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Chemical Engineering and Processing 49 (2010) 1017–1024

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

journa l homepage: www.e lsev ier .com/ locate /cep

new fluidization–suspension combustion technology for coal water slurry

ui Wanga,∗, Xiumin Jiangb, Minxiao Zhanga, Yufeng Mac, Hui Liua, Shaohua Wua

School of Energy Science and Engineering, Harbin Institute of Technology, No. 92, West Da-Zhi Street, Harbin, Heilongjiang 150001, ChinaSchool of Mechanical Engineering, Shanghai Jiao Tong University, Minhang District, Shanghai 200240, ChinaShengli Power Plant, Shandong, Dongying 257087, China

r t i c l e i n f o

rticle history:eceived 23 March 2009eceived in revised form 9 June 2010ccepted 14 July 2010vailable online 21 July 2010

a b s t r a c t

Slagging is a major operating problem in application of the atomization–suspension combustion tech-nology for burning coal water slurry (CWS) fuel in small and low height industrial boilers. Thefluidization–suspension combustion is a new alternative for replacement of oil, which is capable ofsolving the slagging problems. In addition, it can be successfully applied to CWS-fired boilers withcapacity smaller than 35 t/h. About 530,000 medium and small scale industrial boilers with low boiler

eywords:luidization–suspensionWSombustion technologyoilererformance

efficiency in China provide the technology a very promising prospect. The principles and contents of CWSfluidization–suspension combustion technology are introduced in detail in this paper. And a new type of14 MW fluidization–suspension CWS-fired boiler was developed, the performance of which showed thatboiler efficiency was 91.53%. Emission of SO2 and NOx was 346.1 mg/m3 and 469.5 mg/m3, respectively.From the application, the CWS-fired boiler showed good features such as high efficiency, low pollu-tant emission, good load regulation, good CWS quality adaptability, steady operation and convenient

maintenance.

. Introduction

Coal water slurry (CWS) is a coal-based liquid fuel, which can besed as a substitute fuel in oil-fired boilers. Thus, the CWS technol-gy has a great prospect in China to reduce China’s oil consumption.hina has approximately 530,000 medium and small scale indus-rial boilers in operation, with the average capacity of 2.5 t/h andfficiency of only about 65%. Besides the low efficiency, the emis-ions such as particulates, SO2 and NOx from the small scale boilersre high. A considerable economical benefit and better environ-ental performance can be achieved if these boilers are retrofitted

y efficient and clean coal technologies.The combustion technologies of CWS [1–4] have been studied

xtensively. The results reveal that the atomization–suspensionombustion [5,6] has been successfully applied in large-scale boil-rs, in spite of the fact that it has a better economical performanceo burn pulverized coal directly. But for the industrial boilers withapacities smaller than 35 t/h, most of the previous research wasocused on the atomization combustion and seldom achieved high

nough efficiency. It was reported that a 2.8 MW grate boiler [7]chieved an efficiency of 80% after retrofitted for burning CWS byre-combustor and atomization combustion. A 4 t/h grate boiler8] was retrofitted for firing fine-coal water slurry by multi-stage

∗ Corresponding author. Tel.: +86 451 86412318; fax: +86 451 86412528.E-mail address: wanghui hb@163.com (H. Wang).

255-2701/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rioi:10.1016/j.cep.2010.07.009

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

atomization for CWS feeding. The testing results showed that themaximum furnace temperatures were from 1400 ◦C to 1500 ◦C, andthe boiler efficiency can be as high as 81%. A new developed 4.2 MWD-type boiler [9] for burning CWS with a colliding-type atomiza-tion nozzle achieved boiler efficiency of 88% and slagging occurredin the bottom of the furnace. A retrofitted 7 MW D-type boiler [10]to burn CWS equipped with a versatile burner which can fire bothCWS and oil by atomization in a pre-combustor gained combus-tion efficiency over 96% and boiler efficiency of about 83%. A 10 t/hdemonstrative fluidized bed boiler [11] with overbed feeding inspecific shape, non-overflow and non-drainage achieved an aver-age combustion efficiency of 95.4% and a boiler efficiency of 77.7%.From all those applications it may be concluded that the CWS fuelburned in small and low height industrial boilers did not get enoughresidence time to completely burn out, which leads to a low boilerefficiency. Besides, the high temperature in the small furnaces ishard to control, which will lead to local slagging inevitably andrisk the operation stability. That is because the slagging problemis a dilemma for the atomization–suspension technology. Slag-ging happens or not depends on combustion temperature, nozzleaerodynamics and furnace structure. To make the boiler operatestable and reliable, about 1500–1600 ◦C in the small furnace has

to be assured. But the temperature is above the melting point ofcoal ash, which makes slagging happens easily. In addition, theauxiliary equipments such as atomization, slag-removal made thesystem complicated and had potential risk of the system. From theoperation data of the CWS-fired atomization–suspension boilers

ghts reserved.

1018 H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024

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etrofitted from the oil-fired ones, only 75% of the design capac-ty pre-retrofitting can be obtained due to the limitation of furnaceutlet temperature. Some small scale boilers fail to burn CWS fuely atomization technology due to the small furnaces [12,13].

Furthermore, some detailed researches were conducted on thetomization combustion. Manfred et al. [14] reviewed the devel-pment of CWS as an alternative boiler fuel through a series testsn 10–20 MW boilers using atomization technology. Miccio et al.15] carried out an experimental study on the influence of theype of CWS parent fuel and the velocity of injecting air. Rankint al. [16] described the demonstration work of burning CWS in aoiler designed to burn oil. The Coal Research Establishment of Eng-

and, Australian CSIRO and Japanese Sumitomo Company carriedut combustion tests in fluidized bed boiler to burn CWS [7]. The0 t/h fluidized bed boiler of the Sumitomo Company started run-ing in 1980 and obtained boiler efficiency of about 65% [7]. Thosearly tests in small coal-capable front wall and tangentially firedtility boilers showed that two of major problems to be addressedre atomizer durability and poor carbon conversion.

As a new type CWS combustion technology, fluidization–uspension is clean and efficient for burning CWS, solving thelagging problem and simplifying the system. In this paper, mainrinciples and design concept of the technology are introduced inetail. Results of the utility tests in a 14 MW boiler are presented.

. Fluidization–suspension combustion technology of CWS

.1. Principles of CWS fluidization–suspension combustionechnology

Fluidization–suspension combustion technology of CWS is aew type technology combining both fluidized bed combustion anduspension combustion. The flow chart [17] of a boiler system usinghe technology is shown in Fig. 1.

The system includes CWS storage and feeding system,gnition system and dust separating system. Comparing withWS atomization–suspension combustion technology, the air-tomization (or steam-atomization), CWS filtering, high-pressureeeding and slag-removal are not needed, so that the total systems simplified and the reliability gets improved.

The principles are as follows: high density solid particles areelected to be the bed material. The CWS fuel is delivered to granu-ating device located on the top (or on the front wall) of the furnacend is fed at about 0.4 MPa onto the bed surface with tempera-ures of 850–950 ◦C. The CWS droplets experience drying stagend the volatile matters release and burn [18]. Under the statef fluidization, CWS particles will either be fragmented or formgglomerates [19] in the dense bed. The small particles will be car-ied upward to the dilute section to finish suspension combustion.

he flue gas enters a separating device inside the furnace and theeparated fines reenter the bed and burnout. The bed material andarge CWS agglomerates carried with the flue gas will be separatednd returned to dense section via its feedback channel, which canecrease the loss of bed material, enhance the complete combus-

Fig. 2. Scheme of CWS granulating device.

tion of CWS agglomerates, and intensify the mixing and burnoutperformance. At the same time, thermal NOx production is effec-tively controlled and slagging in the furnace is avoided because ofthe low combustion temperature (850–950 ◦C). The limestone canbe fed with the bed material or mixed with CWS fuel to react withSO2 and reduce the emission. The temperature in the separatingdevice happens to be the optimal temperature of desulfurization,so the objects of high efficiency and low pollution can be simulta-neously realized.

The technology is characterized by following features: (1) It usesa granulating device to feed the CWS in the shape of drops withdiameter of 4–10 mm, and eliminates the need of atomization andfiltering systems in atomization–suspension technology. (2) It usesan internal circulating combustion gas–solid separator set insidefurnace to separate the large particles and agglomerates leading toa high combustion efficiency. (3) It can solve the slagging problemsin atomization–suspension technology.

2.2. Components in fluidization–suspension combustiontechnology

2.2.1. CWS granulation-feeding systemBecause of better combustion conditions for CWS in fluidiza-

tion–suspension combustion boiler, diameter of CWS drops couldbe a little bigger without the risk of decreasing the combustion per-formance. Thus, the expensive high-pressure atomization devicesin the atomization–suspension technology, which are easy to block[20], are not necessary. To guarantee better fluidization combus-tion characteristics, the particles formed by CWS drops must meeta specific size distribution. So feeding CWS fuel steadily and reliablyinto furnace are critical in the fluidization–suspension combustiontechnology.

The granulating device is developed to meet the demands, whichis shown in Fig. 2 and composed of cooling air tube, flame detector,CWS inlet tube, dredge hole with seal bolt, sweeping and washingtube, and CWS granulating tube. The sweeping and washing tube(connected with water or compressed air) is used to clean the gran-ulating tube when stop feeding. The dredge hole on the top of thegranulating tube is used to clear blocks that cannot be swept orwashed, which will be sealed when operating. And the granulatingtube is offset placed in the cooling air tube (or a cooling air box) tomake the air control CWS flow and avoid solidifying and blocking,since the offset assembled structure can form velocity difference ofcooling air in the eccentric annulus, which benefits for granulatingthe CWS flow into acceptable size distribution. The flame detector

is mounted to observe the state of combustion and CWS feedingwhen operating.

The features of the device are as follows: (1) The simple struc-ture with no moving parts can make it work reliably and easy to

ing and Processing 49 (2010) 1017–1024 1019

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Table 1Design parameters of coal water slurry boiler.

No. Name Units Value

1 Rated output MW 142 Circulating water flux kg/h 266,6673 Inlet water temperature ◦C 704 Outlet water temperature ◦C 1155 Discharge flue temperature ◦C 1506 Boiler efficiency % 897 Fuel consumption kg/h 30558 Boiler heating surface m2 652.8849 Air flux of induced fan m3/h 58,000

10 Air pressure of induced fan Pa 330011 Air flux of blower m3/h 24,000

H. Wang et al. / Chemical Engineer

aintain. (2) The CWS feeding stability and uniformity withoutlocking can be guaranteed, and the landing position and flow fluxf CWS could be controlled easily. (3) The low air consumption andir pressure requires no special air source. (4) The formed 4–10 mmWS particles meet the demands of the fluidization–suspensionombustion, in particular the relatively big drop diameter lowersequirements [21] for CWS and need no filtering equipments. (5) Itas good flexibility to different fuel types and can be used to feedany kinds of slurry fuels.

.2.2. Technology of continuously running without slag-removalIn the conventional fluidized beds, keeping a relatively steady

ed material height during the operation is very important. For thexed air flow, an excessive thick bed material layer will make flu-

dization difficult, maybe even worse to form stack and slag, andn extra thin layer will add possibility of the air short-passinghe layer and the temperature falling, even lead to extinguish. Sohe frequent slag-removal is necessary in order to keep a steadyed material layer. But for fluidization–suspension combustion ofWS, it leaves little ash in the bed and the slagging is avoided,herefore it needs no slag-removal in the fluidization–suspensionombustion process of CWS. In addition, the dewatered and driedWS spheres will float and burn on the upper bed material layerecause of their lower density comparing with quartz sand. In short,he merits of technology of continuously running without slag-emoval are as follows: (1) Losses of incomplete combustion andlag-removal are decreased. (2) The make up requirement of bedaterial is decreased. In the technology, the ash left in the fur-

ace is of small amount and cannot make up for the bed materialoss, so the bed material has to be made up through adding quartzand. Accordingly, some man-power or feeding device is needed.

ithout slag-removal, the bed material loss discharged with slag isvoided, then the make up requirement is lowered, so the operatingost and complexity of combustion devices is decreased.

.2.3. Internal circulating combustion technology in the furnaceBecause of the wide size distribution of bed material, some fine

articles will be carried out from the furnace with the flue gas. Ifothing is done, small particles will be carried to convective heatransfer surfaces. This may: (1) increase the mechanical incompleteombustion loss because some fine fuel particles are carried outogether, (2) increase the complement quantity of bed material, (3)ncrease the particle concentration in the flue and speed up thettrition of convective heating surfaces.

For these reasons, an internal circulating combustion gas–solideparator is set inside the furnace of the CWS fluidization–uspension combustion boiler. The big particles in the flue will beeparated and returned to the furnace to burn again, and in thenternal separating device the combustion happens, too. In addi-ion, the overall boiler volume changes little as adding separator inhe furnace, so it can be used to retrofit some oil-fired boilers.

The device named high temperature internal circulating com-ined vortex separator [22] is shown in Fig. 3. It is horizontal andomposed of several groups of vortex separating cells (two cellsorm one group). For each group, the first cell is a coarse powdereparator, which has one flue entrance and two feedback outlets:he flue entrance is up on one side, one feedback outlet is on theenter of the opposite side and the other one on the center of theottom. The second cell has one flue outlet and a feedback out-

et: the flue outlet is up on the opposite side of the flue entrance,he feedback outlet is on the center of the bottom wall. In the

roup between the first and the second cells, a central passage isormed by an annular baffle plate, which is a plate ring of specificidth fixed on the inner circle. The vortex separator is placed at

he upper part of the dilute section and connected to the furnaceutlet. The flue carrying ash and some bed material particles goes

12 Air pressure of blower Pa 14,00013 Flux of circulating pump m3/h 63014 Head of circulating pump m 50

upward to dilute section from the dense section, and tangentiallyenter the first cell of the vortex separator at a specific velocity andform a high-speed rotating vortex flow. The solid particles in theflue will be concentrated to the wall and trapped by the feedbackoutlets under the effect of centrifugal force, and the high temper-ature flue goes into the second cell via the hollow baffle plate tocontinue its high-speed vortex flow and separate solid particlesfurther. The separated solid particles trapped by the feedback out-lets are returned to dense section to finish circulating combustionand make up for the loss of bed materials. The vortex separatorcan obtain a separating efficiency of over 90% for particles downto 0.25 mm and has many merits such as low resistance pressure,attrition resistance, small volume, simple structure and easy instal-lation, low requirements of manufacture, big flow flux for flueseparating and perfect separating performance, easy to be enlargedand combined according to the capacity of boiler, etc.

2.2.4. High efficiency and low cost technology of desulfurizationand reduction of NOx

The processes of desulfurization and reduction of NOx can beachieved at low cost through directly adding sorbents such aslimestone into furnace or mix them into CWS fuel and staged-airarrangement [23], which is a general feature of CFB, too. For thefluidization–suspension technology, the internal circulating com-bustion technology can significantly increase the residence time ofthe desulfurization agent in the furnace and improve its utilizationratio and reduce the operating cost.

3. The 14 MW vertical CWS fluidization–suspension boiler

3.1. Design data of boiler

The main design parameters are shown in Table 1.

3.2. Design fuel

The design fuel is CWS made of Datong bituminous coal. Thedetailed analytical data of design CWS and CWS in tests are givenin Table 2 and size distribution of coal powder in it is shown inFig. 4.

3.3. Design principles

3.3.1. Selection of bed material

The selection of density and size distribution of inert bed

material will directly affect the fluidization quality of CWSfluidization–suspension combustion boiler, further the character-istics of combustion and heat transfer. Considering the features ofdifferent density fluidized bed and power consumption of fans, the

1020 H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024

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uartz sand with density of 2300 kg/m3 [24,25] is selected. The sizeistribution of fresh bed material is shown in Table 3 and the massverage diameter is 2.52 mm.

.3.2. Selection of fluidization air velocity and cross-section heatoad

Based on the density and size distribution of the bed mate-ial and parameters of air, the critical fluidization velocity cane calculated using empirical equation. The actual fluidizationelocity used in operating boiler is 6.47 m/s. From the features ofWS fluidization–suspension combustion technology, the designedross-section heat load is 1106.53 kW/m2 and the volume heat loads 237.96 kW/m3.

.3.3. Heat balance and combustion processCombustion process of CWS drops and coal is quite different

ecause CWS needs big latent heat of vaporization and carriesut agglomeration combustion. The latent heat of vaporizationccounts for more than 47% total ignition heat of the CWS fuel. Andhe evaporation time is about 0.05–0.1% total residence time in

he dense section [26]. The porosity formed by evaporation is 0.59hen the mass fraction of coal in CWS is 50% [2]. When the fed CWS

annot mix with bed material in time, big agglomerates are possi-le to be formed. Sometimes the burned chars will stick together,oo. By different-density combustion technology, the agglomerates

Fig. 4. Size distribution of coal powder in CWS.

able 2roximate and ultimate analyses of design CWS and CWS in tests.

Proximate analysis

Mar (%) Aar (%) Var (%) Qar,net (kJ/kg)

Design CWS 32.9 5.64 30.96 18,877CWS in tests 35.40 7.08 36.94 17,700

e combined vortex separator.

are kept floating and burning on the upper layer in the bed. Theagglomerates will break into fragments under the effect of bedmaterial. There were not any clusters found in the dilute sectionthus far. The CWS fuel can still obtain good combustion efficiencyso long as the agglomerate diameter does not exceed 25 mm [27].After the evaporation finishes, the volatile will start to release andburn immediately to form a flame surrounding the CWS spheresbecause of the high temperature in the bed material. The combus-tion processes of volatile and carbon have some overlap. For a singleCWS particle, when the combustion of volatile has finished 90%, thecarbon has already burned about 10% [26]. When the evaporationand volatile releasing nearly ends, the carbon will form a spherewith certain strength. From the data of a strong coking fine-coalslurry [26], the compressive strength of a 6 mm sphere is about2 MPa and the strength of the same size sphere of another weakcoking coal slurry is about 0.5 MPa. For the core of the CWS sphere,the strength changes little until it is nearly burned out. The burningbehavior of CWS spheres in the fluidized bed is a layer-by-layer,inward flaking-off combustion [19]. With the fluidization of bedmaterial and CWS fuel, relatively small particles formed by frag-mentation or attrition and small agglomerates are carried into thesuspension space to continue combustion. At the same time, theincomplete burned volatile matters and combustible componentof particles separated from the combined vortex separator will goup to burn in the suspension space from the dense section, too.

3.3.4. Selection and control of working temperature in the furnaceThe furnace temperature is an important parameter in boiler

designing. It has extremely important influence on the processesof steady and high efficiency combustion, slagging avoidance,desulfurization and removal of NOx. The operating temperature isdetermined to be 850–950 ◦C by comprehensive analysis of thoseinfluences.

3.3.5. Determination of circulating ratio and separating efficiencyCirculating ratio is of great importance for the circulating

fluidization mode, which affects combustion, heat transfer andattrition of boiler and fans power a lot. For a CWS fluidization–suspension combustion boiler with the temperature of dense sec-tion is 850 ◦C, a relatively low circulating ratio is selected to

maximize the combustion efficiency and minimize the power con-sumption. The circulating ratio is mainly achieved by the regulationof separator, so the separator is designed carefully to assure aseparating efficiency as high as possible, primarily the operatingstability and security are guaranteed.

Ultimate analysis

Car (%) Har (%) Oar (%) Nar (%) Sar (%)

50.57 3.27 6.13 0.93 0.5647.43 2.93 6.02 0.80 0.34

H. Wang et al. / Chemical Engineering an

Table 3Size distribution of bed material.

No. Aperture ofscreen (mm)

Averagediameter ofparticle (mm)

Percentage onmass basis (%)

1 0.71–1 0.855 0.152 1–2 1.5 12.2

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3 2–3 2.5 73.24 3–4 3.5 14.35 4–5 4.5 0.15

.3.6. Preventive means of internal attrition in the furnaceIn circulating fluidized bed boiler, the attrition to internal heat-

ng surfaces is a very important problem for the high concentrationf fuel or bed material. Measures are taken to consider designarameters, material and local strengthening comprehensively.ttrition-preventive fins and bricks or casting materials were used

n the dense section where the flue velocity was very high. Becausehe attrition of convective heating surface is proportional to thearticle concentration in the flue and proportional to 3.6 timesf the flue velocity, the flue velocity turns to be more critical,o a relatively low velocity is selected in the design. In addition,he upward flowing flue is opposite to the gravity of particles,hich will reduce the influence of the particles and benefit for the

ttrition prevention. To make the security better, uniform distri-ution plate and attrition-preventive capping plate are used in theue segregation area. An air cushion backflow area covered withttrition-resistance casting material is set up in turning chamber ofpper furnace to decrease the collision momentum of particles. And

n the horizontal internal gas–solid separator, inner lining madef attrition-resistance material is coated to increase resistance ofttrition.

.4. Whole structure of the 14 MW CWS fluidization–suspensionoiler

A 14 MW CWS fluidization–suspension boiler is used for com-unity heating in winter, whose general arrangement diagram is

hown in Fig. 5. And its features are as follows:The vertical boiler has two horizontal drums and its water circu-

ation is combined by controlled circulation and natural circulation.he heat load of the boiler is 14 MW and the pressure of outlet waters 1.0 MPa (gauge pressure). The temperatures of outlet water, inlet

ater and cold air are respectively 115 ◦C, 70 ◦C and 20 ◦C. Theesign fuel is CWS.

The outside dimensions of the boiler are 7860 mm ×490 mm × 9660 mm (height × width × depth). The furnace isivided into dense and dilute sections. The height of dense section

s 1350 mm with a trapezoid air distributor at the bottom. Someuried heating surface composed of Ф51 mm× 5 mm tubes withttrition-preventive rings are installed in the dense section. Oneroup of the internal high temperature combined vortex separators installed at the outlet of furnace to achieve the circulatingombustion. All the air needed enters the furnace from two ways:ne as primary air goes via funnel caps to bottom furnace toeep the bed material fluidizing; and the other one as secondaryir goes via ejectors installed at the top of the dense section,ntering the dilute section to reinforce burning completely andeduce the releasing of NOx. The CWS fuel is fed from front wall inrop shape into the dense section by the granulating device. Therops are heated by the fluidizing bed material and burn rapidly,

hen the hot flue carrying some bed material and fragments orgglomerates goes into the separator via flue outlet window. Theig particles trapped are fed back to dense section again to achievehigh efficiency circulating combustion. The separated flue goes

hrough the anti-slag tubes in the turning chamber and passes the

d Processing 49 (2010) 1017–1024 1021

convective tube groups located between the upper and bottomdrums, then via the ash separator to the stack.

From the heat calculation of the 14 MW fluidization–suspensionboiler, the outlet temperature of the furnace is 950 ◦C, then behindthe anti-slag tubes is 869 ◦C, then 209.5 ◦C behind the convectiontubes, and the exhaust gas temperature is 150 ◦C.

In order to make the boiler structure compact, decrease the radi-ation heat loss and increase the tightness, the boiler wall is selectedto be light-duty type, i.e. from the inner wall, it is made in sequenceof firebrick, plates of aluminum silicate fiber and berg meal bricks. Asteel layer is used to cover the bricks from outer at last. Some mate-rial feeding inlets, manhole doors, peep holes and measuring pointsof temperature are set up in the front wall. An oil gun is installedin the air box under the distributor plate to achieve ignition underbed when startup.

To ensure the safe and economical operation of the boiler, somethermal measuring instruments and automatic controlling devicesare equipped to remote control the forced and induce fans and mon-itor parameters such as feeding water pressure, air box pressure, airbox temperature, sub-atmospheric pressure, fluidized bed temper-ature, exhaust flue temperature, feeding water temperature, flowflux of feeding water, feeding quantity of CWS, outlet water tem-perature and oxygen concentration in the flue, etc.

4. Results and discussion

In order to investigate the overall performance of the boiler, andanalyze its technical and economic characteristics, experimentsunder the rated condition are carried out. Measurements includeboiler capacity, boiler efficiency using input–output method andheat loss method, and the releasing characteristics of the flue,etc. The criterion used are standards of The People’s Repub-lic of China: (1)“Test code for industrial boiler” (GB10180-94),(2)“Monitoring and testing method for energy saving of industrialboilers” (GB/T15317-94), (3)“General specification for industrialboilers” (ZB/T10099-1999), (4)“Measurement method of smokeand dust emission from boilers” (GB5468-91) and (5)“The determi-nation of particulates and sampling methods of gaseous pollutantsfrom exhaust gas of stationary source” (GB/T16157-96), etc. Thetest results show that every technical and economic indicatorreaches the expected design requirements and national standards,and the boiler efficiency has reached the international advancedlevel.

4.1. Efficiency of the CWS fluidization–suspension combustionboiler

The objective of tests is to evaluate the overall heat engineeringperformance of CWS fluidization–suspension combustion boiler.Two tests under rated condition were carried out. The instrumentsused are shown in Table 4, the fuel used in the tests can be seenin Table 2, the data of heat engineering performance and the boilerefficiency based on input–output method and heat loss method areshown in Tables 5–7, respectively.

From the tables, the main characteristics can be seen:

(1) Characteristics of boiler capacity. The required boiler capacityis realized and a relatively high boiler efficiency of 91.53% isachieved. The improvement of boiler efficiency is significant

[17] comparing with the conventional industrial boilers [8–11],even with boilers retrofitted with atomization–suspensiontechnology [7], which is about 80%. The CWS granulating deviceis reliable and the operation of the boiler is steady without anyslag formed in the furnace [17].

1022 H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024

Fig. 5. 14 MW vertical CWS fluidization–suspension combustion boiler.

Table 4Measuring instruments used for boiler tests.

Measuring items Instrument Specification Precision

Heat value of fuel Jingying automatic calorimeter SDACM3000 0.01%Fuel consumption Platform scale – 0.02 kgCarbon content in fly ash Muffle furnace – 0.1%Flux of circulating water Ultrasonic flowmeter FBL 0.2%Inlet water temperature Standard glass thermometer – 0.1 ◦COutlet water temperature Standard glass thermometer – 0.1 ◦CDischarge flue temperature Combustion efficiency analyser KM9101 1 ◦CAnalysis of flue contents Combustion efficiency analyser KM9101 1%Temperature of boiler surface Far infrared thermoscope IRT-1200 0.1 ◦C

Table 5Data obtained in boiler heat engineering performance test.

Test Capacity (MW) Heat efficiency of positivebalance (%)

Heat efficiency of inversebalance (%)

Discharge fluetemperature ( ◦C)

Excess air ratioat flue exit

Content of combustiblematters in fly ash (%)

(

TD

1 14.067 91.214 88.16982 14.231 91.835 88.5464Average capacity: 14.149 MW; average heat efficiency: 91.53%

2) Combustion characteristics of CWS. High utilization ratio ofCWS has been obtained in the fluidization–suspension com-bustion boiler. In the two experiments under rated condition,the combustible matter content in the ash are 3.32% and

3.33%, respectively, and the mechanical incomplete com-bustion loss are 0.4614% and 0.4528%, respectively. Theresults of so low the combustion losses verified not onlythe fluidization–suspension combustion in the furnace couldobtain high combustion efficiency, but also the internal high

able 6ata obtained in boiler efficiency test using input–output method.

No. Items

1 Circulating water rate of boiler2 Inlet water temperature of boiler3 Outlet water temperature of boiler4 Enthalpy of boiler inlet water5 Enthalpy of boiler outlet water6 Capacity of hot water boiler7 Fuel consumption rate8 Heat efficiency of positive balance

170.0 1.5417 3.32165.0 1.5307 3.33

temperature vortex separator could improve the combustionconditions of CWS and make it burn completely.

The CO concentration in the exhaust flue are 0.0010% and0.0011%, RO2 (i.e. CO2 and SO2) concentration are 11.6% and

11.7%, O2 concentration are 6.9% and 6.8%, and the chemi-cal incomplete combustion loss q3 are 0.0058% and 0.0063%,respectively. The low concentration of O2 and CO and highconcentration of RO2 in the exhaust flue shows gaseous com-bustible matters have obtained sufficient combustion because

Units Rated test 1 Rated test 2

kg/h 379,150 378,850◦C 70.1 71.4◦C 102.0 103.7kJ/kg 293.49 298.94kJ/kg 427.05 434.17MW 14.067 14.231kg/h 3136.64 3151.83% 91.214 91.835

H. Wang et al. / Chemical Engineering and Processing 49 (2010) 1017–1024 1023

Table 7Data obtained in boiler efficiency test using heat loss method.

No. Items Units Rated test 1 Rated test 2

1 Combustible content in fly ash % 3.32 3.332 Ash ratio of fly ash to inlet total ash (mass percentage) % 100 1003 Mechanical incomplete combustion heat loss % 0.4514 0.45284 RO2 in discharge flue % 11.6 11.75 O2 in discharge flue % 7.6 7.56 CO in discharge flue % 0.0010 0.00117 Excess air ratio at discharge flue – 1.5417 1.53078 Theoretical air volume N m3/kg 4.8038 4.80389 RO2 volume N m3/kg 0.8874 0.8874

10 Vapor volume in discharge flue N m3/kg 0.8834 0.882511 Dry flue volume in discharge flue N m3/kg 7.2910 7.238212 Discharge flue volume N m3/kg 8.1744 8.120713 Chemical incomplete combustion heat loss % 0.0058 0.006314 Cold air temperature ◦C 10 1015 Discharge flue temperature ◦C 170.0 165.016 Dry flue average specific heat at constant pressure in discharge flue kJ/(N m3 ◦C) 1.3314 1.331017 Enthalpy of discharge flue kJ/kg 1888.746 1820.75218 Enthalpy of cold air kJ/kg 97.715 97.01819 Heat loss of discharge flue % 10.073 9.6945

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(4) Blackness of flue. Because the concentration of dust and com-bustible matters is very low, the Ringelmen blackness of theflue is certainly low and the value is 0–1 level.

Table 8Comprehensive analysis table of air pollutant emission from boiler.

No. Items Units Value

1 Flue temperature ◦C 862 Dynamic pressure of flue Pa 803 Total pressure of flue Pa −20004 Flow velocity of flue m/s 13.35 Flux of flue m3/h 39,7806 Moisture of flue % 6.57 Excess air ratio – 2.238 Dust concentration before inducing mg/m3 99.69 (6% O2)9 Dust concentration after inducing mg/m3 123.5 (6% O2)

10 SO2 concentration mg/m3 346.1 (6% O2)11 NO concentration mg/m3 469.5 (6% O )

20 Dissipated heat loss21 Total heat loss22 Inverse balance efficiency of boiler

of their fully mixing with oxygen in the furnace of thefluidization–suspension boiler.

3) Heat loss of exhaust flue. In spite of the fact that high watercontent in CWS increases the quantity of exhaust flue, the boilercan still achieve good combustion and burnout performance ofthe CWS fuel under relatively low excess oxygen conditions inthe furnace. So the excess air ratio needed is relatively small,which means the heat loss of exhaust flue q2 is not very high.

The boiler efficiency is about 90%, which is relatively highbecause of the lower mechanical incomplete combustion lossand exhaust flue heat loss. Along with the better burningstability and slagging performance than that of atomization–suspension CWS boilers [17,28], the fluidization–suspensionCWS boiler was proved to be successful.

.2. Results and analysis of environmental performance of boiler

Through the test of environmental performance of boiler, theesign and operational quality of sections such as main boiler body,ust removal and desulfurization system are examined to obtainata for further improvement. The test is carried out only one time

n the boiler equipped with an ordinary TLS water-bath duster. Theesults are shown in Table 8 and the analysis is as follows:

1) SO2. From the “Emission standard of air pollutants for coal-burning oil-burning gas-fired boiler (GB13271-2001)”, thenational standards for SO2 emission is 1200 mg/m3 in I timeperiod and 900 mg/m3 in II time period. The results of test with-out adding desulfurizing agents in the desulfurization systemshow that the SO2 concentration in the flue is only 346.1 mg/m3,which is much lower than the national standards. The mainreason should be the coal used in CWS is a low-sulfur washedcoal and its ash is of weak alkaline which has self-desulfurizingeffect in the process of low temperature combustion. Accord-ingly, the SO2 emission of a 2 t/h CWS-fired fluidized bed boileris 518.21–647.79 mg/m3 [28], 730 mg/m3 for a 4 t/h boiler [29],352–598 mg/m3 for a 65 t/h boiler [30], 473–503 mg/m3 for a

130 t/h boiler [31], 941–1070 mg/m3 for a 220 t/h boiler [32],627.4 mg/m3 for a 230 t/h boiler [33].

2) NOx. The concentration of NOx in the flue is 469.5 mg/m3, whichis a typical result for the technology. The CWS combustionat low temperature guarantees low concentration of thermal

% 1.3 1.3% 11.8302 11.4536% 88.1698 88.5464

NOx and staged air feeding controls the production of NOx, too.Accordingly, the NOx emission results from above boilers are425–585 mg/m3 for the 65 t/h boiler [30], 632–721 mg/m3 forthe 130 t/h boiler [31], 425–538 mg/m3 for the 220 t/h boiler[32], 495.1 mg/m3 for the 230 t/h boiler [33].

(3) Dust. Even though the dust separator used is of an ordinary TLStype, the dust concentration in the exhaust flue is very low,which is only 123.5 mg/m3. It is mainly due to the CWS fuel haslow ash content, which makes the inlet concentration of sep-arator is only one third of that of a coal-fired boiler. In someother applications with bag-type dust collectors, the dust con-centration measured is lower than 30 mg/m3, which makes thefluidization–suspension combustion boiler very appropriate tobe used in regions with stricter environmental requirements.Something has to be mentioned is that the dust concentrationafter the inducing fan is measured to be higher than that beforethe fan might be due to measurement error. The dust concen-tration of a 220 t/h CWS-fired boiler is 140–147 mg/m3 [32], alittle higher than the 14 MW boiler.

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12 CO concentration mg/m3 25 (6% O2)13 Oxygen content in flue % 11.614 Releasing rate of dust kg/h 3.69915 Releasing rate of SO2 kg/h 13.76816 Ringelmen blackness of flue Level 0–1

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In brief, the CWS fluidization–suspension combustion boiler hasood environmental characteristics and the emission is lower thanational standards for type II regions. The boilers will obtain bet-er performance if the desulfurizing agents such as limestone aredded.

.3. Application of CWS fluidization–suspension combustionoiler

Since 2002, the combustion technology of fluidization–uspension is used in more than 30 boilers and of more than 10ypes. The capacity is concentrated in the range of 2.8–35 t/h, andne 75 t/h utility boiler was retrofitted to 60 t/h successfully.

In Shengli Oil Field of Shandong province, there are more than 20oilers in operation using the CWS fluidization–suspension com-ustion technology. The total capacity is 400 t/h and the biggestne is 60 t/h. In the single year of 2002, the burned CWS was5,400 t, and 24 million Yuan was saved. In 4 years’ operationince 2002, the burned CWS was 297,000 t, and 135,000 t of oilas saved. The boilers were very successful which burns steadily

nd is easy to be operated. Among the 530,000 industrial boilers,bout 76% of them could be considered to be retrofitted using theWS fluidization–suspension combustion technology and it giveshe technology a promising prospect.

In addition, the internal vortex separating device used inhe combustion technology is of great importance in achiev-ng the performance, but it has no application in boilers biggerhan 35 t/h, so careful design is needed to scale-up CWS-fireduidization–suspension boilers.

. Conclusion

Using the fluidization–suspension combustion technology, theWS fuel can be burned in the small and low height furnaces withigh efficiency and good environmental performance. It solved theroblems appeared in the combustion of CWS in boilers smallerhan 35 t/h such as burning stability and slagging in local sec-ions. An application of 14 MW boiler showed that it obtained goodeat engineering performance and environmental performance forashed-coal CWS. The boiler efficiency is 91.53%, emission of SO2

nd NOx are 346.1 mg/m3 and 469.5 mg/m3, respectively. The boilersing the technology showed good features such as high efficiency,

ow pollutant emission, good load regulation, good CWS qualitydaptability, steady operation and convenient maintenance. Theombustion technology of CWS has been used successfully in morehan 20 boilers and it will have a good prospect.

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