A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines

Abstract

The fundamental combustion and emissions properties of advanced biofuels are reviewed, and their impact on engine performance is discussed, in order to guide the selection of optimal conversion routes for obtaining desired fuel combustion properties. Advanced biofuels from second- and third-generation feedstocks can result in significantly reduced life-cycle greenhouse-gas emissions, compared to traditional fossil fuels or first-generation biofuels from food-based feedstocks. These advanced biofuels include alcohols, biodiesel, or synthetic hydrocarbons obtained either from hydrotreatment of oxygenated biofuels or from Fischer–Tropsch synthesis. The engine performance and exhaust pollutant emissions of advanced biofuels are linked to their fundamental combustion properties, which can be modeled using combustion chemical-kinetic mechanisms and surrogate fuel blends. In general, first-generation or advanced biofuels perform well in existing combustion engines, either as blend additives with petro-fuels or as pure “drop-in” replacements. Generally, oxygenated biofuels produce lower intrinsic nitric-oxide and soot emissions than hydrocarbon fuels in fundamental experiments, but engine-test results can be complicated by multiple factors. In order to reduce engine emissions and improve fuel efficiency, several novel technologies, including engines and fuel cells, are being developed. The future fuel requirements for a selection of such novel power-generation technologies, along with their potential performance improvements over existing technologies, are discussed. The trend in the biofuels and transportation industries appears to be moving towards drop-in fuels that require little changes in vehicle or fueling infrastructure, but this comes at a cost of reduced life-cycle efficiencies for the overall alternative-fuel production and utilization system. In the future, fuel-flexible, high-efficiency, and ultra-low-emissions heat-engine and fuel-cell technologies promise to enable consumers to switch to the lowest-cost and cleanest fuel available in their market at any given time. This would also enable society as a whole to maximize its global level of transportation activity, while maintaining urban air quality, within an energy- and carbon-constrained world.

Introduction

As stated by MacLean and Lave [1], the use of gasoline or diesel in an internal combustion engine is likely to remain the most cost-effective overall ground-based transportation propulsion system for the near future. However, concern over greenhouse-gas emissions and the potential for rapid increases in petroleum prices due to supply-demand constraints motivates a search for alternative transport fuels, as well as high-efficiency conversion technologies that can obtain the maximum motive power (energy) out of the available chemical fuels. Liquid fuels remain attractive for transportation because of their high energy density: a teaspoon of gasoline, diesel or jet fuel contains chemical energy equivalent to the kinetic energy of a 1000 kg vehicle being driven at 100 km/h [2].

The first developments in biofuels for transportation applications were based on the well-established processes of converting plant sugars into ethanol via fermentation, and the upgrading of vegetable oils via transesterification. Ethanol is a high-octane fuel compatible with the majority of spark-ignition, or gasoline-fueled, internal combustion (IC) engines on the road today, as long as it is blended with gasoline at a reasonable level [3]. The high-level of interest in ethanol as a biofuel was motivated by the seemingly easy merging with existing infrastructure and the relatively low cost of producing the fuel due to an existing and proven alcohol production industry.

Similarly, biodiesel gained importance for compression-ignition, or diesel, engines due to its relative ease of manufacture. The use of plant oils as a transportation fuel was suggested by the visionary Rudolf Diesel at the very dawn of our current internal-combustion-engine driven society, when he stated:

“the fact that fat oils from vegetable sources can be used may seem insignificant today, but such oils may perhaps become in course of time of the same importance as some natural mineral oils and tar products are now….they make certain that motor-power can still be produced from the heat of the sun, which is always available for agricultural purposes, even when all our natural stores of solid and liquid fuels are exhausted” [4].

Vegetable oils, in raw form, can be used as fuels for IC engines, but engine wear problems and poor combustion performance, as well as the related higher emissions levels, motivate extra fuel processing to convert the triglycerides in the raw oils to the fatty-acid esters known as biodiesel [3], [5], [6], [7]. Traditionally, methanol has been used during the trans-esterification reaction producing fatty-acid methyl esters (FAME), but ethanol or propanol have been suggested for improving the cold-flow performance of biodiesel fuels [6], [7], and use of bioethanol can improve the sustainability of the resulting biodiesel [8], [9], but this may come with associated cost increases.

Increasing concerns over the environmental impacts of so-called first-generation biofuels [10], [11], [12], [13], [14], which include ethanol from corn and biodiesel from soybean or other edible oils, has motivated efforts to find other feedstocks. The first efforts in this area focused on conversion of nonedible oils [15], from crops such as camelina [16], [17] or jatropha [15], [18], [19], [20], [21], into biodiesel or synthetic hydrocarbons, while current efforts are focusing on finding ways of converting the cellulosic fraction of the biomass into a suitable fuel source [11], [12], [13], [14], [22], [23], [24], [25]. Such cellulosic-derived fuels are commonly termed second-generation biofuels [12], [13], [14], [26]. While ethanol is a possible fuel product from cellulosic biomass [13], [14], [22], [23], [24], [25], there has been a significant increase over the past few years in the number of potential new fuel molecules, including methanol [27], [28], butanol [14], [25], [29], [30], [31], longer-chain alcohols [32], furan-based molecules [33], [34], [35], and bio-derived synthetic hydrocarbons [13], [23], [35], [36], [37], [38], [39]. Third-generation biofuels are considered to be those fuels that are derived from algae feedstocks [40], [41], [42], [43], [44], [45], [46], which could be FAME [45], [47] or synthetic hydrocarbons obtained from upgrading algal oils [26], [47], [48]. Production of synthetic hydrocarbon fuels, which store primary energy from solar or nuclear sources as chemical energy, directly from water and recycled carbon dioxide, after its capture from exhaust stacks or the atmosphere, has also been proposed [49], [50], [51], [52]. These synthetic “solar fuels” [50], [51], [52] will have similar combustion properties to bio-derived synthetic hydrocarbons of the same molecular composition.

Within the aviation community, it has become clear that the conversion of the global aircraft fleet to operate on a new fuel molecule is unacceptable from a cost and logistical perspective. Therefore, all biofuels for aviation use must be “drop in”, a term that means that the fuels must meet the general requirements of the ASTM specification for jet fuel (ASTM D1655) [53], [54]. A standard has been created (ASTM 7566) that allows blends of up to 50% of synthetic hydrocarbon alternative jet fuels to be used when mixed with conventional commercial jet fuel, but, currently, only for select processes from specific feedstocks [55]. These synthetic hydrocarbon fuels can be obtained either through Fischer–Tropsch synthesis [36], [38], [39], [53], or by hydrotreatment of triglycerides from vegetable oils, or animal fats, to remove all oxygen content from the fuel [36], [53], [56], [57].

The present review discusses the combustion properties of a range of advanced biofuels and the impact of these different properties on engine performance and exhaust pollutant emissions. The conversion technology employed, along with the biomass feedstock in some cases, determines the chemical content and molecular structure of the fuel molecules, which will affect the combustion chemistry and the resulting pollutant emissions when they are used in engines. It is important to clarify that this is only one of the many factors that will be important in the decision to choose a specific biofuel, and its associated conversion process and feedstock(s). The technical, economic and environmental issues surrounding the production of these advanced biofuels, including the availability, cost and sustainability of the feedstock [11], [46], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], the efficiency and cost of the associated biological or thermochemical biomass-to-biofuel conversion technology [8], [12], [13], [14], [15], [20], [22], [23], [24], [25], [27], [28], [29], [73], [74], [75], and the life-cycle greenhouse gas emissions [11], [50], [56], [57], [62], [65], [66], [70], [71], [76], [77], [78], are complex topics that have been extensively discussed and reviewed elsewhere. In addition to biofuels, bioenergy can be harnessed by burning biomass in power stations to generate electricity which is then used in electric-drive cars [79], or even by directly burning biomass dusts within internal-combustion engines [80]; however, discussion of these topics is outside the scope of the present paper. Biomass can also be gasified into producer gas or synthesis gas (syngas), or be anaerobically digested to form biogas (bio-methane). These are of current interest as potential fuels for stationary power generation, e.g. [81], but are not currently considered feasible transportation biofuels within the context of this review.

The basic types of advanced biofuels are discussed in Section 2, followed by the combustion properties of such fuels, grouped by the engine type that they are most compatible with, in Section 3. The review is intended as a high-level overview of the combustion and emissions properties of a broad range of advanced biofuels for non-combustion specialists. Many of the topics herein have been reviewed in detail in past publications and preference is given to these sources when available. The review will also discuss advanced-engine and fuel-cell technologies that can more-efficiently convert the chemical energy stored in the fuel into motive power (energy), i.e., increase fuel efficiency, while simultaneously producing lower exhaust emissions than traditional spark ignition (SI) or compression-ignition (CI) engines to meet future emissions regulations and improve urban air quality. The optimal properties of fuel blends for future vehicles will be determined through advanced-engine research, which already suggests changes from traditional gasoline and diesel specifications due to the move to low-temperature combustion. The potential operational improvements, in terms of efficiency and emissions, and fuel requirements of a selection of advanced engine and fuel-cell concepts are discussed in Section 4.