Thesis: Alternative Fuels for Commercial Aviation

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¶ … alternative fuels as well as the presently available fuels and those which are still in the developmental phases. There has been an increase in concern over degradation of the environment over the past two decades and "of the various options open to society to reduce the environmental burden, technology is widely considered as the most attractive option." (Kemp, nd) the world's economic system is dependent on coal, oil and natural gas for approximately 90% of the world's energy supply. It is stated that "conversion and end-use technologies" transform energy into useful forms of energies and services. Alternative energy supply technologies have become available as well. Reviewed in this study are the various fuel energy types and the characteristics of each of these.

ALTERNATIVE FUELS for COMMERCIAL AVIATION

Introduction

This work reviews the requirements for alternative fuels as well as the presently available fuels and those which are still in the developmental phases. The work of Kemp (nd) entitled: "Technology and the Transition to Environmental Sustainability: The Problem of Technological Regime Shifts" states that the present environmental problems "call for more environmentally benign technology." There has been an increase in concern over degradation of the environment over the past two decades and "of the various options open to society to reduce the environmental burden, technology is widely considered as the most attractive option." (Kemp, nd) the world's economic system is dependent on coal, oil and natural gas for approximately 90% of the world's energy supply. It is stated that "conversion and end-use technologies" transform energy into useful forms of energies and services. Alternative energy supply technologies have become available as well.

I. Available Alternative Aviation Fuels

The work of Daggett, Hendricks, Walther, and Corporan (2008) entitled "Alternate Fuels for Use in Commercial Aircraft" states that an investigation by the engine and aircraft Research and Development (R&D) communities has been ongoing in regards to "the practicality of using alternative fuels in near-term, midterm and far-term aircraft." (Daggett, Hendricks, Walther, and Corporan, 2008) p.5 in the present there appears to be the use of "drop in" jet fuel replacement which is comprised by kerosene and synthetic fuel blend whereas future "midterm aircraft may use a biojet and synthetic fuel blend in ultra-efficient airplane designs. Future far-term engines and aircraft in 50-plus years may be specifically designed to use a low- or zero-carbon fuel." (Daggett, Hendricks, Walther, and Corporan, 2008) p.5

Jet fuels that are synthetic are stated to be manufactured through use of a "Fisher-Tropsch process from coal, natural gas, or other hydrocarbon feedstocks. These fuels are very similar in performance to conventional jet fuel but contain almost zero sulfur and aromatics." (Daggett, Hendricks, Walther, and Corporan, 2008) p.5 According to Daggett, Hendricks, Walther, and Corporan the outcome may be "lower particulate exhaust emissions.' (2008) p.5 Additionally, synthetic fuels exhibit excellent low-temperature properties, maintaining a low viscosity at lower ambient temperatures." (Daggett, Hendricks, Walther, and Corporan, 2008) p. 5

Furthermore synthetic fuels are stated to "exhibit excellent low-temperature properties, maintaining a low viscosity at lower ambient temperatures." (Daggett, Hendricks, Walther, and Corporan, 2008) This results in improvement of thermal stability properties and ultimately less fuel deposits in the system. Synthetic fuels are good in performance and for this reason have been used in the form of Sasol fuel at the Johannesburg airport. For this reason it will be easy to supplement current jet fuel supplies with synthetic derived fuel." (Daggett, Hendricks, Walther and Corporan, 2008)

Capture of CO2 produced during the process of manufacturing and permanent sequestering of CO2 would result in synthetic fuel becoming a good near-term supplement according to Daggett, Hendricks, Walther, and Corporan (2008). p.6 Any mid-term solutions, or those in the next ten to fifty years is held to be comprised by alternate fuels making up "a much larger percentage of jet fuels. These fuels may also involve the blending of biofuels with the synthetic fuel." (Daggett, Hendricks, Walther and Corporan, 2008)

Daggett, Hendricks, Walther and Corporan (2008) report that presently "...almost all alternative fuels present some challenges to implement when compared with conventional kerosene jet fuel." Fundamental requirements stated for a commercial jet fuel include those as follows: (1) a low weight per unit heat of combustion (BTU) to allow the transport of revenue producing payload and (2) a low volume per unit heat of combustion to allow fuel storage without compromising aircraft size, weight, or performance. (Daggett, Hendricks, Walther and Corporan, 2008)

The various types of fuel include those as follows:

(1) Hydrogen Fuel. -- H2, publicized as the most environmentally benign alternative to petroleum, has its own drawbacks and is not a source of energy in itself. H2 production needs an abundantly available source of energy, such as electrical power, produced from nuclear fusion and a large source of clean water. Although combustion of H2 emits no carbon dioxide (CO2) emissions and is lightweight, its production, handling, infrastructure, and storage offer significant challenges. The volumetric heat of combustion for LH2 is so poor that it would force airplane design compromises. The use of LH2 (or methane) will also require an entirely new and more complex ground transportation, storage, distribution, and vent capture system.

(2) Other Liquefied Fuels. -- the liquefied petroleum gases, propane and butane, are not cryogens, but they have many of the same storage and transfer problems associated with a cryogen. in-depth studies of these fuels have not been conducted because the natural supply is not sufficient to support a worldwide aviation fleet. Manufacturing propane or butane offers no availability, cost, or environmental advantage as a replacement for conventional jet fuel.

(3) Alcohols. -- the alcohols (methanol and ethanol) have very poor mass and volumetric heats of combustion and are not satisfactory for use as a commercial aircraft fuel. Even though they are not useful for commercial aviation, their widespread production and use could influence the supply and cost of conventional jet fuel by freeing up additional petroleum resources for aircraft. Their production might have merit in that context

(4) Biofuels" these are combustible liquids that are manufactured from renewable resources such as plant crops or animal fats. Crops with high oil content such as soybeans, rapeseed (canola), and sunflowers are the starting materials used to produce bio-oils or bio-blending components that can be mixed with petroleum fuels. The oil is obtained by first cleaning, cracking, and conditioning the beans. The beans are subsequently compressed into flakes. The oil is then extracted from the flakes by a solvent extraction process. The primary components of bio-oils are fatty acids. The first process in utilizing these bio-oils is to crack and convert the raw oil into an ester. These esters can be used directly or can be modified into a variety of products. The ester from soybeans is called SME (soy methyl ester) and from rapeseed, RME. One of the challenges of using SME in a commercial aircraft is its propensity to freeze at normal operating cruise temperatures. By selecting specific fatty acids and the method of esterification, different properties, such as freezing point, can be obtained. Another option is to use a separation process to enable a lower freezing point for bio-jet fuel. Another challenge of SME is the stability of the oil over time. Currently, it is advised that the product be used within 6 months of manufacture. The lack of product consistency and storage stability -- as exhibited by the cloudiness are common problems of biofuels. For these reasons, SME is usually blended with petroleum diesel and limited to a 20% blend. For biofuels to be viable in the commercial aviation industry, significant technical and logistical hurdles need to be overcome. However, the task is not insurmountable, and no single issue makes biofuel unfit for aviation use. Aircraft equipment manufacturers and regulatory agencies will require a great deal of testing before biofuels can be approved. With adequate development, biofuels could play some role in commercial aviation fuel supplies.

(5) -- Jet fuels produced from synthesis processes are somewhat different from petroleum-based jet fuel and are currently being investigated by the aviation industry. The positive attributes of this fuel include a cleaner fuel with no sulfur, higher thermal stability, and possible lower particulate engine emissions. The negative attributes include poorer lubricating properties, lower volumetric heat content, possible contributor to fuel system elastomer leakage, and increased CO2 emissions during its manufacture. these can be made into synthetic transportation fuels by two routes. One method is a direct liquefaction technique; however, this is complex and expensive. The other, most favored process, is partial oxidation, or the Fischer-Tropsch (FT) process. The feedstock, such as coal, is mined and crushed, then converted into carbon monoxide (CO), H2 gases, and ash. The ratio of CO to H2 is adjusted before the mixture goes into a synthesis unit to produce the jet fuel. Large quantities of energy are used in this process that can result in the release of large quantities of CO2 into the atmosphere. The process can be considered only as a long-term, viable alternative to… [END OF PREVIEW]

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