**11. Future direction**

There emerged several candidates that compete with alcohol fuels in the twenty-first century liquid fuel field. Green hydrogen and green electricity are the most prominent players. Whether alcohol fuels can compete with these two players will depend on the future progress in dealing with key required target: CO2 reduction, environmental cleanness, convenience in existing infrastructure, and price competitiveness. Moreover, energy-related focal points nowadays are

sustainability, suitability for carbon-free (green energy) status, and alleviation of polluting materials such as fine dust, which are primarily emerging and ecologically important topics.

Replacement of fossil fuels with alternative clean fuel will eventually lead to green energy-based sustainable society. However, currently available technology is not up to the level of commercially viable standard for social acceptance in terms of CO2 evolution and fine dust, etc., which must be comprehensively overcome.

Currently available elementary and applied technology can be utilized to synthesize liquid fuel in various forms. Although synthesis of liquid fuel is more costly than direct mining of petroleum, the application of currently available technology to alcohol fuel synthesis should be tried to make ways that can be economically feasible and lucrative when commercialized. In a sense, it is rather a problem-solving for cost-effective technology rather than the technology itself. Mass production of lignocellulosic ethanol necessitates economically competitive technology rather than the barely profitable or only technically feasible technologies.

Actually, bio-alcohol such as bio-ethanol is an industry of low unit cost. Without installing a proper scale of plant size, cost competitiveness with other liquid fuels must be quite low [13]. Actual plant construction cost remains high because of the inherent limitation of using low-energy density raw feedstock and of complex nature involved in converting into alcohol fuels. Among these, pretreatment/ detoxification and hydrolysate conditioning processes are especially costly. Such auxiliary processes have to be developed in such a way that overall process cost can be dramatically reduced through the introduction of more energy-efficient and process simplification [13].

Another important aspect is that soils and climates in much of Africa have similar characteristics to those in Brazil [31]. Africa and South America have a great potential in increasing bio-energy products including alcohol fuels.

In short, alcohol fuels should work as an energy source that can minimize the environmental impact as lower than natural gas at all applications while opening more applicable places as well as manufacturing a cheaper liquid fuel that can be used in big scales also in developing countries where plentiful but low-grade raw materials exist in plenty.
