Superior fuel economy, higher torque and durability have led to the diesel engine being widely used in a variety of fields of application, such as road transport, agricultural vehicles, earth moving machines and marine propulsion, as well as fixed installations for electrical power generation. However, diesel engines are plagued by high emissions of nitrogen oxides (NO_x), particulate matter (PM) and carbon dioxide when conventional fuel is used. One possible solution is to use low-carbon gaseous fuel alongside diesel fuel by operating in a dual-fuel (DF) configuration, as this system provides a low implementation cost alternative for the improvement of combustion efficiency in the conventional diesel engine. An initial step in this direction involved the replacement of diesel fuel with natural gas. However, the consequent high levels of unburned hydrocarbons produced due to non-optimized engines led to a shift to carbon-free fuels, such as hydrogen. Hydrogen can be injected into the intake manifold, where it premixes with air, then the addition of a small amount of diesel fuel, auto-igniting easily, provides multiple ignition sources for the gas. To evaluate the efficiency and pollutant emissions in dual-fuel diesel-hydrogen combustion, a numerical CFD analysis was conducted and validated with the aid of experimental measurements on a research engine acquired at the test bench. The process of ignition of diesel fuel and flame propagation through a premixed air-hydrogen charge was represented the Autoignition-Induced Flame Propagation model included ANSYS-Forte software. Because of the inefficient operating conditions associated with the combustion, the methodology was significantly improved by evaluating the laminar flame speed as a function of pressure, temperature and equivalence ratio using Chemkin-Pro software. A numerical comparison was carried out among full hydrogen, full methane and different hydrogen-methane mixtures with the same energy input in each case. The use of full hydrogen was characterized by enhanced combustion, higher thermal efficiency and lower carbon emissions. However, the higher temperatures that occurred for hydrogen combustion led to higher NO_x emissions.

To become sustainable, the production of electricity has been oriented towards the adoption of local and renewable sources. Distributed electric and thermal energy generation is more suitable to avoid any possible waste, and the Micro Gas Turbine (MGT) can play a key role in this scenario. Due to the intrinsic properties and the high flexibility of operation of this energy conversion system, the exploitation of alternative fuels and the integration of the MGT itself with other energy conversion systems (solar field, ORC, fuel cells) represent one of the most effective strategies to achieve higher conversion efficiencies and to reduce emissions from power systems. The present work aims to review the results obtained by the researchers in the last years. The different technologies are analyzed in detail, both separately and under a more complete view, considering two or more solutions embedded in micro-grid configurations.

The authors discuss in this paper the potential of a micro gas turbine (MGT) combustor when operated under unconventional\ud conditions, in terms of variation in the fuel supplied. The authors' expertise in the field of micro-combustor has addressed, in\ud recent years, some topics of current interest, as the fuelling with gaseous and liquid biofuels and the NOx reduction through the\ud optimization of the combustor.While the previous authors' works were mainly referring to a tubular combustor of a 100kW MGT,\ud the present proposal deals with an annular, reverse flow combustor of a 30 kW MGT. In this case the particular location of both\ud the injectors and of the secondary and diluting holes allows the combustor process to develop under nearly RQL conditions. This\ud is of special interest when supplying the combustion chamber with low LHV fuels. In addition, recent authors' papers have\ud demonstrated that the integration of the MGT with a solar field leads the combustor to require a decreased fuel/air equivalence\ud ratio because of the higher air entry temperature. Under these aspects, the existence of a Rich region within the combustor may\ud be helpful for the early phases of the oxidation process. The pollutant formation rates should be effectively controlled by the\ud secondary (Quick-mix) and diluting (Lean) air flows.The authors' methodology relies on an advanced CFD approach that makes\ud use of a reaction scheme coupled with an accurate study of the turbulent interaction of the reacting species. Extended kinetic\ud mechanisms are also included in the combustion model. A preliminary set-up of the model will be based on the combustion\ud analysis with boundary conditions provided by thermodynamic analysis of the micro-turbine.\ud Several computational examples are discussed, namely:\ud - The analysis of the combustor response with reduced equivalence ratios or changes in the inlet air conditions;\ud -The comparison of combustion efficiency and pollutant production with different fuels

Compression ignition engines will still be predominant in the naval sector: their high efficiency, high torque, and heavy weight perfectly suit the demands and architecture of ships. Nevertheless, recent emission legislations impose limitations to the pollutant emissions levels in this sector as well. In addition to post-treatment systems, it is necessary to reduce some pollutant species, and, therefore, the study of combustion strategies and new fuels can represent valid paths for limiting environmental harmful emissions such as CO₂. The use of methane in dual fuel mode has already been implemented on existent vessels, but the progressive decarbonization will lead to the utilization of carbon-neutral or carbon-free fuels such as, in the last case, hydrogen. Thanks to its high reactivity nature, it can be helpful in the reduction of exhaust CH₄. On the contrary, together with the high temperatures achieved by its oxidation, hydrogen could cause uncontrolled ignition of the premixed charge and high emissions of NO_x. As a matter of fact, a source of ignition is still necessary to have better control on the whole combustion development. To this end, an optimal and specific injection strategy can help to overcome all the before-mentioned issues. In this study, three-dimensional numerical simulations have been performed with the ANSYS Forte^® software (version 19.2) in an 8.8 L dual fuel engine cylinder supplied with methane, hydrogen, or hydrogen–methane blends with reference to experimental tests from the literature. A new kinetic mechanism has been used for the description of diesel fuel surrogate oxidation with a set of reactions specifically addressed for the low temperatures together with the GRIMECH 3.0 for CH₄ and H₂. This kinetics scheme allowed for the adequate reproduction of the ignition timing for the various mixtures used. Preliminary calculations with a one-dimensional commercial code were performed to retrieve the initial conditions of CFD calculations in the cylinder. The used approach demonstrated to be quite a reliable tool to predict the performance of a marine engine working under dual fuel mode with hydrogen-based blends at medium load. As a result, the system modelling shows that using hydrogen as fuel in the engine can achieve the same performance as diesel/natural gas, but when hydrogen totally replaces methane, CO₂ is decreased up to 54% at the expense of the increase of about 76% of NO_x emissions.

The authors discuss in this paper the potential of two power plant concepts for distributed generation, based on the integration of a cogenerating micro gas turbine with a solar panel array. The first one relies on the adoption of a parabolic trough network with an intermediate thermal carrier, while the second one considers the direct heating of the working air in a solar tower system. The first solution also includes a bottoming organic Rankine cycle (ORC) plant, so that it is mainly addressed to the power output increase. The second one involves a relevant temperature increase of the air entering the combustor, so allowing a direct fuel energy saving, whose amount is strongly variable with both the solar irradiance and the eventual part-load operation. In addition, the latter solar-assisted scheme involves noticeable variations in the conditions for the combustion development. This suggested the authors to proceed with a detailed CFD analysis of the combustion, after a preliminary thermal cycle study for highlighting the main benefits from the solar integration of the power plant.