NOx formation and reduction by a coal, a lignite, an anthracite and a petroleum coke in conditions of cement plant calciner

摘要

The cement industry is a high energy consumer. This energy is mostly provided by the combustion of pulverized carbonaceous solid fuels. Up to 60 % of this fuel income may be injected into the calciner. However, the combustion of solid fuels produces pollutants, particularly NOx. Several reduction technology were developed. The most promising one, in term of cost and efficiency, is the reburning. It consists in a secondary fuel injection, creating a fuel rich zone favorable to NOx reduction. The fuel injection in cement plant calciners may be compared to this technique. The solids interact with NOx at different levels : through gas phase reactions with volatile matters released during the pyrolysis, and through solid-gas heterogeneous reactions : i.e. char oxidation that produces N-species and NO reduction at the char surface. A coupled experimental and modeling protocol was developed to determine the relative contribution of these different phenomena. The used fuels are of four distinct types, commonly used in cement plants: a lignite, a coal, an anthracite and a petcoke. Thus, the elementary heterogeneous reactions – devolatilisation, char oxidation and NO reduction by char – were characterized by specific experiments and modelings. One observes a large disparity between the four fuels in terms of kinetic parameters and quantity of volatile matters released. Finally, the experiments and modelings representative of the reactions occurring simultaneously during reburning in calciner conditions were performed. It appears that the effect of NO reduction in the gas phase is of the same order of magnitude that the effect of reduction by char after a particle residence time of 2 s. The NO reduction by char increases continuously with the temperature, whereas the gas phase reduction presents singularities in function of the temperature for the high volatile fuels: The NO reduction is lower at 900C than at 800 and 1000C, in case of lignite and coal. A detailed chemical analysis of these singularities was carried out and enabled to determine the main reaction paths occurring during NOx formation and reduction in the gas phase.