OCR Text |
Show It is very difficult to determine the mixing and diffusion process of fuel and air in the flame. Therefore, the mixing and diffusion condition of fuel was observed, although in a non combustion state, by measuring the CO2 concentration distribution in the combustion chamber by injecting CO2 in place of a fuel during combustion. However, the air is heated as during combustion. As with the NOx measurement, COz was sampled at each point in the combustion chamber and the CO2 concentration distribution was analyzed using a gas chromatograph (Yamagimoto, G-3800TFP). It is known that the amount of thermally formed NOx increases abruptly when the flame temperature increases. The flame temperature increases if high temperature air combustion is simply conducted. In order to suppress thermally formed NOx, therefore, the target cannot be achieved with the conventional diffusion combustion type burner. In this experiment, therefore, combustion of such a type that the combustion air and the fuel are completely separately introduced into the combustion chamber by utilizing the fact that the air temperature is higher than the self ignition temperature of fuel. Although there is no parameter that accurately expresses the degree of flow and mixing in the combustion chamber, it might be thought that empirically, the intensity of recirculating currents formed is expressed by the rate of inflow at the inlet of combustion chamber. Therefore, the quotient obtained by dividing the volume flow calculated from the flow rate and temperature of supplied air by the cross sectional area of inlet was defined as the inflow rate at the combustion chamber and the effect of the inflow rate on the amount of NOx emission was investigated. For each of the four fuel injection positions, Fig.6 shows the effect of the fuel injection position on the amount of NOx emission when the air temperature is kept at 1,150'C. When the fuel nozzle position is fixed at position 1, the amount of NOx emission increases with increasing air flow rate at the inlet. It might be thought that this is mainly because the high temperature air and fuel are supplied to the lump flame formed at the center of recirculating current, which reaches a very high temperature locally, resulting in the formation of a large amount of thermally formed NOx. When the fuel nozzle position is fixed at 2, the amount of NOx emission is generally large compared with the nozzle position 1 and does not show a monotonous increase relative to the increase in the air flow rate at the inlet. When the fuel nozzle position is fixed at 3, the change in the amount of NOx emission relative to the increase in the air flow rate at the inlet has a tendency completely different from that observed at the nozzle positions 1 and 2. In other words, the amount of NOx emission is large when the volume of supplied air is small and the air flow rate at the inlet is low. The amount of NOx emission decreases when the air flow rate at the inlet increases and strong recirculating currents are formed. When the fuel nozzle position is fixed at 4, the change in the amount of NOx emission relative to the change in the air flow rate is very small and besides the emission level is very low on the whole. In this case, the amount of NOx emission is not much related to the air flow rate at the inlet and the fuel jet joins the recirculating current, moving through the whole combustion chamber with the recirculating current while producing combustion or reaction. At this time,the light orange flame spreads throughout the combustion chamber and heat generation seems to occur in a wide range. However, the flame does not emit a strong white ray, nor does it burn brightly. It became apparent from the observation of flames that the type of flame in the combustion chamber is greatly influenced by changing the fuel supply position. The principal cause of this change in the type of flame is the difference in the diffusion and mixing of fuel. It can be said that in the type of flame that burns during mixing with the air near the fuel jet as observed in the conventional burner, a large amount of NOx is |