Second generation biofuels
Second-generational biofuels created from inedible Parts of Plants Promise Near-term Alternatives to Oil. Biofuels can be made from anything that is, or ever was, a plant. First-generation biofuels derive from edible biomass, primarily corn and soybeans (in the U.S.) and sugarcane (in Brazil). They are the low-hanging fruits in a forest of possible biofuels, given that the technology to convert these feedstocks into fuels already exists(180 refineries currently process corn into ethanol in the U.S.). Yet first-generation biofuels are not a long term solution. There is simply not enough available farmland to provide more than about 10 percent of developed countries’ liquid-fuel needs with first-generation biofuels. The additional crop demand raises the price of animal feed and thus makes some food items more expensive – though not as much as the media hysteria last year would indicate. And once the total emissions of growing, harvesting, and processing corn and factored into the ledger, it becomes clear that first-generation biofuels are not as environmentally friendly as we would like them to be.
Second-generation biofuels made from cellulosic material- colloquially, “grassoline”- can avoid these pitfalls. Grassoline can be made from dozens, if not hundreds, of sources: from wood residues such as sawdust and construction debris, to agricultural residues such as cornstalks and wheat straw, to “energy crops” – fast growing grasses and woody materials that are grown expressly to serve as feedstocks for grassoline. The feedstocks are cheap, abudant and do not interfere with food production. Most energy crops can grow on marginal lands that would not otherwise be used as farmland. Some, such as the short-rotation willow coppice, will decontaminate soil that has been polluted with wastewater or heavy metals as it grows.
Huge amounts of cellulosic biomass can be sustainably harvested to produce fuel. According to a study by the U.S. Dept. of Agriculture and Dept of Energy , the U.S. can produce at least 1.3 billion dry tons of cellulosic biomass every year without decreasing the amount of biomass available for our food, animal feed or exports. This much biomass could produce more than 100 billion gallons of grassoline a year- about half the current annual consumption of gasoline and diesel in the U.S. Similar projections estimate that the global supply of cellulosic biomass as an energy content equivalent to between 34 billion to 160 billion barrels of oil a year, numbers that exceed the world’s current annual consumption of 30 billion barrels of oil. Cellulosic biomass can also be converted to any type of fuel- ethanol, ordinary gasoline, diesel, even jet fuel. Scientists are still much better at fermenting corn kernels than they are at breaking down tough stalks of cellulose, but they have recently enjoyed an explosion of progress. Powerful tools such as quantum-chemical computational models allow chemical engineers to build structures that can control reactions at the atomic level. Research is done with an eye toward quickly scaling conversion technologies up to refinery scales. And although the field is still young, a number of demonstration plants are already online, and the first commercial refineries are scheduled for completion in 2011. The age of grassoline may soon be at hand.
The most promising approaches involve subjection of the biomass to extremes of pH and temperature. We are developing a strategy that uses ammonia- a strong base- in one of our laboratories. In this ammonia fiber expansion (AFEX) process, cellulosic biomass is cooked at 100 degrees C with concentrated ammonia under pressure. When the pressure is released, the ammonia evaporates and is recycled. Subsequently, enzymes convert 90 percent or more of the treated cellulose and hemicellulose to sugars. The yield is so high in part because the approach minimizes the sugar degradation that often occurs in acidic or high temperature environments. The AFEX process is “dry to dry”: biomass starts as a mostly dry solid and is left dry after treatment, undiluted with water. It thus can provide large amounts of highly concentrated, high-proof ethanol.
AFEX also has the potential to be very inexpensive: a recent economic analysis showed that, assuming biomass can be delivered to the plant for around $50 a ton, AFEX pretreatment, combined with an advanced fermentation process called consolidated bioprocessing, can produce cellulosic ethanol for approximately $1 per gallon of equivalent gasoline energy content, probably selling for less than $2 at the pump.
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