BIOMETHANE is gaining interest, since it is a renewable surrogate of natural gas from fossil origin, and can be used both as a fuel in the transport sector and for high efficiency power generation, exploiting the existing natural gas infrastructure. In order to accomplish such results, biomethane has to be produced starting from biogas (a mixture of methane and carbon dioxide) performing a separation between its components, denominated biogas upgrading. There is a wide range of technologies available to that extent, but none emerged as clear benchmark for the typical sizes of biogas producing plants (500-1000 Nm3/h), due to the large variability in both costs and performances reported in the literature and by vendors. The development of newer adsorption materials has raised interest in the application of Pressure & Temperature Swing Adsorption (PTSA) to biogas upgrading, since such process exhibits moderate energy consumption (of both thermal and electric power) and operational easiness (no solvent flow has to be handled). The present work aims at evaluating the potential of PTSA processes applied to biogas upgrading potential, with special focus on novel sorbent (alumina impregnated with amines). To such extent, the performances of PTSA processes are compared to the ones of two other benchmark technologies: water scrubbing and chemical scrubbing. Water scrubbing is actually the most diffused upgrading technology, it uses water to absorb carbon dioxide and it is simple and effective. On the other hand, water scrubbing shows large energy consumption in order to regenerate the solvent. Chemical scrubbing looks promising, since the use of chemical solvents (generally amines) allows higher CO2 capture capacity and consequently a reduced energetic consumption. On the other side, such technology is more expensive and more difficult to operate. A methodology for the comparison of biogas upgrading technologies, considering energetic, environmental and economic aspects is established: a common framework for the analysis is defined, in which the energy demand of the upgrading process is satisfied by burning part of the biogas available in a Combined Heat and Power unit. Then, for the three different upgrading technologies i) accurate first principle model are developed/tuned to solve the mass and energy balances of the plants. ii) an economic model is established, to estimate the cost of every equipment unit ii) a multi-objective optimization is performed to identify the optimal trade-off curve between energetic efficiency and cost of each technology. The optimization of absorption based technologies is carried out with available optimization tool, whereas an ad-hoc optimizer, named MO-MCS is developed for tackling the optimization of TPSA processes. The optimisation of adsorption based processes is a well-known problem, since heuristics and engineering considerations may allow to determine some of the decision variables, while others, such as the duration of time steps, are extremely difficult to predict, given the dynamic nature of such processes. Moreover, constraints on the purity and recovery of the products have typically to be considered, and the solution of the model describing equations is computationally intensive. The proposed MO-MCS algorithm is global and make use of models of the objective functions in order to save computational time. Such algorithm is applied in this work to determine the optimal working conditions of two proposed adsorption cycles. Finally, the comparison between the optimal performance trade-off curves is carried out, and conclusions are drawn. The advantage of this approach is that the optimal operating range of each technology is explored, and the results are not biased by the uncertainty of cost related data. On the other side, the final choice of a technology from a user entails the selection of a single configuration among the ones laying on the optimal trade-off curves. Once the trade-off curves between efficiency and cost are known, they represent a valuable tool for the final user who can select the best configuration based on application specific conditions (operating costs, valorisation price of biomethane and electric power). The TPSA process with the novel material, shows the lower specific equipment cost when adopting a six-step adsorption cycle, followed by water scrubbing technology and by chemical scrubbing. From the efficiency point of view while, the TPSA processes with a simpler four-step cycle outperforms chemical scrubbing, amine, the six-step configuration and water scrubbing.+ Also a benchmark adsorbent material (Zeolite X13) is considered, showing unsatisfactory performances. The methane slip is taken as an indicator of environmental performances, since venting methane raises environmental concerns due to its considerable Global Warming Potential. The methane slip allowed by chemical scrubbing process is the lowest, trailed by six-step TPSA, while water and the four step-TPSA show similar methane slips.

Il biometano riscontra crescente interesse, poichè è un sostituto del metano e può essere usato sia nel settore dei trasporti che in quello di generazione di potenza. Per permettere tale applicazione, il biometano deve essere prodotto partendo dal biogas (una miscela di di anidride carbonica e metano). Esistre un ampio ventaglio di tecnologie disponibili per questo scopo, ma nessuna si è affermata come riferimento del settore. per la taglia tipica delgli impianti di produzione di biogas (500-1000 Nm3/h), a causa di una grande variabilità nei coati e nelle prestazioni. Lo sviluppo di nuovi materiali ha destato interesse nell'ambito dei cicli di adsorbimento. Questo lavoro di tesi presenta il confronto tra un processo TPSA basato su un materiale innovativo e un sistemi tradizionali basati su scrubbing ad acqua o su ammine in soluzione. Viene proposto un metodo di ottimizzazione specifico per trattare i cicli di adsorbimento. Un constesto comune è definito per confrontare tutte le tecnologie, dove un motore a combustione brucia parte del biogas per sostenere i consumi delle tecnologie di upgrading. Il confronto è basato su due indici, uno economico (costo di investimento) e uno energetico (efficienza di energia primaria). Lo slip di metano è usato come indicatore delle performance ambientali. Lo scrubbing ad acqua è risulta la soluzione più economica e vantaggiosa.

Biogas upgrading by absorption and adsorption technologies

CAPRA, FEDERICO

Abstract

BIOMETHANE is gaining interest, since it is a renewable surrogate of natural gas from fossil origin, and can be used both as a fuel in the transport sector and for high efficiency power generation, exploiting the existing natural gas infrastructure. In order to accomplish such results, biomethane has to be produced starting from biogas (a mixture of methane and carbon dioxide) performing a separation between its components, denominated biogas upgrading. There is a wide range of technologies available to that extent, but none emerged as clear benchmark for the typical sizes of biogas producing plants (500-1000 Nm3/h), due to the large variability in both costs and performances reported in the literature and by vendors. The development of newer adsorption materials has raised interest in the application of Pressure & Temperature Swing Adsorption (PTSA) to biogas upgrading, since such process exhibits moderate energy consumption (of both thermal and electric power) and operational easiness (no solvent flow has to be handled). The present work aims at evaluating the potential of PTSA processes applied to biogas upgrading potential, with special focus on novel sorbent (alumina impregnated with amines). To such extent, the performances of PTSA processes are compared to the ones of two other benchmark technologies: water scrubbing and chemical scrubbing. Water scrubbing is actually the most diffused upgrading technology, it uses water to absorb carbon dioxide and it is simple and effective. On the other hand, water scrubbing shows large energy consumption in order to regenerate the solvent. Chemical scrubbing looks promising, since the use of chemical solvents (generally amines) allows higher CO2 capture capacity and consequently a reduced energetic consumption. On the other side, such technology is more expensive and more difficult to operate. A methodology for the comparison of biogas upgrading technologies, considering energetic, environmental and economic aspects is established: a common framework for the analysis is defined, in which the energy demand of the upgrading process is satisfied by burning part of the biogas available in a Combined Heat and Power unit. Then, for the three different upgrading technologies i) accurate first principle model are developed/tuned to solve the mass and energy balances of the plants. ii) an economic model is established, to estimate the cost of every equipment unit ii) a multi-objective optimization is performed to identify the optimal trade-off curve between energetic efficiency and cost of each technology. The optimization of absorption based technologies is carried out with available optimization tool, whereas an ad-hoc optimizer, named MO-MCS is developed for tackling the optimization of TPSA processes. The optimisation of adsorption based processes is a well-known problem, since heuristics and engineering considerations may allow to determine some of the decision variables, while others, such as the duration of time steps, are extremely difficult to predict, given the dynamic nature of such processes. Moreover, constraints on the purity and recovery of the products have typically to be considered, and the solution of the model describing equations is computationally intensive. The proposed MO-MCS algorithm is global and make use of models of the objective functions in order to save computational time. Such algorithm is applied in this work to determine the optimal working conditions of two proposed adsorption cycles. Finally, the comparison between the optimal performance trade-off curves is carried out, and conclusions are drawn. The advantage of this approach is that the optimal operating range of each technology is explored, and the results are not biased by the uncertainty of cost related data. On the other side, the final choice of a technology from a user entails the selection of a single configuration among the ones laying on the optimal trade-off curves. Once the trade-off curves between efficiency and cost are known, they represent a valuable tool for the final user who can select the best configuration based on application specific conditions (operating costs, valorisation price of biomethane and electric power). The TPSA process with the novel material, shows the lower specific equipment cost when adopting a six-step adsorption cycle, followed by water scrubbing technology and by chemical scrubbing. From the efficiency point of view while, the TPSA processes with a simpler four-step cycle outperforms chemical scrubbing, amine, the six-step configuration and water scrubbing.+ Also a benchmark adsorbent material (Zeolite X13) is considered, showing unsatisfactory performances. The methane slip is taken as an indicator of environmental performances, since venting methane raises environmental concerns due to its considerable Global Warming Potential. The methane slip allowed by chemical scrubbing process is the lowest, trailed by six-step TPSA, while water and the four step-TPSA show similar methane slips.
BOTTANI, CARLO ENRICO
CONSONNI, STEFANO
30-ott-2017
Il biometano riscontra crescente interesse, poichè è un sostituto del metano e può essere usato sia nel settore dei trasporti che in quello di generazione di potenza. Per permettere tale applicazione, il biometano deve essere prodotto partendo dal biogas (una miscela di di anidride carbonica e metano). Esistre un ampio ventaglio di tecnologie disponibili per questo scopo, ma nessuna si è affermata come riferimento del settore. per la taglia tipica delgli impianti di produzione di biogas (500-1000 Nm3/h), a causa di una grande variabilità nei coati e nelle prestazioni. Lo sviluppo di nuovi materiali ha destato interesse nell'ambito dei cicli di adsorbimento. Questo lavoro di tesi presenta il confronto tra un processo TPSA basato su un materiale innovativo e un sistemi tradizionali basati su scrubbing ad acqua o su ammine in soluzione. Viene proposto un metodo di ottimizzazione specifico per trattare i cicli di adsorbimento. Un constesto comune è definito per confrontare tutte le tecnologie, dove un motore a combustione brucia parte del biogas per sostenere i consumi delle tecnologie di upgrading. Il confronto è basato su due indici, uno economico (costo di investimento) e uno energetico (efficienza di energia primaria). Lo slip di metano è usato come indicatore delle performance ambientali. Lo scrubbing ad acqua è risulta la soluzione più economica e vantaggiosa.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10589/136233