The field known vaguely as clean tech is full of promising ideas for clean power production that haven’t panned out, such as tidal power, concentrating solar towers and gigawatt hamster wheels. (OK, I made up that last one.)

From New Hampshire’s point of view, the biggest of these disappointments has to be cellulosic biofuel.

The idea behind this awkwardly named technology is to take the cellulose from inedible parts of vegetation, like all the branches that have no value when our forests are harvested, and turn it into ethanol that can be burned to run machinery. Voila! A renewable alternative to petroleum.

This ideas seems feasible – we already do it with corn kernels, making use of nature’s preparatory work – and it was a hot topic when George W. Bush was president.

New Hampshire’s connection came from Dartmouth researcher Lee Lynd and a startup called Mascoma Corp. (named after the New Hampshire lake) that was going to use chemicals and heat and enzymes to manufacture lots of cellulosic biofuel and fight climate change. I wrote about visions of turning Northern New England into the Saudi Arabia of cellulosic biofuel, because I love a cliche as much as the next writer.

Fast forward to 2020, alas, and nobody, including Mascoma Corp., has been able to make the technology work at scale without spending a fortune. Perhaps worse, doubts have been cast on whether it actually would fight climate change.

I had pretty much forgotten about cellulosic biofuel until recently, when Lynd and other researchers released a paper arguing that the technology still has a role in developing a carbon-free economy. Making not as big a role as once hoped and maybe not a role for northern forests, but a role, nonetheless.

“Ten years ago expectations were very high. But the technology maturity was over-estimated – everybody got fooled and fooled each other. I started a company that was part of the dynamic, shall we say,” said Lynd, professor of engineering and biology in the Thayer School at Dartmouth.

He’s no longer associated with Masoma but, while staying with Dartmouth, will direct a research laboratory in Brazil to pursue the idea of making ethanol from fast-growing grasses in that wet and sunny climate, without putting pressure on food-growing land or natural systems.

Why is it so hard? The plant kingdom has spent a few million years evolving cellulose so that it won’t break down, so that stems and wood will remain strong. (Existing biofuels are made from seeds like corn kernels, which are easy to break down because plants want them to be eaten, to spread their genetics.) Outmaneuvering cellulose’s defenses proved much harder than expected.

“The field got a bit fixated on one approach. It’s a little bit as if in the battery field somebody said it’s got to be lead-acid forever and nobody tried lithium-ion batteries,” Lynd said.

Lynd thinks combining a couple of steps can bring the costs down to feasible levels. That leaves a bigger question, however, of whether the technology is worth it from an environmental point of view.

Should we really be melting down plants, preventing them from absorbing more carbon, to make a fuel that’s burned? Many critics say no, which has squelched funding for the field. This debate will sound familiar to New Hampshire because it’s just like the debate over whether to burn wood chips for electricity.

The new paper published in the Proceedings of the National Academy of Sciences, has 13 authors from a host of institutions, argues that we should, in certain circumstances. Lynd said the paper has been in the works for several years.

It contains detailed arguments based on technical matters about such things as how to measure carbon storage, the climate effects of reforestation, opportunity cost (what else could have been done with vegetation that’s turned into biofuel) and supply chains.

It acknowledges, Lynd said, that cellulosic biofuel has limitations.

“We’re no longer looking to biofuels to make light-duty transportation, or carbon-neutral electricity. That  still leaves roughly half of transportation that’s going to be difficult to power by hydrogen or electricity, including ocean shipping and aviation,” he said.

I’m afraid the report also deflates my hopes for a biofuel revolution in the Great North Woods, saying that even when cutting down existing forests for biofuel might make sense in the long run, it can result in “ecosystem carbon debts requiring several years to several decades or more to overcome” at a time when we don’t have decades to reduce our climate impact.

Nonetheless, the report argues that critics of biofuel have over-estimated the climate mitigation effects of reforestation, which makes biofuels look worse in comparison, and have overlooked the advantages of using perennial crops such as switchgrass.

“We need to understand what is the initial land cover; what is the natural land cover if you do restoration; what is technology used; and to include carbon capture and storage,” Lynd said. “If you make wise choices, none of them prevent you from getting greenhouse-gas mitigation.”

With climate change speeding up, creating worst-case scenarios for wildfires, Atlantic Ocean storms, glacier melt and natural methane release, turning cellulose into a petroleum alternative needs to be part of how we redesign the industrial world.

“We need everything,” Lynd said.