**Acknowledgements**

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

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 com-

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

relatively high electrical efficiencies [182]. From all the commercially available technologies

The primordial source of biomass for energy is derived from stands and forests. Due to the wide range of stand structures, the amounts of biomass available for energy are also quite

, gasification is the one that presents higher efficiencies [182].

[184]. They are a promising technology suitable for CHP plants below 100 kW<sup>e</sup>

, respectively) [174].

,

and achieve

be within the range of 18–33% (for installed capacities of 10 to 50 MW<sup>e</sup>

bustion engine [177]. They are used in smaller systems than steam turbines [178].

*5.3.3. CHP applications*

32 Renewable Resources and Biorefineries

for sizes below 2 MW<sup>e</sup>

**6. Conclusions**

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