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Show Corrosion of the superheater can be avoided by means of using suitable materials and corrosion-avoiding design. As for materials, low al loy steel (i.e. STBA12) or Inconel R anf IncoloyR can be used, in addition to the normal boi ler-grade steel pieps. The most important factor in superheater design is to lower the pipe-wal I temperature. To accompl ish this, a superheater location away from the reducing atmosphere which also enables to lower flue gas temperature before it enters the superheater. 2. 2 IMPROVEMENT IN POWER GENERATION EFFICIENCY BY REBURNING Reburning, injection of natural gas into the furnace, reduces the excess air rate from 80% to 40% whi Ie reducing CO emission down to 25 ppm. It lowers the exhaust gas loss and improves boi ler efficien~y and increases power output. Furthermore, if the installation is within the NOx 50 ppm I imit area, the need for de-NOx catalytic converter wi I I be el I iminated, reducing the in-house power use and increasing available power for sale. 2. 3 REPOWERING Repowering means adding a new equipment to an existing power plant in order to increase its output. This usually involves addition of gas turbine which uses natural gas, and the waste heat is uti I ized by the existing steam turbine, forming what is cal led a combined cycle. The gas turbine increases the power output whi Ie waste heat uti I izaiton increases efficiency. This repowering concept can be appl ied to MSW burning power generators. It recovers waste heat from gas turbine in the form of steam, which augument the steam from the MSW burning boi ler. It thus increases the power generation output, and raises it efficiency. It is particularly effective when the steam from MSW burning boi ler can be heated by a free standing superheater using gas turbine waste heat which improves cycle effeciency. 3. COMPARATIVE STUDY OF POWER GENERATION by MSW only, · with REBURN, and REBURN + REPOWERING METHODS 3. 1 MSW-only Burning Power Generation In Japan, the net thermal efficiency of MSW burning power generation is less than 15% based on the lower heat value. The reason for this low number, in addition to the explanation which appeared earl ier, are as fol lows. CD Stable incineration of MSW was the main target, and I ittle effort was made toward higher power output. GD The temperature of superheated steam was kept below 300°C for fear of hightemperature corrosion which is caused by the cholorides generated during MSW incineration. (In US and Europe, temperature in excess of 400°C is being used. It is evident that the life of steam pipe shortens as temperature goes up (Figure 10), so an appropriate design change is required.) ~ Due to the above, gas temperature at superheater inlet is capped at 650°C also, the gas flow is set at a low speed to avoid corrosion and errosion, I imiting the desing parameter of the superheater. @D In order to appease local residents, no visible exhaust is al lowed to appear from the smokestack, even though it may be just steam. Air-cooled condensers are used for this reason, which decreased the vacuum. 1 0 |