Bioethanol and Bioethylene

Biomass- based ethanol currently substitutes around 3% of the global gasoline consumption. It has a higher octane rate compared with gasoline (98 vs. 80), but has a lower volumetric energy content (67% of gasoline). Therefore, per kilometer driven, around 20% more ethanol is required - 15] , Favorable political conditions for biofuels have stimulated a significant increased in global production. Currently, the major producers are Brazil (sugarcane) and the USA (corn). In Brazil, the integrated production of sugar and ethanol provides a certain degree of flexibility, depending on market demand. The production may be shifted from 55/45 sugar/ethanol to 45/55 sugar/ethanol. At the standard 50/50 ratio, roughly 67 kg sugar and 47 l ethanol is obtained per ton sugar cane. Most production plants are energy self- sufficient due to the use of the internal by-product bagasse. One ton of sugar cane yields between 240 and 280 kg bagasse (humidity ca. 50%) with an energy content that can replace 580 kWh by using a combined heat and power unit. The state-of-the-art integrated production allows a bagasse surplus of 7-15%, which might be sold on the fuel market [16, 17]. Aside from biofuels, ethanol can also be used as a chemical feedstock. A commercially proven system is the catalytic dehydration into ethylene; a bulk petrochemical. So-called 'bioethylene' could provide access to the C4-building blocks such as butadiene, 2-butene, etc. Through metathesis 2-butene and ethylene can be converted into propylene; the key C3-building block. Most advances are currently geared towards conversion of ethanol via catalytic dehydration and the subsequent polymerization into bio-based PE.

Braskem has announced the construction of a 200 kton bioethylene production site in Triunfo, Brazil with a planned completion for end 2010. The demand for such 'green' plastics could reach 2 Mton over the 2010-2019 period [18]. Nonetheless, this is still small compared with the global production of fossil-based plastics based on, for example PE and polypropylene (PP) which has reached the 245 Mton threshold [19] .

Table 12.5 reveals the impact of CO- emissions by shifting from fossil-based to bio -based processing systems. Currently, due to the high energy intensity of cultivating wheat and other temperate carbohydrate-based crops, only the tropical sugar cane can lead to a reduction in CO2 emissions in the case of PE production.

432 I 12 CO2-Neutral Production-Fact or Fiction? Table 12.5 Emissions for the production of chemicals/fuels and for their end use.

Figures given

in g CO2eq MJ

Figures given

in kg CO2eq kg 1

product

Gasoline (Crude oil)

Bioethanol (Sugarcane)

Bioethanol (Wheat)

(Sugarcane)

Bio PE (Wheat)

Production of raw materials

(Biomass)

(Naphtha)

0.6 (Biomass)

2.2 (Biomass)

Processing

7.0

1.1

31.9

0.5

0.5

0.5

Distribution

1.0

2.3

1.5

0.2

0.2

0.2

Total emissions

12.5

21.4

73.7

2.2

1.3

2.9

Combustion

74.4

CO2 cycle (71.4)

CO2 cycle (71.4)

(Incorporated in product)

(Incorporated in product)

(Incorporated in product)

Total emissions

(Burning)

86.9

21.4 (92.8)

73.7 (144.1)

Source: Based on [20], [21], [22] and own estimations.

Source: Based on [20], [21], [22] and own estimations.

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