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Chemical-Looping Combustion of Solid Fuels using Manganese Ores as Oxygen Carriers Matthias Schmitz, Carl Linderholm, Peter Hallberg, Sebastian Sundqvist, and Anders Lyngfelt Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02440 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016

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Chemical-Looping Combustion of Solid Fuels using Manganese Ores as Oxygen Carriers Matthias Schmitza*, Carl Linderholma, Peter Hallberga, Sebastian Sundqvistb, Anders Lyngfelta a

Department of Energy and Environment, Chalmers University of Technology, S-41296

Göteborg, Sweden b

Department of Chemical and Biological Engineering, Chalmers University of Technology, S-

41296 Göteborg, Sweden KEYWORDS Carbon Capture and Storage (CCS), Chemical-Looping Combustion (CLC), Oxygen Carrier, Manganese ore, Biomass, Petcoke

ABSTRACT In chemical looping combustion (CLC), the choice of the oxygen carrier material is crucial with respect to overall system performance and cost. Materials based on manganese ores are promising candidates due to their favourable thermodynamic properties, high availability and low price. As these ores tend to be comparably soft and prone to attrition, the challenge is to find materials which combine the abovementioned advantages with sufficient mechanical durability. In this study, three manganese materials were screened for their suitability as oxygen carriers in the chemical looping process. The materials were subjected to continuous operation with fuel in a 10 kW chemical looping unit and evaluated in terms of gas conversion, carbon capture efficiency and particle lifetime. All oxygen carriers showed good performance and reached more

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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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than 90% gas conversion at relevant conditions. Particle lifetime based on fines production was in the range of 99 to 284 hours, which is a considerable improvement compared to a manganese ore previously tested in this unit. One material was ruled out as potential candidate for up-scaling due to agglomeration tendencies.

1 Introduction In the context of rising anthropogenic greenhouse gas emissions and the resulting increase in the average temperature on the planet, carbon capture and storage (CCS) is a means to separate CO2 from combustion processes and prevent it from entering the atmosphere. Within CCS, chemical looping combustion (CLC) is a technology to efficiently burn gaseous, liquid or solid fuels with inherent CO2-separation. While other CO2 capture technologies rely on an active gas separation step, the energy penalty associated with such a process can be avoided in CLC. In the chemical looping process, the fuel is not mixed with the nitrogen contained in air. Instead, oxygen is transferred to the fuel by a solid oxygen carrier, typically a metal oxide. As both high rates of mass and heat transfer are needed to make the process work, the concept can be realized using two fluidized bed reactors connected by cyclones and loop seals. In the air reactor, the oxygen carrier particles are oxidized, after which they are transported to the fuel reactor, where the fuel is oxidized by the particles before they are transported back to the air reactor, thus closing the loop. While operation with gaseous fuels has been in the focus of CLC research in the first years, most work in the last years was done with solid fuels. Using oxides of copper and iron, Lewis et al.1 were the first to study solid-fuel CLC. Fifty years later new studies emerged2-4, involving the same oxides. Leion et al. investigated different fuels


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and oxygen carriers in a small laboratory fluidized bed, e.g.5-7. Lyngfelt recently presented a review on chemical-looping combustion with solid fuels8. Gas-solid reactions take place at much higher rates than solid-solid reactions, which is why solid fuels have to be gasified by steam or CO2 if they are to be used in chemical looping combustion. Fuel devolatilization and the gasification process are carried out in the same reactor as the subsequent gas-solids reaction with the oxygen carrier. The gaseous volatiles and gasification products react with the oxygen carrier to form CO2, H2O and, if a sulphurous fuel is burnt, SO2. In CLC, gasification takes place in an atmosphere with high concentrations of CO2 and H2O, which is an advantage compared to conventional gasification. Leion7 showed that the presence of an oxygen carrier clearly accelerates gasification. For fuel particle sizes normally used in CLC (< 250 µm), drying is very rapid, devolatilization is essentially complete in a matter of seconds, whereas gasification may take several minutes, also at fuel-reactor temperatures as high as 970°C. Equations (1) - (6) describe the principle of solid fuel gasification and subsequent chemical looping combustion: Solid fuel char (C) + tar + gas (H2, CO, CO2, CH4, CxHy)

(1)

C + H2O → CO + H2

(2)

C + CO2 → 2 CO

(3)

Reaction of volatiles and gasification products with oxygen carrier particles: MexOy + H2 → MexOy-1 + H2O,

(4)

MexOy + CO → MexOy-1 + CO2,

(5)


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4 MexOy + CH4 → 4 MexOy-1 + CO2 + 2 H2O,

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(6)

where methane represents all hydrocarbons released during devolatilization. To date, most operational experience in solid-fuel chemical looping combustion has been gained with ilmenite, an iron-titanium material. Ilmenite is comparably cheap, mechanically durable and non-toxic9-14 and has therefore become the state-of-the-art oxygen carrier in chemical looping combustion15. However, gas conversion and carbon capture are lower than with copper- or manganese-based materials16, 17. Consequently, the search for viable alternatives featuring high reactivity, low cost and toxicity and mechanical durability is an important issue. For solid fuel combustion, the cost aspect is especially important as oxygen carrier material will be lost in ash separation. Manganese ores have a high manganese content, are cheap and abundant. Such materials have previously been shown to possess important qualities as oxygen carrier. Sundqvist et al.18 found that gas conversion for different manganese ores was 2.7 to 6 times faster compared to ilmenite. Linderholm et al.19 used a mixture of ilmenite and manganese ore in a 100 kW chemical looping combustor and found that the fraction of unconverted gases could almost be halved by addition of manganese ore. However, it has also been observed that some manganese materials are more sensitive to attrition forces than ilmenite20. The objective of this work is to analyse the performance of three manganese materials in the chemical looping combustion process, to evaluate their lifetime and to compare the findings with ilmenite.


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2 Experimental 2.1 10 kW pilot The pilot is based on interconnected fluidized-bed technology. In the riser, which constitutes the upper part of the air reactor, high gas flows in combination with a small cross-section area ensure high gas velocities which provide the driving force for the circulation. The entrained oxygen carrier particles enter a cyclone, where they are separated from the air flow and fall into the fuel reactor via a loop seal to avoid gas mixing. The fuel reactor is designed as a bubbling bed and consists of several parts: in the main section, fuel is oxidized to CO2 and H2O. The char remaining after devolatilization is gasified followed by oxidation of the gasification products by the oxygen carrier. The main section is usually fluidized with steam. The particles are forced to pass under a vertical wall, see Figure 1. The carbon stripper (CS), which is fluidized by nitrogen, is supposed to separate coal and oxygen carrier particles by making use of their different densities. The particles entrained in the carbon stripper are then reintroduced to the low velocity section via a small loop seal, whereas the reduced particles enter the air reactor again.


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Figure 1. 10 kW solid fuel chemical looping combustor. TC 1-3 mark thermocouple positions Fuel is introduced into the fuel-reactor bed via a coal screw and a fuel chute. The operating temperature is measured via three thermocouples located in the air reactor, fuel reactor and air reactor cyclone. Fluidization behaviour, solids circulation and inventory can be estimated from


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numerous pressure measurements. The exhaust gas streams from both air and fuel reactor are passively cooled before entering filter bags (air reactor exhaust) or a water seal (fuel reactor exhaust). The water seal is used both to collect condensate from fuel conversion and steam fluidization and to impose a hydrostatic pressure on the fuel reactor exhaust, thus creating a pressure difference between the outlets of the fuel reactor and air reactor. This is necessary to avoid inadequate pressure differences over the loop seals connecting the reactors. A part of the exhaust gas streams is cooled, filtered for removal of fines, led through gas conditioning systems to condense remaining steam and then analysed by infrared- (CO, CO2 and CH4), thermal conductivity- (H2) or paramagnetic analysers (O2). Apart from that, gas samples can be withdrawn and analysed in detector tubes. As the heat losses are higher than the thermal power generated by fuel addition, the unit is enclosed in an electrically heated furnace, which also is used to heat up the unit initially. During heat-up, all parts of the unit are fluidized by air before switching to steam-/nitrogen fluidization of the fuel reactor, the loop seals and the carbon stripper. Previous operational experience in this unit has been achieved using different natural minerals as oxygen carrier – mainly ilmenite and manganese ores9, 11, 20-24.

2.2 Data evaluation 2.2.1 Conversion performance As in conventional combustion processes, the aim in chemical looping combustion is to convert the fuel to the fullest extent possible. Therefore, performance evaluation is based on the quantification of non-converted fuel species. These can escape the fuel reactor in three different ways: as unconverted gas, as char particles to the air reactor or as elutriated char.


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The unconverted gas species in the fuel reactor exhaust are made up from both volatile fuel compounds and gasification products. The oxygen demand,

Ω =

0.5 , + 2 , + 0.5 ,

(7)

Φ , + , + ,

describes the fraction of oxygen which after conversion in the fuel reactor is lacking to achieve full conversion and would have to be added in an oxygen polishing step. In this definition, Φ is the molar ratio of oxygen needed to oxidize the fuel to moles of carbon contained in the fuel 9. Thus, the denominator represents the amount of oxygen that would have been needed to achieve full conversion of the fuel that has been transformed to gaseous compounds. In comparison to char gasification, devolatilization is a fast process. This means that when the fuel flow in a solid fuel chemical looping combustor is switched off, the remaining coal particles are devolatilized before the char residual is gasified. The resulting char oxygen demand is lower than the oxygen demand during continuous operation and corresponds to the oxygen demand which could be reached when using a completely devolatilized fuel. The reason for that minimum is that the oxygen demand is higher for volatiles as compared to the syngas generated by char gasification. As an alternative to oxygen demand, the gas conversion, = 1 − Ω , can be calculated. Both oxygen carrier and unconverted fuel particles move in a continuous flow from the fuel- to the air reactor. Some char particles will therefore escape the fuel reactor before they can be fully gasified. These particles will eventually be oxidized by air in the air reactor and thus evade the carbon capture process. The oxide oxygen efficiency is the proportion of the amount of oxygen used for reoxidation to the total oxygen consumption in the air reactor:


8

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=

0.21 − , − , 0.21 − , − 0.21 ,

(8)

A precise calculation of that performance indicator is possible because it only depends on gas concentrations. Because the oxygen not used for oxidizing the oxygen carrier is used for oxidizing the char, provides a good estimation of the carbon capture efficiency. This is further explained in12. Therefore, will be referred to as carbon capture efficiency in the following. The reactor system is mainly designed to evaluate gas conversion and carbon capture efficiency and thus neither has a reliable on-line fuel flow measurement or the possibility to monitor char elutriation during experiments, making the closure of the carbon balance complicated. Experiments in a 100 kW solid fuel unit have shown carbon elutriation rates of up to 35% with ilmenite as oxygen carrier25. However, the cyclone efficiency in both the 10 and 100 kW reactors is poor and the riser height of around 2 m in the 10 kW unit results in considerably shorter residence times for elutriated fuel particles than in industrial size units. Against this background, Lyngfelt and Leckner 26 assume the char conversion to be 97% in the fuel reactor of a 1000 MW unit, with the remainder reacting in the post-oxidation chamber. The results on char elutriation from this reactor system have thus little relevance for the larger scale, which is why the amount of unconverted elutriated char particles in the fuel reactor exhaust was not measured. The circulation index, CI, is used as a measure of solids circulation in the reactor. CI is based on measurements of the pressure drop over the riser and the air reactor temperature and gas flow.


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= ∆! " ∙ $ ∙

% + 273 273

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(9)

2.2.2 Lifetime of the oxygen carrier During operation, some of the oxygen carrier particles break into smaller pieces. Repeated oxidation and reduction processes with the associated phase changes in combination with highvelocity collisions with the bottom plate and cyclone walls demand a high attrition resistance of the oxygen carrier particles. To quantify the loss of oxygen carrier, all particles elutriated from the air reactor and caught in the downstream filters are wet-sieved and dried in an oven. Particles smaller than 45 µm are called fines. These are not returned into the reactor system. The mass of elutriated particles is determined before and after the sieving and drying process with the balance being the production of fines, ∆mfines during a certain period of time, ∆t. Based on that, the production of fines Lf can be calculated to

() =

∆*)+,- 1 ∙ ∆. *"

(10)

with mI being the total solids inventory. The corresponding oxygen carrier lifetime can be expressed as:

./+)- =

1 ()

(11)

Fines are mostly produced during operation with fuel, but also during hot conditions without fuel addition. In this study, it is assumed that all fines are produced during fuel operation which slightly underestimates the lifetime of the oxygen carrier materials. It should be noted that the oxygen carrier is also exposed to ash fouling which may cause loss in reactivity and ultimately


10

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particle deactivation. The rate of ash fouling is not known, but should depend on the fuel used. It cannot be ruled out that fouling leading to significant loss of reactivity occurs well before 1/Lf is reached, in which case the lifetime of the carrier is governed not by the attrition rate but by the rate of fouling.

2.3 Fuels Wood char and petcoke were used as fuel in this study. The wood char is produced by subjecting wood chips of both hard- and softwood to 450°C for 8 h in the absence of oxygen. The asreceived fuel has a size of approximately 5 mm, which is reduced to 200-1000 µm when the fuel is passed through the feeding screw. The petcoke has a mass-weighted mean diameter of 79 µm. Table 1 shows composition and heating value of the fuels. Component

Wood char (%)

Petcoke (%)

Comment

Fixed carbon

73.9

81.5

as received

Volatiles

16.7

10.0

as received

Moisture

3.9

8.0

as received

Ash

5.5

0.5

as received

C

86.9

88.8

maf

H

3.2

3.1

maf

O

9.5

0.5

maf

N

0.4

1.0

maf

S

0.03

6.6

maf

LHV (MJ/kg)

29.8

31.8

as received

maf: moisture and ash free


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Table 1. Composition of the fuel: proximate and ultimate analyses.

2.4 Oxygen carriers Before heat-up and operation with fuel, the 10 kW unit is filled with 15-20 kg of oxygen carrier particles. While two of the tested materials are untreated manganese ores (“Mesa”, “Mangagran”), the third (“Sinfin”, a portmanteau created from the words “sintered fines”) is a material from manganese ore handling. The materials were chosen with respect to chemical composition and mechanical properties such as crushing strength to cover a wide range of potential oxygen carrier materials. All materials were crushed and calcined at 950˚C for 24 hours at Chalmers, while sieving into three size fractions (355 µm) was done by an external contractor. The long calcination duration is part of the handling routine used at Chalmers in order to ensure fully oxidized oxygen-carrier particles and thus minimizing the risk of agglomeration during the first heat up of the material. Such a long calcination time would not likely be required in large-scale applications. During calcination, the Mesa and Sinfin materials formed soft agglomerations, which could easily be taken apart mechanically. In the case of Mesa, the particle size increased during calcination while the density was essentially unchanged. The particle size distributions for the different oxygen carriers are depicted in Figure 2.


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0.40 0.35

Mass fraction [-]

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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0.30 0.25 0.20 0.15 0.10 0.05 0.00 355

Size interval [µm] Mangagran

Mesa

Sinfin

Figure 2. Particle size distribution for calcined particles prior to operation Mangagran and especially Mesa contained a higher share of particles >250 µm than Sinfin. The average size for Mangagran, Mesa and Sinfin material was 173, 200 and 150 µm, respectively. Table 2 shows the elemental compositions of the materials and a previously tested manganese ore called “Buritirama” is included for comparison. Mangagra

Buritiram Sinfin

Element

Mesa

n

a

Mn

66.36

48.86

39.88

43.61

Si

1.37

4.15

6.91

3.55

Al

3.11

4.76

0.42

2.86

Ca

0.08

0.29

2.48

0.14

Fe

2.95

5.68

13.57

5.58


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K

0.69

0.98

0.75

1.57

Mg

0.05

0.15

0.49

0.37

Na

0.04

0.07

0.17

0.00

P

0.11

0.11

0.04

0.00

Ti

0.09

0.25

0.05

0.22

Ba

0.27

0.21

1.11

1.61

Cu

0.05

0.08

0.07

0.02

S

0.06

0.07

0.11

0.05

Zn

0.06

0.10

0.01

0.22

Sum

75.29

65.76

66.04

59.81

LOI

2.4

-1.8

0.8

13.4

LOI: loss on ignition (1000˚C) Table 2. Elemental compositions of the tested manganese materials and a reference material. Numbers represent mass-%.

3 Results and discussion 3.1 Operational overview The following Table 3 summarizes in chronological order the periods in which steady state with respect to gas concentration and solids circulation was achieved. Tests 1-23 were made with Sinfin, tests 24-38 with Mangagran and tests 39-44 with Mesa. Basic operational data such as air reactor gas flow, fuel flow and fuel reactor temperature and the resulting key performance indicators are shown to provide an overview. In general, a better performance was achieved in the beginning of test days, when the solids inventory was highest due to addition of elutriated or


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fresh particles. A more detailed analysis of the results for each oxygen carrier is given in the following subchapters.

Test # 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

AR flow [ln/min] 150 165 180 180 200 240 200 200 220 180 200 180 160 140 230 180 180 180 200 180 200 210 230 150 160 185 210 160 180 210 230 230 230 160 140

Fuel Wood char Wood char Wood char Wood char Wood char Wood char Petcoke Petcoke Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char Wood char

TFR [°C] 957 957 962 971 972 971 971 973 970 969 970 970 968 968 970 969 970 969 969 970 971 1001 1000 971 971 969 973 958 972 971 970 971 971 971 971

CI [kPa/(l/min)] 324 417 531 576 866 1360 824 681 824 474 766 580 409 329 1074 254 406 350 432 420 530 523 522 467 447 629 763 451 621 825 1047 922 896 431 340

ΩOD [%] 6.2 7.6 8.2 8.6 8.7 8.2 12.8 12.0 8.0 5.7 4.7 6.1 7.6 9.8 5.9 7.2 7.5 8.7 9.1 4.6 6.2 6.0 5.9 6.5 7.2 6.7 6.7 6.8 7.4 7.0 6.4 5.8 6.3 6.6 8.2

ηOO [%] 94.0 94.6 93.6 94.6 94.7 91.4 66.5 77.9 84.8 90.6 89.6 92.5 93.9 95.6 90.4 95.6 94.6 94.7 94.9 94.5 95.0 95.5 94.7 96.4 94.4 91.9 90.7 91.8 90.7 86.6 82.4 82.8 86.6 90.2 94.8

Steady state time [min] 48 22 43 17 9 39 17 18 27 27 33 20 23 17 30 10 73 62 18 30 20 32 15 20 97 53 98 73 68 35 26 21 17 62 40

Oxygen carrier Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Sinfin Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran Mangagran


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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

36 180 Wood char 37 180 Wood char 38 180 Wood char 39 180 Wood char 40 160 Wood char 41 150 Wood char 42 160 Wood char 43 160 Wood char 44 180 Wood char Table 3. Test compilation

971 999 970 889 887 891 892 925 925

681 688 573 888 569 1178 1232 950 1238

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7.9 7.3 6.6 10.3 11.4 8.4 8.9 7.8 7.2

88.3 92.0 88.3 77.9 83.9 76.9 76.0 87.5 82.6

45 23 70 203 20 32 15 117 153

Mangagran Mangagran Mangagran Mesa Mesa Mesa Mesa Mesa Mesa

3.2 Performance evaluation 3.2.1 Sinfin The Sinfin manganese material was used for three experimental days, in which 14.6 h of continuous operation with fuel were conducted. Besides wood char, petcoke was used as fuel to compare the oxygen carrier’s performance with ilmenite, which had been tested with the same fuel in the same unit11, 20, 27. The fuel power with wood char was 5 kW, except on the first two tests, where it was 2.7 kW. Due to problems with the fuel feeding system, the fuel power with petcoke was difficult to control and ended up in the range of 8-20 kW.


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Carbon capture efficiency [%]

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Energy & Fuels

Oxygen demand [%]

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Figure 3. Oxygen demand and carbon capture efficiency during operation with Sinfin oxygen carrier and wood char as fuel. The arrows mark an increase in the air reactor gas flow Figure 3 shows oxygen demand and carbon capture efficiency using Sinfin oxygen carrier and wood char as fuel. During the depicted 2-hour period, operation was stable with a fuel reactor temperature of 970°C, except for a high-temperature test at 1000°C, which lasted around 45 minutes, see box in the figure. Increasing the temperature to 1000°C slightly improved the gas conversion. No oxygen carrier material was added during the test. In the beginning of the experiment, the oxygen demand dropped below 4%. However, a trend towards higher oxygen demand, i.e., lower gas conversion is observable throughout the period. To keep the solids circulation stable, the air reactor flow was increased four times during the test period, marked by arrows in Figure 3. With that measure, it was possible to stabilize the oxygen demand at around 6.5-7%. The general trend of a high gas conversion in the beginning of each test period and after refilling of elutriated particles could be observed for all tests with Sinfin material. This could be interpreted as an indicator that the solids inventory used in the tests was at the lower limit of


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Energy & Fuels

what is needed to keep adequate circulation when operating with this particular oxygen carrier. It shall be noted that the 10 kW unit does not feature a continuous recirculation of elutriated oxygen carrier or make-up stream of new material. This leads to a steady decrease of the solids inventory during operation and makes it difficult to maintain a constant circulation. In an additional experiment, petcoke was used as fuel for around one hour. In contrast to the bigger wood char particles, this fuel was fed to the reactor system as particles with an average diameter of 79 µm. During the experiment, a piece of metal which probably had been in the fuel container, was jammed in the coal screw and slightly deformed it. This led to a higher fuel flow, which had to be balanced by a lower revolution speed of the screw. Until this problem was identified, the fuel flow was higher than intended and reached around 28 kW at times, with an average of 17 kW over the whole experiment. The results thus have to be interpreted with the background of a low oxygen carrier-to-fuel ratio. Figure 4 shows a period of 20 minutes, in


which stable operation was reached. The average fuel power during that period was 12.5 kW.

Oxygen demand [%]

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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Figure 4. Oxygen demand and carbon capture efficiency for operation with Sinfin oxygen carrier and petcoke as fuel An oxygen demand of 12% and a carbon capture efficiency of 78% could be reached on average in that period, which means that the oxygen carrier performs worse with petcoke than with wood char. The char oxygen demand, as defined in section 2.2.1, was 5.1%. The higher oxygen demand for petcoke compared to wood char is consistent with previous measurements28. Figure 5 shows average values for oxygen demand and carbon capture efficiency for all test periods with Sinfin. 20

100

16

80

Carb. capt. eff. Oxygen demand

12

Pet coke experiments

60

8

40

4

20

0

0 0

2

4

6

8

10

12

14

16

18

20

22

24

Test no. Figure 5. Average values for oxygen demand and carbon capture for all Sinfin test periods Tests 22 and 23 were conducted at a higher temperature. In tests 7 and 8, petcoke was used as fuel. Experiments were spread over three days, separated by vertical dashed lines in the diagram. After day one and two and in addition prior to test 17, fresh oxygen carrier material was added to the reactor to compensate for attrition losses. A clear correlation between higher solids inventory


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and better gas conversion, i.e., lower oxygen demand can be seen; during tests 1-5, the gas flow to the air reactor and with it the circulation index was increased. However, the oxygen demand increased as well instead of decreasing as was the case with the other tested oxygen carriers. It is not until test 6, when the gas flow to the air reactor was increased to an unusually high value, that this trend is broken. The same is true for tests 16-19, in which the air reactor flow was first held constant and then increased. The oxygen demand then dropped distinctly in test 20, before which elutriated particles were added. An even stronger increase in oxygen demand than in those cases was detected during tests 11-14, when the air reactor flow was gradually decreased. Here, the two effects which opposed each other in the first two examples, namely decreasing solids inventory and increasing circulation index, both act in the same direction of worse gas conversion. As a side effect of this superimposition, no clear trend can be seen when plotting the oxygen demand against the circulation index. However, the expected trend of a decreasing carbon capture efficiency is still present, see Figure 6. The figure contains all tests shown in Table 3 apart from tests 7 and 8, which were conducted with pet coke as fuel, and tests 22 and 23, which were high-temperature tests.


20

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12

100

80 8 60

40 4 Carb. capt. eff.

20

Oxygen demand 0

0 0

400

800

1200

1600

Circulation index [kPa/(l/min)] Figure 6. Oxygen demand and carbon capture efficiency versus circulation index for Sinfin

3.2.2 Mangagran Mangagran was used for three days, during which a total experimental duration of 16 h was

12

100

10 90 8 6

80

4

Carb. capt. eff. Oxygen demand

2 0

70

60 0

20

40

60


reached. The average fuel power was 3.6 kW, with one short test at around 2.7 kW.

Oxygen demand [%]

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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80 100 120 140 160 180 200 220 240 260 280 300

Time [min]


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Figure 7. Oxygen demand and carbon capture efficiency during operation with Mangagran oxygen carrier and wood char as fuel. The arrows mark an increase in the air reactor gas flow Figure 7 shows oxygen demand and carbon capture efficiency using Mangagran oxygen carrier and wood char as fuel during a 5-hour period. The fuel reactor temperature was kept stable at around 970°C. At three occasions (at t=53, 156 and 211 minutes, marked by arrows in the figure), the solids circulation was increased, every time followed by a decrease in both oxygen demand and carbon capture efficiency. No oxygen carrier material was added to the reactor during the 5-hour period. The fuel flow was stopped at t=150 minutes for about 5 minutes. Fuel was refilled to the tank at t=185 minutes, resulting in some fluctuations of the fuel flow. Due to operational problems with the steam generators, the fuel reactor had to be co-fluidized with nitrogen for a short period of time starting at around t=227 minutes. The char oxygen demand after the two fuel stops was 4.7 and 4.4%, respectively. Figure 8 shows average values of oxygen demand and carbon capture efficiency in all test periods with Mangagran.


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Figure 8. Average values for oxygen demand and carbon capture for all Mangagran test periods The abovementioned 5-hours experiment represents tests 24-27 in this picture. Tests 28-38 were conducted during another day. Fresh oxygen carrier particles were refilled before test 28 and test 34. In both cases, oxygen demand stayed more or less at the same level as before although the air reactor gas flow was decreased both times, see Table 3. Test 37 was conducted at a higher fuel reactor temperature (around 1000˚C) and as expected, overall performance increased. A further decrease in oxygen demand was seen when returning to a lower temperature, which can probably be explained by the dropping fuel level in the tank and the consequent decrease in fuel flow. The same applies for the comparison of tests 31 and 32. In general, a higher air reactor gas flow, causing a higher solids circulation, resulted in both lower oxygen demand and lower carbon capture efficiency. This correlation is shown in Figure 9. As was the case with Sinfin, the high-temperature test (no. 37) is not included in this figure.


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12

100

80 8 60

40 4 20

Carb. capt. eff. Oxygen demand 0

0 200

400

600

800

1000

1200

Circulation index [kPa/(l/min)] Figure 9. Oxygen demand and carbon capture efficiency versus circulation index for Mangagran

3.2.3 Mesa A first attempt to use Mesa ore in the 10 kW unit had to be aborted due to agglomeration of the oxygen carrier. During that test, defluidization of parts of the reactor had occurred. As it was unclear if a general agglomeration tendency of the material or the temporary defluidization had caused this failure, another test with Mesa ore was conducted, starting at lower temperatures to try to avoid agglomeration. A total of 11.5 hours of operation was reached at an average fuel power of 3.9 kW.


24


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Oxygen demand [%]

Page 25 of 39

Figure 10. Oxygen demand and carbon capture efficiency during operation with Mesa oxygen carrier and wood char as fuel. The arrows mark an increase in the air reactor gas flow. Note axis break from t=90 min to t=190 minFigure 10 shows oxygen demand and carbon capture efficiency using Mesa ore as oxygen carrier and wood char as fuel during almost six hours of operation, corresponding to tests 41 to 44. During the first period until t=87 minutes, the fuel reactor temperature was around 890°C, which was increased to 925°C in the second period starting at t=193 minutes. The system performance improved slightly by increasing the temperature. The char oxygen demand as defined in section 2.2.1 was 4%. At comparable fuel feeding rates, the CO2 concentration in the fuel reactor exhaust gases was lower as compared to the other oxygen carriers. Although the carbon capture was lower with Mesa, the carbon lost to the air reactor does not make up for the lower CO2 concentration in the fuel reactor, which indicates a poorer solid fuel conversion. That outcome was expected due to the comparably low fuel reactor temperature. In addition, the CO2 concentration decreased with


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time. During the second test period, particles were refilled at t=346 minutes, which can be seen as a decrease in oxygen demand. This effect is masked by refilling the fuel tank at around t=360 minutes, which influenced the fuel flow as described in section 3.2.2. When attempting to increase the fuel reactor temperature further to reach the same temperature level as for the other oxygen carriers, circulation ceased and the experiment had to be aborted. Upon opening of the reactor, a block of sintered particles was found in the fuel reactor. Higher temperatures and other parameter changes could therefore not be tested. Figure 11 and Figure 12 give an overview of all conducted experiments with Mesa ore as oxygen carrier and the correlation of system performance and circulation index. Tests 43 and 44 were made at a fuel reactor temperature of 925°C, the rest at 890°C

Figure 11. Average values for oxygen demand and carbon capture for all Mesa experiments


26

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16

100

80

12

60 8 40 4

20

Carb. capt. eff. Oxygen demand 0

0 400

600

800

1000

1200

1400

Circulation index [kPa/(l/min)] Figure 12. Oxygen demand and carbon capture efficiency versus circulation index for Mesa

3.3 Comparison of the oxygen carriers Table 4 gives a comparison of the time-weighted averages of the performance of the three oxygen carriers tested. In the biomass experiments, the lowest oxygen demand was achieved for operation with Mangagran, the highest carbon capture with Sinfin. It should be observed that the performance numbers for Mesa were achieved with lower operating temperatures. At higher temperatures, this material probably would have shown better performance. However, due to the mentioned agglomeration problems, this was not possible to test. Comparing the petcoke experiments with Sinfin as oxygen carrier to older experiments in the same unit with ilmenite, the performance of Sinfin is clearly better. With ilmenite as oxygen carrier using the same fuel in the same 10 kW unit, an oxygen demand of 20% and a carbon capture efficiency of 66% were reached20. Previous results with Buritirama manganese ore also showed better performance than ilmenite with 15% oxygen demand and 94% carbon capture 27. However, with Buritirama both fuel power and solids circulation were considerably lower than in


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the current study, which explains the higher value for carbon capture efficiency. The lifetime of Buritirama manganese ore was lower as well, see section 3.4. With petcoke, Buritirama and Sinfin have a significantly lower oxygen demand as compared to ilmenite. Further, when using wood char, the other two manganese ores show an oxygen demand close to that of Sinfin, which leads to the conclusion that all four manganese materials are significantly more reactive than ilmenite. While quantification of the char gasification rate was not possible in this study due to char losses to the fuel reactor exhaust as described in section 2.2.1, it has been shown before that it is typically higher for manganese ores than for ilmenite29. Mangagran

Sinfin

Mesa

Ilmenite

Buritirama

970°C

970°C

890/925°C

970°C20

960°C27

7.0

7.2

10.1/7.4

-

-

90.5

93.1

78.1/84.7

-

-

-

12.4

-

20

15

-

72.5

-

66

94

Oxygen demand (wood char) [%] Carbon capture (wood char) [%] Oxygen demand (petcoke) [%] Carbon capture (petcoke) [%] Table 4. Performance comparison of all oxygen carriers

3.4 Lifetime calculation The lifetime of the particles can be calculated according to equations (10) and (11) based on operation duration, total amount of fines produced and the solids inventory. For all oxygen


28

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carriers, an average solids inventory of 17.5 kg during operation was assumed. The resulting lifetimes are depicted in Table 5. Mangagran

Sinfin

Mesa

Buritirama 20

Hot fluidization > 600˚C [h]

29.2

32.3

23.1

n.a.

Fines produced [kg]

2.57

0.90

2.06

3.8

Fuel operation time [h]

16.0

14.6

11.6

10.5

Loss of fines [%/fuel hour]

0.92

0.35

1.01

2.1

Lifetime [Fuel hours]

109

284

99

48

Table 5. Relevant data for lifetime calculations All oxygen carriers showed higher lifetime than the previously tested Buritirama manganese ore. Of all tested materials, Sinfin clearly had the highest lifetime, 284 hours based on fines production during fuel operation, although the size distribution of Sinfin was such that a high percentage of the fresh material consisted of particles in the range 90-125 µm, i.e., close to our definition of fines.

3.5 Effect of operation on particles Whereas operation was rather smooth with Mangagran and Sinfin, the experiments with Mesa had to be aborted earlier due to the build-up of an agglomeration blocking about half of the fuel reactor cross section. When the reactor was emptied after operation with Mesa, a distinct change in average particle size had occurred. Figure 13 shows the change in the particle size distributions after operation for all materials. For the Mesa material, the fraction of material >355 µm increased significantly during operation.


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0.15

Change in size fraction [-]

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0.1 0.05 0 -0.05 -0.1 -0.15 355

Size [µm] Sinfin

Mesa

Mangagran

Figure 13. Change in particle size distributions after operation for all materials Upon closer analysis, it was found that a considerable mass fraction of the particles in the fuel reactor had formed clusters with a diameter of more than 500 µm. Figure 14 displays a light microscope picture of this size fraction, clearly showing microagglomerates consisting of several individual particles.


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Figure 14. Microagglomerates >500 µm formed after operation with Mesa Formation of these microagglomerates changes the fluid-dynamic properties of the material, leading to difficulties to achieve adequate circulation and likely contributing to the eventual failure in operation. Also for Mangagran, a noticeable increase in the size fraction >355 µm can be seen. However, in the case of Mangagran, this did not lead to operational problems, also because no big agglomerates were formed. For Sinfin, a few soft agglomerates were found upon reactor opening. Those fell apart when not handled carefully and are thus not seen as problematic. The mass fraction of particles sized 90125 µm decreased during operation, most likely due to the production of fines. Compared to Mesa and Mangagran, the share of particles >355 µm was small. Figure 15 shows light microscope images of calcined particles and used particles from the air reactor for all three materials. For Mangagran, there is a colour shift, from light to dark brown.


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The surfaces of the particles show signs of attrition in the form of cavities. For Sinfin, no major differences between fresh and used particles can be distinguished. As mentioned above, smaller Mesa particles showed a tendency to form microagglomerates, which are also found in the size range 180-212 µm.

Fresh Sinfin particles, 125-180 µm

Used Sinfin particles, 180-212 µm

Fresh Mangagran particles, 125-180 µm

Used Mangagran particles, 180-212 µm


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Fresh Mesa particles, 125-180 µm

Used Mesa particles, 180-212 µm

Figure 15. Pictures of “fresh” (calcined) and used particles Table 6 shows the density and crushing strength of calcined and used particles. The tap density is acquired by putting a 125-180 µm sample of particles in a graduated cylinder and measure its mass and volume. The volume is read after tapping the cylinder. Crushing strength is measured on particles in the size 180-212 µm as the average of 30 tests. Density [kg/m3]

Crushing strength [N]

Fresh Mangagran

1.89

2.6

Used Mangagran

2.0

2.6

Fresh Sinfin

2.05

3.9

Used Sinfin

1.95

3.2

Fresh Mesa

2.01

5.3

Used Mesa

2.02

5.7

Table 6. Comparison of “fresh” (calcined) and used particles: density and crushing strength. Neither density nor crushing strength of the materials changed significantly during the experiments. As established in30, crushing strength and attrition resistance are only moderately


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correlated. In this study, Mesa, the material with the highest crushing strength, had the shortest lifetime while Sinfin, which features the lowest crushing strength, performed best in that respect.

4 Discussion The differences in gas conversion between the oxygen carriers tested were fairly small. The Sinfin material, which was tested with pet coke as well, performed similarly as compared to Buritirama ore, which had been tested previously. Altogether, it can be concluded that the main differences between the tested manganese materials lie in their mechanical properties and durability rather than in their performance. With the data available, the reason for the sintering tendency of the Mesa oxygen carrier cannot be identified with certainty. To draw a conclusion as to that reason, further comparisons with other manganese ores would be necessary. Concerning the comparison between manganese materials and ilmenite, the findings are consistent with previous tests in the same unit and other chemical looping reactors, which showed that manganese materials generally allow for a higher gas conversion and carbon capture efficiency. The same studies show that the lifetime of these materials is an issue. In a 100 kW chemical looping unit, the lifetime of ilmenite as oxygen carrier was found to be around 700 hours based on fines production25, which is more than twice as high as the projected lifetime for Sinfin, the most durable manganese material tested in this study. However, ash fouling and ash removal rather than attrition might limit the lifetime as mentioned in the introduction of this paper.. Lyngfelt and Leckner26 assume a lower lifetime of ilmenite in a utility-scale chemical looping plant, 100-300 hours, due to interactions with ash. In the current study, Sinfin reached a projected lifetime of 284 hours based on fines production.


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Lyngfelt and Leckner also conducted a study of potential costs of large-scale CLC: Ilmenite and manganese ores are compared in an example where ilmenite has a price of 175 €/ton and a lifetime of 200 hours, whereas manganese ore has a price of 225 €/ton and a lifetime of 100 hours, leading to a cost of 2 and 5 €/ton CO2, respectively. This can be compared to an assumed cost for CO2 compression of 10 €/ ton CO2 from the same study. It should be stressed that this is an example, and that uncertainties in both lifetime and price are large. Assuming a similar lifetime of for instance 100 h for both materials would reduce the difference in cost from 3 to 1 €/ton CO2. On the other hand, the same study indicates that a reduction in oxygen demand by 5%-units would save around 2.5 €/ton CO2 through both lower investment and operating cost for the air separation unit required in the oxygen polishing step. As can be seen in the present study, manganese materials have the ability to reduce the oxygen demand significantly. Thus, it is clear that manganese ores have a potential for reducing costs, provided that their price does not differ too much from ilmenite. An interesting option is to mix these materials, as previously done by Linderholm et al.19, who almost halved the oxygen demand using a mixture of ilmenite and manganese ore.

5 Conclusions A 10 kW chemical looping combustor has been operated for a total of around 42 hours with three different manganese materials called Mangagran, Sinfin and Mesa as oxygen carriers and wood char as fuel. All materials showed high reactivity, resulting in low oxygen demand. As compared to a previously tested manganese ore, production of fines was lower. The main findings are:


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•

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At 970°C fuel reactor temperature and using wood char as fuel, Mangagran and Sinfin reached low oxygen demands (7.0 and 7.2%) and high carbon capture efficiencies (90.5 and 93.1%) at stable operating conditions.

•

The Mesa material could not be operated at the same temperatures as Mangagran and Sinfin due to its tendency to form both micro- and macroagglomerates already at lower temperatures. Consequently, the material was discarded as a suitable candidate.

•

The lifetime deduced from production of fines for Mesa, Mangagran and Sinfin was 99, 109 and 284 hours, respectively. For Sinfin, this is almost six times more than what was achieved with the previously tested Buritirama ore. This is a clear indication that some manganese ores should be able to combine high gas conversion with reasonable lifetime.

•

As for the comparison of Mangagran and Sinfin, the former had a lower lifetime and formed a higher fraction of microagglomerates while the latter’s performance was more easily influenced by changes in the system’s solids inventory.

•

Compared to previous tests with the state-of-the-art oxygen carrier ilmenite in the same unit using the same petcoke as fuel, Sinfin showed significantly better performance; the average oxygen demand with petcoke was 12.4% compared to 20% for ilmenite. As the two other ores, Mangagran and Mesa, showed an oxygen demand very similar to Sinfin, it is evident that all the tested manganese materials are superior to ilmenite with respect to reactivity.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union’s Research Fund for Coal and Steel (RFCS) research program under grant agreement number [CT-201200006]. ABBREVIATIONS AND ACRONYMS: AR, air reactor; CCS, carbon capture and storage; CLC, chemical looping combustion; FR, Fuel reactor; LHV, lower heating value; , circulation index; $, molar flow; () , loss of fines; ∆*)+,- , mass of fines produced; *" , solids inventory; MexOy, oxidized oxygen carrier; MexOy-1, reduced oxygen carrier; ∆! " , pressure drop over air reactor riser; ∆., time of fuel operation; ./+)- , average particle lifetime; , , volume fraction of substance 0; , gas conversion; , carbon capture efficiency; Ω , oxygen demand.

6 References 1. 2. 3.

4.

Lewis, W., Gilliland, E., and Sweeney, M., Gasification of carbon Chemical Engineering Progress, 1951, 47(5): p. 251-256. Lyon, R.K. and Cole, J.A., Unmixed combustion: an alternative to fire. Combustion and Flame, 2000, 121: p. 249-261. Cao, Y., Casenas, B., and Pan, W.P., Investigation of chemical looping combustion by solid fuels. 2. Redox reaction kinetics and product characterization with coal, biomass, and solid waste as solid fuels and CuO as an oxygen carrier. Energy and Fuels, 2006, 20(5): p. 1845-1854. Scott, S.A., Dennis, J.S., Hayhurst, A.N., and Brown, T., In situ gasification of a solid fuel and CO2 separation using chemical looping. Aiche Journal, 2006, 52(9): p. 33253328.


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Leion, H., Jerndal, E., Steenari, B.M., Hermansson, S., Israelsson, M., Jansson, E., Johnsson, M., Thunberg, R., Vadenbo, A., Mattisson, T., and Lyngfelt, A., Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers. Fuel, 2009, 88(10): p. 1945-1954. Leion, H., Mattisson, T., and Lyngfelt, A., The use of petroleum coke as fuel in chemicallooping combustion. Fuel, 2007, 86(12-13): p. 1947-1958. Leion, H., Mattisson, T., and Lyngfelt, A., Solid fuels in chemical-looping combustion. International Journal of Greenhouse Gas Control, 2008, 2(2): p. 180-193. Lyngfelt, A., Chemical-looping combustion of solid fuels - Status of development. Applied Energy, 2014, 113: p. 1869-1873. Berguerand, N. and Lyngfelt, A., Chemical-looping combustion of petroleum coke using ilmenite in a 10 kWth unit-high-temperature operation. Energy and Fuels, 2009, 23(10): p. 5257-5268. Cuadrat, A., Abad, A., García-Labiano, F., Gayán, P., de Diego, L.F., and Adánez, J., The use of ilmenite as oxygen-carrier in a 500Wth Chemical-Looping Coal Combustion unit. International Journal of Greenhouse Gas Control, 2011, 5(6): p. 1630-1642. Linderholm, C., Cuadrat, A., and Lyngfelt, A. Chemical-looping combustion of solid fuels in a 10 kWth pilot- Batch tests with five fuels. 2011. Markström, P., Linderholm, C., and Lyngfelt, A., Operation of a 100kW chemicallooping combustor with Mexican petroleum coke and Cerrejón coal. Applied Energy, 2014, 113: p. 1830-1835. Ströhle, J., Orth, M., and Epple, B., Design and operation of a 1MWth chemical looping plant. Applied Energy, 2014, 113(0): p. 1490-1495. Thon, A., Kramp, M., Hartge, E.-U., Heinrich, S., and Werther, J., Operational experience with a system of coupled fluidized beds for chemical looping combustion of solid fuels using ilmenite as oxygen carrier. Applied Energy, 2014, 118(0): p. 309-317. Lyngfelt, A., 11 - Oxygen carriers for chemical-looping combustion, in Calcium and Chemical Looping Technology for Power Generation and Carbon Dioxide (CO2) Capture, Paul Fennell and Ben Anthony, Editors. 2015, Woodhead Publishing. p. 221254. Adánez, J., Gayán, P., Adánez-Rubio, I., Cuadrat, A., Mendiara, T., Abad, A., GarcíaLabiano, F., and de Diego, L.F., Use of Chemical-Looping processes for coal combustion with CO2 capture. Energy Procedia, 2013, 37(0): p. 540-549. Schmitz, M., Linderholm, C., and Lyngfelt, A., Performance of Calcium Manganate as Oxygen Carrier in Chemical Looping Combustion of Biomass in a 10 kW pilot, in 3rd International Conference on Chemical Looping, 2014: Göteborg. Sundqvist, S., Arjmand, M., Mattisson, T., Leion, H., Rydén, M., Lyngfelt, A. . Screening of different manganese ores for chemical-looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU). in 11th International Conference on Fluidized Bed Technology, CFB 2014. 2014, Beijing; China. Linderholm, C., Schmitz, M., Knutsson, P., and Lyngfelt, A., Chemical-Looping Combustion in a 100-kW Fluidized-Bed System using a Mixture of Ilmenite and Manganese Ore as Oxygen Carrier. Submitted for publication, 2015. Linderholm, C., Lyngfelt, A., Cuadrat, A., and Jerndal, E., Chemical-looping combustion of solid fuels – Operation in a 10 kW unit with two fuels, above-bed and in-bed fuel feed and two oxygen carriers, manganese ore and ilmenite. Fuel, 2012, 102(0): p. 808-822.


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21.

22.

23.

24. 25.

26. 27.

28.

29.

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Chemical-Looping Combustion of Solid Fuels ... - ACS Publications

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