How Carbon-neutral E-fuels Work

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-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.

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.

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]:

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.