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Catalytic and gas–solid reactions involving HCN over limestone
Author(s) -
Jensen Anker,
Johnsson Jan Erik,
DamJohansen Kim
Publication year - 1997
Publication title -
aiche journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.958
H-Index - 167
eISSN - 1547-5905
pISSN - 0001-1541
DOI - 10.1002/aic.690431118
Subject(s) - chemistry , catalysis , calcination , inorganic chemistry , selectivity , sulfation , combustion , fluidized bed , coal combustion products , organic chemistry , biochemistry
In coal‐fired combustion systems solid calcium species may be present as ash components or limestone added to the combustion chamber. In this study heterogeneous reactions involving HCN over seven different limestones were investigated in a laboratory fixed‐bed quartz reactor at 873–1,173 K. Calcined limestone is an effective catalyst for oxidation of HCN. Under conditions with complete conversion of HCN at O 2 concentrations above about 5,000 ppmv the selectivity for formation of NO and N 2 O is 50–70% and below 5%, respectively. Nitric oxide can be reduced by HCN to N 2 in the absence of O 2 and to N 2 and N 2 O in the presence of O 2 . At low O 2 concentrations or low temperatures, HCN may react with CaO, forming calcium cyanamide, CaCN 2 . The selectivities for formation of NO and N 2 O from oxidation of CaCN 2 is 20–25% for both species. The catalytic activity of limestone for oxidation of HCN decreases with increasing degree of sulfation. Simultaneously the selectivity for formation of NO decreases while tht for N 2 O increases. The catalytic activity of sulfated limestone in‐creases with decreasing SO 2 concentration, indicating a competition between SO 2 and HCN for sites on the surface. The results indicate that heterogeneous oxidation of HCN is important in calciners and fluidized‐bed combustors with limestone addition or when burning coals with an ash with a high catalytic activity.