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Ammonia is a carbon-free fuel, so it has potential to reduce carbon dioxide emission from power plants when used as a fuel. However, combustion characteristics of ammonia are notably different from hydrocarbon fuels, especially regarding NOx emission . The nitrogen atom of the ammonia molecule may cause high NOx emission. Therefore, special techniques to reduce NOx emission are essential for gas turbine combustors which burn ammonia and natural gas. The results of our previous study  showed the characteristics of NOx emission in single-stage combustion. In this study, the concept for low-emission combustion in two-stage combustion has been examined numerically and experimentally.
Numerical study was conducted to investigate the combustion characteristics in two-stage combustion. A reactor network model comprising two perfectly stirred reactors (PSR) was calculated with CHEMKIN-PRO to evaluate NOx emission in short computation time. The reaction mechanism constructed by Tian et al.  was used. In the primary zone, methane/ammonia mixture is supplied as fuel. Then, only ammonia is supplied to the secondary zone (Fig.1). Air, methane and ammonia flow splits to primary and secondary zone are changed as parameters while total amounts of methane, ammonia and air flow rate are fixed.
Figure 2 shows the numerical results in terms of NOx concentration in secondary zone against equivalence ratio in primary zone for three ammonia flow splits. These results revealed that there are two methods to achieve low NOx combustion, namely rich-lean combustion and combination of lean-burn and secondary ammonia supply. In the first method, NOx is produced only in the primary zone in fuel-rich condition and burned gas is diluted in the secondary zone with secondary air. Therefore, the NOx formation in the primary zone is dominant for emission. In the second method, lean combustion condition in the primary zone leads to low temperature and low oxygen concentration in the secondary zone. These conditions lead to low NOx concentration at the combustor outlet.
These predicted combustion characteristics have been examined experimentally. Experimental results showed that NOx emission behavior is in good agreement with the numerical results.
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