| 1. |
- Fagerström, Jonathan, et al.
(författare)
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Influence of peat ash composition on particle emissions and slag formation in biomass grate co-combustion
- 2014
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 28:5, s. 3403-3411
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Tidskriftsartikel (refereegranskat)abstract
- Co-combustion by fuel blending of peat and biomass has shown positive effects on operational problems. However, peat ash compositions vary considerably, and this has been shown to affect the potential for operational problems in different fuel-blending situations. The present work used three different peat types with the objective to elucidate how the variation in peat ash composition influences both particle emissions and slag formation during co-combustion with three different biomasses in a small-scale pellet boiler. Estimations of potassium release and slag formation were performed and discussed in relation to fuel composition in the (K2O + Na2O)–(CaO + MgO)–(SiO2) system. All tested peat types reduced the fine particle emissions by capturing potassium into the bottom ash as one or several of the following forms: slag, sulfates, chlorides, and alumina silicates. However, there were considerable differences between the peat types, presumably depending upon both their content and mineral composition of silicon, calcium, aluminum, and sulfur. Some general important and beneficial properties of peat type in co-combustion situations with biomass are defined here, but the specific blending proportion of peat should be decided on an individual basis for each scenario based on the relative contents in the fuel mixture of the most relevant ash-forming elements.
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| 2. |
- Fagerström, Jonathan, et al.
(författare)
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Reduction of fine particle- and deposit forming alkali by co-combustion of peat with wheat straw and forest residues
- 2010
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Ingår i: Proceedings of the International Conference on Impact of Fuel Quality on Power production and the Environment..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Combustion of ash rich biomass fuels like forest residues and wheat straw often cause severe fouling/deposits and high emissions of PM1, mainly related to alkali transformation. Due to technical and air pollution aspects, primary process/fuel related measures for reduction of volatilized alkali could therefore be of importance. Peat has been used extensively in e g Sweden and Finland since the early 80th due to its positive ash chemical effects. Earlier research with co-combustion of peat and biomass has mostly been focused on fluidized bed boilers and aspects of bed agglomeration and deposits/corrosion. It has also been shown that the content and form of ash forming elements in different peats can vary significantly. The objective with this work was to determine the potential reduction of fine particle- and deposit forming alkali during co-combustion of forest residue and wheat straw with four different peat types in a small scale (15 kW) grate fired pellet boiler. The results showed that significant reduction of fine particle- and deposit forming alkali is possible, either simply by "dilution" of K content (e.g for wheat straw) or by "capturing" of K to bottom ash/slag (e.g. for forest residues), most probably caused by reaction of K vapour from the biomass with reactive Si or clay minerals from the peat. The alkali reduction potential for different biomass fuels and peat mixtures is dicussed in relation to the slagging tendencies and general ash transformation processes
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| 3. |
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| 4. |
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| 5. |
- Näzelius, Ida-Linn, et al.
(författare)
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Fuel Indices for Estimation of Slagging of Phosphorus-Poor Biomass in Fixed Bed Combustion
- 2017
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 31:1, s. 904-915
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Tidskriftsartikel (refereegranskat)abstract
- The market for solid biofuels will grow rapidly during the coming years and there will be a great demand for raw materials. This will force the existing fuel base to also cover wooden materials of lower qualities as well as agricultural raw materials and residues, which often show unfavorable ash melting temperatures. This may lead to combustion related problems. Thus, for the utilization of lower quality fuels, it is important to be able to predict potential fuel ash related problems such as slagging. In light of this, the first objective of the present paper was to evaluate the applicability of previously defined indices for slagging of biomass fuels (phosphorus-poor) in fixed bed combustion. The evaluation showed that none of the previously suggested indices in the literature are suitable for qualitative (nor quantitative) prediction of slagging during fixed bed combustion of P-poor biomass fuels. Hence, a second objective was to develop improved novel fuel indices that can be applied to estimate the slagging of phosphorus-poor biomass in fixed bed combustion. The novel fuel indices give a qualitative prediction of the slagging tendency in biomass fixed bed combustion but still needs additional work to further extend the compositional range as well as to fine-tune the indices’ boundaries.
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| 6. |
- Näzelius, Ida-Linn
(författare)
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Influence of peat addition to biomass pellets on combustion characteristics in residential appliances
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The biomass pellet consumption, both in the industry and in residential appliances, has increased during the latest years and this is expected to continue. In order to handle the demand, more ash rich (>0,5 wt-%) raw materials have been introduced as energy wood and other forest based fuels. In connection with this, ash related problems as fouling, slagging and corrosion have occurred. However introduction of peat as a co-combustion fuel has turned out to have positive effects on these problems. Earlier research shows that introduction of peat into biomass results in reduced bed agglomeration, fouling and corrosion. Nevertheless the previous works has mainly focused on fluidized bed boilers with bed agglomeration and deposit formation in focus. Experimental work in small and residential combustion appliances is still scarce (e.g. in grate firing boilers and burner’s).In this work the influence of peat addition to biomass pellets on combustion characteristics here defined in terms of slagging characteristics, operational maintenance, gaseous- and particulate emissions in residential appliances, were determined. Six peat samples representing a broad variation in ash forming matter of Scandinavian peats were chosen in this study. They were co-pelletized with sawdust, energy wood, forest residue as well as wheat straw and combusted in a P-labeled underfed commercial pellet burner (15 kW) installed in a reference boiler.The NO and SO2 emissions were generally higher when introducing peat. However these emissions are both fuel and combustion specific and the increment has not been further investigated to conclude its origin. The particle emissions were in all cases totally dominated by fine (<1 μm) particles and contained high concentrations of K. When adding peat containing high amounts of Si and/or clay minerals into the wood derived fuels a clear reduction of emitted fine particles were shown. The likely mechanism is that the reactive Si and/or the clay minerals from the peat react with K vapor from the biomass forming K-silicates that will stay in the coarse ash fractions, hence capturing the K. The reduction was most obvious for the biomasses short in reactive Si as sawdust and forest residue. In fact a K reduction of up to 70 % was shown when adding a Si- or clay-rich peat to forest residue. However, when adding a peat with low ash content and high Ca content the reduction was diminished. It was further shown that addition of peat to the K and Si rich wheat straw did not generally affect the reduction of fine particle- and deposit forming K i.e. only a dilution effect of ash forming matter occurred. The slagging tendency was increased in all cases when peat was introduced hence also the operational maintenance was disturbed. However, great differences between different peat assortments existed in this aspect. High Si together with low Ca contents in the peat resulted in the most severe slagging, whereas a peat with higher Ca/Si ratio gave a significant lower slagging tendency. The sawdust was generally more affected by the peat addition than the other studied biofuels.In order to receive a considerable particle reduction in parallel with a manageable slagging tendency when using "problematic" biomass fuels in small scale burners and grate boilers, co-combustion with a carex based peat with a high ash content and relatively high Ca/Si ratio is suggested.
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| 7. |
- Näzelius, Ida-Linn, et al.
(författare)
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Influence of peat addition to woody biomass pellets on slagging characteristics during combustion
- 2013
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 27:7, s. 3997-4006
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Tidskriftsartikel (refereegranskat)abstract
- Upgraded biofuels such as pellets, briquettes, and powder are today commonly used in small as well as large scale appliances. In order to cover an increasing fuel demand new materials such as bark, whole tree assortments, and peat are introduced. These materials have higher ash content which is why they are potentially more problematic compared with stem wood. In general, few studies can be found regarding cocombustion of peat and biomass and in particular where the slagging tendencies are discussed. The overall objective of this study was therefore to determine the influence of peat addition to woody biomass pellets on slagging characteristics. Two different peat assortments (peat A and B) were copelletized separately in four different dry matter levels (0-5-15-30 wt %) into stem wood and energy wood, respectively. Peat A was a traditional Scandinavian fuel peat, with a high ash and Si content (carex), and peat B had a low ash content and relatively high Ca/Si ratio (sphagnum) chosen for its special characteristics. The produced pellets were combusted in a commercial underfed pellet burner (15 kW) installed in a reference boiler. The collected deposits (bottom ash and slag) from the combustion experiments were chemically characterized by scanning electron microscopy (SEM) combined with energy-dispersive X-ray analysis (EDS) and X-ray diffraction (XRD) regarding the elemental distribution and morphology and phase composition, respectively. In addition, the bottom ashes were characterized according to inductively coupled plasma atomic emission spectroscopy (ICP-AES). To interpret the experimental findings chemical equilibrium model calculations were performed. The slagging tendency increased when adding peat into the woody biomasses. Especially sawdust with its relatively low ash and Ca content was generally more sensitive for the different peat assortments. Cofiring with the relatively Si and ash rich peat A resulted in the most severe slagging tendency. A significant increment of the Si, Al, and Fe content and a significant decrement of the Ca content in the slag could be seen when increasing the content of peat A in both woody biomasses. The slagging tendency increased when adding peat A because high temperature melting Ca-Mg oxides react to form more low temperature melting Ca/Mg-Al-K silicates. The slagging tendency was significantly lower when adding the more ash poor peat B, with relatively high Ca/Si ratio, into the woody biomass fuels compared with the peat A mixtures. The slag from the peat B mixings had a slightly higher Ca content compared with the Si content and a clearly higher content of Ca compared with the peat A mixtures. There were still Ca-Mg oxides left in the bottom ash i.e. a less amount of sticky low temperature melting K-silicate rich melt was formed when peat B was added to the woody biomasses.
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| 8. |
- Näzelius, Ida-Linn, 1985-
(författare)
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Slag formation in fixed bed combustion of phosphorus-poor biomass
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To handle a great demand for biomass, alternative biomasses beyond stem wood are being introduced into the solid fuel combustion market, fuels with generally higher (>0,5 wt-%) ash content and different fuel ash compositions compared to stem wood, such as forest residue, bark, grass and straw. Unfortunately, combustion of these alternative fuels often causes more ash related problems such as fouling, slagging and higher particle emissions compared to combustion of stem wood. Many research studies have been conducted regarding ash melting and ash sintering in biomass combustion. However, literature discussing slagging of biomass ash is rather scarce, especially relating to fixed bed combustion. The majority of the biomass fuels available on the market today are phosphorus-poor and this thesis emanates from those. The overall objective was to obtain knowledge of slag formation in fixed-bed combustion of phosphorus-poor biomass, based on bench- and full-scale experiments, chemical analysis of produced ash fractions, chemical equilibrium calculations, viscosity estimations and statistical evaluations.This thesis investigates slagging of [phosphorus-poor] biomass in fixed bed combustion. 85 fuels and 10 different burner/boiler technologies were utilized. The results in this thesis highlight the importance of the ash forming elements Si, Ca, K and Alin the slag formation process in fixed bed combustion of phosphorus-poor biomass. Increased Ca/Si ratios in the fuel reduce slag formation due to formation of more temperature stable phases, i.e. Ca/Mg-oxides and/or formation of carbonate melts with lower viscosity (not sticky) that are less prone to forming slag. A high Al/Si ratio increases the possibility of forming solid and thermally stable K−Al silicates that can reduce slag formation.The fraction of ash melt, along with viscosity, are critical for slag formation and these parameters vary between different fuels. Four classes were defined according to their slagging potential; 1) No slag: fuel composition and the bottom ash contains low Si and K contents and higher Ca content. Fuel examples: non-contaminated stem- and pulpwood/energy wood, 2) Minor slagging tendency: fuel compositions show increased Si compared to non-slagging fuels and the bottom ash contains lower Ca, but increased Si content and approximately unchanged K content compared to the former category. Fuel examples: stem wood, bark and logging residue with increased Si-content due to light contamination. 3) Moderate slagging tendency: fuel composition contains further increased Si content. Increased share of formed silicate melt and higher viscosity (more sticky) compared to minor slagging fuels. Fuel examples: mostly contaminated woody fuels and grass and straws with relatively high amount of Ca. and 4) Major slagging tendency: Fuel composition contains high Si and K content. Sticky K-silicates causes major increase in slagging tendency. Fuel examples: different types of grass and straw fuels.The burner/boiler technology can affect whether slagging will induce major problems in the burner or not. However, long residence times and high temperatures for the combustion residues in the hot part of the fuel bed are technical prerequisite for increased slag formation.This thesis developed two qualitative fuel indices for predicting slagging in fixed bed combustion of phosphorus-poor biofuels – one index for fraction of fuel ash that forms slag and one index for sintering category of the formed slag. Both novel indices deliver acceptable results and are more reliable than previous indices found in the literature. Importantly, the fraction of fuel ash that forms slag index outperforms the sintering category for qualitative prediction of the problematic slagging potential of a certain fuel. Additional work is needed to further widen the compositional range as well as to fine tune the indices’ boundaries.
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| 9. |
- Näzelius, Ida-Linn, et al.
(författare)
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Slagging in Fixed bed Combustion of Phosphorus-Poor Biomass : Critical Ash Forming Processes and Compositions
- 2015
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 29:2, s. 894-908
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Tidskriftsartikel (refereegranskat)abstract
- Slagging in combustion facilities are not welcomed, as it may cause technical and operational problems as well as extra costs. Increased understanding of the critical slagging sub-processes makes it easier to suggest semi-empirical models and fuel indexes for prediction of slagging tendency of different fuels. That could open the biomass market for potentially more troublesome raw materials. The objective of this work was to determine critical ash forming processes and compositions in fixed bed combustion of phosphorus-poor biomass fuels. This was achieved by performing a systematic review of data and experience gathered from combustion experiments in a small grate burner of 36 different biomasses, chemical analysis of their bottom ashes and slags. The paper presents a discussion of the slagging tendency in phosphorus-poor biomass by combining three different slagging classifications ending up with a proposed starting point for a new slagging index. The slag (ash particles > 3.15 mm) formed during the combustion experiments has been described according to fraction of fuel ash that forms slag (wt-%), visual sintering category (1-4) and viscosity predictions. The results explain that both the fraction of melt and its viscosity is critical for the slag formation process in phosphorus-poor biomasses. Additionally, fuels with low Si/K ratio along with higher Ca concentration may form a low viscous carbonate melt not prone to form slag. Increased Si and lowered Ca concentration will increase the amount of formed silicate melt formed as well as its viscosity, thus resulting in a more sticky melt.
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| 10. |
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| 11. |
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| 12. |
- Rebbling, Anders, 1980-, et al.
(författare)
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Prediction of slag related problems during fixed bed combustion of biomass by application of a multivariate statistical approach on fuel properties and burner technology
- 2020
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Ingår i: Biomass and Bioenergy. - : Elsevier. - 0961-9534 .- 1873-2909. ; 137
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Tidskriftsartikel (refereegranskat)abstract
- Slag is related to the melting properties of ash and is affected by both the chemical composition of the fuel ash and the combustion parameters. Chemical analysis of slag from fixed bed combustion of phosphorus-poor biomass show that the main constituents are Si, Ca, K, O (and some Mg, Al, and Na), which indicates that the slag consists of different silicates. Earlier research also points out viscosity and fraction of the ash that melts, as crucial parameters for slag formation. To the authors’ knowledge, very few of the papers published to this day discuss slagging problems of different pelletized fuels combusted in multiple combustion appliances. Furthermore, no comprehensive classification of both burner technology and fuel ash parameters has been presented in the literature so far. The objective of the present paper was therefore to give a first description of a qualitative model where ash content, concentrations of main ash forming elements in the fuel and type of combustion appliance are related to slagging behaviour and potential operational problems of a biomass fuel in different small- and medium scale fixed bed appliances.Based on the results from the combustion of a wide range of pelletized biomass fuels in nine different burners, a model is presented for amount of slag formed and expected severity of operational problems. The model was validated by data collected from extensive combustion experiments and it can be concluded that the model predicts qualitative results.
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| 13. |
- Rebbling, Anders, et al.
(författare)
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Waste Gypsum Board and Ash-Related Problems during Combustion of Biomass : 2. Fixed Bed
- 2016
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 30:12, s. 10705-10713
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Tidskriftsartikel (refereegranskat)abstract
- This paper is the second of two describing the use of shredded waste gypsum board (SWGB) as an additive during combustion of biomass. The focus of this paper is to determine whether SWGB can be used as a fuel additive providing CaO and SO2/SO3 for mitigation of ash-related operational problems during combustion of biomass and waste derived fuels in grate fired fixed bed applications. The former study in this series was performed in a fluidized bed and thus allow for comparison of results. Gypsum may decompose at elevated temperatures and forms solid CaO and gaseous SO2/SO3 which have been shown to reduce problems with slagging on the fixed bed and alkali chloride deposit formation. Three different biomasses, spruce bark (SB), reed canary grass (RG), and wheat straw (WS), were combusted with and without addition of SWGB in a residential pellet burner (20 kWth). Waste derived fuel with and without the addition of SWGB was combusted in a large scale grate-fired boiler (25 MWth). The amount of added SWGB varied between 1 and 4 wt %. Ash, slag, and particulate matter (PM) were sampled and subsequently analyzed with scanning electron microscopy/ energy dispersive spectroscopy and X-ray diffraction. Decomposition of CaSO4 originating from SWGB was observed as elevated SO2 emissions in both the large scale and small scale facilities and significantly higher than was observed in the fluidized bed study. Slag formation was significantly reduced due to formation of calcium-silicates in small scale application, but no conclusive observations regarding calcium reactivity could be made in the large scale application. In the small scale study the formation of K2SO4 was favored over KCl in PM, while in the large scale study K3Na(SO4)2 and K2Zn2(SO4)3 increased. It is concluded that SWGB can be used as a source of CaO and SO2/SO3 to mitigate slag formation on the grate and chloride-induced high temperature corrosion and that fixed bed applications are likely more suitable than bubbling fluidized beds when using SWGB as an additive.
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| 14. |
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| 15. |
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| 16. |
- Öhman, Marcus, et al.
(författare)
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Slag formation during combustion of biomass fuels with low phosphorus content
- 2011
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Ingår i: From research to industry and markets. - Florence : ETA - Renewable Energies. - 9788889407554 ; , s. 1267-1270
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Konferensbidrag (refereegranskat)abstract
- Slag and ash deposition problems have more than occasionally been observed in biomass fueled plants in general and especially in pellet fired plants. These problems can lead to reduced accessibility as well as performance of the combustion appliances. Slag formation including ash transformation mechanisms, measures for prevention and prediction during combustion of biomass fuels has been studied by the research groups at Luleå University of Technology, Umeå University and Energy Technology Center for over one decade now. Several (about 40) different biomass fuels/fuel mixtures have been studied and many different fuel additives/co-combustion fuels have been used to combat slag formation. About 20 scientific papers and 10 technical reports have been published by the research groups within the topic of slag formation during combustion of biomass fuels with low phosphorus content. The objective of the present work was to summarize the general experiences and conclusions of our research within the area of slag formation during combustion of fuels with low phosphorus and varying silicon content. Silicon rich fuels, i.e. fuel ash dominated by silicate-alkali chemistry (e.g. straw fuels), generally shows relatively high slagging tendencies. Exceptions to these general trends exists i.e. fuels with very high Si/K-ratios. Wood derived fuels with a relatively low inherent silicon content shows low or relatively moderate slagging tendencies. However, severe contamination of sand material to woody biomass fuels may greatly enhance the slagging tendencies. Fuel additive addition to problematic woody biomass and straw significantly reduces the slagging tendencies. When adding limestone and kaolin to the problematic wood and straw fuels the composition of the formed slag is changed from relatively low temperature melting silicates to high temperature melting silicates and oxides. The standard ash fusion tests and chemical equilibrium calculations gives useful qualitatively information of the slagging tendencies. However, both methods must be further improved before quantitatively results can be used. The use of different fuel indexes to predict the slagging tendencies could be an interesting option. More research within this field is, however, needed.
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