Environmental impact and sustainability study on biofuels for transportation applications

Renewable and Sustainable Energy Reviews 67 (2017) 277–288

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Environmental impact and sustainability study on biofuels for transportation applications Wei-Ru Chang a,nn, Jenn-Jiang Hwang b,n, Wei Wu c a

Department of Landscape Architecture, Fu Jen Catholic University, New Taipei City, Taiwan Department of Greenergy, National University of Tainan, Tainan, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan b

art ic l e i nf o

a b s t r a c t

Article history: Received 31 March 2015 Received in revised form 24 April 2016 Accepted 8 September 2016

A review on lifecycle analysis of energy consumption and greenhouse gas (GHG) emission for various biofuel vehicles has been performed. Four potential vehicular biofuels are simulated: corn ethanol, switchgrass ethanol, soybean biodiesel, and bio-hydrogen from corn ethanol. A fuel-cycle model developed at Argonne National Laboratory, called the GREET model, is employed to evaluate the biomassto-tank (BTT) energy and emissions impacts of various biofuels. The fuel economies of three types of vehicles, i.e., flexible fuel vehicles (FFVs), diesel vehicles (DVs), and fuel cell vehicles (FCVs) are also determined using the simulation tools in MATLAB/Simulink. The effects of replacing conventional gasoline vehicles (GVs) by the aforementioned biofuel vehicles on the lifecycle GHG emission and energy consumption are examined. The results showed that the FFVs fueled with an ethanol fuel blend of 85% switchgrass ethanol and 15% gasoline (E85) have the greatest benefits in GHG emission reduction by 59.4%, but suffer from 101.3% total energy consumption compared to the baseline system. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Lifecycle assessment Biofuel Flexible fuel vehicle Biomass-to-wheels efficiency

Contents 1. 2.

3.

4. 5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Biofuel pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1. Bioethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1.1. First-generation bioethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1.2. Second-generation bioethanol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1.3. Social concerns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 2.2. Biodiesel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 2.3. Bio-hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Vehicle technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 3.1. Flexible fuel vehicles (FFVs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 3.2. Diesel vehicles (DVs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 3.3. Fuel cell vehicles (FCVs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 5.1. Fuel economy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 5.2. Biomass-to-tank efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 5.3. Stage and total energy consumption and GHG emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 5.4. Fossil fuel and petroleum uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 5.5. Relative change in total energy and GHG emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286

n

Corresponding author. Corresponding author. E-mail addresses: [email protected] (W.-R. Chang), [email protected] (J.-J. Hwang). nn

http://dx.doi.org/10.1016/j.rser.2016.09.020 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

other potential octane enhancers, such as MTBE, are discouraged or prohibited.

1. Introduction Currently, the transportation sector accounts for approximately 50% of the global oil consumption and produces approximately 25% of the global energy-related CO2 emissions [1]. Therefore, improving energy security and decreasing vehicle contributions to greenhouse gas (GHG) emissions and air pollution become primary goals compelling governments to seek alternatives to the petroleum fuels currently dominating transportation. Over the past few decades, several fuel candidates have emerged, such as liquefied petroleum gas (LPG) [2–5], compressed natural gas (CNG) [6–9] and electricity for electric vehicles (EVs) [10–15]. The above fuel alternatives have some benefits over petroleum, but they also show a number of drawbacks that reduce their ability to capture the market share. For example, they all require significant vehicle modifications and a new fueling infrastructure. As a result, except in a few places, both automakers and fuel suppliers are disinclined to make substantial investments in such an uncertain market. In contrast, biofuels have the potential to overcome the traditional barriers mentioned above. They have been regarded as sustainable options for reducing petroleum-dependence and GHG emissions in the transportation sector. In addition, biofuels could share the existing distribution infrastructure with little modification. In fact, many countries are implementing the use of biofuels. Low-percentage bioethanol blends, such as 10% bioethanol in conventional gasoline (known as E10), are already dispensed in many refueling stations worldwide [16], with a high level of compatibility with materials and equipment. In addition, biodiesel is also currently blended with conventional diesel in many countries, ranging from 5% (BD5) in France to 20% (BD20) in the USA, and is used as a neat fuel (100% biodiesel) by some trucks in Germany [17]. Expanding the use of biofuels would support several major policy objectives:


Energy security: Biofuels can readily replace petroleum fuels







and, in many countries, can provide a domestic rather than an imported source of transport fuel. Even if imported, ethanol or biodiesel will likely come from regions other than OPEC (Organization of Petroleum Export Countries), creating a broader global diversification of supply sources of transport fuels. Environment: With lower GHG emissions over the whole fuel chain, biofuels are generally more climate-friendly than petroleum fuels. Either in their neat form or as blends with conventional petroleum fuels, vehicles running on biofuels emit less of some pollutants that exacerbate air quality problems, particularly in urban areas. Reductions in some air pollutants are also achieved by blending biofuels, though some other types of emissions (e.g., NOx) might be increased this way. Fuel quality: Refiners and automakers have become interested in the benefits of ethanol to boost fuel octane, especially where

The objective of the present paper is to study the lifecycle performance of biofuel vehicles using simulation tools. A biomassto-wheels (BTW) analysis is performed to assess the potential benefits in energy savings and GHG-emission reductions for applying biofuels to the transportation sectors. As shown in Table 1, three types of biofuel vehicles, i.e., flexible fuel vehicles (FFVs), diesel vehicles (DVs), and fuel cell vehicles (FCV), are studied to investigate the effects of replacing conventional gasoline vehicles (GVs) on the GHG emission and the total energy consumption. First, the fuel economy is determined by using the simulation tools in MATLAB/Simulink [18]. Then, a comprehensive analysis of fuelcycle energy consumption and GHG emissions for various biofuels is conducted using the GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) code. As shown in Table 2, three types of biomass-feedstock are studied in the pre-

Table 2 Assumptions of biofuel production. Feedstock

Items

Unit

Corn

CO2 emissions from domestic land use g/bushel change by corn farming CO2 emissions from foreign land use g/bushel change by corn farming Corn farming energy use Btu/ bushel Ethanol production energy use: dry Btu/ mill gallon Ethanol production energy use: wet Btu/ mill gallon

Switchgrass CO2 emissions due to domestic land g/dry ton use change by switchgrass farming CO2 emissions due to foreign land use g/dry ton change by switchgrass farming Switchgrass farming energy use Btu/dry ton EtOH yield from switchgrass fermen- gallons/ tation plant dry ton kWh/dry Electricity co-product in switchgrass fermentation plant ton Soybean

Soybean farming: energy Use Soyoil extraction: energy Use Transesterification: bio-oil se Transesterification: Energy Use

Btu/ bushel Btu/lb of soyoil lb of bio oil/lb fuel Btu/lb fuel

Assumptions 447 285 9142 26,856 47,409  433 541 123,700 90 205 16,560 3551 1.04 1844

Table 1 Biofuel vehicles associated with fuel pathways. Vehicles

Biofuels

Flexible Fuel Vehicles (FFVs)

Bioethanol

Diesel Vehicles (DVs) Fuel Cell Vehicle (FCVs)

Biodiesel Bio-hydrogen

Feedstock

Production process

Group

Specification

Starch-based biomass Lignocellulosic biomass Soybeans Corn ethanol

Corn Switchgrass

Fermentation Trans-esterification Reforming

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sent work, i.e., corn, switchgrass, and soybean. The analysis comprises four fuel pathways, corn-to-ethanol, switchgrass-to-ethanol, soybean-to-biodiesel, and corn-based hydrogen. The pathway of petroleum-to-gasoline is also discussed for comparison. In conjunction with the vehicle fuel economy and the fuel-pathway results, the lifecycle performance in energy consumption and GHG emission of the biofuel-based transportation system are determined. The results obtained in the present work could enrich the information available for the public, industry and government to make biomass-informed decisions at this important policymaking time.

2. Biofuel pathways 2.1. Bioethanol Bioethanol is an alcohol that is usually produced from biomass materials. Conventionally, it is produced from fermentation of glucose, in which oxygen is insufficient for normal cellular respiration, and anaerobic respiration takes place in yeasts, thus converting glucose into ethanol and carbon dioxide, C6H12O6-2CH3CH2OH þ 2CO2

(1)

The glucose usually comes from starch crops such as corn and wheat or from sugar crops such as sugarcane and beet. Recently, cellulosic biomass, derived from nonfood sources such as herbaceous and woody biomass or waste residues, is also being developed as a feedstock for ethanol production. The former based on sucrose- or starch-biomass is known as first-generation bioethanol, whereas the later based on lignocelluloic biomass is called second-generation bioethanol [19]. They are described in the following discussion.

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2.1.1. First-generation bioethanol Fig. 1 shows the production routes for first-generation bioethanol, which contains two types of feedstocks, i.e., the sucrosebased biomass and the starch-based biomass [20]. For the sucrose-based biomass, such as sugarcane or beet, the juices are first mechanically pressed from the cooked biomass and fractionationated. Then, yeasts metabolize the sucrose to ferment the hexoses into ethanol. Finally, the ethanol is separated and recovered by distillation. As for the starch-based biomass, the starch-crop grains such as corn, cereal, barley or wheat are pretreated by crushing and milling. Then, the meal of the starch crops is hydrolyzed enzymatically into hexose. The yeasts convert the mash hexose into ethanol biochemically. The hydrous ethanol is subsequently purified by distillation. In general, from the fermentation process forwards, the process is almost identical for both the starch-based route and the sucrose-based route. In general, the starch-based route consumes more energy than the sucrose-based route due to an additional step of converting of starch to glucose. Overall, using either sucrose or starch is a mature technology to which few significant improvements have been recently made. However, the development of these routes continues with stepby-step improvements and new technology solutions could still emerge now and then. Some improvements focus on the optimization of energy integration and finding value-added solutions for co-products deemed as “wastes” in existing production facilities. For example, the overall economic and environmental efficiency of starch-based processes are largely influenced by the value of coproducts such as dried distiller's grains with solubles (DDGS) and fructose. 2.1.2. Second-generation bioethanol The second-generation bioethanol is produced from lignocellulose that is composed mainly of cellulose, hemicellulose

Fig. 1. Conversion routes for sugar and starch feedstocks [20].

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Fig. 2. Bioethanol productions from lignocellulose via the biochemical route [20].

and lignin, which accounts for approximately 50% of the biomass in the world. The process diagram of the lignocellulose-to-ethanol via the biochemical process is shown in Fig. 2. General lignocellulosic materials for ethanol production are agricultural and forest residues such as corn stover, switchgrass, miscanthus grass species, and wood chips [21–23]. In addition, feedstocks based on the potential recovery of biomass from urban municipal solid waste (MSW) streams are also popular. As shown in Fig. 2, the lignocellulose-to-ethanol processes consist of three main steps,


Pretreatment of feedstocks: The process steps of feedstock




pretreatment include feedstock harvesting, handling, recovery and transport; size reduction; fractionation of the polymers; separation of the solid lignin component; and end product recovery Enzymatic hydrolysis: The biochemical-conversion step uses enzymes [24] to deconstruct the lignocellulose into its base polymers and break down cellulose and hemicellulose into monomeric sugars including glucose and xylose. Note that in




addition to the enzymatic hydrolysis, acid agents (concentrated or diluted acid) have been used in the hydrolysis steps. However, the acid hydrolysis suffers from equipment corrosion and low glucose yield; there is little research on this technology [25,26]. Fermentation: These monomeric sugars are then fermented into ethanol.

An overview of the status of the technology development of each process is given in Table 3. For example, combustion of process residues followed by the capture of some of the heat for onsite use and possibly power generation is one traditional solution, but this is often carried out inefficiently with disposal of the waste biomass. Following privatization of the power sector in many countries, the ability to export excess power to the grid has resulted in incentives for onsite construction of more efficient combined heat and power (CHP) systems that generate approximately 10 times the electricity needed onsite. In the design of large-scale bioethanol production plants, the waste lignocellulosic

W.-R. Chang et al. / Renewable and Sustainable Energy Reviews 67 (2017) 277–288

raw materials arising from the processing of sugarcane, corn, small cereal grains and so forth could possibly be better-utilized onsite as feedstock to produce additional ethanol. The cellulose undergoes enzymatic hydrolysis to produce hexoses such as glucose. Pentoses, mainly xylose, are produced from the hemicellulose, thereby fully utilizing the feedstock. 2.1.3. Social concerns The feedstocks of first-generation bioethanol are also used as food for humans and animals, such as sugarcane in Brazil and corn in the USA, which is usually not the case for the second-generation bioethanol. In fact, the use of first-generation bioethanol has led to a public debate about the ethical issues of using food for fuels [27– 30], which can harm food security, following a peak in agricultural commodity prices in 2007–2008. Although the analyses suggest that a combination of high oil prices, poor harvests and use of commodities by financial investors most likely had a considerably higher impact on food prices than biofuel production did [31], food security remains a critical topic for the design of sound biofuel policies. Nevertheless, it would only be reasonable to produce bioethanol from nonfood resources such as household waste, straw, corn-stoves and wood. These resources are often considered to be waste materials and are thus significantly less valuable than edible ones; not only will the use of these resources not interfere with food production, it will also greatly reduce GHGs and feedstock costs from the ethanol production [32]. Therefore, there is intense global research on developing second-generation bioethanol; however, only a few companies have reached the point of demonstrating the process in a pilot plant. In addition, there have also been concerns over the amount of fossil fuels used in the production of the bioethanol [33,34]. 2.2. Biodiesel Biodiesel is a renewable fuel manufactured from vegetable oils or animal fats for use in diesel-engine vehicles (DV). Chemically, it is comprised of a mix of mono-alkyl esters of long chain fatty acids, and has technically been defined as a mono-alkyl ester [35]. In general, biodiesel can be used in a standard diesel engine in contrast to the vegetable oils that have to be used in a fuel-converted diesel engine. It can be used alone, or blended with petrodiesel. Fig. 3 shows the process diagram of the production of biodiesel. The major production step is referred to as a transesterification reaction, which involves the reaction of methanol with the triglycerides of the base oil to form the corresponding fatty acid methyl esters (FAME) and glycerin as indicated in the following reaction scheme:

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resulting ester, at the cost of a less efficient transesterification reaction. Note that raw or refined plant oil, or recycled greases that have not been processed into biodiesel, are not biodiesel and should be avoided. Fats and oils (triglycerides) are much more viscous than biodiesel, and low-level vegetable oil blends can cause long-term engine deposits, ring sticking, lube-oil gelling, and other maintenance problems that can reduce engine life. 2.3. Bio-hydrogen Hydrogen herein is produced by reforming bioethanol [36,37]. As the products of biomass conversion are mainly hydrogen rich gases, they have been used in conventional internal combustion engines or gas turbine to provide power or heat. However, it would be more effective to use fuel cells to convert hydrogen energy into electricity efficiently, cleanly, and silently. It is anticipated that distributed reforming of biomass-derived liquid fuels could be commercial during the transition to hydrogen and used in the mid- and long-term time frames [38]. Biomass resources can be converted to ethanol, biodiesel, or other liquid fuels, which can be transported at relatively low cost to a refueling station or other point of use and reformed to produce hydrogen. Biomass-derived liquids, such as ethanol and biodiesel, can be produced at large, central facilities located near the biomass source to take advantage of economies of scale and reduce the cost of transporting the solid biomass feedstock. The liquids have a high energy density and can be transported with minimal new delivery infrastructure and at relatively low cost to distributed refueling stations or stationary power sites for reforming to hydrogen. The reaction pathways and thermodynamics of steam-ethanol reforming (SER) have been extensively studied recently [39–43]. Most processes converting ethanol into hydrogen involve a reaction with water in the steam-reforming reaction [44,45], which is very similar to reforming natural gas. The ethanol is reacted with steam at high temperatures in the presence of a catalyst to produce a reformate gas composed of mostly of hydrogen and carbon monoxide.

C2H5OH+H2O → 2CO + 4H2

HR = 260 kJ−mol−1

(3)

This reaction is strongly endothermic, requiring a flame to heat the reactor to the 800 °C necessary to achieve high conversion at residence times of one second. Then, the carbon monoxide created in the SER is further reacted with high-temperature steam to produce additional hydrogen via the water-gas shift (WGS) reaction.

(2)

In addition to methanol, ethanol can also be used to produce fatty acid ethyl ester (FAEE) biodiesel and higher alcohols such as isopropanol and butanol have also been used. Using alcohols of higher molecular weights improves the cold flow properties of the

CO+H2O → CO2 +H2

HR = –40 kJ−mol−1

(4)

In the above reactions, the WGS reaction generally goes to equilibrium at the high temperature required for oxidation

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Table 3 Status of each sub-process involved in biochemically converting lignocellulose to bioethanol. Sub-process

Objectives

Pretreatment







Fractionation Enzyme production

Enzymatic hydrolysis Hexose fermentation

Ethanol recovery Lignin recovery and applications

State of development


Demonstration/commercial Properly size the material
Needs optimization for difProduce ideal bulk density ferent feedstocks and Remove dirt and ash downstream processing Rapid depressurization to explode fiber.
Open the fiber structure Cyclone to separate solids from R&D vapors Cost and processing rate are Commercial -but needs furkey factors. ther cost reductions to reach USD 0.02–0.03/liter of ethanol. Produce C6 and C5 sugars. Reduce viscosity.
Standard yeast Commercial.
Pentose fermentation
Standard yeast is not suitable.
New microorganisms dictate yield and rate.
This affects feedstock demand/unit of product and capital expenditure on plant Distillation to obtain 99.5% ethanol. Separate lignin and other solids. Combust for heat and power or to produce biomaterial co-products

Waste treatment

Early demonstration Research/pilot plant moving towards commercialization

Commercial


Research/pilot plant
Co-products to improve economic performance. Research/commercial

reactions. This reaction has a favorable hydrogen equilibrium only at lower temperatures and with large amounts of water added. Finally, the hydrogen is separated out and purified. The energy efficiencies for hydrogen pathways from corn-ethanol reforming are summarized in Table 4.

3. Vehicle technologies Three types of biofuel vehicles are examined in the present study. The first one is the flexible-fuel vehicle, which is an alternative fuel vehicle with an internal combustion engine (ICE) designed to run on more than one fuel, usually gasoline blended with ethanol fuel, and both fuels are stored in the same common tank. The second one is the diesel vehicle running with biodiesel blend. The last one is the fuel cell vehicle, powered by the renewable hydrogen from biomass. 3.1. Flexible fuel vehicles (FFVs) FFVs are based on dual-fuel systems that supply both fuels into the combustion chamber at the same time in various calibrated proportions. The most common fuels used by FFVs are unleaded gasoline and ethanol fuel. As ethanol FFVs became commercially available during the late 1990s, the common use of the term “flexible-fuel vehicle” became synonymous with ethanol FFVs [46– 50]. In the USA, FFVs are also known as “E85 vehicles”. In Brazil, the FFVs are popularly known as “total flex” or simply “flex” cars. In Europe, FFVs are also known as “flexifuel” vehicles. Automakers, particularly in Brazil and the European market, use badging in their FFV models with the some variant of the word “flex”, such as Volvo Flexifuel, Volkswagen Total Flex, Chevrolet FlexPower or Renault Hi-Flex, Ford sells its Focus model in Europe as Flexifuel

and as Flex in Brazil. In the USA, only since 2008 did FFV models feature a yellow gas cap with the label “E85/Gasoline” written on the top of the cap to differentiate E85s from gasoline only models. In general, FFVs experience no loss in performance when operating on E85. However, because ethanol contains less energy density (by volume) than gasoline, FFVs typically obtain approximately 25–30% fewer miles per gallon (MPG) when fueled with E85 [51]. 3.2. Diesel vehicles (DVs) Diesel-powered vehicles typically obtain 30–35% more MPG than comparable gasoline vehicles. Diesel engines are inherently more energy efficient, and diesel fuel contains 10% more energy per gallon than gasoline. Biodiesel is most often used as a blend with regular diesel fuel, and can be used in many diesel vehicles without any engine modification. Therefore, biodiesel and conventional diesel vehicles are one in the same. The most common biodiesel blend is B20, which contains 20% biodiesel and 80% petroleum diesel. B20 is popular because it has a good balance of cost, emissions, coldweather performance, materials compatibility, and ability to act as a solvent. In addition, B20 and lower-level blends generally do not require engine modifications. Engines operating on B20 have similar fuel consumption, horsepower, and torque to engines running on petroleum diesel. Moreover, B20 has a higher cetane number (a measure of the ignition value of diesel fuel) and higher lubricity (the ability to lubricate fuel pumps and fuel injectors) than petroleum diesel. Biodiesel contains approximately 8% less energy per gallon than petroleum diesel. For B20, this could mean a 1–2% difference, but most B20 users report no noticeable difference in performance or fuel economy [20]. 3.3. Fuel cell vehicles (FCVs) A fuel cell vehicle is a type of alternative fuel vehicle that uses hydrogen and oxygen from the air to electrochemically produce electricity in fuel cells, powering its on-board electric motor. All fuel cells are made up of three parts, i.e., an electrolyte, an anode and a cathode [52,53]. In principle, a fuel cell works like a battery, producing electricity that can run an electric motor. However, it can be refilled with hydrogen instead of requiring recharging [54]. In addition, compared to the conventional GVs, a fuel cell vehicle emits few pollutants, producing mainly water and heat, although the production of hydrogen would create pollutants unless the hydrogen were produced using only renewable energy [55]. Although there are currently no fuel cell cars available for commercial sale, over thirty prototypes and demonstration cars have been released since 2009 [56]. Automobiles such as the GM Equinox, Honda FCX Clarity, Toyota FCV-R, and Mercedes-Benz F-Cell [57] are all pre-commercial examples of fuel cell vehicles. Actually, several of the car manufacturers have announced plans to introduce a production model of a fuel cell car. Toyota has stated that it plans to introduce FCVs at a price of around USD 50,000 [58] and Mercedes Benz announced in 2011 that it plans to move up the production of the Mercedes-Benz F-Cell to 2014 [59].

4. Methodology The present work employs the GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) code developed by Argonne National Laboratory to examine the lifecycle energy and GHG emissions of various biofuel/vehicle technologies. The details of the model have been described elsewhere [60–66] and are not elaborated on in this work. In the present model,

W.-R. Chang et al. / Renewable and Sustainable Energy Reviews 67 (2017) 277–288

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Fig. 3. Production pathway of biodiesel [20].

Table 4 Energy efficiencies for hydrogen pathways from corn-ethanol reforming. Items

Data

Refueling station production efficiency: EtOH as feedstock 67.5% Refueling stations compression efficiency for station produced GH2: NG 85.5% compressors Refueling stations compression efficiency for station produced GH2: 93.9% Electric Compressors

probability-based distribution functions are developed to describe energy use and emissions for individual operations in fuel production and transportation processes, as well as vehicle operations. The CO2-equivalent GHG emissions are calculated by considering the global warming potentials (GWPs) of three types of GHG emissions, i.e., 1 for CO2, 23 for CH4, and 296 for N2O, which are recommended by the Intergovernmental Panel on Climate Change (IPCC) for the 100-year time span [67]. Fig. 4 shows the scope of a well-to-wheels analysis for biofuel/ vehicle systems. The boundary of the present fuel-cycle model could be divided into two stages, i.e., the biomass-to-tank (BTT) stage and the tank-to-wheels (TTW) stage. The BTT stage starts with the feedstock production (farming, harvesting and transportation) and ends with biofuels available in the fuel tank of vehicles.

The TTW stage covers all vehicle-operation activities. Note that the present study considers the operation-related energy and emissions only. That is, the energy and emissions related to operational activities for the fuel process and vehicle are included. Those of infrastructure-related energy consumption and GHG emissions, such as energy and emissions associated with building roads, plants, and plant equipment, are not included for any of the pathways evaluated. There has been a vigorous debate about the extent to which biofuels lead to GHG reductions and about the emissions associated with land-use changes (LUC) caused by biofuel production [68–70]. When biofuel production involves LUC then there may be additional emission impacts, either positive or negative, that must be taken into account in calculating the GHG balance. The land-use change could be:


Direct, as when biofuel feedstocks are grown on land that was previously forest;


Indirect, when biofuel production displaces the production of other commodities, which are then produced on land converted elsewhere (domestic or foreign land). For biofuels to provide the envisaged emission reductions in the transportation sector, it is essential to avoid large releases of GHG caused by LUCs. However, emissions related to current

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Fig. 4. Scope of a biomass-to-wheels analysis for biofuel/vehicle systems.

biofuel production generate only approximately 1% of the total emissions caused by land-use change globally [71], most of which are produced by changes in land use for food and fodder production, or other reasons. Table 2 gives the assumptions of GHG emissions due to LUC of the investigated biofuels [72]. Several comprehensive and up-to-date analyses of the issues involved have been published elsewhere [71,73].

5. Results and discussion 5.1. Fuel economy Essentially, fuel economy is another type of TTW efficiency of a vehicle, which is a central factor in determining the BTW energy consumption and GHG emissions associated with a biofuel/vehicle system. In general, the fuel economy of a vehicle has three types, i.e., a city mode, a highway mode, and a combined mode [74]. The city mode means that a vehicle is started in the morning after being parked all night and driven in stop-and-go traffic in urban area. The highway mode is a mixture of rural and interstate highway driving in a warmed-up vehicle, representing a typical longer trip in free-flowing traffic. The combined-mode fuel economy is based on a combination of both of the aforementioned modes. Table 5 shows the fuel economies for four types of sedans, i.e., GV, FFV, DV and FCV. The GV serving as the baseline case for comparison is the Ford Focus FWD, which has a fuel economy of 31 mpg in the combined mode. To standardize the comparison, the flexible fuel vehicle selected for assessment was the Ford Focus FFV. It is clearly seen that the fuel economy for FFV is less than that of the GV type, typically approximately 25%. The DV selected for comparison was the Volkswagen Golf, which has a fuel economy of 34 mpg in the combined mode. As for the FCVs, they have not reached the mass market yet, and only a limited number is available for sale or lease to demonstration fleets in areas with a readily accessible hydrogen supply. In general, the fuel economy for FCVs is dependent on the automatic control [75], operating temperature [76–78], and many other operating characteristics [79–81]. In this work, the Honda Clarity [82] was selected for the lifecycle analysis. As shown in Table 5, the fuel economy for the FCV is considerably higher than that of the GV because the energy efficiency of the fuel cell engine is significantly higher than that of the internal combustion engine. 5.2. Biomass-to-tank efficiency Fig. 5(a) and (b) show the energy intensity (Btu/mmBtu-biofuel) and the carbon intensity (gGHG/mmBtu-biofuel) of various biofuels, respectively. As shown in this figure all biofuels have

higher energy intensities than conventional gasoline. The energy intensity of E85, blended with either corn ethanol or switchgrass ethanol, is higher than that of E10. In addition, E85 blended with corn ethanol consumes more energy than that of E85 blended with switchgrass ethanol. This is because the corn-to-ethanol process consumes more energy than the switchgrass-to-ethanol process in feedstock and fuel processes. As for the carbon intensity, the negative value for E85 blended with switchgrass ethanol means that the feedstock process of switchgrass could capture the carbon. The highest carbon intensity for bio-hydrogen from cornethanol reforming is of course caused by its highest energy intensity. Fig. 6 further shows the BTT efficiency of the six different biofuel-pathways. The well-to-tank (WTT) efficiency for the fuel pathways from petroleum to gasoline serves as the baseline case for comparison. As shown in this figure, the BTT efficiencies for all biofuel pathways are lower than the WTT of the gasoline from a petroleum refinery. Among the six biofuel-pathways, switchgrass ethanol for E10 has the highest BTT efficiency (78.7%), while the bio-hydrogen from corn ethanol has the lowest BTT efficiency (26.8%). Two inefficient processes, corn-to-ethanol fermentation and hydrogen production from corn-ethanol reforming, are the subjects of the lowest BTT efficiency. 5.3. Stage and total energy consumption and GHG emissions Fig. 7(a) and (b) show the energy consumption and GHG emissions for different stages in the lifecycle of the biofuel/vehicle systems, respectively, which includes the feedstock, fuel, and vehicle operation stages. The overall energy consumption and GHG emissions are also displayed by purple symbols in the plot. To place on the common basis, the energy consumption and GHG emission in the following discussion are based on the per unit travel distance of vehicles. As shown in Fig. 7(a), the FFVs fueled by E85 blended with corn ethanol have the most significant energy consumption in both the fuel stage and the vehicle-operation stage, resulting in the largest total energy consumption (up to 11,126 Btu-mile  1). Basically, the energy analysis of the corn ethanol is based on the energy consumption in the biomass harvest and the energy consumption of the ethanol process. The energy consumption in the feedstock stage accounts for the energy required for farming and processing corn into ethanol in addition to the energy in the corn kernels, which results in a significant total energy use for ethanol production. The DV/B20 derived from soybean biomass has a slightly higher energy consumption than the GV/gasoline during the combined feedstock and fuel stages. Fortunately, the higher fuel economy for the DV/B20 has reduced its total energy consumption. As for the FCV/H2, its excellent fuel economy (i.e., TTW efficiency) can offset the significant energy

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285

Table 5 Fuel economies of various types of vehicles. Type

Brand

Fuel Economy, MPGa

Photo

City/Hwy

Combined Mode

Gasoline Vehicle

Ford Focus FWD

27/38

31

Flexible Fuel Vehicle

Ford Focus FFV

20/28

23

Diesel Vehicle

Volkswagen Golf

30/42

34

Fuel Cell Vehicle

Honda Clarity

60/60

60

a

Fuel Economy Guide 2013, EERE, US DOE US EPA.

Fig. 6. Biomass-to-tank efficiency for various fuel pathways.

Fig. 5. Energy intensity and carbon intensity of various biofuel pathways. (a) Energy Intensity. (b) Carbon Intensity.

consumed by the fuel processes in the combination of fermentation and reforming. The total energy consumption of FCV/H2 is thus higher than that of the GV/gasoline. Attention is now turning to the results of stage GHG emissions for various biofuel/vehicle systems shown in Fig. 7(b). Although, the FFV fueled by E85 blended with corn ethanol has a significant GHG emission in the vehicle-operation stage, the carbon could be captured significantly during the feedstock stage, which would compensate the GHG emissions during the vehicle-operation stage. Consequently, the FFV fueled by E85 blended with corn ethanol has the lowest total GHG emissions among the various biofuel/vehicle systems. In addition, the FCV/H2 does not emit any GHGs during the vehicle-operation stage and thus the fuel stage covers all GHG emissions. Basically, the fuel process for corn ethanol consumes an amount of fossil fuels (resulting in GHG

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Fig. 7. Stage energy consumption and GHG emission for various biofuel/vehicle systems, (a) energy consumption and (b) GHG emissions. (a) Stage energy consumption. (b) Stage GHG emissions.

emissions) and the cornfields produce a large amount of N2O emissions from nitrogen nitrification and de-nitrification as well. As a result, the FCVs fueled with the bio-hydrogen derived from corn ethanol are the second best in the GHG emission reduction among the various biofuel/vehicle systems. 5.4. Fossil fuel and petroleum uses Fig. 8 illustrates the effect of replacing the GV/gasoline with various biofuel/vehicle systems on oil dependence, which is shown by the reduction rates in fossil fuel and petroleum compared to the GV/gasoline in Fig. 8(a) and (b), respectively. As shown in Fig. 8(a), all biomass-derived fuels discussed here offer a certain reduction (5.0–68.5%) in fossil energy use, which result from the fact that the four renewable fuel options are a nonfossil feedstock. Among them, the FFV fueled with E85 blended with switchgrass ethanol could offer the largest reduction in fossil-fuel use, a 68.5% reduction. It is further shown in Fig. 8(b) that except for GV/E10, all other biofuel/vehicle systems offer significant petroleum savings. Petroleum energy used in the biomass-based fuel cycle comes entirely from the BTT stage, primarily from diesel fuel used for farming equipment and for the trucks and locomotives needed to transport feedstock and fuel. Like fossil energy use, the petroleum use associated with FFV/E85, DV/B20 and FCV/H2 is lower than that of GV/CG. Although the soybean diesel has a large amount of co-products that are assumed to replace petroleum fuels, the DV/ B20 provides less petroleum saving credits than the FFV/E85. However, the ethanol blended with 90% gasoline in volume (GV/

Fig. 8. Fossil fuel and petroleum reduction rate for various biofuel/vehicle systems (a) fossil fuel, and (b) petroleum. (a) Fossil fuel. (b) Petroleum.

E10) has a negligible effect on the petroleum reduction rate compared to GV/CG, from either corn or switchgrass. In contrast, FFV/E85 could reduce the petroleum use by over fifty percent. As for the FCV/H2 from corn ethanol, the petroleum reduction rate can reach as high as 94.7% because there is no use of petroleum in the TTW stage. 5.5. Relative change in total energy and GHG emissions Fig. 9 shows the relative changes in the total energy consumption and GHG emissions for various biofuel/vehicle systems relative to those of GV/gasoline systems. As clearly shown in Fig. 9, except for the DV/B20, all biofuel/vehicle consume more energy than the GVs. The DV/B20 could reduce total energy consumption by 10.1% compared to the conventional GV/gasoline. In contrast, the total energy consumption for the FFVs fueled with E85 blended with corn and switchgrass ethanol are more that double that which is consumed by the conventional GVs, typically approximately 121.1% and 101.3% for corn ethanol and switchgrass, respectively. As for the GHG emissions, however, the FFVs fueled with switchgrass-blended E85 can reduce emissions by more than 59% because the co-products are used to produce heat and power, replacing fossil fuels. It also noted that the FCVs fueled with biohydrogen from corn ethanol reforming have the benefit of reduction in the GHG emission by 31.1%, but suffer from the increase in the total energy by 37.4% compared with the GVs. 6. Conclusion This paper has carried out an assessment of lifecycle energy

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References

Fig. 9. Relative change in total energy and GHG emissions for various biofuel/vehicle systems, (a) total energy and (b) GHG emissions.

consumption and GHG emission of various biofuel vehicles. Several biofuel options for transportation are discussed, including bioethanol from corn and switchgrass, biodiesel from soybeans, and bio-hydrogen from corn ethanol. The biofuel-to-wheel (BTW) efficiencies for various vehicular systems are compared with that of the conventional GVs. Major conclusions from the lifecycle assessment research presented here are described below. 1. FFVs/E85 blended with switchgrass ethanol show the greatest capability in minimizing GHG emissions. 2. FFVs/E85 blended with corn ethanol is not recommended because of significant energy consumption and GHG emissions in the feedstock stage, which is even higher than those of the GVs. 3. FCVs fueled with the bio-hydrogen from corn-ethanol reforming offer a low GHG emission but suffers from a significant energy consumption. 4. Even with an amount of processed fossil fuels is required during biomass farming and alternative fuel production processes, the four renewable fuel/vehicle options can still achieve a certain extent of reduction in fossil energy and petroleum usage. Actually, there are many research for production of biofuels from various sources has not been completely executed in the field. The authors will conduct the experiments and detailed study in future work for the benefit of the society.

Acknowledgments The author Jenn-Jiang Hwang would like to thank the National Science Council of Taiwan, for financially supporting this research under contract number 100-2221-E-024-014-MY2.

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