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This study focuses on formulating the most sustainable concrete by incorporating recycled concrete aggregates and other products retrieved from construction and demolition (C&D) activities. Both recycled coarse aggregates (RCA) and recycled fine aggregates (RFA) are firstly used to fully replace the natural coarse and fine aggregates in the concrete mix design. Later, the cement rich ultrafine particles, recycled glass powder and mineral fibres recovered from construction and demolition wastes (CDW) are further incorporated at a smaller rate either as cement substituent or as supplementary additives. Remarkable properties are noticed when the RCA (4–12 mm) and RFA (0.25–4 mm) are fully used to replace the natural aggregates in a new concrete mix. The addition of recycled cement rich ultrafines (RCU), Recycled glass ultrafines (RGU) and recycled mineral fibres (RMF) into recycled concrete improves the modulus of elasticity. The final concrete, which comprises more than 75% (wt.) of recycled components/materials, is believed to be the most sustainable and green concrete mix. Mechanical properties and durability of this concrete have been studied and found to be within acceptable limits, indicating the potential of recycled aggregates and other CDW components in shaping sustainable and circular construction practices. The authors wish to acknowledge the financial support from EU Horizon 2020 Project VEEP ‘‘Cost-Effective Recycling of C&DW in High Added Value Energy Efficient Prefabricated Concrete Compo-nents for Massive Retrofitting of our Built Environment” (No.723582).

Gasification is considered to be a promising way to use biomass with high efficiency in combined heatand power production, for the production of second generation biofuels and in the chemical industry.Especially allothermal fluidized bed steam gasification produces a medium calorific, nitrogen free gassuitable for a variety of downstream processes. In general the raw product gas has to be cleaned fromcondensable hydrocarbons (tar) and conditioned (e.g. adjustment of the H2/CO-ratio) before downstreamuse. The operating conditions of the gasification reactor have a large impact on the quality of the productgas. Hence first steps to a product gas low in tar content can be undertaken directly in the reactor. In thisstudy the capability of influencing the tar content and gas composition by changing temperature (750?840 C), steam to biomass (S/B) ratio (0.8?1.2) and pressure (0.1?0.25 MPa) in an allothermal bubblingfluidized bed steam gasifier is investigated. It is found that rising temperature reduces the total tar contentand affects especially heterocyclic and light aromatic compounds. At atmospheric pressure the naphthalenecontent increases slightly with increasing temperature in contrary to pressurized gasificationwhere naphthalene decreases significantly with increasing temperature. An increase in the S/B ratio leadsto a decreasing total tar content, this tar reduction according to a higher steam content is higher at highertemperatures. Increasing pressure leads to increasing total tar content mainly due to naphthalene, theeffect is most distinct for low S/B ratios.

Abstract The storage of heat in aquifers, also referred to as Aquifer Thermal Energy Storage (ATES), bears a high potential to bridge the seasonal gap between periods of highest thermal energy demand and supply. With storage temperatures higher than 50 °C, High-Temperature (HT) ATES is capable to facilitate the integration of (non-)renewable heat sources into complex energy systems. While the complexity of ATES technology is positively correlated to the required storage temperature, HT-ATES faces multidisciplinary challenges and risks impeding a rapid market uptake worldwide. Therefore, the aim of this study is to provide an overview and analysis of these risks of HT-ATES to facilitate global technology adoption. Risk are identified considering experiences of past HT-ATES projects and analyzed by ATES and geothermal energy experts. An online survey among 38 international experts revealed that technical risks are expected to be less critical than legal, social and organizational risks. This is confirmed by the lessons learned from past HT-ATES projects, where high heat recovery values were achieved, and technical feasibility was demonstrated. Although HT-ATES is less flexible than competing technologies such as pits or buffer tanks, the main problems encountered are attributed to a loss of the heat source and fluctuating or decreasing heating demands. Considering that a HT-ATES system has a lifetime of more than 30 years, it is crucial to develop energy concepts which take into account the conditions both for heat sources and heat sinks. Finally, a site-specific risk analysis for HT-ATES in the city of Hamburg revealed that some risks strongly depend on local boundary conditions. A project-specific risk management is therefore indispensable and should be addressed in future research and project developments.

Deposits formation on heat transfer surfaces is one of the main problems associated to biomass co-combustion. It reduces plant efficiency and availability and increases maintenance costs. It is obvious that an increasing amount of low-temperature melting components in fuel ash accelerates and aggravates this process. Research is done to evaluate the validity of thermal analysis methods to characterise fusion of biomass and waste ashes. Laboratory ashes from a set of biomass and waste fuels are leached in successive steps. The original and the leached ashes are analysed by Thermo-Mechanical Analysis (TMA). Traces obtained from TMA show to be promising ash fingerprints to classify deposition tendencies. Additionally Simultaneous Thermal Analysis (STA) is performed on selected samples. Furthermore, improved chemical equilibrium calculations are proposed to predict the proportion of melted species resulting from combustion of biomass fuels. The model takes into account the reactivity of the inorganic matter in the fuel as issued from ash leaching.

The transition process towards renewable energy systems is facing challenges in both fluctuating electricity generation of photovoltaic and wind power as well as socio-economic disruptions. With regard to sector integration, solutions need to be developed, especially for the mobility and the industry sector, because their ad hoc electrification and decarbonization seem to be unfeasible. Power-to-fuel (P2F) technologies may contribute to bridge the gap, as renewable energy can be transferred into hydrogen and hydrocarbon-based synthetic fuels. However, the renewable fuels production is far from economically competitive with conventional fuels. With a newly developed agent-based model, potential developments in the German energy markets were simulated for a horizon of 20 years from 2016 to 2035. The model was constructed through a participatory modeling process with relevant actors and stakeholders in the field. Model findings suggest that adjusted regulatory framework conditions (e.g., exemptions from electricity surtaxes, accurate prices for CO2-certificates, strong start-up subsidies, and drastic emission reduction quotas) are key factors for economically feasible P2F installations and will contribute to its large-scale integration into the German energy system. While plant capacities do not exceed 0.042 GW in a business-as-usual scenarios, the above-mentioned adjustments lead to plant capacities of at least 3.25 GW in 2035 with concurrent reduction in product prices.

LCAs of electric cars and electrolytic hydrogen production are governed by the consumption of electricity. Therefore, LCA benchmarking is prone to choices on electricity data. There are four issues: (1) leading Life Cycle Impact (LCI) databases suffer from inconvenient uncertainties and inaccuracies, (2) electricity mix in countries is rapidly changing, year after year, (3) the electricity mix is strongly fluctuating on an hourly and daily basis, which requires time-based allocation approaches, and (4) how to deal with nuclear power in benchmarking. This analysis shows that: (a) the differences of the GHG emissions of the country production mix in leading databases are rather high (30%), (b) in LCA, a distinction must be made between bundled and unbundled registered electricity certificates (RECs) and guarantees of origin (GOs); the residual mix should not be applied in LCA because of its huge inaccuracy, (c) time-based allocation rules for renewables are required to cope with periods of overproduction, (d) benchmarking of electricity is highly affected by the choice of midpoints and/or endpoint systems, and (e) there is an urgent need for a new LCI database, based on measured emission data, continuously kept up-to-date, transparent, and open access.

Torrefaction is a promising biomass upgrading technology as it makes biomass more coal alike and offers benefits in logistics and handling operations. Gasification is an attractive thermochemical conversion technology due to its flexibility in the product gas end-uses. Therefore, it is valuable to investigate whether additional benefits are foreseen when torrefaction is coupled with gasification. Therefore, two commercial torrefied wood fuels and their parent materials are gasified at 800–850 °C under atmospheric steam-oxygen circulating fluidized bed gasification conditions and magnesite as bed material. The torrefied feedstocks consisted of wood residues torrefied by Topell at 250 °C (Topell black), and mixed wood and wood residues torrefied by Torrcoal at 300 °C (Torrcoal black). The gasification results show that torrefaction resulted in an increased gas quality, as it yielded higher H2 and CO contents, a decrease of the CO2 content, increased gas yield and a significant decrease of the total tar content for both feedstocks. For the Torrcoal samples, torrefaction resulted in a decrease in the carbon conversion efficiency (CCE). In addition, the cold gas efficiency (CGE) remained approximately the same due to the increase in the H2 and CO contents. The Topell samples showed an increase in the CCE and CGE upon torrefaction, but this could be attributed to a significant grinding in the screw feeder. It is generally concluded that both torrefied fuels may offer benefits as a feedstock for steam-oxygen blown circulating fluidized bed gasification, in particular in terms of gas quality and yield.