Residence Time and Temperature CFD Study for Incinerators

BACKGROUND

Incinerators play a crucial role in various industries, including power generation, cement production, animal waste management, general waste management, and cremation services. All these applications are subject to the environmental requirements outlined in the Industrial Emissions Directive (IED) 2010/75/EU. Article 50, Clause 2 of this directive states:

"Plant shall be designed, equipped, built and operated in such a way that the gas resulting from the incineration of waste is raised, after the last injection of combustion air, in a controlled and homogeneous fashion and even under the most unfavourable conditions, to a temperature of at least 850 °C for at least two seconds." 

If hazardous waste (defined as waste with more than 1% halogenated organic substances) is incinerated, the minimum temperature requirement increases to 1100 °C. Our clients approached us to perform detailed Computational Fluid Dynamics (CFD) studies to ensure their plants operate within these standards. Where requirements were not met, we provided recommendations to achieve compliance. 

THE CHALLENGE

Element Digital Engineering developed a full 3D model of our client's asset, including the kiln (primary combustion zone), the secondary combustion zone, and regions of the post-combustion zone where residence time and temperature are measured in practice. Traditionally, the client used coal as fuel. In an effort to reduce environmental impact, a range of cleaner alternative fuels was considered. 

Using the chemical composition of these fuels, we calculated flame temperatures and determined the percentage split of the flue gas for each fuel. Additional calculations regarding material porosity were conducted in regions where the post-combustion flue gas would pass through loose material used in the cement production process. 

A passive scalar was set up as a non-invasive method to measure the time the flue gas spends in regions where residence time is measured in practice. Measurement points were established as planes in the model, averaging values to reflect multiple physical measurement points and produce realistic CFD modelling results. 

The analysis revealed that for two out of the three fuels considered, the average residence time exceeded two seconds, and the minimum temperature at the second measurement region was above 850 °C. It was concluded that residence time is sensitive to the inlet temperature, which affects the density and, consequently, the flow velocity. The initial mass flow rate at the inlet also impacts residence time, requiring a delicate balance between these factors for an optimal solution. 

THE SOLUTION

The refractory lining was identified as a key factor in heat losses within the post-combustion region. To retain heat and maintain the necessary temperature to meet directive requirements, the thermal conductivity of the refractory lining must be appropriately low, minimizing overall heat rejection (quantified by the Heat Transfer Coefficient in the model). 

For the case that initially failed to meet the requirements, we suggested adjustments in the mass flow rate of secondary airflow to compensate for the low residence time. Although these adjustments resulted in a decrease in fluid temperature, the residence time increased to exceed two seconds while maintaining the fluid temperature above 850 °C. This enabled our client to present these findings to the environmental agency as evidence that their plant now operates within the IED directive requirements.