**5.2. Feedstock characterization**

Biomass for energy uses comes from various sources. Generically, it can be divided into forest, agricultural, and residual biomass. From these three categories, biomass from forestry is by far the most significant source of biomass for energy production. In 2014, it generated more than 87% of the world biomass feedstock, while agriculture contributed with 10% and municipal solid wastes and landfill gas with 3% [1].


**5.3. Conversion technologies**

*5.3.1. Heating applications*

*5.3.2. Power applications*

Combustion is by far the most common way of converting forest biomass into energy [154]. It is performed in batch or continuous systems, depending on the scale, and to produce heat, power, or combined heat and power. The focus of this chapter is not on the traditional equip-

Solid Biomass from Forest Trees to Energy: A Review http://dx.doi.org/10.5772/intechopen.79303 31

Depending of the scale, different combustion equipment can be used. In Europe, most of the biomass is burned in small-size units for *household heating*, whose scale is typically of the order of a few kWth. Equipment such as stoves, fireplaces, furnaces, and boilers are used to produce heat (a description can be found in [156, 157]). The most common fuels are firewood, wood pellets, and wood chips. The conversion efficiencies depend on the equipment. The traditional open fireplaces have efficiencies lower than 20% [158] and should not be considered a heating solution. At the high end of the range, wood pellet boilers can achieve efficiencies of more than 90% [159]. The scale of nondomestic applications is very variable and can go up to several MWth. Heat can be produced in main activity heating plants or in industrial facilities. It is in Europe that most *district heating* is used [160]. Most of the biomass heat sold by the European energy sector comes from CHP plants. Biomass heat-only plants are important in small-scale district heating systems [161]. The combustion technologies used in district heating power plants are mainly fixed bed, bubbling fluidized bed, and circulating fluidized bed furnaces (a description can be found in [157, 162, 163]). Fixed-bed boilers are less efficient (60–90%) than fluidized bed boilers (75–92%) [164]; they present lower costs and are typically used for smaller capacities than fluidized bed boilers [157]. Heat distribution losses have to be taken into account to know the overall efficiency of district heating. Several parameters affect heat losses, such as linear heat density, pipe diameter, or temperature level [165]. In the industrial sector, *process heat* is typically generated by boilers, dryers, kilns, furnaces, and stoves. Wood and wood-upgraded fuels (*e.g.*, torrefied pellets and charcoal) can be burned to provide the broad spectrum of temperatures required by the industries [166]. For low and medium temperature process heat, mainly boilers are used, while for high temperature process heat, direct heat is supplied [167]. The equipment used for direct heating is very diversified and dependent on the process itself. For example, Ref. [168] and Ref. [169] describe the equipment used in the iron and steel industry, while Ref. [170] in the cement, lime, and magnesium oxide industries. The combustion technologies used for indirect process heating are similar to the ones used in district heating. The industries that use biomass for process heat generation are mainly those that generate biomass residues (*e.g.*, pulp and paper and the wood and wood products industries). An example of a sector that does not produce biomass residues but uses

solid biomass for the partial substitution of fossil fuels is the cement industry [171].

The primary combustion technologies used in biomass-fired power plants are similar to that of district heating and industrial plants with indirect heating applications: fixed and fluidized

ment to burn wood, but a review can be found, for example, in Ref. [155].

**Table 1.** Forest biomass fuel properties [153].

Biomass from the forest sector (*e.g.*, fuelwood, forest residues, and wood industry residues) is mostly used as raw material and not subjected to an upgrading process. However, the use of upgraded biomass has been gaining importance and, for example, pellets are one of the fastest growing bioenergy carriers [1]. Some advantages of upgraded forest biomass over raw biomass are the fact that it is more uniform and convenient to use and especially well suited when biomass is consumed in a place far away from its production site. As a disadvantage it has a higher cost compared to the correspondent raw biomass fuel [149].

The most relevant properties in terms of energy conversion for some forest biomass fuels are presented in **Table 1**. Due to the variability for a specific species, they should be considered as illustrative. Untreated wood is characterized by low carbon content and high volatile matter and oxygen contents when compared to solid fossil fuels. This leads to the lower heating values of wood, which in combination with its low density results in low values of energy density. The lower heating value of oven-dry wood of different species does not have a large variation [150]. However, in practice, in many applications wood is not oven-dried and contains a certain amount of water. Typically, fresh timber has a moisture content between 50 and 60%, while timber stored for a summer and for several years have, respectively, 23–35% and 15–25% water content [150]. The lower heating value of wood fuels is very dependent on the water content of the fuel. The more water content the wood has, the lower is its energy content. The ash content of wood is typically low [151], but it can be significantly higher in bark [152]. Additionally, the harvesting process can introduce inorganic materials in the feedstock.
