Reduction Of Recycled NO Research Articles

Reduction of recycled NO over char during oxy-fuel fluidized bed combustion: Effects of operating parameters

To investigate the effects of operating parameters on the recycled NO reduction over char during oxy-fuel fluidized bed combustion, tests with two types of char were carried out in a bubbling fluidized bed system. The reduction ratios of recycled NO were calculated based on the test results. It was found that char combustion obviously affected the recycled NO reduction in the oxy-fuel fluidized bed combustor. The reduction of recycled NO was promoted by the increase of oxygen concentration in the feeding gas. The reduction ratio of recycled NO could be improved by the higher reaction temperature. Although the smaller sizes of char particles were conducive for char combustion, char particle sizes had little effects on the reduction of recycled NO. The prolonging of gas residence time favored the reduction of recycled NO over char. In addition, the effects of various char types on the recycled NO reduction were also evaluated in this study.

The characteristics of recycled NO reduction over char during oxy-fuel fluidized bed combustion

To explain the reduction process of recycled NO over char during oxy-fuel fluidized bed combustion, the characteristics associated with the reduction of recycled NO were investigated in the present study. The experimental results were obtained using a bubbling fluidized bed. The effects of typical impurities within the recycled flue gas on the reduction of recycled NO were also studied. It was determined that significant reduction of the recycled NO over char occurred under oxy-fuel fluidized bed combustion conditions. The apparent activation energy of the recycled NO reduction reaction was much lower when combustion occurred. The competition between the NO formation from char nitrogen and the reduction of recycled NO over char determined the final NO concentration of the flue gas. The recycled NO concentration had little influence on the reduction ratios during oxy-fuel fluidized bed combustion when NO formation from char nitrogen was considered. Recycled SO2 can enhance the reduction of recycled NO over char during oxy-fuel combustion while the low concentration of recycled CO had a little effect on the process of recycled NO reduction. The reduction ratios of recycled NO increased as the recycled SO2 concentration increased. Moreover, significant capture of recycled SO2 was observed during oxy-fuel fluidized bed combustion.

Homogeneous and heterogeneous contributions of CO2 and recycled NO to NO emission difference between air and oxy-coal combustion

High concentration of CO2 and recycled NO-containing flue gas are known to affect the total NO emission during oxy-coal combustion through both homogeneous and heterogeneous reactions. To evaluate each contribution, this work investigated the NO emission behavior in air and oxy-coal combustion atmosphere (O2/CO2 and O2/recycled flue gas) with three different oxygen concentrations (21%, 27% and 32%). Three typical Chinese coals and their derived chars were employed and all of the experiments were conducted in a drop tube furnace at a temperature of 1373K. The results show that the conversion ratio of coal nitrogen (coal-N) to NO increased with decreasing coal rank in all atmospheres. The NO emission rate in oxy-coal was lower than that in air combustion. The heterogeneous effect of CO2 mitigated NO emission in oxy-coal combustion, whereas homogeneous contribution of CO2 may have increased NO formation due to a great amount of coal-N was released in the form of volatile nitrogen (volatile-N) at a high concentration of CO2. The reduction of recycled NO was the major reason for the lower NO emission in oxy-coal combustion, but the main removal route differed. The reduction of recycled NO through homogeneous reactions was more important for both the lignite and the high volatile content bituminous coal, whereas the heterogeneous reactions were more important for the anthracite NO removal.

Oxyfuel technology: NO reduction during oxy-oil shale combustion

Nitrogen oxides are one of the major environmental problems arising from fossil fuels combustion. Oxyfuel combustion is one of the most promising clean coal technologies for pulverized fuel-fired power plants to control and avoid CO2 emissions; with this technology NOx emissions are significantly reduced.Due to the importance of oil shale utilization in Jordan and all over the world, this study continues with our previous work on oil shale combustion. Unstaged and staged air-firing and oxyfuel combustion were carried out using Jordanian oil shale collected from El-Lajjun area. The reduction of simulated recycled NO has been investigated in a 20kW vertical reactor. The actual situation has been simulated by injecting NO in the reactor through the burner. It was found that the simulated recycled NO reduction efficiency is affected by the media and the mode of firing. The simulated recycled NO is reduced efficiently (60–70%) during unstaged oxyfuel combustion as well as air-firing of El-Lajjun oil shale. The reduction of the injected NO is more efficient with staging compared to unstaged combustion mode for both air-firing and oxyfuel combustion. The reduction of the injected NO for combustion in air ranges from 61% (without staging) to 100% (with staging) whereas for combustion in 27% O2/73% CO2, it ranges from 57% (without staging) to 100% (with staging). Moreover, El-Lajjun oil shale ashes are characterized as being high in CaO and SiO2, low in Al2O3, SO3, and P2O5 and with minor amounts of F2O3, MgO, Na2O, K2O, and TiO2.

Experimental study of NO and NO2 formation in a PF oxy-fuel firing system

NO and NO2 formation in PF oxy-fuel firing was investigated for a high volatile bituminous coal and a high-sulfur lignite coal. Results obtained under various oxy-fuel atmospheres were compared with those produced during air combustion. Experiments involving NO injection under dry and under wet conditions were also conducted in order to simulate dry and wet flue gas recycles respectively. NO emissions were observed to be significantly lower during all oxy-fuel cases for both the high volatile bituminous coal and the lignite coal in comparison to air combustion. Overall trend was an increase in NO emissions with increasing inlet oxygen concentration for the oxy-fuel cases. The presence of steam (wet flue gas recycle) decreases NO formation from fuel nitrogen as well as reduces recycled NO. The efficiency of steam in recycled NO reduction wanes however with increasing inlet oxygen concentration. NO2 formation in oxy-fuel environment increases as a result of increasing oxygen partial pressure but these increases are not substantial. Steam addition to the oxy-fuel oxidant gases yields substantial increases in NO2/NOX ratios for high volatile bituminous coal in all cases while for lignite coal, the ratio remained low.

Reduction of Recycled NOx by Simulated Coal Volatiles in Oxy-Fuel Combustion

The reduction of recycled NO by simulated coal volatiles (CH4 and NH3) during oxy-fuel combustion has been studied in a flow reactor. Special emphasis was given to the effects of different variables on the reduction of recycled NO, and to the comparison with results obtained in air-fired combustion. The experiments were conducted in a temperature range of 873–1273 K. The stoichiometries varied from fuel-rich to fuel-lean. The results showed higher temperature favored the reduction of recycled NO under fuel-rich conditions, and the recycled NO reduction obtained in CO2 was much lower than that obtained in N2. However, under stoichiometric and fuel-lean conditions, the recycled NO reduction decreased with increasing temperature, and a slightly higher recycled NO reduction was obtained in CO2 compared to that in N2. Higher recycled NO concentration accelerated its reduction. The recycled NO reduction decreased with increasing CO2 concentration under fuel-rich conditions, while it increased under fuel-lean conditions. Higher CO maximum concentration was obtained in the CO2 case due to the CO2 + H ⇔ CO + OH reaction. But a decrease in the CO concentration was observed when the CO2 concentration reached a certain level. This indicates there are other important reactions responsible for the effect of a high concentration of CO2 besides CO2 + H reaction.

Factors Affecting NO Reduction during O2/CO2 Combustion

O2/CO2 combustion is a promising technology to facilitate carbon capture in pulverized coal-fired power plants. The reduction of recycled NO, influence of CO2 concentration on the conversion ratio ...

Computational Fluid Dynamics Modeling of NOxReduction Mechanism in Oxy-Fuel Combustion†

Oxy-fuel combustion is a promising carbon capture technology in which both the conversion of fuel-N to NO and the reduction of recycled NO contribute to lowering of final NO exhausted from the coal combustion system. Combustion characteristics for both air and oxy-fuel conditions were numerically investigated in a pilot scale test facility for an Australian sub-bituminous coal. On the basis of De Soete’s mechanism, the additional reactions for formation of fuel-NO and reduction of recycled NO were added into computational fluid dynamics (CFD) codes, using user-defined functions (UDFs). The NOx predictions in air and oxy-fuel combustion were compared to experimental data. The NO emission in oxy-fuel condition is predicted to be significantly lower than that in air combustion, even without recycled NO. The effect of the nitrogen partitioning ratio between volatile and char on the NOx emission was also investigated.

Effects of coal properties on recycled-NO x reduction in coal combustion with O 2/recycled flue gas

The emission of NO x and the reduction of recycled-NO x in coal combustion with O 2/recycled flue gas were experimentally studied in an electrically heated up flow combustor under low recycling ratio with three kinds of coal: a high volatile bituminous, a medium volatile bituminous and a semi-anthracite. The recycled flue gas was simulated with the mixture of CO 2 and NO x . NO x emissions from the combustion of the three coals were firstly investigated under the recycling ratio ranging from 0 to 0.4 but without recycled-NO x . The results show that the relative release rate of fuel-nitrogen to volatile matter and the partitioning of volatile-N and char-N are more critical than volatile matter content and nitrogen content in coal for NO x emissions, especially in the fuel lean combustion. The experimental data on recycled-NO reduction indicate that the reduction efficiency increased with fuel equivalence ratio and recycling ratio. In the fuel rich region, the reduction efficiency reached as high as 80% at a fuel equivalence ratio of 1.4. The experiments on the replacement of NO recycle by NO 2 recycle indicated that the same reduction efficiencies of recycled-NO 2 as those of recycled-NO were reached for the high volatile bituminous, while the reduction efficiencies of recycled-NO 2 were always lower than those of recycled-NO for the semi-anthracite coal.

The reduction of recycled-NO x in coal combustion with O 2/recycled flue gas under low recycling ratio

The emission and reduction of NO x in coal combustion with O 2/recycled flue gas were investigated in an electric heated up-flow-tube combustor with a high volatile bituminous coal. The recycling ratio of flue gas ranged from 0 to 0.4, and the concentrations of recycled-NO in inlet gas were 0, 500 and 1000 ppm. Reduction efficiency of the recycled-NO x changed both with equivalence ratio and recycling ratio. More than 60% recycled-NO was reduced when equivalence ratio ϕ>1.4. Less recycled-NO was reduced under lower recycling ratio than that under higher recycling ratio. HCN and NH 3 were also detected with GC-FTD. The results indicated that HCN emissions decreased with the increase of recycled-NO concentration in inlet gas. No obvious influence of temperature on the reduction of recycled-NO was found. The experiments on recycled-NO 2 demonstrated a consistent trend similar to the NO recycle tests.

NO x reduction mechanism in coal combustion with recycled CO 2

Coal combustion with CO 2 O 2 is one of several promising new technologies associated with mitigating the CO 2 rise in the atmosphere. In coal combustion with recycled CO 2, the amount of NO x exhausted from the system is reduced to less than one third of that with combustion in air. This result is associated with decreased conversion of fuel-N to NO x and reduction of recycled NO x in the flame zone. The effects of CO 2 concentration, reduction of recycled NO x, and interaction between fuel-N and recycled NO x on the decrease of the final NO x exhausted from the coal-combustion system with recycled CO 2 have been separated for appropriate NO x-reduction mechanisms.