**1. Introduction**

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476 Biofuels - Status and Perspective

Fossil fuels continue to be the primary global energy sources, suppling approximately 80% of global demand. However, such energy sources can cause the greenhouse effect with the generation of harmful gases such as COx, NOx, SOx, CxHx and other organic compounds. These pollutants are released into the atmosphere as a result of fossil fuel combustion [1]. Renewable technologies, such as biofuels, present possible alternative sources of energy that are carbon neutral.

Approximately 68% of global biodiesel supply is produced by five countries: Brazil, Germany, USA, France and Argentina. Brazil is among the largest producers and consumers of biodiesel in the world, with a production capacity of 8539 million liters [2]. Several factors favour the cultivation of different plants for biodiesel production in Brazil, especially the climatic condition, and the availability of arable land. The increase in the production of biodiesel makes Brazil a strategically important country for the whole world, due to the depletion of already known fossil energy reserves.

Biodiesel is a biofuel obtained by the transesterification of raw materials such as animal fats and vegetable oils. In Brazil, sources like soybeans and palm are economically attractive for biodiesel production due to their abundance. Approximately 80% of the biodiesel produced in Brazil is derived from soybean [3]. In Brazil, the addition of 2% biodiesel in diesel fuel has been mandatory since 2008, this amount was increased to 6% in July 2014 and then increased to 7% in November 2014 [4].

The large surpluses of glycerol that are generated in this process require new commercial uses to be identified. Brazil will become a major producer and consumer of biodiesel due to strong

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strategy for biofuel production. This is possible due to exceptional conditions for the cultiva‐ tion of oilseeds for oil extraction. The selection of feedstock depends strongly on potentialities of each region [5].

When biodiesel is produced from vegetable oils and animal fats through transesterification process, high amounts of waste are created. This waste chiefly consists of crude glycerol, which has limited commercial value, unless expensive purification processes are performed. Even when purified, there is such a large global overproduction that traditional markets find difficult to absorb it. Each ton of biodiesel produced generates approximately 100 kg of crude glycerol. However, the amount of crude glycerol generated in biodiesel production can vary from 1% to 85% (v/v), depending on operating conditions of the transesterification plants. Thus, to overcome these issues it is necessary to discover new uses for this significant residue.

Ethanol has been primarily produced from sugarcane in Brazil since 1975, encouraged by the implementation of the National Program for Alcohol (1975–1985). Brazil produces (in 2011/2012) nearly 571 million tons of sugarcane, which is processed by sugar mills to produce 36.9 million tons of sugar and 22.9 billion liters of ethanol [6]. The ethanol could potentially be used in the esterification process of biodiesel [7] to develop cheaper and more environmentally friendly processes. Several studies have been developed to obtain hydrogen gas, as a renew‐ able energy source that can be generated from waste glycerin, a byproduct of biodiesel production.

In this sense, this chapter presents a comparative study on biological hydrogen production from crude glycerol, the microorganisms involved in the biological processes for hydrogen production and some of the strategies applied in the literature for the improvement of these systems. It will contribute to innovation in research into the reuse of crude glycerol, in a sustainable manner, thus leading to potential cost reduction and clean energy generation.
