• Energy Research

Experimental and computational activities were performed to determine the possible improvements of total environmental impact of a wood-open fireplace. Two models of fireplaces will be discussed, both with the same power and with only a difference relative to the geometry of the internal elements of the heat exchanger. In the second model the two blades have been substituted with one pipe that represents a possible and cheaper solution in the way of optimization. The optimization process has been conducted up to the determination of a third configuration with a better thermal efficiency. While combustion emissions are difficult to reduce where only a minimum control of actual combustion conditions can be ensured in such domestic appliances, a significant improvement can be considered for the heat recover efficiency. This improvement can be achieved through a detailed analysis of the thermal conditions realized and can require modifications to the heat exchanger geometry profitable from the manufacturing point of view too. © 2006 Elsevier Inc. All rights reserved.

The paper deals with the direct integration between a Stirling engine and a fluidized bed combustor for micro-scale cogeneration. A pilot-scale facility integrating a fluidized bed combustor (up to 60 kWt ) and a gamma-type Stirling engine (0.5 kWe) was set up and tested to demonstrate the feasibility of this solution and investigate the critical concerns of the combined system. The Stirling engine was installed at a lateral wall of the combustor in contact with the bed region. An experimental campaign has been carried out to assess the performance of the innovative integrated system. The paper reports on the experimental results that can be summarized in: i) very high combustion efficiency with biomass feeding, ii) enhanced heat transfer rate to the engine, iii) conversion to electric power close to the upper performance limit of the engine. Moreover, aspects of the dynamics of the integrated system were investigated and mathematically modeled, the dynamics of the thermal system being dominated by the fast response of the Stirling engine, which rapidly reacts to the slow changes of the fluidized bed. Under the tested conditions (i.e., a thermal power of about 13 kWt), only a limited fraction of power (2 kWt) was subtracted from the engine to the combustor, leading to a dynamic response of the system with a thermal time constant of the order of 10 min.

This poster will illustrate the new insights gained on the fate of char and ash particles in entrained flow gasifiers, upon particle interaction with the reactor walls, by conducting numerical simulations at different levels of detail. Several computational studies, supported by experimental evidences, have been conducted to investigate the swarm dynamics of these particles, always present in large number especially close to the internal walls of the gasifiers (Ambrosino et al., 2013; Marra et al., 2017). The possibility that different regimes can establish, taking into account simple solid-solid interactions, was already postulated in the past (Montagnaro and Salatino, 2010). The effective mechanisms and governing parameters are now emerging from detailed simulations focused on the dynamics of multiparticles confined systems. The results allow to identify the key role of both rheological properties, with a predominant effect of the coefficient of restitution of the gasified material during its transformations, and operative parameters, having in this case the momentum flux and the number density of the particle phase a decisive effect on the kinetic energy dissipation. As a result, a new and different interpretation of the very good conversion performances measured in entrained flow gasifiers is obtained.

Limit combustion phenomena, such as ignition, are rather sensitive to chemical kinetics and these properties are therefore used to physically characterize the behaviour of different fuels. In the framework of bifurcation theory, the ignition and extinction phenomena for combustion occurring in a Perfectly Stirred Reactor correspond to saddle-node bifurcation points and leads to the classical S-shaped steady-state curve. Then, the location of ignition and extinction conditions and their dependence on the main parameters (like pressure, equivalence ratio, residence time or inlet temperature in reactors) can be reformulated as a problem of bifurcation analysis. Even when the reactive mixture is described by a simple surrogate, but in conjunction with very complex and detailed chemical mechanism, with several hundreds of species and thousands of chemical reactions, the computation of the bifurcation diagram becomes computationally very demanding. In this work we explore this issue. The several steps required to formulate a complete continuation algorithm are analysed from a computational point of view and convenient formulations or approaches are introduced to made viable this kind of analysis even adopting desktop class computers. It is shown that the adoption of a Broyden type corrector in the continuation algorithm outperform for this problem the usually adopted Newton type correctors. Consequently, we introduce a suitable algorithm to investigate ignition, extinction and linear stability of the air-fuel mixtures in PSR. The algorithm is based on the well-known Keller pseudo-arclenght continuation method in order to compute steady state solution curve. The solutions stability and then ignition and extinction states are identified by test functions based on the numerical eigenvalues of the Jacobian of the governing system of equations. The algorithm is implemented in the numerical computing environment Matlab coupled with the CANTERA Toolbox for managing of complex chemical kinetic mechanisms and species properties. The algorithm is thus easily applicable to chemical schemes available in the standard CHEMKIN format. To demonstrate the capability of the resulting method, the characteristic S-Shaped curve, including non-stable branches, for different Air-Jet Fuels mixtures have been computed.

A new concept system is presented for the combined production of heat and electric power. Two renewable energy sources are used, i.e., direct solar (thermodynamic solar) and biomass (indirect solar energy). Biomass combustion is conducted using a fluidized bed combustor. A second source of energy, given by the direct irradiation of the bed with a concentrated solar radiation, is integrated in the same system. A Scheffler mirror is planned to carry out sunlight irradiation of the fluidized bed in a fixed focal point. A Stirling engine, integrated in the fluidized bed, converts heat into electricity. A working prototype, referred to as MEGARIS, has been developed very recently and is presently under test. The proposed paper is mainly devoted to automatic control and load regulation of the MEGARIS prototype. A simple dynamic mathematical model has been developed for the fluidized bed acting at the same time as solid biomass combustor, solar light receiver and heat exchanger toward the Stirling engine. For control and regulation purposes, the bed temperature has been taken as the controlled variable; the process variables acting as disturbances have been identified, e.g., the sunlight power; the main manipulated variable has been considered the solid biomass feed rate; the system parameters that are subject to uncertainty have been identified, e.g., the power dissipated through insulation. The control and regulation strategy relies on a feedback architecture. Different theoretical approaches and sophistication levels have been proposed and tested for the loop controller, i.e., PID, Fuzzy Logic and predicting model-based. Both the dynamic model and the controller algorithms have been developed in Matlab scripts and implemented in Simulink codes. A systematic simulation has been carried out for both the open loop and closed loop system responses to those changes that are realistically expected in the main process variables or in uncertain parameters. In parallel, the dynamical analysis of the mathematical model of the prototype has been carried out with Matcont. The results obtained so far are very encouraging to achieve flexibility and controllability of the MEGARIS prototype.

The paper introduces a numerical tool based on a predictor-corrector continuation algorithm to obtain the bifurcation analysis of a perfectly stirred reactor with detailed reaction mechanisms.Each step of the continuation algorithm is reviewed and adapted to handle reaction mechanisms with hundreds of species and thousands of reactions. Particularly, the adoption of a Broyden solver in the predictor-corrector algorithm and a new formulation of the test functions are proposed. The implementation in Matlab and the adoption of the CANTERA Toolbox, make the tool easily applicable to reaction mechanisms available in CHEMKIN format.To validate and demonstrate the capability of the tool, the full equilibrium curves have been obtained for three different cases, having increasing number of species and reactions: methane-air (GRIMech.1.2), simple surrogates of Jet-A in air (JetSurF2.0) and a ternary surrogate of Jet-A in air (CRECK). The tool gets performances that make affordable the computations even with desktop computers.

This paper introduces a generalized formulation for the entropy production analysis. The use of relations valid under the hypothesis of validity of the principle of detailed balance, that is violated in the remarkable case of irreversible reactions, is avoided by using more general expressions for the entropy generation due to the chemical reactions. As a result, the new formulation is applicable to generate reduced mechanisms involving both irreversible reactions or ad hoc estimated reverse rates.

This paper presents a new concept for the integration of a combustion chamber based on renewable fuels and a Stirling Engine (SE). Biomass is adopted as fuel to be burned in a fluidized bed (FB) combustor in the preferred form of wood pellets. This combustion technology has proven to be flexible, efficient and clean in a variety of configurations. The heat generated by combustion is used as source for the Stirling engine that converts part of the heat into mechanical and then electric energy. The novelty of the proposed setup consists in placing the heat exchanger directly into the sand bed of the fluidized combustor. This choice is motivated by the very high heat exchange coefficients attainable with the fluidized bed and by the absence of fouling due to the cleaning action exerted by the fluidized sand. A prototype system has been built to prove these concepts. The fluidized bed combustor has a square section of 290x290 mm2 in the bottom and 390x390 mm2 in the top; the total height is 1850 mm. All components are made in stainless steel. The combustor is equipped with an electric pre-heater, devices for temperature, pressure and flow measurements as well as a dedicated gas analyzer. The thermal power can be varied in the range 15-40 kW by changing the operating conditions of the fluidized bed and the fuel feeding rate. The typical operation temperature of the bed is in the range 800-850 °C. The heater of the Stirling Engine, produced by El.Ma. Srl, is immersed in the bed section of the combustor. A specifically designed support and connector, able to dump the engine oscillations and to allow the same engine to follow the movements of the insertion point determined by the thermal expansion of the combustor, has been constructed. First tests of heating-up, combustion and co-generation were performed, providing positive confirmations about the technical solutions adopted for the prototype and the possibility to achieve better performance when the SE heater is placed in contact with the hot flue gases. The system is the core of a larger research project named MEGARIS, aimed at developing a micro-cogeneration system, which is exclusively based on renewable energy sources and able to guarantee heat and electric power continuously 24 hours a day.