Influence of CaO on urea pyrolysis in the selective non-catalytic reduction deNOx process

CaO affects urea pyrolysis when coalescing with melting urea, which decreased the performance of urea-SNCR deNO(x) process. The influence of CaO on urea pyrolysis was studied with a thermogravimetric analyzer, a fixed bed reactor, and DRIFTS. The mechanism of CaO was also analyzed. Experimental results showed that CaO decreased the pyrolysis rate of urea, especially in the presence of O-2. NH3 production in urea pyrolysis was not influenced by CaO, whereas HNCO in the gaseous products disappeared and CO2 production was increased remarkably. NO, N2O, and CO were also produced in the presence of O-2. DRIFTS experiments showed that Ca (NCO)(2) and CaCN2 were produced and the composition of the solid products varied with the atmosphere. Mechanism analysis showed that CaO initially reacted with melting urea and biuret to produce Ca(NCO)(2) and release NH3 and H2O. Ca(NCO)(2) decomposed to CaCN2 and CO2 with increasing temperature, and CaCN2 can exist stably in the absence of O-2 because of its high thermostability. In the presence of O-2, Ca(NCO)(2) can be oxidized to N2O, CO2, and N-2, whereas CaCN2 can be oxidized to NO, CO, and N-2.