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Title: Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NOx combustion modifications

Abstract

Many coal-fired boilers retrofitted with low NOX firing systems are experiencing significant operational difficulties due primarily to (1) increased carbon in the fly ash or (2) increased water wall wastage. This paper presents the results of a computational investigation of a wall-fired boiler that has recently been retrofitted with low NOx burners and overfire air. The focus of this paper is the effect of the retrofit on unburned carbon/NOx and the potential for applying operational and design modifications to minimize unburned carbon without adversely impacting NOx emissions. Although the magnitude of an increase in unburned carbon after a low NOx retrofit is system and coal dependent, it is often the case that reduction in emissions of nitrogen oxides is accompanied by a corresponding increase in the amount of unburned carbon in fly ash. Since low NOX firing systems increase the residence time of coal/char particles in a fuel rich environment, it might be expected that there is insufficient time under high temperature, oxidizing conditions to ensure complete carbon oxidation in a low NOx firing system. A relatively straight forward consideration of the effect of temperature and oxygen concentration on coal particle pyrolysis/oxidation can be used to provide a qualitative understandingmore » of this effect. However, the complex flow patterns and highly nonlinear physical and chemical phenomena in a boiler make it difficult to predict carbon-in-ash (c-i-a) levels without the use of advanced computational tools. The sensitivity of c-i-a to burnout is pointed out in a figure for a coal with 10% ash. Although c-i-a changes slowly for low burnout, it changes very rapidly at the extent of burnout typical of a boiler. When the extent of burnout drops only slightly, from 99.5 to 99 percent for example, the c-i-a doubles, from 4 to 8%, which in many situations would be unacceptable. The importance of fuel efficiency and ash disposal/recycle emphasizes the need for understanding and addressing this issue. The computational tools used by Reaction Engineering International (REI) have been developed to address the operational and design considerations of a wide range of combustion systems including utility boilers, pyrolysis furnaces, rotary kilns, waste incinerators, flash smelters, and smelting cyclones. The current models simulate reacting flows and particles, including gaseous diffusion flames, pulverized-coal flames, liquid sprays, coal slurries, injected sorbents, and other oxidation/reduction systems. In particular, emphasis has been placed on simulating coal combustion and pollutant formation.« less

Authors:
; ;  [1];  [2];  [3]
  1. Reaction Engineering International, Salt Lake City, UT (United States)
  2. Electric Power Research Inst., Palo Alto, CA (United States)
  3. Mississippi Power Co. Fuel Services, Gulfport, MS (United States)
Publication Date:
OSTI Identifier:
349140
Report Number(s):
CONF-980985-
ISBN 1-890977-15-2; TRN: IM9924%%140
Resource Type:
Conference
Resource Relation:
Conference: 15. annual international Pittsburgh coal conference, Pittsburgh, PA (United States), 14-18 Sep 1998; Other Information: PBD: 1998; Related Information: Is Part Of Fifteenth annual international Pittsburgh coal conference: Proceedings; PB: [1500] p.
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; BOILERS; ASHES; NITROGEN OXIDES; AIR POLLUTION ABATEMENT; BURNERS; MODIFICATIONS; CARBON; CONCENTRATION RATIO; MATHEMATICAL MODELS

Citation Formats

Davis, K A, Eddings, E G, Heap, M P, Facchiano, A, and Hardman, R R. Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NOx combustion modifications. United States: N. p., 1998. Web.
Davis, K A, Eddings, E G, Heap, M P, Facchiano, A, & Hardman, R R. Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NOx combustion modifications. United States.
Davis, K A, Eddings, E G, Heap, M P, Facchiano, A, and Hardman, R R. 1998. "Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NOx combustion modifications". United States.
@article{osti_349140,
title = {Computational investigation of carbon-in-ash levels for a wall-fired boiler after low-NOx combustion modifications},
author = {Davis, K A and Eddings, E G and Heap, M P and Facchiano, A and Hardman, R R},
abstractNote = {Many coal-fired boilers retrofitted with low NOX firing systems are experiencing significant operational difficulties due primarily to (1) increased carbon in the fly ash or (2) increased water wall wastage. This paper presents the results of a computational investigation of a wall-fired boiler that has recently been retrofitted with low NOx burners and overfire air. The focus of this paper is the effect of the retrofit on unburned carbon/NOx and the potential for applying operational and design modifications to minimize unburned carbon without adversely impacting NOx emissions. Although the magnitude of an increase in unburned carbon after a low NOx retrofit is system and coal dependent, it is often the case that reduction in emissions of nitrogen oxides is accompanied by a corresponding increase in the amount of unburned carbon in fly ash. Since low NOX firing systems increase the residence time of coal/char particles in a fuel rich environment, it might be expected that there is insufficient time under high temperature, oxidizing conditions to ensure complete carbon oxidation in a low NOx firing system. A relatively straight forward consideration of the effect of temperature and oxygen concentration on coal particle pyrolysis/oxidation can be used to provide a qualitative understanding of this effect. However, the complex flow patterns and highly nonlinear physical and chemical phenomena in a boiler make it difficult to predict carbon-in-ash (c-i-a) levels without the use of advanced computational tools. The sensitivity of c-i-a to burnout is pointed out in a figure for a coal with 10% ash. Although c-i-a changes slowly for low burnout, it changes very rapidly at the extent of burnout typical of a boiler. When the extent of burnout drops only slightly, from 99.5 to 99 percent for example, the c-i-a doubles, from 4 to 8%, which in many situations would be unacceptable. The importance of fuel efficiency and ash disposal/recycle emphasizes the need for understanding and addressing this issue. The computational tools used by Reaction Engineering International (REI) have been developed to address the operational and design considerations of a wide range of combustion systems including utility boilers, pyrolysis furnaces, rotary kilns, waste incinerators, flash smelters, and smelting cyclones. The current models simulate reacting flows and particles, including gaseous diffusion flames, pulverized-coal flames, liquid sprays, coal slurries, injected sorbents, and other oxidation/reduction systems. In particular, emphasis has been placed on simulating coal combustion and pollutant formation.},
doi = {},
url = {https://www.osti.gov/biblio/349140}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Dec 31 00:00:00 EST 1998},
month = {Thu Dec 31 00:00:00 EST 1998}
}

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