**3. Feedstock for alcohol fuels**

Alcohols fuels can be made from all available organic materials. Natural gas, coal, biomass, and organic wastes are good sources. Alcohol fuels have been synthesized from corn and sugar cane as major raw materials, but focal issues nowadays are synthesis and production of alcohol fuels from non-food crops and agricultural residues. Non-food lignocellulosic biomass includes energy crops, cellulosic residues, and wastes.

Grain-based ethanol as a first generation has been tried to change to the secondgeneration cellulosic ethanol and other advanced cellulosic biofuels. Cellulosic ethanol has identified as a key biochemical route of converting biomass to fuels after the 2000s [8]. Algae-based third-generation feedstock for alcohol fuels emerged as a candidate that can provide a vast raw material for future alcohol fuel industry. **Figure 2** illustrates the generations of raw feedstock for the alcohol fuel production and also shows the most apparent material that is being utilized in different countries.

Definitely there exists a clear difference between developing countries and developed countries in the priority choice, but basic understanding should be identical: use the locally available, underutilized feedstock, and choose the feedstock that tipping fee is available to treat the feedstock like municipal/industrial wastes. However, when wastes are involved as feedstock, it should be noted that not-in-my-backyard (NIMBY) problem occurs as a norm in almost every countries nowadays.

The European Commission has recently resolved by voting against utilization of biofuels synthesized from biomass of food crop sources by the year 2030. Intensive interdisciplinary efforts are anticipated for timely commercialization of cellulosic bio-ethanol, which is the second-generation bio-alcohol.

Agricultural waste typically contains a relatively high content of alkali metals (potassium and sodium) and other inorganic elements including calcium, magnesium, and sometimes chlorine and sulfur. When applying thermal methods in

**Figure 2.**

*Key raw materials for bio-ethanol production in different countries (modified figure from Ref. [11]).*

*Alcohol Fuels: Current Status and Future Direction DOI: http://dx.doi.org/10.5772/intechopen.89788*

converting these wastes to alcohol fuels, alkali metal components act to produce low-melting salts that will cause plugging and other ash-related problems during the process. In contrast, fermenting method can reduce the tendency of ash problems, which is a beneficial aspect in actual manufacturing process.

In particular, rice husk contains ash content of over 90%, and rice straw consists of more than 30% as silica, although there is a variation with rice stock, climate, and geographical environment. Such inorganic contents work as a barrier to thermal conversion process, and fermenting can be a more appropriate way in converting this biomass feedstock.

## **3.1 First generation: grain feedstock**

Starch and carbohydrates have been used as a first-generation raw material to produce ethanol. During the year 2013, more than 90% of bio-ethanol had been produced from the starch and carbohydrates. Corn, grain, and cassava are major such crops. Downside issues are the destruction of environment during the crop cultivation and ethanol production as well as the use of valuable food resources as fuel production. Therefore, at current situation, large agricultural countries like the United States, Brazil, and China are major production places of biofuels including alcohol fuels. In the United States, 95% of ethanol has been produced from the starch in corn grain [7].

### **3.2 Second generation: lignocellulosic biomass**

Recently, the production of bio-ethanol from grain-based raw materials is gradually becoming limited, and the second-generation bio-ethanol production from non-grain-based biomass is now receiving a gradually increasing priority.

Bio-ethanol is currently becoming a solid option as automobile fuel, and it has been usually produced from starch of corn and cassava or sugary contents of sugar cane and sugar turnip. Bio-ethanol is also produced from lignin cellulose-based material of crop wastes. Sugar and starch are readily convertible to bio-ethanol but their availability is limited and they are costly. Therefore, work is underway to investigate into various processes to produce bio-ethanol from lignocellulose-based raw materials to utilize their abundant amount in nature and to meet the economic viability in the market [11]. Wood chips or crop residues are common lignocellulosic feedstock (**Figure 3**).

Non-edible xylem parts that constitute most of the botanical stocks or cellulose are used to produce ethanol. Rice straws, weeds, and other shrubbery are good examples as raw material for alcohol production, and valuable food resources are not wasted in this case. However, a large-scale forest or farmland is still used and the low-production efficiency is a problem. Also, economically, viability is not satisfactory yet and is not applied at measurable proportion [13].

The main obstacle of using lignocellulosic biomass resides in the difficulty in extracting the essential parts from the hard-binding components of lignin, hemicellulose, and cellulose in plants as shown in **Figure 3**.

High-growth productivity of lignocellulosic crops compared to corn and sugarcane is one of the key factors that bio-alcohols can be produced economically in the future. **Figure 4** clearly shows the high growth rates in lignocellulosic crops like sorghum, energy cane, and water hyacinth.

#### **3.3 Third generation: algae species**

Sea algae grow relatively faster than most of the land-based plants as shown in **Table 4**, and they are good source of raw material to produce alcohol fuels.

**Figure 3.** *Three key components of lignocellulose [12].*

**Figure 4.** *High-growth productivity of lignocellulosic crops to corn and sugarcane [12].*

They do not require large-scale farmland to cultivate, and non-edible algae are also a good source of bio-ethanol. Due to their fast growth rate, large-scale farming for 4–6 times cropping per year is possible and their carbon dioxide sequestration is 3–7 times more effective than that of grains. However, large-scale acquisition of the raw sea algae and its economic viability remains to be overcome before commercialization. Most of algae-related efforts are still under R&D probing stage.

Fundamental background to try algae species for biofuel production relies on their higher efficiency in converting solar energy than higher plant biomass.

### *Alcohol Fuels: Current Status and Future Direction DOI: http://dx.doi.org/10.5772/intechopen.89788*


#### **Table 4.**

*Current biofuel yields from various biomass [14].*

**Figure 5.** *Biofuel production sequence from microalgae [14].*

However, actual cultivation of microalgal biomass is not easy, rather quite challenging and still expensive than growing crops. It is a similar situation as comparing the product that has been updated for several decades and the one that is starting to experience initial trial and errors.

**Figure 5** illustrates the typical biofuel production procedures in which the basic process is identical with the hydrolysis/fermentation/separation parts of bio-ethanol production, except the feedstock cultivation and harvesting parts.
