Energy Fuels 2010, 24, 4738–4742 Published on Web 02/10/2010
: DOI:10.1021/ef901273n
Mass and Energy Balances of Wet Torrefaction of Lignocellulosic Biomass† Wei Yan, Jason T. Hastings, Tapas C. Acharjee, Charles J. Coronella,* and Victor R. Vasquez Department of Chemical and Metallurgical Engineering, University of Nevada, Reno, 1664 North Virginia Street, MS0170, Reno, Nevada 89557 Received November 1, 2009. Revised Manuscript Received January 20, 2010
Solid handling of diverse lignocellulosic biomass feedstock is very challenging for thermochemical conversion to renewable fuels. Wet torrefaction is a pretreatment process to convert biomass to energydense solid fuel, with relatively uniform handling characteristics. The fuel value of the produced solid may be as much as 36% higher than that of the original biomass. In the process, biomass is reacted with hot compressed water at the temperature of 200-260 °C. The mass and energy balance in wet torrefaction were established for these conditions. Products include pretreated solid, precipitates (simple sugars and sugar derivatives), volatile acids, and gases (carbon dioxide). With increasing temperature, the mass of the solid decreases, the fuel value of the solid increases, and the quantity of gas increases. The heat of reaction for each temperature was estimated from an energy balance. The uncertainty analysis also showed that the temperature slightly affected the heat of reaction, which is very close to zero.
A wealth of research have been conducted in the wet torrefaction of lignocellulosic biomass,5-9 but relatively few address the comprehensive mass and energy balance involved in the wet torrefaction. Mass and energy balances are of significant importance for the economic design and optimization of a new process. To date, there is no such analysis published in the literature. In this study, the primary objective is to establish the mass and energy balance in the wet torrefaction, to allow for a detailed techno-economic analysis of the process and determine its commercial viability. In the process known as wet torrefaction, biomass is treated in hot compressed water, resulting in three product groups: solid fuel, aqueous compounds, and gases.10-12 The reaction temperature is in the range of 200-260 °C, and the pressure is the saturated vapor pressure. The solid product contains about 55-90% of the mass and 80-95% of the fuel value of the original feedstock. Aqueous compounds, consisting primarily of monosaccharides, furfural derivatives, and organic acids, make up approximately 10% by mass of the feedstock. Gas products make up the balance.13-15 The solid products have some promising properties, which may be more suitable for thermochemical conversion than the original feedstock.12 Wet-torrefied biomass has increased fixed carbon (proximate analysis) and atomic carbon (ultimate analysis), indicating that the biomass has been transformed into a fuel with properties resembling low-rank coal. In addition, reduced equilibrium moisture content indicates that the pretreated
1. Introduction The Energy Independence and Security Act (EISA) of 2007 requires that renewable fuels collectively supply at least 36 billion gallons of U.S. motor fuels by 2022. Today, the U.S. produces nearly 9 billion gallons of ethanol annually, only about a quarter of the renewable fuels called for by 2022.1,2 To achieve this goal, lignocellulosic biomass is considered as the most important potential feedstock to produce the secondgeneration biofuels for years to come. Although lignocellulosic biomass is not expensive, logistics of these feedstocks, including harvesting, handling, and transportation, could be very cost-intensive. Moreover, the specific chemical properties of lignocellulosic biomass2-4 prevent biomass gasification and fast pyrolysis from being technically and economically ready for widespread commercialization, such as coal application. For example, some challenges associated with biomass are related to high volatile content, high oxygen content, and high ash content in some cases. Consequently, there is a need of pretreatment technologies to convert lignocellulosic biomass to an intermediate solid fuel, which is more suitable for thermochemical conversion. † This paper has been designated for the Bioenergy and Green Engineering special section. *To whom correspondence should be addressed. Telephone: þ1-775784-4253. Fax: þ1-775-784-4764. E-mail:
[email protected]. (1) Biofuels create green jobs: Growing transportation fuels and the nation’s economy. Office of the Biomass Program, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C., 2008. (2) Huber, G. W.; Dale, B. E. Sci. Am. 2009, July, 52. (3) Biomass multiyear program. Office of the Biomass Program, Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C., 2008. (4) Yu, Y.; Lou, X.; Wu, H. W. Energy Fuels 2008, 22, 46. (5) Mochidzuki, K.; Sakoda, A.; Suzuki, M. Thermochim. Acta 2000, 348, 69. (6) Petchpradab, P.; Yoshida, T.; Charinpanitkul, T.; Matsumura, Y. Ind. Eng. Chem. Res. 2009, 48, 4587. (7) Goudriaan, F.; Beld, B.; Boerefijn, F. R.; Bos, G. M.; Naber, J. E.; Wal, S.; Zeevalkink, J. A. Proceedings of the Progress in Thermochemical Biomass Conversion, Tyrol, Austria, 2000.
r 2010 American Chemical Society
(8) Allen, S. G.; Kam, L. C.; Zemann, A. J.; Antal, M. J., Jr. Ind. Eng. Chem. Res. 1996, 35, 2709. (9) Funke, A.; Ziegler, F. Proceedings of the 17th European Biomass Conference, Hamburg, Germany, 2009. (10) Ando, H.; Sakadi, T.; Kokusho, T.; Shibata, M.; Uemura, Y.; Hatate, Y. Ind. Eng. Chem. Res. 2000, 39, 3688. (11) Sasaki, M.; Adschiri, T.; Arai, K. Bioresour. Technol. 2003, 86, 301. (12) Yan, W.; Acharjee, T. C.; Coronella, C. J.; Vasquez, V. R. Environ. Prog. Sustainable Energy 2009, 28, 435. (13) Sun, Y.; Cheng, J. Bioresour. Technol. 1992, 39, 107. (14) Bobleter, O. Prog. Polym. Sci. 1994, 19, 797. (15) Petersen, M.; Larsen, J.; Thomsen, H. M. Biomass Bioenergy 2009, 33, 834.
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Table 1. Mass Distribution in the Wet Torrefaction of Loblolly Pinea mass in (g)
mass out (g)
temperature (°C)
wood
water
pretreated wood
acetic acids
precipitates
water
gas
200 230 260
1.000 1.000 1.000
4.93 (0.06) 4.98 (0.03) 4.99 (0.02)
0.83 (0.00) 0.75 (0.01) 0.63 (0.02)
0.01 (0.00) 0.03 (0.00) 0.06 (0.00)
0.14 (0.01) 0.10 (0.01) 0.09 (0.0)
4.86 (0.04) 4.99 (0.07) 5.01 (0.08)
0.09 (0.01) 0.12 (0.04) 0.20 (0.10)
a
Reactants and products are given per the mass of dry wood. Uncertainty is shown in parentheses.
solid is more hydrophobic and can be easily stored to accommodate seasonal availability. In addition, wet-torrefied biomass is very friable, contains lignin, and might be pelletized for feeding to a thermochemical conversion process.12 In this study, wet torrefaction of loblolly pine was carried out in the temperature range of 200-260 °C and at the saturated vapor pressure (225-680 psi). After the pretreatment, the products were separated and quantitatively measured. The heat of formation of biomass solid (including feedstock and pretreated biomass) was first determined from the ultimate analysis and the heat of combustion. In the end, the heat of reaction at selected temperatures was estimated from the energy balance, and uncertainty analysis of ΔHr was performed as well.
Figure 1. Mass balances for wet torrefaction of woody biomass.
(Pittsburgh, PA) for full determination of C, H, N, S, and O.16 This work was performed at the Desert Research Institute (DRI; Reno, NV). 2.4.2. Heat of Combustion. The heat of combustion for biomass solid was measured in a Parr 1241 adiabatic oxygen bomb calorimeter (Moline, IL), which is fitted with a continuous temperature recording system.17 2.4.3. Heat Capacity. The heat capacity of biomass solid was determined using a Perkin-Elmer Pyris 1 differential scanning calorimeter (Waltham, MA).18 Nitrogen is used as the purging gas, and the flow rate was maintained at 20 mL min-1. The heat capacity of biomass solid was measured for temperatures ranging from 25 to 100 °C. 2.4.4. Analysis of Aqueous Compounds. Precipitates (nonvolatile aqueous compounds) in the liquid product were analyzed by GC/MS. Organic acids are measured separately by IC. Both analyses were performed at the DRI.
2. Experimental Section 2.1. Biomass. As a typical lignocellulosic biomass, loblolly pine was acquired from the state of Alabama. On a mass basis, it consists of 11.9% hemicellulose, 54.0% cellulose, 25.0% lignin, 8.7% extractives, and 0.4% ash.12 The pine sample was milled to the particle size range (0.6-1.2 mm) and dried at 105 °C for 24 h before wet torrefaction. 2.2. Wet Torrefaction. Wet torrefaction of loblolly pine was performed in a Parr Series 4560 benchtop reactor (reactor volume of 100 mL) (Moline, IL). The temperature of the reactor was controlled using a proportional-integral-derivative (PID) controller. The reactor pressure was not controlled, approximately in accordance with the water vapor pressure. For each run, a mixture of loblolly pine and water in a mass ratio of 1:5 was loaded to the reaction vessel. The mixture was stirred manually to ensure complete wetting. Nitrogen was passed through the reactor for 10 min to purge oxygen from the reactor. The reactor was heated to the desired temperature and maintained at that temperature for 5 min, after which the reactor was rapidly cooled off by immersion in an ice bath. All experiments were performed at least 3 times. 2.3. Quantitative Measurement of Reaction Products. When the reactor reaches room temperature, the gas product was not collected but was released into the atmosphere. The quantity of gas product was calculated by balance. The liquid sample and wet pretreated solid product were separated by vacuum filtration using a Buchner funnel with Whatman filter paper (grade 3, 0.6 μm). The wet pretreated solid product was dried at 105 °C for 24 h. The composition of aqueous compounds (except volatile acids) was determined by gas chromatography/mass spectrometry (GC/MS). The composition of volatile acids was determined by ion chromatography (IC). The amount of precipitates (nonvolatile aqueous compounds) was obtained by drying the liquid product at 105 °C for 24 h. The amount of volatile acids is calculated by the product of volatile acid concentration and total volume of total mass loss in both drying processes. The amount of output water was determined by total mass loss, excluding the amount of volatile acids. 2.4. Analytical Methods. Both raw biomass feedstock and pretreated solid product were characterized by ultimate analysis, heat of combustion, and heat capacity measurements. Prior to the analysis, the biomass solids were dried at 105 °C for 24 h. The aqueous compounds are characterized by either GC/MS or IC. 2.4.1. Ultimate Analysis. Ultimate analysis of biomass solid was carried out with a FlashEA 1112 elemental analyzer
3. Results and Discussion 3.1. Mass Balance in Wet Torrefaction. Figure 1 shows input and output streams in the experimental batch wet torrefaction process. The input stream includes the loblolly pine and water. According to the separation procedure (described in the Experimental Section), the output is divided into five streams: pretreated solid, precipitates, volatile acids, water, and gases. Both pretreated solid and precipitates are obtained upon the drying of wet pretreated solid and filtrate, respectively. In both drying processes, the mass loss is only due to volatile acids and water evaporating at the drying conditions. The concentration of volatile acids is determined with IC. The amount of volatile acids (mainly acetic acid) can be estimated by the product of its concentration and volume of total mass loss from both dryings. Thus, the water in the output is the difference between the total mass loss and the amount of volatile acids. In this study, the quantity of gas is determined by balance and not measured directly. The mass balance for wet torrefaction is shown Table 1. The gas product accounts for 9-20% with an increasing temperature. The temperature also significantly affects mass yield and characteristics of the pretreated solid according to (16) American Society for Testing and Materials (ASTM). ASTM D3176-89. ASTM International, West Conshohocken, PA, 2002. (17) Operating instructions for the 1241 oxygen bomb calorimeter. Parr Instruments, Moline, IL, 203M, p 1. (18) Chiu, L. F.; Liu, H. F.; Li, M. H. J. Chem. Eng. Data 1999, 44, 631.
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Table 2. IC Analysis of Organic Acids in the Liquid Producta temperature (°C)
acetic acid
formic acid
methanelactic sulfonic glutaric acid acid acid
succinic acid
200 230 260
0.0100 0.0246 0.0550
0.0045 0.0085 0.0150
0.0017 0.0004 0.0026 0.0004 0.0078 0.0008
0.0005 0.0005 0.0008
a
0.0002 0.0004 0.0012
Table 3. Ultimate Analysis of Biomass Solid biomass solid loblolly pine wet torrefied loblolly pine
The data are given per the mass of dry wood.
temperature (°C) C (%) H (%) N (%) S (%) O (%) n/a 200 230 260
50.25 54.72 56.05 72.07
5.97 6.03 5.94 4.90
0 0.14 0.09 0.16
0 0 0 0
43.34 39.11 37.92 22.89
The values of x and y can be easily determined from the ultimate analysis of the biomass solids. Using the measured heat of combustion and heat of formation of the products (CO2 and H2O), the heat of formation of the biomass solids can be determined, summarized in Table 4. 3.3. Determination of the Heat of Reaction. Estimation of the heat of reaction (ΔHr) is important for the proper design of commercial-scale reactors for wet torrefaction of lignocellulosic biomass. To estimate the heat of reaction, it is necessary to make some assumptions. First, the gas is assumed to consist mainly of CO2. Measurements performed at the DRI indicate that CO2 accounts for over 90% in the gas product. It is also confirmed in the literature.7 Second, acetic acid is selected as the representative for volatile acids because of its dominant presence (see Table 2). Third, the heat of formation of all precipitates measured is accounted for by the heat of formation of a single representative compound. For example, glucose is chosen as a representative of all precipitates formed at 200 °C. This is reasonable because all C5 and C6 sugars have similar heats of formation and heat capacities (on a mass basis). At higher reaction temperatures (230 and 260 °C), 5-hydroxymethy furfural (5-HMF) was selected as the representative compound, owing to its predominant concentration in the precipitates. The heat of reaction is determined by the difference of the heats of formation of the products and reactants at each temperature (see Figure 3). The reference condition for the enthalpy calculations is 25 °C and 14.7 psi. The analytical results show that the mole fraction of total organic acid in the aqueous solution is about 0.5% (mol/mol). The enthalpy of mixing of acetic acid in water is negligible when its mole fraction in water is below 1%19 and is neglected in these calculations. Therefore, the heat of reaction is summarized in Table 5. It is obvious that the heat of reaction is quite small and barely endothermic. The magnitude of the heat of reaction is less than 2% of the heat of combustion of the untreated biomass. With an increasing temperature, the reaction seems to become less endothermic. Wet torrefaction in the selected operation conditions12 showed that the temperature (rather than the water biomass ratio, particle size, and reaction time) is the significant factor affecting the yield and characteristics of pretreated biomass. The fiber analysis shows that the decomposition of major fractions in lignocellulosic biomass increases when the temperature rises. For instance, the conversion of hemicellulose, cellulose, and lignin are 100, 64.2, and 22.9% at 260 °C compared to 97, 22.1, and 1.4% at 200 °C. It means that the main reaction that occurred in wet torrefaction is the hydrolysis of hemicellulose and cellulose. These hydrolysis reactions are believed to be endothermic.20 However, the reaction rates of monosaccharide degradation increase dramatically at higher temperatures, confirmed by the results of Figure 2. At 260 °C, the degradation of monosaccharides
Figure 2. Composition of precipiates in the aqueous output stream at various temperatures. Each mass fraction is reported as a fraction of the dry biomass feed.
ultimate analysis and fuel value measurements.12 From IC analysis results, organic acids are also produced and account for 2-9% of raw biomass (Table 2). Among these acids, volatile acids (acetic and formic acids) are dominant, with their amounts increasing with the rising temperature. The amount of precipitates decreases with an increasing temperature, from 14% at 200 °C to 9% at 260 °C. Figure 2 shows that precipitates consist of mainly monosaccharides (e.g., xylose, mannose, galactose, arabinose, and glucose) and their degradation products (e.g., 5-hydroxymethyl furural). One dominant reaction is the hydrolysis of hemicellulose and cellulose to monosaccharides.6,7 The more reactive fraction, hemicellulose, easily hydrolyzes even at low temperature (200 °C), and less reactive cellulose reacts more significantly with an increasing temperature (230260 °C). On the other hand, thermal degradation of monosaccharides becomes increasingly significant with an increasing temperature. 3.2. Determination of the Heat of Formation for Biomass Solids. The heat of combustion (ΔHc) of the biomass solids is accurately measured with a calorimetric bomb. The higher heat of combustion is equal to the difference between the heat of formation (ΔHf) of the products (carbon dioxide and liquid water) and the heat of formation of the reactants (biomass solid and oxygen). Table 3 shows the ultimate analysis results of the biomass solids. The chemical formula of biomass solid is represented as CHxOy and the combustion of biomass solid can be expressed as
ΔHc x y x H2 OðlÞ CHx Oy ðsÞ þ 1 þ - O2 ðgÞ f CO2 ðgÞ þ 4 2 2
(19) Nagata, I.; Gotoh, K.; Tamura, K. Fluid Phase Equilib. 1996, 124, 31. (20) Kunihisa, K. S.; Ogawa, H. J. Therm. Anal. Calorim. 1985, 30, 49.
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Table 4. Chemical Formula, Heat of Combustion, and Heat of Formation of Biomass Solid biomass solid loblolly pine wet torrefied loblolly pine
temperature (°C)
heat capacity (J g-1 K-1)
n/a 200 230 260
3.30 6.30 6.30 6.30
chemical formula
heat of combustion (kJ g-1)
heat of formation (kJ g-1)
-19.520 -21.081 -22.057 -26.512
-5.490 -5.480 -4.812 -4.124
CH1.426O0.647 CH1.322O0.536 CH1.272O0.507 CH0.816O0.238
Table 5. Enthalpy and Heat of Reaction in the Wet Torrefaction of Loblolly Pinea enthalpy in (kJ g-1) temperature (°C) 200 230 260 a
wood
water
-4.92 (0.52) -74.64 (0.89) -4.82 (0.52) -74.64 (0.42) -4.72 (0.52) -74.12 (0.22)
enthalpy out (kJ g-1) pretreated wood -3.65 (0.47) -2.63 (0.45) -1.65 (0.45)
acetic acid
precipitates
water
gas
-0.08 (0.00) -0.94 (0.08) -73.56 (0.59) -0.77 (0.13) -0.19 (0.00) -0.29 (0.03) -74.78 (1.08) -1.04 (0.36) -0.42 (0.00) -0.25 (0.00) -74.41 (1.21) -1.86 (0.87)
heat of reaction (kJ g-1) 0.56 (0.72) 0.53 (0.75) 0.25 (0.92)
All data are obtained on the basis of 1 g of biomass feedstock. Uncertainty is shown in parentheses.
Figure 3. Calculation flow diagram for the heat of reaction in the wet torrefaction.
Figure 5. Frequency distributions of the heat of reaction at three temperatures. Figure 4. Mass and energy balances in wet torrefaction of loblolly pine at a temperature of 260 °C for 5 min.
includes instrumental error from each measurement and error propagation produced from the calculation. The distribution of error could be Gaussian or uniform depending upon the variables. In this study, we use the Monte Carlo method. The basic steps are the specification of the probability distributions for the random variables, a sampling method to choose random values from these distributions, recalculation of the heat of reaction using these random values, and then performing a statistical analysis of the results. As an example of mass balance in wet torrefaction, there are three repeated runs for each temperature. In each run, the error could be instrumental error for the directmeasured variable or error propagation for the indirectmeasured variable. The statistical distributions for these variables were also determined as well. A Monte Carlo simulation was performed 5000 times for each independent experimental run. Since the experiments were done three times at each temperature, 15 000 calculations are combined in the final uncertainty analysis (see Table 1). We performed an uncertainty analysis of ΔHr estimations using the same approach. Figure 5 shows the results as frequency distributions for three temperatures. We see that the impact of the errors on the estimates of the heat of the reaction is significant in the sense that the distributions overlap substantially and do not provide a clear distinction of the temperature effect on ΔHr. The results show that mostly the heat of reaction for the pretreatment conditions studied is not significant and tends to be around zero on average. One could argue that there is a mild trend for the
dominates and produces a large amount of products, such as 5-HMF. Recent research has shown that the degradation of sugars is exothermic.21 Thus, it is possible that wet torrefaction could be exothermic if the reaction is held for a long period of time, so that the thermal degradation of sugars (produced by the endothermic hydrolysis reactions) is significant. There are some similarities between dry and wet torrefaction. Prins et al.22 reported a mass and energy balance for dry torrefaction of beech wood at 250 °C for 30 min and at 300 °C for 10 min, respectively. Acetic acid and carbon dioxide are produced in significant quantities during dry torrefaction. The heat of dry torrefaction is reported to be 0.087 and 0.124 kJ g-1, respectively. Hence, the heat of reaction of both pretreatment processes seems to be of the same magnitude. 3.4. Uncertainty Analysis of the Heat of Reaction Calculation. In the case shown in Figure 4, the estimated heat of reaction is relatively small (0.25 kJ) compared to the energy in the reactants to the reaction temperatures. Therefore, the errors associated with each variable may play a significant role in determining if the reaction is endothermic or exothermic. The heat of reaction ΔHr calculation involves several steps that include the use of variables containing errors. The error (21) Knezevic, D.; van Swaaij, W. P. M.; Kersten, S. R. A. Ind. Eng. Chem. Res. 2009, 48, 4731. (22) Prins, M. J.; Ptasinski, K. J.; Janssen, J. J. G. F. Energy 2006, 31, 3458.
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heat of reaction decreasing with an increasing temperature, but statistically, it would be difficult to justify such a conclusion given the effects of the random errors in the calculations.
distribution. In the temperature range, the heat of reaction was close to zero. Uncertainty analysis of the heat of reaction estimation was also performed to show that the effect of the temperature on the heat of reaction is not significant.
4. Conclusions
Acknowledgment. This work was supported by the U.S. Department of Energy, Award DE-FG36-01GO11082. The authors acknowledge meaningful discussions with Larry Felix from the Gas Technology Institute (GTI), Kent Hoekman from the Desert Research Institute (DRI), and Craig Einfeldt from the Changing World Technologies (CWT). The authors also acknowledge some assistance by Jeremey Riggle (DRI), Barbara Zielinska (DRI), and Joko Sustrino [University of Nevada, Reno (UNR)] in chemical characterization and analysis.
Wet torrefaction can convert lignocellulosic biomass into a solid fuel, which may become a more favorable feedstock for further thermochemical conversions. In this study, the mass and energy balance of the wet torrefaction process are established in the range of conditions studied. The temperature in the range of 200-260 °C has a significant influence on the reaction mechanism, solid fuel character, and product
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