Paulo Brito

Renewable hydrogen purification is a critical yet often underemphasised step in enabling its use as a clean energy carrier. Hydrogen produced from biomass-based thermochemical and biological routes typically contains CO₂, CO, CH₄, H₂S, and other impurities that must be removed to meet stringent requirements for fuel cell, industrial, and grid-injection applications. This review provides a critical and up-to-date assessment of renewable hydrogen purification technologies, focusing on their suitability for variable and impurity-rich renewable hydrogen streams. Established benchmark technologies, including pressure swing adsorption and cryogenic separation, are described, with emphasis on their operating principles, material innovations, and process integration strategies. Recent advancements in inorganic, polymeric, and mixed-matrix membranes are highlighted, with particular focus on how advanced porous materials enhance selectivity, permeability, and flexibility. Additionally, a comparative techno-economic assessment is presented, evaluating each purification method based on technology readiness level, capital and maintenance costs, energy efficiency, and operational lifespan. By incorporating recent research trends, this approach facilitates the selection and design of purification systems that are not only efficient and scalable but also cost-effective, tailored to both decentralised and centralised renewable hydrogen production.

The growing demand for carbon-based energy materials requires sustainable alternatives to fossil fuels. This study explored the production and characterization of char obtained from refuse-derived fuel (RDF) and biomass blended pellets in varying proportions (0%, 15%, 25%, 50%, and 100% RDF). The objective was to evaluate their potential as high-energy-density solid fuels while addressing operational challenges related to ash behavior. Chars were produced at 400 °C for one hour in a muffle furnace in closed crucibles. A set of analytical techniques (calorimetry, infrared spectroscopy, thermogravimetry, inductively coupled plasma, and X-ray fluorescence) was employed to assess physicochemical properties. RDF content strongly affected mass yield, energy yield, and thermochemical behavior. Among the tested formulations, char with 50 and 25% of RDF (C_RDF50:BW50 and C_RDF25:BW75) ignited at lower temperatures (≈150 °C) and showed high flammability (C) values (1.97–2.03 × 10⁻⁵), indicating greater flammability. They also reached higher combustion temperatures (716–746 °C), suggesting improved thermal stability during the final combustion stage. Both chars presented increased high heating values (18–19 MJ/kg, dry basis) and a few surface functional groups. This supports a lower devolatilization rate, meaning that although ignition is easy, combustion remains stable and controllable. All chars showed very high acid–base indices, indicating a strong tendency for ash melting. However, low slag viscosity and alkalinity values suggest viscous, poorly mobile slag, reducing adhesion and buildup on reactor surfaces. This study combines thermogravimetric combustion analysis with ash chemistry–based slagging and fouling indices to provide an integrated assessment of the operational behavior of RDF–biomass-derived char fuels. The results highlight the technical feasibility of chars produced from RDF and biomass blended pellets, whose thermal properties make them promising candidates for use as solid fuels.

Mobile pyrolysis systems offer a practical pathway for the decentralized valorization of biomass waste, addressing the high logistical and economic burdens of transporting low-density, moisture-rich feedstocks to centralized facilities. By operating directly at the source, these systems convert diverse agricultural and forestry residues into biochar, bio-oil, pyrogas, and wood vinegar, while reducing transport volumes and associated emissions. Reported mobile reactors process between 4 kg per batch and 10 t/day, achieving biochar yields of 33–44 wt.% at 400 °C and bio-oil yields of 55–68 wt.% in fast pyrolysis at 500–550 °C, demonstrating performance comparable to stationary installations. This review synthesizes current mobile pyrolysis technologies, including reactor configurations, feedstock suitability, operational constraints, and recent advances in automation, real-time monitoring, and machine learning-based optimization. The agricultural and industrial applications of pyrolysis products are examined, with emphasis on soil health enhancement, biopesticide activity, renewable gas generation, and carbon sequestration. Emerging international projects and commercial efforts are highlighted, illustrating growing interest in flexible, low-carbon pyrolysis solutions for rural waste management and distributed bioresource utilization, while outlining the technological gaps that remain to be addressed.

Syngas, a renewable fuel primarily composed of hydrogen and carbon monoxide, is emerging as a viable alternative to conventional fossil fuels in internal combustion engines (ICEs). Obtained mainly through the gasification of biomass and organic waste, syngas offers significant environmental benefits but also presents challenges due to its lower heating value and variable composition. This review establishes recent advances in understanding syngas combustion, chemical kinetics, and practical applications in spark-ignition (SI) and compression-ignition (CI) engines. Variability in syngas composition, dependent on feedstock and gasification conditions, strongly influences ignition behavior, flame stability, and emissions, demanding detailed kinetic models and adaptive engine control strategies. In SI engines, syngas can replace up to 100% of conventional fuel, typically at 20–30% reduced power output. CI engines generally require a pilot fuel representing 10–20% of total energy to start combustion, favoring dual-fuel (DF) operation for efficiency and emissions control. This work underlines the need to integrate advanced modelling approaches with experimental insights to optimize performance and meet emission targets. By addressing challenges of fuel variability and engine adaptation, syngas reveals promising potential as a clean fuel for future sustainable power generation and transport applications.

The demand for clean energy to improve waste valorization and enhance resource utilization efficiency has been increasingly recognized in the last few years. In this context, the co-carbonization of different waste streams, aiming at solid fuel production, appears as a potential strategy to address the challenges of the energy transition and divert waste from landfills. In this work, refuse-derived fuel (RDF) samples were subjected to the co-carbonization process with low-quality animal fat waste in different proportions to assess the synergistic effect of the mixture on producing chars with enhanced fuel properties. Dry (DC) and hydrothermal carbonization (HTC) tests were conducted at 425 °C and 300 °C, respectively, with a residence time of 30 min. The RDF sample and produced chars with different animal fat incorporation were analyzed for their physical, chemical, and fuel properties. The results demonstrated that increasing the fat proportion in the samples leads to an increase in mass yield and apparent density of the produced chars. Furthermore, char samples with higher fat addition presented a proportional increase in high heating value (HHV). The highest values for the HHV corresponded to the char samples produced with 30% fat incorporation for both carbonization techniques (27.9 MJ/kg and 32.9 MJ/kg for dry and hydrothermal carbonization, respectively). Fat addition also reduced ash content, improved hydrophobicity in hydrochars, and lowered ignition temperature, although additional washing was necessary to reduce chlorine to acceptable levels. Furthermore, fat incorporation reduced concentrations of elements linked to slagging and fouling. Overall, the results demonstrate that incorporating 30% fat into RDF during DC or HTC is the most effective condition for producing chars with improved physical, chemical, and fuel properties, enhancing their potential as alternative solid fuels.

Portugal exhibits a large deficit in cereals with an import/export ratio of about 18%. Alentejo is a southern vast plain region, which is the largest cereal producer in the country, with about 80% of the total cereal area. The region also shows a huge local energy deficit with a ratio of about 17% between spent and produced energy. In this context, this work used GIS modeling based on available digital geographical information on soil and topographic conditions in Alentejo for estimating optimal production areas of four main classes of cereal and legume classes, which were wheat, barley, oat/lupin, and triticale/broad bean. The estimated areas were validated by 199 sample points in the field and allowed to quantify a potential of bioenergy production from straw biomasses based on yields of biomass net calorific values of 18 MJkg⁻¹ and yields of 6, 9, 6, and 9 tons/ha for the four classes in the order indicated. The estimated areas allocated to the cereal and legume classes covered approximately four municipalities in the region. The total modeled area in Alentejo for the four cultivation classes was 44,980 ha. The results showed that even if 50% of the estimated total straw biomass produced was used for animal feed, the estimated bioenergy production of the remaining half biomass would be of about 2940 TJy⁻¹, or about 12.5% of the actual regional energy production, which is an energy amount able to supply 35 organic Rankine cycle (ORC) 2.5 MW cogeneration units and 347 boilers with 125 kW thermal power, delivering renewable electricity to the grid, and heating facilities as diverse as buildings, nursing homes, or horticultural greenhouses. More than 160 kton of CO₂ fossil emissions would also be avoided, delivering a contribution to mitigating effects of climate change. By contributing to the reduction of the large cereal dependence and the carbon emissions of the country, the proposed strategy would contribute to increasing the decentralized bioenergy production for applications in buildings and local facilities, significantly boosting the socio-economic dynamics of rural areas involved.