Tar Removal in an Entrained Flow Cracker (EFC) with application to Biomass Gasification

Abstract

A project is carried out to develop a so called Entrained Flow Cracker (EFC) for tar removal downstream of a gasifier making use of cheap and active materials as catalysts. These catalysts convert tars in the producer gas to useful gases. In this reactor the fine catalyst is mixed intensively with the producer gas and then separated and recycled to the reactor and or to the gasifier. In the first stage of the project screening of catalysts was carried out in a fixed bed tubular reactor. Calcined dolomites and olivine are considered as such active and inexpensive catalysts for hot gas cleaning of a gas produced by biomass gasifiers. In addition to them, other cheap catalysts were investigated such as FCC catalyst, char and ashes. The efficiency of tar removal for these catalysts was compared with that of an inert material (silica sand) and with that of high activity catalyst (commercial nickel catalyst). Tar was modeled using naphthalene and its decomposition over the different catalysts was modeled by naphthalene reaction with CO2 and steam. Tests were carried out in a fixed bed tubular reactor at a temperature of 900 C under atmospheric pressure and a gas residence time in the catalyst bed of 0.3-0.4 s. For the selected catalysts, kinetic studies were carried out in the temperature range of 700 to 900 C and atmospheric pressure. A simple first order kinetic model was used to describe the tar conversion. The gas composition and tar content in the gas were analyzed downstream the reactor using solid-phase adsorption (SPA) sampling method in combination with gas chromatography and mass spectrometry (GC/MS). The results of the fixed bed experiments will be used in the design of the entrained flow reactor. INTRODUCTION Biomass can be converted to energy carriers by biological or thermochemical processes. Among the various thermochemical processes gasification has attracted significant interest. This is due to the higher efficiencies produced by this technology either for small or large-scale systems. One of the most important technical barriers in biomass gasification development is the removal of tars. These tars are always present in producer gas from the gasifier as a side product. If tar content in the exit gas is high, then it will condense in the gas transfer lines and cause severe plugging problems resulting in serious operational interruptions. For engines and turbines, high concentration of tar can damage them or lead to unacceptable level of maintenance. For most people working on gasification, high temperature catalytic tar removal is technically and economically interesting approach for gas cleaning. Such approach is intuitively interesting because it has the potential to increase conversion efficiencies while simultaneously eliminating the need for collection and disposal of tars. CATALYSTS Calcined dolomites and olivine are considered as active and inexpensive catalysts used for hot gas cleaning of a gas produced by biomass gasifiers. In addition to these catalysts, other catalysts are investigated such as FCC catalyst, char and ashes. Tar removal over calcined dolomites (MgO.CaO) is mostly due to steam and dry (CO2) reforming reactions [1]. The reacting network for the overall tar elimination over calcined dolomite is not known yet, but at least includes: reactions of steam and CO2, thermal cracking and hydrocracking of tars [2]. Simell et al., related the activity of dolomite to the large pore size, CaO surface area of the corresponding calcinates presence of active metal such as iron and may be due to its relatively high alkaline (K, Na) content [3]. The main problem with dolomite is its fragility [4]. It has a lower mechanical strength than olivine [5,6]. Delgado et al., (1996), presented the factors that influence the deactivation of dolomite: space time or space velocity, Particle diameter and temperature [7]. Simell et al., (1999), concluded that thermal reactions at a temperature of 900 C did not have a measurable effect on the decomposition rates of tar model component [8]. Olivine consists mainly of a silicate mineral in which magnesium and iron cations are embedded in the silicate tetrahedral. Natural olivine is ((Mg,Fe)2SiO4) [8]. Its mechanical strength is comparable to that of silica sand, even at high temperatures, it presents a higher attrition resistance than dolomite [4,5]. Olivine is available in the market at about the same price as that for dolomite (120 Euro per metric ton) [4]. FCC catalysts used nowadays are composite catalysts, made of zeolite component and a matrix component consisting mostly of amorphous silica alumina. FCC catalysts are readily poisoned by substances whose molecules react with the catalyst acidic sites. Basic nitrogen compounds and alkaline metals present in the feedstock show such poisoning effect. Corella et al., (1998) tested an “in equilibrium” spent catalyst in a fluidized bed and found that FCC catalyst was quickly elutriated from the bed [9]. Herguido et al., (1992) tested an “in equilibrium” spent FCC catalyst in a 15 cm i.d. rider-gasifier with a stable fluidized bed of sand at its bottom. Tar was reduced from 78 to 9 g/Nm with recirculation and continuous regeneration of the catalyst [10]. Charcoal is produced by heating wood under limited access of oxygen. When wood is heated slowly to 280°C, an exothermic reaction occurs. In the usual carbonization procedure, heating is prolonged to 400 to 500°C in the absence of air. Sjostrom et al., (1999) [11] run cogasification of birch wood with both Daw Mill coal and Polish bituminous coal in a pressurized fluidized bed reactor. They found that both tar and ammonia yields seemed lower in the cogasification experiments compared with the yields from gasification of the individual fuel. Joseph et al. (1996) [12] note that an entrained-flow vortex gasifier, with re-injection of char ensures that a larger proportion of tars and higher molecular weight hydrocarbons are cracked. Ash is a waste product from the gasification process. The chemical composition of ash determines the physical properties of the material such as softening, melting points or vaporization points [13]. The autocatalytic effect of ash is due to its minerals content [14]. The analysis of wood ash after gasification showed that there was a high content of alkali metals. Hauserman [14] investigated the use of wood ash as a possible catalyst for the production of hydrogen by steam gasification of wood and coals. The catalyst was directly added to the feed by dry mixing. He found that the addition of 20 wt.% wood ash increased the reactivity of the steam gasification reaction of the bituminous coal at 700 C by a factor of almost 9 and that of wood by a factor of 32 relative to the uncatlyzed results.