Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Online Measurements of Alkali Metals during Start-up and Operation of an Industrial-Scale Biomass Gasification Plant Dan Gall,*,† Mohit Pushp,‡ Anton Larsson,§ Kent Davidsson,‡ and Jan B. C. Pettersson*,† †
Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, SE-412 96 Gothenburg, Sweden RISE Research Institutes of Sweden AB, Borås, Sweden § Göteborg Energi AB, Gothenburg, Sweden ‡
ABSTRACT: Alkali metal compounds may have positive influences on biomass gasification by affecting char reactivity and tar reforming but may also disturb the process by formation of deposits and agglomerates. We herein present results from online measurements of alkali compounds and particle concentrations in a dual fluidized bed gasifier with an input of 32 MWth. A surface ionization detector was used to measure alkali concentrations in the product gas, and aerosol particle measurement techniques were employed to study concentrations and properties of condensable components in the gas. Measurements were performed during start-up and steady-state operation of the gasifier. The alkali concentration increased to approximately 200 mg m−3 when fuel was fed to the gasifier and continued to rise during activation of the olivine bed by addition of potassium carbonate, while the alkali concentration was in the range from 20 to 60 mg m−3 during steady-state operation. Addition of fresh bed material and recirculated ash had noticeable effects on the observed alkali concentrations, and K2CO3 additions to improve tar chemistry resulted in increased levels of alkali in the product gas. Addition of elemental sulfur led to reduced concentrations of CH4 and heavy tars, while no clear influence on the alkali concentration was observed. The study shows that alkali concentrations are high in the product gas, which has implications for the fluidized bed process, tar chemistry, and operation of downstream components including coolers, filters, and catalytically active materials used for product gas reforming. reforming of tars to permanent gases,11,12 and this is the primary reason why K2CO3 is sometimes added to a gasification process.13 Considering the role of alkali in gasification processes, online monitoring of alkali concentrations is beneficial for understanding and controlling the process. However, while many studies have dealt with online alkali measurements in combustion, experimental results from studies under gasification conditions are relatively scarce. Laser-based methods including excimer laser induced fragmentation fluorescence (ELIF)14 and tunable diode laser absorption spectroscopy (TDLAS)15 have been applied to determine concentrations in situ. Other online methods rely on continuous extraction of product gas and include inductively coupled plasma optical emission spectroscopy (ICP-OES)16 and molecular beam mass spectrometry (MBMS).17 Recently, a volatility tandem differential mobility analyzer (VTDMA) method was used to study alkali and tar in a 2−4 MWth gasifier.18 The latter method is based on thermal stability of condensed material and may potentially be used to distinguish between the different types of alkali compounds. Online measurements have also been performed using19 surface ionization (SI)methods, and this is the method of interest in the present study. The SI method has been used in combustion studies,20,21 and a few applications to the more challenging conditions in gasification have been reported.22−24 Olsson et al. used SI techniques for online alkali detection in
1. INTRODUCTION The replacement of fossil fuels with biofuels to reduce the net emissions of CO2 is a strong driving force for current research and development in the energy sector. Biofuels can be produced using gasification in which a biogenic feedstock is thermochemically treated to produce a gas that may subsequently be converted to a range of useful products including transportation fuels. Biomass gasification is currently being refined to achieve higher efficiencies and reduce product costs,1−3 and one promising technique is dual fluidized bed (DFB) gasification.4−6 The product gas that exits the gasifier consists of permanent gases, tar, and inorganic species such as alkali metal compounds in gaseous or particulate form. The gas needs gas cleaning and upgrading before a useful product is generated, and this process becomes simpler and less costly if the gas is of good quality already as it leaves the gasifier. Tar compounds are undesirable because they condense as the product gas is cooled and may result in clogging of downstream equipment.7 Tar formation also constitutes a loss, as the compounds instead could have been converted to valuable gases. Alkali on the other hand is known to cause agglomeration and deposits, with subsequent corrosion problems.8−10 Downstream equipment such as catalysts, coolers, and filters operating at temperatures below the condensation points of the gaseous alkali compounds are likely to develop deposits and thereby suffer from lower efficiency. As the temperature of the product gas decreases, alkali compounds nucleate and form submicron-size particles, adding to the total particle load. However, alkali released during gasification may have some advantageous features, e.g., by affecting the gasification rate of char.2 Alkali metals may also enhance the © XXXX American Chemical Society
Received: October 14, 2017 Revised: December 12, 2017 Published: December 13, 2017 A
DOI: 10.1021/acs.energyfuels.7b03135 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels pressurized fluidized bed gasification and for characterization of alkali emission from associated filter ash and fluidized bed samples.22 Kowalski et al. used a similar SI instrument and detected alkali species released from single grass pellets in a small laboratory-scale gasifier.23 Wellinger et al. used an alternative SI setup to increase the total area of the hot filament where ionization of alkali occurs and to reduce problems with turbulent flows and deposits caused by tars and particles,24 and the instrument was successfully applied in timeresolved measurements in a laboratory-scale BFB gasifier fed with wood pellets. A majority of earlier studies focused on alkali in laboratoryscale gasifiers and limited information are available concerning the conditions in industrial-scale processes. The overall aim of the present study is to characterize the alkali concentrations during operation of a 32 MWth DFB biomethane facility using online methods. Surface ionization and aerosol particle measurement techniques are used to study time-resolved concentrations and properties of alkali metal compounds and other condensable components. Of particular interest is the influence of changes in operational conditions on the concentration of condensable material, including start-up of the gasifier and the addition of alkali compounds, sulfur, bed material, and recirculated ash to the system.
with the bed material to the combustor. The combustion reactor is a circulating air-fluidized bed. Here, the char is combusted together with energy-rich off-streams from downstream processes as well as part of the product gas, which is fed back to the boiler so as to maintain and control the temperature of the process. The warm bed material is circulated back to the gasifier via a cyclone, thereby providing the heat required for the fuel conversion. The produced gas is a mixture of mainly H2, CO, CO2, H2O, CH4, C2H4, and a large variety of organic compounds, referred to as tar. The bed material is olivine, which acts both as a heat carrier and as an active material enhancing the biomass reforming. After so-called “activation” whereby the bed material chemisorbs compounds such as Ca and K, the olivine can contribute to a decrease of the total tar yield.26−29 2.2. Gasifier Operational Conditions. Two measurement campaigns were performed at GoBiGas during different operational stages of the gasifier. The first campaign (A) was carried out as the gasifier was being started after a maintenance stop, and it covers the start-up procedure and load increase until a subsequent shut-down, while the second campaign (B) was performed when the gasifier was in steady-state operation mode. The gasifier was fed with wood chips during both campaigns, and the results from a fuel analysis performed prior to measurement campaign B are given in Table 1. During the measurement campaigns, potassium carbonate, olivine, recirculated ash, and elemental sulfur were added, to induce changes in the product gas composition. Additions of K and olivine were determined by the operational procedure, in order to maintain the activity of the bed material with respect to tar reduction. The additions of S and recirculated ash were separated by 18 h of operation and may therefore be considered as independent. The operational settings used in campaigns A and B are summarized in Table 2. Potassium carbonate dissolved in water (40%mass of K2CO3) was added to the combustion side of the process. Potassium compounds act as a catalyst for most of the tar species, with somewhat higher activities for polycyclic aromatics and ketone/furan oxygenates.30 The amount of potassium to be added is based on an observed correlation between the concentrations of tar and methane, as has been described previously.13 Briefly, as the methane concentration increases, so does the tar concentration and vice versa. An increase in methane concentration is counteracted by increasing the feed of K2CO3, the flow of which can be regulated in the range of 0−30 l h−1. During start-up, a saturated K2CO3 solution was introduced for 10 min per hour, instead of continuous addition, in an attempt to saturate the potassium sinks and rapidly activate the bed material. Potassium and sodium are also added to the process via the fuel and via ash recirculation. The recirculated material mainly consists of small bed material particles that have exited the process with the flue gas and become captured in the disengagement zones of the flue gas cooling system. During cooling of the flue gas, alkali vapors condense on the existing aerosol particles, making them extra rich in alkali. Furthermore, as shown by Marinkovic et al.27 the effect of potassium can be further enhanced by adding sulfur to the combustor. Therefore, elemental sulfur in the form of granules was added to the combustion reactor at rates of 0−1.2 kg h−1. Note that sulfur is never added without potassium to the process, as this would have no positive effect.27 2.3. Extraction and Dilution System. Figure 2 shows a schematic of the gas extraction system used to sample and dilute the product gas from the upper part of the gasifier. The probe is composed of stainless steel (SS 316L) and is heated with an electrical ribbon. The outer part of the probe and the first diluter are covered by a heating box to prevent cold spots. The heated box contains a fan, a 1-kW heater element, and a temperature control unit. The probe and the box were designed in-house to fit the specific requirements of the sampling position. The probe is connected through an airtight housing on a
2. EXPERIMENTAL METHODS The measurements were performed at the 32-MWth (thermal input) GoBiGas gasifier, which is a unique large-scale facility used to produce biogas of a quality similar to commercial natural gas. The gasifier, the extraction and dilution system, the online instrumentation, and the methodology are described in the following subsections. 2.1. Description of the Gasifier. The GoBiGas gasifier has been described in detail by Alamia et al.25 The DFB gasification unit consists of a combustion reactor in which heat for the gasification is produced, and a gasifier where the fuel is converted into gas. A schematic of the gasification part is provided in Figure 1, and the reader is referred to
Figure 1. Simplified schematic of the GoBiGas DFB section. The sampling position is indicated by the blue circle. A detailed description of the GoBiGas plant is provided in ref 25. ref 25 for a more detailed description of the GoBiGas plant. The fuel is primarily fed to a BFB gasifier fluidized with steam where the fuel is devolatilized and partially gasified. Unconverted char is transported
Table 1. Ash Composition of the Fuel Used during Measurement Campaign B, Shown as Fractions of Dry Matter total ash (%)
S (%)
Cl (%)
Al, mg/kg
Fe, mg/kg
Ca, mg/kg
K, mg/kg
Mg, mg/kg
Na, mg/kg
Si, mg/kg
0.4
