1. Introduction
In recent times, there has been an increase in the interest in bioenergy due to the growing awareness of the problems of climate change. The increase in global energy demand together with the high cost of fossil fuels and their associated environmental problems has caused the need for increased research into renewable energy sources, including biomass [
1,
2,
3]. Among the renewable energies, biomass plays a very important role in the new energy framework, as forest and agricultural residues are produced in relatively great quantities all over the world, and its high energy content is managed, out of the inconveniences that other renewable sources have, such as the sun and/or the wind, which are subjected to temporary availability for exploitation. According to several reports [
4], biomass contributes about 11% to the global amount of primary energy and is the fourth biggest resource exploited in the world [
5]. In the Autonomous Community of the Basque Country (ACBC, Spain), biomass is the most commonly used renewable source of energy [
6]. For example, in 2015, the renewable energy consumption was 5.06 million MWh of which 85% was biomass [
7]. Nowadays, bioenergy only provides 4.9% of electricity and heat generation in the ACBC. However, the energy exploitation of biomass could reduce the Basque external energy dependence, which is currently 93.1%, higher than that of any of the European Union countries, except Luxembourg.
Projections suggest increasing the participation of cogeneration and renewable energies for electric generation will increase bioenergy from 20% in 2015 to 31% in 2025. With this increment, it will be possible to contribute to the reduction of 1.6 Mt of CO
, with biomass being one of the most relevant renewable energy sources (Energy Strategy 3E-2025 [
8]). Several studies suggest that the use of forest biomass is an available strategy to help compensate for greenhouse gas emissions (GGE) [
9,
10,
11,
12,
13,
14]. If biomass comes from agricultural or forest residues, the reduction in the emissions of CO
exceeds 80% in comparison to fossil fuel [
15]. The energy use of the residues generated by forest mass is, at the very least, an interesting alternative, especially in Biscay, a province belonging to the ACBC, where more than 60% of the surface is forest. This province has the highest relative quantity of wood volume in Spain, with an average standing timber stock of 177 m
/ha [
16]. These data suggest that the forest biomass residues could be an abundant fuel source for bioenergy projects, replacing a portion of the fossil fuel in energy facilities.
The forestry management of the forests of Biscay, which are mainly private, is essential for the ecological sustainability and the supply of wood products. Sustainable management of the woods plays an essential role in environmental protection. However, little is known about the biomass reserves that are now available in the woods of this area, which could sustain the bioenergy industry. Its quantification is essential to determine the structure, functioning, and dynamics of these ecosystems, as well as to determine the carbon sequestration in the vegetation and evaluate its use as an energy resource [
17,
18]. One of the main problems faced by researchers when dealing with bioenergy is the difficulty in accurately estimating the available resources. The inability to completely address the capacity of indigenous biomass resources and their probable contributions to energy continues to be a severe constraint to the complete realization of the bioenergy potential. Traditional forest inventories have provided a great quantity of data about biomass estimation and the growth of trees. Therefore, they have been widely used by different authors [
19,
20].
The object of this research project is to develop a methodology for the evaluation of the forest biomass used in energy production and the cartography of the resources by means of the Geographic Information Systems (GIS) in Biscay (Spain). To this end, it involves determining the quantity of forest biomass residue (
) that is available and usable as an energy source coming from the forestry treatments of the main local forest species. As a rule, logging operations only eliminate the marketable part of timber. The rest of the biomass is normally left to rot in the reaping or unloading site, but its energy use could reduce the risk of wildfire [
21]. Although industrial by-products, such as sawdust and woodchips, are available and can be alternative fuel sources, they are not taken into account for their use as biofuel in this research as they are currently widely used in the area by birch plywood and wood fibreboard industries. Only the availability of primary residues is taken into consideration in the calculations, and its assessment considers the different stages through which the full rotation of the forest species is developed and the forest biomass generated in each stage.
In this research, we consider environmental protection and sustainable development; thus, the estimation of residual biomass usable for energy purposes is analysed, not only in terms of economical aspects (land inclination) but also environmental ones, considering the reduction of greenhouse gas emissions (GGE) in the biomass combustion with respect to fossil fuels. However, biomass combustion provokes gas emissions and particles (PM) which can severely affect the atmosphere and human health [
3,
22,
23], and it is therefore essential to carry out estimations so as to determine the emissions of pollutant gases produced in energy assessment.
Today, the main problem regarding the use of forest residues as energy sources arises from the lack of available information about the traceability of the biomass to be used in the installation of energy exploitation. This obliges them to have an analytical infrastructure, sometimes complex, to determine the quality of the biofuel. Because of this, in this research, as a second objective, we aimed to estimate the levels of gas emissions (CO, CO
, CH
, NO
and SO
) and dust in the use of forest residues as fuel, using the methodology of emission factors [
24]. To obtain this information, it was necessary to obtain information about the properties of
as a fuel by determining its chemical and fuel properties.
4. Discussion
The precise estimation of the availability of forest residues for bioenergy is very important for the sustainability of biomass supply in energy installations. The use of forest biomass is considered renewable if the extraction rate does not exceed the rate of its natural regeneration, as indicated by the results obtained in the study. We estimated that the total annual growth of timber biomass of
P. radiata and
E. gobulus in Biscay is 489.53 Gg year
(dry matter), and the extraction of timber resources of these species is 270.73 Gg year
. These data show that only 55.3% of the total annual growth of forest biomass is used, as it is in most of the countries in the European Union, whose reports show that only between 60 and 70% of the annual forest resources are used [
54]. Estimations of
P. radiata residue (
) ranged from 0.312 to 1.165 Mg ha
year
and 0.784 to 1.243 Mg ha
year
(dry mass), respectively. Previous research on the residues of different forest species has shown very similar results. Dominguez et al. [
55] estimated 0.91 Mg ha
year
(dry matter) in the residue of
P. radiata in Navarra (Spain), and Zabalo [
56] found values of
between 0.71 and 1.47 Mg ha
year
(dry matter) in
E. globulus plantations in Huelva (Spain).
The results of the proximate analysis of the
(
Table 6) showed reduced moisture values below 10%, which is considered optimal for combustion processes [
47]. The
samples had average ash contents of around 3 and 4 %. These values are higher than the ash content of timber biomass (0.4–0.5) [
49]. Arteaga et al. [
50] measured ash contents of 0.3 and 0.18% in timber samples of
P. radiata and
E. globulus, respectively. These results show that the ash content is higher in the branches fraction than in the biomass from the timber stage [
57,
58] which decreases its quality as a fuel residue compared to wood. However, the ash content of the samples analyzed is much lower than that presented by some coals [
51].
High percentages (>70%) of VM were obtained from the analyzed forest residues. Such high values show the potential of these percentages in gasification processes, which is much higher than the content in coal volatile matters (30.52 and 44%) [
48,
51]. The fixed content ranged from 14.92 to 18.55% for the
of
E. globulus and
P. radiata, respectively. Thus, the percentage of fuel ratio (FR) was higher in
P. radiata (0.26) than in
E. globulus (0.21). These results show that the residue of
E. globulus is a fuel with easier ignition [
59]. In Filipe dos Santos et al. [
60], values of around 18% of FC were obtained for the branches and needles of maritime pine. In relation to the timber fraction, Arteaga-Pérez et al. [
50] found values of 15.26 and 16.20% for the FC of timber of
P. radiata and
E. globulus, respectively.
Carbon (C) and oxygen (O) are the main components of solid fuels. In the ultimate analysis (
Table 6), percentages of C higher than 50% were obtained in the residues of
P. radiata and
E. globulus, slightly higher to those found by Arteaga-Pérez in the timber of these species (48.84 and 48.72%). Another reference [
50] gave the C weight percentages as 51.0 and 44.8% for stems and 52.0 and 45.5% for branches of
P. radiata and
E. globulus, respectively. The percentage of oxygen in both types of residual forest biomass analyzed was very similar, about 41%, slightly lower than the timber fraction [
50]. In comparison, for coal, the content of C is much higher than the biomass fractions, and the content of oxygen is much lower. As an average, the percentage in C is higher than 70% and the oxygen is lower than 13% [
48]. For the elementary analysis, hydrogen (H) values of around 6% were obtained for the residues of the two species. Other authors found similar results with percentages of H in the range of 5.6–6.9% in the different fractions of forest biomass (see
Table 6). The nitrogen (N) and sulphur (S) content of fuels affects the emission of atmospheric pollutants (NO
and SO
). The percentages of nitrogen and sulphur in the residues of
P. radiata and
E. globulus were 1.67%, 0.36% and 1.34%, 0.23%, respectively (
Table 6). These values are higher than those found by other authors in different biomasses [
24,
46,
47,
48,
50]. Some studies have shown that the leaves of the species studied have the highest percentages of nitrogen and sulphur in relation to the biomass fraction [
30,
60]. Another reference [
51] gave the N weight percentage as 1.55%, and in [
48], the N amount was shown to be 1.9% in different types of coal. In [
51], the sulphur content was around 0.26% in Australian bituminous coal (WH), while in [
48], a value of 0.5% was reported for coal.
The
analysed for the
of
P. radiata and
E. globulus resulted in very similar values of 20.75 and 20.96 (MJ kg
) respectively (
Table 6). Similar
values were presented in Filipe dos Santos et al. [
60] in different fractions of maritime pine. Likewise, we obtained very similar values for the net calorific value (
), 19.45 MJ kg
and 19.48 MJ kg
, for
P. radiata and
E. globulus respectively. In this respect, Kollmann [
61] claimed that the calorific value of dry timber varies so little that it can be considered to have an average value of 4500 cal kg
. Nevertheless, other studies have shown that the calorific value of the chips of the forest residues is slightly higher than that of the chips from the tree and the trunk [
47,
50,
62]. Compared to forest biomass, coal has a much higher
, more than 27 MJ kg
.
Broadly speaking, the
(kg Mg
fuel) of
of
P. radiata and
E. globulus was higher than that obtained in the timber biomass of these species [
24] or the residues from other species, but lower than the
of hard coal [
53], except for NO
. The results show that the
of
P. radiata is fuel with a higher
E of pollutant gases (CO, CH
, CO
, NO
, and SO
) in relation to the
of
E. globulus. However, its dust emission is lower (see
Table 7). These results are consistent with studies undertaken in the timber biomass of these two species [
52]. The
E of CO, CH
, and CO
in the residues have average values of around 60 kg Mg
, 3 kg Mg
, and 1500 kg Mg
, respectively, with the
of
P. radiata (2.8%, 2.7%, 2.6%) being slightly higher than those of
E. globulus.
5. Conclusions
The main conclusion reached in the study is that the analysis of biomass properties can generate information that can be used to optimize the management and use of biomass to generate energy. The results obtained in this study indicate that the of both P. radiata and E. globulus has good energetic properties. The high calorific value of (20.75 to 20.96 MJ kg) reveals the considerable potential of this residue to be utilized as an important source of energy.
The elemental analysis indicated that a high carbon content (50.12 to 51.56%) is stored in the
of
E. globulus and
P. radiata. This shows the importance of this species in the global cycle of carbon. Despite the fact that the bioenergy based on these residues is not emission-free, its use can help to mitigate climate change. If the forest residues are burnt in order to obtain energy, their carbon content is immediately released. On the contrary, if the forest residues are not burnt, they decompose and emit carbon gradually, so that their emission is not avoided. On the other hand, unless fossil fuels are burnt, carbon remains stocked in the ground and it is not released [
63], which is an advantage of the use of biomass fuel in relation to fossil fuels. It also adds value to waste materials [
64]. The determined emission factors indicate a reduction in gas emissions, namely CO (23–25%), CO
(22–25%), SO
(87–91%), and dust (11–38%), and an increase in NO
by 11–37% compared to hard coal.
Finally, despite its small extension, the study area contains a large percentage of forest land, similar to that of Northern European countries. The study was carried out only on the two predominant species (P. radiata and E. globulus) because they represent more than 90% of the short species with possible energy use. This study could be generalized to other European countries (e.g., Finland, Austria, Sweden) with a strong presence of fast forest species. The use of forest residue as a renewable energy source is an excellent solution for the socio-economic development of disadvantaged areas, rural or peripheral, which, in most cases, contain most of the forest areas. This would also reduce the risk of fire.