Article pubs.acs.org/EF
Influence of Torrefaction on Single Particle Combustion of Wood Zhimin Lu,*,† Jie Jian,† Peter Arendt Jensen,‡ Hao Wu,‡ and Peter Glarborg‡ †
School of Electric Power, Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization, South China University of Technology, No. 381 Wushan Road, Tianhe District, Guangzhou 510640, China ‡ Department of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads 229, 2800 Lyngby, Denmark S Supporting Information *
ABSTRACT: This study focuses on the influence of torrefaction on the char reactivity, char yield, and combustion time of 3−5 mm spherical wood particles in a single particle combustion reactor (SPC) operating at a nominal temperature of 1231 °C. The devolatilization times were reduced and the char burnout times were increased with increasing torrefaction degree. The devolatilization time depended linearly on the particle mass. The torrefaction pretreatment resulted in a marked increase in char yield and char particle density but no intrinsic reactivity change as determined by thermogravimetric analysis. The char yield and char particle density increased from 9 wt % and 123 kg/m3 for raw particles to 14 wt % and 259 kg/m3 for particles pretreated at 290 °C for 4 h. The results of this study demonstrate that the higher char yield and density are the main reasons for the longer combustion time of torrefied wood.
1. INTRODUCTION Over the past decade, there has been an increasing interest in using torrefaction technology for pretreatment of biomass, in order to improve the grindability,1−3 hydrophobicity,3,4 carbon content,5,6 and energy density7,8 of biomass fuels. Because of these advantages, torrefied biomass is believed to be more suitable than raw biomass for dedicated combustion or cocombustion in suspension-fired power plants, requiring fewer modifications.9,10 While the torrefaction process and its effect on upgrading of biomass fuels have been extensively investigated, limited work had, until recently, been reported on high-temperature conversion of torrefied biomass. This situation has improved over the past few years with studies on the impact of torrefaction on devolatilization,11,12 combustion/char oxidation,8,10,13−17 and gasification.12,18 Decreased conversion of torrefied biomass has been reported for both combustion13,16 and gasification18 conditions. Bridgeman et al.8 found that the average ignition times and volatile combustion times were reduced and the char burnout times were increased after torrefaction. There are indications that torrefied biomass devolatilize slower than untreated biomass.11,12 It is widely recognized that torrefaction leads to higher char yields, independent of whether the biomass fuels are tested in a low heating rate environment15,20,21 or in high heating rate reactors,11,14,18,19 and independent of the biomass type (willow,14,15 spruce,20,21 birch,20 palm kernel shell,11 beech,19 pine shell,18 olive stones,18 or wheat straw18). Studies in high temperature and high heating rate environments indicate that the char conversion time for torrefied biomass increases, compared to that of raw biomass, under both gasification18,19 and combustion conditions.8,10 The increased char conversion time can be attributed to lower char reactivity, higher char yield, or changes of mineral matter content and morphology in the char. The effect of torrefaction on biomass © 2016 American Chemical Society
char reactivity seems to depend on the heating rate during devolatilization.15 For chars formed under low heating rate conditions, the impact of torrefaction on the reactivity is generally small,15,18,20 while thermogravimetric analysis (TGA) studies of high heating rate chars of torrefied biomass indicate a lower reactivity than that of the untreated feedstocks.14−16 However, the findings also depend on the biomass type,18 and more work is required to obtain conclusive results. Wood char is a highly heterogeneous material, with its structure, chemical composition, and properties depending on the original wood and on the thermal history of the conversion. Despite the recent progress, more work is required to clarify how the torrefaction pretreatment influences the combustion properties of biomass. The objective of this work is to evaluate the influence of torrefaction on the combustion time, char reactivity, and char yield of single biomass particles (3−5 mm) under conditions simulating a suspension-fired boiler. The time required for devolatilisation and char burnout of raw and torrefied wood particles is determined in a single particle reactor (SPC) at 1231 °C. In addition, by extracting char particles from the SPC reactor, the char reactivity, char yield, and char density of raw and torrefied wood particles are quantified. In this way, the char combustion time can be related to the char yield and char properties. The char oxidation results are interpreted in terms of a simple model for external diffusion control.
2. MATERIALS AND METHODS 2.1. Materials. Spherical wood particles (Schima hardwood) with diameters of 3, 4, and 5 mm were used as feedstock in this study. The gross calorific value, proximate analysis, and ultimate analysis of the Received: April 6, 2016 Revised: June 9, 2016 Published: June 15, 2016 5772
DOI: 10.1021/acs.energyfuels.6b00806 Energy Fuels 2016, 30, 5772−5778
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Energy & Fuels fuel are listed in Table 1. As can be seen, Schima wood is low in ash and alkali and alkaline earth metal content.
Table 1. Gross Calorific Value, Proximate and Ultimate Analyses of Schima Wood parameter
unit
value
gross calorific value moisture ash volatiles fixed carbon carbon (C) hydrogen (H) nitrogen (N) sulfur (S) oxygen (O) chlorine (Cl) aluminum (Al) calcium (Ca) iron (Fe) potassium (K) magnesium (Mg) sodium (Na) phosphorus (P) silicon (Si)
MJ/kg (as received) wt % (as received) wt % (as received) wt % (as received) wt % (as received) wt % (dry basis) wt % (dry basis) wt % (dry basis) wt % (dry basis) wt % (dry basis) wt % (dry basis) mg/kg (dry basis) mg/kg (dry basis) mg/kg (dry basis) mg/kg (dry basis) mg/kg(dry basis) mg/kg (dry basis) mg/kg (dry basis) mg/kg (dry basis)
18.7 5.6 1.2 75.6 17.7 49.6 6.1
