Eucalyptus globulus coppices in Portugal: influence of site and percentage of residues collected for energy

Studies that quantify forest bioenergy potentials hardly address the questions of site quality, proportion and type of residues removed from the stands, and environmental impacts of those removals. However, those factors are important for energy-potential results and forest sustainability. This study compares, in terms of residual biomass availability for energy production and of sustainability, different locations, site indices, and forest management strategies in Eucalyptus globulus stands for pulp and paper in northern and central Portugal. A growth and production simulator was used to calculate the availability of residues and the area needed to supply a biomass-fired power plant under a variety of scenarios. Regions with more rainfall generate more residues, but site index and quantity and type of residues harvested are the most important factors. Under the different scenarios analyzed, the amount of residues potentially harvested range from 0.7 to 4.3 Mg ha−1 a−1, the upper bound corresponding to a scenario where stumps are valorized. The maximization of residue removal maximizes the bioenergy produced but has to be considered prudently. Studies indicate that stump removal has limited effect on Eucalyptus globulus stand productivity, diversity, and system sustainability, but impacts of residue removals increase with a decrease in site index

Biogas in Portugal: status and public policies

Portugal has recently implemented a number of measures and monitoring mechanisms that seek to promote renewable energy sources as a way of protecting the environment and assuring a sustainable development. In its policy for renewable energies, 39% of the electricity from renewable energy sources was defined as target for 2010. Biogas arises as a useful energy source by its direct conversion into heat and electricity or by injection into a natural gas network. This work reviews the Portuguese public policies on biogas, the current status of biogas production and its potential implementation as a relevant energy source. In Portugal, over the last decade there has been a big growth of the biogas produced from anaerobic digestion, both in electricity production (67 GWh in 2008) and installed power (12.4 MW in 2008). However, the current installed capacity and energy produced from this renewable source does not reflect its huge potential

A new look on the anomalous thermal gradient values obtained in South Portugal

It is well known that soil temperatures can be altered by water circulation. In this paper, we study numerically this effect by simulating some aquifers occurring in South Portugal. At this location, the thermal gradient values obtained in boreholes with depths less than 200 m, range between 22 and 30 ºC km-1. However, there, it is easy to find places where temperatures are around 30 ºC, at depths of 100 m. The obtained thermal gradient values show an increase one day after raining and a decrease during the dry season. Additionally, the curve of temperature as function of depth showed no hot water inlet in the hole. The region studied shows a smooth topography due to intensive erosion, but it was affected by alpine and hercinian orogenies. As a result, a high topography in depth, with folds and wrinkles is present. The space between adjacent folds is now filled by small sedimentary basins. Aquifers existing in this region can reach considerable depths and return to depths near the surface, but hot springs in the area are scarce. Water temperature rises in depth, and when the speed is high enough high temperatures near the surface, due to water circulation, can be found. The ability of the fluid to flow through the system depends on topography relief, rock permeability and basal heat flow. In this study, the steady-state fluid flow and heat transfer by conduction and advection are modeled. Fractures in the medium are simulated by an equivalent porous medium saturated with liquid. Thermal conductivity values for the water and the rocks can vary in space. Porosities used have high values in the region of the aquifer, low values in the lower region of the model and intermediate values in the upper regions. The results obtained show that temperature anomaly values depend on water ascending velocity, permeability values and depth of the aquifer. Comparing the results of our model with the measured values we can obtain information about aquifer depth and temperature

Potential of porous media combustion technology for household applications.

Households are major energy consumers and have a significant contribute to the World’s final energy consumption and CO2 emissions. Among the energy end-use in buildings lies cooking. The energy saving potential of cooking appliances is large and an investment on the development of more efficient and less polluting stoves and ovens is necessary. Porous medium combustion, already commercially available for several other applications, is a promising technology that can be applied also to household cooking. This paper reviews the research works done in the field. The number of papers dedicated to this specific application is relatively low, and most of them concentrate on experimentally proving the advantages of porous burners when compared to conventional solutions. The influence of burner characteristics and operating conditions are analysed in a few studies. However, there is still a considerable scope for the development of enhanced porous burners for household applications

Numerical investigation of the heat transfer characteristics of propane/air flames impinging on a cylindrical surface

To better understand the complex phenomena occurring inside furnaces with direct flame impingement and extend the existent studies to situations with multiple enclosed jets, numerical simulations of a radial array of four turbulent confined flame jets impinging on a cylinder are presented. The three-dimensional, incompressible, steady, averaged equations for the transport of mass, momentum, energy and species were solved. Turbulence was modeled with the realizable k-ε model, combustion with the finite-rate/eddy-dissipation model and radiation with the finite-volume scheme. The reference condition simulated is based on the operation of an existent industrial furnace, but the Reynolds number, excess air ratio, impinging distance, cylindrical target diameter and reactant temperature were varied in order to analyze their influence on the fluid flow and heat transfer and to determine a correlation for the area-averaged Nusselt number. The curvature of the target and the interaction of adjacent opposed wall jets lead to the increase of the heat transfer to the target. For the parameter values considered, the excess air ratio, Reynolds number and reactant temperature are of primary importance to enhance heat transfer and reduce the power required, while the target curvature is of secondary importance and the least influential parameter is the impinging distance

Energia da biomassa

Valorização energética de biomassa

Radiation statistics in homogeneous isotropic turbulence

An analysis of the interaction between turbulence and radiation in statistically stationary (forced) homogeneous and isotropic turbulence has been carried out. A direct numerical simulation (DNS) code was used to generate instantaneous turbulent scalar fields, and the radiative transfer equation (RTE) was solved to provide statistical data of the radiation intensity and absorption coefficient, as well as correlations related to radiative emission and absorption. In addition, the time-averaged RTE was solved and the mean radiation intensity, mean absorption coefficient, and mean radiative emission were computed and compared with those derived from the statistical data. An analysis of the number of samples required to achieve statistically meaningful results is presented, and the influence of the optical thickness of the medium, mean and variance of the temperature and variance of the mean molar fraction of the absorbing species are studied. The moments of the radiation intensity, Planck-mean and incident-mean absorption coefficients, and emission and absorption correlations relevant to the turbulence – radiation interaction (TRI) are calculated. It was found while turbulence yields an increase of the mean blackbody radiation intensity, it causes a decrease of the time-averaged Planck-mean absorption coefficient. The absorption coefficient self-correlation is small in comparison with the temperature self-correlation, and the role of TRI in radiative emission is more important than in radiative absorption. The correlation between fluctuations of the absorption coefficient and fluctuations of the radiation intensity is small, which supports the optically thin fluctuation approximation (OTFA), and justifies the good predictions often achieved using the time-averaged RTE

Promoting Sustainability: Wastewater treatment plants as a source of biomethane in regions far from a high-pressure grid. A Portuguese case study

Wastewater treatment plants (WWTP) located in regions far from a high-pressure grid can produce renewable biomethane, which can partially substitute the natural gas locally consumed. However, the economic viability of implementing biomethane plants in WWTP has to be guaranteed. This paper uses the discount cash flow method to analyze the economic viability of producing biomethane in a WWTP located in Évora (Portugal). The results show that, under the current conditions, it is unprofitable to produce biomethane in this WWTP. Since selling the CO2 separated from biogas may result in an additional income, this option was also considered. In this case, a price of 46 EUR/t CO2 has to be paid to make the project viable. Finally, the impact of potential government incentives in the form of feed-in premia was investigated. Without selling CO2, the project would only be profitable for feed-in premia above 55.5 EUR/MWh. If all the CO2 produced was sold at 30 EUR/t CO2, a premium price of 20 EUR/MWh would make the project profitable. This study shows that the economic attractiveness of producing biomethane in small WWTP is only secured through sufficient financial incentives, which are vital for developing the biomethane market with all its associated advantages

Design of a trigeneration system using a high temperature fuel cell

Fuel cells are one of the technologies available for CHCP, combined heat, cooling and power production, systems. They offer several advantages over more conventional systems, but they still need to overcome a number of barriers until they are readily available for commercialization. At this stage, it is important to fund demonstration projects that experiment with fuel cell technology in pre-commercial situations. In this context, a CHCP system, using a high temperature fuel cell (SOFC) and an absorption chiller, was designed in order to meet the energetic demands of a hospital for electricity, cooling, heating and hot water. The hospital load profile was determined taking into consideration the hourly energy consumption for four different typical days in the year. The CHCP system was designed so that the fuel cell meets the electrical demand of the hospital and, since the SOFC did not produce enough thermal energy, a boiler was considered. The artificial thermal efficiency of the CHCP system is 68%. The investment analysis is presented and it is concluded that, at the present and at three other scenarios, the system is not financially feasible. Despite this conclusion, it is important to invest on demonstration projects to help fuel cells reaching commercialization

Techno-economic analysis of a hybrid solar-biomass heating system for the cork industry. What makes it profitable?

The industrial sector is an important energy consumer, which accounted for 29% of the world’s final energy consumption in 2015 [1]. In the European Union this share is slightly lower, 25% in 2016, and in Portugal 27% [2]. Moreover, its energy consumption is still dominated by fossil fuels and industry is therefore an important contributor to the emission of CO2 into the atmosphere. Looking at the penetration of renewable energies in the industrial sector, in 2016 in the EU28, only 8% of the final industrial energy consumption was supplied by renewable energies, being biomass presently the only renewable energy source with some expression and mainly in the biomass residue generating industries. This paper focuses on the cork industry, a sub-sector where Portugal is an important player worldwide. Many industrial units already use biomass as an energy source; however, part of it could be replaced by solar energy, freeing biomass for other uses. The use of solar energy is technically possible, and for some of the industrial processes can be provided with commercially available solar collector. However, this is not done. One of the reasons is that the solar systems are costly and the industry has energy systems with a long life and is sensitive to costs. According to [3], the implementation of solar systems in the industry requires the installation of pilot plants with the help of public funds or incentives. This work presents a techno-economic viability study for the implementation of a hybrid solar-biomass systems to provide process heat for the boiling of cork. As expected, for the proposed scenario, the implementation of the system is not profitable. The critical incentives in terms of CAPEX that would be required to implement the system with no financial losses are identified. Although the study focuses on a particular industrial sub-sector, it sheds light on the required incentives for the promotion of renewable process heat in industry