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Enrico Zio - One of the best experts on this subject based on the ideXlab platform.

  • Uncertainty propagation in a model for the estimation of the ground level concentration of dioxin/furans emitted from a Waste Gasification plant
    Reliability Engineering and System Safety, 2013
    Co-Authors: Giovanna Ripamonti, Giovanni Lonati, Piero Baraldi, Francesco Cadini, Enrico Zio
    Abstract:

    In this paper we compare two approaches for uncertainty propagation in a model for Environmental Impact Assessment (EIA). A purely Probabilistic (PMC) and a Hybrid probabilistic-possibilistic Monte Carlo (HMC) method are considered in their application for the estimation of the ground levels concentration of dioxin/furans emitted from a Waste Gasification plant. Under the condition of insufficient information for calibrating the estimation model parameters, HMC is shown to be a valid way for properly propagating parameters uncertainty to the model output, without adopting arbitrary and subjective assumptions on the input probability distribution functions. In this sense, HMC could improve the transparency of the EIA procedures with positive effects on the communicability and credibility of its findings.

  • Uncertainty propagation in a model for the estimation of the ground level concentration of dioxin/furans emitted from a Waste Gasification plant
    Reliability Engineering & System Safety, 2013
    Co-Authors: Giovanna Ripamonti, Giovanni Lonati, Piero Baraldi, Francesco Cadini, Enrico Zio
    Abstract:

    International audienceIn this paper we compare two approaches for uncertainty propagation in a model for Environmental Impact Assessment (EIA). A purely Probabilistic (PMC) and a Hybrid probabilistic-possibilistic Monte Carlo (HMC) method are considered in their application for the estimation of the ground levels concentration of dioxin/furans emitted from a Waste Gasification plant. Under the condition of insufficient information for calibrating the estimation model parameters, HMC is shown to be a valid way for properly propagating parameters uncertainty to the model output, without adopting arbitrary and subjective assumptions on the input probability distribution functions. In this sense, HMC could improve the transparency of the EIA procedures with positive effects on the communicability and credibility of its findings

  • Uncertainty propagation methods in dioxin/furans emission estimation models
    2011
    Co-Authors: Giovanna Ripamonti, Giovanni Lonati, Piero Baraldi, Francesco Cadini, Enrico Zio
    Abstract:

    In this paper we propose a comparison between two different approaches for uncertainty propa-gation in Environmental Impact Assessment (EIA) procedures. Both a purely Probabilistic (PMC) and a Hy-brid probabilistic-possibilistic Monte Carlo method (HMC) are applied on an estimation model of dio-xin/furans emission from a Waste Gasification plant. The analysis shows that when input variables affected by scarcity of information are present, HMC seems to be a valid alternative method that properly propagates un-certainty from data to output avoiding arbitrary and subjective assumptions on the input probability distribu-tion functions. HMC could improve the transparency of the EIA procedure with positive effects on the com-municability and credibility of its predictions.

Chris Chapman - One of the best experts on this subject based on the ideXlab platform.

  • Reforming of tars and organic sulphur compounds in a plasma-assisted process for Waste Gasification
    Fuel Processing Technology, 2015
    Co-Authors: Massimiliano Materazzi, Richard Taylor, Luca Mazzei, Paola Lettieri, Chris Chapman
    Abstract:

    Abstract Waste Gasification is considered a valuable and sustainable solution to the production of clean energy (via gas turbines or gas engines) and bio-fuels, such as synthetic natural gas and bio-hydrogen, provided that the syngas produced in the gasifier is free of condensable tars and organic sulphur contaminants that cause equipment fouling and deactivation of catalytic stages downstream. In particular, catalytic reaction stages are highly sensitive to specific trace contaminants (e.g. PAHs, thiophenes, etc.), necessitating the use of additional cleaning operations to remove these residues to levels where the catalyst degradation is acceptable. In this work, the use of thermal plasma (coupled with primary Waste treatment) to completely reform tars and organic sulphur compounds to simple gaseous products (predominantly H2and CO) is assessed. To this end, a 20-hour Waste Gasification run was performed on a two-stage fluid bed-plasma demonstration plant to investigate the tar evolution in the syngas, with special attention on the chemistry of generic and sulphur-substituted aromatics within the plasma stage. The organic fraction in the gas phase was found to be completely reformed under plasma conditions, leaving essentially CO, H2and H2S as ultimate products. In particular, reduction efficiencies typically exceeded 96%v/v for complex organics (e.g. PAH) and thiophenes were observed. The syngas, after a tertiary simplified gas cleaning process, is suitable for high efficiency power generation, or conversion to a fuel gas capable of injection into national or industrial supply grids.

  • Technical aspects and thermodynamic evaluation of a two-stage fluid bed-plasma process for solid Waste Gasification
    2013
    Co-Authors: Massimiliano Materazzi, Chris Chapman, Luca Mazzei, Paola Lettieri, Richard Taylor
    Abstract:

    This study focused on the thermodynamic assets of using a two-stage process for solid Waste Gasification over the conventional single fluid bed approach. The study effectively demonstrated that the two-stage Gasification system significantly improves the gas yield of the system and the carbon conversion efficiency, which are crucial in fluid bed systems, whilst maintaining high energy performances. INTRODUCTION Most of the Gasification systems from Waste are based on high-temperature techniques that use oxygen as a source of heat or as partial oxidation agent. Among all Waste Gasification technologies, fluidized bed reactors are the most promising, for a number of reasons (1). In particular, the enhanced flow mixing between reactants, the nearly constant temperature and the great operating flexibility of fluidized bed reactors make it possible to utilize different types of feedstock, including biomass and solid Wastes. These gasifiers usually work as ‘‘partial combustors’’, and a portion of the carbon present in the fuel is combusted to support pyrolysis and Gasification reactions. Because of the relatively low temperature used to prevent agglomeration and sintering of bed material, the gas that is produced by a standard fluid bed gasifier (FBG) has tars and other condensable organic species that are technically difficult and costly to remove. Furthermore, the bottom ash/char that is generated in the gasifier or pyrolysis fluid bed reactor may contain high levels of carbon, heavy metals and organic pollutants which lower the conversion efficiency of the process and limit any secondary usage. Tar generation and ash disposal represent the strongest barrier for use of FBG for Waste treatment, whereas sufficing for both is only possible with expensive cleaning systems and further processing. The use of plasma systems has increasingly been applied with thermal Waste treatment for its ability to completely decompose the input Waste material into a tarfree synthetic gas and an inert, environmentally stable, vitreous material known as slag. The principal advantages that plasma offers to thermal conversion processes, besides the already mentioned tar/ash related issues absence, are a smaller installation size for a given Waste throughput, and the use of electricity as energy source, characteristics which permit the technology to treat a wide range of low calorific value materials including various hazardous Waste, such as PCBs, medical Waste, and low-level radioactive Wastes. Its efficient application in the treatment of general Waste is still under debate though, due to the power required to convert the solid Waste to a gas. Only additions of combustion heat supplied by the Waste feedstock or a fuel additive make the process suited to large Waste streams. In applying the plasma technology to the gaseous products from a fluid bed gasifier, an advanced two-stage thermal process is able to achieve efficient cracking of the complex organics to the primary syngas constituents whilst limiting the electrical energy demand of the process. The purpose of this work is to model a fluidized Waste Gasification system followed by a plasma converter in order to identify the relevant parameters in the design and operation of such an innovative technology and to compare single stage fluid beds with two-stage systems to determine if there are meaningful differences among them. The feedstock consists of different types of refuse derive fuel (RDF) produced from a combination of residual municipal, commercial and industrial Wastes. TWO STAGE Gasification CONCEPTS The physical and chemical processes which take place between the Gasification agents and the fresh solid feed in the conversion route to synthesis gas are complex, influenced by varying feed, process design and operating conditions; nonetheless, the Gasification chemistry may be considered as a two distinct conversion mechanisms. When biomass particles are rapidly heated at high temperature (above 600 °C) in the reactor, more than 80% of their (dry) mass is rapidly converted into permanent gases and organic vapours, leaving only a variable amount of char and few mineral ashes in the solid phase. This first step is usually referred to as pyrolysis, wherein water vapour, organic liquids and non-condensable gases, such as CO, H2, CO2, are separated from the solid carbon (i.e. char) and ash content of the fuel. The vapour/liquid product comprises mostly of polyaromatic hydrocarbons (PAHs) and tar (i.e. dark, oily, viscous material, consisting mainly of heavy organic and mixed oxygenates). Subsequently, the volatiles and char undergo a second Gasification step and they modify their composition due to the occurrence of several reactions becoming the final syngas (see Table 1). Most of these reactions are endothermic and require a consistent amount of energy to proceed. Reaction name Biomass Gasification Energy (kJ/mol) Exothermic: Combustion 2 2 (Char / Volatiles) C O CO + AE -398.3 Partial oxidation 2 (Char / Volatiles) C 1 2O CO + AE -123.1 Water gas shift 2 2 2 CO H O H CO + + -40.9 CO methanation (I) 2 4 2 CO 3H CH H O + + -217.0 CO methanation (II) 2 4 2 2CO 2H CH CO + + -257.0 Endothermic: Pyrolysis 4 2 2 Biomass Char Volatiles CH CO H N AE + + + + + +200-400 Methane steam reforming 4 2 2 CH H O CO 3H + + 206.0 Water gas/steam carbon 2 2 2 (Char / Volatiles) C H O CO H + AE + 118.4 Boudouard 2 (Char / Volatiles) C CO 2CO + AE 159.9 Table 1. Typical Gasification reactions (1) The distinction between primary and secondary conversion is based on the different times of conversion of the various processes. Experimental studies have shown that as a result of the rapid heating of the fuel, 90% of devolatilization takes place in a matter of milliseconds, whereas the reminder of Gasification processes (mainly heterogeneous reactions) take one or two orders of magnitude longer time (2). From this general concept originates the idea of dividing the Gasification process in two different reactor design arrangements, namely ‘single-stage’ and ‘multi-stage’ groups. The aim of a ‘single-stage’ fluid bed gasifier is to convert organic substances entirely in one reactor. Depending on the type of operation, the solid fuel is injected into the hot environment, together with oxygen and steam. As the fuel particles devolatize, the hydrocarbons volatiles undergo gas-phase reaction with the most reactive species in the ambient gas, that is, oxygen. Thus, the oxygen supplies the required heat by reacting with the reactive volatiles (3). The two-stage concept design physically separates the principal unit operations of pyrolysis-preliminary Gasification zone from the final conversion zone, involving two different levels of heat intakes. Most of this type of advanced thermal processes eliminates char Gasification as a limiting process step and, consequently, the efficiency of the process depends on how the conversion is organized. In a single stage process, the residual char reacts heterogeneously with the steam and CO2 with a slow and highly endothermic process that is often accelerated to practical rates by the use of additional oxygen to keep the temperature high. The concept of two-stage Gasification is based on providing longer residence time whilst making a more efficient use of the oxygen required to support the endothermic steam reactions. Figure 1 shows the effects of oxygen availability within the Gasification reactions on the syngas calorific value, with a maximum achieved at a stoichiometric ratio (the ratio between the oxygen available and that required for complete combustion) of around 0.4, a value that depends on the composition of the RDF/Waste being utilised as a feedstock. 0 100 200 300 400 500 600

  • Thermodynamic modelling and evaluation of a two-stage thermal process for Waste Gasification
    Fuel, 2013
    Co-Authors: Massimiliano Materazzi, Richard Taylor, Luca Mazzei, Paola Lettieri, Chris Chapman
    Abstract:

    AbstractTar generation and ash disposal represent the strongest barrier for use of fluid bed Gasification for Waste treatment, whereas sufficing for both is only possible with expensive cleaning systems and further processing. The use of plasma within an advanced two-stage thermal process is able to achieve efficient cracking of the complex organics to the primary syngas constituents whilst limiting the electric power demand. This study focused on the thermodynamic assets of using a two-stage thermal process over the conventional single-stage approach. These include, for example, the fact that the primary thermal Waste decomposition is performed in conditions of optimal stoichiometric ratio for the Gasification reactants. Furthermore, staging the oxidant injection in two separate intakes significantly improves the efficiency of the system, reducing the plasma power consumption. A flexible model capable of providing reliable quantitative predictions of product yield and composition after the two-stage process has been developed. The method has a systematic structure that embraces atom conservation principles and equilibrium calculation routines, considering all the conversion stages that lead from the initial Waste feed to final products. The model was also validated with experimental data from a demonstration plant. The study effectively demonstrated that the two-stage Gasification system significantly improves the gas yield of the system and the carbon conversion efficiency, which are crucial in other single stage systems, whilst maintaining high energy performances

Abel Rouboa - One of the best experts on this subject based on the ideXlab platform.

  • thermodynamic evaluation of portuguese municipal solid Waste Gasification
    Journal of Cleaner Production, 2016
    Co-Authors: Nuno Couto, Valter Silva, Abel Rouboa
    Abstract:

    Abstract Through a massive population growth, Waste management has become one of the main concerns of our time. In Portugal the growing volume of municipal solid Waste has become a central problem for municipalities, due to lack of space and the high costs to solve it. Gasification has received a renewed interest in the municipal solid Waste treatment since it limits dioxins formation, presents higher efficiency and requires less expensive gas cleaning equipment when compared to available methods gas. However, to make to process more appealing to both private sector and government institutions is necessary to overcome some concerns related to the process. To expedite the mainstream of this technology a first and second law analysis conducted on Portuguese municipal solid Waste was carried out. The thermodynamic method was coupled with a previously developed numerical model. Said model was validated using data from a pilot scale plant. Both energy and exergy values were investigated in order to evaluate Portuguese municipal solid Waste as an energy source. Optimal operating point was found at 900 °C for an equivalent ratio of 0.25. Tar content energy values decreased as high as 80% when temperature was increased to 900 °C. A comparison between several Gasification efficiencies was investigated.

  • municipal solid Waste Gasification in semi industrial conditions using air co2 mixtures
    Energy, 2016
    Co-Authors: Nuno Couto, Valter Silva, Abel Rouboa
    Abstract:

    The Gasification of MSW (municipal solid Wastes) using CO2 as a gasifying agent has been object of growing interest in recent years. Although quite limited, studies have shown that CO2 can behave as a catalyst and accelerate the thermal cracking of volatiles as well as minimize tar formation, and even give a positive contribute to environment. Despite these promising features, it is still necessary to develop mathematical models able to assist the advance of this technology. A previously published numerical model validated for numerous substrates (including MSW) and operating conditions in a pilot scale plant was used as a baseline to study MSW Gasification with air-CO2 mixtures. Real MSW data from Oporto metropolitan area were used as model inputs and numerical results were validated against experimental ones. Results demonstrate that increasing CO2 content boosts carbon conversion, CO2 conversion, and cold gas efficiency while mitigating tar production. Also, due to the ability to tailor H2/CO ratio, air-CO2 mixtures can be used for catalyst-based Fischer–Tropsch synthesis and particularly for the production of specific chemicals such as urea, methanol and acetic acid.

Soon-mo Hwang - One of the best experts on this subject based on the ideXlab platform.

  • Hydrogen recovery from the thermal plasma Gasification of solid Waste.
    Journal of hazardous materials, 2011
    Co-Authors: Youngchul Byun, Moohyun Cho, Jae Woo Chung, Won Namkung, Hyeon Don Lee, Sung Duk Jang, Young-suk Kim, Jin-ho Lee, Carg-ro Lee, Soon-mo Hwang
    Abstract:

    Thermal plasma Gasification has been demonstrated as one of the most effective and environmentally friendly methods for solid Waste treatment and energy utilization in many of studies. Therefore, the thermal plasma process of solid Waste Gasification (paper mill Waste, 1.2 ton/day) was applied for the recovery of high purity H(2) (>99.99%). Gases emitted from a Gasification furnace equipped with a nontransferred thermal plasma torch were purified using a bag-filter and wet scrubber. Thereafter, the gases, which contained syngas (CO+H(2)), were introduced into a H(2) recovery system, consisting largely of a water gas shift (WGS) unit for the conversion of CO to H(2) and a pressure swing adsorption (PSA) unit for the separation and purification of H(2). It was successfully demonstrated that the thermal plasma process of solid Waste Gasification, combined with the WGS and PSA, produced high purity H(2) (20 N m(3)/h (400 H(2)-Nm(3)/PMW-ton), up to 99.99%) using a plasma torch with 1.6 MWh/PMW-ton of electricity. The results presented here suggest that the thermal plasma process of solid Waste Gasification for the production of high purity H(2) may provide a new approach as a future energy infrastructure based on H(2).

Massimiliano Materazzi - One of the best experts on this subject based on the ideXlab platform.

  • Waste Gasification processes for SNG production
    Substitute Natural Gas from Waste, 2019
    Co-Authors: Hermann Hofbauer, Massimiliano Materazzi
    Abstract:

    Abstract Gasification is the conversion of solid or liquid feedstock into useful and convenient synthetic gas (or syngas) that can be burned to release energy or used for production of high-value chemicals and fuels, including synthetic natural gas (SNG). When biomass or Waste are used as Gasification feedstock, large part of the biogenic carbon is retained in the final product, making it a perfect renewable (i.e., “bio”) alternative to fossil fuels. Bio-SNG, for example, is a product which can be fed into an already existing and also well-functioning infrastructure all over Europe and also in many other parts in the world. Therefore, bio-SNG is one of these products that has a large distribution potential and also allows a fast transition from a fossil-based to a renewable-based energy carrier. The production technology for SNG from syngas is also well known since decades and available in large scale which makes this process a possible candidate for frontrunner. The question is only, whether the reliable technologies for Waste Gasification and gas upgrading for supply of a suitable syngas ready for bio-SNG production can be established in the near future. This chapter will give an overview of the state of the art of Waste Gasification and will present promising technologies for further development.

  • Reforming of tars and organic sulphur compounds in a plasma-assisted process for Waste Gasification
    Fuel Processing Technology, 2015
    Co-Authors: Massimiliano Materazzi, Richard Taylor, Luca Mazzei, Paola Lettieri, Chris Chapman
    Abstract:

    Abstract Waste Gasification is considered a valuable and sustainable solution to the production of clean energy (via gas turbines or gas engines) and bio-fuels, such as synthetic natural gas and bio-hydrogen, provided that the syngas produced in the gasifier is free of condensable tars and organic sulphur contaminants that cause equipment fouling and deactivation of catalytic stages downstream. In particular, catalytic reaction stages are highly sensitive to specific trace contaminants (e.g. PAHs, thiophenes, etc.), necessitating the use of additional cleaning operations to remove these residues to levels where the catalyst degradation is acceptable. In this work, the use of thermal plasma (coupled with primary Waste treatment) to completely reform tars and organic sulphur compounds to simple gaseous products (predominantly H2and CO) is assessed. To this end, a 20-hour Waste Gasification run was performed on a two-stage fluid bed-plasma demonstration plant to investigate the tar evolution in the syngas, with special attention on the chemistry of generic and sulphur-substituted aromatics within the plasma stage. The organic fraction in the gas phase was found to be completely reformed under plasma conditions, leaving essentially CO, H2and H2S as ultimate products. In particular, reduction efficiencies typically exceeded 96%v/v for complex organics (e.g. PAH) and thiophenes were observed. The syngas, after a tertiary simplified gas cleaning process, is suitable for high efficiency power generation, or conversion to a fuel gas capable of injection into national or industrial supply grids.

  • Technical aspects and thermodynamic evaluation of a two-stage fluid bed-plasma process for solid Waste Gasification
    2013
    Co-Authors: Massimiliano Materazzi, Chris Chapman, Luca Mazzei, Paola Lettieri, Richard Taylor
    Abstract:

    This study focused on the thermodynamic assets of using a two-stage process for solid Waste Gasification over the conventional single fluid bed approach. The study effectively demonstrated that the two-stage Gasification system significantly improves the gas yield of the system and the carbon conversion efficiency, which are crucial in fluid bed systems, whilst maintaining high energy performances. INTRODUCTION Most of the Gasification systems from Waste are based on high-temperature techniques that use oxygen as a source of heat or as partial oxidation agent. Among all Waste Gasification technologies, fluidized bed reactors are the most promising, for a number of reasons (1). In particular, the enhanced flow mixing between reactants, the nearly constant temperature and the great operating flexibility of fluidized bed reactors make it possible to utilize different types of feedstock, including biomass and solid Wastes. These gasifiers usually work as ‘‘partial combustors’’, and a portion of the carbon present in the fuel is combusted to support pyrolysis and Gasification reactions. Because of the relatively low temperature used to prevent agglomeration and sintering of bed material, the gas that is produced by a standard fluid bed gasifier (FBG) has tars and other condensable organic species that are technically difficult and costly to remove. Furthermore, the bottom ash/char that is generated in the gasifier or pyrolysis fluid bed reactor may contain high levels of carbon, heavy metals and organic pollutants which lower the conversion efficiency of the process and limit any secondary usage. Tar generation and ash disposal represent the strongest barrier for use of FBG for Waste treatment, whereas sufficing for both is only possible with expensive cleaning systems and further processing. The use of plasma systems has increasingly been applied with thermal Waste treatment for its ability to completely decompose the input Waste material into a tarfree synthetic gas and an inert, environmentally stable, vitreous material known as slag. The principal advantages that plasma offers to thermal conversion processes, besides the already mentioned tar/ash related issues absence, are a smaller installation size for a given Waste throughput, and the use of electricity as energy source, characteristics which permit the technology to treat a wide range of low calorific value materials including various hazardous Waste, such as PCBs, medical Waste, and low-level radioactive Wastes. Its efficient application in the treatment of general Waste is still under debate though, due to the power required to convert the solid Waste to a gas. Only additions of combustion heat supplied by the Waste feedstock or a fuel additive make the process suited to large Waste streams. In applying the plasma technology to the gaseous products from a fluid bed gasifier, an advanced two-stage thermal process is able to achieve efficient cracking of the complex organics to the primary syngas constituents whilst limiting the electrical energy demand of the process. The purpose of this work is to model a fluidized Waste Gasification system followed by a plasma converter in order to identify the relevant parameters in the design and operation of such an innovative technology and to compare single stage fluid beds with two-stage systems to determine if there are meaningful differences among them. The feedstock consists of different types of refuse derive fuel (RDF) produced from a combination of residual municipal, commercial and industrial Wastes. TWO STAGE Gasification CONCEPTS The physical and chemical processes which take place between the Gasification agents and the fresh solid feed in the conversion route to synthesis gas are complex, influenced by varying feed, process design and operating conditions; nonetheless, the Gasification chemistry may be considered as a two distinct conversion mechanisms. When biomass particles are rapidly heated at high temperature (above 600 °C) in the reactor, more than 80% of their (dry) mass is rapidly converted into permanent gases and organic vapours, leaving only a variable amount of char and few mineral ashes in the solid phase. This first step is usually referred to as pyrolysis, wherein water vapour, organic liquids and non-condensable gases, such as CO, H2, CO2, are separated from the solid carbon (i.e. char) and ash content of the fuel. The vapour/liquid product comprises mostly of polyaromatic hydrocarbons (PAHs) and tar (i.e. dark, oily, viscous material, consisting mainly of heavy organic and mixed oxygenates). Subsequently, the volatiles and char undergo a second Gasification step and they modify their composition due to the occurrence of several reactions becoming the final syngas (see Table 1). Most of these reactions are endothermic and require a consistent amount of energy to proceed. Reaction name Biomass Gasification Energy (kJ/mol) Exothermic: Combustion 2 2 (Char / Volatiles) C O CO + AE -398.3 Partial oxidation 2 (Char / Volatiles) C 1 2O CO + AE -123.1 Water gas shift 2 2 2 CO H O H CO + + -40.9 CO methanation (I) 2 4 2 CO 3H CH H O + + -217.0 CO methanation (II) 2 4 2 2CO 2H CH CO + + -257.0 Endothermic: Pyrolysis 4 2 2 Biomass Char Volatiles CH CO H N AE + + + + + +200-400 Methane steam reforming 4 2 2 CH H O CO 3H + + 206.0 Water gas/steam carbon 2 2 2 (Char / Volatiles) C H O CO H + AE + 118.4 Boudouard 2 (Char / Volatiles) C CO 2CO + AE 159.9 Table 1. Typical Gasification reactions (1) The distinction between primary and secondary conversion is based on the different times of conversion of the various processes. Experimental studies have shown that as a result of the rapid heating of the fuel, 90% of devolatilization takes place in a matter of milliseconds, whereas the reminder of Gasification processes (mainly heterogeneous reactions) take one or two orders of magnitude longer time (2). From this general concept originates the idea of dividing the Gasification process in two different reactor design arrangements, namely ‘single-stage’ and ‘multi-stage’ groups. The aim of a ‘single-stage’ fluid bed gasifier is to convert organic substances entirely in one reactor. Depending on the type of operation, the solid fuel is injected into the hot environment, together with oxygen and steam. As the fuel particles devolatize, the hydrocarbons volatiles undergo gas-phase reaction with the most reactive species in the ambient gas, that is, oxygen. Thus, the oxygen supplies the required heat by reacting with the reactive volatiles (3). The two-stage concept design physically separates the principal unit operations of pyrolysis-preliminary Gasification zone from the final conversion zone, involving two different levels of heat intakes. Most of this type of advanced thermal processes eliminates char Gasification as a limiting process step and, consequently, the efficiency of the process depends on how the conversion is organized. In a single stage process, the residual char reacts heterogeneously with the steam and CO2 with a slow and highly endothermic process that is often accelerated to practical rates by the use of additional oxygen to keep the temperature high. The concept of two-stage Gasification is based on providing longer residence time whilst making a more efficient use of the oxygen required to support the endothermic steam reactions. Figure 1 shows the effects of oxygen availability within the Gasification reactions on the syngas calorific value, with a maximum achieved at a stoichiometric ratio (the ratio between the oxygen available and that required for complete combustion) of around 0.4, a value that depends on the composition of the RDF/Waste being utilised as a feedstock. 0 100 200 300 400 500 600

  • Thermodynamic modelling and evaluation of a two-stage thermal process for Waste Gasification
    Fuel, 2013
    Co-Authors: Massimiliano Materazzi, Richard Taylor, Luca Mazzei, Paola Lettieri, Chris Chapman
    Abstract:

    AbstractTar generation and ash disposal represent the strongest barrier for use of fluid bed Gasification for Waste treatment, whereas sufficing for both is only possible with expensive cleaning systems and further processing. The use of plasma within an advanced two-stage thermal process is able to achieve efficient cracking of the complex organics to the primary syngas constituents whilst limiting the electric power demand. This study focused on the thermodynamic assets of using a two-stage thermal process over the conventional single-stage approach. These include, for example, the fact that the primary thermal Waste decomposition is performed in conditions of optimal stoichiometric ratio for the Gasification reactants. Furthermore, staging the oxidant injection in two separate intakes significantly improves the efficiency of the system, reducing the plasma power consumption. A flexible model capable of providing reliable quantitative predictions of product yield and composition after the two-stage process has been developed. The method has a systematic structure that embraces atom conservation principles and equilibrium calculation routines, considering all the conversion stages that lead from the initial Waste feed to final products. The model was also validated with experimental data from a demonstration plant. The study effectively demonstrated that the two-stage Gasification system significantly improves the gas yield of the system and the carbon conversion efficiency, which are crucial in other single stage systems, whilst maintaining high energy performances