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Biofuels and Biomass
Influence of K/C ratio on gasification rate of biomass chars Oskar Karlström, Meheretu Jaleta Dirbeba, Mario Costa, Anders Brink, and Mikko Hupa Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02288 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018
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Energy & Fuels
Influence of K/C ratio on gasification rate of biomass chars Oskar Karlströma*, Meheretu Jaleta Dirbeba a, Mario Costa b, Anders Brink a, Mikko Hupa a a
Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Turku, Finland
b
IDMEC, Mechanical Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal *corresponding author, email:
[email protected], phone: +358 2 215 3275
Keywords: catalyst, gasification, biomass, char, agricultural biomass, potassium Abstract The present study investigates the influence of the K/C ratio on the gasification rate during char conversion. Chars of five agricultural biomasses are investigated: kiwi branches, olive branches, grape pomace, torrefied grape pomace and sugarcane bagasse. Chars of the biomasses were produced in N2 in a single particle reactor at 800 °C and subsequently gasified in CO2 in a TGA at 800 °C. The sugarcane bagasse char had a high Si content and a lower reactivity, which often is attributed to the presence of non-catalytic silicates. SEMEDX analysis of the initial Si-rich char revealed that most of the K was not bound as silicates. Around 30-60% of the K was lost during the char gasification of the chars. The K/C ratio increased as the char conversion proceeded. The instantaneous char gasification rate increased until the K/C ratio was around 0.1 for four of the chars. For coal chars and pure carbons it has been suggested that the catalytic char gasification rate is saturated when the K/C ratio is 0.1. The results of the present study imply that catalytic char gasification rate may become saturated at high degrees of conversion. From 0 80% of conversion, the chars showed a similar catalytic behavior: the rate increased almost linearly with the K/C ratio.
1. Introduction In combustion and gasification of solid fuels, the char residue reacts simultaneously with CO2, H2O and O2 [1,2]. Biomass chars are generally more reactive towards O2, CO2 and H2O as compared to coal chars [3]. Unless the particles are sufficiently small, such as in pulverized fuel combustion, the reactions between biomass char-C and O2 are limited by external mass transfer. The slower biomass char gasification reactions by CO2 and H2O are in general limited by chemical kinetics or by the combined effects of pore diffusion and kinetics [4-8]. The reactions between CO2 and carbon are frequently expressed as [1,9-11]: C + CO2 ⇌ C(O) + CO(g)
(R1)
C(O) → CO
(R2)
where C(O) is a carbon-oxygen surface complex. For carbon, the CO2 dissociation reaction (1) is accelerated by a redox cycle due to the involvement of catalytic species such as K and Ca [12]. A postulated mechanism for such catalytic gasification is the following [12-15]: CO2 + * ⇌ CO + O*
(R3)
O* + Cf ⇌ C(O) + *
(R4)
C(O) → CO
(R5)
Here [*] represents the “empty” and oxygen containing metal species active for oxygen transfer to a free carbon site Cf resulting in a carbon-oxygen surface complex C(O). In biomass chars, compounds
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of K and Ca, and also some other metals, influence the reactivity [16,17]. K and Ca ions have different charges and exist in different forms in the char matrix and, as a result, the catalytic activities differ for Ca and K. In some biomasses, Ca exists as oxalates and it is not likely that large salt particles in the fuel can influence the char reactivity. Most biomasses contain both K and Ca, and it is difficult or even impossible to separate the influence of K and Ca on the char reactivity. Several groups have demineralized biomasses and doped them with controlled amounts of K and Ca [18-20]. In general, higher contents of doped catalysts give a higher reactivity. Perander et al. [18] showed, however, that dependent on which procedure that was used to dope Ca, the reactivities differed significantly although the char-Ca contents were the same. Dupont et al. [21-22] showed that there is a good correlation between the gasification reactivity and the K content for a range of biomasses, even if they emphasize that Ca, as well as other factors, may influence the gasification reactivity. Other elements influencing the catalytic gasification reactivity are for example Na and Mg [19-20], which are generally less available in biomasses as compared to K [23]. In spite of the extensive research on char gasification reactivity, the following two factors influencing the reactivity have not been, to the authors’ knowledge, emphasized in the same context: (i) loss of catalyst and (ii) accumulation of catalytic compounds on the surface as the carbon is consumed. The accumulated catalytic compounds can continue to participate in the char gasification reactions via complex migration [12-15]. As the carbon is consumed, the ratio of catalytic elements to carbon increases as a function of the degree of char conversion. However, since for example loss of K occurs, it is difficult to predict how the catalyst/C ratio changes as a function of char conversion. In the present study, instantaneous char gasification rates are determined and compared to K/C ratios of the chars throughout the char conversion process. The objectives are to evaluate whether the char reactivity varies as a function of the K/C ratios during the char conversion and whether the dependency is similar for various biomasses. 2. Experiments 2.1 Parent fuels Chars of five agricultural biomasses were investigated: kiwi branches (KB), olive branches (OB), grape pomace (GP), torrefied grape pomace (TGP) and sugarcane bagasse (SCB). Torrefaction may influence the reactivity of the char residue [24], and torrefied grape pomace has previously not been analyzed in this context. The samples were dried at 105 °C for 24 h and ground to particle sizes
