Optimal tuning of a thermo-chemical equilibrium model for downdraft biomass gasifiers

A thermo-chemical equilibrium model is applied to predict the released syngas composition, char, tar content and temperature in biomass gasifiers. The accuracy of the model results is improved by proper calibration, namely by modifying the equilibrium constants through correction factors that represent the degree of approach of the analyzed system to equilibrium. To this aim, the developed model is coupled with a genetic algorithm (Moga II), to search for the optimal correction factors able to minimize the error between the computed and the experimentally measured product yields and temperatures. The approach is repeated to simulate the thermal treatment of different biomasses with increasing carbon content, from straw to sawdust. The possibility to resort to a unique set of correction factors for different biomasses is explored, that would allow the model being of particular interest for engineering applications, to trace the design guidelines for gasification systems

A phenomenological model of a downdraft biomass gasifier flexible to the feedstock composition and the reactor design

The development of a one-dimensional (1D) phenomenological model for biomass gasification in downdraft reactors is presented in this study; the model was developed with the aim of highlighting the main advantages and limits related to feedstocks that are different from woodchip, such as hydro-char derived from the hydrothermal carbonization of green waste, or a mix of olive pomace and sawdust. An experimental validation of the model is performed. The numerically evaluated temperature evolution along the reactor gasifier is found to be in agreement with locally measured values for all the considered biomasses. The model captures the pressure drop along the reactor axis, despite an underestimation with respect to the performed measurements. The producer gas composition resulting from the numerical model at the exit section is in quite good agreement with gas-chromatograph analyses (12% maximum error for CO and CO2 species), although the model predicts lower methane and hydrogen content in the syngas than the measurements show. Parametric analyses highlight that lower degrees of porosity enhance the pressure drop along the reactor axis, moving the zones characterized by the occurrence of the combustion and gasification phases towards the bottom. An increase in the biomass moisture content is associated with a delayed evolution of the temperature profile. The high energy expenditure in the evaporation phase occurs at the expense of the produced hydrogen and methane in the subsequent phases

Thermodynamic-based method for supporting design and operation of thermal grids in presence of distributed energy producers

District heating networks are well-established technologies to efficiently cover the thermal demand of buildings. Recent research has been devoting large efforts to improve the design and management of these systems for integrating low-temperature heat coming from distributed sources such as industrial processes and renewable energy plants. Passing from a centralized to a decentralized approach in the heat supply, it is important to develop indicators that allow an assessment of the rational use of the available heat sources in supplying heating networks, and a quantification of the effect of inefficiencies on the unit cost of heat. To answer these questions, Exergy Cost Theory is here proposed. Thanks to the unit exergetic cost of heat, energy managers can (i) quantify the effects of thermodynamic inefficiencies occurring in the production and distribution on the final cost of heat, (ii) compare alternative systems for heat production, and (iii) monitor the performance of buildings’ substation over time. To show the capabilities of the method, some operating scenarios are compared for a cluster of five buildings in the tertiary sector interconnected by a thermal grid, where heat is produced by a cogeneration unit, an industrial process, and distributed heat pumps. Results suggest that moving from the centralized production of heat based on fossil fuels to a decentralized production with air-to-water heat pumps, the unit cost of heat can be decreased by almost 30% thanks to the improvement of thermodynamic efficiency. In addition, the analysis reveals a great sensitivity of unit exergetic cost to the maintenance in substations. The developed tool can provide thermodynamic-sound support for the design, operation, and monitoring of innovative district heating networks

Optimization of the Efficiency in a Syngas Powered Si Engine Through Numerical Studies Related to the Geometry of the Combustion Chamber

The combustion process occurring in an alternative Spark Ignition (SI) engine powered with bio-syngas from biomass gasification was previously studied by authors through the development of two different numerical models: a 0-1D model developed in the GT-Suite® environment, aimed at gaining a first look upon the main features of the heat release by the syngas and engine performances; a 3D Computational Fluid Dynamics (CFD) model developed within the AVL FireTM software reproducing the engine combustion cycle within a Reynolds Averaged Navier Stokes (RANS) schematization and employing a detailed chemical reaction mechanism to highlight the interaction between the fluid dynamics and the kinetics of the specific biofuel oxidation chain. The numerical results were validated with respect to experimental measurements in a baseline condition, where the presence of a relatively high amount of CO in the exhaust gases was noticed as related to an engine low combustion efficiency, mainly due to the peripheral spark plug position that determines the persistence of residual gases on the opposite side of the combustion chamber wall. The proposed work presents a numerical analysis made through the developed models on the effects of proper changes in the spark plug position. A multi-objective optimization problem is conducted also by varying the Start of Spark (SOS) and the mixture air-to-fuel ratio with the aim of reducing the engine environmental impact without affecting its performances. A centrally mounted spark, along with a correct calibration of the SOS and mixture ratio, allows a reduction of more than 90% of CO emission with respect to the baseline condition without penalizing the engine brake power and efficienc

CFD modelling of a spark ignition internal combustion engine fuelled with syngas for a mCHP system

Micro Combined Heat and Power (mCHP) powered with biomass is nowadays a technology attracting increasing interest to develop a local supply chain to produce, process and valorise the available material in territorial areas as much as possible circumscribed, with a considerable reduction also of the CO2 related to transportation. Application for biomass powered mCHP produces environmental benefits by reducing primary energy consumption and associated greenhouse gas emissions and complies with the need for increased decentralization of energy supply. Of particular relevance is mCHP based on biomass gasification due to the negligible particulate matter release with respect to combustion. The present work describes a 3D CFD model of the spark ignition (SI) internal combustion engine (ICE) fuelled with syngas installed in the mCHP pilot system ECO20 manufactured by the Italian company Costruzioni Motori Diesel S.p.A. (CMD). The considered system is made of a gasifier combined with proper syngas cleaning devices, an ICE and a generator to deliver a maximum electrical and thermal power of 20 kW and 40 kW, respectively. For the proper initialisation of the 3D CFD model, the syngas composition is experimentally characterised using a gas-chromatograph on samples collected under real operation. The calculated pressure cycle is verified by comparison with the one calculated through a properly developed 1D ICE model. Main goals of the performed numerical analysis are to study into detail the combustion process and to assess the engine performance characteristics related to the use of syngas

The “INNOVARE” Project: Innovative Plants for Distributed Poly-Generation by Residual Biomass

The valorization of residual biomass plays today a decisive role in the concept of "circular economy", according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project "INNOVARE-Innovative plants for distributed poly-generation by residual biomass", funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise

The "INNOVARE" project: Innovative plants for distributed poly-generation by residual biomass

The valorization of residual biomass plays today a decisive role in the concept of "circular economy", according to which each waste material must be reused to its maximum extent. The collection and energy valorization at the local level of biomass from forest management practices and wildfire prevention cutting can be settled in protected areas to contribute to local decarbonization, by removing power generation from fossil fuels. Despite the evident advantages of bioenergy systems, several problems still hinder their diffusion, such as the need to assure their reliability by extending the operating range with materials of different origin. The Italian project "INNOVARE-Innovative plants for distributed poly-generation by residual biomass", funded by the Italian Ministry of Economic Development (MISE), has the main scope of improving micro-cogeneration technologies fueled by biomass. A micro-combined heat and power (mCHP) unit was chosen as a case study to discuss pros and cons of biomass-powered cogeneration within a national park, especially due to its flexibility of use. The availability of local biomasses (woodchips, olive milling residuals) was established by studying the agro-industrial production and by identifying forest areas to be properly managed through an approach using a satellite location system based on the microwave technology. A detailed synergic numerical and experimental characterization of the selected cogeneration system was performed in order to identify its main inefficiencies. Improvements of its operation were optimized by acting on the engine control strategy and by also adding a post-treatment system on the engine exhaust gas line. Overall, the electrical output was increased by up to 6% using the correct spark timing, and pollutant emissions were reduced well below the limits allowed by legislation by working with a lean mixture and by adopting an oxidizing catalyst. Finally, the global efficiency of the system increased from 45.8% to 63.2%. The right blending of different biomasses led to an important improvement of the reliability of the entire plant despite using an agrifood residual, such as olive pomace. It was demonstrated that the use of this biomass is feasible if its maximum mass percentage in a wood matrix mixture does not exceed 25%. The project was concluded with a real operation demonstration within a national park in Southern Italy by replacing a diesel genset with the analyzed and improved biomass-powered plant and by proving a decisive improvement of air quality in the real environment during exercise

Modelling approaches to biomass gasification: A review with emphasis on the stoichiometric method

Worldwide escalating energy consumption of recent years, due to the earth population growth and the spreading of industrialization, has resulted in an increased concern about the environmental impact of energy conversion systems. Heavy exploitation and extensive use of fossil fuels have indeed also led to envisage their foreseeable depletion, thus opening the way to the use of alternative fuels as biomass. Among thermo-chemical treatments of biomass, gasification is particularly attractive for its release of syngas (or producer gas), suitable of being used in various combustion systems, including internal combustion engines. In principle, biomass contaminants and heavy hydrocarbons can be removed during syngas cleaning, before the actual combustion process, thus leading to an overall cleaner conversion process. At present, demonstrating operational feasibility and effectiveness of gasification technologies and proving long term sustainability, also through the enhancement of fuel flexibility, are recognized as key elements for the development and market diffusion of biomass energy systems. In fact, although gasification has been known for a long time, its control has long requested serious efforts by researchers and manufacturers. Nowadays, new perspectives are imaginable thanks to the use of simulation tools that may reveal particularly useful to improve gasification efficiency and increase the quality of the producer gas. In recent years, several numerical models have been indeed proposed to characterise and predict such a complex process, where drying, pyrolysis, gasification and combustion take place simultaneously. This article presents a general overview of gasification models available, with emphasis on those based on the stoichiometric method. Although this last may seem too restrictive under some circumstances, equilibrium models are useful to predict the maximum yield attainable by a reagent system, since they reproduce an ideal gasification performance. Due to their simplicity and the reduced computational time, these models are suitable of being employed in a first stage of an analysis or within optimization procedures, where the influence of a number of parameters has to be investigated or a choice of the optimal biomass to be treated for a certain scope is to be made

Numerical analysis of a compression ignition engine powered in the dual-fuel mode with syngas and biodiesel

Biomass gasification for the release of a syngas and its use in combined heat and power (CHP) generation systems are attracting realities in the European market, due to the perspective to provide energy to remote districts by using local renewable sources, as residuals of forest practices or agro-food industries. The syngas produced from biomass gasification is a feasible alternative to traditional fuels in internal combustion engines at the micro and small power scales, although the quality of the produced gas is poorer in terms of calorific value and laminar flame speed. Therefore, as a direct consequence, proper modifications and optimizations of engines are needed to enhance energy efficiency and reduce the environmental impact. In the present work, a numerical model for the simulation of a compression ignition engine fueled in the dual-fuel mode with syngas and biodiesel is presented. The aim is to highlight the main influences on the combustion process related to the use of syngas and the effects of different biomass moisture contents on power output and main pollutants emission. The used extended coherent flamelet model for turbulent combustion is preliminary validated on the basis of experimental data of engine pressure cycles collected under only biodiesel fueling