Predicting Bed Agglomeration Tendencies for Biomass Fuels Fired in FBC Boilers: A Comparison of Three Different Prediction Methods

Experimental determination of bed agglomeration tendencies of some common agricultural residues in fluidized bed combustion and gasification

Bed agglomeration characteristics of palm shell and corncob combustion in fluidized bed

Waste in oil palm biomass is one of the renewable energy sources that can be converted into energy. The availability of oil palm biomass which is a renewable energy source is currently very adequate. This research will specifically analyze the differences in the level of thermal efficiency and heat transfer rates of two different types of biomasses. In addition, this comparative analysis was also carried out when testing the modification of the perforated plate with the standard plate or without modification. The combustion test was carried out in a fluidized-bed combustor (FBC) combustion chamber with data measurements using a Digital Thermometer brand Hot-Temp HT-306. Palm oil solid waste biomass such as palm kernel shells and oil palm fronds were used as testing fuel in this study. The results show that the average level of thermal efficiency for palm kernel shell (PKS) and oil palm midrib (OPM) fuels, when tested with a modified hollow plate, is 33.59% and 28.31%, respectively. Meanwhile, the results of the average thermal efficiency at the time of testing the standard plate were 19.77% PKS and 28.29% OPM. The results of the heat transfer rate test for standard plates with PKS fuel are 7363.53 W/m2, which is lower than after modification, which is 7762.38 W/m2. Meanwhile, the results of combustion using OPM fuel were higher when testing the standard plate at 7289.84 W/m2 compared to 7162.81 W/m2 when testing with a modified perforated plate. Overall, testing with the application of modified perforated plates can increase the thermal efficiency and heat transfer rate in the FBC chamber.KeywordsEfficiency thermalHeat transfer rateOil palm fuelFBCCombustion

Much bio-energy can be obtained from wood pruning operations in forests and fruit orchards. Several spatial studies have been carried out for biomass surveys, and many linear programming models have been developed to model the logistics of bio-energy chains. These models can assist in determining the best alternatives for bio-energy chains. Most of these models use network structures built up from nodes with one or more depots, with arcs connecting the depots. Each depot is a source of a certain biomass type. Nodes can also be biomass storage points or production facilities (e.g., power plants) where biomass is used. The arcs in the networks represent transport between depots. In order to combine GIS spatial studies with linear programming models, it is necessary to build a network from a digital map of biomass production centers, such as orchards. Biomass collection points should therefore be defined as sources in the delivery network model. In this work, a mathematical calculation method is developed to select the actual biomass collection points on a map. The database for this model is composed of area surveys of forest and agricultural biomass storage points given in GIS maps (shape files). The limits of the area studied and different types of biomass are defined and located in different layers of the GIS maps. These energy-biomass production maps are overlaid with a 1 km × 1 km grid of the area studied. The result is a grid in which the different types of total available biomass in each quadrant are known. Harvesting and collection costs are also defined. The connections between all n × m quadrants of the area studied are defined by the available road network. Every quadrant is associated with a point on the road network. The selection criteria for sources of biomass (sub-areas) are the following: firstly, a minimum production of available biomass type is required; and secondly, harvesting and collection costs should be minimal. The algorithm provides the location of points where biomass from the associated area can be concentrated. These biomass collection points are then taken as source nodes in the network during the implementation of the logistics models. In the next step, the network is analyzed by linear programming techniques to supply the optimal position of energy plants or factories, given the available biomass sources.

In this study, sludge combustion was carried out in a bench-scale fluidized bed combustor to investigate the formation of agglomerates, followed by a thorough identification of bed materials and agglomerates using scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDX), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Agglomeration invariably became more severe at higher temperatures with degraded sand as the bed material compared to fresh sand. Two major crystalline components, hematite (Fe2O3) and quartz (SiO2), were detected in agglomerates, showing that the eutectics of iron and silicate with the viscous phase might be one of the promoters of bed agglomeration. SEM/EDX analysis revealed the agglomeration progress, suggesting the role of Si (from sand particles) and P, Mg, and Ca (from sludge) in initializing the formation of sticky surface of sand particles to act as a glue matter, followed by the formation of an agglomerate bridge contributed most likely by Fe, then by Al, K, and Na. The bed agglomeration observed was possibly attributed to both melt-induced and coating-induced mechanisms. Furthermore, the behavior of mineral matters in sludge combustion was simulated by means of thermodynamic multiphase multicomponent equilibrium (TPCE) calculations. The thermodynamically major low melting point species formed in sludge combustion were predicted. In combining the experimental and computational results, it is believed that alkali phosphates (KPO3 and NaPO3) and the eutectics of Fe2O3 and SiO2 might play the most important role in bed agglomeration, by forming low melting point compounds, in the course of sludge combustion.

The bed agglomeration characteristics during combustion of typical biomass fuels were determined in a bench-scale bubbling fluidized-bed reactor (5 kW) using olivine and quartz sand as bed material. The fuels studied include willow, logging residues, wheat straw, and wheat distiller’s dried grain with solubles (wheat DDGS). Bed material samples and agglomerates were analyzed by means of scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), for morphology and elemental composition. Furthermore, bed ash particles were separated by sieving from the bed material samples and analyzed for elemental composition by SEM–EDS and for determination of crystalline phases by powder X-ray diffraction (XRD). Chemical equilibrium calculations were performed to interpret the experimental findings of layer formation and reaction tendencies in both bed materials. Significant difference in the agglomeration tendency between olivine and quartz was found during combustion of willow and logging residues. These fuels resulted in inner layers that were more dependent on the bed material composition, and outer layers that have a composition similar to the fuel ash characteristics. The elemental composition of the inner layer formed on the quartz bed particles was dominated by Si, K, and Ca. In the olivine bed, the inner layer consisted mainly of Mg, Si, and Ca. Chemical equilibrium calculations made for both bed materials showed a low chemical driving force for K to react and be retained by the olivine bed particles, which is in accordance to the experimental findings. For the quartz case, the inner layer was found responsible for the initiation of the agglomeration process. The composition of the fewer and more porous agglomerates found after the experiments in the olivine bed showed neck composition and characteristics similar to the individual bed ash particles found in the bed or outer bed particle coating composition. For DDGS (rich in S, P, K, and Mg) and wheat straw (rich in Si and K), no significant differences in the bed agglomeration tendency between olivine and quartz bed materials were found. The results show that the bed particle layer formation and bed agglomeration process were associated to direct adhesion of bed particles by partly molten fuel ash derived K–Mg phosphates for DDGS and K-silicates for wheat straw.

The use of biomass fuels for energy production through combustion has a growing application worldwide mainly for two reasons: first, the utilization of biomass for energy contributes to mitigate emission of green house gases; second, its use decreases the dependence of imported fossil fuels in Europe. The objective of this work was to study the combustion behaviour of two endogenous biomass species: cardoon (cynara cardunculus) and arundo (arundo donax), which were specially produced in energy crops plantations. Mixtures of cardoon and a forestry biomass specie (eucalyptus) were also studied to evaluate potential benefits from synergies between both biomass fuel types. The results showed that the utilization of cardoon, in pelletized form, and loose arundo as feedstock, did not give rise to any operational problems related with the feeding system. It was verified that the mono combustion of cardoon could pose problems at industrial scale in fluidised bed systems, considering the high levels of HCl and NOx emissions obtained and tendency to sinter the bed sand material. The addition of the forestry biomass to cardoon appeared to prevent the bed agglomeration problem. Furthermore, both the NOx and SO2 emissions were found to decrease at the same time suggesting potential synergy of blending different types of biomass regarding pollutant emissions and in bed agglomeration problems.Keywordsenergy cropscynara cardunculusarundo donaxfluidised bed combustiongaseous pollutantsbiomass

Following the recent Bovine spongiform encephalopathy (BSE) experiences, thermal treatment of meat- and bonemeal (MBM) in existing fluidized bed combustion (FBC) plants for refuse-derived fuels (RDFs) has evolved as an interesting disposal and disintegration method. However, only a limited number of studies have previously been performed for combustion of MBM in fluidized beds. The objectives of the present work were, therefore, to determine the bed agglomeration tendencies of these materials during combustion in fluidized beds and to evaluate the effects of dolomite and kaolin addition to the fuel mix, as well as to elucidate the overall ash transformation mechanisms governing the potential bed agglomeration and fouling processes. By controlled agglomeration experiments in a 5 kW bench-scale fluidized bed reactor, the fuel-specific critical agglomeration temperatures in normal quartz bed material were determined for the different fuel/additive mixtures. All collected samples of bed materials, final bed agglomerates, and cyclone ashes were analyzed using SEM/EDS and XRD. The results indicated that the MBM fuels could be expected to be problematic concerning bed agglomeration in normal quartz beds, while kaolin and possibly dolomite addition could be used to reduce this risk to moderate levels. A significant elemental fractionation between the bed material and the cyclone ash was obtained. Apatite (Ca5(PO4)3(OH) or potentially some other calcium phosphates are elutriated from the bed and enriched in the fly ash, while sodium and potassium are enriched in the bed material. The characteristics and the corresponding melting behavior estimations of the necks formed between agglomerated bed particles suggest that silicate melts are responsible for the bed agglomeration. Results from XRD analysis of the fly ash formed from the fuels used in the present study indicated that the risk for melt-related fly ash problems seem relatively small.

Entrained flow pyrolysis and gasification of selected biomass – an experimental and modeling study

Thermo-chemical conversion of biomass for sustainable aviation fuel/fuel additives

Experimental study on the sintering characteristics of biomass ash

Study on burning oil palm kernel shell in a conical fluidized-bed combustor using alumina as the bed material

The fluidized bed combustion of a biomass residue (olive husk) common in the Mediterranean area was investigated in a bench-scale reactor. The focus of the study was the high propensity of this fuel to have bed agglomeration problems during combustion as a consequence of the high potassium content of the ash. Temperature and pressure profiles in the bed were followed as a function of time during steady combustion tests at different operating conditions. Bed defluidization characteristic times were measured and correlated to the fuel ash buildup on the bed sand particles. In addition, a diagnostic tool based on the measurement of the dynamic pressure signal inside the bed was tested for its ability to predict bed agglomeration. On the basis of SEM/EDX analysis of agglomerate samples discharged from the bed after defluidization had occurred, the mechanisms of fuel ash−bed particle interaction and agglomerate formation are discussed.

Combustion of Oil Palm Shells in a Fluidized-bed Combustor Using Dolomite as the Bed Material to Prevent Bed Agglomeration

Fluidized bed combustion (FBC) technology was commercialized in the 70s. Both bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) technology are capable of handling a wide variety of solid fuels. Natural sand is typically used as the fluidizing material. However, the properties and behavior of some solid fuel ash may limit the use of these fuels due to bed agglomeration problems. Natural sand contains several minerals, typically mainly consisting of 20–50 wt.-% of plagioclase (NaAlSi3O8 + CaAlSi3O8), 10–30 wt.-% of potash feldspar (KAlSi3O8), and 25–100 wt.-% of quartz (SiO2). Biomass based fuels contain high amounts of alkali. Ash high in alkali may react with the free quartz of the natural sand, producing an alkali silicate mixture with low melting point. This mixture may act as an adhesive between fluidized bed particles and may, in the worst-case, result in serious fluidization problems. This problem can be avoided by using AGGLOSTOP™ quartz-free bed material. Four different bed materials were tested in a 15 kW laboratory-scale FBC test rig with plywood residue, which is known to cause severe fluidization problems in FB boilers. Two of the tested bed materials were quartz-free. When quartz-free bed materials were used, no signs of bed agglomeration were observed. The other two bed materials containing free quartz caused total defluidization at a temperature of around 750°C after about half an hour of operation. The concept of using AGGLOSTOP™ quartz-free bed material with high alkali fuels has been successfully applied in two industrial scale BFB boilers (15 and 74 MWth). The use of AGGLOSTOP™ fluidized bed material enables energy production in FB boilers based on high alkali fuels, which were earlier impossible to utilize due severe bed agglomeration problems. This paper focuses on the bed agglomeration phenomenon by discussing the results from laboratory and industrial-scale boilers and presents a new solution to extend the use of high alkali fuels in FB boilers.