The Oberlack definition of the MILD combustion limit for premixed systems was derived under the assumption of lean combustion and a one-step reaction. In this study, a generalization of this definition is presented by removing the lean combustion assumption, which leads to a more comprehensive relation between the Damk & ouml;hler number and temperature, defining the so-called S-curve. The transition of the S-curve to a monotonic function, indicating MILD conditions in the generalized formulation, reveals a dependency on the kinetic parameters of the reaction (reaction orders) and the equivalence ratio. Unlike the previous definition, the proposed solution applies across a broader range of conditions, from rich to lean mixtures, incorporating variations in combustion conditions and the reactivity of the analyzed system. Analytical solutions are not available due to the strong non-linearity of the model; therefore, the results are obtained numerically and are presented as plots and approximation functions, all valid in a wide range of parameter values and applicable to various fuels. The proposed methodology is adaptable to different parameter ranges if needed. Finally, two practical examples, based on hydrogen and methane, illustrate the findings. The results show that reaction orders and the equivalence ratio significantly influence the limit curve defining the MILD combustion regime, with dependencies on the combustion conditions and the chosen fuel.

This study explores the selection of a heat recovery system for cogeneration units based on gas engines supplying the district heating system in Opole in order to enhance the efficiency and sustainability of the system. The proposed modifications focus on utilizing low-temperature (LT) waste heat from engine cooling circuits and improving exhaust heat recovery. The research examines retrofitting three cogeneration engines (total thermal capacity of 7.6 MW) by integrating water-to-water heat pumps to upgrade low-temperature waste heat (55–45 °C up to 700 kW), enhancing heat supply to the district heating network. Additionally, a second stage of economizers is evaluated to maximize condensation-based exhaust heat recovery from the existing 95–135 °C system. These system modifications increase the overall thermal capacity up to 9–9.1 MW. To maintain heat supply during cogeneration unit shutdowns (due to failures or electricity price fluctuations), an auxiliary air-to-water cascade heat pump provides an additional 0.8–1 MW. With increasing electricity price volatility, these system modifications provide crucial operational flexibility. Computational simulations confirm that the hybrid configuration successfully upgrades waste heat while strictly maintaining the existing engine return water safety limit. The evaluation demonstrates high economic profitability alongside stable emission reductions. This research presents a case study in optimizing heat recovery in cogeneration-based district heating networks, demonstrating practical and scalable applications for sustainable energy systems.

District heating systems are central to Europe’s decarbonisation strategy and its 2050 climate-neutrality objective. However, district heating is deeply embedded in the socio-economic system and the built environment. This makes compliance with policy targets at the local level particularly challenging. The issues are attributable to two factors. Firstly, the process is characterised by a high degree of complexity and multidimensionality. Secondly, there is a scarcity of local resources (e.g., land, surface waters, waste heat, etc.). In Bucharest, Romania, the largest district heating system in the European Union, the process of decarbonisation represents a particularly complex challenge. The system is characterised by large physical dimensions, high technical wear, heavy dependence on natural gas, significant heat losses and complex governance structures. This paper presents a strategic planning exercise for aligning the Bucharest system with the Energy Efficiency Directive 2023/1791. Drawing on system data, investment modelling, and local resource mapping from the LIFE22-CET-SET_HEAT project, the study evaluates scenarios for 2028 and 2035 that shift heat generation from natural gas to renewable, waste heat, and high-efficiency sources. The central objective is the identification of opportunities and issues. Options include large-scale heat pumps, waste-to-energy, geothermal and solar heat. Heat demand profiles and electricity price dynamics are used to evaluate economic feasibility and operational flexibility. The findings show that the decarbonisation heat supply in Bucharest is technically possible, but financial viability hinges on phased investments, interinstitutional coordination, regulatory reforms and access to EU funding. The study concludes with recommendations for staged implementation, coordinated governance and socio-economic measures to safeguard heat affordability and system reliability.

The Ranque-Hilsch vortex tube (RHVT) is a device that separates a pressurized inlet stream into two decompressed streams of different temperature, flowing to the so-called hot and cold outlets. In this study, the flow structures within the vortex tube were examined qualitatively. The examination considered both compressible and incompressible fluids, using pressurized air and water as working fluids. A parametric study was conducted, in which the fluid inlet pressure and the vortex tube length were varied. Three tubes, with the same diameter but differing lengths (100, 180, and 240 mm) were utilized. The flow inside the tube was investigated using a flow visualization technique, which was employed in a variety of configurations and setups contingent on the specific fluid conditions under examination. The visualization process required the use of aerosol injection in the case of air, and kalliroscope particles in the case of water investigation. The research enabled the visualization of the flow structure within the vortex tube, thereby significantly advancing the comprehension of the underlying physical processes. The findings of the experimental research demonstrated the existence of phenomena of considerable scientific value. The internal vortex and its spatial and temporal structure observed in the RHVT were consistent with literature data. This was achieved despite the so-far established consensus that this type of research is challenging and not entirely reliable. In the course of water-based investigation, the cavitation phenomenon was observed in the vicinity of the internal vortex. This discovery is likely to be the first of its kind and may contribute significantly to the advancement of research on the Ranque-Hilsch phenomenon.

The paper presents possibilities of using waste heat from a coal mine to help decarbonize the local district heating system. Two sources of waste heat: underground water and ventilation air have been studied. A technological structure of the heat recovery system based on heat pumps has been designed, and the available heat generation potential possible to integrate with the district heating network has been estimated at 198 TJ/a. Energy and environmental performance of the system has been evaluated for two approaches: net – only accounting for heat pump electricity intensity, and gross – adding the electricity consumption of the fixed elements of the coal mine: pumps and fans. Net results show an exergy efficiency of 0.44–0.52, a CO2 emission reduction of 9 713 to 10 933 Mg/a and a dimensionless, exergy-based thermoecological cost from 4.92 to 6.2, i.e. below the existing cost 6.91 of heat production. The results indicate that underground water is thermodynamically more favorable compared to the ventilation air; and the calculated results for water are positive even if the gross energy consumption is taken into account.

The paper presents a critical analysis of a commercial pre-feasibility study for hydrogen generation from sewage sludge generated in a wastewater treatment plant sized Population Equivalent (PE) = 160,000 Three commercial offers based on hydrothermal carbonization, gasification/plasma gasification and gas separation, are analysed and validated in terms of chemical reactions and energy balance. The obtained hydrogen yield equals 10 kg H-2/Mg sludge for Technology#2 and 5.5 kg H-2/Mg sludge for Technology#3, while the process efficiency equals 31.4% and 41.2%, respectively. The offer for Technology#1 is inconsistent. The potential market can be built in the transport sector, based on buses (8-12/day) or locomotives (1-2/day) and complemented by passenger cars (16-22/day). Hydrogen surplus can be disposed into the natural gas network at a city gate station, with target concentration <2%. Alternatively, it is possible to install a fuel cell unit for on-site electricity generation of 1780 MWh/a sold to the public grid. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

The ongoing energy transition comprises the transformation of space heating, by increasing the share of district heating, where heat production is based either on renewables, or on waste heat recovery. The existing transmission infrastructure of natural gas, being a major source of waste heat, is also awaiting its transformation by adapting to hydrogen -rich gas or pure hydrogen. The paper quantifies the available waste heat for 4 compressor stations, and then estimates the potential for future, hydrogen -based scenarios. The registered industrial data was subject to a thermodynamic model to estimate time -dependent waste heat production, simultaneously, local demand for heating was estimated. It was found that waste heat can cover up to 80% of the in-house demand, and 51%-77% of the space -heating demand in nearby municipalities. Further, it was found that adding 10% hydrogen to the existing network increases the WH generation by 20%, while replacing the existing system with a 100% H 2 transportation infrastructure multiplies the available WH from 6 to 12 times per unit of the transported energy, In the pure hydrogen scenario, the compression waste heat corresponds to about 4%-7% of the national -scale demand for space heating in case of Poland, providing a significant contribution to the ongoing decarbonization.

The paper presents design considerations for experimental investigation of the Ranque-Hilsch vortex tube effect (VT).The research aims to find physical mechanisms governing the thermal and mass separation of non-homogenous gases entering the VT.Also, an approach to study phase-change phenomena inside the VT will be undertaken to evaluate the applicability of the VT in refrigeration systems.For this purpose, it is essential to have a mutually validated experimental setup and numerical model.The 1st generation test rig was based on a VT of 36 mm inner diameter and up to 1.62 m length.With inlet air supply at 3 bar abs, it was possible to achieve outlet temperature values from 0.1 C at the cold side to 42.3 C at the hot side, and the maximum air flow was 123 kg/h.The 2nd generation should be reduced in size for two reasons.First, lighter and heavier gases (helium, carbon dioxide) will be supplied from pressurized cylinders and mixed with air or nitrogen.The flow rate should be minimized to enable a continuous operation from the pressurized cylinders.Second, to apply higher inlet pressure, it is required to reduce the flow channels to maintain the gas flow below the technical limit of about 60...100 kg/h.The proposed test rig configuration is based on two supply options.In the first option, pure gases can be supplied from batteries (bundles) of pressurized cylinders, and then mixed to achieve the desired composition.The second option can be obtained by connecting the test installation to an air compressor, which is more suitable for test operation or for multiple-parameter measurements required to obtain boundary conditions for numerical modelling.Moreover, the test rig will be extended to include a multi-phase flow unit with a liquid dosing and separation equipment.

Wydawnictwo SIGMA-NOT wydaje czasopisma fachowe informujące swoich czytelników o najnowszych osiągnięciach naukowych i nowoczesnych rozwiązaniach technicznych w Polsce i na świecie, popularyzuje problemy techniczne oraz poszerza wiedzę i kulturę techniczną.

This paper presents the possibility of energy storage in natural gas transmission networks using two strategies. Proof-of-concept calculations were performed under a steady-state assumption, and the more promising option was additionally modeled in a transient approach. The first strategy is based on a dedicated compressor–expander system installed at two ends of a pipeline. An electric-driven compressor increases the gas pressure in periods of peak electricity generation, while a gas expander allows energy recovery at a later stage. The compressor–expander distance determined by the inlet flow velocity of 5 m/s and a 4–5 h time shift ranges from approx. 75 to 120 km. The system provides a synergy effect, which allows to exceed 100% storage efficiency by reducing transmission losses. Storage efficiency obtained from the simplified model ranges from 70% to 128% for the performed case study. The second option uses existing compressors and pressure letdown stations expanding the gas to the distribution pressure. Here, gas pre-heating required prior to the expansion reduces the storage efficiency to about 30–40%. The dedicated machinery option was also evaluated using a transient model, which reports a lower efficiency if applied to the same assumptions. The system redesigned with the transient model is characterized by a longer storage duration (about 12 h) and a promising efficiency of 103.5%. Further research is needed to find the optimum design system parameters and to solve the detected problem of simultaneous compressor–expander operation which introduces idle work to the designed system.

Waste heat plays a significant role in obtaining the 4th and 5th generation of District Heating (DH) System in cities. This article presents the possibilities of integrating selected waste heat emitters into DH, with the objective of meeting the demand for heat for the selected residential area (approx. 4000 inhabitants) in the city of Gliwice (180 000 inhabitants). The total heating demand of the studied area was estimated at 19 800 GJ including both space heating and domestic hot water. The maximum thermal power was estimated at approx. 2.45 MW. The demand was calculated on the basis of registered metering values for individual buildings which were processed and summarized due to the lack of collective meters for the district. A detailed data classification, correction and completion procedure was elaborated to deal with non-uniform and low-quality data registration. Two industrial objects with waste heat generation were examined to be integrated with the local DH network. The waste heat generation potential equals 9.0 MW for plant #1 and 0.9 MW for plant #2. Apart from the constant generation declared by the industrial entities, realistic profiles including possible shaft-work and maintenance periods were created. It has been shown that the total heat demand for selected residential areas can be covered by integrating waste heat into the current DH network. Depending on the waste heat generation profile, the local area heat demand can be covered entirely or to a large degree (coverage factor ranges from 72 to 100%). The waste heat utilization factor ranges from 6.3 to 8.3%. To manage the remaining waste heat potential, it is required to build additional district heating pipelines and nodes connecting to the existing network to receive an additional 7.45 MW thermal power. The potential of waste heat recovery is significant at the scale of a medium sized city: integrating two large industrial emitters allows up to 13% decarbonization of heats production in the local district heating plant.

The treatment of growing production of municipal sewage sludge has become a significant global problem. Drying of digested sewage sludge is a promising alternative to sludge disposal at dumping sites. The research objective of this study was to find the optimal heat source for a sludge drying plant in a large municipal sewage treatment plant (people equivalent: 250,000). Two boundary heat supply cases were analyzed in the paper: cogeneration of heat and power (CHP) units, internal combustion (IC) engines fired with natural gas, and plant supplied with a gas boiler. The aim of the research was to find the optimal size of the cogeneration unit cooperating with the gas boiler as heat sources for a given drying plant case with the maximum net present (NPV) value as the objective function. The results of the conducted optimization show higher profitability of cases with larger cogeneration unit. For the basic assumptions, the maximum NPV is obtained for the largest analyzed CHP unit: 1300 kW of thermal power output. Sensitivity analyses show that the varying gas and electricity prices can relocate the NPV maximum towards smaller CHP sizes. A supplementary energy analysis shows that implementing larger CHP units yields a higher energy efficiency of the system, up to 0.52.

This article concerns natural gas liquefaction using high-pressure potential, which is available in pressure letdown stations. The article proposes an integration of a pressure letdown station with a natural gas liquefaction line which enables partial exergy recovery and decreasing natural resources consumption to produce LNG. Exergy recovery is carried out by a replacement of the pressure reduction valve by a turboexpander, which can recover exergy from high-pressure natural gas flowing through a pressure reduction stage. The recovered energy may be used to drive a natural gas liquefaction unit, coupled with the reduction station. A case study concerning an existing pressure letdown station includes two chosen model of a turboexpanders with low and high internal efficiency and several natural gas liquefaction units possible to integrate. The varying size of the liquefaction unit corresponds to a different degree of utilization of energy generated in the expander. Turboexpander produces electric power supplying the liquefaction unit, however, it requires the use of additional energy to heat the gas before the reduction stage, which increases the thermoecologic cost of natural gas transferred to distribution network. Energy, exergy and thermoecological cost analysis was carried out for three system design configurations and for six sizes of the liquefaction line with different turboexpander efficiency. The first configuration included a basic configuration with the pressure letdown station and the liquefaction unit, the second configuration also included an integration with a waste heat source (ICE exhaust gases), and the third configuration used multi-stage expansion. Energy efficiency of the integrated expansion-liquefaction system varies from 35.40% to 66.64%, while its exergy efficiency ranges from 15.75% to 46.33% depending on the size of the liquefaction unit and the gas liquefaction method. It was proved that it is possible to reduce thermo-ecological cost of LNG by 8.2%. Reducing raw material consumption needed for LNG production increases natural gas thermo-ecological cost by only 1%. A preliminary economic analysis based on prices of energy carriers was done. It was found that the boundary price was estimated at 0.0246 €/kWh for one of chosen systems.

This special issue of Journal of Energy Resources Technology presents selected scientific output of the 5th Conference on Contemporary Problems of Thermal Engineering CPOTE 2018, which was held on Sept. 18–21, 2018, in Gliwice, Upper Silesia, Poland.Since its first edition in 1998, the CPOTE Conference has become a cyclic event gathering the most recognized scientists in the areas of thermodynamics, energy and environmental engineering. Compared to other conferences dedicated to thermodynamics and energy, CPOTE focuses on exergy analysis to a larger extent. Instead of convincing nonspecialists that exergy does represent a useful tool for evaluation of thermodynamic quality, the CPOTE Conference provides a dedicated space for advance discussion among the World’s leading experts on exergy.Most editions of CPOTE were given a special dedication. The first edition (1998) was dedicated to the 50th jubilee of Professor Jan Szargut’s scientific activity. The second edition (2004) celebrated centenary traditions of the Institute of Thermal Technology reaching back to its origins in Technical University of Lwów (now Lviv, Ukraine). The third edition (2012) was devoted to the jubilee of Professor Andrzej Ziębik’s 50 years academic activity. The fourth CPOTE Conference (2016) was given a special topic, Sustainable Energy Systems for Our Common Future, and this is also maintained for the fifth and the upcoming issues.The recent fifth Conference held in 2018 in Gliwice was a special one, devoted to the memory of Professor Jan Szargut (1927–2017), one of the founders of modern exergy analysis. His life, first of all, reflects the complex history of the 20th century: he was born in 1927 in the formerly Polish city of Lwów (now Lviv, Ukraine) and was then resettled in the formerly German city of Gleiwitz (since 1945 Gliwice, Poland). Although Poland was his fatherland, his actual dwelling was the University with the scientific field of thermodynamics. The field was more than fertile for Professor Szargut, who documented his research with more than 290 scientific articles and 21 books published in English, German, Russian, and Polish.Selected papers presented during CPOTE 2018 Conference were invited to ASME Journal of Energy Resources Technology special issue dedicated to the Conference. We hope the readers will find this special issue a valuable read.

The paper discusses the problem of reference temperature for exergy calculation using an example of a combined cold, heat and power unit integrated with biomass gasification. Depending on the ambient temperature and the demand for heating and cooling, several modes of operation are considered. The performance of the system is evaluated by exergy analysis for various reference temperatures. First, the annual average temperature of 8.5°C is applied. Then, monthly variation of the reference temperature between −0.5 and 18.6°C (Katowice, Poland) is considered. It has been shown that a constant reference temperature yields misleading results with numerical error between 5% for high-temperature media to almost 90% for cooling agents, in particular for cooling agents, where the useless return water has apparently more exergy than the cold ice water. In conclusion, a variable reference temperature is recommended for exergy analysis of systems including cooling or low-temperature heating agents.

Shale gas and tight gas exploration and extraction processes create potential threats to the environment. In Poland, no comprehensive guidelines for environmental risk assessment have been prepared so far. This paper presents a proposal of environmental risk assessment methodology which can be used for corporate risk management procedures during exploration and extraction of unconventional hydrocarbons in Poland. The most frequent environmental threats that may occur during the exploration and exploitation of unconventional hydrocarbon deposits include degradation of soils through construction of drilling rigs and access roads, landforms change, local soil pollution caused by fuels, cleaning agents and materials used to prepare drilling fluids, rubble, cement, gravel, pollution of surface and underground water as a result of emergency discharges of sewage, infiltration of pollution from waste reservoirs, disturbance of hydrogeological equilibrium through significant water intake, noise and atmospheric pollution resulting from the combustion of fuels. To check the level of these threats' six exploration sites form Pomeranian and Carpathian region of Poland (3 wells of shale gas and 3 wells of tight gas) have been evaluated in detail, and the risk quantification has been made. Because of a local, short-term and reversible environment impact, the environmental risks for the exploration and extraction processes of unconventional hydrocarbons have been found to be medium or negligibly small. It is recommended that using the same methodology for other regions of Poland where we can find unconventional hydrocarbons and it can be enriched in dedicated application with spatial maps to give the investors a quick feedback on the potential environmental risks. Key words: environmental risk assessment, sustainable development, shale gas, tight gas drilling rig, environment pollution.