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Moderate or intense low-oxy dilution oxy-fuel (MILD-oxy) combustion can synergistically reduce the emissions of CO2and NOx, and its industrial implementation is carried out under dry and wet recycle modes. In this work, a series of numerical simulations on pulverized coal MILD-oxy combustion under the typical dry and wet recycle modes are performed, aiming at fully evaluating the combustion behaviors affected by the oxidizer O2concentration (21–30%) under different recycle modes. The results show that increasing O2concentration weakens the in-furnace flue gas recirculation and promotes the combustion intensity, while a more uniform temperature distribution is obtained under dry recycle mode than under wet recycle mode. The diffusion/kinetics-controlled MILD regime is realized as the O2concentration increases. Nevertheless, the diffusion effect is enhanced due to the accelerated reaction rate under a high O2level. The gasification reaction is strengthened with the increased O2concentration. Compared with wet recycle mode, the burnout of coal char is slowed down under dry recycle mode because of the lower burnout rate. The fuel-NO conversion ratio is significantly reduced with the increased O2concentration, while wet recycle mode shows more potential in achieving low NOxemission than dry recycle mode. This work is expected to provide new insights into the comprehensive understanding of MILD-oxy combustion.

As promising functional materials, ionic liquids have been widely used in flue gas purification and separation. Previous studies reported that ionic liquid based mixtures can be used for mercury removal due to their high economic efficiency and environmental friendliness. In comparison with ionic liquid based mixtures, a supported ionic liquid phase (SILP) adsorbent shows significantly improved mercury removal performance because of its greatly enhanced gas/liquid interfacial area. In this study, a novel supported task-specific ionic liquid phase adsorbent [C4mim][FeCl4]–SiO2was prepared for high efficiency elemental mercury capture from flue gas. The Hg0removal performance was investigated in a bench-scale fixed bed reactor, and the reaction mechanism was proposed based on the temperature programmed desorption (TPD) test. The results showed that Hg0removal was mainly due to the oxidation by [C4mim][FeCl4]. The addition of [C4mim][FeCl4] on SiO2in a certain composition range enhanced the Hg0removal performance, and the optimized ionic liquid loading is 30%. The Hg0removal by the adsorbent was favored at higher temperatures. TPD results showed that the mercury compound formed on the adsorbent was HgCl2. The Hg0removal mechanism involves combined physisorption and chemisorption, which consists of Hg0oxidation to HgCl2by the ionic liquid and physical adsorption of HgCl2on the porous adsorbent.

In oxy-combustion power plants, flue gas impurities such as NOxmust be removed before CO2recovery. Currently, sorbents or catalysts as well as a flue gas treatment device are required for the application of the traditional NOxcontrol technologies. To produce NOx-lean oxy-fuel-derived CO2, an attractive option is using the existing CO2compression and purification units (CPUs) as NO removal units instead of traditional selective catalytic reduction technology. The special condition in the CO2CPU system, high pressure, was one of the most crucial operating parameters for NOxremoval in the CO2compression process. Appropriate pressure along with sufficient residence time of the pressurized denitrification system should be provided to achieve cost-efficient NOxremoval. In this work, to simulate the dynamic process of flue gas denitrification at elevated pressures, a two-stage compression process simulation model was built based on the experimental results, and good agreement between experimental and simulated data was obtained. The optimization of the pressure of the compression process as well as residence time showed the feasibility of the system eliminating 94% of NOxwith the emission concentration of 48 ppm (100 mg/m3) at the optimized pressure of 2.6 MPa and residence time of 223 s. This work demonstrated the possibility of the design and optimization of pressurized denitrification in an oxy-fuel CO2compression system.

Oxy-fuel combustion is considered as one of the most promising options for capturing CO2from coal-fired power plants. Oxy-fuel flue gas contains traces of gaseous elemental mercury (Hg0) impurity, which accumulate in the CO2compression units, causing severe damage to the aluminum heat exchangers. It is of great interest that the co-capture of flue gas impurities, such as nitrogen oxide (NOx) and elemental mercury, could be achieved in a CO2compression system at a high pressure. Currently, most of the studies associated with Hg0removal were conducted on traditional gas-cleaning devices at atmospheric pressure. It has been demonstrated that both NOxand SO2from flue gas played a significant role in Hg0reactions at atmospheric pressure in O2/CO2(oxy-combustion) or O2/N2(air-fired) conditions. However, the effect of those acidic gases on elemental mercury removal in the oxy-fuel compression system at high pressures was unclear. In this work, gas-phase Hg0oxidation by NOxand SO2was experimentally performed at the simulated O2/N2and O2/CO2atmospheres at pressures up to 2.5 MPa. It was found that Hg0removal was dependent upon the concentration of NO2in flue gas and primarily achieved by gas-phase reactions between Hg0and NO2. High pressure dramatically enhanced the oxidation of NO to NO2. Thus, the flue gas impurity Hg0could be efficiently removed through reacting with NO2in the CO2compression purification units. SO2hardly reacted with Hg0but affected the reaction of Hg0and NO2. The presence of SO2inhibited mercury removal especially at high pressures because of the decreased NO2concentration through the reaction with SO2and the decomposition of HgO to Hg0by SO2. The presence of CO2made the Hg0removal rate less dependent upon the pressure. The removal behavior of Hg0in the O2/CO2atmosphere was similar to that in the O2/N2atmosphere at pressures lower than 1.0 MPa. With the increasing pressure, the removal rate of Hg0in the O2/CO2atmosphere was lower than that in the O2/N2atmosphere.

Cocombustion technology of pulverized coal blending with NH3has a promising use in large-scale CO2reduced emission. However, the unstable flame and high NOx emissions during the cocombustion process retard its wide application. In order to overcome these limits, a new scheme of advanced moderate or intense low-oxygen dilution (MILD) combustion technology implemented in pulverized coal/NH3cocombustion is proposed in this work, and a numerical study on cocombustion characteristics of pulverized coal blending with different NH3proportions (0% ∼ 50% NH3, by caloric) is performed. Results show that the achievement of a pulverized coal/NH3MILD combustion regime is preferred at a high NH3case. As NH3proportions increase from 0 to 50%, the furnace temperatures are decreased due to the enhancement of heat transfer between flue gas and the furnace wall. The ignition time and temperature of coal particles are decreased by 52.1 and 33.0%, respectively, while the particle burnout time is prolonged by 52.3% and the char consumption rate decreases linearly. In addition, MILD combustion shows a good potential to reduce NOx conversion during pulverized coal/NH3cofiring. Although the proportion of NH3reaches up to 50%, NOx conversion ratio is only 2.1%. These results are expected to provide several new insights into stable flame and low NOx emissions of pulverized coal/NH3cocombustion.