**5.3. Conversion technologies**

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

## *5.3.1. Heating applications*

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

4.00 4.80 11.40 15.20 5.00 3.87 4.70

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

has a higher cost compared to the correspondent raw biomass fuel [149].

**Eucalyptus wood**

*Proximate Analysis (wt% dry)*

30 Renewable Resources and Biorefineries

*Ultimate Analysis (wt% dry)*

Moisture content (wt%, on wet base, as received)

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

**Poplar wood**

**Willow wood**

Fixed carbon 18.80 13.05 13.73 14.53 26.60 19.40 12.65 Volatile matter 80.40 80.99 73.18 84.87 71.80 80.00 83.64 Ash 0.80 1.16 1.68 0.60 1.60 0.60 3.71

Carbon 51.20 47.05 43.06 49.38 53.90 51.80 49.12 Hydrogen 6.00 5.71 5.49 6.17 5.80 6.10 7.82 Oxygen 41.69 41.00 38.36 43.55 38.26 41.19 38.77 Nitrogen 0.20 0.22 0.44 0.28 0.40 0.30 0.56 Sulfur 0.02 0.05 0.00 0.01 0.03 0.01 0.02

LHV (MJ kg−1) (dry) 18.50 18.19 18.05 17.97 20.10 19.56 17.42

**Beech wood**

**Bark (pine)** **Wood chips (pine)**

**Pellets (wood)**

inorganic materials in the feedstock.

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].

#### *5.3.2. Power applications*

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 bed boilers. Additionally, pulverized combustion is also used; it is used as well in industrial applications, but not so commonly [157]. Pulverized biomass-fired boilers are very efficient but require a considerable amount of fuel pretreatment [172] when the biomass is not already generated in fine particles (*e.g.*, in sawmills or cork industry). As far as secondary technologies are concerned, today biomass-fired power plants are mostly based on steam turbines [173]. The electrical efficiencies of these plants depend on the size of the power plant and tend to be within the range of 18–33% (for installed capacities of 10 to 50 MW<sup>e</sup> , respectively) [174]. Higher efficiencies in larger systems have been reported in the literature [172]. The size of biomass power plants is typically much smaller than that of fossil fuel power plants due to the restricted availability of local biomass sources and transport costs. Co-firing of wood and coal is a strategy to reduce greenhouse gas emissions, improving the overall efficiency of power plant with no need for a continuous supply of biomass [175]. It enables the advantages of the larger coal-fired power plants, while partially using a renewable energy source. Gasification of forest biomass into syngas followed by combustion of the syngas is an interesting alternative to combustion only systems, which offers higher efficiencies especially for smaller capacity power plants [176]. The most mature technology is gasification coupled with an internal combustion engine [177]. They are used in smaller systems than steam turbines [178].

variable. Higher quantities per unit area are attained in energy plantations. Pure or mixed even-aged high forests managed for timber potentially originate larger amounts of forest residues when compared with the other types of stands. The renewed interest of biomass as a source of energy brought about the challenge of its estimation. Remote sensing is a useful tool that enables a more cost-efficient evaluation and monitoring when compared with the forest inventory approach. Forest biomass is a very versatile renewable energy source, yet its share on the world energy supply is relatively small. It is mainly converted to energy in combustion systems used for heat generation, but CHP and electricity production are also common. For most applications, the use of raw biomass is adequate, but it might be necessary and/or more

Solid Biomass from Forest Trees to Energy: A Review http://dx.doi.org/10.5772/intechopen.79303 33

The work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 696140 [TrustEE - Innovative market based Trust for Energy Efficiency investments in industry] and National Funds through FCT – *Fundação para a Ciência e Tecnologia*, under the projects UID/AGR/00115/2013 and UID/EMS/50022/2013. The work reflects only the authors' view and the Agency and the Commission are not respon-

\*, Isabel Malico2,3 and Adélia M. O. Sousa<sup>1</sup>

Mediterranean Agricultural and Environmental Sciences (ICAAM), Institute of Research and

2 Department of Physics, School of Sciences and Technology, University of Évora, Évora,

3 LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

1 Department of Rural Engineering, School of Sciences and Technology, Institute of

Advanced Information (IIFA), University of Évora, Évora, Portugal

sible for any use that may be made of the information it contains.

The authors declare that there is no conflict of interest.

\*Address all correspondence to: acag@uevora.pt

appropriate to upgrade it.

**Acknowledgements**

**Conflict of interest**

**Author details**

Portugal

Ana Cristina Gonçalves<sup>1</sup>

#### *5.3.3. CHP applications*

Combined heat and power is the simultaneous generation of electricity and useful heat. It is a much more efficient way to burn forest biomass than biomass-fired power plants, since the overall efficiencies of CHP plants is much higher (global efficiencies above 85% can be achieved [179]). CHP biomass systems have an important application in industries that generate wood residues, such as the pulp and paper and wood industries [180, 181]. The other important CHP application is district heating plants [160]. CHP power plants for capacities above 2 MW<sup>e</sup> are dominated by burning biomass in steam turbines (Rankine cycle) [182]. Steam turbines are a mature technology and applied in a wide range of powers. However, in small decentralized plants their electrical efficiency is low [159]. In this case, CHP plants should be operated in a heat-controlled mode with low power-to-heat ratios [159]. For systems smaller than 2 MW<sup>e</sup> , the biomass CHP conversion technologies are not so well established [182]. In this power range, one of the commercial technologies available is the organic Rankine cycle (ORC). Its electric efficiency is relatively low, but the investment and maintenance costs are lower than that of the conventional Rankine cycles [183]. Another commercially available technology for small capacities is the steam piston engine [159]. Its nominal efficiency is comparable to that of steam turbines, having in efficiency little variation at partial load, contrary to steam turbines that have low part-load efficiencies [159]. Stirling engines are not commercially available yet [184]. They are a promising technology suitable for CHP plants below 100 kW<sup>e</sup> and achieve relatively high electrical efficiencies [182]. From all the commercially available technologies for sizes below 2 MW<sup>e</sup> , gasification is the one that presents higher efficiencies [182].
