Contaminants as particulate matter, sulfur, chlorine and tar should be removed from biosyngas to avoid damaging solid oxide fuel cells. However, there is no sufficient information on tar effect since they might be potentially used as a fuel, or they might cause performance losses and irreversible damages. Therefore, this study aims to assess whether tar can be reformed inside the SOFC and used as fuel. Short-duration experiments were conducted on Ni-GDC cells operating with simulated biosyngas containing different concentrations of representative tar compounds from biomass gasification. While benzene and ethylbenzene could be regarded as additional fuels even at concentrations as high as 15 g/Nm³, naphthalene and phenanthrene act as contaminants for the SOFC electrochemical and catalytic reactions, even at concentrations of 0.3 and 0.05 g/Nm³. However, the effect on these reactions appeared almost completely reversible. Solid carbon deposited on the SOFC ceramic housing in proximity of the inlet. Post-mortem analysis should be performed to asses the tar effect on the cell anode.

Limited overall efficiency and excessive complexity can hinder the competitiveness of biomass gasifier solid oxide fuel cell micro combined heat and power systems. To overcome these problems, hydrocarbons direct internal reforming is analysed as a strategy to increase efficiency and reduce system complexity. To the same end, two biosyngas heating-up strategies are compared: catalytic partial oxidation and afterburner off gases utilization. A comprehensive approach combining thermodynamic equilibrium calculations, experimental measurements, and system modelling was used. The gas cleaning unit should operate at 400 °C to decrease H₂S and HCl below 1 ppmv. A tar amount of 120–130 g Nm⁻³ dry biosyngas for woodchips and 190 g Nm⁻³ for straw pellets was measured and 2-methoxyphenol, hydroxyacetic acid and hydroxyacetone were selected as representative compounds. With direct internal reforming the cathode air flow rate decreases from approximately 90 kg h⁻¹ to 60 kg h⁻¹. This leads to an increase of around 1% point in electrical efficiency and of even 5–6% points in thermal efficiency. Direct internal tar reforming seems therefore an advantageous strategy. The catalytic partial oxidation unit increases the system overall efficiency but reduces the electric efficiency from roughly 38%–30% and is therefore not advised.

High temperature biosyngas cleaning is more efficient when the end user operates at elevated temperature, as in biomass gasifier solid oxide fuel cell systems. However, there is not much experience with this technology and low temperature gas cleaning is usually adopted. This paper advances current knowledge by presenting the results from the investigation of side reactions catalysed by commercially available sorbents involving biosyngas main components, and the results obtained with the pilot plant developed within the Horizon2020 project “Flexifuel-SOFC”. K₂CO₃, used for HCl removal, appeared catalytically active towards the water gas shift reaction. Under conditions representative of a real system, the residence time was not sufficient for the gas composition to reach thermodynamic equilibrium. ZnO–CuO, used for H₂S removal, showed a catalytic activity significantly higher. Both sorbents seemed not active towards the methanation reaction. The pilot plant tests confirmed the occurrence of the WGS reaction in the HCl removal reactor. The sorbents decreased H₂S and HCl below the target value of 1 ppmv for H₂S and 5 ppmv for HCl. The catalytic activity of sorbents and the heat released by these reactions should be carefully considered in the design phase of high temperature gas cleaning units.

Tar compounds have been defined as Achilles’ heel in biomass gasification. Catalytic reforming solves problems caused by tar by converting them into H₂ and CO. Most of the research has focused on secondary and tertiary tar reforming while some information on primary tar can be derived from bio-oil reforming. However, these studies use humidified N₂, Ar or He as gas carrier. Therefore, in this work, three catalysts are compared for reforming 40 g/Nm³ acetic acid as main primary tar compound from biomass updraft gasification using simulated biosyngas as gas carrier. The catalysts were tested over a 72 h period between 680 and 750 °C with a gas composition of 35.0 vol% H₂O, 2.3 vol% CO, 19.5 vol% CO₂, 3.6 vol% CH₄, 24.0 vol% H₂ and 15.6 vol% N₂. Olivine completely converted acetic acid, but a considerable amount of carbonaceous deposits was found on the catalyst and the catalytic activity decreased over time with 0.2 g/Nm³ hydroxyacetone measured in the last day of testing. Dolomite showed promising performances by completely converting acetic acid and accumulating carbonaceous deposits only in the low temperature part of the catalyst bed. The carbonaceous deposits could be suppressed increasing the steam content to 50.1 vol% and the temperature to 900 °C. However, the catalyst became excessively brittle. The metal-based catalyst out-performed the naturally-occurring catalysts by completely converting acetic acid with almost no carbonaceous deposits accumulation. These results are expected to help the further development of tar reformers, and the commercialisation of biomass updraft gasifiers based systems.

Integrated biomass gasifier solid oxide fuel cell systems are an alternative to fossil-fuel-based combined heat and power generators. However, biosyngas contaminants represent a bottleneck for small-scale systems. In this work, we present the results of experiments on the effects of H₂S, HCl, and acetic acid as model primary tar on Ni-GDC SOFC. First, the effects of 17-128 g/Nm³ dry basis acetic acid were studied. On a second cell, 0.8 and 1.3 ppm(v) H₂S were added to the simulated biosyngas anode flow. After a full recovery, the cell was exposed to 42 g/Nm³ acetic acid and 0.8 ppm(v) H₂S. On a third cell, 3.4, 20, and 50 ppm(v) HCl were tested and, after a recovery period, 42 g/Nm³ acetic acid and HCl were added. Even 0.8 ppm(v) H₂S caused an immediate voltage drop. H₂S affected CH₄ reforming and water-gas shift reaction. Differently, even 50 ppm(v) HCl appeared not to significantly affect these reactions. Acetic acid increased the cell voltage but caused carbon deposition at the cell inlet. The voltage increase seemed not to be affected by H₂S or HCl, and no acetic acid was measured at the cell outlet, indicating that these contaminants do not affect the primary tar conversion.

Charge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends.