The flame radiation enhancement in gas fired furnaces significantly improves the thermal efficiency and also reduces flame temperature, which results in NO emission reduction. In this research, the effects of the inlet natural gas fuel temperature on the flame luminosity and the NO emission in a boiler were investigated experimentally. The Exhaust gases temperature and nitrogen oxides emissions were measured for different inlet fuel temperatures. Also, the resultant radiation of flame luminosity was measured by a photovoltaic module. Results show that by increasing the inlet gas temperature up to 240oC, NO concentration and exhaust gases temperature increases. But, with further increasing of the inlet gas temperature from 240oC to 310oC, NO concentration and exhaust gases temperature decreases significantly as a result of an increase in flame luminosity. This special behavior, which disputes with common expectation, also happens in highly preheated air combustion technology.

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