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Time-dependent layer formation on K-feldspar bed particles during fluidized bed combustion of woody fuels Hanbing He, Nils Skoglund, and Marcus Öhman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02386 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017
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Time-dependent layer formation on K-feldspar bed particles during fluidized bed combustion of woody fuels Hanbing Hea,*, Nils Skoglunda,b, Marcus Öhmana a
Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-971 87
Luleå, Sweden b
Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics,
Umeå University, SE-901 87 Umeå, Sweden. Keywords: layer formation; K-feldspar; bed particle; fluidized bed; combustion; woody fuels Abstract: Despite frequent reports on layer characteristics on quartz bed particles, few studies have focused on the layer characteristics of K-feldspar bed particles. The layer characteristics of Kfeldspar bed particles was therefore investigated by collecting bed material samples of different ages from fluidized bed combustion of woody fuels in large-scale bubbling and circulating fluidized bed facilities. Scanning electron microscopy/energy-dispersive spectroscopy was used to analyze the layer morphology and elemental composition. Bed particles aged 1 day displayed a thin layer rich in Si, Ca and Al. Inner layers had a more homogeneous composition than the outer layers, which instead were more heterogeneous and sometimes contained discernible fuel ash particles. The outer layer was thinner for K-feldspar bed particles sampled from circulating fluidized bed, compared to particles from bubbling fluidized bed. The concentration of Ca in the inner layer increases towards bed particle surface, the molar ratio of Si/Al is maintained, and the molar ratio of K/Al decreases compared to the K-feldspar. The inner layer thickness for quartz and K-feldspar bed particles collected at the same operation conditions was found to be similar. No crack layers, as have been observed in quartz particles, were found in the core of the K-feldspar bed particles. The results suggest that the diffusion and reaction of Ca2+ into/with the feldspar particle play an important role on the inner layer formation process. 1 ACS Paragon Plus Environment
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1. Introduction Biomass is continuously gaining interest both due to its potential to reduce CO2 emissions and its increasing contribution to the global energy supply. Biomass can be obtained from different sources and used for production of power, heat, and fuels through different thermochemical conversion processes. Some biomass fuels contain a significant amount of ash-forming elements such as potassium, calcium, silicon, and chlorine. During the thermochemical conversion process these elements will undergo different ash transformation reactions.1,2 The release, transformation, transportation, and sintering of these elements/produced ash compounds could lead to different ashrelated operational problems during the combustion process.1-7 This reduces the efficiency of the plant, may generate additional costs for maintenance, and impede the effective use of biomass. Currently, fluidized bed combustion is one of the most common thermal energy conversion technology used in heat and power production from biomass.3,8 One of the remaining technology problems concerning efficient use of biomass in this technology is related to bed agglomeration which has the potential to cause total bed defluidization.5,6,9 It has previously been suggested that agglomeration during combustion of woody biomass is related to the formation of sticky layers on bed particles.10-12 Most of the studies on bed agglomeration and bed particle layer formation in fluidized combustion and gasification of woody biomass have focused on quartz as model bed material, since quartz is a major component in natural sand which is commonly used in full-scale plants. Sticky bed particle layers due to formation of alkali-rich silicates can cause quartz bed particles to agglomerate upon collision.10,12-14 Accompanying the layer build-up, cracks in the layer will appear and subsequently lead to the formation of crack layers in the core of the quartz particle, resulting in the fragmentation of old quartz particles.15,16 This could eventually lead to bed material deposition in cyclones and return legs.15 In addition to these negative effects of layer formation, the formation of Ca-rich layers has been found to have a positive effect on improving the syngas quality by reducing tar content in 2 ACS Paragon Plus Environment
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fluidized bed wood gasification.17 Furthermore, the bed particle layer thickness and composition differs depending on quartz particle age.18-20 Compounds with low melting temperature on quartz bed particles were found to form at the initial stage of layer formation, but the amount of melt decreases as more Ca reacts/diffuses into the layer, causing the layer growth to follow a Ca2+ diffusion-controlled solid-phase reaction process.20 Natural sand commonly contains K-feldspar besides quartz. The potential application of K-feldspar as bed material in replacement of current olivine during dual fluid bed steam wood gasification is being considered due to its comparable catalytic activity, thermal stability, lower price and the absence
of
hazardous
impurities.21-23
The
formation
of
bed
particle
layers
during
combustion/gasification of woody biomass has been observed after interaction between ash-forming matter in biomass and K-feldspar bed particles.21-24 However, few studies have focused on Kfeldspar as bed material and its layer formation characteristics. De Geyter et al.24 found multiple layers consisting of an inner- and outer layer on K-feldspar particles during bubbling fluidized bed bark combustion. The inner layer was mainly composed of Si, Al and K (except O), whereas lower concentrations of Si, Al and K and higher concentrations of Mg were found in the outer layer compared to the inner layer.24 In comparison to quartz, a thinner layer thickness on K-feldspar was observed during combustion of bark for 40 h, indicating lower layer growth rate on K-feldspar particles.24 Due to the lack of a chemical driving force enabling K-gaseous attack/reaction with the feldspar particle, De Geyter et al.24 suggested that molten K-silicates from surrounding ash may adhere to the bed particle surface and the melt formed will probably be in equilibrium with the feldspar together with solid K-silicates. Kuba et al.21 found increasing Ca concentration and decreasing K concentration in the layer towards the K-feldspar particle surface and a substitution mechanism rather than a K-attack toward Si, as known for quartz particles, was suggested. Berguerand et al.22,23 found that using K-feldspar as bed material in dual fluid bed gasification of wood displayed no signs of agglomeration. A quite thin layer was already observed after a duration of 2.5 h and after 4 days Ca was present in an observable extend in the layer. Furthermore, an 3 ACS Paragon Plus Environment
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increasing Ca concentration in the layer with bed particle age was observed.23 It was also found that gaseous K-compounds had a higher reaction affinity for quartz particles compared to K-feldspar.23 Previous research related to layer formation on K-feldspar particles has been mainly focused on layer characteristics of relatively young bed particles. The objective of this work is therefore to determine the time-dependent layer characteristics on K-feldspar bed particles during fluidized bed combustion of typical wood-derived fuels, i.e. woody fuel mixtures and softwood. Samples of bed material were collected from large-scale experiments at several different times starting with a completely fresh bed. The morphologies and elemental compositions of the layers on K-feldspar bed particles were examined by scanning electron microscope combined with energy-dispersive spectroscopy (SEM/EDS) to reveal the layer characteristics with bed particle age. The layer characteristics were further compared with quartz particles present in the same bed sample.
2. Methods and Methodology The methodologies used in the present study include the following: (1) Sampling of bed material from full-scale combustion of woody biomass at different times starting with a fresh bed, (2) Kfeldspar layer composition and -morphology were analyzed using SEM/EDS to determine changes as a function of time. 2.1. Bed Material Sampling Bed sampling was carried out according to previous study detailing the time-dependent layer formation characteristics for quartz bed particles. For clarity, the sampling procedure previously described by He et al19 is outlined below. 2.1.1 30 MWth Full-Scale Bubbling Fluidized Bed (BFB30, Ahlström) The bed material consisted of around 80% quartz with the remainder consisting of K-feldspar (KAlSi3O8) and smaller shares of mica, where the average particle size was 700 µm. About 1 kg of bed material was collected under normal operation conditions at 1, 3, 5, 13 and 23 days after a 100% fresh bed was introduced. The primary air flow represented approximately 40% of the total air flow 4 ACS Paragon Plus Environment
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and dry flue gases contained about 4% O2. The bed exchange rate was the same as during normal operation (see Table 1 for further details). A typical wood-based fuel mixture for the combustion site was used in this study (see Table 2). 2.1.2. 90 MWth Full-Scale Circulating Fluidized Bed (CFB90, Foster Wheeler) As in the BFB case, about 1 kg of bed material was collected after different operation times (1, 5 and 11 days) after a 100% fresh bed replacement. The bed material consisted of around 80% quartz with the remainder consisting of K-feldspar (KAlSi3O8) and smaller shares of mica with a size of 200-500 µm. The excess O2 concentration was 3-4% in the dry flue gases and the primary air flow represented 50-60% of the total air flow. The bed material exchange rate was kept the same as during normal operation (see Table 1 for details). Sawdust was used as fuel. In both plants, used bed material flowed out continuously through a pipe at the bottom of the furnace and was deposited in the ash container. The bed material samples were collected directly after the bottom ash screw before it fell down into the ash container. The sampling campaigns were performed while the respective full-scale plants were running as constant as possible and with the same fuel blends for the whole duration of the experiments. This was verified by determining fuel ash and moisture content in the fuel samples taken every day during the experiments. The fuel samples were further compiled to a general sample for which the main ash-forming elements were determined, see Table 2. 2.2. SEM/EDS Analyses of Bed Material Bed particles from BFB30 and CFB90 with a particle sieving size of 500-850 µm and 200-500 µm, respectively, were collected after gentle sieving. Prior to analysis with SEM/EDS, cross-sections of the K-feldspar bed particles were obtained by encasing bed samples in epoxy resin followed by dry polishing. By analyzing only the largest bed particle cross sections with the thickest layers as determined using back-scattered electron (BSE) images, it was possible to reduce both the effect of varying bed particle age caused by the continuous bed exchange in the full-scale plants as well as 5 ACS Paragon Plus Environment
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the effect of non-central cross-sections. At least 10 commonly found K-feldspar bed particles from respective bed samples were subjected to analysis of bed particle layer thickness and composition. 3-4 EDS line analysis evenly distributed over the bed particle’s periphery were chosen to determine elemental composition along the layer. Different layer types were identified using SEM images obtained in backscattering mode, and elemental compositions were determined using EDS in the center of the respective layers to determine their average composition. The layer thickness for each layer was estimated by using line analysis on 3-4 different sections on the bed particle.
3. Results 3.1. Layer Characteristics for K-feldspar Bed Particles from BFB30 Images of bed particle cross sections from BFB30 with ages ranging from 1 day to 23 days are shown in Figure 1. The brighter bed particles in the figures are K-feldspar particles, while the darker grains are quartz bed particles. From the images in panels (a)-(e) of Figure 1, the bright peripheral line surrounding the darker core of bed particles constitute layers. For older particles (Figure 1d-e) both inner- and outer layer can clearly be distinguished. It is evident that an increasing bed particle age results in an increased layer thickness. No crack layers, as have been observed in quartz particles,15,16 were found in the core of the K-feldspar bed particles. The average molar composition, on a C- and O free basis, in the middle of inner and outer layers for bed particles with ages of 3, 5, 13 and 23 days are shown in Figure 2. The inner layer is dominated by Ca, Si and Al (except O), while the outer layer is rich in Ca, Si and Mg (except O). Increasing Ca content in the inner layer is observed with bed particle age. Typical layer morphology and compositional profiles along the layers on the K-feldspar bed particles with ages from 1 day to 23 days are illustrated in Figure 3. Layer formation was observed on all bed particles from BFB30 for all ages between 1 and 23 days. Bed particles with an age of 1 day displayed only an inner layer. Both an inner layer and an outer coating layer could be observed for all bed particles with an age of 3 days or older. Similar to quartz bed particles, inner layers had a 6 ACS Paragon Plus Environment
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more homogeneous composition than the outer layers, which instead were more heterogeneous and sometimes contained discernible fuel ash particles.19 An irregular interface between the inner layer and the core of the feldspar particle was also observed, more frequently so with increasing bed particle age. Bed particles older than 3 days also displayed cracks in the inner layer. The molar ratio of K/Al decreases at the layer formation boundary whereas the Si/Al molar ratio is constant and the Ca concentration increases for all bed particles. The Ca concentration increases sharply outwards towards the outer layer. All analyzed elemental composition profiles along the layer are shown in Figure 4. The composition trends for all layers are similar. Increasing concentration of Ca and decreasing concentration of Si, Al and K towards the outer layer were observed in the inner layer, while nearly constant concentration gradients were found in the outer layer. The Si/Al molar ratio in the inner layers was about 3 independent on particle age and spatial location, i.e. corresponding to the same ratio as in KAlSi3O8 present in the original K-feldspar particle. It is noteworthy that the K/Al ratio in the inner layer is reduced at the boundary of layer formation, which indicates that Ca2+ substitutes K+ in the feldspar particle. In addition to the previously shown typical bed particle layers, around 15% of the K-feldspar bed particles with age of 13 days and around 30% of the bed particles with age of 23 days were found to have different layer morphology and composition, as illustrated in Figure 5. Compared to the typical bed particle layers shown in Figure 3, the interface between layer and bed particle is not as clearly distinguished, and the concentration gradients of Ca and Si at the interface was reduced, i.e. a more homogenous particle layer characteristics were found. In addition, cracks in the inner layers cannot be observed as frequently as in BFB bed particle layers. The average layer thickness on typical K-feldspar bed particles from BFB30 with different ages is displayed in Figure 6. The measured average total layer thickness is 4, 12, 18, 33 and 38 µm for bed particle with ages from 1 day to 23 days, respectively. The corresponding inner layer thickness after 7 ACS Paragon Plus Environment
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3 days is 6, 8, 13 and 16 µm. The initial layer growth rate of a few micrometers per day is relatively high for either inner or outer layer, but it eventually decreases to reach an accumulated total thickness of about 38 µm. 3.2. Layer Characteristics for K-feldspar Bed Particles from CFB90 Cross-sections of bed particles with ages of 1, 5, and 11 days taken from CFB90 are shown in Figure 7. A lighter K-feldspar layer surrounding the darker core of bed particle was found for all the bed particles. For older particles (Figure 7b-c) both inner- and outer layer can be distinguished and an increasing bed particle age clearly increases total layer thickness. The average elemental compositions in the middle of the layer for bed particles with ages of 5 and 11 days are shown in Figure 8. The result shows that the inner layer predominantly consists of Ca, Si and Al (except O) and the outer layer mainly consists of Ca, Si and Mg (except O). Compared to bed particles with age of 5 days, the concentration of Ca in the inner layer is higher for bed particles with age of 11 days. Since only quite thin layers were found in bed particles with age of 1 day (Figure 9a), no elemental composition is provided. For K-feldspar bed particles of ages 5 and 11 days, the layer consists of an inner and outer layer (Figure 9b-c). Similar to the findings from BFB particles, cracks are found in the inner layer for bed particles older than 5 days. For all bed particles with different ages, the concentrations of Si and Al decrease close to bed particle/inner layer interface while maintaining a constant molar ratio, whereas the Ca concentration increases sharply towards the outer layer. The K concentration close to bed particle/inner layer interface decreases together with a decreased K/Al molar ratio. All analyzed elemental composition profiles along the layer are shown in Figure 10. The composition trends for all layers are similar. The increasing concentration of Ca and decreasing concentration of Si, Al and K towards the outer layer were observed in the inner layer, similar to the findings from BFB’s. The constant Si/Al ratio in the inner layers independent on particle age and 8 ACS Paragon Plus Environment
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spatial location and decreased K/Al ratio towards outer layer were also observed, again suggesting a substitution of K+ by Ca2+ in the aluminium silicate structure. Average total layer thickness for bed particles with different ages from CFB90 are shown in Figure 11. Layers with an age of 1 day were too thin to be accurately measured and were therefore omitted. The average total layer thickness is 16 µm after 5 days, and increases further to 18 µm in bed particles with an age of 11 days, corresponding to the inner layer thickness 7 and 9 µm, respectively. Bed particles from CFB have thinner layers compared to the bed particles from BFB with similar ages, especially the outer layer.
4. Discussion K-feldspar bed particles with an age of 1 day displayed only thin inner layers, whereas bed particles older than 3 days had both an inner layer containing cracks and an outer more particle-rich layer, i.e. a similar morphology to analyzed quartz bed particles in same bed samples.16 The elemental composition of the inner layer was dominated by Ca, Si and Al (except O), and the Ca concentrations in the inner layer increased with particle age. As shown for other bed particles, an increasing layer thickness with particle age was found. On the basis of experimental results, processes responsible for layer formation on K-feldspar bed particle are discussed and compared with previously investigated quartz bed particles from same bed samples.19,20 In combustion of woody fuels, K mainly exists as K-silicates and/or in gaseous compounds. It is unlikely that the gaseous K-compounds react with the K-feldspar particle because of lack of chemical driving forces for reaction.25 Instead, molten K-silicates from fuel ash may adhere to the bed particle surface followed by a subsequent incorporation of Ca2+ into the ash melt. This promotes the transport of Ca2+ to the feldspar bed grain surface, thereby promoting the substitution of K+ in feldspar with Ca2+. The initial layer formation is thereby enhanced by the presence of this molten phase. This initial melt formation on the K-feldspar particle differs from that on quartz that has been discussed to be caused by reaction with gaseous K-compounds.19 However, the 9 ACS Paragon Plus Environment
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incorporation of Ca2+ is reduced due to the formation of solid-phase compounds. Therefore, a decreasing concentration gradient of Ca in the inner layer was observed with increasing particle age. The Ca-compounds deposited on the bed particle surface without diffusion into the inner layer leads to the outer layer formation and its growth. Therefore, two layers can be found for bed particles older than 3 days. The outer layer is much thinner for particles from CFB, i.e. in similarity with findings for quartz.19 It can be expected that bed particles in CFBs are exposed to a notably higher attrition compared to BFBs due to higher flue gas and particle velocities, which likely explains the thinner outer layers observed. In the former case, less Ca-compounds will be available to diffuse/react with the bed particle. The layer thickness and the growth rate of the inner layer on K-feldspar bed particle are similar to that of quartz, where the layer formation process has shown to be diffusion-controlled after an age of about 1 day.19 For the process of a diffusion-controlled solid-state reaction in sphere, with an assumption of constant diffusion coefficient, the equation can be expressed as Fick’s second law.26 For a diffusion system with constant diffusion coefficient DA, the diffusion distance (layer thickness) can be expressed as = t, where α is a parameter related with Ca concentration in the outer layer and t is time. If the layer growth rate was dominated by diffusion for bed particles in this study, the layer thickness L should be linearly correlated to √ , and the slope would correspond to the layer growth rate coefficient. The inner layer thickness as function of t0.5 for bed particles from BFB30 are shown in Figure 12. The inner layer thickness falls on a straight line with the slope 3.5. The linear relation between layer thickness and t0.5 indicates a diffusion-controlled process during inner layer growth. The similar average compositions in inner layers dominated by Ca and Si for both quartz and K-feldspar bed particle could indicate a similar inner layer growth rate. The constant Si/Al ratio around 3, an increasing concentration of Ca, and a reduced K/Al ratio in the inner layer together suggest that the diffusion/reaction of Ca2+ in/with the bed particle leads to the formation of more thermodynamically stable solid phases accompanied with expelling of K+ 10 ACS Paragon Plus Environment
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from the feldspar particle. The formed compounds will, based on SEM/EDS results, consist of Casilicates as well as Ca-Al-silicates where some Si may have originated from the fuel ash. It is likely that CaSiO3 and CaAl2Si2O8 (anorthite) are initially formed, and thereafter transformed to more Carich silicates, e.g. Ca2SiO4 due to continuous addition of Ca2+. Cracks in the inner layer were found for K-feldspar bed particles older than 3 days. The structural strain induced by formation of different phases of Ca- and Ca-Al-silicates within the inner layer could probably be the process responsible for these crack formations, similar to quartz. The reduction of K/Al molar ratio suggests that Ca2+ substitutes K+ in the K-feldspar during formation of the inner layer. The results suggest that the diffusion and reaction of Ca2+ into/with the K-feldspar particle play an important role in the inner layer formation process. Based on the experimental results, a three phases formation process of the layer is deduced and shown in Figure 13. Initially, molten Ksilicates from surrounding fuel ash could adhere to the K-feldspar bed particles. The surrounding individual ash particles, containing Ca-compounds and Mg-compounds, may thereafter be trapped by the melt. At phase 1, an inner layer is formed when Ca-compounds attached on the ash melt diffuse into the melt, leading to the initial inwards growth of thin layer with composition rich in Ca, Si and Al. The Ca- and Mg-compounds adhering to the fuel particle without diffusion into the inner layer are main components of the very thin outer layer. At phase 2, the molten phase disappears due to continuous diffusion of Ca2+ into the melt and driving force for diffusion decreases. Therefore, more Ca2+ will accumulate outside of the inner layer, forming a readily discernible outer layer. This typically takes place for bed particles older than ∼3 days. Finally, in phase 3 – typically present for bed particles older than ∼2 weeks – only small amounts of Ca2+ can reach the interface between unreacted bed particle and inner layer due to long diffusion distances. Therefore, relatively slow layer growth rate will be observed at this stage.
5. Conclusions
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Major differences in layer characteristics were found for K-feldspar bed particles of different ages. Bed particles younger than one day display only a thin layer rich in Si, Ca and Al. Bed samples with operational duration of several days displayed both an inner, homogenous layer containing cracks and an outer particle-rich layer. The outer layer was thinner for K-feldspar bed particles sampled from CFB compared to particles from BFB. K-feldspar and quartz bed particles collected at the same operational conditions displayed similar inner layer thicknesses. No crack layers, as have been observed in quartz particles, were found in the core of the K-feldspar bed particles. The concentration of Ca in the inner layer increases towards bed particle surface, while a reduced K/Aland a similar Si/Al ratio are maintained, indicating that K+ in the feldspar is substituted by Ca2+ leading to formation of Ca-Al-silicates and Ca-silicates in the inner layer. The results suggest that the diffusion and reaction of Ca2+ into/with the K-feldspar particle play an important role in the inner layer formation process in combustion of woody-type fuels with K-feldspar as bed material, and a layer formation mechanism with three phases is proposed.
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Table 1. Operational data and bed materials for the experimental campaigns
Plant
Abbreviation
Bed consumption (wt-% of bed/day)
Bed mass (ton)
Bed particle size (µm)
Bed temp. (°C)
Fuel
Campaign duration (days)
23
11
Full-scale BFB 30 MWth
BFB30
20
