Elsevier

Combustion and Flame

Volume 158, Issue 7, July 2011, Pages 1255-1263
Combustion and Flame

NOx formation and reduction mechanisms in staged O2/CO2 combustion

https://doi.org/10.1016/j.combustflame.2010.11.006Get rights and content

Abstract

The purpose of this study was to investigate the NOx formation and reduction mechanisms in staged O2/CO2 combustion and in air combustion. A flat CH4 flame doped with NH3 for fuel-N was formed over the honeycomb, and NOx formation characteristics were investigated. In addition, chemiluminescence of OH* distribution was measured, and CHEMKIN-PRO was used to investigate the detailed NOx reduction mechanism. In general, the NOx conversion ratio decreases with decreasing primary O2/CH4 ratio, whereas NH3 and HCN, which are easily converted to NOx in the presence of O2, increases rapidly. Therefore, a suitable primary O2/CH4 ratio exists in the staged combustion. Our experiments showed the primary O2/CH4 ratio, which gave the minimum fixed nitrogen compounds in O2/CO2 combustion, was lower than in air combustion. The NOx conversion ratio in O2/CO2 combustion was lower than in air combustion by 40% in suitable staged combustion. This could be explained by high CO2 concentrations in the O2/CO2 combustion. It was shown that abundant OH radicals were formed in O2/CO2 combustion through the CO2 + H  CO + OH, experimentally and numerically. OH radicals produced H and O radicals through H2 + OH  H + H2O and O2 + H  OH + O, because a mass of hydrogen source exists in the CH4 flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH3 and HCN. NOx formed by the oxidation of NH3 and HCN was converted to N2 because the oxidation occurred in the fuel-rich region where the NOx reduction effect was high. In fact, the oxidation of NH3 and HCN in the fuel-rich region was preferable to remaining NH3 and HCN before secondary O2 injection in the staged combustion. A significant reduction in NOx emission could be achieved by staged combustion in O2/CO2 combustion.

Introduction

CO2 has attracted attention as one of the unfavorable greenhouse gases, which is the main contributor to global warming. The Convention on Biological Diversity has met annually since 1995 to assess progress in dealing with climate change; the Conference of the Parties (COP) is the governing body of the Convention. The Kyoto Protocol was adopted at the Third Conference of the Parties (COP3) to the United Nations Framework Convention on Climate Change held in Kyoto in 1997. Under the Protocol, Annex I countries agreed to reduce greenhouses by 5% from 1990 level in the period from 2008 to 2012. The protocol was later ratified and implemented in February 2005. Since then, the effect of greenhouse gases on global climate change has been acknowledged by many governments, and the reduction of emissions of these gases is drawing increasing attention.

Over the past decade, coal has been an important energy resource for meeting the demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. However, modern pulverized coal fired power plants are also some of the largest single point emitters of greenhouse gases, in particular CO2. As pulverized coal power plants have such a large impact on CO2 releases to the atmosphere, there is a compelling need to explore the best near-term strategy to improve the existing coal power plants to capture CO2. Recently, Carbon Capture and Storage (CCS) technologies have gained huge interest as a promising option, which has the potential to reduce CO2 emissions drastically.

Figure 1 shows the schematic of a pulverized coal-fired boiler with CCS. Because the existing coal power plants of coal–air combustion have low concentration of CO2 (13–15%) in the flue gas, CO2 needs to be separated from the flue gas. However, the capture of CO2 from such dilute mixtures using amine stripping is relatively expensive [1]. On the other hand, O2/CO2 combustion seeks to dramatically increase the CO2 concentration in the product flue gases. During oxy-fuel combustion, oxygen is separated from air and then mixed with a recycled stream of flue gas, leading to an increase in CO2 from about 13–15% to 95% by volume. Therefore, direct CO2 recovery becomes possible without additional energy consumption. Both the energy efficiency and economy are expected to be better than with air combustion although pure O2 must be produced for this process [2].

O2/CO2 combustion has some other advantages over the conventional coal–air combustion, such as lower NOx emission as well as easier CO2 removal. The review by Normann et al. [3] shows the emission control of NOx in O2/CO2 combustion. Experimental studies on NOx emission characteristics in O2/CO2 combustion have been conducted by many researchers [3], [4], [5], [6], [7], [8], [9]. Some studies have shown that O2/CO2 combustion has high potential for reducing NOx emission owing to its recycling process even when a equivalence ratio in a combustor is assumed to be uniform [4], [5]. It means that further NOx reduction is possible if O2/CO2 combustion is combined with a local condition control of equivalence ratio such as staged combustion. Some researchers have investigated staged O2/CO2 combustion characteristics [10], [11]. They showed that staged combustion was as effective only for NOx reduction in O2/CO2 combustion as in air combustion, however, results for staged O2/CO2 combustion are available on the semi-industrial scales. Therefore, NOx formation and reduction characteristics in staged O2/CO2 combustion have not been investigated in detail. It seems to be different from staged air combustion because the properties of the combustion process differ between air and O2/CO2 combustion. The high concentration of CO2 may affect the combustion characteristics through the chemical reaction of CO2. Some researchers have investigated the chemical effects of high CO2 concentration in gas phase to understand the detailed reaction mechanism [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. OH is an active radical which is formed during combustion, and it is involved in many combustion reactions. Earlier, hydroxyl radical chemiluminescence has been measured in various flames [22], [23], [24], [25]. However, there are few studies that measure OH* radical profiles in O2/CO2 combustion.

The purpose of this study was to investigate NOx formation and reduction mechanisms in staged O2/CO2 combustion and air combustion. A flat CH4 flame doped with NH3 for fuel-N was used, and measurements of not only NO, HCN and NH3, but also chemiluminescence of OH* radical, were performed. In addition, CHEMKIN-PRO was used to investigate a detailed NOx reduction mechanism.

Section snippets

Experiment

Figure 2 shows a schematic diagram of the experimental apparatus. A laminar, flat flame of CH4 was formed on a honeycomb structure whose each rectangular porous size is 2 mm. Although most studies of O2/CO2 combustion deal with solid fuels, typically coal [4], [5], [6], [7], [8], [10], [11], [12], [13], [14], [15], [16], a large part of the fuel conversion in the combustion process occurs in the gas phase. In this research, detailed NOx formation and reduction mechanisms were investigated by

Model simulation

NOx formation mechanisms in primary combustion were investigated at λprimary of 0.7 with detailed chemical reaction kinetics. The conservation equations for steady plug flow were solved using CHEMKIN-PRO [28]. The adopted reaction mechanism was a GRI-Mech 3.0 [29], which consisted of 53 species and 325 elementary reaction steps. For simplification, a plug flow model was used to express the primary combustion in this research. Supplementary material shows the comparison between plug flow model

Results and discussion

Figure 4 shows measured temperature profiles of primary combustion at λprimary = 0.7. Because flat flame is formed around x = 0 mm, gas temperature rapidly increases at this point. Gas temperature in air combustion is higher than that in O2/CO2 combustion because of the higher heat capacity of CO2.

Figure 5 shows measured temperature profiles of primary air combustion (λprimary = 0.7) and staged air combustion (λprimary = 0.7, λglobal = 1.2). Gas temperature of staged combustion increases around the

Conclusion

NOx formation and reduction mechanisms in staged O2/CO2 combustion and air combustion was investigated. Primary O2/CH4 ratio, which yielded the minimum fixed nitrogen compounds in O2/CO2 combustion, was lower than in air combustion. The lowest NOx conversion ratio in O2/CO2 combustion was lower than it in air combustion by 40%. This could be explained by a high CO2 concentration, which was one of the most important features of O2/CO2 combustion. It was shown that abundant OH radicals were

Acknowledgments

This study was partially supported by a JSPS Grant-in-Aid for Scientific Research (A) (No. 21246035), JST Strategic International Cooperative Program and J-Power.

References (31)

  • D. Singh et al.

    Energy Convers. Manage.

    (2003)
  • B.J.P. Buhre et al.

    Prog. Energy Combust. Sci.

    (2005)
  • F. Normann et al.

    Prog. Energy Combust. Sci.

    (2009)
  • K. Okazaki et al.

    Energy

    (1997)
  • H. Liu et al.

    Fuel

    (2003)
  • Y.Q. Hu et al.

    Fuel

    (2001)
  • H. Stadler et al.

    Proc. Combust. Inst.

    (2009)
  • H. Liu et al.

    Fuel

    (2005)
  • A. Molina et al.

    Proc. Combust. Inst.

    (2007)
  • T. Suda et al.

    Fuel

    (2007)
  • D. Toporov et al.

    Combust. Flame

    (2008)
  • S.P. Khare et al.

    Fuel

    (2008)
  • F. Liu et al.

    Combust. Flame

    (2001)
  • F. Liu et al.

    Combust. Flame

    (2003)
  • J. Giménez-López et al.

    Combust. Flame

    (2010)
  • Cited by (91)

    View all citing articles on Scopus
    View full text