Stergios Vakalis

Thermochemical valorization and characterization of household biowaste

Valorization of municipal solid waste (MSW), by means of energy and material recovery, is considered to be a crucial step for sustainable waste management. A significant fraction of MSW is comprised from food waste, the treatment of which is still a challenge. Therefore, the conventional disposal of food waste in landfills is being gradually replaced by recycling aerobic treatment, anaerobic digestion and waste-to-energy. In principle, thermal processes like combustion and gasification are preferred for the recovery of energy due to the higher electrical efficiency and the significantly less time required for the process to be completed when compared to biological process, i.e. composting, anaerobic digestion and transesterification. Nonetheless, the high water content and the molecular structure of biowaste are constraining factors in regard to the application of thermal conversion pathways. Investigating alternative solutions for the pre-treatment and more energy efficient handling of this waste fraction may provide pathways for the optimization of the whole process. In this study, by means of utilizing drying/milling as an intermediate step, thermal treatment of household biowaste has become possible. Household biowaste has been thermally processed in a bench scale reactor by means of torrefaction, carbonization and high temperature pyrolysis. According to the operational conditions, fluctuating fractions of biochar, bio-oil (tar) and syngas were recovered. The thermochemical properties of the feedstock and products were analyzed by means of Simultaneous Thermal Analysis (STA), Ultimate and Proximate analysis and Attenuated Total Reflectance (ATR). The analysis of the products shows that torrefaction of dried household biowaste produces an energy dense fuel and high temperature pyrolysis produces a graphite-like material with relatively high yield.

Utilisation of biomass gasification by-products for onsite energy production.

Small scale biomass gasification is a sector with growth and increasing applications owing to the environmental goals of the European Union and the incentivised policies of most European countries. This study addresses two aspects, which are at the centre of attention concerning the operation and development of small scale gasifiers; reuse of waste and increase of energy efficiency. Several authors have denoted that the low electrical efficiency of these systems is the main barrier for further commercial development. In addition, gasification has several by-products that have no further use and are discarded as waste. In the framework of this manuscript, a secondary reactor is introduced and modelled. The main operating principle is the utilisation of char and flue gases for further energy production. These by-products are reformed into secondary producer gas by means of a secondary reactor. In addition, a set of heat exchangers capture the waste heat and optimise the process. This case study is modelled in a MATLAB-Cantera environment. The model is non-stoichiometric and applies the Gibbs minimisation principle. The simulations show that some of the thermal energy is depleted during the process owing to the preheating of flue gases. Nonetheless, the addition of a secondary reactor results in an increase of the electrical power production efficiency and the combined heat and power (CHP) efficiency.

Household food waste dehydration technique as a pretreatment method for food waste minimisation

The results of the examination of the physicochemical characteristics of domestic food waste (DFW) pretreated with the domestic food waste drying method are presented within this research. Domestic waste drying is considered to be an innovative approach which involves the source separation and dehydration of household food waste in order to reduce its mass and volume at source through the efficient removal of its moisture content. An innovative waste drying unit was developed from scratch in order to perform this research. The use of the system revealed that the average mass reduction of food waste reached 78.52%w/w. The average decrease of its volume was recorded equal to 58.77%. The method of domestic waste drying was found to be an attractive pretreatment method and could be used as an inextricable part of a waste management strategy for bioethanol production as its use preserves the carbohydrate content of DFW.

Development and application of the compression frictional treatment method for sterilizing and valorizing organic waste

ABSTRACT Confined animal feeding operations are producing large amounts of organic waste with large fractions being mixtures of manure and sawdust. On one hand, the inhomogeneity of the material and the high water content restrict the optimal application of thermal treatment methods. On the other hand, the high concentration of bacteria in these mixed waste streams is an issue of concern. This study introduces a novel process for treating manure and woody biomass mixtures and upgrading them into valuable products. It is defined as compression frictional treatment (CFT) and takes place continuously in a rotary compression unit. Fresh poultry droppings and fresh anaerobic digester offal from a cattle feedlot were processed by means of CFT. Moisture was decreased by 52% and heating value was increased by 27% for treated poultry litter. The corresponding values were 63% moisture decrease and 25% increase for the heating value of cattle manure. On the aspect of bacteria reduction, the Escherichia coli and the Enterobacteriaceae family of bacterium were reduced by 94% and 91%, respectively.

Modeling the emissions of a dual fuel engine coupled with a biomass gasifier—supplementing the Wiebe function

There is a growing market demand for small-scale biomass gasifiers that is driven by the economic incentives and the legislative framework. Small-scale gasifiers produce a gaseous fuel, commonly referred to as producer gas, with relatively low heating value. Thus, the most common energy conversion systems that are coupled with small-scale gasifiers are internal combustion engines. In order to increase the electrical efficiency, the operators choose dual fuel engines and mix the producer gas with diesel. The Wiebe function has been a valuable tool for assessing the efficiency of dual fuel internal combustion engines. This study introduces a thermodynamic model that works in parallel with the Wiebe function and calculates the emissions of the engines. This “vis-à-vis” approach takes into consideration the actual conditions inside the cylinders—as they are returned by the Wiebe function—and calculates the final thermodynamic equilibrium of the flue gases mixture. This approach aims to enhance the operation of the dual fuel internal combustion engines by identifying the optimal operating conditions and—at the same time—advance pollution control and minimize the environmental impact.

Assessing the removal of heavy metals in industrial wastewater by means of chemical exergy

aNational Technical University of Athens, School of Chemical Engineering, Unit of Environmental Science & Technology, 9 Iroon Polytechniou Str., GR-15780 Athens, Greece, Tel. +30 6947930517; emails: stergios.vakalis@outlook.com (S. Vakalis), konmoust@yahoo.com (K. Moustakas), malamis.dimitris@gmail.com (D. Malamis), angeluoi81@gmail.com (A. Sotiropoulos) bNational Technical University of Athens, School of Civil Engineering, Department of Water Resources and Environmental Engineering, 9 Iroon Polytechniou Str., GR-15780 Athens, Greece, email: smalamis@central.ntua.gr