Thermodynamic analysis of small-scale polygeneration systems producing natural gas, electricity, heat, and carbon dioxide from biomass

ABSTRACT: Agricultural greenhouses are still heavily dependent on fossil fuel-based products despite the abundant residual biomass at their disposal. This paper presents two novel decentralized systems that can convert biomass simultaneously into synthetic natural gas (SNG), electricity, useful heat, and a CO₂-rich stream. To do so, the electricity and H₂/O₂ production features of reversible solid oxide cells (RSOCs) are exploited. A steam dual fluidized bed (DFB) gasifier is used in the first proposed system, while the second one adopts a simpler oxygen/steam-blown downdraft gasification approach. Thermodynamic simulations using Aspen Plus software reveal that the total polygeneration process efficiency could reach 86.6%, with a CO₂ generation capacity exceeding 275g per kilogram of biomass input. If not used inside the greenhouse atmosphere to enhance crop growth, this high-purity CO₂ stream could be sequestered/liquefied to render the process carbon negative. The flexibility of the polygeneration systems is investigated through parametric analysis, where maximum SNG efficiencies that are on par with large-scale plants are obtained. The possibility of storing surplus electricity from intermittent sources as chemical energy in SNG is also highlighted

Diffusive-thermal instabilities in unstrained H₂O-diluted syngas diffusion flames

ABSTRACT: A new version of the unstrained diffusion flame burner that can be operated with gaseous fuels containing high vapor content is introduced. Being a good approximation of the classical chambered diffusion flame solution, the flames generated are nominally unstrained, unlike common research burners where hydrodynamic effects are significant. This permits quantitative comparison with theoretical models that are often based on this simple configuration, and paves the way for fundamental experimental studies with vaporized fuels. In this paper, the capabilities of the new burner design are exploited to study diffusive-thermal instabilities (DTIs) in H2O-diluted H2-CO-CH4-CO2 mixtures. H2O dilution can be significant in biomass-derived syngas mixtures that are not cooled prior to combustion, and that are often burned directly to lower losses as waste heat and pollutant emission in practical combustors. Flammability limits are first presented for a broad range of fuel blends, where the destabilizing effect of H2O dilution is discussed. Instability maps in terms of the Damköhler number are then provided to illustrate the different types of superimposed cellular-pulsating instabilities that onset from the simultaneous presence of H2 with high diffusivity, and CO/CH4 with much lower mobility. The characteristics of these peculiar instabilities are highly dependent on the H2O dilution fraction, which increases both the fuel blend and oxidizer Lewis numbers. The degree of cellularity superimposed in the pulsating multi-fuel flame is reduced at higher water content, as the number of observed cells decreases. The opposite effect is observed on the pulsation frequency, which increases at higher water concentrations

Production and Combustion of Syngas in Biomass Polygeneration Systems

RÉSUMÉ: Il semble paradoxal que malgré la biomasse abondante à leur disposition, les serres agricoles dépendent encore fortement des produits à base de combustibles fossiles pour leurs activités énergivores. Pourtant, la biomasse résiduelle présente dans les zones agricoles comme les copeaux de bois ou la paille de blé, ressources qui ne sont pas en concurrence avec les cultures vivrières, est une ressource d’énergie renouvelable neutre en carbone. En plus d’être simplement brûlée, la biomasse peut être convertie par gazéification en produits à valeur ajoutée pour satisfaire les principaux besoins énergétiques des serres et de plusieurs autres tâches énergivores en milieu agricole. Alors que la littérature sur de tels systèmes de conversion se concentre principalement sur les implémentations à grande échelle, les bioraffineries décentralisées à petite échelle qui pourraient être intégrées dans des serres sont rarement discutées. Par conséquent, le premier objectif de cette thèse est de concevoir et d’analyser les performances de nouvelles conceptions de systèmes de biomasse qui pourraient produire du biocarburant, de l’électricité, de la chaleur et un flux de dioxyde de carbone de haute pureté. Ce dernier, en plus d’être nécessaire dans les atmosphères de serre pour améliorer la croissance des cultures, est facilement séquestrable, permettant ainsi la capture et le stockage du carbone. Des modèles thermodynamiques sont mis en oeuvre en utilisant le simulateur ASPEN Plus pour concevoir quatre nouveaux systèmes de polygénération, qui diffèrent soit par le type de gazéifieur utilisé, soit par le biocarburant synthétisé. Pour ces derniers, le méthanol et le gaz naturel renouvelable sont considérés. Ces biocarburants peuvent être utilisés pour alimenter des machines dans ou autour des serres, et les quantités excédentaires peuvent être vendues par les agriculteurs, fournissant une nouvelle source de revenus. Des piles à combustible réversibles à oxyde solide sont également incorporées dans les systèmes. Ces composants polyvalents peuvent produire de l’électricité de manière efficace et propre lorsqu’ils sont alimentés en gaz de synthèse dérivé de la biomasse, et produisent H2 et O2 par l’électrolyse de la vapeur d’eau lorsqu’ils sont alimentés avec une partie de l’électricité générée. Ce H2 est essentiel pour conditionner le gaz de synthèse avant le réacteur de biocarburant, tandis que le O2 pur est utilisé dans le gazéifieur pour générer un gaz de synthèse sans N2. Les conditions de fonctionnement favorables sont identifiées par une analyse paramétrique, qui a montré que l’efficacité globale du processus de polygénération pourrait atteindre 85%. Un élément clé pour atteindre ces rendements élevés est le brûleur de gaz de synthèse, qui exploite l’énergie du gaz résiduel après les étapes de production de biocarburant et d’électricité, et produit de la chaleur qui est localement demandée en grande quantité par les serres. L’énergie chimique du gaz de synthèse restant pourrait atteindre 35% de l’énergie initiale de la biomasse. ABSTRACT: It is paradoxical to realize that despite having abundant biomass at their disposal, agricultural greenhouses are still heavily relying on fossil fuel-based products for their energy-intensive activities. Residual biomass found in agricultural areas such as wood chips or wheat straw, resources that do not compete with food crops, is a carbon-neutral and renewable energy resource. Moreover, in addition to simply being burned, biomass can be converted via gasification into value-added products to satisfy the main energy demands of greenhouses and of several other energy-intensive tasks in agricultural settings. While the main focus in the literature for such conversion systems is large-scale implementations, decentralized smallscale biorefineries that could be integrated in greenhouses are rarely discussed. Therefore, the first objective of this thesis is to design and analyze the performance of novel biomass system designs that could produce biofuel, electricity, heat, and a high-purity carbon dioxide stream. The latter is needed within the greenhouse atmospheres to enhance crop growth, with surplus amounts being readily sequesterable to enable carbon capture and storage. Thermodynamic models are implemented on the ASPEN Plus process simulator to design four novel polygeneration systems, which either differ in the type of gasifier used or biofuel synthesized. For the latter, methanol and synthetic natural gas are considered. These biofuels can be used to power machinery in or around greenhouse facilities, and the excess amounts can be sold by farmers, providing a new revenue stream. Reversible solid-oxide fuel cells are also incorporated in the systems. These versatile components can produce electricity in an efficient and environmentally friendly manner when supplied with biomass-derived sygnas, and produce H2 and O2 via steam electrolysis when fed with part of the generated electricity. This H2 is key for conditioning the syngas before the biofuel reactor, while the pure O2 is used in the gasifier to generate an energy-dense N2-free syngas stream. The favorable operating conditions are identified through parametric analysis, where it is revealed that the overall polygeneration process efficiency could reach 85%. A key component in attaining such high efficiencies is the syngas burner, which harnesses the energy of the residual gas following the biofuel and electricity production stages, and produces heat that is locally demanded in large quantities by greenhouses. The chemical energy of the leftover syngas could reach 35% of the initial biomass energy

Thermodynamic analysis of novel methanol polygeneration systems for greenhouses

This work presents the modeling and thermodynamic analysis of two novel small-scale polygeneration systems that are capable of simultaneously converting residual biomass to methanol (MeOH), electricity, heat and a CO2-rich stream for agricultural greenhouses. The first system is based on a downdraft gasifier, while the second relies on a dual fluidized bed (DFB) gasifier. Both configurations leverage the H2, O2, and electricity generation capabilities of reversible solid oxide cells (RSOCs). The Aspen Plus process simulator is used to model the thermodynamic performance of the proposed polygeneration systems, which operate at total efficiencies ranging from 83.9 to 85.0%. From a biofuel and electrical efficiency perspective, the system based on a DFB gasifier is superior, providing the added benefit of enabling carbon capture and storage, as a N2-free stream with a molar purity exceeding 90% CO2 is generated, which could be readily liquefied or sequestered

Experimental characterization of diffusive-thermal instabilities in CO₂-diluted H₂―CH₄―CO unstrained diffusion flames

ABSTRACT: The combustion of multi-fuel mixtures is experimentally studied for the first time in unstrained diffusion flames, where the parasitic hydrodynamic effects present in common research burners are negligible. A broad range of H₂-CO-CH₄ fuels highly diluted in CO₂ is investigated to provide an understanding of the intrinsic diffusive-thermal instabilities (DTIs) that onset in low calorific biomass-derived syngas. For each fuel blend, the burning intensity or the Damköhler number ($D_a$) is gradually reduced, going through the marginal stability state where DTIs first appear, down to the lean extinction limit. Flame stability limits are provided. From the large difference between the Lewis numbers of the multiple fuel species ($Le_i$), the cells that onset due to H₂ are seen to interact and compete with the pulsations from CO and CH₄, leading to superimposed cellular-pulsating instabilities. These are thoroughly characterized by measuring the pulsations amplitude, frequency, cell size, number of cells, and fraction of the flame sheet actively burning. An effective fuel Lewis number ($Le_{F,eff}$) calculated from the fuel mixture composition is introduced and used along with the Damköhler number to map the DTIs observed. At lower $Le_{F,eff}$ and $D_a$, the cellular attributes of the superimposed instabilities dominate, while at larger Lewis numbers and the near marginal stability state, pulsations prevail

A Dynamic Rotor Vertical-Axis Wind Turbine with a Blade Transitioning Capability

This work presents an optimized design of a dynamic rotor vertical-axis wind turbine (DR VAWT) which maximizes the operational tip-speed ratio (TSR) range and the average power coefficient (Cp) value while maintaining a low cut-in wind velocity. The DR VAWT is capable of mimicking a Savonius rotor during the start-up phase and transitioning into a Darrieus one with increasing rotor radius at higher TSRs. The design exploits the fact that with increasing rotor radius, the TSR value increases, where the peak power coefficient is attained. A 2.5D improved delayed detached eddy simulation (IDDES) approach was adopted in order to optimize the dynamic rotor design, where results showed that the generated blades’ trajectories can be readily replicated by simple mechanisms in reality. A thorough sensitivity analysis was conducted on the generated optimized blades’ trajectories, where results showed that they were insensitive to values of the Reynolds number. The performance of the DR VAWT turbine with its blades following different trajectories was contrasted with the optimized turbine, where the influence of the blade pitch angle was highlighted. Moreover, a cross comparison between the performance of the proposed design and that of the hybrid Savonius–Darrieus one found in the literature was carefully made. Finally, the effect of airfoil thickness on the performance of the optimized DR VAWT was thoroughly analyzed