Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel or biomass into carbon monoxide and hydrogen, by reacting raw materials with a controlled amount of oxygen and/or steam. The resulting gas mixture is called synthesis gas or syngas and is itself a fuel. Syngas can be used for heat production and for generation of mechanical and electrical power. Moreover, the syngas may be efficiently converted to dimethyl ether (DME) by methanol dehydration, methane via the Sabatier reaction, diesel-like synthetic fuel via the Fischer-Tropsch process or used to produce methanol and hydrogen. Like other gaseous fuels, syngas gives greater control over power levels than solid fuels, leading to more efficient and cleaner operation. An advantage of gasification is that the produced syngas is potentially more efficient than direct combustion of a fuel, because it can be combusted at higher temperatures or even used in fuel cells. In addition, the high-temperature combustion refines out corrosive ash elements such as chloride and potassium, allowing clean gas production from fuels, otherwise problematic. In this context, the gasification is a technology that has, actually, a great potential in terms of efficiency of biomass into electricity conversion. In particular, by transforming a solid fuel into a gaseous one, by means of the gasification processes, it is possible the use of biomass to power a high-performance system, allowing the achievement of high overall conversion efficiency and the potential reduction of the atmospheric pollution and greenhouse gas emissions, produced by fossil derivates. Biomass gasification is the latest generation of biomass energy conversion processes and is being used to improve the efficiency and to reduce the investment costs of biomass electricity generation through the use of gas turbine technology. High efficiencies (up to about 50%) are achievable using combined cycle gas turbine systems, where waste gases from the gas turbine are recovered to produce steam for a steam turbine. Economic studies show that biomass gasification plants can be as economical as conventional coal fired plants. Moreover, in the context of environmental problems, the gasification process allows producing a gas with relatively high hydrogen content, from 30 to over 45% by volume, which could be recovered by means of membrane technology and used as an attractive alternative energy source for supplying, for example, PEM fuel cells. In the last decades, inorganic hydrogen selective membranes have attracted a great interest in the field of the hydrogen economy development. In particular, dense self-supported palladium and palladium-based membranes are fully hydrogen perm-selective. Therefore, when a syngas stream is supplied to a dense palladium-based membrane reactor, which combines both the hydrogen separation and the syngas conversion via the water gas shift reaction, only hydrogen can permeate through the membrane, which is collected as a high purity hydrogen stream to be used for further applications. Otherwise, supported inorganic palladium-based membranes are not fully hydrogen perm-selective, but they are more resistant to mechanical stress and high temperature than the-self supported ones. Furthermore, these membranes are more economical because constituted by a thin palladium/palladium-alloy layer deposited onto a porous support. However, as a further advantage, inorganic palladium-based membranes and membrane reactors could be useful not only for separating hydrogen but also for providing a stream rich in CO2. In detail, the hydrogen is selectively separated from the other gases and, in the meanwhile, the stream not permeated through the membrane is more concentrated in CO2. Another important aspect of membrane technology in the field of gas separation of streams coming from gasification processes is represented by the use of polymeric membranes. The potential application of this kind of membranes is mainly oriented towards CO2 separation of stream rich in CO2, constituting an alternative technology for CO2 capture with respect to the conventional systems. These membranes have a number of innate advantages over other separation techniques, including simple design with no moving parts, limited maintenance, single-step separation and exceptional reliability. Moreover, these membranes do not need to add chemicals or to regenerate an absorbent/adsorbent. On the contrary, the polymeric membrane technology is not yet commercially used to recover CO2 from syngas, although these membranes seem a natural choice for carbon capture, in particular when pressures and CO2 concentrations are high. Another reason for the limited use of these membranes today is that other conventional and mature technologies are based upon a well-established industrial knowledge. To resume, the main scope of this chapter is, after giving an extensive overview of the main gasification systems, the description of both the benefits and the principal drawbacks of the membrane and membrane reactor technologies applied to the treatment of syngas streams produced from gasification, paying particular attention to the potentialities of both the inorganic and organic membranes.
Membrane and membrane reactor technologies in the treatment of syngas streams produced from gasification processes
Basile A;Iulianelli A;
2011
Abstract
Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel or biomass into carbon monoxide and hydrogen, by reacting raw materials with a controlled amount of oxygen and/or steam. The resulting gas mixture is called synthesis gas or syngas and is itself a fuel. Syngas can be used for heat production and for generation of mechanical and electrical power. Moreover, the syngas may be efficiently converted to dimethyl ether (DME) by methanol dehydration, methane via the Sabatier reaction, diesel-like synthetic fuel via the Fischer-Tropsch process or used to produce methanol and hydrogen. Like other gaseous fuels, syngas gives greater control over power levels than solid fuels, leading to more efficient and cleaner operation. An advantage of gasification is that the produced syngas is potentially more efficient than direct combustion of a fuel, because it can be combusted at higher temperatures or even used in fuel cells. In addition, the high-temperature combustion refines out corrosive ash elements such as chloride and potassium, allowing clean gas production from fuels, otherwise problematic. In this context, the gasification is a technology that has, actually, a great potential in terms of efficiency of biomass into electricity conversion. In particular, by transforming a solid fuel into a gaseous one, by means of the gasification processes, it is possible the use of biomass to power a high-performance system, allowing the achievement of high overall conversion efficiency and the potential reduction of the atmospheric pollution and greenhouse gas emissions, produced by fossil derivates. Biomass gasification is the latest generation of biomass energy conversion processes and is being used to improve the efficiency and to reduce the investment costs of biomass electricity generation through the use of gas turbine technology. High efficiencies (up to about 50%) are achievable using combined cycle gas turbine systems, where waste gases from the gas turbine are recovered to produce steam for a steam turbine. Economic studies show that biomass gasification plants can be as economical as conventional coal fired plants. Moreover, in the context of environmental problems, the gasification process allows producing a gas with relatively high hydrogen content, from 30 to over 45% by volume, which could be recovered by means of membrane technology and used as an attractive alternative energy source for supplying, for example, PEM fuel cells. In the last decades, inorganic hydrogen selective membranes have attracted a great interest in the field of the hydrogen economy development. In particular, dense self-supported palladium and palladium-based membranes are fully hydrogen perm-selective. Therefore, when a syngas stream is supplied to a dense palladium-based membrane reactor, which combines both the hydrogen separation and the syngas conversion via the water gas shift reaction, only hydrogen can permeate through the membrane, which is collected as a high purity hydrogen stream to be used for further applications. Otherwise, supported inorganic palladium-based membranes are not fully hydrogen perm-selective, but they are more resistant to mechanical stress and high temperature than the-self supported ones. Furthermore, these membranes are more economical because constituted by a thin palladium/palladium-alloy layer deposited onto a porous support. However, as a further advantage, inorganic palladium-based membranes and membrane reactors could be useful not only for separating hydrogen but also for providing a stream rich in CO2. In detail, the hydrogen is selectively separated from the other gases and, in the meanwhile, the stream not permeated through the membrane is more concentrated in CO2. Another important aspect of membrane technology in the field of gas separation of streams coming from gasification processes is represented by the use of polymeric membranes. The potential application of this kind of membranes is mainly oriented towards CO2 separation of stream rich in CO2, constituting an alternative technology for CO2 capture with respect to the conventional systems. These membranes have a number of innate advantages over other separation techniques, including simple design with no moving parts, limited maintenance, single-step separation and exceptional reliability. Moreover, these membranes do not need to add chemicals or to regenerate an absorbent/adsorbent. On the contrary, the polymeric membrane technology is not yet commercially used to recover CO2 from syngas, although these membranes seem a natural choice for carbon capture, in particular when pressures and CO2 concentrations are high. Another reason for the limited use of these membranes today is that other conventional and mature technologies are based upon a well-established industrial knowledge. To resume, the main scope of this chapter is, after giving an extensive overview of the main gasification systems, the description of both the benefits and the principal drawbacks of the membrane and membrane reactor technologies applied to the treatment of syngas streams produced from gasification, paying particular attention to the potentialities of both the inorganic and organic membranes.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
