Article pubs.acs.org/EF
Thermochemical Equilibrium Study of Slag Formation during Pressurized Entrained-Flow Gasification of Woody Biomass Charlie Ma,*,† Rainer Backman,‡ and Marcus Ö hman† †
Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-97187 Luleå, Sweden Energy Technology and Thermal Process Chemistry, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden
‡
ABSTRACT: The potential slag formation behavior during pressurized entrained-flow gasification (PEFG) of woody biomass has been studied from a thermodynamic perspective with respect to compositional, temperature, and pressure variations. An ash transformation scheme was proposed on the basis of the melt formation potential that arises when gaseous K species are present with Si and Ca. Databases and models in FactSage 6.4 were used to carry out thermochemical equilibrium calculations within ChemSheet. It was found that increasing pressure and increasing Si content expanded the range of operating conditions that are conducive of melt formation, while increasing temperature and increasing Ca content diminished the range. The results from the calculations compared qualitatively well to experimental results and provide further information needed in the development of PEFG reactors for woody biomass. part and the level of soil contaminants.8 The chemical associations of these elements within the fuel as well as the significant degree of fuel particle fragmentation during hightemperature pulverized firing imply the likelihood of volatilization and dispersion of smaller ash particles (fractionation). This scenario constitutes a potential for melt formation. This analysis differs from the analysis presented by Coda et al.,5 where instead all fuel ash elements were input into a single set of calculations to assess the potential amount of melt formation. The use of multiphase equilibria based on minimization of Gibbs energy has proven useful for analyzing and prediction of ash-related interactions during combustion and gasification processes.9 Thermodynamic databases and models have been developed that are suitable for coal-based ash-forming elements to predict their extent of molten material formation under different O2 partial pressures, compositions, and temperatures relevant to PEFG.10 Similar databases and models have also been used to predict stable multiphase inorganic products during thermal processes associated with black liquor11,12 and other biomass.13,14 The results of the analysis are compared qualitatively to deposits collected from the aforementioned PEFG experiments,5,6 and some practical implications and further developments required are discussed.
1. INTRODUCTION Pressurized entrained-flow gasification (PEFG) of woody biomass is currently being studied as a potentially sustainable and economically viable process to produce fuels and other vital chemicals.1,2 An important aspect during fuel conversion is the transformation of ash-forming elements. The formation of molten ash material can lead to deposition of slag upon the reactor wall during the gasification process.3 Continuous extraction of such ash slag from the reactor is necessary to prevent outlet blockages and to sustain steady production of syngas. One possibility would be to form a steadily flowing slag, consisting of mostly molten ash, to be tapped at the bottom of the reactor as in the entrained-flow gasification process for coal.4 It has, however, been observed from laboratory-scale PEFG experiments of woody biomass that ash deposits consist mainly of CaO-rich particles that exhibit only minor signs of molten slag.5 The high melting point of pure wood ash was attributed to this observation. In contrast, ash deposits from a pilot-scale PEFG reactor firing three woody biomasses with different levels of Si-based soil contaminants showed significant interaction of Ca, Si, and K in high-viscosity slags with differing degrees of sintering.6 Such high-viscosity slag deposits may accumulate and cause reactor outlet blockages.7 Hence, slag formation, intended or otherwise, arising from the formation of molten ash material is crucial to the PEFG process. It forms part of the knowledge regarding ash transformations that is necessary for the development of woody biomass PEFG into a feasible syngas production pathway. In light of this, the objective of this study was to evaluate, from a thermodynamic perspective, conditions during PEFG of woody biomass that may favor melt formation. This was carried out with the aim of providing a guideline and basis for comparison for future experimental work. A sequential ash transformation scheme is presented on the basis of the most abundant inorganic elements in woody biomass, Ca and K, as well as Si, which can be high depending upon the tree species/ © 2015 American Chemical Society
2. MATERIALS AND METHODS 2.1. Ash Transformation Scheme. Two sets of calculations were carried out based on sequential events that are expected to occur during PEFG of woody biomass, depicted in Figure 1. In the primary ash transformation process, as the fuel particle is converted (left panel of Figure 1), significant amounts of K are expected to be volatilized, mainly as K(g) or KOH(g).15 Simultaneously, the suspension-firing regime combined with a relatively high degree of fuel particle Received: April 21, 2015 Revised: June 1, 2015 Published: June 5, 2015 4399
DOI: 10.1021/acs.energyfuels.5b00889 Energy Fuels 2015, 29, 4399−4406
Article
Energy & Fuels
behavior of ash particles varying in composition from Ca-rich to Sirich, which may be consequent from, e.g., adhesive contact between ash particles. Essentially, this part of the study concerned the addition of K to compositions along the SiO2−CaO system as dictated by the gasification atmosphere at the specified conditions. The corrosion of silica glass furnaces by gaseous alkali was studied in a similar manner by Allendorf and Spear.20 2.4. Thermochemical Equilibrium Calculation Software and Databases. Thermochemical equilibrium calculations were carried out in ChemSheet21 using thermodynamic databases and models from the Equilib module of FactSage22 6.4: FACTPS database for pure solids and gases, FToxid for oxide melt solution, and FTpulp and FTsalt for salt melt/solid solutions (Table 2). The selection of the oxide melt phase (SLAGA) with the salt melt phases (LCSO and MELTA) assumes that they are not miscible. The gas phase was treated as a perfect mixture.
Figure 1. Schematic depiction of sequential ash transformation events analyzed thermodynamically. fragmentation is likely to disperse ash particles rich in CaO and/or Si.16,17 This subsequently creates a thermochemical potential for the reaction between the particles with the gaseous K species (right panel of Figure 1), which may lead to the formation of melt or other condensed phases as a result of secondary ash transformations.18,19 The following sections 2.2 and 2.3 detail the two calculation sets. Global equilibrium conditions in the reactor are assumed throughout this study. Although this is a simplification, it provides useful information regarding the potentiality of phases to form as well as their trends under specific operational conditions and ash compositions. 2.2. Concentrations of Gaseous K Species. Estimations of the gaseous K species concentrations were necessary to begin evaluation of the thermodynamic potential for melt formation according to the proposed ash transformation scheme (left panel of Figure 1). The conditions for these atmospheres were calculated for temperatures of 900−1800 °C and pressures of 1−80 bar to cover the plausible range of global PEFG reactor conditions. The oxidant was pure O2 at an oxidant/fuel ratio (λ) of 0.35. The fuel compositions were based approximately on two woody biomasses,8 shown in Table 1. Only K was included as the ash-forming element in this calculation set; i.e., no chemical interactions of K were considered, except those with the gasification atmosphere. This allowed for the deduction of the partial pressures of gaseous K species at the different conditions that may be conducive of melt formation with Ca and Si, which is addressed in section 2.3. Elements S and Cl that can affect the concentrations of gaseous K species have been omitted. S is stabilized predominantly as H2S(g) in reducing atmospheres at equilibrium, and while KCl is stable up to high temperatures, Cl is generally significantly lower than K in woody biomass and would, therefore, only slightly reduce the concentrations of K(g) and KOH(g).8,15 2.3. Interaction of the Gasification Atmosphere with Ca and Si. The resultant partial pressures of K(g), KOH(g), and CO2(g) at the corresponding temperatures and total pressures stated from the previous calculations (section 2.2) were set as inputs in calculations with the introduction of Ca and/or Si. This effectively dictated the condensed products of Ca, K, and Si under the global equilibrium gasification atmospheres and conditions, as depicted in the right panel of Figure 1. With the partial pressures of the gaseous K species fixed, the total amount of K in each calculation was allowed to be in surplus, such that K was saturated within the predicted condensed phases at the corresponding atmospheres and conditions. In practice, this represents the composition on the very surface of the Ca and/or Si particle. Hence, along with the potential for melt formation at the specified conditions and atmospheres, these calculations also indicate the likely trend in changes of those surface melt compositions. In addition to calculations involving Ca and Si individually, Si/Ca molar ratios of 1:2, 1:1, and 2:1 were studied to predict the melt formation
Table 2. FactSage Databases Used in TECs (in Order of Precedence for Duplicate Compounds) database
full name and chemical compounds SLAGA (oxide melt: SiO2, K2O, and CaO) LCSO (K+, Ca2+/CO32− melt) SCSO (K+, Ca2+/CO32− solid solution) MELTA (K+/CO32−, OH− salt melt) pure stoichiometric gas and solid phases
FToxid FTsalt FTpulp FACTPS
3. RESULTS AND DISCUSSION 3.1. Concentrations of Gaseous K Species. The main gaseous K species predicted to be present are K(g) and KOH(g), which collectively comprise >99 mol % of all gaseous species containing K. On the basis of the wood composition, their concentrations vary between approximately 0 and 100 ppm, as shown in Figure 2a. At high pressures and low temperatures enclosed by the dashed line, the concentration of gaseous K species is relatively low, because of stabilization of a KOH− K2CO3 salt melt. This is interpreted as a result of equilibrium chemical condensation reactions under the gasification atmosphere, such as P↑T↓
2KOH(g) + CO2 (g) JoooooK K 2CO3(l) + H 2O(g)
(1)
With increasing temperature, the salt melt is no longer stable and K is predicted to be mainly KOH(g). With further temperature increases, the concentration of gaseous K shifts from KOH(g) to K(g), while increasing pressure stabilizes more KOH(g). On the basis of the changes in composition, this may be represented by the overall equilibrium P↑T↓
2K (g) + 2H 2O(g) JoooooK 2KOH(g) + H 2(g)
(2)
that is consequential from the increase in H2O(g) because of methanation of the syngas: P↑T↓
CO(g) + 3H 2(g) JoooooK H 2O(g) + CH4(g)
(3)
Table 1. Woody Biomass Compositions Used To Estimate Gasification Atmospheres at Different Conditionsa wood bark/twigs a
C (%, ds)
H (%, ds)
N (%, ds)
O (%, ds)
H2O (wt %)
K (g/kg, ds)
52 52
6 6
0.5 0.5
41.5 41.5
7.5 7.5
0.3 3.5
ds = dry substance. 4400
DOI: 10.1021/acs.energyfuels.5b00889 Energy Fuels 2015, 29, 4399−4406
Article
Energy & Fuels
Figure 2. Gaseous K species concentrations after primary ash transformation inside the reactor based on (a) wood and (b) bark compositions.
The equivalent plots based on the bark composition (Figure 2b) share the same trends. However, the concentrations of the gaseous K species are approximately 10 times higher (0−1000 ppm), which also results in broadening of the stability range of the KOH−K2CO3 salt melt. The conditions and corresponding gasification atmospheres based on both fuels were used for the subsequent calculations to study their interactions with Ca and/or Si (described in section 2.3). These results are presented in the following section 3.2. 3.2. Interactions of the Gasification Atmosphere with Ca and Si. Under the conditions and corresponding gasification atmospheres of the wood composition, the left panel of Figure 3a shows that, when only Ca is present, a carbonate melt can form with K at low temperatures and elevated pressures. This salt melt stability region is slightly
increased in comparison to the scenario with no Ca and may be represented as an equilibrium condensation reaction, such as 2x KOH(g) + CO2(g) + (1 − x)CaO(s) P↑T↓
JoooooK K 2x·Ca(1 − x)CO3(l) + x H 2O(g)
(4)
The fraction of CaCO3 is predicted to be very low in the salt mixture compared to the fraction of K2CO3. Because woody biomass usually contains significantly more Ca than K, this means that, in practice, such salt melts are likely to be undersaturated in K and could contain CaO(s). With increasing temperature, gaseous K species would be non-reactive with Ca because the latter is preferentially stabilized as CaO(s) against the atmosphere. The right panel of Figure 3a shows that the scenario is significantly different when only Si is present; an oxide melt can 4401
DOI: 10.1021/acs.energyfuels.5b00889 Energy Fuels 2015, 29, 4399−4406
Article
Energy & Fuels
Figure 3. Predicted regions of stability for salt melt and oxide melt when Ca and Si, respectively, are individually present under gasification atmospheres with gaseous K species. Annotations indicate the molar distribution of total Ca and total Si within each respective phase. White regions indicate the absence of melt.
broadened in comparison to those of the wood composition. The salt melt is similarly dominated by K2CO3, while the share of K2O in the oxide melt is generally greater than that for the wood composition. In addition, polymorphs of SiO2(s) are no longer stable at low pressures. The results of the scenarios with Si and Ca jointly present are shown with a focus on the oxide melt. This is because it is the most dominant phase over most of the range of conditions. The following figures (Figures 4−6) use dotted lines to demarcate the regions of different phase compatibilities, while annotations indicate the approximate molar distributions of total input Ca and Si within the phases. At low temperatures and high pressures for both wood and bark compositions, all of the studied Si/Ca molar ratios show a K2O-rich carbonate melt coexisting with a SiO2-rich oxide melt. Ca is mainly within the carbonate salt melt phase within this region; i.e., the oxide melt is mainly low in Ca, especially at the lowest temperatures. The
form across a broad range of conditions and atmospheres, in an equilibrium, such as 2x KOH(g) + SiO2(s) ⇋ (K 2O)x · SiO2(l) + x H 2O(g)
(5)
The absence of the KOH−K2CO3 salt melt indicates that the oxide melt is more stable. The composition of the oxide melt is sensitive toward variations in the temperature, particularly at low pressures. The fraction of K2O in the oxide melt correlates with the concentration of KOH(g). At pressures of
