Numerical study on oxy-fuel combustion of coal pre-gasification products in cement calciner

With the increasing concerns over global climate change, carbon emissions reduction in the cement industry has become a key focus. This paper investigates the temperature field, species distribution, and raw meal decomposition of pre-gasification oxy-fuel combustion in a 5500 t/d calciner. Numerical simulations are performed for various combustion conditions, including coal-fired, oxy-fuel, and pre-gasification combustion. An optimized pre-gasification staged combustion layout was designed for the cement calciner, and its emission reduction and production increase effects were validated through numerical simulations. It reveals that oxy-fuel combustion improves temperature uniformity and reduces local high-temperature zones. Compared to the air-fired case, the maximum temperature decreased by 71.42 K in the 21 % O₂ concentration oxy-fuel combustion case. An increase in O₂ concentration leads to higher temperatures and temperature non-uniformity. High CO₂ concentrations in oxy-fuel combustion inhibit CaCO₃ decomposition, while increasing O₂ concentration significantly enhances the decomposition ratio. Both oxy-fuel and pre-gasification combustion significantly reduce NOx emissions. Under the oxy-coal combustion, compared to the air-fired case, NOx emissions were reduced by 56.12 % in the 21 % O₂ concentration oxy-fuel combustion case. In unoptimized pre-gasification cases, the fuel burns out more quickly, resulting in an oxidizing atmosphere that slightly increases NOx at the outlet. Moreover, the residence time of CaCO₃ in the lower calciner becomes shorter because the larger flow rate distributed in the reduction zone, causing a decrease in the CaCO₃ decomposition ratio by 5.37 %. Therefore, the fuel inlets of pre-gasification combustion need to be arranged along the calciner to achieve multi-staged combustion to reduce the NOx emission. The optimized pre-gasification layout, along with oxy-fuel combustion, can achieve ultra-low NOx emissions (22.49 ppm) at the calciner outlet and significantly alleviate the issue of reduced raw meal decomposition rate caused by oxy-fuel combustion, reaching a CaCO₃ decomposition ratio of 95.36 % under 30 % O₂ concentration oxy-fuel combustion. In industrial applications, it is recommended to conduct oxy-fuel combustion in the cement calciner with this pre-gasification layout at 30 % O₂ concentration to ensure that the decomposition ratio of cement raw meals remains above 95 %. This study provides a theoretical guide on how oxy-fuel and pre-gasification combustion technologies can be optimized to improve energy efficiency and reduce NOx emissions in cement production.