Article pubs.acs.org/EF
Transformation and Release of Potassium, Chlorine, and Sulfur from Wheat Straw under Conditions Relevant to Dual Fluidized Bed Gasification Placid A. Tchoffor,*,†,‡ Kent O. Davidsson,† and Henrik Thunman‡ †
SP Technical Research Institute of Sweden, SE-501 15 Borås, Sweden Chalmers University of Technology, 41296 Göteborg, Sweden
‡
ABSTRACT: The release and transformation of potassium (K), chlorine (Cl), and sulfur (S) from biomass during thermochemical conversion processes may lead to problems, such as the corrosion and fouling of heat transfer surfaces, agglomeration of bed material, and the poisoning of catalysts used in the downstream processes of gasifiers. To predict and to mitigate effectively these problems, information regarding the quantity and mechanism of the release of these elements under relevant operating conditions is required. In the present work, the release of K, Cl, and S from wheat straw under conditions relevant to dual fluidized bed gasification were quantified in a laboratory-scale bubbling fluidized bed reactor. During the pyrolysis step, the bed temperature ranged from 550 to 900 °C, while the residence time for the fuel in the reactor was fixed at 3 min. The char samples obtained from the pyrolysis step were partially combusted at the same temperature at which they were produced for an additional 3 min. The fractions of the elements released from the fuel were quantified by chemical analysis of the char/residual ash obtained in each experiment and a mass balance across the system. Overall, 75%−62% of the Cl, 59%−67% of the S, and 14%−31% of the K in the virgin wheat straw were released during pyrolysis conducted within the investigated temperature range. The char obtained from the pyrolysis process contained significantly higher amounts of K, Cl, and S than the virgin fuel. Furthermore, the ash content of the char was about 5-fold higher than that of the virgin fuel. This suggests that at combustion-relevant temperatures, complete combustion of the char is more likely to result in severe ash-related problems than combustion of the virgin fuel. Partial combustion of the char resulted in additional release of K, Cl, and S. In addition to the experimental results, the transformation and release of the elements during this process are discussed with the aid of chemical thermodynamic equilibrium modeling and leaching.
1. INTRODUCTION
gasification reactor, most of the fuel is converted during the pyrolysis step, and a part of the char that remains after the pyrolysis is partially gasified with steam. The unconverted char from the gasification reactor, together with the bed materials, is circulated to the combustion reactor where it is oxidized with air to generate the heat needed to raise the temperature of the combustion air to the combustion temperature and to raise the temperature of the circulated bed materials. The hot bed materials are thereafter recirculated to the gasification reactor, so as to deliver the heat for fuel conversion. During thermal conversion of biomass, as occurs in the dual fluidized bed gasification process, K, Cl, and S (among other elements) are released from the fuel. As a result of the various transformations in the gas and solid phases that take place, the compounds formed by these elements may cause undesirable effects in the combustion and gasification reactors as well as in the downstream processes. These effects include ash-related problems, such as corrosion4−8 and fouling8−11 of heat transfer surfaces, and bed agglomeration12−14 caused by alkali chlorides, sulfates, and silicates. An additional and serious problem caused by the presence of inorganic species, especially S, is the poisoning of downstream catalysts.15,16
1.1. Background. Dual fluidized bed gasification offers a means to produce a medium to high energy value gas (11−15 MJ/Nm3) from biomass.1−3 In this system, one of the fluidized bed reactors is operated as a gasification reactor, while the other is operated as a combustion reactor (Figure 1). In the
Received: August 26, 2013 Revised: November 19, 2013 Published: November 21, 2013
Figure 1. Dual fluidized bed gasification process. © 2013 American Chemical Society
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dx.doi.org/10.1021/ef401703a | Energy Fuels 2013, 27, 7510−7520
Energy & Fuels
Article
released.38,39 The organically associated K is released as KOH26 or combined with water-soluble and water-insoluble tars.40 In addition to the release of organically associated K after the devolatilization of biomass, the release of K from the char at temperatures in the range of 700−900 °C can occur through the evaporation of KCl, decomposition of K2CO3, and the release of char-bonded K.23,41 The release of K from char through these pathways is influenced by the atmosphere in the reactor,20,24,41 the ash composition,20,38 and the residence time of the fuel in the reactor.20 As the vapor pressure of KCl increases significantly within and above this temperature range, fuels that have higher Cl/K ratios than Si/K ratios release more K.20 Since the evaporation of KCl from biomass is a mass transfer process, the smaller the fuel particle size and/or the longer the residence time of the fuel in the reactor, the higher will be the evaporation of KCl from the fuel.20 At temperatures approaching 900 °C, K2CO3 decomposes, which leads to the release of K, as shown in reactions 1 and 2.20
The severities of these problems depend largely on the fractions of these elements in the virgin biomass transferred to the vapor, and the fractions that remain in the solid char.17−19 In addition to the fuel properties, such as ash composition, particle size, and the nature of the associations of elements in the fuel, the transformation and release of K, Cl, and S from biomass depend on the operating conditions in the reactor.20 These conditions include the heating rate, the residence time, reactor temperature, and the atmosphere in the reactor. While the pathways for the release of these elements under given temperature intervals and atmospheres may be similar in various reactors, such as fixed and fluidized beds,20 the difference in heating rates and the residence times of the fuels in these reactors may affect the quantity of each element released from the fuel. The heating rate for fuel particles of a given size in the reactor is established by the heat transfer coefficient in the reactor.21 For a given fuel particle size, the external heat transfer coefficient in a fluidized bed is much higher [up to 1000 W/(m2/K)] than that in a fixed bed reactor [up to 100 W/(m2/K)].22 For a given process, the fuel particles in fixed bed reactors have significantly longer residence times than those in fluidized bed reactors. The influences of operating conditions on the transformation and release of K, Cl, and S from biomass during thermochemical conversion processes have been investigated extensively under fixed bed conditions.18,20,23−25 However, there are few similar studies using conditions of fast heating, short residence time, and alternating atmospheres that prevail during dual fluidized bed gasification. The aim of the present work was to obtain quantitative and qualitative information regarding the transformation and release of K, Cl, and S from biomass under dual fluidized bed gasification conditions. Since most of the fuel in this system is converted in the pyrolysis step and the char gasification step occurs relatively slowly, the effect of steam on the release of these elements from biomass will be investigated in a subsequent work. Therefore, this work deals with the pyrolysis of virgin biomass and the combustion of the resulting char. The results obtained are also relevant to stand-alone combustion and gasification processes. 1.2. Theory. 1.2.1. Forms of K, Cl, and S in Biomass. K, Cl, and S may be both organically and inorganically associated in biomass.18,24,26−30 60−90% of the Cl and K in various biomass fuels can be dissolved in water, which implies that these elements are present as salts in the fuel matrix.31−33 The fractions of these elements in biomass that are insoluble in water can be organically bonded in the fuel matrix (or in the case of K, exist as silicates).34 Studies33,35,36 have shown that 40−77% of the S in various biomass fuels is inorganically associated, while the remainder is organically associated. The inorganically associated sulfur in most biomass fuels is mainly present in the chemical form of alkali sulfates, probably K2SO4.34,35,37 1.2.2. K release Pathways. Studies20,23,24,38 have shown that
