Innovative Power Generation Technologies for Improved Household Energy Delivery and Sustainable Future: Classical Solutions from ENEA Research Centre, Trisaia Italy

Abstract

The present communication is focused predominantly on important R&D solutions relevant to renewable energy technologies covering the following: (i) Innovative heat transfer fluid and thermal storage technology based on a molten salt mixture developed by ENEA for large-scale heat storage. The system uses a parabolic trough collector, compared with diathermic oil, which allows higher operating temperature, resulting in significant benefits to the plant’s operation, safety and the environment. (ii) The world’s first solar disk powered by air micro turbine developed by ENEA. (iii) An innovative steam-explosion prototype plant installed at ENEA for the pre-treatment of lignocellulosic biomass and the fractionation of bio components to generate ethanol from lignocellulosic material using hemicellulose and lignin. (iv) The production of hydrogen-enriched biogas using steam as the gasification agent, which helps in obtaining nearly nitrogen-free product gas and with a high calorific value of around 12 MJ/Nm³ dry gas and a high percentage of hydrogen (up to 55%) while using steam as the gasifying agent in the presence of a catalyst. (v) A rotary kiln plant, with the main purpose being to develop and optimize a thermo-chemical process to convert used rubber tyres so as to recover material and energy, as well as other solid products, with high value-added “Activated carbon” and synthesis gas.

1. Introduction

The lack of access to electricity, or “energy poverty”, is an ultimate economic hindrance, as it prevents people from participating in the modern economy. Energy poverty is becoming more relevant in the global debate on energy. Energy and, in particular, clean energy technologies with improved efficiency, as well as the development of a range of environmental-friendly energy technologies, are important topics that need special attention. The implementation of the transition towards a low-carbon economy requires measures for transforming the energy conversion systems.

As a result of oil supply disruptions in the early 1970s, almost all the nations have increased their dependence on foreign oil supplies. Burning vast quantities of fossil fuels, such as coal and oil, has already caused a great deal of harm to the environment, whereas the continued exploitation of fossil fuels has contributed significantly to global warming. The primary goal of developing green sources of energy is to generate power while minimising both waste and pollution, in order to thereby reduce the impact of energy production on the environment.

Scientists who advocate the use of green energy say that using such sources will reduce the rate at which climate change occurs, although it cannot stop or reverse the temperature increase.

In addition to climate challenges, we are now faced with challenges related to high energy prices and the high risk of supply shortages. It has been estimated that every billion kilowatt-hour of renewable electricity produced would reduce carbon emissions by over 200,000 tonnes, sulphur emissions by 10,000 tonnes and particulate matter by 2000 tonnes [1].

With the present policy scenario, according to the most recent analyses, 660 million people will still be lacking access to energy in 2030, most of them in Sub-Saharan Africa, although the region also shows great disparities in access between urban and rural areas. Non-existent and unreliable electricity is not just an issue confined to rural Africa. Even Nigeria—Africa’s largest economy—has an electrification rate of just 54% [2].

In view of the above, the development of novel energy technologies with improved conversion efficiency and a range of environmental-friendly qualities is an important topic that needs special attention form the scientific community worldwide. There is a need to accelerate the development of renewable energy using new technologies that enable improved energy access globally.

The present paper reports on some of the novel energy solutions developed by the ENEA research Centre, Trisaia, which are aimed at improving the process efficiency of already-existing technologies and/or reducing the overall unit costs of production.

2. New Energy Technology Options

Only natural resources have the capability of supporting the survival of mankind and their energy needs in the long term.

Renewable energy, especially solar energy derived from the Sun, geothermal energy derived from heat inside the earth, wind energy, biomass from plants and hydropower from flowing water, should be viewed as the most important instruments for socio-economic development, the eradication of poverty and unemployment, and rural development. The world is already moving in this direction, and we must commit ourselves to the development of pollution-free renewable energy technologies as well.

As far as solar energy is concerned, it is worth noting that the world’s consolidated energy consumption is only 1/10,000 of that available on the surface of the Earth in Sunny countries [3].

Fusing atoms together in a controlled way can release nearly four million times more energy than a chemical reaction, such as the burning of coal, oil or gas, and four times as much as nuclear fission reactions. Energy form nuclear fusion offers a number of advantages that make it useful, but it also carries certain disadvantages that can make it dangerous. The design and development of the International Thermonuclear Experimental Reactor (ITER) Project is aimed at replicating the fusion processes carried out in the Sun to create energy on Earth.

With an estimated cost of about ERU 20 billion (USD 25 billion), the International Thermonuclear Experimental Reactor (ITER) represents a prototype fusion reactor that can be used to generate electricity in a process similar to nuclear fusion. The main objective of ITER is to produce a ten-fold return on energy (Q = 10), or 500 MW of fusion power from 50 MW of input heating power. The project is the result of cooperation between Europe, the United States, China, India, Japan, Russia and South Korea [4]. The ITER Project is more than halfway towards the first test of its super-heated plasma, estimated by 2025, and its first full-power fusion process by 2035.

Fusion offers advantages that make it worth pursuing: its fuels are widely abundant, inexpensive and virtually unlimited, and it has the capacity to operate in a base load manner, which is not often seen in generation methods based on intermittent sources, such as wind or the Sun. The average cost per kilowatt of electricity is expected to be similar to that of modern fission reactors.

Investing in renewable energies such as solar, wind and geothermal is important. Just like in fusion, significant investment in R&D will result in advancements in technology, leading to significant reductions in energy prices.

The ideal future energy source would comprise a mixture of generation methods, instead of an overwhelming reliance on one source.

3. Traditional Classification of Renewables

Instead of fossil fuels, the energy sector is based largely on renewable energy. Two-thirds of total energy supply produced in 2050 is expected to be derived from wind, solar, bio, geothermal and hydro energy. Solar will become the largest source, accounting for one-fifth of total energy supply. Solar PV capacity will increase 20-fold between now and 2050, and wind power 11-fold. The traditional classification of renewable energy sources is presented in Figure 1.

The Sun’s heat also drives the wind, and the energy thus produced is captured with wind turbines. When water flows downhill into rivers or streams, its energy can be captured using hydroelectric dams. Biomass can be used to produce electricity, transportation fuels, or chemicals. The use of biomass for any of these purposes is called bioenergy. Hydrogen can also be found in many organic compounds, as well as water. It is the most abundant element on the Earth.

However, not all renewable energy resources come from the Sun. For instance, geothermal energy involves tapping the Earth’s internal heat for a variety of uses, including electric power production, and the heating and cooling of buildings, while the energy produced by the ocean’s tides is produced by the gravitational pull of the moon and the Sun upon the Earth. In fact, ocean energy comes from a number of sources. In addition to tidal energy, there is also the energy produced by the ocean’s waves, which are driven by both the tides and the wind. The Sun also warms the surface of the ocean more than the ocean’s depths, creating a temperature difference that can be used as an energy source. All these forms of ocean energy can be used to produce electricity.

Solar Thermal Energy at Low- to Medium-Temperature Applications

Solar resources in Europe and across the world are abundant and cannot be monopolised by one country. Amongst the various renewable energy sources, solar energy appears to be a key technological means of implementing the shift to a decarbonised energy supply, and it can be deployed almost everywhere on the planet.

There are two main technologies involved in the exploitation of solar energy. These are solar photovoltaic (PV) and concentrated solar power (CSP) technology.

Solar thermal energy is a technology used for harnessing solar energy to produce thermal energy (heat). Solar thermal collectors are defined as low-, medium-, or high-temperature collectors. Today, low-temperature (<100 °C) thermal solar technologies are reliable and ready for introduction onto the market. Worldwide, they help to meet heating needs, via the installation of several million square metres of solar collectors per year. These technologies can play a very important role in advanced energy saving projects, especially in new buildings and structures. An estimated 25.2 gigawatt-thermal (GWth) of new solar thermal capacity was added in 2020, increasing the global total to around 501 GWth, while the Global CSP capacity grew to 6.2 GW [5].

It is important to involve policymakers, representatives of the solar thermal sector, and other industrial sectors in order to brainstorm the potential contributions that solar heating and cooling technologies can bring to a decarbonised and decentralised energy sector.

A brief summary of the important R&D aspect of RES and low-carbon technologies is presented in Figure 2.

4. Solar Energy for District Heating

District and block heating applications offer good conditions for the use of solar thermal energy in existing buildings, and there are a number of demonstration plants with ground-mounted as well as roof-mounted collector arrays. A major advantage is that a solar plant can be of considerable size, leading to lower specific costs. The largest solar district heating plant features an 18,300 m² collector area (~10 MW thermal capacity) [6].

The main barriers to growth are the lack of cost-effective seasonal heat storage methods, the low costs of alternative fuels, and the lack of confidence in solar heating on the part of thermal utilities. Once cost-effective seasonal heat storage systems are made widely available, large-scale applications will become more competitive, resulting in a strong increase in the potential for solar thermal.

4.1. Combined Domestic Hot Water (DHW) and Space Heating (Combi-Systems)

In Central and Northern Europe, it is now a common practice to install combi-systems able to provide heat both for the production of domestic hot water and for space heating. The collector size of these combi-systems is typically in the range of 7–20 m², and the tank(s) range 300–2000 L [6]. Combi-systems are often more complex than solar systems that supply DHW only. Combi-systems are still rarely used in Southern Europe, though there is great potential for the development of systems to generate space heating in winter, air-conditioning in summer and DHW throughout the year.

4.2. Research and Development Topics of Interest Related to Solar Thermal

Solar energy is a crucial energy source that requires solar capture, conversion, and storage. Topics of interest related research and development for solar thermal energy production in low–medium-temperature applications include:

• I.. Research and development into energy storage for solar energy applications at medium temperatures;

• II.. Research and development into hybrid system using solar, wind and biomass;

• III.. Development of a toolbox including the basic components involved in the construction of small-/medium-sized CS multi-generative power plants to be integrated into buildings. The goal should be to combine and optimize innovative subsystems for the efficient capture of solar energy to generate electricity, produce heat and cold, and drive thermal processes such as desalination or sterilisation, alone or in combination with backups (e.g., gas or biomass);

• IV.. To deploy technologies through technology optimisation, integration and validation, as well as via case studies and impact assessments using small-scale demonstrative units of poli-generative Concentrating Solar (CS) technologies.

5. Generation of Electric Power Using Solar Energy

5.1. Solar Concentration for Power Production

There are four main families of solar-concentrating technology, as shown in Figure 3—namely, parabolic trough collectors, parabolic dishes, linear Fresnel reflectors and solar towers. These are classified by the way they focus the Sun’s rays and the technology used to collect the Sun’s energy [7].

The inherent advantage of CSP technlogies is their unique integrability into conventional thermal plants. All of them can be integrated as “a solar burner” in parallel with a fossil burner into conventional thermal cycles. With thermal storage, solar thermal plants can provide a reliable capacity without the need for a separate backup power plant and without stochastic perturbations of the grid.

5.2. ENEA’s Concentrating Solar Energy Program: Main Objectives

Using parabolic trough collectors, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) has developed an innovative technology chain based on a molten salt mixture functioning as a heat transfer fluid and thermal storage medium. Compared with the use of diathermic oil, this technology allows the operating temperature to be increased, thus resulting in significant benefits to the plant’s operation, safety, and environmental impact [6]. Other considerable advantages are related to the possible integration of this solar plant with other conventional thermoelectric plants and an efficient mode of thermal energy storage, thus increasing the “dispatchability” of the electric power produced against the unpredictability of the solar source. The activities that have been performed by ENEA in relation to this specific topic are summarised in Figure 4.

Of note, the introduction of an adequate energy storage system, the use of an alternative transfer fluid, and the development of a new solar collector design and an innovative receiving tube design comrpise the most important innovations in the system designed by ENEA (Figure 5).

5.3. Molten Salt as Transfer Fluid and Thermal Storage

ENEA introduced a new fluid heat carrier (molten salt comprising a mixture of 60% NaNO₃–40% KNO₃) in order to increase the operating temperature and possibities for heat storage. It has the advantages of a high working temperatiure (oil 380 °C–molten salt 440 °C), as well as the requirements of atmosphereic pressure and a lower volume for thermal storage therefore involving lower costs.

5.4. World’s First Solar Power Plant in Italy

The Archimedes project combines the best modern and future technology, and consists of a solar field, a storage system and a steam generator. It is the first of its kind in the world and was inaugurated in Italy on 15 July 2010 [6].

5.5. Innovations Introduced by ENEA

ENEA introduced a new fluid heat carrier comprising molten salt (a mixture of 60% NaNO₃ and 40% KNO₃) in order to increase the operating temperature and the capacity for storing heat.

Given its chemical properties, the use of thermal oil as a heat transfer fluid (HTF) is environmentally dangerous, as it is inflammable and is not able to operate at temperatures above 400 °C. At higher temperatures, the components are subjected to degradation, and they cannot be used anymore; therefore, the efficiency of the overall steam is reduced. Moreover, due to the aging process, diathermic oil must be replaced every year (even if it is only used at a small amount), and this leads to further costs. Also, CSP plants using thermal oil as the HTF can only operate in the daytime [8].

In order to address the above-cited issues and to mitigate the intermittency of the solar energy source, ENEA, under the guidance of Prof. Carlo Rubbia, introduced a heat storage system based on a mixture of molten salts (60% of sodium nitrate and 40% of potassium nitrate). Technically speaking, the molten salt mixture, which is already widely used as a fertilizer and is available in large quantities at lower costs, is able to provide a number of advantages.

Molten salts can be used both as a heat storage medium or as an HTF able to reach temperatures up to 550 °C. Since molten salts are able to operate at higher temperatures, the volume of storage system required could be reduced by 2/3, leading to a reduction in the size of the storage tanks, entailing a 30% cost reduction. These savings will manifest a roughly 20% decrease in plant costs, compared with typical oil plants with storage. Higher operating temperature could increase the plant’s efficiency up to 6% [9]. Finally, unlike oil, molten salts are an environmentally friendly, non-flammable, stable fluid, and they not cause the degradation of receiving tubes.

Another innovation of ENEA is the design of a new type of concentrator based on thinner mirrors, which reduces construction and installation costs. The use of large-scale heat storage (another innovation in the Archimedes project) enables the plant to provide heat to the steam generator at a constant rate 24 h a day, regardless of variations in solar energy availability. The steam generator consists of “tube and shell” heat exchangers, in which heat is transferred to water to produce super-heated steam for use in a conventional thermoelectric plant.

The 5 MW-capacity solar plant, costing nearly EUR 60 million, has a unique ability to collect and conserve thermal energy from the Sun for many hours at a time, thus enabling the plant to generate electricity during hours with no sunshine and when there is an overcast sky. The cost per kWh produced is around five or six times greater compared to the cost of energy generated using conventional fuels, but 2100 tons of oil are saved and 3250 tons of carbon dioxide emissions are reduced over a year, meaning that the present achievement certainly represents an important milestone [6].

5.6. Solar Disk Powered by Air Micro Turbine

ENEA has presented an innovative design in the form of the world’s first solar disk (see Figure 6); owing to its integration with an innovative micro turbine, around 70 kW of the radiant power that it captures can be converted into as much as 15 kW of electric power (enough to power five apartments).

The novelty of this system employing a solar disk (dish power system) lies in the use of an air micro turbine engine (attempted here for the very first time) as opposed to Sterling, which is commonly used in this type of application. Its ease of operational management and its modularity enable its use in small shopping malls and businesses, supermarkets and schools, both connected to and detached from the power grid. Its solar mirrors are able to focus the solar radiation up to 2000 times on a small focal area.

5.7. Concentrated Solar Tower Power Plant (CSP) Development and Future Trends

CSP power plants can be evaluated from the perspective of their environmental impacts and natural resource requirements. On average, CSP plants occupy 10 acres/MW. From a material point of view, CSP plants require a considerable amount of iron and cement [10].

Depending on the technology used, its scale, and the possible use of a thermal storage unit, the installation costs of CSP plants are in the order of ERU 2000–6000/kW [10].

Ordinary annual maintenance costs for CSP plants are equal to roughly 2% of the initial investment.

Degradations in the energy performance during the whole life-cycle of the plant (varying between 0.5% and 1% per year) lead to the unfavourable absolute economic effects of CSP plants [10].

With the same rated power and under the same environmental conditions, CSP plants produce more energy than PV plants. This implies that the economic return of CSP is greater. With a new installed capacity of 1.2 GW and due to the increase in activity in the coming years, global CSP capacity is expected to reach 40 GW by 2025.

One of the points that could be seen as an advantage for the CSP system is the possibility of heat storage. The CSP system provides the capacity to incorporate simple, efficient, and cost-effective thermal energy storage, as an intermediate step to generating electricity during peak times.

Another important point concerns potential technological breakthroughs related to solar fuels, which could accelerate CSP development.

By comparing the trends of CSP development with those in other renewable sources, we can see that CSP is now at a turning point. The costs of mirrors, vacuum receivers, lenses, the support structure, high-efficiency heat transfer fluids and turbines will certainly affect the initial and operational costs of CSP projects significantly. In the most optimistic scenarios, CSP systems could supply around 10% of global electricity.

In brief, it is not possible to say a priori that one technology is better than another. The choice of which solar technology to adopt depends on the specificity of each application. The authors have here merely highlighted the technological options available.

6. Biomass-Based Power Generation

Using biomass, through a range of technologies, to produce energy, sustainable fuels and bio-based materials and chemicals offers several solutions in this era of energy transition.

In addition to industrial applications regarding bioenergy and sustainable biofuels, bio-based products contribute to resource-efficient, sustainable, low-carbon economies.

Several biomass technologies are currently poised to play an important role in the decarbonisation of the economy, acting as part of a circular economy in the context of sustainable development. As such, sharing knowledge and ideas related to the latest scientific achievements and industrial applications in the biomass sector is of the utmost necessity.

6.1. Areas of Intervention and Innovative Aspects of Biomass Gasification

R&D activities related to biomass are focused on two transformation processes: gasification and steam explosion.

The R&D activities being pursued in the field of thermo-chemical conversion processes are presented in Figure 7. These are based on gasification and pyrolysis, and thus support the attempts of the national and European industrial sectors in scouting out and focusing on advanced new technologies. The use of biomass gasification for biofuel production has also been considered.

6.2. Gasification Activities

A number of experimental plants employing different technologies (fixed bed, BFB, FICFB gasifiers) and of different sizes, i.e., 10 kW_th, 100 kW_th, 500 kW_th and 1 MW_th (Figure 8), are available at ENEA Trisaia. Fixed-bed gasifier plants, with power capacities of 25–30 and 80 kW_e, have been developed, and these are most appropriate for use in developing countries, as well widely at the national level [11].

Focussing on different possible modes, such as using internal combustion engines, gas turbines and high-temperature molten carbonate fuel cells (MCFCs), the plants have been investigated experimentally in an attempt to determine the most promising fields of application (production of fuel gas for power generation). The use of biomass gasification for syngas production via biofuel conversion has also been considered. More recent activities have aimed at the development of gasifiers dedicated to the production of syngas (to be used in an internal combustion engine), synthetic biofuels (Sun-diesel, methanol) and hydrogen.

In the context of its research activities focused on biomass gasification, ENEA has designed, developed, and installed at its research centre Trisaia an industrial-scale FICFB gasifier (used for the thermo-chemical conversion of waste biomass to produce both thermal and electric power) that uses steam as the gasification agent.

6.3. Novely of the R&D Activities on Biomass Gasification at ENEA

• ❖. The use of steam as the gasification agent helps in obtaining a nearly nitrogen-free product gas with a high calorific value of around 12 MJ/Nm³ dry gas.

• ❖. The purification of the product gas is achieved via the inclusion of a high-temperature ceramic filter in the cleaning section. By using a natural catalyst as the bed material and a gasification temperature above 800 °C, the tar content was reduced below 5 g/Nm³.

• ❖. By adding into the reactor a specific catalyst, the hydrogen content in the product gas can be made to exceed 50%, and its quality can be further improved.

7. Production of Ethanol from Lignocellulosic Biomass

The technology required for bio-ethanol production from sugar and starch crops (sugar cane, sugar beet, maize, etc.) is now sufficiently mature. However, in the context of pursuing its diffusion on a large scale while not penalising the alimentation product market, it has been of utmost necessity to find alternative raw materials.

It is in the above context that ENEA has focussed its research on the use of lignocellulosic biomass, which can be specifically cultivated or easily recovered from agricultural, forest or agro-industrial wastes.

These materials, in contrast to the sugar syrups obtainable from reed or beet, must be combined with acids or enzymes to derive the simple sugars required to start fermentation. It is for this reason that the production of ethanol from lignocellulosic biomass is currently somewhat costly when compared with using sugar and starch. In spite of the quality and availability of the raw material, it is necessary to improve the technologies used for the pre-treatment, enzymatic hydrolysis and separation of alcohol from the broth, in addition to the valorisation of the current processes based on lignin and hemicellulose [12].

Steam Explosion for Biomass Treatment

Steam explosion, with its low environmental impact and capacity to produce a highly biodegradable substrate, has been selected as the most appropriate technology for use in biomass pre-treatment. The process is based on the natural ability of water vapour, when subjected to high temperatures and pressures, to penetrate and break the chemical bonding of the polymers, cellulose, hemicellulose and lignin in vegetal materials.

Thus, in brief, steam explosion is a physicochemical pretreatment technology in which the biomass is heated to 160–260 °C in a closed vessel under high pressure (7–50 bar) for a short period of time (30 s–20 min), after which the pressure is quickly released, causing an explosive effect on the cells [13]. The aim is to make available the sugars contained in the feed material, which otherwise could not be metabolised easily by the microorganisms used in the successive stages of bio-conversion.

Two experimental pilot plants, one using the batch version and the other the continuous version, have been developed at Trisaia to study steam explosion. The plant that operates in continuous mode, called STELE, is capable of treating nearly 300 Kg/h of a wide variety of different types of biomass (wheat straw, wood chips, sorghum, kenaf, miscanthus, etc.).

It is equipped with a system for the treatment of the stream of exposed product. Extraction with water, caustic solutions and filters is carried out on a semi-industrial scale to separate cellulose, hemicellulose, and lignin.

There are many current research activities focused on the development of biomass fractionation processes in macro-components (cellulose, hemicellulose and lignin), and the development of chemical and biotechnological processes (enzymatic–fermentative) for the processing of biomass into biofuels and bio-products, as well as scaling-up analysis, and the synthesis and characterisation of composite materials based on the use of renewable sources. Most laboratory research is focused on biorefinery and green chemistry. The basic activities in this field involve the development of new processes for the multi-purpose production of biofuels, biochemicals and bioenergy from lignocellulosic raw materials, oleaginous crops, residual oils, co-products of bioethanol and biodiesel production, and by-products of the food industry. The Steam Explosion Continuous Plant installed at ENEA Trisaia is presented in Figure 9.

A preliminary analysis indicated that when using enzymes costing higher than EUR 1.5/kg, in situ production could be economically advantageous.

8. Thermo-Chemical Processes for the Exploitation of Biomass, Residues and Wastes (Figure 7)

Research and development in the area of thermo-chemical conversion processes, such as gasification and pyrolysis, that produce power, have focused on methods to improve energy-carrying capacity, develop gaseous streams for synthesis and recover energy and materials. Pilot- and industrial-scale plants have been built that focus on biomass, urban and industrial wastes. ENEA supports the national and European industrial sectors in scouting out and focusing on new technologies.

In relation to the exploitation of biomass, key activities are mainly focused on the development of gasification processes for the production of gaseous energy carriers with higher value, which can be used for direct application in Combined Heat and Power (CHP) production or, after proper cleaning and conditioning, as a gas for use in synthesis to produce derived fuels (e.g., hydrogen, SNG, Fischer–Tropsch liquids, methanol, DME). From the produced gas, chemicals can also be synthesised. Different technologies of biomass gasification and gas cleaning and conditioning have been studied.

R&D activities have mainly been focused on the development of small- to medium-sized technologies for power production that allow the exploitation of low-value feedstocks, such as biomass residues from forest management, the agro-industrial and agricultural sectors, and wood industries.

A technology park dedicated to biomass gasification has been developed. Five gasification pilot plants with sizes ranging from 120 kWth to 1000 kWth are now in operation [14,15,16]. They differ in terms of the gasification reactor’s design (i.e., fixed-bed, fluidised-bed and staged gasifiers) and the approach to effective gas cleaning and conditioning used.

The Internally Circulating Bubbling Fluidised Bed (ICBFB) gasification plant (Figure 10) is based on a 1000 kWth reactor; it operates a steam/oxygen gasification process to generate a gas with medium heating value (LHV 9–12 MJ/Nm³dry) [1,2].

The gasifier is based on a patented reactor design featuring a modification that, compared to conventional bubbling fluidised bed reactors, prolong the residence time of the biomass feedstock inside the reaction bed. Recently, at this plant, a filtration system has been integrated directly inside the gasification reactor (Figure 11). The new system uses high-temperature ceramic candles, and has been developed to simplify conventional downstream plant sections used for gas purification. During experimental gasification campaigns, a dust removal efficiency higher than 99 wt. % was achieved with this filtration system.

The updraft fixed-bed gasification plant (Figure 11) employs a 150 kW_th reactor and facilitates a steam/air gasification process that generates a producer gas of LHV 5–6 MJ/Nm³_dry. It has been designed to carry out the gasification conversion of a feedstock with a high biogenic fraction. The plant is equipped with a wet purification system run on biodiesel.

The gas was generated in a stream with a high H₂/CO ratio (>2), which could be used in the synthesis of biofuels (e.g., methanol) or to produce hydrogen of fuel cell grade. To this end, after the biodiesel is scrubbed, the gas stream is directed towards a section for upgrading and CO₂ removal.

The three-staged 500 kW_th gasification pilot plant shown in Figure 12 functions via a three-stage gasification process, each stage of which is carried out in a different unit.

The process commences with the pyrolysis of the supplied biomass, which is performed inside an indirectly heated screw reactor, using part of the product gas as fuel. The pyrolysis gas is then conveyed to a partial oxidation reactor, where tars are mostly cracked and converted into lighter gases, while the pyrolysis char is fed to an open-core downdraft reactor via air/steam primary and secondary lines. The char bed also acts as an active carbon filter for the raw product gas. The ultimate result is a producer gas with a very low tar content, and this process has the capacity to use a wide range of biomass feedstocks (including low-value residues, e.g., AD sludge) as solid fuels.

Innovative processes for the thermal treatment of residues and wastes have been developed in order to:

• Recover carbon fibres and energy from end-of-life composites;

• Produce activated carbon and energy from scrap tyres and waste biomass;

• Produce high-added-value technical ceramics and energy from scrap tyres and waste glass;

• Convert chemical energy from non-recyclable waste (RDF, ASR, manure, sewage sludge, waste plastics) into more flexible energetic vectors, such as char, bio-oil and syngas.

Preliminary investigations into the optimum process parameters for waste/biomass pyrolysis and gasification are typically carried out in a bench-scale continuous rotary kiln with a mass rate of about 1 kg/h (Figure 13). This plant gives useful information regarding the scalingup of the pyrolysis/gasification process applied to tyres, sewage sludge, ASR and digestate.

A pilot-scale rotary kiln system was built to process Automotive Shredder Residue (ASR) and waste biomass at a maximum mass flow rate of 10 kg/h. It was equipped with a gravity settler to collect char. The raw gas was here purified by the gas treatment system, which comprised a spray tower, a panel filter and a scrubber working with alkaline solution.

Also, a rotary kiln plant, with a treatment capacity of 30 kg/h, was built with an industrial partner SICAV srl (see Figure 14). This comprised a 4 m-long rotating drum reactor with an internal diameter of 0.4 m. The main purpose of this plant was to develop and optimize a thermo-chemical process to convert waste/biomass into solid products containing high-added-value “activated carbon” and synthesis gas.

A fixed-bed batch pyrolysis plant with a reactor capacity of 5 m³ was built to recover the carbon fibres from scrap and composite waste materials (Figure 15). The ENEA-patented process allows for the recovery of carbon fibres retaining 90% of the mechanical properties of virgin fibres [17,18]. Moreover, the recovery cost is about 20% lower than the commercial cost. The patented process was validated in the continuous rotary kiln shown in Figure 13. The kiln was sold to an SME that is building an industrial plant for the recovery of carbon fibres, following an ENEA patent.

A fluidised-bed gasifier was built to treat RDF with a mass rate of 10 kg/h. It comprised a raw gas cleaning section and catalytic modules for steam reformation and water gas shifting for hydrogen enrichment.

A technologically integrated platform was developed to dispose of waste tyres, produce a high-added-value material such as nanometric silicon carbide, and recover power. The technological platform was composed of a unit for the production of char via pyrolysis; a unit for the cleaning of syngas; a unit for the carbo-thermal reduction of silica and char using a plasma torch; units for gas separation and argon recovery; an internal combustion engine for power production. The experimental tests showed that nanometric SiC could be produced with a yield of about 70%.

Here, innovative processes for the thermal treatment of residues and wastes were developed in order to:

• Recover carbon fibres and energy from end-of-life composites;

• Produce activated carbon and energy from scrap tyres and waste biomass;

• Produce high-added-value technical ceramics and energy from scrap tyres and waste glass;

• Convert chemical energy from non-recyclable waste (RDF, ASR, manure, sewage sludge, waste plastics) into more flexible energetic vectors such as char, bio-oil and syngas.

Preliminary tests investigating the optimum parameters of waste/biomass pyrolysis and gasification were conducted in a bench-scale continuous rotary kiln with a mass rate of about 1 kg/h (Figure 15). The results obtained will be useful for the scaling-up of the process of the pyrolysis/gasification of tyres, sewage sludge, ASR and digestate.

9. Conclusions

• The transition towards a low-carbon economy requires measures aimed at transforming the energy and transport systems. Renewable energies complemented by a rapid phasing-out of fossil fuels are critical for achieving the climate goals;

• Today, low-temperature (<100) solar thermal technologies are reliable and sufficiently mature for entry into the market. Regarding the generation of electric power using solar energy, we must implement on an industrial scale the novel concepts developed and tested by ENEA, i.e., the Archimedes project, which combines the best modern and future technology, such as solar fields, storage systems and steam generators, solar disks powered by air micro turbines, etc.;

• ENEA has developed and installed fixed-bed gasifier plants, with a power capacity of 25–30 and 80 kWe, which have been shown to be the most appropriate for use in the agricultural sector, with further potential applicability in developing countries;

• In the area of lignocellulosic biomass processing, ENEA has successfully designed and constructed a novel steam explosion pretreatment plant for the optimal use of these raw materials.

Author Contributions

Writing—original draft, V.K.S. and G.B.; Writing—review & editing, C.N.A.; Visualization, C.N.A. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures

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