CEM for N2O emissions control from ammonia-fired power generation
Sep 24 2023
Author: Stephen B. Harrison on behalf of sbh4 GmbH
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Sustainable hydrogen and ammonia will be important power generation fuels in the future and are likely to progressively substitute natural gas. However, their emissions differ to natural gas and introduce continuous emissions monitoring (CEM) challenges that must be addressed to avoid greenhouse gas emissions.
These new fuels and their emissions monitoring requirements will lead to equipment development challenges for solutions providers. They will also create opportunities to serve the market with new gas analysers and CEM systems.
The role of hydrogen in grid balancing
Hydro, wind, and solar power can decarbonise electricity production, but their temporal variability or intermittency results in a mismatch between power supply and demand. Batteries can support grid balancing over very short durations, but they are less cost-effective over the long term.
Converting excess power to a molecular energy store such as hydrogen or ammonia during peak renewable power generation periods and then using the hydrogen or ammonia to generate power on a turbine to meet peak power demand will be one way of balancing the grid over longer durations.
Hydrogen blending with natural gas and pure hydrogen fired gas turbines are being tested. Hanwha Energy tested 59.5% hydrogen blended with natural gas at their 80MWe gas turbine power plant in Daesan, South Korea. A 30% total NOx reduction was reported alongside a 22% reduction in CO2 emissions.
Due to the relationship between H2 blend composition and CO2 emissions reduction, a high proportion of hydrogen is required to achieve significant CO2 emissions reduction. Ultimately, pure hydrogen on the turbine would be the goal.
The hydrogen to Magnum project in the Netherlands proposes to burn pure hydrogen on three Mitsubishi Power M701F 440MW power blocks on the Eemshaven Magnum power plant, operated by RWE. The project will integrate underground hydrogen storage in multiple salt caverns to enable very long duration energy storage for long term grid balancing.
Ammonia as a transportable clean energy vector
Hydrogen is very difficult to transport in large quantities over long distances. Linking energy importers with locations with abundant renewable power that can produce large quantities of low-cost green hydrogen from electrolysis schemes or with CCS may require conversion of the hydrogen to ammonia, which is easier to transport by ship due to the high volumetric energy density of liquid ammonia.
Ammonia can be fired on gas turbines using various power generation cycles. The ammonia fuelled combined cycle can approach the natural gas fired IGCC efficiency. An alternative cycle known as CGHT can marginally improve on the ammonia combined cycle to come very close to the natural gas fired IGCC efficiency. In the CGHT, ammonia is decomposed to hydrogen and nitrogen using heat from the combustion process prior to the cracked ammonia being fed to the burner.
Ammonia-fired turbines are like those used with natural gas. The main changes are in the burner and the process control system, not the turbine itself. Control of NOx emissions in general, and especially nitrous oxide (N2O) which is a very potent greenhouse gas is a focus of development in this area.
Minimising and monitoring nitrous oxide emissions
Partial cracking of 20% to 30% of the ammonia is a solution that several turbine OEMs are testing. The blend of ammonia, hydrogen and nitrogen seems to burn very well.
When using 20 to 20% of cracked ammonia blended with ammonia on a gas turbine, optimisation of the air:fuel ratio is important to ensure efficient combustion with minimum emissions. Tests have shown that using a slightly oxygen rich flame, with an equivalence ratio of 0.8 to 0.9, can eliminate N2O emissions. As the equivalence ratio falls to 0.6 N2O emissions can rise to as much as 200 ppm by volume. At a ratio of 1.3, they would be around 50 ppm.
The National Institute of Advanced Industrial Science and Technology (AIST) is one of the largest public research organizations in Japan. They have conducted tests to validate the use of an ammonia natural gas blend for power generation on a small gas turbine. Co-firing ammonia with hydrocarbons changes the emissions footprint since CO and CO2 may be formed.
In Japan, there is a large pilot project in planning which will co-fire ammonia onto unit 4 of the Hekinan coal fired power plant. The reduction in coal usage on this 1GWe unit will reduce CO2 emissions since the ammonia will be sourced as green or blue ammonia. It will be important to avoid N2O emissions since N2O is an extremely potent greenhouse gas, about 250 times worse than carbon dioxide.
When co-firing ammonia with coal, or natural gas CO and CO2 will be present in the flue gas. This adds to the complexity of the CEM system because CO and N2O have overlapping peaks in the IR spectrum. The CO2 IR absorption peak is also very close to N2O.
The ISO 21258 (Stationary source emissions -Determination of the mass concentration of dinitrogen monoxide) has been developed to enable accurate measurement of N2O in flue gas streams. It confirms NDIR as the reference method and draws attention to the need to convert CO to CO2 and potentially to analyse the CO2 on a separate instrument or channel and then compensate for CO2 if required.
Commercial experience of CEM for nitrous oxide
Fortunately, there is relevant experience to draw on for N2O flue gas analysis. N2O is a regulated greenhouse gas in the European Emissions Trading Scheme (ETS) and is also regulated through US Greenhouse Gas (GHG) legislation. Nitric acid production has dealt with the challenge of CEM for N2O for many years.
Commercial systems exist that can enable simultaneous N2O and NOx analysis. As an example, the Fuji CEMS from Fuji Electric uses NDIR for the N2O analysis. The SWG200 from MRU also uses NDIR as the method for N2O analysis.
MKS offers their MultiGas FTIR with capability for N2O analysis. The Rosemount CT5100 quantum cascade laser gas analyser has also been used for research into N2O emissions from power generation turbines.
Ammonia may also be used as a fuel for aviation, maritime propulsion, and land-based mobility. In these cases, engine development and testing will require N2O CEM systems and they may also be required to be installed on ships to monitor maritime emissions.
The growth of low-carbon fuels for power generation and propulsion will introduce new CEM challenges. It will simultaneously open new opportunities for innovative solutions providers to serve this emerging market.
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