ARTICLE pubs.acs.org/EF
Hydrothermal Carbonization (HTC) of Lignocellulosic Biomass S. Kent Hoekman,* Amber Broch, and Curtis Robbins Division of Atmospheric Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada 89512, United States ABSTRACT: Hydrothermal carbonization (HTC) of biomass involves contacting raw feedstock with hot, pressurized water. Through a variety of hydrolysis, dehydration, and decarboxylation processes, gaseous and water-soluble products are produced, in addition to water itself and a solid char. In this experimental effort, a 2 L Parr stirred pressure vessel was used to apply the HTC process to a mixed wood feedstock. The effects of the reaction conditions on product compositions and yields were examined by varying temperature over the range of 215 295 °C and varying reaction hold time over the range of 5 60 min. With increasing temperature and time, the amounts of gaseous products and produced water increased, while the amount of HTC char decreased. The energy density of the char increased with reaction severity. At reaction conditions of 255 °C for 30 min, the HTC char had 39% higher energy density than the raw biomass feedstock. Aqueous solutions from HTC experiments at lower temperatures (215 235 °C) contained significant levels of sugars. At higher temperatures (255 295 °C), greatly reduced concentrations of sugars were observed, while concentrations of acetic acid increased. A two-step HTC process involving low- and high-temperature regimes may be advantageous to maximize both the recovery of sugars and production of energy-dense char.
1. INTRODUCTION Lignocellulosic biomass is increasingly regarded as a promising, renewable feedstock for the production of heat, chemicals, fuels, and electrical power.1 3 However, in most cases, it is necessary to modify or pretreat the lignocellulosic material by chemical and physical means to satisfy the requirements of biochemical or thermochemical conversion processes. For thermochemical conversion processes (e.g., gasification, pyrolysis, and combustion), the wide diversity of physical shapes, densities, and other handling properties among different lignocellulosic feedstocks creates significant challenges in feeding these materials into process units. To overcome these problems, it is helpful to homogenize all feedstocks to a certain degree and, thereby, minimize handling differences among them. Another purpose of pretreatment prior to thermochemical conversion of solid fuels is densification of the material in terms of mass per volume (kg/m3) and energy content (MJ/kg). Raw lignocellulosic material typically contains approximately 40% oxygen (dry mass basis), which contributes nothing to the heating value of the material. The intention of a pretreatment process is increased carbon content and decreased oxygen content, which together increase the energy density of biomass materials. An additional benefit of some pretreatment methods is improved handling, transportation, and storage of thermochemical feedstocks. The diversity of sources (forest thinnings, agricultural residues, energy crops, etc.) and the seasonality of some introduce challenges with logistics and reliable supply. These problems can be mitigated by pretreatment processes that convert raw lignocellulosic material into a form that is more easily transported and stored. In some respects, the handling logistics of pretreated biomass then become similar to those of coal. One form of biomass pretreatment is commonly employed as an initial step in the production of cellulosic ethanol. Such pretreatment is used to partially break apart the lignocellulosic structure of the feedstock, thereby recovering sugars and other r 2011 American Chemical Society
chemicals and making the cellulose more amenable to further hydrolysis and subsequent fermentation to ethanol.4 7 This is different from our interests in investigating pretreatment processes meant primarily to increase the energy content of the remaining solid residue (or char) and enhance its suitability as a feedstock for subsequent thermochemical processing.8,9 Two principal thermal approaches have been used to convert raw lignocellulosic biomass into higher energy density chars. One approach, called torrefaction, involves mild pyrolysis (typically 200 300 °C) conducted in an inert atmosphere.10 14 The other approach involves treatment of biomass in a hot, pressurized, aqueous environment. Such wet processes have been widely studied and are known by various names, including hydrothermal pretreatment, wet torrefaction, coalification, hot compressed water (HCW) treatment, and hydrothermal carbonization (HTC). In this paper, the name HTC is used, because this appears to be the most widely accepted terminology. Early reviews regarding HTC treatment of biomass were published by van Krevelen15 and Bobleter.16 Several more recent reviews on this topic have also appeared.8,17 19 Because HTC treatment involves pressurized conditions, the process equipment may be more complicated and costly compared to conventional torrefaction. On the other hand, HTC can be carried out more quickly than torrefaction and may more easily accommodate a broader range of feedstocks, because the initial moisture level is not a concern. The solid char produced by thermal pretreatment (wet or dry) is also known by several names, including biochar, biocoal, green coal, and charcoal. To avoid confusion, the term HTC char is used here when referring to the solid product produced from the HTC process. A recent review has discussed the upgrading and use of HTC char in several high-value applications, including Received: December 23, 2010 Revised: March 7, 2011 Published: March 08, 2011 1802
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Figure 2. Temperature and pressure profiles for HTC process. Tahoe Mix feedstock at 255 °C. Zone A, reactor heatup; zone B, reactor hold time; zone C, reactor cooling; and zone D, reactor venting. Figure 1. Schematic of 2 L Parr stirred reactor system used for the HTC process.
catalysis, surface adsorption, and energy storage.20 HTC char and torrefied char have also been investigated as materials for soil amendment and carbon sequestration.20 24 However, one of the most attractive uses of HTC char is as a coal substitute in gasification and combustion applications. Conduct of HTC under supercritical conditions has been shown to rapidly degrade and dissolve lignocellulosic materials, producing noncondensable gases (H2, CO, CO2, CH4, and others) along with a wide variety of water-soluble organic compounds.25 27 In the present study, a milder subcritical HTC process was conducted using a woody biomass feedstock. Such conditions have been shown to primarily decompose hemicellulosic material while retaining most of the cellulose and lignin fractions and to produce a char that exhibits considerable energy densification.28,29 This study focused on characterizing the HTC products, defining accurate mass balances, and determining the effects of process conditions on the products and mass balance.
2. EXPERIMENTAL SECTION 2.1. Parr Reactor System. The HTC reactor and sampling system used a 2 L Parr stirred pressure reactor (model 4522), as shown in Figure 1. During the HTC process, approximately 90 g of biomass was combined with distilled water in an 8:1 water/biomass ratio (w/w). Prior to reaction, residual air was removed from the sealed vessel by repeatedly pressurizing with helium and venting to vacuum. The reactor and contents were then heated while stirring. The reaction temperature was controlled with a National Instruments LabView data acquisition program using two thermocouples: one installed at the outer wall of the reactor and the other inside the reactor. During initial heating, the outer wall temperature was set at a maximum of 380 °C. Once the internal temperature reached the desired set point, the reactor heater was turned off. As the internal temperature dropped below its set point, the heater was turned back on to a lower temperature (approximately 50 °C above the internal set-point temperature). The internal temperature was maintained near the set point in this manner throughout the duration of the test, as shown in Figure 2. The pressure inside the reactor (solid line) was measured and recorded throughout the HTC process and was compared to the saturated steam curve (dashed line), as shown in Figure 2. This figure also indicates approximate times for each phase of
the reaction, including the heating phase, the reaction hold time, the cooling phase, and gas venting (represented by zones “A”, “B”, “C”, and “D”, respectively). After treatment of the biomass at the desired temperature and hold time, the reactor vessel was immediately removed from the heated well and placed in an ice bath to cool the contents and condense all liquids. When the inner reactor temperature reached approximately 15 °C below room temperature, the noncondensable gases were released to a 50 L Tedlar bag through a heated gas line (60 °C) while measuring the flow rate and relative humidity. Once gas flow from the reactor ceased, helium was sparged through the chamber to liberate the trapped gases and collect them in the Tedlar bag. The use of helium as a sparging gas makes subsequent gas chromatography (GC) analysis more convenient. Gases were collected until the Tedlar bag was approximately 3/4 full, which typically resulted in a dilution ratio of approximately 5:1. The LabView software was configured to continuously record the helium flow rate (and total flow rate) to ensure that an accurate volume of gases collected in the Tedlar bag was determined. After removal of the produced gases, the reactor was opened to collect the solid and aqueous products. The reactor contents were transferred into a tared collection vessel, using a measured amount of rinsewater to completely remove all products. The solid and aqueous products were separated by vacuum filtration, and the char was rinsed (with a measured amount of water) to remove as much of the aqueous products as possible. The char product and aqueous solution were weighed independently and stored (under refrigeration) until analysis. 2.2. Feedstock Preparation. The primary biomass feedstock used for these experiments was a mix of Jeffrey Pine and White Fir (called Tahoe Mix) obtained from the Tahoe Forest. As received, the Tahoe Mix chip size was 1 2 in. in diameter. To promote a more homogeneous mixture and provide effective stirring during HTC treatment, the chip size was further reduced using a Wiley Mill with a 6 mm screen (approximately 1/4 in.). The smaller chips were separated through a series of sieves to eliminate particles smaller than 1 mm. The 6 mm chips were then air-dried and stored in a sealed container to await processing. 2.3. Product Characterization. 2.3.1. Gaseous Products. The noncondensable gases collected in the Tedlar bag were analyzed using a SRI 8610C gas chromatograph equipped with a 0.5 mL gas sampling loop and a thermal conductivity detector (TCD). A 6 ft 1/8 in. packed column (Haysep D) was employed, with helium as a carrier gas (flow rate of 11 mL/min). The column temperature was increased from 40 to 200 °C at 50 °C/min and then held for 5 min. These conditions allow for measurement of H2, N2, CO, CH4, and CO2, although N2 and CO partially coelute. Gaseous products collected from the HTC experiments 1803
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Table 1. Gradient Program for Carbohydrate Analysis by IC time (min)
600 mM NaOH (%)
water (%)
0
12
88
5
12
88
6
37
63
15
37
63
17
50
50
51
100
0
64
100
0
12 12
88 88
64.5 80
Figure 3. GC analysis of gaseous products from HTC of Tahoe Mix at 255 °C (30 min hold time). wxere transferred from the Tedlar bags directly into the GC sample loop. Calibration was accomplished using a custom-blended gas mixture containing H2, CO, CO2, and CH4 in helium. A typical chromatogram is provided in Figure 3, which shows that CO2 was the dominant gaseous product. Although the system was purged with helium before use, trace amounts of N2 were often seen. The GC TCD system was relatively insensitive; thus, CH4 and H2 were usually not detectable after dilution in the Tedlar bag, although trace amounts of each are likely formed in the HTC process.19 A late-eluting system peak was observed when analyzing both calibration and product mixtures, although it did not interfere with quantification of the other constituents. 2.3.2. Aqueous Phase Products. After the aqueous and solid HTC products were separated, the pH level and the nonvolatile residue (NVR) content of the aqueous solutions were measured immediately, while further lab analyses [total organic carbon (TOC), sugars, and organic acids] were completed in batches after several weeks of refrigerated storage (in previous work, it was demonstrated that no major degradation of the aqueous products of interest occurred during such storage). The pH of the liquids was measured using a Hanna Instruments HI 8424 portable pH and temperature meter. Replicate samples of the aqueous solution were dried to determine the NVR content. Samples were weighed into drying tins and placed in a convective oven at 105 °C overnight (approximately 18 20 h) to obtain a constant weight. The remaining residue represents and is reported as the NVR content of the aqueous solution. The TOC was determined using a Shimadzu TOC-VCSH instrument (Columbia, MD). This instrument catalytically oxidizes all organic compounds into CO2, which is measured by nondispersive infrared detection (NDIR). The nonpurgeable organic carbon (NPOC) method was used, in which the addition of hydrochloric acid, combined with sparging, removes inorganic carbon (and highly volatile organic carbon) from the sample prior to analysis. A Shimadzu ASI-V autosampler was used to introduce a 25 μL sample, which was treated with 2 M HCl and sparged with air for 1.5 min before introduction into the catalytic combustion chamber, held at 680 °C. The oxidation products were passed through the NDIR detector, where CO2 was quantified by comparing the sample peak areas to a three-point calibration curve generated from analysis of potassium hydrogen phthalate (KHP) at the beginning of each run. A set of sugars was analyzed using high-performance anion-exchange chromatography, with pulsed amperometric detection (HPAEC PAD or simply IC method). The method used was based on similar, literaturereported methods.30,31 A Dionex ICS-3000 system with an ED50A
Figure 4. Sugar analysis of aqueous products from HTC of Tahoe Mix at 215 °C (30 min hold time).
Table 2. Gradient Program for Organic Acid Analysis by IC time (min)
100 mM NaOH (%)
water (%)
0.0
1
99
7.9
1
99
12.9
8
25.9 37.9
12.5 60
87.5 40
42.9
60
40
43.0
1
99
50.0
1
99
92
detector was used for the measurement of carbohydrate and organic acid species (Dionex Corp., Sunnyvale, CA). For carbohydrate analysis, a Dionex CarboPac MA1 guard and analytical column was used with a flow rate of 0.4 mL/min and a column temperature of 25 °C. A 30 μL sample volume was loaded onto the column, and a multi-step linear gradient elution program (shown in Table 1) was used to separate the carbohydrate species. Amperometric detection was performed using a gold working electrode and an Ag/AgCl reference electrode with the standard quadruple potential waveform. Carbohydrate species concentrations were calculated by comparing peak area responses to those generated during a multi-point calibration constructed from authentic standards. A chromatogram from analysis of sugars produced during HTC of a Tahoe Mix sample is shown in Figure 4. A set of organic acids [including lactic acid, acetic acid, formic acid, methane sulfonic acid (MSA), glutaric acid, succinic acid, malonic acid, maleic acid, and oxalic acid] was analyzed using ion chromatography 1804
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Figure 5. Organic acid analysis of aqueous products from HTC of Tahoe Mix at 215 °C (30 min hold time). (IC) following the method by Jaffrezo et al.32 A Dionex IonPac AG11HC guard column (4 50 mm) and AS11 HC analytical column (4 250 mm) were used. The flow rate was held at 1.5 mL/min, while the column temperature was maintained at 25 °C. A 30 μL sample loop delivered the sample onto the column, where a multi-step gradient program (shown in Table 2) was used to elute the organic acid species. After the column effluent passed through the anion self-regenerating suppressor (ASRS 300, 4 mm) with a current of 223 mA, conductivity was measured by a DS3 conductivity cell. Organic acid species concentrations were calculated by comparing peak area responses to those generated during a multi-point calibration constructed from authentic standards. A chromatogram from analysis of aqueous products from HTC of a Tahoe Mix sample is shown in Figure 5. 2.3.3. HTC Char. The wet char collected from the HTC reactor was weighed and air-dried for 48 h, with occasional stirring to promote homogeneous drying. The moisture content of the air-dried char was determined by oven drying (105 °C for 18 20 h). For calculation purposes, all mass lost during both air and oven drying was assumed to be water, although, as discussed later, small amounts of volatile organic products may also have been lost upon drying of the char. The calorific energy content of oven-dried HTC char was measured with a Parr 6200 calorimeter, equipped with a Parr 6510 water handling system. Samples were combusted in an oxygen-rich environment inside a bomb that is contained within a water bath. The increase in temperature of the water bath was used to determine the gross heat of combustion (higher heating value) through a comparison to a calorificgrade benzoic acid standard. A ThermoElectron Flash EA 1112 automatic elemental analyzer was used for direct measurement of C, H, N, S, and O in HTC char. The complete analysis requires two methods, with two separate injections: one for C, H, N, and S analysis and the other for O analysis. For C, H, N, and S determination, the sample is weighed in a tin foil capsule and then dropped into an oxidation/reduction (FeO and Fe) reactor kept at a temperature of 900 1000 °C. The amount of O2 necessary for complete combustion is delivered into the reaction chamber. The exothermic reaction between the sample and O2 temporarily raises the temperature to about 1800 °C, which is sufficient to convert both organic and inorganic compounds into elemental gases that are reduced and separated on a GC column using He as the carrier gas. The produced gases (N2, CO2, H2O, and SO2) are detected and quantified by a thermal conductivity detector (TCD). For oxygen determination, the sample is weighed into a silver foil capsule, then dropped into a reaction chamber containing a nickel-coated carbon catalyst, and held at a temperature slightly above 600 °C. Under these conditions, oxygen is converted to CO, which is routed through a water trap before being passed to the GC column for quantification by the TCD.
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2.4. Mass Balance. A mass balance for each HTC reaction was obtained by quantifying all materials in the gaseous, aqueous, and solid products. All product masses were expressed relative to that of the dry starting biomass. Gaseous products were quantified on the basis of total gas volumes collected in Tedlar bags and GC analyses of the gas compositions. Solid products were quantified gravimetrically, after drying samples of the HTC char at 105 °C. Aqueous products include NVR, volatile organics, and produced water. The NVR content was quantified by evolving all volatiles in an oven at 105 °C for 18 20 h (to attain a constant weight). During this procedure, both water and volatile organics are lost. The two most significant volatile organic compounds are formic acid and acetic acid, which are quantified separately using the ion chromatographic method described above. Finally, the produced water was quantified as the difference between all water inputs and outputs. Water inputs include moisture in the biomass feedstock, water added to the Parr reactor, and all rinsewater. Outputs include water lost during drying of the HTC char, along with the total aqueous solution recovered, minus the NVR content and the volatile organics (formic and acetic acids) dissolved in the aqueous solution.
3. RESULTS AND DISCUSSION A series of HTC experiments was conducted to determine the effects of the reaction temperature and hold time on the treatment of Tahoe Mix wood chips. In one set of experiments, a range of reaction temperatures was used, from 215 to 295 °C at 20° intervals, while the hold time was kept constant at 30 min. In a second set of experiments, the reaction temperature was held constant at 255 °C, while the hold time was varied from 5 to 60 min. A test at 255 °C with 30 min hold time was included in each series of experiments to provide information about experimental repeatability. 3.1. Mass Recovery. Despite the fact that the HTC process has been extensively investigated, detailed mass balances have not been widely reported.19 In this study, a total mass balance of the HTC reaction at each temperature and hold time was carried out by determining the mass of each recovered product fraction and comparing the sum of all products to the total starting mass of feedstock. The recovered products included the solid HTC char, noncondensable gases (CO and CO2), water solubles (NVR and volatile organics), and produced water. Mass recoveries for these HTC experiments are summarized in Table 3 (note that, because of a malfunction in the gas sampling system, no gaseous products were measured from the experiment at 215 °C). The sum of identified products in these experiments varied from approximately 86 to 92%. The “difference” values shown in Table 3 represent the unidentified mass needed to achieve complete mass closure. Most of this “difference” is believed to be water, remaining in the Parr reactor vessel and stirring mechanism after removal of the product mixture. In subsequent tests, tared dry towels were used to wipe out the Parr apparatus following each HTC experiment. This showed that 5 8 g of water was typically left behind, accounting for 6 9% of starting dry feedstock mass. However, because this method was not used consistently throughout the experimental matrix described here, water left behind is categorized as the “difference”. Another issue with respect to mass closure is the loss of volatile organic compounds during drying of the HTC char. We have observed in other work that the same organic materials found in the aqueous product solutions are also adsorbed on the char. This includes formic and acetic acids, which may be present at 1805
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Table 3. Effect of the Reaction Temperature and Time on the HTC Mass Balancea 255 °C reactor temperature
30 min hold time HTP product fraction
a
215 °C
235 °C
255 °C
275 °C
295 °C
5 min
10 min
30 min
60 min
CO2
NA
7.9
7.9 ( 0.2
10.1
11.1
5.3
5.6
7.9 ( 0.2
9.1
CO
NA
0.0
0.6 ( 0.2
0.6
0.7
0.2
0.2
0.6 ( 0.2
0.4
water solubles
13.0
12.6
14.9 ( 0.6
13.0
12.3
14.0
14.1
14.9 ( 0.6
12.9
HTC char
69.1
63.7
50.3 ( 0.5
50.9
50.1
57.7
55.5
50.3 ( 0.5
52.1
produced water
3.3
3.5
12.8 ( 3.4
17.4
16.7
8.5
10.2
12.8 ( 3.4
12.5
sum (% of total)
NA
87.6
86.1 ( 3.5
91.9
91.0
85.7
85.6
86.1 ( 3.5
87.0
difference
NA
12.4
13.9 ( 2.0
8.1
9.0
14.3
14.4
13.9 ( 2.0
13.0
mol CO2/mol H2O
NA
0.26 ( 0.08
0.24
0.27
0.93
0.26
0.22
0.26 ( 0.08
0.30
All results are expressed as a percentage of the starting dry feedstock mass. Results at 255 °C and 30 min are averages ( standard deviation.
significant levels but would likely be lost during drying of the char. In future work, we plan to capture all volatiles removed during drying of the char and quantify the volatile organic compounds that are present. A higher recovery of HTC char was observed at lower temperatures, with nearly 70% of the starting dry mass recovered at a reaction temperature of 215 °C, decreasing to approximately 50% at 295 °C. The mass of recovered char dropped dramatically between 215 and 255 °C and then remained at approximately 50% with higher temperature treatments. These changes in char recovery are comparable to those reported recently by Yan et al., who investigated a similar range of temperature conditions.28,29 As shown in Table 3, the mass recoveries of noncondensable gases (mainly CO2) and produced water increased with an increasing reaction temperature, while the total water solubles remained nearly constant. Broadly speaking, these overall product distributions are consistent with those reported in the literature.8,19 Varying reaction hold time had a similar although smaller effect on mass recovery to changes in the temperature. The HTC char mass recovery was highest at short hold times and decreased with an increasing hold time. The amounts of noncondensable gases and produced water increased with longer hold times, while the water solubles remained nearly constant. The relative effects of the reaction time and temperature on the HTC process have been investigated by others.9,19,28,29 Under the operating conditions typically employed, the temperature appears to have a stronger effect on HTC product distributions than the reaction time. Over the range of temperature and time conditions employed here, approximately 50 70% of the starting feedstock mass was recovered as char, 5 12% was recovered as gases, and 12 15% was recovered as water-soluble products. The amount of produced water is less certain, because of handling losses and the uncertainties inherent in measuring small increments in already large amounts of water. However, the amount of produced water was likely in the range of 10 20%, assuming that most of the “difference” was actually produced water. 3.2. Noncondensable Gases. GC analysis of the noncondensable gases revealed CO2 to be the dominant gaseous species, typically responsible for about 90 95% of the total gas volume. The remaining 5 10% was mostly CO, although trace amounts of CH4 and H2 were likely present. CO2 formation results from decarboxylation reactions, which, along with dehydration, represent two major processes occurring during HTC. To a first approximation, the ratio of decarboxylation/dehydration can be
defined as the mole ratio of CO2/H2O.8,19 This ratio appears to be a function of the feedstock type but is thought to be relatively insensitive to the reaction temperature.33 The results presented in Table 3 show a CO2/H2O ratio between 0.2 and 0.3 for all experiments, except at 235 °C, where analytical measurement problems are suspected. However, if much of the “difference” category is actually produced water (as discussed above), the true CO2/H2O ratio is even smaller, suggesting that dehydration dominates over decarboxylation in the HTC process. Shortening the reaction hold time in the 255 °C experiments noticeably reduced gas production, despite long preheat times. This suggests that much of the decarboxylation process producing CO2 occurs during the HTC temperature hold period itself. Also, note that, in Figure 2, the reactor pressure did not significantly exceed the steam saturation pressure until the temperature was above 200 °C, indicating minimal gas production during the preheating period. 3.3. Aqueous-Phase Products. A summary of aqueous-phase product characterizations for the two sets of HTC experiments is presented in Table 4. Over the range of conditions that we explored, the NVR content and the TOC levels each comprised about 8 12% of the starting dry feedstock mass. As the reaction severity increased (higher temperature or longer time), both NVR and TOC slightly decreased, suggesting that organic materials originally produced under mild conditions continue to react under more severe conditions. This is consistent with the observations of increased gas production (described above) and degradation of sugars (discussed below) with increasing reaction severity. 3.3.1. Sugars. The effects of the reaction temperature and hold time on the production of sugar species are shown in Table 4. “Other” sugars include minor species, such as inositol, erythritol, xylitol, arabitol, sorbitol, trehalose, mannitol, and arabinose. Total sugars were found to diminish rapidly with increasing reaction temperatures, declining from approximately 1.4% at 215 °C to 0.1% at 255 °C. Several sugars were detectable only in samples from the lowest temperature experiments, including galactose, mannosan, trehalose, and arabinose. Glucose and xylose (which coelute in our IC method) and levoglucosan were present in all aqueous-phase products from HTC experiments at temperatures of 255 °C and below. The only sugars observed from experiments above 255 °C were the minor “other” sugars and glycerol, all of which remained fairly constant with changes in the temperature. Increasing the reaction hold time produced effects similar to increasing the temperature, with higher concentrations of sugars present after shorter reaction hold times. 1806
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Table 4. Aqueous Phase Products from HTC of Tahoe Mixa 255 °C reactor temperature
30 min hold time product
215 °C
235 °C
255 °C
275 °C
295 °C
5 min
10 min
30 min
60 min
NVR
10.00
8.78
10.16 ( 0.39
8.36
7.52
10.09
9.88
10.16 ( 0.39
8.53
TOC
9.17
9.17
11.27 ( 1.06
8.47
7.75
11.40
12.02
11.27 ( 1.06
8.56
sugars glucose/xylose
1.02
0.54
0.08 ( 0.03
nd
nd
0.60
0.55
0.08 ( 0.03
nd
galactose
0.18
nd
nd
nd
nd
nd
nd
nd
nd
mannosan
0.05
0.02
nd
nd
nd
0.02
0.01
nd
nd
levoglucosan
0.07
0.06
0.01 ( 0.00
nd
nd
0.08
0.07
0.01 ( 0.00
nd
glycerol others
0.03 0.06
0.04 0.08
0.07 ( 0.03 0.05 ( 0.01
0.06 0.04
0.06 0.04
0.07 0.08
0.08 0.08
0.07 ( 0.03 0.05 ( 0.01
0.10 0.04
total sugars
1.41
0.73
0.22 ( 0.02
0.10
0.10
0.86
0.79
0.22 ( 0.02
0.14
organic acids
a
formic
0.83
1.02
0.89 ( 0.09
0.26
0.06
1.04
1.00
0.89 ( 0.09
0.31
acetic
2.14
2.78
3.85 ( 0.08
4.37
4.75
2.91
3.20
3.85 ( 0.08
4.08
lactic
0.21
0.34
1.47 ( 0.17
1.89
1.98
1.05
1.19
1.47 ( 0.17
1.69
others
0.15
0.19
0.21 ( 0.07
0.33
0.44
0.10
0.14
0.21 ( 0.07
0.23
total acids
3.34
4.33
6.43 ( 0.08
6.85
7.22
5.11
5.52
6.43 ( 0.08
6.31
All results are expressed as a percentage of starting dry feedstock mass. Results at 255 °C and 30 min are averages ( standard deviation.
The rapid degradation of sugars at elevated temperatures is well-documented in the literature.17,18 To maximize sugar recovery requires operation at relatively lower temperatures, typically below 225 °C. In contrast, to maximize energy densification of the HTC char requires higher temperatures (discussed below). This suggests that a two-step process may be advantageous for realizing high sugar recovery in addition to maximizing char densification. 3.3.2. Organic Acids. The effects of the reaction temperature and hold time on the production of organic acids are also shown in Table 4. Organic acids were present in much higher concentrations than were sugars; typically 3 7% of the starting dry feedstock was recovered as organic acids, with formic, acetic, and lactic acids being the dominant species. The remaining minor acids lumped into “others” in Table 4 include glutaric acid, succinic acid, MSA, malonic acid, maleic acid, and oxalic acid. Total acids increased substantially with an increasing reaction temperature up to 255 °C and then remained fairly constant. At temperatures above 255 °C, formic acid levels decreased, while acetic acid and lactic acid production increased. Increasing the reaction hold time at 255 °C increased total acids slightly, because of increased production of acetic and lactic acids, while formic acid levels decreased. The acid content in the aqueous phase products resulted in a pH level near 3.0 for all samples. Varying the reaction temperature and hold time had little effect on pH. It is well-known from the literature that organic acids, particularly acetic acid, are produced by HTC processes. Our results are consistent with the understanding that some acids are produced from direct reaction of the starting biomass, while additional acids are produced by further degradation of intermediate products, such as sugars. Inspection of Table 4 indicates that the sum of identified sugars and acids does not account for all of the TOC within the aqueous phase products especially under the lower temperature reaction conditions. On the basis of other analyses (not reported here) and literature reports,18,19 it is known that many other organic compounds are present in the
aqueous phase. Some of the most significant include furans, furfurals, and phenolic compounds. 3.4. HTC Char Products. Noteworthy properties of the char samples produced in these two sets of HTC experiments are summarized in Table 5. Figure 6 illustrates the effects of the reaction temperature and hold time on HTC char mass recovery and energy content. For clarity, trendlines are fit to the 30 min hold time data for each parameter. Mass recovery is shown as a percentage of the starting dry feedstock mass and is indicated by square symbols (blue squares represent 30 min hold times, and green squares represent other hold times). HTC char mass recovery decreased dramatically with an increasing reactor temperature from 215 to 255 °C and then remained fairly constant up to 295 °C. A small but noticeable decrease in mass recovery was observed with an increasing hold time at 255 °C. The energy content of the HTC char [higher heating value (HHV), expressed on a dry basis] is shown as circles in Figure 6 (red symbols represent 30 min hold times, and yellow symbols represent other hold times). Increasing the reaction temperature clearly increased the energy content of the HTC char. The energy content of the starting dry feedstock (20.3 MJ/kg) is also shown in Figure 6. At a reaction temperature of 255 °C (with a 30 min hold time), the resulting char had an energy content of 28.3 MJ/kg, an increase of 39% compared to the feedstock. The char energy content continued to increase as reaction temperatures exceeded 255 °C, albeit less dramatically. The HTC char produced at 295 °C exhibited an energy content 45% higher than the starting feedstock. A significant effect of the reaction hold time on HTC char energy content was also observed. At a 255 °C reaction temperature, the produced char had energy contents of 25.1 and 26.0 MJ/kg for 5 and 10 min hold times, respectively, but increased to 29.2 MJ/kg at a 60 min hold time. These increases in char energy density compared to raw biomass feedstock are comparable to other reported increases. For example, Yan et al. measured energy density increases up to 36% using loblolly pine,28,29 while Inoue et al. reported increases of 11 73% when treating Konara (oak) wood at 250 350 °C.34 1807
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Table 5. Properties of HTC Char Products from the Treatment of Tahoe Mix Feedstock 255 °C reactor temperature
30 min hold time property energy content (MJ/kg)
feedstock
215 °C
235 °C
255 °C
20.32
22.58
24.27
28.26 ( 0.28
29.02
29.52
69.1
63.7
50.3 ( 0.5
50.9
50.1
mass yield (%) energy densification
1.11
energy yield (%)
1.19
275 °C
1.39 ( 0.01
1.43
295 °C
1.45
5 min
10 min
30 min
25.10
26.04
28.26 ( 0.28
29.17
57.7
55.5
50.3 ( 0.5
52.1
1.23
1.28
1.39 ( 0.01
60 min
1.43
76.7
76.1
70.0 ( 0.9
72.7
72.8
71.2
71.1
70.0 ( 0.9
74.8 71.89
elemental analysis %C
49.02
54.57
60.54
70.06 ( 0.13
70.08
73.01
62.65
62.98
70.06 ( 0.13
%H
5.93
5.89
5.66
5.19 ( 0.07
5.31
5.14
5.43
5.40
5.19 ( 0.07
5.15
%N %S
0.11 nd
0.09 nd
0.13 nd
0.10 ( 0.06 nd
0.16 nd
0.14 nd
0.05 0.03
0.04 nd
0.10 ( 0.06 nd
0.05 nd
%O
41.26
34.89
31.59
23.42 ( 0.42
21.14
19.87
32.31
30.72
23.42 ( 0.42
22.26
0.63
0.48
0.39
0.25 ( 0.01
0.23
0.20
0.39
0.37
0.25 ( 0.01
0.23
atomic O/C ratio
Figure 6. Effects of the reaction temperature and hold time on mass recovery and energy content of HTC char from Tahoe Mix. Hold time = 30 min, except where otherwise indicated. The mass is represented by squares, and the energy content is represented by circles.
Energy density increases of 20 30% have been reported with mild HTC treatment (180 °C) of wheat straw.35 Also, a similar HTC process applied to whole microalgae was found to produce a char having very high energy density (30 32 MJ/kg).36 Another recent study of HTC treatment of microalgae at 250 °C was reported to produce a solid product, although the energy content was not indicated.37 It is also useful to evaluate the produced char in terms of energy densification and energy yield. As defined by Yan et al.,28 the energy densification is the energy content of the char divided by the energy content of the feedstock, while the energy yield is defined as the char mass yield multiplied by the energy densification ratio. The values for mass yield, energy densification, and energy yield for all HTC experiments are provided in Table 5, which reveals that energy densification increases with the reaction temperature, from a low of 1.11 at 215 °C to a high of 1.45 at 295 °C. Similarly (but to a lesser extent), the energy densification increased with an increasing hold time at 255 °C. The total energy yield of the HTC char did not vary greatly over the sets of experiments conducted here, ranging from 70 to 77%. Consistent with previous reports, the highest energy yield was observed at the lowest process temperature.28
Figure 7. Van Krevelen diagram of biochar from HTC of Tahoe Mix. Hold time =30 min, except where indicated. The temperature was at 255 °C for indicated hold times.
The HTC process involves dehydration, condensation, and decarboxylation reactions, resulting in a loss of some carbon, hydrogen, and oxygen. Of these, oxygen loss is of the greatest importance and is most desirable, because it directly increases the energy content. The results of C, H, N, S, and O analyses carried out on each char sample are also presented in Table 5. As the severity of HTC increased, the carbon content of the char increased, while the oxygen content decreased. The hydrogen content varied only slightly with reaction severity. A useful way to examine these changes in atomic C, H, and O compositions is by means of a Van Krevelen diagram, as shown in Figure 7.19,38 In this figure, typical ranges for biomass, peat, lignite, coal, and anthracite are indicated. The starting Tahoe Mix feedstock is shown in the upper right corner of the diagram, within the typical biomass region. Figure 7 illustrates that, with higher HTC process severity, the resulting char becomes increasingly similar to coal. At a reaction temperature of 235 °C, the HTC char possesses H/C and O/C ratios similar to peat, while at 255 °C or higher, the ratios are similar to those of lowgrade coal. The beneficial impact of increasing reaction hold time at 255 °C is also shown in this figure. With the HTC process resulting in little change to the hydrogen content of the char, a similar understanding of the 1808
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Energy & Fuels relationships between HTC conditions and char characteristics can be obtained by merely considering the atomic O/C ratios. As shown in Table 5, this ratio decreases with increasing process severity, going from 0.63 in the feedstock to 0.20 when treated at 295 °C. The behavior of decreasing O/C with increasing process severity has been discussed at length by Ruyter, who indicated that O/C ratios of 0.2 0.3 are indicative of low-grade coals.33 The O/C ratios observed in HTC chars are considerably lower than in conventional, torrefied char. Yan et al. carried out both “wet” and “dry” torrefaction processes using a loblolly pine feedstock.28 Under HTC conditions at 260 °C, it was found that the O/C ratio decreased from 0.65 in the feedstock to 0.24 in the char. With dry torrefaction, an O/C ratio of only 0.54 was obtained when carried out at 300 °C. The increased energy densification and greater coal-like behavior of HTC char represents an important advantage of the HTC process, as compared to conventional torrefaction, when producing fuels for combustion and gasification.
4. CONCLUSIONS HTC of lignocellulosic biomass has been shown to produce a solid char product having considerably higher energy density compared to the raw feedstock. As HTC reaction severity was increased (higher temperature and longer time), the extent of energy densification increased. Results obtained from HTC treatment of a mixed wood feedstock obtained from the Tahoe Forest were largely consistent with those reported in the literature for other woody feedstocks. Although energy densification of the HTC char was found to increase at higher process temperatures, most of the benefit was realized at modest temperatures. At 255 °C, a 39% increase in energy density was achieved; this rose to 45% at 295 °C. Higher temperature conditions also entail higher process pressures. In a commercial application, higher pressures could require use of more complex and costly equipment and could increase handling difficulties. Although a thorough techno-economic analysis is required for confirmation, it appears that a HTC char having desired energy density may be most effectively produced at operating temperatures near 255 °C and pressures near 5 MPa. Under these conditions, the resulting char has coal-like properties and is expected to exhibit favorable behavior with respect to combustion, gasification, and other thermal conversion processes. CO2 was the dominant gaseous product produced during the HTC process, representing approximately 8% of the starting feedstock mass when treated at 255 °C. Various water-soluble organic products were also produced in the HTC process, including sugars and organic acids. Maximum sugar concentrations were observed under low-temperature conditions (215 °C) but were degraded considerably at higher temperatures. To achieve both maximum sugar recovery and energy densification of the char may require a multi-step process involving different temperature regimes. Successful recovery and use of sugars (and other water-soluble organics) could also improve the overall economics of this process.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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’ ACKNOWLEDGMENT Funding support from the U.S. Department of Energy (DOE) under awards EE0000272 and DE-FG36-01GO11082 is gratefully acknowledged. Laboratory analytical support was provided by Stephanie Salke and Mark McDaniel of the Desert Research Institute (DRI). Parr reactor experiments were conducted by Eric Ceniceros and Keri Noack of the DRI. We also acknowledge helpful discussions and guidance by Charles Coronella and Wei Yan of the University of Nevada, Reno, NV, and Larry Felix of the Gas Technology Institute. ’ REFERENCES (1) U.S. Department of Energy (DOE). Biomass Multi-Year Program Plan; Office of the Biomass Program, U.S. DOE: Washington, D.C., 2009. (2) Perlack, R. D.; Wright, L. L.; Turhollow, A. F.; Graham, R. L.; Stokes, B. J.; Erbach, D. C. Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply; Oak Ridge National Laboratory: Oak Ridge, TN, 2005. (3) U.S. Department of Energy (DOE). Roadmap for Bioenergy and Biobased Products in the United States; Biomass Research and Development Technical Advisory Committee, U.S. DOE: Washington, D.C., 2007. (4) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Bioresour. Technol. 2005, 96, 673. (5) Lynd, L. R.; Elander, R. T.; Wyman, C. E. Appl. Biochem. Biotechnol. 1996, 57/58, 741. (6) Cybulska, I.; Lei, H.; Julson, J. Energy Fuels 2010, 24, 718. (7) Petersen, M. O.; Larsen, J.; Thomsen, M. H. Biomass Bioenergy 2009, 33, 834. (8) Funke, A.; Ziegler, F. Proceedings of the 17th European Biomass Conference; Hamburg, Germany, 2009. (9) Kleinert, M.; Wittman, T. Proceedings of the 17th European Biomass Conference; Hamburg, Germany, 2009. (10) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. J. Anal. Appl. Pyrolysis 2006, 77, 35. (11) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. Energy 2006, 31, 3458. (12) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. J. Anal. Appl. Pyrolsis 2006, 77, 28. (13) Arias, B.; Pevida, C.; Fermoso, J.; Plaza, M. G.; Rubiera, F.; Piskorz, J. Fuel Process. Technol. 2008, 89, 169. (14) Pimchuai, A.; Dutta, A.; Basu, P. Energy Fuels 2010, 24, 4638. (15) van Krevelen, D. W. Coal Typology—Physics—Chemistry— Constitution; Elsevier: Amsterdam, The Netherlands, 1993; pp 837 846. (16) Bobleter, O. Prog. Polym. Sci. 1994, 19, 797. (17) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Froling, M.; Antal, M. J.; Tester, J. W. Energy Environ. Sci. 2008, 1, 32. (18) Yu, Y.; Lou, X.; Wu, H. Energy Fuels 2008, 22, 46. (19) Funke, A.; Ziegler, F. Biofuels, Bioprod. Biorefin. 2010, 4, 160. (20) Titirici, M. M.; Antonietti, M. Chem. Soc. Rev. 2010, 39, 103. (21) Glaser, B.; Lehmann, J.; Zech, W. Biol. Fertil. Soils 2002, 35, 219. (22) Lehmann, J.; Gaunt, J.; Rondon, M. Mitigation Adapt. Strategies Global Change 2006, 11, 403. (23) Fowles, M. Biomass Bioenergy 2007, 31, 426. (24) Sohi, S.; Lopez-Capel, E.; Krull, E.; Bol, R. Biochar, Climate Change and Soil: A Review To Guide Future Research; Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water Science: Canberra, Australian Capital Territory, Australia, 2009. (25) Antal, M. J.; Allen, S. G.; Schulman, D.; Xu, X. D.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040. (26) Kruse, A.; Gawlik, A. Ind. Eng. Chem. Res. 2003, 42, 267. (27) Matsumura, Y.; Sasaki, M.; Okuda, K.; Takami, S.; Ohara, S.; Umetsu, M.; Adschiri, T. Combust. Sci. Technol. 2006, 178, 509. 1809
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