Proceedings World Bioenergy 2010

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12 world bioenergy 2010 SLAGGING AND FOULING RISK OF MEDITERANEAN BIOMASSES FOR COMBUSTION Daniel J. Vega-Nieva 1 , Raquel Dopazo 2 and Luis Ortiz 3 . Contact: CÁTEDRA ENCE. University of Vigo (Spain). Forestry School. A Xunqueira Campus. 36005. Pontevedra (Spain). Tel: 1-2: +34/986801948; 3: +34/986801902. Email: 1. DanielJVN@gmail.com 2. dopazo.raquel@gmail.com; 3. lortiz@uvigo.es. ABSTRACT: The interest in biomass combustion has grown exponentially in the last years, as a means for renewable heat and energy promoting local development and mitigating climate change. Various Mediterranean agricultural and forest resources such as olive stone, almond shell or pinecone chips remain large unutilized, despite their potential for being utilized in biomass combustion. New energy crops such as Cardoon, Brassica or Sorghum, are being introduced in Mediterranean countries for Bioenergy production; however, the slagging and fouling risk of many of these potential feedstocks are currently limiting their application in combustion processes given their high alkali, silica or chlorine contents. In this publication, various methods for biomass slagging and fouling hazard monitoring and prediction are presented based on recent studies with Mediterranean biomasses combustion in Spain. Keywords: slagging, fouling, biomass combustion. 1. INTRODUCTION. The interest in biomass combustion has grown exponentially in the last years, as a means for renewable heat and energy promoting local development and mitigating climate change. However, various Mediterranean agricultural and forest resources such as almond shell or pinecone seed shells remain large unutilized. The slagging and fouling risk remain as important barriers that are currently limiting the use of various agricultural residues and potential agricultural energy crops feedstocks such as Cardoon, Brassica, or Sorghum [1], [2], [3]. Slagging occurs in the boiler sections that are directly ex posed to flame irradiation. The mechanism of slagging formation involves stickiness, ash melting and sintering. Slagging de posits consist of an inner powdery layer followed by silicate and alkali compounds [4], [5]. Fouling deposits form in the convective parts of the boiler. The mechanism of fouling is mainly due to condensation of volatile species that have been vaporised in previous boiler sections and are loosely bonded [5] In this publication, various methods for biomass slagging and fouling hazard monitoring and prediction are presented based on recent studies with Mediterranean biomasses combustion in Spain. 2. ASH MELTING BEHAVIOUR: SLAGGING AND FOULING INDICES AND FUSION TEMPERATURES. 2.1. Slagging and Fouling Indexes. Various authors have proposed a series of slagging and fouling indexes to explain the accumulation of ashes into the radiation and convection areas of biomass boilers, respectively. Although originally developed for coal, these indices seem to have potential for application on biomass combustion behaviour prediction (i.e. [6], [7]). Most widely used slagging and fouling indexes and currently proposed thresholds are synthesized below: Base to acid index [1], [6], [7], [8]: Critical value for coal: < 0.75 slagging trend Alkaki Index [9]: where HHV: Higher Heating value (MJ/Kg) at H=0% if index > 0.17 kg alkali /MJ probable fouling if index > 0.34 kg alkali /MJ fouling is certain to occur Slagging index [1], [7], [10]: where S d is % S from elementary analysis if RS < 0.6 low slagging trend if 0.6 < RS < 2 medium trend if 2.0 < RS < 2.6 high trend if RS > 2.6 very high trend
Chlorine Index: Cl content (%) of the sample dry weight. According to [1], [7]: if Cl < 0.2 low slagging trend if 0.2 < Cl < 0.3 medium trend if 0.3 < Cl < 0.5 high trend if Cl > 0.5 very high trend ; whereas [11 ] gives a more conservative threshold: if Cl > 0.1 corrosion and HCl emissions 2.2.Ash Fusion Temperatures. Ashes fusion is a continuous process which can be characterized by the following temperatures, according to norms ISO 540:2008 and DIN 51730:1998-04: Deformation temperature (DT). Temperature at which the first signs of rounding, due to melting, of the tip or edges of the test piece occur. Sphere temperature (ST). The temperature at which the edges of the test piece become completely round, with its height being equal to the width of the base line. Hemisphere temperature (HT). The temperature at which the test piece is approximately hemispherical, with the height being equal to half the base diameter. Flow temperature (FT). The temperature at which the test piece material has spread out so that its height is onethird of that at the hemisphere temperature. A large number of authors have proposed predictive functions of ash fusion temperatures from ash composition for coals (i.e. see [4] for a comprehensive review). Information on such predictive functions for biomass, however, remains scarce (i.e. [12], [13]). .Mineral composition in biomass differs significantly from coal, especially in the amount of potassium, calcium and chlorine, therefore, most appropriate slagging indices definition and threshold calibration, as well as ash fusion predictive functions must be different [9]. Additionally, biomass disintegration laboratory tests have recently been proposed as a complementary means to characterize slagging tendency of biomass fuels, with significant potential to reproduce sintering and fouling behaviour observed in biomass boiler combustion tests [14]. Results using the above mentioned methodologies for characterizing slagging and fouling tendency of various Mediterranean biomass fuels are presented. The effect of ash composition on ash behaviour as characterized by laboratory ash fusion and ash disintegration tests and boiler combustion tests, as well as the potential of biomass ash slagging indices to predict ash behaviour in terms of slagging and fouling hazard is discussed below. 2. SLAGGING AND FOULING RISK OF MEDITERRANEAN BIOMASSES IN COMBUSTION Various recent studies (i.e. [01], [02], [03] and [04]) have recently analyzed the slagging and fouling tendency of several forest and agricultural residues and energy crops by means of laboratory ash characterization tests and combustion tests in Spain. The results of the ash composition of some of the main Mediterranean biomasses analyzed by these studies are synthezised in Table I below. Table I: Ash composition (wt% dry basis) at 550 ºC of various Mediterranean Biomasses from studies [01], [02], [03] and [04] in Spain. Sample SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O P2O5 Ref. Poplar I 2.80 -- -- 33 3.70 0.14 18 -- [14] Poplar II 4.20 0.34 0.36 29 3.00 0.14 16 3.00 [15] Eucalyptus 41 -- -- 18 4.20 1.90 8.70 -- [14] Thistle I 13 -- -- 14 2.50 9.20 15 -- [14] Thistle II 20 -- -- 33 3.30 4.70 6.60 -- [14] Almond shell I Almond shell II Olive stontes 4.60 -- -- 15 1.50 0.30 22 -- [14] 3.50 0.49 0.27 16 2.60 0.49 31 2.40 [17] 24 -- -- 4.40 1.70 0.52 27 -- [14] Brassica I 7.60 0.61 0.28 23 3.20 0.69 21 9.10 [15] Brassica II 1.30 0.16 0.60 25 2.70 1.10 27 8.40 [16] Wheat straw 44 -- -- 8.10 2.40 0.22 18 -- [14] Rice straw 51 -- -- 8.9 3.5 2.8 16 -- [14] Sorghum 40 2.60 1.00 5.50 2.50 0.29 23 2.10 [16] Forest biomasses (i.e. Poplar, Eucalyptus chips) have very beneficial properties for combustion, namely a high Ca content, low silica content, and generally lower K content when compared to most agricultural feedstocks and residues, this resulting in high initial deformation and ash fusion temperatures, generally well above 1200- 1400 ºC, as shown in Table II. It has to be noted, however, that woody fuels can often be contaminated during harvest operations with soil particles, resulting in a higher silica content (i.e. Eucalyptus sample in Table I), which can lower the ash fusion temperatures. This situation has been noted by various authors (i.e. [18]. More careful harvest methods need to be implemented in order to minimize soil contamination. Herbaceous fuels such as brassica, wheat and sorghum have large contents of both silica and potassium, resulting in the potential formation of K silicates (i.e. [4], [8], [19]). These compounds create deposits in the boiler, potentially causing slagging and fouling problems at temperatures above 700-800ºC, as illustrated by the sintering temperatures DT in Table II. Table II. Ash fusibility temperature of various Mediterranean feedstocks from Spain. world bioenergy 2010 13
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