3.1 Introduction

Today’s energy system is unsustainable because of equity issues as well as environ­mental, economic, and geopolitical concerns that have implications far into the fu­ture. Bioenergy is one of the most important components to mitigate greenhouse gas emissions and substitute for fossil fuels (Goldemberg 2000; Dincer 2008). Renew­able energy is one of the most efficient ways to achieve sustainable development.

Plants use photosynthesis to convert solar energy into chemical energy. It is stored in the form of oils, carbohydrates, proteins, etc. This plant energy can be con­verted to biofuels. Hence biofuels are primarily a form of solar energy. For biofuels to succeed at replacing large quantities of petroleum fuel, the feedstock availability needs to be as high as possible.

In the context of climatic changes and of soaring prices for a barrel of petroleum, biofuels are now being presented as a renewable energy alternative. Presently, re­search is being done on microscopic algae, or microalgae, which are particularly rich in oils and whose yield per hectare is considerably higher than that of sunflower or rapeseed.

In recent years, recovery of liquid transportation biofuels from biorenewable feedstocks has became a promising method. The biggest difference between biore­newable and petroleum feedstocks is oxygen content. Biorenewables have oxygen levels of 10 to 44% while petroleumhas essentially none, making the chemical prop­erties of biorenewables very different from those of petroleum (Demirbas 2008; Balat 2009). For example, biorenewable products are often more polar and some easily entrain water and can therefore be acidic.

There are two global transportation fuels — gasoline and diesel fuel. The main transportation fuels that can be obtained from biomass using different processes are sugar ethanol, cellulosic ethanol, grain ethanol, biodiesel, pyrolysis liquids, green diesel, green gasoline, butanol, methanol, syngas liquids, biohydrogen, algae diesel, algae jet fuel, and hydrocarbons. Renewable liquid biofuels for transportation have recently attracted considerable attention in different countries around the world be-

A. Demirbas, M. Fatih Demirbas, Algae Energy DOI 10.

solid waste. On the other hand, “traditional biomass” is produced in an unsustain­able way and is used as a noncommercial source — usually with very low efficiencies for cooking in many countries.

A biorefinery is an integrated plant producing multiple value-added products from a range of renewable feedstocks. This innovative approach responds to chang­ing markets for traditional forest products as well as new products such as energy, chemicals, and materials. The range of feedstocks, processes, and potential products is large; each combination of feedstock, process, and product is characterized by its own unique combination of technical and economic opportunities, emerging tech­nologies, and barriers. Figure 3.2 shows an overview of conversion routes of plant biomass feedstocks to biofuels.