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Metal Casting Technologies : September 2008
Figure 1. Influence of the combustion air temperature and the oxygen concentration on the combustion efficiency [ 1 ] result of this procedure, the flame becomes nearly invisible. The original intention lies in a substantial reduction of NOx values and a more homogeneous distribution of heat (a diluted flame still holds the same heat quantity). Although no nitrogen is introduced during an oxyfuel combustion, the high flame temperature aids the formation of NOx , which originates from nitrogen in the fuel or the leak air. This is different from the combustion of air fuel mixtures, where air consists of 79% nitrogen. With the flameless process, the NOx emissions are lowered below the legal government and TA-Luft limit values. The mode of operation of a diluted or mild combustion can be seen in Figure 3. With a recirculation of the exhaust gas, a dilution of the flame is achieved, which leads to a reduction of the temperatures at the mouth of the burner as well as a reduction of the NOx Figure 2. Theoretical flame temperature for the stoichiometrical combustion of natural gas plotted against the oxygen concentra- tion of the combustion air [ 1 ] is lower and more evenly distributed due to the strong internal recirculation of the furnace atmosphere in front of the burner mouth. values. This effect is reached with both air and oxygen burners. The temperatures of the oxygen burners are at the same level as the conventional air burners. Flameless leads to a homogenization of the temperature inside the furnace The distribution of the temperature inside the furnace can be seen. For the flameless Oxy-fuel combustion, the temperature Figure 3. Diluted combustion – a possibility to reduce NOx [ 3 ] METAL Casting Technologies September 2008 85