How Biofuels are Produced?


Bioethanol and biodiesel are the two most promising alternative liquid transportation fuels that are being produced commercially worldwide. The conversion technologies for producing these two biofuels are quite standardized. Depending on feedstocks, some additional upstream pretreatment may be adopted to prepare the feedstocks in the form that can be readily converted into biofuels

Bioethanol


Ethanol is produced through anaerobic fermentation of sugars by yeast, Saccharomyces cerevisiae (Fig. 2). In most cases, 6-carbon sugars are the common substrate for the fermentation. Five-carbon sugars derived from plant material (lignocellulosic biomass) can also be fermented by other microbes. Sugar-based feedstocks such as sugar cane, sweet sorghum, and sugar beet provide readily sugars for fermentation. Molasses, a by-product from a sugar factory containing roughly 49% sucrose, can be diluted and utilized similarly as the sugar-based feedstocks. On the other hand, starch-based feedstocks such as corn, cassava, and sorghum grain require enzymatic hydrolysis to release sugars prior to fermentation. Complex feedstocks such as lignocellulosic biomass require more complicated upstream pretreatment for releasing fermentable sugars.

Sugar-rich juice derived from sugar-based feedstocks is primarily composed of disaccharides such as maltose or sucrose (C12H22O11) which can be readily hydrolyzed by enzymes present in the yeast, namely sucrose or invertase (Fig. 1). The resulting sugars are monosaccharides such as glucose or fructose (C6H12O6) which are converted into ethanol by anaerobic fermentation (Fig. 2).



Fig. 1 Enzymatic hydrolysis of disaccharide into monosaccharide by yeast




Fig. 2 Glucose fermentation by yeast for ethanol production


Regarding the stoichiometry, one mole of sucrose produces four moles of ethanol and four moles of carbon dioxide. As a result, theoretically, one ton of sucrose can produce 163 gallons of ethanol. However, under normal plant operating conditions, about 141 gallons of ethanol per ton of sucrose can be expected from sugarcane.

Unlike sugar-based feedstock, starch-based feedstocks require starch hydrolysis (liquefaction and saccharification) to produce fermentable sugars after the feedstocks are milled and mashed. The stoichiometry of starch hydrolysis and ethanol fermentation is illustrated in Fig. 3. Thus, theoretically, ethanol yield per bushel (56 lbs) of corn containing ~75% starch is 3.63 gallons. This is equivalent to 490.7 L of ethanol per dry ton of corn. However, in actual dry-grind corn ethanol process, the conversion efficient is around 71.6-77% resulting in ethanol yield of about 2.6 to 2.8 gallons per bushel of corn. This corresponds to an actual ethanol yield of 351 to 378 L per dry ton of corn. Similarly to cassava chips with a starch content of 69.7%, the theoretical ethanol yield is 455 L per dry ton cassava, whereas the actual ethanol yield is around 400 L per dry ton cassava. Sorghum grains contain 50 to 75% starch and the ethanol yield is within the periphery of corn.



Fig. 3 Stoichiometry of ethanol conversion from starch-based feedstocks


Biodiesel


Biodiesel is made through a chemical reaction called transesterification (Fig. 5) where fats and oils are converted into a mixture of fatty acid methyl esters (FAME) in a presence of alcohol (commonly methanol) with base catalyst (mostly potassium hydroxide).



Fig. 4 Transesterification reaction for biodiesel production


Oil seed with the oil content exceeding 20% is considered as an oil feedstock for biodiesel production. Most of the biodiesel currently available in the market is generally derived from plants (food/feed crops) including soybean, rapeseed, and palm oil, whereas biodiesel derived from animal fat (tallow-based biodiesel) is viable only in the countries having the large livestock sector like Paraguay and Uruguay. However, due to significant land-use and concerns over food security, the trend for biodiesel production has moved towards the use of non-agricultural feedstocks and non-food oil feedstocks such as waste oil, Jatropha, and algae.

Unlike ethanol production, the calculation of biodiesel yield is more complicate due to a variety of fatty acids present in oil/fat feedstock and its purity. In general, 10 pounds of fat/oil can be converted to 10 pounds of biodiesel with a co-production of 1-pound glycerol. In actual operating condition, 1.5 gallons of biodiesel can be produced per bushel of soybean (60 lbs), which is equivalent to 66 gallons per acre (44 bushel of soybeans per acre).

Biofuels Residues/Wastes


Along with biofuels, the production processes also generate low-value residues and wastes. Some of the residues/wastes are:

  • Stillage from dry-grind corn, cassava, and sorghum-based ethanol plants
  • Vinasse from sugarcane, sweet sorghum, and molasses-based ethanol plants
  • Bagasse from sugarcane-based ethanol plants
  • Lignocellulosic stillage and lignin from lignocellulosic-based ethanol plants
  • Seed cake and glycerol from biodiesel plants
  • Oil-extracted algal cells from algal-based biodiesel plants


  • The low-value residues have been generated in considerable amounts as a result of the increasing biofuel production in recent years. Generally, biofuel residues/wastes are:

  • Low value
  • Low pH (normally ~ pH 4 resulted from optimal pH of ethanol fermentation)
  • Dark colored
  • Odor nuisance
  • High impurity
  • High organic matters
  • Recalcitrant to biodegradation

  • Given their characteristics, treatment is necessary prior to their disposal to minimize adverse impact on environment. However, cost associated with treatment process could be quite high. Considering biofuel as a single source of revenue, biofuel production remains infeasible.