Cement Energy and Environment

.. Technology Trend REBURNING: HOW TO REDUCE NOx EMISSIONS IN DIFFERENT TYPES OF INLINE CALCINERS Several techniques can be used to reduce NOx emissions in the cement industry. Primary measures consist in optimization of the kiln and calciner combustion and should be comprehensively studied and implemented before the implementation of secondary measures, which consist mainly of treatment of the exhaust gases (SNCR and SCR) and necessarily present higher operational costs. One of the best options for reducing NOx relies on the calciner optimization by means of reburning. This technique can reduce the NOx formation in the calciner and create conditions to reduce, within the calciner, the NOx previously formed in the kiln. The potential for NOx emissions reduction by reburning depends on the calciner configuration and on its operational conditions. As the NOx formation/ destruction reactions are complex and involve several parameters, it is rather difficult to predict whether or not design modifications or changes in operational parameters would result in suitable NOx reduction rates. The knowledge of the internal fluid dynamics, temperature profile, local volatiles concentration, local oxygen concentration, combustion, and calcination efficiencies is essential to optimize the reburning conditions. In this regard, mineral interactive computational fluid dynamics (MI-CFD) modeling - developed by Cinar Ltd., is an effective modelling and simulation tool as it allows comprehensive understanding of the features mentioned together with the NOx formation and reduction profiles. 1. Introduction Nitrogen oxides (NOx) are atmospheric pollutants that contribute to acid rain and photochemical smog. In combustion systems, NOx refers to NO and N02, as N20 emissions are negligible. The major part of emissions consists of NO which is oxidized to N02 in the atmosphere. In the clinkerization process, NOx is formed mainly by two mechanisms: thermal NOx and fuel NOx' Thermal NOx is formed by breaking the bonds of molecular nitrogen and oxygen when submitted to temperatures higher than 1600°C and this is the main cause for the NOx generated in the kiln, where the gas temperatures are mostly higher than 1800°C. In calciners, the temperatures are not sufficiently high to form thermal NOx. The prevailing mechanism in this case is the fuel NOx, in which the fuel-bound nitrogen is oxidized to NO. The conversion of the fuel nitrogen to NO depends mainly on the local oxygen concentration near the fuel injection. The main intermediate compound of this reaction is HCN, which can react with 0 2, forming NO or, in the absence of oxygen, react with NO forming N2. Fuel NO, is also formed in the kiln but its contribution is less important than thermal NOx. Prompt NOx, is not significant as thermal NOx and fuel NOx in Industrial process. It consist of the formation of amines and cyano by the reaction between fuel hydrocarbon radicals and molecular nitrogen, which are then oxidized to NO. 2. Measures to reduce NOx emissions Environmental agencies worldwide are continuously imposing strictly low NOx emissions limits on industries and the cement industry needs to develop, implement and improve the technology to reduce NOx emission. The techniques commonly employed may be divided into primary and secondary measures. Primary measures involve combustion optimization in the kiln and in the calciner. The most commonly used are low •0:! CH, prompt NOx, burners, staged combustion reburning. Secondary and 1 Mechanisms involved in NO. formation and reduction G o) Fuel-N__..., HCNINH 1 I • HCN •O, tlwn11ol •NH, N , ' NO r measures are the ~ I implementation of .<: ~ technologies to treat ------------------------------------ ~ the exhaust gases. In the cement industry, 29