• Energy Research

The feasibility and operation performance of the gasification of rice straw in an atmospheric fluidized-bed gasifier was studied. The gasification was carried out between 700 and 850 °C. The stoichiometric air-fuel ratio (A/F) for rice straw was 4.28 and air supplied was 7-25% of that necessary for stoichiometric combustion. Mass and power balances, tar concentration, produced gas composition, gas phase ammonia, chloride and potassium concentrations, agglomeration tendencies and gas efficiencies were assessed. Agglomeration was avoided by replacing the normal alumina-silicate bed by a mixture of alumina-silicate sand and MgO. It was shown that it is possible to produce high quality syngas from the gasification of rice straw. Under the experimental conditions used, the higher heating value (HHV) of the produced gas reached 5.1 MJ Nm(-3), the hot gas efficiency 61% and the cold gas efficiency 52%. The obtained results prove that rice straw may be used as fuel for close-coupled boiler-gasifier systems.

Most biogas plants in the world run under psychrophilic conditions and are operated by small and medium farmers. There is a gap of knowledge on the performance of these systems after several years of operation. The aim of this research is to provide a complete evaluation of a psychrophilic, low-cost, tubular digester operated for eight years. The thermal performance was monitored for 50 days, and parameters such as pH, total volatile fatty acid (tVFA), chemical oxygen demand (COD) and volatile solids (VS) were measured every week for the influent and effluent. The digester operated at a stabilized slurry temperature of around 17.7 °C, with a mean organic load rate (OLR) equal to 0.52 kg VS/m3digester *d and an estimated hydraulic retention time (HRT) of 25 days. The VS reduction in the digester was around 77.58% and the COD reduction was 67 ± 3%, with a mean value for the effluent of 3.31 ± 1.20 g COD/Lt, while the tVFA decreased by 83.6 ± 15.5% and the presence of coliforms decreased 10.5%. A BioMethane potential test (BMP) for the influent and effluent showed that the digester reached a specific methane production of 0.40 Nm3CH4/kg VS and a 0.21 Nm3CH4/m3digester d with 63.1% CH4 in the biogas. These results, together with a microbiological analysis, show stabilized anaerobic digestion and a biogas production that was higher than expected for the psychrophilic range and the short HRT; this may have been due to the presence of an anaerobic digestion microorganism consortium which was extremely well-adapted to psychrophilic conditions over the eight-year study period.

Bioelectrochemical systems (BESs) is a term that encompasses a group of novel technologies able to interconvert electrical energy and chemical energy by means of a bioelectroactive biofilm. Microbial electrosynthesis (MES) systems, which branch off from BESs, are able to convert CO2 into valuable organic chemicals and fuels. This study demonstrates that CO2 reduction in MES systems can be enhanced by enriching the inoculum and improving CO2 availability to the biofilm. The proposed system is proven to be a repetitive, efficient, and selective way of consuming CO2 for the production of acetic acid, showing cathodic efficiencies of over 55% and CO2 conversions of over 80%. Continuous recirculation of the gas headspace through the catholyte allowed for a 44% improvement in performance, achieving CO2 fixation rates of 171 mL CO2 L−1·d−1, a maximum daily acetate production rate of 261 mg HAc·L−1·d−1, and a maximum acetate titer of 1957 mg·L−1. High-throughput sequencing revealed that CO2 reduction was mainly driven by a mixed-culture biocathode, in which Sporomusa and Clostridium, both bioelectrochemical acetogenic bacteria, were identified together with other species such as Desulfovibrio, Pseudomonas, Arcobacter, Acinetobacter or Sulfurospirillum, which are usually found in cathodic biofilms. Moreover, results suggest that these communities are responsible of maintaining a stable reactor performance.

The biological stabilization process of cattle and poultry manure was studied using thermogravimetric analysis and (13)C nuclear magnetic resonance. The stabilization processes carried out were composting, anaerobic digestion and a mixed process (partly aerobic, partly anaerobic). It was observed from the analyzed samples that the biological stabilization processes reduce the volatile content of the bio-wastes and increase the degree of aromaticity. The stabilization of cattle manure by means of aerobic processes was able to further oxidize and enriched in aromatic compounds the bio-waste when compared with the digestion process. On the other hand, the stabilization of poultry manure resulted in a greater aromatization under the digestion process. Stabilized samples with a high degree of aromaticity presented a lower volatile content accompanied by a reduction in the intensity of the differential thermogravimetry peak registered under an inert atmosphere, indicative of the thermal decomposition of the organic matter. The thermal decomposition of all the analyzed materials (fresh and stabilized samples) commenced at around 200 degrees C but for the digested poultry manure, which decomposition initiated close to 250 degrees C. All stabilized samples yielded a lower degree of volatilization to that one observed in fresh samples.

Research was carried out with the aim of monitoring anaerobic digestion processes using thermal analysis with the aid of mass spectrometry so as to define the stability of the digestate obtained. Three different systems were investigated under varying conditions. The digestion of waste sludge from a pharmaceutical industry (PI) and the digestion of cattle manure (CM) were evaluated under mesophilic conditions. The co-digestion of a mixture of primary sludge (PS) and the organic fraction of municipal solid wastes (OFMSW) was studied under thermophilic conditions. Temperature-programmed combustion tests were carried out to investigate the degree of stabilization of samples throughout the digestion processes. The derivative thermogravimetry (DTG) profiles obtained for the mesophilic digestion of PI waste showed a decrease at low temperatures and an increase at high temperatures in the intensity of the peaks recorded as the stabilization process proceeded. These results are in accordance with those obtained by the present authors in their previous work on the mesophilic digestion of primary sludge and OFMSW. In contrast, the DTG profiles obtained from the stabilization process of CM and thermophilic codigestion of PS and OFMSW showed a reduction in peaks at high temperatures. When the stabilization products obtained from CM by anaerobic digestion and by composting processes were compared, it was observed that the composting process was capable of further decomposing materials readily oxidized at low temperatures and increasing the presence of structurally more complex substances. The evolution of the differential thermal analysis (DTA) signal recorded simultaneously showed considerable similarity to the mass/charge (m/z) signal 44 registered by the mass spectrometer. The use of mass spectrometry helped to clarify the inner workings of the digestion process.

This study seeks to understand how the bacterial communities that develop on biocathodes are influenced by inocula diversity and electrode potential during start-up. Two different inocula are used: one from a highly diverse environment (river mud) and the other from a low diverse milieu (anaerobic digestion). In addition, both inocula were subjected to two different polarising voltages: oxidative (+0.2 V vs. Ag/AgCl) and reductive (-0.8 V vs. Ag/AgCl). Bacterial communities were analysed by means of high throughput sequencing. Possible syntrophic interactions and competitions between archaea and eubacteria were described together with a discussion of their potential role in product formation and current production. The results confirmed that reductive potentials lead to an inconsistent start-up procedure regardless of the inoculum used. However, imposing oxidative potentials help to quickly develop an electroactive biofilm ready to withstand reductive potentials (i.e. biocathodic operation). The microbial structure that finally developed on them was highly dependent on the raw community present in the inoculum. Using a non-specialised inoculum resulted in a highly specialised biofilm, which was accompanied by an improved performance in terms of consumed current and product generation. Interestingly, a much more specialised inoculum promoted a rediversification in the biofilm, with a lower general cell performance.

Power-to-gas technology makes use of surplus electricity by its conversion and storage in the form of a gas. Currently power-to-gas schemes based on biological processes are of great interest. Microbial electrosynthesis (MES) cells are biological systems that produce biogas via microbial action and a supply of electrical energy. The OxyMES scheme proposed is a power-to-gas system that seeks to neutralize the CO2 emissions of a standard industrial process through the hybridization of oxy-fuel combustion and bioelectrochemical processes that produce CH4 (in cathode) and O2 (in anode). This oxygen is used for oxycombustion in an industrial C-fuel boiler. The energy balance analysis yielded a power-to-gas efficiency in the MES cell close to 51%, and the overall performance of the OxyMES integrated system was close to 60% for a cell with a Faradaic efficiency of 80%, CO2-to-CH4 conversion rate of 95%, and ¿Vcell = 1.63 V. With the proper sizing of the CO2, O2, and biogas process tank system, it is possible to achieve 100% autonomy, free from external feedstock supplies. © 2022 Elsevier Ltd

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.