assessing alternative and supplementary sources … a fuel suitability study to establish a valid...

ASSESSING ALTERNATIVE AND SUPPLEMENTARY SOURCES TO BAGASSE FOR BIOMASS ENERGY GENERATION, BELIZE

ELECTROWATT-EKONO OY P.O. Box 93 (Tekniikantie 4 A) FIN-02151 Espoo Finland Domicile Espoo, Finland Trade Reg No. 338.595 Tel. +358 9 469 11 Fax +358 9 469 1981 E-mail: [email protected] Date February 6, 2005 Ref. No 60D5193.Q060.001 Page 1 (40)

ELECTROWATT-EKONO OY Ref: No 60D5193.Q060.001

Date: April 22, 2005 Page 2 (40)

CONTENTS

1 Background information ............................................................................................................. 3

2 Assessing suitability of alternative biomass fuels...................................................................... 4

2.1 Gereral ................................................................................................................................... 4

2.2 Ash forming components in the fuels ..................................................................................... 6 2.2.1 Bagasse from the sugar manufacturing process ............................................................. 6 2.2.2 Sugar cane tops and trash left in the field after harvesting ............................................ 9 2.2.3 Elephant grass .............................................................................................................. 10 2.2.4 Wild cane ..................................................................................................................... 11 2.2.5 Jatropha Curcas (physic nut)........................................................................................ 12

2.3 Impact of chlorine, potassium and sulfur on chlorine induced corrosion ........................... 13 2.3.1 Potassium, chlorine, sulfur and silicon in biofuels ...................................................... 13 2.3.2 Chlorine induced corrosion due to biofuels ................................................................. 16 2.3.3 Heavy metals in trace elements.................................................................................... 17

2.4 Recommendations for suitable fuel mixtures ....................................................................... 21 2.4.1 Recommendations with biofuels .................................................................................. 21 2.4.2 Recommendations with material selection .................................................................. 24

3 Waste bagasse store ................................................................................................................... 25

3.1 Methodology to evaluate energy volume.............................................................................. 25

3.2 Assessing energy value of the store...................................................................................... 27

3.3 Recommendations for energy use of the bagasse store........................................................ 30

Appendices:

1. Laboratory analyses 2. Collecting biomass samples (pictures) 3. Waste bagasse store at the sugar mill (pictures) 4. Schematic drawing of inclined grate boiler



1 BACKGROUND INFORMATION General

The Belize Sugar Industries Ltd (BSI) has a firm intention to invest in a cogeneration plant in Belize. Both the Power Purchase Agreement and the Environmental Compliance Plan has been approved and signed. The Belize Cogeneration Project (BELCOGEN Limited) is expected to burn some 420,000 tons of bagasse from 1.25 million tons of cane to produce electrical and thermal energy for the BSI, parasitic load for the cogeneration plant and excess generation is sold to the grid. Of the installed capacity of 25 MW, 13.5 MW is to be exported to the gird as base load all year round. The annual electrical energy exported will be up to 106 GWh. In 2007 the project is expected to go online and deliver some 20% of the load demanded by the national grid.

The original cogeneration models have indicated that some 7,000 tons of Bunker C oil may have to be burnt to guarantee the base load delivery to the grid. This however is undesirable for two major reasons being that the cost and availability of Bunker C is uncertain and as a hydrocarbon the burning of Bunker C is considered environmentally unfriendly. The present configuration will now use the fossil fuel more efficiently in engine generators.

There is a strong incentive to find alternative sources of energy to the fossil fuel required to bridge the energy gap that is needed to guarantee the base load grid supply. An option is to find biomass alternatives, which in itself have its challenges. Annual grasses and other biomass products that would be simply shredded and burnt are found to cause fouling of boiler tubes, which drastically reduced the boiler thermodynamic efficiency.

Any alternative biomass will require its own preparation plant and shall have to be suitable for combustion in high efficiency, suspension-fired boilers designed primarily for the combustion of bagasse.

A fuel suitability study to establish a valid biomass alternative to fossil fuel is required in order that BSI can develop and complement a biomass energy programme.

Detailed boiler specifications are so far not known for this study. This means that it is impossible to specify in practice the best fuel mixture to be used in the forthcoming new boiler, because fuel suitability is directly linked with the boiler type and materials to be used, live steam values, fuel prices and availability. In this study it is assumed that the boiler steam pressure level will be 960 psia (65 bar) and boiler steam temperature will be 905 0F (485 0C).

Replacing fossil fuel (7 000 tons of bunker C oil) with biomass (other than bagasse) would mean that some 5% of the total annual fuel need should be covered with alterna-tive biomass types, while the rest would be bagasse from the sugar milling process.



Objective of the study

The objective is to find out the best supplementary biomass fuel(s), as an alternative to fossil fuel, for biomass energy generation in Belize. The alternative fuel types have been identified by BSI

In order to be able to meet the objective of the study consultant has:

- Assessed the quantity and calorific value of the existing waste bagasse stored at site as well as made recommendation about the energy use of the store.

- Tested samples of the selected biomass types to determine their fuel characteristics and properties.

- Made recommendations about suitable fuel mixtures that could be used in electricity production based on grate boiler combustion technology (to produce high pressure steam of 905 0F, 960 psi, 128 t/h).

- Made recommendations as to what could be the best supplementary biomass source to replace fossil fuel without notable risks in boiler operation.

Methodology used

Consultant has elaborated the suitability of selected biomass types to be used as a mix-ture in energy production. This review has been based on consultant’s experiences, laboratory analyses, and on interviews with selected boiler manufacturers.

2 ASSESSING SUITABILITY OF ALTERNATIVE BIOMASS FUELS

2.1 Gereral Samples of the potential biomass-based fuel types were analysed in a laboratory in Sweden and following elements were clarified:

- Moisture content - Ash content - Gross calorific value, including a calculation of Net calorific value - Elemental analysis C-H-N (O calc. included) - Chlorine - Sulphur - Trace elements from original sample (Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, K,

Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Se, Si, Sn, Ti, V, Zn) Laboratory analyses have been presented as a whole in appendix 1.



Table 1 Summary of the characteristics of the selected biomass fuels Elephant

grass Bagasse Wild caneSugar cane

topsJatropha

Curcas-nutsMoisture % 68.0 50.3 64.5 74.6 8.5Ash cont. % db *) 11.8 8.6 5.7 6.3 4.6Sulphur S % db 0.16 0.03 0.27 0.1 0.18Chlorine Cl % db 0.55 0.02 0.28 0.53 0.06Carbon C % db 43.4 44.8 46.8 46.2 58.4Hydrogen H % db 5.4 5.4 5.7 5.7 8.1Nitrogen N % db 0.8 0.3 0.9 0.9 3Oxygen O (calc.) % db 37.9 40.9 40.3 40.3 25.7Aluminium Al Al mg/kg, dm 130 2500 69 190



2.2 Ash forming components in the fuels Different soil types affect the uptake of ash components (e.g. Si, K, Ca, Mg, Na, Al, P, S, Cl) in biofuels. Silicon is often a major ash forming element, especially when the ash level is high in the biofuel.

Because fuel composition strongly impacts combustion behaviour, the knowledge of the behaviour and the fate of the elements such as K, Ca, Na and Si, S, Cl during combus-tion are important for the understanding of deposit formation and corrosion. Potassium (K) and chlorine (Cl) are the elements which can cause ash related problems during combustion such as slagging, fouling, and corrosion in the boiler and convection area.

There are several commercial programs to calculate the thermodynamic equilibrium states of different elements of the fuel during combustion. These programs calculate the amounts of compounds at equilibrium at certain temperatures or pressures. However, one should keep in mind that in the real combustion systems with varying fuel composi-tions and temperatures, the equilibrium states hardly give exact values prevailing in a boiler. However, equilibrium analysis can give an estimate on the mechanisms of trace elements in combustion systems. They offer a fast way to see the trends when some components in the fuel or the combustion temperature will be increased or decreased.

The biofuels from Belize contain between 4.6 and 11.8 wt.% ash 1 on dry basis. The ash content of bagasse is known to vary from factory to factory and over time at any given factory, depending on local weather and field harvest conditions.

Estimation of melting behaviour of ashes includes a variety of methods. To predict ash melting temperatures from element compositions of fuels it was assumed that the ash forming elements were fully oxidized.

2.2.1 Bagasse from the sugar manufacturing process The main ash forming components in the bagasse are estimated and presented in figure 1.

1 wt.% ash = weight percent of ash (on dry basis)



Ash forming elements in BagasseAsh forming components totally: 71,07 g/kg DS

Ash measured (at 550oC): 8,6 wt.%DS (Difference 1,5 wt.%)

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65.00

70.00

75.00

80.00

Al2O3 BaO B2O3 CaO Cl Fe2O3 K2O MgO MnO2 Na2O P2O5 SiO2 TiO2

wt.%

or m

ole-

%

wt.%-ashmole-%(ash)

Figure 1 Main ash forming components in bagasse. The main component in bagasse (about 75 wt.%) is silica (SiO2) and then comes cal-cium oxide (CaO, about 10 wt.%). The amount of alkali oxides (e.g. K2O and Na2O) is low. Also the content of chlorine is low, only 0.02 wt.dry basis (i.e. 200 mg/kg DS).

Figure 1 shows also that the ash content measured after heating in air to 550oC (accord-ing to the Swedish standard) gives 8.6 wt.% DS 2 and the amount based on the main ash forming components reveal only 7.1 wt.% DS. There is a difference in wt.% units of 1.5 wt.% DS. This means that the ash from bagasse contains quite a lot of ash (100*1.5/8.6 i.e. 17 %) originating from organic compounds.

The ash components of biofuels are normally presented in the CaO-K2O-SiO2 ternary diagram with solidus 3 lines, as presented in figure 2. The calculated composition of the ash in the case of bagasse is 11 wt.% CaO, 2.7 % K2O and 86.3 % SiO2 when the ash consists of these components only. This means that it is restricted to the SiO2 rich cor-ner of the ternary diagram. The first melting temperature in this system is 720oC, while small addition of calcium oxide increases this value to 1080oC. The presence of potas-sium oxide or even calcium oxide to silica can give rise to the formation of silicate glasses at much lower temperature than 1300oC, which will eventually dissolve all the quartz or other form of silica until the melt is saturated with silica as can be seen in fig-

2 DS = Dry Substance 3 solidus lines = below the solidus lines no liquid exists the composition being entirely crystalline



ure 3. This means that sticky temperature of ash decreases with an increase of potas-sium in the fuel.

The ternary diagram CaO-K2O-SiO2 is useful for predicting melting behaviour of bio-fuels. Thermodynamic calculations with the aid of FactSage program give the share of fused ash (i.e. molten ash) at different temperatures. According to these calculations with the FactSage program the amounts of fused ash at 900oC, 1000oC and 1100oC are 10%, 11.6 % and 13 %, respectively. The minimum, critical limit for a sticky ash is normally 15 %. This means that bagasse does not form sticky ash at normal combustion temperatures of grate boilers.

Figure 2 Partial section from the ternary phase diagram CaO-K2O-SiO2.

In this section bagasse locates close to the SiO2 corner; about 86 % SiO2, 11 % CaO and 3 % K2O.



Figure 3 Binary diagram K2O-SiO2 Binary diagram K2O-SiO2 shows that potassium oxide (K2O) drastically decreases the melting (i.e. fusion) point of silica (SiO2). There are several potassium silicates (K2O·SiO2, K2O·2SiO2 and K2O·4SiO2) having melts at about 760oC. It should be pointed out that the use of phase diagrams is a theoretical method and the accuracy of this method declines with the number of ash forming components. Most biomass fuels contain a wide variety of elements.

2.2.2 Sugar cane tops and trash left in the field after harvesting The main ash forming component with sugar cane tops were estimated in a similar manner as with bagasse.


Date: April 22, 2005 Page 10 (40)

Ash forming components in Sugar Cane TopsAsh froming components totally: 5,76 g/ kg

Ash measured (at 550oC): 6,3 wt.% DS, Difference 0,54 wt.%

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35.00

40.00


wt.%

or m

ole-

%

wt.%-ashmole-%(ash)

Figure 4 Main ash forming components with sugar cane tops.

In this case the ternary system CaO-K2O-SiO2 would be 49% SiO2, 33% K2O and 18% CaO. If the sugar cane tops would be used alone as a fuel the amount of fused ash would be, according to the thermodynamic calculations with the FactSage program, at 1100oC about 71 % of the total amount of ash. Due to this high amount of fused ash the bottom ash during combustion will be very sticky.

The difference in the ash amounts measured and calculated in this case is only 0.54 wt.% DS. This means that the amount of ash originating from organic compounds is about 8 % (100*0.54/6.3).

Sugar cane tops contain about 25 times more chlorine than the bagasse, this means that chlorine does enrich to leafs of sugar cane. The sugar cane tops contain chlorine as much as 5300 mg/kg DS compared to 200 mg/kg DS in the bagasse.

2.2.3 Elephant grass Main ash forming elements in the elephant grass are presented in figure 5.


Date: April 22, 2005 Page 11 (40)

Ash forming components in Elephant GrassAsh forming components totally: 10,804 g/ kg DS


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55.00

60.00


wt.%

or m

ole-

% in

ash

wt.%-ashmole-%(ash)

Figure 5 Main ash forming component in elephant grass.

In this case the ternary system CaO-K2O-SiO2 would be 67 % SiO2, 26 % K2O and 7 % CaO. If this elephant grass were used alone as a fuel the amount of fused ash according to the thermodynamic calculations with the FactSage program would be about 83% at 900oC, 1000oC and 1100oC. Due to the high amount of fused (liquid) ash the bottom ash will be easily flowing on the grate during combustion.

The difference in the ash amounts measured and calculated in this case is 1 wt.%. This means that the ash originating from organic compounds is about 8% (100*1/11.8).

Chlorine content in the elephant grass was even higher than in the sugar cane tops i.e. as high as 5500 mg/kg DS.

2.2.4 Wild cane Main ash forming components with the wild cane are presented in figure 6.

In this case the ternary system CaO-K2O-SiO2 would consist of 32% SiO2, 51% K2O and 16% CaO. If this wild cane were used alone as a fuel the amount of fused ash ac-cording to the thermodynamic calculations with the FactSage program would be 82%, 97% and 99% at 900oC, 1000oC and 1100oC respectively. These figures mean an easily flowing bottom ash during combustion.


Date: April 22, 2005 Page 12 (40)

Ash forming components in Wild CaneAsh forming components totally: 25,89 g/ kg


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35.00


wt.%

or m

ole-

%

wt.%-ashmole-%(ash)

Figure 6 Main ash forming components with wild cane.

Chlorine content in wild cane was not as high as in the sugar cane tops. It was in this case 2800 mg/kg DS.

Difference between the measured ash content (5.7 wt.% DS) and the calculated one (2.6 wt.%) was extremely high. The difference was as high as 3.1 wt.% units of DS. This means that the ash originating from organic compounds is as high as 54 % (100*3.1/5.7).

2.2.5 Jatropha Curcas (physic nut) Main ash forming components with the jatropha curcus nuts are presented in figure 7.

In this case the ternary system CaO-K2O-SiO2 would consist of 9 % SiO2, 52 % K2O and 39 % CaO. If this Jatropha Curcas (physic nut) would be used alone as a fuel the amount of fused ash according to the thermodynamic calculations with the FactSage program is at 900oC, 1000oC and 1100oC 28 %, 35 % and 37 %, respectively. These amounts of fused ash mean a little sticky bottom ash during combustion.

Chlorine content of these nuts is low – only 600 mg/kg DS.


Date: April 22, 2005 Page 13 (40)

Ash forming components in Jatropha Curcus NutsAsh forming components totally: 43,32 g/kg

Ash measured (at 550oC): 4,6 wt.% DS, Difference 0,27 % wt.%

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30.00

35.00


wt.%

or m

ole-

%

wt.%-ashmole-%(ash)

Figure 7 Main ash forming components with jatropha curcus nuts.

The difference in the ash contents (measured and calculated) is also very low i.e. 0.27 wt.%. This means that the ash originating form organic compounds is less than 6 %.

2.3 Impact of chlorine, potassium and sulfur on chlorine induced corrosion

2.3.1 Potassium, chlorine, sulfur and silicon in biofuels Potassium is the dominant source of alkali in most biofuels, and it is normally bound to chlorine as potassium chlorine (KCl). Potassium can be bound either biologically or non-biologically and the non-biologically bound alkali exhibits much lower volatility than the biologically bound alkali. The high portion of volatile alkalis in biofuels makes the ash and its behaviour very difficult to predict, because alkali metals or inorganic compounds of alkali metals are easily released into the gas phase.

Chlorine is the major factor in deposit formation, since it facilitates the release of many inorganic compounds, in particular potassium. Potassium chloride is among the most stable high-temperature, gas-phase, alkali containing compound. Chlorine concentration in the fuel often dictates the amount of alkali vaporized during combustion more strongly than the potassium concentration in the biofuel. In the absence of sulfur, chlo-rides are present on the heat transfer surface. If chlorine is not present, alkali hydroxides


Date: April 22, 2005 Page 14 (40)

are the major gas phase species in combustion gases. Organic chlorine may also be re-leased into the gas-phase as gaseous hydrogen chloride (HCl) during combustion.

High temperatures promote alkali and chlorine release. More alkalines are released into gas phase as the temperature increases. Higher chlorine content in the fuel also in-creases alkali vaporization. High moisture content favors alkali hydroxide formation and also enhances HCl formation.

Silicon in the fuel ash captures alkali because it forms alkali silicates, which have low melting points. One example of this can be seen in the binary phase diagram K2O-SiO2, figure 2, which shows potassium silicates melting at temperatures lower than 800oC. However, silicon is effective only when the quantity of sulfur is low. Alkali silicates are formed when the sulfur-to-alkali molar ratio, S/(K+Na), is below 0.5. When excess sul-fur is available alkali sulfates will be formed.

Table 2 S/(K+Na) molar ratios in different raw materials and their mixes.

Bagasse Elephant grass

Jatropha curcus nuts

Sugar cane tops

Wild cane

100 % 0.147 0.096 0.188 0.087 0.396

95% bagasse + 5 % X

0.147 0.190 0.247 0.208 0.300

This means that there is not enough sulfur in these biofuels in order to form potassium sulfates. Thus, chlorine in the fly gas will be either HCl or KCl. Potassium sulfates are known to be less corrosive to low alloy steels than potassium chlorides. If potassium chlorides are present in the fly ash deposits an enhanced corrosion on low alloy steels may take place already at temperatures 500oC or higher.

The concentration of HCl in the gas phase is dependent also on the sulfur content in the fuel. Part of the sulfur released as gaseous SO2 may react with alkali chlorides (KCl) and hydroxides (KOH) and alkali sulfates and gaseous HCl will be formed:

2 KCl + SO2 + H2O + ½ O2 = K2SO4 + HCl

Thus it has been suggested that the sulfur-to-chlorine molar ratio (S/Cl) in the fuel should be at least 2 in order to retard potassium (or sodium) chloride induced corrosion in boilers, table 3. If this ratio is low it means that there will be potassium chlorides pre-sent in the deposits and the deposit will be corrosive depending on the temperature of the metal surface on which the deposit sticks. If there are any liquid compounds present in the ash deposit there is a high risk for chloride corrosion.


Date: April 22, 2005 Page 15 (40)

Table 3 S/Cl molar ratios in different raw materials and their mixes.

Bagasse Elephant grass

Jatropha curcus nuts

Sugar cane tops

Wild cane

100 % 1.64 0.32 3 0.19 0.96

95% bagasse + 5 % X

1.64 0.86 1.87 0.81 1.4

One can see that the S/Cl molar ratio is lower than 2 except with 100% of Jatropha cur-cus nuts. Especiallly elephant grass and sugar cane tops combusted alone without ba-gasse can generate big problems during operation of the boiler.

Thus in biofuels the contents of alkalines (K+Na), chlorine and sulfur in the raw materi-als are of importance, figure 8.

BagasseElephant

Grass JatrophaCurcus-nuts Sugar cane

tops Wild cane

S

Cl

K+Na0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

cont

ent

(wt.%

in D

S)

Alkali, chlorine and sulfur (wt.% in DS) in the raw materials

Figure 8 Alkalis, chlorine and sulfur in the raw materials. Observe the low con-

tents of bagasse compared to the other materials.


Date: April 22, 2005 Page 16 (40)

There is still one element which can have impact on the formation and behavior of fly ash. This element is calcium. Calcium may capture sulfur when it forms calcium sulfate (CaSO4). Thermodynamic calculations show that calcium sulfate is more stable than po-tassium or sodium sulfates at lower temperatures. However, at higher temperatures the chlorides are more stable than the sulfates.

In conclusion, if equilibrium is established in the gas phase, chlorides formed at higher combustion temperatures will be converted to sulfates (if enough sulfur is present) in the flue gas with decreasing temperature. Thus mostly sulfates will be stable in the de-posits, formed in the low temperature regions of the boiler i.e. the superheater or con-vection sections. The equilibrium amount of chlorides (mostly KCl) will be small. If the gas is in the nonequilibrium state, more chlorides are found in the deposits.

What will be the components in the fly ash and in the flue gas phase? Will these com-ponents generate corrosion or not? Thermodynamic equilibrium calculations can be used to predict the flue gas and fly ash compositions generated in biofuel combustion. These calculations have many approximations concerning interactions of the reacting components. Moreover, these calculations assume that time is not limiting the reactions but the equilibriums will be reached even they may take in reality several years to reach. In real processes this is not the case and thus the results of these thermodynamic calcu-lations should be used as estimates of trends than as the exact values of compositions.

2.3.2 Chlorine induced corrosion due to biofuels Clorine-induced corrosion in biofuel-fired boilers can be caused by gas phase attack and/or by the presence of chlorides in the deposits. Alkali compounds, especially chlo-rides, lower the melting point of the deposits (both in bottom ash and in flue ash), be-cause they form low melting systems with several alkali compounds. If chlorides form liquid phase in the deposits, corrosion rate of steel in a heat transfer surface (e.g. super-heaters) will be drastically accelerated. This corrosion due to liquid phase in the depos-its will strongly increase with increasing the temperature.

The major mechanisms of ash deposition during combustion of biofuels are related to the types of inorganic material in the fuel and the combustion conditions. Both the ash deposition rate and the ash deposits as such are important factors in the operation of a boiler.

The fouling in a boiler is dependent on the melting behavior of the fly ash. If the first melting temperature is low and the fly ash forms sticky deposits on a wide temperature range there will be deposits on superheaters and economizers.

As the vapor pressure of potassium chlorides even at 500oC is high, potassium in the biofuel may form volatile chlorides and accelerate corrosion drastically. This corrosion is called chlorine induced corrosion.


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The situation with different fuels from Belize, when 5 % of bagasse will be substituted with an other biofuel, is shown in figure 9.

Basa

sse

5% El

ephe

nt gr

ass

5% Ja

troph

a Cur

cus N

uts

5% Su

gar C

ane T

ops

5% W

ild C

ane

SCl

K+Na0

0.05

0.1

0.15

0.2

0.25

wt.% DS

Alkali, chlorine and sulfur (wt.% DS) in the fuel (Bagasse + 5 % an other raw material)

Figure 9 Alkali, chlorine and sulfur in the final fuels with co-combustion of ba-

gasse.

Looking at the chlorine contents in the mixes the best biofuel after bagasse (0.02 wt.% Cl in DS) would be jatropha curcus nuts (in the mix 0.022 wt.% Cl in DS) and then comes wild cane (in the mix 0.033 wt.% Cl in DS).

Also the potassium content in these mixes is lower than in the elephant grass and sugar cane tops, so that jatropha curcus nuts and wild cane can be acceptable fuels in consid-eration of potassium chlorides. As shown earlier the risk of fouling, slagging and corro-sion in a boiler can be indicated by the amount of potassium chlorides in the fuel.

2.3.3 Heavy metals in trace elements Heavy metals such as Pb and Zn, which belong to the so called trace elements of the fu-els, can increase corrosion of superheaters. In the presence of heavy metal chlorides


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(PbCl2 or ZnCl2) liquid phases are formed, preferentially low melting eutectics and peri-tectics 4. There eutectics are usually formed from KCl-ZnCl2 system, which has eutectic melting temperatures at about 260oC. This means that heavy metals together with high chlorine and high potassium may generate severe corrosion if they are present in the fu-els. However, the amounts of these heavy metals are detrimental to the fate of trace elements during combustion.

Figures 10-14 show the contents of trace elements in these biofuels and also when ba-gasse is combusted with 5 wt% of some other biofuel from Belize.

Trace Elements (mg/kg DS) in the Biofuels

02.5

57.510

12.515

17.520

22.525

27.530

32.535

37.540

As Cd Co Cr Cu Hg Mo Ni Pb Sb Se Sn V Zn

mg/

kg D

S

Bagasse mg/kg DSElephant Grass mg/kg DSJatropha Curcus-nuts mg/kg DSSugar cane tops mg/kg DSWild cane mg/kg DS

Figure 10 Trace elements in the raw materials.

Figure 10 shows that bagasse contains more iron, chromium and nickel than the other biofuels. These trace elements in bagasse most probable originate from erosion of steel components during preparation of the bagasse fuel. The amount of sand in bagasse is high and it makes the fuel aggressive to wear and erosion. Steel is soft compared to sand because the hardness of sand is about 1200 Hv 5 compared to the one of steel (i.e. 200-300 Hv). The risk of wear and erosion should be kept in mind when the handling of the fuel and the ash will be planned in Belize.

Figures 11 to 14 show the contents of trace elements in the biofuels under study.

4 eutectics and peritectics = at the eutectic point the melt is such a composition that the eutectic reaction (Melt = A crystals + B

crystals) takes place and at the peritectic point the peritectic reaction (Crystal A = Crystal B + Melt) takes place. 5 Hv = Vickers hardness value


Date: April 22, 2005 Page 19 (40)

0

1

2

3

4

5

6

7

8

9

10m

g/ k

g D

S


TRACE ELEMENTS: Bagasse + 5% Elephant Grass

Bagassemg/kg DS

Bagasse+5%ElephantGrassmg/kg DS

Figure 11 Trace elements of the fuel when 5 % elephant grass is combusted with

bagasse.

0

1

2

3

4

5

6

7

8

9

10

mg/

kg D

S


TRACE ELEMENTS; Bagasse + 5% Japrotha Curcus NutsCl: from 200 to 220 mg/kg DS

Bagassemg/kg DS

Bagasse+5% JatrophaCurcus Nutsmg/kg DS

Figure 12 Trace elements of the fuel when 5 % jatropha curcus nuts are com-busted with bagasse.


Date: April 22, 2005 Page 20 (40)

0

1

2

3

4

5

6

7

8

9

10m

g/ k

g D

S


TRACE ELEMENTS: Bagasse + 5% Sugar Cane Tops

Bagassemg/kg DS

Bagasse+5% SugarCaneTopsmg/kg DS

Figure 13 Trace elements of the fuel when 5% sugar cane tops are com busted with bagasse.

0

1

2

3

4

5

6

7

8

9

10

mg/

kg

DS


TRACE ELEMENTS: Bagasse + 5% Wild Cane

Bagassemg/kg DS

Bagasse+5% WildCane mg/kgDS

Figure 14 Trace elements of the fuel when 5% wild cane is combusted with ba-

gasse.

These figures show that the amount of zinc only slightly increases with the biofuels added to bagasse compared to the situation when only bagasse is combusted. The


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amount of zinc in bagasse is only 8.1 mg/kg DS. The amount of chromium is rather high and it may form chlorides, too, and thus increase corrosion in the boiler.

2.4 Recommendations for suitable fuel mixtures

2.4.1 Recommendations with biofuels The maximum steam temperature is 905oF (i.e. 485oC), which means the maximum metal temperature in the hottest superheaters under radiation might be even 530oC. However, this maximum temperature without radiation, for instance when the hottest superheater is behind a nose of the boiler, is only 505 or 510oC. In order to avoid corro-sion of the superheaters the deposits of fly ash on the superheaters should not contain any liquid compounds. This means that the lowest melting point of the ash deposits should be higher than 530oC.

Thermodynamic calculations with the “HSC Chemistry program” show that, if there is a risk even for a short period of time for using any of these biofuels alone without any bagasse at all, the fuels sugar cane tops or elephant grass should not be chosen. These fuels will form flue gas and fly ash rich with gas-phase KCl, Figures 15-18.

Elephant Grass to Bagasse

0

10

20

30

40

50

60

70

0 % 20 % 40 % 60 % 80 % 100 %% of Elephant grass to Bagasse

kg a

t 110

0oC

/ t D

S co

mbu

sted

HCl (g)

KCl (g)

KOH (g)

K2SO4

K-silicatesSiO2

Figure 15 Estimates of chlorine and potassium containing compounds and sili-cates formed when one ton of dry solids (elephant grass + bagasse) is combusted at 1100oC according to the thermodynamic calculations with HSC5.


Date: April 22, 2005 Page 22 (40)

Sugar Cane Tops to Bagasse

0

10

20

30

40

50

60

70

0 % 20 % 40 % 60 % 80 % 100 %% of Sugar cane tops to Bagasse

kg a

t 110

0oC

/ t D

S co

mbu

sted

HCl (g)

KCl (g)

KOH (g)

K2SO4

K-silicatesSiO2

Figure 16 Estimates of chlorine and potassium containing compounds and sili-cates formed when one ton of dry solids (sugar cane tops + bagasse) is com busted at 1100oC according to the thermodynamic calculations with HSC5.

Japhotra Curcus Nuts to Bagasse

0

10

20

30

40

50

60

70

0 % 20 % 40 % 60 % 80 % 100 %% of JapC Nuts in Bagasse

kg a

t 110

0oC

/ to

n D

S co

mbu

sted

HCl (g)

KCl (g)

KOH (g)

K2SO4

K-silicatesSiO2

Figure 17 Estimates of chlorine and potassium containing compounds and sili-cates formed when one ton of dry solids (jatropha curcus nuts + ba-gasse) is combusted at 1100oC according to the thermodynamic calcu-lations with HSC5.


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Wild Cane to Bagasse

0

10

20

30

40

50

60

70

0 % 20 % 40 % 60 % 80 % 100 %% Wild cane to Bagasse

kg a

t 110

0oC

/ t D

S co

mbu

sted

HCl (g)

KCl (g)

KOH (g)

K2SO4

K-silicatesSiO2

Figure 18 Estimates of chlorine and potassium containing compounds and sili-cates formed when one ton of dry solids (jatropha curcus nuts + ba-gasse) is combusted at 1100oC according to the thermodynamic calcu-lations with HSC5.

Neither japrotha curcus nuts nor wild cane should be used alone, because then the total amount of gas-phase KCl will become too high. Because ash deposits often contain un-burned material the ash will then accumulate higher amounts of condensed potassium compounds and get sticky and aggressive.

However, jatropha curcus nuts and wild cane can be used if they are added in small amounts in a steady flow to the bagasse going into the boiler. The amount of jatropha curcus nuts or wild cane in the bagasse flow should not exceed 30 % even momentarily. The changes between the mixes and the clean bagasse should be done smoothly.

Before each planned shutdown of the boiler only bagasse should be combusted for at least 12 hours. Also the start-ups should be done with bagasse only for a minimum time of 12 hours.

The tests with straw show that during normal drying of the straw the content of organi-cally bound alkali can decrease. This could be the case with wild cane, too. The wild cane taken to the chemical analysis was fresh and thus contained 64.5 % moisture. As such it is very difficult to be combusted and therefore it should be always mixed with bagasse.

Jatropha curcus nuts contains only 8.5 % moisture and thus it has a heat value about 50% higher than the heat value of bagasse which has a moisture content of 50.3 %.´The


Date: April 22, 2005 Page 24 (40)

heat caloric value for the jatropha curcus nuts showed 27.07 MJ/kg db and bagasse about 18.08 MJ/kg db. Wild cane showed a caloric value of about 18.9 MJ/kg.

2.4.2 Recommendations with material selection Material selection for the superheaters should be done with care when the design of the boiler is known in more detail. Corrosion rates of superheaters made of basic ferritic steels, e.g. St. 35/8 or 15 Mo3, which hardly contain any chromium at all can become too high to be accepted. More corrosion resistant steels are more expensive but due to their longer lifetime they are compatible. The corrosion rate of the superheaters is strongly dependent on the maximum steam temperature. If the steam temperature is in-creased the risk for corrosion drastically increases.

Elephant grass, sugar cane tops and even jatropha curcus nuts combusted alone even for a shorter period of time in the boiler will be detrimental to the refractory because they release too much alkali compounds. The degradation of refractory depends on the qual-ity of the refractory used in the lower part of the boiler.

Thus the corrosion risks for different steels and refractory grades should be considered already when the boiler is designed. If these risks are neglected there can be several sudden stops and failures during the first year of operation and they will become more expensive than the more corrosion resistant materials. Also the overhauls due to these sudden failures are always much more expensive than the emphasis to the corrosion risks during design and erection of the boiler.


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3 WASTE BAGASSE STORE

3.1 History At the end of the first crop of 1967 a relatively small heap of surplus bagasse started to accumulate at the south-western end of the bagasse house. Because of low throughput and total cane ground the surplus bagasse pile was just put on open fire at that location. This practice was followed up to 1983 when the incinerator was built and put into op-eration. This alleviated problems with farmers that suffered from the surplus bagasse burning while delivering their cane to the factory.

The change to the use of an incinerator was also timely as in 1984 the operations of the Libertad and Tower Hill factories were amalgamated to only the Tower Hill Factory. Therefore, a much larger quantity of cane was ground in the Tower Hill Factory and consequently, a much larger amount of bagasse was being disposed of.

An average of 15 to 20 tons of bagasse per hour is left over to be disposed of. In full operation the incinerator consumes about 12 to 15 tons of surplus bagasse and up to 8 tons per hour goes to the waste pile. From 1983 to present this pile exists and is almost 8 metres (25.5 feet) deep from the deepest point.

3.2 Methodology to evaluate energy volume The dimensions of the waste bagasse area were estimated to be 380m (length) and 162m (width). The depth of the pile was approximately 8m in the outer end line and close to the ground level in the inner end line (figure 16). The stock area was divided into six equal segments as shown in figures 15 and 16.


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380 m

162 m

.81 m

81 m

127 m

127 m

127 m

x (B1)

x (B2)

x (C1)

x (C2)

x (A1)

x (A2)

Figure 15 Bird’s view of bagasse stock

Fuel samples were taken from different depth levels (0.1m, 0.5m, 1m, 1.5m, 2m and 3m) from the center of each segment as shown in figure 16.

C1C2

B1B2

A1A2

0.1 m0.5 m1.0 m1.5 m2.0 m

3.0 m

Figure 16 Cross section of stock area and location of biomass sample points


Date: April 22, 2005 Page 27 (40)

3.3 Assessing energy value of the store Thirty fuel samples were taken by BSI to facilitate the evaluation of the energy content of the bagasse pile. The moisture and ash contents of each sample were analyzed in BSI’s own laboratory and the results are presented in the following table.

Table 4 Special analysis of accumulated bagasse

Sample point / Depth Ash % Moisture % A1- 10cm 2,74 51,60 A1- 1m 1,84 71,70 A1- 1.5m 3,02 66,64 A1- 2m 2,89 67,96 A1- 3m 3,47 65,17 A2- 10cm 9,18 60,91 A2- 50cm 13,34 58,95 A2- 1m 5,03 65,92 A2- 3m 6,09 65,85 B1- 10cm 4,31 68,79 B1- 50cm 4,48 65,96 B1- 1m 4,01 69,83 B1- 1.5m 2,43 71,66 B1- 2m 4,26 67,80 B1- 3m 2,12 68,12 B2- 10cm 17,67 63,76 B2- 50cm 2,53 71,09 B2- 1m 3,87 67,54 B2- 1.5m 2,37 67,23 B2- 2m 1,77 67,25 B2- 3m 2,06 64,80 C1- 10cm 12,00 68,66 C1- 50cm 12,32 66,48 C1- 1m 6,23 71,92 C1- 1.5m 5,17 75,27 C1- 2m 3,17 68,46 C1- 3m 3,08 69,17 C2- 10cm 8,66 50,88 C2- 50cm 7,66 51,60 C2- 1m 6,75 51,21 C2- 1.5m 6,01 50,49 C2- 2m 6,21 46,54 C2- 3m 9,36 58,25

Surprisingly the fuel sample analysis shows very contradictory numerical values. At most fuel sample points the ash content intends to reduce the deeper the sample was taken. The Consultant is not able to explain this phenomenon as the hypothesis was that the ash content would increase rather than decrease the deeper the sample was taken.


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Instead, the moisture content seemed to rise while moving from the surface to the deeper parts of the pile. The average MC rose from 60.8% (10 cm depth) to 65.2% (3 m depth).

No conclusions could be made based on the analysis done. The Consultant has esti-mated the energy content of the bagasse stored based on the very thin information on this subject. There are hardly any studies available on bagasse-based biomass decompo-sition and behaving in anaerobic circumstances when piled outdoors. However based on existing know-how about other biomass type behavior and on the chemical composition of bagasse we can draw some assumptions.

Sugar cane juice contains a fair amount (8 to 16%) of sucrose, while the dry bagasse (about 28% of the wet weight of sugar cane with 50-60% moisture content) is lignocel-lulosic, i.e., consists of fermentable sugars: 72%, lignin: 19% and extractives: 7% (among others like wax). Bagasse is one of the easiest decomposable lignocellulose species.

Anaerobic digestion is the bacterial fermentation of organic material, like bagasse, in oxygen free conditions. Anaerobic digestion produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas.

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant.

Anaerobic process description in a controlled environment

Organic waste is put in an airtight container called a digester so that the process could occur. Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane. State-of-the-art systems report producing biogas that is more than 95 percent pure methane.

The process of anaerobic digestion consists of three steps.

The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to methane gas.

Process temperature affects the rate of digestion and should be maintained in the meso-philic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is pos-sible to operate in the thermophilic range (135 to 145 degrees F), but the digestion proc-ess is subject to upset if not closely monitored.

Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odour and water pollution such as in large dairies, anaerobic digestion re-duces the odour and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.


Date: April 22, 2005 Page 29 (40)

Assessing energy volume of the pile

Total volume of the bagasse pile was estimated to be 170 000 – 180 000 m3 (assuming average depth of the pile as 4 m) including the day’s/week’s storage of fresh bagasse (covered store by the mill).

According to an estimation 6 made by the consultant, the energy content of bagasse de-creases rapidly during storing as illustrated in the following figure 17.

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5Time stored in years

NC

V , M

J/kg

(as

rece

ived

)

Figure 17 Development of energy content of bagasse as a function of time (Note: rough estimation made by the consultant)

The NCV of fresh bagasse as received is approximately 7.3 MJ/kg. According to the consultant’s estimation the one-year storage time would reduce the NCV to 3 MJ/kg. In practice it is recommended that 3 MJ/kg could be the lowest NCV content in fuel that would be used in co-firing with fresh bagasse. One-year excess bagasse output from BSI is typically about 30 000 tons.

Figure 18 shows estimated waste bagasse volumes (tons) at different fuel energy con-tent levels (NCV, MJ/kg).

6 Not based on any research or facts thus does not give basis to make firm conclusions


Date: April 22, 2005 Page 30 (40)

0

20 000

40 000

60 000

80 000

100 000

120 000

140 000

160 000

180 000

0 1 2 3 4 5 6 7 8Fuel NCV: MJ/kg

Bag

asse

, ton

s

Suitable / Recommended part for co-firing

Figure 18 Volume and energy content of waste bagasse (Note: rough estimation made by the consultant)

3.4 Recommendations for the recovery of the bagasse store as a supplementary fuel Fuel samples must be taken and NCV measured to verify the hypothesis set by the

consultant regarding the fermentation of bagasse.

Only bagasse that is less than one year old should be used.

Bagasse should be added in small amounts in a steady flow to the fresh bagasse going into the boiler.

Research work should be done to develop a suitable storing method for fresh bagasse

Research work should be done to verify the waste bagasse use potential as soil im-provement material.


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APPENDIX 1 Laboratory analyses

Bagasse (fresh)


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Elephant Grass (fresh)


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Jatropha Curcas nuts


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Sugar cane tops


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Wild Cane (fresh)


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APPENDIX 2 Collecting biomass samples

Elephant Grass (Belize, Nov. 2004)

Wild Cane (Belize, Nov. 2004)


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Sugar cane tops (Belize, Nov. 2004)

Bagasse (BSI, Orange Walk Town, Nov. 2004)


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APPENDIX 3 Waste bagasse store at the sugar mill

(BSI, Orange Walk Town, Nov. 2004)


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APPENDIX 4 Schematic drawing of an Inclined Grate Boiler

assessing alternative and supplementary sources … a fuel suitability study to establish a valid...

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