Co-firing of refuse derived fuels with coals in cement kilns: combustion conditions for stable sintering

Usage of refuse derived fuels (RDFs) in cement clinker processes has grown considerably in the last decade. It has been demonstrated that RDFs can partially substitute fossil fuels without compromising clinker quality. The paper considers RDF properties which are essential for stabilisation of the sintering zone in the clinker burning process. An energy replacement indicator, defined as the RDF thermal input to the fossil fuel thermal input ratio, is used. Comminution of RDF to particle scales of pulverised fuel is economically not yet feasible and RDF is burnt at particle sizes in the order of several millimetres. Therefore, RDF combustion rates are orders of magnitudes smaller if compared to pulverised coal combustion rates. This shifts the flame zone downstream the rotary kiln and flame temperatures are reduced. A simple kiln combustion model has been used to examine the effects of RDF properties on sintering zone temperatures. Refuse derived fuels with calorific values below 20 MJ kg(-1) and coarse particle sizes burn at adiabatic temperatures below 2000 degrees C. To compensate for this, the energy replacement ratio has to be increased to 114%. Calculations have shown several options to increase gas temperatures in sintering zones. The most effective would be the comminution of RDFs to pulverised fuel sizes. In this case, a further reduction of the RDF calorific value would be acceptable.