Elsevier

Fuel

Volume 80, Issue 13, October 2001, Pages 1851-1855
Fuel

The reduction of recycled-NOx in coal combustion with O2/recycled flue gas under low recycling ratio

https://doi.org/10.1016/S0016-2361(01)00048-5Get rights and content

Abstract

The emission and reduction of NOx in coal combustion with O2/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-NOx 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 NH3 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-NO2 demonstrated a consistent trend similar to the NO recycle tests.

Introduction

On the third Conference of the Parties to the Framework Convention on Climate Change held in Kyoto in December 1997, the so-called Annex-1 countries agreed to reduce greenhouse emissions by 5% in the period 2008–2012 compared with 1990 levels. As one of the promising new processes associated with reducing CO2 emissions, coal combustion with oxygen, which separates nitrogen from the combustion air in advance, has attracted considerable attention [1], [2], [3], [4], [5], [6], [7], [8]. Besides easier CO2 removal due to high concentrations of CO2 in the flue gas, this process also has some other advantages over the conventional process of coal combustion with air, such as high energy efficiency and better economy [1], [2], [3], lower emissions of NOx and SOx [2], [8], [9] and so on.

Flue gas recirculation was proposed for this new process in order to lower the flame temperature to within the withstanding temperature of the materials used in combustors, boilers and gas turbines. The behavior of NOx emissions in this process has been studied using O2/flue gas [4], [10], or O2/CO2 as a model gas [11]. Kimura and others [4] reported a reduction of NOx in the exhaust to less than about one-third of that exhausted with conventional air combustion. Okazaki and Ando [11] examined separately the effects of CO2 concentration, reduction of recycled-NO in the flame zone, and interaction between fuel-N and recycled-NOx on the decrease of the final NOx exhausted from the coal-combustion system with recycled CO2. They concluded that the conversion ratio from fuel-N to exhausted NOx is reduced to less than one-fourth of that with air combustion, and that the effect of a reduction of recycled-NO in the furnace was dominant and amounted to 50–80%. In their experiments, they fixed the oxygen concentration at about 21%; the recycling ratio was as high as 80%. There are few reports on the case of low recycling ratio. As Garay [2] pointed out, however, the fuel reduction (or combustion efficiency) could increase with the flue gas temperature and oxygen concentration. Since CO2 possesses a higher specific heat than N2, to reach the same adiabatic flame temperature as in air, oxygen concentration should be as high as 42% [12]. Nozaki et al. [10] found that particulates were formed in the secondary stream when oxygen concentration was less than 40%, and therefore injected 15–20% of total oxygen into the furnace directly. Consequently, 21% oxygen in the feed gas was not the only selection for the process. The main objective of this paper is to investigate the behavior and mechanism of recycled-NOx reduction in pulverized-coal combustion with O2/CO2 under higher oxygen concentration or lower recycling ratio of flue gas.

Section snippets

Experimental

The experiments were performed in an entrained-flow reactor described in detail previously [8]. The combustion chamber consisted of a cylindrical alumina tube with an inner diameter of 28 mm, which was heated by an element of SiC. The heated part was 300 mm long. Four Pt/Pt–13%Rh thermocouples were placed at different positions along the axis of the tube. O2/recycling flue gas was simulated with the mixture of O2, CO2 and NOx, which were separately supplied from gas cylinders and regulated by

The effect of recycling ratio and fuel equivalence ratio

The experiments were performed under different fuel equivalence ratio (ϕ) and flue gas recycling ratio at 1373 K. The fuel equivalence ratio was changed from 0.3 to 1.4. The flue gas recycling ratios were 0, 0.2 and 0.4. The concentrations of recycled-NO in inlet gas (O2+CO2) were 0, 500 and 1000 ppm. In all the cases, the emitted NOx (in ppm) in flue gases increased initially with ϕ in the fuel-lean region, and then declined dramatically as ϕ approached and exceeded the stoichiometric point. The

Conclusion

The results of the investigation showed that the reduction efficiency of recycled-NO changed both with equivalence ratio and flue gas recycling ratio. Less than 10% recycled-NO was reduced when ϕ<0.5; the reduction efficiency was higher than 60% when ϕ>1.4. The reduction efficiency increased with recycling ratio of flue gas. Few changes in NH3 concentration with recycled-NO concentration in inlet gas and recycling ratio was observed. The emission of HCN decreased with the increase of

Acknowledgements

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

References (26)

  • S Nakayama et al.

    Energy Convers Mgmt

    (1992)
  • N Kimura et al.

    Energy Convers Mgmt

    (1995)
  • J Andries et al.

    Energy Convers Mgmt

    (1996)
  • Y Shao et al.

    Energy Convers Mgmt

    (1996)
  • Y Hu et al.

    Fuel

    (2000)
  • H Liu et al.

    Fuel

    (2000)
  • T Nozaki et al.

    Energy

    (1997)
  • K Okazaki et al.

    Energy

    (1997)
  • H Spliethoff et al.

    Fuel

    (1996)
  • P.N Garay

    Proceeding of Second American Conference on Electric Power

    (1994)
  • S Takano et al.

    IHI Engng Rev

    (1995)
  • T Kiga et al.

    Energy Convers Mgmt

    (1997)
  • K Shirakawa et al.

    Proceeding of Conference on the Utilizing Technologies of Coal (Japan)

    (1992)
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