Article pubs.acs.org/EF
Microwave Torrefaction of Douglas Fir Sawdust Pellets Shoujie Ren,† Hanwu Lei,*,† Lu Wang,† Quan Bu,† Yi Wei,† Jing Liang,† Yupeng Liu,† James Julson,‡ Shulin Chen,† Joan Wu,† and Roger Ruan§ †
Bioproducts, Sciences and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, Washington 99352, United States ‡ Department of Agricultural and Biosystems Engineering, South Dakota State University, Brookings, South Dakota 57007, United States § Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota 55108, United States ABSTRACT: Microwave torrefaction of Douglas fir pellets was investigated and the effects of process conditions on the yields of products were determined. The reaction temperature and time significantly influenced the yields of torrefied biomass, bio-oil, and noncondensable gases. Three linear models were developed to predict the product yield, as a function of reaction temperature and time. Gas chromatography/mass spectroscopy (GC/MS) analysis for bio-oils showed that the bio-oils were mainly composed of furans, phenolics, sugars, ketones/aldehydes, and organic acids. The amounts of each compound varied with the reaction conditions. Over the reaction temperature of 275 °C, noncondensable gases were mainly composed of CO2 and CO and its yields and compositions were significantly influenced by the temperature. Higher heating values (HHVs) of torrefied biomass were 20.90−25.07 MJ/kg, which is an ∼6%−31% increase, compared to the HHV of raw biomass. The energy yields of torrefied biomass, 67.03%−90.06%, implied that most energy was retained in the torrefied biomass. One linear model as a function of reaction temperature and time was developed to predict the energy yield. Mass and energy balance analysis showed that the total energy recovery was ∼79%−88%, implying that the microwave torrefaction of biomass is practical and energyefficient.
1. INTRODUCTION
to pelletization and pyrolysis, which have efficiencies of 84% and 64% respectively.3 Torrefaction is a mild thermal treatment method that is operated at atmosphere pressure with the absence of oxygen. The temperature of torrefaction ranges from 200−300 °C.4 During torrefaction, water is removed and biomass is dried deeply. The hemicelluloses are decomposed to volatiles and cellulose and lignin are dehydrated and partially decomposed. The structure of cellulose and lignin are modified. The main product of torrefaction is torrefied biomass with coproducts of bio-oil and noncondensable gases. Advantages of torrefied biomass were reported in the literature.4−12 Torrefied biomass has a very low moisture content, which is generally ∼1−6 wt %.4 The O/C ratio of torrefied biomass is also decreased. Prins et al. reported that torrefaction decreased the O/C ratio of wood from 0.7 to 0.5−0.6, and the degree of decrease was dependent on the reaction temperature and time.5 The higher heating value (HHV) of torrefied biomass is increased to 20− 24 MJ/kg, which is 10%−30% higher than those of fresh biomass.6,8 Approximately 90% of energy content in initial biomass is retained in ∼70 wt % of torrefied biomass.4 The energy density of torredied biomass is greatly improved. Torrefied biomass also has high grindability, because the biomass structure is changed during torrefaction.4,9 The power consumption in size reduction for torrefied biomass can be decreased by ∼80%−90%.4 These characteristics of torrefied
Biomass including wood, crop residue, and energy grass are enormous and renewable energy sources. Over 1.3 billion dry tons of potential biomass is available annually, and more than 6% of total energy consumption is from renewable sources in which biomass contributes ∼47% in the United States.1 With reducing fossil energy and increasing energy demand, biomass is playing an important role in energy supplies. However, biomass has some disadvantages that retard its utilization. Biomass has low bulk density, which is ∼0.1−0.3 t/ m3 for wood and 0.02−0.2 t/m3 for straw.2 Low bulk densities lead to increased cost for transportation and storage. Water content in biomass is generally larger than 15%, which results in deterioration during storage and influences biomass conversion efficiency. The energy contents of biomass are much lower than those of coals and crude oil, partly due to high oxygen content.2 Therefore, it is difficult to efficiently and consistently apply biomass as a source of fuel. Biomass pretreatment and treatment is required before conversion to high-energy biofuels. There are several types of technologies to improve biomass density and energy content or convert biomass to high energy fuels; these methodologies include pelletization, torrefaction, and pyrolysis. Pelletization can increase the bulk density of biomass to 0.5−0.7 t/m3 and increase the energy density to 7.8−10.5 GJ/m3.3 Fast pyrolysis converts biomass to bio-oils, which are easy to transport. However, current pyrolysis oil (biooil) is acidic, viscous, reactive, and thermally unstable. Torrefaction is considered as a very promising technology, with its high process efficiency, which is up to 94%, compared © 2012 American Chemical Society
Received: April 17, 2012 Revised: August 8, 2012 Published: August 8, 2012 5936
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
China) with a rated power of 1000 W was used at a 600 W power setting. Two hundred grams (200 g) of Douglas fir pellets were placed in a half-liter quartz flask inside of the microwave oven. The oven reached desired temperatures after ∼6 min with the average heating rate of 40 °C/min. The experimental setting is shown in Figure 1. Two parallel bulb condensers, each one-half meter long, were used for condensation. The temperature of the cooling water in the condensers was 0−5 °C. The system was purged with nitrogen on a flow rate of 1000 mL/min for 20 min prior to microwave torrefaction to create an oxygen-free background. The temperature of biomass was measured by an infrared sensor through a dead-end quartz tube, which was penetrated to the central of the flask. After reaching desired reaction temperatures, the microwave reactor equipped with automatic temperature/power control used a minimum power (e.g., 0−100 W) to maintain the desired reaction temperatures. During torrefaction condensable volatiles were condensed into liquids as bio-oils and noncondensable gases escaped at the end of the condensers, where they were either burned or collected for analysis. Torrefied biomass was left in the quartz flask. The weight of noncondensable gases was calculated by difference using the following equation:
biomass benefit further processes, such as combustion and gasification.5,10−12 Torrefied biomass can generate electricity with a similar efficiency to coal.12 Prins et al. investigated gasification for torrefied biomass and concluded that the quality of syngas was improved compared with direct biomass gasfication.5 However, in the torrefaction process, ∼10%− 20% of the energy of the biomass is maintained in two coproducts: torrefaction bio-oil and noncondensable gases. Characterization of bio-oil and noncondensable gases from torrefaction is important. It will be helpful to understand the mechanism of biomass torrefaction and recover energy by investigating the potential utilization for bio-oil and noncondensable gases. Microwave heating is a thermochemical treatment of biomass with microwave irradiation. The major advantage of the microwave heating process over conventional heating methods is the nature of internal fast and uniform heating by microwave irradiation.13,14 Microwave heating also triggers thermochemical conversion reactions rapidly in relatively large-sized biomass materials that will help reduce the process cost, because it is not necessary to grind the biomass to very fine particles prior to pretreatment.14−16 Microwave heating has been widely used in processing plant residues, 15,17−20 wood,13,14,16 and sewage sludge21,22 to produce high-energy fuels. The objective of this study was to investigate the microwave torrefaction of Douglas fir pellets. The effects of torrefaction conditions on the yields of torrefied biomass, noncondensable gases, and torrefaction bio-oil were determined and regression models as functions of reaction temperature and time were established to predict products yields. Heating values of torrefied biomass were investigated to determine the energy yield. Compositions of bio-oil and noncondensable gases were determined by GC/MS and GC, respectively. The mass and energy balances were also studied.
weight of noncondensable gases = initial wood pellet mass − bio‐oil mass − torrefied biomass mass
2.3. Experiment Design. A central composite experimental design (CCD) was used to optimize the process conditions in microwave torrefaction.23 Two independent variables, reaction temperature (X1, °C) and reaction residence time (X2, min), and three dependent variables, the yields of bio-oil, noncondensable gases, and torrefied biomass were chosen. For statistical calculations, the variables Xi were coded as xi, according to eq 2: xi =
2.1. Materials. Douglas fir sawdust pellets were purchased from Bear Mountain Forest Products, Inc. (USA). The pellets were made from 100% natural Douglas fir wood sawdust. The pellets had an average diameter of 6 mm and an average length of 10 mm. The mass density (bulk) and energy density (bulk) were ∼680 kg/m3 and 13.2 GJ/m3, respectively. Proximate and elemental analyses of Douglas fir pellet was shown in Table 1. The higher heating value (HHV) of Douglas fir pellet was 19.4 MJ/kg. 2.2. Microwave Apparatus. A Sineo MAS-II batch microwave oven (Sineo Microwave Chemistry Technology Company, Shanghai,
(2)
Yi = b0 + b1X1 + b2X 2 + b11X12 + b21X 2X1 + b22X 2 2
(3)
where Yi denotes the predicted responses, X1 and X2 are independent variables, b0, b1, b2, b11, b22, and b21 are regression coefficients. 2.4. GC/MS Analysis for Bio-oil. Chemical compositions of biooils were determined using an Agilent 7890A GC/MS system (Agilent Technologies, USA) with a DB-5 capillary column. The gas chromatograph was programmed at 45 °C for 3 min and then the temperature was increased at a rate of 5 °C/min to 300 °C and finally held with an isothermal for 5 min. The injector temperature was 300 °C, and the injection size was 1 μL. The flow rate of the carrier gas (helium) was 0.6 mL/min. The ion source temperature was 230 °C for the mass selective detector. The compounds were identified by comparing the spectral data with the NIST Mass Spectral library. 2.5. GC Analysis for Noncondensable Gases. Chemical compositions of noncondensable gases were determined by a Carle AGC 400 gas chromatography (GC) system with a thermal conductivity detector (TCD). The details of experiment setting were described in a previous report.24 2.6. Heating Value Determination. The heating values of torrefied Douglas fir pellets and bio-oils were determined by the Poultry Laboratory at the University of Arkansas. Noncondensable gases
Table 1. Proximate and Elemental Analyses of Douglas Fir Pellet
Proximate Analysis (wt %) moisture volatile matter fixed carbon ash Elemental Analysis (wt %) carbon hydrogen nitrogen oxygen higher heating value, HHV (MJ/kg)
Xi − X 0 ΔX
where xi is the dimensionless value of an independent variable, Xi the real value of an independent variable, X0 the real value of the independent variable at the center point, and ΔX the step change. The central point of independent variables was set at 275 °C and 15 min and the step changes were chosen at 25 °C and 5 min. A total of 11 experiments with 4 axial points (α = 1.41) and 3 replications at the center points was employed for the optimization of microwave torrefaction (see Tables 2 and 3). The second-degree polynomial (eq 3) was calculated to estimate the response of dependent variables.
2. MATERIALS AND METHODS
characteristics
(1)
Douglas fir pellet 4.82 76.08 18.89 0.21 47.9 6.55 0.08 45.57 19.4 5937
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
Figure 1. Diagram of laboratory-scale microwave oven setting.
was used to fit the data for bio-oil, noncondensable gases, and torrefied biomass. Equation 3 was reduced by using backward statistical analysis, and eqs 4−6 were obtained with its significant terms (P-value < 0.0001). The correlation coefficients of determination (R2) was 0.96, 0.93, and 0.97 respectively for bio-oil, noncondensable gases, and torrefied biomass, implying that the linear regression models accurately represented the experiment data and can be used to predict the production yields for microwave torrefaction of biomass. Three-dimensional (3D) surface response profiles were developed for the yields of bio-oil, noncondensable gases, and torrefied biomass versus torrefaction conditions (Figure 2). The yield of bio-oil was within the range of 13.12−36.35 wt % and the yield of noncondensable gases was within the range of 3.55−11.32 wt %; each increased with the increase of reaction temperature and time. The maximum yield of bio-oil was observed at a temperature of 300 °C and a reaction time of 20 min. The maximum yield of noncondensable gases was found at 310 °C and 15 min. In contrast, the yield of torrefied biomass ranged from 52.61 wt % to 83.15 wt % and decreased as the reaction temperature and time each increased. The minimum and maximum torrefied biomass yields were observed under the conditions of 300 °C and 20 min, and 250 °C and 10 min, respectively. The amount of mass loss is an important parameter in torrefaction, because too much mass loss has negative effects on torrefied biomass production. The mass loss of biomass during torrefaction is highly correlated to the reaction temperature, according to previous reports. At temperatures below 250 °C during torrefaction, mass loss occurs with the decomposition of
Table 2. Coded Levels of Independent Variables in the Experimental Design level
reaction temperature (°C)
reaction time (min)
−α = −1.41 −1 0 1 α = 1.41 ΔX
240 250 275 300 310 25
8 10 15 20 22 5
3. RESULTS AND DISCUSSION 3.1. Response Surface Analysis. Reaction temperature and time were chosen as independent variables to investigate their effects on product yields, because they have significant influences on biomass thermal conversions. The detailed experimental conditions and results are shown in Table 3. Using these results of experiments obtained, the following three linear model equations for bio-oil yield (eq 4), noncondensable gases yield (eq 5), and torrefied biomass yield (eq 6), as a function of reaction temperature (X1 = 240−310 °C) and reaction time (X2 = 7.9−22 min) and respectively: Ybio‐oil = −69.72 + 0.32X1 + 0.018X 2 (4) Ynoncondensable gases = −24.37 + 0.10X1 + 0.18X 2
(5)
Ytorrefied biomass = 194.09 − 0.43X1 − 0.55X 2
(6)
It was found that the model terms X12 (P-value = 0.86, 0.65, 0.98), X11 (P-value = 0.80, 0.079, 0.76), and X22 (P-value = 0.61, 0.72, 0.76) were insignificant (P-values > 0.05) when eq 3 Table 3. Summary of Experimental Design and Results Code value
Actual value
Yield (%)
run
reaction temperature x1
reaction time x2
reaction temperature X1
reaction time X2
bio-oil
noncondensable gases
torrefied biomass
DFT1 DFT2 DFT3 DFT4 DFT5 DFT6 DFT7 DFT8 DFT9 DFT10 DFT11
−1 1 −1 1 −1.41 1.41 0 0 0 0 0
−1 −1 1 1 0 0 −1.41 1.41 0 0 0
250 300 250 300 240 310 275 275 275 275 275
10 10 20 20 15 15 7.93 22 15 15 15
13.12 32.57 17.66 36.35 14.06 32.62 22.08 26.79 23.41 25.56 24.56
3.74 8.33 5.82 11.04 3.55 11.32 5.61 7.29 5.75 6.25 7.15
83.15 59.11 76.52 52.61 82.39 56.07 72.31 65.93 70.85 68.20 68.29
5938
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
(35 wt %) and light damage of lignin with small weight mass loss
3.2. GC/MS Analysis for Bio-oil. The bio-oil yield in Douglas fir pellet torrefaction was ∼13.12 wt % to 36.35 wt %. The composition of bio-oil was determined by GC/MS to obtain an insight of the torrefaction reactions and can be used to investigate potential utilization for bio-oil. The main components of bio-oils were furans, guaiacols, ketones/ aldehydes, sugars, and organic acids (see Figure 4). 5939
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
chain hydrocarbons, such as ethylene, ethane, and propane, were also observed at reaction temperatures over 275 °C in small amounts (0.5%−1% (v/v)). These observations were consistent with the previous reports.5,27 Hydrogen was not detected under all torrefaction conditions, possibly because of the mild biomass decomposition conditions with the relatively low reaction temperature during torrefaction. The compositions of noncondensable gases at different reaction temperatures are shown in Figure 5.
Figure 4. Distribution of chemicals in bio-oil.
The furans ranged in area percentage from 18.49% to 29.07% and were primarily composed of ß-methoxy-(S)-2-furanethanol, furfural, 2-furanmethanol, and tetrahydro-2,5-dimethoxy-furan. The largest amount of chemical in furans was ß-methoxy-(S)-2furanethanol, which contributed an area percentage of ∼10− 20.55% in bio-oils. Sugars in bio-oils ranged in area percentage from 2.74% to 17.9% and were mainly composed of 1,6anhydro-ß-D-glucopyranose, 1,4:3,6-dianhydro-α-D-glucopyranose, and D-mannose, and the amounts of these sugars varied with the reaction conditions. The organic acids ranged in area percentage from 3.23% to 5.59% and increased as the reaction temperature increased. Acetic acid was the acid that was present in the largest amount, which was ∼1.10−2.06 area% in bio-oils. Previous reports indicated that the lignin of biomass was slightly decomposed during torrefaction and a small amount of phenolic chemicals was detected in bio-oils.5,7 In this research, GC/MS analysis of torrefaction bio-oils showed that a large amount of phenolic chemicals (such as 2-methoxyphenol, 2methoxy-4-(1-propenyl)-phenol, 2-methoxy-4-methylphenol, and 4-ethyl-2-methoxyphenol) was observed. The total phenolic chemicals in the bio-oil ranged from 33.66 area% to 49.02 area%. The smallest amount of phenolics was observed at the reaction temperature of 240 °C and a reaction time of 15 min. The amount of phenolics was not significantly changed with the reaction temperature and time over 250 °C. The large amount of phenolics observed in bio-oil indicates that the lignin of the Douglas fir pellet was partially damaged and decomposed during microwave torrefaction. 3.3. GC Analysis for Noncondensable Gases. Noncondensable gas was one of the main byproducts in the microwave torrefaction. The yields of noncondensable gases ranged from 3.55 wt % to 11.32 wt % and increased as the reaction temperature and time each increased. The noncondensable gases were collected at the end of condensers and analyzed using a gas chromatograph. The composition of noncondensable gases was highly correlated with the reaction temperature. At reaction temperatures of 240 and 250 °C, only a small amount of CO2 was observed. At torrefaction temperatures over 275 °C, the composition of noncondensable gases was mainly composed of CO2 and CO. The amount of CO2 ranged from 30% to 52% (v/v), and the amount of CO ranged from 18% to 28% (v/v); each increased significantly as the reaction temperatures increased. The methane and short-
Figure 5. Compositions of noncondensable gases at different reaction temperatures.
3.4. Heating Value Analysis for Torrefied Douglas Fir Pellet. Torrefaction can remarkably improve the heating value of biomass. The higher heating value (HHV) was 30% improved, compared to the raw material in previous reports.6,8 In this research, the HHVs of torrefied biomass ranged from 20.3 MJ/kg to 25.4 MJ/kg and increased as the reaction temperature and time each increased (see Figure 6). The highest HHV of torrefied biomass was 31% more than that of raw biomass, which was observed at a reaction temperature of 300 °C and a reaction time of 20 min. The heating values of torrefied biomass from this study were significantly greater than those of raw biomass and torrefied woody biomasses, based on nonpelletized raw biomass, and were similar to those of combined torrefaction and pelletization (TOP) pellets (combined torrefaction and pelletization) and coals.27−29 The mass density (bulk) of torrefied biomass ranged from 488 kg/ m3 to 621 kg/m3, which was slightly lower than that of wood pellets but significantly higher than that of woody biomass.27−29 The mass density (bulk) decreased with the increasing severity of torrefaction. The energy density (bulk) was ∼11.6−13.0 GJ/ m3, which was significantly higher than those of woody biomass and other torrefied wood biomasses.27−29 The energy yields of torrefied biomass were calculated according to the HHVs and mass yields. The energy yields of torrefied biomass were 67.03%−90% (see Figure 7). The highest energy yield was observed at the reaction temperature of 239 °C and reaction time of 15 min. The lowest energy yield was observed at the reaction temperature of 310 °C and reaction time of 15 min. The energy yields of torrefied biomass were used to fit eq 3 to estimate the effect of process conditions. The following linear model equation for energy yield, as a function of reaction temperature (X1, °C) and reaction time (X2, min), ranging from 240 °C to 310 °C and from 7.9 min to 22 min, respectively, was obtained: Yenergy = 176.87 − 8.56X1 − 0.98X 2
(7) 2
The correlation coefficient of determination (R ) was 0.96, implying that the linear regression models accurately 5940
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
Figure 6. Higher heating values (HHVs) of torrefied biomasses.
the reaction temperature of 275 °C, the energy yields of torrefied biomass decreased to 80.24%−82.05%. Further increasing the temperature to over 300 °C, the energy yields of torrefied biomass were significantly reduced to 67.69%− 71.02%. The energy loss of biomass was from 9.4% to 32.31%, which increased as the reaction temperature increased. The degree of carbonization was calculated by the ratio of the HHVs of torrefied biomass to the HHVs of raw biomass. The degree of carbonization significantly increased with the reaction temperature and time. The highest degree of carbonization was 1.3, which was obtained at a reaction temperature of 300 °C and a reaction time of 20 min. The lowest degree of carbonization was 1.08, which was obtained at a reaction temperature of 250 °C and a reaction time of 10 min. These results showed that most of the energy of raw biomass was maintained in less mass of torrefied biomass. 3.5. Mass and Energy Balance. Mass and energy balance at different reaction temperatures were calculated based on the actual laboratory-scale microwave torrefaction system (see Tables 4 and 5). Torrefied biomass was the largest mass output,
Figure 7. Comparison of mass yield and energy yield of torrefied biomass.
represented the experiment data and can be used to predict the energy yield for the microwave torrefaction of Douglas fir pellets. The surface response profile was developed for the yields of energy versus torrefaction conditions (Figure 8). The energy yields of torrefied biomass decreased with the increase of reaction temperature, but they were not significantly influenced by the reaction time. At the temperature range of 239−250 °C, the energy yields of torrefied biomass were 85.46%−90.6%. At
Table 4. Mass Balance of Microwave Torrefaction for Douglas Fir Pellet at Different Reaction Temperatures Mass In (g)
Mass Out (g)
torrefaction temperature (°C)
Douglas fir pellet
torrefied biomass
bio-oil
noncondensable gases
250 275 300
200 200 200
166.30 141.69 118.21
26.23 46.81 65.14
7.47 11.50 16.65
which was ∼118−166 g and decreased with the increase of the torrefaction temperature from 200 g of initial biomass input. Coproducts, noncondensable gases, and bio-oil contributed a mass output of ∼26−65 g. In the energy balance, the consumption of electricity was the total energy used to heat 200 g of raw biomass from 25 °C to the desired torrefaction temperatures and maintaining temperatures to complete the torrefaction by microwave heating. As shown in Table 5, the consumption of electricity was less than 6.5% of the total energy input. The energy output included the energies that were maintained in products and the heat lost from hot products (bio-oil, noncondensable gases, and torrefied biomass) cooling from reaction to room temperature and heat loss from the reactor to environment during the reaction. Most energy was maintained in torrefied biomass ranged from
Figure 8. Response surface profiles for the energy yield of torrefied biomass versus torrefaction conditions. 5941
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
Table 5. Energy Balance and Energy Recovery of Microwave Torrefaction for Douglas Fir Pellets at Different Reaction Temperatures Energy In (kJ) torrefaction temperature (°C)
Douglas fir pellet
250 275 300
3880 3880 3880
Energy Out (kJ)
electricity
torrefied biomass
225 256.5 261
3475.67 3175.27 2748.38
Energy Recovery (%)
bio-oil
noncondensable gases
heat loss in reactor and products cooling
in torrefied biomass
in all products
113.05 274.78 482.04
10.3 22 50.6
506.00 664.44 859.99
84.67 76.76 66.37
87.67 83.94 79.23
ature. The higher heating values (HHVs) of torrefied Douglas fir pellets, which ranged from 20.3 MJ/kg to 25.4 MJ/kg, were significantly improved; these HHV values increased as the reaction temperature and time each increased. The energy yields of torrefied biomass ranged from 67.03% to 90% and decreased as the reaction temperature increased. The mass density (bulk) of torrefied pellets was slightly lower than that the raw pellet, but the energy density (bulk) was significantly higher than that of woody biomass and torrefied biomass. The total energy recovery was ∼79%−88%, which indicated that the microwave torrefaction of biomass is practical and viable, with the high energy efficiency.
66 to 85% and decreased with the increase of torrefaction temperatures (Table 5). The energy in coproducts (bio-oil and noncondensable gases) ranged from 3.0% to 12.9% and increased with the increase of torrefaction temperatures. These energies from coproducts were sufficient to replace the electricity consumed in the torrefaction theoretically, except the torrefaction process at 250 °C. The energy recovery from the torrefied biomass and coproducts, bio-oil and noncondensable gases, was about 79 to 88% which decreased with the increase of the process temperature. For energy output, ∼10%−20% was lost due to the hot products cooling and heat loss from the reactor to the surrounding environment. However, the heat of hot products can be recovered as a heat source for the process and the heat lost also can be reduced by improving the insulation of the reactor. These results indicate that the microwave torrefaction of biomass is practical and viable, with the high energy efficiency. Microwave torrefaction significantly improved the energy density of biomass in this research and previous reports, which is comparable to the conventional torrefaction.30,31 There are also other benefits in microwave torrefaction. Microwave torrefaction does not require predrying for the biomass, because the moisture is a good acceptor for microwave irradiation.32 This helps to reduce the cost for equipment and energy consumption. Microwave heating is a fast and uniform heating method that can be used to treat large-sized biomass such as pellets. It indicated that the microwave torrefaction for biomass pellets has potential merits for saving the cost of transportation and size reduction. Microwave torrefaction for biomass pellets produced high-quality solid fuels that had the same properties as those of the TOP pellets and coals. Its high energy density, high mass density (bulk), low moisture content, and hydrophobic make the torrefied pellet easy to handle and transport.27,28 According to the findings of this research and previous reports, the microwave torrefaction for biomass is energy-efficient and profitable.30,31
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: 509-372-7628. Fax: 509-372-7690. E-mail: hlei@tricity. wsu.edu. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported in part by Pacific Northwest National Laboratory and the Office of Research at Washington State University. The authors also thank the scientists of Pacific Northwest National Laboratory (PNNL), Alan Zacher and Todd Hart, for their contributions and assistance on noncondensable gases analysis.
■
REFERENCES
(1) Perlack, R. D.; Wright, L. L.; Turhollow, A. F.; Graham, R. L.; Stockes, B. J.; Erbach, D. C. Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply; Technical Report for U.S. Department of Energy and the U.S. Department of Agriculture; Oak Ridge National Laboratory, Oak Ridge, TN, April 2005. (2) McKendry, P. Bioresour. Technol. 2002, 83, 37−46. (3) Uslu, A.; Faaij, A. P.C.; Bergman, P. C. A. Energy 2008, 33, 1206−1223. (4) Bergman, C. A. P.; Kiel, H. A. J. Torrefaction for biomass upgrading. Presented at the 14th European Biomass Conference and Exhibition, Paris, France, 2005. (5) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. J. Anal. Appl. Pyrolysis 2006, 77, 35−40. (6) Pimchuai, A.; Dutta, A.; Basu, P. Energy Fuels 2010, 24, 4638− 4645. (7) Medic, D.; Darr, M.; Shah, A.; Potter, B.; Zimmerman, J. Fuel 2012, 91, 147−154. (8) Bridgeman, T. G.; Jones, J. M.; Shield, I.; Williams, P. T. Fuel 2008, 87, 844−856. (9) Arias, B. R.; Pevida, C. G.; Fermoso, J. D.; Plaza, M. G.; Rubiera, F. G.; Martinez, J. J. P. Fuel Process. Technol. 2008, 89, 169−175. (10) Lipinsky, E. S.; Arcate, J. R.; Reed, T. B. Prepr. Pap., Am. Chem. Soc., Div. Fuel Chem. 2002, 47, 408−410. (11) 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-f low
4. CONCLUSIONS In this study, the microwave torrefaction of Douglas fir pellets was conducted to investigate the effects of process conditions on the yields and characteristics of products. The yield of torrefied biomass decreased with the increase of reaction temperature and time, while the yield of bio-oil and noncondensable gases were increased. Three linear equations as a function of reaction temperature and time were developed to predict the yield of torrefied biomass, bio-oil, and noncondensable gases based on the central composite experimental design and surface response analysis. The biooils from the microwave torrefaction of Douglas fir pellets contained some valuable chemical compounds, such as furans, guaiacols, ketones/aldehydes, and sugars. The noncondensable gases were mainly composed of CO2 and CO, and their amounts were significantly influenced by the reaction temper5942
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
Energy & Fuels
Article
gasif ication of biomass, Report ECN-C-05-067, ECN, Petten, The Netherlands, 2005. (12) Bergman, P. C. A.; Boersma, A. R.; Zwart, R. W. H.; Kiel, J. H. A. Torrefaction for biomass co-firing in existing coal-fired power stations “BIOCOAL”, Report ECN-C-05-013, ECN, Petten, The Netherlands, 2005. (13) Miura, M.; Kaga, H.; Tanaka, S.; Takanashi, K.; Ando, K. J. Chem. Eng. Jpn. 2000, 33, 299−302. (14) Miura, M.; Kaga, H.; Sakurai, A.; Kakuchi, T.; Takahashi, K. J. Anal. Appl. Pyrolysis 2004, 71, 187−199. (15) Lei, H.; Ren, S.; Julson, J. Energy Fuels 2009, 23, 3254−3261. (16) Ren, S.; Lei, H.; Wang, L.; Bu, Q; Chen, S.; Wu, J.; Julson, J.; Ruan, R. J. Anal. Appl. Pyrolysis 2012, 94, 163−169. (17) Yu, F.; Deng, S.; Chen, P.; Liu, Y.; Wan, Y.; Olson, A.; Kittelson, D.; Ruan, R. Appl. Biochem. Biotechnol. 2007, 136−140, 957−970. (18) Wang, X.; Chen, H.; Luo, K.; Shao, J.; Yang, H. Energy Fuels 2008, 22, 67−74. (19) Huang, Y.; Kuan, W.; Lo, S.; Lin, C. Bioresour. Technol. 2010, 101, 1968−1973. (20) Salema, A. A.; Ani, F. N. Bioresour. Technol. 2011, 102, 3388− 3395. (21) Dominguez, A.; Menendez, J. A.; Inguanzo, M.; Pis, J. J. Bioresour. Technol. 2006, 97, 1185−1193. (22) Tian, Y.; Zuo, W.; Ren, Z.; Chen, D. Bioresour. Technol. 2010, 102, 2053−2061. (23) Box, G. E. P.; Wilson, K.B. J. R. Stat. Soc. B 1951, 13, 1−45. (24) Elliott, D. C.; Hart, T. R.; Neuenschwander, G. G. Ind. Eng. Chem. Res. 2006, 45, 3776−3781. (25) Chen, W.; Kuo, P. Energy 2011, 36, 803−811. (26) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. J. Anal. Appl. Pyrolysis 2006, 77, 28−34. (27) Tumuluru, J. S.; Sokhansanj, S.; Hess, J. R.; Wright, C. T.; Boardman, R. D. Ind. Biotechnol. 2011, 7, 384−401. (28) Bergman P. C. A. Combined torrefaction and pelletisation: The TOP process. ECN Report ECN-C-05-073, ECN, Petten, The Netherlands, 2005. (29) van der Stelt, M. J. C.; Gerhauser, H.; Kiel, J. H. A.; Ptasinski, K. J. Biomass Bioenergy 2011, 35, 3748−3762. (30) Lei, H.; Ren, S.; Julson, J.; Wang, L.; Bu, Q.; Ruan, R. In 6th ASME 2011 International Manufacturing Science and Engineering Conference, MSEC 2011, Vol. 1; ASME: New York, 2011, pp 685−692. (31) Wang, M. J.; Huang, Y. F.; Chiueh, P. T.; Kuan, W. H.; Lo, S. L. Microwave-induced torrefaction of rice husk and sugarcane residues. Energy 2012, 37, 177−184. (32) Thostenson, E. T.; Chou, T. W. Composites, Part A 1999, 30, 1055−1071.
5943
dx.doi.org/10.1021/ef300633c | Energy Fuels 2012, 26, 5936−5943
