Annually, a huge amount of waste from plant biomass and animal manure is produced from agriculture and animal farming. Many studies provide information on the biomethane potential of agricultural and livestock wastes, but only a few studies have investigated the application of the substrates in combination. The objective of the study is to enhance the fermentation process in the digester for biogas production, obtained from animal and plant waste substrates. In four batch processes for three months, the temperatures and the residence time of the substrates in the fermenter were analyzed. Simultaneously, electricity and thermal energy were produced via cogeneration units, which were exported to the public grid and city heating network. The plant substrate is a silage mixture of corn and wheat waste. The animal substrate is a mixture of beef and pig manure. Animal and vegetable waste raw materials are collected and transported to the site, located in the region of southeastern Bulgaria. The total annual consumption of animal and plant waste is 17,971 t/year. The enhancement of the process leads to the production of 1,506,000 Nm³ CH₄/a of methane, the generation of which requires 299.63 MWh/a of electricity and 649.09 MWh/a thermal energy.

The focus of the present study is a small biogas power plant for anaerobic fermentation of several types of animal waste raw materials used for biogas production. The impact of some of the characteristics of substances such as composition, temperature, humidity, and pH of the mixture in the bioreactor has been considered. The above is vital for optimizing the fermentation process, and also to improve the biogas production process. The plant is located in Northeastern Bulgaria and the raw liquid manure is supplied by several neighboring small farms. The annual quantities of raw waste are as follows: cow manure - 1252 t / a; chicken manure - 427 t / a and pig manure - 639 t / a. The manure is collected in a preliminary tank and then pumped to the bioreactor. The fermenter itself is a hermetically sealed and thermally insulated tank where constant temperature is maintained. It is equipped with a stirring system, which helps the mixing and homogenization of the substrate. The tests were performed during three charges of the installation. The fermentation takes approximately 23 up to 25 days. The experiments were performed during the summer and autumn seasons when the ambient air temperature varies from 28 to 45 °C. The biogas can be used as an energy carried as the obtained organic fertilizer is suitable for agriculture purposes.

This study presents an analysis of the influence of weather conditions on the performance of a multicrystalline silicon photovoltaic module, which operates under constant resistive load and is situated near a limestone quarry. The quarry is a significant source of dust, and hence the focus of the study is on the weather factors influencing the presence of soiling on the module’s surface. The analysis encompasses a three-week period, during which the global horizontal irradiance and wind speed were recorded at 10-min intervals by an on-site weather station. The current, voltage, and back temperature of the module were also measured. Supplementary weather data were obtained from the Copernicus Atmosphere Monitoring Service and the NASA POWER databases. The primary objective is to assess whether any influence of the observed weather conditions on the presence of soiling can be inferred from the recorded data. The contribution is in part intended to test how different techniques can be used to extract useful information on the weather-related effects from somewhat limited data, assembled from various sources, while dealing with the underlying uncertainties. The analysis indicates a persistent deterioration of the module’s performance because of soiling and its subsequent improvement due to a favourable weather event.

A model of the dynamics of steam bubbles generating on a solid surface in porous structures and a steam-generating wall (substrate) is developed. The model is based on the filming and photography with speed camera SKS-1M. The removal of high heat fluxes (up to 2х10⁶ W/m²) is provided by the combined action of capillary and mass forces with application of intensifiers. An analytical model is developed based on the theory of thermoelasticity. The limiting state of a poorly heat-conducting porous coating and a metal substrate has been determined. The heat fluxes were calculated from the time of spontaneous appearance of the steam nucleation (10^-8) up to the time of material destruction (10² ÷ 10³ s). The destruction of the coating under the action of compression forces occurs in much earlier time than the tension forces. The intervals of the heat flux within which such destruction occurs are different for the quartz coating q_max ≈ 7х10⁷ W/m², q_min ≈ 8х10⁴ W/m² and for granite coating q_max ≈ 1х10⁷ W/m², q_min ≈ 21х10⁴ W/m². Experimental units, experimental conditions, the results of the heat exchange crisis and the limiting state of the surface are presented, and critical heat fluxes are calculated. The investigated capillary-porous system, operating under the combined action of capillary and mass forces, has the advantage over pool boiling, thin-film evaporators and heat pipes.

This study reveals the possibilities of the sustainable usage of pellets produced from waste biomass based on the thermal properties of processed raw materials. For this study, a thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and differential thermal analysis (DTA) were performed to better understand the thermal properties of the pellets. This study covered two types of wood pellets with different ratios of waste material: a kind of pellet made from a combination of wood and sunflower residues, and two types of pellets made from sunflower husks. The analysis revealed that the wood pellets offered the best thermal stability and high energy values, making them the preferred choice for heating systems. Mixed pellets showed a lower thermal capacity and combustion efficiency, showing possibilities for further optimization. Sunflower-husk pellets demonstrated a high calorific value, but their application was restricted by a significantly higher ash content and worse environmental impact compared with the first two types of pellets. In addition, the outputs from this study revealed that pellets composed of wood reduced their mass the most at temperatures in the range of 310 to 323 °C. In comparison, the mass loss of sunflower husk pellets was 35.6%/°C (at a 5 °C/min temperature gradient) lower than wood pellets and about 42%/°C lower at a 10 °C/min temperature gradient. These findings highlight the importance of pellet production and technology usage to achieve better sustainability and better thermal properties of the pellets.

The present study analyzes the combustion process of mixed biomass pellets in a domestic boiler. For the purposes of the research, experimental measurements of flue gases are combined with numerical simulations based on computational fluid dynamics (CFD). Special attention is given to the impact of the ratio between primary and secondary air on the combustion process, emission characteristics, and thermal balance. The results show that an air distribution ratio of 60/40 (primary/secondary) leads to more complete combustion, reducing emissions of carbon monoxide (CO) and nitrogen oxides (NOx), while also improving the efficiency of the boiler. The analysis of the numerical modeling results shows that CO emissions decrease by 12% and NOx emissions by 27%. The calculated model is validated using experimental data on flue gas temperature, oxygen (O₂) and carbon dioxide (CO₂) concentrations, and combustion efficiency, and a high degree of correspondence between theoretical and actual measurements is established. The simulations reveal the dynamics of the temperature field, the movement of flue gases, and the role of turbulence in the combustion chamber. Optimization of the air distribution is proven to improve the combustion process and reduce the harmful emissions generated. The obtained results highlight the potential of mixed biomass pellets as a sustainable alternative to conventional fuels, provided that combustion parameters are precisely regulated. They can serve as a foundation for the enhancement of biomass-based heating systems in order to achieve higher efficiency and environmental sustainability. A market research study is also conducted, revealing that mixed pellets are preferred due to their high calorific value, low cost, and low ash content.