A silk purse from a sow’s ear? Two key questions answered with sustainable fuels
How can aviation fuel be sustainable?
The idea of producing fuel from sustainable sources is often greeted with scepticism, and quite rightly so. How can it ever be sustainable to burn fuel in the atmosphere releasing carbon dioxide in large quantities?
The answer is that the Avioxx process uses raw materials which would otherwise be burnt or disposed of in landfill, and this would normally lead to carbon emissions. So, the carbon in the wastes will indeed be emitted, but correspondingly less fossil fuel will be burnt.
There is not currently an alternative to reducing aviation CO2 output other than sustainable fuels because current battery technology is too heavy for electric planes and hydrogen is too volatile as an alternative aviation fuel.
Furthermore, the benefits of cleaning up the waste materials rather than sending them to landfill or incineration offer a double-edged sword in environmental benefits. Other green fuel alternatives include biofuels which are fuels made from crops that are grown. The challenge with these is that all the fertile farmland is currently used for food production and therefore more fertile land would be required and this leads to its own negative environmental impacts such as deforestation.
The costs and environmental impact of replacing the global fleet of aircraft into hydrogen or electric planes is not just infeasible based on currently technology but also comes with its own environmental cost of manufacturing new planes. Transition of aircraft needs to happen over the longer term and full lifecycle of an aircraft which is about 30 years.
How is sustainable fuel made?
We need to understand what fuels are and why they work. Liquid fuels like petrol, diesel and aviation fuel have a very high energy density and are easily stored and handled. Historically they were derived from crude oil by distillation in a crude distillation unit in a refinery. The heated crude oil separates into three main parts:
- Gases like hydrogen, methane and so on, which are normally used in the refinery itself.
- Paraffiwns, such as kerosene, which condense from the gases, and
- A whole range of other materials such as phenols, tars and bitumen which remain as liquids.
Molecular weights
The fraction of condensed gases (ii) contains a mixture of materials, and these can be separated out by further distillation steps. They will end up as the fuels we are familiar with but will still consist of mixtures of carbon compounds of different sorts, characterised according to boiling point and flammability. Petrol has smaller molecules made of carbon and hydrogen. It vaporises and ignites easily.
Diesel has larger molecules which have a higher boiling point. It needs to be compressed with air in order to ignite. Aviation fuel is designed to combine high energy density and a low tendency to ignite for safety reasons. The various fuels are the subject of tight internationally recognised specifications, but remain mixtures of molecules of various types because they are derived from crude oil by distillation.
The Fischer-Tropsch reactor
So how do we make fuels from waste? Some time ago, back in the 1920s two German scientists (Fischer and Tropsch) found that if you pass a mixture of carbon monoxide and hydrogen gases over iron beads at about 300C and 20bar pressure, the carbon and hydrogen combine to form a mixture of paraffins, leaving water as the by-product.
This mixture can be further processed to produce materials which have similar compositions to the traditional fuels. This principle has been used over many years to make fuels when crude oil is difficult to source (such as in South Africa during the trade embargo, and in Nazi Germany). In these cases, the carbon monoxide and hydrogen were made by treating coal with air and steam, in a process known as gasification.
The unique Avioxx process
The Avioxx process uses the same principles to treat organic wastes, but crucially involves fuel cells to generate power, electrolysers to make extra hydrogen and oxygen to react with the waste feedstock. These features ensure a wide variety of wastes can be processed to create the optimum mixture of carbon monoxide and hydrogen for entry to the second stage of the process.
So, in summary, the wastes are broken down into carbon monoxide and hydrogen, (often referred to as synthesis gas or “syngas”), and then built up to form paraffins and other organic compounds which collectively match the traditional fuels.
There are several very significant opportunities in developing this idea. Firstly, the feedstock is essentially free, and indeed is often seen as a costly disposal problem. Secondly, the aviation industry, faced with an obligation to address the growing need to mitigate its climate change impact, can better manage the transition to long term sustainability options whilst continuing to use its considerable fuelling infrastructure and flight platforms. The third opportunity is, for me, the most exciting one, and that is in the development of liquid fuels.
The second part of the fuel-making process, often referred to as Fischer-Tropsch, remains very much as it has existed over many years. It produces a waxy “FT Crude” and this has to undergo further treatment in order to yield the fuel of the precise quality. This is because the reaction at the start of the reactor is more intense than further on, and quite a wide variety of molecules are formed in the polymerisation reaction.
This wax covers the surface of the iron catalyst beads and blocks their use. It also covers the walls of the reactor and stops the heat from getting away. Controlling such reactors is known to be problematical. Our reactor design team are confident that their novel approach can lead to significant improvements in both yield and operability.
Use of software and machine learning to categorise waste
Waste materials are by their very nature difficult to classify and specify which means that the quality of the feedstock varies quite considerably over time. This poses big problems for operators because the fuel product needs to meet stringent specifications. The Avioxx process is specifically designed to accommodate this variability by providing generous flexibility plus a control system based on AI and Machine Learning. Basically, the raw material is analysed in real time using component recognition software, and from this the composition and heat content is estimated.
The AI system then determines the optimal route, flows and energy requirements in order to achieve the desired gas composition at the entry to the FT section. This is essential to allow low manning and stable, economic operation.
If you want to learn more about Avioxx and our green fuel movement then contact us on info@avioxx.com.