Article pubs.acs.org/EF
Time-Dependent Crack Layer Formation in Quartz Bed Particles during Fluidized Bed Combustion of Woody Biomass Hanbing He,*,† Nils Skoglund,†,‡ and Marcus Ö hman† †
Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-971 87 Luleå, Sweden Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden
‡
ABSTRACT: Bed agglomeration during combustion and gasification of woody biomass fuels in quartz beds has been frequently studied, and chemical mechanisms responsible for bed agglomeration have been suggested. However, few studies have focused on the bed material deposition on walls, in cyclones, and return legs in fluidized bed combustion. Part of these bed material depositions originates from sticky fragments of alkali-rich silicates formed after crack formation in older quartz bed particles. The crack layer formation in quartz bed particles in fluidized bed combustion of woody biomass was therefore investigated by collecting bed material samples of different ages from full-scale bubbling and circulating fluidized bed facilities. Scanning electron microscopy/energy-dispersive spectroscopy was used to analyze the crack morphology and composition of the layer surrounding the cracks. For quartz bed particles with an age of some days, a crack in the quartz bed particle was observed in connection to the irregular interface between the inner layer and the core of the bed particle. The crack layer composition is similar for quartz particles with different ages and for samples taken from different fluidized bed techniques. Their composition is dominated by Si, K, Ca, and Na (except O). These crack layers become deeper, wider, and more common as bed particle age increases. The crack layers eventually connect with each other, and the whole quartz particle is transformed into smaller quartz cores surrounded by crack layers, which were observed in particles older than 1 week. From the characterization work, a crack formation process including three phases is proposed on the basis of the presumption that the initial crack layer formation resulted from the presence of induced cracks in the inner quartz bed particle layer. Fragmentation after the third phase is likely responsible for the formation of sticky alkali silicate deposit formation, and a weekly complete exchange of the bed is therefore recommended to avoid problematic deposits in combustion of woody-type biomass in fluidized bed combustion.
1. INTRODUCTION Biomass, thought as a potentially CO2-neutral and renewable energy resource, has attracted much attention worldwide in recent years. In the past decade, many combined heat and power (CHP) plants have been designed for using biomass. Among these technologies, fluidized bed combustion (FBC) is an important process for utility-scale biomass CHP plants as a result of its advantages, including flexibility toward the fuel, high combustion efficiency, and low environmental impact.1,2 However, problems experienced in the FBC of woody fuels are often related to agglomeration of bed particles resulting in bed defluidization as well as bed material deposition on furnace walls, in cyclones, and return legs.3−6 To solve these problems in combustion of woody biomass, a deeper understanding of the formation mechanisms of these ash-related problems is of essential importance. Many researchers have studied the formation mechanisms of the adhesive material in FBC using different types of biomass fuels.7−16 Ash-derived compounds with low melting temperatures, such as alkali silicates, have been reported to be responsible for bed agglomerate formation.17−20 For woody biomass, the onset of bed agglomeration is attributed to ash deposition, which interacts with bed material by a combination of (i) attachment of small particles, (ii) condensation of gaseous alkali species (KCl and KOH), and (iii) chemical reaction of gaseous alkali on the surfaces.7 By subsequent sintering, the inner layer of the coating is probably © 2017 American Chemical Society
homogenized and strengthened and the melting behavior of this silicate layer seems to control the adhesive forces.7 If the adhesive force is larger than the separation force between bed particles, agglomerates form.14 Therefore, one basic idea behind remedies to agglomeration is to minimize the presence of compounds with low melting temperature. The layer formation on a quartz bed particle has been extensively studied for FBC using woody biomass. It has been shown that an inner layer is primarily responsible for bed agglomeration in combustion of woody biomass fuels.8,9,16 The use of quartz bed materials in the operation of FBC may be accompanied by bed agglomeration problems and may also contribute to the formation of bed material deposition.4−6 Although bed agglomeration has been reported in scientific literature during combustion and gasification of woody biomass fuels and the responsible chemical bed agglomeration mechanisms have been elucidated, only very limited studies have previously focused on the bed material deposition. Previous work suggests that entrained alkali-silicate-rich fragments from old quartz bed particles are responsible for bed material deposition in cyclones and return legs in circulating fluidized bed (CFB) combustion of woody biomass fuels.4 Received: November 11, 2016 Revised: January 18, 2017 Published: February 1, 2017 1672
DOI: 10.1021/acs.energyfuels.6b02980 Energy Fuels 2017, 31, 1672−1677
Article
Energy & Fuels Table 1. Summary of the Bed Material and Operational Data for the Sampling Campaign plant BFB at 30 MWth CFB at 90 MWth
bed particle size (μm)
bed temperature (°C)
abbreviation
bed mass (ton)
bed consumption (wt % of bed/day)
BFB30
20
