Estimation of Liquid Fuel Yields from Biomass by Cenários Cana

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

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 H2, 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/m2 · 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 standalone H2Bioil-B process, where a portion of the biomass is gasified to provide H2 for the fast-hydropyrolysis/hydrodeoxygenation 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 H2 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 H2, 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 * Corresponding author telephone: (765) 494-2257; fax: (765) 4940805; e-mail: agrawalr@purdue.edu. 5298

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

hydrocarbon fuels to satisfy the remainder of the transportation 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 H2 would require huge development in alternate infrastructure along with major technical innovations. A simple fact that favors the use of biomass for transportation 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 H2, 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/m2 · 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). Therefore, when compared to other solar energy conversion and recovery processes such as electricity or H2, 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. 10.1021/es100316z

 2010 American Chemical Society

Published on Web 06/07/2010

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 yield numbers are for high energy density liquid fuel with the exception of fast-pyrolysis which is for low-energy density bio-oil. In this paper, we estimate the annual solar incident energy on a given land area that could be recovered as high energy density liquid fuel for dedicated fuel crop cases. These calculated “sun-to-fuel” (S2F) yields for different processes provide us the much needed overall efficiencies of converting solar energy to liquid fuel. We evaluate self-contained as well as augmented thermochemical processes, where supplementary energy is used to increase the S2F yield from a given land area (8, 15). This information is useful in assessing how to maximize liquid fuel production from a given land area. We also propose a novel self-contained biomass to liquid fuel thermochemical process. This process, when experimentally developed, has the potential to provide significantly greater S2F yield of high energy density oil than any current self-contained thermochemical process. An additional benefit of this S2F yield study is that the potential of different processes can also be compared in terms of the quantity of liquid fuel that could be produced from a unit quantity of biomass fed to each conversion process. This is helpful for understanding of how to increase the liquid fuel production from a given quantity of SAW biomass. Even though we discuss increased fuel production in the context of thermochemical processes, it should be emphasized that the calculated liquid yields provide us with potential liquid fuel yields against which all suitable processes, including those based on biochemical routes, can be measured. It is not our purpose to imply that thermochemical routes are superior to other alternative biomass conversion routes. Liquid Fuel Yield from Self-contained Biofuel Processes. We briefly discuss here the existing as well as some novel self-contained biomass to liquid fuel processes for their liquid

fuel yield. We define a self-contained process as the one where very little, if any, energy is used from a nonbiomass source. The calculation details for all the processes are given in the Supporting Information (SI). In Figure 1, we show estimated values of the overall annual oil yield by different processes using dedicated fuel crops and the total sunlight falling on 1 m2 of land area. The results are 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 collection rates of biomass: 1.5, 3, 5, and 6.25 kg/m2 · y, the results reported in Figure 1 are for the biomass collection rate of 3 kg/m2 · y. By collection rate, we refer to the annual amount of biomass that arrives at a processing plant from 1 m2 of land area. In one scenario, a fraction of the land area may be assigned to directly recover additional energy needed to grow, harvest, and transport the biomass to the plant gate. If this additional energy is a small fraction of the biomass energy arriving for processing, then the biomass collection rate will be close to the biomass growth rate. Alternatively, if most of the additional energy is utilized as electricity and heat, then since these forms of energy are recovered from solar energy at an order of magnitude higher efficiency than biomass, the fraction of the land area dedicated for the additional energy will be relatively small. In this case, the biomass collection rate will again be nearly equal to the biomass growth rate. Thus, in Figure 1, for a biomass energy content of 17 MJ/kg, a collection rate of 3 kg/m2 · y implies that from the total 6307 MJ/m2 · y of solar energy, 51 MJ/m2 · y are available as biomass energy at the plant for further processing VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5299

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

energy content (MJ/kg)

energy content (kJ/mol C)

carbon loss with 100% efficient conversion (%)

assumed conversion efficiency (%)

total carbon loss (%)

switchgrass poplar sugar

17.2 19.6 14.1

485 455 423

24.6 32.8 42.8

75 75 97

43.4 49.6 44.5

The energy content of gasoline ) 604.1 kJ/mol C.

to produce liquid fuel. In this paper, we report all energy contents as lower heating values (LHV) unless specified otherwise. From the available biomass energy, the liquid fuel yield is calculated based on overall process conversion efficiency. This means that in our S2F calculations, we do not account for any auxiliary power that the plant may use. It is assumed that such auxiliary power is either absent or a small fraction of the total rate at which energy is processed within a plant. Thus, in a thermochemical plant, major energy inputs such as the heat needed to dry the biomass, electricity needed to circulate hot gases, etc. are produced from the biomass feed to the plant. Similarly, for a fermentation based biochemical plant, the heat energy needed for separation of ethanol or butanol is assumed to be supplied from the lignin portion of the biomass. If within a chemical process, excess energy is generated which cannot be converted to liquid fuel, we do not take credit for such energy in the S2F calculations. Our S2F calculations do account for all the major energy conversion steps, and the numbers in Figure 1 provide a good representation of the efficiency of each process in recovering annual incident solar energy as liquid fuel. Another useful piece of information that can also be derived from the S2F calculations in Figure 1 is the amount of liquid produced per ton of biomass. For each process, information regarding the amount of biomass used and the corresponding energy content of the fuel produced is available. Since all processes do not produce the same product, a convenient unit to compare processes on the same basis is ethanol gallon equivalent (ege) per ton of biomass (18). For this calculation, we divide the energy content of the fuel 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 to get ege per ton of biomass. This information is useful as it allows us to compare different processes on the basis of biomass conversion to liquid fuel. This is particularly important when the available biomass is limited. When biomass is processed through a conventional gasifier followed by a Fischer-Tropsch diesel (FTD) process, the overall process efficiency of the chemical conversion process can vary between 41 and 50% (19). This leads to a recovery of 20.9-25.5 MJ/m2 · y of energy as biofuel, which also translates into 87-106 ege of liquid fuel per ton of biomass. For the biochemical routes using enzymes and microbes, the reported efficiency values for the lignocellulosic biomass is in the 35-50% range with 75-105 ege/ton of biomass (18, 20, 21). This implies that energy content of the recovered ethanol is similar to that of diesel produced in the gasification/FT route. On the other hand, conventional fast-pyrolysis is quite efficient (65-77%) (22, 23) in converting biomass to a lowenergy density liquid fuel referred to as bio-oil and recovers 33.1-39.5 MJ/m2 · y of solar incidence energy. The bio-oil from fast-pyrolysis has extremely high oxygen content (∼35-40 wt %), however, and its energy content is only half that of petroleum and similar to that of the original biomass (∼17 MJ/kg). Bio-oils do not easily blend with petroleum products and tend to polymerize and condense with time during shipment and lead to gumming in downstream 5300

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

reactors. This necessitates the subsequent upgrading of this bio-oil by hydrodeoxygenation (HDO) using H2 in the presence of a catalyst (15, 24, 25). A recent study by Pacific Northwest National Laboratory (PNNL) reports modeling results for a three step process of fast-pyrolysis, hydrodeoxygenation, and hydrocracking to produce gasoline and diesel-like liquid fuel from lignocellulosic biomass (26). One of our goals in this paper is to study processes that have a potential to eliminate three-step processing and the handling challenges associated with the fast-pyrolysis based route for producing upgraded oil. For this purpose, we will first discuss augmented processes where H2 needed to remove oxygen from the biomass to directly produce high energy density oil is supplied from a nonbiomass source. Liquid Fuel Yields from Augmented Biofuel Processes Using Supplementary Hydrogen. A key feature of all the three self-contained processes discussed so far is that biomass is the sole energy source being converted to liquid fuel. As seen from Table 1, this inherently limits the biomass carbon atoms that can be recovered as high energy density liquid fuel. The energy content on a per carbon basis for biomass sources such as Switchgrass, Poplar, and sugars is only twothirds of the energy content of the molecules composing gasoline. This means that when biomass carbon molecules are upgraded to a high energy density liquid fuel such as gasoline, even for a 100% energy efficient conversion process, about one-third of the biomass carbon atoms will be rejected as low energy molecules such as CO2. With reasonably optimistic process conversion efficiencies, we find from Table 1 that roughly only half of the biomass carbon is recovered as high energy density liquid fuel. Most likely, the lost carbon will be in its low-energy state as CO2. Considering the fact that solar energy to biomass carbon is a low-efficiency process, it is attractive to find processes that will economically either reduce or eliminate the loss of condensed biomass carbon as CO2 during the biomass to liquid fuel conversion process. It is possible to envision thermochemical as well as biochemical conversion processes using supplementary sources of energy such as heat, H2, or electricity to increase production of high energy density liquid fuel per ton of biomass. We have focused here on thermochemical routes. It is worth noting that for self-contained biomass conversion processes both biochemical as well as gasification/FTD processes were found to provide similar yields. Therefore, we expect that the yields calculated from thermochemical processes using supplemental energy should provide good target estimates for the corresponding non-thermochemical processes as well. Although synergistic augmented biofuel processes using supplemental energy from fossil fuel or nuclear are described elsewhere (8, 15), in this work we are interested in the sun-to-fuel yield to create alternative scenarios for a future where most transport needs are met through renewable energy. Therefore, in this analysis, we have used solar energy as the supplemental energy source. In order to avoid difficulties associated with the handling of bio-oil from a fast-pyrolysis reactor prior to its HDO treatment, it will be best to pyrolyze biomass in the presence of H2 and a catalyst as shown in a process alternative in

FIGURE 2. H2Bioil process where solar H2 is used for fast-hydropyrolysis of biomass and subsequent HDO of the pyrolyzer product stream. Figure 2. Ideally one would like to choose a reactor design, catalyst, and operating conditions to directly produce deoxygenated high energy density liquid fuel in one step. However, it is likely that the much higher temperature, most likely in excess of 400 °C, and much shorter contact time (∼1 s) that will be needed for the fast-hydropyrolysis step will not be the optimal conditions for the level of deoxygenation required. Therefore, if needed, the exhaust from the fasthydropyrolysis reactor, after removal of any char present and adjustment of the temperature, can be directly sent through an HDO reactor to allow the needed contact time with the HDO catalyst at its preferred operating conditions. The thrust is to avoid some of the steps associated with fastpyrolysis such as total condensation of bio-oil, the associated problems with subsequent handling, and then revaporization for HDO. Bridgwater and Peacock describe a large number of fast-pyrolysis reactor configurations, and many of them can be adapted for fast-hydropyrolysis (23). We use the term “H2Bioil” to refer the processes that are based on fasthydropyrolysis/HDO of biomass with supplemental H2 and directly produce high energy density liquid fuel. It should be noted that a few groups have reported labscale hydropyrolysis of biomass in fixed-bed reactor mode (27-34). In some cases, lower char and higher oil yields were observed when H2 is present during pyrolysis (29, 30, 34). Also, less intense degradation reactions have been reported under H2 than under an inert atmosphere (33). These early experiments provide favorable conceptual support for hydropyrolysis to produce oil with low oxygen content. However, unlike fast-pyrolysis experiments, all the reported hydropyrolysis data in the literature are from fixed-bed reactors. Fast pyrolysis experiments are generally not conducted in a fixed-bed mode due to the need for rapid heating-cooling and the requirement of a very short residence time in the reactor. Thus, there is a need to conduct hydropyrolysis experiments in a mode similar to that for fast-pyrolysis with a possibility of immediate downstream HDO. However, unlike fast-pyrolysis, the fast-hydropyrolysis reactors will possibly operate at a much higher pressure. Such experiments in the presence of H2 could provide higher yields of deoxygenated oil per unit of biomass. In the absence of experimental data for a fast-hydropyrolysis operation mode, it is difficult to make an accurate estimate for the oil yield. However, it is possible to make a reasonable estimate for yields that could be achieved. The fixed bed hydropyrolysis studies do report decrease in char production that could lead to higher oil yield. Elimination of the bio-oil revaporization step prior to the HDO reactor

and the possibility of better heat integration could also contribute to improved process efficiency and yield. In light of these factors, in our calculations, the carbon yield in the oil from the H2Bioil process was taken to be ∼70%, which is same as reported in the literature for the bio-oil from a fastpyrolyzer (35). When bio-oil from a fast-pyrolyzer is upgraded through HDO, the experimentally observed energy content of the upgraded oil is found to be in the range of 38-43 MJ/kg (26). For our calculations, we have assumed that, with enough solar H2 and advances in catalysis and processing conditions, the energy content is likely to be closer to the upper end value of ∼42 MJ/kg of oil. Furthermore, the carbon and oxygen in the oil from the H2Bioil process were taken to be 86.5 and 1.25 wt % with the rest being hydrogen. On this basis, the estimated yield of oil from the H2Bioil process is 215 ege/ton of biomass (see the SI for details). Note that this number is slightly lower than the 230 ege/ton of biomass reported earlier (15) as a slightly lower and more probable value of the oil LHV is used in this work. For the estimation of H2 consumption, we assumed that two H2 molecules are needed to remove an oxygen atom. As shown in the detailed calculations provided in the SI, this leads to an estimated H2 requirement of ∼1240 L at STP/L oil (0.11 kg H2/L oil), which is higher than the highest experimental value reported in the literature to upgrade bio-oil from a fast-pyrolyzer (∼1144 L at STP/L upgraded oil) (25). This gives us a confidence that our estimate of H2 consumption is conservative, and in an actual process, the H2 consumption may be lower. From the calculations for Figure 1, out of 6307 MJ/m2 · y of solar energy, the H2Bioil process is estimated to use 231 MJ/m2 · y of energy to make H2 and the rest to grow biomass. Using a sun to electricity conversion efficiency of 15% and an electrolyzer efficiency (based on LHV of H2) of 50.7%, 17.6 MJ/m2 · y worth of energy is estimated in solar H2. It is estimated that from this H2Bioil process one can obtain 49.8 MJ/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 through a thermochemical process rather than electrolysis, the overall yield from solar energy to fuel will still be in the neighborhood of 49.8 MJ/m2 · y. A recent study reports ∼30% high heating value (HHV) based efficiency to H2 conversion from the net solar energy soaked up by the thermochemical reactor (14). When one accounts for radiation losses etc., the solar energy to 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 VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5301

FIGURE 3. H2Bioil-B process where a portion of the feed biomass (32-42%) is fed to the gasification zone to provide H2 for fast-hydropyrolysis and HDO of the remaining biomass fed to the hydropyrolysis zone. supply H2, but this decrease will have a small incremental impact on the overall yield of 49.8 MJ/m2 · y as oil. Results are also available for a gasification/FT based hybrid hydrogen-carbon (H2CAR) process where almost all the biomass carbon is recovered as liquid fuel by using supplementary H2 (8). Figure 1 shows that the estimated recovery of solar energy as liquid fuel from the H2CAR process is indeed the highest of any process. However, this increase in oil production comes at a steep increase in demand for solar H2 at about 0.33 kg H2/L oil. In fact, the inefficiency of the H2CAR process is reflected in the expected production of only 66.9 MJ/m2 · y of liquid fuel from the input of 72 MJ/m2 · y of solar H2 as seen from Figure 1. If energy efficiency were the only criteria, one would be better off using H2 via fuel cell vehicles which are more efficient than the oil based internal combustion 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 or a biochemical process, it will be more informative to compare it with a self-contained version that is also based on fasthydropyrolysis. Liquid Fuel Yields from a Self-Contained Fast-Hydropyrolysis Based Process. The self-contained version of the H2Bioil process can be created by gasifying a portion of the biomass to supply the needed H2 for the fast-hydropyrolysis/ HDO of the remaining biomass portion. In order to consolidate biomass gasification/fast-hydropyrolysis in one reactor unit, we propose the novel fast-hydropyrolysis reactor configuration 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 mixture which is sufficient to hydropyrolyze and hydrodeoxygenate the remaining fraction of the biomass that is directly fed to the hydropyrolysis zone. The hot gas from the gasifier is directly injected in the pyrolyzer zone. If needed, the temperature of the exhaust gas prior to its injection in the pyrolyzer zone may be adjusted. Also, if required, a hot or a cold recycle stream may be injected between the gasifier and the pyrolyzer zone to provide better temperature control in the pyrolyzer section of the reactor. The majority of the CO from the gasification zone is expected to provide H2 through the water-gas-shift (WGS) reaction. Generally, the WGS reaction can be conducted at high temperatures of 350-500 °C using an iron oxide based catalyst (36). In this temperature range, formation of H2 is preferred and most of the CO can be converted. Of course, the use of in situ water-gas-shift catalysts in the fast5302

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

hydropyrolysis and the HDO reactors can provide an effective means to control the H2 partial pressure during biomass conversion. As H2 is consumed, there will be driving force for CO reaction with H2O that is formed as a byproduct of the HDO reaction. Furthermore, an interesting but as yet unanswered question is whether CO can play a direct role in pyrolysis or deoxygenation. It should be noted that for the H2Bioil-B process, in situ biomass gasification will provide hot syngas for the biomass fast-hydropyrolysis process, leading to an increase in energy efficiency. An important advantage of this process over conventional HDO is that it will avoid the need for pure hydrogen and thus save the energy and capital cost associated with hydrogen purification and recovery. Due to expected improvements in energy efficiencies due to process integration, as compared to a process using three separate steps of fast-pyrolysis, gasification and HDO, we expect higher liquid fuel yield from the H2Bioil-B process (26). In calculations for Figure 1, we find that for a biomass to H2 conversion efficiency of 50% (1), nearly 42% of the biomass will be gasified to provide the needed hydrogen to yield 30.1 MJ/m2 · y of solar energy as liquid fuel. This translates into a carbon efficiency, energy efficiency, and liquid fuel yield of 40.6%, 59.1%, and 125.4 ege/ton of biomass. For a higher biomass gasifier efficiency of 75% to hydrogen, the carbon efficiency, energy efficiency, sun-to-fuel recovery, and liquid fuel yield estimates are 47.2%, 68.6%, 35 MJ/m2 · y, and 146 ege/ton of biomass, respectively (refer to the SI for calculation details). The calculated yield of 125-146 ege/ton of biomass is highest among the hitherto known self-contained biomass to high energy density liquid fuel processes (roughly 50% higher liquid fuel yield than the self-contained gasification/FT or current biochemical processes). However, the use of the organic residue from a biochemical process to produce additional liquid fuel via gasification/FT process has been suggested in the literature (20, 37) and has the potential to produce a competitive 77 gallons of gasoline and 11 gallons of diesel equivalent oil per ton of biomass (equivalent to ∼136 ege/ton of biomass) (20). The process by ZeaChem first produces acetic acid by fermentation of sugars (38). The acetic acid is then converted to an ester which is reacted with H2 to form ethanol. The H2 is produced via gasification of the lignin residue. It is claimed that the nth plant will yield 135 ege/ton of biomass. The proposed H2Bioil-B uses only a thermochemical route and has the potential to be built on small scale while providing high yields of a high energy density liquid fuel product.

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. Since we have S2F recovery estimates for various selfcontained as well as augmented processes, they provide us an opportunity to make some interesting observations: Observations from S2F Recovery Estimates. From the various S2F recovery estimates shown in Figure 1 for a biomass collection rate of 3 kg/m2 · y, we note that (1) While S2F recoveries shown for the self-contained fastpyrolysis and H2Bioil-B are similar, the energy content on a per unit mass basis of the bio-oil from fastpyrolysis is less than half of the oil from H2Bioil-B and it cannot be used without spending additional energy to upgrade it. (2) For the self-contained processes, the energy recovered as high energy density oil is in the range of 18 to 35 MJ/m2 · y. When compared to the energy content of 51 MJ/m2 · y for original biomass, the energy efficiency for conversion of biomass varies over a wide range of 35-68%. The efficiency range of 35-50% for the gasification/FT and biochemical based processes is similar. It seems that fast-hydropyrolysis/HDO based processes have the potential to be among the most efficient of the self-contained processes. This provides an 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-contained processes is less than 0.56%. This is not surprising as the efficiency of collecting dedicated fuel crop was ∼0.81%. Nevertheless, it does point out that the solar energy recovered as liquid fuel via the dedicated fuel crop and self-contained processing is quite low. (4) The augmented processes hold the promise to substantially improve S2F efficiencies (0.8-1.06%). As a matter of fact, the energy content in the fuel can be similar (49.8 vs 51 MJ/m2 · y) or even higher (66.9 vs 51 MJ/m2 · y) than that in the biomass. On the basis of the S2F yields, the liquid fuel produced by the augmented

processes using the same total land area can be oneand-one-half to three times that of the self-contained processes. (5) It is interesting to compare incremental oil productions and H2 consumption for the two routes: (i) the H2CAR process as compared to the self-contained gasification/FT process and (ii) the augmented H2Bioil process with the self-contained H2Bioil-B process. We find that the energy content of the incremental oil produced per unit of energy in the H2 used (MJ in incremental oil produced/MJ in H2 used) is much more for the H2Bioil based route (0.9-1.2) than for the H2CAR route (∼0.65). This shows that the fast-hydropyrolysis/HDO based process is very efficient in increasing the biomass carbon recovery. However, its carbon recovery is limited to ∼70%. In order to get nearly ∼100% carbon recovery, the H2CAR process essentially converts CO2 to liquid fuel. Conversion of CO2 to liquid fuel is an energy intensive process and is reflected in the high H2 requirements for the H2CAR process. This also demonstrates the energy penalty associated with very high recoveries of biomass carbon. (6) We can draw another important conclusion from Figure 1. When additional liquid fuel is needed, it should preferably be first produced by using augmented processes in conjunction with SAW biomass rather than through growing dedicated fuel crops. Such a step would make more efficient use of sunlight falling on a given land area. This is possible because heat or H2 are recovered from sunlight at a much higher efficiency than a dedicated fuel crop, and at least 65% of the energy in H2 can be recovered as the incremental liquid from the SAW biomass. In Figure 4, we show estimated S2F yields for four different biomass collection rates. While the expected range for the yields are calculated in the SI for a number of conversion processes, we only show upper estimates in the bar graphs. VOL. 44, NO. 13, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

5303

As expected, increased growth rates and hence collection rates for biomass have a profound impact on S2F yields. For all processes except H2CAR, the increase in liquid fuel rate is directly proportional to the increase in biomass collection rate. For the H2CAR process, large quantities of H2 are needed and high biomass growth rates lead to relatively larger fraction of the land area being needed for the H2 production. This results in less than proportional increase in liquid fuel yield with the increase in biomass growth rates. It is informative to 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 solar light being collected as liquid fuel. This tells us that, even under a highly favorable process condition, the dedicated biomass carbon route to liquid fuel provides only a small portion of the incident solar energy as liquid fuel. This has a strong implication on the land area requirements when dedicated biomass is to be grown for liquid fuel production.

Discussion In this work, we have estimated the sun-to-fuel yield for liquid hydrocarbon fuel via the biomass carbon route. This is relevant when dedicated biomass is to be grown and harvested for the production of liquid fuel. In such a case, one of the goals is to increase the collection of solar energy incident on a given land area as liquid fuel. The S2F yield for the self-contained processes that mainly rely on the biomass to supply all the energy need is quite low. Even at a high biomass collection rate of 6.25 kg/m2 · y, only ∼1.16% of the solar energy is estimated to be recovered as liquid fuel. Two main contributing factors to the low S2F yield are (i) the relatively low recovery of solar energy as biomass energy and (ii) the fact that less than 50% of the biomass carbon recovery in the liquid fuel is from self-contained processes. We find that the S2F yield can potentially be increased by a factor of 1.5-3 when all the available land area is not used to grow dedicated fuel crop but the solar energy falling on a portion of the land area is harnessed as hydrogen which is then used in novel augmented biomass conversion processes to increase biomass carbon yield as liquid fuel. Such augmented processes are estimated to have higher S2F yield because hydrogen is harvested from solar energy with a much higher efficiency than biomass. In this paper, we have examined a number of alternative augmented processes that provide different levels of biomass carbon recovery by using a wide range of supplemental H2 mass per unit of liquid fuel produced. A process such as H2CAR, based on gasification/FT chemistry, can recover nearly 100% biomass carbon but will need approximately 0.33 kg H2/L oil produced. On the other hand, fasthydropyrolysis/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 augmented H2Bioil process is that it has led to an efficient self-contained H2Bioil-B process concept. We have proposed a process whereby a portion of the biomass is gasified to produce H2/ CO containing hot gas which in turn is used for the fasthydropyrolysis/HDO of the remaining biomass. This H2Bioil-B process, after successful experimental demonstration, could result in a high energy density liquid fuel yield that is greater than other known self-contained processes. The model calculations indicate possible yields of 125-146 ege/ton biomass. This yield range can be taken to represent a good achievement target for any efficient self-contained conversion process. The low S2F yields for the dedicated fuel crops point us to the urgency of developing processes that are efficient and increase the liquid fuel yield from an available quantity of the SAW biomass. This stresses the need to decrease the 5304

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

release of SAW biomass carbon as CO2 during the conversion process. Use of supplemental energy such as H2, heat, or electricity, which are recovered at much higher efficiencies from solar energy, to increase liquid fuel yield from SAW biomass will decrease the land area needed to grow dedicated biomass for the incremental liquid fuel. The development of efficient augmented processes will not only help in increasing S2F yields for the dedicated fuel crop cases but will also be valuable in increasing liquid fuel from a given quantity of SAW biomass.

Acknowledgments We thank the US Department of Energy (Grant No. DE-FG 3608GO18087) and AFOSR (Grant No. FA 9550-08-1-0456) for partial support of this work. We also thank Eric Smoldt for his valuable help with the figures.

Abbreviations ege FT FTD H2Bioil H2Bioil-B H2CAR HDO WGS

ethanol gallon equivalent Fischer-Tropsch Fischer-Tropsch diesel hydrogen bio-oil process H2Bioil process where needed H2 is produced from a portion of biomass hybrid hydrogen-carbon process hydrodeoxygenation water-gas shift

Supporting Information Available Assumptions, definitions, and basis for calculations along with estimation and modeling details for both the selfcontained and augmented processes. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) NRC. The NRC report-The Hydrogen Economy-Opportunites, Costs, Barriers, and R&D Needs; The National Academies Press: Washington D C, 2004. (2) Tarascon, J. M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414 (6861), 359– 367. (3) Kintner-Meyer, M.; Schneider, K.; Pratt, R. Impact Assessment of Plug-in Hybrid Vehicles on Electric Utilities and Regional U.S. Power Grids, Part 1: Technical Analysis; Pacific National Laboratory: Richland, WA, 2007; http://www.ferc.gov/about/ com-mem/wellinghoff/5-24-07-technical-analy-wellinghoff. pdf. (4) Davis, S. C.; Diegel, S. W. Transportation Energy Data Book, edition 28 ORNL-6978; Oak Ridge National Laboratory: Oak Ridge, TN, 2009; http://cta.ornl.gov/data/index.shtml. (5) BP. Statistical Review of World Energy; London, June 2008. (6) NRC. America’s Energy Future Panel on Alternate Liquid Transportation Fuels-Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts; National Academies Press: Washington, D.C., 2009; p 300. (7) Perlack, R. D.; et al. Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply; DOE/GO-102995-2135 ORNL/TM-2005/66, 2005; http://feedstockreview.ornl.gov/pdf/billion_ton_vision. pdf. (8) Agrawal, R.; et al. Sustainable fuel for the transportation sector. Proc. Natl. Acad. Sci. U.S.A. 2007, 104 (12), 4828–4833. (9) Dismukes, G. C.; et al. Aquatic phototrophs: efficient alternatives to land-based crops for biofuels. Curr. Opin. Biotechnol. 2008, 19 (3), 235–240. (10) King, R. R. Multijunction cells: Record breakers. Nat. Photon 2008, 2 (5), 284–286. (11) King, R. R.; et al. 40% efficient metamorphic GaInP/GaInAs/Ge multijunction solar cells. Appl. Phys. Lett. 2007, 90 (18), 183516. (12) Sargent and Lundy LLC Consulting Group Chicago, I. Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts; NREL/SR-550-34440, 2003; www. osti.gov/servlets/purl/15005520-kLbVbt/native/. (13) 1996 Status Report on Solar Trough Power Plants - Experience, Prospects and Recommendations to Overcome Market Barriers of Parabolic Trough Collector Power Plant technology; Pilkington

(14) (15) (16)

(17)

(18) (19) (20)

(21)

(22)

(23)

(24)

(25) (26)

Solar International GMBH, Cologne, Germany, 1996; ISBN 3-9804901-0-6. Diver, R. B.; et al. Solar Thermochemical Water-Splitting FerriteCycle Heat Engines. J. Solar Energy Eng. 2008, 130 (4), 041001. Agrawal, R.; Singh, N. R. Synergistic Routes to Liquid Fuel for a Petroleum deprived future. AIChE J. 2009, 55 (7), 1898–1905. Knaupp, W. Power rating of photovoltaic modules from outdoor measurements. Photovoltaic Specialists Conference, 1991, Conference Record of the Twenty Second IEEE, 1991; Vol. 1, pp 620624. King, D. L. Photovoltaic module and array performance characterization methods for all system operating conditions; Proceedings of NREL/SNL Photovoltaics Program Review Meeting, Nov 18-22, 1999, Lakewood, CO; AIP Press: New York, 1997; SAND-96-2848C, 1996. Schubert, C. Can biofuels finally take center stage. Nat. Biotechnol. 2006, 24 (7), 777–784. Tijmensen, M. J. A.; et al. Exploration of the possibilities for production of Fischer-Tropsch liquids and power via biomass gasification. Biomass Bioenergy 2002, 23, 129–152. Greene, N.; et al. Growing Energy: How Biofuels Can Help End America’s Oil Dependence; Natural Resources Defense Council: New York, Dec 2004. Mark Wright, M.; Brown, R. C. Comparative economics of biorefineries based on the biochemical and thermochemical platforms. Biofuels, Bioprod. Biorefining 2007, 1 (1), 49–56. Mohan, D.; Pittman, C. U.; Steele; P. H. Pyrolysis of Wood/ Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20 (3), 848–889. Bridgwater, A. V.; Peacock, G. V. C. Fast pyrolysis processes for biomass. Renewable Sustainable Energy Rev. 2000, 4 (1), 173. Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev. 2006, 106 (9), 4044–4098. Elliott, D. C. Historical Developments in Hydroprocessing Biooils. Energy Fuels 2007, 21 (3), 1792–1815. Jones, S. B.; et al. Production of Gasoline and Diesel from Biomass via Fast Pyrolysis, Hydrotreating and Hydrocracking: A Design Case; Pacific National Laboratory: Richland, WA, PNNL-18284 Rev. 1, 2009.

(27) Gercel, H. F.; Putun, A. E.; Putun; E. Hydropyrolysis of Extracted Euphorbia rigida in a Well-Swept Fixed-Bed Tubular Reactor. Energy Sources 2002, 24, 423–430. (28) Pu ¨ tu ¨ n, A. E.; et al. Fixed-bed pyrolysis and hydropyrolysis of sunflower bagasse: Product yields and compositions. Fuel Process. Technol. 1996, 46 (1), 49–62. (29) Pu ¨ tu ¨ n, A. E.; et al. Oil production from an arid-land plant: fixedbed pyrolysis and hydropyrolysis of Euphorbia rigida. Fuel 1996, 75 (11), 1307–1312. (30) Dilcio Rocha, J.; Luengo, C. A.; Snape, C. E. The scope for generating bio-oils with relatively low oxygen contents via hydropyrolysis - a versatile technique for solid fuel liquefaction, sulphur speciation and biomarker release. Org. Geochem. 1999, 30, 1527–1534. (31) Rocha, J. D.; et al. Hydropyrolysis: a versatile technique for solid fuel liquefaction, sulphur speciation and biomarker release. J. Anal. Appl. Pyrolysis 1997, 40-41, 91–103. (32) Pindoria, R. V.; et al. A two-stage fixed-bed reactor for direct hydrotreatment of volatiles from the hydropyrolysis of biomass: effect of catalyst temperature, pressure and catalyst ageing time on product characteristics. Fuel 1998, 77 (15), 1715–1726. (33) Pindoria, R. V.; et al. Structural characterization of biomass pyrolysis tars/oils from eucalyptus wood waste: effect of H2 pressure and sample configuration. Fuel 1997, 76 (11), 1013–1023. (34) Pindoria, R. V.; et al. Hydropyrolysis of sugar cane bagasse: effect of sample configuration on bio-oil yields and structures from two bench-scale reactors. Fuel 1999, 78, 55–63. (35) Ringer, M.; Putsche, V., Scahill, J. Large Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis; NREL/TP-510-37779, National Renewable Energy Laboratory: Golden, CO, 2006; http://www.nrel.gov/docs/fy07osti/37779.pdf. (36) Farrauto, R. J.; Bartholomew, C. H. Fundamentals to Industrial Catalytic Processes; Chapman and Hall: London, 1997. (37) Lynd, L. R. Energy Myth Three - High L and Requirements and an Unfavorable Energy Balance Preclude Biomass Ethanol from Playing a Large Role in Providing Energy Services, in Energy and American Society- Thirteen Myths; Sovacool, B. K., Brown, M. A., Eds.; Springer: New York, 2007; p 75-102. (38) Zeachem. Zeachem technology overview. www.Zeachem.com/ technology/overview.php(accessed January 2010).

ES100316Z

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

5305