The positive effects of bed material coating on tar reduction in a dual fluidized bed gasifier

Abstract

The utilization of biomass for the substitution of fossil fuels to reduce greenhouse gas emissions in biomass steam gasification plants is a promising technology for the production of electricity, heat, and fuels for transportation. Experience from industrial scale dual fluidized bed steam gasification plants showed a modification of the bed material due to the interaction of the bed material (olivine) with biomass ash components and additives. In this paper the influence of bed material modification on the gasification properties of used olivine from an industrial scale plant in Güssing is compared with the case of fresh olivine. The trials were carried out under similar conditions in a pilot plant at the Vienna University of Technology. The pilot plant trials showed an increase in hydrogen and carbon dioxide in the product gas with the used bed material while the content of carbon monoxide in the product gas decreased. The exothermal water–gas shift reaction is enhanced by the used bed material, resulting in a lower energy demand for the gasification. Tar content was decreased by around 80% for tars detected by gas chromatography–mass spectrometry (GCMS) and the composition of the tar showed less components during the trial with used bed material.

The results obtained with the used bed material at the 100 kW pilot plant are in good agreement with those for the 8 MW industrial plant in Güssing, confirming good scale-up properties from the 100 kW plant to industrial scale plants.

Introduction

Biomass can play a significant role by substituting for fossil fuels which cause greenhouse gas emissions in the production of electricity and second generation biofuels [1]. Biomass is a renewable source and releases the same amount of carbon dioxide during energy usage or natural decomposition as it aggregates during the growth period. Using woody biomass for steam gasification is a promising technology that has been successfully demonstrated since late 2001.

The basic principle of biomass gasification is the thermal decomposition of biomass using a gasification agent such as steam, air, or carbon dioxide to produce a combustible gas consisting mainly of hydrogen, carbon monoxide, carbon dioxide, and methane. If air is used as a gasification agent a high amount of nitrogen is present in the producer gas, which reduces its heating value. The gas can be utilized for electricity production, fuel synthesis or synthetic natural gas.

Dual fluidized bed (DFB) steam gasification for biomass was developed by the Institute of Chemical Engineering, Vienna University of Technology [2]. A pilot plant (100 kW) is in operation at the Institute of Chemical Engineering, Vienna University of Technology, with various research activities concerning bed material [3], [4], [5] or different fuels [6], [7]. This process was brought to the market by the first plant in Güssing (Austria; 8 MW^th) in 2001, followed by plants in Oberwart (Austria; 8.5 MW^th), Villach (Austria; 15 MW^th), Ulm (Germany; 11.5 MW^th), Götheborg (Sweden; 32 MW^th), which commenced operation recently or are currently in the realization phase [4]. Various research activities concerning further utilization of the product gas in addition to production of heat and electricity are ongoing at the plants in Güssing and Oberwart [8], [9].

The advantage of DFB steam gasification is the usage of steam as a gasification agent to avoid the introduction of nitrogen into the producer gas. It results in a producer gas with a high caloric value of 12–14 MJ/Nm³ (referred to as dry gas) and a high hydrogen content. To supply the thermal energy that is required for the endothermic gasification reactions in the bubbling bed a separate combustion zone with a fast fluidized bed is used. The bed material, which is heated up in the combustion zone, is transferred to the gasification zone.

The main gasification reactions are shown in Table 1. These reactions are considered as equilibrium reactions with variable equilibrium conditions according to Le Chatelier’s principle depending on gas concentrations, temperature and pressure. Since various reaction paths are possible, an enhancement of reaction paths is possible through the use of catalysts.

Undesired by-products of biomass gasification are described by the collective term “tars”. Tars are defined as follows: “The organics, produced under thermal or partial-oxidation regimes (gasification) of any organic material, are called ‘tars’ and are generally assumed to be largely aromatic” [10].

These tars cause operational problems when cooled down and condensed at heat exchangers, plugging pipes, and so on. Various research activities focus on reduction of these tars before further utilization of the product gas. The chemical reactions concerning tar reduction are described in Table 2 for toluene as a model component. Similar reactions can be described for the other tar components. These chemical equations allow a wide range of reaction schemes and show the complexity of the topic.

To decrease the amount of tar in the product gas, catalytic material is used in the fluidized bed.

The use of olivine as a bed material has become state of the art in industrial scale DFB steam gasification plants. Additionally calcium-rich additives such as calcite or dolomite are used for further reduction of the tar content in the producer gas. Experience from the industrial scale plant in Güssing, Austria, showed that biomass ash and additives interact with bed material, building calcium-rich layers around the particles. The inner layer is homogeneous, composed mainly of calcium and silicate, while the outer layer has a similar composition to the fly ash of the plant [13]. Fig. 1 shows a micrograph of a used bed material particle and the results of energy-dispersive X-ray (EDX) spectroscopy are shown in Table 3. The positive effects of this calcium-rich layer on the tar reduction and the influence on the gas composition are known from the experience of the plant in Güssing. Experience also showed that the catalytic properties of the bed material improve with a higher retention time of the bed material in the system. This effect has not yet been described in detail or quantified.

The significant increase in calcium in the surface layer of the bed material leads to the assumption that the catalytic properties of the used bed material are dominated by the calcium-rich surface. Small amounts of potassium are also detected.

The positive effect of olivine on tar reduction has been reported by various authors with regard to various fuels [4], [14], [15], [16], [17], [18], [19], [20], [21]. A better catalytic activity of the olivine can be achieved by calcination of olivine [14], [22]. Even better conversion is achieved by the use of modified olivine such as Ni-olivine, Fe-olivine [4], [20], [23], [24], [25], [26] or synthetic catalysts [27], [28], [29].

Dolomite also showed good results with regard to the catalytic activity for reducing tars [3], [4], [18], [30], [31], [32], [33].

A comparison of dolomite and inert bed material was carried out by Ruoppolo et al. [27]. They reported a 50% higher tar conversion in a fluidized bed using dolomite as bed material compared to quartzite as well as a simplification of tar components.

Corella et al. [18] reported that the tar removal efficiency with dolomite was ∼1.4 times better compared to olivine in a fluidized reactor. In a previous publication the authors concluded [34] that the introduction of dolomite into the fluidized bed gives better results compared to a down-stream usage of dolomite in a second reactor. The polymerization of tars is suspected to take place when using a gasification reactor with silica sand and a down-stream reactor with dolomite.

A laboratory scale quartz tube reactor was used by Simell et al. [11] to study the decomposition of toluene, which was used as a model component for tar over dolomite and nickel catalyst. They concluded that the dry reforming reaction (Formulas (15) and (16)) and steam reforming reaction (Formulas (9) and (10)) take place with both catalysts. The presence of steam inhibits the dry reforming reaction but tar decomposition is carried out with steam reforming. The presence of CO on dolomite strongly inhibited the tar decomposition on dolomite.

Alarcón et al. [35] studied the catalytic activity of a mixture of CaO with MgO for naphthalene steam gasification in a fixed bed. While pure MgO and CaO achieved carbon conversion of 54% and 62%, respectively, a mixture of 10% CaO and 90% MgO showed the highest carbon conversion, 79%. A catalytic synergy between the two oxides was described.

Similar results were published by Delgado et al. [36]. The authors studied the catalytic activity of calcined dolomite, calcite, and magnesite in a fixed bed reactor reforming product gas of a fluidized bed gasification reactor. The fixed bed reactor was loaded with the examined catalyst. They reported better gas yields and tar reduction with dolomite followed by calcite and magnesite.

Kyotani et al. [37] investigated the mechanism of calcium catalysis of carbon gasification with oxygen. They found that calcium enhances the formation of CO₂ in carbon gasification with O₂. The process was explained as follows: O₂ dissociatively chemisorbs on CaO particles to form CaO(O). The active oxygen from CaO(O) quickly migrates to the carbon surface to form C(O). When active sites around CaO are occupied by C(O), oxygen reacts with the C(O) at the active site to CO₂ which is released leaving an active site on the CaO.

Nair et al. [38] studied the tar removal of biomass-derived fuel gas by pulsed corona discharge with respect to the decomposition scheme of naphthalene. They concluded that the most favorable pathway for tar decomposition is the direct attack of oxygen radicals.

This theory is also confirmed by studies by Chen and Yang [39] on alkali and earth alkali metals where the authors catalyzed gasification reactions of graphite by CO₂ and H₂O. They reported the formation of C–O–M groups, where M denotes for the metal. Oxygen radicals have their origin in CO₂ and H₂O. In earlier studies they found that particles are more active than single C–O–K groups [40].

However, the usage of bed materials with better catalytic activity than olivine is desired. Alternative bed materials to olivine often face problems with attrition (e.g. dolomite [4], [19]) or their preparation is expensive (e.g. synthetic catalysts). The disposal of wastes of alternative bed materials such as nickel coated bed material is also problematic.

An exact statement about the influence of the formation of a calcium-rich layer on the catalytic properties of the bed material is not available. To study this effect, used bed material from the industrial scale gasification plant in Güssing, Austria, was used in the 100 kW DFB steam gasification pilot plant at the Institute of Chemical Engineering, Vienna University of Technology, and compared with the results of the utilization of unused olivine under the same conditions.

This paper summarizes the latest investigations of the catalytic properties of used olivine with a calcium-rich layer with regard to gas composition and tar reduction, and the results from an industrial scale plant are compared with results from the pilot plant.