• Energy Research

Biogas is produced from organic materials, typically containing methane (CH4) with content varying between 50% and 70%, and carbon dioxide (CO2) ranging from 30% to 50%. It may also include hydrogen (H2) with appropriate source material. Biogas is commonly utilized for power or heat generation in combined heat and power systems. The Homogeneous Charge Compression Ignition (HCCI) engine is well-suited for this purpose due to its efficient and rapid combustion process, with a lean air-fuel mixture. However, to limit the high pressure rise rate resulting from simultaneous premixed combustion in the engine cylinder, the combustion process must be controlled. The dilution inherent to this combustion mode helps in limiting the maximum cylinder temperature, and so the nitrogen oxides emissions. Moreover, particulate emissions can be lower than with diesel engine due to the homogeneous charge. Nonetheless, hydrocarbon (HC) and carbon monoxide (CO) levels are greater than those observed with spark ignition (SI) combustion. Additionally, high intake temperatures are needed to operate HCCI engines, depending on compression ratio, intake pressure, fuel type, and equivalence ratio. This paper focuses on an experimental study involving a modified diesel engine initially built for passenger vehicles, adapted to run in HCCI mode. Different fuel mixtures containing methane, hydrogen, and carbon di oxide with a fixed equivalence ratio of 0.4 were tested to reproduce an innovative biogas composition naturally containing hydrogen. The intake temperature and pressure were set according to the fuel mixtures to properly phase the combustion onset. A combustion analysis has been conducted to determine the effect of fuel compo sition on the combustion process. For each fuel type, the optimal intake pressure and temperature for achieving best combustion timing were identified, resulting in a peak indicated efficiency of 40% and specific NOx emissions as low as 0.1 g/kWh. Results show the effect of both CH4/H2 ratios and CO2 content as well as intake conditions on the load (IMEP), efficiencies, engine stress (i.e. pressure gradient) and emissions.

Organic Rankine Cycle (ORC) power plants are characterized by high efficiency and flexibility, as a result of a high degree of maturity. These systems are particularly suited for recovering energy from low temperature heat sources, such as exhaust heat from other plants. Despite ORCs having been assumed to be appropriate for stationary power plants, since their layout, size and weight constraints are less stringent, they represent a possible solution for improving the efficiency of propulsion systems for road transportation. The present paper investigates an ORC system recovering heat from the exhaust gases of an internal combustion engine. A passenger car with a Diesel engine was tested over a Real Driving Emission (RDE) cycle. During the test exhaust gas mass flow rate and temperature have been measured, thus calculating the enthalpy stream content available as heat addition to ORC plant in actual driving conditions. Engine operating conditions during the test were discretized with a 10-point grid in the engine torque–speed plane. The ten discretized conditions were employed to evaluate the ORC power and the consequent engine efficiency increase in real driving conditions for the actual Rankine cycle. N-pentane (R601) was identified as the working fluid for ORC and R134a was employed as reference fluid for comparison purposes. The achievable power from the ORC system was calculated to be between 0.2 and 1.3 kW, with 13% system efficiency. The engine efficiency increment ranged from 2.0% to 7.5%, with an average efficiency increment of 4.6% over the RDE test.

Real-world driving conditions are essential for measuring pollutant emissions and fuel consumption of vehicles. Real driving emission measurement was recently required for heavy-duty vehicles, previously tested only on the engine dynamometer over standardized operating conditions. This paper describes the results of an experimental campaign carried out on three different EURO VI urban buses fuelled by natural gas, aimed at measurement of carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO2), particulate matter (PM) emissions and fuel consumption (FC) in real driving conditions. Buses, used for public transport in 3 different Italian cities, were equipped with a portable emission measurement system (PEMS) and tested over line routes, representative of small, medium, and large urban areas. The actual driving cycles were analyzed to understand the differences in exhaust emissions and fuel consumption between the vehicles. Cold start conditions were also investigated. The results showed that the main differences in pollutant emissions between tested buses can be attributed to the calibration of the fuel supply system and to the driving cycle characteristics. Cold-start strongly impacted CO and NOx emissions because of the effect of temperature on catalytic converter efficiency, with cold over-emissions strongly dependent on the ambient temperature at engine start.

The exhaust heat of energy conversion systems can be usefully recovered by Organic Rankine Cycles (ORC) instead of wasting it into the environment, with benefits in terms of system efficiency and environmental impact. Rankine cycle technology, consolidated in stationary power plants, has not yet spread out into transport applications due to the layout limitations and to the necessity of containing the size and weight of the ORC system. The authors investigated an ORC system bottoming a compression ignition engine for marine application. The exhaust mass flow rate and temperature, measured at different engine loads, have been used as inputs for modeling the ORC plant in a Simulink environment. An energy and exergy analysis of the ORC was performed, as well as the evaluation of the ORC power at different engine loads. Two different working fluids were considered: R1233zd(e), an innovative fluid belonging to the class of hydrofluoroolefin, still in development but interesting due to its low flammability, health hazard, and environmental impact, and R601, a hydrocarbon showing a benchmark thermodynamic performance but highly flammable, considered as a reference for comparison. Three plant configurations were investigated: single-pressure, dual-pressure, and reheating. The results demonstrated that the dual-pressure configuration achieves the highest exploitation of exhaust heat. R1233zd(e) produced an additional mechanical power of 8.0% with respect to the engine power output, while, for R601, the relative contribution of the ORC power was 8.7%.

The paper describes a numerical study of the combustion of hydrogen enriched methane and biogases containing hydrogen in a Controlled Auto Ignition engine (CAI). A single cylinder CAI engine is modelled with Chemkin to predict engine performance, comparing the fuels in terms of indicated mean effective pressure, engine efficiency, and pollutant emissions. The effects of hydrogen and carbon dioxide on the combustion process are evaluated using the GRI-Mech 3.0 detailed radical chain reactions mechanism. A parametric study, performed by varying the temperature at the start of compression and the equivalence ratio, allows evaluating the temperature requirements for all fuels; moreover, the effect of hydrogen enrichment on the auto-ignition process is investigated. The results show that, at constant initial temperature, hydrogen promotes the ignition, which then occurs earlier, as a consequence of higher chemical reactivity. At a fixed indicated mean effective pressure, hydrogen presence shifts the operating range towards lower initial gas temperature and lower equivalence ratio and reduces NOx emissions. Such reduction, somewhat counter-intuitive if compared with similar studies on spark-ignition engines, is the result of operating the engine at lower initial gas temperatures.

Abstract The use of biogas in internal combustion engines presents many advantages. Nevertheless, the CO2 content in the fuel negatively affects the combustion process, reducing combustion speed and stability. The presence of hydrogen in the biogas would improve its combustion characteristics. This paper investigates the combustion of biogas in a Controlled Auto Ignition (CAI) engine by means of numerical simulations. A model of the combustion system was developed for analyzing the use of biogas in a Chemkin environment. The biogas considered in this paper naturally contains hydrogen as a result of a new anaerobic digestion process. The CAI internal combustion engine model allowed the evaluation of engine performance and emissions. Different hydrogen contents were investigated to find the optimal fuel composition for reducing nitrogen oxides emission. Exhaust Gas Recirculation (EGR) was adopted for controlling in-cylinder temperature and therefore fuel auto-ignition. In fact, the presence of very reactive chemical species in the EGR, such as nitric oxide (NO), resulted to have an important impact on the onset of combustion. The results demonstrated that biogas containing hydrogen allow a reduction of NOx emissions with respect to a conventional biogas. An optimal biogas composition was identified, allowing a 32% reduction in NOx emission compared to the conventional biogas. The reaction mechanism for NO formation did not change with biogas composition, with an important contribution coming from prompt NOx formation mechanism. The main differences were in the reaction rates, higher for the conventional biogas.

Abstract The increasing diffusion of micro-cogeneration systems is raising the need for studying their environmental impact in order to assess their sustainability. The adoption of the systems for the combined production of heat and power may provide a significant reduction of global impact in terms of carbon dioxide emissions with respect to the separate production of electricity and heat. However, a comprehensive environmental evaluation of this technology should take into account as well the impact due to the presence of plants spread over the territory that could increase the local pollution, in particular due to nitrogen oxides and carbon monoxide, and thus could worsen the local air quality. In this paper the nitrogen oxides and carbon monoxide emissions of a residential building-integrated micro-cogeneration system were evaluated; a 6.0 kWel natural gas fuelled internal combustion engine-based micro-cogeneration unit was coupled with a multi-family house compliant with the transmittance values suggested by the Italian Law. The analyses were carried out by using the whole building simulation software TRNSYS upon varying the city where the building is located (four Italian cities representative of different climatic regions were considered) as well as the control logic of cogeneration device (electric and thermal load-following strategies). The simulated performance of the proposed system was compared with those of a conventional system composed of a natural gas-fired boiler (for thermal energy production) and a power plant mix connected to the central electric grid (for electricity production) in order to assess the suitability of the cogeneration-based system in reducing the local emissions.