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Introduction
Based in Northern Germany, PTB (Physikalisch-Technische Bundesanstalt) is Germany’s highest authority in metrology and ISO standards. Located on the wooded outskirts of Braunschweig, PTB plays a critical role in supporting the country’s transition to a decarbonized society, helping to reduce CO2 emissions from combustion engines used in factories, boats, airplanes, and cars.
The Challenge: Reducing CO2 Emissions without Nuclear Power
As Germany phases out nuclear power plants, the focus has shifted to alternative energy carriers, mainly hydrogen (H2) and ammonia (NH3), which can store energy generated by wind and solar power. While hydrogen shows promise, it is challenging to produce and liquefy. Ammonia offers potential as it contains one hydrogen atom, but it requires further research. PTB’s mission includes exploring the use of these molecules as energy sources and resolving the complexities associated with their use, including precise measurement of fuel consumption in diverse applications such as factories, households, and vehicles.
The Approach: Laser Diagnostics for Real-Time Combustion Analysis
In response to these challenges, Dr. Zhechao QU’s department at PTB focuses on understanding the species generated by combustion engines through precise measurements. The first priority is to reduce uncertainty in the measurements, aligning as closely as possible with ISO standards. The second is replicating harsh industrial temperature and pressure conditions in a laboratory setting to simulate combustion.
Because combustion reactions occur rapidly, in less than a millisecond, PTB employs laser diagnostics, which offer greater speed and accuracy than traditional methods such as mass spectrometry. Although useful for analyzing exhaust samples, PTB’s FTIR instrument is too slow and invasive for real-time measurement in harsh environments. Instead, PTB uses Quantum Cascade Lasers (QCLs), which emit in the “fingerprint region” of the target species (e.g., NO, NH3, N2O, NO2). For hydrogen measurements, PTB uses Interband Cascade Lasers (ICLs) that emit around the 2µm wavelength, where hydrogen has a weak absorption line. PTB developed a kilometer-long multipass cell setup to compensate for this weak absorption.
PTB’s Innovation: The Shock Tube Setup
To achieve its research goals, PTB developed a shock tube setup. In this system, a piston compresses a helium and argon gas mixture up to 100 bars, rupturing a diaphragm and generating a shock wave. This shock wave passes through a gas cell containing a prepared mixture, such as Argon, O2, and H2. The cell is sometimes heated for up to an hour to prevent condensation. Argon is used instead of nitrogen to avoid interference with measurements of nitrogen-based species (e.g., NO, N2O). The shock wave ignites the gas mixture, raising the temperature to more than 10,000 Kelvin.
PTB uses a high-speed camera to monitor the combustion flame, and a laser beam is passed through windows in the shock tube to measure the species present using absorption spectroscopy. This innovative setup enables PTB to accurately simulate industrial combustion conditions and obtain real-time measurements of the resulting molecular species.
The mirSense Advantage
Dr. Zhechao QU noted that the mirSense QCL system, used for NO measurements (~5.2µm wavelength), provided several key advantages: its high power at the required wavelength and its ability to pulse the laser at 1 MHz, significantly faster than traditional function generators, which are typically limited to tens of kHz. mirSense’s software was also easy to use, allowing PTB to adjust pulse length and QCL chip temperature efficiently.
PTB built a custom water-cooling system to maintain the QCL’s base plate temperature at approximately +20°C, ensuring stable operation in their air-conditioned laboratory environment. This highly controlled setup has allowed PTB to conduct extensive experiments, pushing the boundaries of combustion analysis.
Results and Future Research on Reducing CO2 Emissions
PTB’s groundbreaking experiments with intrapulse absorption spectroscopy are paving the way for cleaner, more efficient combustion technologies. Their research is expected to lead to new insights into hydrogen and ammonia use as energy carriers, with the goal of reducing CO2 emissions and contributing to global decarbonization efforts. PTB is preparing a manuscript to share the results of these experiments, advancing the field of combustion engine analysis.
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