Proceedings of The South African Sugar Technologists' Association - June 1987

THE EFFECT OF FUEL MOISTURE CONTENT ON THEPERFORMANCE OF A TYPICAL BAGASSE FIRED

WATERTUBE BOILERBy NORMAN MAGASINER

John Thompson Africa (Pty) Limited, Bellville, Cape

AbstractMoisture content has a significant influence on the per-

formance of a bagasse fired watertube boiler. Recent im-provements in bagasse drying, baling and transportingtechniques have resulted in greater emphasis being placedon the use of bagasse for off-crop power generation at mills

o

o

FIGURE 1 General arrangement of 70 t h-' bagasse fired boiler

and in centralised power stations. This has resulted in theneed to evaluate boiler performance under a much widerspectrum of fuel moistures. In the paper the performance ofa typical sugar mill boiler is analysed over the fuel moisturerange a to 56%. Performance limitations are defined and

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Proceedings of The South African Sugar Technologists' Association - June 1987

and forced draught fans. The speed controlled induceddraught fan is turbine driven. The boiler. is bottom sup-ported. Grit and ash hoppers are water sluiced,

Performance characteristics, as a function of load withairheater only and with airheater plus future economiser on50% moisture bagasse are illustrated in Figure 2. Installingthe economiser reduces the final gas temperature by ap-proximately 45'C at MCR which results in an efficiency im-provement of 2,6% on the GCV. This in tum translates intoa fuel saving of approximately 4%. The steam temperaturewhich varies by less than 10' over a 4: I turndown ratiodrops by about 7' to 8'C.

--- Air heater only--- Air heater and economiser

80604020

LEGEND

o

700

1100

~ 1000>~

z~cr::::> BOOLL

IntroductionMoisture content has a significant influence on the per-

formance of a bagasse fired watertube boiler. Under normalmilling conditions moisture content varies between 47% and56%. Whilst superficially this may seem a narrow band, thespecific moisture content, i.e., the moisture content per unitmass of dry fibre increases rapidly in this range. 46% morewater must be evaporated per unit mass of dry fibre at 56%moisture than at 47% moisture.

Recent improvements in bagasse drying, baling and trans-porting techniques have resulted in greater emphasis beingplaced on the use of bagasse for off-crop power generation.This has resulted in the need to evaluate boiler performanceover a much wider spectrum of fuel moistures.

Below 47% factors other than moisture begin to dominateperformance characteristics. With drier fuels combustion andfurnace leaving temperatures increase. This can result ingrate level slagging and severe fouling of heating surfaces.The furnace gas leaving temperature then becomes the crit-ical parameter.

In this paper the performance of a typical sugar mill boileris analysed over the fuel moisture range 0 to 56%. Perfor-mance limitations are defined and preferred features whichshould be incorporated in plants designed specifically forcentral power station applications indicated.

preferred features, which should be incorporated in plantsdesigned specifically for central power station applications,indicated.

80604020

EVAPORATION (tons I hr )o

60

70

80

100

Zo>- 0u 0 ....Z-

~>uuU::~~

....J~

FIGURE 2 Boiler performance characteristics on 50% moisture ba-gasse vs load.

70000 kg h-'2600 KPa (g)350'ClO5'C200m30'C90%Milledbagasse

Moisture content 50%Ash content 2,0%Sucrose 2,5%Gross calorific value 9332 kJ kg-INett calorific value 7 459 kJ kg"The plant was designed to be installed in an existing boiler-

house in which the fore/aft dimensions were very restricted.This dictated locating the grit collector between the main-bank and heat recovery equipment and the ID fan inboardof the airheater.

The unit is a conventional three pass John Thompsonboiler with three roll bagasse feeders, dump grate stoker andtangent tube watercooled combustion chamber with a re-fractory band in the bagasse ignition zone. A cross flowdrainable superheater is placed in a separate pass with pro-vision for bypassing grits. The three pass convection bankincludes anti-erosion baffles'. A two pass parallel flow air-heater provides hot air for combustion while the duct layoutallows for the future installation of a mild steel bare tubeeconomiser. Electric motors direct drive the secondary air

Description of Plant

The boiler shown in Figure I, which is presently beingerected at Lonrho Sugar Corporation Limited's, BritanniaMill in Mauritius was used as the basis for the exercise. Itsdesign characteristics are as follows:Evaporation at MCROperating pressure at mainsteam stop valveFinal steam temperature at mainsteam stop

valve at MCRFeedwater temperatureAltitude above mean sea levelAmbient Air TemperatureRelative humidityFuel

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Proceedings of The South AfricanSugar Technologists' Association - June 1987

The effect which varying steam temperature has on a typ-ical 3 MW turbo alternator set is illustrated in Figure 1Extra superheater heating surface must be added to maintainpower output with an economiser.

120o 10 20 30 40 so 60

Curve A : Condensing setCurve B : Back pressure set (100kPa)

110

Boiler capacitywith alternativefurnace leavinggas temperaturessuperimposed

I ISuperheateroutlet temperature

I II300

80

90

U 400o

0:: 100U

~

0.. 350~UJ.....

AB

3: 3100.:J(

I-::::>a..~ 3000oa::UJ3:~ 2900

FIGURE 3 Net power output of nominal 3 MW turbo alternator setas function of steam temperature for constant streamflow.

STEAM TEMPERATURE 1C)

o

300

400

100

uo

0.. 200~UJ.....

360350340330320

FIGURE 4 Boiler performance as function of fuel moisture.

FUEL MOISTURE (Ofo)

Expected Performance on Varying FuelsThe expected performance of the unit on fuels with mois-

ture contents ranging from 0 to 56% is illustrated in Figure4. Experience in Mauritius shows that when the furnace leav-ing temperature exceeds I 050'C, furnace slagging and su-perheater fouling become a problem. The critical temperatureis a function of the ash chemistry. The ratio of alkali metaloxides to silica in the ash provides a first approximation ofwhat this temperature is likely to be. In certain areas ofNataland Queensland where this ratio is an order of magnitudehigher than it is in Mauritius, higher furnace leaving tem-peratures can be tolerated.

zo

t>~z-UJ>-u~~LLLLUJ

80

70

60o 10 20 30 40 so 60

To avoid exceeding the critical temperature, the airheatermust be bypassed when burning low moisture fuels. Theperformance curves shown in Figure 4 have been generatedon the assumption that the airheater is completely bypassedin the moisture range 0% to 34%, is progressively broughtinto operation in the 34% to 47% moisture range and is fullyutilised in the range 47% to 56%.

Figure 5 shows how the dry ash free fibre consumptionper ton steam varies with moisture when the limiting factorsreferred to above are imposed on the unit. The effect whichinstalling an economiser has, is superimposed. Figure 5 alsoillustrates how fan power consumption varies with and with-out an economiser. These curves have been generated onthe assumption that all fans are damper controlled. Powersavings of up to 25% are possible with variable speed drives.

In the 47% to 56% moisture range grate rating as well asID. fan capacity become limiting factors. For a given grateratmg unburnt carbon loss increases rapidly as moisture con-tent increases; hence the need to derate. The unit is designedto e~sure that maximum gas velocities are compatible withmaximum grate heat release rates. The ID fan speed mustbe increased by 24% to permit operating at 57 t h-Ion 56%moisture fuel.

Desirable Features for Power GenerationThe expected moisture content of the fuel plays an im-

portant role in the design of the plant. If a supply of lowmoisture bagasse can be guaranteed, the unit should be de-signed specifically for this fuel. The alternative is a unit de-signed for a greater degree of flexibility in fuel quality. The

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Proceedings of The South African Sugar Technologists' Association - June 1987

AcknowledgementsI wouldliketo thank John Thompson Africa (Pty) Limited

for allowing me to publish this paperand my colleague Ber-nard Nicholls for his assistance in its preparation.

fuels some form of airheatingis required. This can be eithera steam heated airheater or a tubular airheater placed up-stream of the economiser. These units can be brought intoservice to improve combustion stability when fuel moisturecontent dictates their use.

A 2 to 3 m high refractory band in the ignition zone pro-motescombustion stability when burning high moisture fuels.Heavy slagging can occur when moisture levels drop below45%. In a unit designed predominantly for drier fuels therefractory band should be omitted. Refractory backed openpitched tubing in the ignition zone can be used as a com-promise solution.

Furnaceheating surface shouldbe proportioned to ensureminimum slagging and boiler fouling when burning the.lowest moisture fuel contemplated. Thiswill resultin a largerfurnace than wouldbe required to burn highmoisturefuels.

For superheat temperatures of up to 420C uncontrolledsuperheaters are acceptable provided the turbine to whichthe boiler is to be coupled can accept the expected temper-ature deviations. The fall offin turbineefficiency withsteamtemperaturemust beevaluatedagainst the extracostofsomeform of steam temperature control. Control becomes moreimportant at higher temperatures. Variable speedcontrolofthe ID and possibly eventhe FD fanisdesirable to maximisepowersent out. Someform of closed loop fuel/airratio con-trol is also desirable.

ConclusionsFuelmoisturecontent plays a significant rolein the design

and performance of bagasse fired boilers. For plant to op-.erate successfully the factors which limit performance mustbe clearly understood and cognizance taken of them.

60

60

50

50

40

4030

20 30

20

FUEL MOISTURE (%)

10

10

---Without economiser---With economiser

LEGEND:

o

o

180

160

400

480

220

140

360

440

200

240

320

280

0:LU

~o11.

Zu,

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0:LU11.

I- CZ 00:-=>-lD~LU

o:~lDLLLU

I-LU(/)LUlJ..ff:oIZ(/)0I-

>-0:o

FIGURE 5 Boiler fuel and fan power consumption as function offuel moisture content.

dividing line is about 40% moisture. Below this figure econ-omiser heat recovery surface is preferred to minimise thepossibility ofgratelevel slagging and furnace and convectionsurface fouling while to make provisionfor highermoisture

REFERENCESI. Magasiner, N; Naude, DP and McIntyre, P (1984). Operating expe-

rience on single and three pass boilers in the cane sugar industry withparticular reference to erosion and drum water level stability. Proc.S.A/r. Sug. Technol. Ass. 58 123-128.

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