Influence of Bed Material, Additives, and Operational Conditions on

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Biofuels and Biomass

Influence of bed material, additives and operational conditions on alkali metal and tar concentrations in fluidized bed gasification of biomass Mohit Pushp, Dan Gall, Kent Oskar Davidsson, Martin C. Seemann, and Jan B. C. Pettersson Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Energy & Fuels

Submitted to Energy & Fuels

Influence of bed material, additives and operational conditions on alkali metal and tar concentrations in fluidized bed gasification of biomass

Mohit Pushp1, Dan Gall2, Kent Davidsson1, Martin Seemann3, and Jan B. C. Pettersson2,*

1

RISE, Research Institutes of Sweden, Sweden

2

Department of Chemistry and Molecular Biology, Atmospheric Science, University of Gothenburg, SE-412 96 Gothenburg, Sweden

3

Department of Space, Earth and Environment, Division of Energy Technology, Chalmers University of Technology, Sweden.

*

To whom correspondence should be addressed. E-mail: [email protected]; Tel. +46 31 7869072.

1 ACS Paragon Plus Environment

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Abstract Gasification of biomass results in release of tar and alkali metal compounds that constitute a significant challenge to the optimization of the gasification process. Here we describe on-line measurements of alkali, condensable tar and particle concentrations in product gas from a 2-4 MWth dual fluidized bed gasifier, with the aims to characterize typical concentrations and contribute to the understanding of alkali-tar interactions. The influence of bed material, additives and operational parameters on the concentrations is investigated. Alkali concentrations are measured with a surface ionization detector, and particle and tar concentrations are determined using aerosol measurement techniques. The gasification of wood chips using quartz or olivine as bed material results in an alkali concentration of 30 250 mg m-3, and the observed alkali levels are consistent with a significant release of the fuel alkali content. Addition of ilmenite to a quartz bed and additions of K2SO4 and K2CO3 to an olivine bed influence both alkali and heavy tar concentrations. The additions result in changes in alkali concentration that relaxes to a new steady state in tens of minutes. The concentration of condensable tar is lower for the olivine bed than for the quartz bed, and tends to decrease when potassium or sulfur is added. The concentration of condensable tar compounds is anticorrelated with the alkali concentration when a quartz bed is used, while no clear trend is observed with an olivine bed. An increase in steam flow rate results in a substantial decrease in heavy tar concentration from a quartz sand bed, while the alkali concentration increases slightly with increasing flow rate. This is in contrast to the alkali concentrations observed when using an activated olivine bed, where concentrations are higher and tend to decrease with increasing steam flow rate. The study confirms that several primary methods are available to optimize the alkali and tar behavior in the gasifier, and suggests that on-line monitoring is needed to systematically change the operational conditions and to study the underlying processes.


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1. Introduction Gasification may be used to transform biomass into synthesis gas (syngas) and bio-oils, which may subsequently be upgraded into a range of products.1 The wider applicability of the products and a potentially high energy efficiency make gasification an attractive alternative to conventional biomass combustion.2 Biomass gasification processes are currently being investigated and developed for increased efficiency. Some of the challenges are associated with the presence of condensable material in the product gas, and the behavior of these different components is the main interest of the present study.

Tar and inorganic compounds are two classes of condensable compounds that are released to the gas phase during biomass gasification. The transformation of the fuel into light gases and char is usually incomplete, and additional hydrocarbons are formed under typical gasification conditions. These byproducts consist of a wide range of polycyclic aromatic compounds and various oxygenated hydrocarbons, and are collectively referred to as tar.2 Tar formation decreases the syngas yield and thereby the overall plant efficiency. Condensation of tar may also cause damage or operational issues to down-stream process equipment when the temperature drops below 400 °C,3 and tar should thus be removed or at least reduced to improve the process efficiency. Primary methods to reduce tar concentrations by treatments inside the gasifier are usually more cost- and energy-efficient than secondary methods involving gas cleaning downstream of the gasifier. The primary tar reduction methods include the use of additives, changes of operating parameters and changes in bed material composition.4

In addition to tar, several inorganic compounds are readily released to the gas phase during biomass gasification.5 The inorganic compounds may include KOH(g) and KCl(g), which


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have high vapor pressures at typical gasifier temperatures.6 The alkali compounds originate from the biomass, which naturally contains potassium and sodium compounds required by the living material. Similar to the behavior of tar, alkali compounds condense as temperature is reduced downstream of the gasifier and may cause slagging, fouling and corrosion.7 Alkali may also interfere with catalysts used to chemically transform syngas into products.8 However, the presence of alkali compounds may also have beneficial effects on both tar formation and char cracking.9-13

Several earlier studies focused on the effect of alkali metal compounds on biomass pyrolysis and gasification.13-18 Addition of alkali metal compounds has been observed to lower tar concentrations,12,13 while removal of alkali from the fuel prior to gasification increases the tar yield and lowers the char gasification rate.17 The detailed role of alkali metals in tar formation processes, however, remains unclear. Laboratory-scale investigations indicate that alkali metals influence the primary pyrolysis step by inhibiting glycosidic bond cleavage.15,16 This reduces the formation of anhydrous sugars, which may further react to form tars, and instead favors formation of lighter products. Carbon - metal bonds such as those formed with alkali metals are also known to be readily hydrogenated,19 which may constitute a catalytic hydrogenation pathway in a reducing environment.

Both alkali compounds and tar interact with the bed material used in fluidized bed (FB) applications. Quartz sand has traditionally been used as bed material, but other minerals including olivine, ilmenite, dolomite and bauxite have been investigated in order to improve tar chemistry and overall efficiency of the gasification process.20 Olivine mainly consists of (Mg,Fe)2SiO4 and has an effect on tar reduction,21-23 which combined with a high mechanical strength makes it suitable for FB gasification.24 Ash components form a surface layer on


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olivine bed particles that is active with respect to fuel conversion, and the build-up of the layer may be referred to as bed activation.25 In comparison, ash layer formation on quartz sand is relatively slow,26,27 and quartz to a larger extent absorbs alkali compounds.28 Another material of interest is the mineral ilmenite, which mainly consists of FeTiO3, and has been shown to have good tar decomposition properties,28 but also has a high oxygen-carrying capacity that may make it less suitable for syngas production.29 In addition, potassium has been shown to reacts with ilmenite by diffusing into the solid forming KTi8O16, and ilmenite may thereby influence the availability of potassium in a fluidized bed.30

Recent alkali measurements in a 32 MWth dual fluidized bed (DFB) gasifier showed gasphase alkali concentrations in the range of tens of parts per million (ppm) during gasifier operation, and the concentration reached hundreds of ppm during start-up and activation of an olivine fluidized bed by addition of potassium carbonate.31 The alkali concentrations are thus relatively high, with potential effects on both bed composition and tar reactivity. In the present study, on-line alkali, tar and particle measurements are performed under semiindustrial conditions in a 2-4 MWth DFB gasifier. The overall aims are to characterize the influence of primary tar reduction methods on alkali concentrations, and to contribute to an improved understanding of alkali-tar interactions in the DFB system. We investigate the influence of fluidized bed composition (quartz and olivine), steam fluidization rate, and the effects of additives including ilmenite, potassium carbonate, potassium sulfate and elemental sulfur on measured concentrations of alkali, tar and particles.


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2. Methodology

Measurements were carried out at a DFB gasifier by continuous extraction of product gas, and on-line analysis of alkali, tar and particle concentrations. The methodology including the gasifier and its operating conditions, the extraction and dilution system, and the online instrumentation is described in the following subsections.

2.1 The dual fluidized bed gasifier The 2-4 MWthermal DFB gasification system consists of a bubbling fluidized bed (BFB) gasifier connected to a circulating fluidized bed (CFB) boiler.29 The fluidized beds are connected by two loop seals for recirculation of bed material. The seals are fluidized with steam to prevent contamination of the gas between the two reactors, and the gasifier can be turned on and off without affecting the boiler.32 Bed material is heated in the boiler before entering the BFB gasifier, and the heat results in drying, devolatilisation and steam gasification of the feedstock within the gasifier.32,33 The released gases exit at the top of the reactor, and remaining char follows the bed material back to the combustor.

The operating conditions during the experiments are summarized in Table 1. The gasifier was fed with wood pellets at a feeding rate of 295 – 300 kg h-1 and the chemical composition of the fuel is given in Table 2. Experiments were performed with quartz and olivine as bed material, both provided by Sibelco AB. The quartz sand (Baskarpsand 28) had an average particle size of 280 µm and a SiO2 content of over 90%. The olivine (Vanguard 180-355) had a particle size distribution between 180-355 µm and a composition of 49 wt-% MgO, 41.7 wt% SiO2 and 7.4 wt-% Fe2O3. The beds were fluidized with steam (140 °C, 1.2 bar) at a standard flow rate of 160 kg h-1, which was altered to 100, 220 and 280 kg h-1 to examine the


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effect of steam flow rate on measured parameters. The gasifier was operated at a temperature of 820 °C and a fuel flow rate of 300 kg/h to the gasifier.

The added ilmenite, iron titanium ore provided by Titania AS, was collected from the secondary cyclone29 during a test with ilmenite as bed material that was performed before the experiment described here. The added material had a particle size distribution between 15 and 100 µm. The fine ilmenite particles were stored in a silo with feeding arrangement and pneumatically injected to the inlet of the loop seal leading to the gasifier. The potassium salts and sulfur in elemental form were added as pure chemicals via the same system.

2.2 Extraction and dilution of product gas Measurements were performed in the product gas channel 5 m downstream of the gasifier. The temperature at the sampling point was 750 °C, which was thus lower than the gasifier temperature (820 °C). The experimental setup is schematically illustrated in Figure 1. The probe consisted of a 1.0 m stainless steel tube (inner diameter 4 mm), which was electrically heated to 400 °C. The probe inlet was located in the center of the product gas channel (diameter 0.2 m) with a sharp nozzle directed towards the gas stream. A gas line was connected to the probe exit, to allow dilution of the sample gas with hot nitrogen in order to fine-tune the dilution ratio. A cyclone (Dekati Ltd.) was used to collect particles larger than 2.5 µm to avoid blockage of downstream nozzles before the sample gas reached two ejector diluters. The ejector diluters (Diluter DI-1000, Dekati Ltd.) were connected in series downstream of the cyclone. The first diluter was kept at 350 °C to prevent tar and water from condensing. The inlet of the second diluter was kept at the same temperature, while the rest of the diluter was at 110 °C. After the second diluter the sample gas was cooled to the surrounding temperature (30 °C). The sampled gas was transported in a 3 m copper tube


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(inner diameter 4 mm) to a manifold where it was distributed to the different analyzers. The dilution factor was determined by measurements of the ratio of the CO concentrations in the gasifier and downstream of the dilution system, and the dilution factor was typically 600-900 during the experiments.

2.3 On-line measurement methods A surface ionization detector (SID) was used to measure the total alkali concentration (K + Na) in the extracted gas. The surface ionization (SI) method has been described in detail elsewhere31,34,35 and is only briefly presented here. The method relies on the efficient ionization of elements with a low ionization potential in contact with a hot metal surface.34 An atom adsorbed on a hot metal surface has a certain probability of being thermally desorbed in ionic form, and the ionization probability, β, can be described by the Saha-Langmuir equation;36

β=

1 , g (1 − r0 )  IP − φ  1+ 0 exp  g + (1 − r+ )  RT 

(1)

where g0/g+ denotes the statistical sum ratio of neutral atoms and ions (g0/g+ = 2 for alkali metals). The reflection coefficients r0 and r+ are generally close to zero. IP, ϕ, R and T denote ionization potential, surface work function, the gas constant and surface temperature, respectively. For most elements IP > ϕ, which means that the emission of neutral species from the surface dominates over ion desorption. However, the ion emission is strongly favored when IP < ϕ. Figure 2 illustrates the calculated ionization probability for a few selected elements with low ionization potentials on a 1500 K platinum surface. For K (IP = 432 kJ/mol) on a Pt surface (ϕ ≅ 530 kJ/mol),37 β = 1.00 and the corresponding value for Na (IP = 8 ACS Paragon Plus Environment

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496 kJ/mol) is 0.89. The application of the SI method for on-line measurements also includes aerosol particle transport to and decomposition on the hot surface, which have been investigated in earlier studies.35,38,39

The employed SID consists of a resistively heated platinum filament (diameter 0.35 mm, purity 99.997%) and an ion collector housed in a cylindrical stainless steel chamber with an inner length of 70 mm and a diameter of 70 mm.39 The filament is biased at a voltage of +350V, and ions formed at the filament diffuse to the stainless steel collector plate positioned parallel to the filament at a distance of 3 mm. The chamber is equipped with a quartz window in order to enable temperature readings with an optical pyrometer. The ion current at the collector was measured using a current amplifier (Model 427, Keithley Inc.) and the output from the amplifier was logged at a sampling rate of 1 Hz. A sample flow rate of 0.7 L/min through the SID was maintained by an external pump during the experiments. The response of the SID to alkali salt particles with known properties and concentration was studied in laboratory experiments using a previously described laboratory setup.18,31 The measured current when a KCl aerosol with different mass concentrations was led through the instrument was used to deduce the alkali mass concentration using the following relationship:

C = (y + 0.358) / 0.22,

(2)

where C is the estimated mass concentration of alkali (µg m-3) and y is the SID signal strength (in nA). The instrument has a similar response to potassium chloride, carbonate and hydroxide particles, while sodium salts have an approximately 10% lower detection probability reflecting the difference in surface ionization potential between Na and K on the hot Pt filament. The lower detection probability for sodium salts has not been accounted for in the


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determination of total alkali (Na + K) mass concentrations, and the effect on reported values is considered minor since the content of Na is about a tenth of that of K (Table 2).

A scanning mobility particle sizer (SMPS) was used to characterize the size and concentration of aerosol particles during the experiments. The SMPS system consists of a differential mobility analyzer (DMA; Model 3071, TSI Inc.) and a condensation particle counter (CPC; Model 3010, TSI Inc.). The DMA is extensively used in aerosol research and has been described in detail elsewhere.40 Briefly, a polydisperse aerosol entering a DMA is charged with a bipolar diffusion charger, which produces an aerosol with a well-defined charge distribution. Particles are subsequently separated based on their electric mobility in the gas phase. The instrument is used to determine the particle number concentration as a function of particle size by scanning the electric field within the instrument. The CPC is used to continuously monitor the number concentration of particles leaving the DMA. The SMPS provided particle number size distributions in the size range 0.012-0.552 µm. In addition, an optical aerosol spectrometer (Dust Monitor Model 1.108, GRIMM Aerosol Technik GmbH) was employed for on-line analysis of particles in the size range 0.575-8.75 µm.

The thermal stability of aerosol particles was analyzed by occasionally passing the particles through a furnace upstream of the on-line instruments. A bypass line made it possible to alternate between sampling through the furnace and bypassing it. The furnace consisted of a 2.0 m stainless steel tube with an inner diameter of 4 mm within a compartment that was electrically heated to temperatures up to 850 °C.41 The steel tube had a smoothly bent curled shape within the heated zone of the furnace. The furnace temperature was measured at a central position with a thermocouple (type K, Pentronic AB). After exiting the furnace the flow was cooled to ambient temperature over a distance of 0.4 m before entering the SID,


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SMPS and optical spectrometer. The change in particle size as a function of furnace temperature reveals the thermal stability of the condensed material, and is used to deduce the chemical composition of condensable components. The method has recently been applied in biomass gasification studies.31, 41

In addition to the instruments described above, a CO analyzer (Rosemount X-Stream X2GK) was used to measure the CO concentration in the diluted sample gas, and the dilution ratio was determined by comparing the obtained values with the product gas concentration measured by an online Multi-Component Gas Analyzer (MLT4 Emerson). Process parameters such as flows, pressures and temperatures were exported from the operating and control system of the plant.

2.4 Data analysis Particle concentrations are reported as particle number concentrations, as determined with the SMPS and the optical aerosol spectrometer. Particle number concentrations are also converted to mass concentrations by assuming that particles are spherical and have a density of 1.2 g cm3

. The aerosol particles are expected to be internally mixed, consisting of heavy tar

compounds (with more than three aromatic rings), alkali compounds, and a non-volatile fraction, and the assumed density is based on the heavy tar component. The actual density is not known, and the particle mass concentration may be higher than that reported here. The reported alkali concentration is derived from the observed ion current and converted to mass concentration following the method described above. Both the particle mass and alkali concentrations are corrected for dilution, using the CO ratio between the diluted sample gas and the product gas.


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3. Results Online measurements were performed in the product gas downstream of the gasifier during experiments using quartz and olivine as bed materials. Results from the individual experimental cases are described below, followed by a comparison between the results obtained under different operating conditions.

3.1 Biomass gasification using quartz sand with ilmenite additions Figure 3 shows an overview of measured alkali and particle concentrations as a function of time during biomass gasification with a quartz sand bed. Two short injections of ilmenite were conducted during the time period and the injections are marked in the figure. In addition, a period when the steam fluidization rate was varied in the range from 100 to 280 kg h-1 is indicated.

Results from particle measurements are displayed as total mass concentrations in the particle diameter size ranges 0.012-0.552 µm (SMPS data) and 0.575-8.75 µm (optical spectrometer data), and as the sum of the two fractions. Note that the observed particle concentrations include both particles originating from the gasifier and gas phase components that condensed to form aerosol particles during gas extraction and quenching, and the importance of these different fractions is further discussed below. The total particle mass concentration varies in the range from 0.8 to 2.0 g m-3 during the experiments depending on the operating conditions. The mass concentration is dominated by submicron particles with a diameter below 0.552 µm when the steam fluidization rate is 100 or 160 kg h-1, while the fine and coarse particle fractions make similar contributions to the total when the steam fluidization rate is raised to 280 kg h-1. The alkali concentration simultaneously measured with the SID varies in the range


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from 30 to 110 mg m-3, with the highest concentrations being observed at elevated steam fluidization rates. Alkali compounds thus make a minor yet non-negligible contribution to the total sampled particle flux.

The first ilmenite injection consisted of 25 kg of ilmenite powder dosed to the gasifier during 5 min, which was 83 min later followed by an injection of 82 kg during 22 minutes. The injections lead to an immediate decrease in total particle mass concentration, and the concentration remains low during more than an hour after each injection. The concentration of large particles measured with the optical spectrometer instead increases during the ilmenite powder injections, and considering the product gas flow from the gasifier the integrated mass of the first and second peak is 6 and 23 g, respectively. Loss of added ilmenite as particles with a diameter < 10 µm is thus negligible compared to the added amounts of ilmenite. The alkali concentration is strongly affected by the ilmenite additions as seen by a decrease as new material is being added to the bed, followed by slow resilience during the following hour (Figure 3). The alkali kinetics during these events are further discussed in Section 3.3.

Figure 4a shows a typical particle size distribution observed with the combined SMPS and the optical spectrometer instruments under the conditions presented in Figure 3. Three major components (modes) can be distinguished in the data. The majority of the experimentally determined particle size distributions are well fitted with a linear combination of three lognormal distributions as illustrated in Figure 4a. Figure 4b shows the results of measurements performed under similar conditions as in Figure 4a except that the aerosol was allowed to pass through a furnace at 400 °C before reaching the instruments. The mode with the smallest particle sizes disappears completely when the aerosol is heated in the furnace. This mode is attributed to the nucleation and condensation of heavy tar compounds during the extraction


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process. The results for the tar-containing mode are consistent with earlier studies carried out in the same gasifier showing that the tar compounds tend to reversibly evaporate when heated to 100-400 °C.41 The condensed tars consist of compounds with typically more than three aromatic rings, while compounds with lower boiling points remain in the gas phase.41

The intermediate mode in Figure 4a is expected to mainly consist of inorganic compounds that condense to form sub-micron particles during the initial stages of gas extraction and quenching where the temperature is reduced from 750 °C in the product gas to 400 °C in the extraction line.42 The major compounds are expected to be alkali hydroxides, while alkali chlorides are likely to be less important considering the low chlorine content of the fuel (Table 2).43 The coarse particle mode is typical for materials that remain in the condensed phase during the gasification process and the main components in this mode are expected to be bed material, fly-ash and fly-char particles.44 Depending on conditions, some of the tar and volatile inorganic compounds could also be expected to condense on the surface of these coarse particles.45 However, furnace experiments suggest that condensation on coarse mode particles is of limited importance under the conditions studied in Figure 4. An observed reduction in mass concentration for the two modes with larger particles can in the case presented in Figure 4 be explained by particle losses during transport through the furnace,41,45 which was quantified in separate laboratory experiments with thermally stable particles.

All particle size distribution measurements during the time period covered by Figure 3 were fitted with three lognormal modes as illustrated in Figure 4, and the mass concentration for each mode was calculated. The resulting mass concentrations for the three modes as a function of time are illustrated in Figure 5. The mode associated with heavy tar compounds is observed to decrease during the course of the experiments, while the mode associated with


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volatile inorganic compounds is relatively stable. All three modes are affected by the ilmenite additions, which results in a reduction in the concentrations of heavy tars and alkali. The mode with volatile inorganic compounds shows minor changes, which may be associated with the observed decrease in alkali concentrations. As explained earlier, the pronounced increase in the mode associated with bed material and fly-ash particles is also observed during the short time periods when ilmenite is added to the bed. The likely explanation is that a fraction of the ilmenite powder contains particles in the 1-10 µm range, or form particles by initial attrition.

Figure 5 also includes a time period from 13:20 to 15:00 when the sampled aerosol was repeatedly allowed to pass through the furnace at a temperature of 380-400 °C. The temperature was chosen to completely evaporate condensed tar compounds from the aerosol particles.41 The heat treatment results in loss of the heavy tar mode, while the two other particle fractions are not affected, which confirms the attribution of the different modes to different material fractions in the product gas.

To further investigate the thermal stability of the particles, the furnace temperature was linearly ramped from 30 to 800 °C with a heating rate of 20 °C min-1. Figure 6 illustrates the effect of furnace temperature on normalized alkali and particle mass concentrations in the size range 0.012-0.552 µm (SMPS data). The particle mass concentrations decrease in the temperature range 100-400 °C due to evaporation of heavy tars in agreement with earlier volatility tandem DMA (VTDMA) studies.41 The alkali concentration decreases markedly in the 500-600 °C range and a small fraction remains stable up to 800 °C. Comparison with earlier VTDMA data for evaporation of sub-micrometer alkali salt particles suggests that the observed decrease around 500 °C is consistent with the thermal stability of K2CO3-containing 15 ACS Paragon Plus Environment

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particles.41 As described in earlier studies KOH(g) in the product gas is transformed into K2CO3(s) during gas extraction and cooling,31,41 and the results thus consistent with that the observed alkali signal is due to KOH(g) in the product gas.

The alkali concentration of approximately 70 mg m-3 measured with the SID directly prior to the furnace experiments (Figure 5), corresponds to a KOH concentration of 100 mg m-3 in the product gas. A major fraction of the intermediate and coarse mode particles (Figure 4a) thus consists of alkali salts and the alkali measurements with the SID are consistent with the observed thermal stability of particles.

3.2 Biomass gasification using olivine beds Experiments were also performed with olivine as bed material. Compared to quartz, olivine may help to actively reduce the tar formation since it has an effect on the reactions taking place in the gasifier.20-25 Olivine experiments were performed during two consecutive days and the main experimental results are displayed in Figure 7. The experiments start with fresh olivine bed material on day one and the bed is activated by additions of K2CO3 to enhance the tar removal efficiency. The alkali concentration varies in the range from 180 to 250 mg m-3 during the first day, and the concentration is thus considerably higher than in experiments with a quartz sand bed. The alkali and particle concentrations follow the same pattern during day one and appear to be closely correlated. An addition of K2CO3 around 11:00 led to a rapid increase of the CO concentration in the combustor, which stopped the process for safety reasons during 20 min before it was restarted. The potassium addition led to an increased alkali concentration and slow decay of the signal over the following hours. Addition of elemental sulfur to the CFB combustion unit started at 13:15 and continued until the next


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morning. This did not result in a directly measurable effect on the alkali concentration in the product gas.

The bed material in the gasifier was fluidized using flue gas from the combustor during the night between day 1 and 2. On the second day, the alkali concentration is initially 20-25% of the concentration during the afternoon of day 1 (Figure 7). To further investigate the influence of alkali and sulfur additions, two K2SO4 injections were performed during day 2. The first injection consisted of 3.5 kg and the second of 1.5 kg. The first injection started with a rapid injection of 1.5 kg followed by a 20 s pause, and the remaining material was more slowly dosed during 5 min. The SID signal increases rapidly when K2SO4 is injected and the signal then slowly decays during tens of minutes. The reason for the increase in alkali concentration from 13:30 and onwards has not been conclusively determined, but the beginning of the change coincides with the addition of a new batch of fuel to the silo used to feed biomass into the gasifier. An injection of 1.5 kg of K2CO3 during 5 min is also carried out after the two injections of K2SO4, which only results in a minor transient change in alkali concentration.

3.3 Alkali kinetics during transient operating conditions We next focus on the alkali kinetics related to the addition of material to the gasifier. The two ilmenite additions to the quartz bed (Figure 3) results in a substantial reduction in alkali concentration that progresses during the injection periods and then slowly recovers on the time scale of tens of minutes. Figure 8a shows the change in alkali concentration directly after the two ilmenite injection in further detail. Time zero corresponds to the end of each ilmenite injection period. Figure 8a includes fits of the experimental data with an exponential decay function:


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‫ܫ = ܫ‬଴ ሺ1 − ݁ ି௞௧ ሻ + ‫ܫ‬௕௔௖௞௚௥௢௨௡ௗ

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(3)

where I is the alkali concentration, and I0 and Ibackground are the initial concentration and a background level, respectively. k is an apparent rate coefficient for the return of the alkali concentration to a new steady state after the ilmenite injection. For the first ilmenite addition the alkali concentration changes with a time constant τ = 1/k = 25 ± 6 min, and the second addition shows a similar behavior with a time constant of 28 ± 3 min. Error limits are given as 95% confidence intervals. The ilmenite additions result in an instantaneous reduction in alkali concentrations that suggests that alkali is lost to the fresh ilmenite that is added to the bed, but influences of other physicochemical changes and transport processes cannot be ruled out based on the present data.

The effects of potassium sulfate additions to the olivine bed on the alkali concentration (Figure 8b) were treated in a similar way. Figure 8b shows the decay of the alkali signal after two consecutive additions of K2SO4 (see also Figure 7). Figure 8b includes least-square fits of the experimental data with an exponential decay function:

‫ܫ = ܫ‬଴ ݁ ି௞௧ + ‫ܫ‬௕௔௖௞௚௥௢௨௡ௗ .

(4)

The experimental results are well described by an exponential decay with a time constant τ = 1/k = 6.1 ± 0.2 min for the first addition and τ = 9.5 ± 0.2 min for the second addition. The observed time scales are of importance in any attempts to optimize the gasification process using additives, and both transport (mixing) processes and changes in chemical composition of bed material and the plant refractory may be considered as responsible for the observed changes. 18 ACS Paragon Plus Environment

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Energy & Fuels

3.4 Influence of steam fluidization rate on alkali and tar concentrations The experiments with both quartz and olivine beds include changes in steam fluidization rate (Figures 5 and 7). Figure 5 illustrates that the measured parameters during experiments with quartz sand are affected by changes in steam fluidization rate in the range from 100 to 280 kg h-1. An increase in steam fluidization rate results in a significant decrease in heavy tar concentration and also a decrease in the intermediate mode related to inorganic compounds, while the concentration of coarse particles shows a marked increase. The analysis of particle size distributions using three modes, as described for the quartz sand experiments, could not be repeated for the olivine experiments. The main reason is a shift and change in intensity of the two modes with smallest particle diameters that make them difficult to separate in the analysis. The heavy tar concentration in the gas was instead determined by heat treatment of the sampled particles in the furnace upstream of the on-line instruments. The observed alkali and heavy tar concentrations at different steam flow rates are summarized in Figure 9. In case of a quartz bed, an increase in steam flow rate results in a substantial decrease in heavy tar concentration and a minor increase in alkali concentration. This is in contrast to the alkali concentrations observed when using an activated olivine bed where the concentration is higher than observed with the quartz bed and tend to decrease slightly with increasing steam flow rate. In neither of the two bed material cases can the observed changes be explained by a pure dilution effect when the steam flow rate is increased.

3.5 Alkali-tar correlations Figure 10 summarizes the observed correlations between alkali and heavy tar concentrations during the experiments. In the case of a quartz bed, the time-resolved data in Figure 5 allow us to study the correlation between alkali and heavy tar concentrations during an extended


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time period. The results obtained with a steam flow rate of 160 kg h-1 indicate that the heavy tar concentration is anti-correlated with the alkali concentrations. The trend is further emphasized when the data for steam flow rates of 220 and 280 kg h-1 are taken into consideration. The results obtained during the transient ilmenite additions deviate from the general trend. The available results obtained for the olivine bed are more limited and were obtained by using a furnace to desorb condensed heavy tars prior to the on-line instruments. The olivine results illustrate that the heavy tar concentration and in particular the alkali concentration vary considerably during the activation and operation of the gasifier. The alkali concentration decreases by more than 75% for the activated olivine bed compared to the fresh bed, and the corresponding change for heavy tars is a reduction of about 50%.

4. Discussion

The results reveal that a substantial fraction of the measured particle concentration consists of heavy tars that condense during product gas extraction. During operation with quartz sand the concentration of heavy tar compounds is typically around 1 g m-3 with a steam flow rate of 160 kg h-1, which is consistent with previous findings.26 A substantially lower tar concentration is observed when the quartz is replaced by fresh olivine, and the concentration is further decreased as the bed material is activated. The present findings using activated olivine also agree with recent results from a 32 MWth DFB gasifier where a heavy tar concentration of 150 mg m-3 was observed during steady-state operation.31 The lower tar concentration for olivine compared to quartz is consistent with the previously observed effect of activated olivine that reduces the amount of condensable organics available in the gas stream.22-23,46


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Energy & Fuels

The observed alkali concentration ranges from 35 to 105 mg m-3 when quartz is used. In case of olivine, values up to around 250 mg m-3 are observed with fresh bed material, while values of 30 - 120 mg m-3 are obtained after bed activation. The latter values agree favorably with recent results from an industrial scale facility using olivine as bed material.31 Typical values from 20 to 60 mg m-3 were observed during steady-state operation and higher values were observed during bed activation. The present study thus confirms the relatively high alkali concentrations observed on the industrial scale.

The alkali input from the biomass entering the gasifier is estimated to be 135 g h-1 of K and 12 g h-1 of Na based on the fuel analysis (Table 2). This gives a theoretical alkali (K+Na) concentration of approximately 180 mg m-3 using a wet gas flow from the gasifier of 815 m3 h-1, and assuming that all of the fuel alkali evaporates and leaves with the product gas. The estimate is comparable to the observed concentrations and efficient release of the fuel alkali is thus sufficient to explain the typical levels during the campaign. The observed alkali flow from the gasifier thus corresponds to 19 - 58% of the inflow of fuel alkali when a quartz bed is employed. Similar values are obtained for an activated olivine bed, while a larger alkali release is required to explain the alkali levels before activation of the bed. Note that alkali is also being transported with the bed material to and from the combustion side of the dual bed setup, and may be released from the bed material and other deposits under changing conditions.

Norheim et al.7 have performed equilibrium calculations of the trace element composition in biomass gasification. Comparison with the present results suggests that the observed alkali concentrations are close to or above the saturation pressure of KOH in equilibrium with a


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condensed phase mainly consisting of K2CO3. In the present case, the temperature decreases from 820 °C to 750 °C when the product gas is transported from the fluidized bed to the sampling point. Due to the high alkali concentrations in the gasifier, this temperature drop is likely to results in condensation of KOH and nucleation of new particles, and alkalicontaining particles can thus be expected to form before reaching the sampling line.

The highest alkali concentrations are observed with fresh olivine as bed material, while lower concentrations are observed with activated olivine. This indicates that the alkali vapor pressure over fresh olivine surfaces is initially high, but decreases as a layer grows on the bed particles. Studies aiming at the characterization of the condensed phases have been carried out and indicate that the outer layer is rich in Ca and may inhibit formation of alkali silicates that are instead found in the inner layer.47 Further studies should attempt to link the properties of the condensed phases to the evaporation of alkali, and we conclude that dedicated studies of alkali interactions with bed materials under gasification conditions will be required to develop a more detailed understanding of the processes in operation.

Addition of ilmenite to the quartz bed leads to a pronounced decrease in both tar and alkali concentrations (Figure 5). Earlier research has shown that potassium diffuses into the ilmenite structure forming KTi8O16,30 and the present results suggest that the uptake process is fast and efficient under the current conditions. The estimated K uptake by the added ilmenite corresponds to 2.7 g per kg of ilmenite in the two addition experiments (Figure 5). These values should be considered as a lower limit since the residence time may not be sufficient to reach saturation.


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Energy & Fuels

The addition of K2SO4 to the gasifier induces a steep increase in the alkali signal, which indicates that decomposition of the salt and evaporation of alkali components into the product gas proceed rapidly. Earlier equilibrium calculations showed possible decomposition of K2SO4(s) to form K2CO3(s) and KOH(g) under the conditions that prevail in the gasifier.25 K2CO3 addition is expected to increase the gas phase alkali concentration in a similar way, by partial dissociation into K2O and CO2, followed by K2O reaction with steam to form KOH.48 An earlier study at an industrial-scale gasifier operated with olivine under similar conditions showed less clear effects of various additives,31 and the present study illustrate the need for systematic studies on a smaller scale.

The observed anti-correlation between tar and alkali concentrations for a quartz bed (Figure 10) suggests that higher alkali levels limit the formation of heavy tar compounds or promote tar decomposition, and the positive effect of alkali on tar reduction is in line with the results from earlier studies.12,13,17 The present study does not allow us to identify a mechanism responsible for the observations and open questions remain concerning the role of alkali interactions with the bulk bed material and ash layers on the surface of bed particles, as well as any potential reaction pathways in the gas phase. We also note that no clear alkali-tar correlation trend is observed in the limited data set for the olivine system, which appears to display a distinctly different behavior compared to quartz.

5. Conclusions

Measurements of alkali, heavy tar and particle concentrations were performed in a 2-4 MWth DFB biomass gasification plant. The time-resolved measurements allowed us to follow the


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influence of changes in operational condition, including the use of additives to modify tar removal and activate bed material. Alkali concentrations are in general high when using both quartz and fresh and activated olivine as bed material, with values from 30 to more than 200 mg m-3 depending on the detailed conditions. Using olivine increases the alkali vapor pressure compared to quartz. The observed concentrations are consistent with release of a major fraction of the fuel content of potassium and sodium during the gasification process. Both alkali and heavy tar concentrations are comparable to values recently obtained in a 32 MWth gasifier operated with olivine as bed material and using similar processes to activate the bed material.31 The gas-phase alkali is concluded to mainly consist of KOH(g), which is also in agreement with the earlier large-scale experiments.

Additions of ilmenite to a quartz bed and additions of K2SO4 and K2CO3 are observed to influence both alkali and heavy tar concentrations. The additions result in changes in alkali concentrations that relax to a new steady-state situation in typically tens of minutes. The detailed mechanisms behind the observed kinetics have not been identified and the roles of physicochemical and transport processes within the DFB setup should be addressed in further studies. In particular, future tracer experiments may help to follow and determine the fate of alkali compounds in the system.49

We observe an interesting anti-correlation between the alkali and heavy tar concentrations in the case of a quartz sand bed, while the corresponding behavior is not observed when using olivine as bed material. One potential explanation is a direct gas-phase reaction between KOH and hydrocarbons, but alternative routes that involve heterogeneous processes cannot be ruled out and the reasons for the observed correlations remain to be determined.


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Energy & Fuels

The present study confirms that several primary methods are available to change and control tar and alkali concentrations in DFB biomass gasification. An improved mechanistic understanding is a key to the successful application of these methods, and on-line monitoring of tar and alkali concentrations is essential in any further optimization of the industrial processes.

Acknowledgements This work was performed within the Swedish Gasification Centre supported by the Swedish Energy Agency. Frida Almkvist is gratefully acknowledged for technical assistance during the experiments. We gratefully acknowledge the assistance from personnel at Chalmers University of Technology during the experiments.


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References (1) Sansaniwal, S. K.; Pal, K.; Rosen, M. A.; Tyagi, S. K. Recent advances in the development of biomass gasification technology: A comprehensive review. Renew. Sustainable Energy Rev. 2017, 72, 363–384 (2) Milne, T. A.; Abatzoglou, N.; Evans, R. J. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; technical report NREL/TP-570-25357; National Renewable Energy Laboratory: Golden, CO, 1998; Vol. 570. (3) Li, C.; Suzuki, K. Tar property, analysis, reforming mechanism and model for biomass gasification—An overview. Renew. Sustainable Energy Rev. 2009, 13, 594–604. (4) Devi, L.; Ptasinski, K. J.; Janssen, F. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003, 24, 125-140. (5) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M.; Zhou, J. The fate of inorganic constituents of biomass in fluidized bed gasification. Fuel 1998, 3, 135-146. (6) Tchoffor, P. A.; Davidsson, K. O.; Thunman, H. Transformation and Release of Potassium, Chlorine, and Sulfur from Wheat Straw under Conditions Relevant to Dual Fluidized Bed Gasification. Energy Fuels 2013, 27, 7510-7520. (7) Norheim, A.; Lindberg, D.; Hustad, J. E.; Backman, R. Equilibrium Calculations of the Composition of Trace Compounds from Biomass Gasification in the Solid Oxide Fuel Cell Operating Temperature Interval. Energy Fuels 2009, 23, 920-925. (8) Moud, P. H.; Andersson, K. J.; Lanza, R.; Engvall, K. Equilibrium potassium coverage and its effect on a Ni tar reforming catalyst in alkali- and sulfur-laden biomass gasification gases. Appl Catal B Environ 2016, 190, 137-146. (9) El-Rub, Z. A.; Bramer, E. A.; Brem, G. Review of Catalysts for Tar Elimination in Biomass Gasification Processes. Ind. Eng. Chem. Res. 2004, 43, 6911-6919.


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(10) Shahbaz, M.; Yusup, S.; Inayat, A.; Patrick, D. O.; Ammar, M. The influence of catalysts in biomass steam gasification and catalytic potential of coal bottom ash in biomass steam gasification: A review. Renew. Sustainable Energy Rev. 2017, 73, 468–476 (11) Sutton, D.; Kelleher, B.; Ross, R. H. J. Review of literature on catalysts for biomass gasification. Fuel Process. Technol. 2001, 73, 155−173. (12) Kim, Y.-K.; Park, J.-I.; Jung, D.; Miyawaki, J.; Yoon, S.-H.; Mochida, I. Lowtemperature catalytic conversion of lignite: 3. Tar reforming using the supported potassium carbonate. Ind. Eng. Chem. Res. 2014, 20, 9–12. (13) Patwardhan P. R.; Satrio J. A.; Brown, R. C.; Shanks B. H. Influence of inorganic salts on the primary pyrolysis products of cellulose. Bioresour. Technol. 2010, 101, 4646– 4655. (14) Le Brech, Y.; Ghislain, T.; Leclerc, S.; Bouroukba, M.; Delmotte, L.; Brosse, N.; Snape, C.; Chaimbault, P.; Dufour, A. Effect of Potassium on the Mechanisms of Biomass Pyrolysis Studied using Complementary Analytical Techniques. ChemSusChem 2016, 9, 863−872. (15) Di Blasi, C.; Galgano, A.; Branca, C. Influences of the Chemical State of Alkaline Compounds and the Nature of Alkali Metal on Wood Pyrolysis. Ind. Eng. Chem. Res. 2009, 48, 3359–3369. (16) Nowakowski, D. J.; Jones, J. M. Uncatalysed and potassium-catalysed pyrolysis of the cell-wall constituents of biomass and their model compounds. J. Anal. Appl. Pyrolysis 2008, 83, 12–25. (17) Jiang, L.; Hu, S.; Wang, Y.; Su, S.; Sun, L.; Xu, B.; He, L.; Xiang, J. Catalytic effects of inherent alkali and alkaline earth metallic species on steam gasification of biomass. Int. J. Hydrog. Energy 2015, 40, 15460-15469


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(18) Guan, G.; Kaewpanha, M.; Hao, X.; Abudula, A. Catalytic steam reforming of biomass tar: Prospects and challenges. Renew. Sustainable Energy Rev. 2016, 58, 450–461. (19) Friedman, S.; Kaufman, M. L.; Wender, I. Alkali Metals as Hydrogenation Catalysts for Aromatic Molecules. Div. of Fuel Chemistry, 1964, Aug. 30-Sept. 4. (20) Vilches, B. T.; Marinkovich, J.; Seemann, M.; Thunman, H. Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite. Energy Fuels 2016, 30, 4848−4857. (21) Koppatz, S.; Pfeifer, C.; Hofbauer H. Comparison of the performance behaviour of silica sand and olivine in a dual fluidised bed reactor system for steam gasification of biomass at pilot plant scale. Chem. Eng. Sci. 2011, 175, 468– 483. (22) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Pretreated olivine as tar removal catalyst for biomass gasifiers: investigation using naphthalene as model biomass tar. Fuel Process. Technol. 2005, 86, 707– 730. (23) Devi, L.; Craje, M.; Thune, P.; Ptasinski, K. J.; Janssen, F. J. J. G. Olivine as tar removal catalyst for biomass gasifiers: Catalyst characterization. Appl. Catal., B, 2005, 294, 68–79. (24) Corella, J.; Toledo, J. M.; Padilla, R. Olivine or Dolomite as In-Bed Additive in Biomass Gasification with Air in a Fluidized Bed: Which Is Better? Energy Fuels 2004, 18, 713-720. (25) Marinkovic, J.; Thunman, H.; Knutsson, P.; Seemann, M. Characteristics of olivine as a bed material in an indirect biomass gasifier. Chem. Eng. J. 2015, 279, 555-566. (26) Israelsson, M.; Vilches, B. T.; Thunman, H. Conversion of Condensable Hydrocarbons in a Dual Fluidized Bed Biomass Gasifier. Energy Fuels, 2015, 29, 6465−6475.


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(27) He, H.; Ji, X.; Boström, D.; Backman, R.; Öhman, M. Mechanism of Quartz Bed Particle Layer Formation in Fluidized Bed Combustion of Wood-Derived Fuels. Energy Fuels 2016, 30, 2227−2232. (28) Lind, F.; Berguerand, N.; Seemann, M.; Thunman, H. Ilmenite and Nickel as Catalysts for Upgrading of Raw Gas Derived from Biomass Gasification. Energy Fuels 2013, 27, 997−1007. (29) Larsson, A.; Israelsson, M.; Lind, F.; Seemann, M.; Thunman, H. Using Ilmenite To Reduce the Tar Yield in a Dual Fluidized Bed Gasification System. Energy Fuels 2014, 28, 2632−2644. (30) Corcoran, A.; Marinkovic, J.; Lind, F.; Thunman, H.; Knutsson, P.; Seemann, M. Ash Properties of Ilmenite Used as Bed Material for Combustion of Biomass in a Circulating Fluidized Bed Boiler. Energy Fuels, 2014, 28, 7672−7679. (31) Gall, D.; Pushp, M.; Larsson, A.; Davidsson, K. O.; Pettersson, J. B. C. : Online measurements of alkali metals during start-up and operation of an industrial-scale biomass gasification plant. Energy Fuels 2017, DOI: 10.1021/acs.energyfuels.7b03135. (32) Thunman, H; Seeman, M. C. First Experiences with the new Chalmers Gasifier. Proceedings of the 20th International Conference on Fluidized Bed Combustion, Vol. II (2009), p. 659-663. (33) Larsson, A.; Seemann, M.; Neves, D.; Thunman, H. Evaluation of Performance of Industrial-Scale Dual Fluidized Bed Gasifiers Using the Chalmers 2–4-MWth Gasifier. Energy Fuels, 2013, 11, 6665–6680. (34) Zandberg, É. Y.; Ionov, N. Surface Ionization. Usp. Fiz. Nauk, 1959, 57, 581-623. (35) Davidsson, K. O.; Korsgren, J. G.; Lönn, B.; Engvall, K.; Pettersson, J. B. C. A surface ionization instrument for on-line measurements of alkali metal components in combustion. Energy Fuels 2002, 16, 1369-1377.


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(36) Kingdon, K. H.; Langmuir, I. A Method for the Neutralization of Electron Space Charge by Positive Ionization at Very Low Gas Pressures. Phys. Rev. 1923, 21, 380. (37) Holmlid, L.; Möller, K. Cesium ion desorption from oxygen and carbon adlayers on platinum surfaces with nanosecond time resolution: Variation of desorption parameters with time available for surface diffusion and degree of surface heterogeneity. Surf. Sci. 1985, 149, 609-620. (38) Hagström, M.; Jäglid, U.; Pettersson, J. B. C. Desorption kinetics at atmospheric pressure: alkali interactions with rhodium and steel surfaces. Appl. Surf. Sci. 2000, 161, 291299. (39) Korsgren, J. G.; Pettersson, J. B. C. Collision Dynamics and Decomposition of NaCl Nanometer Particles on Hot Platinum Surfaces. J. Phys. Chem. B 1999, 47, 10425–10432. (40) Knutson, E. O.; Whitby, K. T. Aerosol classification by electric mobility: apparatus, theory, and applications. J. Aerosol Sci. 1975, 6, 443-451. (41) Gall, D.; Pushp, M.; Davidsson, K. O.; Pettersson, J. B. C. Online Measurements of Alkali and Heavy Tar Components in Biomass Gasification. Energy Fuels 2017, 31, 81528161. (42) Jensen, J. R.; Nielsen, L. B.; Scultz-Moller, C.; Wedel, S.; Livbjerg, H. The Nucleation of Aerosols in Flue Gases with a High Content of Alkali - A Laboratory Study. Aerosol Sci. Tech. 2000, 33, 490-509. (43) Tchoffor, P. A.; Moradian, F.; Pettersson, A.; Davidsson, K. O.; Thunman, H. Influence of Fuel Ash Characteristics on the Release of Potassium, Chlorine, and Sulfur from Biomass Fuels under Steam-Fluidized Bed Gasification Conditions. Energy Fuels 2016, 30, 10435−10442.


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(44) Gustavsson, E.; Lin, L.; Seeman, M. C.; Rodin, J.; Strand, M. Characterization of Particulate Matter in the Hot Product Gas from Indirect Steam Bubbling Fluidized Bed Gasification of Wood Pellets. Energy Fuels 2011, 25, 1781-1789. (45) Hinds, W. C. Aerosol Science and Technology: Properties, Behavior, and Measurement of Airborne Particles; John Wiley & Sons, Inc: New York, 1999. (46) Kirnbauer, F.; Wilk, V.; Kitzler, H.; Kern, S.; Hofbauer, H. The positive effects of bed material coating on tar reduction in a dual fluidized bed gasifier. Fuel 2012, 95, 553-562. (47) Kuba, M.; He, H.; Kirnbauer, F.; Skoglund, N.; Boström, D.; Öhman, M. Thermal Stability of Bed Particle Layers on Naturally Occurring Minerals from Dual Fluid Bed Gasification of Woody Biomass. Energy Fuels 2016, 30, 8277−8285. (48) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Transformation and Release to the Gas Phase of Cl, K, and S during Combustion of Annual Biomass. Energy Fuels 2004, 18, 1385−1399. (49) Svane, M.; Hagström, M.; Davidsson, K. O.; Boman, J.; Pettersson, J. B. C. Cesium as a Tracer for Alkali Processes in a Circulating Fluidized Bed Reactor. Energy Fuels 2006, 20, 979−985.


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Table 1 Summary of operational conditions during the measurements at the Chalmers 2-4 MWth DFB gasifier.

Bed material

Steam/Fuel (kg h-1)

Quartz

100/300

Quartz

160/300

Quartz

160/300

Quartz

220/300

Quartz

280/300

Olivine

100/300

Olivine

160/300

Olivine

160/300

K2CO3

Olivine

160/300

K2SO4

Olivine

220/300

Additive

Ilmenite


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Energy & Fuels

Table 2. Fuel analysis. Values are averages on dry basis of four samples taken during the measurement campaign. Element

Content (wt-%)

C

50.8

O

43

H

6.2

Ca

0.096

N

0.06

K

0.045

Si

0.018

Mg

0.017

Mn

0.013

P

0.007

Al

0.006

Fe

0.006

Na

0.004

Ba

0.001

S

Influence of Bed Material, Additives, and Operational Conditions on

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