Slagging Behavior of Wood Ash under Entrained-Flow Gasification

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Energy & Fuels 2007, 21, 3644–3652

Slagging Behavior of Wood Ash under Entrained-Flow Gasification Conditions Beatrice Coda, Mariusz K. Cieplik, Paul J. de Wild, and Jacob H. A. Kiel* Energy research Centre of the Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands ReceiVed May 16, 2007. ReVised Manuscript ReceiVed August 31, 2007

The overall objective of the work described in this paper was to determine the behavior of wood ash under entrained-flow gasification conditions. Experimental work in atmospheric and pressurized entrained-flow gasification simulators, combined with thermodynamic equilibrium calculations, has shown that wood ash is not prone to form a molten slag at typical operating conditions of (pressurized, dry-feed, oxygen-blown) entrained-flow gasifiers, in spite of the presence of a relatively high amount of low-melting alkaline elements. This appears mostly due to the formation of mainly high-temperature-melting compounds (e.g., CaO) and only a small fraction of Ca silicates, which are characterized by a lower melting temperature. Phosphor and silicon may contribute to creating a higher melt amount, whereas low-melting alkali metal compounds are mostly partitioned into the vapor phase. Experiments, as well as modeling work performed for three types of wood, have shown consistent results. Addition of a fluxing agent is a promising option to improve the slagging behavior of wood-based systems by reducing the melting point of the slag. Moreover, thermodynamic calculations have shown that slag recycle may represent a feasible option in order to obtain sufficient slag coverage of the refractory wall despite the low ash content of woody fuels (typically 1 order of magnitude lower than in coal). In the present work, the determination of slag viscosity, a parameter critical for continuous operation of a slagging gasifier, has been addressed as well. The results of modeling work, showing the inapplicability of predictive formulas developed in the past for coal slags to wood-based slags, underline that further work is required to allow for a quantitative assessment of the slag viscosity as a function of slag composition and temperature.

Introduction Biomass is considered to be the most important renewable energy source for the coming decades. In addition to the interest in this renewable energy source from environmental (greenhouse gas emissions) and sustainability considerations, the use of biomass for the production of heat, power, transportation fuels, and chemicals is also a way to increase fuel diversification and decrease the dependence on imported oil. Entrained-flow (EF) gasification has been identified as one of the most promising technologies for large-scale conversion of a variety of biomass streams for applications aimed at (integrated) production of power, hydrogen, and liquid transportation fuels (e.g., Fischer– Tropsch diesel) thanks to, e.g., its advanced development status, large scale, and superior gas quality (e.g., the absence of tars).1–3 However, application of biomass in EF gasifiers similar to those employed in coal gasification (i.e., oxygen-blown slagging-type gasifiers) is not straightforward and requires R&D, especially * Corresponding author: e-mail [email protected]; tel +31 224 564590; fax +31 224 568487. (1) Calis, H. P. A.; Haan, H.; Boerrigter, H.; Van der Drift, A.; Peppink, G.; Van der Broek, R.; Faaij, A.; Venderbosch, R. H. Preliminary technoeconomic analysis of large-scale synthesis gas manufacturing from imported biomass. In Pyrolysis and Gasification of Biomass and Waste, Expert Meeting, 30 Sept–1 Oct 2002, Strasbourg, France; Bridgewater, A. V., Ed.; CPL Press: Newbury, 2003; pp 403–418. (2) Van der Drift, A.; Boerrigter, H.; Coda, B.; Cieplik, M. K.; Hemmes, K. Entrained-flow gasification of biomass; Ash behaviour, feeding issues, system analyses. ECN report ECN-C--04-39, Petten, The Netherlands, 2004. (3) Zwart, R. W.; Boerrigter, H.; van der Drift, A. Energy Fuels 2006, 20, 2192–2197.

regarding biomass pretreatment (e.g., torrefaction4), biomass fuel feeding, and ash behavior. Large-scale experience with EF gasification of biomass is limited to a small number of cogasification tests in the NUON coal-fired IGCC plant (Buggenum, The Netherlands)5 and some short experiments in the pilotscale gasifier of Future Energy (Freiberg, Germany).6 This paper presents results of the characterization of slag behavior for selected woody biomass—beech, willow, and a wood mixture—under simulated (pressurized, dry-feed, oxygenblown) EF gasification conditions. Since the ash content in the woody fuel is very low (about 1 wt %) and characterized by a high alkaline earth and alkali metal content, the utilization of wood as a fuel in entrained-flow gasifiers designed for coal requires careful adaptation. This is primarily due to the fact that the design is based on a higher fuel ash content (typically >6 wt %) and an operating temperature, where coal ash can form a liquid slag (typically 1300–1500 °C). Experimental Section The approach comprised a combination of experimental and modeling work. Experiments were performed in an atmospheric (4) Bergman, P. C. A.; Boersma, A. R.; Kiel, J. H. A. Torrefaction for biomass conversion into solid fuel. Proc. (in print) 15th European Biomass Conference & Exhibition, Berlin, Germany, May 7–11, 2007. (5) Wolters, C., Kanaar, M., Kiel, J. H. A. Co-gasification of biomass in the 250 MWe IGCC plant “Willem Alexander Centrale”. Presentation at theBiomass Gasification workshop at the 2nd World Biomass Conference, Rome, Italy, May 10–14, 2004. (6) Volkmann, D. Future Energy GmbH, Updates on Technology and Projects. Proc. Gasification Technologies Conference, Washington, DC, Oct 4–8, 2004.

10.1021/ef700247t CCC: $37.00  2007 American Chemical Society Published on Web 10/24/2007

Slagging BehaVior of Wood Ash

Energy & Fuels, Vol. 21, No. 6, 2007 3645

Figure 2. Experimental temperature/residence time profiles. Table 1. Burner Gas Composition ln/min

Figure 1. Schematic representation of the top section of the LCS test rig.

as well as a pressurized entrained-flow reactor, both equipped with an integrated, premixed, and multistage flat-flame gas burner. Both reactors have been applied extensively in previous studies on pulverized-fuel combustion and entrained-flow gasification.7–10 Since the atmospheric facility is much more easy to operate, most experiments were conducted in this facility, while the pressurized facility was applied to investigate the pressure effect. Laboratory-Scale Combustion and Gasification Simulator. The top section of the applied atmospheric laboratory-scale test rig (laboratory-scale combustion and gasification simulator, LCS) is depicted schematically in Figure 1. It has been designed to mimic pulverized-fuel combustion and dry-feed, oxygen-blown EF gasification conditions in terms of particle heating rate, reaction atmosphere, and temperature history. The ring-shaped, concentric, staged gas burner is used to simulate the high initial heating rate, resulting in explosive devolatilization and serves as a source for the reaction atmosphere. The alumina reactor, placed in a twostage electrically heated furnace, is designed to further mimic the temperature history, downstream of the flame front. To mimic EF gasification, the flame front temperature can be set as high as 2600 °C, while the reactor/furnace can withstand 1750 °C. Under these conditions, residence times up to ∼200 ms allow for high degrees of conversion when firing biomass fuels.11 For this experimental campaign, the reactor furnace temperature was set at 1450 °C, while the flame temperature reached ∼2050 °C (see Figure 2). In Table 1, an overview of the corresponding gas composition applied to fire the LCS burner is given. The slagging behavior of the ash was characterized by means of a deposition probe (Figure 3), on top of which an uncooled alumina deposition plate was mounted. This probe was used in many previous studies to evaluate the slagging and fouling behavior of different fuels under combustion as well as gasification conditions.7,8 (7) Korbee, R.; Boersma, A. R.; Cieplik, M. K.; Heere, P. G. Th.; Slort, D. J.; Kiel, J. H. A. Fuel characterisation and test methods for biomass co-firing - ECN contribution to EU-project ENK5-1999-00004 Combustion Behaviour of “Clean” Fuels in Power Generation (BioFlam). Report ECNC--03-057, ECN, Petten, The Netherlands, 2003. (8) Korbee, R.; Cieplik, M. K.; Kiel, J. H. A. Cost-effective screening of biomass materials for co-firing. In Van Swaaij, W. P. M., Fjällström, T., Helm, P., Grassi, A., Eds.; Proc. 2nd World Biomass Conf. Rome, Italy, May 10–14, 2004; pp 1330–1333. (9) Kiel, J. H. A.; Eenkhoorn, S.; Heere, P. G. T. Ash behaviour in entrained-flow gasification: preliminary studies. ECN report ECN-C--99037, Petten, The Netherlands, 1999. (10) Dacombe, P. J.; Jacobs, J. M.; Kiel, J. H. A. Ash Formation in Entrained-Flow Co-Gasification of Coal and Biomass. ECN report ECNCX-03-033, Petten, The Netherlands, 2003. (11) Bos, A.; Eenkhoorn, S.; Jacobs, J. M.; Kiel, J. H. A. Pressurised Entrained-Flow Gasification Simulator - Design, Construction and Commissioning - executive summary. Report ECN-C--98-061, ECN, Petten, The Netherlands, 1998.

inner burner outer burner shield gas (ring)

CH4

O2

0.46

1 1.66

N2

CO2

CO

H2

14.68

9.08

1.49 1.67

In this particular study, the probe was set at two different positions along the reactor axis, viz. at 300 and 760 mm from the gas burner corresponding to particle residence times of approximately 80 and 220 ms, respectively. In some experiments, (pretreated) SiC deposition plates were applied alternative to Al2O3 plates. The purpose of this was to simulate the impaction of (partly converted) fuel particles onto a refractory (untreated coupons) or into a slag layer (pretreated plates), allowing char to react further in a manner most realistic for the simulated EF conditions. Pressurized Entrained-Flow Gasification Simulator. Similar experiments as those performed in the LCS were conducted in the pressurized entrained-flow gasification simulator (PEFG simulator). The PEFG simulator is equipped with an integrated, premixed, and multistage flat-flame gas burner as well, but it contains six independently controlled heating sections (Tmax ) 1600 °C) to create the required temperature history for the fuel particles. To help creating a strongly reducing zone, the burner is equipped with a third annular ring for additional CO/H2/H2S supply, which can be preheated electrically up to 1000 °C. The operating pressure can be set up to 20 bar. Particle sampling between residence times of 10–2000 ms is possible with a fast-quenching probe. Alternatively, a deposition probe may be used for slagging/fouling tests. The PEFG simulator has been applied extensively in the past to investigate coal mineral transformations under entrained-flow gasification conditions. In Figure 4 the reactor, including the threestage flat-flame gas burner, is shown schematically.9 The operating conditions have been chosen as close as possible to the conditions set in the atmospheric installation LCS, in terms of temperature profile and residence time. Therefore, the main difference between the experiments in the two installations was

Figure 3. Photograph of the deposition probe and substrates.

3646 Energy & Fuels, Vol. 21, No. 6, 2007

Coda et al.

Figure 4. Schematic of the ECN pressurized entrained-flow gasification simulator (PEFG simulator).

the operating pressure (1 and 10 bar in the LCS and the PEFG simulator, respectively). Experiments in the PEFG simulator were conducted with (uncooled) alumina deposition plates. After each test, the deposition plate was collected and subjected to SEM-EDX analysis. Fuels. Experiments in the LCS were performed with three different types of wood: beech (and torrefied beech10), willow, and a wood mixture that is commonly utilized in a Dutch power station. The fuel ash compositions are shown in Table 2. The feeding rate was kept constant at 2.0 g/h throughout all the experiments. The experiments in the PEFG simulator were performed with torrefied beech only. In the case of the wood ashes, standard ash fusion tests were performed according to DIN 51730. Thermodynamic Modeling. Slagging/melting tendencies of the selected fuels have been studied using a thermodynamic equilibrium model (FACTSAGE12) and applying it to a hypothetical (pressurized) entrained-flow gasification system. The thermochemical calculation package consists of thermodynamic properties database and calculation and manipulation modules that enable one to access and manipulate pure substances and solution databases. Chemical equilibrium calculations can be made for a system which has been (12) Bale, C. W.; Chartrand, P.; Degterov, S. A.; Eriksson, G.; Hack, K.; Ben Mahfoud, R.; Melançon, J.; Pelton, A. D.; Petersen, S. Calphad 2002, 26, 189–228.

Table 2. Fuel Composition fuel

beech

torrefied beech

willow

wood mixture

moisture [% w/w a.r.] volatiles [% w/w d.a.f] ash [% w/w d.b.]

Proximate 9.0 5.0 84 77 1.0 1.3

5.0 84 1.9

5.0 85 1.5

C [% w/w d.b.] H [% w/w d.b.] O [% w/w d.b.]a N [% w/w d.b.] S [% w/w d.b.] Cl [% w/w d.b.]

Ultimate 49.0 6.20 43.3 0.20 0.02 0.004

50.0 6.10 42.9 0.60 0.06 0.02

49.3 6.10 44.3 0.30 0.02 0.019

Al [mg/kg d.b.] Ca [mg/kg d.b.] Fe [mg/kg d.b.] K [mg/kg d.b.] Mg [mg/kg d.b.] Na [mg/kg d.b.] P [mg/kg d.b.] Si [mg/kg d.b.] a

56.1 5.70 40.4 0.20 0.02 0.007

Ash Composition 48.0 42.0 3100 3900 47.0 42.0 1150 1450 370 460 9.00 8.00 90.0 106 170 182

Calculated by difference.

60.0 5720 68.0 2894 524 210 708 618

95.0 2450 110 970 252 53.0 190 417


Figure 5. SEM micrograph of an (uncooled) alumina deposit plate (top view) after a 2 h LCS beech gasification test (probe position 300 mm from the gas burner).

uniquely defined with respect to temperature, pressure (or volume), and composition. Employing a sophisticated Gibbs energy minimization algorithm and thermochemical functions,13 concentrations of chemical species are calculated when specified elements or compounds react or partially react to reach a state of chemical equilibrium. The calculations focus on slag formation at typical operating temperatures of an EF gasifier and on the distribution of the inorganic constituents over a wide range of temperatures, corresponding to a theoretical temperature profile of an EF gasifier, with a temperature above 2000 °C in the near burner zone. In this way, the transformation paths of the inorganic constituents of the fuel can be followed closely. To study the slagging behavior of the wood ashes, the model that fits best the chemical composition of the wood ashes has been chosen from the several slag models within FACTSAGE. The gas phase and condensed solids have been modeled as ideal mixtures. The EF gasification reactor has been modeled by introducing in the hypothetical reactor a predefined portion of wood with appropriate oxygen and steam rates, to simulate the operating conditions of commercial dry-feed, oxygen-blown EF gasifiers. The oxygen rate has been adjusted to an equivalence ratio (ER) of 0.25, i.e., 25% of the amount of oxygen that would be needed for stoichiometric combustion of the fuel, taking into account the oxygen content of the fuel itself. The steam rate has been chosen to be 0.1 kg/kg of fuel.2 In a sensitivity analysis, the effect of the ER in the range 0.1–0.4 on the slagging behavior of beech ash has been addressed, showing little if any effect. Therefore, it was decided to keep the ER constant throughout all the experiments. While most calculations have been performed at atmospheric pressure, the impact of the operating pressure has been assessed as well, since in practice EF gasifiers operate at a much higher pressure (e.g., 20–30 bar).

Results and Discussion Characterization of Slagging Behavior: LCS and PEFGSimulator Tests. Figure 5 shows the top view of the deposit plate after a 2 h beech gasification experiment. The experimental results reveal that wood ash is not prone to form a molten slag at typical operating conditions of slagging gasifiers (e.g., 1300–1500 °C). On the surface of the plate, single (clusters of) ash particles rather than a uniform melt/ash layer can be observed. Figure 6 shows a single beech ash particle (see Table 3 for the SEM/ EDX-derived chemical composition), in which the original, (13) Eriksson, G.; Hack, K. Metall. Trans. B 1990, 21B, 1013–1023.


Figure 6. Details of Figure 5. For chemical analysis data at point (a), see Table 3.

fluffy or spongy wood structure is still recognizable. Shifting the deposit probe position from 300 to 760 mm (corresponding to changing the residence time from 80 to 220 ms) did not show a different melting behavior of the ash. Only a small fraction of the particles showed melting behavior. These molten entities showed an increased (a few % w/w) concentration of silicon alongside with calcium (and oxygen), while the nonmolten entities were almost exclusively composed of Ca and O. Therefore, it may be assumed that the melt is most likely due to the formation of Ca silicate compounds (Figure 7). The slagging behavior for torrefied beech was basically identical to that of beech: the absence of a molten slag layer and the occurrence of ash particles resembling the original (torrefied) wood fuel and composed mostly of CaO. However, in this case melting due to Ca–Si interactions was less evident. For willow, it appears that the higher fuel phosphorus content enhances the melting behavior of the slag. Figure 8 shows a typical cluster of ash particles from a willow gasification test: a molten slag structure—enriched in phosphor (Table 3/Figure 8, point 8b)—is recognizable, whereas the nonmolten structure (8a) is composed predominantly of CaO. For the wood mixture (Figure 9), the slightly higher Si content of the fuel leads to clusters with a greater extent of melt due to the formation of Ca silicate compounds having a lower melting temperature than CaO. However, as silicon is most probably present as discrete sand particles and not homogenously distributed in wood itself, the observed melting is of incidental nature (compare points 9a and 9b). It was observed that changing the deposit substrate from alumina to SiC did not influence the melting behavior of the ash. However, when a pre-existing melt was present on the SiC plate (basically: SiO/SiO2), wood ash particles were homogeneously incorporated in this melt (Figure 10). This may have practical implications in a commercial-scale gasifier. The highmelting wood ash particles, once impacted in the pre-existing slag layer at the wall of the gasifier, will be completely incorporated; the residual ash components will chemically interact with the slag. Figure 11 shows a cross section of a deposit plate from a torrefied beech experiment in the PEFG simulator at 10 bar. To this purpose, the deposit plate was cut and embedded in an epoxy resin. The micrograph shows again the absence of any


Coda et al. Table 3. SEM/EDX Analysis Results

Figure/analysis point test rig fuel

6/a LCS beech

8/a LCS willow

8/b LCS willow

9/a LCS wood mixture

9/b LCS wood mixture

11/a PEFG simulator torrefied beech

O [% w/w] Al [% w/w] Ca [% w/w] Cl [% w/w] Fe [% w/w] K [% w/w] Mg [% w/w] Na [% w/w] P [% w/w] S [% w/w] Si [% w/w]

36.5 0.23 61.2 0.08 0.01 0.00 0.05 0.00 1.59 0.28 0.10

42.8 3.20 45.5 0.02 0.41 0.12 0.24 0.00 3.35 0.00 4.31

28.9 1.22 44.7 0.00 1.06 0.09 5.91 0.00 17.55 0.00 0.63

42.1 0.10 55.4 0.09 0.01 0.00 0.13 0.09 1.93 0.00 0.15

24.6 0.64 64.9 0.00 1.42 0.00 0.19 0.00 0.93 0.00 7.29

48.5 0.39 44.5 0.03 1.77 0.00 0.00 0.00 0.07 0.32 4.46

slag layer, similarly to what was found under atmospheric conditions. Furthermore, the occurrence of single, isolated particles was observed, whose structure resembled closely the spongy, porous species observed in the atmospheric-pressure experiments. EDX analysis of these particles indicates the presence of CaO-rich structures. Also, a large number of particles have a high Ca and Si content, but no substantial melting could be observed. Thermodynamic Equilibrium Modeling. Slagging BehaVior of Beech at ER 0.25 and 1 bar. In general, the experimental results correlate well with the predictions from the thermodynamic equilibrium calculations performed using FACTSAGE software. Figure 12 presents the phase distribution of the ash elements as a function of temperature upon gasification of beech. The weight percentages are related to the total amount of ash elements in the system. Only 13–25% w/w of the ash elements are predicted to form a liquid slag in the temperature range 900–2300 °C. At the lowest temperature of 800 °C a maximum of ∼33% w/w is calculated due to the formation of liquid calcium and potassium carbonates. Despite the fact that these liquid carbonates are not stable above and below 800 °C, their possible formation near 800 °C might pose a serious problem because of fouling at the entrance of the syngas cooler. At the highest temperature of 2400 °C, all ash elements are in the gas phase. Moving down to lower temperatures, condensation of CaO is predicted to commence in the temperature window 2100–2200 °C, while in between 1800 and 1900 °C, other condensed species will begin to form. The alkalis will exclusively form gaseous species such as elemental K, KCl, and KOH in the temperature range 900–2400 °C. Solid (condensed) CaO is the dominant compound in the system. At 1000 °C, ∼70% w/w of all the calcium in the system is condensed as CaO. This amount of condensed Ca attributes to 48% w/w of all the ash elements. The other ash elements are distributed among other condensed solids (17% w/w in the form of calcium silicate, magnesium oxide), liquid slag (13% w/w), and the gas phase (22% w/w, predominantly as gaseous alkali species such as K, KCl, and KOH/K2O). Effect of the Oxygen/Fuel Ratio. The effect of the oxygen fuel ratio in the range ER ) 0.1 to ER ) 0.4 on the slagging behavior of beech seems to be minor as can be seen from Figure 13. At temperatures below 900 °C and above 1800 °C slightly less liquid slag is predicted to form for ER ) 0.1 when compared to ER ) 0.25 and ER ) 0.4. Also, the temperature at which liquid carbonates are formed is shifted toward somewhat lower values. Because of these slight differences, it has been decided to perform all other calculations at ER ) 0.25. Effect of Operating Pressure. In the first part of the modeling work, the pressure in the FACTSAGE calculations was set to 1 atm in order not to deviate from the conditions of the

atmospheric LCS installation in which the majority of the experiments were performed. Additional calculations were conducted to evaluate the impact of pressure (up to 60 atm). This pressure influence for beech wood is shown in Figure 14. In the temperature range 1200–1800 °C it appears that operating at increased pressures is only slightly beneficial as far as the slag amount is concerned. Nonetheless, solidification of CaO begins still in the range 2100–2200 °C. Also at high pressure, this solidified phase represents the major ash transformation path. However, high pressure might increase the fraction of molten phase below 1200 °C. A critical issue of operating at such high pressure is the possible formation of alkali carbonates (Na2CO3 and K2CO3) and CaCO3, which are known to form low-melting-point eutectics. Indeed, the equilibrium calculations predict that the increase of molten phase with increasing pressure is exclusively due to the formation of carbonates in the slag at low temperatures. The temperature range in which these carbonates are predicted to form a stable liquid phase shifts toward higher values at higher pressures. These results indicate the importance of taking into account the eventual formation of alkali carbonates and CaCO3 for proper gas cooling system design of the gas quenching/cooling system, as these molten carbonates might lead to fouling problems on heat exchanger surfaces. In general the thermodynamic calculations are in agreement with experimental results, which indicate that a change in pressure from 1 up to 10 bar does not significantly influence the slag amount and composition, as well as the distribution of the ash-forming constituents in the high-temperature region of the gasifier. Comparison of the Slagging BehaVior for Beech, Torrefied Beech, Willow, and the Wood Mixture. Figure 15 compares the phase distribution of the ash elements for the four wood fuels upon entrained flow gasification at atmospheric pressure and ER ) 0.25. While the (weight) fraction of liquid slag for beech and torrefied beech is only minor, FACTSAGE predicts a significant fraction of the ash elements to form liquid slag for willow and the wood mixture. Correspondingly, a lower fraction of condensed pure solids has been calculated for the latter two when compared to the two beech fuels. For the wood mixture and willow ∼45 wt % of the ash elements is predicted to form a liquid slag at 1400 °C, against only 20 wt % for the two beech fuels. The higher degree of melting that has been calculated for willow is in agreement with the experimental results. In general, the higher slagging tendency for willow and the wood mixture is probably correlated to their phosphorus and silicon content that is significantly higher when compared to the beech fuels. AdditiVe Requirement. Since wood ash alone is not prone to melt sufficiently under typical operating temperatures of entrained-



Figure 7. SEM micrograph of a deposit after a 2 h LCS beech gasification test (probe position 760 mm from the gas burner). Figure 9. SEM micrograph of a deposit after a 2 h LCS gasification test with the wood mixture (probe position 300 mm from the gas burner); for chemical analysis data at points (a) and (b), see Table 3.

Figure 8. SEM micrograph of a deposit after a 2 h LCS willow gasification test (probe position 760 mm from the gas burner); for chemical analysis data at points (a) and (b), see Table 3.

flow slagging gasifiers, additives will be required to lower the melting point of the ash. In this respect, additives rich in quartz or clay(s) may enhance the overall fluidity of the slag. Experiments performed with beech wood mixed with high-purity quartz sand, as well as thermodynamic equilibrium calculations and standard ash fusion test, have shown that adding silica on a molar ratio Si:Ca ) 1:1–2:1 [mol/mol], corresponding to 464–928 g of quartz/kg of fuel ash, may be sufficient to decrease the melting point of the ash system down to typical operating temperatures. In the equation, Ca represents the fuel Ca content, while Si stands for the Si content of the additive. Figure 16 shows the results of the thermodynamic equilibrium predictions for beech that has been fluxed with silica (quartz) and clay with a simulated composition of kaolinite. The weight fraction of liquid slag in the condensed phases has been plotted as function of temperature. When adding clay (with a share of Al:Si ) 1:1 mol/mol) in an amount such that the molar ratio Siclay:Cafuel ash ) 1:1 mol/mol, all ash forming constituents will form a liquid melt at 1400 °C, according to thermodynamic predictions. This is due to the fact that Ca will be effectively encapsulated in the Al/Si-based matrix. Experimentally, a layer of molten slag (Ca silicate) was found on the

Figure 10. SEM micrograph of homogeneous slag on a pretreated SiC deposit plate (top view) after a 2 h LCS beech gasification test (probe position 300 mm from the gas burner).

deposit probe plate for a molar ratio Si:Ca ) 2:1. When adding quartz, alkaline earth (and, to a lesser extent, alkali) metals will tend to be incorporated in the silicon-based melt. Figure 17 shows a qualitative comparison between thermodynamic calculations and experimentally determined ash fusion temperatures for an averaged wood ash. The melting temperature according to thermodynamic calculations has been defined as the temperature where 70% w/w of the total condensed phases is in a liquid form. The results show that there is an optimum value of flux addition where the melting temperature is brought to a minimum, while higher flux addition will lead again to an increase of the melting temperature of the slag. Slag Viscosity. Next to determining the composition and melting tendency of the ash, there is a need to characterize slag properties, such as viscosity and temperature of critical viscosity, to estimate whether continuous operation of slagging EF gasifiers at a temperature range of 1300–1500 °C is feasible. For a slagging gasifier to operate well, the slag must be removed continuously, and the critical condition for efficient removal of


Coda et al.

Figure 14. Influence of operating pressure on slagging behavior of beech.

Figure 11. SEM micrograph of a deposit (cross section) after a 2 h PEFG simulator test with torrefied beech (probe position 300 mm from the gas burner); for chemical analysis data at point (a), see Table 3.

Figure 12. Calculated equilibrium phase distribution versus temperature for beech upon oxygen-blown gasification at ER ) 0.25 and atmospheric pressure.

Figure 15. Influence of fuel ash composition on slag formation.

Figure 16. Influence of flux additive quality and quantity on slagging behavior of beech.

Figure 13. Influence of oxygen concentration on slagging behavior of beech.

the slag is that the slag remains in the liquid phase throughout the system. The tapping temperature must be high enough to avoid crystallization of the slag in the tapping hole, but at the same time not too high, since this has a negative impact on the cold gas efficiency. It is generally reported that viscosity of

the slag should be less than 15–25 Pa · s, typically 8–15 Pa · s.14 Too low values are also undesirable because then slag flow velocities in the gasifier become too high, and a proper slag layer with sufficient thickness can no longer be guaranteed. A number of empirical correlations relating the viscosity of the slag to its composition and temperature have been reported in literature.15,16 However, these correlations have been developed primarily for coal ash. Some, like the Watt–Fereday (14) Watt, J. D.; Fereday, F. J. Inst. Fuel 1969, 42, 99–103.


Energy & Fuels, Vol. 21, No. 6, 2007 3651 Table 4. Slag Composition and Viscosity and Temperature of Critical Viscosity

beech (no flux)

beech (fluxed with clay, molar ratio Siclay:Alclay:Cabeech ) 2:2:1)

slag composition as determined by FACTSAGE [% mol/mol] CaO 61.4 27.8 24.9 38.6 SiO2 MgO 6.4 8.3 Al2O3 4.4 24.1 FeO 2.8 0.4 Fe2O3 0.06 0.001 0.04 0.6 K2O Na2O 0.008 0.1

Figure 17. Comparison of predicted and experimental wood blend ash melting behavior upon doping with SiO2.

correlation,14 do not include alkali oxides (only CaO, SiO2, Al2O3, and Fe oxides) and appear therefore not applicable to wood systems. The Urbain–Kalmanovitch model17 (also known as modified Urbain model) might be suitable for wood/biomassbased slags, since next to calcium, aluminum, and silicon, it also incorporates the oxides of Fe, Mg, Na, K, and Ti. Phosphorus, however, is not considered. In order to evaluate the predictive capabilities of the Urbain– Kalmanovitch model, a number of slag compositions as calculated by FACTSAGE for the entrained-flow gasification of beech and clay-fluxed beech (at 1400 °C, atmospheric pressure, and ER ) 0.25) have been used as an input for slag viscosity calculation. In addition, the temperature of critical viscosity (Tcv)—defined as the temperature at which the viscosity of the slag begins to increase rapidly upon cooling as a consequence of crystallization—has been calculated for the same systems, based upon the Brockelie viscosity model.18 As mentioned above, none of the models includes phosphorus; therefore, the input data had to be normalized to a phosphorusfree basis. Results of the calculations are presented in Table 4. As can be seen from Table 4, a viscosity of 0.3 Pa · s is predicted for beech and 4.3 Pa · s for beech fluxed with clay (molar ratio Si:Ca ) Al:Ca ) 2). Both values are fairly low, implying that a proper slag layer cannot be built up. On the other hand, the predicted Tcv values are in the range 1500–1600 °C, which suggests that the slags would be rock solid at 1400 °C, which obviously is in disagreement with the calculated slag viscosities as well as the experimental data discussed earlier in this paper. Clearly, more accurate slag viscosity correlations, derived biomass-based slags, are required to allow better predictions. Conclusions In order to utilize wood as a fuel for entrained-flow slagging gasifiers, one critical issue is the slagging behavior of the wood (15) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. Prog. Energy Combust. Sci. 2001, 27, 237–424. (16) Browning, G. J.; Bryant, G. W.; Hurst, H. J.; Lucas, J. A.; Wall, T. F. Energy Fuels 2003, 17, 731–737. (17) Kalmanovitch, D. P.; Frank, M. An EffectiVe Model of Viscosity for Ash Deposition Phenomena;Engineering Foundation Conference on Mineral Matter and Ash Deposition from Coal; United Engineering Trustees Inc.: Santa Barbara, CA, 1988. (18) Bockelie, M. J.; Denison, M. K.; Chen, Z.; Linjewile, T.; Senior, C. L.; Sarofim, A. F. CFD-Based Models of Entrained-Flow Coal Gasifiers with emphasis on Slag Deposition and Flow. Proceedings of Colloqium on Black Liquor Combustion and Gasification, Park City, UT, May 13–16, 2003.

viscosity at 1400 °C, according to the Urbain–Kalmanovitch model17 [Pa · s] 0.3 4.3 critical viscosity temperature according to Brockelie et al.18 [°C] 1583 1517

ash. Typically, entrained-flow slagging gasifiers have been designed for coal, with a much higher ash content (>6% w/w) and a different ash composition (e.g., wood ash contains more alkali and alkaline earth metals). For a proper adaptation of slagging gasifiers to wood/biomass firing, it is necessary to increase the knowledge base on the behavior of wood ash under representative operating conditions. It has been the objective of the present work to accomplish this by a combination of experimental and modeling work. A comparative analysis, including test runs and thermodynamic equilibrium calculations, shows that wood ash is not prone to form a molten slag at typical operating conditions of (dryfeed, pressurized, oxygen-blown) entrained-flow gasifiers. Ca is the dominant inorganic component in wood ash. The absence of slag formation at typical gasifier operating temperatures is related mostly to the formation of high-melting-temperature compounds (e.g., CaO), and the low abundance of Ca silicates, which have a relatively low melting temperature. Higher concentrations of phosphorus and silicon in the fuel ash may contribute to the formation of a higher amount of liquid slag. Experiments as well as modeling work performed for three types of wood have shown consistent results. An increase in operating pressure (up to 60 bar) does not influence significantly the slag amount nor its composition in the temperature range of 1200–1800 °C. However, thermodynamic calculations indicate that upon gasifying wood at high pressures a significant amount of liquid carbonate slag can form at the temperatures below 1200 °C, thus creating potential fouling problems for a downstream heat-exchanger. This may be more pronounced for high-alkali fuel concentrations. In order to use wood as a feedstock for slagging gasifiers and to overcome its poor slagging behavior, flux addition (quartz-based or clay-based compounds) appears a promising option, as this leads to the formation of a liquid molten slag at typical gasifier temperatures. In particular, experimental and modeling work performed in this study identified silica- and/or clay-based material as possible fluxing agents to lower the melting point of the ash and, therefore, produce a liquid slag in the temperature range 1400–1500 °C. A crucial issue in the design and operation of a slagging entrained-flow gasifier is the protection of the refractory wall of the reactor. Normally this protection is realized by the running slag itself, flowing down to the bottom of the gasifier. Generally, slagging entrained-flow gasifiers have been designed for a fuel ash content of at least 6% w/w. In the case of wood gasification,


a solution has to be found to cope with the much lower ash content of typically 1% w/w or less. In this respect, slag recycle might be an interesting option. In the present work, also the issue of slag viscosity determination—a critical parameter for continuous operation of a slagging gasifier—has been addressed. The slag should have a viscosity value in the range 8–15 Pa · s, and the gasifier operating temperature should be higher than the temperature of critical viscosity. Extensive research work (of both experimental and predictive nature) has been done in the past decades to characterize coal ash slags.19–21 However, this knowledge (19) Hurst, H. J.; Noval, F.; Patterson, J. H. Fuel 1999, 78, 439–444. (20) Browning, G. J.; Bryant, G. W.; Lucas, J. A.; Wall, T. F. IFRF Combust. J. 1999, Commun. No. 199901. (21) Jak, E.; Degterov, S.; Hayer, P. C.; Pelton, A. D. Fuel 1998, 77, 77–84.

Coda et al.

appears to be insufficient when dealing with biomass (wood) ash that is characterized by high alkali and alkaline earth metals. Further research work is required to assess quantitatively the characteristics of the slag flow. Here, the determination and prediction of slag viscosity should get specific attention. Moreover, optimal strategies for flux addition and slag recycle should be developed. Acknowledgment. The Agency for Research in Sustainable Energy (SDE) is acknowledged for financial support. The authors thank P. Zuideveld, E. Wesker, and H. J. P. Haan (Shell Global Solutions International) for the fruitful discussions and F. Mehlhose and M. Schingnitz (SIEMENS, formerly Future Energy GmbH, Leipzig, Germany) for performing the ash fusion tests. EF700247T

Slagging Behavior of Wood Ash under Entrained-Flow Gasification

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