An Experimental Study on the Conversion of CaO to CaSO₄ During Diesel Particulate Filter Regeneration

The deposition of ash on a diesel particulate filter (DPF) results in an increase in pressure drop across the aftertreatment system, which leads to reduced capacity for soot and an increase in frequency of regeneration to eliminate the soot. To estimate the amount of ash and design a reduction method for the accumulated ash on the DPF, a deeper understanding of the ash generation mechanism is required. Previous studies have found that ash sampled at exhaust manifolds comprises metal sulfate and some oxides, whereas ash accumulated on DPFs primarily comprises typical metal sulfates. Although metal oxides are converted to the metal sulfates, the conversion mechanism is not well understood. Herein, the formation mechanism of calcium sulfate (CaSO₄), which is the main component of ash on DPFs, from calcium oxide (CaO) was investigated using a flow reactor. CaO was made to react with SO₂, SO₃, O₂, and H₂O, when passed through a diesel oxidation catalyst. Quantitative analysis of the ash components in the products was performed using ion chromatography. The concentrations of SO₂, SO₃, and H₂O during the reaction were analyzed by Fourier-transform infrared spectroscopy. Based on the experimental results, it was found that CaSO₄ mainly formed from the reaction of CaO with SO₂ and SO₃; whereas, H₂O inhibited CaSO₄ formation owing to the change in physicochemical properties of CaO.