Elsevier

Bioresource Technology

Volume 198, December 2015, Pages 325-331
Bioresource Technology

Effect of biomass-sulfur interaction on ash composition and agglomeration for the co-combustion of high-sulfur lignite coals and olive cake in a circulating fluidized bed combustor

https://doi.org/10.1016/j.biortech.2015.09.016Get rights and content

Highlights

  • Bed agglomeration was not seen during the tests.

  • Sulfur in coal prevented agglomeration in the bed.

  • High-sulfur content in the coal helped to hinder “Potassium Silicate” formation.

  • It is advantageous to add high-S coals to the combustor for olive cake combustion.

  • Limestone addition transfer K from Arcanite to Potassium Calcium Sulfate phase.

Abstract

This study aimed to investigate the effect of biomass-sulfur interaction on ash composition and agglomeration for the co-combustion of high-sulfur lignite coals and olive cake in a circulating fluidized bed combustor. The tests included co-combustion of 50–50% by wt. mixtures of Bursa-Orhaneli lignite + olive cake and Denizli-Kale lignite + olive cake, with and without limestone addition. Ash samples were subjected to XRF, XRD and SEM/EDS analyses. While MgO was high in the bottom ash for Bursa-Orhaneli lignite and olive cake mixture, Al2O3 was high for Denizli-Kale lignite and olive cake mixture. Due to high Al2O3 content, Muscovite was the dominant phase in the bottom ash of Denizli Kale. CaO in the bottom ash has increased for both fuel mixtures due to limestone addition. K was in Arcanite phase in the co-combustion test of Bursa/Orhaneli lignite and olive cake, however, it mostly appeared in Potassium Calcium Sulfate phase with limestone addition.

Introduction

Fluidized bed combustion technology has an advantage of burning different types of fuels in the same combustor. Due to this advantage, most of the fluidized bed combustors all over the world are recently designed to operate with different solid fuels. However, mixing different fuels may cause some operational problems in the system and may also lead to complete shutdown of the system. When two or more fuels are burned in a fluidized bed combustor, chemical reactions and/or physical interactions may occur between ash particles due to different ash characteristics which result in operational problems such as agglomeration of bed material, slagging, fouling and corrosion of heat exchanger tubes (Hupa, 2008). Although these operational problems are main concerns for the combustion of all types of solid fuels, they are particularly important for the combustion of biomass containing high alkali and alkaline earth metals (Szemmelveisz et al., 2009).

Although the agglomeration problem was firstly studied for coal combustion (Reddy and Mahapatra, 1999, Vuthaluru and Zhang, 1999, Reddy and Mahapatra, 1999, Vuthaluru and Zhang, 1999, Vuthaluru and Zhang, 1999, it became more important when several biomass fuels started to be co-combusted with coals (Arvelakis et al., 2001, Lin et al., 2003, Zheng et al., 2007, Vamvuka et al., 2008, Toscano and Corinaldesi, 2010, Yang et al., 2011, Silvennoinen and Hedman, 2013, Duan et al., 2015). Alkali metals, K and Na, in biomass play an important role in the formation of agglomeration during co-combustion of biomass in fluidized beds (Hupa, 2012, Silvennoinen and Hedman, 2013). These alkali metals can react with silica sand (bed material) at temperatures in the range of 700–900 °C forming low-melting point eutectics like alkali silicates (Scala and Chirone, 2006, Scala and Chirone, 2008, Lokare, 2008). Alkali silicates can form a molten layer on particle surfaces making the particles sticky during combustion (Hupa, 2012). When the system operates for a long time, the sticky layer on the surface of the particles causes formation of permanent bonds between sand particles (Scala and Chirone, 2008). The particles in the bed hit and adhere onto each other due to this sticky layer, causing the particles increase both in size and number. If this is allowed to continue, agglomeration of the whole bed material takes place and total shutdown of the combustor cannot be avoidable. Although the formation of silicates is not well defined, a solid–gas reaction between silica and alkali chloride vapors is given as a possible way which forms liquid alkali silicate (K2SiO3) (Hupa, 2012). If there is no alkali chloride available, silica can react with other alkali vapors such as alkali hydroxides forming alkali silicate (Hupa, 2012).

For the combustion of biomass with ash containing high amounts of K or Na, reaction of SiO2 with alkali oxides or salts are possible as reported by Hupa (2012). Quartz sand, which is mainly composed of SiO2, has a melting temperature of around 1450 °C (Lin et al., 1997). SiO2 can react with alkali oxides or salts in the ash, forming eutectic mixtures with melting temperatures of 874 °C for alkali oxides and 764 °C for alkali salts (Grubor et al., 1995). These temperatures are lower than the melting temperature of SiO2 as well as the melting temperatures of individual components (891 °C for K2CO3 and 851 °C for Na2CO3).

The main precaution to prevent total agglomeration is lowering the alkali content in the combustor by continuously removing the ash formed and replacing the bed material with fresh silica sand (Hupa, 2012). It is also important to use alternative bed materials in order to minimize the silica content which eventually reacts with alkalis (Hiltunen et al., 2008). Different types of materials are used as an alternative to silica sand. Dolomite, alumina and limestone (Ninduangdee and Kuprianov, 2015), porous alumina (Shimizu et al., 2006), blast-furnace slag and olivine sand (Davidsson et al., 2008) can be used as alternative bed materials. Using these alternative bed materials may eliminate the formation of molten silicates. However, silica sand is commonly preferred bed material even with the high-alkali fuels due to high cost of alternative bed materials (Hupa, 2012). Moreover, some problems such as high attrition and entrainment rates, chemical instability, and plugging of air nozzles and windbox were reported during the usage of these alternative bed materials (Khan, 2007). Another measure to take against agglomeration is to add some materials such as kaolin, dolomite, lime, and alumina into the bed. However, these materials have limited usage due to low efficiency and high operational cost. Co-combustion of high-alkali biomass with high-sulfur coals is another solution to solve the bed agglomeration problem during combustion of high-alkali biomass. Sulfur in the coal may react with alkali oxides forming alkali sulfates. Consequently, Si cannot find enough alkali metals to form viscous alkali silicates.

Before using any fuel in real applications, it is a good practice to estimate the tendency of the fuel ash to agglomerate. The agglomeration tendency of a fuel ash is estimated by using some indices. Bed Agglomeration Index (BAI) is the one that is commonly used. It is the ratio of iron oxides to the sum of potassium and sodium oxides in the fuel ash. The ratio is given in

Eq. (1). If the ratio is less than 0.15, it is possible that the agglomeration will be seen (Bapat et al., 1997).BAI=%(Fe2O3)%(K2O+Na2O)

The BAI is directly related to melting temperature of the fuel ash. If K and Na contents are high in fuel ash, this lowers the BAI and increases the tendency of ash to agglomerate. On the other hand, Ca and Mg contents in the ash were reported to have an increasing effect on BAI due to increase in the melting temperature of the fuel ash (Loo and Koppejan, 2008).

In the literature, while burning biomass itself or co-combusting of several biomasses with coal, some operational problems regarding bed agglomeration or defluidization of the bed have often been reported. Control methods such as use of alternative bed materials (sillimanite, bauxite, calcite, magnesite, silica, and alumina), mineral additives (clay, kaosil, bauxite, and carbonate), pre-treatment (water washing, Al and Ca pre-treatment) of fuels for mitigating particle agglomeration were extensively studied (Vuthaluru and Zhang, 1999, Liu et al., 2007, Sun et al., 2008, Vamvuka et al., 2008). Agglomeration tendency of fluidized bed combustor ashes were also investigated in several studies (Anthony and Jia, 2000, Brus et al., 2005, Zevenhoven-Onderwater et al., 2006, Liu et al., 2009). Although there are several studies about the ash-related problems for fluidized bed combustion of biomass in the literature, there is no study conducted with olive cake and high-sulfur Turkish lignites to investigate ash and agglomeration characteristics of these fuels. This study aims to fill this gap. This study also aims to investigate the effect of limestone addition on agglomeration characteristics as well as the effect of sulfur-biomass interaction on the ash composition by burning olive cake with Bursa-Orhaneli and Denizli-Kale lignites in a CFBC.

Section snippets

Characteristics of fuels

Proximate and ultimate analyses of Bursa-Orhaneli lignite, Denizli-Kale lignite and olive cake are given in Table 1. In all tests, the fuel particle size was 1–2 mm. Olive cake was selected as biomass because of its high alkali content in ash (K2O content of olive cake ash was about 50% by wt. on d.b. as given in Table 3).

Laboratory-scale Circulating Fluidized Bed Combustor (LAB-CFBC) and experimental conditions

The experimental setup consists of a circulating fluidized bed combustor, a fuel feeding system, electrical heaters, and two cyclones, flue gas cooling unit and a bag filter.

Co-combustion of Bursa-Orhaneli lignite and olive cake without and with limestone addition

The XRF results of BAsh, FAsh-C and FAsh-BF for the co-combustion tests of Bursa-Orhaneli lignite and clive cake without (B-OC) and limestone (B-OC-L) are given in Table 3. XRF analyses of the fuel ashes are also given in Table 3 for comparison.

When Table 3 is investigated, it can be seen that Al2O3, CaO, Fe2O3, MgO, and SiO2 are the major oxides in the Bursa-Orhaneli lignite ash. However, K2O, CaO, and SiO2 are the major oxides in the olive cake ash. When these results are compared with the

Conclusion

Sulfur in coal is very advantageous while co-combusting coal with biomass having high-K content (like olive cake) to minimize the agglomeration in a fluidized bed combustor. The results showed that K was in the Arcanite phase in the bottom ash for the co-combustion of olive cake with Orhaneli lignite. On the other hand K was found in the Muscovite phase for the co-combustion with Kale lignite. The presence of K and Si in Muscovite phase in the bottom ash was confirmed with XRD analysis.

Acknowledgement

The financial support provided to this project by the Turkish Scientific and Technical Research Council-TUBITAK (Project Code: KAMAG-105G023) is greatly appreciated.

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