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UGARCANE BIOMASS RESIDUES:

A CUBAN BIOENERGY ALTERNATIVE FOR SUSTAINABLE, ENVIRONMENTALLY-SAFE DEVELOPMENT

W. Alonso Pippo, Institute of Materials and Reagents, University of Havana, Cuba

J. del Rey Ocampo, Institute of Materials and Reagents, University of Havana, Cuba

Abstract

The search for alternative energy sources has been a constant objective since the middle of the last century. Today it is very important for developed as well as underdeveloped countries. Sugarcane-producing countries have in the sugarcane residuals produced by their sugar agricultural industries a great source of renewable energy. Sugarcane biomass residues have an extraordinary potential for use as biofuels and electricity that could be used to supply rural areas and remote communities. Moreover, the use of sugarcane-derived bioenergy would diminish the emission of greenhouse effect gases like CO2, thus contributing to sustainable development. Starting from open literature sources, this work presents a systematic study of Cuban sugarcane biomass production and the new technologies, advantages and drawbacks involved in the use of sugarcane-based fuels. Using data on the production of sugarcane biomass in Cuba from 1970-1998, the study also includes an assessment of the pollutants associated with its use as a fuel source.

1.Introduction

Since the early 19th century Cuba has been one of the biggest sugar-exporting countries in the world. Until 1959 the main market for Cuban sugar was the United States. However as a result of the political belligerency between the U.S. and Cuba following the Cuban Revolution, the U.S., in 1962, imposed a commercial blockade on trade with Cuba, ending Cuban sugar exports to the U.S. and the sale of U.S. fertilizers, spare parts, transportation equipment, and other goods used in sugar production, to Cuba. After the imposition of the blockade, Cuban sugar exports shifted to the former Soviet Union and Eastern European socialist countries. Almost three decades later, in 1991, this market abruptly disappeared with the collapse of Communism in the Soviet Union and Eastern Europe. Since 1991, Cuba has been struggling to rebuild its sugar production and market on a new basis. It has been a difficult time for the Cuban sugar agro-industry because of the lack of fertilizer, spare parts and other goods.

The effects of the Soviet collapse are evident in Figure 1., which shows Cuban sugarcane production and its derivates from 1970 to 1998. It is easy to notice the difference after 1991 when sugarcane production fell from 80.106 tons to 40.106 tons in 1998. Accordingly, any assessment of the potential of Cuban sugarcane biomass should rely on production data during the 1980s, the last period of normal growth for the sugar industry in Cuba

The Cuban sugar-agro industry has begun a recovering process in the last decade. The most important aspects of this recovery are the:

2. Availability of Cuban Sugarcane Biomass as an Energy Source

2.1 Sunlight Conversion Potentiality into Sugarcane Biomass

The surface area of the Cuban archipelago is approximately 110,860 km2 [2]. About 60% of Cuban land is arable. On average, 12% of Cuban land is dedicated to sugarcane production. (See Table 1). On the other hand, solar radiation in Cuba is quite homogeneous. The direct solar radiation average (Rs) may be assumed as 5kWh/m2 per day (18MJ/m2 per day) [3], [4]. For sugarcane, the percentage of sunlight utilization to conversion into organic carbon compounds on an annual basis is about 2% [5]. This is one of highest percentages of utilization for perennial grasses in the world. Sugarcane has a higher yield than other varieties of plants (about 80tons (dry) /ha.yr)

Figure 1. Cuban sugarcane production and its derivates 1970-1998 [FAO Statistical Database]

The annual available energy from sugarcane biomass may be estimated by the formula:

EAvailable - possible energy from sugarcane (Exa Joules, EJ) (yearly) EJ=1018.joule; PJ= 1015.joule

Scane- harvested surface of sugarcane (km2/y)

Rs- daily direct solar radiation annual average in Cuba (6.57 PJ/km2 . yr)

P%- percentage of sunlight utilization

Table 1 shows the values of Scane from 1970 to 1991. Using the above-mentioned formula, and considering P=2%, the EAvailable annual values, from 1970 to 1991 were calculated. The amount of EAvailable for the whole period was:

Annual average:

2.2 Sugarcane Biomass Residues Energy Yield

After the milling process, one ton of sugarcane yields approximately 100kg of sugar, 125kg of bagasse (dry basis) and leaves 240kg of sugarcane agricultural residues (RAC) with 30% moisture content. However, because of the harvesting method, only 23% of the RAC—that is, just 55kg RAC/ton of sugarcane—can be exploited to produce energy [7]. On the other hand, from an energy point of view about 60% of bagasse is needed to feed boilers during milling season, which means you need to use only 50 kg of bagasee for each ton of sugarcane to obtain an energy surplus. The caloric value of the bagasse and the RAC are 15MJ/kg (dry) and 10MJ/kg, respectively. This way it is possible to obtain 750MJ from bagasse and 378 MJ from RAC from each ton of sugarcane, in other words, 1128 MJ/ton. The data in Figure 1. calculates bagasse and RAC energy yield averages for the period 1970-1991, expressing both in tons of equivalent oil (EO). Table 2 shows the available total energy EA of bagasse and RAC during the 1970-1991 period, their yearly averages, and their useful energy EU. Comparing EAvailable=36.88 EJ with EA=5116 PJ from Table 2, it is easy to appreciate that the first one is seven times larger than the second. It is evident that in biomass residues there is an important potential for energy growth. In the period analyzed, even when sugarcane biomass residues were used with very low efficiency, it could have been possible to obtain an energy production of about 38 million EO tons, which would represent more than double the amount of oil consimmed in Cuba in 1988, the highest level in history. This alternative energy source would have saved Cuba several million dollars. Since the period under study, 1970-1991, the Cuban sugarcane agro-industry has become far more efficient in its use of sugarcane biomass residues and in utilizing its potential as a clean energy source. No longer are large quantities of bagasse wasted through low-efficiency burning in sugar mill boilers. Now, as a rule, during milling season all bagasse is used to produce steam and electricity to partially satisfy sugar mill energy needs. In summary, to achieve an energy surplus from sugarcane biomass residues it has been necessary to increase the industrial efficiency of sugarcane milling.

3. Environmental Issues

3.1 Sugarcane Biomass Reducing CO2 Emissions
In the above-mentioned period, the sugar agro-industry consumed yearly about 20% of the total imported oil by Cuba, nearly 1.5.106 tons of oil [7]. There are many studies of the CO2 emissions resulting from oil combustion that analyze different combustion conditions and fuels found throughout the world. The 1998 FAO report on CO2 emission in Nicaragua’s sugar mills determined that 0.75 kg of CO2 is emitted to produce a kWh using oil "Bunker C"[10]. The Department of Energy (DOE) of Ireland uses the figure of 3.3 tons of CO2 for each ton of fuel oil [11]. It is also possible using simple chemistry to calculate the amount of CO2 emitted. The atomic weight of carbon is very close to 12, and the atomic weight of oxygen is 16. Thus the atomic (molecular) weight of CO2 is 12 + 2x16 = 44, which means that each unit of carbon that is burned produces 44/12, or 3.67, units of carbon dioxide.

Table 1. Sugarcane production and land surface for its harvest [1]

Year

Surface

106 ha.

Surface

km2

Sugarcane

Production

106ton

Year

Surface

106 ha

Surface

km2

Sugarcane

Production

106ton

1970

1.251

12510

52.2

1981

1.327

13270

73.1

1971

1.181

11810

44.3

1982

1.200

12000

69.7

1972

1.072

10720

48.2

1983

1.349

13490

77.4

1973

1.103

11030

50.4

1984

1.347

13470

67.4

1974

1.180

11800

52.4

1985

1.328

13280

68.5

1975

1.223

12230

53.8

1986

1.3280

13280

68.5

1976

1.137

11370

60.4

1987

1.3580

13580

70.8

1977

1.236

12360

69.7

1988

1.2970

12970

73.74

1978

1.312

13120

77.3

1989

1.3500

13500

81.0

1979

1.391

13910

64.0

1990

1.4200

14200

81.8

1980

1.209

12090

66.6

1991

1.4520

14520

68.5

Besides, it is accepted that 84-86 percent of oil weight is carbon and 99 percent of carbon is burned during combustion. Then: 3.67x 0.85x0.99=3.088 (roughly three times, by mass, of burned oil.) Using the above data, for the whole period 1970-1991, the amount of oil consumed in Cuba for sugar production was 33.106 tons. The Cuban agro-sugar industry could significantly reduce CO2 emissions if it could it could become self-sufficient, that is, self-generate the energy it needs using sugarcane biomass, instead of relying on fossil fuels. It is well known that plants in general during their metabolism absorb CO2 and convert it into biomass. Therefore from an environmental point of view it is accepted that energy use of biomass is innocuous in terms of CO2 emmision. Using biomass as basic fuel during milling season would have avoided the emission of 99.106 tons of CO2 during the period under study. Furthermore, the bagasse and RAC could have substituted for 38.106 tons of oil (Table 2), which would have prevented 114.106 tons of CO2 emmissions. Currently, the efficient use of sugarcane biomass has the potential to reduce the emmission of 9.6.106tons of CO2 yearly. The savings would be significant. In the year 2000, in the EU, each ton of CO2 emmitted resulted in an average cost of around 8ˆ [12]. Finally, one of the greatest advantages of using sugarcane biomass as a fuel source is that it produces virtually no sulfur emissions, a cause of acid rain.

Table 2. Sugarcane Biomass Residues Energy 1970-1991

Sugarcane Residues

1991

EA

1970

EA

Year Average

1991

EU

1970

EU

Year Average

(PJ)

106

ton EO

(PJ)

106

ton EO

(PJ)

106

Ton EO

(PJ)

106

ton EO

Bagasse

2720.53

63.09

123.66

2.86

1088.21

25.23

49.46

1.14

Trash (RAC)

2395.51

55.55

108.89

2.52

548.45

12.72

24.93

0.57

Total Biomass Residues

5116.04 118.64   232.55 5.38    1636.66 37.95    74.39   1.71

3.2 Expelled Ashes

The sugarcane harvest method using storing/cleaning centers is widespread in Cuba. These centers accumulate a large amount of RAC. Part of it is used for livestock feed, but most RAC is burned, emitting not only CO2, but also large quantities of ashes. Figure 2. shows the chemical composition of RAC. The RAC ash content is 8%. The amount of RAC burned for whole period 1970-1991 was 90.106tons. This means that 7.2.106 tons of ashes—nearly 320,000 tons a year—were expelled. Therefore, not only was approximately 13.106 tons of EO lost through the burning of RAC, but the environment was poisoned as well. The incineration of similar quantities of residuals during milling seasons further pollutes the atmosphere in the vicinity of sugarcane farms.

Because of the low efficiency of sugar mills, 40% of bagasse is also burned. Noting from the above data the amount of bagasse used and amount of ash in its composition, 4%, (see Figure 3), we can calculate that some 8,12.106tons of ashes were expelled through bagasse combustion during the studied period. The effects of the wasteful and environmentally harmful use of sugarcane biomass residues between 1970-1991 are summarized below in Table 3.

Figure 2. RAC Chemical Composition

Figure 3. Bagasse Chemical Composition

3.3 Sugarcane Biomass Residues Fire Risks

To convert bagasse and RAC into energy, biofuels, or any other energy carrier it is necessary to store them in bulk outside of the milling season. The most common method of bagasse storage is unprotected storage, storing it in bleakness from 26 up to 52 weeks in piles that have an average height of 10-12 meters. The density of bagasse is 128kg/m3 while that of RAC is120 kg/m3. From this, we can calculate that a sugar mill with a capacity of 5000 tons of sugarcane daily would need to store 275 tons of bagasse (supposing a consumption of 60% to maintain the industrial process) and approximately 303 tons of RAC, meaning in the first case a volume of 1953 m3 and in the second 2292 m3. Assuming the pile shape as a truncated cone with an average height of 10m. and a radius of 10m., the pile volume would be 2053.m3, and its surface 314 m2. Such a sugar mill would need to store a pile a day. There are 155 sugar mills in Cuba. (Of course, their different milling capacities produce bagasse and RAC piles smaller or larger than our hypothetical average.) Nonetheless, calculating only for 100 sugar mills, and for a normal milling season (150 days), the needed surface, only for bagasse, would be 471ha. For RAC we can project the same figure. To store both would require a total surface area of 942 ha.

Studies of bagasse storage indicate that after a long storage period it loses around 10-20% [13] of its weight. The fundamental cause of this is bacterial action (mainly thermophiles mesophiles) [14], which in a rich polysaccharides material like bagasse begin to reproduce and increase their metabolic temperature. This process increases the risks of self-combustion, which can lead to the break out of fires. Preventing the break out of fires amongst these large and numerous sugarcane biomass residue piles is a time-consuming, labor-intensive and extremely costly task. Therefore it is very important that sugarcane biomass residues be converted into liquid or gas energy carriers directly at the sugar mill in order to ensure their easy, safe and inexpensive handling.

Table 3. Main environmental aspects concerning sugarcane biomass residues

Sugarcane Biomass Residues

Wasted Energy EO

(106 ton)

Emitted

CO2 (106ton)

Ash (106ton)

Whole

Average

Whole

Average

Whole

Average

Bagasse

25.23

1.14

75.69

3.42

8.12

0.369

RAC

12.72

0.57

38.16

1.71

7.2

0.327

Total

37.95

1.71 113.85 5.13 15.32

0.696

4. Technical, Financial and Social Aspects of Sugarcane Biomass Residues Energy

Use

4.1 Technical and Financial Impediments

Utilizing the spare bagasse from sugar production is the sine qua non to reducing the costs of power production or any energy carrier derived from sugarcane biomass residues. This is because 60-70% of the energy costs depend on the cost of feedstock. Achieving trash surpluses is not possible without changing the traditional technologies of cogeneration in sugar mills. The widespread cogeneration system in the Cuban sugar mills employs the contra-pressure turbine. Modernization of the sugar industry could be carried out by introducing cogeneration systems like the Integrated Gasification Combined Cycle (IGCC) [15], Condensing Extraction Steam Turbine (CEST), or Boiler Integrated Gasifier Gas Turbine Combined Cycle( BIG/GTCC) [16]. Table 4 compares cogeneration systems for a medium size Brazilian sugar mill [17]. Any of the above-mentioned cogeneration systems is much more efficient than the contra-pressure turbine still used in Cuba. However these systems require a steep initial investment that is beyond the means of Cuba to undertake alone and thus would require the participation of joint-venture foreign investors. The biggest obstacles to the introduction of these improved technologies are that:

Table 4 Cogeneration Systems Comparison.

Parameter

Unit

Cogeneration System

BIG/GTCC

CEST

Total Generated Power

MW

32,583

18,537

Consumed Power

MW

4,875

4,875

Electricity /ton sugarcane

kwh

217

124

Heat/ ton sugarcane

kwh

182

182

Total Energy /ton sugarcane

kwh

399

306

Surplus electricity

kwh

162

91

A no less important technology is biomass flash pyrolysis. Through bagasse flash pyrolysis it is possible to obtain an energy carrier known as bio-oil, which can be easily stored and handled like liquid fossil fuels, using existing pipeline infrastructure. However to date, the technology for converting bagasse into bio-oil in not yet commercially viable.

 

4.2 Social Aspects

There is a famous popular expression: "There is not country without sugar.” Old though it may be, this expression remains true today, at least as regards Cuba. Around 20% [1] of industrial workers still continue to be associated in one way or another with the sugar industry in Cuba. Overall, more than a million people out of the four million Cubans in the workforce are linked to the sugar agro-industry. Over the last 40 years, to the method of harvesting sugarcane has changed considerably. Before 1959, sugarcane was harvested entirely by hand, by some 350,000 machete-swinging workers, macheteros. Today agricultural machines harvest about 80% of the sugarcane. Also, during the 1980s a new harvesting method, using storing/cleaning centers was introduced in Cuba. At present, this is the most common way of carry out the cleaning of the harvested crop in sugar mills. Nonetheless, while the harvesting and production of sugar has been mechanized to some degree, the geographical, technological and social bases of sugar production in Cuba still resemble in many respects those of the mid-19th century.

5. Conclusions

The treated issues and points of view are authors’ personal opinions. In brief summary, it can be concluded that:

 

6.References

  1. FAOSTAT. Agriculture Statistical Data Base 1999. http://apps.fao.org/default:html
  2. Anuario Estadistico de Cuba 1986. Comite Estatal de Estadisticas1987. Cuba
  3. Enrico Turrini “El Camino del Sol “CUBASOLAR. 1999
  4. Average daily solar radiation Annual www.ok.solar.com/technical/dayl_solar_radiation7.gif
  5. Braun G. Bioenergia para el desarrollo. In: SEMINARIO INTERNACIONAL GENERACION COMERCIAL DE ENERGIA ELECTRICA EN LA AGROINDUSTRIA CANERA. Guatemala 1994.
  6. James A. Duke. “Handbook of Energy Crops” 1983
  7. Seminario Internacional de energia de la cana de azucar.” Ponencia Central energia potencial de la cana de azucar” La Habana 7-9 Noviembre 2000
  8. Energia Renovable. Comision Nacional de Energia 1987
  9. ICIDCA-GEPLACEA-PNUD. Manual de Derivados de la cana de azucar. Coleccion GEPLACEA. Mexico.1988
  10. Estudio de la generacion de electricidad en los ingenios de Nicaragua FAO 1998
  11. DOE Ireland Emission CO2 http://www.iol.ie/~eceal1/Data/Emmision%20Factors.html
  12. EcoSecurities Ltd. “ Carbon Trading. What does it mean for Renewable Energy”. REFOCUS Jan/Feb.2002 pag.24-27
  13. STUDY OF COMPOSITIONAL CHANGES IN BIOMASS FEEDSTOCKS UPON STORAGE (RESULTS) D. K. Johnson, et al. National Renewable Energy Laboratory Golden, CO. 80401.1998
  14. Kirk-Othmer Encyclopedia of Chemical Technology, (3rd ed.), Vol. 3, 434 p. Wiley and Sons, New York, 1978. Resource Information. Clearinghouse 1999. http://rredc.nrel.gov/biomass.html
  15. Kari Salo, “Gasification Technologies for Sugarcane Biomass”. International Workshop Energy in the Sugar Cane Agro industry. Convention Center Havana 7-9 November 2000
  16. Eric D. Larson, “ Biomass Integrated-Gasifier /Gas turbine Combined Cycle Technology for Sugarcane Processing Industries: Possibilities for Cuba” International Workshop Energy in the Sugar Cane Agro industry. Convention Center Havana 7-9 November 2000
  17. Electo Silva Lora, et al, “Electricidade a partir do bagaco de cana” Biomassa Energia dos Tropicos em MINAS GERAIS. Pag. 59-81. Belo Horizonte 2001

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