Article pubs.acs.org/EF
Biomass Hydropyrolysis in a Pressurized Fluidized Bed Reactor David C. Dayton,* John Carpenter, Justin Farmer, Brian Turk, and Raghubir Gupta Center for Energy Technology, RTI International, 3040 Cornwallis Road, Research Triangle Park, North Carolina 27709, United States ABSTRACT: Biomass pyrolysis and hydropyrolysis have been studied in a high-temperature, high-pressure fluidized bed reactor system. The reactor system can be operated at reaction temperatures up to 982 °C (1800 °F) and pressures up to 4 MPa (600 psi). Baseline biomass pyrolysis experiments with an inert heat transfer material were conducted at various pressures and hydrogen partial pressures to determine the effect of these variables on product yields and quality, defined by the amount of oxygen in the hydrocarbon-rich liquid product. Biomass hydropyrolysis was performed at temperatures between 375 and 400 °C at 2 MPa (300 psi) with selected hydroprocessing catalysts. The most promising catalyst was exposed to 2.9 kg of woody biomass for a total of 21.7 h time on stream over a 10 day period. The cumulative mass balance during this period was 83%, and the overall C4+ yield was 16 wt %. This corresponds to 31% of the energy in the input biomass feedstock recovered in the hydrocarbon-rich liquid that contained an average of 4.2 wt % oxygen on a dry basis.
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INTRODUCTION In the past several years, development efforts have been transitioning to advanced biofuel technologies to produce hydrocarbons that are drop-in replacement intermediates or fuels in the existing petroleum refining and distribution infrastructure.1−3 The demand for ethanol as an oxygenated blendstock in reformulated gasoline is currently being satisfied by corn ethanol production. Consequently, the market for additional lignocellulosic ethanol production in the future requires an increased demand for fuel ethanol. This can occur by (1) increasing the ethanol content in reformulated gasoline, (2) increasing the number of E85 vehicles in the transportation fleet, and (3) displacing corn ethanol. In January 2011, the U.S. Environmental Protection Agency (EPA) partially granted a waiver to increase the ethanol content of commercial gasoline to greater than 10 vol % and up to 15 vol % ethanol for use in light-duty motor vehicles from model year 2001 and newer (http://www.epa.gov/otaq/regs/fuels/additive/e15/). The increase in E85 vehicles requires additional fuel infrastructure to add new fuel dispensers at filling stations around the country at a considerable cost. A promising alternative is the development of conversion technologies that produce infrastructure-compatible advanced biofuels. These advanced biofuels are hydrocarbon-based fuels that can be processed, blended, and distributed in the existing petroleum infrastructure. Not only does this avoid the oxygenated blend limit for fuel ethanol, it also opens up additional markets beyond fuel for gasoline-powered light-duty vehicles that includes the diesel and jet fuel-powered segments of the transportation sector and the alternatives for petroleumbased products. Pyrolysis is a thermochemical processing option for producing liquid intermediates from biomass that can be upgraded into hydrocarbon fuels. Traditional biomass flash pyrolysis processes have demonstrated a roughly 70% liquid product yield; however, this pyrolysis oil product has limited use without significant upgrading or refining. Unfortunately, the physical and chemical properties of the current fast biomass © 2013 American Chemical Society
pyrolysis oils make them unsuitable for integrating into the refinery. Adverse properties of conventional pyrolysis oil include (1) thermal instability and high fouling tendency, (2) corrosiveness due to high carboxylic acid content (pH 2.2−2.4, typically), (3) immiscibility with refinery feedstocks, and (4) metals (K, Na, and Ca) and nitrogen content, which foul or deactivate refinery catalysts. Traditional hydroprocessing, including hydrocracking over solid acid catalysts and hydrodeoxygenation (HDO) in the presence of a catalyst and high-pressure hydrogen, can be adapted for biofuel production. Although both of these processes have the potential for producing hydrocarbon biofuels, it should be noted that both hydrocracking and hydroprocessing are accompanied by the loss of hydrogen (as H2O) and carbon (as carbon dioxide [CO2] or carbon monoxide [CO]) from the bio-oil.4,5 The relative contribution of HDO and decarboxylation needs to be balanced to optimize hydrocarbon yields while minimizing H2 requirements. The current status of bio-oil upgrading to fuels has recently been reviewed by Elliott.6 In general, biomass needs to be deoxygenated and is hydrogen-deficient when it comes to producing hydrocarbon fuels. The chemical formula of biomass is generally represented as CH1.5O0.7, whereas gasoline (octane) has a H:C ratio of 2 with no oxygen. Biomass pyrolysis oil is essentially liquid wood with a similar chemical formula (CH1.7O0.5) that requires additional hydrogen for upgrading to hydrocarbon biofuels. Upgrading bio-oil into hydrocarbon transportation fuels requires removal of oxygen and an increase in the hydrogento-carbon ratio. Therefore, one of the challenges in developing biofuel technologies is efficient utilization of hydrogen while maximizing carbon (energy) conversion efficiency in the product. Received: March 1, 2013 Revised: May 30, 2013 Published: May 31, 2013 3778
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differential across the high-temperature reactor, avoiding the need for expensive materials of construction. Ground biomass feedstock (180−300 μm sawdust) was loaded into a pressurized feed hopper that was balanced with the reactor pressure. A dosing screw metered the biomass into an eductor where it was entrained with nitrogen and pneumatically conveyed into the reactor system and injected into a fluidized bed of hot solids (SiC sand or catalyst). The height of the solids in the reactor is represented by the shaded area in Figures 1 and 3. The fluidized bed is effectively a densephase mixing zone that rapidly heats the biomass to achieve effective biomass devolatilization and subsequent vapor phase hydropyrolysis. Biomass feed rates were on the order of 2−4 g/min, and total gas flow was on the order of 100−200 slm. Biomass pyrolysis and hydropyrolysis were conducted at temperatures between 375 and 450 °C and pressures between 0.33 MPa (50 psig) and 2 MPa (300 psig). The temperature was measured in the fluidized bed at the bottom of the reactor and in the exit gas stream at the top of the reactor. For all experiments, the temperature differential between the bed and the exist gases was consistently less than 25 °C. The thermocouple located at the exit of the reactor verified that the biomass/catalyst/products mixture reached the reactor temperature. The heat transfer rate between the large excess of solids and the biomass was a minimum of 200 °C/s if the entire length of the riser was required to heat the biomass. More likely, the biomass was heated to temperature in the dense-phase mixing zone at the bottom of the riser, which would result in a heating rate closer to 1000 °C/s. Reactants were fed into the reactor through a set of concentric tubes. A nitrogen purge was added through the outer annulus while biomass entrained in nitrogen was added through the inner annulus. Hydrogen was fed through the inner tube. The cross section of the bottom of the hydropyrolysis reactor is shown in Figure 2 to highlight the gas and solid inlets. One liter of bed material creates a 2−3 foot mixing zone at the base of the reactor that was aerated with nitrogen introduced through a porous frit to create a fluidized bed. The riser tube extends into the fluidized bed, creating a narrow, higher velocity region that results in transport of solids to the top of the reactor. Catalyst from the outer region enters the bottom of the inner bed as catalyst is entrained from the top of the fluid bed into the riser. The reactants (hydrogen and biomass) with nitrogen are directly fed through the reactor inlet at the base of the riser section into the bottom of the fluidized bed. The section where biomass is fed is contained in the lower section of the riser tubing, limiting the reactants to the fluid bed in that section. The gas exiting the top of the fluid bed has a high enough velocity to entrain some of the catalyst and most to all of the char, product vapors, and reaction gases into and through the riser. The reacting mixture has an additional residence time of approximately 2−3 s in the 10 ft riser section before entering the top section. A disengagement baffle is positioned at the top of the riser to separate the solids from the product gases by differences in momentum. The product stream is redirected toward the reactor base such that the solids, with greater momentum, fall back down into the bed, while the product gas, char, and very fine particulates have a low enough momentum to exit the top of the reactor. In this manner, the bed solids continuously circulate within the reactor and the space between the riser and reactor wall effectively creates a standpipe for the circulating solid as it re-enters the fluidized bed. The lighter fine bed material and biomass char entrained in the product gases and hydropyrolysis vapors exit the top of the 12 ft tall reactor through a heated transfer line. Solids were separated from the product gases in a cyclone separator designed to achieve 90% solids collection efficiency for particles larger than 10 μm at an operating temperature up to 500 °C and a pressure up to 4 MPa (600 psig), respectively. Solids are collected in the cyclone dipleg, and the particlefree product vapors and gases were directed into the condensation system. The condensation system was designed and fabricated to maximize bio-oil collection efficiency. Product vapors and permanent gases are cooled in a tube-in-tube heat exchanger to remove sensible heat from the gas/bio-oil vapor mixture. The cooled product gases were then
One technology option that attempts to meet these requirements is hydropyrolysis. Biomass and catalyst are combined at elevated temperatures and a high hydrogen partial pressure to hydrodeoxygenate biomass pyrolysis vapors and produce low-oxygen-containing hydrocarbon-rich liquid product. Several groups have demonstrated the technical feasibility of producing hydrocarbon liquids from biomass at hydropyrolysis conditions.7−11 The economics of hydropyrolysis also compares well to other biofuel technologies considering the higher pressures involved and the high hydrogen demand.8,9,12,13
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EXPERIMENTAL METHODS
Biomass Feedstock. The biomass feedstock used in the hydropyrolysis studies was a woody residue provided by Catchlight Energy LLC to the National Advanced Biofuels Consortium. It was predominantly loblolly pine chips that were air-dried and coarseground using a hammermill with a 1/4 in. screen. After drying and grinding, the average moisture content of the 1/4 in. material was 8.1%. The material was milled again for laboratory experiments to a final particle size distribution between 180 and 300 μm. The woody biomass feedstock proximate and ultimate analyses are presented in Table 1. This material was used without any additional processing or drying.
Table 1. Loblolly Pine Feedstock Analysis proximate analysis ash volatile matter fixed carbon HHV (BTU/lb) ultimate analysis C H N O S
wt %, dry 0.93 80.8 18.3 8700 wt %, dry, ash free 50.1 6.2 0.15 43.5 0.027
Catalyst. Three different heat transfer media were investigated in this study. An inert bed material (SiC) was used to evaluate noncatalytic biomass pyrolysis at elevated temperatures and pressures as a baseline. Two different catalysts (A and B) were used for catalytic biomass hydropyrolysis. Catalyst A is a commercially available hydrotreating catalyst received as 0.5 mm extrudates that were crushed and sieved to obtain a 106−125 μm size distribution required for the fluidized bed reactor system. Prior to use, this catalyst was reduced in the reactor at 450 °C for 3 h in 14.3% H2/balance N2. Catalyst B is a commercially available spray-dried FCC material that was just sieved to the desired particle size distribution. Catalyst B did not require reduction but was dried at 120 °C in an oven overnight prior to use. Hydropyrolysis Reactor System. The biomass hydropyrolysis studies were conducted in a bench-scale high-temperature, highpressure reactor system, as shown in Figure 1. The reactor system can be operated at reaction temperatures up to 982 °C (1800 °F) and pressures up to 4 MPa (600 psi). The 12 ft tall hydropyrolysis reactor consists of a 10 ft long, 1/2 in. riser tube inserted into a 1 1/2 in. pipe that is electrically heated with six independently controlled resistance heating elements. The heaters are enclosed in a layer of hightemperature thermal insulation, thereby reducing the heat transfer from the inner reactor shell to the water-cooled outer wall of the pressure vessel. A cross section of the reactor vessel is shown in Figure 2. The water-cooled outer shell was maintained below the critical temperature for safe operation at pressures up to 4 MPa (600 psig), and the pressure inside the reactor was balanced by nitrogen introduced on the shell side of the vessel to minimize the pressure 3779
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Figure 1. Process flow diagram of the bench-scale hydropyrolysis reactor system (shaded region represents the fluidized bed in the system). A slipstream of the noncondensable product gases was analyzed with an Agilent 3000a MicroGC capable of detecting permanent gases through C6 hydrocarbons every 3.5 min. The remaining noncondensable product gases are vented. The product gas composition was mostly nitrogen (82.5%) and H2 (17%) because of the large amount of carrier gas needed to circulate and entrain the solids and the nitrogen added to the shell side to maintain the elevated system pressures. The online microGC analysis of product gas was used to determine the concentrations of CO, CO2, CH4, and C2−C6 hydrocarbons. Analytical Methods. The elemental composition (C, H, N, S, and O) of all solids and liquids collected was determined off-line in a Thermo Scientific Flash 2000 Organic Elemental Analyzer. Solid samples included those separated in the cyclone and solids recovered from the bed. The liquid samples collected from each of the impingers and in the ESP were also analyzed separately. The chemical composition of the liquid samples was determined with an Agilent Technologies 5975C GC/MS to identify the organic products. The water content of the liquids was determined by Karl Fischer titration. Mass balances were determined for all solid, liquid, and gas-phase products in relation to the input biomass and hydrogen. Gas product yields were determined by online MicroGC analysis. High carrier gas flow rates diluted product gases such that, in some instances, C2−C6 hydrocarbons were not detected. These light gases along with light liquid products were not captured adequately by the condensation system and likely constitute the unaccounted mass in each trial. Solids collected in the exit cyclone were analyzed to determine char yields and bed carryover. Char yield was calculated as the difference between the weight of the cyclone solids plus the weight of the solids in the bed and the initial weight of the bed material. The char sample was oxidized to determine the ash content of solids, and bed carryover was
Figure 2. Cross section of reactor and inlets. directed into a series of impingers cooled in ice and dry ice to collect condensed bio-oil. An electrostatic precipitator (ESP) downstream of the impingers was used to collect any condensation aerosols that formed and escaped the impingers. A back-pressure regulator was used to control the overall system pressure before the product gases were vented. The process flow diagram of the hydropyrolysis reactor system is shown in Figure 1. 3780
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2−4 g/min into the hydropyrolysis reactor. Inlet gas flows and biomass feedrate were varied to establish a 2−3 s residence time in the riser section of the reactor. The inert bed material was SiC. Typical pyrolysis temperatures reported in the literature range from 300 to 600 °C where the temperature that results in maximum bio-oil yield is dependent on feedstock and system.14 A series of flash pyrolysis experiments in the hydropyrolysis reactor system at atmospheric pressure validated that the maximum bio-oil yield was obtained between 400 and 450 °C. Noncatalytic Pressure Effects. The effect of pressure and excess hydrogen partial pressure on biomass pyrolysis was systematically studied at 400 °C and pressures between 50 and 300 psig. An excess of hydrogen (hydrogen partial pressures of 7, 21, and 43 psi) based on the stoichiometric amount of oxygen introduced with the biomass was used as the carrier gas. The experimental results presented in Table 2 indicate that, as the biomass pyrolysis pressure is increased, the liquid yields increase and the char and gas yields decrease. Higher pyrolysis pressure favors liquid production where liquid yield is increased from 41.7% at 50 psig to 49.5% at 150 psig. Liquid yield at 300 psig was lower than that at 150 psig, likely due to inefficient collection of liquid as both gas and char yields continue to decrease with increasing pressure. This is consistent with the lower mass closure at 300 psig compared to 150 psig. The effect of pressure and excess hydrogen on gas composition during biomass pyrolysis is also shown in Table 2. Methane and hydrogen are not detected in the product gases at any pressure at 400 °C. Only CO and CO2 were at high enough concentrations to be detected in the large excess of inert gas required to operate the reactor system. The molar ratio of CO to CO2 increases from 1.1 to 2.3 with increasing pressure from 50 to 300 psig. The results in Table 2 indicate that the gas yields from biomass pyrolysis with and without added hydrogen in the absence of a catalyst were very similar. The presence of excess hydrogen does affect the gas composition as a lower CO/CO2 ratio is clearly observed in biomass pyrolysis in excess hydrogen at 300 psig. The char composition was insensitive to process conditions. The average char composition was 74 ± 3 wt % carbon, 4 ± 0.2 wt % hydrogen, 18 ± 3 wt % oxygen, and 4 ± 1 wt % ash. Char produced under varying pressures in excess hydrogen have similar compositions to the char produced in the absence of hydrogen. The decrease in solids yield in the presence of
Figure 3. Expanded view of reactor bottom (shaded region represents the fluidized bed in the system). determined by physical separation based on different particle sizes for the catalyst and char particles. Liquid products were collected at the outlet of the heat exchanger, in the impingers, and in the ESP. The condensation system was weighed before and after to determine the total liquid yield. The water-rich portion of the liquid products was collected in the impinger train, and the oil-rich portion was typically collected in the ESP. The liquid product was centrifuged to separate fine particles, and the dried solid weight was subtracted from the total liquid yield. An acetone wash of the ESP walls was also analyzed by GC/MS to determine the composition of the uncollected organic fraction.
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RESULTS AND DISCUSSION The effects of pressure and hydrogen partial pressure on biomass pyrolysis were systematically investigated using the hydropyrolysis reactor system. Loblolly pine was fed at a rate of
Table 2. Product Yields and Gas Composition from Biomass Pyrolysis at Various Bed Materials, Total Pressure, and Hydrogen Partial Pressure products (wt %) reactor bed material SiC SiC SiC SiC SiC SiC catalyst catalyst catalyst catalyst catalyst a
A A B B B
gas composition (vol %)
temperature (°C)
pressure (psig)
P[H2] (psi)
gas
solids
liquid
total
CH4a
COa
CO2a
400 400 400 400 400 400 400 450 450 400 450
50 150 300 50 150 300 300 300 50 300 300
0 0 0 7 21 43 43 43 7 43 43
9.9 7.8 4.9 9.8 7.6 7.2 8.6 20.8 19.8 17.8 19.4
40.5 36.1 29.2 17.5 17.5 19.9 18.6 10.1 30.9 17.9 12
41.7 49.5 45.7 42.8 42.9 52.9 38.2 42.2 47.5 29.6 35.4
92.1 93.4 79.8 70.1 68.0 80.0 65.4 73.1 98.2 65.3 66.8
ND ND ND ND ND ND 86.2 100 4.2 ND ND
52.2 64.5 70.2 54.9 60.0 50.4 ND ND 71.7 81.1 81.0
47.8 35.5 29.8 45.1 40.0 49.6 ND ND 19.4 18.9 19.0
ND: not detected. 3781
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Table 3. Liquid Product Composition from Biomass Pyrolysis at Various Bed Materials, Total Pressure, and Hydrogen Partial Pressure yield (wt %) reactor bed material SiC SiC SiC SiC SiC SiC catalyst catalyst catalyst catalyst catalyst
A A B B B
composition (wt % - dry basis)
temperature (°C)
pressure (psig)
P[H2] (psi)
liquid
bio-oil
H2O
400 400 400 400 400 400 400 450 450 400 450
50 150 300 50 150 300 300 300 50 300 300
0 0 0 7 21 43 43 43 7 43 43
41.7 49.5 45.7 42.8 42.9 52.9 38.2 42.2 47.5 29.6 35.4
25.8 33.2 30.2 26.6 29.5 42.5 0.4 0.4 0.5 0.3 0.4
15.9 16.3 15.5 16.2 13.4 10.4 37.8 41.8 47.0 29.3 35.0
carbon
hydrogen
oxygen
53.5 6.5 40.0 55.4 6.2 38.4 54.4 6.4 39.2 54.8 6.4 38.8 55.3 6.4 38.4 59.0 6.5 34.6 not enough organic product for analysis
conventional bio-oil. Instead, the liquid was clear to yellowish in color with a distinct hydrocarbon odor. Small sample size and low collection efficiency prohibited a visible fractionation between bio-oil and water, suggesting a high water content. Karl Fischer titrations confirmed that the liquid products from these experiments were greater than 99 wt % water. Further process optimization is required to improve the desired product yield with the catalysts able to increase the removal of oxygen from the biomass as water. The permanent gas compositions measured during catalytic biomass hydropyrolysis are presented in Table 2. The permanent gas composition is highly dependent on the catalyst. No CH4 is detected at high pressure (300 psig) with catalyst B; however, CH4 is the dominant product gas component with catalyst A. A higher CO/CO2 ratio is observed with catalyst B, a cracking catalyst, compared to the gas composition from noncatalytic biomass pyrolysis in excess hydrogen, indicating that there may be increased decarbonylation and/or a catalytic effect on the water-gas shift equilibrium. CO and CO2 concentrations decreased significantly with time on stream for catalyst A, consistent with water production via hydrodeoxygenation. Additionally, given that catalyst A is a hydrotreating catalyst, it is very likely that this catalyst has significant methanation activity under these experimental conditions. This would explain the low carbon oxide concentrations and high methane concentration in the product gases. The objective of the hydropyrolysis catalyst is to affect the chemistry during the flash pyrolysis to produce a liquid product with improved properties, mainly lower oxygen content. Both catalysts A and B demonstrate an ability to affect the chemistry, resulting in a significant increase in gas and water yield while minimizing increases in solids yield. Both catalysts demonstrated such activity that the yield of desired product was minimal with increased light gas products. Controlling the activity through catalyst optimization and process conditions is critical to developing hydropyrolysis as a technology for producing liquid bio-oils. Recent work in the field has demonstrated the potential to achieve bio-oil yields between 25 and 30 wt % from the hydropyrolysis of woody biomass under proper conditions.9,15 A balance between additional carbon deposition and gas yields with lower oxygen content must be achieved to generate a useful bio-oil product on an economically attractive scale. Catalyst B was discounted because of the high measured solid yield. Catalyst A was tested at a lower temperature over longer times to investigate
hydrogen is an interesting product shift that should be noted happens in the absence of catalyst. Further investigation is required to determine if this decrease is the result of chemical processes during biomass devolatilization or hydrodynamic bed effects from the addition of hydrogen to the fluidization gas. The influence of pressure and excess hydrogen on liquid product composition is presented in Table 3. Higher bio-oil yield was achieved with increasing pyrolysis pressure, which is consistent with the general trend of liquid yield at various pressures. The elemental composition of the bio-oil on a dry basis remains essentially unchanged within the pressure range investigated in this study. Liquid product compositions from biomass pyrolysis with and without excess hydrogen are detailed in Table 3. As with the char and gas compositions, no difference is observed in the liquid products between inert and excess H2 conditions at 50 and 150 psig. Bio-oil yield continues to increase when pressure is increased to 300 psig and the oxygen content is slightly decreased. Increasing pressure resulted in a higher liquid yield, whereas the opposite effect was measured with increasing temperature. Increasing pressure or the addition of excess hydrogen in the absence of a catalyst does not promote deoxygenation of the liquid product. Hydropyrolysis Catalyst Effect. Biomass was injected into a fluidized bed of the catalyst at 400 and 450 °C with 14.3 vol % H2 in N2 used as the carrier gas at 300 psig total pressure (P[H2] = 43 psi.). A summary of the catalytic biomass hydropyrolysis results with two catalysts (A and B) are presented in Table 2. The product yields for the catalytic biomass hydropyrolysis experiments are presented in Table 2. The presence of catalyst during hydropyrolysis resulted in higher gas yields compared with the results from biomass pyrolysis in excess hydrogen without catalyst (300 psig, P[H2] = 43 psi, 400 °C, no catalyst.) The lowest gas yield during catalytic hydropyrolysis was measured with catalyst A at 400 °C and 300 psig. It is worth noting that the product yields are reported without subtracting the added hydrogen in the carrier gas to the inputs. A net production or consumption of hydrogen was not evident from the gas compositions measured during catalytic biomass hydropyrolysis. The solids yield with both catalysts decreased with increasing temperature. Solids yield includes both char and carbon (coke) deposited on the catalyst. The high solids yield with catalyst B at lower pressure (50 psig, P[H2] = 7 psi, 450 °C) results from increased coke formation. The catalytic hydropyrolysis liquids produced did not contain a dark viscous organic fraction characteristic of 3782
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Table 4. Summary of Daily Mass Closure for Hydropyrolysis with Catalyst A day 1
2
3
4
solids 20.9 0.0 69.3 17.6 gas 25.9 34.3 38.9 33.8 liquid 21.7 29.8 19.1 37.4 organic 2.3 0.5 5.3 9.6 aqueous 19.5 29.4 13.9 27.7 product yields in wt % biomass fed, O wt % in wt % of organic liquid organic composition O wt % (DAF) not enough organic product for analysis 9.6 H/C 1.2
5
6
7
8
9
10
27.6 31.8 27.3 8.2 19.1
22.1 23.8 33.4 8.4 25.0
18.4 23.5 38.5 9.5 29.0
17.8 20.7 40.4 9.1 31.2
16.9 22.1 44.4 11.5 33.0
20.0 19.7 42.2 12.5 29.8
2.7 1.3
2.7 1.3
2.2 1.2
4.0 1.3
5.1 1.3
4.4 1.3
Table 5. Average Daily Product Gas Yields (as wt % of Biomass Fed) Measured during Hydropyrolysis with Catalyst A day CO CO2 CH4 C2 C3 C4 C5 C6
1
2
3
4
5
6
7
8
9
10
2.1 0.8 2.1 2.9 7.6 2.7 7.6 0.0
3.2 2.8 2.6 4.3 9.6 3.2 8.3 0.3
3.7 2.7 2.9 4.6 11.4 3.7 9.5 0.4
3.3 3.1 2.6 3.9 9.3 3.2 8.7 0.3
3.2 3.2 2.2 3.1 9.7 2.7 7.3 0.5
3.0 2.4 1.8 3.1 6.4 1.4 5.4 0.4
3.5 2.8 1.9 3.2 6.7 1.3 3.6 0.4
3.8 2.8 1.6 2.6 5.4 1.0 3.5 0.0
4.7 3.6 1.7 2.5 5.2 1.0 3.4 0.1
4.6 3.6 1.5 2.0 3.9 0.8 2.9 0.6
were not significantly different during the first three days compared to the remaining days, so excessive cracking because of high catalyst activity is unlikely. Poor liquid collection efficiency resulted when the heat exchanger was fouled, as indicated by an increased temperature on the shell side. On the basis of the carbon and oxygen balances, it appears that a fraction of the aqueous phase was not being collected. Most of the carbon is accounted for in the products, but the oxygen balance is low. The aqueous liquid stream contains the majority of the oxygen, mostly as water. The elemental composition and water content of the aqueous and organic liquid products are shown in Table 6.
equilibrium catalyst activity and increase the bio-oil product yield. Biomass Hydropyrolysis Results with Catalyst A. Extended testing with catalyst A was performed to collect experimental data for proof-of-concept validation and to collect enough liquid product for detailed analysis. The major accomplishments include over 21 h of loblolly pine hydropyrolysis with catalyst A and the production of ∼200 mL of hydrocarbon-rich (4.2 wt % O) liquid product. The cumulative time on stream was accomplished in 10 days at 375 °C and 300 psig (42.9 psig H2). At the beginning of the campaign, 909 g of catalyst A was loaded into the reactor and reduced in hydrogen. Over the 10 day campaign, a total of 2.9 kg of loblolly pine was consumed and the overall mass balance was 83%. The mass closure includes 22% collected solids (char, catalyst fines, and carbon deposition on the catalyst), 8.7% biocrude, 25.2% water, and 26.8% gas (CO, CO2, CH4, and C2−C6 hydrocarbons). The overall C4+ yield was 16 wt % with 29.4% carbon recovery and 31% energy recovery in the products. The overall carbon closure (96%) was determined from the elemental composition of the product streams. Roughly 41% of the input carbon from the biomass feedstock was in the product gases, and 39% of the input carbon was in the collected solids (char + carbon deposition on the catalyst). Fifteen percent of the input carbon was captured in the organic liquid phase, and only 1% of the input carbon was lost to the aqueous phase. The oxygen balance was only 74% with 56% of the input oxygen collected in the aqueous phase. Products were recovered and analyzed daily to evaluate process stability and catalyst activity as a function of time on stream. The mass balances determined at the end of each day are presented in Table 4, and the average gas compositions for each day are presented in Table 5. Very little organic liquid was collected during the first three days of testing with catalyst A. This may have been because of inefficient product collection. The gas and aqueous liquid yields
Table 6. Elemental Composition of Liquid Products (DAF) C (wt %) H (wt %) O (wt %) water content wt % H2O
organic
aqueous
86.7 9.1 4.2
55.7 7.3 37
0.5
97.0
During days 4−10, the liquid collection efficiency was improved by ensuring the heat exchanger was clean before each day to maximize heat transfer out of the vapors. The organic and aqueous phase liquid yields were relatively consistent over this six day period. The chemical composition of the liquid products was also fairly constant. The oxygen content of the organic liquid ranged between 2% and 5%, and the H/C ratio was 1.2 to 1.3. A more detailed chemical composition of the liquid products was determined by GC/MS analysis. A summary of the chemical composition of the aqueous and organic liquid products is presented in Table 7. The organic phase has a very high aromatics content split equally between benzene derivatives and substituted naphthalenes and higher PAHs. 3783
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in the hydrocarbon-rich liquid that contained an average of 4.2 wt % oxygen on a dry basis. Hydrogen consumption could not be directly measured given the large excess of nitrogen required to maintain system pressure. Hydrogen consumption can be correlated to the stoichiometric amount of water produced during hydropyrolysis, but the oxygen balance in the experiments was low, suggesting that the aqueous fraction of the liquid product is underestimated. Nevertheless, the hydrogen consumption is a key parameter impacting the operating cost of the process. The hydroprocessing catalyst was effectively used as received and reduced in hydrogen prior to exposure to biomass. There has been no effort at this point in developing an optimized hydropyrolysis catalyst, so the preliminary results with a standard hydroprocessing catalyst are very encouraging. The organic liquid yield increased over the 10 day experimental period as the time on stream increased for the catalyst. The higher initial gas yields and lower initial liquid yields suggest that the catalyst was too active at the beginning of the experimental period. The organic liquid yield increased with time on stream, suggesting that the catalyst activity was decreasing but was more appropriate for hydropyrolysis. Future catalyst development activities will focus on understanding the catalyst changes occurring during this initial time on stream and determining the steady-state catalyst performance. Understanding the role of the catalyst in biomass hydropyrolysis will inform the development of optimized formulations that maximize organic liquid yield and minimize carbon loss to char and carbon deposition on the catalyst. Future studies will also target direct measurement of hydrogen consumption. Optimized organic liquid yields and hydrogen consumption are basic input data required for process models to evaluate the economic viability and commercial potential of biomass hydropyrolysis.
Table 7. Chemical Composition of Liquid Products Based on Area % in the GC/MS aliphatics aldehyde/ketone acids aromatics benzene derivatives PAHs phenols other
organic
aqueous
3 0.6 0.6 94 47 45 2 2
0 28 14 46 2 5 39 12
The carbon distribution in the organic liquid is plotted in Figure 4. The low relative concentration of phenolics, acids, and
Figure 4. Carbon distribution in organic liquid product.
aldehydes/ketones is consistent with the measured oxygen content of the organic liquid product. The total acid number (mg KOH/g biocrude) for the organic phase was below 10, based on a pH = 11 end point titration. The aqueous product is mostly water (95%), and the water-soluble organics are mostly phenols, acids, aldehydes, and ketones.
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CONCLUSIONS In the absence of a catalyst, the effect of pressure and hydrogen partial pressure on the biomass pyrolysis product distribution and bio-oil composition was minimal. Increasing pressure suggests a trend toward higher liquid yields with less char formation. Improved heat transfer or better liquid collection efficiency may explain this result. Improved deoxygenation of pyrolysis vapors and reduced char formation was the hypothesis being tested by adding hydrogen to the inert pyrolysis atmosphere. Hydrogen was thought to be a potential source of radicals to enhance hydrodeoxygenation and terminate thermal cracking reactions before carbon is produced. Char yields were suppressed in the presence of hydrogen; however, minimal effects on gas yields and liquid product compositions were observed. When a hydroprocessing catalyst was used in the reactor with hydrogen added to the pyrolysis atmosphere, the product distribution and bio-oil composition were quite different compared to the fast pyrolysis products. Biomass hydropyrolysis was performed at temperatures between 375 and 400 °C at 2 MPa (300 psi). The liquid product contained a larger fraction of water, but the organic fraction of the bio-oil had a much lower oxygen content compared to conventional fast pyrolysis oil. The most promising catalyst was exposed to 2.9 kg of woody biomass for a total of 21.7 h on stream over a 10 day period. The cumulative mass balance during this period was 83%, and the overall C4+ yield was 16 wt %. This corresponds to 31% of the energy in the input biomass feedstock recovered
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was conducted as part of the Hydropyrolysis Strategy Area in the National Advanced Biofuels Consortium (NABC) led by the National Renewable Energy Laboratory and the Pacific Northwest National Laboratory under the direction of Dr. Thomas Foust, Director. Funding for the NABC is provided by the Department of Energy’s Energy Efficiency and Renewable Energy Office of Biomass Programs. Catalyst samples were provided through the NABC by Albemarle Corporation. Biomass feedstock was provided by Catchlight Energy LLC and prepared by Iowa State University as part of the NABC feedstock activities.
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