Slag Formation during Entrained Flow Gasification: Silicon Rich Grass

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Biofuels and Biomass

Slag Formation during Entrained Flow Gasification: Silicon Rich Grass Fuel with KHCO Additive 3

Per Holmgren, Markus Broström, and Rainer Backman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02545 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

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Slag Formation during Entrained Flow Gasification: Silicon Rich Grass Fuel with KHCO3 Additive

Per Holmgren 1 , Markus Broström* 1 , Rainer Backman 1 1

Umeå University, Department of Applied Physics and Electronics, Thermochemical Energy Conversion Laboratory, SE-901 87 Umeå, Sweden *

Email address of corresponding author: [email protected]

Abstract Prediction of ash particle adherence to walls, melting and flow properties are important for successful operation of slagging entrained flow gasifiers. In the present study, silicon rich reed canary grass was gasified at 1000 °C and 1200 °C, with solid KHCO3 added at 0, 1 or 5 %wt to evaluate the impact and efficiency of the dry mixed additive on slag properties. The fuel particles collided with an angled flat impact probe inside the hot reactor, constructed to allow for particle image velocimetry close to the surface of the probe. Ash deposit layer build-up was studied in situ, as well as ash particle shape, size and velocity as they impacted on the probe surface. The ash deposits were analyzed using SEM-EDS, giving detailed information on morphology and elemental composition. Results were compared with thermodynamic equilibrium calculations for phase composition and viscosity. The experimental observations (slag melting, flow properties and composition) were in good qualitative agreement with the theoretical predictions. Accordingly, at 1000 °C no or partial melts were observed depending on potassium/silicon ratio; instead high amounts of additive and temperature of at least 1200 °C was needed to create a flowing melt.

1 Introduction Ash melting and flow properties are important aspects for stable operation of slagging entrained flow gasifiers. Fuel composition is determining for ash properties and slag flow, and fuels rich in silicon are difficult from a slag flow perspective since ash dominated by SiO2 generally have high melting points, high viscosities and large solid fractions

1, 2

. Solid ash

particles could open up for the option of running entrained flow gasifiers in dry mode, but

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small amounts of alkali, potassium being most relevant for biomass, is always present in the fuel, forming varying amounts of a sticky but non-flowing silicate melt in equilibrium with the SiO2 at a wide temperature interval relevant for gasification. Silica-rich fuels have been successfully co-fired with coal 3 and work has been done co-combusting powered straw and woody biofuels 4-6 in different reactors. Work has also been done on modelling deposit buildup from bio-mass fuels in entrained flow reactors (EFR) 7, though detailed models on how gas-phase ash-forming elements interact with solid-phase ash elements locked in a fuel particle is still lacking 8, 9. Emerging ideas of designing fuel mixtures or using additives to form preferred slag compositions primarily suggest increasing the alkali content of the fuels 10. From a chemical equilibrium point of view this is rather straight forward, but a number of aspects need to be considered for successful implementation: i) If using alkali containing additives, will it react efficiently with the ash if added as dry carbonate powder to the fuel? ii) Are reactions by gas phase alkali at a limited partial pressure efficient in an entrained flow geometry? iii) Do silicon rich ash particles react with gaseous alkali already in the flame, or is residence time on the reactor wall needed to reach local equilibrium? v) What temperatures and gaseous alkali partial pressures are needed to achieve a flowing slag? In order to provide new knowledge on the aspects mentioned above, the objective of the present study was to assess the efficiency of a dry potassium additive, as well as the rheological changes induced by a KOH(g)-enriched atmosphere surrounding silicon rich ash. Theoretical slag composition was compared with the composition of experimentally produced slag. Reed Canary Grass (RCG) powder, used as a silicon rich model fuel, and KHCO3, a dry mixed additive, was gasified in an entrained flow reactor. Reed canary grass (RCG) is a silicarich fuel that is also low in phosphorous, avoiding the difficulties involved in interpreting silicate-phosphate mixtures. Alkaline earth metals are also known to alter slag properties and


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the release of alkali if incorporated in silicate slags 11, 12, or similarly in phosphate dominated systems

13, 14

. However, the RCG used had also low contents of Ca and Mg, allowing for

further simplifications in the interpretations by considering mainly the K2O-SiO2 binary system from the thermodynamic perspective. Optical in-situ measurements, SEM-EDS analysis of deposits, and thermodynamic phase and viscosity calculations were used for planning of the experiments and for evaluation of the results. 2 Experimental An entrained flow reactor (EFR) allows for high heating rates and temperatures similar to those found in large-scale appliances. It was used in the present study to carry out particlewall interaction experiments gasifying pulverized RCG. The RCG powder was sieved to 250500 µm (well below the maximum size allowed for successful entrained flow gasification of biomass powder

15

), and the global oxygen-to-fuel ratio was kept at approximately 0.6. The

KHCO3 (99.5%, CAS# 298-14-6, Merck KGaA, Germany) used as fluxing agent was dry mixed with the fuel powder at 0, 1 and 5%wt resulting in molar K/Si ratios of 0.1, 0.3 and 1.3, according to Table 1 (fuel compositions determined using ICPOES/ICPMS according to EN 14775/15104/15289/15290/15297). Both raw fuel and additive were dried at 105 °C before use. KHCO3 was chosen as source of potassium since at the prevailing conditions it is assumed to relatively easily decompose into KOH, the dominating potassium compound released during combustion and gasification of biomass, and therefore a suitable additive in systematic lab scale studies. Also, KHCO3 is not as hygroscopic as e.g. K2CO3, and therefore more easily added to the fuel in exact proportions. Size, shape, velocity and vector changes were monitored by in-situ PIV for particles colliding with a non-cooled impact plate at different angles. Experiments were performed at two different reactor temperatures (1000 and 1200 °C). Collected ash/slag samples were analysed by SEM/EDS and interpretations were supported by thermodynamic equilibrium calculations.


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Table 1. Fuel compositions, on dry basis. sample

RCG

RCG + 1%wt KHCO3

RCG + 5% wt KHCO3

1

5

473,300

469,699

455,995

H

58,400

57,909

56,040

N

5,780

5,722

5,499

O

423,600

424,064

425,832

738

731

702

Cl

388

384

369

Al

108

107

103

Ca

1,040

1,029

989

Cu

5.42

5.37

5.16

Fe

68

67

65

2,060

5,910

20,559

Mg

397

393

378

Na

34

34

32

P

663

656

631

Si

12,600

12,472

11,987

Zn

20

20

19

KHCO3 additive

%wt

0

Ash (550°C)

%wt

3.80%

C

mg/kg

S

mg/kg

K

2.1 Entrained Flow Reactor

Details on the reactor system can be found elsewhere 16, though in these experiments the inner reactor tube consisted of a main alumina ceramic tube (DEGUSSIT AL23) with 70 mm inner diameter and a total height of 2100 mm. Located 700 mm from each end were perpendicular ports enabling insertion of an ash sampling probe and also providing optical access. The inner diameter of the optical ports were 25 mm on the CCD camera side, and 40 mm on the LED


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flash side (Figure 1a). Windowed endcaps prevented gas leakage while allowing for direct observation of the ash sampling probe surface. The central tube rested on a cooling ring surrounding the exhaust in order to protect downstream equipment by cooling mechanical parts, product gas and particles. The reaction tube was heated by five pairs of individually controlled electrical heaters. Gas and fuel particles were supplied from the top by use of a flat flame McKenna-style burner with internal water cooling 16, custom-fabricated by Holthuis & Associates, USA. The face of the burner was divided into three zones. A center tube with an inner diameter of 6 mm was used to introduce carrier gas and fuel particles. Surrounding this was the 50 mm primary gas supply zone. An outer shroud zone with a diameter of 70 mm was used to supply N2 to protect the reactor tube and adapter near the burner, and to enable tuning and manipulations of the near-burner flow. A loudspeaker operating at 1.4 kHz was also attached to the fuel feeder tube, preventing fuel particles from forming agglomerates and clogging the feeder, similarly to as described by others 17. Ash was collected on a 25 mm diameter alumina plate, inclined so that particles would have a 30° impact angle (Figure 1b). The probe assembly (Figure 1c) had a built in type-S thermocouple (TC) located inside the probe, in contact with the collection plate.


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a

b

c

Figure 1. Reactor and probe assembly. a) ceramic (Al2O3) reactor tube, b) positioning of the probe visualized by cutout, and c) probe assembly with dimensions in mm.

The primary gas flow supplied was 9 ln/min of air. A flow of 1 ln/min N2 was supplied through the shroud section. 0.75 ln/min air was used as carrier gas for the solid particles. The electrically heated furnace was set to 1000 or 1200 °C measured by the controlling TC:s outside of the reactor tube. Fuel was supplied to the reactor by a calibrated loss in weight feeder at 200 g/h. Reynolds number of the gas flow was calculated to approximately 150, i.e. within the laminar regime for a tube (Re < 2300). Particle diameter and velocity allowed calculations of the particle Reynolds number (Rep) and Stokes number (Stk), where Rep < 10 indicates a laminar flow around the particle and Stk < 1 implies that the particle follows the gas flow. The average Rep and Stk were 0.8 (1%wt KHCO3, 1200°C), less in all other cases, indicating these particles were not common. Figure 4b displays the circularity distribution of all captured particles, with 1 representing a perfect circle (in fact a sphere, but monitored as a circle in 2D by the PIV). No clear trends can be found, and a possible interpretation is that gas-phase interactions with the ash are not fast enough to substantially allow for complete melting and low viscosity melts to form while the particles are still suspended in the gas flow. On the other hand, the velocity observations in Figure 3b indicate that some reaction, likely a change in density, have already occurred while the ash particles were suspended in the gas flow.


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a

b

25%

20%

Particle shape distribution

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15%

10%

5%

0% 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

Circularity 0%wt KHCO₃, 1000°C 1%wt KHCO₃, 1000°C 5%wt KHCO₃, 1000°C

0%wt KHCO₃, 1200°C 1%wt KHCO₃, 1200°C 5%wt KHCO₃, 1200°C

Figure 4. Spherical particles observed (a), and particle shape distribution (b).

SEM-EDS analysis of sampled material is summarized in Figure 5. This gives a first indication on the efficiency of potassium capture in the silicate dominated particles. SEMEDS analysis also showed varying but minor amounts of Ca, Mg P, and Al (aluminum only at the probe surface), but at comparably low concentrations (calcium typically 4 %at, phosphorous 2 %at, and other elements < 2 %at), and even though calcium and phosphorous at these levels has some impact on slag properties, the further interpretations is based on the K2O-SiO2 binary system.


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Figure 5. Ternary diagram of fuel and deposit compositions

The phase chemical information supplies important information for interpretation of the experimental observations. Figure 6 shows that ingoing fuel composition and samples agreed fairly well without the additive and at the low level (1%wt). Increasing to 5%wt additive increased potassium content even more, but it did not correspond to the total flow composition at equilibrium. Overall lower capturing efficiencies were found for the high temperature experiments.


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Figure 6. Binary diagram showing phase boundaries, samples analyzed (symbols), and ingoing fuel mixtures (dashed lines).

All samples from fuels without additives (both temperatures), and the 1200°C samples with 1%wt additive ended up in the SiO2-melt two phase area. All liquid compositions follows the liquidus line and its composition therefore depends on temperature only. The melt/solid fraction within the two-phase are, however, depends on the total composition. In practice, the small amount of potassium in the fuel leads to formation of a small amount of melt together with solid SiO2, possibly explaining the stickiness observed also for these experiments. The samples in liquidus and mixed regions correspond well to the particle structure of the straw ash particles, with varying amounts of gluing melt keeping the structure together. At 1%wt addition, 1000 °C, most samples were found in the liquidus area, but spreading from Krich, more melted, towards Si-rich with larger fraction of apparently un-melted compounds.


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SEM-EDS results are global on single particle level, and taking that in account while at the same time considering possible surface attacks by gaseous potassium species forming potasium-rich melt, it can be explained why the deposit collected was not entirely molten in this case. RCG + 5%wt KHCO3, 1000 °C, resulted in a mixture of solid and liquid sample, corresponding to the region close to the intermediate K2O·2SiO2, which is an intermediate phase that is congruently melting at relatively high temperature. The slag core had a homogeneous structure with bubbles, suggesting it had been molten, but the composition corresponded to the intermediate K2O·2SiO2, and it did not flow off the probe, indicating it may have resulted from a liquid phase by either dissolving or evaporation of potassium. The surface of the deposit was richer in potassium, had a more molten appearance, and a composition corresponding to the liquidus area (see Figure 7).

Figure 7: Melt formed after gasification at 1000 °c of RCG + 5%wt KHCO3 additive. K/Si atomic ratio as analyzed by SEM-EDS indicated at the surface and deep in the melt.

The effect of the K2O·2SiO2 intermediate is further illustrated by the melt fractions (Figure 8) calculated for the K2SiO3-SiO2 system presented in Figure 6, supporting the experimental


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observations on the complexity of adding K to the Si rich fuel. Besides the SiO2 containing region to the right, the share of solids around the K2O·2SiO2 is significant at 1000 °C.

100 Amount liquid (%wt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 0.5

0.6 0.7 0.8 0.9 SiO2/(SiO2+K2O) (mol/mol) 1000 °C 1200 °C

1.0

Figure 8. Melt fractions calculated for a part of the K2O-SiO2 system. Most of the RCG + 5%wt KHCO3, 1200 °C, sample was entirely molten, in agreement with the composition being well into the liquidus area. It was flowing off the probe, probably dripping approximately once every 10 seconds, seen as small fluctuations of the slag surface. Stickiness and flow properties of the samples (K2O-SiO2 binary system) was estimated from viscosities calculated by FactSage, Figure 9.


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40

30

20 ln(Pa·s)

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10

0

-10 0.5

0.6

0.7 0.8 SiO2/(SiO2+K2O) (mol/mol) 1000 °C

0.9

1.0

1200 °C

Figure 9. Viscosities calculated for a part of the K2O-SiO2 system. Full line indicates no crystallization, dotted line indicates crystallization.

Overall, higher SiO2 content of the melt leads to a higher degree of polymerization and thus higher viscosity. Viscosity is increasing with decreasing temperature, as expected. In the mixed SiO2-liquid area (> approximately 0.85), viscosity increases rapidly if assuming an entirely melted system. If taking crystal formation into account, the viscosity instead follows the dashed lines since liquid composition is constant, as previously discussed. However, the efficient viscosity of the mixed liquid-solid is heavily affected by the presence of solid particles; especially further to the right where the solid fraction is high. Existing models cannot accurately predict slag properties in this mixed region 22. The small jump in the dashed 1000 °C curve around 0.65 is also caused by a solid fraction in the melt changing the liquid composition. The apparently fully dissolved RCG + 5%wt


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KHCO3, 1200 °C, sample corresponds to approximately 20 Pa·s, which is in qualitative agreement with the flow observed. Thermochemical equilibrium and viscosity calculations were also performed using the complete fuel composition, assuming complete mixing of all elements, see Figure 10. Even without additive the ash is predicted to contain some amounts of melt, but with a very high viscosity (approximately 10k). This could explain why particles adhered to the probe, despite a dry (non molten) appearance. Using 1%wt additive instead gave almost full melts, but with predicted viscosities that are still high enough to prevent flowing behavior, explaining the fast buildup of deposits (Figure 2b). The experiments using 5%wt additive were predicted to be almost fully molten, but only the runs at 1200 °C could be expected to exhibit flowing behavior in the time frame of the experiments (approximately 30 Pa·s), in qualitative agreement with the experimental findings. In the experiments with the KHCO3 additive, the main source of potassium in the melt was supplied from the surround gas, giving a gradient in expected viscosity from the melt surface to the probe-melt interface, supported by the calculations presented in Figure 10 and observed in Figure 7.


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100%

1.E+05

90% 1.E+04

70% 60%

1.E+03

50% 40%

1.E+02

30% 20%

Viscosity (Pa·s)

80% Melt fraction (%wt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.E+01

10% 0% 900

1000

1100 Temperature (°C) Melt fraction, 0% KHCO₃ Melt fraction, 5% KHCO₃ Viscosity, 1% KHCO₃

1200

1.E+00 1300

Melt fraction, 1% KHCO₃ Viscosity, 0% KHCO₃ Viscosity, 5% KHCO₃

Figure 10. Expected melt fraction (dashed lines) and viscosity (solid lines) for RCG + 0/1/5%wt KHCO3 additive mix. Calculations based on complete fuel composition, except for phosphorus.

4 Conclusions A powder of silicon rich reed canary grass was gasified with and without a potassium rich additive in order to assess the efficiency of the additive, and the resulting slag properties. It was found that the dry-mixed additive efficiently decomposed and reacted with the silicon rich ash, both in the flame and in already deposited ash. However, complete melting of ash particles was observed only on the probe. It was found that increased melt fraction coupled with high viscosity increased deposit growth rate, and that the influence of gas-phase potassium appeared to have a large impact on particle impact velocities, probably due to melt formation. The deposition built-up rate was found to depend on both temperature and added gas-phase potassium. At 1000 °C deposit build-up increased with increased potassium levels, while at


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1200 °C deposit build-up decrease with increasing potassium levels, due to formation of a flowing melt. Slag layer properties were in qualitative agreement with calculations predicting and explaining thermochemical phase compositions and viscosities. For the present fuel mixtures, controlling the slag composition to the flowing regions would probably be difficult in an industrial setting. At 1000 °C viscosities are high, and the system is sensitive to variations in composition since small fluctuations may cause large solid fractions. With high potassium addition and high (1200 °C) temperature, an apparently flowing and possibly manageable slag was formed.

5 Acknowledgements The authors gratefully acknowledge financial support from the Swedish strategic research program Bio4Energy, the Swedish Research Council grant number 2014-5041, and the Swedish Energy Agency through the Swedish Gasification Center and through the project Catalytic Entrained Flow Gasification of Biomass.

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Slag Formation during Entrained Flow Gasification: Silicon Rich Grass

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