The history, genotoxicity and carcinogenicity of carbon-based fuels and their emissions: part 4 - alternative fuels.

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MUTREV-8085; No. of Pages 17 Mutation Research xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Mutation Research/Reviews in Mutation Research journal homepage: www.elsevier.com/locate/reviewsmr Community address: www.elsevier.com/locate/mutres

Review

The history, genotoxicity and carcinogenicity of carbon-based fuels and their emissions: Part 4 – Alternative fuels Larry D. Claxton * LDC Scientific Services, 6012 Brass Lantern Court, Raleigh, NC 27606, United States

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 May 2014 Received in revised form 27 June 2014 Accepted 28 June 2014 Available online xxx

Much progress has been made in reducing the pollutants emitted from various combustors (including diesel engines and power plants) by the use of alternative fuels; however, much more progress is needed. Not only must researchers improve fuels and combustors, but also there is a need to improve the toxicology testing and analytical chemistry methods associated with these complex mixtures. Emissions from many alternative carbonaceous fuels are mutagenic and carcinogenic. Depending on their source and derivation, alternative carbonaceous fuels before combustion may or may not be genotoxic; however, in order to know their genotoxicity, appropriate chemical analysis and/or bioassay must be performed. Newly developed fuels and combustors must be tested to determine if they provide a public health advantage over existing technologies – including what tradeoffs can be expected (e.g., decreasing levels of PAHs versus increasing levels of NOx and possibly nitroarenes in ambient air). Another need is to improve exposure estimations which presently are a weak link in doing risk analyses. ß 2014 Elsevier B.V. All rights reserved.

Keywords: Alternative fuels Genetic toxicology Carbonaceous fuels Emissions Health Cancer

Contents 1.

2. 3. 4. 5. 6. 7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alternative types of vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Conventional internal combustion engine vehicles (conventional ICEVs) . . . . . . 1.1.1. Conventional hybrid electric vehicles (HEVs). . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2. Plug-in electric vehicles (PHEVs and BEVs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3. Types of alternative fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Conventional alternative fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1. Other biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2. Coal-derived products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3. Shale oil or kerogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4. Genetic toxicology of biofuels: carcinogenicity/mutagenicity of the raw (unprocessed) product The carcinogenicity and mutagenicity of biofuel emissions – at the point of release . . . . . . . . . Synfuel and related products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Electricity – the other alternative? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of emission controls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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* Tel.: +1 919 839 8978. E-mail addresses: [email protected], [email protected], [email protected] http://dx.doi.org/10.1016/j.mrrev.2014.06.003 1383-5742/ß 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: L.D. Claxton, The history, genotoxicity and carcinogenicity of carbon-based fuels and their emissions: Part 4 – Alternative fuels, Mutat. Res.: Rev. Mutat. Res. (2014), http://dx.doi.org/10.1016/j.mrrev.2014.06.003

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1. Introduction Before the availability of inexpensive fossil fuels, our society depended on plant biomass to meet its energy needs. The discovery of crude oil, in the 19th century, produced an inexpensive liquid fuel source that aided in the industrialization of the world and the improvement of living standards. In the mid-1800s, biomass supplied more than 90% of U.S. energy and fuel needs. Today, because of declining petroleum reserves, increased demand for petroleum, political and environmental concerns, countries feel that it is important to develop other economical and energyefficient sources to supplement or replace fossil fuels. Huber et al. [1] informs us that the U.S. could produce 1.3 109 metric tons of dry biomass/year using its agricultural and forest resources and still meet its food, feed, and export demands. While the U.S. consumes 7 109 barrels of oil/year, this alternative biomass has the energy content equivalent to 3.8 109 boe (barrels of oil energy equivalent). Many developing countries use biomass as a primary energy source, and other more developed countries use biomass to meet a significant percentage of their energy demands (Sweden, 17.5%; Finland, 20.4%; and Brazil, 23.4%). The laws of supply-and-demand economics forecast that as petroleum reserves dwindle, the price of petroleum products will increase, and biofuels eventually will be cost-competitive or even cheaper than petroleum-derived fuels. CO2, H2O, light, air, and nutrients are the components needed for biofuel production. In addition, energy to power the vehicles needed for producing and distributing the biomass-derived fuels is needed. The effect on food production also must be considered. Biomass (produced not only on agricultural land but also on forest, aquatic, and arid lands) provides a wide range of starting chemical constituents. Biotechnology and genetic engineering along with classical plant breeding is expected to deliver more plants with faster growth rates, and plants that require less energy input. In an editorial for Science, Koonin [2] reasons: ‘‘Credible studies show that with plausible technology developments, biofuels could supply some 30% of global demand in an environmentally responsible manner without affecting food production. To realize that goal, so-called advanced biofuels must be developed from dedicated energy crops, separately and distinctly from food. . . . There are major technological challenges in realizing these goals. . . . The combination of modern breeding and transgenic techniques should result in achievements greater than those of the Green Revolution in food crops, and in far less time.’’ To generate biomass fuels, the process usually can be described as three steps: (1) production of the biomass, (2) processing of the biomass, and (3) use of the biomass fuel [3]. For example: vegetable oil plants (rapeseed, soy, palm, sunflower, etc.) are: (1) harvested, (2) extracted and processed, and (3) the bio-oils are used for diesel fuels and in the cogeneration of electricity. For biodiesel, the plant materials not only have to be extracted but also esterified and purified. For bioethanol, the plant materials undergo fermentation and distillation. Biogases usually are produced from manure, biowaste, and corn, which undergo anaerobic digestion with the produced gas used for the cogeneration of electricity. See the reviews by Huber et al. [1], Leal et al. [4], Tsai and Wu [5], Kerckhoffs and Renquist [6], Philp et al. [7], and Azadi et al. [8] for more information on the science of biomass production. Biofuels can be thought of as either primary or secondary biofuels, and secondary biofuels may be subdivided into first-, second-, or thirdgeneration fuels (Table 1). In spite of increases in cost and with efforts to reduce pollution, crude oil remains the raw material for the production of today’s fuels because crude oil production and distribution has kept pace with demand. In addition, world reserves have actually expanded as a result of ongoing technological progress [11]. Internal

combustion engines (ICEs) operating with gasoline and diesel fuels have powered almost all light-duty vehicles (LDVs) for a century. The dominance of LDVs using petroleum fuels instead of steam and batteries has been due to petroleum fuels continuing to have low cost, high energy density, a good distribution system, and the ability to operate for long distances in a wide range of environmental conditions [11]. Goldemberg [12] reported that the British Petroleum Statistical Review of World Energy showed that ‘‘at constant production and consumption, the presently known reserves of oil will last around 41 years, natural gas 64 years, and coal 155 years.’’ They also reported that in 2007 the world energy use was distributed as follows: oil, 35.03%; coal, 24.59%; gas, 20.44%; traditional biomass, 8.48%; nuclear, 6.33%; modern biomass, 1.91%; solar, 0.53%; small hydroelectric, 0.41%; wind, 0.32%; geothermal, 0.23%; hydro and other renewables, 1.73%. Therefore, fossil fuels (oil, coal, and gas) represent 80.1% of the total world energy supply. The most likely alternatives to fossil fuels are renewable sources such as hydroelectric, biomass, wind, solar, geothermal, and marine tidal [12]. In 2012, the revival of shale gas meant that the US had the largest increase in oil production outside Organization of Petroleum Exporting Countries (OPEC) for the third year in a row [13]. Renewable energy for power generation rose by 17.7% and was driven by wind energy (+25.8%) which accounted for more than half of the renewable power generation for the first time, with the US and China showing the largest increments [13]. Because of the Fukushima Daiichi nuclear power plant accident in Japan on March 11, 2011 (which triggered a magnitude 9.0 earthquake, caused a tsunami, and caused a month-long discharge of radioactive materials into the atmosphere), world nuclear power generation declined by 4.3% worldwide. Japanese nuclear output declined by 44.3% [13]. Disaster reignited the international debate on the future of nuclear energy [14]. In Germany, the federal government decided to temporarily shut down the old-generation nuclear reactors and reexamine the safety of all national nuclear power facilities [14]. German output from nuclear power plants fell by 23.2% [13]. The LDV fleet is responsible for about half the petroleum consumed in the United States [11]. Many technologies, with widely varying levels of current capability, cost, and commercialization, can reduce LDV petroleum consumption. However, any transition efforts aimed at extensive reduction in petroleum utilization are likely to take decades. The demand for biofuels is partly driven by the need to replace fossil fuels for: (1) cost, reducing any dependence on higher-priced fossil fuels; (2) environmental concerns, lessening the impacts that come from the use of non-renewable resources; and (3) political concerns, having less dependence upon nations that are of political concern. Present national goals being set are so ambitious that Europe and the U.S. are unlikely to produce enough biofuels to meet the targets being set by developed countries. Because biofuels are from renewable resources, research may not examine the potential health effects of these fuels adequately. Initially, programs will depend upon first generation biofuel crops (food crops such as maize, sugarcane, soybeans, rapeseed, and palm oil). Second generation biofuel crops (from nonfood crops such as wood chips and switch grasses), while competing with food crops for land and water are likely to have even less research aimed at health effects (e.g., emissions from their use). The National Research Council’s Committee on Transitions to Alternative Vehicles and Fuels [11] recent analysis (while exploratory and having significant uncertainty) indicated that the costs and benefits of replacing petroleum consumption with alternative fuels will be substantial, and these analyses also suggested that policy will play a major role in achieving these reductions [11]. Many biological-source fuels will undergo either biochemical or thermo-chemical conversion when being produced. Biochemical conversion uses enzymes and/or


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Table 1 Categories of alternative fuels. Category

Method used to categorize

Examples

Primary biofuels

Unprocessed fuels primarily for heating, cooking or electricity production Fuels produced by the processing of biomass and are able to be used in vehicles and various industrial processes. Secondary biofuels can be categorized into three generations on the basis of different parameters, such as the type of processing technology, type of feedstock, or their level of development Conventional technologies using conventional sources

Firewood, wood chips, pellets, animal waste, forest and crop residues, landfill gas

Secondary biofuels

Secondary biofuels: 1st generation

Secondary biofuels: 2nd generation

Conventional technologies but this generation is based on novel starch, oil, and sugar crops

Secondary biofuels: 3rd generation

Use of microbiological or newer methods to produce fuels

Natural gas products

Natural gas is a mixture of low molecular weight hydrocarbons with methane (CH4) the main one. The advantages of natural gas are: it is clean burning, and it is commercially availability to end users. Because of its volatility it must be stored either in a compressed gaseous state (CNG) or a liquefied state (LNG). Propane, also known as liquefied petroleum gas (LPG) has been used (in light and medium-duty vehicles) for over 60 years. Propane usually is a by-product of natural gas processing and crude oil refining. Synfuel, synthetic fuel, is any liquid fuel derived from coal or from natural gas P-series fuels are blends of natural gas liquids, ethanol, and methyltetrahydrofuran (MeTHF). P-series fuels are clear, colorless, liquid blends designed to be used alone or mixed with gasoline in any proportion. These fuels are not currently being produced in large quantities and are not widely used Synfuel, synthetic fuel, is any liquid fuel obtained from coal or from natural gas Coal energy converted to electricity Electricity produced from some form of NG, hydroelectric sources, wind, wave energy, etc. Electrical energy stored in batteries or capacitors (modern capacitors are electrochemical capacitors or ECs (also called supercapacitors or ultracapacitors) which like other capacitors physically store charge. Conventional capacitors store charge on low-surface-area plates, but ECs store charge in an electric double layer set up by ions at the interface between a high-surface area carbon electrode and a liquid electrolyte) Uses sun energy collector cells Hydrogen is a clean energy that can be found everywhere. The technology needed to use it is neither simple nor cheap; however, manufacturers are releasing the first hydrogen car models Steam: external combustion heat engines based upon Rankine’s cycle. Steam engines burn a fuel which produces heat which is used to raise the temperature of the working fluid (e.g., water) in a confined space, which increases the pressure, and exerts a force against a piston in the engine Steam engines generally use less volatile fuels

P-series fuels

Coal products (including synfuels) Electricity: coal generated Electricity: non-carbonaceous types of generation Electricity: battery powera

Solar energya Hydrogena

Other types of alternative fuelsa

Fermentation of starch (e.g., wheat, barley) or sugars (e.g., sugarcane, sugar beet, etc.) to yield bioethanol and butanol. Transesterification of oil crops (rapeseed, soybeans, palm, coconut, used cooking oil, etc.) to give biodiesel Bioethanol and biodiesel produced from conventional technologies but based on novel starch, oil and sugar crops such as Jatropha or Miscanthus Using lignocellulosic materials (e.g., straw, wood, and grass) to produce bioethanol or biobutanol Microalgae to give biodiesel, bioethanol, or hydrogen Seaweeds to give bioethanol Microbes to give hydrogen Liquid natural gas (LNG); compressed natural gas (CNG); synthetic natural gas (SNG); propane (liquefied petroleum gas or LPG)

P-series fuels

Synfuel Electricity Electricity Electricity

Solar panels Hydrogen fuel and engines

Mechanical energy or electricity

Based on information provided by Dragone et al. [9] and Colomar et al. [10]. a Not reviewed in this series but given to make information in this table more complete.

microorganisms to break down lignocellulose into base polymers, and then into monomeric sugars including glucose and xylose, which can be fermented into ethanol. Thermo-chemical conversion exposes lignocellulosic biomass to severe heat in the presence of air or oxygen to make a synthetic gas, which is cleaned and used as a chemical building block to make a range of fuels [3]. In 2006, the staff at Popular Mechanics compared the cost and the fuel needed to travel from New York to California (Table 2) using gasoline and some alternative fuels [15,16]. The results are

very instructive. First, it is possible to develop automobiles (and other vehicles and engines) that use alternative fuels. Next, because of the energy content of each fuel, different mpg or mpge (miles per gallon of gasoline equivalent) would result from the use of each type of fuel. With the source of each fuel being different, cost for a fuel will vary. Since the projections were based on 2006 values, costs in the present market would be expected to be different. Table 2 does not factor in other factors such as the cost of the automobile, availability and the distribution of the fuel,


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Table 2 Popular Mechanics 2006 virtual trip (3000 miles) across the United States using gasoline and alternative fuels [15,16]. Type of fuel and automobilea

Mpg or mpgeb, fuel needed

Approximation of what starting material would be used

Fuel cost per trip in 2006 U.S. dollars, approximate fuel cost/mile (percent cost relative to a gasoline engine)

Gasoline – 87 octane, 2006 Honda Civic Hydrogen fuel, GM HY-Wire M85/methanol, 1998 Tauris M85 FFV

33 mpg, 90.9 gal 41 mpge, 73 gge 14 mpg, 214 gal

$212.70, $0.071 (100) $804.00, $0.268 (377) $619.00, $0.206 (290)

E85/ethanol, 2005 Tauris FFV

17 mpg, 176 gal

B100/biodiesel, 2006 Golf TDI CNGc, Honda Civic GX Electricity, 1997 Honda EV Plus

44 mpg, 68.2 gal 32 mpgeb, 88 gge 202 mpge, 16.4 gge

4.5 barrels of crude oil 16,000 ft3 of hydrogen 18,190 ft3 of natural gas and half barrel crude oild 53 bushels of corn and half barrel of crude oild 16.5 gal of used vegetable oil 10,650 ft3 of natural gas 1 ton of coal

$425.00, $0.142 (203) $231.00, $0.077 (108) $110.00, $0.037 (52) $60.00, $0.02 (28)

The data for this table was summarized and calculated from the supplemental material supplied with the main article [16]. a Cars chosen for this virtual cross-country trip were as close in size and weight as possible. b To compare the alternative fuels, the energy content in gallon of gasoline equivalents, gge (the amount of fuel with the same energy content as a gallon of gasoline), was calculated. c Compressed natural gas. d The half barrel of crude oil is because there is a need to add 15% of a petroleum product that would help in the starting of the automobile.

changes in the cost of a fuel over time, differences due to regional availability, etc. 1.1. Alternative types of vehicles The National Academy of Sciences (NAS) [11] said that the different types of vehicles that can be expected to conserve energy include: (1) more efficient internal combustion engine vehicles (ICEVs), (2) hybrid electric vehicles (HEVs), (3) plug-in hybrid electric vehicles (PHEVs), (3) battery electric vehicles (BEVs), (4) fuel cell electric vehicles (FCEVs), and (5) compressed natural gas vehicles (CNGVs). Plug-in electric vehicles (PEVs) include BEVs and PHEVs collectively. ICEVs, HEVs, and PHEVs engines can use fuels produced from petroleum, biomass, natural gas (NG), or coal; while, BEVs, FCEVs, and CNGVs only operate on their specific fuel. For all types of vehicles, there are some important crosscutting issues (e.g., weight reduction and improvements in aerodynamic resistance). 1.1.1. Conventional internal combustion engine vehicles (conventional ICEVs) There are multiple technological methods for improving the efficiencies of ICEVs. Included in these improvements are direct injection systems (better fuel vaporization), turbocharging (increasing torque and power output and allowing engine downsizing), friction reduction, increasing transmission efficiency, and vehicle weight reduction. A NAS report [11] says that over the past twenty years, reductions in engine friction has occurred at the rate of about 1% per year. Even greater reductions, however, are possible. For example, ‘‘laser texturing can etch a microtopograph on material surfaces to guide lubricant flow’’ [11]. EPA apportioned the energy losses and efficiencies to engine thermal efficiency, friction, pumping losses, transmission efficiency, torque converter losses, and accessory losses [11]. Today’s diesels are about 15–20% more efficient than gasoline engines; however, the efficiency advantage of the diesel will decrease in the future as gasoline engines improve [11]. Current diesels have a much higher level of technology than gasoline engines in order to deal with diesel drivability, noise, smell, and emission concerns. Diesel engines commonly have direct fuel injection, sophisticated turbocharging systems, and cooled exhaust gas recirculation (EGR) systems. As this same level of technology becomes common in the gasoline engine, the efficiency advantage of the diesel engine will decline. In addition, future development of the homogenous charge compression ignition engines will blur the distinction between gasoline and diesel engines.

1.1.2. Conventional hybrid electric vehicles (HEVs) HEVs combine an ICE, electric motor(s), and a battery (or ultracapacitor). Most HEVs have a ‘‘stop–start’’ system that shuts off the engine when idling and restarts it rapidly when the accelerator is depressed. Such hybrids need a higher capacity battery and starter motor than ICEVs. Stop–start systems are rapidly growing and are likely to be universal by 2030 [11]. Other approaches are: (1) having two electric machines connected via a planetary gearset to the engine and the powertrain, (2) having waste heat recovery systems, (3) having components that are more efficient (improved designs and control strategies), and (4) improving drivetrains. 1.1.3. Plug-in electric vehicles (PHEVs and BEVs) HEVs, which include PHEVs and BEVs, rely on battery power for propulsion. These types of vehicles are in production by some manufacturers, and other manufacturers are introducing electric vehicles over the next several years. Improvements in battery technology will be critical to the success of electric vehicles. PHEVs can travel up to 40 miles on electricity. When driven beyond the charge depletion mode of the first 40 miles, the vehicles operate as conventional hybrid vehicles. A BEV has no combustion engine, a significant cost savings relative to PHEVs. However, currently a SUV might require 100 kWh for a range of 200 miles. In the future, a battery of 78 kWh of available energy would give a range of 300 miles. Unfortunately, current technology and costs make this prohibitively expensive, heavy, and bulky for most applications. In addition, the unit cost of batteries is expected to decline with increased production and development; and the energy storage (in kilowatt-hours) required for a given vehicle range will decline with vehicle load reduction and improved electrical component efficiency. Several advanced battery technologies (e.g., lithiumair) are being developed that would address some of the drawbacks of lithium-ion batteries, but their potential for commercialization by 2050 is still highly uncertain [11]. If the use of petroleum is greatly reduced, the retirement of crude oil production and a decline in distribution infrastructure will follow. With production and distribution infrastructure being changed, much of the distribution systems and filling stations will become obsolete, and this could have drastic effects on the economy [11]. Large increases in fuel economy are possible; therefore, a NAS report says that manufacturers will need incentives or regulatory standards or both to apply widely the new technologies [11]. For example, as of February 2010, the United States had approximately 136 million cars, 110 million trucks, and 1 million buses (247,000,000 registered road vehicles).


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There were 159,006 ‘‘retail gasoline outlets.’’ This gave 1553 vehicles/gasoline outlet [11]. At the same time, the United States had only 1327 NG filling stations – with the majority for CNG (only 60 of these were for LNG). For NG vehicles, a NAS report estimates that approximately 19 million vehicles could be fueled by CNG without significantly affecting the price and supply of domestic NG [11]. In addition, electric vehicles estimates ranged up to approximately 10 million total vehicles on the road; however, this value is highly speculative. Hydrogen vehicles may number 2 million vehicles on the road by 2025. This discussion is important because nearly 160 thousand older gasoline sites will need monitoring due to health concerns and nearly an equal number of new sites may represent new or additional health concerns. 1.2. Types of alternative fuels Remembering that alternative fuels include alcohols, hydrogen, compressed or liquid natural gas, and gasoline and diesel derived from coal, NG, biomass, electric power, and fuels derived from crude oil or unconventional oils, most think of biofuels when considering alternative fuels. Biofuels are alternative fuels produced from vegetable oils and animal fats. The term ‘‘Conventional biofuel’’ usually refers to ethanol derived from starch of corn grain (corn-grain ethanol). The term ‘‘Advanced biofuels’’ (newly applied biofuels) usually refers to renewable fuels other than corn-grain ethanol and includes types of biofuels derived from such renewable biomass such as cellulose, hemicellulose, lignin, sugar, or any other starch that is not from corn, biomass-based diesel, and coprocessed renewable diesel. Biofuels normally consist of the monoalkyl esters formed by a catalyzed reaction of the triglycerides in the oil or fat with a simple monohydric alcohol. The reaction conditions generally involve a trade-off between reaction time and temperature. Much of the complexity of the process originates from contaminants in the feedstock (and found as impurities in the final product). Developed processes that produce biofuels from high free fatty acid feedstocks include starting materials such as recycled restaurant grease, animal fats, and soap stocks. Huber et al. [1] provide a useful review of the chemistry, catalysts, and engineering associated with the synthesis of transportation fuels from biomass. They show, for example, that the conversion from cellulosic biomass (e.g., wood, crop residues, grasses, sugar cane, water hyacinth, etc.) can proceed along any of three paths. The first path is gasification, which produces Syngas, which in turn can be used to produce alkanes, methanol, and hydrogen. The second path is pyrolysis of biomass materials to produce bio-oils (e.g., aromatics, tars, alcohols, aldehydes) which supplement liquid fuels. The third path uses hydrolysis, which produces aqueous sugars and lignin. The aqueous sugars can be fermented to yield ethanol, can be dehydrogenated to provide aromatic hydrocarbons, or go through other processing to give liquid alkanes or hydrogen. A National Research Council (NRC) report [11] gives the potential for reducing petroleum consumption and reducing GHG emissions by the U.S. LDV fleet by 80% by 2050. This report examines the technologies that could achieve these two goals and the barriers that might hinder their adoption. Four pathways could contribute to attaining both goals: (1) highly efficient ICEs, (2) vehicles operating on biofuels, (3) vehicles operating on electricity, or (4) vehicles operating on hydrogen. The report acknowledges that NG vehicles could contribute to the goal of reducing petroleum consumption. The writers of this document said, ‘‘Driving costs per mile will be lower, especially for vehicles powered by NG or electricity, but vehicle cost is likely to be a significant issue for consumers for at least a decade.’’ Because it is impossible to know which technologies will succeed, regulatory,

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industry, and other groups must establish policies that are ‘‘broad, robust, and adaptive’’ [11]. This is especially true for policies concerning human health. The highest gains are expected when the first goal (efficient ICEs) is combined with the other goals (e.g., use of an alternative fuels) and additional goals (e.g., load or weight reduction). Making any goals for reducing dependence on petroleum fuels harder to achieve is the expectation that the number of LDVs and the vehicle miles traveled (VMT) will nearly double from 2005 to 2050 [11]. Direct replacement biofuels (direct replacements for gasoline or diesel fuels) produced from some type of biomass could lead to large reductions in petroleum use and emissions. While they can be introduced without major changes in the fuel-delivery infrastructure or engine design, the achievable production levels are uncertain [11]. The petroleum fuels, gasoline and diesel, would almost be eliminated from the fuel mix when the U.S. petroleum goal to reduce dependence upon petroleum fuels by 80% by 2050 is reached. LDVs account for almost half of petroleum use in the United States, and about half of that fuel is imported [17]. Biofuel production is expected to increase because of the Renewable Fuel Standard 2 (RFS2) passed as part of the 2007 Energy Independence and Security Act (EISA) [11]. 1.2.1. Conventional alternative fuels 1.2.1.1. Ethanol. Ethanol is regarded as a primary conventional fuel. Ethanol (C2H5OH), a volatile liquid fuel used to replace refined petroleum, can be obtained from many feedstocks including cereals, sugarcane, and sugarbeets plus cellulose materials, namely, wood and vegetable remnants [18]. One advantage of ethanol is that it can lower the concentration of aromatic products found in high octane gasolines [18]. Many researchers and engineers consider ethanol as a less than optimal biofuel because it is corrosive and extremely hygroscopic. In addition, ethanol creates many problems such as its transport and incorporation in gasoline. The power energy of this molecule is insufficient to provide the energy needed to fly an airplane. Its low energy yield is because ethanol is already a partially oxidized derivative of carbon. Because the European vehicle market is very focused on diesel engines, this molecule represents only a moderate interest there [19]. The largest effort to employ ethanol is a Brazilian program, started in the 1970s [12]. See the Goldemberg paper [12] described above for more information. The ‘‘new renewable energy sources’’ amount to 16 exajoules (1 EJ = 1018 J), or 3.4% of the total. Renewables are also more labor intensive, requiring more workforces per unit of energy than conventional fossil fuels (3). Although technologically mature, some of the renewable sources of energy are more expensive than energy produced from fossil fuels. This is particularly the case for the ‘‘new renewables.’’ However, a simple calculation shows that expanding the Brazilian ethanol program by a factor of 10 (i.e., an additional 30 million hectares of sugarcane in Brazil and in other countries) would supply enough ethanol to replace 10% of the gasoline used in the world. This land area is a small fraction of the more than 1 billion hectares of primary crops already harvested on the planet. Conversion to ethanol does allow the phasing-out of lead additives and MTBE and the reducing of sulfur, particulate matter, and carbon monoxide emissions. 1.2.1.2. Methanol. Although methanol (CH3OH) is mainly obtained by synthesis from NG, it can be produced from a wide range of raw materials (dry biomass in general, coal, etc.) [18]. Compared to ethanol, methanol is less volatile, can be extinguished with water, pollutes less, and it also has no sulfur content [18]. In addition to decades of use in motor racing, the public in California used


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methanol for 25 years. According to the Energy Information Administration (EIA) [20], the dramatic increases in NG prices (the starting material for most of the methanol) prompted methanol’s decline at least in part. Methanol is one of the alternative fuels currently pursued by China. Methanol has a high octane number (114) and has about half of gasoline’s volumetric energy content (2.01 gal CH3OH = 1 gge). Methanol can be used with gasoline in different proportions (e.g., neat or 100% methanol, 85% methanol with 15% gasoline, and 85% gasoline with 15% methanol). Methanol also can be made from coal. With the abundant resources of NG and coal in the United States, the supply of methanol would be ensured. Because methanol is less volatile than gasoline, it has a fire safety advantage. Methanol has some drawbacks. Methanol is hygroscopic, a solvent for some plastics, and corrodes aluminum. Therefore, methanol is incompatible with some automotive materials. The major concerns with methanol as an automobile fuel focus on environmental and health issues. Although ingested methanol is well understood, there is insufficient data about the health effects of inhaled and skin-penetrated methanol. Predictions show that methanol will be plentiful and cheap; therefore, methanol will likely remain under consideration as an alternative fuel. 1.2.2. Other biofuels Within this review, the other biofuels are renewable fuels other than corn-grain ethanol and methanol. Usually, they are chosen to achieve a 50% reduction in GHG emissions. Advanced biofuels include biofuels derived from such renewable biomass as cellulose, hemicellulose, lignin, sugar, or any other starch that is not from corn, biomass-based diesel, and co-processed renewable diesel. 1.2.2.1. Vegetable oils. Vegetable oil can be obtained from more than 300 different plant species. Pure (refined and deslimed) vegetable oils can be used directly in some diesel engines or mixed with fossil diesel fuels. Pure vegetable oil, however, cannot be used in direct-injection diesel engines. Some pure vegetable oils can be used as lubricants and as hydraulic oils. These oils are mainly contained in fruits and seeds with the highest oil yields obtained from tree crops, such as palms, coconuts, and olives. However, there are a number of field crops containing oils. Vegetable oils can also be used in the esterified form achieved by cracking procedures or by treating the oil with an alcohol transesterification process. Transesterification is usually a less expensive way of transforming the large, branched molecular structure of the bio-oils into smaller, straight-chain molecules of the type required in regular diesel combustion engines. Rape oil methyl-ester (RME) and sunflower methyl-ester (SME) are two frequently used biodiesel fuels derived from their corresponding oil seeds. 1.2.2.2. Biodiesel or FAME. A common example of a biofuel is biodiesel. Biodiesel is produced by chemically reacting a vegetable oil or animal fat with an alcohol such as methanol. In addition to methanol, other alcohols (e.g., propanol, isopropanol, butanol, and pentanol) can be used [21]. After reaction requiring a catalyst and usually a strong base (such as sodium or potassium hydroxide), this reaction produces methyl esters (generally known as fattyacid methyl ester or FAME). It is these esters that came to be known as biodiesel [21], and the process is called transesterification. Knothe in the Belgian Congo described how ethyl esters were used as diesel fuel substitutes as early as 1937 [21,22]. Since the carbon (in the vegetable oil or animal fat) originated from carbon dioxide in the air, many believe that biodiesel contributes much less to global warming than fossil fuels. Diesel engines operated on biodiesel have lower emissions of carbon monoxide, unburned hydrocarbons, particulate matter, and air toxics than when operated on petroleum-based diesel fuel. Although the United

States produces 35.3 billion pounds per year of vegetable oil and animal fat (combined) and could provide 4.6 billion gallons of biodiesel, the on-highway diesel fuel consumed yearly in the United States is about 33 billion gallons. All of the vegetable oil and animal fat produced in the U.S. would only provide 13% of the current demand for on-highway diesel fuel [21]. Although FAME can be produced from many types of oils, RME is the most common in Europe and SME in the U.S. Undesirable byproducts (e.g., glycerin and water) are removed from the fuel. FAME can be used pure (B100) in diesel engines, but it is more often used as a mix with diesel fuel. In summary, biodiesel is produced by transesterification of triglycerides from vegetable oils usually with methanol [19,23,24], resulting in a fuel with similar properties as mineral oil derived fuels [24,25]. See other publications if there is a desire to learn more about the production of biodiesel [21,26]. 1.2.2.3. Algae fuels. Algae fuel is referred to as a third-generation biofuel [19]. The membrane components of microalgae, similar to those of animal and vegetable oils, contain lipids and fatty acids as sources of energy. In addition, algal oils can be processed in a manner similar to other biodiesel fuels. Even though the use of algae to make fuel was discussed more than 50 years ago, efforts to use algal oils did not begin until the 1970 oil crisis when the US Department of Energy (USDOE) began research efforts in Golden, Colorado [27]. From 1990 to 2000, the Japanese government funded algae research looking for an alternative fuel. However, none of these approaches have proven to be economical on a large scale. The USDOE program closed in 1996 [27]. The closure of these programs is due in part because these systems could not compete with the cheap crude oil of the late 1990s [28]. However, because genetic engineering has improved and the cost of fossil fuels may increase, the production of algal oils can yield more oil per acre of land than other biofuels. Therefore, algal oils may be examined again [19,27,29]. Articles on the genotoxicity and carcinogenicity of algal oils or their emissions were not found. 1.2.2.4. Natural gas, liquid natural gas, and synthetic natural gas. An option to other alternative fuels is NG. NG can be liquefied by refrigerating it to approximately 259 8F at atmospheric pressure. Liquefaction of the gas will ‘‘shrink’’ about 625 cubic feet of gas into 1 cubic foot of liquid [30]. This makes the transport of NG feasible. For residential, commercial, and industrial sectors, NG which is predominantly methane (CH4) has proven itself as an important energy source. Liquefied natural gas (LNG) is NG that has been converted to liquid form for ease of storage and/or transport. Synthetic natural gas (SNG) is a proposed alternative to NG and LNG. NG currently provides 24% of the energy used by United States homes (1). There are several opportunities, direct and indirect, to use natural gas in LDVs, including producing electricity for PEVs and producing hydrogen for FCEVs. In addition, the USDOE estimates that in the coming decades the United States’ NG demand for electricity generation will increase [31]. Estimates also suggest that NG supply will increasingly come from imported LNG [31]. Additional supplies of NG could come domestically from the production of synthetic natural gas (SNG) via coal gasification– methanation [31]. There was a surge in construction of naturalgas-fired power plants: between 1992 and 2003, coal-fired capacity increased only from 309 to 313 GW, and natural-gasfired capacity more than tripled, from 60 to 208 GW [31]. Adding to this was the EIA prediction of continued low NG prices through 2020, lower capital costs, shorter construction times, and generally lower air emissions for natural-gas-fired plants allowing power generators to meet the clean air standards [31]. Demand from electricity generators is projected to grow and the U.S. demand can


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only be met with alternative sources of NG, such as imported LNG or SNG. NG is extracted from wells and sent to processing plants where water, carbon dioxide, sulfur, and other hydrocarbons are removed. The produced NG then enters the transmission system [32]. The U.S. transmission system includes some storage of NG in underground facilities (such as reconditioned depleted gas reservoirs, aquifers, or salt caverns) to meet seasonal and/or sudden short-term demand. From the transmission and storage system, some NG goes directly to large-scale consumers, like electric power generators. The rest goes into local distribution systems that deliver it to residential and commercial consumers via low-pressure, small-diameter pipelines. The use of liquefied natural gas (LNG) adds additional life-cycle stages to the NG lifecycle stages noted above: (1) after NG is produced and processed to remove contaminants, the NG is transported by pipeline a relatively short distance and liquefied. Liquefaction plants are generally located in coastal areas of LNG exporting countries, (2) LNG ocean tankers transport the LNG to the United States, and (3) upon arriving, the LNG tankers offload their cargo and the LNG is regasified [32]. At this point, the regasified LNG enters the U.S. NG transmission system. The coal life-cycle is usually simpler than the NG life-cycle, consisting of three main steps: (1) coal mining and processing (U.S. coal is produced from surface mines, 67%; or underground mines, 33% and the mined coal is processed to remove impurities), (2) transportation (via rail, 84%; barge, 11%; and trucks, 5%), and (3) use (90% of the coal used in the United States is used by the electric power sector) [32]. The life-cycle of SNG can be considered a combination of the coal life-cycle and the NG life-cycle [32]. Coal is mined, processed, and transported to the SNG production plant. Then the syngas is produced by gasification and converted, via methanation. The SNG is then sent to the NG transmission system, described above, and on to consumers. If CNGVs can be made competitive (with respect to both vehicle cost and refueling opportunities), CNGVs offer a quick and economical way for reducing petroleum use. With currently envisioned technology, some think that sufficient biofuels are likely be produced by 2050 to meet the United States’ goal of 80% reduction in petroleum use [11]. Environmental issues associated with shale gas extraction (fracking) must be resolved, including leakage of NG, itself a potential contaminator of groundwater [11]. NG and coal conversion to liquid fuel (GTL, CTL) may be used as a direct replacement for petroleum gasoline, but the GHG emissions from these fuels are slightly greater than those from petroleum. Therefore, these fuels will play only a small role in reducing petroleum use even if GHG emissions are reduced simultaneously. The NAS committee [11] finds that ‘‘Meeting the study goals requires a massive restructuring of the fuel mix used for transportation. Petroleum-based fuels must be largely eliminated from the fuel mix. Other alternative fuels must be introduced such that the average GHG emissions from a gallon equivalent of fuel are only about 40% of today’s level.’’ The volume of economic NG from shale deposits within the United States has been increasing rapidly. Based on 2009 estimates, the probable NG reserves would provide about 86 years of usable NG if the consumption rate stays at 2009 levels. In 2011, this estimate increased to 90 years of probable reserves existing based on 2010 consumption. Many earlier estimates for alternative fuels did not include NG as a possible source for LDV fuel for mainly two reasons: (1) a belief that only a very limited domestic supply existed and (2) the use of NG would cause a substantial price increase in electricity and residential heating costs. Because of increasing domestic production, NG now is a viable option for providing transportation fuels [11]. The most efficient use of NG is direct use as CNG. The main advantages of CNG include: (1) 1 gge of CNG is cheaper than 1 gal of petroleum-based gasoline, (2) the needed technology is proven and

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available, (3) fuel distribution pipelines are in place and can meet initial requirements, and (4) emissions are lower when compared to petroleum-based gasoline. The main disadvantages for CNG include: (1) CNG fuel stations are few and expensive to build, (2) there is much design work that must be done on vehicles that use CNG (e.g., engines, trunk space, range), and (3) CNG vehicles are not economical [11]. 1.2.2.5. Hydrogen. The hydrogen reciprocating engine has the potential of replacing conventional gasoline and diesel engines in part due to reducing dependence on petroleum and other less regulated emissions. Before 2050, the cost and operating costs of FCEVs could be lower than the cost of an equivalent ICEV. As a fuel, hydrogen has several desirable properties (e.g., a high flame velocity, high ignition temperature, and low ignition energy). Concerns for hydrogen engines revolve around NOx and PM emissions from the lubricating oil and from operating concerns such as backfire, pre-ignition, and knocking [33]. Other papers that can be consulted are Crabtree et al. [34], Mao [35], Premkartikkumar et al. [36], Verhelst et al. [37], Kumar et al. [38], and Verhelst [39]. 1.2.3. Coal-derived products Faced with crude oil shortages and dwindling supplies of NG, many argued that we must utilize our vast coal energy reserves [30]. Coal is the United States’ most abundant energy source, with enough proven reserves to meet energy needs for hundreds of years [30]. The energy in coal can be utilized in three ways [30]: (1) direct burning: in the past coal was used directly for residential and industrial heating. However, the transporting and distributing of coal to the residential and industrial market was very costly, and the environmental impact was extremely high; (2) the burning of coal to generate electrical power: this is a viable alternative only if power generation plants are fitted with after-treatments that protect the environment and the public’s health. Another disadvantage to this alternative is that the thermal efficiency of generating electricity from any fossil fuel is quite low, ranging from 35 to 42% for the best of designs. Nonetheless, society’s need for electrical power will mandate the use of coal to generate power until alternatives are developed and accepted; (3) the conversion of coal into a substitute natural gas (SNG or synfuel products) for residential and industrial heating: coal gasification is about 70% thermally efficient. Therefore, a coal gasification plant is much more environmentally desirable than a coal-fired power plant [30,40]. 1.2.3.1. Synfuels and coal gasification. The commercial gasification of coal has been practiced for nearly one hundred years. Lowpressure coal gasifiers date back to the 1920s [30]. Most coal gasification plants involve using chemical reactions which combine the carbon from the coal with hydrogen from steam to form methane, which constitutes 97 vol% of the product SNG. The amount of coal used for producing the energy from a SNG plant is very specific to the particular coal being used [30,40]. Synthetic fuels, for purposes of the Synthetic Fuels Corporation, were defined as any solid or liquid that could be used as a substitute for crude oil or natural gas and that was produced by chemical or physical transformation – primarily from coal, oil shale, or tar sands, including heavy oil (Energy Security Act Conference Report, No. 96824, June 19, 1980, 22) [41]. Synthetic fuels did not include solar, wind, renewable, or nuclear. With the dwindling of petroleum and NG supplies during the 1970s, came an awareness of an opportunity to develop synthetic fuels from coal. In 1980, President Carter and the U.S. Congress created the $20 billion Synthetic Fuels Corporation. The goal was to use coal to produce 700 million barrels of oil per year by 1992. The


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corporation spent $2 billion on demonstration projects in California, Louisiana, and North Dakota. But management scandals and battles between the corporation and the White House during the Reagan Administration, and the falling price of oil caused Congress to stop funding. Experts said the only thing that would revive synfuels was $100-a-barrel oil [42]. Therefore, a multibillion-dollar U.S. effort to turn coal into gasoline ended in the 1980s because the effort was plagued by mismanagement, political wrangling, and falling oil prices [42]. In addition, environmentalists concerned about the impact of additional coal mining cheered the end of the synthetic fuels program, which was aimed at cutting U.S. dependence on oil from the Middle East [42]. China is building up to 23 synfuel plants. With today’s economy and dwindling crude oil reserves, a few U.S. energy companies have plans for synfuels plants that would produce millions of barrels of the alternative fuels annually. The synfuel process begins by turning coal into gas, which creates carbon monoxide and hydrogen gases. Traditional synfuel plants make syngas from coal; however, industry contemplates the using of large amounts of plant biomass along with coal and storing in the ground the CO2 emitted during the production of synfuels. The gas would then be catalyzed into various liquid fuels such as diesel fuel, jet fuel, or chemical feedstocks. The use of cooler gasifiers could generate problems, because lower temperatures mean that less of the feedstock (coal or biomass) is converted into syngas [42]. The remaining toxic, carbonaceous muck and its disposal also present an undesirable cost. Some processes produce mainly sulfur, ash, ammonia, and slag as by-products. A process, called the Lurgi process, also produces other by-products (tars and oils) through the volatilization and condensation of organic compounds from the feed coal. These tars and oils present some unique problems in the process stream cleanup which in turn could present potential health hazards for workers and the general population [43]. In contrast to the CO2 from a standard generating plant which must be separated from other flue gases, the CO2 that synfuel plants create, could take advantage of the fact that the process creates a concentrated CO2 stream that can simply be injected into deep underground formations. But the amount of CO2 needed to be stored by a new generation of synfuels plants is much larger than any current projects. Even if the CO2 generated could be stored, many say that the effects of expanding coal mining could be harmful to the environment [42]. Because many publications describe the non-genotoxic aspects of biofuels and other alternative fuels, the reader may have a need to consider these other sources of information. Included in these publications are Brazon [44], Koonin [2], Priddy [41], Ros et al. [45], Kaniyal et al. [46], Li and Li [47], Hoö¨k et al. [48], Tchapda and Pisupati [49], and Yang et al. [50]. 1.2.4. Shale oil or kerogen Geologically speaking, oil shale is not shale, and oil shale contains virtually no oil. Instead, it is sedimentary rock containing a material called ‘‘kerogen’’ [30]. When heated, this solid organic material yields substantial amounts of hydrocarbon crude oil and gas (typically 10–60 gal of crude oil per ton of shale). Eons ago, oil shale began in a manner similar to crude oil, when organic matter was deposited in large lakes; however, the oil shale deposits were not subjected to the heat and pressure essential for forming petroleum [30]. Instead, the organic matter was transformed into the solid hydrocarbon kerogen and locked into a marlstone matrix. The geological term for our Western oil shale is ‘kerogenous marlstone’. Marlstone is a rock containing clay materials and calcium and magnesium carbonates, with approximately the same composition as marl. Marl was originally an old term loosely applied to a variety of materials; most are loose, earthy deposits containing 35–65% clay and 65–35%

carbonate (see http://en.wikipedia.org/wiki/Marl or http://www. answersingenesis.org/articles/cm/v22/n2/geology). A typical oil shale contains about 15 wt% kerogen and 85 wt% of carbonates, feldspars, quartz and clays. Deposits in the western United States are thick and yield more than 25 gal of crude shale oil per ton of rock (‘‘rich’’ shale oil deposits). Over 15 nations around the world have extensive shale oil reserves, the largest of which are: USA (2000 billion barrels), Brazil (342 billion barrels), and Zaire (103 billion barrels) [30]. Some of these deposits yield nearly 100 gal per ton of rock [51]. At current national oil consumption rate of about 18 million barrels a day, the Green River Formation (deposits in the USA states of Colorado, Utah and Wyoming) represents about a 300 year supply of in-place oil. Beginning as early as 1840 in France and Scotland, commercial shale oil production was practiced for many years. Beychok [30] estimates that the disposal wastes from shale oil processes will be 54,000 tons/day. Presently, the only way found to recover oil from the kerogen is to heat (retort) the kerogen to very high temperatures, and then to treat with hydrogen which creates a stable synthetic crude oil. The synthetic crude can be refined into gasoline and other petroleum products in a manner similar to conventional crude oils [51]. The average percentages for chemical classes are: n-alkanes, 3.4–3.9%; branched and cyclic alkanes, 23.6–30.3%; aromatic oils, 2.7–3.3%; resins, 54.4–57.4%; and asphaltenes (including fatty acids), 9.0– 12.5% [52]. The n-alkanes, resins, and asphaltenes are not likely to contain a detectable amount of carcinogens. The cyclic alkanes and aromatic oils may contain detectable amounts of mutagens and carcinogens [30]. 2. Genetic toxicology of biofuels: carcinogenicity/mutagenicity of the raw (unprocessed) product During the late 1970s and 1980s, Oak Ridge National Laboratory (ORNL) did seminal research for the USDOE examining synfuels. Much of their efforts were documented in ORNL reports [53–56]. In a 1979 progress report [54], Epler reported that it was feasible to use short-term mutagenicity assays to isolate and identify the potential biohazards of complex materials. He noted that fractionation procedures are used to characterize the mutagens present with the bioassay being used as a tool to follow the activity and guide the separations. The mutagenicity tests were intended to function as (a) predictors of profound long-range health effects such as mutagenesis and/or carcinogenesis, (b) a mechanism to rapidly isolate and identify hazardous biological agents in a complex mixture, and (c) as a measure of biological activity that could correlate base-line data with changes in process conditions. He said, ‘‘The investigator can accumulate information on the actual compounds responsible for the biological effect. Thus, the mutagenicity tests will also aid in identifying the specific hazardous compounds involved and in establishing priorities for more definitive . . . testing, for mutagenesis and carcinogenesis.’’ The researchers at ORNL also recommended a ‘‘tier system’’ to test the mutagenicity and carcinogenicity associated with fuel technologies. Using such systems, their results implicated chemicals in the basic (ether-soluble) and the neutral fractions of coalderived fuels as potential genetic hazards. They showed that: (1) alkaline constituents of petroleum substitutes are major contributors to the Salmonella mutagenic activity, (2) subfractions of ether-soluble bases from shale- and coal-derived oil had concentrated the bioactive constituents into a subfraction contained in 0.5 wt% of the starting oil, and (3) nitrogen heterocyclics were the principal constituents of the active subfraction. They also summed the fractions to estimate the biological activity of the starting material. Applying mutagenicity testing of fractionated mixtures (from natural crude and synthetic oils, aqueous condensates, process waters, leachates, and organic extracts of raw materials,


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wastes, and particulate materials) illustrated that specific activities (revertants per mg of fraction) can be summed to yield an estimate of the mutagenic potential of the crude starting material. The total activity varied from process to process, but basic and neutral fractions consistently contained the bulk of the mutagenic activity. Metabolic activation with rat liver extracts was routinely required for activity. In a 1980 report [53], Epler et al. examined emerging processes for producing energy from fossil fuels, namely a low Btu industrial gasifier. The electrostatic precipitator (ESP) tar samples were mutagenic in the Salmonella assay (TA98, +S9) and showed (1) the mutagenic activity varied with different sampling dates, (2) most mutagenic activity remained mainly in the basic fraction indicating aromatic amines as the suspect agents, and (3) some ESP tars exhibited significant mutagenic activity in the neutral (PAH fractions). This report also outlined some initial efforts by Hsie to examine the mutagenicity and carcinogenicity of energy related pollutants in cultured mammalian cell systems and by L.B. Russell to examine in vivo screening for gene mutations in mouse germ cells and somatic cells. In the 1981 report [56], Epler et al. reported on their efforts with raw and hydrotreated coal (H-coal) product liquids. Direct coal liquefaction was one of the technologies investigated in an effort to generate clean and cost-competitive fuels from coal. To generate environmentally friendly products, hydrogenation was used. Hydroprocessing results in at least two major improvements: the reduction of aromatic content and the reduction of heteroatom content by hydrogenolysis of heteromolecules. Salmonella (TA98, +S9) results from chemical class fractions showed that any effects were generally reduced as the severity of hydrotreatment was increased. The data revealed that coal-derived distillates generated by the H-coal process are highly carcinogenic to mouse skin, but wide differences were seen due to the material composition. The 1986 report [55], presented results for lifetime C3H mouse skin tumorigenicity assay of an H-coal series of oils and considered the relationships between tumorigenicity, chemistry, and processing. The report also documented the physical and chemical properties of the oils tested. Results of the lifetime tumorigenicity assay demonstrated that even low-severity hydrotreatment reduced tumorigenicity; however, higher severity hydrotreatment led to no further reduction in tumorigenicity and may actually have increased tumorigenicity slightly. Citing work done by the University of Idaho [57,58], Howell and Weber [59], the U.S. Bureau of Mines and the USEPA [60,61], Kalligeros et al. [62] said the following, ‘‘It is well known that biodiesel is non-toxic, contains no aromatics, has higher biodegradability than fossil diesel, is less pollutant to water and soil and does not contain sulfur. It offers safer handling in the neat form and shows reduced oral and dermal toxicity, mutagenic and carcinogenic compounds. It is the most suitable fuel in environmentally sensitive areas (national parks, lakes, rivers) or in confined areas where environmental conditions and worker protection must meet high standards (underground mines, quarries).’’ See below for a summary of the published Kalligeros et al. [62] efforts. To be a viable alternative, a biofuel should provide a net energy gain, have environmental benefits, be economically competitive, and be producible in large quantities without having negative impacts (e.g., reducing food supplies). Hill et al. [63] compared ethanol to biodiesel using such criteria. They found that: Even dedicating all U.S. corn and soybean production to biofuels would meet only 12% of gasoline demand and 6% of diesel demand. Both biofuels when replacing petroleum would influence food supplies. Ethanol yields 25% more energy than the energy invested in its production, whereas biodiesel yields 93% more.

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Biodiesel also generates less air pollutants per net energy gain than ethanol. Transportation biofuels such as synfuel hydrocarbons or cellulosic ethanol, if produced from low-input biomass grown on agriculturally marginal land or from waste biomass, could provide much greater supplies and environmental benefits than food-based biofuels. 3. The carcinogenicity and mutagenicity of biofuel emissions – at the point of release Clark et al. [64] used the Salmonella assay to evaluate the use of alcohol additives on the mutagenicity of particulate exhaust extracts. In this study, four typical gasoline engine vehicles were operated on either 3 or 4 fuels (gasoline alone, 10% ethanol/90% gasoline, 10% methanol/90% gasoline, and a commercially available ‘‘gasohol’’). The gasohol contained a different gasoline than that used in the ethanol blended fuels. Although all particulate extracts were mutagenic in strains TA100 and TA98, a decrease mutagenic response was seen in the nitroreductase deficient strains TA98NR and TA98DNPR. The decreases in the nitroreductase deficient strains indicate that nitro-PAHs may be responsible for part of the mutagenicity. The response was higher in TA98 than in TA100 indicating that the majority of mutagens would be characterized as frame shift mutagens. The addition of S9 significantly decreased the mutagenic response in all the samples tested. Carbon monoxide and particulate emission rates decreased with the use of alcohol fuel blends. Lower values for revertants per mile were seen with the addition of alcohols to the gasoline, but this was mainly a result of decreased particle mass per mile. Because of the small changes seen with the addition of ethanol or methanol to unleaded gasoline, the authors concluded that ethanol or methanol added (at 10%) to unleaded gasoline does not appear to significantly alter the genetic toxicity of particulate exhaust products. Gra¨gg [65] reported on the testing of two CNG fueled Volvo buses equipped with oxidation catalysts. Using a transient driving cycle that simulates a city bus driving pattern (average speed, 22.5 km/h; maximum speed, 58.2 km/h; per cent idling, 22%; and driving distance 11 km), the NOx emission, CO emission, and the emission of particulate matter was low. Most of the HC was methane. The catalyst was not as efficient converting methane as converting CO. One of the buses underwent testing for unregulated emissions (alkenes, aldehydes, polycyclic aromatic hydrocarbons) and for Salmonella mutagenicity (TA98 and TA100, S9). The emission of alkenes, aldehydes, PAHs, and mutagenicity were low [65]. The PAH emission was dominated by the semivolatiles phenanthrene and pyrene. The source of the pyrene and the phenanthrene emission was not discernible. Bu¨nger et al. [66] examined the mutagenic and cytotoxic effects of diesel engine exhaust emissions from a passenger car using biodiesel fuel (RME) and exhaust emissions of a petroleum-derived diesel fuel. After collecting the emitted particles on PTFE-coated glass fiber filters and extracting the filters with dichloromethane, the extracts’ mutagenicity was tested in the Salmonella assay (TA97a, TA98, TA100, and TA102). The extracts’ cellular toxicity was examined using mouse lung fibroblasts (L929) in the neutral red assay. In the Salmonella assay, a significant increase of mutations resulted when using emission extracts from both fuels, but the petroleum-derived fuel gave results that were significantly higher when compared to biodiesel fuel emissions. In some comparisons, the biodiesel extracts showed slightly higher toxic effects in the neutral red assay. These results indicated a higher level of mutagenicity for the emissions of the petroleum-derived diesel fuel compared to biodiesel. The authors attributed this to the lower content of polycyclic aromatic compounds (PAC) in biodiesel exhaust.


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Knowing that several PAHs are carcinogenic in rodents, and PAHs are associated with various types of human cancer, Karahalil et al. [67] studied young workers exposed to PAHs in engine repair workshops. They compared exposed workers with non-exposed workers for urinary 1-hydroxypyrene (1-OHP), sister-chromatid exchange (SCE), and micronucleus (MN) levels. 1-OHP excretion is clearly an internal indicator of PAH exposure [68–72]. The mean value SE of urinary 1-OHP excretions from workers was 4.71 0.53 mmol/mol creatinine and the mean value for controls was 1.55 0.28 mmol/mol creatinine. Thus, a 4-fold increase in urinary 1-OHP excretion was seen. The mean values SE of SCE frequency per cell in peripheral lymphocytes from the workers and controls were 4.47 0.09 and 4.06 0.16, respectively. The mean SE for MN (%) frequencies in peripheral lymphocytes from the workers and controls were 1.87 0.04 and 1.56 0.06, respectively. Bu¨nger et al. [73] compared a RME biodiesel fuel to a common fossil diesel fuel in a test tractor. Filter-sampled particles were extracted and tested using the Salmonella/microsome assay. The extracts from the fossil diesel fuel emissions were significantly more mutagenic in TA98 and TA100 than the particulate extracts of the engine emissions when biodiesel fuel was used. The extracts of fossil diesel fuel showed a fourfold higher mutagenic effect in TA98 (and a twofold effect in TA100) than extracts of the biodiesel fuel emissions. The authors attributed this lower mutagenic response of the biodiesel to lower emissions of PAHs. However, the biodiesel extracts produced a fourfold stronger toxic effects on mouse fibroblasts cell line L929 at ‘‘idling’’ but not at ‘‘rated power’’ than fossil fuel diesel extracts [73]. A study (also reported in 2000) of Bu¨nger et al. [74] examined the organic fraction of particulate emissions for polynuclear compounds and mutagenic effects. Four different fuels were used (two biodiesel fuels, RME and SME; and two fossil diesel fuels, with the normal levels of sulfur (DF) and a low sulfur content fuel). The soluble fraction was analyzed for PAHs. Again, the Salmonella assay (TA98 and TA100) was used. Compared with normal fossil diesel fuel, the exhaust particles of the low sulfur fuel and the two biodiesel fuels contained less insoluble material (mainly carbon cores). Although individual polynuclear aromatic compounds varied widely among the different exhaust extracts, the total concentrations of PACs were approximately double for the fossil diesel and soy biodiesel fuels when compared with low sulfur and rapeseed fuels. The results indicated that diesel exhaust particles from two biodiesel fuels and the low sulfur fossil fuels contained less black carbon, contained less total polynuclear aromatic compounds, and are significantly less mutagenic in comparison with higher sulfur fossil fuel. Two of the most used biodiesel fuels are RME and SME. Knowing that it is necessary to judge the environmental and health effects that derive from the use of biodiesel in combustion engines, Krahl et al. [75] compared regulated (CO, HC, NOx, particulate matter, benzene, methane, nitrous oxide, and aldehydes) and some nonregulated emissions from different blends of RME and fossil diesel fuel (DF). The tests were carried out using a Fendt type 306 LSA tractor with a direct-injecting diesel engine. A comparison of the particulate matter emissions (particle size and the particle number distributions) from DF to those from RME was done. To estimate the genotoxic effects of diesel fuel and biodiesel particulate matter, their mutagenic potencies were determined. They also studied whether biodiesel influences ozone formation by quantizing ozone precursors (e.g., ethene and formaldehyde). Although hydrocarbon (HC) emissions declined with increasing RME content, the CO emissions increased slightly. Independent of the catalytic converter, particulate matter rose with an increasing percentage of RME, but non-linearly. However, the organic insoluble matter decreased with an increasing RME. While varying the blend had almost no effect on NOx emissions, the catalytic converter doubled these

emissions. Although benzene was absent in RME, the emission of benzene increased with the amount of RME. In recent years it has become clear that determining particle number and particle size distribution may be more important than determining mass alone, because small particles reach pulmonary alveoli and deposit there. In comparison to diesel fuel, RME reduced hydrocarbon emissions but increased aldehyde emissions; therefore, the authors found it impossible to compare the ozone-formation potential of the two fuels. Analyzed for 16 PAHs, DF and SME had higher total PAH concentrations than RME. Concentrations of the individual compounds varied from fuel to fuel. When using TA98, DF revertant frequency was twice as high as for RME and SME. The revertant frequency for DF and SME without a catalytic converter was also significantly elevated for tester strain TA100. The catalytic converter lead to decreased mutations for all particulate extracts. In addition, Krahl et al. [76] used a test engine and two agricultural tractors run on a RME (biodiesel) or conventional diesel fuel plus blends of the two. Their work compared for RME and DF: (1) mutagenic potentials using the Salmonella assay, (2) particle mass as well as particle size and particle number distributions, (3) influence of emissions on ozone formation, and (4) different blend levels. Because of their median dynamic diameter (0.1–0.3 mm) the particles could be readily inhaled with about 10% deposited in the alveolar region of the lungs [76,77]. Generally, emissions of regulated compounds changed linearly with the blend level. The known positive and negative effects of biodiesel varied accordingly. Overall, no optimal blend was found. Increasing biodiesel content of the fuel caused a linear increase in benzene emissions in the agricultural five-mode engine test, an effect that may be explained from previous studies on precombustion chemistry. In using the test engine, it was found that PM from biodiesel had significantly reduced mutagenic potential compared with that from diesel fuel; although in this work, PM masses were found to be reproducibly higher for biodiesel from rapeseed oil compared with conventional diesel fuel. Ozone precursors increased 10–30% when using biodiesel compared with conventional diesel fuel. Emissions of aldehydes and alkenes are mainly responsible for this effect. N2O emissions increased when using a catalytic converter. However, even the combustion of ‘‘green’’ fuels such as biodiesel leads to emissions of hazardous gaseous compounds and particulate matter that may affect human health. Other studies by Krahl and colleagues [78–81] examining the effects of fuel composition upon regulated emissions and the mutagenicity of exhausts may be of interest to the reader [82]. Scientists have examined many species for the production of biodiesel. For example, Cardone et al. [83] compared the performance of Brassica carinata oil-derived biodiesel with a commercial rapeseed oil-derived biodiesel and petroleum diesel fuels by examining engine performance, regulated exhaust emissions, and unregulated exhaust emissions. B. carinata is an oil crop that can be cultivated in coastal areas of central-southern Italy, where it is more difficult to achieve the productivity potentials of the most common rapeseed cultivated in continental Europe (Brassica napus). Experimental tests used a turbocharged direct injection, passenger car diesel engine. The unregulated exhaust emissions were characterized by determining the soot, the soluble organic fraction content of the particulate matter, the content and speciation of polycyclic aromatic hydrocarbons, aldehydes, and ketones. B. carinata and a commercial biodiesel behaved similarly as far as engine performance and regulated and unregulated emissions were concerned. Compared with petroleum diesel fuel, the engine analysis did not show any appreciable variation of output engine torque values. However, there was a significant difference in specific fuel consumption data. The biofuels produced higher levels of NOx concentrations and lower levels of PM with respect to the petroleum diesel fuel. When


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compared with petroleum diesel fuel, biodiesel emissions contain less soot, and a greater fraction of the particulate soluble fraction. The analysis and speciation of the soluble organic fraction of biodiesel particles suggest that the carcinogenic potential of the biodiesel emissions were probably lower than that of petroleum diesel. The performance of B. carinata biodiesel is quite similar to commercial biodiesel making B. carinata a promising oil crop that takes advantage of Mediterranean agricultural areas [83]. Because of the drawbacks associated with individual biofuels, a number of other chemical substrates have been investigated as alternative liquid transportation fuels (e.g., for example, fatty acids, fatty alcohols, biobutanol, biopropanol, acetone, and methanol) [19]. The equation for transesterification is given by Gerpen [21]. Chen and Wu [84] examined the level of submicron particles (0.008–1 mm) emitted from a direct injection diesel engine using either biodiesel or a commercially available petroleum–diesel fuel. The biodiesel fuel was a SME. Although the emitted particle sizes for both fuels were about the same, the diesel engine using biodiesel reduced (24–42%) the total number concentration and the total mass concentration (40–49%) of submicron particles. This would indicate that the exposure to submicron particles would be less when using soy-based biofuels. Before 2003, RME had not been directly compared to a Swedish low sulfur diesel fuel. Therefore, Krahl et al. [85] decided to compare the regulated and some non-regulated emissions of three fuels: biodiesel (RME), a common diesel fuel (DF), and Swedish low sulfur diesel fuel (MKI). They also reported the mutagenicity results observed. The test engine was a four cylinder Daimler-Chrysler engine OM 904 LA. For the first time, Swedish diesel fuel MKI and RME were compared after combustion in a diesel engine. MKI showed a higher mutagenic potential in the Salmonella assay than RME, and the results of four modes of the 13-mode test indicate a tendency to slightly more advantages for RME versus MKI and DF. Kalligeros et al. [62] examined the exhaust emission and fuel consumption measurements from a single cylinder, stationary, diesel engine (a Petter engine, model AV1-LAB). The engine was fueled with pure marine diesel and mixtures containing 10%, 20%, and 50% of biodiesel methyl esters from either sunflower oil or olive oil. The two biodiesel fuels performed in a similar manner. The biodiesel fuels decreased particulate matter, carbon monoxide, hydrocarbon, and nitrogen oxide emissions but had a slight increase of the volumetric fuel consumption. In an abstract of research that used a LDV, Bu¨nger et al. [86] compared the Salmonella mutagenicity of the particulate emissions while using four different fuels (two were designed diesel fuels, one was a biodiesel, and one was a fossil diesel fuel). While only the biodiesel fuel showed a decrease in the mass of particle emissions, the level of mutations was higher for the fossil diesel fuel emissions than for the emission extracts of the other three fuels. Bu¨nger et al. [25] used the Salmonella mutagenicity assay (TA98 and TA100) to examine the particle extracts and the gas phase condensates from the following fuels: a common type of fossil diesel fuel, a NG derived fuel, a RME fuel, a rapeseed oil fuel, and a preheated rapeseed oil fuel. Compared to the common diesel fuel, the rapeseed oil fuel emissions significantly increased (up to 59 in TA98 and up to 22.3 in TA100) the mutagenicity of the particle extracts. In addition, the gas phase condensates of the rapeseed oil and the preheated rapeseed oil emissions were more mutagenic than the reference fuel. The RME emission extracts had a moderate but significantly higher mutagenic response (TA98, +S9; TA100, S9). The NG derived fuel samples did not differ significantly from the common diesel fuel. The authors concluded that the increase of mutagenicity using rapeseed oil fuels as diesel fuels compared to the reference common diesel fuel ‘‘causes deep concern on future usage of this biologic resource as a replacement of established diesel fuels.’’

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In a 2012 review, Bu¨nger et al. [87] compared the combustion of fatty acid methyl ester (FAME) fuels with common fossil diesel fuels for legally regulated and non-regulated emissions as well as for toxic effects. They reviewed 62 publications on the chemical analyses of diesel engine emissions, 18 in vitro toxicological studies, and a small number of human studies and animal experiments. In most studies, the use of biodiesel fuels reduced legally regulated emissions of carbon monoxide, hydrocarbons, and particulate matter but increased the production of nitrogen oxides. The non-regulated emission aldehydes (e.g., formaldehyde) increased, and polycyclic aromatic hydrocarbons decreased. There were lower levels of mutagenicity compared to fossil fuel exhaust. More recent studies of fossil diesel fuel showed lower levels of mutagenicity. This may be caused by decreasing levels of sulfur in present-day diesel fuels and new control technologies in diesel engines [87]. 3.1. Synfuel and related products Although Germany and South Africa were operating commercial scale coal gasification plants in the 1970s, little information existed on possible health effects associated with gasification processes. Physical, chemical, and toxicological characterization of fugitive emissions and potential waste effluents were needed for evaluating potential health risks. In 1982, Vick and Epperly examined the genotoxicity of a liquefied coal process used to make a synthetic fuel [88]. Four process streams were evaluated using the Salmonella assay and the Syrian hamster embryo morphologic transformation assay. Three high boiling liquids (>200 8C) were active in both assays, but a hydrotreated naphtha sample (50% in both particle and vapor-phase with both fuels by when OC was used. Vaporphase mutagenicity was not detected with the biodiesel samples; and only a very low level of mutagenic activity was detected when the D2 fuel and the OC were used. The OC caused a slight shift in the particle size to smaller particles for the diesel fuel; therefore, a larger percentage of the particles would reach lung alveoli, but fewer particles would be emitted. Use of the biodiesel fuel in this study, did not increase any of the potentially toxic, health-related emissions that were monitored as part of this study. For PAH compounds, seven of the nine monitored compounds were found in the D2 and biodiesel samples obtained both with and without the OC (i.e., all). Only one nitro-PAH compound (1-nitropyrene) was found at quantifiable levels. The levels of the soluble organic fraction and volatile compounds decreased with OC use; however, the amount of PAH and nitro-PAH per unit mass actually increased with OC use for the biodiesel fuel [107]. Bu¨nger et al. [108] knew from earlier studies that particle emissions from diesel engines contain PAHs and that these compounds cause Salmonella mutagenicity. Therefore, they explored what effect nitrogen oxides and an oxidation catalytic converter had on the mutagenicity. The engine was fueled with common fossil diesel fuel, low-sulfur diesel fuel, or one of two biodiesel fuels (RME or SME) and run at five different load modes in two series with and without installation of an oxidation catalytic converter. The mutagenicity of the extracts was examined with the Salmonella assay (TA98 and TA100). Although the lowest numbers of revertant colonies were with the biodiesel fuels (without oxidation catalytic converter), the number of revertant colonies also was low in extracts of low sulfur diesel fuel emissions. Usually, engine operation with the oxidation catalytic converter led to a reduction of the mutagenicity. However, direct-acting mutagenic effects were significantly increased for rapeseed biodiesel and soy biodiesel under heavy duty conditions. Direct-acting mutagenicity also increased for common fossil diesel fuel and low sulfur diesel fuel under heavy duty conditions when emissions were treated with the oxidation catalytic converter. Because the oxidation catalytic converter increased formation of direct-acting mutagens (probably nitroaromatics), the authors expressed concern over the use of oxidation catalytic converters with diesel engines [108]. California Air Resources Board (CARB) researchers [109–111] reported that tailpipe emissions from a compressed natural gas (CNG) fueled transit bus without after-treatment had levels of toxicants (e.g., formaldehyde, nanoparticles, and Salmonella

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mutagenicity) that in some cases were greater than that found with a similar diesel bus equipped with either an oxidation catalyst (OC) or a particulate filter using a ultra-low sulfur diesel fuel. Because of these initial results, CARB researchers explored the effectiveness of an OC for a CNG-engine bus [112,113]. Results showed that an OC reduced total PM, total hydrocarbons, carbon monoxide, and formaldehyde emissions. 1,3-Butadiene emissions were reduced to levels below detection. Toxic aromatic HCs such as benzene appeared reduced by the catalyst, but the results were not statistically significant. The OC had little effect on methane and NOx levels [112]. For a CNG bus without an OC, formaldehyde was 86–92% of all carbonyl emissions. In contrast, an OC was 96% effective in reducing formaldehyde from the emissions from a Detroit Diesel bus without after-treatment [112]. In another report by CARB researchers, after-treatment reduced emissions for both CNG and diesel heavy-duty engines; however, the magnitude of these reductions differed by pollutant and driving cycle [113,114]. Only the Diesel bus and the uncontrolled CNG bus had mutagenic activity (Salmonella typhimurium TA98, +S9) in the volatile phase [113]. The OC reduced the mutagenic activity in both the volatile and PM phase [113]. 6. Other studies One approach to reduce the nation’s dependence on coal and other fossil fuels is for utility companies to cofire (either directly or indirectly) a percentage of their total fuel requirements using biomass. Recognizing the tremendous coal requirements to produce electricity and process steam in the United States, even small biomass co-firing rates would have a significant impact both environmentally and economically. Therefore, Klasson and Nghiem [115] examined the possibility of using zoo animal waste for a biomass source. Because animal waste has the greatest potential for methane production (yielding from 0.3 to 0.4 m3 methane/kg volatile solids) when compared to other types of biomass, methane yields from animal waste have been studied (although these studies have targeted primarily toward domesticated animals). Klasson and Nghiem used elephant and rhinoceros dung to investigate the feasibility of generating methane from the dung of zoo animals. They observed that the methane yield for this dung was approximately 0.033 L biogas/g dung (0.020 L CH4/g dung). The Knoxville Zoo produces 30 cubic yards (23 m3) of herbivore dung per week and cost of disposal of this dung is US$105/week. The estimated weight of this dung is approximately 1050 ton/year. This annually generated dung could potentially generate 17,400 m3 methane, and the energy value for this methane is 6.6 108 kJ (6.2 108 Btu). Because biomass represents a clean fuel source which can reduce SOx, NOx, and methane, the authors noted that such co-firing supports policies of the Clean Air and Energy Policy Act [115]. Readers may want to review other documents related to this subject [116–118]. Many herders in the Tibetan Plateau still follow traditional lifestyle practices, including living in tents and burning yak dung for fuel. Kang et al. [119] reported on a study of indoor air quality in the nomadic tents in the Nam Co region, inland Tibetan Plateau. The results showed very high concentrations of total suspended particles (TSP), averaging at 4.45 mg/m3 during the cooking/ heating period (with daily value of 3.16 mg/m3). Elevated concentrations of toxic elements (Cd, As, and Pb) were found within the tents, averaging 3.16 mg/m3, 35.00 mg/m3, and 81.39 mg/m3 for a day, respectively, which was more than 104– 106 times higher than the outdoor air level in the Nam Co area [119]. Such fuels, including dried cow dung, coconut shell and husk, rice and other cereal straws, tobacco stalks, and jute sticks are often burned in open fires or unvented cookstoves inside the home.


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Table 3 Summary of selected mutagenicity results (S. typhimurium, TA98 S9) from Bell and Kamens [125] in revertants per mg extract. Fuel Dried dunga Coconut shell Crop residue Pine Red oak Peat a

S9 0.16 1.56 0.04 0.48 0.09 0.25

4.

+S9 0.33 2.61 0.28 1.20 0.81 0.38

5. 6.

An average of samples from two sources.

The resulting human exposures to combustion gases and particles from indoor burning can be 100-fold higher than pollutant levels typically experienced in polluted urban air [120–124]. Dichloromethane extracted 97% (370.0 mg extract/379.8 mg particles) of the mass of the cow-dung-smoke particles and 35.3% (50.0 mg extract/141.8 mg particles) of the coconut-shell-smoke particles. In studies at UNC [125], it was observed that soft, slow-burning fuels often produce particles with higher extractable organic content than do hard, fast-burning fuels. Bell and Kamens also examined these fuels using the Salmonella bioassay using TA98, S9 (Table 3). 7. Summary Charles [126] in a public policy paper questioned the use of biofuels as a replacement for petroleum fuels. While looking at the proposed benefits and disadvantages of biofuels, Charles says that the use of biofuels may inhibit the development and maturation of needed longer-term technologies [126]. In a scientific news article, Dan Charles wrote, ‘‘Last week, the California Air Resources Board (CARB) adopted a low-carbon fuel standard that requires greater use of fuels that cause lower greenhouse emissions, compared with gasoline. . .. Corn-based ethanol doesn’t meet that test and won’t benefit from the new standard, CARB says, because diverting corn into ethanol production increases deforestation and the clearing of grasslands’’ [127]. CARB, using a Purdue University model, developed and concluded that corn-based ethanol produces slightly more greenhouse emissions than does gasoline, with about 30% of those emissions occurring as farmers clear land for crops. One researcher found that corn ethanol produces twice the greenhouse gas emissions of gasoline, for every mile driven, once emissions from land conversion are counted [128]. This finding is important because additional toxic compounds may also be emitted into the atmosphere. 8. Conclusions I infer from the literature the following conclusions: 1. Emissions from ‘‘alternative’’ carbonaceous fuels are mutagenic and carcinogenic (i.e., they are positive in bioassays and/or contain known mutagens and carcinogens). 2. Depending on their source and derivation, ‘‘alternative’’ carbonaceous fuels before combustion may or may not be genotoxic; however, in order to know their genotoxicity, appropriate chemical analysis and/or bioassay must be performed. In most cases, both bioassay and chemical analysis must be performed in order to quantify the level of genotoxicity or to rule out any significant level of genotoxicants. 3. Although it is not possible, in most cases, to examine all possible fuels used in all possible combustors (engines), the testing of typical fuels in a variety of combustors will provide guidance for

7.

8.

estimating the health effects associated with a particular type of fuel (or engine). Because results are limited, results cannot be used to infer differences for vehicles as a whole. That is, for example, not all diesel engines produce the same results (quantitatively or qualitatively). Other factors (e.g., after-treatment durability, deterioration, vehicle maintenance, etc.) have not been adequately explored. Newly developed fuels and combustors must be tested to determine if they provide a public health advantage over existing technologies – including what tradeoffs can be expected (e.g., decreasing levels of PAHs versus increasing levels of NOx and possibly nitroarenes in ambient air). Investigators should determine if after-treatments will decrease toxicants over extended periods rather than just immediately following instillation. Because mutation is linked to heritable conditions, birth rates, birth defects, and many other health conditions, researchers and decision makers should not limit their efforts to the cancer process when mutagens are identified.

Much progress has been made in reducing the pollutants emitted from various combustors (including diesel engines and power plants) by the use of alternative fuels; however, much more progress is needed. Not only must researchers improve fuels and combustors, but also there is a need to improve the toxicology testing and analytical chemistry methods associated with complex mixtures. Another need is to improve exposure estimations which presently are a weak link in doing risk analyses. Conflict of interest statement The author declares that there are no conflicts of interest. Acknowledgements Special thanks go to Mutation Research and the editors (David DeMarini and Mike Waters) and the journal’s reviewers for their encouragement and assistance. I want to think all researchers and decision makers worldwide who have contributed to the efforts that have helped to protect the public’s health through protecting the air breathed, the water ingested, and the soils contacted. As John Locke once said, ‘‘It is one thing to show a man that he is in an error, and another to put him in possession of the truth.’’ References [1] G.W. Huber, S. Iborra, A. Corma, Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering, Chem. Rev. 106 (2006) 4044–4098. [2] S.E. Koonin, Getting serious about biofuels, Science 311 (2006) 435. [3] A. Finco, M. Padella, Biofuel sector overview, in: A. Finco (Ed.), Biofuels Economics and Policy. Agricultural and Environmental Sustainability, FrancoAngeli, Milano, Italy, 2012. [4] M.R.L. Leal, M.V. Galdos, F.V. Scarpare, J.E. Seabra, A. Walter, C.O. Oliveira, Sugarcane straw availability, quality, recovery and energy use: a literature review, Biomass Bioenergy 53 (2013) 11–19. [5] W.-T. Tsai, P.-H. Wu, Environmental concerns about carcinogenic air toxics produced from waste woods as alternative energy sources, Energy Sources Part A: Recovery Util. Environ. Effects 35 (2013) 725–732. [6] H. Kerckhoffs, R. Renquist, Biofuel from plant biomass, Agron. Sustain. Dev. 33 (2013) 1–19. [7] J.C. Philp, R.J. Ritchie, J.E. Allan, Biobased chemicals: the convergence of green chemistry with industrial biotechnology, Trends Biotechnol. 31 (2013) 219–222. [8] P. Azadi, O.R. Inderwildi, R. Farnood, D.A. King, Liquid fuels, hydrogen and chemicals from lignin: a critical review, Renew. Sustain. Energy Rev. 21 (2013) 506–523. [9] G. Dragone, B.D. Fernandes, A.A. Vicente, J.A. Teixeira, Third generation biofuels from microalgae, in: A. Mendez-Vilas (Ed.), Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology, Formatex, Portugal, 2010, , Available at: http://repositorium.sdum.uminho.pt/ bitstream/1822/16807/1/3067.pdf.


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The history, genotoxicity, and carcinogenicity of carbon-based fuels and their emissions. Part 2: solid fuels.

The history, genotoxicity, and carcinogenicity of carbon-based fuels and their emissions: 1. Principles and background.

The history, genotoxicity, and carcinogenicity of carbon-based fuels and their emissions. Part 3: diesel and gasoline.

Market-driven considerations affecting the prospects of alternative road fuels.

Preliminary ecotoxicity assessment of new generation alternative fuels in seawater.

Issues for storing plant-based alternative fuels in marine environments.

Dangerous properties of petroleum-refining products: carcinogenicity of motor fuels (gasoline).

Hedgehog fuels gut regeneration.

Temperature effects on particulate emissions from DPF-equipped diesel trucks operating on conventional and biodiesel fuels.

Fossil fuels' future.

Inflammation fuels tumor progress and metastasis.

Metabolic fuels and reproduction in female mammals.

Solar energy for electricity and fuels.

Gas fermentation for commodity chemicals and fuels.

Epigenetic noise fuels cancer evolution.

The future of oil: unconventional fossil fuels.

Well-to-Wheels Greenhouse Gas Emissions of Canadian Oil Sands Products: Implications for U.S. Petroleum Fuels.

PETA fuels animal lab improvements.

Monolithic cells for solar fuels.

Cytoplasmic methylation fuels leukocyte invasion.

Air emission from the co-combustion of alternative derived fuels within cement plants: Gaseous pollutants.

Long-term sampling of dioxin-like substances from a clinker kiln stack using alternative fuels.

Extrafloral nectar fuels ant life in deserts.

Consideration of black carbon and primary organic carbon emissions in life-cycle analysis of Greenhouse gas emissions of vehicle systems and fuels.