Article pubs.acs.org/EF
Potassium Retention in Updraft Gasification of Wood Joseph Olwa,*,†,‡ Marcus Ö hman,† Pettersson Esbjörn,§ Dan Boström,∥ Mackay Okure,‡ and Björn Kjellström⊥ †
Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-97187 Luleå, Sweden Department of Mechanical Engineering, Makerere University, Box 7062, Kampala, Uganda § Energy Technology Centre, SE-94128 Piteå, Sweden ∥ Energy Technology and Thermal Process Chemistry, Umeå University, SE-90187 Umeå, Sweden ⊥ Exergetics AB, SE-61991 Trosa, Sweden ‡
ABSTRACT: The release of compounds of K with producer gas during biomass gasification is known to play significant roles in fouling and high-temperature corrosion in boilers and high-temperature heat exchangers as well as blades in gas turbines that use producer gas as fuel. These phenomena are a major setback in the application of biomass fuel in combination with advanced process conditions. Updraft gasification provides gas filtering by the fuel bed with a gas cooling effect, conditions anticipated to create an avenue for K retention in the gasifier. The objective of this study was to determine the K retention potential of such gasifiers during wood gasification. Samples for the determination of the fate of K compounds included in the feedstock were collected from the generated producer gas using Teflon filters and gas wash bottles and also from wall deposits and ash residues. Analyses of samples were carried out using inductively coupled plasma−atomic emission spectrometry/mass spectrometry and Xray diffraction methods. The finding was that about 99% of K was retained in the gasifier. K was found in the ash samples as a crystalline phase of K2Ca(CO3)2(s) (fairchildite). A possible reaction mechanism leading to the formation of K2Ca(CO3)2 is discussed in the paper. The 1% K understood as released, equivalent to 1200 ppbw content of K entrained in the producer gas stream, exceeds a known limit for application of the gas in conventional gas turbines. This would suggest application of the gas in an externally fired gas turbine system, where some limited K and other depositions in the heat exchanger can be relatively easy to handle.
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INTRODUCTION
production loss and, in worst cases, shutdown of the whole plant. In this study, K was of interest because its compounds are the main initiators of high-temperature corrosion, fouling, and slagging. Generally, the content of K in biomass is much higher than that of Na,11 with the latter occurring as a minor species in wood and playing an insignificant substitution role for the former in the thermal degradation process. It is known from tests carried out on wood combustion and other gasification processes that K may form K silicates, K2SO4, and KCl, K2CO3, and KOH at temperatures above 500 °C.9,12 The speciation of K compounds that can be formed is dependent upon the availability of the other main ash-forming elements that are found in wood, e.g., Ca, Si, S, and Cl. During thermal conversion of biomass, K will initially react readily with the ubiquitous water vapor to a more stable and relatively volatile hydroxide. Thus, KOH is the primary compound formed through which all subsequent K compounds are formed. Part of KOH reacts with silica to form K silicate, and because there is a low quantity of Si in the fuel, the ensuing formation of K compounds in the order of the sulfates, chlorides, and carbonates occurs depending upon the quantity of available K left after the formation of the preceding compounds in the mentioned order.7,13 KCl and KOH are known to react, in gas
Wood gasifiers are equipment used for a controlled combustion of wood in limited supply of air as the oxidizing medium to generate a combustible product gas. Other oxidizing media, such as oxygen and steam, or a combination of any two media can be used in the gasification process. There are some prospects in the integration of wood gasifiers with gas turbines for electricity power generation in small capacity range (below 1 MW), where steam turbines are not economically attractive.1 However, a major challenge with the integration relates to the presence of alkali, in particular K, in the wood fuel. K is inherently present in a considerable quantity in wood, together with other ash-forming elements, such as Ca, Mg, Mn, Si, S, Cl, Fe, Al, and P. Most wood fuels do not have significant quantities of Fe, Al, and P. Details on ash-forming elements of wood and their mode of occurrence are discussed in the works of Werkelin et al.,2 Miles et al.,3 and Zevenhoven.4 K in wood is bound as an organically associated ion-exchangeable metal ion and water-soluble salts.2,5 It is understood that a significant amount of K is vaporized during wood gasification, and they react mainly in the gas phase with anionic species from the fuel and air to form inorganic compounds of carbonates, chlorides, sulfates, oxides, and hydroxides.2,3 Studies have shown that the main cause of high-temperature corrosion and low ash-melting temperatures of some biomass fuel is related to the quantity of K in the fuel.3,6−10 The effects of these phenomena are the main causes of unscheduled maintenance operations, leading to © 2013 American Chemical Society
Received: June 24, 2013 Revised: September 30, 2013 Published: October 14, 2013 6718
dx.doi.org/10.1021/ef401179f | Energy Fuels 2013, 27, 6718−6724
Energy & Fuels
Article
and condensed phases, with SO3 to form K2SO4. SO3 is formed from the oxidation of biomass fuel S. KCl and K2CO3 are known to be stronger high-temperature corrosion initiators than K2SO4.6,14 The prominent effect of the sulfation reaction in abating high-temperature corrosion is presented in a study by Boström et al.,15 where the sulfation of alkali chloride by ammonium sulfate was shown to reduce the corrosion of a ferrous alloy by about 70%. Ash and volatilized potassium species are the detrimental entities fomenting fouling and corrosion in boilers and high-temperature heat exchangers; primary fuel/gasification-related measures to reduce the amount of alkali in the producer gas would imply both technical and economic benefits for plant owners. Therefore, a substantial retention of K in coarse ash fractions, e.g., in the bottom ash of the gasifier, can significantly offset the hightemperature corrosion tendency of wood fuel. Studies have been carried out on alkali retention in different thermal processes, and one such study conducted with a smallscale cyclone gasifier using wood powder realized about 40− 60% K retention.16 A similar study with a cyclone gasifier using bagasse attained about 70% K retention.8 In yet another study conducted with a fluidized-bed gasifier using bagasse and bana grass, it was mentioned that 40 and 80% retentions, respectively, were attained.17 Although the form in which K is retained is not mentioned, of the three studies presented, the best retention in the gasification process of 80% would release 20% of K into the gas phase. This amount of K released and entrained in the gas stream is way above the required level for gas quality (equivalent of 20 ppbw of K in the gas stream) recommeded for firing even the most tolerant gas turbine.18 For comparison purposes, a study with wood pellets in a small-scale fixed-bed combustion process (15 kW) captured up to 96% of K in the bottom ash and slag. In that study, the main crystalline ash compound of K detected was fairchildite,19 which also had been identified in another wood combustion process.20 Although studies have been conducted with cyclone and fluidized-bed gasifiers, there is currently no literature on the K retention potential of the updraft gasifier to the knowledge of the authors. It is likely that an updraft gasifier having a combustion process beneath the fuel bed could provide favorable conditions to retain more K than the discussed processes above. Therefore, the updraft gasifier was chosen for this study because of its anticipated potential to retain K (a fact attributed to the cooling effect of the fuel bed), simplicity in small-scale application, high thermal efficiency, and possible integration to gas turbines. The objective of this study was to determine the quantity of K retainable in updraft gasification of wood fuel and to identify processes leading to that retention. It is also possible with the results of this study to assess the producer gas quality from the gasifier for integration to gas turbines. Experiments were carried out with wood pellets to generate producer gas with fuel feed and airflow rates chosen to resemble those used in a study by Di Blasi et al.21 This was to allow for some comparison to a bench-scale gasification experiment. The gas was analyzed for contents of alkalis and gas composition. The ash and wall deposits were also analyzed.
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Table 1. Composition of Main Ash-Forming Elements of the Wood Pellets on a Dry Basis dry substance (wt %) ash (wt %) calculated higher heating value (MJ/kg) C (wt %) H (wt %) O (wt %) N (wt %) S (wt %) Cl (wt %) Si (wt %) Al (wt %) Fe (wt %) Ca (wt %) Mg (wt %) Mn (wt %) P (wt %) Na (wt %) K (wt %)
92.0 0.3 20.3 51.9 6.0 41.8
