Removal mechanism of organic sulfur in coal assisted by microwave irradiation

In order to clarify the effect of microwave field combined with peracetic acid (PAA) on the removal of organic sulfur in coal, four demineralized coals including Shanxi Linfeng (LF), Ningxia Ningdong (ND), Shanxi Lingshi (LS) and Henan Luoyang (LY) coal were selected, and three sulfur-containing model compounds including benzyl mercaptan (BM), benzo(b) thiophene (BT) and diphenyl sulfoxide (DS) were used as well. Each test, the microwave with the power of 100 W irradiated coal for 1-5 min combined with PAA. The change of sulfur form in the solid phase was analyzed by X-ray photoelectron spectroscopy (XPS). The concentration of SO₄^2- in the liquid phase after desulfurization was analyzed by ion chromatography (IC), and the change of sulfur form in the extract was analyzed by gas chromatography/mass spectrometry (GC/MS). The results show that the higher the organic sulfur content, the greater the desulfurization rate. The maximum desulfurization rates of LY and LS are as high as 55.06% and 45.78%, respectively, and the maximum desulfurization rates of ND and LF are 31.24% and 28.21%, respectively. It is found that the organic sulfur as mercaptan in coal is easier to remove than that as thiophene and sulfoxide, and the sulfur form gradually transforms to a high valence state during desulfurization. The sulfur-containing bond is broken in the microwave field and the sulfur can be oxidized to SO₄^2- by PAA finally.