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The presented paper focuses on the combustion analysis of a highly oxygenated, viscous and economically viable fuel in an internal combustion micro gas turbine. Results suggest that environmental benefits in terms of NOx emissions and PM emissions are significant as concentrations of both species are reduced. Based on the analysis presented in the study, altered local air-fuel ratio suppresses NOx formation and PM formation resulting in 10-fold lower PM emission than same experimental apparatus with diesel fuel. The study further analyses and identifies three key mechanisms that are responsible for this reduction: altered oxygen profile during mixture preparation, prevention of soot precursors formation and promotion of soot oxidation reactions. The influence of tested fuel is also perceivable through higher CO and THC emissions. Parametric analysis within the selected experimental space suggests that operational parameters of the micro gas turbine can be adjusted to reduce concentrations of these two species without negatively affecting other pollutants. Significant benefits of the fuel in terms of environmental impact indicate that further investigation and development of this approach might provide a new alternative energy source for stationary use as well as for mobility in sectors where heavy duty gas turbines are used as prime movers.

Commonly applied 3D CFD models lack suitable combustion description of fuels with complex composition, such as encountered in wastewater sludge specific applications. This significantly influences their prediction capability, in particular when used in small, confined combustion volumes. To tackle this challenge, it is necessary to introduce detailed chemical kinetic models for specifically tailored fuel surrogates, which ensure physically accurate description of local thermodynamic conditions and heat release rates. However, necessity to increase the level of detail of models needs to be balanced by their computational expenses to preserve their applicability when solving real engineering problems. With an aim to fill this gap, the present study significantly extends the existing surrogate model methods for applications in small-scale systems to include also reduced combustion kinetic mechanisms. The innovative extension applies Simulation Error Minimization Connectivity Method and involves tailoring of kinetic mechanisms to variable thermodynamic conditions and variable surrogate compositions, specific to sewage sludge combustion. Suitability of the proposed approach is confirmed with the 3-D CFD simulations maintaining similar level of accuracy within the design space of reduced mechanisms as well as in off-design conditions, while maintaining sufficient flexibility to adapt to different types and compositions of the sludge. Thereby, reduction of the in-model applied detailed ethanol and propene mechanisms from 47 and 71 species down to 33 and 34 species, respectively, was demonstrated together with linearly dependent decrease in computational time. The proposed model extension and resulting surrogate combustion model thus for the first time offer an efficient tool for affordable and accurate virtual design of small-scale combustion systems using fuels with complex chemical composition.

Conventional combustion has been exploited for power generation purposes since ancient times. Considering all relevant analyses of primary energy consumption, it is obvious that worldwide it will still play a major role in heat generation and will only slowly be replaced in generation of electricity. Despite unpreceded breakthroughs, growing capacity and large success of environmentally more acceptable technologies in terms of global warming potential as well as their growing share in the primary energy consumption, relevant analyses do not predict a significant midterm decrease of combustion-generated energy (International Energy Agency, 2019). In this view, the main scientific focus should be the development of clean combustion approaches which are, emission wise, on par or even supersede currently available renewable energy technologies.

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.

The combustion of a viscous biofuel, liquefied wood (LW) produced via solvolysis of lignocellulosic biomass in acidified glycols, has been studied in a small gas turbine rig. The test rig includes a modified injection line which is compatible with acidic, viscous biofuels allowing fuel preheating and two pilot injectors, and a re-designed combustion chamber. The link between fuel properties and combustion performance of liquefied wood is investigated by burning the biofuel at different blending ratios with ethanol. Exhaust emissions have been compared to reference measurements with diesel fuel and ethanol. Combustion analysis is supported by the investigation of the engine operating parameters and the main emission species at different electrical loads. The experimental study reveals that it is possible to establish efficient operation of the micro gas turbine while utilizing liquefied wood-ethanol blends with high share of liquefied wood.

Power generation units based on the bio-syngas system face two main challenges due to (i) the possible temporary shortage of primary sources and (ii) the engine power derating associated with the use of low-energy density fuels in combustion engines. In both cases, an external input fuel is provided. Hence, complementing syngas with traditional fuels, like natural gas, becomes a necessity. In this work, an experimental methodology is proposed, aiming at the quantification of the impact of the use of both natural gas and syngas in spark ignition (SI) engines on performance and emissions. The main research questions focus on investigating brake thermal efficiency (BTE), power derating, and pollutant emission (NOx, CO, THC, CO2) formation, offering quantitative findings that present the basis for engine optimization procedures. Experimental measurements were performed on a Toyota 4Y-E engine (a 4-cylinders, 4-stroke spark ignition engine) at partial load (10 kW) under different syngas energy shares (SES) and at four different spark ignition timings (10°, 25°, 35° and 45° BTDC). Results reveal that the impact of the different fuel mixtures on BTE is negligible if compared to the influence of spark advance variation on BTE. On the other hand, power derating has proven to be a limiting factor and becomes more prominent with increasing SES. An increasing SES also resulted in an increase of CO and CO2 emissions, while NOx and THC emissions decreased with increasing SES.

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.

A pure tire pyrolysis oil produced from waste tires was utilized in a modern 4-cylinder, turbocharged and intercooled, automotive Diesel engine. Due to its low cetane number, cetane improvers, external energy addition or increased compression ratios are generally required for its use in Diesel engines. Successful utilization of pure tire pyrolysis oil is also achievable with the addition of pilot injection but limited to mid- to high-load operating range. The first objective of the present study is therefore focused on further extension of the operating range towards lower loads by novel combined application of the exhaust gas recirculation and tailored main injection strategy. As the second objective, the article provides for the first time an in-depth analysis of the particulate emissions of the tire pyrolysis oil measured with two different methods. In this area it identifies and reasons challenges related to determination of the particulate emissions for alternative fuels. The original contribution of the presented approach thus arises from the holistic assessment of interactions between the exhaust gas recirculation ratios, injection parameters and combustion as well as gaseous and particulate emissions formation phenomena.

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.

Finding alternative value-added usage for glycerol is imperative as existing uses are inadequate due to the vast excesses of glycerol generated mainly as a result of increased biodiesel production. This paper explores a mid-term, cost-efficient, technically viable utilisation in power generation applications and exposes the nature of its combustion. Blended with methanol to avoid the heat loss and fuel coking problems associated with preheating, glycerol was combusted in a model swirl-stabilised gas turbine burner utilising a standard pressure-swirl injector for fuel atomisation. Stable flames resulting from the tested blends – 70/30 and 50/50 combinations of methanol/glycerol by volume – were achieved over an equivalence ratio (ER) range between 0.29 and 0.51. The upper and lower limits of stable operating ER for the 70/30 case were accompanied by significant flame lift-off from the nozzle exit orifice – a phenomenon that was not pronounced in the 50/50 case. Also, the temporal variation of CH* species integral intensity suggested a reduction in the fluctuation of heat release rate, hence better flame stability, as ER increased for both blends. Overall, the 50/50 case showed greater flame stability compared with the 70/30 case based on the CH* chemiluminescence data and spectral analysis by means of fast Fourier transform.

Although various modelling approaches exist for the simulation of solid fuel combustion, no specific model has been developed for the accurate description of gas-phase combustion in small-scale combustion devices. This is particularly limiting in scenarios when volatile-rich, complex and incompletely described solid fuels such as sewage sludge are used. To address this issue, an accurate description of combustion from the fuel bed onwards is required as well as an improved description of emitted volatiles. This paper introduces an innovative surrogate-based combustion model that combines data on sludge devolatilisation and measured combustion characteristics to offer a new surrogate composition. The composition includes heavy hydrocarbon species to accurately describe combustion evolution. A sensitivity analysis revealed that H2 contributes significantly to combustion evolution, while the most robust surrogate composition is obtained when ethanol is used as a leading representative of heavier hydrocarbons. The model can be used to produce suitable surrogates for the main sludge combustion interval, offering the required improvement in fuel descriptions and accuracy of simulations in the vicinity of the fuel bed. Hence, this model is particularly suitable for the optimisation of temperature, heat release rate, and concentration field in combustion chambers with limited volumes.

To further expand the knowledge base on the use of innovative fuels in the micro gas turbines, this paper provides insight into interrelation between specific fuel properties and their impact on combustion and emission formation phenomena in micro gas turbines for stationary power generation as well as their impact on material corrosion and deposit formation. The objective of this study is to identify potential issues that can be related to specific fuel properties and to propose counter measures for achieving stable, durable, efficient and low emission operation of the micro gas turbine while utilizing advanced/innovative fuels. This is done by coupling combustion and emission formation analyses to analyses of material degradation and degradation of component functionality while interpreting them through fuel-specific properties. To ensure sufficiently broad range of fuel properties to demonstrate the applicability of the method, two different fuels with significantly different properties are analysed, i.e. tire pyrolysis oil and liquefied wood. It is shown that extent of required micro gas turbine adaptations strongly correlates with deviations of the fuel properties from those of the baseline fuel. Through the study, these adaptations are supported by in-depth analyses of impacts of fuel properties on different components, parameters and subsystems and their quantification. This holistic approach is further used to propose methodologies and innovative approaches for constraining a design space of micro gas turbine to successfully utilize wide spectra of alternative/innovative fuels.

Presented paper deals with adaptation procedure of a microturbine (MGT) for exploitation of refuse derived fuels (RDF). RDF often possess significantly different properties than conventional fuels and usually require at least some adaptations of internal combustion systems to obtain full functionality. With the methodology, developed in the paper it is possible to evaluate the extent of required adaptations by performing a thorough analysis of fuel combustion properties in a dedicated experimental rig suitable for testing of wide-variety of waste and biomass derived fuels. In the first part key turbine components are analyzed followed by cause and effect analysis of interaction between different fuel properties and design parameters of the components. The data are then used to build a dedicated test system where two fuels with diametric physical and chemical properties are tested – liquefied biomass waste (LW) and waste tire pyrolysis oil (TPO). The analysis suggests that exploitation of LW requires higher complexity of target MGT system as stable combustion can be achieved only with regenerative thermodynamic cycle, high fuel preheat temperatures and optimized fuel injection nozzle. Contrary, TPO requires less complex MGT design and sufficient operational stability is achieved already with simple cycle MGT and conventional fuel system. The presented approach of testing can significantly reduce the extent and cost of required adaptations of commercial system as pre-selection procedure of suitable MGT is done in developed test system. The obtained data can at the same time serve as an input for fine-tuning the processes for RDF production.

To achieve a near zero emission footprint of combustion in power generation, introduction of fuels with low global warming potential, namely low-carbon or carbon neutral fuels and simultaneous reduction of harmful emissions through implementation of advanced combustion concepts is necessary. The study addresses this challenge experimentally by proposing a new approach which combines the benefits of highly oxygenated waste derived fuels, here represented by glycerol, and an introduction of external exhaust gasses recirculation (EGR) aimed for further reduction of NOx emissions. Thus, the recognized role of the high oxygen content in glycerol can positively influence the well-known penalties of EGR, which are commonly perceivable through elevated CO and soot emissions. The measurements were performed with an experimental gas turbine equipped with an exhaust heat regeneration system and feedback loop for 8% and 13% EGR content in compressor intake air. The proposed system layout represents a technically viable and cost-efficient approach for upgrading existent gas turbine setups with a goal to improve their emission footprint. Results confirm that with 8% and 13% EGR rate, NO [sub] x, CO and soot can be reduced simultaneously, thus improving the CO- NO [sub] x and soot- NO [sub] x trade off approximately 2-fold for each species. Additionally, underlying phenomena responsible for observed improvements while increasing EGR rate are identified as an increased soot reactivity, a competing effect of EGR related dilution and an increased primary air temperature together with spray related parameters linked to low stoichiometric ratio of glycerol.

The current work investigates the impact of a paraffinic fuel on combustion and emissions of a diesel engine, examining alternative injection strategies for the full exploitation of the fuel characteristics. The paraffinic fuel used was the HVO (Hydrotreated Vegetable Oil) produced by Neste Oil with the brand name NEXBTL. The study was conducted on a light-duty turbocharged and aftercooled common-rail diesel engine, with both HVO and a conventional diesel fuel. Four steady-state operating points were examined, at low and medium engine speeds (1500 and 3000 rpm) and loads (35 and 100 Nm), typical of daily driving of a passenger car. The key contribution of the study is a comprehensive analysis of the phenomena influencing the crank angle resolved in-cylinder parameters, as well as interlinking the effects of different variations of injection pressure (default and 300 bar higher), pilot injection timing (default and ± 5°CA) and main injection timing (default and ± 2°CA) on gaseous emissions and particulate matter (PM). The findings have shown that at default engine settings the use of HVO results in up to 40% reduction of engine-out PM and HC emissions without appreciable changes in NOx emissions. The significant reduction of engine-out PM levels, facilitates the adoption of measures for NOx emissions limitation. The latter are reduced by up to 20% when the main injection timing is retarded (by 2° CA in the present study), while PM emissions are still kept well below the respective diesel fuel levels.

The paper focuses on the implementation of a comprehensive and robust optimization procedure for a synthesis gas fired four-cylinder, spark ignited, 2.2 L industrial engine used in combined heat and power applications. Innovatively designed workflow is for the first time incorporating also a thorough operational stability analysis for evaluation of the engine operation durability while using off-design fuels. Design constraints of the engine operational space are set after in depth investigation of knock phenomena, cycle to cycle variations, emission formation phenomena and engine performance parameters. These are derived from experimental data, obtained from the engine, equipped with newly designed components. Throughout the paper, results obtained with synthesis gas are benchmarked to natural gas. With significant emphasis laid on analysis of lean operation conditions, as a measure to reduce environmental footprint of energy generation, a newly proposed optimum operation points reveal a possibility to obtain TA-Luft and EPA emission limits already with stoichiometric mixture. This allows to achieve a remarkably low power de-rating factor of only 16.5% and omission of any aftertreatment system. Therefore, findings of this study represent a significant improvement of current control strategies and enable further increase in specific power and thus economic attractiveness of distributed power generation techniques at enhanced durability while using low-carbon and renewable fuels.

The work presented here characterises the influences of different waste-derived fuels on the combustion process in a microturbine. The two most common types of solid waste are converted into liquids by liquefaction in polyhydric alcohols and by pyrolysis to produce the novel fuels LW (liquefied wood) and TPO (tire pyrolysis oil). Baseline results were obtained with diesel fuel and with raw polyhydric alcohols otherwise used in wood liquefaction process. The original contribution of this analysis is the in-depth comparison of the combustion and emission phenomena of LW and TPO under different operating conditions. The results revealed significantly higher CO (carbon monoxide) and THC (total hydrocarbons) emissions for LW due to its reduced atomisation ability and increased spray penetration in line with the physical and chemical properties of the fuel. The combustion properties of TPO resembled those of diesel fuel in terms of CO and THC. NO x (nitrous oxides) emissions reflected the elemental composition of the fuels. In addition, pronounced soot formation is observed when utilizing TPO, whereas for LW, the opacity measurements are surprisingly low. These results indicate that TPO can be exploited directly in gas turbines, whereas LW requires adaptations in the fuel supply system and a narrower range of operating conditions.

The paper reports foremost the results of a successful combustion of an innovative lignocellulosic biofuel in a gas turbine. The fuel was processed through liquefaction of lignocellulosic materials with polyhydroxy alcohols in an acid catalyzed reaction. The liquefaction process features: high efficiency, high liquid yields and inexpensive, easily available process equipment. For the purpose of this analysis the following were developed: an experimental gas turbine with internal combustion chamber, a preheated pressurized fuel supply system with swirl-air fuel injector and a heat exchanger to obtain high primary air temperatures. The paper gives results on the emissions of CO, THC, NOx and soot. For the purpose of benchmarking the turbine was also run on diesel fuel. The paper presents analyses of the underlying phenomena with which it aims to provide guidelines for improvements in the fuel processing and in the experimental equipment. It has been shown that direct utilization of this innovative lignocellulosic biofuel gives promising results. Although the CO and THC emissions are higher compared to the benchmark diesel results it has been shown that both emissions decrease with increased turbine inlet temperature and with the increased fuel preheat temperature, due to a very high viscosity of the fuel. It is additionally shown that NOx emissions are low and comparable to those of the diesel fuel, whereas soot emissions are very low for both fuels.

After initial studies, further research work on the combustion properties of second generation biofuels, obtained through solvolysis in polyhydroxy alcohols is oriented towards different types of liquefied wood that exhibit several favorable properties. In this study, different types were obtained by altering the reactant ratios of the fuels. These were focused on increased wood content and elevated pH value that would increase the techno-economic attractiveness of the fuel. Three different types of fuels were tested in a laboratory scale gas turbine, and evaluated through CO, THC and NOsubx emissions measurements, while varying multiple operating parameters. To achieve sufficient atomization quality, the high viscosity of the fuels was reduced by preheating to 100 °C. To speed up the droplet evaporation process and additionally to resemble conditions present in commercially available systems, high temperatures of primary air were employed by the use of exhaust gas heat regenerator. CO and THC emissions were found to be highly dependent on wood content and turbine inlet temperature, whereas with partial neutralization of the fuel this dependency was less pronounced and only NOx concentrations were influenced by altered elemental composition of the fuel. Results indicate it is possible to maintain successful combustion in microturbines even with fuels that exhibit higher pH value and reduced reactivity and with fuels containing higher amounts of lignocellulosic biomass. V želji po nadomeščanju fosilnih goriv je v zadnjem času vedno več pozornosti usmerjene v biogoriva in goriva, proizvedena iz različnih vrst odpadkov. Zaradi svojih fizikalnih in kemičnih lastnosti sta na tem področju izpostavljena predvsem biodizel in etanol, ki ju je mogoče brez obsežnih adaptacij uporabiti v motorjih z notranjim zgorevanjem. Kot tipična predstavnika biogoriv prve generacije ju pesti predvsem njuna konkurenčnost s surovinami, namenjenimi pridelavi hrane. Ena izmed možnosti za izogibanje prehrambnim surovinam je izkoriščanje odpadne lignocelulozne biomase kot surovine za goriva motorjev z notranjim zgorevanjem.

Tire pyrolysis oil (TPO) represents a promising waste-derived fuel for Diesel engines with its main deficiency being lower cetane number compared to Diesel fuel. Until now, successful utilization of the TPO in Diesel engines was possible only by increasing its cetane number, increasing compression ratio of the engine or preheating intake air or operation. This study shows the foremost results of utilizing the pure TPO in a modern turbocharged and intercooled Diesel engine without any of the aforementioned aids, which significantly facilitates its use and boosts its conversion efficiency to mechanical work. This was achieved by the tailored injection strategy that includes pilot injection, which was previously not utilized in combination with the TPO. The study reveals that with additional tailoring of the pilot injection, further optimization of thermodynamic parameters can be achieved while operating the turbocharged and intercooled Diesel engine in a wide operating range under the use of pure TPO. Discovered phenomena are supported by interpretation of interactions between the injection parameters and combustion as well as emission formation phenomena of the pure TPO.