99-0087

Paper No. FBC99-0087

Advancement of 10T/H Fluidized Bed Boiler BurningFujian Anthracite With Extremely Low Volatile

Proceedings of the 15th International Conference onFluidized Bed Combustion

May 16 - 19, 1999Savannah, Georgia

Copyright ©1999 by ASME

Advancement of 10T/H Fluidized Bed Boiler BurningFujian Anthracite With Extremely Low Volatile

Changsui Zhao, Yufeng Duan, Xiaoping Chen, Xin Wu, Shuzhi Wu,Wenxuan Wang and Chao Huang

Thermoenergy Engineering Research InstituteSoutheast University

Nanjing 210096, P. R. ChinaTelephone: +86(0)25-3793453

Telefax: +86(0)25-7714489E-mail: [email protected]

ABSTRACT

The anthracite in Fujian Province, China with extremely low volatile content about2%~4% is very difficult to burn in grate firing boilers, and operation conditions are verypoor, such as steam output well below the nominal capacity and very low burning-outrate. Burning Fujian anthracite in specially designed bubbling fluidized bed (BFB) boilerswith capacities between 4 T/H and 20 T/H are still unsatisfactory. In order to increaseboiler output and utilize local coal more efficiently, a 10 T/H BFB boiler burning Fujiananthracite was retrofitted with several special techniques, in terms of underbed feeding ofrecycling fly ash, vortexing secondary air injection, continuous bottom ash removal andadding immersed tube surface. The boiler performances before and after the retrofittingwere measured and compared. The experimental results indicate that steam output of theboiler is boosted from 7~8 T/H to more than 14 T/H, unburned carbon content (UBC) inash from the convective banks drops from 31.39% to 3.89%, UBC in ash from themulticyclone drops from 38.87% to 22.19%, and UBC in fly ash drops from 35.3% to18.07%. The boiler thermal efficiency increases from 67.27% to 82.93%. Boiler operationbecomes more stable. Particulate emission is substantially lessened because of separationby the vortexing secondary air. The retrofitting was completely successful. The techniqueused and experiences obtained in the retrofitting can be widely applied in industrial BFBboilers.

INTRODUCTION

There is abundant anthracite reserve in China. Fujian Province is one of the provinceswhere the largest amount of anthracite reserve is. In addition, there is not any bituminousreserve. Fujian anthracite has extremely low volatile content, typically 2%~4% on dry ashfree basis. It is a very difficult task to utilize local anthracite for industrial, residential andinstitutional needs efficiently and reliably. Traditionally, a lot of industrial grate firingboilers are specially designed for burning Fujian anthracite. But the situation is verydisappointing, steam output is well below the nominal capacity and unburned carboncontent in both bottom ash and fly ash is quite high, in extreme case even high than fixed

carbon content in raw coal unthinkably. Since 1970's, hundreds of bubbling fluidized bedboilers with capacities between 4 T/H and 20 T/H have been developed and operated inFujian province. In order to burn local anthracite with low reactivity, boilers are designedto have following features. The freeboard is almost adiabatic except for a small amount ofwater tubes on the roof of the furnace. The gas velocity in the freeboard is very low,typically less than 1m/s. The freeboard temperature can be kept at high level. Particlesfrom the bed have long residence time in the freeboard. There is a U-shaped inertialseparator at the exit of the furnace. Fly ash particles escaped from the furnace burn furtherand are collected in the separator and then recycled back into the furnace. There is amulticyclone between the convective banks and the economizer. Fly ash particles collectedin the banks and the multicyclone are supposed to be recycled back into the bed surfaceseparately. Unfortunately, ash particles from the banks and the multicyclone can not berecycled because of lack of effective way. Operation conditions of the fluidized bed boilersare still unsatisfactory and boiler performances are below expected. Major commonproblems for the boilers are inadequate steam output, poor boiler thermal efficiency andlow availability. As demonstration of modification, a 10 T/H fluidized bed boiler in anagricultural chemical company was retrofitted with fly ash underbed feeding, vortexingsecondary air and continuous bottom ash removal. The measurements show that there is abig improvement in boiler performances, in terms of load, thermal efficiency andavailability of the boiler.

BOILER DESCRIPTION

Boiler unit 3 in Shanming Agricultural Chemical Company Lt. is a bubbling fluidized bedboiler with the capacity of 10 T/H. The design fuel is Fujian anthracite with volatilecontent Vdaf of 2.84%. The schematic diagram of the boiler is shown in Fig. 1. The crosssection of the bed is 2.4m×1.38m and expanded to 4.6m×3.9m in the cross section of thefreeboard with a transition section. The total height of the furnace from the distributor tothe roof is 7.85m. Two rows of vertical immersed tubes are arranged on each side of thebed. There is not any water-wall tube in the furnace. Only are 18 water tubes located onthe furnace roof and four connecting water tubes across the freeboard. The U-shaped fluegas duct just located at the furnace exit separates coarser fly ash particles from the gas byinertial force when particle laden flue gas takes a U-turn there and the gravity. Collectedparticles are sent back to the freeboard via nozzles. Some particles are separated by thegravity in a convective bank section between two drums. Finer particles are collected in ahoneycombed multicyclone. Fly ash particles collected in the banks and the multicycloneare supposed to be recycled back into the bed surface to get higher burn out rate. Passingthrough an economizer and two sets of air preheaters in the back pass, the flue gas is ventinto the atmosphere through an induced fan.

The ultimate analysis and low heating value of the design coal as received by weight arelisted in Table 1. The specifications of the boiler are shown in Table 2. No limestone is fedinto the bed for desulfurization because of very low sulfur content in the coal fired.

Table 1. Coal ultimate analysis(*dry ash free basis)C H O N S M A Volatile* LHV% % % % % % % % kJ/kg73.3 1.18 0.58 0.14 0.15 10.8 13.85 2.84 25197

Figure 1. Schematic diagram of the 10T/H boiler

Table 2. Boiler specificationsDesigndata

Operation databefore retrofitting

Operation dataafter retrofitting

Steam output T/H 10 9.63 14.4Steam pressure Mpa 1.3 1.06 1.3Steam temperature °C 194 185.5 194UBC in bottom ash % 2 3.16 2.86UBC in fly ash % 42 35.3 18.07Fraction of bottom ash 0.44 0.48 0.42Fraction of fly ash 0.56 0.52 0.58Exhaust gas(EG) temperature °C 157 204.8 151Excess air coefficient in EG 1.6 2.2 1.6CO content in EG % NA 0.0139 0.17Heat loss due to EG % 7.61 13.8 7.48Heat loss due to CO % 1 0.10 0.87Heat loss due to UBC % 7.44 16.94 5.42Combustion efficiency % 91.56 82.96 93.71Boiler thermal efficiency % 81.62 67.27 82.93

Operational practice showed that outer row of vertical inbed tubes on each side of thebed could not be vigorously scoured by the bed materials. Inbed heat transfer surface areaof 12.9 m2 is not sufficient for the nominal capacity of the boiler. The bed level can not beincreased further because of limited pressure head of the forced fan. Normally, the steamoutput of the boiler is about 7 T/H to 8 T/H. Unfortunately, it is failed to recycle the ashparticles collected in the tube banks and in the multicyclone back into the furnace due tolack of effective recycling device, such as J-valve, L-valve and non mechanical loop sealand lack of conveying air with high enough pressure head. Even if the particles can berecycled into the bed surface, it is still useless to increase the burning out rate of theparticles. The reason is that the temperature of the particles is only 200°C , and it takesquite long time to increase particle temperature to the point at which unburned carbonstarts to burn. Within their residence time in the freeboard, the particles are carried out ofthe furnace before being heated up to burning temperature. Measurement results beforeretrofitting shown in the middle column of Table 2 indicate that unburned carbon contentin fly ash is 35.3%. The ash particles collected in the tube banks and the multicyclone areneither recycled nor discharged. The heat loss caused by UBC in fly ash is 16.94%.Therefore, the combustion efficiency of Fujian anthracite in the boiler is only about 83%,the boiler thermal efficiency is as low as 67%. Furthermore, the bottom ash is manuallydisposed every two hours. The air static pressure in wind box, i.e., the bed level isperiodically varied. The range of variation in the bed height is up to 200 mm. It causesperiodical variation in the flow rate of the primary fluidizing air, which is harmful to stableand complete combustion in the bed. Operation conditions under full load and overloadalso show that the temperature in the freeboard is very high, sometimes reaches 1100°C,which is close to the softening temperature of coal ash, that causes slagging on the roof.

RETROFITTING

In order to increase steam output of the boiler to meet the company's need for processsteam and to utilize local coal more efficiently, several modification measures are taken forthe boiler. First of all, the vertical immersed tube bank of 12.9 m2 is replaced by inclinedimmersed tube bank of 18.39 m2 with 15° related to the horizon. In that case, heat transfercoefficient between the immersed tubes and the bed increases. Second, two new fly ashunderbed recycling systems (Lan et al.,1987) are installed for sending the fly ash particlescollected in the convective banks and in the multicyclone, respectively. The systemconsists of the ash hopper, the stand pipe, the U-shaped valve, the injector, the underbedrecycling nozzle and the Roots fan. The ash particles from the hopper and the stand pipe,controlled by the U-shaped valve, are injected into the bed via pneumatic conveying pipesand the underbed feeding nozzle which is located just above the air distributor. Aftergetting into the bed, ash particles at the temperature of 200 °C are heated up very quicklybecause intensive heat and mass transfer between ash particles and high temperature bedmaterials in the dense phase bed. During the residence time period in the bed, unburnedcarbon in ash particles starts to burn and continues to burn in the freeboard. In this wayvery high burning out rate can be reached when particles are recycled once. On the otherhand, recycling of the fly ash particles collected in the convective banks exposes the

formerly immersed part of the banks. It contributes to the increased output of the boilerand also the restrained abrasion of the banks. Thirdly, the vortexing secondary airtechnique (Nieh et al., 1992, Xu et al.,1993) is employed. Four nozzles of secondary airfrom the forced fan are arranged on both side walls in the freeboard at the properelevation. The secondary air jets injected from the nozzles are tangential to an imaginarycircle. The nominal ratio of the flow rate of the secondary air to the total flow rate is12.5%. Swirling flow formed by the secondary air with high velocity causes entrainedparticles move towards the wall. Therefore, an internal recirculation of coarse particles isformed and the residence time of finer particles is prolonged. As a result, the burning outrate of entrained particles increases and particle elutriated from the furnace in the flue gasis lessened. The flue gas temperature can be controlled by the flow rate of the secondaryair injected. Finally, an air-cooled vibratory feeder is refitted for bottom ash removal. Theflow rate of bottom ash removal can be regulated by the current of the vibrator. Automaticcontinuous removal of the bottom ash makes it possible to keep the bed level constant,and thus combustion process in the bed becomes stable. It also diminishes the possibilityof bed collapsing induced by manual disposal.

RESULTS AND DISCUSSIONS

The comparison tests were carried out in February 1989 before retrofitting and in March1998, respectively. The experimental results are shown in the right two columns of Table2. The ultimate analysis and the low heating value of coal tested before retrofitting arelisted in Table 3. The proximate analysis and the low heating value of coal tested afterretrofitting is listed in Table 4.

Table 3. Coal analysis data as receivedC H O S N A M LHV

%wt %wt %wt %wt %wt %wt %wt kJ/kg54.09 1.01 2.57 0.98 0.50 32.57 8.28 18976

Table 4. Coal analysis date, as receivedfixed carbon ash content moisture volatile low heating value

63.53% 24.89% 6.92% 4.66% 22181kJ/kg

Some findings can be drawn from Table 2. Steam output of the boiler is almost doubled,from 7~8 tons per hour just before the retrofitting to over 14 tons per hour at the nominalsteam parameters, over 2 tons per hour more than the objective output. Controllable andstable fly ash recycling leads to notable drop of unburned carbon content in fly ash from35.3% to 18.07% and the boiler thermal efficiency increases by 15.66%. The gastemperature at the exit of the furnace is controlled between 930 °C and 950 °C at themaximum load of the boiler. No slagging problem on the furnace roof exists anymore.Sure enough, firing Fujian anthracite in circulating fluidized bed with high recirculationrate may reach better combustion efficiency more than 94% and better boiler thermalefficiency more than 83%, but for small-scale industrial boiler the capital cost and

operation cost of CFB boiler are higher than bubbling fluidized bed boiler. Successfulretrofitting of the 10 T/H BFB boiler sets an example for hundreds of industrial BFBboilers firing local anthracite in Fujian or other low volatile anthracite in China.

In the test before retrofitting, four kinds of fly ash particles collected in the U-shapedinertial separator, the convective banks, the multicyclone and the water-membraneseparator were sieved. Their particle size distributions are shown in Fig.2. The sizedistribution of fly ash particles collected in the water-membrane separator after retrofittingis shown in Fig.3. The cut sizes d50 and d99 for 99% separation of four kinds of collectedash before retrofitting and the ash collected in the water membrane separator after retro-fitting are listed in Table 5. It can be seen that the ash collected in the multicyclone isthe finest, ts d50=67µm and d99=190µm. Two kinds of ash collected

Table 5. The cut sizes d50 and d99 of fly ash particlesbefore retrofitting Retrofitted

U-separator convective banks multicyclone water membrane Water membraned50, µm 118 117 67 99 93d99, µm 384 398 231 520 330

Figure 2. Ash particle size distribution Figure 3. Ash particle size distribution

in the U-shaped separator and the convective banks by the gravity and the inertia are thecoarsest, corresponding d50 are 118µm and 117µm, their d99 are 384µm and 231µm,respectively. The fly ash in the water membrane separator is quite coarse because whenthe ash hoppers for previous two stage separators are full and no ash is removal, some

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coarse ash particles are collected in the final stage separator. After retrofitting d50 and d99

of the fly ash in the water membrane separator are still not satisfactory. It indicates thatthe separation performance of the multicyclone is not very good. The particle sizedistributions of fly ash can also be expressed in form of probability density of particle sizeP(d), as shown in Fig.4 and Fig.5.The peak probability density is located at 70µm for themulticyclone ash and 100µm for the other three kinds of ash before retrofitting. The peakprobability density for the water membrane separator ash after retrofitting is about 95µm.

Figure 4. Probability density distribution Figure 5. Probability density distribution of particle size of particle size

Unburned carbon contents in collected ash are analyzed with the particle size and shownin Fig.6 and Fig.7. Figure 6 shows that the diameter of the particles with the maximumcarbon content is about 30µm for the ash in the water membrane separator and that isbetween 50µm and 60µm for the other kinds of ash. When the size is smaller than thepeak, the carbon content decreases obviously. When the size exceeds the size related tothe peak value, the carbon content drops sharply first. After reaching a valley size,it increases gradually. Plotting multiplication of the size probability density and thecarbon content versus the particle size, we can get distribution of the unburned carbonloading P(d)•C(d) with the particle size. The carbon loading curves for the tests beforeand after the retrofitting are depicted in Fig.8 and Fig.9, respectively. It implies that theheat loss due to unburned carbon is mainly caused by the ash particles with the diametersbetween 40µm and 120µm. Therefore, the key to creasing the combustion efficiencyfurther is to manage to separate more effectively the ash particles with the size mentionedabove and recycle them to the bed. It seems that the existing multicyclone of the boiler issupposed to be improved.

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Figure 6. Unburned carbon content Figure 7. Unburned carbon content distribution distribution

Figure 8. Unburned carbon loading curves Figure 9. Unburned carbon loading curves

CONCLUSIONS

1. The retrofitting of the boiler with several practical techniques is very successful. Itsexperiences can be applied to hundreds of industrial BFB boilers firing Fujian anthraciteand other small- scale BFB boilers firing low volatile anthracite in China.

2. The performances of the retrofitted boiler are very satisfactory. The steam output isalmost doubled and beneficial to meeting the need of the company for process steam. The

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combustion efficiency and the boiler efficiency increase by 10.75% and 15.66%,respectively.

3. The unburned carbon loading distribution curves indicate that for the boiler tested theheat loss due to UBC is mainly caused by the fly ash particles with the diameter between40µm and 120µm.

4. The effect of the vortexing secondary air is manifested as increasing combustionefficiency, lessening particulate emission from the furnace as well as controlling freeboardtemperature.

5. Stable and continuous bottom ash removal is beneficial to stability of both steamparameters and the coal combustion.

REFERENCES

Lan Jixiang, et al., The Experimental Investigations of Fine Ash Recycle in a AFB BurningLean Coal. Proc. of 9th Int'l Conf. on FBC, 1987:1096∼1100.

Nieh S.,et al, Measurements of Gas-Particle Flows and Elutriation of a 18'' I. D. ColdVortexing Fluidized-Bed Combustion Model. Powder Technogy, 1992, 69:139∼146.

Xu Yiqian, et al., Combution Characteristic and NOx Emission Control in VortexingFluidized Bed. Proc. of 12th Int'l Conf. on FBC, 1993: 123∼128.

NOMENCLATURE

A Ash content, % C Carbon content, % C(d) Carbon content in particles eith diameter of d, % CO Carbon monoxide, % dp Particle diameter, µm d50 Cut size, µm d99 Particle diameter for 99% separation, µm H Hydrogen content, % LHV Low heating value, kJ/kg M Moisture content, % N Nitrogen content, % O Oxygen content, % P(d) Probability density of particle with mean diameter of d S Sulphur content, %

99-0087

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