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High Temperature Elemental Losses and Mineralogical Changes in Common Biomass Ashes

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High Temperature Elemental Losses and Mineralogical Changes in Common Biomass Ashes Fuel 85 (2006) 783–795 www.fuelfirst.com High temperature elemental losses and mineralogical changes in common biomass ashes P. Thy a,*, B.M. Jenkins b, S. Grundvig c, R. Shiraki d, C.E. Lesher a a Department of Geology, University of California, One Shields Avenue, Davis, CA 95616, USA b Department of Biological and Agricultural Engineering, University of California, One Shields Avenue, Davis, CA 95616, USA c Department of Earth Sciences, Aarhus University, DK-8000 A˚ rhus C, Denmark d Interdisciplinary Center for Plasma Mass Spectrometry and McClellan Nuclear Radiation Center, University of California, Davis, CA 95616, USA Received 22 December 2004; received in revised form 16 August 2005; accepted 16 August 2005 Available online 15 September 2005 Abstract The elemental losses from ashes of common biomass fuels (rice straw, wheat straw, and wood) were determined as a function of temperature from 525 8C to below 1525 8C, within the respective melting intervals. The experimental procedure was chosen to approach equilibrium conditions in an oxidizing atmosphere for the specific ash and temperature conditions. All experiments were conducted in air and used the ashes produced initially at temperatures of 525 8C as reactants. Losses during the initial ashing at 525 8C were negligible, except for a K2O loss of 26% for wood and a Cl loss of 20% for wheat straw. Potassium losses are positively correlated with temperature for all fuel ashes. The K2O loss for wood ash commences at 900–1000 8C. Carbonate is detected in the wood ashes to about 700–800 8C and thus cannot explain the retention of K2O in the ashes to 1000 8C. Other crystalline phases detected in the wood ashes (pericline and larnite) contain little or no potassium. Petrographic examinations of high temperature, wood ash products have failed to reveal potassium bearing carbonates, sulfates, or silicates. The release of potassium, thus, appears to be unrelated to the breakdown of potassium-bearing crystalline phases. The straw ashes show restricted potassium loss compared to wood ash. The potassium content declines for both straw ashes from about 750 8C. Cristobalite appears in the straw ashes at about 700–750 8C and is replaced by tridymite in the rice straw ash from about 1100 8C. Sylvite (KCl) disappears completely above 1000 8C. The Cl content starts to decline at about 700 8C, approximately at the same temperature as potassium, suggesting that the breakdown of sylvite is responsible for the losses. The K–Cl relations demonstrate that about 50% of K (atomic basis) released from breakdown of sylvite is retained in the ash. The presence of chlorine in the ash is, therefore, best attributed to the presence of sylvite. Potassium is easily accommodated in the silicate melt formed at temperatures perhaps as low as 700–800 8C from dehydration, recrystallization, and partial melting of amorphous components. Loss of potassium persists for ashes without remaining sylvite and points to the importance of release of potassium from partial melt at temperatures within the melting interval for the fuel ashes. q 2005 Elsevier Ltd. All rights reserved. Keywords: Biomass; Wood; Rice straw; Wheat straw; Ash; Slag; High temperature; Mineral transformations; Alkali metal losses 1. Introduction fireside surfaces, especially superheaters. Such deposits retard the rate of heat transfer, reduce efficiency, and increase Knowledge of the retention or volatilization of alkali metal corrosion of metal surfaces. More advanced technologies that and halogen elements in ash during combustion of biomass offer substantial increase in thermal efficiency, such as biomass fuels is important for an efficient and economically viable integrated gasifier combined cycles, may in some cases prove large-scale utilization of many field crop residues and future even more sensitive to fouling. energy crops. Combustion in biomass fueled boilers, using Annual growth plant materials, including straws and stoker-fired grate furnaces, fluidized beds, and suspension grasses, contain high concentrations of alkali and alkaline units, can lead to a rapid build-up of unmanageable deposits on earth metals, silicon, and chlorine [1–3]. Potassium is the alkali metal present in largest concentration and its volatilization during firing is directly linked to formation of tenacious surface * Corresponding author. Tel.: C1 530 752 0359; fax: C1 530 752 0951. deposits. Chloride is an important facilitator for the deposition E-mail address: [email protected] (P. Thy). of alkali components. At flame temperature, potassium is 0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. deposited as KCl or KOH dependent on the availability of doi:10.1016/j.fuel.2005.08.020 chlorine. At lower temperatures, KCl may react with sulfur or 784 P. Thy et al. / Fuel 85 (2006) 783–795 carbon components of the gas to produce either sulfates or as fuel in California due to problems such as fouling, slaging, carbonates that readily precipitate on boiler surfaces. Sulfur and in fluidized beds also agglomeration of the bed medium can also react with potassium in surface deposits to form observed with straw in operating power boilers. Straw is used sulfates [3–6]. It is commonly, perhaps intuitively, assumed in boilers elsewhere, especially Europe, where designs have that chlorine is easily evaporated and acts as a high temperature been adopted that better allow the ash to cool below the transport media for potassium [3,7]. This appears to be liquidus prior to encountering cross-flow heat transfer surfaces. supported by the presence of sylvite crystals in some boiler Although heating rates and furnace gas compositions in deposits [8]. Controlled and simplified laboratory experiments operating boilers were not entirely matched in this study, have shown that volatilization of potassium, within slag results are relevant to the formation of slag and agglomerates melting temperatures, is strongly controlled by the fuel often found under oxidizing conditions. composition and that potassium is retained in molten rice straw ash, but evaporated from molten wood ash [9]. Use of a 2. Fuel selection and ash preparation straw component in fuel blends was predicted to restrict fouling [10]. These findings require a systematic study of the elemental The samples selected for the ashing experiments are three losses during combustion and ash and slag formation. The commonly available biomass materials in California. The halogen elements have been suggested to play an important first is a mixed conifer (white fir and ponderosa pine) whole- role in the elemental flux in combustion systems, but such tree chip fuel obtained from Wheelabrator-Shasta Energy effects have not been investigated in controlled and simplified Company, Inc., Anderson, California. The trees were laboratory experiments [9–11]. harvested from the northeastern slopes of Mt. Shasta. This We present here the result of a pilot study of the relatively clean and high quality fuel is one of many types compositional and mineralogical effects on ash and slag by received at the plant. The second sample is a medium grain burning fuel at temperatures from within the ashing range to Japonica (variety M202) rice straw from Colusa Country, near complete melting. In particular, we evaluate the co- California. The third sample is a wheat straw from Yolo variation of potassium and chlorine concentrations for three County, California. Neither of the straw fuels is currently commonly available biomass fuels in California (wood, rice used as boiler fuel in California, although several facilities straw, wheat straw). Of these three, currently only wood is used are permitted to use straw. Table 1 Compositions of fuels used as starting materials Wood Rice straw Wheat straw Ultimate analysis (%, wet basis) Carbon 48.54 38.50 43.81 Hydrogen 5.22 3.56 5.10 Nitrogen 0.07 0.55 0.58 Sulfur 0.02 0.06 0.12 Oxygen 34.66 29.23 33.50 Moisture 10.4 7.5 7.8 Chlorine !0.012 0.648 0.402 Potassium 0.075 0.996 0.593 Ash (%, dry basis) 1.2 22.1 9.8 Wood Volatile free Rice straw Volatile free Wheat straw Volatile free Ash analysis (% oxides) SiO2 9.35 14.01 75.38 82.13 57.47 70.28 TiO2 0.13 0.19 0.01 0.01 0.05 0.06 Al2O3 3.12 4.68 0.09 0.10 0.77 0.94 Fe2O3 1.14 1.71 0.10 0.11 0.39 0.48 MnO 1.76 2.64 0.27 0.29 0.07 0.09 MgO 4.93 7.39 1.64 1.79 1.82 2.23 CaO 32.06 48.05 1.60 1.74 2.8 3.42 Na2O 0.39 0.58 0.14 0.15 0.8 0.98 K2O 10.72 16.06 11.95 13.02 16.55 20.24 P2O5 3.13 4.69 0.61 0.66 1.05 1.28 L.O.I. 27.59 7.97 14.91 Total 94.32 100.00 99.76 100.00 96.68 100.00 SO2 0.69 0.67 1.66 Cl !0.065 3.18 3.58 CO2 6.24 0.22 0.19 Ultimate analyses by Hazen Research, Inc., Golden, CO, except that Cl and K determined at UCD by INAA. Ashing temperature was 525 8C. Ash composition determined at University of Aarhus, except SO2 and CO2 determined by Hazen and Cl determined at UCD by INAA. L.O.I. is the loss-on-ignition determined from heating the ash to 950 8C. P. Thy et al. / Fuel 85 (2006) 783–795 785 Fig. 1. Back-scattered electron image (BSE) and elemental X-ray dot-maps of Ka lines for Si and K of rice straw ash. Two different types of particles are present: elongated highly porous particles of biogenic origin and irregular silicic particles of likely terrigenous origin.
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