History of alcohol combustion. Image courtesy of Hyun Ho Hwang.           

Alternative fuels from traditional or non-traditional sources have a promising future in the transportation industry. Amongst these are alcohol fuels that contain anywhere from one to five or more carbon atoms. Mani Sarathy, Ph.D., Assistant Professor and Principal Investigator of Combustion and Pyrolysis Chemistry (CPC) at the Clean Combustion Research Center at KAUST, says they are considered promising because they can be produced by various methods, including both renewable and fossil based feedstocks. 

In a paper titled "Alcohol Combustion Chemistry" published in "Progress in Energy and Combustion Science" and a book chapter on "Biofuels from Lignocellulosic Biomass: Higher Alcohols", Prof. Sarathy and his team documented the results of the alcohol combustion in reactors, shock tubes, rapid compression machines, and research engines. They also studied the theoretical aspects of alcohol combustion chemistry and used this information to develop a model that simulates alcohol combustion. From their study, they were able to gather specific aspects of alcohol combustion to provide a better understanding of combustion in engines including such factors as ignition quality, flame speed, and emissions. The collection of experimental targets and semi-predictive models will help design more efficient internal combustion engines operating on alcohols.

Prof. Sarathy, Ph.D., sat down with us to provide additional insight into what he learned from his findings, the challenges both he and his team encountered, and what his team's findings mean for the future of alcohol combustion.

Your research and review of alcohol combustion factors within the publication were extensive. Could you describe the process and work that goes into creating a scientific paper like this? 

The paper took about 1.5 years to write up. We read 570 papers on alcohol combustion dating back to the early 1900s. Then we had to synthesize all existing knowledge to paint a clear picture of the current state of the art. We finally identified gaps that are in the literature and used simulations to predict how alcohols would burn at conditions that have never been tested before. 

Can you briefly outline the findings of your paper?

The combustion of alcohols actually dates back nearly a thousand years, when it was first discovered by Islamic scholars in the 11th century. These fuels have been used for various applications (lighting, heating, transportation, etc.) over the centuries. However, it is only in the past 10-20 years that alcohol combustion has been studied as a science. These studies reveal many fascinating properties of alcohol fuels, including high resistance to autoignition, decreased particulate matter emissions, and other unique properties that can actually help the design of new high efficiency low emission engine technologies. In addition, we also found that alcohol fuels can have negative consequences, such as increased emissions of carcinogenic aldehydes, which has implications on air quality regulations and fuel formulation/blending strategies.

What in your paper is new and previously undiscovered, or at least undeveloped?

One major development was the ability to predict a wide range of fundamental properties and emissions profiles arising from the combustion of all industry relevant alcohols. The model had to include detailed knowledge of alcohol pyrolysis and oxidation chemistry at the atomic level, in order to reproduce available experimental combustion data. Once we had the model, we were able to show how alcohol combustion is very different than combustion of typical hydrocarbon fuels and how these differences can be used to design better fuels for better engines.

In our paper we show that most of the theoretical and experimental science on alcohol combustion only occurred in the past decade. Prior to this, there was a lack of information. With the body of information sufficient, we were able critically review all the knowledge and make new discoveries.

How did the KAUST Clean Combustion Research Center (CCRC) help enable this intensive research?

When you read 500+ papers covering a range of topics from quantum chemistry to experimental engine research, you need to be able to comprehend all that information and discuss it with experts in the field. The CCRC has expertise in virtually all areas of combustion research, so interactions with them were invaluable. In addition, the CCRC has a range of experimental capabilities (shock tubes, rapid compression machines, flame experiments, laser diagnostics, molecular beam mass spectrometers, etc.), which have all been used to interrogate alcohol combustion. Having access to all these facilities allowed me to really understand the literature and what the limitations of each study were.

What was the most surprising finding?

There is a relatively new phenomenon affecting high efficiency gasoline engines (downsized turbocharged spark ignition engines) called superknock. This phenomenon occurs when a fuel/air mixture ignites in the cylinder prior to the main spark firing. The ignition kernel then grows into a flame kernel, eventually leading to a very heavy knock or super knock that severely damages the engine. We found that certain alcohols can significantly decrease super knock tendency due to their high resistance to autoignition and low laminar flame speed. These findings can help fuel blenders develop optimal fuels or additive packages for new high efficiency gasoline engines.

How much of an impact your paper will have in the Alcohol Combustion community and in the scientific community as a whole?

Our work is expected to guide future research on experimental and theoretical combustion research on alcohols. It will also enable fuel producers to identify the best alcohols needed for various engine applications.

What’s you long term goal here at KAUST? What do you hope to achieve?

We are trying to create a fundamental understanding of how engines can be improved using basic knowledge of how fuels burn. This fundamental knowledge is used to create computer models that simulate the burning process. The way the engines were designed in the past was mainly empirical…some guys that are really good with tools tweak the engine to find the optimal design and operating conditions. Industry is quickly changing this model of engine development towards using computer simulation to predict how an engine will operate as various parameters (including the fuel) change.

Our simulations can inform producers and refiners about what the best fuels may be and how they can tailor their products to increase marketability in the transportation sector. We are also doing fuel systems and life cycle analysis to ascertain the economic and environmental sustainability of various fuels in their relevant combustion devices. These metrics are equally as important as the fundamental combustion metrics that we are studying. By synergizing combustion models with sustainability models, we can make informed decisions on what is the best fuel to burn from a technical as well as economic and environmental standpoint.

By David Murphy, KAUST News 

Click here to read this news item on KAUST Discovery where it was originally published.