1 Introduction

The severe impacts of human-induced climate change are nowadays undeniable. Carbon neutrality must be foreseen for the middle of the century, to ensure that global warming does not jeopardize living resources as presently known. Urgent reduction of anthropogenic sources of greenhouse gases involves fossil fuel combustion as their largest source [1]. For a rapid decarbonization of the global energy system, the IEA presents a roadmap based on seven key pillars [2], bioenergy being one of them. According to the IPCC, bioenergy should represent 19% of the total primary energy supply (TPES) if net zero CO2 emissions (NZE) from the energy system are to be achieved in 2045 [1]. In the IEA 2050 NZE scenario, bioenergy contributes with 20% of the total energy needs [2].

In addition to the energy sector, bioenergy can also contribute to decrease greenhouse gas emissions from other sectors. In the agriculture sector, the digestate from anaerobic digestion can be used, avoiding the production of mineral fertilizers [3] and landfill disposal of manure [4]. In the waste management sector, landfill disposal of the organic fraction of municipal solid waste is avoided [5]. Moreover, bioenergy technologies can link different sectors, increasing the overall environmental performance of regional industrial ecosystems [6]. Their role in industrial symbioses, as ‘anchor tenant’ or as ‘upcycling tenant’, has been demonstrated by many case studies around the world, e.g. Finland [6], Brazil [7], Sweden [8], and the USA [9].

In any of the IPCC scenarios limiting warming to 1.5 or 2°C, bioenergy potentials of 50 to 250 EJ yr−1 from dedicated biomass production and 5 to 50 EJ yr−1 from residues, both forecasted for 2050, cannot be disregarded [1]. The residues’ bioenergy potential is thus important and does not raise concerns with biodiversity decrease or with increase in competition for land and water, both raised by large-scale dedicated biomass production.

Nevertheless, published scenarios show very divergent results for the bioenergy contribution, ranging from 27 to 1546 EJ yr−1, raising questions on the considered assumptions [10]. As reported in a review of 66 studies on bioenergy scenario construction [11], bioenergy potential results markedly depend on their more or less conservative approach. At one end, some scenarios consider neither bioenergy crops, to avoid resources competition, nor solid biogenic waste and residues that can be used for materials recovery. At the other end, some scenarios do not even clarify the underlying assumptions.

In Portugal, different models, mostly foreign and thus limited in their adjustment to context-specific policies [12], and different assumptions have been implemented, resulting in different outcomes. The Roadmap for Carbon Neutrality 2050 (Rnc2050) [13], using the model TIMES_PT, estimates a contribution from bioenergy of 25 PJ yr−1 in 2050, while OTIMBIO, a project of the Directorate-General of Energy and Geology using the JANUS model, estimates a contribution of 120 PJ yr−1 [14]. Besides the different models, the inclusion of different energy vectors and different uses for biomass explain the differences between these figures.

The biomass cascading use principle (energy production being not the main priority) should be considered in energy potential estimations for biomass [15]. Non-energy demand needs to be increasingly considered for the future, such as for chemicals, building materials, and peat substitutions, and the suitability of biochemical or bioenergy technologies should be evaluated [16]. Yet current models tend to neglect these demands which are already in use, as well as their growth, resulting in an overestimated supply and underestimated alternative demand. In the present contribution, bioenergy quantification relies on the identification of the biomass potential uses.

Moreover, there are high expectations on bioenergy and in its ability to fill in the gaps from the intermittent nature of the wind and solar electrical energy generation. A recent review presents bioenergy technologies that can provide short- to long-term flexibility services to the grid [17]. Nevertheless, bioenergy is ‘often used as a wildcard to close any remaining gaps’ [18] and is allocated according to exogenous goals, regardless of the demands for different energy carriers, namely, electricity, heat, biogas, and fuels. Therefore, in the present contribution, the bioenergy potential is assessed taking into consideration diverse bioenergy conversion routes, albeit selecting those technologies most suitable for each type of biomass under examination. Moreover, taking a conservative approach, feedstocks supplied by bioenergy crops are left out of this assessment, thus restricting it to feedstocks from biological residues.

Feedstock availability is decisive for bioenergy planning, and, because it is context-specific, bioenergy potential assessments should also be. Climate conditions dictating the most suitable crops, residues associated to the economic tissue, feedstock prices, networking opportunities, economic agents’ concerns, land and renting prices, and gap filling needs in the intermittences of the renewable energy grid already in place, all vary according to the geographic location, social environment, and techno-economic fabric of a region and influence bioenergy system implementation and configuration. The access to and the acceptance of biotechnologies by a given community can work as a lever or an obstacle [19]. Even differences in personality traits and risk cognitions of the farmers can lead to uneven developments of bioenergy production, as suggested for explaining disparities across Germany [20]. Regional-specific assessments are therefore important to support decision-makers to produce location-adapted policies. Moreover, they are the basis for the needed bottom-up approaches that can lead to a harmonized development of national feedstock strategies [19].

The region of Lafões, object of the present case study, has a great feedstock potential for bioenergy from residues due to its economic fabric. Its intensive poultry industry, besides generating manure that can be used to produce energy, needs heating energy for breeding pavilions during winter. Its intensive wood industry also generates residues and needs energy for operations. Industrial parks, potential consumer of surplus energy, have been expanding in the region due to the convenient location in the centre of Portugal and good road accesses. The region is currently lacking in terms of area coverage by the district’s heating grid, but the expansion of this grid to cover all three municipalities is foreseen until 2027, rendering possible the distribution of the bioenergy produced in the form of heat. Because Lafões is a mountainous region, wind energy is well implemented, and bioenergy can fill in gaps and extra needs for heating or transportation.

The case study of the region of Lafões is here used to illustrate the application of the proposed methods, where theoretical and technical biomass and bioenergy potential values are estimated and discussed, for which equations, parameters, and their values are gathered from disperse sources in the literature (Online Resource 1 – Equations and parameters used for the calculations), and data sources and assumptions are analysed in detail, providing a clear understanding of method applications. It adds to the methodology of bioenergy potential assessment by considering efficiencies and conversion losses that some assessments still fail to consider. Its novelty lies in providing a clear and user-friendly step-by-step guideline for estimation of regional biomass and bioenergy potentials that combines biomass categorization and biomass and bioenergy quantification methods. In order to do this, in ‘Section 2’, the biomass and bioenergy potential assessment methods are introduced, namely, the theoretical framework and calculation methods, starting with the bioresidues characterization (Section 2.1), then justifications for the selection of suitable energy conversion technologies are given (Section 2.2), and finally the biomass and energy potential assessment methods are explained (Sections 2.3 and 2.4, respectively). The results of the assessment are given in Section 3 and discussed in Section 4. Section 5 gathers the main conclusions and what remains to be achieved in the context of this study.

2 Materials and methods

The methodology proposes four steps to assess the biomass and bioenergy potentials, illustrated in Fig. 1 and described in the following sections. In brief, the biomass categorization is done in four levels: the bioenergy conversion technologies are identified considering their suitability for each biomass; the biomass potential assessment follows the biomass cascading use principle, i.e., it does not consider potentials already in use for materials and energy (used technical biomass potential) or not yet in use but where the priority should be materials (mobilizable for other than energy); and the bioenergy potential assessment considers efficiencies and conversion losses.

Fig. 1
figure 1

Steps for biomass and bioenergy potentials assessment based on [21]. Boxes in black are not accountable for bioenergy potential.

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2.1 Categorization and characterization of the bioresidues of Lafões

The definition that ‘biomass is a term for all organic material that stems from plants (including algae, trees and crops)’ [22] was given when bioenergy was mainly obtained from fuelwood and was maintained within the first generation of biomass-to-energy conversion, when only forest and energy crops were considered as raw materials. In the second generation of bioenergy conversion, residues from different production processes must also be considered as feedstock. Also, biomass types are categorized differently, depending on the purpose and the field within which it is being analysed (e.g. lignocellulosic and non-lignocellulosic [23], or conventional resources, wastes, and plantation [24]).

This heterogeneous categorization allows a subjective interpretation of the biomass being assessed and results between studies often cannot be compared. In an attempt to standardize biomass potential assessments, a four-level categorization is proposed by Brosowski et al. [25]. The latter was followed as the theoretical framework for the categorization of the bioresidues of Lafões. In short, level 1 introduces a designation and description for the biomasses, and the remaining levels are used to group them in categories by origin, up to level 4, ending in the following higher-order groups: agricultural by-products; residues of forestry and wood industries; municipal waste; industrial residues; and residues from other sectors. The latter two were left out of the present study.

In level 1 of the Lafões residue categorization, 12 biomasses are identified. The residues covered are biomass from apple and blueberry orchards, olive grove and vineyard pruning, cattle, pig and poultry manures, biomass from eucalyptus and pine forest cleaning, the biodegradable and green fractions of municipal solid waste, and sewage sludge from public wastewater treatment plants. The resulting framework is given in Table 1.

Table 1 Four-level categorization of the bioresidues of Lafões
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The subcategories of category 1.1 consider the residues of the Lafões’ agricultural cultivated area, namely, its top four residues. They account for 84 % of permanent crops and for 58% of the total cultivated area, namely, apple orchards (7%), blueberry orchards (13%), vineyards (15%), and olive groves (22%). Being perennial, these crops demand annual pruning that generates notable amounts of lignocellulosic residues. The stalks and rootstock, both resulting from cutting of the trees at the end of their production cycle, were left out of this assessment, since they are produced in much smaller amounts. Within rotation crops, 95% are fodder, occupying 30% of the total cultivated area. Nevertheless, because it is assumed that in Lafões they do not generate residues, they are not considered.

For the subcategories of category 1.2, horse, sheep, and goat manures are not contemplated, because of the low number of bred animals in the region. In terms of the number of animals, poultry is dominant (99.7% of the total farmed in the region), but cattle and pigs farming produce high amounts of manure and thus are counted as residue categories. Livestock farming residues include liquid and solid manure fractions which vary among animal classes and can be collected mixed or separated, depending on the breeding techniques. These manures have high moisture and organic matter contents, including lignocellulosic biomass, fibres and proteins.

Category 2.1 has two subcategories, the biomasses from eucalyptus and maritime-pine forestry, which are lignocellulosic residues with low moisture content. They result from fire prevention operations or commercial exploitation and together represent 92% of the total area of forestry residues production. The residues of all other forest classes identified in the region’s Land Cover Map were thus left out of the present assessment.

The single subcategory of category 3.1 is the biodegradables from households, restaurants, and canteens and has high moisture, protein, lipid, and carbohydrate contents. It represents 26% of the municipal solid waste. It is currently collected by mechanical treatment of the unsorted municipal waste and sent for anaerobic digestion outside the region of Lafões. The biomass from public and private greenery areas (the single subcategory considered for category 3.2.) represents 15% of the municipal solid waste. It is collected by the producers and delivered through public and private operators to the urban solid waste management system (SGRU), being finally sent for composting outside Lafões. The category of sewage sludge from public wastewater treatment (the single subcategory of category 3.3), is the solid product discharged by the 23 wastewater treatment plants operating in the region of Lafões and is the municipal bioresidue with the highest moisture content.

2.2 Suitable bioenergy conversion technologies for Lafões

There are two main process types used to convert bioresidues into energy: biochemical and thermochemical. The choice between these two conversion processes depends mainly on the type of biomass and the desired energy carrier, though environmental guidelines, the legal framework, and economic conditions also influence the decision [26]. The most considered biochemical processes are fermentation and anaerobic digestion, while combustion, gasification, and pyrolysis are the most considered thermochemical processes. Because the residues being here considered are generally poor in readily bioavailable carbohydrates, fermentation will not be considered.

Anaerobic digestion is a well-established process to recover nutrients and produce biogas from organic residues. The process starts with the breakdown of complex matter into proteins, lipids, and carbohydrates, followed by enzymatic hydrolysis that converts them into soluble products (monosaccharides, amino acids, and long chain fatty acids). These can cross the cell membrane of acidogenic microorganisms to be converted into volatile fatty acids such as propionate, valerate, and butyrate, which are further converted by acidogenic microorganisms to acetate and hydrogen, which are finally converted to CH4 and CO2 by acetoclastic and hydrogenotrophic methanogenic microorganisms (archaea), respectively.

Multiple steps of the anaerobic digestion sequence are subject to microbiological activity inhibition, interrupting the process, so care must be taken when choosing the feedstock. The lignocellulosic fraction of the residues of categories 1.1 and 2.1 (see Table 1) is very hard to biodegrade, since its lignin matrix is resistant to enzymatic hydrolysis. Therefore, unless a pre-treatment is applied, these residues should not be considered for anaerobic digestion. The residues in categories 1.2 and 3.1 to 3.3 have higher moisture content; are often rich in proteins, lipids, and carbohydrates; and have low lignin content, being thus suitable for this process.

Direct combustion is the oldest bioenergy conversion process, and it is still the main energy source in many developing countries, used for heat production and cooking, despite the low energy efficiency of the process and the indoor air pollution associated to it. For this, small-scale combustion domestic furnaces with 10% efficiency are still being used. In large-scale combustion in thermal power plants, grate combustion (both fixed and moving grate), fluidized bed combustion, rotary hearth furnace combustion, and burner combustion are used to produce heat, electrical power being further obtained when a generator is operated, already with an efficiency range between 20 and 40% [24].

Direct combustion consists basically in burning biomass under excess oxygen (within a temperature range of 700–1400 °C [27]), which means that energy is consumed to vaporize the water content. Therefore, residues with high moisture content, such as those in categories 1.2, 3.1, and 3.3, are less suitable for this process. All the other residues here considered are suitable for this energy conversion process, but its low conversion efficiency leads to a search for other alternatives such as co-firing of biomass with fossil fuels [24].

Gasification is considered an alternative to combustion, where the oxidant can be air oxygen, steam, or CO2, being run at a lower temperature range (500–1300 °C [27]) with higher efficiency [24]. The first step of the process is the evaporation of the surface and inherent moisture of the biomass, followed by volatilization heating the system and allowing further conversion into syngas, which is the target product of the process [24]. Again, because the process involves spending energy in drying, the residues suitable for this process are those considered for combustion.

The energy products originated from combustion are generally heat and electricity, while gasification generates a fuel gas. Pyrolysis is a more versatile process in terms of its end-products, since it is possible to obtain pyrolytic oil (or bio-oil), fuel gas, and biochar. It is achieved at a lower temperature range than gasification (380–830 °C [27]), in anaerobic conditions, where there is also the evaporation of the water content first, followed by the decomposition of the organic substrate, e.g., hemicellulose, cellulose, and lignin, in this order [24]. Again, because it also demands energy for water evaporation, it is most suitable for biomass with low moisture content.

2.3 Biomass potential assessment methodology

Analysis of the biomass potential has different levels, depending on the constraints considered. An explanation of these different levels can be found in several literature sources [21, 28]. In short, the theoretical potential represents all the biomass produced in optimal conditions. It is therefore the maximum contribution of biomass for material or energetic uses within a given region and period of time. The technical potential considers restrictions or constraints to biomass collection, namely, environmental, legal, or technical. The used technical potential is that which is already in use for material, energetic, or non-differentiable purposes. What remains after subtracting the later from the technical potential is the mobilizable technical potential, meaning the biomass which is still available for energy and materials. Following the biomass cascading use principle, the mobilizable potential for uses other than energy should be subtracted from the biomass still available, finally resulting in the biomass mobilizable for energy.

Table 2 shows a summary of the parameters used to calculate the theoretical and technical biomass potentials. Whenever a range of values is reported for a given parameter (or more than one value is found in different sources), a conservative approach is adopted, assuming the lower end value. If a regional-specific value is not found, the value from a context as similar as possible is sought. The following are considerations on data collection for each residue category, as well as its potential uses, which should be taken in the light of each local context. The equations and the parameter values, used for the calculations, are provided in Online Resource 1 (Online Resource 1 – Equations and parameters used for the calculations).

Table 2 Parameters for calculation of the theoretical and technical biomass potentials
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To assess the biomass potential of crops pruning, the area of each crop is needed and discretized data for each municipality were provided by the Centre Region Directorate for Agriculture and Fisheries (DRAPC). The publicly available data from national entities are frequently aggregated by level II regions of the Nomenclature of Territorial Units for Statistics (NUTS II), and thus, to perform a discrete analysis, local entities should be involved.

To obtain the theoretical potential values, the area of each crop is multiplied by its residue productivity. Residue-specific yield values were found in the literature [29], but it is unclear if they consider constraints in biomass collection, and therefore whether the indicated potential is theoretical or technical. In the DBFZ database [30], separate figures are presented for yield and recovery rate, clearly identifying the potential being assessed.

Regarding the used technical potential, the only certainty is that no crop residue amount in the region is diverted to the energy conversion route. These residues are burnt under open air or used as matrix for compost, no exact information being available on the amounts taking each of these routes. Therefore, it cannot be stated that the latter is mobilizable for energy uses and the calculated results are taken to represent used technical potential.

The publicly available livestock manure data are again aggregated by NUTS II regions and do not allow a discrete analysis. The Directorate-General for Food and Veterinary (DGAV) of Viseu could however supply discretized data for the number of animals in each municipality. Livestock production can use a variety of techniques, involving the duration of the production cycle which affects the number of cycles per year, the barn time (the solely parameter of the technical biomass potential), and the materials used in livestock beds. The most representative techniques for the region have to be identified first, resorting to local data–privileged key-actors (e.g. veterinaries of the municipalities) that hold context-specific and up-to-date data. The number of animals or places, cycles per year, and the amount of manure produced per animal in each class are presented in the Portuguese ministerial ordinance no. 259/2012 [31]. Nevertheless, the regional-specific figures for cycles provided by DGAV did not always match the ordinance values, so a ratio between these two was used to adjust the manure production values.

The dry matter content of each class is needed since the manure production values are provided on a fresh matter basis. This content depends on the type of husbandry, e.g. solid (straw-based) manure versus liquid manure. The husbandry-type values considered are regional-specific and were consulted at the Portuguese National Statistics Institute (INE) for cattle and pigs [32] and provided by DGAV for chickens. For solid manure, average values of dry matter content were obtained for a sample of 270 farms in France [33]. These are used here, except for the pig class, for which the DBFZ database value [30] is selected, since it is lower. For the dry matter content of liquid manure, the value in the DBFZ database is considered [30].

The technical biomass potential of the livestock manure is achieved when the barn time is considered, also provided by DGAV. The used technical potential can be admitted as equal to the total technical potential, since all livestock residues in the region are being sent for composting. Nevertheless, anaerobic digestion is the suitable technology for their energy potential recovery and can still provide composting services in the form of digestate. Thus, the resultant biomass potential can be regarded as entirely mobilizable for energy.

The potential of forestry residues in the region of Lafões was already assessed and published [34]. There, to obtain the forestry biomass potential, the effective area of each forest classes (classified according to the Land Cover Map [35]) is multiplied by its biomass yield and by its vegetation horizontal projection, as suggested in the literature [36]. To compute the effective area, the accessibility criterion was taken into consideration and areas with slopes greater than 20% were removed, as well as those with distances to a road greater than 3 km.

An additional criterion is inserted in the present assessment, namely, conflicts with nature conservation areas (NCA), implying the removal of sites of community importance and special conservation areas from the effective area. For this, geo-referenced data analysis through GIS-based methods (using ArcGIS version 10.5) was implemented.

The yield parameter, since it considers the percentage of residues that should be left on the soil for environmental reasons, can be seen as a technical potential parameter. Nevertheless, here it is considered as part of the theoretical potential, while the accessibility and NCA criteria are used to compute the technical potential. In the DBFZ database, a recovery rate value is also provided for forest cleaning operations, but because in Portugal a complete cleaning is mandatory due to the wildfire risk, this value is disregarded (this decision must be context-specific).

These residues are currently burned under open air, so the used technical potential is null and all of it can be regarded as mobilizable. Since pyrolysis has biochar as co-product, the biomass can still be used as bioadsorbent (the most proficient technology of the cascade for residues with high lignin content [16]), and therefore, it respects the biomass cascading use principle, since the technology allows that the use as material is not impeded by the use as energy. Combustion and gasification do not respect the biomass cascading use principle, since no biochar or any other material with relevant value is produced, and a range of values will be presented for these technologies.

Within the data reported by the INE on the produced amounts of municipal solid waste, its biodegradable fraction appears to be null. This is because in Portugal the organic fraction is generally not collected separately. To obtain data on the amounts of this biodegradable fraction and of green waste a study was consulted [37], which was carried out in the framework of a program to support the development of bioresidues collection systems in the district where the region of Lafões is located. The study uses the values reported in the municipal waste registration map (MRRU), where a physical characterization of the unsorted stream of municipal solid waste is presented, carried out according to the guidelines in the Portuguese ministerial ordinance no. 851/2009 [38].

The amounts are presented as received, so the dry matter content values of the biodegradable fraction and of garden waste are required, and they were taken from the DBFZ database [30]. For assessing the technical biomass potential, no values were found in the literature for the inorganic fractions of biodegradable municipal waste and of garden waste in Portugal. Thus, the values for Germany in the DBFZ database [30] were used (the highest in the database ranges). The biomass potential was considered as entirely used for both wastes, since they are already being sent to an energy recovery plant.

Concerning sewage sludge, the volumes of treated wastewater per year in Lafões are disclosed by the INE [32], and these were multiplied by the sludge productivity in wastewater treatment plants that include the secondary stage, taken from the literature [39]. It is assumed that all the sludge can be recovered with no technical restrictions, and the theoretical potential is thus equal to the technical potential. Since at present all sludge in the region is being sent to landfills, the resultant biomass potential can be regarded as entirely mobilizable for energy since anaerobic digestion is the considered option, therefore still providing composting services.

2.4 Energy potential assessment methodology

The theoretical energy potential of a biomass can be assessed through its higher heating value (HHV), if combustion is the intended energy conversion process, since it measures the maximum heat released in this process. If anaerobic digestion is the targeted process, stoichiometric equations, such as the Buswell and Mueller equation or the Boyle equation [40], are used to assess the theoretical maximum biogas potential. The latter will of course be higher than the measured potential, since ca. 2–5% of this metabolic energy is used for microbial growth and maintenance [41].

This concept of theoretical energy potential (or just energy potential [41]) differs from the concept of technical energy potential (or energy yield [41]), because it does not consider the given operational conditions of a specific energy production facility where the conversion takes place [42]. In the present assessment, both potentials (theoretical and technical) are quantified.

To quantify the theoretical energy potential, residue-specific or category-specific energy conversion factors are used, adopted from the literature or from the DBFZ [30] and Phyllis [43] databases. In some cases, energy conversion factors are not available, and gas or bio-oil yields associated to a given energy production facility are considered. In these cases, the technical energy potential is being assessed instead of the theoretical energy potential. In both cases (using conversion factors or gas/oil yields), energy consumption or efficiency values for the overall energy production process are used to quantify the final technical energy potential.

When conversion factors (or gas/oil yields) could not be found for a particular biomass, an average value or a value for a biomass that belongs to the same category was assumed. To assign energy consumption or efficiency values, a fixed process layout was assumed for each technology, regardless of the residue being assessed. For parameters for which a range of values is reported, again a conservative approach was adopted, namely, by considering the low end for energy outputs and the high for energy inputs.

The equations to compute the theoretical and technical energy potential, the conversion factors (or gas/oil yields) of each residue category, and the energy consumption/efficiency values of each technology are provided in Online Resource 1 (Online Resource 1 – Equations and parameters used for the calculations).

3 Results

The results of the biomass potential assessment at the different levels (theoretical, technical, used and mobilizable) are presented in Table 3. Together, the four categories produce 107,431 t DM year−1 of biomass, theoretically. Since 3917 t DM year−1 is not accessible, it results in a technical biomass potential of 103,513 t DM year−1. Of these, 1752 t DM year−1 are already in use, meaning that 101,762 t DM year−1 are mobilizable for bioenergy purposes.

Table 3 Theoretical, technical, used, and mobilizable biomass potentials of the residues of Lafões
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The category of agricultural by-products produces the second highest biomass amount, most of it from livestock production, one of its two subcategories. The other, biomass from orchards, groves, and vineyards pruning, presents a biomass potential two orders of magnitude lower. Moreover, it is a used technical potential, resulting in a neglectable contribution to the biomass potential of the overall category. The mobilizable biomass potential of this category is therefore that of its sub-category livestock, 41,339 t DM year−1. The category of forestry residues shows the highest technical biomass potential of 60,302 t DM year−1 (65% from eucalyptus and 35% from maritime pine), all of it mobilizable. The category with the lowest technical biomass potential is municipal waste with 1751 t DM year−1 (54%, 39%, and 7% from biodegradable waste, green waste, and sewage sludge, respectively), almost all not mobilizable since only the one with the lowest biomass potential, sewage sludge, is mobilizable.

The top three residues in terms of biomass provision, representing 94% of the total mobilizable biomass potential of Lafões, are maritime pine forestry residues (21%), poultry manure (35%), and eucalyptus forestry residues (39%).

The results of the subsequent bioenergy potential assessment are presented in Table 4. A theoretical bioenergy potential of 318 TJ year−1 was obtained for biochemical conversion, 42% being consumed in process operations, resulting in a technical bioenergy potential of 185 TJ year−1. Among thermochemical conversion options, combustion gives the highest theoretical bioenergy potential, but due to higher losses in process operations ends up providing the least energy, with a technical potential of 239 TJ year−1. Gasification shows the second highest technical bioenergy potential, 381 TJ year−1, only exceeded by the technical potential of 543 TJ year−1 from pyrolysis.

Table 4 Theoretical (Theor) and technical (Tech) bioenergy potential of the residues of Lafões through conversion by anaerobic digestion, combustion, gasification, and pyrolysis
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Nevertheless, according to the biomass cascading use principle, when assessing residues with high lignin content the biorefinery technologies should be considered, namely, the production of bioadsorbents [16]. This means that the bioenergy technical potential values for combustion and gasification in the region of Lafões should rather be represented with the ranges 3–239 TJ year−1 and 5–381TJ year−1, respectively, where the lower end value of the range respects the cascading principle. In this same line of reasoning, it is better to represent the bioenergy theoretical potential with the ranges 11–957 TJ year−1 and 2–686 TJ year−1 for combustion and gasification, respectively.

According to the present methodology, when energy conversion factors are not available and gas or bio-oil yields associated with a given energy production facility are considered (which was the method selected for pyrolysis), the technical energy potential is being assessed and therefore is not possible to present a result for the theoretical bioenergy potential for pyrolysis, so this option was not used for comparison purposes.

Adding up the best results for the two conversion routes, namely anaerobic digestion and pyrolysis, the technical bioenergy potential of the residues of Lafões is 724 TJ year−1, 26% of it from anaerobic digestion. The category of agriculture by-products contributes 25%, with a technical potential of 178 TJ year−1 (177 TJ from anaerobic digestion of livestock manure and 1 TJ from pyrolysis of biomass from pruning). The category of forestry residues shows a technical bioenergy potential of 538 TJ year−1 (74% of the total) from pyrolysis of the two residues included in it (344 of which are from eucalyptus). At the bottom is the category of municipal waste, with a technical bioenergy potential of 8 TJ year−1 (2% of the total), mainly from anaerobic digestion of biodegradable and green waste (3.3 and 4.5 TJ year−1, respectively).

In terms of the top individual residues for bioenergy provision in the Lafões region, again three of them represent 96% of the total technical energy potential but following a distribution other than that for biomass potential. Specifically, poultry manure has the lowest contribution (12%), followed by maritime pine (27%) and eucalyptus (48%) forestry residues.

4 Discussion

The presented results show that Lafões can recover more energy from its bioresidues through the thermochemical conversion options, when compared with the biochemical route. The region is characterized by high values of biomass and bioenergy potentials from livestock farming, more suitable for the biochemical route, and from forestry residues, more suitable for the thermochemical route. Taking only the top three residues (poultry manure and pine and eucalyptus forest residues), anaerobic digestion of poultry manure can provide 154 TJ year−1 against 538 TJ year−1 achieved with the pyrolysis of forest residues.

The residue materials other than the top three can still deliver up to 32 TJ year−1. Of this, only 9 TJ year−1 are already in use, through composting of woody agriculture by-products and processing of municipal biodegradable waste and green waste outside Lafões, so the remaining 23 TJ year−1 (from cattle and pig manure and sewage sludge) should be included in design scenarios for integrated energy systems. For example, there are poultry production facilities nearby cattle production facilities, and, since there are no significant changes in feedstock collection costs, anaerobic co-digestion of the two manures is a reasonable option. Moreover, it can reduce microbial inhibition issues from the low C:N ratio value of poultry manure (around 8), since cow manure has a typical C:N ratio of 18 [44].

It is uncertain how much of the regional energy demand can be covered by the assessed residue energy potential, since publicly available data on total energy demand are not discretized by municipality. The INE indicates a value of 406 TJ only for electrical energy demand in the three Lafões municipalities in 2020 [32]. The calculated residue yearly potential of 724 TJ thus represents 178% of this electrical energy consumption. If it could ultimately be converted using CHP processes with an electrical yield of 40%, 71% of the 2020 electrical energy demand could be covered. Given the strong presence of manufacturing industries in Lafões, heat is expected to be an energy vector under high demand, but an assessment of the industrial energy needs, and the features of the production processes (e.g., their temperature ranges) is necessary for deciding the best allocation of resources.

The methodology as applied in the present assessment has limitations that should be highlighted. First, complete availability of the mobilizable potential for energy purposes is assumed, neglecting the growth of non-energy biomass demand and thus resulting in an overestimated bioenergy potential. Second, although the methodology foresees the improvement of assessments by including the energy demand of the conversion processes, not always considered in published reports, it is defined for this case study that burdens with feedstock provision are allocated to waste management, thus neglecting feedstock collection costs (energetic, economic, and environmental) for both conversion routes. It is therefore only possible to estimate the increase in energy production when compared to that of the current waste management practice in the region.

A detailed comparison between conversion route technologies and the prioritizing of actions is premature. To do such comparison, the cost of feedstock collection for each route should be computed in, which implies considering the context-specific exploitation characteristics of the feedstock in the region. One of these characteristics is an irregular topography associated with a smallholding character in forest production and exploitation, dictating a dispersed location and ownership of the production of feedstocks suitable for the thermochemical conversion route. Part of the feedstock production for the biochemical conversion route is also somewhat dispersed, due to a significant presence of small-scale poultry production units. These constitute a barrier for the development of bioenergy systems. Medium- to large-scale production facilities are also represented, but their weight in the region still remains to be assessed. The other feedstock suitable for biochemical conversion, sewage sludge, comes from more centralized facilities.

There are benefits in economic performance associated with large-scale power plants, which are difficult to obtain due to the dispersed and small-scale ownership of forestry residues [45]. In addition, the entities that are presently in charge of the disposal of forestry residues mainly have no commercial exploitation in view and hardly any investment capacity. This means that regardless of its expectable economic performance, is it not possible to implement the thermochemical route unless other economic agents take over or intervene.

The expectations laid on bioenergy to confer more flexibility to the regional energy mix, filling in the gaps of the intermittent wind-based electricity generation, should also be considered in identifying the residue/technology that demands priority action. The provision of flexibility services in short and medium term implies investments in CHP units. This is an obstacle in the thermochemical conversion route, due to the aforementioned limited investment capacity of the region’s forestry sector. The fulfilment of long-term flexibility services implies the upgrading of fuels, e.g., biogas to biomethane, gas from gasification to biobased synthetic natural gas or the bio-oil from pyrolysis. This requires investments that only the large-scale economic agents dealing with poultry manure can secure.

In terms of energy vectors, a high demand for heat is expectable in the region, and thus the bio-oil from pyrolysis can lose priority. This leaves the other two less efficient options of thermochemical conversion, both with better energetic performances than the biochemical conversion route. Nevertheless, the latter appears to benefit from regional-specific characteristics, such as feedstock distribution and collection and placement of investment capacity, as well as the ability to meet regional energy vector needs.

Therefore, any improvement of the environmental and economic performances, to be achieved through the implementation of bioenergy technologies, seems to be possible only through the biochemical conversion route, since it assumes the additional role as “anchor and/or upcycling tenant” by supplying agriculture fertilizer, replacing chemical fertilizers, and optimizing the environmental performance [46], thus respecting the biomass cascading use principle. Although pyrolysis-producing biochar also improves the environmental performance [16], the financial barriers to the implementation of the thermochemical conversion route, mentioned above, render this technology unattainable.

But more barriers can be foreseen for the mobilization of the bioenergy potential in the region. The above-mentioned use of its co-products, such as the organic fertilizer, faces challenges not only in terms of transport, handling, storage, and field applications, but also in terms of their market acceptance due to a negative public perception of digestates [46]. In what concerns the implementation itself of bioenergy systems, legislative, financial, research, teamwork, local support, and personnel training barriers can be predicted [47], some being already experienced in the region. The postponing of the Biomethane Action Plan under REPowerEU is one example.

One of these barriers was experienced within the present assessment, namely the hesitation to provide information on feedstock availability (described as a local support barrier [47]). Achieving optimal potential for this role requires the assessment of all available feedstocks, namely considering the industrial residues category. The characteristics of the latter and their current disposal practices can point to new priorities and the investment capacity of the economic agents associated with them can also influence decisions.

When planning the next steps to mobilize the assessed potentials, more information on the environmental and economic performances of the conversion routes is necessary. For example, avoiding emissions caused by current context-specific feedstock management systems can cause action on a specific residue to become a priority, with the implementation of the associated specific route. That is the case of the present open-air burning of forestry residues, but can also apply to manure composting or the landfilling of sewage sludge.

Beyond action priorities, the ambitious goals of the RED II [48] and the current RED II revision proposal [49] by the European Commission (EC) require a massive mobilization of biogenic residues and feedstocks for the production of advanced biofuels, and thus all residues and conversion routes are a priority. The National Energy and Climate Plan (NECP) of Portugal (resolution of the Council of Ministers no. 53/2020 [50]) outlines the need for ca. 7 PJ of advanced biofuels in 2030, and the present assessment results show that roughly 18% of this requirement could be met with the 1 PJ year−1 produced from the residues of Lafões. The EC proposal points to even higher requirements of advanced biofuels and the analysis of the biomass potentials should therefore be extended to the entire territory in all member states.

Moreover, when pursuing circular economy and industrial symbiosis objectives, the co-processing of complementary substrate mixtures can be necessary, and here, there is a marked lack of knowledge on expectable yields and potentials. Mixtures of maritime-pine forestry residues with poultry manure have been shown, albeit at a very low Technology Readiness Level (TRL), to lead to improved performance of the biochemical conversion route [34, 51]. Sewage sludge with its high biomass potential is gaining visibility for the possible implementation of symbioses. Wastewater sludge treatment can be redesigned to enable the application of the digestion process to mixed feedstocks that are complementary in chemical composition but are hard to mix due to density differences. The yields and potentials of such symbiotic conversion routes remain largely unknown and the performance increment from the management of complementarities is presently disregarded. In any case (individual or symbiotic routes), the assessment of their performance should be made also through life cycle analyses (LCA) and life cycle costing (LCC).

5 Conclusion

The application of this multistep methodological assessment shows the possibility of fulfilling all electrical energy demands of the case-study region of Lafões with the energetic conversion of its bioresidues. However, the assessment of the role of this bioenergy in providing flexibility services to the regional heat and electrical power sectors must be performed. The potential utilization of the residues bioenergy in combination with other renewables or in applications which are difficult to defossilize must be weighed. An assessment of the energy performance of all options including LCA and LCC should be performed.

The assessment identified three top residues produced in Lafões, namely poultry manure and pine and eucalyptus forestry residues. The biochemical conversion route, anaerobic digestion, leads to the worst energy performance, but with indications that it will provide better environmental and economic performances than the thermochemical options. The feedstocks suitable for biochemical conversion, poultry manure, can benefit from economies of scale due to the operation of large-scale facilities in the region. Nevertheless, the production of woody industrial residues in the region is non-negligible, and further assessment of the industrial residues category can lead to changes in the prioritization of conversion routes.

Beyond the scope of the present assessment, the development of complete and updateable resource databases, using the potential level-based methods outlined in the present methodology, should be carried out at countrywide or even wider levels, but still considering the regional contexts. Assessments are often focused solely on estimating theoretical or technical potentials, an insufficient approach for a future bioeconomy that increasingly requires the consideration of conflicting use interests. The present assessment improves on previous reports in considering the fractions already in use, such as biomass demand for non-energy (material) uses, aiming to assess the mobilizable energy potential, but there is still room for broadening its scope.

Bioenergy research is reporting needs to clarify yield definitions and provide more complete assessments of potentials. This is already the case of the bioenergy conversion routes assessed here, but it is especially important when new conversion routes are addressed, arising from the management of mixtures and their complementarities. The verification and benchmarking of results reported for experimental efforts, as well as their upscaling, recommends such clarification.