• Energy Research

Anaerobic digestion is an established technological option for the treatment of agricultural residues and livestock wastes beneficially producing renewable energy and digestate as biofertilizer. This technology also has significant potential for becoming an essential component of biorefineries for valorizing lignocellulosic biomass due to its great versatility in assimilating a wide spectrum of carbonaceous materials. The integration of anaerobic digestion and pyrolysis of its digestates for enhanced waste treatment was studied. A theoretical analysis was performed for three scenarios based on the thermal needs of the process: The treatment of swine manure (scenario 1), co-digestion with crop wastes (scenario 2), and addition of residual glycerine (scenario 3). The selected plant design basis was to produce biochar and electricity via combined heat and power units. For electricity production, the best performing scenario was scenario 3 (producing three times more electricity than scenario 1), with scenario 2 resulting in the highest production of biochar (double the biochar production and 1.7 times more electricity than scenario 1), but being highly penalized by the great thermal demand associated with digestate dewatering. Sensitivity analysis was performed using a central composite design, predominantly to evaluate the bio-oil yield and its high heating value, as well as digestate dewatering. Results demonstrated the effect of these parameters on electricity production and on the global thermal demand of the plant. The main significant factor was the solid content attained in the dewatering process, which excessively penalized the global process for values lower than 25% TS.

Anaerobic digestion is a biological process that transforms high-strength organic effluents into biogas with multiple benefits. However, concurrent with organics’ biological transformation, a liquid phase with a high solid content is also derived from this process. Valorizing this fraction is not an easy task if an agronomic application cannot be considered as a suitable option. The thermal valorization of this fraction allows for energy extraction but also gives rise to additional capital investment and increases the energy demand of the global process. In addition, the thermal treatment of digestate has to deal with a mineralized material. The changes in organic matter due to anaerobic digestion were studied in the present manuscript, by evaluating the thermal behavior of samples, activation energy, and organic transformation using Fourier transform infrared (FTIR) spectroscopy. Digested samples of a mixture composed of manure and glycerin (5% v/v) were studied. The stabilization caused a dramatic decrease in aliphatic compounds, greatly increasing the mineral content of the sample. Results from differential scanning calorimetry (DSC) indicated an energy content of 11 kJ/g for the feed material and a reduction to 9.6 kJ/g for the long-term stabilized sample. The activation energy of the feed was 249.5 kJ/mol, whereas this value was reduced to 70–80 kJ/mol for digested samples. If the valorization route selected for digestates is thermal conversion, the lower energy content and more complex structure of these materials (higher content of lignin and protein-type compounds) must be carefully evaluated.

The high amount of sludge produced from wastewater treatment plants (WWTPs) requires final disposal, forcing plant operators to search for alternatives without exerting an excessive energy demand on the global plant balance. Future revisions of the WWTP Directive will probably set additional constraints regarding the land application of sludge. Therefore, thermal treatment may seem a logical solution based on the additional energy that can be extracted from the process. The purpose of the present manuscript is to assess the integration of anaerobic digestion of sewage sludge and subsequent gasification using SuperPro Designer V13. Mass and energy balances were carried out, and the net energy balance was estimated under different scenarios. The integration of the process showed an electricity power output of 726 kW (best scenario, equivalent to 4.84 W/inhab.) against 411 kW (2.7 W/inhab.) for the single digestion case. The thermal demand of the integrated approach can be fully covered by deviating a fraction of gaseous fuels for heat production in a burner. Transforming syngas into methane by biological conversion allows densifying the gas stream, but it reduces the total energy content.

Wastewater treatment plants are essential in improving life quality by degrading organic matter, reducing contamination, and therefore greatly impacting human activities. The role of these critical treatment units can be further promoted by integrating new biological processes that currently are still under experimental scale. The co-digestion of sewage sludge and food waste has been proposed as an efficient way to increase plant treatment capacity and energy recovery. The assessment of hydrogen production along with food waste co-digestion is carried out in the present manuscript. Assessing several parameters is necessary to implement a new biological process in an operating plant, and quantifying its effects on the plant's overall performance is crucial. The implications associated with the extra equipment needed to handle additional waste material were evaluated. Results indicated that a conventional unit may treat a 10 % addition of food waste (expressed as VS) without experiencing severe modifications in process parameters, thus obtaining 16 % extra energy. However, the increase in food waste by over 10 % translates into substantial plant modifications requiring the installation of digesters with higher volumes and handling an additional amount of sludge. Another relevant factor is the lower energetic content of biogas when mixed with hydrogen. The increase in food waste until 50 % VS in the mixture reduced the biogas lower heating value to 15.5 MJ/m3. Future research will deal with an economic analysis of the approach and the effect on engine performance when dealing with a fuel mixture with different combustion properties.

[EN] In the present manuscript, the energy efficiency and economic feasibility of different digestion configurations were evaluated by considering a double turbocharged engine, Jenbacher type JGS 320 GS-BL. Scenarios considered a single farm producing the biogas needed to run the engine (Scenario 1). The second scenario assumed a centralized system treating manure from the surrounding farms (Scenario 2). The third scenario considered partial decentralization with farms treating locally produced wastes, and biogas being transported and valorized in a centralized engine (Scenario 3). Centralized valorization showed the best results. However, this scheme is inappropriate due to the size of the farm needed to support this configuration. The transport of wastes to a centralized treatment unit showed similar efficiency values but the economic feasibility was adversely affected. The worst performance was found for the decentralized configuration with efficiency in the range of 39–43%, much lower values than those obtained from previous cases (58%) with null economic feasibility due to the high costs associated with the transport of biogas either by truck or through a piping system. SI

Preprint. Submitted version [EN] The need to accommodate power fluctuations intrinsic to high-renewable systems will demand in the future the implementation of large quantities of energy storage capacity. Electromethanogenesis (EM) can potentially absorb the excess of renewable energy and store it as CH4. However, it is still unknown how power fluctuations impact on the performance of EM systems. In this study, power gaps from 24 to 96 h were applied to two 0.5 L EM reactors to assess the effect of power interruptions on current density, methane production and current conversion efficiency. In addition, the cathodes where operated with and without external H2 supplementation during the power-off periods to analyse how power outages affect the two main metabolic stages of the EM (i.e.: the hydrogenic and methanogenic steps). Methane production rates kept around 1000 mL per m2 of electrode and per day regardless of the duration of the power interruptions and of the supplementation of hydrogen. Interestingly, current density increased in the absence of hydrogen (averaged current density during hydrogen supplementation was 0.36 A·m-2 , increasing up to 0.58 A·m-2 without hydrogen). However current was less efficiently used in the production of methane with no hydrogen supplementation. Nevertheless, when the electrical power was restored after the power interruption experiments, performance parameters were similar to those observed before. These results indicate that EM is resilient to power fluctuations, which reinforces the opportunity of using EM as a technology for renewable energy storage. NO

The combination of phytoremediation of soils contaminated by potentially toxic elements with energy production by combustion of the generated biomass can be a sustainable land management option, combining the production of renewable bioenergy with soil restoration while minimising energy consumption and CO2 emission. In this work, plant biomass from phytoremediation of soils contaminated by potentially toxic elements was studied as solid biofuel for combustion by thermal analysis and biomass composition. Six plant species were grown in two soils with differing degrees of contamination: Brassica juncea, Cynara cardunculus, Atriplex halimus, Nicotiana glauca, Dittrichia viscosa, Retama sphaerocarpa and Salvia rosmarinus. The composition of the plant biomass was characterised chemically and thermogravimetric analyses were performed for the mass loss (TG), derivative curves of mass loss (DTG) and temperature difference (DTA) signal. The cellulose concentration correlated with the parameters of the thermal analysis in the low temperature range (150-350 °C), while lignin correlated with the thermal parameters of the second peak in the high temperature range. Salvia rosmarinus and R. sphaerocarpa showed the best combustion characteristics according to the thermal profile and mineral residue results. The accumulation of potentially toxic elements in B. juncea grown in heavily contaminated soil led to a higher amount of residue at 750 °C, with a global activation energy lower than the one obtained when this species was grown in a soil with lower contamination. Therefore, the most beneficial combination of soil phytoremediation and energy production (combustion) that can be suggested would depend on the level of soil contamination: in heavily contaminated soil, phytostabilisation using R. sphaerocarpa and S. rosmarinus; in slightly contaminated soil, B. juncea due to its high energy of activation, although the concentrations of potentially toxic elements in the residue must be controlled, as well as possible particulate matter emissions during combustion.

Herein, the valorization of vegetable and fruit waste was assessed via hydrothermal carbonization (HTC) and anaerobic digestion (AD) in terms of product characterization and energy requirements. HTC was conducted at reaction temperatures between 150 ºC and 190 ºC, and residence times between 20 min and 40 min. The increase in the process severity resulted in hydrochars with higher carbon contents and higher energy densification ratios. AD was performed in two different ways. i.e., batch and semi-continuous reactions. From the batch experiments a methane yield of 300 L CH4/kg VS was obtained, while for the semi-continuous, the average specific methane production estimated (for HRTs from 75 to 50 days) was 213 ± 32 L CH4/kg VS. To estimate the energy requirements, mass and energy balances were performed considering the basic stages of each process to obtain a suitable biofuel material. In this sense, it was concluded that for this specific waste, AD was a more suitable process with a positive energy net balance. On the contrary, HTC presented a negative energy net balance being required 1.29 MJ/kg of fresh food waste. A combined HTC-AD treatment may be an efficient method to take advantage of both technologies leading to higher energy efficiencies and other valuable products. Khadija Metyouy would like to thank the University of León for her scholarship (within the framework of Erasmus). This work has been partially supported by the grant FJC2021-047672-I, co-financed by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR Funds. A big thank you to all the team of the Grupo Ingeniería Química, Ambiental y Bioprocesos (IQUIMAB) Universidad de León and especially for Marta E. Sánchez and Jorge Cara-Jiménez for sharing information and knowledge from their experiences. Peer reviewed