Low-NOx and high-burnout combustion characteristics of a cascade

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Energy and the Environment

Low-NOx and high-burnout combustion characteristics of a cascade-arch-firing, W-shaped flame furnace: Numerical simulation on the effect of furnace arch configuration Haiqian Wu, Min Kuang, Jialin Wang, Xuehui Hu, Sili Wu, and Chuyang Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03872 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Environmental Science & Technology

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Low-NOx and high-burnout combustion characteristics of a cascade-arch-firing, W-shaped flame

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furnace: Numerical simulation on the effect of furnace arch configuration

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Haiqian Wu, Min Kuang, Jialin Wang, Xuehui Hu, Sili Wu, Chuyang Chen

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Faculty of Maritime and Transportation, Ningbo University, Ningbo 315211, P.R. China

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ABSTRACT: A cascade-arch-firing low-NOx and high-burnout configuration (CLHC) was proposed as a

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solution for the W-shaped flame furnace's incompatible problem of strengthened low-NOx combustion

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and high burnout. Numerical simulations verified by industrial-size measurements of a 600 MWe

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W-shaped flame furnace, were used to confirm the CLHC's low-NOx and high-burnout characteristics and

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again, evaluate its cascade-arch configuration's effect on the gas/particle flow, coal combustion, and NOx

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formation. The furnace with the existing low-NOx combustion art showed NOx emissions of about 900

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mg/m3 at 6% O2 and carbon in fly ash of about 5%. In applying CLHC as a replacement for the prior art,

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numerical simulations at typical cascade-arch configurations of CL = 1/5, 1/4, and 1/3 (CL signifying the

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ratio of the lower arch depth to the total arch depth) showed that as CL increased, both the flow field and

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combustion symmetry initially improved but then deteriorated. In conjunction with improvement in both NOx

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emissions and burnout, the CL = 1/4 setting achieved the best furnace performance with NOx emissions 707

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mg/m3 at 6% O2 and carbon in fly ash of 5.5%. In comparison with the prior low-NOx art, CLHC reduced further

17

NOx

emissions

by

22%

and

almost

maintains

Corresponding

author: Tel.: +86 574 87609538; Fax: +86 574 87605311. Email address: [email protected] (M. Kuang) 1

ACS Paragon Plus Environment

the

burnout

rate.


18

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1. Introduction

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NOx are extremely toxic and involve in the formation of acid rain and photochemical smog. NOx emissions

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from power plant's coal combustion are relatively concentrated and huge, resulting in the NOx reduction in this

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field having attracted widespread attention.1,2 Anthracite and lean coal reserves are abundant in the world, but

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unfortunately, their low volatile content and poor reactivity require high ignition temperature, stable combustion

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conditions, and long burnout time. W-shaped flame furnaces (also called down-fired furnaces), which are

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designed especially for burning these fuels, have been widely used in China in the past 30 years. However,

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real-furnace operations showed that there existed various problems such as unstable combustion, asymmetrical

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combustion, poor burnout (carbon in fly ash of 8–15%), severe slagging, and particularly high NOx emissions

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(typically in the range of 1000–2000mg/m3 at 6% O2).3–6 Accordingly, various solutions were developed for

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improving furnace performance, such as burning blended fuels to improve combustion stability,7,8 positioning

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fuel-lean coal/airflow nozzles far from the furnace central part9 or shutting down near-wall burners10 to weaken

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slagging, inclining downward wall-air jets to improve burnout,11,12 and regulating asymmetric air distribution,

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partially dealing secondary air ports, or inclining downward staged air to achieve symmetrical combustion.13–15

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On the strictest pollutant emission standards for coal-fired power stations coming into force in China at July

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1, 2014, the allowed NOx levels are 200 mg/m3 at 6% O2 for W-shaped flame furnaces. In conjunction with the

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increasingly flourishing ultra-low emission requirements (NOx emissions below 50 mg/m3 at 6% O2), the above

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high-NOx emission problem is particularly prominent in China, which owns the largest W-shaped flame furnace

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market in the world. A conventional, feasible solution of the high-NOx problem is to adopt the combination of

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in-furnace low-NOx combustion and flue gas denitrification. In the low-NOx combustion aspect, the effect of

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deep-air-staging low-NOx combustion is remarkable and widely reported. Both Cañadas et al.16 and Fueyo et al.17

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found that greatly adjusting the air distribution between the arch air to the wall air could reduce NOx emissions by

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20–35% but significantly affected burnout. Accordingly to the Foster Wheeler (FW) Company,18 a low-NOx 2


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combustion technology consisting of applying fuel-preheat nozzles and feeding vent air through overfire air (OFA)

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ports could reduced NOx emissions by as much as 50%, but unfortunately, a pulverizing system improvement for

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advancing the pulverized-coal fineness was necessary to compensate the essentially resulted burnout loss. Leisse

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et al.19 applied a low-NOx combustion solution of graded swirl burners and OFA on Babcock & Wilcox (B&W)

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W-shaped flame furnaces to reduce NOx emissions by 40% (achieving levels of 857–1060mg/m3 at 6% O2), plus

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a worsen burnout performance with OFA in service. Yang et al.20 trialed a proposed hot-air packing technology on

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a bench-scale W-shaped flame furnace for a low-NOx and high-burnout performance. Through the application of

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a combined high-efficient and low-NOx technology in both FW and B&W W-shaped flame furnaces,21–23 Li's

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group gained NOx emissions of 697–1057 mg/m3 at 6% O2 and carbon in fly ash of 7.54–11.4%. By positioning

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fuel-lean nozzles in the near-wall side, redirecting staged air, and introducing the separated OFA to regulate a

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novel combustion system for a 600 MWe FW W-shaped flame furnace, Ma et al.24,25 found that NOx emissions

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reduced by 42–50% to achieve levels of about 760mg/m3 at 6% O2, accompanied by a certain increase in the

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burnout loss. Based on a multiple-injection and multiple-staging combustion technology (MIMSCT) applied in a

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600 MWe supercritical boiler furnace, Kuang et al. optimized an accepted furnace performance with NOx

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emissions of 867mg/m3 at 6% O2 and carbon in fly ash of 5.4%.26–28 Although strengthening further air staging by

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opening OFA fell again NOx emissions to 503–696 mg/m3 at 6% O2, carbon in fly ash surged to 13–15%.29

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The above published work shows that those prior low-NOx combustion arts based on deep air staging

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have already achieved a significant progress in NOx reduction for W-shaped flame furnaces, However,

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still high NOx emissions of about 800 mg/m3 at 6% O2 and the resulted negative effect on burnout (i.e.,

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carbon in fly ash maintaining at 5–8%) suggest that the comprehensive furnace performance is still

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unsatisfactory. Moreover, the costly flue gas denitrification, which is destined to meet the current

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emission regulation and the subsequent ultra-low emission requirements, necessitates the upstream

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low-NOx combustion to further reduce in-furnace NOx emissions as much as possible and meanwhile 3



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maintain high burnout for lowering the denitration cost. On the above incompatible problem of

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strengthened low-NOx combustion and high burnout and the need of lower NOx emissions for controlling

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denitration cost, a cascade-arch-firing low-NOx and high-burnout configuration (CLHC) is proposed in

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this work as a solution for W-shaped flame furnaces. It should be noted that CLHC is born out of the

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application experience gained from several stages of the prior MIMSCT. This CLHC layout develops the

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conventional, single-stage double furnace arches of W-shaped flame furnaces into a newly four-arch,

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cascade-arch-firing pattern with upper and lower furnace arches. A comprehensive ultra-low NOx

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combustion system is regulated in CLHC, which is composed of bias combustion, deep air staging, flue gas

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recirculation, and fine pulverized-coal reburning. Aiming at (i) revealing the CLHC’s high-burnout and

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low-NOx combustion characteristics, (ii) confirming its availability in resolving the low-NOx combustion

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and high-burnout compatibility problem, and (iii) evaluating the furnace-arch configuration effect on the

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gas/particle flow, coal combustion, and NOx formation with CLHC, industrial-size measurements and

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numerical simulations were initially performed for the above 600 MWe MIMSCT furnace at normal

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full-load operation. Thereafter, in applying CLHC in the furnace as a replacement of MIMSCT, the

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verified numerical simulation methodology was used to calculate the gas/particle flow, coal combustion, and

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NOx formation by varying the upper furnace depth. In this form, a preliminary, acceptable cascade-arch

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configuration setup was established based the best furnace performance in symmetrical combustion, low NOx

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emissions, and high burnout. It should be noted that a series of CLHC’s optimizations referring to the

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furnace configuration, combustion system layout, and air/fuel distribution, which are a systematic task

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necessitating lots of efforts in future studies, deviate from the major objective in this work where CLHC

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is only at its proposed stage. Therefore, the attention is not focused on pursuing an elaborate cascade-arch

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configuration setup that can be used in a real-furnace environment, with only several typical cascade-arch

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configuration setups established for numerical simulations. However, the information gained from this work is 4


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still useful to deepen the understanding about how to establish more low-NOx and high-burnout combustion in

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W-shaped flame furnaces compared with the prior arts.

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2. Methodology

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2.1 600 MWe MIMSCT W-shaped flame furnace and its improved version with CLHC

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As mentioned previously, aiming at the compatibility problem of strengthened low-NOx combustion and

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high burnout, CLHC is developed based on our group’s many years of work on W-shaped flame furnaces and

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the application experience gained from several stages of the prior MIMSCT. In order to confirm the CLHC’s

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high-burnout and low-NOx combustion advantages compared with the prior arts, the 600 MWe supercritical

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MIMSCT W-shaped flame boiler furnace involved in the previous work27,29,30 was selected to perform both

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industrial-size measurements and numerical simulations at normal full-load operation. The subsequent efforts

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were devoted to numerical simulations of the gas/particle flow, coal combustion, and NOx formation of the

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furnace with CLHC. Supporting Information provides schematics of the furnace configuration and combustion

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system of the 600 MWe MIMSCT furnace. The conventional two furnace arches in a one-stage configuration

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divide the furnace into two zones: the octagonal lower furnace (the fuel-burning zone) and rectangular upper

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furnace (the fuel-burnout zone). The fuel rich/lean combustion concept is regulated by using louver

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concentrators to divide the primary air/fuel mixture into fuel-rich and fuel-lean coal/air flows. Burner groups are

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symmetrically positioned on the front and rear arches, with each group consisting of fuel-rich coal/air flow

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nozzles, fuel-lean coal/air flow nozzles, inner secondary-air ports, and outer secondary-air ports. The direct-flow

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inner/outer secondary-air jets are used to regulate the first and second stages of airflow injection and air-staging

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conditions in the preceding stage combustion zone. Corresponding to burner groups, staged-air slots (in a group

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pattern) and OFA ports are symmetrically located at the lower parts of lower and upper furnaces, respectively.

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The horizontal staged air and inclined OFA (fed into the primary combustion and burnout zones, respectively),

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are used to regulate the third and fourth stages of airflow injection and deepen the air-staging conditions. This 5



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deep-air-staging combustion layout is based on the so-called multiple-injection and multiple-staging combustion

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technology (MIMSCT), whose detailed description has already appeared in the literature.27,30 Table 1 lists the

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furnace's conventional operation parameters mainly in the airflow and fuel supplying at full load.

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Referring to the MIMSCT concept and maintaining the main furnace dimensions, the 600 MWe furnace

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equipped with CLHC as a replacement of MIMSCT is illustrated in Figure 1. Aside from the conventional

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lower and upper furnace zones having the same roles as those in the prior MIMSCT art, the CLHC’s lower

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furnace has our furnace arches positioned in an upper/lower two-stage pattern, which is used to from a

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cascade-arch-firing, W-shaped flame configuration. With the upper furnace arch dominated in the

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furnace-arch roles and the lower one as a supplement, CLHC is an ensemble incorporated with bias

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combustion, deep air staging in a strengthened multiple-injection and multiple-staging version, flue gas

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recirculation, and fine pulverized-coal reburning. Its larger upper furnace arch is used to position primary

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burners and OFA ports arranged in groups, with the latter obliquely arranged in the side close to the furnace

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throat. Each primary burner group consists of the extraverted fuel-rich coal/air flow nozzles and vertical

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inner/outer secondary-air ports, with the outer secondary-air ports supplying the major secondary air share for

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the primary burners. Compared with the prior MIMSCT, CLHC uses a higher fuel-rich coal/air flow rate and

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airflow velocity to maintain the downward coal/air flow penetration because of a reduction in the inner

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secondary air, thereby lowering the coal concentration (Table 1). The smaller lower arch is used to arrange the

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intilted fuel-lean coal/gas flow nozzles, which are called auxiliary burners and correspond to the fuel-rich coal/air

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flow nozzles on the upper arch. The fuel-lean coal/gas flow is formed via the following procedure: (i) Fine

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pulverized-coal as the fuel. With CLHC, the pulverized-coal particles in the original MIMSCT’s fuel-lean

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coal/air flow are separated as the fuel (i.e., fine pulverized-coal having a median particle size of about 15 μm).

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The discharged airflow is then mixed with a small part of the total secondary to form the horizontal hopper air

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fed at the upper part of the hopper. This explains why the hopper air’s temperature is lower than those of 6


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secondary air, staged air, and OFA (Table 1); and (ii) Low-temperature gas flow as the conveying medium. The

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low-temperature exhaust gas (about 150 °C) initially sucked from the electrostatic precipitator outlet and then

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pressurized by a recirculating fan is taken as the conveying medium to carry the high-speed fine pulverized-coal

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injection into the lower part of the lower furnace. In light of (i) the absence of real-furnace exhaust gas data

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(because of no in-service CLHC W-shaped flame furnace up to now) and (ii) CLHC born out of MIMSCT, the

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corresponding exhaust gas data gained through the mentioned industrial-size measurements of the MIMSCT

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furnace are thus assigned to the fuel-lean coal/gas flow, whose detailed parameters are shown in Table 1.

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Obviously, the fuel-lean coal/air flow utilizes the reductive exhaust gas to feed the fine pulverized-coal (with

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high specific surface area and strong reducibility) directly into the high-temperature coal/air flame in the lower

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part of the lower furnace, acting as dual low-NOx combustion roles of flue gas recirculation and char reburning.

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In comparison to the conventional wall- or arch-arrangement pattern mainly for fuel combustion (such as the

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B&W22,23 and MIMSCT furnaces), here the fuel-lean coal/gas flow, acting as dual low-NOx combustion roles of

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flue gas recirculation and char reburning, is technically located on the newly added lower furnace arch,

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thereby expanding the reaction space for flue gas recirculation and char reburning. This form can achieve a

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low-NOx combustion performance as much as possible and meanwhile increase the flame fullness. In the lower

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part of the front and rear walls between the upper and lower furnace arches, sharply-inclined staged air with its

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air share lower than that of the MIMSCT furnace is supplied for organizing air staging and adjusting the flame

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travel. The combination of three aspects of (i) the fine pulverized-coal directly fed into the lower furnace’s

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high-temperature coal/air flame for improving burnout and significantly lowering NOx, (ii) the downstream

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hopper air used to strengthen air staging, improve burnout, and avoid the downward flame washing over the

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hopper wall and thermal fatigue problem,31 and (iii) the inclined OFA in the furnace throat for strengthening

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further the air staging conditions and completing burnout in the upper furnace, are helpful to enable CLHC for

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an improved low-NOx and high burnout combustion performance compared with the prior MIMSCT art. More 7



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details about the CLHC’s combustion partition and performance improvements in low-NOx and high-burnout

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compared with MIMSCT can be found in Supporting Information. Table 1 lists the full-load fuel and air supply

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parameters of the furnace with CLHC.

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2.2 Industrial-size measurements

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In order to acquire real-furnace data used for numerical simulation verification and meanwhile reveal

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the combustion performance and NOx emissions with the prior low-NOx art, real-furnace measurements

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were performed in the 600 MWe MIMSCT furnace at normal full-load operation. With major operational

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parameters and coal properties listed in Table 1, local gas temperatures and species concentrations, carbon in fly

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ash, and NOx emissions were acquired for the furnace. Details in these measurements are listed as follows. A 3i

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hand-held pyrometer (a type of noncontact infrared thermometer made by Raytek, Santa Cruz, CA), with

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a measurement range from 600 to 3000 °C, accurate to within 1 °C and with an error of ±30 °C, was used

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to measure the furnace's general gas temperature distribution through those symmetrical observation ports in

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the front and rear sides (Supporting Information giving out the measuring port layout). At each port, ten times

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of measurements were taken successively within 5 min and the average value was adopted. This

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temperature-acquiring pattern at each port was usually used to obtain the highest gas temperature along the

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measuring direction in the zone from the wall to the peripheral burners. In the near-wall region surrounding

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several ports along the flame travel in the front-half furnace, a thermocouple device (with a

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0.3-mm-diameter and 4-m-length platinum/platinum fine wire thermocouple located in a 4-mm-diameter

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twin-bore stainless steel sheath) was used to acquire local gas temperature distribution patterns. Meanwhile, a

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3-m-long water-cooled stainless steel probe (for sucking high-temperature gas samples and then cooling them

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quickly) plus a Testo 350M gas analyzer (with measurement errors of 1% for O2, 5% for CO, and 50 ppm for

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NOx) was used to obtain local gas species concentrations. Carbon in fly ash and NOx emissions were acquired by

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sampling fly ash and flue gas at the air preheater exit. More detailed introductory material about the 8


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industrial-size measurement methods, measurement errors and uncertainties analysis, and configuration

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parameters of the water-cooled stainless steel probe, can be found in the literature.9,21,26,27,32

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2.3 Numerical simulations

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Numerical simulations of gas/particle flow, coal combustion, and NOx formation were carried out for both

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the MIMSCT and CLHC W-shaped flame furnaces at normal full-load operations. As mentioned previously, the

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former simulations (parameters listed in Table 1) are mainly used to compare with the real-furnace results for the

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numerical simulation validity. In contrast, the latter simulations are to (i) reveal the CLHC’s coal combustion

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and NOx formation characteristics, (ii) confirm CLHC’s availability in resolving the incompatible problem of

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strengthened low-NOx combustion and high burnout, and (iii) evaluate the cascade-arch configuration’s effect on

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the overall furnace performance. As shown in Figure 1, with the total furnace arch depth L0 kept constant,

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adjusting the upper arch depth forms three cascade-arch configuration settings differentiated by a dimensionless

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coefficient CL, where CL = L1/L0 and L1 is the lower arch depth. Accordingly, numerical simulations

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corresponding to CLHC were performed at three typical settings of CL = 1/5, 1/4, and 1/3, respectively, with the

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same coal as that of the MIMSCT furnace and the calculated parameters listed in Table 1.

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Fluent 15.0 was used to perform the above numerical simulations. Details about the numerical

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simulations are as follows. The gas turbulence was calculated by using the realizable k–ε model.33 The

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Lagrangian stochastic tracking model was applied to the gas/particle two-phase flow simulation. The

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radiation heat transfer was taken into account by the P1 model,34 which included particle radiation

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interaction, with the particle emissivity and scattering factor set at 1.0 and 0.9, respectively. Volatile

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combustion was simulated by the probability density function theory (PDF) in the non-premixed

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combustion model. Char combustion was simulated by the kinetic/diffusion-limited model, and the

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devolatilization mechanism was modeled by using the two-competing-rates model.35 In the aspect

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referring to the tested kinetic parameters of pyrolysis and combustion of the calculated used (Table 1), the 9



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pairs of activation energy and pre-exponential factor for Devolatilizations I and II in the pyrolysis process

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were 85.49 kJ/mol + 2.764 × 105 and 111.27 kJ/ mol + 2.741 × 106, respectively, while that of char

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combustion was 79 kJ/mol + 2 × 10-3. Only NO production was taken into account in the NOx calculation,

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and the prompt-NO formation was ignored. The NO simulation was calculated by a post-processing

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program, where the thermal-NO calculation was based on the extended Zeldovich mechanism (N2 + O →

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NO + N, N + O2 → NO + O, N + OH → NO + H).36 The fuel-NO formation was simulated by using the De

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Soete’s model,37 where the char N directly converted to NO, and the volatile N converted to intermediates, such

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as CHN and NH3 (CHN/NH3 = 9:1).38,39 In addition, NO reduced by char was modeled by the Levy's method.40

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The coupling of pressure and velocity fields was solved by the SIMPLE algorithm of pressure correction.

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By using the first order finite difference method, the gas-phase conservation equations were solved with

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successive under-relaxation iterations until the solution satisfied a pre-specified tolerance. Because the

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above simulation methods are conventional and most of which can be found in the literature,12,25,36,39,40 more

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details are not repeated here. Details on boundary conditions, pulverized-coal size distribution, and other

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important settings in the present simulations are the same as those in the published work.41

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In light of (i) the W-shaped flame furnace's symmetrical configuration along the furnace centerline

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in the furnace breadth direction and (ii) the relatively independent combustion conditions on the

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longitudinal cross-section of each burner group, a half of the furnace along the furnace breadth was taken

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as the computation domain for saving the calculation time and maintaining the calculation accuracy as

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much as possible. The structured grid was used in most of the furnace area, and the grid was refined in

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important zones. The unstructured grid was used only in the hopper. Based on grid-dependent tests used to

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perfect the numerical simulations regarding with the mesh grid preparation, the number of grid cells for

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MIMSCT and CLHC W-shaped flame furnaces were set to 2,090,000 and 2,550,000, respectively.

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Supporting Information shows the representative grid-dependent test results of the CLHC furnace at its CL = 1/5 10


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setting, where the longitudinal velocity component along a cross-section H = 25 m is compared among three grid

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cell setups of 3,625,000, 2,025,000, and 2,550,000. Their particularly similar velocity distributions and

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computational time saving thus fix the mesh consisting of 2,550,000 grid cells for the CLHC furnace's

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simulations. The computation domain and grid division of both the MIMSCT and CLHC furnaces are given

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out in Supporting Information. Considering that the attention for a W-shaped flame furnace is usually focused

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on the change along its longitudinal section, typical calculated results were extracted along a vertical

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cross-section through a burner's centerline in the middle part of the model's furnace breadth. Again, aiming at

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disclosing the combustion performance and NOx formation with varying CL, only those simulated results closely

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related to coal combustion and boiler performance were provided, such as the in-furnace flow field, gas

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temperature field, gas species (O2, CO, and NO) concentration field, and important parameters at the furnace

235

outlet (i.e., O2 content, CO and NOx emissions, and carbon in fly ash).

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3. Results and discussion

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3.1 Real-furnace results of the MIMSCT furnace and validation of numerical simulation

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The numerical simulation validity is verified by a detailed comparison of the MIMSCT furnace's measured

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and calculated data referring to the overall in-furnace gas temperature pattern, gas temperature and gas species

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distributions in the near-wall region, carbon in fly ash, and NOx emissions, as shown in Figure 2 and Table 1.

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According to Figure 2a, both the experimental and calculated in-furnace gas temperature patterns show the same

242

temperature distribution characteristics. That is, gas temperatures are symmetrical in the front- and rear-half sides

243

and meanwhile, increase initially but then decrease along the flame travel (penetrating in turn the furnace zones

244

corresponding to the typical ports 1–4). The reported symmetrical W-shaped flow-field formation with

245

MIMSCT42 explains here the symmetrical combustion occurrence. The mentioned increase-to-decrease

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temperature trend along the flame travel is attributed to the fact that the furnace zones located at typical ports 1–4

247

(Figures 1 and 2) correspond in turn to the burner-outlet zone, preceding stage combustion zone, primary 11



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combustion zone with staged air aiding combustion, and upper part of the hopper zone where the flame begins to

249

turn upward. In addition, the calculated temperatures are well consistent with the real-furnace data in the front-

250

and rear-half sides, with temperature gap levels ranging in 20–50 °C and the calculation discrepancy less than

251

3% at most of ports. In panels b and c of Figure 2, the calculated and measured gas temperatures and species

252

concentrations in the near-wall region also show the same distribution characteristics and change trends in two

253

aspects: (i) Port 1 is close to the burner outlets, resulting in its relatively lower gas temperature levels. Affected

254

by the low-temperature coal/air flow fed by the near-wall burner, its gas temperature initially increases and then

255

decreases with the distance from the wall. Ports 3 and 4 correspond to the primary combustion zone and its

256

downstream upper part of the hopper, respectively. With the combustion intensity weakening along the flame

257

travel from port 3 to port 4, the temperature appears its peak value at port 3 and then decrease to some extent at

258

port 4. Meanwhile, although gas temperature in both two ports increases with the distance, the increase trend is

259

more obvious at port 3; and (ii) In port 2 corresponding to the preceding stage combustion zone, the essentially

260

lower O2 concentration increases slightly and CO concentration

261

wall, while the NOx concentration initially increases and then decreases. These changes in the combustion zone

262

are mainly attributed to the MIMSCT's technical advantages of good staging combustion in the burner zone and

263

the postponed secondary air mixing with the ignited coal/air flow for effectively inhibiting the fuel-NOx

264

formation, plus the latter release of combustion products. Panels b and c of Figure 2 also shows that the

265

calculated gas temperatures and species concentrations in the near-wall region well agree with the measured data

266

in both levels and change trends, except for the relatively larger O2-concentration deviation at port 2. Their

267

calculation deviations generally lower at levels below 3%.

increases apparently with the distance to the

268

As listed in Table 1, the measured optimal furnace performance at full-load low-NOx operation is

269

characterized as NOx emissions of 867 mg/m3 at 6% O2 and carbon in fly ash of 5.4%. According to the

270

published work,29 although strengthening the deep-air-staging conditions by opening OFA continuously 12


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decreased NOx emissions to 500–700 mg/m3 at 6% O2, carbon in fly ash surged to 13–15%. The

272

observations in the MIMSCT furnace are consistent with the aforementioned incompatible problem of

273

low-NOx combustion and high burnout. The still higher NOx emissions and significantly worsen burnout

274

under ultra-low NOx combustion conditions are attributed to the absence of additional low-NOx

275

combustion solution except the bias combustion and deep air staging. Sharply opening OFA to strengthen

276

further the deep-air-staging conditions, on the one hand, deepens again the essentially oxygen-lean

277

combustion in the primary combustion zone to apparently raise the unburnt rate, on the other hand,

278

decreases the downward coal/air flux to affect the burnout because of the flame travel shortening in the

279

lower furnace. Table 1 also shows that the calculated NOx emissions of 905 mg/m3 at 6% O2, carbon in

280

fly ash of 5.1%, and O2 content at the furnace exit are all in good agreement with the measured data, with

281

the calculation deviations less than 5%. In light of the above high consistence in the calculated and measured

282

data referring to the in-furnace gas temperatures and symmetrical combustion pattern, gas temperature and

283

species concentration distributions in the near-wall region, and key parameters affecting the furnace performance

284

such as NOx emissions and carbon in fly ash, the numerical simulation methodology is reliable and can be used

285

to acquire the gas/particle flow, coal combustion, and NOx formation in the CLHC furnace.

286

3.2 Numerical results of the CLHC furnace at different CL settings

287

Figure 3 shows the in-furnace flow fields with varying CL. As mentioned previously, CLHC is an improved

288

low-NOx combustion technology based on the prior MIMSCT concept and its application in this work maintains

289

the main MIMSCT's furnace dimensions, thereby inheriting well the prior MIMSCT's characteristics in

290

regulating symmetrical combustion. Accordingly, at two settings of CL = 1/5 and 1/4 with the smallest and

291

moderate upper-arch depths, respectively, symmetrical W-shaped flow fields form in the furnace, despite the

292

former setup showing a little poorer flow-field symmetry than the latter. In the aspects of regulating a

293

symmetrical flow-field pattern, maintaining good burnout, mitigating slagging, and avoiding the hopper wall's 13



Page 14 of 41

294

thermal fatigue, the CLHC's application results in Figure 3 have the following characteristics: (i) In the upper

295

arch zone, the fuel-rich coal/air flow is redirected to penetrate downward towards the secondary-air side because

296

of the high-speed secondary air's entrainment. After a certain penetration depth, the fuel-rich coal/air flow

297

gradually mixes in turn with the inner and outer secondary air to form the downward primary coal/air flow. This

298

behavior strongly avoids an early mixing of secondary air into the ignited coal/air flow, thereby advancing

299

ignition and greatly inhibiting fuel-NOx. Thereafter, the downward primary coal/air flow turns to penetrate

300

downward in the near-wall region and suppresses staged air, generating a sharply inclined staged air jet-flow

301

around the periphery of primary coal/air flow. After reaching the lower arch zone, the downward primary

302

coal/air flow initially intersects strongly with the auxiliary burner's high-speed fuel-lean coal/gas flow prior to the

303

redirection towards the furnace center, and then mixes with the horizontal hopper air in the upper part of the

304

hopper before turning upward. Consequently, the pulverized-coal staging directed by the main and auxiliary

305

burners along the top-down direction, forms the two-stage W-shaped flame pattern. In the upper arch zone

306

between the downward primary coal/air flow and OFA, two strong recirculation zones symmetrically form in the

307

front- and rear-half sides, which are helpful to ignition and stable combustion by entraining the high-temperature

308

recirculating gas towards the fuel-rich coal/air flow nozzle outlet; (ii) Using the same technical principle of

309

MIMSCT, the CLHC’s downward primary coal/air flow is carried in turn by three high-speed jets of secondary

310

air, staged air, and the fuel-lean coal/gas flow to finally penetrate the upper and middle parts of the hopper. In

311

this form, the flame travel extends to improve burnout. In the furnace-throat zone where the high-speed OFA is

312

ejected, OFA reaches the furnace center to mix well with the unburnt particles entrained in the upward gas flow,

313

favoring a good burnout environment in the upper furnace; and (iii) The downward primary coal/air flow is

314

initially surrounded by the high-speed secondary air and staged air in the near-wall region, so as to avoid the

315

behavior of flame washing over the front and rear walls, and then lifted in turn by the high-speed fuel-lean

316

coal/gas flow in the lower arch zone and the horizontal hopper air in the upper part of the hopper, resulting an 14


Page 15 of 41


317

obvious air layer covering over the hopper wall. These characteristics are helpful to mitigate slagging in the

318

lower furnace and avoid thermal fatigue of the hopper wall.

319

Figure 3 also shows that increasing CL strengthens the recirculaing gas flow below the upper arch and

320

meanwhile shrinks the recirculation zone. This occurs because the upper arch zone decreases and the upward gas

321

flow's interactional extrusion enhances in the furnace central part. The less symmetrical flow field at CL = 1/5

322

than at CL = 1/4 may because that a smaller lower-arch space at CL = 1/5 makes the wall's jet-flows (i.e., staged

323

air and the fuel-lean coal/gas flow) closer to the downward primary coal/air flow. In consequence, the transverse

324

impulsion of the wall's jet-flows on the downward primary coal/air flow strengthens, resulting in the

325

aforementioned interactional extrusion aggravating. Aided again by the asymmetric upper-furnace configuration

326

effect (with the furnace nose and furnace outlet located in the rear side), the above circumstances thus weaken

327

the flow-field symmetry. As increasing extensively CL to CL = 1/3, an obviously deflected flow field generates,

328

with the upward gas flow and OFA fully towards the rear-half side and the flow in the front-half side dominating

329

in the furnace. This flow-field deflection is attributed to the fact that the extensively smaller upper-arch zone at

330

CL = 1/3 significantly shrinks the furnace’s central space, thereby strengthening the aforementioned interactional

331

extrusion and competition between the upward gas flows as entering the furnace throat. Aided by the

332

asymmetric upper furnace's guiding effect, the flow-field deflection thus occurs.

333

Figure 4 shows the in-furnace gas temperature and species (O2, CO, and NO) concentration fields of the three

334

CL settings. The acquired characteristics are analyzed in combination with the flow fields in Figure 3. It can be seen

335

that gas temperature and species concentration fields initially show acceptable symmetrical patterns at the first two

336

settings with a small and moderate CL, respectively, but then significantly deflect at the extensive CL = 1/3 setting.

337

The symmetry sequence of CL = 1/4 > CL = 1/5 > CL = 1/3 fitting for all panels of Figure 4, is attributed the fact

338

that the combustion symmetry in a W-shaped flame furnace strongly depends on the flow-field symmetry.

339

Consequently, the flow-field deflection at CL = 1/3 leads to its corresponding asymmetric combustion, and the 15



Page 16 of 41

340

more symmetrical flow field at CL = 1/4 than at CL = 1/5 thus favors its highest combustion symmetry. Detailed

341

analysis on the characteristics of gas temperature and species concentration fields is as follows.

342

(i) The former two symmetrical-combustion settings show three special zones characterized as high

343

temperature, low O2, high CO, and relatively low NO in the lower furnace; two large circular zones striding

344

across the upper and lower arch zones are symmetrically located in the front- and rear-half sides, and an

345

elongated zone is located in the furnace central part and extends from the hopper's upper part to the upper

346

furnace's middle part. The former two large special zones which are essentially consistent with the two large

347

recirculation zones below the upper arch in Figure 3, account for their gas temperatures of 1800–1950 K and O2,

348

CO, and NO concentrations of < 1%, 6–11 × 104 ppm, and 200–500 ppm, respectively. With oxygen-lean

349

combustion in the primary combustion zone and furnace's central part under the deep-air-staging conditions, the

350

recirculation zone thus directs the high-temperature and low-O2 reductive recirculating gas to participate in the

351

coal combustion in both the preceding stage and primary combustion zones. The low-temperature fresh coal/air

352

flow in the preceding stage combustion zone and intense deep-air-staging combustion in the primary combustion

353

zone lead to higher gas temperature, lower O2, and higher CO and NO in the lower part than in the upper part of

354

the recirculation zone. Compared with the former two special zones, the special zone in the furnace central part

355

show characteristics of relatively lower gas temperature, higher O2, lower CO, and higher NO, and again, along

356

the upward gas travel its gas temperature, CO, and NO initially increase but then decrease, while O2 shows an

357

opposite change trend. These observations occur because this special zone is filled with the high-temperature

358

upward gas emanating from the primary combustion zone in the front- and rear-half sides. The weak internal

359

turbulence of the high-temperature upward gas and absence of such a behavior with strong recirculating gas

360

participating in coal combustion as in the special zones below arches, explain here the different characteristics of

361

gas temperature and species concentrations. The above change trends along the upward gas travel are attributed

362

to the fact that in the lower part, the low-temperature hopper air reacts with parts of the unburnt particles before 16


Page 17 of 41


363

turning upward to raise gas temperature, consume O2, and generate CO and NO, while in the furnace throat and

364

upper furnace, the high-speed OFA mixes well with the unburnt particles entraining in the upward gas to

365

complete burnout. The considerable low-temperature OFA supply with a relatively low combustion share,

366

generates the latter decrease in gas temperature and content of CO and NO plus a O2 concentration increase.

367

(ii) Along the flame travel, a reductive atmosphere with high-temperature, low-O2, high-CO, and high-NO

368

characteristics forms in the side close to the furnace center because of the intense coal combustion, while in the

369

region close to the wall, a low-temperature and high-O2 oxidative atmosphere forms because of the successive

370

protection of the outer secondary air, staged air, fuel-lean coal/gas flow, and hopper air. These circumstances

371

enable the combustion to present an air-surrounding-fuel pattern. The strengthened deep-air-staging conditions

372

gradually supply air on the need of coal combustion to realize low-NOx and high-burnout combustion while

373

avoiding slagging on the front and rear walls and in the hopper. Again, the formed moderate-temperature

374

environment (~1100 K) near the hopper wall can prevent the hopper wall's thermal fatigue problem as appearing in

375

the MIMSCT furnace.27 As the ignited coal/air flow in the upper arch zone is carried in turn by the outer

376

secondary air and staged air to enter the primary combustion zone located below staged air, the intense coal

377

combustion aided by the timely air supply results in gas temperature, CO, and NO continuously increasing while

378

O2 decreasing along the flame travel. As a result, gas temperature and CO reach the highest levels in the lower

379

part of the primary combustion zone, and O2 decreases to the lowest value below 0.5%. Meanwhile, NO reach

380

its highest levels above 1000 ppm because of the combination of fuel-NO release in the preceding combustion

381

stage and much thermal-NO formation during the intense char combustion in the primary combustion stage. In

382

the lower arch zone, the weak reductive fuel-lean coal/gas jet-flow sharply fed slantwise by the auxiliary burners,

383

initially shows the relatively low-temperature and moderate-O2 characteristics (800–1100 K and 6–9%,

384

respectively) with the same CO and NO levels as the recirculating flue gas because the recirculating flue gas

385

delivering the fine pulverized-coal to form the fuel-lean coal/gas flow (its component shown in Table 1). The 17



Page 18 of 41

386

relatively low-temperature and moderate-O2 atmosphere favors the subordinated fine pulverized-coal

387

combustion and dominated NO reduction in the reburning zone and the slagging prevention in the lower arch

388

zone. Obviously, ,the high NO of 800–1100 ppm from the primary combustion zone is sharply reduced to about

389

500–600 ppm after the downward flame intersecting with the fuel-lean coal/gas flow to form the upward gas

390

flow, accompanied by a gas-temperature decreases. This change is attributed to the flue gas recirculation effect

391

(i.e., lowering the local O2 content to weaken the fine pulverized-coal combustion intensity) and fine

392

pulverized-coal reburning of the fuel-lean coal/gas flow. Consequently, the thermal-NO formation is inhibited

393

during the subordinated fine pulverized-coal combustion and again, a lot of the existing NO from the primary

394

combustion zone are reduced to N2. Thereafter, the hopper air, also partially used as oxidant to favor the unburnt

395

particle combustion, lifts the downward flame before turning upward in the middle and upper parts of the hopper.

396

In the furnace throat, OFA mixes well with the upward gas for completing burnout in the upper furnace.

397

(iii) At the CL = 1/3 setting, because of the flow-field deflection characterized as upward gas flow

398

deflecting towards the rear-half side and the flow in the front-half side dominating in the furnace (Figure 3), an

399

asymmetric combustion pattern forms with the deflected upward flame extruding the downward flame in

400

the rear-half side. In consequence, the aforementioned high-temperature and low-oxygen special zone

401

only develops below the front and rear arches and again, the one below the front arch is clearly stronger

402

than the left below the rear arch in the form of showing a larger space, higher temperature, lower O2,

403

higher CO, and lower NO. This indicates that the coal combustion status is better in the front-half side

404

than in the opposite side to release more heat, consume more O2, and generate more CO in the low-O2

405

environment. Moreover, the higher CO for NO reduction, lower O2, and dominant flame pattern in this

406

side providing a greater reaction space reburning, develop lower NO in this side than in the rear-half side.

407

Figure 4 also shows that, with increasing CL from CL = 1/5 to CL = 1/4, the special zone's symmetry below

408

the front and rear arches improves and a more regular pattern plus a lengthened longitudinal span appears for the 18


Page 19 of 41


409

special zone in the furnace central part. In the three special zones and primary combustion zone, such a CL

410

increase raises gas temperature and CO but decreases O2 and NO. Again, gas temperature decreases in the

411

hopper but increases in the upper furnace. Finally, levels of O2, CO, and NO all decrease at the furnace outlet,

412

suggesting an improved combustion performance with lowered NOx emissions at CL = 1/4 than at CL = 1/5. An

413

explanation of these observations is as follows. With increasing CL to shrink the upper arch zone, the downward

414

coal/air flame is closer to the high-temperature furnace center and combustion symmetry improves, thereby

415

strengthening coal combustion in the furnace. However, this CL increase shortens the flame travel in the lower

416

furnace at CL = 1/4, resulting in a decreased combustion share in the hopper to lower gas temperatures. The

417

aforementioned improved special zone in the furnace central part and more symmetrical OFA’s mixing with the

418

upward gas in the throat zone, favor the unburnt particle’s combustion in the upward gas and then gain higher

419

gas temperatures and lower O2 in the upper furnace. In conjunction with the more symmetrical and strengthened

420

combustion in the lower furnace, the furnace outlet’s performance thus improves with a decrease in both O2 and

421

CO. The simultaneously lowered NO levels at CL = 1/4 are attributed to a combination of (i) the lower O2 in the

422

aforementioned zones, (ii) higher CO owing a stronger NO reduction capacity in the primary combustion zone’s

423

lower part and reburning zone, and (iii) more symmetrical combustion conditions to reduce local high temperature

424

zones (favoring the thermal-NO generation). With continuously increasing CL to CL = 1/3, the formed asymmetric

425

combustion pattern obviously deteriorates the overall combustion status to affect burnout and generate local

426

high-temperature zones in the lower furnace and again, the deflected combustion in the upper furnace intensifies

427

further the final unburnt loss. As a result, gas temperature decreases and levels of O2, CO, and NO emission all

428

raise at the furnace outlet. For the fuel-lean coal/gas flow located in the lower arch zone (Figure 1), its ignition is

429

mainly affected by the surrounding gas temperature levels in the primary combustion zone. With increasing CL,

430

the initially increasing but then decreasing gas temperature levels nearby the fuel-lean coal/gas flow (Figure 4a)

431

thus initially shorten but then lengthen the ignition distance, thereby showing an initially improved but then 19



Page 20 of 41

432

worsened ignition performance. The above comprehensive comparison suggests that among the three typical

433

cascade-arch configuration setups, the CL = 1/4 setting develops the most reasonable gas temperature and species

434

concentration patterns in the furnace and the optimal furnace performance in gas temperature, O2 concentration,

435

burnout, and NO emissions at the furnace outlet.

436

Along the selected longitudinal cross-section for the simulated data extraction, the average value of all the

437

node data at any furnace height is taken to graph profiles of gas temperature and species concentrations with the

438

furnace height, as shown in Figure 5. Because the lower furnace's central part is mostly filled with the

439

stable upward gas flow where gas temperature and species concentrations varying slightly in the furnace

440

height direction (Figure 4), the following description on the change trends in Figure 5 conventionally lets

441

alone the contribution of the lower furnace's central part. Figure 5 shows that, although the combustion

442

symmetry varies with CL, trends of gas temperature and species concentrations with the furnace height are

443

similar for all three settings, and the details are as follows.

444

i) As striding in turn across the hopper, the reburning zone, and primary combustion zone along the furnace

445

height (i.e., the region from the hopper bottom to the lower arch's upper edge), gas temperature initially raises

446

rapidly but then keeps constant before finally decreasing, O2 initially increases slightly (except for the

447

asymmetric-combustion CL = 1/3 setting) but then decreases rapidly before finally maintaining unchanged, CO

448

initially increases rapidly but then decreases slightly, and NO first varies slightly but then increases rapidly

449

before finally decreasing. These observations are explained as follows. In the middle and lower parts of the

450

hopper generally filled with the weak-turbulence stagnant gas (Figure 3), the hopper air can also diffuse deep

451

into the hopper during it mixing with the downward flame before turning upward, resulting in the low and stable

452

levels of gas temperature, CO. and NO plus the relatively high O2 in the middle and lower parts of the hopper. In

453

the hopper air zone, the hopper air favors the combustion of partial unburnt particles to release heat and consume

454

O2, thereby increasing gas temperature and O2. CO remains particularly low levels all the time because of the 20


Page 21 of 41


455

moderate O2 of 6–8%. The slightly fluctuated NO profile in this stage is attributed to the low gas temperatures

456

and a limited char combustion share without much thermal-NO formation. After the furnace height reaching the

457

upstream zone above the hopper air, these curves stride in turn across the reburning zone (the exothermic

458

reaction of NO reduction via char and endothermic reaction of partial fine pulverized-coal combustion) and

459

primary combustion zone (the low-temperature fuel-lean coal/gas flow supply and intense coal combustion for

460

heat release) with the furnace height, resulting in gas temperature initially increasing but then remaining

461

unchanged before finally decreasing, because of the weakened combustion intensity in the upstream of the

462

primary combustion zone. O2 initially decreases rapidly because of coal combustion and then varies slightly due

463

to the balance of the oxygen supplement from staged air and the fuel-lean coal/gas flow's residual O2 and the O2

464

consumption of coal combustion. The CO profile's initial rapid increase stage is attributed to the strong reductive

465

atmosphere created in turn by the downstream reburning in the reburning zone and the upstream intense coal

466

combustion in the primary combustion zone, while its latter decrease stage occurs because of the combustion

467

intensity weakening in the upstream of the primary combustion zone. In this stage, the increase-to-decrease NO

468

trend along the flame travel is explained by the behaviors that the upstream staged air supply into the primary

469

combustion zone initially generates a large amount of NO, and then the downstream flue gas recirculation and

470

fine pulverized-coal reburning significantly reduce NO.

471

ii) In the region from the lower arch’s upper edge to the furnace throat’s OFA jet-flow (equaling to

472

the region from the preceding stage combustion zone the burner outlet zone), with increasing the furnace

473

height, gas temperature initially increases but then decreases rapidly, O2 slightly decreases but then

474

increases rapidly, and both CO and NO initially increase but then decrease rapidly. This occurs because

475

that the lower part of the region corresponds to the preceding combustion stage with the staged-air

476

injection. The combination of the O2 supplementation, cooling effect, and the air-to-gas dilution of the

477

low-temperature staged air and the heat release, O2 consumption, and product generation of coal 21



Page 22 of 41

478

combustion in the preceding combustion stage, thus develops here the lower gas temperatures, higher O2,

479

and lower CO and NO than those in the upstream middle part of the region without a low-temperature jet

480

supply. As mentioned above, in the middle part of the region without a low-temperature injection, the

481

delayed outer secondary air mixing with the ignited coal/air flow maintains conditions characterized as

482

relatively higher gas temperatures, lower O2, and higher CO and NO (because of the much fuel-NO

483

generation in the preceding combustion stage). In the upper part of the region, there exist the

484

low-temperature coal/air flow fed in the burner outlet zone and much low-temperature OFA ejected in the

485

furnace throat, resulting in the lower gas temperatures, CO, and NO and the highest O2 in the region.

486

These fuel and air supply characteristics in the upper, middle and lower parts of the region generate the

487

above change trends of gas temperature and species concentrations with the furnace height.

488

iii) In the region from the upper part of the OFA zone to the furnace outlet (i.e., the upper furnace taken as

489

the burnout zone), with increasing the furnace height gas temperature initially increases but then fluctuates for a

490

long time before decreasing sharply. Meanwhile, both O2 and CO initially decreases but then varies slightly,

491

while NO generally remains unchanged all the time. These observations are explained by the following reasons.

492

In this region where OFA mixes well with the upward gas, the entrained unburnt particles and

493

high-concentration CO completely combust to release heat and consume O2. In conjunction with the upper

494

furnace's additional function of acting as an radiant heat-exchange zone, the gradually decreasing combustion

495

proportion with the furnace height in this section finally results in gas temperature initially raising but then

496

fluctuating plus a final decrease stage. The limited combustion share occurring in a relatively lower-temperature

497

environment in the upper furnace cannot generate a large amount of thermal-NO to raise the NO levels.

498

Figure 5 also shows that with increasing CL from CL = 1/5 to CL = 1/4, gas temperature, CO, and NO all

499

decrease and O2 increases in the hopper, while the opposite change trends appear for both the lower furnace's

500

middle part and the upper furnace. This occurs because of the improved symmetrical combustion and the slightly 22


Page 23 of 41


501

shortened flame travel moving upward the flame kernel (Figure 3). Consequently, the combustion share falls in

502

both the lower furnace's lower part and the hopper to decrease the O2 consumption and NO production, and

503

raises in the furnace central part and upper furnace to increase heat release, O2 consumption, and CO generation

504

in low-O2 zones. Again, the lower O2 in the upper furnace and improved symmetrical combustion in reducing

505

local high-temperature zones favor the lower NO levels in the upper furnace at CL = 1/4. With continuously

506

increasing CL to CL = 1/3, the obvious change is that in the upper furnace, gas temperature decreases while the

507

O2, CO, and NO concentrations all increase significantly. This change is attributed to the asymmetric

508

combustion pattern where the upward flame deflects towards the rear-half side to apparently deteriorate the coal

509

combustion in this side, resulting in the increased unburnt particles entering the upper furnace. With the upward

510

gas flow in the upper furnace also deflecting towards, OFA mixes poor with the entrained CO and unburnt

511

particles, thereby decreasing the upper furnace's combustion share to reduce gas temperature and increase O2 and

512

CO. The relatively high O2 in the upper furnace and asymmetric combustion favoring the local high-temperature

513

zone formation, facilitate the thermal-NO formation to raise the overall NO levels in the upper furnace. The

514

above comprehensive comparison among three setting suggests that the CL = 1/4 setting has the most reasonable

515

gas temperature and species concentration profiles along furnace height and the optimal comprehensive indexes

516

of gas temperature, O2 content, burnout performance, and NO emission at the furnace outlet.

517

As mentioned above, a W-shaped flame furnace is dominated by the combustion in its lower furnace and

518

its lower furnace's central part is mostly filled with the stable upward gas flow. Accordingly, on the mentioned

519

data-extraction cross-section, a longitudinal AB line (Figure 1) close to the right edge of the primary burners in

520

the front-half side of the lower furnace is selected to compare gas temperatures and species concentrations

521

patterns, as shown in Figure 6, so as to further deepen the understanding of the cascade-arch configuration's

522

effect on coal combustion and NOx formation. In view of the AB line crossing in turn the burner outlet zone,

523

preceding stage combustion zone, primary combustion zone, reburning zone, and hot air zone along the flame 23



Page 24 of 41

524

travel, the NO distribution along the AB line can disclose the major NO-formation zones. Along the AB line, the

525

upper arch zone equals to the range from the upper arch to the lower arch, and the lower arch zone ranges from

526

the lower arch to the hopper. A detailed description and discussion about the change trends of gas temperature

527

and species concentrations along the AB line is as follows. In the upper arch zone along the downward flame

528

travel direction, gas temperature of the latter two settings initially increases slightly but then remains unchanged,

529

and O2 shows very low levels with slight fluctuations. On the contrary, at the lowest CL = 1/5 setting gas

530

temperature fluctuates all the time, and O2 initially increases but then decreases. Again, all three settings show

531

that CO generally increase significantly and NO initially remains unchanged but then increases (with the

532

exception of an initial decrease stage at the asymmetric-combustion CL = 1/3 setting). These observations occur

533

because that under conventional conditions, the upper arch zone of the AB line (located between the primary

534

burner's right side and the furnace center) is mainly affected by the high-temperature recirculation zone below

535

the arch (Figure 3), thereby showing high gas temperatures, low O2 content with levels varying slightly in the

536

downward flame direction. Again, coal combustion in the high-temperature and low-O2 atmosphere continues to

537

release a large amount of CO, resulting in a continuous CO increase. The occurrence of the delayed outer

538

secondary air mixing with the ignited coal/air flame in the preceding combustion zone, a low-O2 atmosphere

539

regulated here by the deep-air-staging combustion, and the high-temperature recirculating gas dominating this

540

zone, finally develop low NO levels with slight fluctuations. Here the CL= 1/5 setting's different change trend is

541

attributed to its largest distance between the AB line and primary burners among three settings and the

542

unsatisfactory OFA jet-flow symmetry below the throat zone. In the front-half side's larger OFA zone, the

543

low-temperature OFA can react with the high-temperature recirculating gas, resulting in the decrease-to-increase

544

gas temperature and increase-to-decrease O2 change trends and CO first maintaining unchanged but then

545

increasing. In the lower arch and hopper zones along the downward flame travel, gas temperature finally

546

changes little in the lowermost part of the AB line after an initial slight increase but then a rapid decrease, and O2 24


Page 25 of 41


547

undergoes in turn two increase-to-decrease stages to generate two O2 peaks respectively in the lower arch and

548

hopper zones. Meanwhile, CO reaches the lowest value in the hopper and then tends to maintain unchanged after

549

rapidly decreasing in the lower arch zone. NO initially undergoes a significant increases-to-decrease stage in the

550

lower arch zone and then changes little in the hopper. These phenomena are explained by the fact that along the

551

downward flame travel, here the AB line passes in turn through the primary combustion, reburning, and hopper

552

air zones. In the primary combustion zone, the staged-air injection and intense coal combustion result in gas

553

temperature increasing and O2 initially raising but then decreasing. In the reburning zone, the low-temperature

554

fuel-lean coal/gas flow injection and endothermic char reburning for NO reduction decrease gas temperature

555

levels. In the hopper air zone with the low-temperature hopper air owing dual roles of cooling and participating

556

the unburnt particle's combustion, gas temperature thus decrease initially but then changes little, and O2 first

557

increases but then decreases. A combination of the oxygen-lean combustion in the primary combustion zone

558

regulated by the deep air staging, the high-concentration CO also reducing NO in the reburning zone, and the

559

hopper air injection, finally leads to CO initially increase but then drops rapidly to particularly low levels. On the

560

AB line, the high NO levels of 800–1400 ppm appears only in the primary combustion zone below the lower

561

arch, while the left low-NO levels of 200–400 ppm occupy the upper arch zone, reburning zone in the lower part

562

of the lower arch zone, and hopper air zone. This NO distribution pattern, on the one side, is related to the fact

563

that the distance between the downward flame travel and AB line continuously decreases with the flame

564

penetration, on the other side, indicates the NO formation and reduction in four aspects: (i) The delayed

565

secondary air mixing with the coal/air flow in the burner zone and the overall deep-air-staging conditions in the

566

furnace effectively inhibit the thermal-NO formation in the preceding combustion stage; (ii) The staged-air

567

injection and high-temperature environment formed by intense combustion leads to a large number of

568

thermal-NO in the primary combustion zone; (iii) The fine pulverized-coal reburning environment constructed

569

by the fuel-lean coal/gas flow achieves a significant NO reduction; and (iv) The hopper air strengthens air 25



Page 26 of 41

570

staging and meanwhile favors burnout without raising NO levels. These four aspects thus confirm the CLHC's

571

availability in controlling further the NO generation and improving burnout.

572

Figure 6 also shows that the CL effect on the calculated results on the AB line is mainly related to the

573

combustion symmetry and the flame travel in the front-half side. At the extensive CL = 1/3 with an

574

asymmetric combustion pattern characterized as the flame deflecting upward towards the rear wall, the

575

larger distance between the AB line and high-temperature flame center and lower temperatures in the

576

primary combustion zone because of its weaker combustion intensity (Figure 4), result in gas

577

temperatures on the AB line are lower than those of two symmetrical-combustion settings. The resulted

578

more unburnt particles entering the hopper air zone, makes O2 in the middle and lower parts of the AB

579

line initially lower but then higher than that of the other two settings. With increasing CL, CO first

580

increases but then decreases in the upper arch zone while continuously decreases in the lower arch and

581

hopper zones. Again, in the obvious NO-varied region from the lower part of the upper arch zone to the

582

upper part of the hopper (crossing in turn the primary combustion and reburning zones), NO initially

583

increases but then decreases with CL. This also occurs because that the front-half OFA diffusion in the

584

upper arch zone is stronger at CL = 1/5 than at CL= 1/4 and in the asymmetric combustion at CL = 1/3, the

585

deflected flame towards the rear wall weakens the combustion intensity in the primary combustion zone

586

(Figure 4). In the primary combustion zone below the lower arch on the AB line, the highest gas

587

temperatures at the same O2 levels finally leads to the highest NO at the CL = 1/4 setting.

588

The calculated O2 content, burnout parameters, and NOx emissions at the furnace outlet are listed in Table 1.

589

As increasing CL to decrease the upper arch's space, O2 at the furnace outlet, CO and NOx emissions, and carbon

590

in fly ash all initially decrease but then increase. Among the three setups, the most symmetrical-combustion

591

setting of CL = 1/4 shows the best furnace performance (characterized as the lowest O2 in exhaust gas, burnout

592

loss, and NOx emissions) to reach an optimal low-NOx and high-burnout combustion pattern with NOx emissions 26


Page 27 of 41


593

of 707 mg/m3 at 6% O2 and carbon in fly ash of 5.5%. As mentioned above, this occurs because as CL increases

594

from CL = 1/5 to CL = 1/4, the combustion symmetry improves and gas temperatures in the primary combustion

595

zone increase (Figure 4) to strengthen the overall coal combustion in the furnace. Meanwhile, the lower O2 and

596

higher CO in the primary combustion zone and furnace center enhances the reductive atmosphere to inhibit the

597

NO production. The more symmetrical OFA jet-flows in the throat at CL = 1/4 (Figure 3) develop good mixing

598

conditions for OFA and the upward gas flow, thereby increasing the combustion share and O2 consumption in

599

the upper furnace to improve burnout and decrease O2 in exhaust gas. The improved symmetrical combustion in

600

reducing the local high-temperature zones in the lower furnace and lower O2 levels in the upper furnace are

601

favorable for reducing the thermal-NOx production, finally achieving the lowest NOx emissions at the furnace

602

outlet. Increasing further CL to CL = 1/3, the asymmetric combustion formation deteriorates the overall

603

combustion performance in the form of weakening the rear-half coal/air combustion in the lower furnace and

604

enabling OFA and the upward gas flow fully to deflect towards the rear wall in the upper furnace. In

605

consequence, poor burnout develops in the whole furnace and the local high-temperature zones increases in the

606

lower furnace. In conjunction with the high O2 levels in the upper furnace because of poor burnout decreasing O2

607

consumption, NOx emissions at CL = 1/3 surge to the highest levels among the three settings. In comparison with

608

the calculated results (NOx emissions of 905 mg/m3 at 6% O2 and carbon in fly ash of 5.1%, respectively)

609

corresponding to the prior MIMSCT art, CLHC reduces further NOx emissions by 200 mg/m3 at 6% O2

610

(equaling to a 22% reduction rate) and almost maintains the original burnout rate at its optimal CL = 1/4 setup.

611

Based on the simulation deviation of NOx emissions (i.e., the MIMSCT's calculated results higher than the

612

measured data by 4.4%), the estimated real-furnace NOx emissions with CLHC can be lowered to ultra-low

613

levels of about 670 mg/m3 at 6% O2. This significant NOx reduction occurs because, based on the

614

deep-air-staging concept of MIMSCT, CLHC partitions the one-stage furnace arch into a two-stage pattern with

615

upper and lower arches for improving the combustion symmetry and strengthening the coal combustion in the 27



Page 28 of 41

616

lower furnace and meanwhile, introduces the additional low-NOx combustion solutions of flue gas recirculation

617

and fine pulverized-coal reburning. Consequently, the NOx generation is significantly inhibited and again, those

618

generated NOx are sharply reduced to N2 by the fine pulverized-coal and high-concentration CO in the reburning

619

zone, thereby achieving such low levels of NOx emissions. In the left high-burnout aspect, CLHC maintains high

620

burnout by means of improving the combustion symmetry, strengthening the coal combustion in the lower

621

furnace (Figures 2a and 3a), and forming a well OFA mixing morphology with the upward gas flow.

622

In summary, the comparison of industrial-size measurements and numerical simulations of a 600 MWe

623

MIMSCT W-shaped flame furnace confirmed the numerical simulation validity. For the proposed CLHC

624

solution destined to resolve the prior art's incompatible problem of strengthened low-NOx combustion and high

625

burnout, numerical simulations were performed in the furnace with CLHC so as to evaluate the cascade-arch

626

configuration effect on the gas/particle flow, coal combustion, and NOx formation characteristics. It was found

627

that CLHC gained an improved symmetrical combustion pattern with combustion strengthened in the lower

628

furnace. The apparently prolonged secondary air mixing with the ignited coal/air flow in the burner zone plus the

629

strengthened deep-air-staging conditions in the furnace, significantly inhibited the fuel-NO formation in the

630

preceding combustion stage. Although a large amount of thermal-NO formed in the primary combustion zone

631

due to the staged-air injection and high gas temperatures, a significant NO reduction by the fuel-lean coal/gas

632

flow developed in the downstream reburning zone, and the hopper air favored burnout without increase NO

633

levels. These characteristics thus confirmed the CLHC's availability in inhibiting the NO formation and

634

improving burnout. With increasing CL, the combustion symmetry initially improved but then deteriorated

635

significantly. Meanwhile, the O2 content in exhaust gas, CO and NOx emissions, and carbon in fly ash all

636

decreased at first but then increased. Consequently, among the three typical cascade-arch settings, the moderate

637

CL = 1/4 setup showed the most symmetrical combustion, lowest O2 content in exhaust, highest burnout rate, and

638

lowest NOx emissions and reached an optimal low-NOx and high-burnout combustion performance with NOx 28


Page 29 of 41


639

emissions of 707mg/m3 at 6% O2 and carbon in fly ash of 5.5%. Compared with the prior MIMSCT art (NOx

640

emissions of 905 mg/m3 at 6% O2 and carbon in fly ash of 5.1%, respectively), CLHC reduced NOx emissions

641

by about 200 mg/m3 at 6% O2 (a reduction rate of 22%) and meanwhile maintained burnout. In view of a series

642

of CLHC’s optimizations referring to the furnace configuration, combustion system layout, and air/fuel

643

distribution (a systematic task necessitating lots of efforts in future studies) being absent in this work

644

where CLHC is only at its proposed stage, here CL = 1/4 can be taken as a preliminary, acceptable

645

cascade-arch configuration setup for CLHC with a good furnace performance of symmetrical combustion, low

646

NOx emissions, and high burnout.

647

Acknowledgments

648

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (Grant No.

649

LY18E060002), National Key Research & Development Plan of China (Contract No. 2016YFC0205800), Scientific

650

Research Foundation of Graduate School of Ningbo University, and K.C. Wong Magna Fund in Ningbo University.

651

Supporting Information

652

Figures S1, S2, and S3. Notes introducing the combustion partition and performance improvement of CLHC

653

compared with those prior low-NOx arts. This material is available free of charge via the Internet at http://pubs.acs.org.

654

References

655 656 657 658

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(3) Luo, Z.; Wang, F.; Zhou, H.; Liu, R.; Li, W.; Chang, G. Principles of optimization of combustion

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deep-air-staging combustion under real-furnace conditions in a 600 MWe down-fired supercritical boiler

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by strengthening the staged-air effect. Environ. Sci. Technol. 2014, 48 (20), 12419–12426.

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multiple injection and multiple staging combustion technology in a 600 MWe supercritical down-fired

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furnace under deep-air-staging conditions. Environ. Sci. Technol. 2014, 48 (1), 837–844.

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burners. Environ. Sci. Technol. 2010, 44 (3), 1130–1136.

737 738 739 740

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742

(36) Hill, S. C.; Smoot, L. D. Modeling of nitrogen oxides formation and destruction in combustion

743 744 745 746 747 748 749 750 751 752 753

systems. Prog. Energy Combust. Sci. 2000, 26 (4–6), 417–458. (37) De Soete, G. G. Overall reaction rates of NO and N2 formation from fuel nitrogen. Symp. (Int.) Combust. 1975, 15 (1), 1093–1102. (38) Visona, S. P.; Stanmore, B. R. Modeling NOx release from a single coal particle II. Formation of NO from char-nitrogen. Combust. Flame 1996, 105 (1–2), 92–103. (39) Nikolopoulos, N.; Nikolopoulos, A.; Karampinis, E. Numerical investigation of the oxy-fuel combustion in large scale boilers adopting the ECO-Scrub technology. Fuel 2011, 90 (1), 198–214. (40) Levy, J. M.; Chan, L. K.; Beer, J. M. NO/char reactions at pulverized-coal flame conditions. Symp. (Int.) Combust. 1981, 18 (1), 111–120. (41) Kuang, M.; Li, Z.; Xu, S.; Zhu, Q. Improving combustion characteristics and NOx emissions of a down-fired 350 MWe utility boiler with multiple injection and multiple staging. Environ. Sci. Technol. 2011, 45 (8), 3803–3811.

754

(42) Kuang, M.; Li, Z.; Zhu, Q. Chen, L.; Zhang, Y. Gas/particle flow characteristics, combustion and

755

NOx emissions of down-fired 600 MWe supercritical utility boilers with respect to two configurations of

756

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Energy

Fuels,

2012,

32


26

(6),

3316–3328.

Page 33 of 41


757

Table 1. Coal Characteristics, Operational Parameters, and Major Real-Furnace and Simulated Results of

758

the 600 MWe W-Shaped Flame Furnace Respectively with MIMSCT and CLHC. Proximate Analysis, Wt % (As Received) Ultimate Analysis, Wt % (As Received) volatile fixed net heating moisture ash carbon hydrogen oxygen nitrogen sulfur matter carbon value (MJ/kg) 19.28 3.36 7.40 6.59 33.29 52.72 51.89 1.88 1.38 0.80 Operation parameters and the measured/calculated results

MIMSCT

quantity

experiment

fuel-rich coal/air flow

fuel-lean coal/air flow

fuel-lean coal/ gas flow

secondary air

staged air

OFA

hopper air

temperature (°C) mass flow rate (kg/s) velocity (m/s) coal concentration (kg coal/kg air) temperature (°C) mass flow rate (kg/s) velocity (m/s) coal concentration (kg coal/kg air) temperature (°C) gas mass flow rate (kg/s) velocity (m/s) coal concentration (kg coal/kg gas) temperature (°C) mass flow rate (kg/s) velocity (m/s) temperature (°C) mass flow rate (kg/s) velocity (m/s) temperature (°C) mass flow rate (kg/s) velocity (m/s) temperature (°C) mass flow rate (kg/s) velocity (m/s)

coal feed rate (t/h) total rate of air (kg/s) O2 at the furnace exit (%) CO in flue gas (ppm) carbon in fly ash (%) NOx emissions (mg/m3, 6% O2) 759 760 761 762

CLHC's simulations

simulation

CL=1/5

CL=1/4

98 45.72 15.0

98 72.96 22.2

1.40

0.88

98 68.58 22.6

N/A

CL=1/3

0.17 153 146.57 N/A

40.6 0.08

359 234.9 37.4 359 182.9 37.4 359 87.8 37.4 N/A 272.3 619.9 2.9 33 5.4 867

3.1 60 5.1 905

3.46 259 5.8 857

359 230.79 36.1 359 123.98 36.0 359 123.98 37.2 207 68.19 27.3 272.3 619.9 3.09 78 5.5 707

3.68 161 5.9 971

Note: With a gas recirculation rate of 20%, the flue gas component is fixed at O2 = 5.25%, N2 = 75.55%, CO2 = 13.1%, SO2 = 3300ppm, H2O = 5.69%, NO = 380ppm, and CO = 338ppm.The hopper air is the mixture of a small part of secondary air and the air separated from the former fuel-lean coal/air flow of the MIMSCT furnace. Consequently, its temperature is lower than that of secondary air.

33



Page 34 of 41

Furnace height H (×1000 mm)

one burner will wa group ll

right sidewall

lw wil

rear wall

all

b. horizontal furnace cross-section and 24 concentrators

upper furnace

corresponding to six millers labelled in turn as A -F

12512 x

5577

OFA port

25

upper furnace arch

upper arch

front (rear) wall

y

furnace center zone

fuel-rich coal/air flow A secondary air

staged air 20

lower furnace arch zone

26680

lower furnace arch zone

upper furnace arch zone

A4 A2

30

E4 E2

F4 F2

35

C4 C2

D4 D2

40

all

C3 C1

B4 B2

23666

lw

E3 E1

B3 B1

45

D3 D1

50

wil

A3 A1

F3 F1

unit of dimension: mm

55

left sidewall

ll

upper furnace arch zone

a lw wil

lower arch

fuel-lean coal/gas flow

lower furnace L0

front wall 23666

L1 rear wall

fuel-rich coal/air flow nozzle fuel-lean coal/gas flow nozzle inner secondary air port outer secondary air port OFA port

lower arch

15 hopper air

c. nozzle and port layout pattern for each burner group (six groups on each arch, with each group fed primary coal/air mixture by two concentrators listed above) B

hopper symmetry axis 569

5

staged-air slot 105

3373

48

wing wall

882

10

2190

front or rear wall

764 765 766

0

d. staged-air slot layout pattern on the front and rear walls

a. vertical furnace cross-section

Figure 1. Schematics of furnace configuration, burner layout pattern, and combustion partition of the 600 MWe cascade-arch-firing, W-shaped flame furnace.

34


Page 35 of 41


port 11 port 867867 1156 1283 11561283

front front 1314 1314 1205 wall wall 1205 port 22 port

1124904 1237 1124 904 1237

831 1253 1454 831 1454

1129 1129

rear rear wall wall

1304 1193 1193 1304

1328 1201 1328

833 1442 1246 833 14421246

1100 1100

1106 1106

calculated calculated data data

measureddata data measured

767

(a) overall gas temperatures in the front and rear parts of the furnace (using the infrared thermometer) measured data

o

1000 900 800 700 0.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

calculated data

1300

port 3

1200 1100 1000 900 800

measured data

port 4

1200 1100 1000 900 800

700 0.0

0.3

0.6

0.9

1.2

1.5

distance to the wall (m)


769

measured data

o

port 1

1100

calculated data

1300

gas temperature ( C)

1200

calculated data

o


1300


768

1.8

700 0.0

2.1

0.3

0.6

0.9

1.2

1.5

1.8

2.1


calculated data

measured data

9 8 7 6 5 4

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 distance to the wall (m)

calculated data

700

measured data 2

600 1.8

500 1.6

400 300

1.4

200 100 0

1

1.2

0.8 0.6 0.4


NOx concentration (dry volume ppm)

O2 concentration (dry volume %)

10

CO concentration (dry volume ppm)

(b) gas temperature distributions in the near wing-wall regions at ports 1, 3 and 4 (using the thermocouple device)

770

774

front front wall wall

795 1233 795 12781233 1278

1135 1135 port4 4 port

773

rear rear wall wall

1287 12871212 1212

1224 8458451224 1306 1306 port port 33

771 772

869 1207 869 1134 1134 1207

1245 1085820 12451085 820

550

calculated data

measured data

500 450 400 350 300 250


(c) gas species concentrations in the near wing-wall region at port 2

Figure 2. Comparison of measured and calculated gas temperatures and component concentrations in the near wing-wall region of the 600 MWe W-shaped flame furnace with MIMSCT.

35



Page 36 of 41

60 55 50

Velocity Magnitude

45

50 45 40 35 30 25 20 15 10 5

Furnace height (m)

40 35 30 25 20 15 10 5

776 777 778

0

0 CL=1/5 Furnace depth(m)

CL=1/4

CL=1/3

Figure 3. Calculated flow fields of the cascade-arch-firing, W-shaped flame furnace at different CL settings.

36


Page 37 of 41


11

125 0

CL=1/4

1 35 0

10 43

125 7

CL=1/3

13

13

17 00

1

71

15 00 50

14

1400

1400

CL=1/5

0 30

00

1

50

1171

1743

1 800

7 25

16

14 16

1 66 5 16

18

00 16

00

00

1171

43

00 18 9 15 00 1 17 1 65 16 8 14 1 6 1 47 1

50 13 0 5 12 0 5 11

1400 13 0 0

1 60 0

14

10

00

16 00 15 00

16 00 7 1 1 846

84

86

17 00

10

0

50

50

0 18 19

00

19

19

1 70 0 1500

00

1 6 65

18 00

00

58

16

1084

10 10 71 20

9 84 2

17 93 3 4 10

18

7

30

85

80

87

1 0 43

11

2

16

00

184

3

19

96

00

927

17

00

16

15

17

4 89 1 95

7 46 79

1665

16.67

14

1176.33

5

13

1900

1 1 5 0 1 18

1 6 00

1238.05

00

00

1 50 0

7 94 9 9 8 0 16 1 1050198 1

1950798

13

11 3 0

00

1 257 10 43

780

(a) calculated gas temperature fields (K) 15

0.3

4

18

4

CL=1/5

3.4

CL=1/4

00

6

CL=1/3

2.7

5

10

2

6

1 5 11

0. 1

15

4

1

0.5

11 50

11

2 3

5

11 8. 5 11

6

3

9. 8

9.2

14 0 0

14

5

5

3 2

2

8

0.1

2

11

6

8

9. 8

8 10 6

0.3

0.1

0. 1

2.7 3.4 0.1 4 6 2.7

0.3

1

10

15

2

8.8

8

6 8

2

5. 6.1 9

10

7

2.7

8.8

6

8.3 8.4

7

8

10

4 7 3 10 1 2 10 7 2 2 18 3 1 12 10 4 1 8 0.5 5 6 6 4 1 14 9 3 1 9 7

8.2 8.5

7

11

6.9 7.8

4

9.2

5

6.4

8.4

3.4

12 1 4 10 11

8. 4 9. 4

50

2

7.83

10

6.6

6. 6.3 1

5

9

7.4

12

3.4

15

50

3 3

12

13

5

4

15

781

6

6

2.7

6

782

(b) calculated O2 concentration (%)

783

14

11

10 37

9


6

6 00 Environmental Science & Technology

10

1

2

0 60

0

13

00

9 1 000 00 10 0

10

1 00

30 0

11 8 46

0.3

90

0.3

0 30 4 62 80 0 0 10

46

0.2

0. 1

0

15

0 300

5.4

3

1E -0 7

2

2

0.2

28 36 12 00 0.0

60

5. 4

9 00

8

50

-06

02

13 25 07

60 0 10 0 11 0 00

30 0 60 0

1 53

5.06005E-06

005E 5.06

8. 46

E06

00 0.

3

2 00 30

0.2

15

0.2

0.2

50

-07

00

602 1E

06

11

E-

30 9 00 0 20 0 600

5. 4

02

100

4

63

9 00 63 02

1 18

0.2 0.35 0 .3

0.2

8.4 2.1

5.06 005E -06

0.4

0.2

1 3 4

5.4

0.4

46

0.1

0.8

0.3

7.3

2.1

0.2

00

1

0.4 2.4 1.2

11

0.2 0.3

3

3

0.2

0

8.5 -05 5E 96 4.1

CL=1/3

1.5

2

80

CL=1/4

3

CL=1/5

00

1.5

2

2

Page 38 of 41

1

2

0.8

784 785

5 52

(c) calculated CO concentration (×100 ppm)

786

50

55

2

200 4 56

80 0

9 65 0

5

80

0

20

0

80

65 9 80 12 0 00

9

53

0 50

(d) calculated NO concentration (ppm) 3 57

38

7

788

500

5 54 7

0

0 45 90 4

4 71

787

44

00

65

50

9 53

3 57

11

44

6

7 00

4

2

1 63

38 200

20 0

35

7

80 0

00

5 44 0 0 5 38 8 348

489 30 0 261

47

44

0 50 5 11 44 00

0

44 6

65 3010

2 00

1 00 0

1

20

1 600 3 9 5

10

7

2 00

8

446

35

2 00 63 1 35 7 1 000 0 9 8 0 10 0 53 0

35 7

5 44 0 20 15 6

3

24

53 9 63

1 00

29

7

3

57

7

2

7

65 2

30 0 5 23 57 2 0 0 1 65 2 50 0 20 700 4 23 0 11 00 0 0 9 55 2 700 5 52 7 00 4 23 471 423

35

00

4 46 47 2

16

11

0

35

47

1

7

471 5 64 0 70

4 89

20

16

57

2 6 54 60 623

1 4 7 89 3 36 8

56 4 0

CL=1/3 4 46

0.0002

3 68 55 3 89 2

2 00

44 6

56

CL=1/4

5 23

47 2

7

CL=1/5

1 600

5. 4 89 4

38

3

16 00

52

789

Figure 4. Calculated gas temperature and component concentration fields at different CL settings of the

790

cascade-arch-firing, W-shaped flame furnace.

38


7 00 4 46

44

5

9

5

Page 39 of 41


60

60

CL=1/5

40

upper furnace 30

upper arch zone lower arch zone

10

0 800

791

CL=1/4

50

CL=1/3

Furnace height (m)

Furnace height (m)

50

20

CL=1/5

CL=1/4

furnace hopper

CL=1/3

40

upper furnace 30

upper arch zone 20

lower arch zone furnace hopper

10

0 1000

1200

1400

1600

1800

2

2000

4

Temperature (K) Temperature 60

10

12

14

16

CL=1/5

CL=1/4

50

CL=1/3

40

upper furnace 30

upper arch zone

20

40

10

0

upper furnace

upper arch zone 20

furnace hopper

CL=1/3

30

lower arch zone 10

CL=1/4

50

Furnace height (m)

Furnace height (m)

8

60

CL=1/5

792

6

O22 concentration O concentration (%) (% )

lower arch zone furnace hopper

0 150

300

450

600

750

100 200 300 400 500 600 700 800 900

900

CO concentration (×100 ppm) CO concentration (×100ppm)

NO concentration NO concentration(ppm) (ppm)

793

Figure 5. Calculated mean gas temperature and component concentration distributions along the furnace

794

height at different CL settings.

39



26

26

CL=1/5 CL=1/4

24

Page 40 of 41

CL=1/5 CL=1/4

24

CL=1/3

CL=1/3 22

Furnace height (m)

Furnace height (m)

22

upper arch zone 20 18

lower arch zone

16 14

10

lower arch zone

18 16

furnace hopper

furnace hopper

14

12

795

upper arch zone 20

12

1000

1200

1400

1600

1800

10

2000

0

2

4

6

8

CL=1/5

26

12

CL=1/5

26

CL=1/4

CL=1/4 CL=1/3

24

CL=1/3

24

22

Furnace height (m)

Furnace height (m)

10

O concentration (% O22 concentration (%))

Temperature(K) (K) Temperature

upper arch zone 20

lower arch zone 18 16 14

22

upper arch zone

20 18

lower arch zone 16 14

furnace hopper

furnace hopper 12 10

796

12

0

200

400

600

800

1000

10 200

1200

400

COconcentration concentration(×100ppm) (×100 ppm) CO

600

800

1000

1200 1400

NO concentration (ppm) NO concentration (ppm)

1600

797

Figure 6. Calculated gas temperature and component concentration distributions along the selected AB

798

line.

40


Page 41 of 41


799

Table of Contents (TOC) Art fuel rich combustion inner/outer secondary air (first air staging)

OFA

(fourth air staging)

(second air staging)

staged air fuel-lean coal/gas flow

recirculating gas

(flue gas recirculation and reburning)

hopper air (third air staging)

800

41


Low-NOx and high-burnout combustion characteristics of a cascade

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