To make better biofuels, researchers add hydrogen
Researchers at Idaho National Labs are developing a technology, with roots in nuclear energy, for squeezing more fuel from agricultural residue using clean electricity.
Research on nuclear energy and hydrogen has yielded what backers say is a technology that could replace U.S. oil imports with biofuels made from agricultural by-products.
Scientists at Idaho National Laboratory have been working for the past year and a half on a process to convert biomass, such straw or crop residue, into liquid fuels at a far higher efficiency than existing cellulosic ethanol technologies.
Rather than one single development, the technology--named bio-syntrolysis--ties together multiple processes, but it has electrolysis, or splitting water to make hydrogen, at is starting point. When combined with a carbon-free electricity source, the approach could deliver a carbon-neutral biofuel, according to models done at INL which has done research for decades in.
Bio-syntrolysis is one of a dizzying number of technologies being developed with the hopes of replacing gasoline, although none have successfully been done at scale. Researchers at INL recognize there remain technical barriers, but its recent computer models show that the technique has better potential than today's biofuel processes.
The key advantage is that bio-syntrolysis would extract far more energy from available biomass than existing methods, said research engineer Grant Hawkes. Using traditional ethanol-making techniques, about 35 percent of the carbon from wood chips or agricultural residue ends up in the liquid fuel. By contrast, the bio-syntrolysis method would convert more than 90 percent of that carbon into a fuel, he said.
"That means if you gather up a kilogram of biomass from a field, you're going to get two and half times the liquid fuel from bio-syntrolysis than you would from cellulosic ethanol. If biomass is a precious commodity, this way you'll get more out of it," Hawkes said.
An often-quoted Department of Agriculture and Department of Energy study (click for PDF) estimated that the U.S. produces enough biomass to meet
"This is the only process available that will give us all the liquid fuel we currently need that's carbon neutral with the all the biomass that's available," he said.
Although it's a compelling vision, there are a number of technical hurdles to making bio-syntrolysis commercially viable and environmentally beneficial.
To reduce carbon emissions significantly over other biomass-to-liquid processes, the INL technology requires a lot of carbon-free electricity--1,000 megawatts of electricity would yield enough 25,000 barrels of fuel a day, enough for almost one million people, according to INL models. A full-size nuclear reactor could produce 1,000 megawatts, but even large-scale wind farms or solar plants are substantially smaller.
The approach also relies on tying together different technologies, some of which are relatively immature in terms of commercial deployment. Making familiar biofuel processes cost effective is hard enough: after years of research and pilot projects, ethanol from wood chips or grasses still isn't produced at commercial scale.
Innovation in integration
Hawkes coined the term bio-syntrolysis to represent the combination of technologies researchers have been working with. To make a liquid fuel, they are using biomass to make a synthetic fuel via electrolysis of water.
Here's how it would work: a high-temperature electrolyzer would split steam into oxygen and hydrogen. Oxygen would be fed to a biomass gasifier, a machine that heats agriculture waste at high temperatures to produce synthesis gas, a combination of carbon monoxide and hydrogen. That synthesis gas, along with the hydrogen from the electrolyzer, would be fed to a refiner to make liquid fuels that could replace gasoline, diesel, or jet fuel.
The biggest technology breakthrough in this design is the high-temperature electrolysis, which originally came from a program to study how nuclear reactors could be used to make hydrogen. But hydrogen-powered vehicles face a, including on-board storage and the infrastructure to cleanly produce and to distribute hydrogen.
By contrast, if the hydrogen was used to make hydrocarbon fuels, they could be distributed through the existing channels and be used with existing autos, including hybrid-electric vehicles.
The jump from hydrogen research to biofuels happened when Hawkes thought to make biomass the heat source for INL's high-temperature electrolysis, rather than the heat from a nuclear reactor. By making that switch, the electrolyzer can operate on biomass and electricity alone, rather than rely on a nuclear reactor.
"We feel each that each one of these technologies is individually proven but nobody has ever taken them and hooked them together to make one process," said Hawkes.
There are some commercially available biomass gasifiers and a few facilities turning synthesis gas into liquid fuel using coal as a feedstock. But coal-to-liquids has a high carbon footprint, even compared with gasoline, said Hawkes. If a renewable or carbon-free source, such as hydro power, can be used through bio-syntrolysis, the resulting fuel would have very low emissions, he said.
Storing hydrogen on plants
So far, INL researchers have done experiments using available commercial products and they have modeled the overall efficiency on computer. To build a high-temperature electrolyzer, they have purchased commercial fuels cells and modified them to work in reverse, so they produce hydrogen and oxygen from electricity.
"There is no need for any great discovery but there is a need for development of materials and electrolyzers and just the will the put all the different sources together," said Steve Herring, a research fellow at Idaho National Labs.
The projected cost of the fuel would be $2.50 a gallon to produce, which is not cheaper than today's gasoline. But the primary advantage is the fuel is domestically sourced, low-carbon, and available at a predictable price, Herring said. One of the rationales for the technology is that biomass to make fuel will become a scarce commodity, making techniques that can squeeze more energy from existing crops more compelling.
INL researchers imagine that a single location to collect biomass, run the gasifier and the electrolyzer. Fuel could be refined on site or shipped to existing facilities. The ash from the gasifier would contain many soil nutrients, such as potassium, that could be redistributed onto the fields that the biomass was collected from.
Why not simply use the hydrogen from the electrolyzer in fuel-cell vehicles? Hawkes and Herring said that the technical limits on hydrogen right now make this an approach that could be deployed without having to wait for technical breakthroughs in hydrogen vehicles.
"It's our observation that the best way to store hydrogen is to hook it onto a carbon atom from biomass now and make it a hydrocarbon fuel," said Hawkes.