Published: December 25th, 2016
Experimental methods for fast pyrolysis of lignocellulosic biomass to produce bio-oils and for the catalytic hydrotreating of bio-oils to produce fuel range hydrocarbons are presented. Hot-vapor filtration during fast pyrolysis to remove fine char particles and inorganic contaminants from bio-oil was also assessed.
Lignocellulosic biomass conversion to produce biofuels has received significant attention because of the quest for a replacement for fossil fuels. Among the various thermochemical and biochemical routes, fast pyrolysis followed by catalytic hydrotreating is considered to be a promising near-term opportunity. This paper reports on experimental methods used 1) at the National Renewable Energy Laboratory (NREL) for fast pyrolysis of lignocellulosic biomass to produce bio-oils in a fluidized-bed reactor and 2) at Pacific Northwest National Laboratory (PNNL) for catalytic hydrotreating of bio-oils in a two-stage, fixed-bed, continuous-flow catalytic reactor. The configurations of the reactor systems, the operating procedures, and the processing and analysis of feedstocks, bio-oils, and biofuels are described in detail in this paper. We also demonstrate hot-vapor filtration during fast pyrolysis to remove fine char particles and inorganic contaminants from bio-oil. Representative results showed successful conversion of biomass feedstocks to fuel-range hydrocarbon biofuels and, specifically, the effect of hot-vapor filtration on bio-oil production and upgrading. The protocols provided in this report could help to generate rigorous and reliable data for biomass pyrolysis and bio-oil hydrotreating research.
Our society depends heavily on fossil fuels (e.g., oil, natural gas, coal, etc.). These resources are not sustainable energy sources and are being depleted at a rapidly increasing rate, leading to concerns regarding dwindling fossil-fuel resources, environmental consequences of CO2 emission, and economic problems.1,2,3,4 There is increasing demand for alternative and sustainable energy sources. Biomass is the only renewable and carbon-neutral resource for production of liquid fuels (biofuels) and carbon-based chemicals to replace fossil fuels in the current energy production and conversion system.3,4
Lignocellulosic biomass (e.g., woods, grass, energy crop, agricultural waste, etc.), which is currently the most abundant and least expensive biomass source, has attracted the most attention as a way to produce biofuels via various thermochemical and biological routes.3,4 Three primary routes have been the focus of the recent research: 1) biochemical or chemical conversion to sugars followed by aqueous-phase catalytic and biochemical processing to biofuels; 2) gasification to synthesis gas followed by catalytic conversion to biofuels or alcohols; and 3) pyrolysis or liquefaction to liquid bio-oils followed by catalytic upgrading to biofuels.3,4
The first route can only utilize the cellulose and hemicellulose portion of lignocellulosic biomass. Pyrolysis integrated with upgrading is considered to be a near-term viable technology for the direct production of biofuels.
Pyrolysis is the thermal decomposition of lignocellulosic biomass at temperatures between 400 and 550 °C in the absence of oxygen.4,5,6 A number of reactions, such as depolymerization, dehydration, and C−C bond cleavage, occur during pyrolysis and lead to the formation of a complex mixture of more than 200 oxygenated compounds.4,5,6 Bio-oils in high yields (up to 75 wt% of dry feed) could be produced with up to 70% of the energy stored in the biomass feedstocks retained.4,5 However, direct use of the produced pyrolysis bio-oil as transportation fuels in standard equipment is problematic because of the high oxygen and water content, which lead to different physical and chemical properties such as high viscosity, corrosiveness, poor volatility, low heating value, and poor stability.6,7,8,9 Therefore, extensive oxygen removal is required to upgrade bio-oils to fuel-range hydrocarbons. Catalytic hydrotreating using solid catalysts in hydrogen is the most common route to upgrade bio-oil by oxygen removal through hydrodeoxygenation and hydrogenation reactions.6,7,8,9
Currently, one of the primary challenges for pyrolysis followed by hydrotreating is to achieve long-term steady operation, especially for the hydrotreating process in which the thermal instability of the bio-oil and inorganic and sulfur residues in bio-oil cause significant catalyst deactivation.10,11 The thermal instability of bio-oil has been addressed by low-temperature hydrogenation to stabilize the active species in bio-oil.11,12 Cleanup of bio-oil by removing inorganic residues, which could catalyze repolymerization of bio-oil fractions and deactivate hydrotreating catalysts by deposition, may be valuable. Hot-vapor filtration is one of the techniques to effectively reduce the inorganic content in bio-oil by removing char particulates during pyrolysis.13,14,15 Hot-vapor filtration is used downstream of the pyrolysis reactor to separate char fines from the pyrolysis gas/vapor stream at high temperature before condensation of the vapors.13,14,15
We report here the protocol used at the National Renewable Energy Laboratory (NREL) for biomass fast pyrolysis both with and without hot-vapor filtration to produce bio-oils using a fluidized-bed reactor and at Pacific Northwest National Laboratory (PNNL) for bio-oil hydrotreating to produce biofuels in a continuous-flow packed-bed catalytic reactor. The configurations of the reactor systems, the operating procedures, and the processing and analysis of feedstocks, bio-oils, and biofuels are described in detail. Results of pyrolysis processing of a representative biomass feedstock with or without hot-vapor-filtering and hydrotreating of the produced bio-oil are also presented along with an assessment of the impact of hot-vapor filtration.
1. Fast Pyrolysis with Hot Vapor Filtration
2. Catalytic Hydrotreating of Bio-oil
Note: The bio-oil samples produced at NREL were shipped to PNNL for catalytic hydrotreating on a hydrotreater system.
The fast pyrolysis of a representative herbaceous biomass, switchgrass, with or without hot-vapor filtration and the catalytic hydrotreating of the product bio-oil are used as an example for the process reported here. More details of these experiments can be found in detail in our recent publication.15
Hot-vapor-filtered fast pyrolysis
Table 1 shows bio-oil, char, and gas yields produced with and without the hot-vapor filter for a typical herbaceous feedstock. For the control experiment without hot-vapor filtration, the vapors passed though the filter housing but the filter was not installed. This kept the residence time in the two experiments the same so any difference is due to the filter only. The bio-oil yields were 52% to 56%, indicating successful conversion of the major portion of the biomass to liquid intermediate oils. A photo of a representative bio-oil sample is shown in Figure 4. The mass balance closures were 86% to 90%. Light vapors that were not properly collected in the condensation train were one source of mass loss. Pyrolysis oils contain several low-boiling-point compounds, such as hydroxyacetaldehyde (boiling point 20.2 °C), that are difficult to condense. Adding a second dry-ice trap will improve the recovery of the light condensable compounds. Performing experiments with higher biomass feed rates than reported here will improve recovery of the light vapors by increasing the vapor concentration prior to condensation. The escape of light condensable compounds can be verified by gas chromatography-mass spectroscopy analysis of the exit gas. The mass balances were relatively low for the herbaceous feedstock, likely because of escaping light char, which was produced from the switchgrass in relatively large quantities. Cracking reactions occur at the filter so inclusion of the hot-vapor filter reduced the oil yields and increased the gas yields.
Table 2 and Figures 5 and 6 show the analysis results of herbaceous feedstock and the bio-oils produced. Hot-vapor filtering reduced the ash residue in the bio-oil from 1.45% to below the detection limit. Various inorganics, such as aluminum, calcium, iron, potassium, magnesium, sodium, phosphorous, and silicon, were observed in the bio-oils, and they mainly originate from the biomass feedstock. Hot-vapor filtering significantly decreased the inorganic contents in the bio-oil, indicating that hot-vapor filtering was a powerful protocol for effectively reducing the trace element content in the bio-oils by removing char and ash particulates. Hot-vapor filtering also decreased the carbon content and increased the oxygen content in the bio-oils. Woody feedstocks have low ash contents compared to herbaceous feedstocks, and lower reductions in the bio-oil ash and inorganic contents are observed.15
Catalytic hydrotreating of bio-oil
The analytical results of the produced bio-oil were consistent with the fact that bio-oils produced from such process are not of sufficient quality for direct use in internal combustion engines. Therefore, upgrading of bio-oil is required. The two bio-oil samples were upgraded by catalytic hydrotreating in the hydrotreater system under the conditions discussed above.
Bio-oils are known to plug the hydrotreating reactors as chars or polymerization products of active species in bio-oils accumulate in the catalyst bed. Therefore, the pressure drop across the catalyst beds during the hydrotreating tests is an important indicator of accumulating chars or polymerization products. The hot-vapor filtered bio-oil performed nearly flawlessly for 60 hours TOS in the hydrotreating test. However, the non-filtered bio-oil had ∼5 wt% undissolved solids, which separated out in the pump and were not treated. Even with these untreated solids, there was still a pressure drop buildup after 50 hours TOS, probably because of the residual solids in the non-filtered bio-oil plugging the packed catalyst bed.
Tables 3 and 4 and Figures 5 and 7 list the yield of the products for bio-oil hydrotreating at different TOSs. Phase-separated liquid products, including an upgraded oil phase and an aqueous phase, and gaseous products, including CH4, C2H6, C3H8, C4H10, CO, and CO2, were produced. Figure 4 shows a photo of a reparative upgraded oil sample. Table 5 shows the analysis results of upgraded oil and Figure 5 compares the elemental analysis results of the bio-oil and the upgraded oil. Hydrotreating was very effective at reducing oxygen, sulfur, and nitrogen and adding hydrogen significantly from the bio-oil feed. The oxygen content in the upgraded oil was ∼2.0 wt%, which was significantly lower than 35 to 40 wt% of oxygen in the bio-oil feed. The hydrogen-to-carbon ratio of the upgraded oil was ∼1.7, compared to ∼1.3 for the bio-oil feed. The trend of density of the upgraded oil, which increased from 0.81 to 0.83 g/ml over the period of the test, suggests a mild catalyst deactivation over the 60 hour TOS.
As shown in Figure 7, comparisons of hydrotreated products between the hot-vapor filtered and non-filtered bio-oil showed that hot-vapor filtered bio-oil led to a slightly higher water-to-upgraded oil ratio, which is consistent with the higher oxygen content in the hot-vapor-filtered bio-oil feed. The properties of upgraded oil for the two bio-oils were very similar. The major difference between the hydrotreating of hot-vapor-filtered and non-filtered bio-oil was that the used catalyst beds of hot-vapor-filtered bio-oil showed much less deposition of inorganics compared to the catalyst beds used with non-filtered bio-oils.
Figure 1. Schematic for the 5-cm fluidized-bed pyrolysis reactor system. There is a hot-vapor filter, a condensation system, and a gas-measurement system. Please click here to view a larger version of this figure.
Figure 2. Schematic of the mini-reactor hydrotreater system. (MFC: mass flow controller; RD: rupture disc; PT: pressure transducer; PI: pressure indicator (gauge); BPR: back-pressure regulator; PR: pressure regulator) Please click here to view a larger version of this figure.
Figure 3. Schematic of the catalyst bed in the mini-hydrotreater reactor. The temperature profile of the catalyst bed is shown in the left and the position of catalysts of each stage is shown in the right. Please click here to view a larger version of this figure.
Figure 4. Photos of a representative bio-oil samples (left) and a representative upgraded oil sample (right). Please click here to view a larger version of this figure.
Figure 5. Comparison of elemental analysis results of the herbaceous feedstock (switchgrass), the bio-oil produced with hot-vapor filtration, and the upgraded oil. Carbon, hydrogen, and oxygen content did not change much after fast pyrolysis of biomass, however, oxygen content decreased significantly and hydrogen content increased after bio-oil hydrotreating. Please click here to view a larger version of this figure.
Figure 6. Comparison of oil yield, carbon efficiency, and some properties of bio-oil from hot-vapor filtered and non-filtered pyrolysis. This demonstrates the impact of hot gas filtration of pyrolysis vapors prior to condensation. Hot gas filtration eliminates inorganic residues but it also affects pyrolysis oil yield [3-LM] and oil properties such as oil oxygen content. Please click here to view a larger version of this figure.
Figure 7. Comparison of hydrotreating results of bio-oils from hot-vapor filtered and non-filtered pyrolysis. Hot-vapor filtered bio-oil leads to a slightly higher water-to-fuel ratio and the properties of upgraded fuel for the two bio-oils are very similar. The major difference between hydrotreating of the two pyrolysis oils is that the catalyst bed of hot-vapor-filtered bio-oil showed much less deposition of minerals. Please click here to view a larger version of this figure.
Table 1. Yields of major pyrolysis products (bio-oil, char, and gas) and mass balance closures for pyrolysis of an herbaceous feedstock (switchgrass) with and without hot-vapor filtration.
Table 2. Analysis of the representative herbaceous feedstock (switchgrass) and bio-oil produced with and without hot-vapor filtration.
Table 3. Yield of major hydrotreating products at the different TOSs for hot-vapor filtered and non-filtered representative bio-oil.
Table 4. Produced gas composition during the hydrotreating of representative bio-oils.
Table 5. Analysis of the upgraded oil products from the hydrotreating of representative bio-oils.
In this paper, we described a detailed procedure for converting lignocellulosic biomass to fuel-range hydrocarbons via fast pyrolysis and catalytic hydrotreating. The NREL pyrolysis reactor system with a 5-cm inner diameter fluidized-bed reactor and the PNNL hydrotreater system with a 1.3-cm inner diameter fixed-bed catalytic reactor and their operation procedures are described in detail. These reactor systems could be used to conduct pyrolysis and hydrotreating tests in an efficient and safe manner. We used representative herbaceous feedstocks to produce liquid bio-oils in the pyrolysis reactor system, and then, the bio-oils were processed in the hydrotreating system with a two-stage catalyst bed including sulfided Ru/C and CoMo/Al2O3 as catalysts to produce fuel-range liquid hydrocarbons. The process also is applicable to pyrolysis of a wide range of biomass feedstocks including wood, grass, and corn stover and then upgrading the produced bio-oil to produce biofuels.16 The hydrotreater and hydrotreating process also could be used for upgrading other biomass-generated intermediates such as liquefaction oil (bio-crude) from biomass such as wood and algae.
Maximizing bio-oil yield during pyrolysis requires heating the biomass rapidly to sufficient temperature to achieve maximum volatilization of the biomass. For most biomass, this means temperatures of 500 to 600 °C. A fluidized bed provides rapid heat transfer from the sand to the biomass, providing a high heating rate. The use of small particles also provides a higher heating rate. Typically a few percent higher bio-oil yield is achieved with biomass ground to <0.5 mm than with biomass ground to <2 mm. Maximizing yield also means minimizing thermal cracking of the vapors by keeping the residence time at temperature low (1 to 2 seconds). Pyrolysis vapors contain compounds with a wide range of boiling points. Thus, the hot piping tends to become fouled with liquid, repolymerized vapors and char. To avoid this condition, keep the auger temperature below 100 °C and all surfaces between the reactor and condensation train above 400 °C to avoid fouling, but below 500 °C to minimize thermal cracking. Thorough coverage with heat tape is necessary to prevent cold spots and provide a uniform temperature. Sewn insulation pads with closures on them generally provide more uniform coverage, thereby resulting in more uniform temperature. It is important that the temperature drops rapidly in the first condenser to minimize the opportunity for repolymerization of high-boiling-point materials, which could lead to blockage of the condenser inlet. It also is necessary to use dry ice in the second condenser to maximize liquid recovery and prevent damage to gas-measurement and analysis instruments.
Some enhanced features were not mentioned in the basic fast pyrolysis procedure. It is useful to have a pressure gauge or transmitter near the reactor inlet. In addition, it is useful to measure differential pressure across the reactor and cyclone and to measure the final pressure and temperature at the dry test meter (to enable accurate volume calculations). It also is helpful to have additional thermocouples in the pyrolysis bed to verify that the bed is fluidizing uniformly enough to provide uniform temperatures. Typically, <5 °C spread is seen vertically through the bed. It also is useful to have nested-loop temperature control on the reactor. When a larger amount of oil is needed, it is helpful to install a valve on the bottom of the char receiver and mount a secondary char receiver below that, which in turn has a valve at the bottom with a jar loosely mounted to it. This makes it possible to empty the char receiver into the secondary receiver and finally down into the jar so that continuous operation can be maintained for many hours. Vibration is helpful to the operation. Manual pounding of the pipes can be used, but an automatic vibrator provides more reliable agitation. These can be operated continuously on the lock hopper and auger port to maintain a smooth feed flow through the feeder. Also, using an automatic vibrator on the secondary char receiver during char draining makes that operation much more reliable. Hot-vapor filtration enhances cracking and reduces bio-oil yield as shown above. Keeping the temperature of the filter low but still above condensation temperature (>400°C) minimizes cracking. An inert surface on the filter also may reduce cracking. The filter area needs to be large to reduce pressure drop.
The major limitation of the fast pyrolysis process is that the produced bio-oil has some major problematic properties such as high viscosity, corrosiveness, poor volatility, low heating value, and chemical instability, which limits their direct utilization and causes some issues during their upgrading. 6,7,8,9 A variant of fast pyrolysis, catalytic fast pyrolysis, wherein fast pyrolysis is integrated with a catalysis process to upgrade the pyrolysis vapor, and hydropyrolysis, wherein fast pyrolysis conducted in the presence of reactive gases such as H2, can lead to a higher quality bio-oil but suffer higher operational complexity and low product yield.4,8
Two-stage catalytic hydrotreating showed good processing results for converting bio-oil to fuel-range hydrocarbons. Bio-oils are known to be chemically unstable because of the presence of active species such as carbonyl and phenolic compounds that could undergo repolymerization and condensation at a low temperature, leading to a high propensity for forming carbonaceous materials and consequent catalyst deactivation and even plugging of catalyst bed. Therefore, the first stage hydrogenation step was critical for the process, and was used to stabilize bio-oil by hydrogenation of carbonyls and phenolics at a relative low temperature by using a proper hydrogenation catalyst. The performance of the hydrogenation catalyst was the key of the long-term stability and operability of the process. Oxygen removal by hydrodeoxygenation occurred at the second stage by a sulfide-based hydrotreating catalyst. The yield and properties of produced final oil product depended on the catalysts and conditions used in the second stage. Maximizing the yield of liquid final fuels could be achieved by using catalysts capable of generating C-C bonds, such as alkylation function, and optimized reaction parameters including reaction temperature, pressure, and space velocity. The major limitation of the hydrotreating process is that, because of some problematic properties in bio-oil such as chemical instability and the presence of contaminants17, the lifetime of hydrotreating catalysts, especially the first step hydrogenation catalysts, are still limited, which makes the overall process costly. Maximizing the lifetime of the catalysts used could be achieved by using more robust catalysts; optimized reaction parameters including reaction temperature, pressure, and space velocity; or pretreatment to lower the content of the active species or contaminants in bio-oil feeds.
The hydrotreater was operated at high pressures and reactor temperatures with flammable gases and liquids involved. Therefore, safety rules and procedure should be strictly followed.
The authors declare that they have no competing financial interests.
This work was supported by the U.S. Department of Energy (DOE) under Contract DE-AC36-08-GO28308 at NREL and Contract DE-AC05-76RL01830 at PNNL. The authors gratefully acknowledge the support of the DOE's Bioenergy Technologies Office.
|Mill to pass 2 mm screen
|Sand for bed material
|Screen to 300-500 microns
|Split tube furnace 3.75 ID X 24 L
|Custom-built at NREL
|2" diameter, height 17", dual staggered plate distributor, 316SS, Auger port is 2.5 cm above distributor and is cooled with air or water, there is a coiled 1/4" 304 SS tube below the distributor to pre-heat the gas
|Custom-built at NREL
|Custom-built at NREL
|1 L capacity
|Cyclone secondary receiver
|Custom-built at NREL
|1 L capacity
|Hot vapor filter
|316SS, 10 inches long, 2.0 MICRON
|2-neck round-bottomed flasks
|Allen Scientific Glassware, NREL-built electrodes
|2" diameter 10" long ground electrode, glass enclosed, stop-cock on bottom
|High-voltage power supply
|Spellman High Voltage
|30 kV max, 0.5mA
|Dry test meter
|with IMAC counter
|MS5A, PBQ, CP-Sil columns
|thermal conductivity detector
|Non-Dispersive Infrared Spectrometer
|Carbon monoxide 0-5%, 0-25%, carbon dioxide 0-5%, 0-20%, methane 0-5000 ppmv, 0-3%
|Mass flow controller
|Celerity (now Tylan)
|0-20 SLM reactor bottom, 0-10 SLM auger, 0-2 slm purges, 0-5 slm air
|Auger Manufacturing Specialists
|3/8" Dia SS RH Auger 18"
|Motor for Auger
|Gearmotor-Parallel Shaft, 94RPM, 1/15 HP, TEFC, 115VAC
|Feeding system: Motor for hopper
|7KB4-7-100H Motor Ac Helical Gearbox 3PH 0.25 kW 1.4/0.82A
|Feeding system: Hopper and Loss in weight feeder
|with K10S controller
|Feeding system: Valves
|151 bar at 37°C and 6.8 bar at 232°C
|4550T152 and similar
|Extreme-Temperature (1400°F), heavy insulation for use on metal
|Custom-built at NREL
|1/4" PFA and stainless steel tubing, 1.4 m tall
|Ru on carbon catalyst
|Fabricated at PNNL
|7.6 wt% Ru on carbon
|3% Co and 9% Mo on Al2O3 catalyst
|Cobalt oxide, typically 3.4-4.5%, Molybdenum oxide typically 11.5-14.5% on alumina
|Syringe pump, 500 ml cylinder capacity
|Mass flow controller
|Digi-Sense Advanced Temperature Controller, 115V
|Dual element heating tape, 1/2in x 12ft, 936 watts, 120VAC w/ 2-prong plug
|Digital pressure gauge
|High Accuracy Digital Pressure Gauge, with Data Logging Capability
|Back pressure regulator
|Gas flow meter
|Dry Cal, Definer 220 Low Flow
|Hydrotreating reactor, cross, tee, fittings
|Combustible gas sensor
|COMBUSTIBLE GAS DETECTION SENSOR, 24 VDC POWER, ANANLOG 4-20 MADC OUTPUR WITH MODBUS, NO RELAYS
|H2S TOXIC GAS SENSOR MODULE, 24VDC POWER, ANALOG 4-20 MADC OUTPUT WITH MODBUS, NO RELAYS
|Fume Hood Monitor FHM10
|Four-channel micro-GC with molecular sieve, Plot U, Alumina, and Stabilwax columns
|Lab-view based monitering and controlling system
|Custom-built at PNNL
|Using National Instruments parts and Labview software
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