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Experimental research on flameless combustion with coal preheating technology Wen Liu, Ziqu Ouyang, Xiaoyang Cao, and Yongjie Na Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00719 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018
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Energy & Fuels
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Experimental research on flameless combustion with coal preheating technology
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Wen Liu†,‡, Ziqu Ouyang†,*, Xiaoyang Cao†, Yongjie Na†,‡
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†
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100190, China
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‡
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ABSTRACT: As a novel combustion technology, flameless combustion is attractive to
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achieve high efficient and low NOX utilization of fuels. In the paper, a new method to
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realize flameless combustion of pulverized coal based on coal preheating technology
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was adopted. Experimental research on flameless combustion was implemented with
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pulverized coal in a 30 kW coal preheating and combustion test rig. In the study, coal
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preheated characteristics, combustion behavior and formation mechanism of NOX are
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discussed. The results show that the temperature of pulverized coal can be preheated
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up to above 850 oC, and most of the volatile are released during preheating process.
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Only 33.90% of the fixed carbon in pulverized coal is converted into coal gas and
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most remains in the high temperature coal char, and 91.14% of the fuel-N released
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from pulverized coal is converted to N2. Stable flameless combustion can be achieved
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with preheated fuels in the down-fired combustor, and no obvious flame front can be
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Institute of Engineering Thermophysics, Chinese Academy of Science, Beijing
University of Chinese Academy of Sciences, Beijing 100049, China
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observed through the looking-fire-windows, and the luminosity is uniform inside the
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down-fired combustor. The temperature profile in combustion zone is uniform with
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normalized temperature fluctuation less than 10%. NOX is generated near the fuel
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nozzle and then reduced in the main combustion zone, and the NOX emission is 67
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mg/Nm3 (6% O2). The formation of NOX is inhibited by rational air distribution, and
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the combustion efficiency reaches up to 96.4%.
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KEYWORDS: coal preheating; flameless combustion; NOX emission
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1. INTRODUCTION
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Flameless combustion is a novel combustion mode different from conventional
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combustion, for it is characterized by no visible flame front and uniform temperature
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profiles1, 2. Since 1990s, flameless combustion has demonstrated its great potential to
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control NOX, therefore it is considered as one of the most promising technique for
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de-NOX combustion and draws more and more attention. Flameless combustion can
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be achieved by preheating air up to 1000 oC and the reactants are diluted in a
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low-oxygen atmosphere, therefore it is also called high temperature air combustion
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(HTAC)3-5 or moderate & intense low oxygen dilution (MILD)6-9. Colorless
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distributed combustion10-12 (CDC) is another name of flameless combustion for its
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flame front is invisible.
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Within two decades, flameless combustion technique has been rapidly developed 2
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Energy & Fuels
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with gaseous fuel, and industrial application has been achieved. With the development
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of flameless combustion, solid fuel flameless combustion is also gaining ever
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increasing attention. The initial experimental investigations on coal flameless
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combustion were launched in the International Flame Research Foundation (IFRF)
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with exhaust gas from the gas turbine13. Since then, more and more studies on solid
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fuel flameless combustion have been implemented. Stadler14 studied the impact of
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gasification on the prediction of NO formation in coal flameless combustion, and
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verified that the prediction of NO emission was up to 40% lower with considering
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gasification than without. Vascellari15 studied the influence of turbulence-chemistry
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interaction on coal flameless combustion and obtained more accurate simulation
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results. Saha16, 17 carried out many experimental researches on Victorian brown coal in
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MILD combustion. Dally7 successfully achieved MILD combustion with sawdust.
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Xing18 achieved flameless combustion of pulverized coal with normal temperature air，
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and concluded that preheating combustion air is not always necessary, and the same
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conclusion was also gained by Dally7. Flameless combustion for solid fuels was
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mainly focused on coals with high volatile content in the past decades and by the
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means of high temperature air combustion or a high speed jet. But nowadays
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nonflammable fuels like anthracite and petroleum coke have also been studied to
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verify the stability and broaden the adaptation range of flameless combustion. In
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Suda’s research19, anthracite was used to demonstrate the effect of coal type on the
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combustion behavior in high temperature air combustion. MILD combustion on 3
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Bowen black coal with either CO2 or N2 as the carrier gas was studied experimentally
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and numerically without any external air preheating in Saha’s researches20, 21. Zhang
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Hai5 developed a novel PRP (primary air enrichment and preheating) burner and an
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internal hot gas recirculation could be formed inside the burner chamber, thus the
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primary air, along with fuel was preheated in the chamber. Fuel preheating might be
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an effective method to establish flameless combustion on solid fuels. Pulverized coal
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preheating technique was deeply studied at the All-Russian Thermal Engineering
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Institute and testified to be effective to control NOX22, 23. But it required an external
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heat source (a natural gas-fired PC burner) to preheat pulverized coal, and the process
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was complicated and preheating temperature was hard to control.
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A novel coal preheating method based on a circulating fluidized bed (CFB) was
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proposed by the Institute of Engineering Thermophysics, Chinese Academy of
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Sciences, and many experiments have been implemented24-27. The results showed that
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pulverized coal was preheated up to above 800 oC without external heat input, and
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NOX emission was as low as 60 mg/Nm3 (@6 vol% O2)27. The aim of this paper is to
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achieve flameless combustion of pulverized coal by applying coal preheating
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technique with a CFB, and to study the preheating and combustion behavior and NOX
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emissions characteristics.
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2. Experiment
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2.1 Test rig. 4
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Energy & Fuels
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The coal preheating and combustion system is shown in Figure 1. The system consists of a preheating burner, a down-fire combustor and some other auxiliaries.
Secondary air
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Looking-fire window
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Tertiary air 4
Primary air
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1 air compressor, 2 coal feeder, 3 preheating burner, 4 down-fired combustor, 5 water cooler, 6 bag
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filter, 7 water tank, 8 chimney.
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Figure1. Schematic diagram of the test rig
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The preheating burner consists of a riser, a cyclone separator and a back-feeder.
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The riser is 90 mm in inner diameter and 1500 mm in height. The primary air, which
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accounts of about 30% of the air required for complete combustion, is supplied into
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the preheating burner from its bottom. Pulverized coal is stored in a stock bin, and is
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fed into the preheating burner by a screw feeder. Pulverized coal is heated up to above
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850 oC by heat released from its partial combustion, and then enters into the
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down-fired combustor for complete combustion. Since the excess air coefficient is
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very low in the preheating burner, coal gasification reaction should be taken into
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account. The gas outflows the preheating burner is mainly composed by N2, CO, CO2, 5
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H2 and CH4, which can be named as coal gas. The preheated fuel contains high
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temperature coal char and coal gas.
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The down-fired combustor is 300 mm in inner diameter and 3500 mm in height
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with a preheated fuel nozzle and the secondary air nozzles on the top. The preheated
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fuel nozzle is 36 mm in inner diameter and located at the center, and the secondary air
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nozzles are 7 mm in inner diameter and located on both sides of the preheated fuel
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nozzle. The schematic diagram of the nozzles is shown in Figure 2. Air-staged method
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is used in the research to reduce NOX emission. The tertiary air is injected into the
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down-fired combustor at a position of 500 mm, 1000 mm and 1500 mm below the
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secondary air nozzle, respectively. To observe the flame, six looking-fire-windows are
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set, and located at the position of 80 mm, 250 mm, 600 mm, 900 mm, 1200 mm and
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1600 mm below the preheated fuel nozzle. The top two are 15 mm×150 mm in square
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shape, and the others are round with a diameter of 63 mm. Compressed air is used to
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sweep the fouling on the quartz glass. Since the fouling speed is so slow that
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compressed air is not frequently used, and the flow is small enough, the effect of
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compressed air to flame is not taken into consideration.
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Energy & Fuels
Preheated fuel nozzle Secondary air nozzle
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Figure 2. The nozzle structure on the top of down-fired combustor
2.2 Sampling point.
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Five K-type thermocouples are set in the preheating burner at the position of 100
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mm, 500 mm and 1450 mm from the bottom of riser, one is at the back-feeder and one
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is at the outlet of the preheating burner; Ten S-type thermocouple are set at the
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position of 100 mm, 250 mm, 400 mm, 600 mm, 800 mm, 1200 mm, 1600 mm, 2100
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mm, 2600 mm and 3200 mm down from the preheated fuel nozzle along the
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centerline of the down-fired combustor. In Saha’s research16, corrected combustion
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temperature was calculated considering the thermal radiation on temperature
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measurement and the results showed that the maximum radiation correction is 4.5%
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and most correction is
