estimation of liquid fuel yields from biomass

8
Estimation of Liquid Fuel Yields from Biomass NAVNEET R. SINGH, W. NICHOLAS DELGASS, FABIO H. RIBEIRO, AND RAKESH AGRAWAL* School of Chemical Engineering and Energy Center at Discovery Park, Purdue University, West Lafayette, Indiana 47907 Received January 29, 2010. Revised manuscript received April 18, 2010. Accepted May 7, 2010. We have estimated sun-to-fuel yields for the cases when dedicated fuel crops are grown and harvested to produce liquid fuel. The stand-alone biomass to liquid fuel processes, that use biomass as the main source of energy, are estimated to produce one-and-one-half to three times less sun-to-fuel yield than the augmented processes. In an augmented process, solar energy from a fraction of the available land area is used to produce other forms of energy such as H 2 , heat etc., which are then used to increase biomass carbon recovery in the conversion process. However, even at the highest biomass growth rate of 6.25 kg/m 2 · y considered in this study, the much improved augmented processes are estimated to have sun-to-fuel yield of about 2%. We also propose a novel stand- alone H 2 Bioil-B process, where a portion of the biomass is gasified to provide H 2 for the fast-hydropyrolysis/hydrodeoxy- genation of the remaining biomass. This process is estimated to be able to produce 125-146 ethanol gallon equivalents (ege)/ ton of biomass of high energy density oil but needs experimental development. The augmented version of fast-hydropyrolysis/ hydrodeoxygenation, where H 2 is generated from a nonbiomass energy source, is estimated to provide liquid fuel yields as high as 215 ege/ton of biomass. These estimated yields provide reasonable targets for the development of efficient biomass conversion processes to provide liquid fuel for a sustainable transport sector. Introduction The liquid hydrocarbons currently used by cars, buses, trucks, trains, and airplanes provide high volumetric energy density fuels along with the associated ease of use. However, for the era when fossil fuels will either be limited or when the transition will have to be made to a sustainable economy in which energy needs are primarily met with renewable resources, we need to critically examine the viability of meeting the needs of the entire transportation sector with resources such as biomass, electricity, hydrogen, etc. For both electricity and H 2 , the high energy density methods for on-board storage are currently unavailable, and the search for solutions is proving to be quite challenging (1, 2). In the meantime, with the given energy density of batteries, partial use of electricity through plug-in hybrid electric vehicles (PHEVs) still requires large quantities of liquid hydrocarbon fuels to satisfy the remainder of the transporta- tion sector needs (3). For example, replacement of the current internal combustion engine based light duty vehicles (LDVs) in the United States with PHEVs containing batteries that can provide a driving distance of about 64 km (PHEV40) between successive charges would decrease the liquid fuel required for the LDVs from 8.9 to 3.4 million barrels per day (Mbbl/d) (3). In spite of this huge reduction, the remaining oil demand for the total U.S. transportation sector, including heavy duty vehicles (HDVs) such as air planes, trucks, trains, etc. will still be quite large at 8.3 Mbbl/d (4, 5). Replacing this massive remaining oil need with electricity or H 2 would require huge development in alternate infrastruc- ture along with major technical innovations. A simple fact that favors the use of biomass for trans- portation fuel is that a sizable quantity is available as sustainable waste biomass, e.g. waste coproduct from food crop plants, forest residues, etc. Recent studies estimate that such sustainably available waste (SAW) biomass in the USA could be 349 million Tons/y or more (6, 7). A recent NRC report estimates that, with the appropriate use of the land under the U.S. Conservation Reserve Program, 149 million Tons/y of dedicated fuel crops could be produced by 2020 (6). If this land is readily available as spare land with no other competing demand, and the dedicated fuel crops could be collected with minimal energy input, then a total of 498 million Tons/y of biomass could potentially be sustainably available in the USA. However, this amount of biomass will provide only about one-fifth to one-fourth of the 8.3 Mbbl/d of oil with the currently practiced biomass to liquid fuel technologies (6-8). Clearly, compared to the quantity of oil needed, the amount of sustainably available biomass is limited. If additional land is to be used to grow dedicated biomass for liquid fuel, then it is informative to compare the efficiency at which incident solar energy is harvested in dedicated fuel crops vs other secondary forms of energy such as H 2 , heat, and electricity that could be recovered from the same land area. In most cases, less than 1% of the incident solar energy is stored in biomass (1-3). Even at the upper expected range of biomass growth rate of 6 kg/m 2 · y, the efficiency of collection of solar energy as biomass energy will be in the neighborhood of 2% (9). Inspection of the efficiencies of the currently available technologies tells us that generally the recovery of solar energy as biomass is an order of magnitude lower than that of sun to heat (50-70%), electricity (10-42%), or hydrogen (5-27%) (10-14). There- fore, when compared to other solar energy conversion and recovery processes such as electricity or H 2 , this translates into a need for larger land area to recover the same amount of solar energy as biomass (8). Due to the relatively low efficiencies for growing dedicated biomass for transportation fuel and the limited availability of SAW biomass, there is a need to critically examine the amount of liquid fuel that can be produced from a given quantity of biomass. First, we need to understand the potential of self-contained process where biomass is the main source of energy and liquid fuel production is maximized. Second, we need to develop augmented processes that will synergistically use supplemental energy available at much higher efficiencies to increase liquid fuel production. This will provide us the potential to efficiently increase liquid fuel production from a given quantity of biomass and thereby an assessment of the degree to which biomass can play a role in a transportation fuel infrastructure. * Corresponding author telephone: (765) 494-2257; fax: (765) 494- 0805; e-mail: [email protected]. Environ. Sci. Technol. 2010, 44, 5298–5305 5298 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 13, 2010 10.1021/es100316z 2010 American Chemical Society Published on Web 06/07/2010

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Page 1: Estimation of Liquid Fuel Yields from Biomass

Estimation of Liquid Fuel Yields fromBiomassN A V N E E T R . S I N G H ,W . N I C H O L A S D E L G A S S ,F A B I O H . R I B E I R O , A N DR A K E S H A G R A W A L *

School of Chemical Engineering and Energy Center atDiscovery Park, Purdue University, WestLafayette, Indiana 47907

Received January 29, 2010. Revised manuscript receivedApril 18, 2010. Accepted May 7, 2010.

We have estimated sun-to-fuel yields for the cases whendedicated fuel crops are grown and harvested to produce liquidfuel. The stand-alone biomass to liquid fuel processes, thatuse biomass as the main source of energy, are estimated toproduce one-and-one-half to three times less sun-to-fuel yieldthan the augmented processes. In an augmented process,solar energy from a fraction of the available land area is usedto produce other forms of energy such as H2, heat etc.,which are then used to increase biomass carbon recovery inthe conversion process. However, even at the highestbiomass growth rate of 6.25 kg/m2 ·y considered in this study,the much improved augmented processes are estimated tohave sun-to-fuel yield of about 2%. We also propose a novel stand-alone H2Bioil-B process, where a portion of the biomass isgasified to provide H2 for the fast-hydropyrolysis/hydrodeoxy-genation of the remaining biomass. This process is estimated tobe able to produce 125-146 ethanol gallon equivalents (ege)/tonofbiomassofhighenergydensityoilbutneedsexperimentaldevelopment. The augmented version of fast-hydropyrolysis/hydrodeoxygenation, where H2 is generated from a nonbiomassenergy source, is estimated to provide liquid fuel yields ashigh as 215 ege/ton of biomass. These estimated yields providereasonable targets for the development of efficient biomassconversion processes to provide liquid fuel for a sustainabletransport sector.

IntroductionThe liquid hydrocarbons currently used by cars, buses, trucks,trains, and airplanes provide high volumetric energy densityfuels along with the associated ease of use. However, for theera when fossil fuels will either be limited or when thetransition will have to be made to a sustainable economy inwhich energy needs are primarily met with renewableresources, we need to critically examine the viability ofmeeting the needs of the entire transportation sector withresources such as biomass, electricity, hydrogen, etc.

For both electricity and H2, the high energy densitymethods for on-board storage are currently unavailable, andthe search for solutions is proving to be quite challenging(1, 2). In the meantime, with the given energy density ofbatteries, partial use of electricity through plug-in hybridelectric vehicles (PHEVs) still requires large quantities of liquid

hydrocarbon fuels to satisfy the remainder of the transporta-tion sector needs (3). For example, replacement of the currentinternal combustion engine based light duty vehicles (LDVs)in the United States with PHEVs containing batteries thatcan provide a driving distance of about 64 km (PHEV40)between successive charges would decrease the liquid fuelrequired for the LDVs from ∼8.9 to 3.4 million barrels perday (Mbbl/d) (3). In spite of this huge reduction, theremaining oil demand for the total U.S. transportation sector,including heavy duty vehicles (HDVs) such as air planes,trucks, trains, etc. will still be quite large at 8.3 Mbbl/d (4, 5).Replacing this massive remaining oil need with electricity orH2 would require huge development in alternate infrastruc-ture along with major technical innovations.

A simple fact that favors the use of biomass for trans-portation fuel is that a sizable quantity is available assustainable waste biomass, e.g. waste coproduct from foodcrop plants, forest residues, etc. Recent studies estimate thatsuch sustainably available waste (SAW) biomass in the USAcould be 349 million Tons/y or more (6, 7). A recent NRCreport estimates that, with the appropriate use of the landunder the U.S. Conservation Reserve Program, 149 millionTons/y of dedicated fuel crops could be produced by 2020(6). If this land is readily available as spare land with no othercompeting demand, and the dedicated fuel crops could becollected with minimal energy input, then a total of 498million Tons/y of biomass could potentially be sustainablyavailable in the USA. However, this amount of biomass willprovide only about one-fifth to one-fourth of the 8.3 Mbbl/dof oil with the currently practiced biomass to liquid fueltechnologies (6-8). Clearly, compared to the quantity of oilneeded, the amount of sustainably available biomass islimited.

If additional land is to be used to grow dedicated biomassfor liquid fuel, then it is informative to compare the efficiencyat which incident solar energy is harvested in dedicated fuelcrops vs other secondary forms of energy such as H2, heat,and electricity that could be recovered from the same landarea. In most cases, less than 1% of the incident solar energyis stored in biomass (1-3). Even at the upper expected rangeof biomass growth rate of ∼6 kg/m2 ·y, the efficiency ofcollection of solar energy as biomass energy will be in theneighborhood of 2% (9). Inspection of the efficiencies ofthe currently available technologies tells us that generallythe recovery of solar energy as biomass is an order ofmagnitude lower than that of sun to heat (50-70%),electricity (10-42%), or hydrogen (5-27%) (10-14). There-fore, when compared to other solar energy conversion andrecovery processes such as electricity or H2, this translatesinto a need for larger land area to recover the same amountof solar energy as biomass (8).

Due to the relatively low efficiencies for growing dedicatedbiomass for transportation fuel and the limited availabilityof SAW biomass, there is a need to critically examine theamount of liquid fuel that can be produced from a givenquantity of biomass. First, we need to understand thepotential of self-contained process where biomass is the mainsource of energy and liquid fuel production is maximized.Second, we need to develop augmented processes that willsynergistically use supplemental energy available at muchhigher efficiencies to increase liquid fuel production. Thiswill provide us the potential to efficiently increase liquid fuelproduction from a given quantity of biomass and thereby anassessment of the degree to which biomass can play a rolein a transportation fuel infrastructure.

* Corresponding author telephone: (765) 494-2257; fax: (765) 494-0805; e-mail: [email protected].

Environ. Sci. Technol. 2010, 44, 5298–5305

5298 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 13, 2010 10.1021/es100316z 2010 American Chemical SocietyPublished on Web 06/07/2010

Page 2: Estimation of Liquid Fuel Yields from Biomass

In this paper, we estimate the annual solar incident energyon a given land area that could be recovered as high energydensity liquid fuel for dedicated fuel crop cases. Thesecalculated “sun-to-fuel” (S2F) yields for different processesprovide us the much needed overall efficiencies of convertingsolar energy to liquid fuel. We evaluate self-contained aswell as augmented thermochemical processes, where supple-mentary energy is used to increase the S2F yield from a givenland area (8, 15). This information is useful in assessing howto maximize liquid fuel production from a given land area.We also propose a novel self-contained biomass to liquidfuel thermochemical process. This process, when experi-mentally developed, has the potential to provide significantlygreater S2F yield of high energy density oil than any currentself-contained thermochemical process.

An additional benefit of this S2F yield study is that thepotential of different processes can also be compared in termsof the quantity of liquid fuel that could be produced froma unit quantity of biomass fed to each conversion process.This is helpful for understanding of how to increase the liquidfuel production from a given quantity of SAW biomass.

Even though we discuss increased fuel production in thecontext of thermochemical processes, it should be empha-sized that the calculated liquid yields provide us with potentialliquid fuel yields against which all suitable processes,including those based on biochemical routes, can bemeasured. It is not our purpose to imply that thermochemicalroutes are superior to other alternative biomass conversionroutes.

Liquid Fuel Yield from Self-contained Biofuel Processes.We briefly discuss here the existing as well as some novelself-contained biomass to liquid fuel processes for their liquid

fuel yield. We define a self-contained process as the onewhere very little, if any, energy is used from a nonbiomasssource. The calculation details for all the processes are givenin the Supporting Information (SI).

In Figure 1, we show estimated values of the overall annualoil yield by different processes using dedicated fuel cropsand the total sunlight falling on 1 m2 of land area. The resultsare based on an average annual incidence of 6307 MJ/m2 ·y(1752 kW ·h/m2 ·y) of solar energy in the USA (8, 16, 17).While calculations were done for four different collectionrates of biomass: 1.5, 3, 5, and 6.25 kg/m2 ·y, the resultsreported in Figure 1 are for the biomass collection rate of 3kg/m2 ·y. By collection rate, we refer to the annual amountof biomass that arrives at a processing plant from 1 m2 ofland area. In one scenario, a fraction of the land area maybe assigned to directly recover additional energy needed togrow, harvest, and transport the biomass to the plant gate.If this additional energy is a small fraction of the biomassenergy arriving for processing, then the biomass collectionrate will be close to the biomass growth rate. Alternatively,if most of the additional energy is utilized as electricity andheat, then since these forms of energy are recovered fromsolar energy at an order of magnitude higher efficiency thanbiomass, the fraction of the land area dedicated for theadditional energy will be relatively small. In this case, thebiomass collection rate will again be nearly equal to the biomassgrowth rate. Thus, in Figure 1, for a biomass energy contentof 17 MJ/kg, a collection rate of 3 kg/m2 ·y implies that fromthe total 6307 MJ/m2 ·y of solar energy, 51 MJ/m2 ·y areavailable as biomass energy at the plant for further processing

FIGURE 1. Estimated values of the overall annual biofuel yield from 1 m2 of land area with annual solar incident energy of 6307 MJ/m2 · y. For each process, literature reported or estimated conversion efficiency values are shown in the parentheses. All yieldnumbers are for high energy density liquid fuel with the exception of fast-pyrolysis which is for low-energy density bio-oil.

VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5299

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to produce liquid fuel. In this paper, we report all energycontents as lower heating values (LHV) unless specifiedotherwise.

From the available biomass energy, the liquid fuel yieldis calculated based on overall process conversion efficiency.This means that in our S2F calculations, we do not accountfor any auxiliary power that the plant may use. It is assumedthat such auxiliary power is either absent or a small fractionof the total rate at which energy is processed within a plant.Thus, in a thermochemical plant, major energy inputs suchas the heat needed to dry the biomass, electricity needed tocirculate hot gases, etc. are produced from the biomass feedto the plant. Similarly, for a fermentation based biochemicalplant, the heat energy needed for separation of ethanol orbutanol is assumed to be supplied from the lignin portionof the biomass. If within a chemical process, excess energyis generated which cannot be converted to liquid fuel, we donot take credit for such energy in the S2F calculations. OurS2F calculations do account for all the major energyconversion steps, and the numbers in Figure 1 provide agood representation of the efficiency of each process inrecovering annual incident solar energy as liquid fuel.

Another useful piece of information that can also bederived from the S2F calculations in Figure 1 is the amountof liquid produced per ton of biomass. For each process,information regarding the amount of biomass used and thecorresponding energy content of the fuel produced isavailable. Since all processes do not produce the sameproduct, a convenient unit to compare processes on the samebasis is ethanol gallon equivalent (ege) per ton of biomass(18). For this calculation, we divide the energy content of thefuel produced in Figure 1 by the lower heating value (LHV)of 80.14 MJ/gal of ethanol to get equivalent gallons of ethanol.This number is then divided by the amount of biomass toget ege per ton of biomass. This information is useful as itallows us to compare different processes on the basis ofbiomass conversion to liquid fuel. This is particularlyimportant when the available biomass is limited.

When biomass is processed through a conventionalgasifier followed by a Fischer-Tropsch diesel (FTD) process,the overall process efficiency of the chemical conversionprocess can vary between 41 and 50% (19). This leads to arecovery of 20.9-25.5 MJ/m2 ·y of energy as biofuel, whichalso translates into 87-106 ege of liquid fuel per ton ofbiomass. For the biochemical routes using enzymes andmicrobes, the reported efficiency values for the lignocellulosicbiomass is in the 35-50% range with 75-105 ege/ton ofbiomass (18, 20, 21). This implies that energy content of therecovered ethanol is similar to that of diesel produced in thegasification/FT route.

On the other hand, conventional fast-pyrolysis is quiteefficient (65-77%) (22, 23) in converting biomass to a low-energy density liquid fuel referred to as bio-oil and recovers33.1-39.5 MJ/m2 ·y of solar incidence energy. The bio-oilfrom fast-pyrolysis has extremely high oxygen content(∼35-40 wt %), however, and its energy content is only halfthat of petroleum and similar to that of the original biomass(∼17 MJ/kg). Bio-oils do not easily blend with petroleumproducts and tend to polymerize and condense with timeduring shipment and lead to gumming in downstream

reactors. This necessitates the subsequent upgrading of thisbio-oil by hydrodeoxygenation (HDO) using H2 in thepresence of a catalyst (15, 24, 25). A recent study by PacificNorthwest National Laboratory (PNNL) reports modelingresults for a three step process of fast-pyrolysis, hydrode-oxygenation, and hydrocracking to produce gasoline anddiesel-like liquid fuel from lignocellulosic biomass (26).

One of our goals in this paper is to study processes thathave a potential to eliminate three-step processing and thehandling challenges associated with the fast-pyrolysis basedroute for producing upgraded oil. For this purpose, we willfirst discuss augmented processes where H2 needed to removeoxygen from the biomass to directly produce high energydensity oil is supplied from a nonbiomass source.

Liquid Fuel Yields from Augmented Biofuel ProcessesUsing Supplementary Hydrogen. A key feature of all thethree self-contained processes discussed so far is that biomassis the sole energy source being converted to liquid fuel. Asseen from Table 1, this inherently limits the biomass carbonatoms that can be recovered as high energy density liquidfuel.

The energy content on a per carbon basis for biomasssources such as Switchgrass, Poplar, and sugars is only two-thirds of the energy content of the molecules composinggasoline. This means that when biomass carbon moleculesare upgraded to a high energy density liquid fuel such asgasoline, even for a 100% energy efficient conversion process,about one-third of the biomass carbon atoms will be rejectedas low energy molecules such as CO2. With reasonablyoptimistic process conversion efficiencies, we find from Table1 that roughly only half of the biomass carbon is recoveredas high energy density liquid fuel. Most likely, the lost carbonwill be in its low-energy state as CO2. Considering the factthat solar energy to biomass carbon is a low-efficiencyprocess, it is attractive to find processes that will economicallyeither reduce or eliminate the loss of condensed biomasscarbon as CO2 during the biomass to liquid fuel conversionprocess.

It is possible to envision thermochemical as well asbiochemical conversion processes using supplementarysources of energy such as heat, H2, or electricity to increaseproduction of high energy density liquid fuel per ton ofbiomass. We have focused here on thermochemical routes.It is worth noting that for self-contained biomass conversionprocesses both biochemical as well as gasification/FTDprocesses were found to provide similar yields. Therefore,we expect that the yields calculated from thermochemicalprocesses using supplemental energy should provide goodtarget estimates for the corresponding non-thermochemicalprocesses as well. Although synergistic augmented biofuelprocesses using supplemental energy from fossil fuel ornuclear are described elsewhere (8, 15), in this work we areinterested in the sun-to-fuel yield to create alternativescenarios for a future where most transport needs are metthrough renewable energy. Therefore, in this analysis, wehave used solar energy as the supplemental energy source.

In order to avoid difficulties associated with the handlingof bio-oil from a fast-pyrolysis reactor prior to its HDOtreatment, it will be best to pyrolyze biomass in the presenceof H2 and a catalyst as shown in a process alternative in

TABLE 1. Estimated Carbon Loss for Biomass to High Energy Density Liquid Fuela

biomassenergy content

(MJ/kg)energy content

(kJ/mol C)carbon loss with 100%efficient conversion (%)

assumed conversionefficiency (%)

total carbonloss (%)

switchgrass 17.2 485 24.6 75 43.4poplar 19.6 455 32.8 75 49.6sugar 14.1 423 42.8 97 44.5a The energy content of gasoline ) 604.1 kJ/mol C.

5300 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 13, 2010

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Figure 2. Ideally one would like to choose a reactor design,catalyst, and operating conditions to directly producedeoxygenated high energy density liquid fuel in one step.However, it is likely that the much higher temperature, mostlikely in excess of 400 °C, and much shorter contact time (∼1s) that will be needed for the fast-hydropyrolysis step willnot be the optimal conditions for the level of deoxygenationrequired. Therefore, if needed, the exhaust from the fast-hydropyrolysis reactor, after removal of any char presentand adjustment of the temperature, can be directly sentthrough an HDO reactor to allow the needed contact timewith the HDO catalyst at its preferred operating conditions.The thrust is to avoid some of the steps associated with fast-pyrolysis such as total condensation of bio-oil, the associatedproblems with subsequent handling, and then revaporizationfor HDO. Bridgwater and Peacock describe a large numberof fast-pyrolysis reactor configurations, and many of themcan be adapted for fast-hydropyrolysis (23). We use the term“H2Bioil” to refer the processes that are based on fast-hydropyrolysis/HDO of biomass with supplemental H2 anddirectly produce high energy density liquid fuel.

It should be noted that a few groups have reported lab-scale hydropyrolysis of biomass in fixed-bed reactor mode(27-34). In some cases, lower char and higher oil yields wereobserved when H2 is present during pyrolysis (29, 30, 34).Also, less intense degradation reactions have been reportedunder H2 than under an inert atmosphere (33). These earlyexperiments provide favorable conceptual support for hy-dropyrolysis to produce oil with low oxygen content.However, unlike fast-pyrolysis experiments, all the reportedhydropyrolysis data in the literature are from fixed-bedreactors. Fast pyrolysis experiments are generally not con-ducted in a fixed-bed mode due to the need for rapidheating-cooling and the requirement of a very shortresidence time in the reactor. Thus, there is a need to conducthydropyrolysis experiments in a mode similar to that forfast-pyrolysis with a possibility of immediate downstreamHDO. However, unlike fast-pyrolysis, the fast-hydropyrolysisreactors will possibly operate at a much higher pressure.Such experiments in the presence of H2 could provide higheryields of deoxygenated oil per unit of biomass.

In the absence of experimental data for a fast-hydropy-rolysis operation mode, it is difficult to make an accurateestimate for the oil yield. However, it is possible to make areasonable estimate for yields that could be achieved. Thefixed bed hydropyrolysis studies do report decrease in charproduction that could lead to higher oil yield. Eliminationof the bio-oil revaporization step prior to the HDO reactor

and the possibility of better heat integration could alsocontribute to improved process efficiency and yield. In lightof these factors, in our calculations, the carbon yield in theoil from the H2Bioil process was taken to be ∼70%, which issame as reported in the literature for the bio-oil from a fast-pyrolyzer (35). When bio-oil from a fast-pyrolyzer is upgradedthrough HDO, the experimentally observed energy contentof the upgraded oil is found to be in the range of 38-43MJ/kg (26). For our calculations, we have assumed that, withenough solar H2 and advances in catalysis and processingconditions, the energy content is likely to be closer to theupper end value of ∼42 MJ/kg of oil. Furthermore, the carbonand oxygen in the oil from the H2Bioil process were takento be 86.5 and 1.25 wt % with the rest being hydrogen. Onthis basis, the estimated yield of oil from the H2Bioil processis 215 ege/ton of biomass (see the SI for details). Note thatthis number is slightly lower than the 230 ege/ton of biomassreported earlier (15) as a slightly lower and more probablevalue of the oil LHV is used in this work. For the estimationof H2 consumption, we assumed that two H2 molecules areneeded to remove an oxygen atom. As shown in the detailedcalculations provided in the SI, this leads to an estimated H2

requirement of ∼1240 L at STP/L oil (0.11 kg H2/L oil), whichis higher than the highest experimental value reported in theliterature to upgrade bio-oil from a fast-pyrolyzer (∼1144 Lat STP/L upgraded oil) (25). This gives us a confidence thatour estimate of H2 consumption is conservative, and in anactual process, the H2 consumption may be lower.

From the calculations for Figure 1, out of 6307 MJ/m2 ·yof solar energy, the H2Bioil process is estimated to use 231MJ/m2 ·y of energy to make H2 and the rest to grow biomass.Using a sun to electricity conversion efficiency of 15% andan electrolyzer efficiency (based on LHV of H2) of 50.7%, 17.6MJ/m2 ·y worth of energy is estimated in solar H2. It isestimated that from this H2Bioil process one can obtain 49.8MJ/m2 ·y of oil which, of course, corresponds to the 215 ege/ton biomass.

It is worth noting that if H2 were to be produced througha thermochemical process rather than electrolysis, the overallyield from solar energy to fuel will still be in the neighborhoodof 49.8 MJ/m2 ·y. A recent study reports ∼30% high heatingvalue (HHV) based efficiency to H2 conversion from the netsolar energy soaked up by the thermochemical reactor (14).When one accounts for radiation losses etc., the solar energyto H2 efficiency turns out to be ∼16% based on LHV of H2

(see the SI). This implies that only one-half of the 231 MJ/m2 ·y of solar energy shown in Figure 1 will be needed to

FIGURE 2. H2Bioil process where solar H2 is used for fast-hydropyrolysis of biomass and subsequent HDO of the pyrolyzer productstream.

VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5301

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supply H2, but this decrease will have a small incrementalimpact on the overall yield of 49.8 MJ/m2 ·y as oil.

Results are also available for a gasification/FT based hybridhydrogen-carbon (H2CAR) process where almost all thebiomass carbon is recovered as liquid fuel by using supple-mentary H2 (8). Figure 1 shows that the estimated recoveryof solar energy as liquid fuel from the H2CAR process is indeedthe highest of any process. However, this increase in oilproduction comes at a steep increase in demand for solar H2

at about 0.33 kg H2/L oil. In fact, the inefficiency of the H2CARprocess is reflected in the expected production of only 66.9MJ/m2 ·y of liquid fuel from the input of 72 MJ/m2 ·y of solarH2 as seen from Figure 1. If energy efficiency were the onlycriteria, one would be better off using H2 via fuel cell vehicleswhich are more efficient than the oil based internal com-bustion engines.

It is of interest to compare the performance of H2Bioil,which is an augmented process, with a self-contained process.However, rather than comparing it with a gasification/FT ora biochemical process, it will be more informative to compareit with a self-contained version that is also based on fast-hydropyrolysis.

Liquid Fuel Yields from a Self-Contained Fast-Hydro-pyrolysis Based Process. The self-contained version of theH2Bioil process can be created by gasifying a portion of thebiomass to supply the needed H2 for the fast-hydropyrolysis/HDO of the remaining biomass portion. In order to con-solidate biomass gasification/fast-hydropyrolysis in onereactor unit, we propose the novel fast-hydropyrolysis reactorconfiguration shown in Figure 3 (H2Bioil-B). In this process,depending on the efficiency of gasification section, 32-42%of the total biomass is gasified to produce an H2/CO mixturewhich is sufficient to hydropyrolyze and hydrodeoxygenatethe remaining fraction of the biomass that is directly fed tothe hydropyrolysis zone. The hot gas from the gasifier isdirectly injected in the pyrolyzer zone. If needed, thetemperature of the exhaust gas prior to its injection in thepyrolyzer zone may be adjusted. Also, if required, a hot ora cold recycle stream may be injected between the gasifierand the pyrolyzer zone to provide better temperature controlin the pyrolyzer section of the reactor.

The majority of the CO from the gasification zone isexpected to provide H2 through the water-gas-shift (WGS)reaction. Generally, the WGS reaction can be conducted athigh temperatures of 350-500 °C using an iron oxide basedcatalyst (36). In this temperature range, formation of H2 ispreferred and most of the CO can be converted. Of course,the use of in situ water-gas-shift catalysts in the fast-

hydropyrolysis and the HDO reactors can provide an effectivemeans to control the H2 partial pressure during biomassconversion. As H2 is consumed, there will be driving forcefor CO reaction with H2O that is formed as a byproduct ofthe HDO reaction. Furthermore, an interesting but as yetunanswered question is whether CO can play a direct rolein pyrolysis or deoxygenation.

It should be noted that for the H2Bioil-B process, in situbiomass gasification will provide hot syngas for the biomassfast-hydropyrolysis process, leading to an increase in energyefficiency. An important advantage of this process overconventional HDO is that it will avoid the need for purehydrogen and thus save the energy and capital cost associatedwith hydrogen purification and recovery. Due to expectedimprovements in energy efficiencies due to process integra-tion, as compared to a process using three separate steps offast-pyrolysis, gasification and HDO, we expect higher liquidfuel yield from the H2Bioil-B process (26).

In calculations for Figure 1, we find that for a biomass toH2 conversion efficiency of 50% (1), nearly 42% of the biomasswill be gasified to provide the needed hydrogen to yield 30.1MJ/m2 ·y of solar energy as liquid fuel. This translates intoa carbon efficiency, energy efficiency, and liquid fuel yieldof 40.6%, 59.1%, and 125.4 ege/ton of biomass. For a higherbiomass gasifier efficiency of 75% to hydrogen, the carbonefficiency, energy efficiency, sun-to-fuel recovery, and liquidfuel yield estimates are 47.2%, 68.6%, 35 MJ/m2 ·y, and 146ege/ton of biomass, respectively (refer to the SI for calculationdetails). The calculated yield of 125-146 ege/ton of biomassis highest among the hitherto known self-contained biomassto high energy density liquid fuel processes (roughly 50%higher liquid fuel yield than the self-contained gasification/FTor current biochemical processes). However, the use of theorganic residue from a biochemical process to produceadditional liquid fuel via gasification/FT process has beensuggested in the literature (20, 37) and has the potential toproduce a competitive 77 gallons of gasoline and 11 gallonsof diesel equivalent oil per ton of biomass (equivalent to∼136 ege/ton of biomass) (20). The process by ZeaChemfirst produces acetic acid by fermentation of sugars (38). Theacetic acid is then converted to an ester which is reactedwith H2 to form ethanol. The H2 is produced via gasificationof the lignin residue. It is claimed that the nth plant will yield135 ege/ton of biomass. The proposed H2Bioil-B uses onlya thermochemical route and has the potential to be built onsmall scale while providing high yields of a high energy densityliquid fuel product.

FIGURE 3. H2Bioil-B process where a portion of the feed biomass (32-42%) is fed to the gasification zone to provide H2 forfast-hydropyrolysis and HDO of the remaining biomass fed to the hydropyrolysis zone.

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Since we have S2F recovery estimates for various self-contained as well as augmented processes, they provide usan opportunity to make some interesting observations:

Observations from S2F Recovery Estimates. From thevarious S2F recovery estimates shown in Figure 1 for abiomass collection rate of 3 kg/m2 ·y, we note that

(1) While S2F recoveries shown for the self-contained fast-pyrolysis and H2Bioil-B are similar, the energy contenton a per unit mass basis of the bio-oil from fast-pyrolysis is less than half of the oil from H2Bioil-B andit cannot be used without spending additional energyto upgrade it.

(2) For the self-contained processes, the energy recoveredas high energy density oil is in the range of 18 to 35MJ/m2 ·y. When compared to the energy content of 51MJ/m2 ·y for original biomass, the energy efficiencyfor conversion of biomass varies over a wide range of35-68%. The efficiency range of 35-50% for thegasification/FT and biochemical based processes issimilar. It seems that fast-hydropyrolysis/HDO basedprocesses have the potential to be among the mostefficient of the self-contained processes. This providesan incentive for their experimental demonstration.

(3) On the basis of the incident solar energy of 6307 MJ/m2 ·y, the recovery as liquid fuel for the self-containedprocesses is less than 0.56%. This is not surprising asthe efficiency of collecting dedicated fuel crop was∼0.81%. Nevertheless, it does point out that the solarenergy recovered as liquid fuel via the dedicated fuelcrop and self-contained processing is quite low.

(4) The augmented processes hold the promise to sub-stantially improve S2F efficiencies (0.8-1.06%). As amatter of fact, the energy content in the fuel can besimilar (49.8 vs 51 MJ/m2 ·y) or even higher (66.9 vs 51MJ/m2 ·y) than that in the biomass. On the basis of theS2F yields, the liquid fuel produced by the augmented

processes using the same total land area can be one-and-one-half to three times that of the self-containedprocesses.

(5) It is interesting to compare incremental oil productionsand H2 consumption for the two routes: (i) the H2CARprocess as compared to the self-contained gasifica-tion/FT process and (ii) the augmented H2Bioil processwith the self-contained H2Bioil-B process. We find thatthe energy content of the incremental oil producedper unit of energy in the H2 used (MJ in incrementaloil produced/MJ in H2 used) is much more for theH2Bioil based route (0.9-1.2) than for the H2CAR route(∼0.65). This shows that the fast-hydropyrolysis/HDObased process is very efficient in increasing the biomasscarbon recovery. However, its carbon recovery islimited to ∼70%. In order to get nearly ∼100% carbonrecovery, the H2CAR process essentially converts CO2

to liquid fuel. Conversion of CO2 to liquid fuel is anenergy intensive process and is reflected in the highH2 requirements for the H2CAR process. This alsodemonstrates the energy penalty associated with veryhigh recoveries of biomass carbon.

(6) We can draw another important conclusion fromFigure 1. When additional liquid fuel is needed, itshould preferably be first produced by using aug-mented processes in conjunction with SAW biomassrather than through growing dedicated fuel crops. Sucha step would make more efficient use of sunlight fallingon a given land area. This is possible because heat orH2 are recovered from sunlight at a much higherefficiency than a dedicated fuel crop, and at least 65%of the energy in H2 can be recovered as the incrementalliquid from the SAW biomass.

In Figure 4, we show estimated S2F yields for four differentbiomass collection rates. While the expected range for theyields are calculated in the SI for a number of conversionprocesses, we only show upper estimates in the bar graphs.

FIGURE 4. Sun-to-fuel yield for various conversion processes at biomass collection rates of (a) 1.5, (b) 3, (c) 5, and (d) 6.25 kg/m2 · y.The values shown in the bars are upper estimates.

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As expected, increased growth rates and hence collectionrates for biomass have a profound impact on S2F yields. Forall processes except H2CAR, the increase in liquid fuel rateis directly proportional to the increase in biomass collectionrate. For the H2CAR process, large quantities of H2 are neededand high biomass growth rates lead to relatively larger fractionof the land area being needed for the H2 production. Thisresults in less than proportional increase in liquid fuel yieldwith the increase in biomass growth rates. It is informativeto note that at even the highest collection rate of 6.25 kg/m2 ·y, the S2F yield from the H2CAR process is 119.9 MJ/m2 ·y. This corresponds to ∼1.9% of the annual incident solarlight being collected as liquid fuel. This tells us that, evenunder a highly favorable process condition, the dedicatedbiomass carbon route to liquid fuel provides only a smallportion of the incident solar energy as liquid fuel. This hasa strong implication on the land area requirements whendedicated biomass is to be grown for liquid fuel production.

DiscussionIn this work, we have estimated the sun-to-fuel yield forliquid hydrocarbon fuel via the biomass carbon route. Thisis relevant when dedicated biomass is to be grown andharvested for the production of liquid fuel. In such a case,one of the goals is to increase the collection of solar energyincident on a given land area as liquid fuel. The S2F yield forthe self-contained processes that mainly rely on the biomassto supply all the energy need is quite low. Even at a highbiomass collection rate of 6.25 kg/m2 ·y, only ∼1.16% of thesolar energy is estimated to be recovered as liquid fuel. Twomain contributing factors to the low S2F yield are (i) therelatively low recovery of solar energy as biomass energyand (ii) the fact that less than 50% of the biomass carbonrecovery in the liquid fuel is from self-contained processes.

We find that the S2F yield can potentially be increased bya factor of 1.5-3 when all the available land area is not usedto grow dedicated fuel crop but the solar energy falling ona portion of the land area is harnessed as hydrogen whichis then used in novel augmented biomass conversionprocesses to increase biomass carbon yield as liquid fuel.Such augmented processes are estimated to have higher S2Fyield because hydrogen is harvested from solar energy witha much higher efficiency than biomass.

In this paper, we have examined a number of alternativeaugmented processes that provide different levels of biomasscarbon recovery by using a wide range of supplemental H2

mass per unit of liquid fuel produced. A process such asH2CAR, based on gasification/FT chemistry, can recovernearly 100% biomass carbon but will need approximately0.33 kg H2/L oil produced. On the other hand, fast-hydropyrolysis/HDO based H2Bioil has a potential to recover∼70% biomass carbon with 0.11 kg H2/L oil.

A side benefit of the development of the augmentedH2Bioil process is that it has led to an efficient self-containedH2Bioil-B process concept. We have proposed a processwhereby a portion of the biomass is gasified to produce H2/CO containing hot gas which in turn is used for the fast-hydropyrolysis/HDO of the remaining biomass. ThisH2Bioil-B process, after successful experimental demonstra-tion, could result in a high energy density liquid fuel yieldthat is greater than other known self-contained processes.The model calculations indicate possible yields of 125-146ege/ton biomass. This yield range can be taken to representa good achievement target for any efficient self-containedconversion process.

The low S2F yields for the dedicated fuel crops point usto the urgency of developing processes that are efficient andincrease the liquid fuel yield from an available quantity ofthe SAW biomass. This stresses the need to decrease the

release of SAW biomass carbon as CO2 during the conversionprocess. Use of supplemental energy such as H2, heat, orelectricity, which are recovered at much higher efficienciesfrom solar energy, to increase liquid fuel yield from SAWbiomass will decrease the land area needed to grow dedicatedbiomass for the incremental liquid fuel. The development ofefficient augmented processes will not only help in increasingS2F yields for the dedicated fuel crop cases but will also bevaluable in increasing liquid fuel from a given quantity ofSAW biomass.

AcknowledgmentsWe thank the US Department of Energy (Grant No. DE-FG3608GO18087) and AFOSR (Grant No. FA 9550-08-1-0456)for partial support of this work. We also thank Eric Smoldtfor his valuable help with the figures.

Abbreviationsege ethanol gallon equivalentFT Fischer-TropschFTD Fischer-Tropsch dieselH2Bioil hydrogen bio-oil processH2Bioil-B H2Bioil process where needed H2 is produced from

a portion of biomassH2CAR hybrid hydrogen-carbon processHDO hydrodeoxygenationWGS water-gas shift

Supporting Information AvailableAssumptions, definitions, and basis for calculations alongwith estimation and modeling details for both the self-contained and augmented processes. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

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