OCR Text |
Show Introduction To popularize the regenerative burner system at the earliest possible time, it will be necessary to find a method that is simple and as highly effective as possible to attain a reduction in NOx emission ;hich is an important environmental task. The FDI (Fuel Direct Injection) combustion which Tokyo Gas has been proposing up to now, indeed fulfills this requirement and may be said to be the ultimate low NOx industrial furnace combustion technology. In case of the energy saving at the high temperature industrial furnace such as metal melting, steel heating, etc., the preheated combustion air is much used for recovering the waste heat. However, the use of preheated air in the combustion furnace will generally bring about a large increase in the NOx emission. Thus, there will be the contradiction that seeking to improve the energy efficiency from the context of protecting the environment will, on the other hand, directly lead to an increase in the emission of NOx. It was found, here, that by applying the FOI combustion to these furnaces the NOx emission level was reduced into 150 ppm (02 = 0%). As a result, the use of FDI combustion has been increasing year by year not only in Tokyo Metropolitan Area where there are strict environmental regulations, but also in Europe and the United States where regulations are being strengthened. We are continuing to carry out various kinds of experiments to further heighten the effectiveness of FOI combustion and in this writing an introduction shall be made on an approach to an advanced FOI, in addition to the tests carried out up to now. 1. FDI Combustion Principles of the FOI The principles of FOI combustion are extremely simple. Fuel nozzles and combustion air nozzles are arranged on the wall at a certain distance from each other and fuel and air are injected directly into the furnace at a high velocity (average velocity of 60 mJs - ISO mJs) from both (Fig. 1). At this time, in-furnace gas is thoroughly mixed and the partial pressure of the oxygen lowered. Also, there will be no immediate reaction between the two flows. When a certain amount of time has elapsed, and after the oxygen thoroughly mixes with the gas, combustion will start slowly. Thus, the volume of the flame will be larger than that of a normal flame. Additionally, as some amount of fuel is broken down to carbon, then flame emissivity is increased. Thus, the peak temperature of the flame will become low and the emission of NOx held down. It was common knowledge up to now that stability of ignition and combustion took priority for practical combustion. However, FDI combustion is a method in which the reduction of the NOx takes priority under a condition of high temperature. 1bis represents a completely different concept from that up to now. By high temperature is meant a temperature above that where the fuel will self-ignite with stability. Priority is placed on reducing the NOx with premise placed on strictly controlling this condition. The arrangement of both nozzles as well as the number of nozzles and the angle of the nozzles are optional and it is possible to arrange the air nozzles around the fuel nozzles, or vice versa. This has a close relationship with the Super FOI which will be explained later. Application of the FOI to the regenerati ve system The FDI combustion is most effective for the regenerative system. In particular, the short cycle regenerative system which is much talked about today, is a system that was put into practical use in England over 10 years ago. This is a revolutionary method which obtains the highest waste heat recovery. The regenerative system is made up of a pair of burners and regenerator, as well as a reversing valve, an air blower and an exhaust fan. The regenerator is placed near the burner and high temperature e~aust g~ and co.mbustion air passes through alternately. Small grain size ceramic particles are used as the. regen~ratIon medium. This ~as solved the problem of durability against thennal shock, thermal stress and corrOSIOn which were the weak pomt of the general recuperators. 1brough direct heat exchanging, passing high temperature exhaust gas and combustion air |