Treatment and use of bottom bed waste in biomass fluidized bed combustors R.C.E. Modolo a, ⁎, L.A.C. Tarelho b , E.R. Teixeira b , V.M. Ferreira a , J.A. Labrincha c a Department of Civil Engineering/CICECO, University of Aveiro, Aveiro, Portugal b Department of Environment and Planning/CESAM, University of Aveiro, Aveiro, Portugal c Department of Materials and Ceramics Engineering/CICECO, University of Aveiro, Aveiro, Portugal abstract article info Article history: Received 28 February 2014 Received in revised form 28 March 2014 Accepted 31 March 2014 Available online 25 April 2014 Keywords: Ash Biomass BFB Combustion Heavy metals Waste Bottom bed waste (BBW) from combustion of forest biomass residues was characterized aiming its use as partial substitute of fresh bed sand (FBS) in industrial bubbling fluidized bed combustors (BFBC). BBW particle size distribution, elemental composition (mainly Si, Ca, Al, Na, K, P and Mg) and mineralogy were evaluated considering also the influence of the characteristics of the biomass used as fuel. Biomass combustion experiments were developed using a pilot-scale bubbling fluidized bed combustor. The operating conditions during the experiments were monitored, namely the flue gas composition (CO 2 ,O 2 and CO), temperature and pressure, and compared with data collected from BFBC located in two industrial biomass thermal power plants. Physical, chemical and mineralogical characteristics of the FBS and BBW revealed that practices related with biomass handling at the forest strongly influence the BBW properties, in terms of not only coarse soil particle addition (N 1.0 mm) but also mineralogically. However, the results obtained here indicate that by sieving of BBW it is possible to recover almost 60% of the original BBW particles (size between 0.3 and 1.0 mm), which have properties that allow its reuse as substitute of FBS for bed make-up in industrial BFBC. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Fluidized bed combustion (FBC) is recognized as the most suitable technology for biomass combustion. FBC plants can deal with distinct fuel mixtures (e.g. different types of biomass) but might show some operating problems when the particle size distribution and type/ amount of impurities in the fuel are not adjusted [1]. The combustion temperature inside the bed must be kept relatively low (usually 700–900 °C) in order to prevent ash sintering. Even so, this technology assures high conversion efficiency [2,3]. Problems that might appear during biomass combustion are related to the amount of ashes being generated and their properties, with potential reflex on the energy con- version efficiency. Slagging, fouling, corrosion, and bed agglomeration are unwanted episodes. Additionally, the generated ash also requires a proper environment [4,5]. Several authors consider agglomeration of bed particles as the major problem during biomass combustion in FBC [6–9]. According to Bartels et al. [10] the inappropriate fluidization caused by agglom- eration could lead to the fluidized bed collapse. The bed material agglomeration and sintering are related with the sticky coatings on bed particle formation [10]. These coatings are formed by multiple layers of ash or ash-rich com- pounds. The internal layers seem to be a sign of the composition of the bed material (sand), while the outer layers are essentially defined by the ash characteristics [11]. In addition, certain element segregation to the ash layers on bed sand particles has been observed [12]. The volume of ashes, their characteristics and management have been considered one of the most important issues during thermochem- ical conversion of biomass to energy. The type of biomass in use (i.e., forest wastes, wood, herbaceous, domestic wastes), the amount/nature of impurities (i.e., soil, inorganics), the technology and the operating conditions (e.g., bed material residence time, and temperature) determine the amount and characteristics of the ashes produced during combustion. According to Vassilev et al. [13] approximately 480 million ton of ash from biomass combustion could be generated worldwide annually. This quantity is becoming comparable to that of coal ash, namely 780 million ton produced per year at present [14]. During biomass combustion in bubbling fluidized bed combustors (BFBC) two main types of ashes are produced: bottom bed and fly ashes. The bottom bed ashes are basically composed by a mixture of sand (mainly quartz) particles from the original bed, and by inert/inorganic components (forest soil and small stones) included in the biomass. Additionally unburnt organic components might also Fuel Processing Technology 125 (2014) 170–181 ⁎ Corresponding author at: Campus de Santiago, University of Aveiro, Department of Civil Engineering, 3810-193 Aveiro, Portugal. Tel.: +351 234 370 049, +351 910 740 471 (mobile). E-mail address: regina.modolo@ua.pt (R.C.E. Modolo). http://dx.doi.org/10.1016/j.fuproc.2014.03.040 0378-3820/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc