Effect of CO₂ content on NO reduction during sewage sludge reburning

The effect of CO₂ content(volume fraction 0-35%) on the reducing gas release characteristics from sewage sludge re-burning and the dynamic properties of NO reduction by sewage sludge and char were investigated in a simulated experimental platform of cement pre-calciner. The experimental results show that the reducing gas release from sewage sludge combustion are mainly HCN, NH₃, CH₄ and CO. With the increase of CO₂ content from 0 to 25%, the release of HCN, NH₃ and CH₄ slowly decreased due to the enhancing effect of sludge gasification by CO₂, while the release of CO increased significantly, eventually promoting the NO reduction rate from 51% to 61%. As continually increasing CO₂ content to 35%, the local thermal instability was enhanced due to the radiation absorption of CO₂, and the weakening of gasification resulted in the decrease of CO release. Moreover, HCN release decreased significantly, while NH₃ release did not change much, CH₄ release increased to a certain extent, and the combined effect makes the NO reduction rate gradually decreased to 55%. The results show that sludge re-burning can efficiently reduce NO in flue gas. It is also found that the homogeneous and heterogeneous reduction of NO are concurrence during sludge re-burning, while the experimental studies revealed that the NO reduction rate over the sludge char was only 18%, it implied that sludge denitration is dominated by gas-gas homogeneous reduction.