RECOVERY BOILER SCR – A CHALLENGE AND ANOPPORTUNITY IN RETROFIT CASES, 2017PEERS

SCR – Selective Catalytic Reduction is a well-established and widely used technology for NOx removal from theflue gases of the power boilers. Until now there has not been any need to utilize SCR in the recovery boilers.Sufficient 200 mg/m3 (6% O2, dry gas) is an achievable NOx emission level with primary means only. There arethough indications e.g. from China and from some European countries that the target emission level will be 100mg/m3 (6% O2, dry gas) in the near future. This is not achievable by air staging or even by SNCR in the recoveryboilers with normal kraft black liquors having N2content of 0,1 % or higher in dry solids.In the recovery boiler retrofit cases there are three major challenges for utilizing the SCR. The first one being thetemperature of the flue gas at the SCR reactor inlet. The second challenge is probable SO2in the flue gas, and thethird, too high particulate concentration after the electrostatic precipitator. The temperature of the flue gas should bearound 250 °C and the SO2 preferably below 5 mg/m3to avoid formation of harmful ammonium bisulfate. Dustconcentration should stay below 30 mg/m3n after ESP to avoid blockages of the catalyst. New recovery boilers runbasically with very low SO2 emissions but some variation and peaks may occur. The desired temperature isachievable with certain high power features. Additional electricity production creates good payback to the highpower investments.In this paper a theoretical study is presented for a retrofit case of a recovery boiler where an SCR is utilized to lowerNOx from 200 to 100 mg/m3 (6% O2, dry gas). Necessary high power features are implemented to raise the flue gastemperature high enough for the SCR.

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