• Energy Research

Abstract This paper presents a novel power block concept for flexible electricity dispatch in a Concentrating Solar Power (CSP) plant. The power block is based on intercooled – unfired regenerative closed air Brayton cycle that is connected to a pressurized solar air receiver. The Closed Brayton cycle uses a mass flow regulation system centered on the pressure regulation (auxiliary compressor and bleed valve) in order to control the Turbine Inlet Temperature (TIT). Doing so, the system is able to modulate turbine electricity production according to variations in the solar resource and changes in power electric demand. It has been found that the proposed power block is able to fully cover the electricity demand curve for those days with high solar resource. In case of integrating particles-based high temperature Thermal Energy Storage (TES) system, the power block can extend its production till the next day following the electricity curve demand during summer period. During winter period, the power plant can extend its production for a few hours due to the lower solar resource and the higher electric curve demand load.

On–off valve systems, commonly used in regenerative thermal oxidizer (RTO) plants, generate, during the opening time, a mass flow rate (MFR) which is constant. On the contrary, rotary valve systems, which are increasingly adopted in RTO plants, are characterized by variable MFR profiles. In this work, the energy requirements of two RTO systems, equipped with on–off or rotary valves, were determined using a home-developed numerical code. Energy performances were evaluated by calculating the thermal efficiency and pressure drop within structured or random packed bed RTO systems, at the same mean MFR. The results demonstrated that thermal efficiency was only moderately influenced by the valve system, and is slightly lower for the RTO with on–off valve. On the other hand, the study revealed that energy requirements of all RTO systems were basically unaffected by cycle duration, allowing valve rotational velocity to be freely set to maximize for other technical requirements. On the contrary, pressure drop was greatly influenced by the valve type and increased as variability in MFR function augmented. Moreover, the type of regenerator, structured or random packed bed, affected differently the total energy requirements (basically pumping energy plus auxiliary fuel). Energy requirements of structured and random regenerators were comparable only when volatile organic compounds concentration was lower than typical values encountered in the industrial practise. In other cases, structured regenerators RTO were more competitive. Finally, structured regenerators are usually the best choice when rotating valve distribution systems are adopted. Copyright © 2007 John Wiley & Sons, Ltd.

Abstract Oxy-MILD (Moderate or Intensive Low Oxygen Dilution) combustion is an attractive technology for increasing combustion efficiency and reducing the nitric oxides with respect to the conventional combustion in traditional boilers or furnaces. This technology is a combination of MILD combustion, which exploits the initial preheating of combustion air up to 800÷1300 °C and the simultaneous recirculation of the hot flue gases, and of Oxy-Combustion, which uses pure oxygen (greater than 95% purity) as oxidant instead of air. The combination of both technologies is expected to bring synergetic effects: NO x reduction, CO 2 capture possibility, fuel flexibility and uniformity of heat fluxes and species concentrations. This study focuses, in particular, on the analysis, by a CFD approach, of the NO x emissions for Oxy-MILD combustion of pulverized coal in a new concept of industrial boiler. The boiler, fueled with a high-volatile coal, is equipped with five burners and three outlets of the combustion products, localized on the top wall. Each burner is made-up of a central pulverized coal jet surrounded by six preheated oxygen jets. This configuration allows the achievement of the MILD combustion in pure oxygen. Results show that the goal of NO x reduction with this combustion approach is achieved. A value of about 296 mg/Nm 3 @6%O 2 at the boiler exit is obtained under Oxy-MILD conditions, which is significantly lower than 600-800 mg/Nm 3 @6%O 2 obtained in a traditional boiler.

AbstractFluidized bed reactors are found in a wide range of applications in various industrial operations, including chemical, mechanical, petroleum, mineral and pharmaceutical industries. This work aims to study hydrodynamics and heat transfer between fluidized bed and a heated wall using the Eulerian – Eulerian approach. Gas/particle flow behavior in the riser section of a Bubbling Fluidized Bed has been simulated using a Computational Fluid Dynamics (CFD) combined with a Kinetics Theory of Granular Flow (KTGF). Conservation equations of mass and momentum for each phase has been solved using the finite volume technique. The results of numerical solutions have been compared with results from literature.

Abstract An innovative system for the recovering of energy from tidal currents is proposed. The system is composed of a blade submerged in sea waters and connected to a vertical bar which, moving up and down through the tide action, transfers energy to a double effect piston pump. The latter feeds a pressurized reservoir able to provide water flow rate, at a suitable pressure level, to a hydraulic turbine. The basic configuration involves a four-bar linkage connecting the vertical bar and the piston pump. The system can be easily employed in all those sites whose seabed quickly deepens and whose tidal currents are parallel to the coast. The proposed system is a valid alternative to the current tidal energy converters: its big dimensions are necessary to balance the low efficiencies of the overall energy conversion. At any rate, during the working the seabed is not altered, neither is the aquatic fauna damaged.

Abstract The paper presents an innovative system for the collection of energy from river and tidal currents, designed with the objective of combining high performance, cost-efficiency and simplicity. The proposed system consists of a kinetic turbine able to be immersed inside water currents and kept in equilibrium by the action of a central deflector and a steel cable anchored to the shore. The size and the orientation of the deflector are defined according to the working conditions and desired equilibrium position. The paper also describes the design parameters of a demonstrative installation at Punta Pezzo (Villa San Giovanni, Italy), located in the Strait of Messina. In the selected site, nearby the coast, the peak current speed reaches 3 m/s (6 kn). The turbine and its components have been designed assuming that the machine will always work under maximum power coefficient conditions. This implies a variable rotational speed, so the use of an inverter becomes mandatory. Preliminary performance estimations show that the system can provide electrical power of about 470 kW, with 43% efficiency when the system works under optimal conditions.

Abstract The Conventional coal-fired plants are large contributors to air pollution and greenhouse gas. The combustion generates pollutants such as oxides of sulphur, nitrogen, and carbon as well as fine organic and inorganic particulates. The new technologies able to reduce drastically the pollutant emissions and facilitate to use of coal in an environmentally more friendly way, are commonly known as clean coal technologies (CCT). In this context the CCS technologies play an important role to reduce the CO2 emissions. The only form with truly zero CO2 emissions in existence today is pre-combustion gas separation, namely, the combustion of fuel using oxygen instead of air. It is well known that burning pulverized coal in pure oxygen increases the flame temperatures, thus also increases NOx emissions. Therefore, to moderate the flame temperature and reduce NOx the oxygen is mixed with recycled flue gas (RFG). This approach to reduce CO2 emissions is often called oxy-firing or oxy-fuel combustion. The purified CO2 stream is then compressed and condensed to produce a manageable effluent of liquid CO2, which can be sequestered for storage (CCS) or for use in subsequent processes (CCR). MILD (Moderate or Intensive Low Dilution) or HiTAC (High Temperature Air Combustion) is an innovative combustion technology and probably the most important achievement of the combustion technology in recent years. In MILD combustion the reactions take place in almost the whole volume of the combustion chamber. This leads to temperature and species concentration fields uniform in the chamber. The fuel is oxidized in an environment that contains a substantial amount of inert gases (N2, CO2, H2O) and low oxygen concentrations. This is caused by an internal recirculation of combustion products generated by injecting preheated air jets into the combustion chamber with very high momentum, bringing the temperatures close to the combustion products temperature, reducing the NOx emissions. Because both technologies allow reductions of pollutant emissions, the aim of this work is to demonstrate the advantages of a combination of these two combustion technologies in order to analyze the temperature and specie concentrations field, the CO2 and NOx emissions by means of CFD. The goal is understand if it is possible to combine the MILD combustion and OXY one in order to reduce the NOx emissions, and capture the CO2.