Limestone addition in fluidised bed combustion of coal is a well established technique for sulphur dioxide control. However, calcium oxide interferes with fuel-N conversion by enhancing the NO emission while strongly reducing N2O release. In laboratory experiments, the role of Ca-compounds on combustion and hydrolysis of HCN was studied. It could be shown that CaCN2 is an intermediate product that directs the fuel-N conversion towards the formation of NH3 which is subsequently oxidised to NO. The catalysed hydrolysis of HCN besides its relevance for nitrogen oxide formation is also an explanation for the observed favoured release of NH3 during pyrolysis of low rank coals such as lignites. Because of the increased content of calcium Compounds in the ashes of many lignites the transformation of HCN with water into NH3 is favoured. (C) 2002 Elsevier Science Ltd. All rights reserved.