**6. Environmental aspects**

Global recognition regarding the significant long-term impact due to climate change provides a key foundation for utilizing alcohol fuels, which means that alcohol fuels should be able to accommodate chances in reducing climate change gases(CO2, methane, N2O, etc.). Bio-ethanol is highly effective in reducing greenhouse gas evolution. Corn-based bio-ethanol is reported to generate 43% less greenhouse gases compared to pure gasoline.

From purely theoretical point of view, ethanol can be finally produced from the biomass that is made based on the CO2 absorbed by plants during photosynthesis, and thus it can be called carbon neutral. Unlike hydrocarbons which evolve voluminous amount of CO2 from their internal carbon atoms during combustion, ethanol can be regarded as carbon neutral without generating as much CO2 from internal carbon atoms. In practice, however, significant amount of greenhouse gas evolution is directly and indirectly caused by cultivation of biomass crops and synthesis of alcohol fuels.

The range of CO2 reduction potential is large when alcohol fuels are used. Values range between 0.5 kg CO2-equivalent/liter of ethanol for ethanol produced from wheat and up to 2.24 kg CO2-equivalent/liter of ethanol for ethanol manufactured from sugar cane (**Figure 11**) [25].

According to the result shown in **Figure 11**, among alternative liquid fuels, only cellulosic ethanol, biomass to liquid (BTL), and CCS-involved processes (BTL-CCS, CBFT-CCS, CBMTG-CCS) exhibit the CO2-negative performance in life cycle analysis (LCA) perspective. Carbon capture and storage (CCS) process is not fully economically feasible and technically proven till now; moreover, considering public objection on CCS, connecting the process to CCS is not practical for the time being. In CO2 reduction aspect, cellulosic ethanol is the most reasonable choice as a renewable alternative fuel.

Adding ethanol to gasoline fuel of automobile, oxygen contents of fuel mixture increases and yielding the effect of reducing pollutants evolution. As alcohol fuels are inherently sulfur-free, it suits for cleaner environment. Besides, since ethanol is produced by fermentation with crops that contains starch, its purity is high, and no hazardous combustion by-products such as SO2 or metal oxides are generated during the combustion when compared to the petroleum-based fuel. But, high solubility into water by short carbon chain alcohols such as ethanol and methanol can cause an underground water pollution, although short carbon chain alcohols are well degradable in few days under normal circumstances. This problem can be minimized with the use of higher carbon chain alcohols like bio-butanol.

#### **Figure 11.**

*Estimated carbon dioxide (CO2) emissions over the life cycle of alternative fuels [8, 26]. Note: BTL, biomass to liquid; CBFT, coal and biomass to liquid, Fisher-Tropsch; CBMTG, coal and biomass to liquid, methanol to gasoline; CCS, carbon capture and storage; CFT, coal to liquid, Fisher-Tropsh; CMTG, coal to liquid, methanol to gasoline.*

Forest clearing and chemical fertilizer are involved to grow corn and other grains for the first-generation bio-ethanol, which eventually ends up with CO2 production and countervailing the CO2 reduction amount by bio-ethanol use, sometimes more than the reduced amount. In this regard, the second-generation lignin-based or third-generation algae-based raw material is a better candidate for bio-alcohols.

It is especially noteworthy that the definition of (environment friendly) bioenergy is rather more stringently defined in the EU and United States: more than 35% reduction of greenhouse gas is required to qualify compared to fossil fuels of the same calorific value [27].

The issue of required water amount asks the approach of water-energy nexus in that technology development will follow for the process of better environmentfriendliness and sustainability [13]. As an example, recent water shortage encountered in Chennai, India, might be attributable to global warming, and the water quantity consumed for the production of alcohol fuels is emerging as an important issue. Chennai region went without rain for 200 days in 2018.

The process consuming the largest amount of water is the cultivation of biomass crops. Among the production processes for alcohol fuels, refinery step consumes the largest amount of water. The water quantity consumed for US corn-based ethanol production is approximately equivalent to the water requirement that can sustain 5000 people for 1 year.

Moreover, the refinery process that is going to be extended for the secondgeneration cellulosic ethanol is expected to consume 2.9 times more water needed for corn-based ethanol refinery process. At present, cellulosic ethanol production process consumes about 9.8 L/L-ethanol [13], which is unduly high.
