Toward a Low-Carbon Transportation Future: Part 2

By Tomas Endicott, Processing & Markets Manager

Last week I wrote about carbon intensity and how the GREET model, standardized by the U.S. Department of Energy, quantifies the amount of carbon dioxide (CO2) that is generated when producing different transportation fuels—both fossil fuels and renewable fuels.

Today, let’s talk about factors that contribute to producing low-carbon transportation fuels.

Lifecycle carbon tracks CO2 emissions from feedstock production to combustion.

Carbon dioxide (CO2) emissions are tracked on a lifecycle basis. That is, CO2 is generated at many points in a fuel production pathway: feedstock acquisition, processing, refining, transport. The more carbon efficient each step in a particular fuel production pathway, the lower the carbon intensity of the final fuel. For processing fossil fuels or biofuels, reducing carbon emissions may include using renewable sources of heat and electricity that generate less CO2, such as biogas, wind power and solar power. Acquiring feedstock to produce transportation fuels presents many different pathways, each unique in the lifecycle CO2 emissions it generates.

All feedstocks are not created (carbon) equal.

Fossil fuel feedstocks—crude oil or natural gas—are fairly carbon-consistent no matter what their origin. They all are extracted in enormous volumes from underground. Biofuel feedstocks are incredibly diverse. There are many more variables that contribute to carbon intensity throughout every step of any particular biofuel feedstock production process.

All plants are self-sufficient. Some more than others.

Most plants are photosynthetic. They create hydrocarbons in the form of carbohydrates (i.e starch, sugar, wood) and/or fats (oils) using carbon dioxide (CO2), nutrients, sunlight and water. The plants use these carbohydrates and fats for their own energy, and they “invest” them into their seeds for the next generation. These carbohydrates and fats also are the source of the energy we harvest and convert into biofuels.

All plants also need some amount of nitrogen to grow and thrive. Legumes, like soybeans, alfalfa seed and pongamia seed, are special in that they harness their own nitrogen—the backbone for proteins—through symbiosis with bacteria that live on their roots. These rhizobium bacteria fix elemental nitrogen from the atmosphere and supply it to the plant in a form the plant can use.

Less inputs equals lower carbon intensity.

Non-leguminous plants must derive nitrogen from compounds in the soil. In a natural environment, that source of nitrogen may be composted organic matter or nitrogen compounds deposited in the soil through earthworm activity. Because modern, improved agricultural crops produce such high yields, they require large quantities of commercial fertilizer. Commercial nitrogen fertilizer is synthesized from natural gas, and its production requires significant energy input. As a result, producing commercial nitrogen fertilizer generates carbon dioxide (CO2) emissions, and those emissions are attributed to the lifecycle carbon of the crops that use the nitrogen fertilizer.

Nitrogen is expensive, both in the energy consumed to manufacture and transport it and in the dollars farmers must expend to apply it to their fields. Because nitrogen fertilizers must be applied to non-leguminous crops like corn and canola, producing biofuels from these non-nitrogen-fixing crops is more carbon intensive than producing biofuels from legumes.

By-products provide additional value.

Oilseed crops, like soybeans, canola and pongamia, can provide oil as feedstock for renewable fuels. They also provide another by-product: high-protein meal, which has significant value as livestock feed and as organic fertilizer.

Pongamia seeds are removed from their shells before being processed. These shells are half the weight of the harvested ponamia pods, and they can provide significant biomass to supply renewable, low-carbon heat and power to the pongamia biofuel processing pathway.

Greater yield per acre equals lower carbon intensity.

Because carbon dioxide (CO2) emissions generated while producing crops are spread across the total yield of a particular crop, crops that produce higher yields per acre can be more carbon-efficient. Every trip across a field to till, seed, fertilize, spray or harvest increases CO2 emissions and increases the carbon intensity for a particular crop. Crops with higher yields spread their carbon dioxide (CO2) emissions over larger production.

Growing conditions also affect yield. Logically, crops grown in tropical and sub-tropical environments experience more sunshine and heat, and they have longer growing seasons, so they produce larger yields per acre.

Annual or perennial makes a difference.

Annual crops—those that must be planted every year—require some amount of tillage or application of broad spectrum herbicides (i.e Round-Up) to prepare the seedbed and to minimize weed competition with the cultivated crop. Tillage alone can increase carbon dioxide (CO2) emissions from agricultural fields simply by exposing organic matter in the soil to oxygen, thereby, allowing it to be decomposed aerobically, which generates CO2.
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Simply tilling the ground increases carbon dioxide (CO2) emissions from agricultural fields. Photo by Kai Oberhäuser on Unsplash

Perennial crops are established once and produce for many years. They do not require annual tillage. For large trees like pongamia, annual maintenance is low when the tree canopy prevents sunlight from penetrating to the ground, so nothing can grow there.

Although they require a few years to produce their first crop, yields for perennial crops tend to be much higher per acre than yields for annual crops. Whereas the average yield for soybeans in the U.S. is about 2,700 pounds per acre, perennial pongamia trees can produce more than 10,000 pounds of seeds per acre per year at eight years of age and beyond. The average lifespan of a pongamia tree is at least 25 to 30 years.

How many gallons of oils per acre?

For the purpose of biodiesel or bio-jet fuel production, seeds of different crops have different concentrations of oil—their percentage of oil by weight. Whereas soybeans contain only 16%-18% oil by weight, canola seeds contains more than 40% oil by weight and pongamia seeds contain 30% to 40% oil by weight. Considering the combination of per acre yields and the oil concentration in the seeds of a particular crop determines the amount of biodiesel or bio-jet fuel that can be produced by a given cultivated area. Here is a chart demonstrating the amount of oil per acre produced by different oilseed crops.

More yield per acre equals greater carbon efficiency.

Whereas soybeans produce only 55 to 60 gallons of oil per acre annually and canola produces about 120 gallons of oil per acre annually, mature pongamia trees can produce more than 450 gallons of oil per acre every year.

The CO2 generated to harvest an acre of soybeans or to harvest an acre of pongamia seeds are similar. Mature pongamia trees, however, yield almost four times more seed per acre than soybeans, and they yield about eight times the oil for every acre harvested. Now that’s efficiency!

Maximizing transportation efficiency minimizes CO2 emissions.

To achieve maximum carbon efficiency, transportation fuels need to be produced and moved in large volumes. It is most carbon-efficient to move fuels by pipeline, although pipelines are expensive to build and they have other environmental considerations. Moving a million gallons of fuel on a single ocean-going barge is ten times more efficient than hauling the same volume of fuel the same distance in hundreds of tanker truck loads. The efficiency of moving fuel in 25,000-gallon rail cars lies somewhere between the efficiency achieved by barges and the efficiency attributed to tanker trucks. Renewable fuels, like fossil-based fuels, must be produced on a large scale to achieve transportation efficiency.

Going further on a gallon of fuel reduces CO2 emissions too.

The most efficient gallon of fuel is the one that you never use. Producing low-carbon fuels at scale is only half the battle. Reducing consumption of all transportation fuels is the best carbon-reduction strategy for the transportation sector. Electric cars, hybrids and clean diesel technology are all available today, and all are improving with each new model year. In 2012, President Obama established new Corporate Average Fuel Economy (CAFE) standards which will raise the average fuel efficiency for all new cars and trucks in the U.S. to 54.5 miles per gallon by 2025. Impressive! Currently, the CAFE standard is 35.5 miles per gallon.

Carbon-efficient, sustainable biofuel feedstock, high-protein livestock feed and organic fertilizer from the perennial pongamia tree.

The pongamia tree can provide a unique and substantial contribution to the United States’ sustainable, low-carbon biofuel future. It is a nitrogen-fixing, subtropical tree that is native to India, Indonesia and Australia, and it grows well in Florida and in Hawaii. It is both drought resistant and accustomed to Monsoonal rains. (TerViva’s pongamia orchards in Florida held their own against the wind, rain and flooding from Hurricane Irma last week.) Pongamia can grow on sandy soils, and it is resistant to moderate salinity. It is a perennial tree that is highly productive for both non-edible oil as a feedstock for biofuels and for protein-rich meal for livestock feed and fertilizer.

Imagine our low-carbon transportation future!

TerViva is rolling out pongamia orchards on abandoned citrus land in Florida and on land that formerly grew sugarcane in Hawaii. Imagine a future where 100,000 acres of pongamia trees produce 50 million gallons of biofuel and 340,000 tons of high-protein meal each year. Imagine biomass heat and power produced from a half-million tons of pongamia shells harvested annually. Imagine bio-char from gasified pongamia shells sequestering carbon in the soil for thousands of years—steadily reversing the CO2 increase in the earth’s atmosphere.

Imagine millions of acres of pongamia orchards spread across the sub-tropical areas of Asia, Africa, Mexico and South America providing billions of gallons of biofuel every year. Imagine fuel efficient vehicles that go twice as far on a gallon of fuel so that we consume half the transportation fuel that we do today. With biofuels, electric vehicles and other technologies in the mix, renewable fuels could make up 50% of the total transportation fuel consumption in the U.S. within twenty years.

This is not a pipedream. It is absolutely possible. It is a matter of aspiration, effort and will.

Let’s do it!

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