Article pubs.acs.org/EF
Ash-Forming Matter in Torrefied Birch Wood: Changes in Chemical Association Tooran Khazraie Shoulaifar,*,† Nikolai DeMartini,† Maria Zevenhoven,† Fred Verhoeff,‡ Jaap Kiel,‡ and Mikko Hupa† †
Process Chemistry Centre, Åbo Akademi University, 20500 Turku, Finland Energy Research Centre of The Netherlands (ECN), Unit ECN Biomass, Post Office Box 1, 1755 ZG Petten, The Netherlands
‡
ABSTRACT: Torrefaction is heat treatment of biomass at relatively low temperatures (∼240−300 °C) in the absence of air. The torrefied fuel offers advantages to traditional biomass, such as higher energy density, better grindability, and reduced biological decay. These factors could, for example, lead to increased use of biomass in pulverized coal boilers. Ash-forming elements are present in biomass as water-soluble salts, ion-exchangeable elements, included or excluded minerals, and covalently bound sulfur and chlorine. In this work, we have studied the change in the chemical association of ash-forming elements in birch wood as a function of the extent of torrefaction. The birch wood was torrefied at 240, 255, 270, or 280 °C at ECN, The Netherlands. The raw and torrefied birch wood samples were studied using three different techniques: chemical fractionation, potentiometric titration, and methylene blue sorption. Chemical fractionation was performed on the original wood sample, and the samples of wood were torrefied at either 240 or 280 °C. These results give a first understanding of the changes in the association of ash-forming elements during torrefaction. The most significant changes can be seen in the distribution of calcium, magnesium, and manganese, with some change in water solubility seen in potassium. These changes may, in part, be due to the destruction of carboxylic acid groups, which were measured by both potentiometric titration and methylene blue sorption. In addition to some changes in water and acid solubility of phosphorus, a clear decrease in the concentration of both chloride and sulfur was measured. The results provide new data about chemical changes with regards to the inorganic elements during torrefaction. The decrease in the chloride content should be investigated further with high-chloride-containing fuels. If a significant level of chloride is removed by torrefaction, this would be a significant additional benefit for the combustion of torrefied biomass.
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INTRODUCTION Torrefaction is a thermochemical pretreatment for biomass, which can increase the energy density, improve grindability, reduce the susceptibility to biological degradation, and reduce the hydrophilicity.1 These improved properties can facilitate increased use of torrefied biomass in co-firing and gasification.2 Biomass use in combustion and gasification requires understanding the fate of inorganic species, which can cause fouling, corrosion, and also bed agglomeration in fluidized-bed conversion, which, in turn, lead to a reduction in the efficiency of the system. These problems may cause costly shutdowns of the units. Understanding the form of ash-forming elements in fuels is important in predicting their fate in combustion and gasification systems.3,4 The elemental ash analysis has been a common method to determine the quality and quantity of ash components. This method is based on laboratory-scale ash and is, thus, often nonrepresentative for real ash behavior. Accordingly, it fails to clarify the chemical and physical characteristics of ash-forming matter in the fuels. In this work, a chemical fractionation method is employed to analyze the form of the ash-forming matters in the fuel. This method was originally developed for ash-forming matters in coal5 and then modified and applied to biomass.6,7 According to this method, ash-forming materials are categorized into four groups.6 The first group consists of water-soluble materials, which can be alkali sulfates, carbonates, © XXXX American Chemical Society
and chlorides. Some organically bound alkali may also be leached during the water-leaching stage because of the slightly acidic pH, which results primarily from carboxylic acid sites in the biomass.8 The second group contains ion-exchangeable cations soluble in ammonium acetate buffer (pH 7), which are believed to be organically bound metals.5 The third group consisted of the materials soluble in hydrochloric acid, including carbonates, oxalates, and sulfates of alkali earth metals. The final fraction is the residue. It contains non-soluble materials, which can be insoluble silicate compounds and covalently bound sulfur, chlorine, and phosphorus. Some ash-forming matters are associated with organics, such as alkali and alkali earth metals associated with carboxylic groups.9,10 An earlier study has shown that the content of carboxylic acid sites decreases during torrefaction.11 In some cases, metals are associated with these functional groups; therefore, metals associated with carboxylic groups may form new bonds during torrefaction. Accordingly, the newly formed compounds are expected to be leached in a different fraction in the fuel fractionation test. Special Issue: Impacts of Fuel Quality on Power Production and the Environment Received: March 25, 2013 Revised: July 18, 2013
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dx.doi.org/10.1021/ef4005175 | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Ultimate and Proximate Analyses of Raw and Torrefied Bircha wt % dry
raw birch
torrefied at 240 °C
torrefied at 255 °C
torrefied at 270 °C
torrefied at 280 °C
ash (at 550 °C) moisture content volatile matter HHV (MJ/kg) C (%) H (%) N (%) O (%)c S (%) Cl (%)
0.19−0.21 8.1−7.9 88−87 19.07−18.99 48.8−48.75b 6.4−6.3b 0.13−0.13b 44.15−45.15b 0.0072 bde
0.23 3.20 81.00 21.07 52.5 (52.3) 6.00 (6.00) 0.15 (0.15) 41.7 (41.9) 0.0051 bd
0.35 3.00 76.00 21.53 54.30 (54.10) 6.00 (6.00) 0.15 (0.15) 39.9 (40.0) nad na
0.33 2.90 75.50 22.08 55.3 (55.0) 5.7 (5.8) 0.15 (0.15) 37.1 (37.4) na na
0.38 2.70 72.00 22.73 56.45 5.8 (5.7) 0.15 (0.15) 36.0 (35.9) 0.0050 bd
a
All of the values are given on a dry fuel basis. bRaw birch values are the average of analysis for two separate samples, which have each been analyzed twice; for other samples, duplicate analysis is given in parentheses. cThis is the analyzed oxygen content. dna = not analyzed. ebd = below detection. The initial biomass, the solid final residue of the fuel fractionation, and the three leachates were sent to an external laboratory, where samples were analyzed according to Swedish standards using inductively coupled plasma−atomic emission spectrometry (ICP− AES) and inductively coupled plasma−sector field mass spectrometry (ICP−SFMS). Anion Quantification. IC with a conductivity detector was used to quantify the concentration of Cl−, NO3−, SO4 2−, PO4 3−, and C2O4 2− , in the three liquid leachates (water, ammonium acetate, and acid). A Metrosep A SUPP-5 column was used with an eluent of 1.0 mM NaHCO3 and 3.2 mM Na2CO3 in ultrapure water. For the identification and quantification of the anions, standard solutions of the anions were prepared from the respective high-purity (p.a.) sodium salts. The concentration of carbonate was determined in the original wood, and the wood was torrefied at 240 and 280 °C by coulometric determination, which was performed in a certified external laboratory. Carboxylic Group. The concentration of carboxylic acid sites was determined by two methods: methylene blue sorption14 and potentiometric titration.12 In the methylene blue sorption technique, acid groups with a pKa less than 7.8 are dissociated to their anionic forms and the methylene blue cations bind to the anions. In biomass, these acids are mostly carboxylic acids. This adsorption continues until the saturation point, after which a plateau is reached. This is taken as the total amount of carboxylic groups in the sample.14 For potentiometric titration, 1500 mg of oven-dried sample is added to 150 mL of 1 M solution of NaNO3. Then, 1.5 mL of 0.5 M HNO3 is added to bring the pH to approximately 2. Potentiometric titration is based on the equivalent amount of base, which is used to neutralize the carboxylic group in the sample.12 This value is obtained by the difference between the inflection point of the titrated sample and the titration of a blank, which consists of the same volume of water and acid but no biomass. Prior to the start of the potentiometric titration, the samples are washed with a 0.01 M HNO3 and 0.1 M ethylenediaminetetraacetic acid (EDTA) solution. During the acidwashing process, metals bound to organic sites are replaced by protons and also alkali and alkali earth bases in the sample, such as carbonates, are removed. Otherwise, acid from the HNO3 addition is consumed, resulting in a shift in the inflection point and an artificially low measure of the concentration of carboxylic acid sites. These two methods for measuring the concentration of carboxylic acid sites were successfully applied to torrefied spruce wood in a previous study.11
In this study, chemical fractionation was used to study the distribution of ash-forming matters in birch wood and birch wood torrefied at 240 and 280 °C. Ion chromatography (IC) was used to quantify five different anions, Cl− (chloride), NO3− (nitrate), PO43− (phosphate), SO42− (sulfate), and C2O42− (oxalate), in the different leachates from chemical fractionation. Coulometry was applied to measure the carbonate content in original and torrefied biomass. Potentiometric titration12 and the methylene blue sorption technique13 were applied to quantify the concentration of carboxylic acid sites, which can also bind cations.10 This study provides new information on the fate of metals, chlorine, and sulfur during torrefaction. A better understanding of the ash-forming matters in biomass and its torrefied forms can help in the use of torrefied biomass.
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EXPERIMENTAL SECTION
Torrefaction. Torrefaction was performed in a pilot plant at ECN, the Energy Research Center of The Netherlands. The birch chips used in torrefaction were smaller than 40 × 40 × 10 mm. Dust particles (
