Experimental study on the ash deposition and NO emission of high-alkali coal under the staged O2/CO2 and O2/RFG conditions

Abstract

The oxy-fuel combustion contributes to carbon capture, while the recirculation of flue gas brings about high concentrations of SO₂ and H₂O, which can affect the transformation of minerals in high-alkali coal. The staged oxy-fuel combustion, as an effective method for NO_x reduction, can also change the ash deposition behavior of high-alkali coal. Two kinds of diluting agents, including pure CO₂ for O₂/CO₂ combustion and simulated “recycled flue gas” (CO₂, SO₂, and H₂O) for O₂/RFG combustion, were employed in the present work. The ash deposition and NO emission of high-alkali coal during the staged oxy-fuel combustion were simultaneously studied under O₂/CO₂ and O₂/RFG conditions. The conversion ratios of fuel-nitrogen to NO (C_NO) and ash deposition efficiencies (E_d) at different stoichiometric ratios in primary combustion zone (SR₁) and different oxygen concentrations were obtained. Afterwards, a series of tests were performed to further analyze the ash deposits. The experimental results show that as SR₁ increases from 0.6 to 1.2, C_NO jumps from 2.0 % to 23.5 % (O₂/CO₂ combustion) and from 1.9 % to 19.9 % (O₂/RFG combustion). The additions of SO₂ and H₂O can reduce the NO emission. With the rising SR₁, E_d under the O₂/CO₂ and O₂/RFG conditions decreases from 4.0 % to 2.6 % and from 4.8 % to 2.1 %, respectively. At high SR₁, the CaSO₄ amount declines and the iron contributes less to the ash deposition. In O₂/RFG combustion, the small sticky particles of sodium aluminosilicates on large particle surfaces reduce, and the large particles of calcium aluminosilicates shrink because some calcium produces CaSO₄. Moreover, the exposure of ferrous iron to H₂O helps its oxidization so iron is harder to cause severe adhesion. As O₂ concentration rises from 21 % to 40 %, C_NO shows an upward trend. Meanwhile, E_d under the O₂/CO₂ and O₂/RFG conditions increases from 2.6 % to 3.7 % and from 2.3 % to 2.7 %, respectively. The present work is expected to provide some conducive information for the clean utilization of high-alkali coal and secure operation of boiler, as well as large-scale CO₂ capture.