A. Sotiropoulos

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.

Research on the drying kinetics of household food waste for the development and optimization of domestic waste drying technique

ABSTRACT Domestic food waste drying foresees the significant reduction of household food waste mass through the hygienic removal of its moisture content at source. In this manuscript, a new approach for the development and optimization of an innovative household waste dryer for the effective dehydration of food waste at source is presented. Food waste samples were dehydrated with the use of the heated air-drying technique under different air-drying conditions, namely air temperature and air velocity, in order to investigate their drying kinetics. Different thin-layer drying models have been applied, in which the drying constant is a function of the process variables. The Midilli model demonstrated the best performance in fitting the experimental data in all tested samples, whereas it was found that food waste drying is greatly affected by temperature and to a smaller scale by air velocity. Due to the increased moisture content of food waste, an appropriate configuration of the drying process variables can lead to a total reduction of its mass by 87% w/w, thus achieving a sustainable residence time and energy consumption level. Thus, the development of a domestic waste dryer can be proved to be economically and environmentally viable in the future.

Improvement of home composting process of food waste using different minerals

This article presents the experimental study of the process of composting in a prototype home-scale system with a special focus on process improvement by using different additives (i.e. woodchips, perlite, vermiculite and zeolite). The interventions with different bulking agents were realized through composting cycles using substrates with 10% additives in specific mixtures of kitchen waste materials. The pre-selected proportion of the mixtures examined was 3:1:1 in cellulosic:proteins:carbohydrates, in order to achieve an initial C/N ratio equal to 30. The control of the initial properties of the examined substrates aimed at the consequent improvement of the properties of the final product (compost). The results indicated that composting process was enhanced with the use of additives and especially the case of zeolite and perlite provided the best results, in terms of efficient temperature evolution (>55 °C for 4 consecutive days). Carbon to nitrogen ratios decreased by 40% from the initial values for the reactors were minerals were added, while for the bioreactor tested with woodchips the reduction was slight, showing slowest degradation rate. Moisture content of produced compost varied within the range of 55-64% d.m., while nutrient content (K, Na, Ca, Mg) was in accordance with the limit values reported in literature. Finally, the composts obtained, exhibited a satisfactory degree of maturity, fulfilling the criterion related to the absence of phytotoxic compounds.

Combination of decentralized waste drying and SSF techniques for household biowaste minimization and ethanol production.

The results of the demonstration of an innovative household biowaste management and treatment scheme established in two Greek Municipalities for the production of lignocellulosic ethanol using dehydrated household biowaste as a substrate, are presented within this research. This is the first time that biowaste drying was tested at a decentralized level for the production of ethanol using the Simultaneous Saccharification and Fermentation (SSF) process, at a pilot scale in Greece. The decentralized biowaste drying method proved that the household biowaste mass and volume reduction may reach 80% through the dehydration process used. The chemical characteristics related to lignocellulosic ethanol production have proved to differ substantially between seasons thus; special attention should be given to the process applied for ethanol production mainly regarding the enzyme quality and quantity used during the pretreatment stage. The maximum ethanol production achieved was 29.12g/L, approximately 60% of the maximum theoretical yield based on the substrates sugar content. The use of the decentralized waste drying as an alternative approach for household biowaste minimization and the production of second generation ethanol is considered to be a promising approach for efficient biowaste management and treatment in the future.