It’s hard to believe that much more remains to be discovered about the science of burning biomass to create fuels and chemicals. The process has been around for decades, with many a fine brain focused on grasping details of the biochemistry. Yet, even now, some fundamental problems are unresolved.
Fuels and chemical by-products are created all round the world by the rapid heating method known as pyrolysis. Regardless of where it’s done and who’s in charge, familiar obstacles persist.
Spotlight on lignin
However, research projects underway in America show signs of resolving two major areas of difficulty. One is the role that the water created during pyrolysis plays in fouling up the conversion process. The other is what to do with lignin, the aromatic polymer found in plant cell walls — the woody substance that makes plants stay firm.
Until now lignin has resisted most attempts at being cost-effectively converted into fuels or useful chemicals. However, concerted efforts at a breakthrough are being made by the National Renewable Energy Laboratory (NREL), the US Department of Energy’s primary lab for renewable energy research and development.
“This is an exciting time for lignin research,” said Gregg Beckham, NREL staff engineer. “In the last decade, there has been sustained funding for biofuels research — which is why we’re seeing so much work on lignins from groups all over the world.”
The findings of Beckham and his team were recently published by the National Academy of Sciences. The ‘What to do with lignin?’ question has become more urgent with the intensive development worldwide of fuel-producing biorefineries. This, of course, means that quantities of waste lignin have increased dramatically.
As Beckham points out, scientists have wrestled with the issue for almost a century — and with good reason. “Within biomass, 15-30 per cent is lignins, and yet most of this is just burned for heat and power,” he explained. “Lignin valorization [the scientific process of increasing its value] has been around for many decades – there are papers dating from the 1930s very similar to those of today. A lot of work has been done on high temperature and catalytic depolymerization.”
So, what has NREL brought to the table from three years of research? One key element came from a simple epiphany — something that struck Beckham while he was out for a stroll in the park. “If you go for a hike, you see trees bleached white with rot — the bacteria within fungi breaking down lignins. For me, it was a ‘eureka’ moment.”
And so to thrust of his team’s quest. “There many different ways to break down lignins, including the enzymes nature uses, but you get a heterogeneous mix of molecules — and of low yield. It’s hard to fish individual molecules out of that mix that have value. Upgrading that stuff [for use as fuel] is really difficult too….but biology already figured out how to do this.”
Beckham believes NREL has made significant progress towards understanding the ‘how’ – and it starts with turning to nature. For, in the natural world, some microorganisms have evolved ways to break down complex, lignin-derived aromatic molecules for use as food and energy source – a process known as catabolism.
Scientists have made a detailed study of catabolism in action, using a natural organism Pseudomonas putida KT2440. In their pilot project, they showed that lignin-enriched material derived from biomass pretreatment can indeed be broken down via ‘natural pathways’ into molecules with similar properties to those coming from conventional carbohydrates.
This is a breakthrough — carbohydrate-derived mcl-PHAs can be used to make bioplastics. Beckham’s team says it has demonstrated that using these pathways may point a way towards conquering the barriers that have long prevented lignin being properly exploited as a fuel source.
The researchers went on to show the full potential of mcl-PHAs. Using a catalyst, they were able to convert these to chemical precursors (transform them into another compound) and fuel-range hydrocarbons. This looks like an important battle won — finally, a way of conquering the stubborn heterogeneity of lignins, which, for so long, has been hard to break down.
“We think that’s really cool – we’re really excited about this work,” Beckham said. “But we realize it’s just the first step on a long road to make this technology viable.”
Soon we may see an end to the days of hard-to-deal-with lignin simply being burned off – and, with it, the old biofuels industry adage that ‘You can make anything from lignin, except money.’ High time, too, Beckham says. “New approaches are desperately needed, and we’ll see a lot of innovative ways to valorize lignin appear in the coming years.”
The concept developed at NREL can probably be applied to many different types of biomass feedstocks and combined with various strategies for breaking down lignin — engineering biological pathways and catalytically upgrading the biologically derived product to develop a larger range of valuable molecules.
NREL’s technology transfer office has been working to identify potential licensees. Meanwhile, researchers are collaborating with industry to develop the best catalysts. “We have multiple patents and iterations [projects that refine the process through repetition] in the pipeline,” Beckham stated. “The preliminary economics look very good, and the environmental impact for valorizing lignin could be really beneficial.”
More broadly, Beckham believes the sky’s the limit for this research. “Luckily, a lot of great researchers are working with lignins,” he said. “There’ll be all sorts of fun and crazy technology coming out to find innovative uses.”
Fresh look at pyrolysis
Over in Washington State, the research into biomass pyrolysis has come from a completely different angle. There, scientists at Pacific Northwest National Laboratory (PNNL), backed by the DoE, have been trying to figure out why water — a by-product of the process — can reduce its efficiency so drastically.
The answer? It helps form purities that slow down key chemical reactions. Yet, as with the lignin discovery, it’s easy to ask why this wasn’t stumbled upon earlier.
The chemistry of pyrolysis produces precursors to fuels but also spawns a potentially unwelcome by-product: phenol.This sits in the vat of chemicals and water undergoing various reactions, and gets converted into molecules called ketones. The trouble is that ketones can link up into long chains, fouling up catalysts and interfering with the reactions needed to produce fuel.
For PNNL researchers, the key question was, ‘How does phenol convert to ketone?’ Seeking answers, the team turned to computer simulations, exploring what happens to a common bio-oil by-product. They found that water was turning this into an impurity, disrupting and blocking reactions that eventually lead to fuel production.
Roger Rousseau, a senior scientist at PNNL, admits that even fellow scientists can be skeptical about whether computer modelling will work in reality. “We are just hypothesising,” he notes. “It’s very difficult studying solid-liquid interfaces, and it’s hard to do controlled experiments. But simulation allows you to examine scenarios regarding molecular interaction – to get a better idea of the physical process.”
The work with computers involved simulating phenol interacting with catalysts and water to see, step-by-step, what is going on. To explore water’s role, they also simulated the same reactions in a vacuum, which puts everything but the solid metal catalyst – either nickel or platinum — in vapour form.
The team found that phenol molecules and water molecules randomly bounce or land on the metal surface. (It’s here that bonds break and reform between atoms within molecules by shifting electrons around. In this way, a phenol might transform into a ketone.)
The team found that the presence of water dramatically raised the speed at which phenols converted to ketones. Also, water affected how the metal catalyst carried its electrons. This, in turn, affected how well it catalyzed the reaction between phenol and the hydrogen atoms settling on its surface.
The PNNL team admits that the impact of water surprised them. But they believe it’s a valuable discovery: that what’s been learned can be applied to other catalyst-driven reactions. In the real world, this will make it easier to deal with complex catalysts and create biofuels.
“Impurity [in the process] is unavoidable…we need to make sure it doesn’t build up enough to interfere,” PNNL’s Rousseau explained. “It points out what we can do to help extend the lifetime of the catalysts we’re using to make bio-oil. You’ll always have ketones; you just have to deal with the kinetics and control them.”
PNNL is also working on an electrochemical way of attacking ketones. As Rousseau explains: “Biomass is hydrogen-poor oxygen- rich. If I need a lot of hydrogen to get rid of the ketones, that impacts the cost-efficiency of the process. And if I have to get rid of them at high temperature, I’m competing against the ketones polymerizing and plugging the reactor.”
The next step
So, what’s the business potential for all this? “What might have the most impact is that making ketones from phenols means you’re making species that can polymerize,” Rousseau explained. “Water makes that transformation unavoidable, since it’s so much faster than adding hydrogen to the molecules.”
In addition, polymerization can be good in that it helps you make C-C bonds (bigger molecules to get in the fuel range), Rousseau argues. But it could be bad if it becomes a polymer that coats the catalyst or, worse, plugs the reactor.
“The research tells us we need to find ways of controlling the rate of this process, to tune the size of the molecules and make sure they’re not too big,” Rousseau stated. “So the next step is understanding how to control the kinetics [speed of the chemical process] of ketone polymerization. We want to extend the time we can keep the reactor going — not plug it up or lose catalyst activity, but enhance the yield of organics with the right amount of carbon for fuels.”
In time — and with further research — Rousseau believes we’ll know how to simplify things so we can deal with more systems, and deal with them at a faster rate. “Some aspects we know how to generalise — even if you haven’t got an exact answer, you can use prior knowledge to get to your answer quicker.”
Andrew Mourant is a freelance journalist whose areas of expertise include renewable energy, education and the rail industry. He is a regular contributor to Renewable Energy Focus magazine.