Article pubs.acs.org/EF
Online Measurements of Alkali and Heavy Tar Components in Biomass Gasification Dan Gall,† Mohit Pushp,‡ Kent O. 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, Box 857, 501 15 Borås, Sweden
‡
ABSTRACT: Tar and alkali metal compounds are released during biomass gasification and have a major impact on the operation and performance of gasification processes. Herein we describe a novel method for characterization of alkali and heavy tar compounds in the hot product gas formed during gasification. Gas is continuously extracted, cooled and diluted, which results in condensation of tar and alkali into aerosol particles. The thermal stability of these particles is subsequently evaluated using a volatility tandem differential mobility analyzer (VTDMA) method. The technique is adopted from aerosol science where it is frequently used to characterize the thermal properties of aerosol particles. Laboratory studies show that pure and mixed alkali salts and organic compounds evaporate in well-defined temperature ranges, which can be used to determine the chemical composition of particles. The performance of the VTDMA is demonstrated at a 4 MWth dual fluidized bed gasifier using two different types of online sampling systems. Alkali metal compounds and a wide distribution of heavy tar components with boiling points above 400 °C are observed in the product gas. Implications and potential further improvements of the technique are discussed.
1. INTRODUCTION Biomass is conventionally combusted to generate heat and electricity and may reduce the global CO2 burden when replacing fossil fuels. As an alternative to direct combustion, gasification may be used to transform biomass into other products including synthesis gas (syngas) and bio-oil, which may subsequently be upgraded into transportation fuels and other products.1 The wider applicability of the products together with potentially higher energy efficiency and reduced emissions make biomass gasification an attractive alternative to conventional combustion.1 At the same time, gasification is a relatively complex thermochemical process with several inherent technical challenges that need to be addressed.2 Some of the challenges are associated with the presence of condensable material in the product gas. Of particular interest are tar and inorganic compounds including alkali metal salts that are formed in the gas phase or directly released from the fuel during the gasification process.3,4 Both tar and alkali metal compounds have a range of consequences for the gasifier performance and the product gas quality, and they are the main focus of the present study. Tar consists of a wide range of hydrocarbons and its formation decreases the syngas yield and thereby the overall plant efficiency. It may also cause damage to process equipment by blocking and fouling when the temperature drops below 400 °C.5 Tar removal is therefore a key to the commercial implementation of gasification,3 and removal methods are divided into primary measures related to treatments inside the gasifier and secondary methods involving gas cleaning downstream of the gasifier. Tar removal in the hot gas stream is costly and not always sufficient why primary tar reduction methods including the use of additives, changes of operating parameters, and changes in bed material composition receive increased attention. © XXXX American Chemical Society
Available tar measurement systems can be divided into offline and online methods. Off-line systems extract raw gas and trap tar vapors for subsequent analysis, and solid phase adsorption (SPA) and the tar protocol are widely used methods that provide quantitative measurements in industrialscale applications.6,7 Online measurements based on optical techniques include systems based on laser-induced fluorescence, photoionization detection, ultraviolet visible and Fourier transform infrared spectroscopy.8−12 The optical techniques provide fast data acquisition, but high loadings of tar and particles may cause disturbances and no long-term measurements have been reported. Online mass spectrometry has also been applied to measure a wide range of tar compounds with high sensitivity,13 and soft ionization methods have been used in raw gas measurements to reduce molecular fragmentation in the mass spectrometer.14 In addition to tar-related issues, biomass is rich in potassium and sodium compounds, and some including KOH and KCl are readily released to the gas phase at the operating temperature of a gasifier. In the following we will interchangeably refer to potassium and sodium compounds as alkali, alkali compounds, or alkali metal compounds. Most alkali compounds condense at temperatures below 500−600 °C and may cause slagging, fouling, and corrosion. Alkali may also contaminate catalysts used downstream to chemically transform the synthesis gas into products. In addition, alkali may have positive effects by contributing to tar cracking within the gasifier.15,16 In contrast to tar measurements, only a few alkali measurements in gasification have been reported. Online measurements have been performed using surface ionization methods17 and Received: February 16, 2017 Revised: June 27, 2017 Published: July 5, 2017 A
DOI: 10.1021/acs.energyfuels.7b00474 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels excimer laser-induced fragmentation.18 Other studies include collection of ash particles from filters, cyclones, and bed material for off-line analysis of alkali species, using X-ray fluorescence and X-ray diffraction techniques, to investigate the fate of alkali compounds and their interaction with the bed material.19,20 Here, we evaluate a novel approach for characterization of both alkali and heavy tar compounds in the hot product gas formed in biomass gasification. Product gas is extracted and tar and alkali compounds are allowed to condense and form aerosol particles. The thermal stability of these particles is subsequently evaluated using a technique known as volatility tandem differential mobility analyzer (VTDMA). The VTDMA method is adopted from aerosol science where it is frequently used to characterize the thermal properties of aerosol particles.21−26 The method has been used in a few earlier combustion-related studies,27−29 but has to our knowledge not previously been applied in gasification research. The aim of the present study is to evaluate the performance of a VTDMA used to characterize condensable components in the product gas from biomass gasification. We describe a setup that characterizes the stability of particles at temperatures up to 1000 °C and present results from laboratory studies and a first demonstration at a 2−4 MWth circulating fluidized bed gasifier.
selecting particles within a narrow electrical mobility range, or to determine the particle number concentration as a function of particle size by scanning the electric field within the instrument. In the present setup, DMA 1 (model 3071, TSI Inc.) is used to select aerosol particles with a certain particle diameter (Figure 1). These particles are sent through an oven at a given temperature. The particles exiting the oven are analyzed with DMA 2 (model 3080, TSI Inc.) to determine the size distribution after heat treatment. A condensation particle counter (CPC) (model 3010, TSI Inc.) is used to continuously monitor the concentration of particles leaving DMA 2. All aerosol lines are made of stainless steel with an inner diameter of 4 mm. Bends and tube lengths are minimized to reduce losses and laminar flow is maintained in all parts of the system (maximum Re = 165). The DMAs were operated at a sample flow rate of 1.0 l min−1, and the recirculated clean sheath gas flow rate required for the DMA operation was 10 L min−1 for both DMA 1 and DMA 2. Both DMA inlets have an impactor with a cutoff diameter of 0.5 μm, preventing larger particles from entering the instruments. We typically select particles with a size around 100 nm using DMA 1, and the cutoff diameter consequently has no effect on the present studies. The oven used for heat treatment of particles consists of a stainless steel tube with an inner diameter of 4 mm within an insulated compartment that is electrically heated to temperatures up to 1000 °C. The steel tube has a smoothly bent curled shape within the heated zone of the oven. One objective was to keep the oven construction simple and robust and to experimentally confirm the operation of the setup including the effects of temperature profiles within the oven and losses due to diffusion and thermophoresis. Two different setups with total tube lengths of 0.45 and 2.0 m within the heated zone were therefore used in the present study. The oven temperature was measured at a central position with a thermocouple (type K, Pentronic AB). Additional measurements with a thermocouple at different distances within the tube confirmed that the temperature corresponds to the actual heating temperature experienced by aerosol particles. After exiting the oven, the flow is cooled to ambient temperature over a distance of 0.4 m before entering DMA 2. During experiments the oven is ramped from low to high temperature, and the particle number concentration as a function of particle size is determined with a time resolution of 60 s. The typical total sampling time is approximately 1 h for a complete thermogram describing the stability of the aerosol over an extended temperature range. 2.2. Laboratory Evaluation of the VTDMA. Laboratory evaluation of the VTDMA was performed with pure and mixed alkali salt and organic particles (Figure 1). Synthesized aerosols were produced from commercial products used as received: KCl (≥99.9%), NaCl (99.5%), KOH (98%), NaOH (98%), and K2SO4 (99.9%) (Merck & Co., Inc.), and pyrene (98%) (Sigma-Aldrich Corp.). Alkali salts were dissolved in purified water (Milli-Q-plus), and organic compounds were dissolved in methanol (99.8%). A constant output atomizer (model 3076, TSI Inc.) operated in recirculation mode was used to produce aerosol particles from the solutions. The atomizer generates a constant flow of droplets from a liquid solution using either pressurized air or nitrogen, and the droplets are subsequently dried with a diffusion drier until aerosol particles consisting of the dissolved material remain. The size of produced particles can be modified by changing the concentration of the compound of interest in the solution. A porous cylinder diluter was used to control the particle concentration before the aerosol was distributed to the analyzers. The cylinder diluter consists of a perforated steel tube that is enfolded by several layers of fine mesh, from where the dilution gas is inserted. The sample gas mixes with the dilution gas that flows from the walls to minimize particle losses. The function of the cylinder diluter was confirmed using different types of particles prior to the measurements. 2.3. Application of the VTDMA Method in Indirect Gasification. In addition to laboratory experiments, the VTDMA was tested at a 2−4 MWth dual fluidized bed gasifier at Chalmers University of Technology, Sweden.31 The gasifier was fed with wood pellets at a rate of 295−300 kg/h. The bed was fluidized with steam
2. EXPERIMENTAL METHODS 2.1. The Volatility Tandem Differential Mobility Analyzer. The VTDMA method relies on continuous extraction and quenching of product gas by dilution and cooling, which results in condensation of tar, alkali compounds, and other condensable components into submicrometer aerosol particles.30 In a subsequent step, the thermal stability of these particles is evaluated using the VTDMA method. The method is based on the principle that different condensed components become volatile in characteristic temperature ranges. When temperature increases a fraction of the chemical constituents of an aerosol particle volatilizes, resulting in a reduction in particle size. The temperature at which complete or partial volatilization takes place may provide information about the chemical composition of the aerosol. The VTDMA system is schematically illustrated in Figure 1 together with the setup used to evaluate the performance of the
Figure 1. Schematic illustration of the VTDMA and particle generation (dashed box) setup used in laboratory studies. instrument in the laboratory. The VTDMA system consists of two differential mobility analyzers (DMA) in series separated by an oven in order to study the thermal properties of aerosol particles. The DMA is extensively used in aerosol research and has been described in detail elsewhere.21 Briefly, a polydisperse aerosol entering a DMA is charged with a bipolar diffusion charger, which produces an aerosol with a welldefined charge distribution. Particles are subsequently separated based on their electric mobility in the gas they are suspended in. The instrument may either be used to produce a monodisperse aerosol by B
DOI: 10.1021/acs.energyfuels.7b00474 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels (140 °C, 1.2 bar) at a rate of 160 kg/h during the measurements, and the product gas contained approximately 50% water vapor. Measurements were performed in the main raw gas channel 5 m downstream of the gasifier unit. The temperature in the raw gas was approximately 750 °C, which may be compared with a bed temperature between 800 and 850 °C during the experiments. Two different probes denoted A and B were used to extract raw gas from the gasifier and the setups are schematically illustrated in Figure 2. The two probes differed in their detailed design concerning dilution
When sample gas enters the probe it gradually cools from 750 to 350 °C. Alkali compounds condense and form particles at this stage, while most tars will remain in the gas phase.30 A heated cyclone (350 °C) was subsequently used to remove particles with a particle diameter larger than 2.5 μm directly after the probe. After the cyclone, the extracted gas was diluted and cooled with nitrogen gas in two steps. The first step was the aforementioned porous cylinder diluter used in laboratory studies where nitrogen flowed from the wall to enhance gas to particle conversion and minimize wall condensation losses. The dilution ratio was controlled with gas flow meters regulating the nitrogen flow and measuring the excess gas flow. The dilution ratio in the first diluter was between 1.8 and 8.5 during the experiments. The second diluter was a Dekati ejector diluter (DI-1000) operated at low dilution flow and primarily functioning as a mixing chamber, with a dilution factor of 10−40. Both diluters were heated to 110 °C to prevent condensation of water vapor. The dilution and rapid cooling result in tar condensation on existing surfaces including the previously formed inorganic salt particles. The simultaneous rapid changes in temperature and dilution ratio make the tar condensation process complex, and volatile compounds with a saturation pressure above a certain level will not condense. Probe B was electrically heated to 220 °C and used to extract the raw gas from the center of the raw gas channel. The raw gas was diluted directly in the probe inlet with preheated nitrogen gas. The dilution results in rapid cooling of the gas and condensation of condensable components on existing particles, or nucleation of new particles. After dilution, a bed of granular activated carbon heated to 300 °C was used to adsorb and remove tars in the gas stream, before the gas was cooled down and thereby preventing a major fraction of the tar to contribute to the measured particle load. Particles formed in the extraction systems were transported by a 3 m copper tube (inner diameter 4 mm) to the VTDMA instrument. The VTDMA was operated with similar settings as in the laboratory experiments, including an option to bypass DMA 1 and the oven unit in order to determine the complete particle size distribution in the flow from the gasifier. 2.4. Analysis and Modeling of VTDMA Data. The thermal stability data provided from the VTDMA may be described directly by the measured electrical mobility diameter as a function of oven temperature. Alternatively, particles are assumed to be spherical, and the remaining volume fraction (RVF) is determined according to
Figure 2. Schematic illustration of the experimental setup used during measurements at the Chalmers gasifier. Two different extraction and dilution systems A and B were used to transfer sample gas to the VTDMA setup. and temperature profiles, and in addition probe B included a thermodenuder for tar removal.32 The probes were used separately at different occasions. Probe A consisted of a 1.0 m stainless steel tube (inner diameter 4 mm) that was electrically heated to 350 °C. The probe inlet was located in the center of the raw gas channel (diameter 0.2 m) with a sharp nozzle directed toward the gas stream. The flow velocity was 18.5 m s−1 in the raw gas channel and 13.5 m s−1 in the probe tip, resulting in an isokinetic deviation of 37%. The sub-isokinetic sampling allows some of the large particles which were not originally in the sample volume to travel into the probe due to their inability to follow the gas stream. However, the particles of relevance for this study, with an aerodynamic diameter
