Ammonia contains no carbon, is easy to store and ship, and already has a global production and distribution network. That makes it one of the more practical ways to transport and store hydrogen, a fossil-free fuel to use where process heat or power are needed. The problem has always been burning it.
Ammonia burns slowly and reluctantly, and the nitrogen contained in the molecule tends to end up as nitrogen oxides (NOx), a harmful pollutant, and nitrous oxide (N2O), a greenhouse gas many times more potent than CO2. Gas turbines, built around short and intense flames, make both problems worse. Most research so far has therefore looked at blending ammonia with hydrogen or methane, or partly “cracking” (or splitting) it into hydrogen before burning it, to make it behave.
SINTEF Energy Research and industry partner Siemens Energy published in September, in open access, a solution to this problem. The paper, in the journal Applications in Energy and Combustion Science, reports the first test of a gas turbine burner designed from the start for pure, uncracked ammonia at pressures up to 10 bar, a range relevant to real-world gas turbines. Earlier pure-ammonia experiments reported in the literature stopped at 4-5 bar.
What the results show
The burner was tested at SINTEF’s HIPROX laboratory in Trondheim: a unique high-pressure combustion facility allowing for multiple combinations of fuel mixtures and oxidisers, and with an architecture that makes it possible to reproduce gas turbine conditions. Tests were run at pressures up to 10 bar, and ran stably over a wide range of settings. The reason the pressure is important is because gas turbines run at high pressure and because NOx formation from ammonia has a pressure dependency. Therefore, results from low-pressure laboratory rigs don’t carry over.
At its best operating point, the burner produced 47 parts per million (ppm) of NOx, about 1 ppm of N2O and no measurable unburnt ammonia. In plain terms, the fuel's own nitrogen ended up mostly as harmless nitrogen gas, the greenhouse gas N2O stayed close to zero, and no toxic ammonia escaped with the exhaust. For anyone intending to use ammonia to fire a gas turbine, this is good news.
The team also showed that the burner can be started on natural gas and switched over to ammonia without producing hydrogen cyanide, a toxic by-product seen in other burners. For a power plant, that means a familiar start-up routine rather than a new one.
Lessons from coal
The design borrows an old trick from coal power plants, where nitrogen in the fuel poses the same problem. The remedy there is to burn in two stages: first with too little air, so that the fuel nitrogen forms harmless nitrogen gas rather than NOx, then adding the rest of the air to finish the job. That second stage happens quickly, to prevent the heat from having time to form new NOx.
Managing heat proved to be as important as managing air. Cold surfaces stop N2O from breaking down, and leave ammonia unburnt. The team therefore built the burner with metal walls, cooled only as much as the material requires.
The main components were 3D-printed at Siemens Energy in Finspång, Sweden.
A fruitful industry-research collaboration
The burner was designed at Siemens Energy by a team led by Karl Johan Nogenmyr, the paper’s lead author. The design draws on a decade of research on burning ammonia, including a series of tests carried out since 2021 at SINTEF’s HIPROX facility, led by chief scientist Mario Ditaranto. Many of the effects had already been seen in small laboratory experiments. HIPROX showed how they behave under conditions close to those in a gas turbine, which gave the designers results they could build on with confidence.
“Every lesson from the last few years of ammonia experiments went into this design: give the flame time, stage the air and keep the walls hot. Seeing all of it work together in one burner at 10 bar is a real step towards ammonia-fired power”, says Ditaranto.
The next step for the team is to study heat transfer through the burner walls and measure the time the gases spend in the oxygen-starved “primary zone”. Detailed computer simulations are also planned to help better understand the process.
