Green ammonia co-firing in cement rotary kiln: numerical investigation on clinker quality and NO_x emission by gas–solid interaction

Green ammonia (NH₃) has been seen as a promising carbon-free fuel while the impacts of NH₃ co-firing on clinker quality and nitrogen oxides (NO_x) emissions in cement kiln are not yet clear. A detailed gas–solid interaction model based on industrial cement rotary kiln integrating one-dimensional mathematical description of clinker reaction is established. The combustion process is calculated by Computational Fluid Dynamics (CFD) method to explore the influence of NH₃ fuel quantity on thermal distribution and the pollutants evolution is evaluated. The clinker contents distribution is calculated by the model to estimate cement quality. A brief economic analysis is conducted and the feasibility of the NH₃ co-firing is illustrated. The results show that the flame inside the kiln is shortened and the heat transfer homogenization is decreased as the NH₃ co-firing ratio scales up, weakening thermal intensity inside the kiln. The preheating environment near the rotary kiln burner promotes the rapid generation and reduction of fuel-NO_x. The increased NO_x emissions during the process are mainly thermal-NO_x owing to the higher average temperature in the kiln. The quality of cement clinker is highly sensitive to NH₃ co-firing. The proportion of ammonia co-firing in cement rotary kilns needs to be appropriately selected with an upper limit to reduce carbon emissions while ensuring clinker production efficiency. Although the additional costs caused by co-firing are still relatively high, green NH₃ can play an important role in reducing carbon emissions in the near future.