How Carbon-neutral E-fuels Work


The A3 Sportback TCNG, which will run on e-gas. Want to learn more? Check out these alternative fuel vehicles. Image courtesy Audi

For a while, many of us allowed ourselves to imagine the possibilities: highways and boulevards filled with zero-emission electric vehicles (EVs). We could see such a world, as Ray Bradbury, Isaac Asimov or Philip K. Dick might have imagined it, with fleets of glistening, battery-powered cars streaming through canyons of glass and steel. When Barack Obama promised, in 2008, to put 1 million EVs on the road by 2015, we thought our utopian dreams of the future would soon come true. And we had the all-electric cars to prove it -- the Nissan Leaf, the Tesla Model S and the BMW ActiveE.

But the EV concept has suffered from a certain fragility. Most electric vehicles have a limited range of just 100 or 200 miles (161 or 322 kilometers) on a single charge. And the lack of an extensive public recharging infrastructure makes it difficult to "fill up" when an EV's batteries run low. This has caused concerns among consumers who fear being stranded on the side of the road because they can't plug into the grid. As a result, EV sales have lagged. Only 52,835 plug-ins were sold in 2012, while only about 70,000 have been sold since December 2010. That means EV sales make up less than 1 percent of the approximately 15 million vehicles sold each year [source: Electric Drive Transportation Association].

Not surprisingly, some companies have shifted strategies. Although they continue to work toward a future of alternative fuels, they acknowledge a current internal-combustion reality. Not only that, they see the fossil-fuel economy as an opportunity to make a paradigm shift, instead of as an unfortunate fallback position. Audi is one of those companies. In 2013, the German car company will open the world's first industrial plant to produce something known as e-gas, a carbon-neutral fuel that can power compressed natural gas (CNG) vehicles, including, of course, Audi's very own A3 Sportback TCNG. At the same time, the e-gas project provides a model and an infrastructure to supply electricity for EVs and hydrogen for fuel-cell vehicles when both of those technologies mature into viable alternatives.

Sound too good to be true? Some people think so. Gas 2 blogger Christopher DeMorro wrote this about the e-gas project: "Maybe I am missing something here, but the problem right now is too much atmospheric emissions from automobiles. Merely sustaining the status quo won't do. Let's get off of the carbon standard as much as possible by focusing on vehicles that go further using less energy."

Before you agree or disagree, it might be wise to understand the full scope and scale of the e-fuel concept, starting with Audi's definition of carbon neutrality.

Carbon-neutral Combustion

Like many people and corporations, Audi seems to be working hard to reduce its carbon footprint. If you're not familiar with that term, or its relative, carbon offsets, then you might want to check out How Carbon Footprints Work and How Carbon Offsets Work. In the meantime, here's a quick primer: A carbon footprint measures all carbon dioxide (CO2) emissions produced by a single person or business. Carbon dioxide is the primary greenhouse gas produced by human activities, so that's why it receives most of the attention. But an accurate carbon footprint measurement also accounts for other gases, such as methane and chlorofluorocarbons. Scientists express carbon footprints in tons of CO2 or CO2 equivalents per year.

If you reduce the CO2 emissions related to a certain activity, then you decrease that activity's footprint. If you cut emissions so that they equal the footprint, then you achieve what's known as carbon neutrality. Companies tackle this problem in many different ways. Some build wind farms to replace coal-fired power plants. Others engage in carbon sequestration activities, such as planting trees to trap carbon dioxide in forests and soils. And still others participate indirectly by funding projects that reduce greenhouse gas emissions. All of these methods qualify as carbon offsetting.

Transportation, which often relies on burning fossil fuels to move people and goods, poses a significant challenge in the fight to reduce carbon output. It takes a lot of electricity from a CO2-spewing power plant to build a car -- and even more to extract, refine and deliver the petroleum products to fill its gas tank. And then, once the car gets on the road, its internal combustion engine produces a steady supply of greenhouse gases, which collect in the atmosphere and become part of a great, planet-warming blanket. Electric and hydrogen-powered vehicles could eliminate some of these issues, but they may not be viable solutions for years.

Despite these challenges, Audi has dedicated itself to "balanced mobility," which the company defines as "holistic, CO2-neutral mobility over short, intermediate and long distances" [source: Audi USA]. E-gas, a fuel that can power internal combustion engines, plays an important role in this effort. How can Audi hope to achieve carbon neutrality with an old-school technology? It will actually use carbon dioxide as a raw ingredient to make its engine-friendly fuel. A refuse biogas plant will provide the CO2, and a purpose-built factory in Werlte, Germany, will carry out the necessary chemical reactions. Beginning in 2013, the factory in Werlte will consume 2,800 metric tons of CO2 and will generate 1,000 metric tons of e-gas a year [source: Audi USA]. This, combined with other green practices we'll explore a bit later, will enable Audi to achieve carbon neutrality across its entire value chain.

Chemists and consumers know e-gas by another name: methane, or natural gas, which is already used extensively to heat homes and to power natural gas vehicles. On the next page, we'll take a closer look at this synthetic gas and the chemistry required to produce it.

E-fuel Fundamentals

These are the points that turn up when you search for compressed natural gas fueling stations near HowStuffWorks headquarters on the U.S. DOE’s alternative fuels data center. When we searched at the time of publication, 558 CNG stations turned up in tot
These are the points that turn up when you search for compressed natural gas fueling stations near HowStuffWorks headquarters on the U.S. DOE’s alternative fuels data center. When we searched at the time of publication, 558 CNG stations turned up in tot
Image courtesy Alternative Fuels Data Center/U.S. DOE

E-gas is just a marketing word for synthetic methane, the laboratory version of the same colorless, odorless gas derived from ancient land plants and aquatic organic matter that lies buried in the Earth's crust, above or below oil deposits. While fossil methane takes millions of years to form, synthetic methane comes to life relatively quickly by way of some basic chemistry.

Everything begins with electrolysis, a type of chemical reaction requiring an electric current to pass through a substance to effect a chemical change. In this case, the substance is water, and the change results in the decomposition of water molecules into hydrogen gas and oxygen. The chemical equation would look like this:

2 H2O → 2 H2 + O2

By itself, hydrogen could be a fuel for future cars. In these vehicles, hydrogen is compressed and stored in a large tank. Then the hydrogen reacts with oxygen to produce electricity and water. Unfortunately, fuel-cell vehicles require a more robust infrastructure before they become economically feasible. Until that time comes, Audi takes the hydrogen gas and reacts it with carbon dioxide in a process known as methanation. Here's the chemical equation for that reaction:

CO2 + 4 H2 → CH4 + 2 H2O

In this equation, methane appears as CH4, and it's no different than the swamp gas collecting above a bog or the natural gas extracted at a wellhead. It's just as colorless, just as odorless and just as flammable. More important, it can enter the existing natural gas network without any other modifications, which means it can be distributed to homes, businesses and compressed natural gas (CNG) stations. The auto industry is, of course, most concerned with the last item on this list, for CNG stations deliver fuel to a growing fleet of natural gas vehicles, or NGVs. NGVs won't solve all of our global warming or fossil dependency issues, but they might have a role in the transition. Up next, we'll review this aspect of the Audi solution.

Natural Gas Vehicles

Audi's A3 Sportback TCNG vehicle -- and the concept of powering it with synthetic methane -- seems revolutionary, but it builds on a long history. As soon as a reliable natural gas transmission system became established in the early 1930s, road warriors had the idea to burn it in their cars. In fact, at a technical level, vehicles powered by gasoline and natural gas work on the same principles: Fuel is mixed with air in the cylinder of a four-stroke engine and then ignited by a spark plug to move a piston up and down (see How Natural Gas Vehicles Work for more details). Had the United States and the Middle East not discovered large crude oil fields and a vast supply of cheap, readily available fuel, natural gas vehicles might have become much more popular.

Today, NGVs represent just a tiny fraction of the world automobile market. There are about 120,000 NGVs driving on American roads and more than 15 million worldwide at the end of 2011. Five countries -- Iran, Pakistan, Argentina, Brazil and India -- accounted for almost 70 percent of the global market share in 2011 [source: Natural Gas Vehicles for America]. NGVs are successful in these regions because the areas lack the capacity to refine oil. But more nations will embrace natural gas in the next 10 years, especially as gasoline prices continue to rise. That's because natural gas tends to cost much less than gasoline. By some estimates, the fuel savings associated with NGVs run about 30 percent less, on average, than gasoline-powered vehicles [source: Consumer Reports].

Fuel costs are only one reason NGVs are so attractive -- they also produce far fewer exhaust emissions. According to the U.S. Environmental Protection Agency (EPA), NGVs reduce carbon monoxide by 70 to 90 percent, nitrogen oxides by 75 to 95 percent and carbon dioxide by 20 to 30 percent [source: Natural Gas Vehicles for America]. Add all of this up, and you have a car that has far less impact on air quality and slows the progress of global warming. You have, in fact, the cleanest internal-combustion vehicle on the planet, which is an honor the EPA has bestowed upon the natural-gas version of the Honda Civic.

It's not all birdsong and green meadows, though. Natural gas vehicles cost more to purchase and present refueling issues that almost rival the recharging woes of EVs. There were only about 1,000 NGV fueling stations in the United States as of December 2011, and only half of those were open to the public [source: NGV Global]. You can install a home refueling system, but it adds even more up-front ownership costs. And, of course, you can't escape the fact that NGVs, while they produce far fewer CO2 emissions, still contribute to the warming of the planet.

As a transitional technology, however, natural gas vehicles are an important piece of the puzzle, especially if you can run them on synthetic methane made from carbon dioxide. Remember, that's what Audi will do in its Werlte plant: split water into hydrogen and oxygen via electrolysis, then combine the hydrogen gas with carbon dioxide obtained from a biogas plant. But wait a minute. Doesn't electrolysis require electricity? And doesn't electricity require a power plant? Funny you should ask, because that's the last element of Audi's e-gas project.

Wind, Wheels and Watts

The e-gas plant in Werlte is the first in the world to combine hydrogen electrolysis with methanation (the hydrogen gas and carbon dioxide reaction), but that's not the only innovation. The electricity used in the electrolysis reactions will begin as strong breezes whipping over the North Sea, the site of three large-scale offshore wind farms. There, Audi has built four turbines, each with a capacity of 3.6 megawatts. Together, the turbines should generate 53 gigawatt-hours of electricity each year [source: Audi USA]. This renewable energy will be used in three ways [sources: Audi USA, Fraunhofer-Gesellschaft]:

  1. Audi has earmarked a portion of this energy for its e-tron electric vehicle program. The company estimates it can harness enough energy from its wind turbines to produce 1,000 Audi A1 e-tron models and operate them for 6,210 miles (10,000 kilometers) per year.
  2. The rest of the energy -- about 20 gigawatt-hours -- will flow to the e-gas plant in Werlte. There, the renewable electricity will power the hydrolysis that splits water into oxygen and hydrogen gas. In the future, the hydrogen could find a home in vehicle fuel cells, but in the next several years, it will serve as one of the main ingredients to make synthetic methane, or e-gas. The e-gas will enter the natural gas supply and, ultimately, the fuel tanks of NGVs.
  3. The synthetic methane will also provide a storage function. Right now, wind farms like those in the North Sea often produce more electricity than the power grid needs at that moment. That electricity is lost if it can't be stored for future use. Enter e-gas, which can be diverted into Germany's vast natural gas infrastructure and then pulled back out, when needed, as a fuel source in power generation. By some estimates, the German natural gas network could hold the equivalent of 200 terawatt-hours of electricity -- enough to satisfy consumption for several months.

Audi didn't come up with this concept on its own. The process was developed by the Center for Solar Energy and Hydrogen Research Baden-Württemberg, in cooperation with the Fraunhofer Institute for Wind Energy and Energy System Technology. Other partners include Solar Fuel Technology, which has a facility next to the e-gas plant. Waste heat generated during electrolysis and methanation in the Audi plant will be used in the Solar Fuel Technology facility, tremendously enhancing overall efficiency.

The car company celebrated the topping out of its Werlte facility in December 2012 and expects e-gas production to begin in 2013. This will be followed by delivery of its new Audi A3 Sportback TCNG vehicles, which will begin arriving at dealerships in late 2013. Audi also plans to launch a second TCNG model, based on the A4, in 2015. By then, you can expect to see more and more natural gas vehicles on the road. Both Chrysler and General Motors have launched natural-gas versions of their heavy-duty trucks. And Honda will continue to promote its natural-gas Civic to consumers looking to save a little money and -- just maybe -- the planet.


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Author's Note: How Carbon-neutral E-fuels Work

It's easy to mistake NGVs as a new concept, but they've been around a long time. My father, who worked in the West Texas oil fields in the late 1940s and early '50s, used to tell stories of guys running their beat-up trucks on natural gas. Wonder what those roughnecks would think today of synthetic methane or the new Audi A3 Sportback TCNG?

Related Articles


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  • Audi Worldwide. "Topping-out ceremony for the Audi e-gas plant." Dec. 13, 2012. (Jan. 2, 2013)
  • Consumer Reports. "The natural-gas alternative." March 2012. (Jan. 2, 2013)
  • DeMorro, Christopher. "Audi E-Gas Claims Carbon Neutrality." Gas 2. Oct. 9, 2012. (Jan. 2, 2013)
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