Scholarly article on topic 'Highly attrition resistant oxygen carrier for chemical looping combustion'

Highly attrition resistant oxygen carrier for chemical looping combustion Academic research paper on "Chemical engineering"

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{"Oxygen carrier" / "Chemical looping combustion" / "Spray drying" / "Nickel oxide" / Magnesia / "CO2 capture" / Fluidized-bed}

Abstract of research paper on Chemical engineering, author of scientific article — Jeom-In Baek, Jungho Ryu, Joong Beom Lee, Tae-Hyoung Eom, Kyeong-Sook Kim, et al.

Abstract Chemical looping combustion (CLC) is expected to become one of the most competitive combustion technologies to reduce the cost of carbon capture and the cost of electricity for power generation. An oxygen carrier is used in the CLC to supply oxygen necessary for the fuel combustion. The oxygen carrier requires high reactivity and mechanical strength for a long time use under a cyclic redox reaction at high temperatures. This work was carried out to confirm that Mg-added NiO oxygen carrier with a high attrition resistant and high oxygen transfer capacity could be obtainable by using γ-Al2O3 mixed with MgO as support material. A NiO oxygen carrier was successfully formed by spray drying slurry containing 70 wt% NiO and 30 wt% support on a dry solid basis. The prepared oxygen carrier was characterized in terms of the physical properties related to the fluidization, such as shape, particle size, packing density, and mechanical strength. The prepared oxygen carrier was, especially, notified with its excellent spherical shape and high mechanical strength at a sintering temperature of 1,300 °C. The redox reactivity of the spray-dried oxygen carrier was measured with a thermogravimetric analyzer using 10 vol.% CH4 in CO2 balance as a fuel gas and air as an oxidizing gas. Water vapor addition to the fuel gas was performed to observe its effect on the reactivity. The oxygen transfer capacity of the oxygen carrier at the reaction temperature of 950 °C was over 12 wt%, indicating more than 80% of the total oxygen in the originally added NiO was transferred to the fuel. Water vapor addition made little change in the oxygen transfer capacity. This work indicates that γ-Al2O3 mixed with MgO could be a good support material for a highly attrition resistant spray-dried NiO oxygen carrier containing Mg component.

Academic research paper on topic "Highly attrition resistant oxygen carrier for chemical looping combustion"

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Procedía

Energy Procedía 4 (2011) 349-355 :

www.elsevier.com/locate/procedia

GHGT-10

Highly attrition resistant oxygen carrier for chemical looping

combustion

Jeom-In Baek*, Jungho Ryu, Joong Beom Lee, Tae-Hyoung Eom, Kyeong-Sook Kim,

Seug-Ran Yang, Chong Kul Ryu

KEPCO Research Institute, 103-16 Munji-dong, Yuseong-gu, Daejeon 305-380, Korea

Abstract

Chemical looping combustion (CLC) is expected to become one of the most competitive combustion technologies to reduce the cost of carbon capture and the cost of electricity for power generation. An oxygen carrier is used in the CLC to supply oxygen necessary for the fuel combustion. The oxygen carrier requires high reactivity and mechanical strength for a long time use under a cyclic redox reaction at high temperatures. This work was carried out to confirm that Mg-added NiO oxygen carrier with a high attrition resistant and high oxygen transfer capacity could be obtainable by using y-Al2O3 mixed with MgO as support material. A NiO oxygen carrier was successfully formed by spray drying slurry containing 70 wt% NiO and 30 wt% support on a dry solid basis. The prepared oxygen carrier was characterized in terms of the physical properties related to the fluidization, such as shape, particle size, packing density, and mechanical strength. The prepared oxygen carrier was, especially, notified with its excellent spherical shape and high mechanical strength at a sintering temperature of 1,300 °C. The redox reactivity of the spray-dried oxygen carrier was measured with a thermogravimetric analyzer using 10 vol.% CH4 in CO2 balance as a fuel gas and air as an oxidizing gas. Water vapor addition to the fuel gas was performed to observe its effect on the reactivity. The oxygen transfer capacity of the oxygen carrier at the reaction temperature of 950 °C was over 12 wt%, indicating more than 80% of the total oxygen in the originally added NiO was transferred to the fuel. Water vapor addition made little change in the oxygen transfer capacity. This work indicates that y-Al2O3 mixed with MgO could be a good support material for a highly attrition resistant spray-dried NiO oxygen carrier containing Mg component. (c©) 2011 Published by Elsevier Ltd.

Keywords: Oxygen carrier; chemical looping combustion; spray drying; nickel oxide; magnesia; CO2 capture; fluidized-bed

1. Introduction

The use of fossil fuels has continuously increased to produce energy for economic growth and human activities, and this trend will not be changed until new energy sources are available. Energy-related CO2 emission also increased with the fossil fuel consumption from 18.05 Gt in 1980 to 27.89 Gt in 2006. The CO2 emission will be 40.55 Gt in 2030 [1]. The emission of CO2 raised atmospheric CO2 concentration from about 295 ppm in 1880-1890 to about 315 ppm in 1958 and currently about 390 ppm [2, 3], resulting in the global warming issue. Climate change by global warming leads to ecological disturbances, an increased risk of natural disasters such as drought and flood, more intense storms and heat, sea level rise by glacier melting and sea water expansion, changes in crops, food and water supply, human health problems etc. The technological options which are mainly considered for the mitigation

ELSEVIER

doi:10.1016/j.egypro.2011.01.061

of climate change are (1) to improve energy efficiency with energy saving, (2) to switch to less carbon-intensive fossil fuels like natural gas, (3) to increase the use of low- and near-zero-carbon energy sources such as wind, solar, hydro power, and (4) to introduce CO2 capture and storage (CCS) [4].

Power generation is the largest CO2 emission industry. Fossil-fuel power plants emitted 11.4 Gt CO2, 41% of world total, in 2006 [1]. Advanced power generation systems with carbon capture and storage (CCS) are expected to contribute to the substantial CO2 emission reduction. CO2 capture technologies for the fossil-fuel power plants are largely classified into three categories: post-combustion, pre-combustion and oxy combustion. The main application of post combustion CO2 capture is the pulverized coal (PC) power plants. Pre-combustion CO2 capture is applicable to the integrated gasification combined cycle (IGCC) plants. Oxy-combustion uses gaseous oxygen separated from air to burn fuel, resulting in significant reduction of NOx emission and high CO2 content in the flue gas.

Chemical-looping combustion (CLC) is one of the most promising CO2 capture technologies. CLC combined with IGCC is reported as an innovative CO2 capture technology with highest cost reduction benefits [5]. A CLC process uses an oxygen carrier composed of metal oxide and support to supply oxygen instead of air or gaseous oxygen for fuel combustion. The oxygen carrier circulates two interconnected fluidized-bed reactors, an air reactor and a fuel reactor. The oxygen carrier receives oxygen from air in the air reactor and transfers the oxygen to fuel in the fuel reactor. Therefore direct contact between fuel and air is avoided. Natural gas and syngas from coal gasification are usually considered as fuel in CLC. Recently, direct use of solid fuel in CLC has been studied [6-8]. In CLC system, pure CO2 can be, in principle, obtained by removing water vapor in the flue gas emitted from the fuel reactor.

The efficiency of a CLC process varies greatly with the performance of an oxygen carrier. Sufficient oxygen transfer capacity, high reactivity, high resistance to attrition and agglomeration, long-term durability in reactivity and structural integrity, good physical properties for fluidized-bed applications and low production cost are required for a quality oxygen carrier. The performance of an oxygen carrier largely depends on the combination of a metal oxide and a support, a metal oxide content, and oxygen carrier preparation method. Proper combination of a metal oxide and a support should be selected according to the fuel type because the combustion characteristics are different according to the fuel type for the same metal oxide. To develop a large scale CLC process, a commercial technology for the mass production of oxygen carrier should be also developed, and the oxygen carrier produced by the commercial technology should be characterized in detail because it could have different physical properties and reactivity with those of the oxygen carrier produced in a small scale by laboratory preparation method.

For the combustion of gas fuel, NiO has been usually selected as active component because it showed higher reactivity than other metal oxides. As a support for NiO, Al2O3 has been extensively investigated because it can lead to good reactivity and high mechanical strength, and thermal stability applicable to the CLC processes. The transformation of NiO to NiAl2O4 spinel compound by strong interaction with Al2O3 during sintering process was disadvantage of the use of Al2O3. Therefore, the support materials that are less interactive with NiO have been preferred to increase the reactivity of the NiO oxygen carrier. For example, a-Al2O3, YSZ, NiAl2O4 and MgAl2O4 were recommended to obtain high reactivity due to the lower NiO-support interaction. However, high calcination temperatures above 1400 °C to obtain sufficient mechanical strength are required for the NiO oxygen carriers produced by commercial technology, spray-drying method, using these materials. Although a higher calcination temperature is favorable for higher mechanical strength, it increases NiO-support interaction which affect adversely on the reactivity. The higher calcination temperature will also increase oxygen carrier preparation cost. It was reported that Mg addition in NiO/Al2O3 oxygen carrier has the effect to minimize the sintering of the NiO and to improve the regenerability of the oxygen carrier and stability of spinel structure [9, 10]. NiO/MgAl2O4 oxygen carriers showed reduced NiO-support interaction, higher methane conversion, and less tendency for carbon formation [11, 12].

This work aimed at the development of a Mg-added NiO oxygen carrier with high mechanical strength and high reactivity at a lower sintering temperature. To reduce the sintering temperature, y-Al2O3 mixed with MgO was introduced as support. The physical properties and reactivity of the spray-dried oxygen carrier were characterized to confirm that y-Al2O3 mixed with MgO could be used as support material for quality Mg-added NiO oxygen carrier .

2. Experimental

2.1. Preparation of oxygen carriers

An oxygen carrier was prepared by spray drying method, which can produce spherical particles in a large scale. The composition of raw materials was 70 wt% NiO as an active component and mixture of 25.8 wt% y-Al2O3 and 4.2 wt% MgO as a support. In this work, y-Al2O3 was selected as main component of the raw support materials because it can give higher mechanical strength to the resultant oxygen carrier at a lower sintering temperature compared to other support materials with stable structure such as a-Al2O3 and MgAl2O4. All raw materials used in this work were commercially available products with a powder form. The NiO was fully mixed with the support materials in pure water. The suspension was comminuted with a ball mill to control particle size and to make homogeneous colloidal slurry. The homogenized slurry was pumped through a spray nozzle, and the slurry was atomized into droplets inside a hot-air chamber. The droplets dried while falling down to the bottom of the hot-air chamber. The collected spherical particles (green body) were sintered in air at 1,200 and 1,300 °C in a muffle oven for 5 h after pre-drying at 120 °C overnight.

2.2. Characterization of oxygen carriers

The spray-dried oxygen carrier was characterized for the physical properties related to fluidization and reactivity. The attrition resistance is the most important physical property to be considered first because it decides whether the prepared oxygen carrier can be used in an actual CLC process. The attrition resistance of the sintered oxygen carrier was measured with a modified three-hole air-jet attrition tester based on ASTM D 5757-95, which is the standard fluidized-bed test method for determination of attrition and abrasion of powdered catalysts. The attrition resistance was determined at 10 standard liters per min (slpm) over 5 h, as described in the ASTM method. The attrition index (AI) is the percent of fines generated over 5 h. The fines are particles collected at the thimble, which was attached to the gas outlet.

AI = [total fine collected for 5 h/amount of initial sample (50 g)] x 100 % (1)

Lower AI values indicate better attrition resistance of the bulk particles. The AI (22.5%) of the commercial fluid catalytic cracking (FCC) catalyst, Akzo, was used as a comparison standard.

The shape and morphology of the oxygen carrier were observed using a scanning electron microscope (SEM) (JEOL JSM 6400). The particle size distribution and average particle size of the sample were measured using a MEINZER II sieve shaker according to the directions described by the American Society for Testing and Materials (ASTM) E-11. The packing density of the sample was determined using the Autotap instrument (Quantachrome) described in ASTM D 4164-88. The Brunauer-Emmett-Teller (BET) surface area of the sample was determined by N2 physisorption using an ASAP 2420 (Micromeritics Inc.) automated system. Porosity was determined from Hg intrusion data collected with an AutoPore IV 9500 (Micromeritics Inc.). The distributions of surface area and pore volume by pore size were determined by using the Hg intrusion data.

2.3. Reactivity investigation in TGA

The reactivity of the prepared oxygen carrier was measured with thermogravimetric analyzer (Thermo Cahn TherMax 500). Approximately 20 mg sample was placed on the alumina crucible and heated to the reaction temperature of 950 °C in a nitrogen atmosphere at ambient pressure. After the balance of the TGA was stabilized, the reaction was initiated by switching the inert gas to the fuel gas. To simulate the oxygen carrier circulation between the air reactor and the fuel reactor in an actual CLC process, cyclic reduction and oxidation reaction was carried out. The fuel gas composition was 10 vol.% CH4 in CO2. CO2 was used as balance gas to better simulate an actual fuel reactor, and it has the effect of reducing carbon deposition on the oxygen carrier. Air was used as an oxidizing gas. Nitrogen was introduced after each reduction and oxidation reaction to prevent direct contact between the fuel and the air. The total flow rate of gas for each period was kept at 0.15 slpm. The weight change of sample and the furnace temperature data were continuously recorded once per second by a data acquisition unit. To investigate the effect of water addition, 10 vol.% of water vapor was supplied to the fuel gas.

The reactivity of the oxygen carrier was analyzed in terms of oxygen transfer capacity and oxygen transfer rate. Oxygen transfer capacity is the maximum percentage of mass change by oxygen transfer under the given reduction conditions and it was calculated on the basis of the mass of the fresh oxygen carrier in its fully-oxidized state. The oxygen transfer rate is expressed as the milligram-mole of transferred oxygen per unit gram of oxygen carrier per unit time (mg-mol O-goc-1-s-1). The degree of conversion, X, of a metal oxide is defined as:

X = (2)

mox_mred

where m is the instantaneous mass of an oxygen carrier, and mox and mred are the theoretical masses of the OC in its fully-oxidized and fully-reduced state, respectively, when it is assumed that all oxygen in the initially added NiO in the raw material is transferrable, or there is no interaction between NiO and supports. The difference between mox and mred in eq. 2 is the theoretical maximum amount of oxygen that can react with the fuel. Oxygen utilization is percentage of the degree of conversion during the reduction reaction.

A mass-based conversion, a>, was used to compare the mass change as a function of time. It is defined as:

a =--(3)

The oxygen ratio, the theoretical maximum mass fraction of oxygen that can be transferred to the fuel, of pure NiO is 0.214. Therefore, the theoretical mass-based conversion of the oxygen carrier prepared using 70 wt% NiO is 0.849 in its fully-reduced state.

3. Results and discussion

3.1. Physical properties

The shape and surface morphology of the Mg-added NiO oxygen carrier sintered at 1,300 °C were shown in Figure 1. It had excellent spherical shape which can reduce the attrition loss caused by irregular shape. The surface image magnified by 5,000X indicates that the raw materials were adhered to each other by sintering process. Other physical properties of the oxygen carrier were summarized in Table 1. The attrition resistance of the prepared oxygen carrier sintered at 1,300 °C was much higher than that of commercial FCC catalyst. This temperature to obtain sufficient mechanical strength for fluidized-bed applications is higher than that (1,000-1,100 °C) of the spray-dried oxygen carrier prepared using 70 wt% NiO and 30 wt% y-Al2O3 only [13]. It indicates that Mg addition increases the sintering temperature for sufficient attrition resistance. However, the sintering temperature, 1,300 °C, is lower than that (above 1,400 °C) of other Mg-added spray-dried NiO oxygen carriers prepared using more stable supports, a-Al2O3 mixed with MgO or MgAl2O4 [14-16], indicating that the use of y-Al2O3 has the effect of

Figure 1. SEM images of the spray-dried NiO oxygen carriers.

Table 1. Physical properties of the spray-dried oxygen carriers.

Sintering Average Tapped BET _Specific surface areaa_ Specific pore volume" Hg Attrition

temp. particle density surface / (cm2/g) / (cm3/g) Porositya Index

/ °C / (g/cm3) area Total Mesopore Macropore Total Mesopore Macropore / % / %

/ fjm / (m2/g)

1,200 93 2.1 - - - - - - - - 60.4

1,300 91 2.6 0.7 0.49 0.10 0.39 0.031 0.0003 0.031 7.7 4.1

aCalculated from Hg intrusion data

reducing the sintering temperature. A little high tapped density, low surface area, and low porosity were obtained due to the increased sintering temperature. They could adversely influence on the performance of CLC by increasing the energy required for fluidization and by reducing the active sites exposed on the surface. Most of the surface area and pore volume were provided by macropores, indicating collapse of small pores during sintering at high temperature. The particle size distribution of the prepared oxygen carrier sintered at 1,300 °C is shown in Figure 2. More than 96 wt% of the particles was in the range of 49 to 165 /m, and the highest faction was appeared at around 100 /m.

0.20 -

¡2 0.10 -

0.05 -

0 50 100 150 200 250 Particle size / pm

Figure 2. Particle size distribution of the spray-dried NiO oxygen carrier sintered at 1,300 °C.

3.2. Redox reactivity

The oxygen transfer capacities of the Mg-added spray-dried oxygen carrier sintered at 1,300 °C are presented as a function of number of cycles in Figure 3. The oxygen transfer capacity slightly increased up to third cycle. This increase seems to be caused by the structure change of the oxygen carrier during initial cyclic redox reaction. From the third cycle the oxygen transfer capacity was stabilized and it was around 12.7 wt%, indicating that 85% of the oxygen in the initially added NiO was transferred to the fuel. When water vapor was added in the fuel gas the oxygen transfer capacity slightly decreased, but it was still higher than 12 wt% (oxygen utilization above 80%). The oxygen carrier presented in this work is similar to that of the oxygen transfer capacity of the oxygen carrier prepared using 70 wt% NiO and 30 wt% y-Al2O3 only and sintered at 1,300 °C [13]. This implies that Mg addition made little difference in the oxygen transfer capacity.

Number of cycles

Figure 3. Oxygen transfer capacities as a function of number of cycles for the Mg-added spray-dried oxygen carrier sintered at 1,300 °C.

The mass-based conversion as a function of reaction time was given in Figure 4(a). After one minute from the beginning of the reduction, oxygen transfer capacity was around 10 wt% (oxygen utilization, 64%) and the reaction appears to slow down due to the beginning of the oxygen transfer from NiO which strongly interacted with support. Oxidation of the oxygen carrier was a little faster than reduction. When one minute passed, about 90% of the oxidation reaction completed. The oxygen transfer rate curve as a function of degree of conversion (Figure 4(b)) showed that highest oxygen transfer rate was appeared when the reaction was progressed around 25-30% for both reduction and oxidation. This indicates that there was mass transfer resistance for the reactant gas to be diffused into the center of the oxygen carrier. There was little difference in the mass-based conversion and oxygen transfer rate by the addition of water vapor. In the CLC process, water vapor is produced during fuel combustion. Therefore, the effect of water vapor should be investigated in detail and considered in the process design.

Figure 4. (a) Mass-based conversion as a function of time and (b) oxygen transfer rate as a function of degree of conversion for the reduction and oxidation of the Mg-added spray-dried oxygen carrier sintered at 1,300 °C.

4. Conclusions

The physical properties and reactivity of a Mg-added spray-dried oxygen carrier prepared using NiO (an active component) and y-Al2O3 mixed with MgO (a support material) were investigated to check the suitability of the prepared oxygen carrier for CLC of methane. The prepared oxygen carrier had high attrition resistance at the sintering temperature of 1,300 °C, excellent spherical shape, and proper average particle size along with narrow particle size distribution. Mg addition required higher sintering temperature, which led to a little high tapped density, low surface area, and low porosity. Despite the low surface area and porosity, the prepared oxygen carrier had high oxygen transfer capacity over 12 wt% (oxygen utilization above 80%) and oxygen transfer rate about 0.22 mg-mol O-goc-1-s-1. Water vapor addition into the fuel gas had little influence on the reactivity. The experimental result in this work suggests that y-Al2O3 mixed with MgO could be a good support material to obtain a highly attrition resistant spray-dried NiO oxygen carrier containing Mg component. More detailed effect of Mg addition on the performance of the spray-dried NiO oxygen carrier and fuel conversion in a fluidized-bed reactor will be investigated in our future work.

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