Introduction to Cellulosic Ethanol
Since the beginning of civilization on earth, humans have used biomass for many of their energy needs such as cooking, heating dwellings, lighting, firing clay pots, and processing metals. The industrial revolution, leading to the development of the internal combustion engine for transportation and coal power plants for electricity generation have caused a rapid shift in our energy dependence from renewable resources to non-renewable fossil fuel resources. The processes of industrialization and continuous economic development are driven by energy consumption. The global demand for energy is expected to increase at a faster rate in upcoming years due to rapidly developing economies and partly due to the exponential growth in the world's population.
The energy demand predictions for the Organization for Economic Cooperation and Development (OECD) nations as well as for non-OECD nations are available in the International Energy Outlook report of the U. S. Department of Energy. The world energy consumption from 1990 with predictions till 2035 is shown in the
bar graph in Figure 1.1 . This study forecasts that total world energy use will rise to 619 quadrillion BTU (British thermal unit) in 2020, and 770 quadrillion BTU in 2035 from the 2008 value of 505 quadrillion BTU. Furthermore, much of the growth in energy consumption is expected to occur in countries outside the Organization for Economic Cooperation and Development (non-OECD nations) where demand is driven by strong long-term economic growth. Energy use in non-OECD nations increases by 85 percent in the reference case, as compared with an increase of 18 percent for the OECD economies as shown in Figure 1.1 .
At a time of rapid increase in global energy consumption, energy sources are a critical term in the energy equation. As of 2012, more than 80% of the world's energy needs are fulfilled by fossil fuels and the contributions to global energy demand from different resources are presented in the Global Renewable Energy Share Report; the current shares of principal resources are shown in the pie chart in Figure 1.2 . Total renewable energy share is 16.7%, and these sources can be divided into two groups: traditional renewables and modern renewables. On the global scale, the share of traditional renewables is slightly higher than all the combined modern renewables. Traditional biomass energy sources such as firewood,
geothermal power generation 0.7% Biofuels
which are used primarily for cooking and heating in rural areas of developing countries, could be considered renewable. These traditional renewables account for approximately 8.5% of total final energy consumption. Modern renewable energy is dominated by hydropower for electricity generation and accounts for 3.3%; heat generation using modern biomass-derived fuels such as biogas, geothermal and solar heating accounts for another 3.3%. Biomass- derived transportation biofuel such as bioethanol and biodiesel supplies only 0.7% of the current global energy requirement.
While fossil fuels have become the world's main energy resource and are at the center of global energy demands, its reserves are limited. There are varying estimates of fossil fuel reserves on earth. In spite of all the recent advances in oil exploration technologies, the frequency of new oil and coal discoveries has rapidly diminished in the last twenty years. In cases like shale oil and fracking, much higher efforts and investments are required for extraction of fossil fuel from earth. As a finite resource depletion of petroleum reserves is inevitable, limitations in the supply have resulted in a rapid increase in fuel prices around the globe after the 1970s.
However, according to the World Energy Outlook 2012 predictions, a steady increase in hydropower and rapid expansion of wind and solar power has cemented the position of renewables as an indispensable part of the global energy mix. By 2035, renewables
are expected to account for almost one-third of total electricity output . Solar power is expected to grow more rapidly than any other renewable energy technology. Furthermore, in accordance with International Energy Agency (IEA) 2012 predictions, renewables will become the world's second largest source of power generation by 2015.
Modern renewable energy can substitute for fossil fuels in four distinct markets: power generation, heating and cooling, transport fuels, and rural/off-grid energy services. During the last decade, total global installed capacity of many renewable energy technologies grew at very rapid rates. Solar photovoltaics (PV) grew the fastest of all renewable technologies during this period, with operating capacity increasing an average of 58% annually. It was followed by concentrating solar thermal power (CSP), which increased almost 37%, growing from a small base and wind power, which increased by 26%. The growth of liquid biofuels has been mixed in recent years, with biodiesel production expanding in 2011, and ethanol stable or down slightly compared with 2010. Hydropower and geothermal power are growing globally at rates of 2-3% per year, making them more comparable with global growth rates for fossil fuels. However, in several European countries the growth in these and other renewable energy technologies far exceeds the global average .