Chemical Looping Gasification of a Biomass Pellet with a Manganese

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Environmental and Carbon Dioxide Issues

Chemical looping gasification of biomass pellet with a Manganese ore as oxygen carrier in the fluidized bed Shangyi Yin, Lai-hong Shen, Maksym Dosta, Ernst-Ulrich Hartge, Stefan Heinrich, Ping Lu, Joachim Werther, and Tao Song Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02849 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Chemical looping gasification of biomass pellet with a Manganese ore as oxygen carrier in the fluidized bed Shangyi Yin1, Laihong Shen2, Maksym Dosta3, Ernst-Ulrich Hartge3, Stefan Heinrich3, Ping Lu1, Joachim Werther3, Tao Song1*

1

School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China 2

3

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, China

Institute of Solids Process Engineering and Particle Technology, Hamburg University of Technology, Hamburg 21073, Germany *Corresponding Author: [email protected] (T. Song);

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Abstract – Due to the increasing transport cost of biomass and benefiting from the more

efficient treatment of a compacted, dustless product, the use of pelletized biomass has gained interest over recent years in China. Chemical Looping Gasification (CLG) with circulating oxygen carriers provides a novel process, which integrates biomass gasification with the hot gas conditioning with the aim to obtain pure syngas with low tar amount. The study focuses on the CLG application using a single typical rice husk pellet as fuel which are characterized by high silicon dioxide in ash. Some experiments in a fluidized bed unit with the mixture of quartz sand and an active manganese ore as bed materials, were performed using a single rice husk pellet as fuel and steam as gasifying agent. The objectives of the work are to investigate its CLG performance and bottom ash characterization. Effects of gasification temperature (750-950 °C) and oxygen carrier-fuel ratio on syngas distributions, effective gas content and syngas yield, were investigated. The conversion of the rice husk pellet is much dependent on reaction temperature. A high temperature promoted tar cracking and gasification reactions, leading to a fast carbon conversion. The effective gas content (CO+H2+CH4) during gasification process were in the range of 74.2% to 79.9% under the temperature of 750 °C to 950 °C. Regarding the CLG application of rice husk pellet as fuel, much attention should focus on bottom ash, which was not separately during the process but still keeping the original pellet shape with some irregular pores inside the ash due to the formation of molten grains. The ash demonstrates a rigid skeleton-like structure. The trapped carbon particles inside the molten ash cannot be gasified, thus limiting the fuel conversion. Keywords: Chemical looping gasification; biomass; oxygen carrier; syngas


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1

Introduction

In each year, among the more than 800 million tons of crop straws produced by China, about 80% can be potentially utilized as one kind of energy resource1. Pellets are solid commonly cylindrical objects. Nowadays, pelleting process develops fast in EU and China mainly due to the huge market for wood pellets. In 2009 more than 10 million tons of wood pellets were produced from approximately 650 plants in the EU2. By 2015, EU28 produced 14.2 million of the global total of 228.0 million tons, accounting for 51% of global wood pellets production3. Germany is one of the largest wood pellet markets worldwide in terms of produced and consumed volumes and installed production capacities4. Due to the increasing transport cost of biomass, and benefiting from the more efficient treatment of a compacted, dustless product, the use of pelletized biomass has gained interest over recent years not only in Germany but also in China. The Chemical Looping concept has been developed in many energy and chemical engineering fields5-8, such as combustion, reforming, and hydrogen production or partial oxidation. For application, it needs at least two connected reactors (a fuel reactor and an air reactor), where the oxygen carriers are continuously moved and transported during the reduction and oxidation cycles. Based on chemical looping concept, Chemical Looping Gasification (CLG) with circulating oxygen carriers provides a novel process, which integrates biomass gasification with the hot gas conditioning with the aim to obtain pure syngas with low tar amount. In gasification reactor, i.e. fuel reactor, the biomass drying and fast pyrolysis take place firstly, with a result of most volatiles releasing here. In the presence of oxygen carriers and steam, tar cracking reactions together with combustion reactions with oxygen carriers occur. Then, the resulting char after devolatiliziation is continuously gasified. Accompanied with the gasification, the homogeneous and heterogeneous combustion reactions take place to reduce the oxygen carrier to the reductions phase. The exothermic combustion reactions in the 3 Environment ACS Paragon Plus

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gasification reactor partly provide the heat needed for biomass gasification, but the main heat is transferred from the air reactor by the circulating bed materials particles. The reduced oxygen carrier is circulated to the air reactor where it reacts with oxygen in the air for regeneration. The circulated oxygen carrier transfer oxygen and heat from air reactor to fuel reactor. The way of heat transfer and chemical energy utilization in CLG process is totally differing from the conventional biomass gasification in dual fluidized beds. Most of the biomass is converted in the gasification reactor, thus directly improving the syngas heating value. At the heat needed for the gasification reactor is not from the residual char combustion, but from the exothermic reaction occurring in the air reactor. In the year of 2011, the using of chemical looping method for biomass gasification was proposed by He et al. 9, with some experimental results in a fluidized bed indicating that the conversion of biomass pyrolysis products can be accelerated due to the presence of hematite oxygen carrier. Huang et al.10-11 investigated biomass direct CLG process using an iron-based oxygen carrier and found that oxygen carrier can improve syngas yield, and the tar content decreased from 36.23 g/Nm3 in normal pyrolysis process to 10.25 g/Nm3 in CLG process. Wei et al.12-13 investigated the hydrodynamics of a 10 kWth interconnected fluidized cold model for CLG of biomass, and the following hot operation indicates that the syngas yield, cold gas efficiency, and carbon conversion increased with increasing operating temperature from 670 °C to 900 °C. Zhao et al.14 compared biomass gasification behavior with Fe-, and Cu-based oxygen carriers, and found that the oxygen carrier can enhance biomass conversion and syngas yield as well as lower the tar emission. Afterwards, they developed a Cu-Fe based oxygen carrier for CLG application using sawdust as feedstock with the objective to optimize the operation conditions15-16. A similar tar positive effect on tar cracking during the treatment of biomass gas using oxygen carrier was also found by Keller et al.17. Zeng et al.18 found that in the case of a gasification temperature of 820 °C and a S/B value of 1, the ratio of H2 to CO in the syngas of 2.45 can be achieved. Ge et al.19-20 investigated biomass gasification behavior 4 Environment ACS Paragon Plus

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Energy & Fuels

in a batch fueled reactor and interconnected fluidized beds reactor and found that the presence of oxygen carrier has significant effect on syngas distributions. In a 2-4 MW gasiﬁer, Larsson et al.21 investigated the gasification performance of biomass using the mixture of 12 wt.% natural ilmenite ore and silica sand, and found a decrease of the heating value of the gasiﬁer due to the presence of oxygen carrier, and a decrease in total tar yield. Oxygen carrier in CLG process should be environmentally friendly, low cost, high reactivity and resistance to attrition and agglomeration at high temperatures. Through the previous studies, most of investigations were using Fe-, Cu-Fe, or Fe-Ni as oxygen carriers. Manganese ore as oxygen carrier has been investigated by some previous studies for chemical looping combustion, and results support that it has high activity and catalytic reactivity22-23. Meanwhile, the original biomass as feedstock was employed in the most of the previous investigations. In comparison to the original biomass feedstock, pelletized biomass are usually cylindrical particles with large size and high density. The different fuel properties generally give a completely different conversion behavior. During pellets gasification different subprocesses take place which determine kinetics of chemical reactions as well as alteration of pellet structure. The main microscale processes are: (1) heating of pellets due to the contact with gas or bed material; (2) evaporation of water steam and volatile release (formation of char particles); (3) primary breakage of pellets due to internal stress in the pellet caused by gas streams; (4) attrition and secondary breakage of char particles; (5) further chemical reactions occurring in char particles. Traditionally, biomass pellets have higher volatiles compared to coals and cokes, and biomass chars are typically more porous than those from coals and cokes24-25. Chirone et al.26 investigated the combustion behavior, first and second fragmentation of biomass pellets in fluidized bed. They found that the fragmentation is much influenced by the type of biomass and ash amount, and carbon elutriation rates are very small, and the PSDs of the primary ash particles largely reflected the combustion pattern. 5 Environment ACS Paragon Plus

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Rice husk is an important byproduct in the South region of China. A lot of pellet production industries are located in the region. As well-known, the rice husk sample is rich in the content of SiO2 with some alkali metals. The aims of the work are: (1) to evaluate the CLG performance using a typical rice husk pellet; (2) to investigate the ash properties of rice husk pellet in CLG process.

2

Experimental

2.1

Solid bed materials

One kind of manganese ore from Guizhou province of China was employed for the study. Before tests, the ore was calcined at a temperature of 950 °C for 3 h. The samples after calcination was used for the experiments. The elements were quantitatively determined by XRF (X Ray Fluorescence), as shown in Table 1, and results indicate that the main elements are Mn, Fe, Si, Al and K. XRD test suggests that the main phases in the ore are Mn2O3, Fe2O3 and SiO2, as shown in Figure 1. Assuming the transformation pattern of Fe2O3 to FeO and Mn2O3 to MnO, a theoretical value of oxygen transport capacity (RO, %) can be obtained as 5.1%. During experiments, the bed materials were the mixture of manganese ore and quartz sand. The apparent density (ρs) of the manganese ore and quartz sand were measured as 1600 kg/m3 and 1290 kg/m3 respectively. The particle size distributions (PSDs) determined by a laser detector are shown in Figure 2. The median (dp,50) for the ore is 373 µm and 440 µm for the quartz sand. The Sauter diameters of the ore and sand are 266 µm and 303 µm. Based on Wen & Yu equation27, the minimum fluidization velocity of the ore (umf, ore) is calculated as: 1.34 cm/s @750 °C, 1.32 cm/s @800 °C, 1.29 cm/s @850 °C, 1.24 cm/s @900 °C and 1.19 cm/s @950 °C. Meanwhile, the minimum fluidization velocity of the sand (umf, sand) is calculated as: 1.66 cm/s @750 °C, 1.64 cm/s @800 °C, 1.57 cm/s @850 °C, 1.53 cm/s @900 °C and 1.46 cm/s @950 °C.


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2.2

Solid fuels

Commercial pellet samples produced by Dingliang company in Anhui province using rice husk as feedstock were used in the investigation. The proximate and ultimate analyses are shown in Table 2. The rice husk pellet has a high volatile matter of 64.92 wt.% and ash content of 17.06 wt.%. A single pellet sample with diameter of 9 mm and length of 10 mm was used for each testing case. Its ash contents after calcination at 900 °C for 2 h are shown in Table 3. As indicated in the table, the rice husk sample is rich in the content of SiO2. Besides, some K and Ca are containing in the ash. 2.3

Apparatus and methods

The experiments were performed in a bubbling fluidized bed unit. The quartz tube with an internal diameter of 32 mm is placed in an electrically heated oven. As shown in Figure 3, the facility consists of the fluidized bed, gas feeding system and on-line gas analytical system. A porous plate of metal is placed in the fluidized bed as gas distributor. The steam was fed to the reactor by using a constant flow-type pump and a cast aluminum heater. The steam concentration into the reactor was 50% with the balance being nitrogen. For each experiment, one single pellet placing in the fuel chute was fed by means of two-way-ball valves (V1 and V2). The unit was fluidized by air and heated up to the designed temperature. A steam balanced with N2 was used for gasification, while an air stream was for oxidization. After every gasification stage, oxygen carrier particles were fully re-oxidized with oxygen before starting a new cycle. A detailed operation procedure can be found in our previous work28. During the experiments, the on-line gas analyzer (Gasboard 9020) was used to determine the dry gas concentrations. After cleaned and condensation, four NDIR cuvettes in series were employed to determine the levels of CH4, CO2, CxHy and CO respectively. A thermal conductivity detector (TCD) is used for H2 concentration, while in parallel some of the offgas is measured by electrochemical detector for levels of O2. 7 Environment ACS Paragon Plus

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2.4

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Data evaluation

In the work, several parameters were used to evaluate the CLG performance, such as oxygen carrier-fuel ratio (φ), carbon conversion rate (rC, mol/s), carbon conversion (XC, red, %). Theirs definitions (Eqs.1 to 4) can be clearly found in our previous work28. ϕ=

NO nO

nout =

(1) nN2 ,in

(2)

(1 − ∑ yi,out ) i

rC = ( yCO2 ,out + yCO,out + yCH4 ,out + yCxH y ,out ) ⋅ nout 1

X C , red =

(3)

t

( mbatch × φC , fuel ) / M C

∫ r dt 0 C

(4)

For each case, the weight of a single pellet fed into the reactor is 1.0 g, corresponding to a value of the oxygen (O) needed to fully oxidized the pellet (nO, mol) of 0.0675 mol/g pellet. The effective gas contents (Yg, %) in the syngas can be obtained:

∑(y

CO

Yg =

+ yCH4 + y H 2 )

i

∑ ( yCO + yCH4 + yH2 + yCO2 + yCxH y )

(5)

i

The syngas yield (Ng, Nm3/Kg) is the addition of effective gaseous products (CO, H2 and CH4).

Ng =

(∑ nout yCO + ∑ nout yCH 4 + ∑ nout y H 2 ) i

i

i

(6)

mbatch

where MCO, MCH4, MH2 are the molar weight of CO, CH4 and H2 respectively, g/mol. Three repeated tests were performed with the relative experimental uncertainty for these definitions lower than 5%.


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3 3.1

Results and discussion Gasification performance

For CLG of biomass pellet using Mn ore as oxygen carrier, several complex reactions including biomass devolatilization (R1), tar cracking (R2), remaining char gasification (R3), water gas shift reaction (R4) and their gaseous products oxidizing reactions (R5) were present in the gasification stage. Biomass pellet(s)→ volatiles(g) + char(s)

(R1)

Tar cracking→ CO(g), H2(g), CO2(g) and CH4(g)

(R2)

C(s)+H2O(g)→CO(g)+H2(g)

(R3)

CO(g)+H2O(g)→CO2(g)+H2(g)

(R4)

CO(g), H2(g), and CH4(g)+MexOy(s) → MexOy-1(s) +CO2(g)+H2O(g)

(R5)

MexOy-1(s) +O2(g) →MexOy(s)

(R6)

In the test, one single pellet sample was fed to the reactor with a bed of the mixture of Mn ore oxygen carrier and quartz sand particles. The biomass pellet was converted in the steam atmosphere in the presence of lattice oxygen containing particles. Figure 4 shows the typical measured off-gas concentrations during the CLG of the rice husk pellet at 900 °C with φ as 0.5. As indicated in Table 2, the biomass has a high content of volatiles. During the first devolatilization stage, the volatiles rapidly released, and then were partly oxidized by the active phases of oxygen carriers (R5), resulting in a fast CO2 evolution. Meanwhile a lot of CH4 and CO were releasing to the off-gas, especially for CO. The matter of CO and CH4 were coming from volatiles or hydrocarbons decomposition. Besides, a little CxHy were present in the off-gas. The char remained continuously reacted with steam to produce CO and H2 (R3). Following the gasification stage, the reduced oxygen carrier during the gasification stage was oxidized back by oxygen in the oxidation stage (R6). After one cycle, the reactor was cooled down and the mixture of oxygen carrier and sand back to the process to be reused.


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3.2

Temperature effect

Figure 5 shows the carbon conversion at different temperatures (750 °C, 800 °C, 850 °C, 900 °C, 950 °C). A high temperature promoted the gasification reaction, leading to a fast carbon conversion. Also, it is obvious that the conversion of the rice husk pellet is much dependent on the reaction temperature. This is much due to the slow devolatilization of pellet-based biomass fuels compared to the traditional fuels with small particle size. The rice husk pellet needs much long time to reach final devolatilization. High temperature gives a strong temperature gradient in the pellet, thus promoting the release of volatiles. As one consequence, a strong pressure gradient due to the fast release of volatiles inside the pellet becomes a driven factor to accelerate the carbon conversion of the pellet. The tar cracking reaction (R2) as well as the steam carbon reaction (R3) are both enhanced under high temperatures10,16,29. In the gasification stage, the main gaseous products in the off-gas were CO, H2, CH4, CO2 and little amount of CxHy. The effect of temperature at an oxygen carrier-fuel ratio (φ=0.5) on the accumulated gas concentrations with N2 free is shown in the Figure 6. The gas distributions are much influenced by the complex homogeneous and heterogeneous reactions in the bed. With the increase of gasification temperature, the accumulative concentrations of CH4 and CxHy decrease. CxHy as well as CH4 are sensitive to the temperature, with the consequence of their concentrations decreasing with increasing temperature, due to the enhancing decomposition effect of hydrocarbons with increasing temperature10,16,29. Mainly ascribed to the reactions of (R2), (R3) and (R4), the concentration of H2 increases from 26.5 vol.% at 750 °C decreasing to 31.0 vol% at 950 °C. Due to high content of volatiles in the pellet, the tar cracking rate (R2) was enhanced under higher temperatures16, leading to more H2 generation. Meanwhile, high temperature gave a favorable atmosphere on the gasification reaction (R3) to accelerate H2 formation10-11,29, although it can also push the reaction (R4) to the left side and accelerate the oxygen carrier reduction rate with H2. The 10 Environment ACS Paragon Plus

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increasing CO concentration with increasing temperature is mainly caused by an increasing fuel conversion (R3), enhancing tar cracking (R2) and revers water-gas shift reaction (R4), although the oxidation reaction with oxygen carrier consumed CO. The case of reaction (R4) to the left side with the increasing temperature mainly leads to the decreasing concentration of CO2. Figure 7 shows the temperature effect on the effective gas content and syngas yield in the case of φ as 0.5. The effective gas content increases from 74.2% at a temperature of 750 °C to 79.9% at 950 °C, while the syngas yield increases from 0.13 Nm3/kg at a temperature of 750 °C to 0.33 Nm3/kg at 950 °C. A high temperature promoted the volatile matter decomposition, leading to more volatiles being converted to the effective gasification products as CO, H2 and CH4. 3.3

From gasification to combustion: effect of oxygen carrier-fuel ratio (φ)

During the experiments, the amount of bed materials is determined on the basis of an unexpanded bed height of 50 mm at incipient fluidization. In the case of a constant value of the oxygen needed to fully oxidize the pellet, the parameter of φ is identified by changing the weight fraction of oxygen carrier in the bed materials. In the case of φ>1, it means the oxygen carrier supplied enough lattice oxygen to burn the pellet. However, it should be stated that the definition of φ is closed to the final reduction state of Mn2O3 as well as Fe2O3. Figure 8 shows the effect of oxygen carrier-fuel ratio (φ) on the accumulative gas concentrations during CLG of a single rice husk pellet. At 900 °C, due to the enhancing oxidation between gasification products and oxygen carriers, the concentrations of CO, H2 and CH4 decrease and CO2 increases with increasing oxygen carrier-fuel ratio (φ) from 0.5 to 2. At a higher temperature of 950 °C, it can be observed that the CO and H2 concentration firstly increase in the case of φ increasing from 0.5 to 1. This is mainly ascribed to the increasing amount of oxygen carriers, which reacted with the tar to produce more combustible gases. Meanwhile, part of gases reacted with oxygen carriers, leading to the increase of CO2 11 Environment ACS Paragon Plus

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concentration. Further, when the oxygen carrier-fuel ratio (φ) continuously increases from 1 to 2, more gasification products were oxidized by oxygen carrier particles. As a consequence, CO and H2 concentrations decrease together with the CO2 concentration increasing sharply. Figure 9 shows the effect of oxygen carrier-fuel ratio on effective gas content and syngas yield at 900 °C and 950 °C. Due to the enhancing oxidation of oxygen carrier with syngas, the effective gas content decreases sharply with increasing oxygen carrier-fuel ratio. Correspondingly, the syngas yield decreases sharply at a reaction temperature of 900 °C. At 950 °C, the syngas yield shows a slight increase with the oxygen carrier-fuel ratio increasing from 0.5 to 1, supporting the beneficial effect of oxygen carrier on enhancing tar creaking reaction. 3.4

Multiple redox cycles test

A stable reactivity of the oxygen carrier is one of the most significant parameters for CLG application. At a temperature of 900 °C with a same operation conditions as described in the section of 3.1, twenty redox cycles were performed using the same batch of oxygen carriers. The variations of syngas distributions with the cycle number are shown in Figure 10. The concentrations of CO, H2, CO2, CH4 and CxHy are in the ranges of 32.4-36.5 vol.%, 28.2-31.7 vol.%, 18.3-21.4 vol.%, 13.5-16.0 vol.%, 0.12-1.5 vol.%, respectively. Although there are some fluctuations for the gas concentrations due to experimental error, a suitable suggestion can be made that the oxygen carrier demonstrates a stable reactivity in the present work. 3.5

Characterizations of bottom ash

The left picture of Figure 11 shows the original rice husk pellet sample and bottom ash which were obtained at a temperature of 750 °C with φ as 0.5. The pellet sample was very hard and had a compact structure. After each test, it can be observed that the bottom ash was not separately but still keeping the original shape with some cracks on the surface. During gasification process, the bottom ash, like a big agglomerate, became brittle in comparison to the pellet sample, although it cannot be separated by itself in the fluidized bed. A same 12 Environment ACS Paragon Plus

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phenomenon was also found by Shan et al.30, who investigated the rice husk pellet combustion during oxygen fuel process. After breaking the bottom agglomerates, the presence of some black carbon particles in the ash can be easily seen. The carbon particles inside the ash limited the pellet carbon conversion, giving a carbon conversion lower than 40%, as indicated in Figure 5. According to Table 3, the rice husk pellet is rich in Si and K. High concentrations of Si and K are finely distributed in the fuel organic matrix31. Combustion reactions between oxygen carrier and reducing gases, as well as gasification reactions are present in CLG process. Traditionally, under strong reducing condition, the dominant K vapor is K(g) or K2CO3(g) 32. The existence of K in the ash drives the melting point of ash lower, with a consequence of some carbon particles coated by the molten SiO2. Figure 12 gives the SEM characterizations of the bottom ash, which were characterized with two magnifications of 150× and 10000×. As shown in the upper two images, it can be observed that the surface of the ash was much rougher with sharp wrinkles. When the magnification is extended to 10000×, as shown in the bottom images, the molten grains are connected together to form some irregular pores. In total, the ash demonstrates a rigid skeleton-like structure. The structure was formed due to the release of volatiles at high temperature. The similar characteristic, 0.1-0.3 mm size silica “skeletons” of the rice husk ash due to its high silica content were also clearly observed by Skrifvars et al.33 during fluidized bed combustion tests. The skeleton-like structure results in some carbon being trapped in the skeleton and cannot be further converted. A same phenomenon of “carbon trapped in the skeleton” during rice husk combustion was also pointed out by Rozainee et al.34, who also found that breaking the rigid char skeletons into smaller fragments can be aided due to the attrition, therefore releasing the trapped carbon for further oxidation.


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EDS analysis on the bottom ash was performed with the result showing in Figure 13. The main element in the surface of the ash are Si, K, and Ca. The EDS results support the formation of molten ash is mainly caused by the interaction of Si with K or Ca in the ash. In our previous work29, original rick husk feedstock, not in a pellet form, was used as fuel for CLG investigations. The feedstock has a low density, small size and fast conversion behavior in the fluidized bed. The pelletized rice husk, as very compact fuel, as indicated in Figure 5, needs high temperature to enhance carbon conversion. Traditionally, for the pelletized fuel, the strong pressure gradient due to the fast release of volatiles inside the pellet drives pellet broken during thermal treatment process. However, due to the special characters of the rice husk ash, that is rigid skeleton-like structure, the trapped carbon particles are hard to be converted, leading to a slow and limited carbon conversion. As a very special type of fuel, rice husk has edged shell-like outer cover, which keeps almost intact during combustion35-36. Skrifvars et al.33 found that during fluidized bed combustion process rice husk produced coarse, almost millimeter-sized ash particles with a characteristic shape. The bottom ash remaining in bed with bed materials should be considered. According to the present finding, the bottom ash was not separately but still keeping the original pellet shape. As bed particles circulating system, chemical looping requires more fuel ash elutriated out of fuel reactor. The accumulation of big ash agglomerates in the fuel reactor has potential operating problems, even as breakdown of fluidization. Based on the considerations, one ongoing work is performing, that is to optimize the pellet fuel production with regard to get pelletized fuel breakage during devolatilization stage, and then dispersed ash elutriated out of the fuel reactor. The breakage of pelletized rice husk during devolatilization stage is expected to lower the gasification temperature for high syngas yield. The dispersed remaining ash can be easily elutriated out fuel reactor due to the density difference between oxygen carrier and ash, without bringing operation problems due to the existence of ash agglomerates. 14 Environment ACS Paragon Plus

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4

Conclusions

The study focuses on the CLG application using rice husk pellet as fuel, with the aim of investigating the gasification performance and bottom ash characterization. Some experiments in a fluidized bed unit with the mixture of quartz sand and an active manganese ore as bed materials, were performed using a single pellet as fuel and steam as gasifying agent. The conversion of the rice husk pellet is much dependent on reaction temperature. A high temperature promoted tar cracking and gasification reactions, leading to a fast carbon conversion. The effective gas content (CO+H2+CH4) during gasification process were in the range of 74.2% to 79.9% under the temperature of 750 °C to 950 °C. Regarding the CLG application of rice husk pellet as fuel, much attention should focus on bottom ash, which was not separately during the process but still keeping the original pellet shape with some irregular pores inside the ash due to the formation of molten grains. The ash demonstrates a rigid skeleton-like structure. The trapped carbon particles inside the molten ash cannot be gasified, thus limiting the fuel conversion.

Acknowledgement This work was supported by the National Natural Science Foundation of China (51761135119) and Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJB480006).


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Table 1: Main chemical elements in the ore.

Elements Fe Mn Al Si Ti K Mg

Contents (wt.%) Before calcination After calcination 14.58 15.30 15.48 20.04 5.35 4.84 18.08 16.95 0.16 0.15 1.03 0.97 0.42 0.35


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Table 2: Proximate and ultimate analysis of the biomass pellet sample.

Fuel type Moisture (wt%, as received) Ash (wt%, as received) Volatiles (wt%, as received) Fixed carbon (wt%, as received) Ultimate analysis (wt%, dry) C H N S O

Rice husk 8.06 17.06 64.92 9.96

37.66 4.81 0.42 0.023 31.97


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Table 3: Main chemical elements in the ash.

Elements Si K Ca P Fe Mn Mg Na Al S

Contents (wt.%) 39.26 1.91 0.91 0.56 0.34 0.23 0.36 0.08 0.16 0.32


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

Intensity (CPS)

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4000 3500 3000 2500 2000 1500 1000 500 0

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a

a SiO2 b Fe2O3

c

c Mn2O3 b

a c b

b ac a b c

ba

c b

a b b c aa

2θ (°) Figure 1: XRD pattern of the manganese ore.


b

b

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12 10

Volume difference (%)

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

Energy & Fuels

Mn ore

8 6 Quartz sand

4 2 0

0

200

400

600

800

1000

1200

dP (µm)

Figure 2 PSDs of the bed materials used for the experiments


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Valve 1 Valve 2 Condensation

N2

Vent

O2 H2O

Analyser

Vent

CO2 CO CH4 O2

H2

Mass flow controller

Constant flow pump

T

Steam generator

Figure 3 Sketch of the experimental facility


10 Gasi. stage

9

Gas distributions (vol.%)

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

Energy & Fuels

Oxi. stage

8 7 6

CO

5

O2

4

CH4

3 2

H2 CO2

1 0

0

500

CxHy

1000

1500

2000

2500

24 22 20 18 16 14 12 10 8 6 4 2 0

O2 concentration (vol.%)

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Time (s) Figure 4 A typical off-gas distributions during the CLG of a single rice husk pellet at 900 °C with φ as 0.5.


Energy & Fuels

100 90

Carbon conversion (%)

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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950°C

80

900°C 850°C

70 60

800°C

50

750°C

40 30 20 10 0

0

250

500

750 1000 1250 1500 1750 2000 2250

Time (s)

Figure 5 Carbon conversion at different temperatures with φ as 0.5.


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10

40 35

CO

30

8

H2

6

25 CO2

4

20 CH4

CxHy (vol.%)

CO, CH4, H2 and CO2 (vol.%)

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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2

15 CxHy

10

750

800

850

900

950

0

Temperature (°C) Figure 6 Accumulative gas concentrations during CLG of a single rice husk pellet at different temperatures with φ as 0.5.


Energy & Fuels

1.0 84 0.8

82

3

80 0.6 78 76

0.4

Yg Ng

74

0.2

72 70

Syngas yield (Ng, Nm /Kg)

Effective gas contents (Yg, %)

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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750

800

850

900

950

0.0

Temperature (°C) Figure 7 Effective gas content and syngas yield at different temperatures with φ as 0.5.


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40

Gas concentrations (vol. %)

35 30

(A)

CO H2

25 20 15

CO2 CH4

10 5 0

CxHy

0.5

1.0

1.5

Oxygen carrier-fuel ratio (ϕ)

2.0

40

Gas concentrations (vol. %)

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

Energy & Fuels

35

CO

30

H2

(B)

25 20

CO2

15

CH4

10 5 0

CxHy

0.5

1.0

1.5

Oxygen carrier-fuel ratio (ϕ)

2.0

Figure 8 Accumulative gas concentrations during CLG of a single rice husk pellet at different oxygen carrier-fuel ratio (φ). A: gasification temperature 900 °C; B: gasification temperature 950 °C


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84

0.60 950 °C

0.55

78

0.50

75

0.45

72

0.40

69

0.35

66

0.30

63

0.25

3

81

Syngas yield (Ng, Nm /Kg)

900 °C

Effective gas contents (Yg, %)

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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60

0.5

1.0

1.5

2.0

0.5

1.0

1.5

2.0

0.20

Oxygen carrier-fuel ratio (ϕ) Figure 9 Effect of oxygen carrier-fuel ratio on effective gas content and syngas yield at 900 °C and 950 °C.


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50

CH4

45

Gas concentrations (vol.%)

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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H2

40

CO CO2

CxHy

35 30 25 20 15 10 5 0

0

5

10

15

20

Cycles (-) Figure 10 Variations of gas concentrations during 20-cycle test at 900 °C with φ as 0.5.


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Figure11 Pictures of the pellet sample and bottom ash


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Figure 12 SEM characterizations of the bottom ash


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Figure 13 EDS characterizations of the bottom ash


Page 34 of 34

Chemical Looping Gasification of a Biomass Pellet with a Manganese

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