Alternative fuels - biofuel

Alternative fuels: Aviation biofuel will power the future

Paola Leibbrandt
26.10.2018
1 picture
5 minutes

Our latitudes are increasing in temperature, the glaciers are melting, and resources are running out. What may, several years ago, have seemed like the beginning of a disaster film has now become reality. It is clear to businesses in all sectors that something must be done to protect the climate. With this in mind, the aviation industry is seeking alternatives to reduce CO2 emissions and the use of traditional aviation fuels. The goal is to replace these with alternative fuels as soon as possible. This article describes the potential jet fuels of the future.

Why do we need alternative aviation fuels?

When aviation fuel is burned, the main products are carbon dioxide (CO2) and steam. CO2 is, however, tremendously detrimental to the environment, and aviation produces 2-3 percent of the world’s total climate-damaging greenhouse gas emissions. In order to reduce the environmental problems caused by air traffic, alternative fuels are being developed which will, ideally, be carbon-neutral when combusted.

Aviation biofuel reduces contrails

Condensation trails may look pretty, but they have a negative effect on the climate. At the aircraft cruising altitude of ten kilometres, the troposphere’s temperature quickly drops to a brisk minus 40 degrees Celsius. This cold air cannot absorb the steam from jet engines as well as the warm air closer to the earth’s surface. The steam therefore condenses on the soot particles in the jet blast, causing water droplets to form. These droplets then freeze, creating the visible trails. As long as the condensation trails immediately vaporise (which they do at temperatures above minus 40 degrees Celsius or in dry air), there’s no problem. If the contrails persist, however, they continue to take on additional steam, eventually forming thin ice clouds, known as cirrus clouds. These “artificial” cirrus clouds cannot be distinguished from naturally occurring cirrus clouds, which herald cold fronts. Cirrus clouds absorb some of the sun’s rays, which they reflect back in the form of warmth. At the same time, the cirrus clouds prevent warmth from the earth’s surface from rising, while their shadows cool the air beneath them still further. This is exactly what happens in a greenhouse. Contrails heat up the atmosphere and increase the greenhouse effect. Aviation biofuel leave fewer soot particles in the jet blast, resulting in measurably fewer condensation trails.

What is aviation biofuel and how is it produced?

Aviation biofuel is an alternative form of jet fuel based on organic materials such as hydrogenated vegetable oils, whole plants, animal fats or algae. Renewable energies (like sun, wind, water or carbon dioxide) are also potential sources of fuel, and can be converted to more climate-friendly alternatives by means of various processes. We explain how conventional aviation fuel is produced in this article.

Fischer-Tropsch Synthesis

The Fischer-Tropsch process enables jet fuel to be produced from liquid hydrocarbons which are converted from a synthetic gas made up of carbon monoxide and hydrogen. This process is frequently used to produce alternative fuels, as they can be derived from all carbon-rich materials. These include, for example, plant biomass and even waste.

The Power to Liquid Process

The power-to-liquid process uses sun or wind, water and carbon dioxide to create a synthetic gas. This is then converted to aviation fuel by means of Fischer-Tropsch synthesis.

Solar to Liquid Process

The solar to liquid process uses the sun’s energy to split metallic oxides into metal and oxygen ions in a reactor. During this process, the sun’s rays are bundled by solar thermal collectors to enable the requisite temperature of up to 2,000 degrees Celsius to be achieved. CO2 and steam are added to produce a synthetic gas, which is then processed into aviation biofuel using the Fischer-Tropsch method.

Hydroprocessed Esters and Fatty Acids (HEFA)

The HEFA process is used to create biofuel based on hydroprocessed esters and fatty acids. In this process, animal and vegetable fats and oils (such as waste oil from food processing) are refined in order to create aviation fuel.

Alcohol to Jet Process

This process converts alcohol directly to hydrocarbon chains. These are then processed into biofuel.

Tremendous demands on aviation biofuel

In order to replace traditional fuels, biofuels must meet multiple requirements. The first challenge is cost: producing alternative fuels must be economical. Producing aviation biofuels is currently about twice as expensive as producing conventional fuels. Secondly, the physical properties and quality cannot deviate from the current standards. Aviation fuel is subject to extremely stringent requirements, and these must be fulfilled in order for it to be certified for aviation use. It must, for example, remain liquid at low temperatures of -50 degrees Celsius and have a very low frost point. It is also important that aviation biofuels are compatible with the aircraft’s technical components, meaning that no modifications to the gas turbines are necessary.

When will alternative fuels become the norm?

It is clear that the aviation industry should be experiencing carbon-neutral growth as of 2020. When exactly alternative fuels will become economical, however, remains unclear. Additional factors, such as water consumption and competition from the food industry (which requires space and resources for growing food crops) also play a decisive role in biofuel research.

by Paola Leibbrandt

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