Thin Solid Films 407(2002) 186–191

0040-6090/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž02.00035-4

Stabilization of pulverized coal combustion by plasma assist

Masaya Sugimoto *, Kaoru Maruta , Koichi Takeda , Oleg P. Solonenko , Masao Sakashita ,a, a a b c

Masakazu Nakamurac

Faculty of Systems Science and Technology, Akita Prefectural University, 84-4 Tsuchiya-Ebinokuchi, Honjo, Akita, 015-0055, Japana

Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences, 4y1, Institutskaya av.,b

Novosibirsk 630090, RussiaJapan Technical Information, Service, 1-6 Kojimachi, Chiyoda, Tokyo 102-0083, Japanc

Abstract

Ignition and stabilization of pulverized coal combustion by plasma assist is investigated with a 10 kW plasma torch for threedifferent kinds of coal, such as high, medium and low volatile matter coals. Not only high volatile matter coal but also lowquality coal can be successfully burned with plasma assist. Research for volatile component of coal shows that a highertemperature field is necessary to extract the volatile matter from inferior coal, while their compositions are almost the same.�2002 Elsevier Science B.V. All rights reserved.

Keywords: Pulverized coal combustion; Plasma torch, Fourier transform infrared spectroscopy

1. Introduction

Coal is an indispensable fossil fuel for electric powergeneration. There are several types of coal combustionmethods and the appropriate one is selected for purposeor economical reasons. Pulverized coal combustion isan efficient coal burning method.Coal burning process starts by the extraction of

volatile components contained in coal particles. As thecoal particles are heated, they gradually emit the volatilecomponent to the surroundings and ignition occurs whenthe temperature becomes high enough for mixture of thevolatile component and air. Once ignition starts, flameaccelerates the release of the volatile component of thecoal particles, and they are heated much more quickly.Finally the fixed carbon component begins to burn. Inorder to improve the process by encouraging the heattransfer and volatile component emission, coal is pul-verized so that the total surface area of coal is dramati-cally increased.In a conventional pulverized coal combustion boiler,

coal is fed by primary air and burned with secondaryair in the furnace. When the flame and the radiation

*Corresponding author. Tel.:q81-184-27-2117; fax:q81-184-27-2188.

E-mail address: sugimoto@akita-pu.ac.jp(M. Sugimoto).

from the furnace wall give the coal particle sufficientenergy, stable combustion is sustained in the boiler.However, when the energy for the coal particles heatingis not high enough, such as at the start or whenoverloading of the boiler occurs, heavy oil must bemixed as fuel to support stable combustion. Addition ofheavy oil is also indispensable in inferior coal combus-tion, which is used at many electric power stations inRussiaw1,2x. These are the reasons why a pulverizedcoal boiler must be equipped with a facility for heavyoil combustion. This makes a boiler construction com-plicated, which results in higher extra cost.In order to eliminate additional heavy oil facilities

from a pulverized coal boiler, we are developing a newtype of pulverized coal burner with a plasma torch. Byusing this burner, a boiler can be operated without heavyoil addition in any operating condition. In this burner,pulverized coal is carried to the nozzle exit of the torchby nitrogen gas flow and injected into the plasma jet.The mixture of volatile component and residual coalparticles flows downstream of the plasma jet, wheresufficient air for the complete combustion is supplied.Because of the higher energy density of plasma com-pared with the fossil fuel flame, some of the preferableeffects described below are expectedw1,2x. For example,there is a possibility that some coal particles are crushedinto pieces by heat shock and volatile component emis-

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Table 1Mass fraction of chemical compounds in coal

Characteristics High volatile Medium volatile Low volatilematter coal matter coal matter coal

Moisture content(mass%) 6.8 2.5 2.7Ash content(mass%, dry) 13.1 15.0 14.9Volatile content(mass%, dry) 40.6 27.0 10.5Fixed carbon content(mass%, dry) 46.3 58.0 74.6Total energy(Jyg) 27 500 28 600 30 200

Elemental compositionCarbon(mass%, dry) 69.2 71.7 77.9Hydrogen(mass%, dry) 5.3 4.3 3.4Total sulfur(mass%, dry) 0.6 0.4 0.4Nitrogen(mass%, dry) 1.1 1.2 0.8Oxygen(mass%, dry) 10.8 7.5 2.8

Fig. 1. Particle size distributions of pulverized coals,(a) high volatile,(b) medium volatile and(c) low volatile matter coal.

sion becomes easier. Active ions or radicals in theplasma may promote the fuel combustion. And if theplasma gas is chosen appropriately, such as NH , for3

example, NO and NO precursors react with radicals inx x

the plasma jet, that probably leads to decrease of NOx

emissionw3x.In the present study, ignition and stabilization of coal

combustion with 10 kW plasma torch is demonstrated.Three different kinds of pulverized coal, whose majordifference is volatile content, are used. We also investi-gate volatile component of coal with a thermogravime-tric analyzer and a Fourier transform infraredspectrometer.

2. Experimental

2.1. Analysis of coal property

One of the most important feature of coal for itsignition and stable combustion is the mass ratio ofvolatile matter to dry total mass of coal. Generally,better coal includes much volatile matter and showsgood combustion characteristics. In this research, threedifferent kinds of coal are prepared, as shown in Table1.

All of these coals are pulverized and classified by asieve so that the maximum particle size is not largerthan 100mm. Fig. 1 shows the distribution of coalparticle diameter for these coals.The compositions of volatile component are investi-

gated for each kind of coal. The samples of approxi-mately 14 mg coals are put in a crystal vessel andheated by a thermogravimetric analysis device(Shimad-zu TGA-51). The samples are heated in a nitrogenatmosphere. The volatile component from the heatedsamples are exhausted from the heater through a KBr-glass gas cell, which are set in the sample room of theFourier transform infrared spectrometer(JASCO FT-IR660). The pass length of the gas cell is 10 cm andthe resolution of the obtained spectrum is 4 cm .y1

2.2. Experimental set-up for combustion

The prepared pulverized coal is burned in a combus-tion chamber with a plasma jet. Fig. 2a shows theschematic of the experimental set-up. The dimensionsof the cylindrical combustion chamber are 100 mm ininner diameter and 1900 mm in length. The chambercan be divided into three parts: the refractory part,water-cooling part and steel tube part. In the 950-mm-

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Fig. 2. Schematic of experimental apparatus,(a) experimental set-up and(b) plasma torch and combustion air supply flange.

Fig. 3. Relation between temperature and mass fraction of the samplesduring the heating. The mass of samples are normalized by the initialvalue before heating.

long refractory part, a zirconium tube, inner diameterand thickness 100 mm and 10 mm, respectively, isplaced at the center of the chamber. Zirconium bricksoccupy the room between the zirconium tube and theouter wall of the chamber. The inside of the chamber isobserved through the ports with windows. Some tem-perature probes are inserted from the rest of the meas-urement ports.Exhausted gas is sampled from the port in the steel

tube part, which is 1850 mm away from the top of thechamber. The fraction of O , CO and CO in the flue2 2

gas are monitored to evaluate the status of combustionof coal. The O volume fraction is measured by a2

magnetic oxygen analyzer. Non-dispersive infraredabsorption method is used for the measurements of theCO and CO volume fractions.2

Fig. 2b shows a plasma torch and a stainless-steelflange for combustion air supply set on the top of thecombustion chamber. Pure nitrogen or air is used asplasma gas, whose flow rates are 14.4 slm(standardliter per minute) for nitrogen and 19.2 slm for air. Theexit diameter of plasma nozzle is chosen as 4 mm.Pulverized coal is transported by 16.2 slm nitrogen flowand injected through two orifices in perpendicular to theplasma flow at 5 mm from the exit of the nozzle.Supplied coal flow rate is kept between 30 gymin and50 gymin. Combustion air is injected from an angle of458 to the axis through the ring nozzle of the flange.The inner diameter of this section is 40 mm. Thedistance between the coal injection point and the ringnozzle for combustion air supply is 75 mm. Stoichio-

metric flow rates of air for combustion are 7.17 lyg,7.35 lyg and 7.54 lyg for high, medium and low volatilematter coal, respectively.

3. Experimental results and discussion

Fig. 3 shows the relation between temperature andmass of the samples during the heating. The heatingrate is 208Cymin. The mass of the samples are nor-malized by the ones before heating. The decreasing ratesof the mass increase suddenly at approximately 4008Cwith high volatile coal, and a similar phenomenon is

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Fig. 4. FT-IR spectra of volatile components obtained with high vol-atile matter coal at 400, 600 and 8008C.

Fig. 5. FT-IR spectra of volatile components obtained with mediumvolatile matter coal at 400, 600 and 8008C.

observed at higher temperature at the lower volatilecomponent ratio.The results of FT-IR spectroscopy analysis of the

emitted gas in various temperature and coal are revealedin Figs. 4–6, those are the cases with high, medium andlow volatile matter coal, respectively. Absorption exceptCO and H O appears after the increase of the mass2 2

decreasing rate occurs and therefore indicates that thiscorresponds to the release of the volatile componentstarts. Time evolution of FT-IR spectrum is shown inFig. 7, which is obtained with high volatile matter coalat 6008C. Strong absorption by the molecules that havebonds of C–H and CN, are observed in addition to themolecule of CO and H O.2 2

The analysis of volatile component shows that thereis a difference in the emission pattern of volatile com-position. From the results presented in Figs. 4 and 7,the C–H bond component is released at comparativelylow temperatures, while the CN component emissioncontinues constantly. However, the wavenumber ofstrong absorption are independent on the coal, whichimplies that there is little difference in the compositionof the volatile component among these three kinds ofcoal.

The results also show that higher temperature isnecessary to extract the volatile component from lowvolatile coal. In addition to the fact that it contains lessvolatile matter, this is possibly important reason why itis difficult for inferior coal to be used in a boiler, despitethe total energy is almost equal to that of superior coal.This also indicates the advantage of using the presentburner with a plasma torch for low volatile matterpulverized coal.The results of ignition experiments under various

conditions are shown in Fig. 8. Nitrogen plasma is usedfor the high volatile matter coal. In the shadowed areain Fig. 8, it is impossible to ignite and burn a pulverizedcoal. This result shows that there is an optimal ratio ofair and fuel for the ignition of pulverized coal.Fig. 9 shows the optimal condition for ignition of

each kind of coal. Less input power is required for theignition with air plasma, and the combustion is morestable compared with the cases with nitrogen plasma.The advantage of using the present burner with plasma

for pulverized coal combustion is also shown in Fig. 9.Even in the case of low volatile matter coal, the ignitionis attained with the input power of only 10% of totalcoal energy. Because of the properties of plasma, suchas high temperature and high energy density, the volatile

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Fig. 6. FT-IR spectra of volatile components obtained with low vol-atile matter coal at 400, 600 and 8008C.

Fig. 7. Time evolution of FT-IR spectrum obtained with high volatilematter coal at 6008C. These spectra are observed at 10, 20 and 27min after the temperature reaches 6008C.

Fig. 8. Ignition condition of high volatile matter coal with nitrogenplasma, whereQ , Q , Air , andAir standplasma supplied coal supplied stoichiometry

for plasma input power, total energy of supplied coal, supplied airflow rate and stoichiometric air flow rate for the supplied coal,respectively.

contents of coals are blown out of the particles veryquickly and sufficiently before they meet to the second-ary air, and thus they can be ignited with low powerinput. Since the combustion chamber used in this exper-iment is cylindrical, it is not so efficient for the mixingof fuel and air. Therefore, there is a possibility toimprove the efficiency of the input power for ignitionand combustion with the optimization of a combustionchamber.Air plasma needs less input power for the ignition

and the coal combustion is more stable for all types ofcoals. The power from the coal combustion with air ofplasma gas is estimated approximately 1 kW at themaximum, which is comparable to 30% of minimuminput power for ignition. This result indicates thatcombustion effect with coal and plasma gas contributesto the improvement of coal combustion.

4. Summary

Ignition and stabilization of pulverized coal combus-tion by a newly developed burner with plasma assist isinvestigated with three different kinds of coal and 10kW plasma torch. Analysis of the volatile componentleads to the result that inferior coal that has less volatile

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Fig. 9. Optimal condition for ignition of coal.

matter requires higher temperature for the volatile emis-sion, although the composition of volatile matter seemsto be independent on the coal type. This result impliesthat high temperature field and high energy density ofplasma are appropriate for encouraging the inferior coal

combustion. The pulverized coal combustion experimentwith plasma jet shows that it can be ignited with inputpower of only 10% total coal energy even in the caseof low volatile matter coal. All of the results mean thatpulverized coal combustion with the current burnerplasma assist is a promising method for wide qualityranges of coals.

Acknowledgments

This work was supported by the New Energy andIndustrial Technology Development Organization(NEDO) in the framework of ‘the Proposal-based Inter-national Joint Research Program in 2000’.

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