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Za zniževanje izpustov škodljivih onesnažil se v procesih zgorevanja pogosto vpeljuje visoke deleže recirkuliranih izpušnih plinov. Sestavljeni so iz ogljikovega dioksida, vode, dušika ter posameznih komponent v sledeh, ki zajemajo tudi dušikov oksid (NO). Temeljne raziskave so razkrile, da NO vpliva na kinetiko zgorevanja in s tem na časovno in prostorsko porazdelitev sproščanja toplote, kar potencialno vpliva na proces zgorevanja in tudi nadaljnjo tvorbo izpustov onesnažil. Za prvo aplikativno potrditev teh hipotez naloga analizira proces zgorevanja zemeljskega plina v motorju z notranjim zgorevanjem in prisilnim vžigom ob različnih koncentracijah NO, ki so dozirane v vstopni zrak. Na podlagi poteka tlaka v valju, sproščanja toplote, temperatur ter izpustov onesnažil je v posameznih obratovalnih intervalih mogoče zaznati vpliv že izrazito nizkih koncentracij NO. Učinek NO na zgorevanje je močno odvisen od temperature in sestave gorljive zmesi. Največja občutljivost je opažena pri temperaturah med 1600 in 1900°C ter revnih zmeseh. Pri višjih dodanih koncentracijah NO deluje kot promotor vžiga, saj pospeši tvorbo reaktivnih OH in O radikalov. NO spreminja tudi kinetiko tvorbe izpustov škodljivih snovi, saj rezultati potrjujejo znaten vpliv na dodatno tvorbo NO predvsem v območjih nizkih temperatur. Naloga na ta način predstavlja prvo aplikativno potrditev temeljnih hipotez in predstavlja izhodiščno delo za nadaljnje študije v realističnih procesih zgorevanja. High proportions of recirculated exhaust gases are often introduced in combustion processes to reduce emissions of harmful pollutants. They consist of carbon dioxide, water, nitrogen and individual components in traces, which also include nitric oxide (NO). Fundamental research has revealed that NO has a potential effect on combustion kinetics and thus on the temporal and spatial distribution of heat release, which potentially affects the combustion process as well as the further formation of pollutant emissions. For the first applied confirmation of these hypotheses, this thesis analyzes the process of natural gas combustion in an internal combustion engine and forced ignition at different concentrations of NO doped into the intake air. Based on the pressure trace in the cylinder, heat release, temperature and pollutant emissions, the influence of already extremely low doped NO concentrations can be detected in individual operating intervals. The effect of NO on combustion is strongly dependent on the temperature and composition of the combustible mixture. The highest sensitivity is observed at temperatures between 1600 and 1900 ° C and lean mixtures. At higher added concentrations, NO acts as an ignition promoter as it accelerates the formation of reactive OH and O radicals. NO also changes the kinetics of the formation of emissions of harmful substances, as the results confirm a significant impact on the additional formation of NO, especially in areas of low temperatures. The thesis represents the first applied confirmation of the basic hypotheses and represents initial work for further studies in realistic combustion processes.

The paper presents a first comparative analysis of emission formation phenomena of three different bioliquids, derived from low-cost waste streams while utilizing the same gas turbine-based experimental setup. A consistent and unbiased comparison is ensured by the application of the same experimental test rig featuring only those minor fuel-based adaptations, which are required to ensure the most favorable operation of each of the analyzed fuels. This provides the direct comparative data between combustion performance of liquefied wood, obtained through solvolysis process, glycerol, and waste liquor from nanocellulose production which were previously tested in various combustion systems, hence making a direct evaluation of fuel's suitability difficult. The study focuses on the analysis of all key thermodynamic parameters and significant emission species covering CO, NOx, HC particulate matter, and soot as well as identification of underlying phenomena for observed emission trends. These indicate that for NOx emissions, a good correlation exists to the stoichiometric ratio of the fuels, where a low stoichiometric ratio results in lower NOx emissions, provided that oxygen content is the main diluent and fuel-bound nitrogen is low. As all tested fuels feature oxygen content above 43%, this enables a large improvement in NOx-CO trade off, as CO emissions are reduced with higher peak combustion temperatures while minimally increasing NOx emissions. Similar observations are made for particulate matter–NOx trade off; however, the ash content significantly impacts the particulate matter emission, hence reducing the potential for clean combustion of waste liquor. In the case of glycerol with no ash content, soot emissions are minimal and for an order of magnitude lower than for benchmark diesel fuel, as there are numerous phenomena effectively reducing their formation and increasing their oxidation. The presented research confirms that utilization of bio-intermediates and waste-derived fuels in appropriate combustion setups can, beside a low CO2 footprint, feature also very low emissions of other pollutant species, providing that fuels feature high oxygen content, low ash content, and low nitrogen content. With such approach, it is possible to achieve clean combustion that is fully in line with circular economy guidelines.

Increased public concerns and stricter regulatory frameworks promote the role of bioliquids (liquid fuel for energy purposes other than for transport, including electricity and heating and cooling, produced from biomass). This is a driving force for development and employment of micro-gas turbines (MGTs) due to their ability to combust bioliquids with less favorable properties in a decentralized manner. Gas turbines are characterized by relatively high combustion efficiency at relatively low concentrations of harmful emissions, relatively high effective efficiency and durability when utilizing a common portfolio of gas turbine approved fuels. It is thus desired to preserve these advantages of gas turbines also while burning bioliquids and further relying on their scalability that is crucial to efficient support of decentralized energy production at small scales. To support these objectives, MGT technology needs to allow for utilization of bioliquids with much wider spectrum of physical and chemical properties compared to common commercially available MGTs in a single MGT-based plant. In this view, the present study is providing the first thorough overview of challenges and solutions encountered by researchers across the wide area of bioliquids in MGTs.

This study investigates for the first time the combustion in a micro gas turbine (MGT) of a new bioliquid, a viscous biocrude, which is a liquefied wood (LW) produced via solvolysis of lignocellulosic biomass in acidified glycols. The test rig includes a modified fuel injection line, a re-designed combustion chamber and revised fuel injection positions. The main novelties of this work are: (1) producing of liquefied wood with pure ethylene glycol as a solvent, and methanesulfonic acid as a catalyst, to obtain a bio-crude with lower viscosity and higher lignocellulosics content than previous tested formulations; (2) upgrading raw liquefied wood by blending it with ethanol to further reduce the viscosity of the mixture; (3) utilizing a commercially available MGT Auxiliary Power Unit (APU) of 25 kW electrical power output, with notably reduced extent of adaptations to use the newly obtained fuel mixture. Fuel properties, and their impact on combustion performance using liquefied wood, are investigated by analyzing MGT performance and emissions response at different load and blend ratios. Emissions revealed that the presence of LW in the blends significantly affects CO and NOX concentrations compared to conventional fuels. CO roughly increased from 600 ppm (pure ethanol as fuel) to 1500 ppm (at 20 kW electrical power). The experimental study reveals that it is possible to achieve efficient MGT operation while utilizing high biocrude to ethanol ratios, but a number of adaptations are necessary. The achieved maximum share of liquefied wood in the fuel blend is 47.2% at 25 kW power output. Main barriers to the use of higher share of liquefied wood in these type of systems are also summarized.