Patent ID: 8550017

Claim:
A method of controlling exhaust gas in an oxyfuel combustion boiler having a primary recirculation passage for introduction of a part of exhaust gas in recirculation from downstream of an exhaust gas treatment device into a mill as primary recirculated exhaust gas and for supply of pulverized coal pulverized by the mill to burners of the boiler by said primary recirculated exhaust gas, a secondary recirculation passage for supply of another part of the recirculated exhaust gas to a wind box of the boiler, an air separation unit, a secondary oxygen mixing passage for supply of a part of oxygen produced by the air separation unit to the secondary recirculation passage and a direct supply passage for direct supply of another part of the oxygen produced by the air separation unit to the burners, said method comprising: arranging a NO x density monitor for measurement of NO x density in the exhaust gas at an outlet of the boiler and an unburned combustible measuring device for measurement of unburned combustibles in the exhaust gas from solids obtained by dust collection by the exhaust gas treatment devices; presetting an operating range where NO x density of the exhaust gas is below a NO x limit value, and an amount of unburned combustibles in the exhaust gas is below an unburned combustible limit value based on the NO x density and the amount of unburned combustibles in the exhaust gas obtained by performing a combustion test of changing a rate of supply of directly supplied oxygen to the burners through the direct supply passage to the total amount of oxygen supplied by the air separation unit in accordance with a supply of pulverized coal; and regulating the rate of supply of directly supplied oxygen to the burners to the total amount of oxygen supplied by the air separation unit to maintain the operating range where NO x density of the exhaust gas measured by the NO x density monitor is below the NO x limit value and the amount of unburned combustibles in the exhaust gas measured by the unburned combustible measuring device is below the unburned combustible limit value.