 16 and 17 min for torrefied rice husks. Raw husks with 13.12% moisture and 13.23% fixed carbon reached thermal equilibrium with combustor heat losses at 550 °C. Torrefied husks with much less moisture (3.95%) and a slightly higher heating value (þ9.9%) and fixed carbon (19.73%) releasing more heat at that temperature than that taken up by flue gas, wall losses, and consumed by moisture evaporation. Thus, the bed temperature continued to increase until about 900 °C when all losses matched the heat generation. The volatile content did not change much after torrefaction. The present unit did not use secondary air injection for combustion of the volatiles. If applied, it could increase the temperature in the oxidation zone, but that rise would be similar for both fuels. (iv) The temperature rise in the bed zone was around 300 °C (566-577 °C) for raw rice husk and was 635 °C (905-908 °C) for torrefied rice husk. The difference between these two temperatures was high (335 °C). This higher heating value, higher fixed carbon content, and lower moisture content in the fuel helped in raising the bed temperature.
rise in the moisture content (%)b temperature (°C)
rice husks
sawdust
peanut husks
bagasse
water hyacinth
250 270 300 raw material
4 2.64 2.26 36.85
7.87 3.27 2.16 150.33
11.77 10.89 6.13 118.26
7.63 3.76 2.89 185.99
17.71 14.97 8.86 197.54
The torrefied products of 250, 270, and 300 °C at 1 h were used for the test. b The moisture content is expressed on a dry basis. a
Figure 8. Average temperature profile at different zones in a spoutfluid bed combustor (spout air flow, 5 m3/h; fluidizing air flow, 80 m3/h).
The temperature rise rates in other zones during the first 8 min were also similar for both fuels: 22 °C/min for the oxidation zone, 19 °C/min for the enlargement zone, and 13 °C/min for the gas outlet zone. From the combustion
4. Conclusion Torrefaction greatly improved the thermophysical and combustion properties of the five biomasses studied. Torrefaction 4644
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of biomass yields a solid fuel with lower moisture and higher energy contents compared to those in a fresh biomass. This directly reduces the transportation cost of the biomass fuel. Experimental results show that mass and energy yields of biomass decrease with the increase in the operating temperature and torrefaction residence time. However, the temperature had a stronger impact on the increase in the energy density of the torrefied biomass. The HHV of the torrefied biomass increases much faster with the increase in the operating temperature than that the increase in the residence time in the torrefier. The energy density of the torrefied biomass increased significantly for all five feedstocks studied. The HHV of the bagasse torrefied at 300 °C and 1.5 h was 25.68 MJ/kg of product, which is close to the HHV of lignite (26-28 MJ/kg).
Combustion tests in a spout-fluid bed revealed a number of major differences between torrefied and raw husks. The heat of combustion of torrefied husks was higher than that of raw husks, which resulted in a higher bed temperature. The higher fixed carbon content partially contributed to the higher heats of combustion. A marked improvement in the water-repellant or hydrophobic property of the biomass (from 36.85 to 2.26% for rice husks, from 150.33 to 2.16% for sawdust, from 118.26 to 6.13% for peanut husks, from 185.99 to 2.89% for bagasse, and from 197.54 to 8.86% for water hyacinth) was noted. This has great industrial significance because husks are mostly stored for months before they are burned and, during this period, husks pick up a large amount of moisture.
Table A1. Proximate Analysis of Torrefied Products and Their Comparison to the Raw Materialsa torrefaction temperature (°C) 250
270
300
torrefaction time (h) raw material
1
1.5
2
1
1.5
2
1
1.5
2
moisture content (%) volatile content (%) ash content (%)
13.12 56.71 16.94
3.95 54.76 21.56
3.18 52.91 24.43
Rice Husk 3.10 53.81 23.03
3.69 49.99 24.57
2.82 50.11 24.49
2.78 48.55 25.49
2.67 30.04 31.32
2.57 30.03 34.29
2.05 30.44 34.08
moisture content (%) volatile content (%) ash content (%)
15.43 70.89 1.48
4.40 68.34 2.12
3.74 67.90 1.99
Sawdust 3.51 68.58 1.69
3.56 59.27 2.25
3.62 58.37 2.64
3.26 58.97 2.53
3.31 40.11 3.61
3.04 39.06 3.35
2.80 40.12 3.83
moisture content (%) volatile content (%) ash content (%)
15.71 52.12 18.31
4.61 50.86 24.48
4.29 48.08 25.15
Peanut Husks 3.95 3.88 46.77 45.47 25.16 26.84
3.78 42.95 26.83
3.45 41.27 27.98
3.55 38.82 33.77
3.36 31.61 32.14
3.26 35.67 29.78
moisture content (%) volatile content (%) ash content (%)
24.81 67.31 1.53
4.8 65.49 2.89
3.61 65.69 2.94
4.09 60.03 2.91
3.49 60 3.12
3.31 57.49 3.3
3.81 40.44 4.12
3.23 40.4 4.4
2.68 41.42 5.32
moisture content (%) volatile content (%) ash content (%)
22.69 52.83 15.68
3.85 31.33 58.98
3.77 26.37 50.16
Water Hyacinth 3.31 3.57 27.85 32.15 50.9 51.19
3.34 29.01 53.32
3.04 25.55 55.43
2.82 24.82 58.5
2.77 23.74 65.1
2.72 23.24 65.43
a
Bagasse 3.55 65.41 2.69
The proximate analysis was expressed on a wet basis.
Table A2. Influence of the Temperature and Residence Time on the Mass and Energy Yields of the Torrefied Producta torrefaction temperature (°C) 250
270
300
torrefaction time (h) 1
1.5
2
mass yields (%) energy yield (%)
77.50 86.00
77.00 86.41
76.50 88.42
mass yields (%) energy yield (%)
67.25 72.50
66.75 73.94
mass yields (%) energy yield (%)
72.50 81.10
mass yields (%) energy yield (%) mass yields (%) energy yield (%) a
1
1.5
2
1
1.5
2
Rice Husk 74.25 83.33
73.25 83.42
70.75 84.67
58.25 71.56
55.25 68.56
54.00 67.18
66.00 73.82
Sawdust 59.50 67.15
57.75 69.93
57.00 69.63
42.00 55.12
41.25 54.45
40.75 56.37
71.00 80.75
70.50 86.64
Peanut Husks 67.00 85.19
64.25 81.89
60.00 76.87
55.75 72.26
55.00 71.82
55.00 72.78
63.50 86.50
63.00 87.11
62.50 89.04
Bagasse 60.00 84.97
58.50 83.59
56.00 81.22
44.00 69.50
42.00 69.87
41.50 67.29
78.50 96.10
78.00 97.33
76.00 96.47
Water Hyacinth 77.00 74.50 98.52 97.89
68.00 91.94
70.00 95.04
69.00 95.47
66.50 95.34
product weightHHV
weight product mass yields ð%Þ ¼ rawproduct material weight 100% energy yields ð%Þ ¼ raw material weightHHVraw 100%
4645
