The afterburner that makes biomass power carbon-negative

Imagine a power plant that runs on forestry waste and, instead of adding CO₂ to the atmosphere, removes it. That is the promise of the EU-funded BioNETzero project: combine biomass with a combustion technology that captures its own CO₂, store that CO₂ permanently, and the plant becomes carbon-negative. Because trees absorbed the carbon from the air as they grew, burying it afterwards effectively removes it from the atmosphere.

The technology at the heart of the project is chemical looping combustion, or CLC. Its great advantage is that the fuel never meets air. Oxygen gets to the fuel via a circulating stream of metal oxide particles, so the exhaust contains no nitrogen from the air — just CO₂ and water vapour. Condense out the water, and the CO₂ is essentially ready for storage, without the expensive separation step that burdens most carbon capture schemes.

At least, that is the theory. In practice, there is a snag: the fuel doesn’t burn completely.
 

What is chemical looping combustion? In an ordinary furnace, fuel burns in air, and the CO₂ in the exhaust is diluted by all the nitrogen that air contains (about 78%). Separating the CO₂ afterwards is costly and energy-intensive. Chemical looping combustion sidesteps the problem by splitting the process in two. In one reactor, small particles of metal oxide pick up oxygen from air. They are then circulated to a second reactor, where they hand that oxygen over to the fuel. The fuel burns without ever meeting air, so the exhaust is nearly pure CO₂ and steam. The particles loop back to be recharged with oxygen, over and over.

The leftovers problem

When biomass is converted in a CLC fuel reactor, not every molecule finds an oxygen partner. A small percentage of the fuel leaves the reactor as unburned gases: carbon monoxide, hydrogen, methane and traces of other hydrocarbons.

Those leftovers cannot simply be sent onwards. They represent lost energy, they would contaminate the CO₂ destined for storage, and some of them are pollutants in their own right. So before the exhaust reaches the plant’s steam generator and gas cleaning stages, it passes through one final, small combustion chamber whose only job is to finish what the fuel reactor started. We call this the “oxy-polishing” afterburner.

In BioNETzero, my colleagues and I at SINTEF Energy Research, together with partners at the Silesian University of Technology (SUT) in Poland, have now designed this afterburner for the CLC pilot facility at Tiller in Trondheim, and evaluated it through extensive computer modelling before anything is built in steel.

Designing it turns out to be a delicate balancing act.

Three problems in one small chamber

First, the gas arriving from the fuel reactor is what engineers call low-calorific: the combustible fraction is small and heavily diluted in CO₂ and steam. It burns reluctantly, a bit like trying to keep a candle lit in the wind, and the afterburner must mix oxygen into the gas well enough that the combustion doesn’t simply fizzle out.

Second, the oxygen itself is expensive. Since air would reintroduce the nitrogen that CLC so elegantly avoids, the polishing step must use pure oxygen, which has to be produced industrially. Every kilogram counts, so the afterburner should complete combustion with almost no excess oxygen.

Third, and most interestingly, there is the nitrogen already hiding in the fuel. BioNETzero deliberately targets cheap biomass residues; in the pilot tests, tops and branches left over from forestry, chopped, dried and pressed into pellets. These residues contain more nitrogen than clean wood, and in the fuel reactor much of it turns into ammonia. When that ammonia meets oxygen in the afterburner, chemistry offers two exits: it can become harmless nitrogen gas (N₂), the same molecule that makes up most of the air we breathe, or it can become NO_x, a family of pollutants behind smog and acid rain. The design must push it firmly towards the harmless exit.

NO_x is shorthand for nitrogen oxides, mainly NO and NO₂: gases formed during combustion that contribute to smog, acid rain and respiratory problems. They form in two main ways: from nitrogen in the air at high combustion temperatures (“thermal NO_x“), or from nitrogen chemically bound in the fuel itself (“fuel NO_x“). Using pure oxygen instead of air eliminates the first route. Careful process design is needed to suppress the second.

The counterintuitive fix: less oxygen first

We used an in-house detailed chemical simulation tool that follows hundreds of reactions along the length of the chamber. With it, we tested different ways of adding the oxygen. The best strategy was to add less oxygen at the start of the chamber.

In the recommended design, only about half of the required oxygen is injected at the inlet. This creates a deliberately oxygen-starved first zone, where there is not enough oxygen to go around. In that competitive environment, ammonia molecules are far more likely to react with the nitrogen oxides around them and recombine into inert N₂ than to be fully oxidised into NO_x. Only further along the chamber is the remaining oxygen added, finishing off the carbon monoxide, hydrogen and methane with a small final surplus of oxygen at the end.

The simulations suggest this staging converts between 75% and 83% of the fuel’s nitrogen into harmless N₂, across scenarios ranging from a well-performing full-scale fuel reactor to a heavily loaded pilot case. The dirtiest scenario, with roughly three times more ammonia entering the afterburner, achieved the best conversion rate, because the chemistry could be driven even more aggressively oxygen-lean. Meanwhile, carbon monoxide at the outlet dropped to a few tens of parts per million.

The exhaust arrives the afterburner directly from the fuel reactor at high temperature, about 920 to 950°C — hot enough to ignite on its own, provided that oxygen is available and the mixing is good. A pilot burner to ensure combustion will not be needed in normal operation but will be needed during the start-up phase, while the system is heating up.

Testing the design in a virtual afterburner

Chemistry tells you when and how much oxygen to add; it says nothing about whether the gases will actually mix inside a real, three-dimensional chamber. For that, we turned to computational fluid dynamics (CFD): simulations that divide the chamber into nearly two million small cells and compute the flow, temperature and reactions in each one.

We tested four successive geometries virtually. In the final design, the exhaust from the fuel reactor enters the afterburner off-centre, setting the whole gas stream spinning like water entering a drain at an angle. This swirl does the hard work of mixing oxygen into the reluctant exhaust gas, and it lengthens the path each molecule travels, buying time for combustion to complete. The simulations show the combustible gases are almost entirely consumed before the exhaust leaves the afterburner, with residual concentrations reduced to trace levels.

A small link in a long chain

None of this is glamorous hardware. The oxy-polishing afterburner will never be the centrepiece of a power plant tour. But carbon-negative energy only works if every link in the chain holds: fuel handling, the looping reactors, gas polishing, cleaning, compression and storage. This work closes one of those links on paper; our design now feeds into the project’s work on flue gas cleaning, CO₂ processing and full-scale plant modelling.

Sometimes the road to negative emissions runs through a steel cylinder two metres long, where the smartest thing the combustible gas can do is to be patient and wait for its oxygen.

BioNETzero is an EU-funded project that has received funding from the European Union’s Horizon Europe Research and Innovation Programme under Grant Agreement No. 101146616.