Article pubs.acs.org/EF
Impact of Biomass Ash−Bauxite Bed Interactions on an Indirect Biomass Gasifier Jelena Marinkovic,* Martin Seemann, Georg L. Schwebel, and Henrik Thunman Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, 412 96 Göteborg, Sweden S Supporting Information *
ABSTRACT: The influence of interactions between biomass ash and bauxite bed material on the performance of an indirect gasification system was investigated. With bauxite as the starting material, changes in gasification performance over time were assessed through measurements of gas composition, the amount of tar in the gas, cold gas efficiency, and carbon conversion. As the operation progressed and interactions with ash elements occurred, the bauxite showed increased activities for tar decomposition and carbon conversion. Additional experiments with solid samples that were extracted from the gasifier were performed in a laboratory-scale reactor and revealed the activity of the bauxite toward a water−gas shift reaction as well as an increased ability to transport oxygen. Further analyses of the same solid samples showed that the bauxite had been enriched with ash elements; specifically, catalytically active alkali metals were detected in leachable form. In general, bauxite has been shown as an interesting material from a fuel conversion point of view. However, the effect on fuel conversion has been overshadowed by the high oxygen transport ability of bauxite. as the composition and purity of the produced gas.5−7 Such bed materials need to be durable, as they alternate between two harsh environments: a highly oxidative one in the boiler and a reducing one in the gasifier. If a metal oxide-based material was to be used, this change in environment would mean that part of the produced gas would be lost due to the release of oxygen from the bed material within the gasification part of the unit and its subsequent reaction with the raw gas. This would result in the production of a gas that had a lower chemical energy. In this work, the possibility for optimization of the DFB gasification system using bauxite as the bed material was investigated. The effects of the interactions between biomass ash and bauxite on the performance of the gasification process were assessed by measuring the composition of the produced gas, the amount of tar in the product gas, and bed material changes. As the process parameters were similar throughout the period of operation, the effects on performance could be correlated to the bed material−biomass ash interactions, e.g., the aging of the bed material in the system.
1. INTRODUCTION Conversion of biomass via allothermal gasification is a potential route for energy production.1−3 Depending on the availability of biomass and whether the biomass resources are handled in a sustainable way, a renewable gas that consists of CH4, H2, CO, CO2, and hydrocarbons can be produced. However, the gas composition can vary depending on the operational conditions and the type of bed material used. In particular, different concentrations of heavy hydrocarbon components (tars) determine the quality of the gas, as they are intrinsically linked with both the efficiency and trouble-free operation of the process.4 The dual fluidized bed (DFB) gasification system (Figure 1) is based on a circulating bed material that is heated in the combusting unit and circulated to the gasification unit where it delivers heat for the gasification process. Although an inert bed material is sufficient for the operation of a DFB gasifier, active bed materials are used to influence the fuel conversion as well
2. THEORY In a DFB setup, the bed material is subjected to environmental and temperature changes while it is circulating. Environmental changes occur not only between the two reactors, the boiler and the gasifier, but also locally within the reactors. A simplified scheme for the different environmental regions of the system is presented in Figure 1. In the boiler, the bed material is exposed to an oxidative environment in the upper part of the reactor, whereas in the region close to the fuel feed, the bed material encounters a locally reducing environment. Within the gasifier, Received: January 22, 2016 Revised: April 14, 2016
Figure 1. Schematic of a DFB system with a simplified description of the different environments. © XXXX American Chemical Society
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DOI: 10.1021/acs.energyfuels.6b00157 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
aluminum was found to be a physical event. Therefore, the reactions between aluminosilicates and alkali are a mixture of chemical and physical processes, depending on the concentration of Si in the material.26 Several studies have investigated alkali-aluminum oxide catalysts and their activities.27−29 Even alumina itself has been shown to be active for tar reduction as compared to that of the inert material. Simell et al.4 examined the influences of aluminabased and alumina-silica-based materials on the tar species in the gas derived from biomass gasification. The tests were performed in a fixed bed reactor at 900 °C, and it was concluded that both silica-alumina and activated alumina had activities for tar reduction as compared to that of the inert material; a decrease in concentration was noticed for all the tar groups, whereas the gas composition showed an increased H2/ CO ratio. Amenomiya et al.29 observed improved activity for the the water−gas shift (WGS) reaction with alkali impregnation of an alumina catalyst, and they concluded that the activity increased until surface saturation was achieved. The same group described how reactive oxygen atoms on the surface of alumina became more active in the presence of potassium. Alkali salts have even been investigated as catalysts for carbon and coal gasification.30−33 The effects of the same species on the reduction of tar have also been reported.34
the bed material is exposed to a very reducing environment in the region where the fuel is undergoing devolatilization, whereas it experiences a mildly oxidative zone at the bottom of the bed where the steam is fed into the system. In addition, during its circulation between the two reactors, the bed material is exposed to mildly oxidative environments in the two loop seals. Besides the bed material and its constituents, the ash elements that interact with the bed material and alter its physicochemical properties will, together with other operational parameters, determine the final gas composition. The most interesting ash elements, which are those that are most heavily involved in the reactions with both the bed material and the raw gas, comprise K, Ca, Si, S, and Cl. The content and distribution of the ash elements are biomass dependent.8 The interactions of ash with the bed material can have negative consequences, e.g., agglomeration of the bed,9 or positive consequences, e.g., increased catalytic properties of the bed.10 The pathway for this interaction reflects the type of bed material, composition and content of the biomass ash, temperature, and reductive potential of the surrounding gases. The reactions that involve ash-forming elements are numerous and complex. Chemical equilibrium-based software is often used to describe the aforementioned chemistry within the system.9,11−13 However, in reality, the system is not in a steady state, and the use of minerals as bed materials for the process entails limitations with respect to the selection of all the relevant species that should be included in the calculation. Therefore, this type of software can only be used as an indicative tool and as a supplement to other results. Reflecting on the complexity of the system and the chemical interactions therein, the bed material for the process has to be selected carefully. The bed material needs to have the following features: good mechanical properties (resistance to thermal and mechanical degradation), acceptable chemical properties (catalytic activity, low oxygen-carrying capacity, resistance to chemical degradation, and desirable interactions with ashforming elements), and low price on the market (good availability and low cost for disposal). Natural ores have been widely investigated as bed materials that can be used in DFB systems.10,14−19 Being composed of an oxide, these ores have some catalytic activity. As they are natural materials, the costs related to their availability and disposal are significantly lower than those for a synthetically produced materials. Naturally occurring, aluminum-based materials have been extensively studied as alkali-adsorbing materials in combustion systems, where the following reaction with alkali-chloride20 occurs:
3. EXPERIMENTAL SECTION 3.1. System Description. The measurements were performed in the Chalmers DFB system35 in which the boiler is a circulating fluidized bed and the gasifier is a bubbling fluidized bed that is fluidized with steam. The two reactors are connected via two loop seals. Wood chips are used as fuel for the boiler, and wood pellets are used as fuel for the gasifier. The results of the analyses of the wood chips and pellets used are shown in Table 1. A known flow of helium is introduced into the gasifier as a tracer gas to quantify the fuel conversion. A small stream of the produced raw gas is led from the raw gas line to the gas cleaning system, where heavy hydrocarbons and water are removed. Thereafter, the dry gas composition is analyzed in a micro-GC. Before entering the gas cleaning system, heavy hydrocarbons are sampled from the raw gas. Sampling and further
Table 1. Fuel Analyses of the Wood Chips and Wood Pellets Used in the Process
MCl(g) + Al 2O3 + H 2O(g) → M 2O· Al 2O3 + 2HCl(g)
For this purpose, kaolin, bauxite, and emathlite have been used.21−23 In these studies, all of the tested materials showed good adsorption properties. For this application, kaolin is superior to bauxite because the adsorption of alkalis to kaolinite is irreversible, and the achievable saturation levels are higher.22,24 Bauxite has been shown to be the most sensitive to process fluctuations, which as a consequence has the partial release of the alkali back into the gas phase. Differences in the behaviors of the materials have been attributed to different modes of alkali adsorption.22−24 In the study conducted by Lee et al.,25 adsorption of the alkali onto silicate was shown to be chemical in nature, whereas adsorption of the alkali onto B
weight %
wood chips 1
wood chips 2
wood pellets 1
wood pellets 2
C H O N S Cl ash Al Si Fe Ti Mn Mg Ca Ba Na K P LHV (MJ/kg)
49.7 5.9 44.0 0.16
