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By Nick Eaves, PhD Candidate, University of Toronto

Canada is known for its long, rough winter, so it’s no wonder when the summertime comes around, Canadians rejoice and make the best out of it. Many of us end up going to a cottage to enjoy the outdoors. When I was at the cottage this summer, I started thinking of what made it so great, and I realized, it must have been all the fresh air I was getting.

One of the major sources of air pollution is particulate matter emissions (soot) from vehicles used in the transportation industry. Atmospheric soot, often referred to as black carbon, or particulate, is known to have detrimental effects on both human health and the global environment. Soot particles, can be absorbed deeply into the lungs (<PM2.5), causing respiratory diseases such as asthma and bronchitis. The particles can be small enough to pass into the bloodstream leading to possible heart disease. Polycyclic Aromatic Hydrocarbons (PAHs), which are pre-cursors to soot formation, have also been classified as known carcinogens by the International Agency for Research on Cancer (IARC) . Additionally, soot is an environmental concern as it is the second most important factor in global warming behind carbon dioxide, due to it being a strong absorbent of solar energy. A portion of the sun’s energy is reflected when it hits the atmosphere or earth’s surface; however, an increase in soot reduces the amount of energy that gets reflected, through thermal absorption and heating of the particles.

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Today, most diesel engines that are used for transportation have very high soot emissions. The current solution has been to utilize particulate filters to clean the exhaust. These filters are expensive to purchase and maintain, requiring periodic regeneration cycles to avoid the filters clogging and cracking. Another possible solution is the use of renewable natural gas (RNG), also called biomethane. RNG has lower soot emissions than conventional diesel fuels, eliminating the need for particulate filters.

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How do we make RNG?

The process starts when bacteria decompose biological matter in the absence of oxygen, which is called anaerobic digestion. One of the byproducts of anaerobic digestion is biogas. When the biological matter is organic waste, the biogas is called landfill gas. Biogas is mostly methane and carbon dioxide; however, it contains other components such as hydrogen sulfide and water.  These impurities can be removed with existing technology to result in high-purity natural gas (RNG or biomethane) that can be used in vehicles.

The advantages of RNG

The main advantage of RNG is greatly reduced emissions. Regular natural gas has very low soot emissions along with the lowest carbon intensity of any transportation fuel. These reduced emissions transfer over the RNG as well, which when combined with its renewable origins, combine to make a very sustainable fuel. With these reduced particulate emissions comes a reduced need for costly particulate filters. Infrastructure for vehicles running on natural gas is already being rolled out and this same infrastructure could be used to deliver RNG as well. This would reduce the overhead associated with adopting RNG for transportation vehicles.

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Issues with using RNG

Using RNG for transportation is not without its problems. Methane has a low cetane number, meaning it is very difficult to auto-ignite, or ignite without a spark. Almost all commercial transportation vehicles utilize compression-ignition (instead of spark-ignition) engines. Research has been done, primarily by Westport Innovations Inc, into compression-ignition engines that can work with methane, although it is still an area of investigation. While methane does have very low particulate emissions, the factors that affect the amount of soot emissions from methane (and RNG) are still unclear. While it is known that the emissions are lower, it is not known how much lower.

Predicting emissions from RNG

Trying to predict soot emissions from RNG is a difficult task, as the fundamental mechanisms of how soot forms are still being investigated and debated. Because of the complex nature of soot formation, detailed numerical models are used to understand the factors that affect soot emission levels. Current research, in collaboration with BioFuelNet, is focused on developing models that have predictive capability that can be used by design engineers to make engines with lower emission levels.

My recent HQP exchange to Cambridge University allowed me to learn a new numerical method, Monte Carlo simulation, for modeling soot formation which is bringing my research closer to the goal of having a fully predictive model. If the factors that affect emission levels are understood, designing lower emission engines will be easier.  This will reduce atmospheric particulate and soot levels, reducing the negative health and environmental effects from soot emissions. If this goal is achieved, Canadians may no longer have to go to the cottage to get their fresh air.

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