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Mill conversions to dissolving grade pulp production often necessitate recovery boiler combustion system modifications to increase the boiler’s solids burning capacity while at the same time reducing NOx emissions. Recently, Jansen was awarded several of these capacity upgrade contracts.
In recent years, consumer product market trends in the Pulp & Paper Industry are increasingly favoring dissolving pulp grade products. To meet market demand for these products, several mills are in the process of “retooling” the pulping line from producing conventional kraft pulp to dissolving grade pulp. Typically, this change in product leads to increased quantities of black liquor dry solids (BLDS) generated with potentially higher heating values. Because of the higher heat input rate to the recovery furnace and potential increase in fuel nitrogen content, generation of NOx would be expected to increase, thereby leading to higher stack emission rates.
To improve the recovery boiler’s BLDS throughput capacity and offset potential increases in NOx, optimization of the existing combustion air delivery system is needed and further staging of combustion air supply can be applied by adding a fourth level of air. In the industry, this fourth level of air is named quaternary air (QA) and has been shown to reduce NOx emissions.
At several mills, Jansen was contracted to conduct evaluations of recovery boiler BLDS burning capacity and associated NOx generation before and after making the change from kraft to dissolving pulp. This evaluation typically consists of a site visit for first-hand collection of operating data, followed by process evaluations and mass and heat balance calculations. An important element of the evaluation process is Computational Fluid Dynamics (CFD) modeling of the furnace combustion conditions with the different BLDS supply rates and heating values. CFD modeling is typically conducted for several configurations of combustion air supply and at the conclusion, a parametric evaluation of key performance factors provides valuable input to the engineering design of the air system modifications. The end result is an air system that best meets the client’s needs.