## 3 Processes for Bioenergy Production
### Methods of Biomass Thermochemical Conversion  
Therrochemical biomass conversion involves drying, roasting, pyrolysis, and gasification. If the solid biomass is subjected to a progressive increase in temperature, in the partial or total absence of oxygen, several thermochemical processes take place, which originate other fuels in solid, liquid, or gaseous form [8]. Although these methods have been widely used in recent years, thermochemical conversion can be still considered the basis for the development of the second-generation biofuels. **Combustion** is the most conventional process. Efficient combustion requires the reduction of the water content of the biomass, generally achieved by open-air drying. Combustion of wet material is less energy efficient as energy is used for evaporation of the water content [23; 24]. For the combustion process to be effective (i.e., compensate the energy used for drying with the improvement in the efficiency of the process), the optimum moisture content of plant biomasses (wood chips, plant residues) has been set to be 10%-15% [25]. Pelletizing (the compression and mechanical pressing process) is an effective method to increase density and reduce moisture. Combustion is, from a thermodynamic point of view, a process of conversion of the chemical energy of the fuel (biomass) into heat. The heat is mostly generated by the oxidation of the carbon, in the presence of sufficient oxygen, to CO\({}_{2}\). The biomasses for the production of energy by combustion are generally wood processing residues, and cogeneration is used to produce heat and electricity [23]. A portion of the heat produced is used to produce steam for power turbines connected to electric generators. The remaining heat can be used by industrial or residential users. The efficiency of these plants is in the order of 20%-25% and, therefore, rather modest. Medium-large cogeneration plants can obtain energy efficiencies close to 25%, with about 10 MW of electric power. To generate these powers, the quantities of biomass to be burned become considerable, and complex and costly systems are needed for the elimination of the toxic substances present in the fumes [23]. According to the temperature reached, the thermochemical processes can be classified as:  
Figure 2: Processes for bioenergy production. * Drying, which occurs at temperatures of up to 150 \({}^{\circ}\)C, when water evaporates without substantial chemical modification of the raw material;
* Roasting, which arises between 150 \({}^{\circ}\)C and 280 \({}^{\circ}\)C, when in addition to water, some organic volatile compounds are released, including acetic acid, methanol, and carbon dioxide. The solid residue darkens, becomes very stable microbiologically, and reaches a high energy density;
* Pyrolysis (carbonization), which takes place when temperatures reach maximum values of 550-600 \({}^{\circ}\)C. During this process, the division of the C-C and C-O bonds (decarbonylation and decarboxylation) occurs with the production of gas (mainly carbon monoxide, hydrogen, carbon dioxide, and methane), hydrocarbons (also of high molecular weight, such as tars), phenols, esters, acetic acid, methanol, and water. The resulting solid residue is charcoal (or biochar), while the gas released, if properly cooled, can be divided in two types of product: the non-condensable part and the condensable part, which is liquid at room temperature and is called \"pyrolytic juice\" or \"bio-oil\" [8]. Biochar, with a carbon content greater than 95%, has an LHV of approximately 29.3 MJ kg\({}^{-1}\); the energetic value of bio-oil can range between 7 and 18 MJ kg\({}^{-1}\) according to the water content, while the gas is characterized by an LHV of 4.8 MJ Nm\({}^{-3}\)[11];
* Gasification, which takes place at temperatures between 600 and 1500 \({}^{\circ}\)C, with the total gasification of the biomass. Carbonaceous and organic compounds are converted into fuel gas, also known as syngas, a mixture of mainly carbon monoxide and hydrogen.