Technical aspects and thermodynamic evaluation of a two-stage fluid bed-plasma process for solid waste gasification

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