Effects of coal properties on recycled-NOx reduction in coal combustion with O2/recycled flue gas

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Abstract

The emission of NOx and the reduction of recycled-NOx in coal combustion with O2/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 CO2 and NOx. NOx emissions from the combustion of the three coals were firstly investigated under the recycling ratio ranging from 0 to 0.4 but without recycled-NOx. 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 NOx 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 NO2 recycle indicated that the same reduction efficiencies of recycled-NO2 as those of recycled-NO were reached for the high volatile bituminous, while the reduction efficiencies of recycled-NO2 were always lower than those of recycled-NO for the semi-anthracite coal.

Introduction

Coal combustion with oxygen, separating nitrogen from air in advance, has been proposed as one of the new power plant processes associated with reducing CO2 emissions and has attracted much attention [1]. The CO2 concentration in the flue gas may be as high as 95%, and direct CO2 recovery, therefore, becomes possible without a separation process [2], [3]. Flue gas recirculation was proposed for this process to lower the flame temperature to within an acceptable temperature of the materials used in the combustors. The efficiency and economy of this process have been established by Nakayama et al. [2] and Okawa et al. [4] and are expected to be better than those of conventional air combustion with the amine adsorption method for CO2 recovery from the exhaust gas.

A large decrease of NOx emission in O2/flue gas coal combustion, compared to air combustion processes, has been reported by several previous researchers under high recycling ratios up to 80% (or O2 concentration about 20% in the inlet gas). Okazaki and Ando [3] examined the factors affecting the emissions of NOx in an O2/CO2 combustion system at a maximum flame temperature of 1450 K and an initial O2 concentration of 21 vol% and concluded that the conversion ratio from fuel-N to exhaust NOx was reduced to less than one-fourth of that with air combustion. Kimura et al. [5] did experiments on coal combustion with O2/recycled flue gas on a bench scale test facility with a coal feed rate of 100 kg/h. The experimental results indicated that the NOx conversion ratio from fuel-N in O2/recycled flue gas combustion was less than 10%, much lower than the 30% in air blown combustion. The concentration of O2 in the inlet gas was up to 30%. Takano et al. [6] conducted tests by adding NO to the recycled flue gas with a high volatile bituminous coal and two medium volatile bituminous coals. The oxygen concentration in the inlet gas was controlled at 30%. The results indicated that the reduction efficiency increased with the decrease in oxygen ratio. Fifty percent and more than 90% recycled NO were reduced at the oxygen ratio of 1.2 and 0.8, respectively, and NO in the recycled flue gas can, thus, be almost ignored. They attributed the reduction of NO in the recycled flue gas to the hydrocarbon produced in the volatilizing process of the pulverized coal combustion. Nozaki et al. [7] conducted experiments on O2/CO2 combustion with low volatile and medium volatile bituminous coals and concluded that the recycled NO was rapidly reduced in the combustion zone. The NOx emission for O2/CO2 combustion was lower than that for air combustion although the recycled flue gas contained relatively large amounts of NOx.

Since the specific heats of the main components of flue gas, such as CO2 and H2O, are higher than that of nitrogen, higher O2 concentration in the inlet gas is needed to obtain comparable flame temperature [8] and combustion efficiency [9]. Accordingly, in our previous paper [10], we examined the behavior of NOx emission in O2/CO2 combustion with a high volatile matter bituminous coal under low recycling ratios of flue gas (or high O2 concentrations). The results showed that the reduction efficiency of the recycled-NOx changed, both with equivalence ratio and recycling ratio, under recycling ratios ranging from 0 to 0.4. A high reduction efficiency was obtained in the fuel rich conditions. Less recycled-NO was reduced under lower recycling ratio than that under higher recycling ratio. The objective of the present study is to examine the influence of coal properties on NOx emission and the reduction of recycled-NOx in this process.

Section snippets

Experimental

The experimental reactor and the analysis of the exhaust gas composition have been described in detail in our previous papers [1]. In brief, an entrained flow reactor with an inner diameter of 28 mm was employed, the heated part of which was 300 mm long. O2/recycling flue gas was simulated by a mixture of O2, CO2 and NOx, which were separately supplied from gas cylinders and regulated by mass flow controllers. Low concentration (2%) of NO or NO2 in the CO2 base was used for safety as the source

Results and discussion

Besides the rank of coal, the main factors affecting the reduction of recycled-NOx are the fuel equivalence ratio (φ) and the recycling ratio of flue gas (the ratio of flue gas in inlet gas) in the combustion of coal with O2/recycled flue gas. Accordingly, all the experiments in this study were done under different fuel equivalence ratios and flue gas recycling ratios. The fuel equivalence ratio was changed from 0.3 to 1.4, and the recycling ratios were 0, 0.2 and 0.4. The experimental

Conclusion

Under low recycling ratio or high oxygen concentration, the process of coal combustion with O2/recycled flue gas has the following characteristics on NOx emissions and recycled-NOx reduction:

  • (1)

    The NOx emission index decreases with the increase in fuel equivalence ratio but increases with increasing recycling ratio of flue gas.

  • (2)

    Instead of the volatile matter content and nitrogen content in coal as determined by the standard proximate and ultimate analysis tests, the relative release rate of

Acknowledgements

The authors greatly acknowledge the financial support for this research provided by the Japanese Science Promotion Society (JSPS).

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