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Slagging Characteristics during Residential Combustion of Biomass Pellets ¨ hman,† Erica Lindstro¨m,‡ Dan Bostro¨m,‡ Rainer Backman,‡ Carl Gilbe,*,† Marcus O Robert Samuelsson,§ and Jan Burvall| DiVision of Energy Engineering, Luleå UniVersity of Technology, S-971 87 Luleå, Sweden, Energy Technology and Thermal Process Chemistry, Umeå UniVersity, S-901 87 Umeå, Sweden, Unit for Biomass Technology and Chemistry, Swedish UniVersity of Agricultural Sciences, Box 4097, S-904 03, Umeå, Sweden, and Skellefteå Kraft AB. S-93180, Skellefteå, Sweden ReceiVed February 6, 2008. ReVised Manuscript ReceiVed April 23, 2008
Limited availability of sawdust and planer shavings and an increasing demand for biomass pellets in Europe are pushing the market toward other, more problematic raw materials with broader variation in total fuel ash content and composition of the ash forming elements as well as in their slagging tendencies. The main objective in the present work was therefore to determine the influence of fuel-ash composition on residual ash and slag behavior. Twelve different biomass pellets were used: reed canary grass (two different samples), hemp (two different samples), wheat straw, salix, logging residues (two different samples), stem wood (sawdust) as well as spruce, pine, and birch bark. The different pellet qualities were combusted in a commercial under fed pellet burner (20 kW) installed in a reference boiler. Continuous measurements of O2, CO, CO2, HCl, SO2, and total particle matter mass concentrations were determined in the exhaust gas directly after the boiler. The collected slag deposits, the corresponding deposited bottom ash in the boiler and the collected particle matter were characterized with X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray analysis (SEM/EDS). For biomass fuel pellets rich in silicon (either inherent or contaminated with sand) and low content of alkaline earth metals the main part of the potassium reacted with the silicon rich ash-residual, forming sticky alkali-silicate particles, which were not entrained from the burner and thereby giving rise to/initiating slag formation. Silicon rich fuels, i.e. fuels were the ash characteristics were dominated by silicate-alkali chemistry, therefore generally showed relatively high slagging tendencies. Straw fuels have typically this ash composition but exceptions to these general trends exists (e.g., one of the hemp fuels used in this work). Wood derived fuels with a relatively low inherent silicon content therefore showed low or relatively moderate slagging tendencies. However, contamination of sand material to these fuels may greatly enhance the slagging tendencies.
1. Introduction New and upgraded biomass fuels (i.e., pellets, briquettes and powder) have become more common and especially fuel pellets have proved to be well suited for heating applications for the residential sector (i.e., burners, stoves and boilers). Limited availability of sawdust and cutter shavings together with an increasing demand for wood pellets in Scandinavia are pushing the market toward new and potentially more problematic raw materials with higher ash content. Examples of such raw materials are some bark, logging residues, whole tree assortments, and different straw fuels. Compared to ordinary stem wood, these raw materials have a broader variation in the total fuel ash content, as well as in the composition of the ash forming elements.1 After complete combustion of the fuel particles a major part of ash forming elements will form a solid residue, known as residual ash. Previous work have shown that residual ash formation during * Corresponding author. Phone: +46 911 211027. Fax: +46 911 232399. E-mail:
[email protected]. † Luleå University of Technology. ‡ Umeå University. § Swedish University of Agricultural Sciences. | Skellefteå Kraft AB. S-93180. (1) Nordin, A. Biomass Bioenergy 1994, 6, 339–347.
biomass combustion depends on the amount and composition of ash forming elements as well as on actual temperatures on the grate, the mixing between fuel and oxidizer and residence time on the grate.2 For some previous studied pelletized biomasses part of the residual ash has formed melted aggregates which not are transported out from the burner grate and therefore forms slag.3–5 The amounts of slag produced are affected both by burner and fuel type used whereas the composition and strength of the slag mostly are influenced of the fuel ash composition.3 Ash related operational problems such as slagging in pellets appliances can lead to a reduced accessibility of the combustion system as well as bad publicity for the pellet market. This inspired/motivated us to more closely study the slagging characteristics during residential combustion of several different pellet qualities representing a broad variation of biomass fuels. (2) Frandsen J. F.; Moiraghi L.; van Lith S.; Jensen P. A.; Glarborg P. Aerosols in biomass combustion. IEA Bioenergy, Task 32. Workshop; Graz University of Technology, Graz, Austria, March 18, 2005; 1st ed. ¨ hman, M.; Boman, C.; Hedman, H.; Nordin, A.; Bostro¨m, D. (3) O Biomass Bioenergy 2004, 27, 585–596. ¨ hman, M. Effect of kaolin and limestone (4) Lindstro¨m E.; Bostro¨m D.; O addition on slag formation during combustion of woody biomass pellets 14th European Biomass for Energy Industry and Climate Protection, Paris, France, October 17-21, 2005. ¨ hman, M.; Hedman, H.; Bostro¨m, D.; Nordin, A. Energy Fuels (5) O 2004, 18, 1370–1376.
10.1021/ef800087x CCC: $40.75 2008 American Chemical Society Published on Web 09/03/2008
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Table 1. Main and Ash-Forming Elements of the Used Fuels wheat straw 6.6 asha cal HHVb 18.5 Ca 46.2 Ha 5.6 Oa 40.7 Na 0.9 Sa 0.14 Cla 0.22 Sia 1.45 Ala 0.0061 Fea 0.010 Caa 0.39 Mga 0.086 Pa 0.10 Naa 0.013 Ka 1.19 a
reed canary reed canary grass (ha) grass (la) 10.7 17.9 44.2 5.5 38.8 0.8 0.09 0.05 3.8 0.31 0.11 0.29 0.073 0.090 0.052 0.36
3.1 19.5 48.2 6.0 41.6 1.1 0.12 0.02 0.68 0.081 0.031 0.34 0.078 0.13 0.0089 0.23
hemp (ha)
hemp (la)
Salix
5.9 1.6 2.0 18.91 19.6 19.7 47.2 48.8 48.8 5.7 5.8 6.0 40.4 43.5 42.9 0.8 0.3 0.3 0.08 0.06 0.03 0.03 0.02
