Ind. Eng. Chem. Res. 2007, 46, 1891-1897
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Bench-Scale Fluidized-Bed Pyrolysis of Switchgrass for Bio-Oil Production† Akwasi A. Boateng,*,‡ Daren E. Daugaard,§ Neil M. Goldberg,‡ and Kevin B. Hicks‡ Eastern Regional Research Center, Agricultural Research SerVice, U.S. Department of Agriculture, 600 East Mermaid Lane, Wyndmoor, PennsylVania 19038, and Department of Mechanical Engineering, The UniVersity of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249
The U.S. biomass initiative is counting on lignocellulosic conversion to boost the quantities of biofuels currently produced from starches in order to achieve much needed energy security in the future. However, with current challenges in fermentation of lignocellulosic material to ethanol, other methods of converting biomass to usable energy have received consideration nationally. One thermochemical technique, fast pyrolysis, is being considered by the Agricultural Research Service (ARS) researchers of the USDA for processing energy crops such as switchgrass and other agricultural residues, e.g., barley hulls and alfalfa stems for bio-oil (pyrolysis oil or pyrolysis liquids) production. A 2.5 kg/h biomass fast pyrolyzer has been developed at ARS and tested for switchgrass conversion. The unit has provided useful data such as energy requirements and product yields that can be used as design parameters for larger systems based on the processing of perennial energy crops. Bio-oil yields greater than 60% by mass have been demonstrated for switchgrass, with energy conversion efficiencies ranging from 52 to 81%. The results show that char yielded would suffice in providing all the energy required for the endothermic pyrolysis reaction process. The composition of the noncondensable gas produced has been initially characterized. Initial mass and energy balances have been calculated based on this system, yielding useful parameters for future economic and design studies. 1. Introduction Switchgrass is being considered as a biomass feedstock for the renewable fuel biorefining industry. Several varieties are being cultivated with the aim of finding the best yielding varieties with maximum fuel potential. Genetic engineering is also being applied to develop improved cultivars of switchgrass. Because enzymatic and fermentative conversion of lignocellulosic feedstock to ethanol is still not economically feasible, thermochemical conversion is considered a potential alternative method for converting biomass into useful forms of energy. Thermal conversion of switchgrass includes combustion, gasification, and pyrolysis. Direct combustion of switchgrass to fuel power plants has been investigated. Thus far only cofiring with coal has been demonstrated to be economical, with switchgrass replacing only up to 20% of pulverized coal boiler energy requirements.1 With fossil fuel prices skyrocketing, industries such as fuel ethanol producers are looking at dedicated crops like switchgrass to fulfill their energy needs. Gasification of switchgrass in which the organic matter is converted into combustible gas (syngas) has been demonstrated in the combined heat and power (CHP) industry.2 Pyrolysis, a rapid decomposition of organic materials in the absence of oxygen, yields char, gas, and pyrolysis oil. The latter has potential to be used to power stationary engines or be upgraded to transportation fuels. Several demonstrations of pyrolysis oil combustion have been carried out, including applications such as boilers, diesel engines, and gas turbines.3 Much of these works have been concentrated in Europe and Canada. Also, pyrolysis oils can be cost-effectively transported to centralized * To whom correspondence should be addressed. Tel.: (215) 2336493. Fax: (215) 233-6406. E-mail:
[email protected]. † Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. ‡ Eastern Regional Research Center, Agricultural Research Service. § The University of Texas at San Antonio.
Figure 1. Reactor design and layout.
refineries and can be used as feedstock for the production of Fischer-Tropsch fuels from syngas when the liquids are used as the gasifier feedstock.4 This notwithstanding, several issues including changes in composition during aging, phase separation, and corrosiveness due to excessive water content are some of the storage stability problems that plague bio-oil as a transportation fuel.5 With regard to production, a number of different approaches have been researched to achieve fast pyrolysis conditions using high heating rates and short residence times that have resulted in several reactor designs. Reactors of choice have included fluidized-bed, circulating fluidized bed, ablative, and auger type systems.6 Most of these designs have used wood sawdust as the primary feedstock. Extensive testing on herbaceous grasses has been rare. Previous work at the Department of Energy’s National Renewable Energy Laboratory (NREL) produced and characterized pyrolysis oils from oak and pine wood along with switchgrass in an ablative reactor and investigated droplet combustion characteristics of these oils.3 Boateng et al.7 investigated pyrolysis of Cave-in-Rock switchgrass in an analytical pyrolysis probe coupled with a gas chromatograph-mass spectrometer (PY-GC/MS), but only a qualitative measure of the condensable gas (pyrolysis liquids)
10.1021/ie0614529 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007
Figure 2. Condensers comprising four canisters in series.
Figure 3. Temperature, pressure locations, and residence times (TC ) thermocouple; PT ) pressure tap).
was possible. To our knowledge, little work on fluidized-bed pyrolysis of herbaceous grasses exists despite the success in producing bio-oils from wood. A bench-scale fluidized-bed reactor was developed for the pyrolysis of herbaceous energy crops being developed within the Agricultural Research Service (ARS) of the U.S. Department of Agriculture (USDA). The goal was to address, through experimentation, barriers associated with pyrolysis oil production from energy crops. Potential barriers include reactor performance such as feeding as well as conversion efficiencies with respect to optimum yields and energy balance. Since factors affecting efficiencies include high heating and heat transfer rates, controlled temperature of around 500 °C, and rapid cooling of the vapors produced,6 design and operation of the system
components can be critical. The work reported herein was designed to address some of these challenges. 2. Experimental Section 2.1. Reactor Design. The pyrolysis unit comprised the reactor vessel system and associated auxiliary systems for biomass feeding and injection, char collection, vapor condensation for bio-oil recovery, and instrumentation for data acquisition and control. Some of the design features are described herein. 2.2. Reactor System. The reactor vessel was constructed from a stainless steel pipe and flange arrangement with 7.8 cm nominal diameter and 52 cm length (Figure 1). Silica sand, with nominal diameter of 655 µm, formed the fluidized medium. The
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 1893 Table 1. Analysis of Switchgrass (Cave-in-Rock) Used in the Pyrolysis Experiment
proximate analysis (wt %) moisture ash volatile matter fixed C ultimate analysis (wt %) moisture H C N S O ash heating value (kJ‚kg-1) a
Table 2. Reactor Operating Parameters
as received
dry basis
DAF basisa
2.65 2.54 81.20 13.81
2.61 83.41 13.98
85.65 14.35
2.65 6.63 46.27 0.50 0.00 41.41 2.54 17 935.8
6.81 47.53 0.51 0.00 42.54 2.61 18 568.5
6.99 48.80 0.53 0.00 43.68 19 066.1
DAF, dry ash free.
sand rested on a distributor plate made of pressed wire mesh forming a fixed bed approximately 17 cm high and a quiescent freeboard of 35 cm in height with the same diameter as the vessel for axial exit. The distributor plate was flanged to a plenum of 15 cm diameter allowing a low-pressure drop of about 1.8 kPa. The reactor was heated by two semicylindrical clamshell-type heaters supplied by Watlow and rated at 1625 W each half. Thermal insulation was accomplished with a 5.2 cm thick castable shell around the vessel and the immediate vicinity to minimize heat loss. Fluidization was accomplished by N2 gas administered through a mass flow controller (Alicat Scientific, Tucson, AZ) at typically 70 standard L min-1 and resulting in a minimum fluidization of 0.23 m/s and a bed expansion of about 10%. 2.3. Feed System. The feed system comprised a metering hopper (K-Tron) of 30 L capacity. The feed was dispensed by a twin screw auger system manually controlled with an autocontrol capability. The auger provided the capability to dispense approximately 20 kg/h maximum feed rate to a variable speed controlled injection auger with an interchangeable diameter (1.6-2.5 cm) that ensured quick discharge of ground biomass into the reactor vessel. The purpose of using a variable diameter injector was to compensate for the variability of feedstock and its poor flow characteristics. 2.4. Product Collection System. The char was collected by two cyclone separators mounted in series. They also served as the main gas cleanup system that separated the solids from the gas stream prior to condensation. The upper cylindrical portions of the first and second cyclones were 7.6 and 5.1 cm in diameter, respectively. The tangential inlet ports were designed to ensure a theoretical 50% cut of solids with 10.3 and 3.9 µm, diameter, respectively. The cyclones were followed by an impinger-type condenser train system (Figure 2) comprising four canisters of equal dimension (10 × 20 cm) in series. They were inserted into a water bath chilled with dry ice. Final capture of pyrolysis oil was accomplished using an electrostatic precipitator (ESP) at 30 kV via a 40 kV power supply (Glassman High Voltage Inc., High Bridge, NJ). 2.5. Instrumentation. Data acquisition and control of the reactor system were accomplished by Labview instrumentation and software (National Instruments, Austin, TX). Nine pressure transducers and twelve Type-K thermocouples were used to monitor system temperatures and pressures at strategic locations (Figure 3) to ensure operational stability. Exhaust gas flow rate was measured with a diaphragm gas meter (Actaris Metris Model M250) having a nominal capacity of 7.1 m3 h-1. Noncondensable gas (NCG) was analyzed using a gas chro-
biomass type fluidized bed material particle size of bed material fluidizing gas gas flow rate minimum fluidizing velocity superficial velocity reactor temperature at run biomass feed rate biomass:N2 ratio (wt/wt) feed mean particle size bed pressure
switchgrass (Cave-in-Rock variety) silica sand -20 + 25 US mesh N2 4.81 kg‚h-1 0.23 m‚s-1 0.65 m‚s-1 480 °C 2.22 kg‚h-1 0.46 110 < -27 28.82 23.02
1.3106 0.0294 6449 1877 42.52 30.3 21.70 7.82 0.009 0.4 82.3 18 8.14 6.21
kg‚L-1
Table 4. Pyrolysis Liquid Fractions in the Condenser System bio-oil fractions
Table 5. Physical and Thermal Properties of Pyrolysis Products (Including Water-Soluble and Water-Insoluble Phases)
water, wt %
total acid, mg of KOH g-1
11.5 28.84 33.17 32.25 6.21
0.11 0.19 0.22 0.24 0.40
Pyrolysis products yielded, comprising bio-oil, NCG, and char (Table 3), indicated about 85% mass balance with about 60.7% pyrolysis oil, 13% char, and 11.3% NCG were captured physically. The difference of about 15% between the biomass and the products was unaccounted for and may be attributed to char and the bio-oil trapped in the transfer lines and in the reactor. It is commonplace for char to be carried over to downstream equipment and toward the atmosphere due to cyclone inefficiencies. However, other sources of unaccountable mass might be attributed to biomass and char trapped in the sand bed, the amount of which is always difficult to estimate by gravimetric analysis. Nonetheless, it was assumed that 15% loss was within the margin of engineering error in an industrial production. The distribution of the pyrolysis liquids in the condenser system (Table 4) decreased downstream from canister to canister; however, the bulk of the liquid was collected in the ESP. About 28% settled in the first canister 1.3 s after reaction, compared with less than 4% in the fourth canister when the volatile residence time was 5.7 s (Figure 3). The total temperature drop across the first and the fourth canisters was about 278 °C, indicating a rapid cooling rate well over 60 °C‚s-1. It has been reported that the reactions should be stopped quickly after volatile evolution in order to maximize bio-oil yield.6 To test the temporal effect on fractional condensation, the bio-oil
fractions in the canisters were subjected to water content and acidity tests. The water content in canisters 2, 3, and 4 were about the same at 30 wt % and constituted about 3 times more than that in canister 1. However, the acid number increased almost linearly from canister 1 through 4 (Table 4). By contrast, the water content in the ESP product ranged between 6 and 8 wt % and an acid number of 0.4 mg of KOH/g, about twice the average of the four canisters. Water content for bio-oils produced from wood is reported to be in the 15-30 wt % range.8 3.2. Pyrolysis Liquid Analysis. Full analysis of the liquids produced was carried out for the composite mixture of the liquids collected in the canisters and the ESP separately. Aside from the water (Karl Fischer) and acidity mentioned above, the soluble and insoluble compounds and the chemical compositions of the major constituents of the liquids were determined. Analysis of the liquids’ fuel properties was carried using ASTM methods normally employed for diesel oil. As Tables 4 and 5 indicate, most of the water produced by the pyrolysis reaction was settled by simple condensation heat transfer in the canisters, about 23 wt %, as against about 6 wt % in the ESP. However, the total water-soluble fractions are distributed almost equally in both the condensers and the ESP catch (Figure 5). Most of the ESP bio-oil fractions from switchgrass pyrolysis comprised insoluble fractions, 42.5 wt %, compared to 11 wt % in the condensers. The water-insoluble
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 1895 Table 7. Noncondensable Gas (NCG) Product Composition
Figure 5. Pyrolysis liquid fraction distribution. Table 6. Compositional Analysis of Water Soluble Pyrolysis Liquid Product in wt %
chemical species hydroxyacetaldehyde levoglucosan acetic acid acetol glyoxal formaldehyde cellobiosan elemental analysis carbon hydrogen nitrogen oxygen
condenser-oil composition
ESP-oil composition
17.4 2.4 3.5 2.7 detected detected detected
3.5 6.8 1.9 1.8 detected detected detected
40.78 6.98 0.33 47.25
52.97 6.43 0.38 39.13
fractions, determined by the 2:1 ratio water addition method,9 were mainly pyrolytic lignin compounds. It has been reported that the oxygen content causes bio-oils to have high polarity and therefore the propensity to hold considerable amounts of water in the solution.10 As the water is condensed in the canisters, the polarity of the liquid enhances electrostatic precipitation. The water-insoluble, basically pyrolytic lignin represents high molecular weight oligomeric compounds possibly from low temperature (
