Natural gas is, nowadays, the most used source of energy in the world after coal. It accounted for more than 20% of the world electricity generation and its share is continuously growing. Moreover, the effort to a more sustainable world will lead to a preferential use of natural gas with respect to oil or coal. NG (natural gas) is in fact the greener source among fossil fuel for the production of energy since it will only generate CO2 and a minor portion of NOx, whereas oil and coal generate also SO2. It can be used in more efficient gas turbine plants with combined cycle and even for tri-generation. Another major advantage of NG is its easy transportability and usage as a fuel, made possible by its gaseous nature and by the fact that all the industrialized countries possess a dispatching grid capable of serving directly the final customer. Today methane is used by domestic application for cooking and heating, industrial power and heat production and even for transportation. However, its supply depends on the oil reservoir that are not evenly distributed, meaning that countries like Italy or China have to find external supplier with additional economic and geopolitical costs. Nevertheless, in the case of China is possible to exploit the vast coal deposit and convert it into “Synthetic Natural Gas”, SNG. Coal is gasified and cleaned form poisonous species (like sulfur and heavy metals) and then converted into methane via “methanation” reaction. The methanation process has been used extensively in commercial ammonia plants, where it is the final syngas purification step in which small residual concentrations of CO and CO2 are removed catalytically by reaction with hydrogen over a nickel catalyst. Methanation is also used in the production of SNG from coal-derived syngas, but in this case the concentrations of CO and CO2 are much higher, as a result, the temperature rise in the reactor must be carefully controlled. Two basically different approached can be used to circumvent this problem, namely the use of two or more adiabatic fixed bed reactors with inter-stage cooling, feed staging or the use of recycles. In the thesis work, different layouts have been analysed, among which the one proposed and patented by Amec Foster Wheeler has been adopted, it consists of a staged feed without recycles. The simulation, performed with “Aspen Hysys Dynamics”, focused on both the normal operation scenarios and the critical ones. Different control logics have been tested and validated, they have shown to be able to keep the plant within safe operation both during normal and unexpected situations. Furthermore, on the basis of the results, modifications of the plant have been proposed and implemented to assure additional passive safety. Finally, an integration of the SNG plant has been proposed with the novel technology developed at Politecnico di Milano, AG2S™. This technology aims at producing valuable syngas from waste H2S, coming from gasification, that otherwise would be sent to Claus plant. Integration is interesting thanks to the possibility of using on-site the syngas to produce additional SNG and to the possibility of sharing the burden of costly auxiliary plant like Rectisol ® and the air separation unit, both needed by the gasifier. This had the effect of lowering both capital and operating costs.
Attualmente, il gas naturale è la più usata fonte energetica dopo il carbone coprendo più del 20% del fabbisogno energetico mondiale e la sua quota è in continua ascesa. Grazie inoltre ai recenti sforzi per abbattere l’inquinamento globale, l’uso di GN viene ormai preferito al carbone o al petrolio essendo una fonte molto più pulita. Esso infatti brucia meglio, produce meno NOx e non rilascia alcun composto solforato. Può essere usato nelle più efficienti turbine a gas con ciclo combinato e anche per la tri-generazione. Infine, nei paesi industrializzati che possiedono una rete di dispacciamento, è facilmente trasportabile fin nel luogo di utilizzo senza la necessità di stazioni di rifornimento. Grazie a queste caratteristiche il metano è oggi usato sia in abito domestico per il riscaldamento e la cottura dei cibi, sia in ambito industriale per produrre potenza e calore, sia per i trasporti. Tuttavia il suo approvvigionamento dipende dagli stessi siti estrattivi del petrolio, perciò paesi poveri di giacimenti come l’Italia o la Cina, sono costretti a fare affidamento su altri paesi con costi economici e geopolitici non indifferenti. Nel caso della Cina in particolare, è però possibile sfruttare i vasti giacimenti di carbone per convertirlo in “Synthetic Natural Gas”, SNG. Il carbone viene prima gassificato e pulito dai composti nocivi (zolfo, metalli pesanti) e in seguito convertito in metano tramite la reazione di “metanazione”. La reazione di metanazione è stata inizialmente usata negli impianti per la produzione di ammoniaca come step finale di rimozione di CO2 e CO residua, facendole reagire su letto di Nickel con l’idrogeno presente. Nel caso di metanazione ad alte concentrazioni di CO2 e CO, necessarie per la sintesi di SNG, essendo la reazione fortemente esotermica, bisogna usare particolari approcci per evitare che la reazione vada in run-away. Queste soluzioni sono sia di tipo progettuale (raffreddamento intermedio, parzializzazione del feed, correnti di riciclo) sia di tipo operativo, ovvero riguardanti la conduzione del processo. Nel lavoro proposto sono stati analizzate le diverse strategie progettuali e operative sulla base dei costi e dei benefici apportati. È stata poi scelta e studiata in dinamico la soluzione brevettata da “Amec Foster Wheeler”, in cui è presente la parzializzazione del feed ma non il riciclo. Lo studio, condotto con “Aspen Hysys Dynamics” si è soffermato in particolare sulla gestione dei by-pass e della corrente di vapore per moderare correttamente la reazione nei reattori in serie. Sono state studiate non solo le diverse logiche di controllo che permettessero di variare efficacemente e in sicurezza la produzione dell’impianto, ma anche gli svariati scenari infausti di eventi critici inaspettati. Sulla base dei risultati sono state poi elaborate sia le migliori logiche di controllo capaci di mantenere sempre in sicurezza l’impianto, che i design impiantistici capaci di garantire una sicurezza passiva. Infine, è stata proposta l’integrazione dell’impianto SNG con la nuova tecnologia sviluppata dal Politecnico di Milano, AG2S™. Tale tecnologia mira a produrre prezioso syngas partendo dall’H2S rimosso dal carbone gassificato, il quale sarebbe altrimenti inviato al classico impianto Claus senza ulteriori benefici. L’integrazione è interessante sia grazie alla possibilità di usare il syngas prodotto in loco (convertendolo in ulteriore SNG), sia grazie alla possibilità di condividere alcuni costosi impianti ausiliari già in uso al gassificatore, come il lavaggio Rectisol ® e l’unità di frazionamento aria; permettendo così l’abbattimento sia dei costi di investimento che di quelli operativi.
Dynamic simulation of synthetic natural gas plant and implementation with AG2S technology
NODARI, LUCA
2016/2017
Abstract
Natural gas is, nowadays, the most used source of energy in the world after coal. It accounted for more than 20% of the world electricity generation and its share is continuously growing. Moreover, the effort to a more sustainable world will lead to a preferential use of natural gas with respect to oil or coal. NG (natural gas) is in fact the greener source among fossil fuel for the production of energy since it will only generate CO2 and a minor portion of NOx, whereas oil and coal generate also SO2. It can be used in more efficient gas turbine plants with combined cycle and even for tri-generation. Another major advantage of NG is its easy transportability and usage as a fuel, made possible by its gaseous nature and by the fact that all the industrialized countries possess a dispatching grid capable of serving directly the final customer. Today methane is used by domestic application for cooking and heating, industrial power and heat production and even for transportation. However, its supply depends on the oil reservoir that are not evenly distributed, meaning that countries like Italy or China have to find external supplier with additional economic and geopolitical costs. Nevertheless, in the case of China is possible to exploit the vast coal deposit and convert it into “Synthetic Natural Gas”, SNG. Coal is gasified and cleaned form poisonous species (like sulfur and heavy metals) and then converted into methane via “methanation” reaction. The methanation process has been used extensively in commercial ammonia plants, where it is the final syngas purification step in which small residual concentrations of CO and CO2 are removed catalytically by reaction with hydrogen over a nickel catalyst. Methanation is also used in the production of SNG from coal-derived syngas, but in this case the concentrations of CO and CO2 are much higher, as a result, the temperature rise in the reactor must be carefully controlled. Two basically different approached can be used to circumvent this problem, namely the use of two or more adiabatic fixed bed reactors with inter-stage cooling, feed staging or the use of recycles. In the thesis work, different layouts have been analysed, among which the one proposed and patented by Amec Foster Wheeler has been adopted, it consists of a staged feed without recycles. The simulation, performed with “Aspen Hysys Dynamics”, focused on both the normal operation scenarios and the critical ones. Different control logics have been tested and validated, they have shown to be able to keep the plant within safe operation both during normal and unexpected situations. Furthermore, on the basis of the results, modifications of the plant have been proposed and implemented to assure additional passive safety. Finally, an integration of the SNG plant has been proposed with the novel technology developed at Politecnico di Milano, AG2S™. This technology aims at producing valuable syngas from waste H2S, coming from gasification, that otherwise would be sent to Claus plant. Integration is interesting thanks to the possibility of using on-site the syngas to produce additional SNG and to the possibility of sharing the burden of costly auxiliary plant like Rectisol ® and the air separation unit, both needed by the gasifier. This had the effect of lowering both capital and operating costs.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/136182