Monday, April 06, 2009

How much water to produce biofuels?

Illustration - Fresh water, a resource to preserve. (photo: Wikimedia Commons)

One of the main arguments put forward against biofuels is the high consumption of fresh water to produce them, at least with current technologies. But, as you will see, advanced plug-in hybrid of tomorrow will consume less than one liter of water per day.

To begin, let us look at a midsize car that normally consumes 8 liters of gasoline per 100 kilometers (60 miles) (fuel efficiency of 29 mpg). If we run it with 85% ethanol (E85), the largest commercial concentration, then we will need 11.2 liters of E85 (including 9.6 liters of ethanol) to travel 100 km. The highest amount is required due to the fact that ethanol contains only 2/3 of the chemical energy of gasoline, for a given volume.

Now, if to produce this E85 biofuel we use corn grains from Nebraska, we know that this corn requires 780 liters of irrigation water per liter of ethanol produced (see my book Driving Without Oil for more details). In addition, about 4 to 5 liters of water per liter of ethanol is needed for the manufacturing plant. In total, 785 liters of water are required for each liter of ethanol produced.

Thus, our traditional midsize car running on E85 ethanol in Nebraska consumes 7500 liters (1980 gallons) per 100 km (60 miles). This represents about 4,100 liters (1,082 US gallons) of water a day for someone who will travel 20,000 km (12,420 miles) per year. It is therefore understandable why many environmentalists are not warm to the idea!

But before you throw out the baby with the bathwater, let's see how we can achieve a consumption of less than one liter of water per day!

This requires, of course, the use of energy crops that do not involve watering, which is the case for wild prairie grasses, such as switchgrass. We have seen in a previous post that these perennial plants have roots 3 meters deep that are very effective in capturing soil moisture.

Furthermore, we have also seen in another post that advanced plug-in hybrid cars will consume 4 times less fuel than conventional cars when running on fuel, after their battery is discharged. But as these cars of tomorrow will run 80% of their mileage on electricity, they will actually consume 20 times less fuel.

But we have mentioned earlier that a traditionnal midsize car running with E85 ethanol mixt requires about 10 liters of ethanol per 100 km. The plug-in hybrid cars of tomorrow will therefore need only 0.5 litres/100 km on average (20 times less), which corresponds to 0.28 liters (0.6 pint) of ethanol per day for an annual mileage of 20,000 km (12,420 miles).

Finally, new ethanol plants will reduce their water consumption to about 3 liters of water per liter of ethanol produced, thanks to new technologies such as the one developed by the company Vaperma. This innovative company has developed molecular filters used to separate very efficiently water from ethanol without using distillation. They can save up to 45% of the energy normally used in an ethanol plant, while consuming less water. The decrease in CO2 emissions is also considerable.

Illustration – Vaperma molecular filter used to separate water from ethanol without distillation. The filters are made of polymeric membranes in the form of hollow fibers that are gathered to form the filter cartridge.

Furthermore, for thermochemical biofuel fabrication technologies (pyrolysis, gasification) manufacturing plants consume less than 2 liters of water per liter of biofuel produced.

There is also a new thermo-biological hybrid technology put forward by the company Coskata, which uses microorganisms to convert the syngas (mixture of CO, H2 and CO2) from a thermal process (gasification) into ethanol. According to Coskata, the process consumes less than one liter of water per liter of ethanol produced! This is less than for gasoline. To achieve such performance, Coskata does not use distillation but a molecular filtration process to separate water from ethanol, probably similar to Vaperma filters. In addition, the Coskata process does not include drying of tailings, as is found in the traditional corn ethanol plants, to make the solid protein supplement sold to the livestock industry. The distillation of the beer (fermentation mixture) and drying of tailings are the two processes that consume most water, by evaporation.

Illustration - Coskata thermobiologic process of making ethanol (Source: Coskata).

Ultimately, our advanced plug-in hybrid car traveling 20,000 km (12,420 miles) per year from which 16,000 km (10,000 miles) on electricity, will consume less than one liter (2 pints) of water per day, using biofuels based on tall wild grasses that do not require watering, and manufactured with the best technologies!

To put this water consumption of our future cars into perspective, it is interesting to know the consumption of water per kilogram for different food products. The table below summarizes the results found in the book by David Pimentel and MH Pimentel, Food Energy and Society, CRC Press, 2008. It shows that we need 43,000 liters (11,360 gallons) of water to produce 1 kg (2,2 lbs) of beef, which means more than 6,000 liters (6,000 quarts) of water for a steak of 150 grams (5,3 ounces)! So it is not the 1 liter (1 quart) of water per day consumed by our car that will endanger our freshwater resources. Too large portion of meat in our diet, particularly red meat, is much more worrying in this respect ...

Illustration - Number of liters (1 US gallon = 3.785 liters) of water required to produce one kilogram (2.2 lbs) of different foods, according to the work of David Pimentel, professor of ecology and agriculture at Cornell University.

1 comment: