1. Introduction

Nowadays the technological advancements and the social acceptability of renewable energy technologies consist a controversial issue in a global level of analysis. This issue has also attracted the interest of scientific communities in the domains of social change processes, technological innovativeness, research and development (R&D), and electricity-oriented applicability. In line to this ongoing and active interest of renewable energy technologies’ diffusion in the everyday life of people, the aim of the four-fold literature review was to explore the main socio-economic constrains and perspectives of renewables’ exploitation in a wider context of energy production. Subsequently, the research interest of the paper was focused on biomass sources, due to the wide distribution of agricultural and energy crops worldwide. Particularly, a biomass-focused literature overview was conducted in order to reveal the potential supplementary contribution of biomass exploitation towards a more diversified energy mix that should be operated autonomously from the current carbon-based global energy production. The paper also outlined the key technological issues that are involved in biomass conversion and commercial distribution to the end-users of biofuels derived from agricultural and forestry biomass. At the end of the paper the main outcomes of the preceded analysis were gathered and presented in the light of both the literature and technology overview conducted.

In response to the renewable energy technologies to the “Not In My Back Yard” (NIMBY) social behaviour remains an argumentative issue, according to, firstly, the promotion of renewable energy production as a social change process in developed societies, and, secondly, the socio-psychological aspects involved in peoples’ responses to social change. Furthermore, to perspectives of understanding the social role of renewable energy technologies are strongly associated with a better understanding of peoples’, policy makers’, industrialists’, and media commentators’ responses to such renewable energy technologies ^[1,2]. In these studies, the social acceptability or opposition to renewable energy technologies has been addressed in the light of Environmental Psychology on the concept of place attachment. Particularly, local opposition is reconceived as a form of place-protective action, which is arising whenever technology projects disrupt pre-existing place attachments and threaten place-related identity processes. Therefore, in a campaign in favour of/or against the installation of renewable energy technologies, residents who argued that these technologies would “industrialise” the area, they felt also strongly attached to their residential place and they:

· were most likely to interpret such renewable-based projects as producing negative local outcomes,

· felt negative emotions associated with these projects,

· indicated low levels of projects’ support,

· had undertaken oppositional behaviours such as signing petitions or writing letters to the local authorities,

· expressed a significant positive correlation between place attachment and trust in the local opposition groups, and a significant negative correlation between place attachment and trust in the development company.

In parallel during the last decade of analysis, the technology transfer and the adoption of renewable energy innovation are still limited to qualitative analytical techniques and on empirically-based experiences of local scientific interest, such as (indicatively) in: Mexico ^[3], China ^[4,5,6], Netherlands ^[7], Europe ^[8,9], United States ^[10], and Spain ^[11].

Mallett ^[3] pointed out that active interest in social acceptance of renewable energy innovations should express a wide spectrum of stakeholders including technicians, industry representatives, local government officials. The author concluded that integral forms of technology cooperation are most effective in eliciting social acceptance of renewable energy innovations. Groba and Cao ^[4] investigated the roles of policy, technological innovation, and markets in exporting renewable energy technologies. These authors investigated the development of solar PV and wind energy technology component (WETC) exports from China in the light of the competitive position of the country‘s renewable energy industry. These authors stressed out that governmental energy policy, renewable energy innovation, and knowledge accumulation could drive exports. The authors concluded that high income countries, with large renewable energy markets and demand side policy supported schemes of incentive tariffs, were increasingly importing Chinese solar PV components, whereas trade costs showed a negative impact on solar PV exports, but not WETC. No evidence that bilateral knowledge transfer and indigenous innovation affected exports was proven.

Negro et al. ^[7] analyzed the dynamics of newly emerging innovation systems that stimulate renewable energy technologies. In parallel, the key-parameters of building up of such emerging innovation systems were identified while dynamic functional patterns of such a technology were identified in order to develop such a systemic innovative policy. A systemic-based analysis of renewable energy technologies was also the objective of the study of Tsoutsos and Stamboulis ^[9]. In this study the innovative and sustainable diffusion process of energy production and management was deployed. Particularly, Tsoutsos and Stamboulis ^[9] introduced and alternative approach which integrated the supply- and demand-side perspectives, pointing out that a successful policy for the speedy deployment of renewables should focus on the systemic innovation processes that characterized the development and sustainable diffusion of renewables. The proposed strategic plan aimed at the integration of the innovation dimension into a policy for renewables, contributing to the growth of successful applications and to the development of the affiliated industry of equipment production and services, fostering the deployment of a new technological regime.

In a global context of renewables’ innovativeness, Buckley and Schwarz ^[10] explored the perspectives of the innovative technology of manure gasification in energy production. The environmental benefits of alternatives to land-spreading of traditional manure management, as well as the advantages of gasification versus traditional combustion techniques for high nitrogen fuels, were denoted. The environment overview of the technology introduced was implemented regarding sustainability, manure management, and renewable energy. In parallel, artificial, non-sustainable barriers to renewable energy, and the impact of wide jurisdictional variability are discussed.

In the European context Hervás Soriano and Mulatero ^[11] established the strategic energy technology plan (SET-Plan) as a strategy that can use research and innovation (R&I) to green EU energy sector. The initiatives of such a SET-Plan are outlined as follows:

· programming plans,

· takes stock of existing initiatives in the energy sector,

· fosters a cooperative approach to R&I,

· introduces a high-level SET-Plan steering group to monitor progress,

· creates a supportive information system (the SETIS) to fill the void in policy information, and

· produces estimates of financial needs over the programming period.

Hervás Soriano and Mulatero ^[11] argued that the SET-Plan could serve as a blueprint for R&I strategies to tackle other societal challenges. The effectiveness of the setting strategies were aligned to further clarifying the hierarchy of existing objectives and instruments, introducing specific instruments to pull the demand of new technologies, strengthening links with education and training policies, as well as formalizing links with the governmental structures.

Renewable energy technologies were also related to, both, the research and development (R&D) policies, as well as to the purposes of meeting governmental policy goals. These policies showed that environmental goals could be utmost importance factors for the development of various renewable energy technologies, followed by the economic, energy, and financial goals, since the development of sustained renewable energy projects could overcome high initial projects costs that, in turn, will result in increased renewable capacity and decreased costs ^[12,13,14].

In another study, Sung and Song ^[15] investigated a causality testing between public policies and exports of renewable energy technologies in the South Korean context. The authors revealed a positive relationship between, firstly, government expenditures on R&D and exports rate, and, secondly, between expenditures on R&D and the contribution of renewable energy to the total energy supply (CRES). In parallel, Garces and Daim ^[16] signified the importance of energy policies aiming to accomplish the energy security objectives, while the authors concluded that the US federal and state energy policies, R&D investment, knowledge stock, and knowledge accumulation were not entirely connected to electrical market variables.

Conclusively, in an earlier study, Anderson ^[17] outlined a wide spectrum of policies regarding the costs and the performance profiles of renewable energy technologies in developing countries in the grid and off-grid markets for electricity, including:

· investments in renewable energy that should be helped by competition and regulatory reforms in the energy industry, since such reforms should reduce the subsidies,

· further cost reductions should be appreciable in all key technologies, tax and regulatory policies, and in budgetary allocations for R&D, education, and training since there are positive externalities to investment, in the view of each generation of investments is acting to reduce the costs of future generations.

· environmental advantages of renewable energy are more apparent as developing countries begin to introduce their environmental policies on fossil fuels.

Finally, numerous studies have been devoted to investigate the perspectives of renewable energy technologies in electrification systems in a global level of analysis: Spain ^[18,19], Taiwan ^[20], Malaysia ^[21], United States ^[22], Italy ^[23], Portugal ^[24], Japan ^[25]. These studies manipulated the involvement of renewable energy technologies in the national energy production grids, in the light of projects’ subsidizing, energy pricing, governmental initiatives to energy mix of production, feed-in tariff remuneration modeling and schemes.

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2. Methodology Framework

In this paper a four-fold overview was conducted upon: renewables’ exploitation for energy production in the global and European contexts, as well as biomass exploitation for electricity generation in a global-oriented and technology overview of the relevant literature. To this end, the paper firstly introduces an extended literature overview that holistically outlines a framework of renewable energy technologies in a global context of presentation, following by an extensive literature overview of European policies and challenges in applying renewables for energy production. Subsequently, the paper denoted the perspectives of biomass sources’ exploitation in electricity generation. Moreover, the key-parameters and the technological advancements that apply to agricultural and forestry biomass sources towards biomass conversion and commercial exploitation in the form of biofuels, where also given. Conclusively, the main limitations and challenges -derived from this four-fold literature overview- as well as the proposed orientations for future studies were also outlined at the end of this paper. The main challenge of this paper is to fill the gap of theoretical and technological knowledge between the transition from biomass sources up to their large-scaled/in-field/marketable exploitation that all enable a stable, supplementary, cleaner, and autonomous energy production and electricity generation.

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3. Literature Overview upon Renewables Exploitation for Energy Production in a Global Context

In the relevant literature there are plentiful studies pronouncing the advantages of the renewables (RES) use. These advantages are, indicatively, the: mitigation of environmental problems such as the deterioration of climate change, low operating costs of RES use -that is not affected by the prices of fossil fuels and by the status of the international economy, jobs’ creation, improvement of the quality of life, reduction of health risks, incremental independence from fossil fuels, increased energy security and supply, increase in tourist activity, protection of the natural resources and the accommodated ecosystems, reduction of pollution incidents, as well as positive contribution to the local development ^{[26,27,28,29,30,31,32,33]}.

On the other hand, apart from the advantages of using RES there are also disadvantages of RES exploitation—as these are stressed out in the relevant literature—including the: visual nuisance of RES infrastructure and the accompanying aesthetics distortion of the nearby landscape, desertification of the local flora and fauna species, acoustic nuisance emanating from wind power plants, as well as high installation costs ^[26,34,35].

Energy production from RES could support the strategic energy plan of each country to reduce its energy dependence from energy imports. However, this statement should not be considered as the only factor in reducing energy dependence on fossil-fuelled plants. In particular, the energy dependence of the EU countries is not determined solely by the pace of energy production from renewables, but it is mainly depended on other parameters, such as economic, social and political, as well as the: quantities of national energy consumption, energy efficiency, national policies for energy, the geo-strategic position of each country, as well as the foreign policies and the plans for energy security ^{[36,37,38,39,40,41,42,43]}. In parallel, electricity production from renewables is no merely a scientific endeavour, but it is an indispensable component in the production process, since numerous countries keep investing in green energy ^{[44,45,46,47,48,49,50]}.

In a global context of analysis, renewable energy has been noted as an important factor in accomplishing low carbon emissions in the economic growth of China. Particularly, it was pointed out that there is a strong association between the electricity produced from RES and GDP per capita, with the opening of trade, foreign direct investment, financial development and the accelerated pace of fossil fuels in energy consumption. Nevertheless, it is also noteworthy that in a wider societal context there are published studies that addressed the commercial applicability of renewables in a modeled level of analysis ^[51,52].

Moreover RES infrastructure has been developed geographically in a dispersed manner thus contributing to regional development of energy infrastructure, satisfying the energy needs of small or larger areas, in the contexts of continental or insular, and, urban or agricultural, levels of analysis in a global overview ^{[53,54,55,56,57,58,59,60,61,62,63,64]}. Such a spatial expansion of RES exploitation:

· determines the RES contribution to national energy mix

· promotes the deployment of a more rational and wise use of renewables

· reveals that optimal spatial allocation of renewables is governed by conflicting criteria and strict constraints, such technical and geological constraints as well as by fostering projections where their applicability should cause low environmental burden

· visualizes and balances the relative opportunities and constraints on development arising from a wide range of environmental, industrial and socio-economic factors, such as emissions implications of altering policy constraints, technology and resource availability, modeling decisions, availability of biomass, and carbon capture and sequestration

· supports decision making in regional renewable energy planning by providing information on regional potentials and restrictions to different energy stakeholders, thus envisaging an energy developmental vision that can orient regional energy development towards sustainability

· prevents energy losses resulting from the energy transport over long distances, thus not burdening each national energy distribution system in order to produce excess energy that is partially lost, prior to end-users consumption. The properly designed of transmission and storage capacities for the power sector could foster the achievement of ambitious and long-term emission reduction targets, the lower possible storage requirements and curtailments and a non-escalation of temporal and spatial electricity prices, in case that emission reductions exceed a critical value.

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4. Literature Overview upon Renewables Exploitation for Energy Production in a European Context

In the European context there exists an extensive research production upon the use of renewables for energy production. It is indicatively noted that in the Greek context the utilization of all forms renewables—that are abundant throughout the Greek territory—can be proven highly promising (mainly based on forestry, wind, or solar energy) entrepreneurial ventures. The main issues that are discussed in these studies are the socio-economic and environmental impact of materializing these ventures, involving the report of occasional delays in the start up phase and the concurring disappointments during the development processes of such ventures ^{[65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96]}.

The European Union (EU) Directive 2009/28/EC set out the contribution of RES in final energy consumption to 20% by 2020. This targeted percentage is allocated among the factors of electricity, transport, heating and cooling. In order to achieve the participation of the RES 20% target by 2020, the EU should double the share of electricity from RES from 16% in 2006 to over 30%, while it should be ensured that the share of energy from RES in transport in 2020 is at least 10% of final consumption of energy in transport in each Member State. In this Directive a list of measures to reduce energy consumption and increase the use of RES is presented, while data on the main administrative structures to accelerate this penetration, are also outlined. Each Member State shall adopt a national action plan for RES support schemes and collaboration measures between them or with third countries, including statistical schemes of collaboration and joint projects ^[97]. In a literature overview, the collaboration concept under the renewables introduction to the current energy mix should be determined by the following key-issues ^[98,99,100].

· The rapid energy shift of the current energy mix, from fossil fuels to renewables, and the accompanying significant investments in infrastructure for electricity generation, transmission, and distribution.

· Renewables exploitation for climate change and sustainable economic growth. Thus, international research collaboration upon renewables requires a considerable amount of R&D expenditure and various core technologies, in order to achieve prompt and efficient development.

· In response to the socio-economic issues of food shortages, in combination with rising population and lack of access to electricity needed for development, tangible results have yet to materialize. Therefore, the use of environmentally-friendly renewable energy technologies in line to a strong rationale for political will, collaboration and transparent energy policies, will ensure the betterment of the human well-being.

Moreover, the “Energy Policy for Europe”, as directed by the European Commission, established a future plan and actions’ framework to achieve the main European Community energy objectives relating to sustainability, competitiveness and security of energy supply. Particularly, ten initiatives of this European Energy Action Plan—that compound a policy framework for renewable energy in the European Union, and could track to reach its future renewable energy targets‑involve ^{[101,102,103]}:

· an optimized functioning of the internal energy market

· member States support towards developing solidarity at energy crises and ensuring the safe supply of oil, gas, and electricity energy

· enhancement of the operational responsibilities of the Community Emissions Trading Mechanism of Greenhouses towards achieving reduction of CO₂ emissions and attract investments in clean energy

· development of an energy-saving program at European (also national and international) level(s) of analysis

· extended use of RES

· development of a strategy for energy technology

· development of fuel conversion technologies with low CO₂ emissions

· development of safety and security issues regarding the use of nuclear energy

· common agreement on an international energy policy with unanimously agreed objectives, abiding from all Member States

· deep understanding of energy issues amongst European citizens/consumers/end-users

· The aforementioned initiatives should establish a coherent external energy policy to assure the achievement of the main goals of the EU policy, which are the security of supply and the adverted challenges of climate change. Conclusively, these initiatives aim at assuring a secure, competitive, and sustainable energy market in the EU ^[103].

The main EU policy—among the European policies for RES launched—can be summarized in the so-named 20-20-20 which refers inreducing greenhouse gas emissions by 20% compared to 1990, increased production of renewable energy to 20% and improving energy efficiency by 20% by 2020. To this end, the EU Action Plan sustains the core contribution of European energy policy in line to this goal and outlines specific measures that should be taken to achieve this goal. Specifically, the four key-objectives of the European Action Plan for Energy aim to ^{[101,102,103]}:

· save energy consumption by 20% compared to projections for 2020

· increase the participation rate of RES to 20%, compared to the gross final energy consumption by 2020

· increase the biofuels’ share—that are directed in the transport mobility (gasoline and oil)—by 10% minimum, in order to achieve primary energy savings of 20%, and

· reduce gas emission by 20% by 2020, compared to 1990 gas emissions

The main objectives of European energy policies can be summarized in the following three areas: competitiveness, sustainability, and energy security. These goals’ achievement necessitates the establishment of a new legislative and institutional framework that should: foster investments in energy infrastructure and set clear and predictable targets for electricity production from RES, while research is directed in new technologies that are energy efficient, and new rules are established for the internal market. Particularly, the EU has adopted the Climate and Energy Package (CEP) ^{[104,105,106,107,108]} through which it is planned to operate the emission permits trading mechanism of carbon dioxide emissions from large industrial and power plants, as well as from airline companies. Additionally, through the CEP, a wide spectrum of targets for each Member State was set in order them to, firstly, reduce greenhouse gas emissions within the regulated levels, secondly, outline objectives for the penetration of RES in energy production, and, thirdly, reinforce the statement of strong environmental leadership to achieve global climate deals, under a pressure of the European negotiations and significant changes in the social context ^{[105,106,107]}.

Conclusively, and in line to the roadmap of Energy 2050 published in December 2011, greenhouse gases emissions should be reduced by 80% by 2050 compared to the reference year 1990. This goal should be achieved taking into consideration the energy security, the supply, and the competitiveness of the European market, as well as accomplishing low emissions of gases that affect the greenhouse effect. The roadmap for Energy 2050 has been structured in the light of scheduling a long-term European plan for the energy sector, in collaboration with the agents of the free market and of the Member States ^{[109,110,111,112,113,114,115,116,117,118,119,120,121]}.

Finally, in the light of the aforementioned roadmap of Energy 2050 it is noteworthy that Pang et al. (2014) introduced the concept of strategic environmental assessment (SEA) that could provide a comprehensive framework for assessment of policies and plans upon contemporary environmental issues such as climate change, as well as resource and land use changes. Particularly, Pan et al. ^[122] revealed possibilities for comprehensive sustainability assessment among published energy-environment models and the alignment of renewable energy analysis to landscape and biodiversity issues through land use concerns. To this end, the available tools—including existing energy, research, environmental, and linking models; surveys on publication; as well as a case study on the aforementioned European roadmap of Energy 2050—all showed low concerns on land use, landscapes and biodiversity, and were proven ineffective to provide comprehensive decision support for assessing future energy scenarios. Subsequently, the development of overly new energy models could provide new orientations, developing them to involve renewable energy options with land use, landscape and biodiversity concerns, which could be advanced into powerful SEA tools for an integrated policy assessment. Subsequently the development of more comprehensive decision support tools will integrate main policy concerns while assessing renewable energy options.

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5. Literature Overview upon Biomass Exploitation for Electricity Generation in a Global Context

The literature review at the Scopus database was implemented during the April of 2016 and its outcomes were collected andshown in the following Tables 1-5. The data mining was gathered and presented in a reverse chronological order of publicity following an alphabetical/chronological arrangement: per last name of the first author, and per year. All information presented in these Tables 1-5 was based on both the Scopus database, as well as, those full articles that are available from this web search platform. This search outcome upon RES sources utilization for electricity generation was further classified in the following five groupings: Table 1-Microbiological overview, Table 2-Economics and political initiates’ overview, Table 3-Co-firing, forestry, and CO₂ sequestration overview, Table 4-Biomass gasification overview, and Table 5-Decentralized power overview.

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6. Biomass Exploitation in a Technological Overview

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6.1. Processes of biomass conversion

In the relevant literature, the biochemical and thermochemical processes that are involved in biomass conversion are denoted in the following flowchart of Figure 1. The overall process is carbon neutral, since carbon dioxide is caught from the atmosphere by plants. In parallel, the processes shown convert plant biomass to energy, chemicals and materials. In the consumption path, carbon is turned to carbon dioxide and released to the atmosphere ^[160].

According to the aforementioned Figure 1, the basic four processes of lignocellulosic biomass conversion are the thermal-chemical dissembling, the pyrolysis, the burning/CHP, and the gasification. In parallel these processes can be classified/grouped as biochemical or systematic dissembling: that of thermal-chemical dissembling and pyrolysis, as well as thermochemical processes: that of burning/CHP and gasification. In the following sections of this review paper has been focused on those critical parameters that determine the technological adaptability of biomass sources towards energy production. The limitations and the challenges that are derived from this literature overview were also outlined.

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6.2. Boiler of biomass heating

An initial assessment of biomass heating involves technical and economic considerations. Biomass heating systems sustain higher initial capital cost than fossil fuel systems of equivalent rated capacity. Nevertheless, this financial difference is commonly recouped through annual fuel cost savings. The technical considerations are utmost importance factors when considering the installation of a biomass boiler. An integrated flow chart that is necessary to follow, is outlined in the following Figure 2.

In an economic considerations overview, in order to assess the economic feasibility of biomass heating, there are several important parameters that the investor needs to be aware of, such as:

· Site’s annual energy consumption.

· Heat demand profile (including utilisation patterns).

· Costs associated with fossil fuel use.

A detailed economic evaluation should enable the final decision upon whether the biomass boiler is practical in the first instance. The concluded results consist of:

· Total capital investment required (€).

· Simple payback (years).

· Unit cost of heat from the project (€/kWh).

· Annual energy cost (€/year).

An initial assessment of economic feasibility can be estimated by using some online tools that are available on the internet. Such a tool is that of CarbonTrust ^[161].

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6.3. Schematic processes of biofuels production from biomass sources

Biomass sources have recently attracted a wide interest among countries and scientists, due to their high energy content, while being converted in biofuels. It is indicatively noted that sugarcane is one of the fundamental agricultural crops towards ethanol production, especially in tropical countries ^[162]. In Brazil, sugarcane occupies 8.4 million hectares that corresponds to 2.4% of farmable lands in the country. The gross revenue of this market is about US$ 20 billion (being allocated as: 54% ethanol, 44% sugar, and 2% bioelectricity). Additionally, up to 50% of all vehicles in Brazil are flex fuel cars that is approximately 15 million cars ^[162]. Therefore, Brazil will play a dominant role upon extending the biomass sources towards energy production, whereas sugarcane bagasse and straw are proven promising feed stocks for biomass ethanol. In 2008 Brazil produced 415 million tons of sugar cane residues, 195 million tons of sugarcane bagasse, and 220 million tons of sugarcane straw, whereas for the year 2012 sugarcane production accounted of 590 million tons, which correspond to 178 million tons of bagasse, and 200 million tons of straw ^[162].

The first generation biofuels are currently produced from sugars, starches and vegetable oils, but these products have shown several disadvantages, such as their limited availability by soil fertility and per-hectare yield, while their contribution to savings of CO₂ emissions and fossil energy consumption is restricted by high energy input for their cultivations and conversions ^[163]. In parallel, although current R&D has been focused on agricultural residues such as sugarcane residual biomass, woody biomass (mainly in the forms of hardwoods and softwoods) remains an important feedstock for cellulosic ethanol production. It is estimated that 370 million dry tons of woody biomass can be sustainably produced per year in the US. Woody biomass is also sustainably abundant in many other regions worldwide such as Scandinavia, New Zealand, Canada, Japan, and South America. Besides, short-rotation intensive culture or tree farming offers an almost unlimited opportunity for woody biomass production ^[162]. Moreover, variable other biomass sources that are appropriate sources of energy, as energy crops, contain diversified quantities of cellulose, hemicellulose, lignin, and ash. The following Table 6 presents a typical composition of the aforementioned compounds for various biomass sources ^[164].

As it is presented in Table 2, lignocellulosic biomass is chiefly composed of cellulose, hemicellulose, and lignin. Nowadays, lignocellulosic biomass has been recognized as a source of energy, fuels, and chemicals. In the relevant literature ^[162] during the last four decades of research and development (R&D), it was signified the use of lignocellulosic materials for energy generation via ethanol production. However, the technology required for industrial conversion of lignocellulosic materials into ethanol, has been not fully developed. To this end, a technical solution for efficient use of lignocellulosic materials to ethanol production is the applicability of a biomass pretreatment step. Under the biomass pretreatment, the production of sugar syrups with high concentrations and yields via enzymatic hydrolysis of biomass requires the pretreatment to be efficient to render the material accessible to the relevant enzyme pool ^[162].

Indeed, lignocellulosic biomass seems to be an attractive choice to ethanol production because, firstly, it is the most widespread renewable source available on earth since the overall chemical energy stored in biomass by plants is approximately 6-7 times of total human energy consumption annually, secondly, it is locally available in many countries and, thirdly, it does not compete with food or food industries. However, woody biomass conversion to fermentable sugars is more difficult than that of agro-based biomass because of the presence of more hemicelluloses (being not easily fermentable) and lignin, as well as due to a more condensed and crystallized structure of cellulose in woody biomass ^[163]. Figures 3 and 4 represent the flowchart for bioethanol production from biomass, and the flowchart for biodiesel production from lignocellulosic biomass, respectively ^[164].

According to these Figures 3 and 4, it can be noted that the production of biofuel from cellulose of biomass necessitates that the lignocellulosic biomass should be first dissembled to facilitate the isolation of cellulose from other constituents, such as lignin and hemicelluloses. Subsequently, cellulose macromolecules should be depolymerized, since depolymerization significantly advances the chemical and biological conversions of cellulose to biofuel. Then, the depolymerized cellulose, such as glucose, can be converted to biofuel via biological treatments, and finally the biofuel can be purified. Additionally, biomass can be deoxygenated, since the presence of oxygen reduces the heat content of molecules and creates high polarity, which impairs its blending with existing fossil fuels ^[163].

At the end of this section that outlined the main processes of biofuels production from biomass sources it is noteworthy to refer the hydrogen production processes from biomass sources. To this end in the Table 7, below, various pathways to produce hydrogen from glucose are gathered and presented.

According to this Table 7, the theoretical and practical yields of hydrogen production through chemical catalytic reactions, such as gasification, pyrolysis and hydrolysis—accompanied by aqueous phase reforming (APR)—of glucose are in similar range ^[163]. Particularly, the gasification is conducted at a high temperature of 1000 K in the presence of oxygen and water, while pyrolysis is accomplished at a high temperature but in the absence of oxygen. The main advantage of APR over gasification is the lower extent of undesirable decomposition reaction. Moreover, the APR is carried out at a lower temperature, 400-550 K, and at a medium pressure, 50-70 bar. The water medium of this process enables the occurrence of the hydrogen production reaction. In an industrial applicability viewpoint, although the chemical catalytic reaction seems to have considerably higher hydrogen production yields, the prerequisite of high energy input and poor selectivity toward hydrogen production are limited its advantageous yields of hydrogen ^[163].

However, the biological conversion of cellulose to hydrogen is developed at much lower temperatures, thus the energy input of this process is much lower than that of chemical catalytic processes. Contrarily, the theoretical and practical yields of hydrogen production through this method are considered low (such as in the dark anaerobic fermentation of Table 7). In order to increase the hydrogen yield, a bioelectrochemically-assisted microbial fuel reactor was reported, but this process also needs electricity input and it has not been studied in large scale/in-field applications ^[163]. The next section is further addressing the technological specifications involved in the process of biomass pretreatment.

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6.4. Biomass pretreatment

The complex structure of lignocellulosic biomass makes its utilization in biofuel production a complex process. According to the Table 6, above, the lignocellulosic raw materials are generally composed of 40-50% cellulose, 25-35% hemicelluloses and 15-20% lignin. A pretreatment stage enables the dissociation of the plant cell wall in order to improve the accessibility of chemicals and/or microorganisms to cellulose for possible conversions. The pretreatment processes target at the removal of lignin, which improves the digestibility of cellulose in the subsequent process of hydrolysis. Table 8, below, lists various pretreatment processes of woody biomass in order to improve the performance of fermentation processes in biofuels’ production ^[162].

Another classification of biomass pretreatment that was given from the same authors contained the following processes ^[162]:

· Hydrothermal pretreatments, including liquid hot water (LHW) and steam.

· Steam-explosion pretreatment.

· Mechanical pretreatments, including ball milling, wet disk milling, and extrusion processes.

· Ionic liquid pretreatment.

· Other pretreatment processes, including alkaline-; ammonia-; and biological- pretreaments.

In the text follows, the paper has been focused on a more detailed presentation of the physical, chemical, and biological pretreatments.

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6.4.1. Physical pretreatment

Physical pretreatment consists of the environmentally friendly process of mechanical disruption of lignocelluloses. The functionality of this process is to increase the surface area of biomass and to decrease the crystallinity of cellulose, without causing an expensive mass loss. Other physical methods that break the structure of lignocelluloses are the irradiation using gamma rays, the electron rays and the microwaves. Microwave irradiation has been also applied in food drying chemical synthesis and extraction ^[162].

Another approach to separate the lignocelluloses of woody materials is the physicochemical pretreatment. Hydrothermal treatment, such as hot water pretreatment and steam explosion, is also a suitable method, particularly prior to enzyme hydrolysis. In the hot water pretreatment process water is maintained under pressure at an elevated temperature of 230-240 °C for 15 min in liquid form, thus less inhibitory compounds (such as furfural) are produced in comparison to steam explosion method. However, the viscosity of the spent liquor produced in this method is rather high, which makes its handling process complex. The steam explosion is practically applied in industry via steaming biomass at an elevated temperature of 170 °C. The limitation of the production of inhibitors involves the precise adjustment of the process conditions. Steam explosion has been further classified in variable subcategories, such as ammonia fiber explosion and acid-explosion, in which acid or ammonia is also added to the system during the steaming process ^[162].

In a modelled example of physical pretreatment of biomass, the case of a pellet plant process was comprehensively studied and selected for further analysis in this section ^[165]. The relevant data have been extensively presented from Huang Jack ^[165], whereas a succinct framework of this process is also given below. Pellet plant process consists of a flowchart that enables the end user to be affiliated with the processes involved in the industry of wood pellet production. The projection of such flowchart should be also depicted in a 3D format and it can be a customized pellet plant design. A typical flowchart of a pellet plant process is given in the following Figure 5.

In this Figure 5, a typical wood pellet plant consists of the following list of pellet plant process ^[165]:

Raw materials storage, in which the raw materials of a pellet plant—such as saw dust, wood chips, corn stalks, rice husk, and sugarcane bagasse—will be stored. A typical warehouse of such a raw material has to be moisture-resistant.

Initial size reduction, since the pellet plant process is commonly involving the size reduction of the bigger raw materials such as logs. As the pellet plant process advances, a wood chipper is used in order to cut further the big raw materials into smaller pieces with a diameter no more than 20 mm towards the processes followed.

Drying, is an important process of the pelletizing technology, since strict moisture content of the raw materials is needed. In the drying process, the rotary pellet dryer is used to complete the process.

Grinding, in which the raw materials undergone the second size reduction process in a pellet plant. Grinding is implemented by the use of a hammer mill in order to grind the raw materials into small particles of less than 5mm, in order to improve the wood pellet quality and output, and to lower the energy consumption of the pellet mill. The smaller the raw materials are, the better wood pellets are formulated.

Pelletization, in which the raw materials are pelletized into wood pellet. In a large pellet plant a ring die pellet mill is the appropriate choice to make wood pellets. A larger capacity—than that of one ring die pellet—of the pellet plant can be handled by connecting several ring die pellet mills together.

Cooling, that is the temperature reduction process of a pellet plant needed since the wood pellets remain at an extremely high temperature when they are discharged from the pellet mill. To this end the counter flow pellet cooler can be operated in order to cool down the wood pellet and to make it regain the proper rigidity.

Sieving, in which a separation of the fines from the well-made wood pellet is applied. To this end a pellet screener can the fines from wood pellet products, thus a high quality of fine particles of wood pellet has accomplished.

Packaging, which is the last process in a pellet plant process. Packaging ensures the proper package and storage of the wood pellet products. This process is especially needed for commercial use of the final production.

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6.4.2. Chemical pretreatment

The chemical pretreatment of cellulosic biomass includes oxidizing hydrolysis, acid or alkaline hydrolysis and solvent extraction. In this process, a lime treatment (including treatment of woody biomass at 180°C with lime solutions) has been considered as an effective chemical pretreatment method, due to its low cost and wide use in agro- and wood-based processes. Pretreatment with dilute acid at intermediate temperatures (such as in 160 °C) is usually considered the most cost-effective method to loosen the cell wall matrix via degrading the hemicelluloses of biomass ^[162].

In a modelled example of chemical pretreatment of biomass, the case of an acid pretreatment process was comprehensively studied and selected for further analysis in this section ^[162]. The main issues associated with acid hydrolysis are related to the formation of toxic compounds from biomass degradation and from equipment corrosion. Such toxic products foster inhibition in cell metabolism, when biomass hydrolysis’s is used for bioconversion. Therefore, in the relevant literature appropriate steps that remove these inhibitory compounds and improve the yields in bioconversion processes, have been employed ^[162]. Such a typical mechanism of the acid hydrolysis reaction of lignocellulosic materials sustains the following steps ^[162]:

1. The diffusion of protons through the wet lignocellulosic matrix;

2. The protonation of the ether-oxygen link between the sugar monomers;

3. The breakage of the ether bond and the generation of a carbocation as an intermediate;

4. The solvation of the carbocation with water;

5. The regeneration of protons and the cogeneration of sugar monomers, oligomers, or polymers, depending on the ether connection that is broken;

6. The distribution of products in the liquid phase (if permitted by their shape and size); and

7. The restart of the process from step 2.

Particular specifications of the material to be treated include the sample phase, the structure and physical accessibility (in the case of heterogeneous hydrolysis), the conformation effects, as well as the structure and the substituents of the sugar ring ^[162].

The following Table 9 presents different conditions of acid pretreatment for variable lignocellulosic materials for enzymatic hydrolysis, as well as the cellulose conversion efficiency for hardwood, softwood, sugarcane bagasse and straw. Historically and conventionally, acid pretreatment is proven the main choice for wood pretreatment ^[162].

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6.4.3. Biological pretreatment

Biological pretreatment, such as fungal, is considered a pretreatment of milder operational conditions in comparison to the aforementioned physical or chemical pretreatment. The fungal petreatment of oxidative biodegradation of lignin by white-rot fungi is an indicative process of biological petreatment. The main advantages of biological pretreatment are its low energy input, no chemicals requirement, mild environmental conditions, and environmentally friendly operability. However, the biological pretreatment processes usually necessitate a long retention time, sustain low yield, and are also sensitive to the process conditions of temperature and pH.

In a modelled example of biological pretreatment of biomass, the case of enzymatic pretreatment of biomass was comprehensively studied and selected for further analysis in this section ^[162]. The biological processes of ethanol production are commonly involving hydrolysis—which breaks cellulose to glucose—and fermentation to convert glucose to ethanol. Ethanol can be produced through the processes of separate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF) ^[162].

In the relevant literature, Mingjie etal. ^[166] introduced a novel biological process for cellulosic ethanol production, following the specifications of the SHF and the simultaneous saccharification and co-fermentation (SSCF) processes. These methodological approaches are outlined in the following Figure 6.

The SSCF process was designed for bioethanol production from lignocellulosic substrates by co-fermentation of xylose and glucose. Indeed, SSCF is proven a suitable process design for simultaneous saccharification and co-fermentation for sufficient xylose utilization. According to the suitable biomass sources used, spruce sustains low ratio between xylose and glucose makes total xylose conversion challenging, whereas wheat straw sustains a relatively good ratio between xylose and glucose. However, large amounts of xylose make total xylose conversion challenging ^[166].

The SHF process facilitates the operation of hydrolysis and fermentation processes under separate optimized conditions. It has been reported that pre-steamed agro-based raw materials (such as corn stover) revealed that the ethanol yield (that is ethanol produced per unit mass of dried feedstock g/g) of SHF was higher than that of SSF ^[166]. Moreover, SHF supported a faster hydrolysis rate than SSF under optimized conditions. In the SSF process cellulose was hydrolyzed to glucose by cellulase, while yeast spontaneously fermented glucose to ethanol. The SSF advantages over the SHF process are, firstly, its commonly higher productivity (that is ethanol produced per unit mass of dried feedstock per unit time, g/g.h) than does SHF, since SSF has a shorter operating time and, secondly, the fact that SHF includes less inhibition of enzymes and a longer time of enzymatic hydrolysis. Usually, fermentation temperature and the presence of ethanol are not at optimized conditions for cellulase activities in the SSF process, leading to poor enzyme performance. This difficulty has been approached by the following two processes ^[162]:

· Enzyme immobilization onto a solid support was proposed to improve the enzyme activity at non-optimal conditions (that is for SSF). The higher ethanol yield of the immobilized enzyme process can be attributed due to higher glucose yields as a result of increased enzymatic stability at the non-optimal enzyme conditions required for the SSF. However, this process results in a higher ethanol yield under the optimum conditions of fermentation rather than that of enzyme hydrolysis, due to the facts that, first, the optimum conditions for the immobilized enzyme reaction may not coincide with those for the enzyme in solutions, second, although the same mass of enzyme can be used in the solution and immobilized systems, there is no the same amount of enzyme available to participate in the hydrolysis reactions and, three, enzyme immobilization results in a random orientation of adsorbed enzyme on the silica substrates, inactivation some enzyme due to buried or inaccessible active sites ^[162].

· Semi-simultaneous saccharification and fermentation (SSSF) are also applied to ethanol production. According to this method the hydrolysis process is applied under optimized conditions and it breaks down cellulose to glucose, and subsequently fermentation is conducted on the product of hydrolysis without removing the hydrolystate. Therefore, the SSSF process is an operating mode between the SHF and SSF that sustains the advantages of both SHF and SSF in terms of higher productivity and yield ^[162].

Conclusively, the following Figure 7 depicts an integrated flowchart for ethanol production from biomass, showing different pretreatments options and the composition of the solid pretreated material ^[162]:

Figure 7 shows various pretreatments options that involve the composition of the solid pretreated material under the SSF process. As the pretreatment choice will also be affected by the type of biomass, the envisaged biorefinery model will need to consider the main types of biomass that will be used for the biorefinery operation so as to select an appropriate, and versatile pretreatment method. Nowadays and in different geographic areas, sugarcane and woody biomass are commonly exploited as the main renewable resources to be fed into a biorefinery. However, due to a wide diversification of physical properties and chemical composition, the relevant pretreatments are proven selective and customized. Another conditioning step—that is the wood size reduction prior to the pretreatment—may not be necessary for sugarcane bagasse, therefore the energy consumption and the costs incurred in diversified pretreatment processes are diversified, too ^[162].

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7. Results and Discussion

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7.1. Results and Discussion upon the literature-based overview

According to the aforementioned analysis, the main issues that can be integrated from this literature overview are outlined as follows:

Small-scale socio-economic and environmental contexts of biomass exploitation

· Active interest and social acceptance of renewable energy innovations in the view of a wide spectrum of stakeholders including technicians, industry representatives, local government officials, community representatives, end-users.

· Regional/Municipalities-scaled strategic plans aimed at the integration of the innovation dimension into a policy for renewables.

· Integrated assessment that enables the development of comprehensive decision support tools at both in-grid and off-grid electricity markets.

· Environmental benefits of alternative-to-land-spreading manure management.

Technological and projections’ context of biomass exploitation

· Technology transfer and adoption of renewable energy innovation in the light of social acceptance of renewable energy technologies.

· Roles of technological innovation in exporting renewable energy technologies at both developed and developing countries.

· Dynamics of newly emerging innovation systems that stimulate renewable energy technologies.

· Development of sustained renewable energy projects to overcome high initial projects costs that result in increased renewable capacity and decreased costs.

Large-scale policies context of biomass exploitation

· Subsidizing energy costs and governmental initiatives to energy mix of production, whereas evaluating feed-in tariff remuneration modeling and patterns.

· Governmental expenditures on R&D and exports rates, towards the contribution of renewable energy to a national energy supply.

· Assessment of energy-based and environment-based models, in the light of renewables, landscape, and biodiversity issues.

· Effective of the setting strategies -being aligned to further clarify the hierarchy of existing objectives and instruments- enabling local economies to take advantage of efficient management and access to financial capital.

The main challenges and the future research orientations that are derived from the aforementioned literature review can be outlined in the following issues:

Firstly, future studies could provide an integrated planning that will overcome the socio-economic, environmental, and technicalrisks involved from uncertain inland energy distribution, towards the smooth energy autonomy from regional energy units using regional/endemic biomass sources.

Secondly, detailed data profiles upon economics costing, fiscal policies, and governmental initiatives, should be all ideally coordinated in the light of the aforementioned literature outcomes, towards formulating an integrated long-term strategic plan.

Thirdly, the perspectives of such an integrated long-term strategic plan -parallel to the aforementioned literature-based research and qualitative and quantitative data profiles- are further determined by the environmental, socio-economic, and technical specifications applied in the remote, island or mountainous, areas of applicability. Specifically:

· Environmental specifications should be the climatic conditions of the accommodated plant species’ breeding and the geo-morphologic profile of these agricultural plantations. To this end, the grouping and the typology features of power production from RES in different regional territories can be conducted in the light of geographic information systems (GIS), while observing and analyzing these visualized maps of RES allocation can be implemented in those territories where the majority of RES have been located.

· Socio-economic specifications should be the competitive land uses (such as the so named food-to-energy debate) as well the social acceptability of biomass plantation to energy purposes, including the citizens’ involvement and the support to their family profitability/income from such energy-oriented initiatives.

· Technological specifications should be oriented towards technological advancements’ spur in biological breeding methods, agricultural yields’ maximization, as well as the optimization of installed capacity and machinery of these power plants’ infrastructure, which are also aligned to the aforementioned agricultural yields at each remote territory.

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7.2. Results and Discussion upon the technology-based overview

The final decision upon the appropriate selection of a biomass pretreatment should be made by considering the parameters of high yields for multiple crops, harvesting times, highly digestible pretreated solid, high biomass concentration, no significant sugar degradation, formation of a minimum level of toxic compounds, high yields of sugars after subsequent hydrolysis, fermentation compatibility, operation in reasonably sized and moderately priced reactors, lignin recovery, and minimum heat and power requirements ^[162].

Although the technological advancements’ spur upon the relevant biomass pretreatment processes is an ongoing and dynamic process itself, it in noteworthy that diversified pretreatment methods produce various effects on the biomass in terms of its structure and composition. Indeed, the hydrothermal and acidic pretreatments conceptually remove mainly the biomass hemicellulose fraction and alkaline pretreatments remove lignin, whereas the product of a milling-based pretreatment retains its initial biomass composition. In parallel, the choice of pretreatment as well as its operational conditions determines the composition of the resulting biomass hexose and pentose syrups. Furthermore, cellulose crystallinity is not significantly reduced by pretreatments based on steam, or hydrothermal, or acidic procedures, whereas ionic liquid (IL)-based techniques can shift crystalline cellulose into amorphous cellulose, substantially increasing the enzymatic hydrolysis rates and yields. Moreover, the activity profile of the enzyme blend and the enzyme load for an effective saccharification may be also varied according to the pretreatment. Finally, a low hemicellulase load can be used for a xylan-free biomass and a lower cellulase load is commonly needed for the hydrolysis of a low crystalline and highly amorphous pretreated biomass material ^[162].

Table 10, below, lists the pretreatment options presented in this review paper and its general effects in the biomass composition and structure. All pretreatments support an increase in the surface area, which responds for the increased enzymatic digestibility of the treated materials. However, the substantial decrease in cellulose crystallinity is only observed for the treatments using ball milling and ion liquid (IL). This is an effect of utmost importance for the increased rates and yields of cellulose enzymatic hydrolysis. The acid, LHW, and steam explosion pretreatments are more effective on hemicelluloses and on the modification of the lignin structure, which also cause a higher formation of inhibitors in comparison to milling, extrusion and IL pretreatments ^[162].

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8. Conclusions

The authors of this review paper presented a systematic literature review, offering a fundamental theoretical background of the existing methodologies involved in the exploitation of renewable energy sources in general, and in biomass sources in particular. In parallel, this review paper was focused their research expertise on providing the limitations and the challenges derived from the biofuels production in a large-scale applicability. In the light of the aforementioned sections, in this concluding section the outcomes of the relevant research are classified in four main contexts of reference: technological, economics, societal, and political.

In a technological context it can be signified that biomass sources can be directly used for heat and electricity production. However, this direct utilization of biomass is limited from the uneven geographical distribution of biomass, since a viable transportation grid has to be developed, the bulky nature of biomass (especially that of low heat value) and its accompanying costly and complicated transportation systems, the poor efficiency and the high environmental pollution of engines that utilize biomass sources (including excessive CO₂emissions). In parallel, a portion of the energy stored in biomass is inevitably lost during the conversion process, thus the biofuel product contains ultimately lower energy, than biomass as raw material. Therefore, special provision has to be made in biofuels’ production. In particular, biofuels’ production should sustain a high energy density on a mass, while energy crops have to produce high yields of biomass production, of low carbon footprint throughout the life cycle assessment of the biofuels’ derived.

In an economics context the allocation of RES is actually an economics issue. Indeed, it is not possible to fully consider the exploitation of any energy resource without considering the economy. Subsequently, to a large extent it is economics that determines the future RES-based technologies that will come to dominate the energy sector and where these technologies will be located. In a societal context this is the case in the liberalised energy markets that are now emerging around the world. Moreover, it is the economic forces of demand and supply that determine which RES are exploited by what technology and where this happens. While technology plays an important role in discovering new RES that could be adapted in a specific country, however, when, where and to what extent RES are exploited is almost entirely a question of economics.

In a societal context it is noteworthy that there is a strong ethical dimension to how citizens try to resolve this energy-oriented economic problem. Indeed, how we produce and consume energy has a strong bearing on socio-economics relations. To this end, energy production units are also social units. The act of energy production and electricity generation can form and destroy communities and profoundly affect the quality of life, as well as the freedoms and rights if individuals, especially while considering the limitations upon an excessive use of production resources of land (see also the food-to-fuel debate upon energy crops cultivation), land (see also the optical nuisance and the spatial occupation of spacious agricultural areas in wind farms against pasture agricultural zones), and capital (that is utmost importance resource of production in epochs of economic recession).

In a political context, it is important governmental policies to understand and to be drawn in accordance with the political dimension to the market allocation of resources. To this end the central governmental policies have to be aligned to the principles of the free market efficiency and to adopt into their political regulations in energy concepts such as the opportunity cost, the cost-benefit scenarios, and the marginal analysis in the energy sector. Additionally, in a strategic energy planning overview, there are especially critical factors, such as the understanding of the use, the different forms, the challenging issues, and the inherent limitations of the relevant economic models.

In an environmental context, the economic system of energy production and electricity generation is not a closed system. Contrarily, this economic system requires land, wind, solar irradiation (that is natural raw materials) which it cannot itself produce and it generates waste. Wastes -in the form of pollution that it is created by producers and consumers- are costs of production and consumption that are not reflected in the tariff pricing of energy produced and they leak from the system. These leakages are cost external to the energy pricing process and they are known as externalities. The externalities cost is borne by the whole society and not by the individual energy consumer or energy producer that creates them. Besides, the externalities cost is commonly accompanied by a reduction in environmental quality.

Finally, it is apparent that not only are the answers themselves to the aforementioned issues are complex, but also how these decisions upon energy production and electricity generation from RES are made is itself a critical issue. Conclusively, the authors of this review paper expect that the results of their study would have some theoretical and methodical importance, being especially supportive to all stakeholders (including, customers, end-users, managers, manufacturers) as well as to those companies that are involved in the energy marketplace.

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Conflict of Interest

All authors declare no conflict of interest in this paper.

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