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

- Urea pyrolysis rate was decreased by CaO.
- CaO exerted no effect on NH3 production but remarkably decreased HNCO production.
- CaO reacted with the melting urea or biuret to form Ca(NCO)2.
- Ca(NCO)2 decomposed to CaCN2 and CaCN2 can stably exist in the absence of O2.
- O2 oxidized Ca(NCO)2 to N2O and CO2, whereas oxidized CaCN2 to NO and CO.

CaO affects urea pyrolysis when coalescing with melting urea, which decreased the performance of urea-SNCR deNOx 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 O2. 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 O2. 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 O2 because of its high thermostability. In the presence of O2, Ca(NCO)2 can be oxidized to N2O, CO2, and N2, whereas CaCN2 can be oxidized to NO, CO, and N2.

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