The large-scale production of ethanol as a fuel started in Brazil in 1975, followed by the USA in 1978. The amount of ethanol produced in the world in 2001 was 22,540 billion l (Fig. 6.1); global production was dominated by Brazil and the USA. Brazilian production is based on the fermentation of sugar from sugarcane, whereas the USA used starch extracted from maize. By 2006, global production had increased to 51 billion l (13.5 billion gallons), which represents 4.6% of global petrol consumption.
Bioethanol has the potential to replace 353 billion l of petrol, which is 32% of the global petrol consumption (1103 billion l) when used as 85% addition (E85) (Balat et al., 2008). In 2006, the USA produced 4.85 billion gallons (18.3 × 109 l) of bioethanol which has overtaken the production by Brazil of 4.49 billion gallons (16.97 × 109 l) (Fig. 6.2).
The plants installed in Brazil are small, in the region of 100,000 l in capacity, compared with the large units installed in the USA at around 0.8 billion l per year.
Table 6.4. Ethanol petrol blends used in vehicles.
Fuel Country Bioethanol (%) Modification required Alcool (E95) Brazil 95.5 Engine modifications
needed Gasoline (E25) Brazil 24 None
E10 (gasohol) USA 10 None
E85 USA 85 Duel fuel cars
Oxygenate for petrol replacing MTBE
USA 7.6 None
Used in reformulated petrol
USA 5.7 None
Addition to petrol UK 5 None MTBE, methyl-tert-butylether.
Brazil 38%
USA 24%
EU 7%
Rest 31%
Fig. 6.1. Distribution of production of ethanol worldwide in 2001, from a total of 22,540 billion l. (From Licht, 2006.)
Brazil 33%
USA 36%
EU 15%
Rest 16%
Fig. 6.2. Distribution of ethanol production worldwide in 2006, from a total of 51,000 billion l. (From Balat et al., 2007.)
Table 6.5. Leading ethanol producers in the USA 2006. (From Solomon et al., 2007.)
Company Capacity (109 l/year) Archer Daniels Midland 4.1
VeraSun Energy 0.87
Hawkeye Renewables 0.84 Aventine Renewable Energy 0.57
Cargill Inc. 0.46
Abengoa Corp. 0.42
New Energy Corp. 0.38
Global Ethanol/Midwest 0.36
Total 19.0
Table 6.5 lists some of the largest ethanol producers in the USA, with the largest vol- ume produced at 4.1 billion l (1.08 billion gallons). This difference is in part due to the USA plants being attached to very large maize-processing plants, whereas in Brazil locally grown sugarcane is processed in smaller units avoiding extensive transport.
Ethanol production process in the USA
In the case of sugarcane the sugar can be pressed from the cane and used directly in fermentation to produce ethanol. Starch on the other hand cannot be used by yeasts in fermentation and so has to be converted to glucose before it can be used. This is the main difference between the processes used in the USA and Brazil.
Figure 6.3 outlines the process that is used to obtain glucose from maize. The process was developed initially to produce starch from maize where the starch was used in food formulation, for the production of high fructose maize syrup, a low- calorie sweetener and a wide range of starch products. The harvested maize is soaked in water to loosen the kernels which are passed through a mill which removes the germ or embryo. The wash water is known as maize steep liquor and is one of the components of the medium used to grow penicillin. The separated germ is used for germ meal as a high protein supplement or pressed to extract maize oil (Mazola) used widely in cooking. The kernels, which now contain mainly starch, are ground, washed and centrifuged to remove fibre and gluten which are used in the food indus- try. What remains is starch which can be used in food and other products. Some of the starch is converted into glucose so that it can be processed into a mixture of glu- cose and fructose known as high fructose maize sugar (HFCS) for use as a low-calorie sweetener. The conversion of starch into glucose is carried out by starch-degrading enzymes, the amylases. Starch is synthesized in the chloroplast where glucose molecules are linked together with α-D-1,4 glycosidic linkages to form long chains (Fig. 6.4). At some stages, a branching by enzyme adds a side chain with a α-D-1, 6 linkage which gives starch a more rigid structure. The starch molecules can be bro- ken down either by hydrolysis with acid or by enzymatic breakdown. The enzymatic
Fig. 6.3. The processing of maize in the USA for the production of glucose. (From Scragg, 2005.)
a-Amylase 60–80°C Liquefaction
Cooler Starch
Gluten Hull and fibre
Germ meal Degerminators
Grinding mill
Washing screen Centrifuge
Maize oil Maize steep liquor Steep
tanks Maize
Glucose
Amyloglucosidase 50–60°C
O O O
O
O
O
O
O
O
O O
OH CH2OH HO
HO HO
HO HO HO
HO OH
HO OH
HO HO HO HO OH
CH2OH OH
CH2OH CH2OH CH2OH
CH2OH CH2OH
CH2OH
CH2OH
CH2OH CH2OH
OH
OH OH
OH
OH OH
OH OH OH
OH
HO
HO Galactose
Starch
Cellulose
Hemicellulose
Xylose Arabinose
C O
CH CH2OH
MeO
O CH
CH CH2OH
MeO O
OMe O OMe
OMe C CH CH2OH
OMe O
CHO CH CH2OH
O MeO
CH CH
HOCH2 O
CH CH
OMe
CH
OH MeO OMe
~
~
~
CH2OH CH3O
HO H
Coniferyl alcohol Lignin
O O
O
O O
O O O O
O O
O O O O
Fig. 6.4. The structure of starch, cellulose, hemicellulose and lignin.
process is normally used as it produces fewer by-products. The starch forms a stiff paste with water, which is first liquefied by heating to 60–80°C and the enzyme α-amylase added. The enzyme breaks the long glucose chains in the starch into shorter sections, known as long chain dextrins. In some cases, a high temperature α-amylase is used and the starch heated to 100°C. After a short period of time, the liquefied starch is cooled to 50–60°C and another enzyme, amyloglucosidase, added.
This enzyme converts the dextrins into glucose.
Once glucose has been produced, it can be used as a substrate for ethanol fermen- tation by yeast. Figure 6.5 shows a typical process for the production of ethanol from glucose. The glucose from the starch is mixed with salts, nitrogen and phosphorus
Fig. 6.5. A typical process for the production of ethanol from glucose produced from maize starch.
Glucose + salts
Sterilization
Carbon dioxide
Solid carbon dioxide
‘dry ice’
Yeast Adsorption
column
Fermentation in bioreactor 30°C
Centrifuge
Spray drier
Heat exchanger
Storage tank
Distillation column
Storage tank
Pump
Ethanol Waste
Dry yeast
and the mixture sterilized by heating to 120°C for 2–5 min in a continuous sterilizer.
Once sterilized, the medium is run into a large bioreactor (200,000 l and above), yeast added from a seed bioreactor (1–10% volume of the main reactor) and the culture incubated at 30°C for a few days. Carbon dioxide produced during the fermentation can be adsorbed and used to make solid carbon dioxide, ‘dry ice’. Once fermentation has finished the yeast cells are removed by centrifugation and the medium, sometimes known as ‘beer’, is warmed by passing through a heat exchanger and then distilled.
Distillation is needed to concentrate the ethanol and is the major energy-consuming stage. The fermentation yields about 10% ethanol and it needs to be more than 95%
to be used as E95 or 100% if used as a blend. Heating a 10% ethanol solution will yield a vapour containing more ethanol than water and the remaining solution will contain more water so that a limited amount of enriched ethanol can be obtained.
But with a series of distillations a concentration of 95.6% ethanol can be obtained.
At 95.6% the liquid and vapour have the same concentration so no further concen- tration is possible. The mixture is known as an azeotrope. However, by using a distil- lation column separated by plates a series of separate distillations can be produced and this will give the azeotrope in one distillation. To produce anhydrous ethanol a second distillation is required where benzene is added and this on distillation gives pure ethanol and the benzene can be recovered and used again.
Ethanol production in Brazil
The production of ethanol is considerably simpler in Brazil as there is no starch to process. The sugarcane is harvested and milled to extract sugar (sucrose) and the rest of the plant, known as ‘bagasse’, is retained as it can be burnt in boilers. The sugar can be processed to produce sugar and the residue and molasses used for fermentation or the sugar juice used directly (Fig. 6.6). The sugar and salts are run into 100,000–
400,000 l open bioreactors and inoculated with yeast. After fermentation has ceased, the yeast is removed by flocculation or centrifugation and the liquid distilled. If more than 95.6% ethanol is required a second distillation is carried out with the ethanol blended with fusel oil. The residue from the first distillation can be used as a fertilizer.
The economy of the process is improved greatly as the residue from the sugarcane (bagasse) is used to fire boilers which supply steam for the distillation process.