Introduction

Climate change is one of the most important issues facing our planet. It is caused by greenhouse gas (GHG) emissions released into the atmosphere. The development and utilization of renewable energy sources is intended to guarantee sustainable development and break our current dependency on fossil fuels (Ruiz-Romero et al. 2012; Nagy and Körmendi 2012).

In March 2007, a plan concerning energy use to be followed by every Member State of the European Union was approved by the European Council. The following targets for 2020 were established: a 20 % reduction in GHG emissions from 1990 levels, a 20 % reduction in primary energy consumption by encouraging energy efficiency and raising the share of EU energy consumption produced from renewable resources to 20 %. Directive 2009/28/EC accepted these targets and requested Member States to submit National Renewable Energy Action Plans (NREAP) (MINETUR 2010). Spain drew up its Renewable Energy Plan (REP) 2011–2020, in which the current situation and the proposals for improvements to meet the European Union targets are gathered. This Plan is even more ambitious than the European Plan regarding renewable energy, because it proposes that by 2020 renewable energy should represent 20.8 % of gross final energy consumption in Spain (IDAE 2011).

This paper analyzes the current situation and the perspectives for improvements of forest biomass utilization in Spain to produce thermal energy, which could be used in heating or hot water systems (IDAE 2011; IDAE 2007). Forest biomass was chosen because it is the main bioenergetic resource used in thermal applications (BioPlat 2013). Furthermore, the use of autochthonous forest biomass creates employment and has positive environmental and economic effects such as the reduction of fire risks and savings in replant treatments (BioPlat 2013; Rivera-Tinoco and Bouallou 2010; Schneider et al. 2001).

In the literature, there are many references concerning biomass valuation related to bioenergy or environmental sustainability (Bare 2013; Ziolkowska 2013; Vinodh et al. 2014).

Čuček et al. (2010) defined a method for the synthesis of regional renewable energy supply chains, based on mixed-integer linear programming. The aim is to maximize the economically viable utilization of resources, accounting for the competition between energy and food production. Panepinto et al. (2014) studied the energy production from biomass and its relevance to urban planning and compatibility assessment in Italy. Ubando et al. (2014) presented a fuzzy mixed-integer linear programming model for optimizing a multi-functional bioenergy system with biochar production. A multi-functional bioenergy system is an efficient way for producing multiple energy products from biomass, which results in near-zero carbon emissions. Benjamin et al. (2014) defined a methodology for criticality analysis in integrated energy systems such as polygeneration plants and bioenergy-based industrial symbiosis networks in order to increase efficiency and reduce carbon emissions.

A revision of the results of previous studies that investigated biomass worldwide resources and their bioenergy potential estimation was presented by Long et al. (2013). According to this study, the gross bioenergy potential of the world changes from 64 to 161 EJ per year, depending on the estimation approaches and the bioenergy scenario. For the whole European region, bioenergy potential for agricultural and forest resources are 3.1–3.9 and 1.4–5.4 EJ/year, respectively (De Wit and Faaij 2010). Scarlat et al. (2011) provided an overview of the Norwegian biomass resources for bioenergy use, bioenergy market, and frame conditions through a comparison with Denmark, Finland, and Sweden, which have a leading role in bioenergy production in the European Union. For example, in Norway forest biomass has an estimated sustainable potential for bioenergy production between 86 and 108 PJ/year (Rorstad and Trømborg 2010)and in Demark the sustainable biomass potential is about 105 PJ/year, with the major contribution of wastes (92 PJ/year) and forest resources (8.4 PJ/year) (EEA 2006).

Gómez et al. (2010) have shown that the combined technical potential of agriculture and forestry resources in Spain is 118 PJ/year.

Geographical information systems (GIS) have been considered as tools for understanding the spatial context of biomass availability and costs under the appropriate spatial scale and data. Panepinto et al. (2012) used WEB-GIS tools to valuate biomass as an energy source. Sukumara et al. (2014) proposed a novel framework to answer questions related to environmental impact of sustainable bioenergy use. In particular, some studies have elaborated methodologies with factors based on GIS. Natarajan et al. (2014) studied forest biomass availability in the north of Europe, but limited the cost estimation to average values. Yoshioka et al. (2011) estimated forest biomass and supply costs with GIS in Asia. Paredes-Sánchez et al. (2015) used a methodology based on WEB-GIS to evaluate forest biomass resources and costs under techno-economical and environmental constraints, including harvesting and transport costs.

Renewable energy represented 14.2 % of the total primary energy consumption in Spain during 2013, and about 29 % of that renewable energy was obtained from biomass—4.1 % of the total primary energy (IDAE 2014). About 66 % of the biomass energy supply in Spain comes from forest biomass (IDAE 2011).

In Spain, forest biomass is widely used for heating in the residential sector as pellet (Paredes-Sánchez and Xiberta-Bernat 2010). This thermal application in boilers and heaters is the main conversion technology, where energy efficiency reaches about 90 % (Monteiro et al. 2012).

The main objective of this study is to define which areas of Spain could use forest biomass in the most efficient and economical way to produce thermal energy, based on the amount of biomass found in each province. This study aims to promote the installation of high potential plants able to cover all the thermal or electric needs in a wide area (García-Maraver et al. 2012; Panepinto et al. 2012). The use of this kind of systems would help Spain reach the target of 20.8 % of renewable energy utilization with autochthonous resources.

The novelty of this work is that there are no studies on the potential of forest biomass resources for energy purposes of a country by defining local collection nodes. It is of particular interest that these nodes are defined for each administrative territory or existing autonomous region.

The authors think that the methodology to analyze the potential for bioenergy production presented in this paper is technically sound and useful for any other researchers working in this field and even of interest for the general reader. This potential is quite often ignored or under-reported. It is necessary to think globally but act locally.

Methodology

Spain is known to have a huge amount of biomass from diverse sources (pruning, firewood, splinters, olive pits, nut shells, etc.) that guarantee a suitable, continuous supply anywhere in the country. This study is focused on forest biomass, i.e., broadleaf, conifer, and mixed conifer–broadleaf forests, as being the most suitable kind of biomass for thermal applications (IDAE 2007; INDUROT 2011; BioPlat 2013).

Many studies have evaluated the biomass potential in different regions using GIS tools (Geographic information system) (Schneider et al. 2001; Gómez et al. 2010). This paper uses a GIS application called BIORAISE (BIORAISE 2014). This application assesses forest biomass resources in Spain (with the exception of Canary Islands). Its latest update calculates not only the existing biomass on surfaces with a discrete radius from 1 to 100 km around a particular localization, but also the existing biomass in a province.

BIORAISE assigns an average productivity value (taken from the yield tables of different forest species), to each one of the Corine Land Cover categories included in the European cartography. Information about potential biomass in the study surface can be used to calculate the available biomass introducing several environmental restrictions, as well as transport costs and energetic content (Esteban et al. 2008; Esteban and Carrasco 2011).

To choose the most suitable location of a collection point for processing the available forest resources for bioenergy, this methodology defines a circle of area A c by its middle point and its radius. This circle must extend over the greatest possible inner surface of the province (Paredes-Sánchez et al. 2013). Therefore, biomass in an operational range around a central collection point is studied in Fig. 1.

Fig. 1
figure 1

Definition of the collection area for a given province

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The collection radius of biomass for its supply as an energy resource can vary from 30 km (Ubeda-Delgado and Antolín-Giraldo 1995) to over 100 km, depending on conditions of energy production, harvest, and transport (De Wit and Faaij 2010). The BIORAISE methodology was used to obtain all the parameters (mass, energy, and costs) in each province, considering the maximum inner collection area, in order to achieve the greatest centralized exploitation of the existing resources in each region (BIORAISE 2014).

Potential and available forest biomass

Energy contained in biomass is a key factor affecting energy production from biomass and generally indicated by the Heating Value—the amount of heat generated in a complete combustion under standard conditions (Paredes-Sánchez et al. 2015; Long et al. 2013). The lower heating value (LHV)—discounting the heat of vaporization of the water vapor—is usually employed to calculate bioenergy potential in biomass (specifically in the BIORAISE methodology).

Potential forest biomass is the sum of the exploitation of the entire forest area, broadleaves, conifers, and mix conifers–broadleaves (IDAE 2007; INDUROT 2011). Therefore, the total potential forest biomass (b p), existing in a given province and assessed in annual oven dry tons (o.d.t. /year), is the result of adding the potential biomass of the different species (1):

$$b_{\text{p}} = b_{\text{pb}} + b_{\text{pc}} + b_{\text{pm}} .$$
(1)

The available forest biomass is the biomass remaining after introducing environmental, economical, and technical restrictions to the potential forest biomass. Other factors must also be considered such as its other uses as wood and harvest efficiency, since it is difficult to collect all the resources and it is necessary to leave some remains on the ground for ecological reasons (IDAE 2007; INDUROT 2011).The total available forest biomass (b d), assessed in annual oven dry tons (o.d.t. /year), is obtained by adding the available biomass of the different species (2):

$$b_{\text{d}} = b_{\text{db}} + b_{\text{dc}} + b_{\text{dm}} .$$
(2)

In order to relate the total available biomass in the circle that represents a province with the total potential biomass in that province, the biomass utilization factor has been defined as follows (3):

$${\text{Utilization Factor }}\left( {\text{\% }} \right) = \frac{{b_{\text{d}} }}{{b_{\text{p}} }} \times 100 .$$
(3)

To compare the information obtained in each province, the available biomass ratio has been defined (R bd) as the amount of available biomass per surface unit (4). The considered surface is the area of the circle that represents the province in question.

$$R_{\text{bd}} = \frac{{b_{\text{d}} }}{{A_{\text{c}} }}$$
(4)

Energetic content of the available forest biomass

The energetic content of the available forest biomass (CE) is the sum of the energetic content of the different species:

$${\text{CE}} = {\text{CE}}_{\text{b}} + {\text{CE}}_{\text{c}} + {\text{CE}}_{\text{m}} .$$
(5)

The energetic content calculated using the lower heating value of each species:

$${\text{CE}} = b_{\text{db}} \cdot {\text{LHV}}_{\text{db}} + b_{\text{dc}} \cdot {\text{LHV}}_{\text{dc}} + b_{\text{dm}} \cdot {\text{LHV}}_{{{\text{dm}} .}}$$
(6)

The average energy content (LHV) of the moisture-free biomass has been evaluated at 17.7, 19.0, and 18.3 GJ/o.d.t. in broadleaves, conifers, and mixtures, respectively (BIORAISE 2014).

To better compare the energetic content in each province, the energetic content ratio (R CE) has been defined as the energetic content of the available forest biomass per harvesting surface unit (7):

$$R_{\text{CE}} = \frac{\text{CE}}{{A_{\text{c}} }}.$$
(7)

Harvesting and transport costs of forest biomass

In this paper, the potential resource costs are analyzed, i.e., the costs before undergoing energy transformation, namely harvesting and transport costs.

Harvesting costs have been estimated for the different species (conifers, broadleaves, and mixtures) in each province, taking into account the different operations (felling, piling, drying, compacting, etc.) (Esteban and Carrasco 2011) and the influence of the topography on forest machinery operational costs (BIORAISE 2014).

The mean harvesting cost (C h) of the forest biomass was obtained by averaging the harvesting costs of the different species (8):

$$C_{\text{h}} = \frac{{C_{\text{hb}} b_{\text{db}} + C_{\text{hc}} b_{\text{dc}} + C_{\text{hm}} b_{\text{dm}} }}{{b_{\text{d}} }}.$$
(8)

Once the fuel cost is defined [1.36 €/l in this case (MINETUR 2013)], BIORAISE can calculate biomass transport costs. Transport costs are the costs of carrying the harvested material from the harvesting point to the circle center. It is calculated in a similar way to the harvesting costs (9):

$$C_{\text{t}} = \frac{{C_{\text{tb}} b_{\text{db}} + C_{\text{tc}} b_{\text{dc}} + C_{\text{tm}} b_{\text{dm}} }}{{b_{\text{d}} }}.$$
(9)

The biomass total cost (C) will be the sum of the two costs (10):

$$C = C_{\text{h}} + C_{\text{t}} .$$
(10)

Results and discussion

Potential and available forest biomass

The total forest biomass potential for each province was obtained using the BIORAISE application considering the whole area of each province. This information is shown in Fig. 2, in which Huelva, Navarra, and Huesca are the provinces with highest total potential (more than 533,000 o.d.t./year); while Palencia, Murcia, and Alicante have the lowest potential (less than 96,000 o.d.t./year). The total forest biomass potential for the whole country has been estimated as 13,192,603 o.d.t./year.

Fig. 2
figure 2

Potential forest biomass in Spain by province

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In order to study the collection point of the available forest biomass with BIORAISE, it is necessary to fix a circle that represents each province. Therefore, the real available forest biomass is not taken into consideration, only the available biomass within the circle. This allows us to calculate the maximum potential associated to a centralized harvesting radius with the resources of any given territory. This information can be seen in Fig. 3, which shows that Madrid and Baleares have the smallest amount of available forest biomass, as opposed to Huesca and Cuenca, which have the greatest.

Fig. 3
figure 3

Available forest biomass in Spain by province

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Table 1 shows the biomass utilization factor in each province. Cádiz, Teruel, and Cuenca have the highest utilization factors with values between 20 and 26 %, compared to Madrid, Baleares, and Palencia with values lower than 2.6 %. The Spanish average utilization factor is 11.41 %.

Table 1 Forest biomass utilization factor by province
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Figure 4 shows that A Coruña, Vizcaya, and Pontevedra are the provinces with the most available forest biomass ratio, as opposed to Madrid, Murcia, and Ciudad Real, which have the least.

Fig. 4
figure 4

Available forest biomass ratio

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Energetic content of the available forest biomass

The biomass energetic content ratio can be seen in Fig. 5. It shows that Madrid, Murcia, and Ciudad Real have the lowest ratio, while Pontevedra, A Coruña, and Vizcaya have the highest.

Fig. 5
figure 5

Energetic content ratio of the available forest biomass

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Harvesting and transport costs

Forest biomass harvesting and transport costs by province are arranged in Table 2.

Table 2 Harvesting and transport costs by province
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Figure 6 shows the total costs of forest biomass—the sum of harvesting costs and transport costs. Baleares, Huelva, and Palencia are the provinces with the lowest total costs, with values lower than 57 €/o.d.t. On the other hand, Toledo, Granada, and Almería have the highest costs with values higher than 87 €/o.d.t. The biomass supply to Pellet plants in Europe fluctuates between 83 and 110 €/t (Trømborg et al. 2013; Paredes-Sánchez et al. 2014). These costs make it economically viable to use those resources in the production of densified solid biofuels for its use in bioenergy production (Rosillo-Calle et al. 2007).

Fig. 6
figure 6

Total cost of forest biomass

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Comparison to other European countries

It has not been found values in the literature for the different Spanish provinces, but for the whole country. Esteban and Carrasco (2011) presented values of the bioenergy potential for most of the EU countries. According to this study, the Spanish forest biomass potential was about 10.5 × 106 o.d.t./year in 2011, which agrees with the value found in this study of 13 × 106 o.d.t./year, taking into account the actualization of the survey data. However, according to that paper, the available forest biomass is higher than the value found in this study (4.7 × 106 o.d.t./year to 1.7 × 106 o.d.t./year), due to the operational restrictions considered (biomass in a range around a central collection point). This value is nevertheless 32.3 PJ/year or 4 % of the net electric energy generated in Spain.

The EU country with the highest potential of forest biomass is Sweden with 20.7 × 106 o.d.t./year (almost double than Spain). Finland, Germany, France, and Italy have similar potential than Spain, but the available forest biomass in Finland and France is approximately double than the one in Spain. On the other hand, Denmark and Portugal are the countries with the lowest potential (0.74 and 2.23 × 106 o.d.t./year, respectively) (Esteban and Carrasco 2011).

Regarding the forest biomass energetic content ratio, the mean value in Spain is 134 GJ/km2year, similar to Norway and Sweden. The highest value corresponds to Austria (over twice the value in Spain) and the lowest values to Finland and Germany (half the value in Spain) (Esteban and Carrasco 2011).

It can be said that the values of all the forest biomass and bioenergy parameters in Spain are approximately the mean values in the EU.

With respect to costs, however, they are much higher. For example, the harvesting cost of forest biomass in Spain is 56 €/o.d.t., while for most EU countries this cost is about 25 €/o.d.t. The highest value corresponds to Italy (74 €/o.d.t.) and the lowest to Poland (11.5 €/o.d.t.) (Esteban and Carrasco 2011).

Feasibility of utilizing available forest biomass in Spain

The Spanish energetic content of the available forest biomass calculated is 32.3 PJ/year which means about 8.9 × 109 kWh/year and the average total cost is 73 €/o.d.t. (0.013 €/kWh), although it is estimated that the processing and commercialization can double this value (0.026 €/kWh).

The energy consumption of an average Spanish house is 10,000 kWh/year, of which 47 % is for heating (4700 kWh/year) (International Bioenergy 2013). 46.3 % of the heating consumption in Spain is supplied by electricity, 32 % by natural gas and 14.3 % by gasoil (IDAE 2013). The prices are 0.14, 0.05, and 0.082 €/kWh for electricity, natural gas, and gasoil, respectively (REE 2014; WEBENERGIA 2013; MINETUR 2013). Consequently, the heating costs for an average house in Spain depending on the heating system employed are 658 €/year (electricity), 235 €/year (natural gas), and 385 €/year (gasoil). On the other hand, if the heating system used is the available forest biomass in the local collection nodes of the different provinces, the average house costs should be 122.2 €/year, and its energetic content could cover the whole heating demand of about 1,900,000 houses with significant savings with respect to the other heating systems (between 48 and 81 % savings).

Considering the cost distribution by province, the average total cost oscillates between 57 €/o.d.t. (Baleares, Huelva, and Palencia) and 87 €/o.d.t. (Toledo, Granada and Almería). These differences are significant enough to imply a change greater than 10 % in the savings.

Conclusions

A new methodology is presented to obtain the potential of forest biomass resources for energy purposes of a country by defining local collection nodes. These nodes are defined for each administrative territory or existing autonomous region. The methodology is considered technically sound and useful for any other researchers working in this field and even of interest for the general reader.

Among other results, maps are shown of potential biomass, available biomass, total cost, and energetic content ratio of the available biomass.

Spain has a total potential forest biomass of 13,192,603 o.d.t./year. Huelva and Navarra are the provinces with most potential biomass with values higher than 540,000 o.d.t./year, compared to Murcia and Alicante with less than 85,000 o.d.t./year. As regards to available forest biomass, Huesca and Cuenca have the most (more than 104.000 o.d.t./year), while Madrid and Baleares have the least (lower than 2400 o.d.t./year). The provinces with the highest biomass utilization factors are Cádiz, Teruel, and Cuenca with values between 20 and 26 %, as opposed to Madrid, Baleares, and Palencia with values lower than 2.6 %. The average utilization factor is 11.41 %.

In terms of forest biomass costs, Toledo, Granada, and Almería have the highest total costs with values between 87.67 and 90.38 €/o.d.t; Baleares, Huelva, and Palencia have the lowest costs with values between 52.13 and 56.64 €/o.d.t. The average total cost in Spain is 72.72 €/o.d.t. As for harvesting costs, Huelva, Badajoz, and Baleares have the lowest costs, while Guipúzcoa, Navarra, and Granada have the highest. Transport costs are obviously influenced by the terrain and surface area of the province; Álava, Guipúzcoa, and La Rioja have the lowest costs, and Murcia, Jaén, and Teruel have the highest.

All the forest biomass and bioenergy parameters in Spain are approximately the mean values in the EU. However, cots in Spain are much higher than most EU countries.

Heating consumption of an average Spanish house is about 4700 kWh/year. This energy is basically supplied by electricity, natural gas, and gasoil. Using the available forest biomass in the local collection nodes of the different provinces, significant savings can be obtained with respect to the other systems (between 48 and 81 % savings).

The best provinces to locate the collection nodes are Baleares, Huelva, and Palencia because the forest biomass total costs are the lowest and the savings can be increased more than 10 % with respect to the mean values in Spain.

It is clear that forest biomass should be taken into account as an alternative source of energy in Spain because of its low cost and huge potential.

Biomass can be used to produce electricity and thermal energy. However, its final use depends on the characteristics of the raw material. Economic incentive development is required to motivate the launch of environmental projects that would allow the exploitation of these resources for energy purposes.