A Novel Hybrid Chemical-Looping Oxy Combustor Process for the

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A NOVEL CHEMICAL LOOPING OXY COMBUSTOR PROCESS FOR COMBUSTION OF SOLID AND GASEOUS FUELS: THERMODYNAMIC ANALYSIS Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502389t • Publication Date (Web): 22 Dec 2014 Downloaded from http://pubs.acs.org on December 31, 2014

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A NOVEL CHEMICAL LOOPING OXY COMBUSTOR PROCESS FOR COMBUSTION OF SOLID AND GASEOUS FUELS: THERMODYNAMIC ANALYSIS

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Energy & Fuels ef-2014-02389t.R1 Article 18-Dec-2014 Shah, Kalpit; The University of Newcastle, Newcastle Institute for Energy and Resources Zhou, Cheng; The University of Newcastle, Newcastle Institute for Energy and Resources Song, Hui; The University of Newcastle, Newcastle Institute for Energy and Resources Doroodchi, Elham; The University of Newcastle, Chemical Engineering Moghtaderi, Behdad; The University of Newcastle, Chemical Engineering

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Energy & Fuels

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A

NOVEL

CHEMICAL

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COMBUSTOR PROCESS FOR COMBUSTION OF

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SOLID

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THERMODYNAMIC ANALYSIS

AND

LOOPING

GASEOUS

OXY FUELS:

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Kalpit Shah, Cheng Zhou, Hui Song, Elham Doroodchi, Behdad Moghtaderi ∗

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Frontier Energy Technology

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Discipline of Chemical Engineering, School of Engineering,

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Faculty of Engineering and Built Environment,

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The University of Newcastle, Callaghan, NSW 2308, Australia

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12 13

∗ Corresponding author; Ph: +61-2-4044-9062; Fax: +61-2-4033-9383

E-mail address: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT

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The larger reactor volume, additional oxygen polishing unit and carbon stripper for separation of

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oxygen carrier and ash in the chemical looping combustion (CLC) and/or chemical looping oxygen

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uncoupling processes for solid fuels are anticipated not only to incur operational complexity but also

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increase the capital and operating costs. As an alternative, this paper proposes a novel hybrid process -

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called “Chemical Looping Oxy Combustor (CLOC)”. This novel process provides an integration of

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chemical looping air separation (CLAS) with fluidized bed oxy-fuel combustion and is expected to

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eliminate the need for an additional oxygen polishing unit and carbon stripper. It can be retrofitted to

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any existing coal CFB at low cost. The other advantages of CLOC includes less solid handling issues,

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flexibility in handling low grade coal with high moisture, no/less contamination of oxygen carriers,

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no/less slip of CO2/SOx in air reactor and low energy penalty etc. Also, in CLOC process coal

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combustion will occur in a separate fluidized bed combustor with relatively faster kinetics due to the

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availability of high oxygen concentration (i.e. ~25-28 vol%) which eliminates the need for a larger fuel

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reactor volume. In the current paper, thermodynamic simulations of CLOC process using Cu-, Mn-,

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and Co- based metal oxide oxygen carriers were carried out. Their performances were also compared

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against the conventional air-firing and oxy-firing technologies, e.g. oxy-fuel combustion integrated

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with cryogenic air separation unit (CASU) and chemical looping oxygen uncoupling (CLOU). It was

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identified that CLOC process needs external heat for reduction reactor provided by either direct or

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indirect methane combustion. Moreover, a maximum plant thermal efficiency was achieved for CLOC

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using Cu-based oxygen carrier. The energy penalty of CLOC process, compared with the air-firing

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base case, was found to be approximately 2-3%, about 4-5 times smaller than those of the CASU cases

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and only half of that of the CLOU process, indicating that CLOC offers a promising option for

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combustion of solid fuels.

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KEYWORDS: Chemical looping, oxy combustor, air separation, solid fuel combustion, clean coal

25 2 ACS Paragon Plus Environment

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1

Introduction

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Combustion of fossil fuels for power generation is one of the major sources of CO2 emissions. Among

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the fossil fuels, coal is the most common solid fuel and generates more than 40% of the total global

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electricity. Consequently recent efforts have been concerted towards developing advanced clean coal

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technologies for cleaner power generation. This includes the development of post-combustion capture,

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pre-combustion capture and oxy-firing processes. Several findings on techno-economic assessments of

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these processes are available in the existing literature, indicating the cost of CO2 capture roughly

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estimated at US $20-90/tone of net CO2 capture, of which roughly 60% cost is mainly associated with

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CO2 capture unit operations such as pre- or post- combustion capture unit and oxygen production plant

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etc.1-5 As one of the leading oxy-firing techniques for CO2 enrichment of flue gas, chemical looping

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combustion (CLC) has been extensively studied. So far, chemical looping research is focused on the

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gaseous fuels. Solid fuels, however, is much more difficult for CLC application due to many reasons,

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in particular, the separation of oxygen carrier and coal ashes as well as the slow kinetics for solid-solid

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type of reaction. Therefore, innovation in CLC for more efficient solid fuel combustion has gained

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more interests in the recent years.5-12 The first study on CLC of solid fuels was done by Lewis and his

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co-workers in the 1951 using Cu and Fe oxides.12 Following that today there are a number of

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publications on laboratory and pilot scale CLC units regarding the use of solid fuels.13-23 In recent

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efforts, even a large scale - 1 MWth demonstration plant was commissioned in collaboration with

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several European universities and Alstom at the Darmstadt, Germany.14 To this point, there are two


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key methods, i.e., direct and indirect CLC, for solid fuels as shown in Figures 1 and 2.13 In indirect

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CLC, as shown in Figure 1, coal is firstly gasified to syngas consisting mainly of CO and H2 and then

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burnt in CLC. However, to obtain nitrogen free syngas the gasification needs to be carried out with

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O2/CO2/H2O environment and thus an additional energy intensive air separation unit for the oxygen

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production would be needed. For direct CLC, as highlighted in Figure 2, the solid fuel is directly

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introduced into the fuel reactor where the fuel oxidation takes place by the reduction of oxygen carrier

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particles without any need for air separation.

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CLC uses Fe2O3 or NiO based oxygen carriers. With existing body of knowledge for direct CLC, the

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gasification process has been identified as the major controlling step in this process.13 Char

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gasification is generally a slow process, and the solids exiting from the fuel reactor could contain some

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unburnt char and residual ash together with the oxygen-carrier. The CO2 capture efficiency with direct

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CLC is expected to reduce significantly if non-gasified char particles are bypassed to the air reactor.

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Moreover, SOx emissions may also be realized from air reactor if char/ash slips from the fuel reactor

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to air reactor. In such case, char needs to be separated and recycled back to the fuel reactor. Also,

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residual ash needs to be separated before oxygen carrier and char are recycled to the air and fuel

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reactors respectively. This all would impose an additional requirement of the carbon stripper for solids

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separation. It has also been observed that full oxidation of the fuel reactor outlet stream could not be

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achieved due to the rate limiting gasification step and limited solid-gas contact time. Therefore, to

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complete the oxidation of unburnt gas compounds, a downstream oxygen polishing unit has been


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proposed, that is, injection of pure oxygen to the fuel reactor outlet stream. The requirements of

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oxygen production unit for both indirect and direct CLC, carbon stripper and larger reactor volume

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together with the issues related to solids handling, CO2 capture efficiency and SOX emissions clearly

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suggest that a step change solution is highly needed for the realization of CLC for solid fuels.

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As a step change solution, Lyngfelt and his co-workers12 proposed the principle of chemical looping

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oxygen uncoupling (CLOU). The schematic of CLOU process is similar to direct CLC as shown in

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Figure 2. However, in CLOU process transitional metal oxides such as CuO, Mn2O3 and CoO are used

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instead of Fe2O3 and NiO that are used in CLC. The advantage of such transitional metal oxide is that

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their reduction reaction occurs between 800 and 1000 oC. Thus, the oxygen released during metal

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oxide reduction can be reacted with coal as it occurs in a conventional coal fired CFB furnace. This

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way coal reactivity can be improved to a greater extent which may eliminate/reduce the need for an

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additional oxygen polishing unit. The CLOU process partially resolves the issues related to low char

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conversion and incomplete combustion via releasing oxygen for in situ char gasification followed by

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combustion. However, in CLOU as the solid fuel combustion occurs in a fuel reactor the operating

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temperature cannot be increased above about 1000oC due to the low ash fusion temperature. At this

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temperature, the oxygen concentration of the product stream in fuel reactor with CuO/Cu2O system

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can only reach ~12 vol%. This is much lesser than 25 vol% O2, which is generally recommended for

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the complete combustion in oxy-CFB.24 Therefore, the possibility of eliminating the carbon stripper

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and oxygen polishing unit with CLOU is yet uncertain unless higher solids residence time is provided


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in the fuel reactor. Moreover, the issues related to the solids handing, such as residual char and ash

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separation; and also losses, contamination, deactivation, attrition and fragmentation for oxygen carrier

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and CO2 and SOx emissions through air reactor are still not well solved. Also, flexibility of using low

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grade coal using CLC/CLOU is questionable. All the above stated issues have still initiated the need

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for an alternative solution.

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Keeping above challenges, developments and process configurations in mind, our research group at the

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University of Newcastle has developed a novel Chemical Looping Oxy Combustor (CLOC) process

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where oxygen is produced in a chemical looping manner using Chemical Looping Air Separation

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(CLAS)5-8 or Chemical Looping Oxygen Uncoupling (CLOU)12 principle, followed by the combustion

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of coal in a separate fluidized bed oxy-fuel combustor. The advantages of CLOC over CLC/CLOU are:

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1. It eliminates the need for an additional oxygen polishing unit and carbon stripper.

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2. The problems related to the solids handling and separation are minimised.

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3. The issues of oxygen carrier contamination and CO2 and SOx emissions are also anticipated to

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be resolved.

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4. It can be retrofitted to any existing coal combustor at possible lowest integration cost.

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5. It adds flexibility in handling low grade coal with high moisture in a chemical looping

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configuration.


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6. CLOC is a hybrid coal-natural gas power plant with carbon capture and storage (CCS). Any

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existing or new coal CFB can be integrated with CLAS to increase the power throughput,

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thermal efficiency as well as reduce the overall emission.

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7. Higher coal reactivity can be achieved in CLOC as coal combustion will be carried out in 26

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vol% oxygen environment. Moreover, higher metal oxide reduction rate can be obtained due to

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high reduction temperature. Thus, overall footprint required for the CLOC fuel reactor may be

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significantly lower than CLC or CLOU.

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8. It is expected to have the lowest energy penalty.

1.1

Chemical looping oxy combustor (CLOC) concept

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The CLOC proposed here relies on a chemical looping principle similar to that used in Chemical

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Looping Oxygen Uncoupling (CLOU) proposed by Mattisson and Lyngfelt in 200512 or Chemical

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Looping Air Separation (CLAS) proposed by Moghtaderi and his co-workers.5-8,10-11 The metal oxide

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oxygen carrier (MexOy) releases oxygen based on the difference between the equilibrium and actual

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oxygen partial pressure in the fuel reactor at a given temperature, through

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2 → 2 +

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This is followed by the combustion of the solid fuel in the same reactor by

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+

(1)

→ +


(2)

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Mattisson et al.12 and Shah et al.7 identified the most suitable oxygen carrier candidates for

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CLOU/CLAS, i.e. CuO/Cu2O, Mn2O3/Mn3O4, and Co3O4/CoO systems. The Cu, Mn, and Co oxides

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release oxygen in the fuel reactor through reactions (3)–(5) as shown below. Among these, CuO has

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been considered to be the most promising oxygen carrier due to its higher oxygen transport capacity

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and higher rate of oxygen release.

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4CuO ←→ 2Cu2O + O2 (3)

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6Mn2O3 ←→ 4Mn3O4+O2 (4)

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2Co3O4 ←→ 6CoO + O2 (5)

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Figure 3 illustrates the detailed configuration for CLOC process. Essentially, the CLOC process

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consists of an Integrated CLAS (ICLAS)/CLOU unit and a separate coal combustion CFB reactor with

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an in-situ sulfur removal capability. The description of ICLAS can be found elsewhere.5-8 Power is

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produced using a heat generated from coal combustion in oxy-CFB as well as using the recovered heat

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from the exhaust stream of the oxidation reactor. The hot CO2-concentrated flue gas emitted from oxy-

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CFB is then recycled back (~70%) to the reduction reactor as a sweep gas, with some of it (~30%)

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being emitted and sent to the CO2 Processing Unit (CPU) for capture. On the other hand, the

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separation of coal combustion and oxygen carrier reduction in CLOC process would require external

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energy input for the reduction reactor. This may be met by either natural gas combustion (via either

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direct or indirect integration as shown in Figures 3 (a) and (b), respectively), or by solar thermal


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heating. With this arrangement for CLOC, hybrid coal-solar or coal-natural gas based system with

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reduced coal feed rate can be designed. This design is expected to improve the efficiency as well as

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reduce the overall emission per unit of power generation.

4 5

2

Process Simulations

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Process simulations were carried out using process simulation package – ASPEN PLUS v7.3 to

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evaluate the performance of CLOC and identify the optimum oxygen carrier for CLOC process. Also,

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efficiency calculations were performed for CLOC process and comparisons have been made against

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the conventional air-firing CFB plant, oxy-firing CFB plant with Cryogenic Air Separation Unit

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(CASU), and CLOU. Table 1 summarises the process simulation cases investigated in this study.

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Figure 4 shows the Aspen simulation flowsheet of the CLOC process, in which the process model of

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the ICLAS block is given in detail in Figure 5. The low sulphur Australian coal is adopted as the solid

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fuel in the simulation. Its proximate and ultimate analysis has been shown in Table 2. In CLOC

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process, coal is firstly combusted in a CFB boiler. The oxygen for coal combustion is provided from

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the recycling O2 rich flue gas produced in reduction reactor. Such flue gas, however, contains some

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water introduced from methane combustion. Therefore, a water removal process i.e. condensation, (as

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illustrated by block ‘C‘ in Figure 3) is required before the recycled flue gas can be used in the CFB

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boiler. Power is produced using the heat generated from both coal combustion in the oxy-CFB boiler

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(via the existing steam power cycle) and heat from the oxidation reactor by introducing a new steam


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cycle (NSC). On the other hand, the reduced oxygen carriers are sent to the oxidation reactor, after re-

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oxidation with air, the oxygen carriers are then recycled back to the reduction reactor ready for

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attending next oxygen production cycle. For calculating the overall plant thermal efficiency, the steam

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cycle was simulated in ASPEN HYSYS. The user interface between ASPEN PLUS and ASPEN

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HYSYS models was created to feed data in the steam cycle.

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Table 2 shows the overall plant performances of the conventional 200 MWe air-firing case (i.e. Case

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0). For oxy-CFB combustion integrated with CASU (i.e. Cases 1, 2, 3, and 4), process simulation

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models were established in Figures 6 and 7 for the integrated plant and a conventional CASU unit,

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respectively. As Figure 6 shows, the oxygen produced from CASU unit is directly introduced to the

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CFB-boiler for complete coal combustion. More detailed configuration for compression and

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distillation operation units for CASU can be seen in Figure 7.

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Figure 8 shows the ASPEN PLUS model of CLOU process, in which coal is directly combusted along

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with oxygen carrier in the reduction reactor. About 70% of the flue gas (a figure suggested from

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literature12) is recycled back to the reduction reactor for fluidizing the particles and heat transfer

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purpose. In such process, power is produced mainly using the heat generated from the oxidation

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reactor via a new steam cycle (NSC), together with the energy associated with the exhaust from the

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reduction reactor and excess heat (if any) produced within the reduction reactor. Table 3 gives a list of

19

specifications of the key unit operation blocks used in Aspen Plus models.


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Results and Discussion

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3.1

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A series of typical operating conditions for the key components of CLOC process were specified. The

4

operating conditions include the use of direct methane combustion as the heat source, operating

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temperatures of the oxidation and reduction reactors in the ICLAS unit at 950°C and 1025°C,

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respectively, thermal efficiency of the NSC at 40%, pressure rise of all blowers at 25 kPa, and specific

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power requirement of CPU at 135 kWh per tonne CO2.25 The typical operating temperature of the

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reduction reactor at 1025°C was justified as it is the minimum temperature which can satisfy the

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requirement of 26 vol% O2 in the boiler inlet stream. The typical operating temperature of the

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oxidation reactor at 950°C was chosen so that a fast oxidation rate and a low equilibrium oxygen

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partial pressure at about 4% are present in the oxidation reactor.

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3.2

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Based on the previous work carried out on CLAS and CLOU processes in the literature, Cu-, Mn- and

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Co- oxides were among the best suitable oxygen carriers. For CLOC process, similar to CLOU, the

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major challenge is to produce an oxygen rich (at 26%) product stream in the reduction reactor. It was

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found that coal CFB boiler cannot be operated above about 1000oC due to low ash fusion temperature.

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Under such circumstances, as shown in Figure 9 cobalt oxide oxygen carrier was found to be the most

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suitable for providing desired oxygen concentration at possible lowest operating temperature (i.e.

Typical operating conditions

Identifying the optimum oxygen carrier


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895oC) in the reduction reactor. This is particularly true for CLOU process which uses single CFB

2

boiler for both coal combustion and reduction reactor. However, for CLOC the limitation of ash fusion

3

temperature does not apply to the reduction reactor since it is separated from coal combustion and can

4

operate at highest possible operating temperatures.

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For copper oxide, producing a product stream with 26 vol% O2 requires the reduction reactor to

6

operate at about 1040oC, whereas using manganese oxide the reduction reactor should operate at about

7

915oC (see Figure 9). It is reported in the literature that copper oxide oxygen carriers may have

8

agglomeration problems at such high operating temperature.12 However, reduction in the copper

9

loading on a substrate and increase in the sintering temperature may reduce the agglomeration issues.

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The trials with reduced copper oxide loading and higher sintering temperature are under way at the

11

University of Newcastle. The results of these trials will be published soon. However, from initial

12

experimental observations, it was identified that the reduced copper loading on a substrate may

13

increase the bed inventory but it can be further compensated by the increased reduction reaction rate at

14

higher temperature. For cobalt oxide, thermodynamically by keeping the reduction reactor temperature

15

50oC higher than the oxidation reactor temperature oxygen concentration >25% in the product stream

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can be achieved (i.e. oxidation temperature: 845oC and reduction temperature: 895oC). However,

17

simulation of CLOC process using cobalt oxide as the oxygen carrier did not converge because the net

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reaction between cobalt oxide reduction and methane combustion (i.e. CH4 + Co3O4) is endothermic at

19

894oC (see reaction 6). This leads to an insufficient heat for sustaining the reduction reactor operation.


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Therefore, in CLOC process for cobalt oxide to become feasible, solar/other forms of heat integration

2

is required. In contrast, copper oxide and manganese oxide, due to their exothermic nature of reduction

3

reaction with methane (see reactions 7 and 8), were found to be quite suitable for CLOC process.

4

4Co3O4 + CH4 ←→ 12CoO + CO2 + 2H2O

∆H = + 9.74 kcal/mol CH4 at 894°C

5

8CuO + CH4 ←→ 4Cu2O + CO2 + 2H2O

∆H = - 56.40 kcal/mol CH4 at 1025°C

6

12Mn2O3 + CH4 ←→ 8Mn3O4 + 2H2O + CO2

∆H = - 99.50 kcal/mol CH4 at 900°C

7

Based on the above conclusion, further process simulations were carried out for CuO/Cu2O and

8

Mn2O3/Mn3O4 system integrated with direct or indirect methane combustion, whilst for CoO/Co3O4

9

solar integration was simulated and investigated (see Figure 10). The performance results of CLOC

10

using different oxygen carriers are provided in Table 4. The details include power generation,

11

auxiliaries, and heat and mass balance results. Some key results are also extracted and presented in

12

figure forms to enable a more visualised comparison.

13

It can be seen from Table 4 that heat generation/requirement of the oxidation/reduction reactors is

14

about 60% higher for copper oxides compared to manganese oxide. As a consequence, for the CLOC

15

process using copper oxide, the external energy requirement met by methane combustion is greater and

16

thus it requires lager amount of methane. This also leads to a greater carbon emission at the end of

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CLOC process and therefore more power is consumed in the CPU plant as shown in Table 4. Another

18

significant difference between CLOC process using copper oxide and manganese oxide is the large


(6) (7) (8)

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quantity of manganese oxide requirement, as high as about 2.4 times of copper oxide requirement for

2

the proposed CLOC process. Also, copper oxide due to larger quantity of heat generation in the

3

oxidation reactor generates higher net power than manganese oxide. For cobalt oxide, the amount of

4

oxygen carrier was found to be significantly lesser than those of the other cases simply because it uses

5

solar as the heat source for the reduction reaction and thus does not need to generate extra amount of

6

oxygen for methane combustion.

7

Figure 11 presents the overall plant thermal efficiency of CLOC using different oxygen carriers. It

8

shows that plant thermal efficiency for methane integration cases does not significantly differ from

9

each other, suggesting that efficiency numbers for copper oxide and manganese oxide are similar.

10

Nevertheless as described earlier, it is clear that in practice there are significant differences. Figure 11

11

also shows the maximum plant thermal efficiency was obtained using cobalt oxide as the oxygen

12

carrier and with solar integration, with the second most maximum efficiency achieved by copper oxide

13

with indirect methane integration. However, it was realised from our own preliminary economic

14

analysis that solar integration at such a high temperature (> 800°C) requires the use of solar thermal

15

tower which is cost intensive and immature technology with inherent low capacity factor and heat

16

fluctuation during the day. Therefore, despite the fact that CLOC process using cobalt oxide with solar

17

integration was found to be the most efficient, it is considered to be economically unfeasible at this

18

stage and further investigation was not carried out. Nevertheless, for places which are particularly

19

suitable for hybrid coal-solar oxy-firing power plant and with sufficient cobalt and solar energy


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resources, a small scale CLOC process using cobalt-based oxygen carrier with solar integration may

2

prove to be an attractive option.

3

Comparing direct and indirect methane integration for CLOC, the results in Figure 11 indicates that

4

CLOC with indirect methane integration normally leads to a greater plant thermal efficiency than

5

direct integration. This is believed to be caused by the greater energy/exergy destruction associated

6

with direct methane integration. In the direct integration, the recycled flue gas mixes with methane

7

combustion product, which contains water and CO2, in the reduction reactor. The following steam

8

condensation and water removal processes together with recycled flue gas preheating inevitably incurs

9

more energy losses/exergy destruction compared to indirect configuration where methane combustion

10

occurs outside the reduction reactor. As such, the recoverable heat and power via FWH in the existing

11

steam power cycle for CLOC using indirect methane integration is significantly larger than those under

12

the direct integration (see Table 4). Although indirect methane integration is favourable for CLOC in

13

terms of thermal efficiency, the increased heat transfer issues on reactor design should also be

14

considered before the final configuration of CLOC process can be determined.

15 16

3.3

17

The previous section has identified Cu-based oxygen carriers as the optimum candidate because of

18

their lesser inventory requirements, higher oxygen transport capacity, higher efficiency and higher

19

power production potential. The literature also indicates their higher reactivity and comparable lower

Comparison with air-firing, oxy-firing with CASU integration, and CLOU


Energy & Fuels

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Page 16 of 47

1

material cost. Therefore, the following study focuses on the use of Cu-based oxygen carriers and

2

examines the plant performance of CLOC, which is then compared with those of air-firing, oxy-firing

3

with CASU integration, and CLOU.

4 5

Table 5 gives the detailed power cycle analysis, including power generation, auxiliaries, and heat and

6

mass calculations, for both air-firing and oxy-firing CFB plants operated at the typical operating

7

conditions (i.e. TOXI = 950°C, TRED = 1025°C). It should be noted that for CLOU process, however, the

8

reduction reactor temperature is set at 985°C instead of 1025°C because of the limitation posed by ash

9

fusion temperature in the fluidized bed.

10 11

Figure 12 shows the total energy penalty of oxy-firing CFB plants using various technology platforms.

12

It can be seen in Figure 12 that the energy penalty of CLOC process (i.e. oxy-firing CFB integrated

13

with ICLAS) is only 2.3% compared with the air-firing base case. This is about only 20 - 25% of those

14

of the CASU cases. The CLOC process, in particular, using ICLAS unit to produce oxygen for coal

15

combustion, is found to be responsible for such low energy penalty. Compared with CLOC, CLOU

16

process shows doubled energy penalty. This is mainly because of a relatively larger blower duty as a

17

result of a greater flue gas recycle rate in CLOU process, as well as greater CO2 emission per unit

18

power output in CLOU process (see Table 5). Therefore, thermodynamically CLOC was found to be a

19

better option over CLOU for oxy-firing of coal.


Page 17 of 47

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Energy & Fuels

1 2

Figure 13 gives the power consumption per unit O2 production for various oxygen production plants.

3

As it shows, the ICLAS unit in CLOC process consumes only 62 kWh per tonne of produced O2

4

whereas for CLOU the figure rises to 74 kWh (about 19% greater) per tonne of produced O2. Overall,

5

the specific power consumption of oxygen production process for both CLOC and CLOU was found

6

significantly lower than those of CASU units at 200-400 kWh per tonne O2.

7

Figures 14 and 15 show the plant thermal efficiency and efficiency gain, respectively for various oxy-

8

firing CFB plants. The results agree with previous conclusion and indicate that greater plant efficiency

9

and efficiency gain (compared to the conventional CASU case) are achieved for both CLOC and

10

CLOU. The above plant performance analysis clearly suggests that ICLAS has a technical advantage

11

over the CASU systems when used as the oxygen plant for oxy-firing CFB combustion.

12

It should be noted that though found better in efficiency, in comparison to CLOU, higher capital

13

investment will be required for CLOC mainly due to the addition of new steam cycle. However, due to

14

the increased power throughput and reduced emission per MW due to the introduction of natural gas,

15

higher net present values (NPV) for CLOC may be achievable.

16

3.4

17

Process simulation of CLOC process has shown superior performances to other parallel oxy-firing

18

technologies. For a complete evaluation of CLOC, various operating conditions have also been studied.

19

The impact of key controlling parameters on the CLOC process, including reactor operating

Impact of key controlling parameters


Energy & Fuels

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Page 18 of 47

1

temperatures, pressure drop of fluidized bed, thermal efficiency of NSC, and specific power

2

requirement of CPU unit are evaluated in detail.

3

3.4.1 Reactor operating temperature

4

Typical reactor operating temperatures were specified in the previous study, here the effect of varied

5

reactor temperatures on the overall plant performance of CLOC is examined. Figure 16 shows the

6

effect of reactor temperature on the equilibrium oxygen partial pressure and heat generation. It can be

7

seen in Figure 16 that the net heat generation per mole O2 slightly decreases as temperature increases.

8

This implies that theoretically, the net heat generated in the oxidation reactor would normally surpass

9

the heat required by the reduction reactor. However, due to the second-law of thermodynamics the

10

lower-temperature heat (oxidation reactor) cannot be used at the higher-temperature (reduction reactor).

11

Therefore, external heat supply must be met by either methane combustion or solar thermal heating.

12

Figure 16 also indicates that equilibrium oxygen partial pressure increases with temperature. It was

13

realized that the operating temperature of the reduction reactor at 1025°C cannot be varied much due

14

to the requirement of maintaining a high O2 concentration at about 26 - 28 vol% in the boiler inlet

15

stream. The temperature of reduction reactor can be reduced by adding steam with flue gas but that

16

may increase the exergy losses in the system. For the current work, this option has not been studied. In

17

contrast, the operating temperature of the oxidation reactor may be varied widely (e.g. from 850°C to

18

1010°C), as long as a favourable oxidation reaction rate is obtained and the equilibrium oxygen partial

19

pressure in the reactor is below the actual O2 concentration in air. 18 ACS Paragon Plus Environment

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Energy & Fuels

1

Following the above conclusion, we investigated the effects of oxidation reactor operating temperature

2

between 850°C and 1010°C on the overall plant thermal efficiency (Figure 18) and the specific power

3

requirement of the oxygen plant in CLOC process - mainly the power consumption for the air and flue

4

gas blowers - (Figure 17). As Figure 17 shows, an increasing oxidation reactor temperature has limited

5

impact on the specific power requirement of ICLAS over the range of 850 - 980°C, but tends to

6

increase at 1010°C. This can be explained by the complex interplay between air blower power

7

consumption (dependant on the actual air flow requirement) and the reactor operating temperature. On

8

one hand, the simulation shows that in order to use the air flow to fully recover the heat generated in

9

the oxidation reactor and subsequently use it in the NSC for additional power generation, the air flow

10

requirement is actually greater than that needed when the oxidation reactor operates at the equilibrium

11

state and results in a reactor oxygen partial pressure above that of the equilibrium state. The air flow

12

requirement is therefore predominantly determined by the amount of heat generated in the reactor and

13

the temperature difference between the inlet and outlet air streams. On the other hand, for operating

14

temperatures between 850°C to 980°C the equilibrium oxygen partial pressure in the oxidation reactor

15

is far less than 21 vol% as in the pure air inlet stream. Therefore, oxygen partial pressure in the reactor,

16

after O2 in air is being absorbed by the oxygen carrier, can be easily maintained above equilibrium

17

partial pressure. Since the amount of heat generated in the oxidation reactor and the temperature

18

difference over the oxidation reactor does not vary significantly over the examined temperature range


Energy & Fuels

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Page 20 of 47

1

(850°C to 980°C), the air flow requirement and thus the air blower power consumption over this

2

temperature range are not changed significantly (see Figure 17).

3

In contrast, at 1010°C the equilibrium oxygen partial pressure in the oxidation reactor is increased to

4

around 15 vol%, close to the 21 vol% as in the pure air stream. This poses a negative impact on the

5

oxidation reaction, that is, the actual oxygen partial pressure in the reactor, after O2 in air is being

6

absorbed by the oxygen carrier, can easily reach the equilibrium partial pressure. As a result, the air

7

flow requirement is now mainly determined by the reactor equilibrium oxygen partial pressure, leading

8

to a significantly increased air flow and therefore a noticeable increase in the air blower power

9

requirement (see Figure 17).

10

Comparing with the conventional and advanced CASU unit with either 99% or 95% O2 purity26-30,

11

Figure 17 shows that the ICLAS unit in CLOC process exhibits lower power requirement over a wide

12

range of oxidation reactor operating temperatures with only 61 – 99 kWh per tonne of produced O2.

13

This is about only 14 – 33% and 24 – 50% of the specific power requirement of conventional and

14

advanced CASU units, respectively.

15

The plant thermal efficiency of CLOC, as given in Figure 18, agrees with the previous conclusion and

16

shows negligible variation for oxidation reactor temperatures between 850°C to 980°C while

17

experiences a large decrease at 1010°C. The above results imply that for CLOC using CuO as oxygen

18

carrier, the oxidation reactor temperature should not exceed 980°C. Although as the oxidation reactor

19

temperature between 850°C to 980°C leads to similar thermodynamic performances, the selection of 20 ACS Paragon Plus Environment

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Energy & Fuels

1

an optimum operating temperature will be subjected to both the kinetics of the oxidation reaction and

2

the cost of the overall plant.

3

3.4.2 Pressure drop of Fluidized Bed (FDB)

4

Figure 19 shows the effects of pressure drop of FDB on the specific power requirement of the oxygen

5

plant (Figure 19a) and plant thermal efficiency (Figure 19b) of CLOC process. It shows that as the

6

pressure drop of FDB increases from 25 kPa to 200 kPa, the specific power requirement of the ICLAS

7

unit in CLOC increases significantly and surpasses those of the conventional CASU units with 95%

8

and 99% O2 purity at approximately 150 kPa and 200 kPa, respectively. For advanced CASU units26-30,

9

these critical pressure drop points are reduced to 88 kPa and 115 kPa, respectively. Accordingly, the

10

overall plant thermal efficiency of CLOC was found to decrease as the pressure increases but still

11

remain above those of the oxy-firing CFB plants using convention CASU unit (about 19 – 20 %

12

efficiency).

13

3.4.3 Thermal efficiency of NSC

14

Thermal efficiency of the NSC determines the conversion efficiency of the recovered heat from the

15

oxidation reactor to extra power output, which is another critical parameter that may affect the

16

feasibility of the whole CLOC system. Figure 20 shows the effect of NSC thermal efficiency on the

17

plant thermal efficiency of CLOC, compared with those of the oxy-firing CFB plant with CASU units

18

and air-firing base plant. It shows that the plant thermal efficiency of CLOC increases as the NSC


Energy & Fuels

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Page 22 of 47

1

thermal efficiency increases, which can reach above that of the air-firing base plant (33.8%) if the NSC

2

thermal efficiency rises above approximately 44%. This happens because at such high thermal

3

efficiency the additional power generation in the NSC becomes so large that the net power production

4

of the CLOC system increases significantly, leading to increased plant thermal efficiency. Figure 20

5

also shows that for NSC thermal efficiency above 30%, which is easily achievable, the plant thermal

6

efficiency of CLOC can be always greater than those of the oxy-firing CFB plants with either

7

conventional or advanced CASU.26-30 The above result therefore suggests that a technically superior

8

CLOC system, compared with the oxy-firing CFB using CASU, can incorporate a NSC with thermal

9

efficiency of 30% and above.

10

3.5

11

For a new CLOC process, due to hybrid coal-NG based power plant system some reduction in coal

12

consumption can be achieved. This is good for the countries where there are abundant natural gas and

13

coal available.

14

Table 6 shows the simulation results indicating reduction in coal consumption as well as tje plant

15

overall performances of a 200 MWe hybrid coal-natural gas oxy-firing plant using CLOC with

16

different oxygen carriers. It shows that with the integration of methane, about 50-54% and 36% of coal

17

can be reduced, respectively for using copper oxide and manganese oxide as the oxygen carrier. Such a

18

high reduction in coal consumption implies that a hybrid coal-natural gas oxy-firing plant may become

19

a better option for regions with abundant natural gas and coal resources.

Reduction in coal consumption


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Energy & Fuels

1

The methane requirement for a 200 MWe CLOC plant using manganese oxide as the oxygen carrier

2

was found to be 14% less than that of the copper oxide case (See Table 6). This is in accordance with

3

the previous discussion that CLOC plant using manganese oxide generally requires less external heat

4

and thus less methane input. Interestingly, though, the results in Table 6 also shows that the CLOC

5

using manganese oxide generate 10-12% more greenhouse emission per unit power output than that

6

using copper oxide, resulting into more CPU power consumption. This indicates that copper oxide is a

7

better oxygen carrier for the hybrid plant compared to manganese oxide.

8

4

9

The current paper investigates the feasibility of a novel CLOC process using transitional metal oxide

10

systems. Comprehensive thermodynamic simulations were carried out using ASPEN Plus v7.3. It was

11

identified that CLOC process is technically superior to both oxy-firing CFB with CASU and CLOU

12

process. Moreover, CLOC process when using CoO/Co3O4 oxide system as oxygen carrier was found

13

to be unfeasible due to the endothermic reaction between cobalt oxide and methane. However, using

14

copper based oxygen carriers CLOC process shows the higher plant efficiency via the indirect methane

15

integration. Manganese oxide oxygen carriers is also found to be suitable for use in CLOC process but

16

it would require significantly larger bed inventories (about 2.4 times greater than copper oxide) and

17

solid circulation rate, and was found to have 10-12% higher carbon emission footprint per unit of

18

power output than that of copper oxide oxygen carriers. Overall, it has been concluded that CLOC can

19

be a promising step change solution for the chemical looping combustion of solid fuels.

Conclusion


Energy & Fuels

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

1

ACKNOWLEDGMENT

2

The Authors would like to appreciate the financial supports they received from the University of

3

Newcastle, the NSW Coal Innovation, and Glencore (formerly Xstrata) Pty Ltd.

4 5

NOMENCLATURE

6

CASU = Cryogenic Air Separation Unit

7

CFB = Circulating Fluidized Bed

8

CLAS = Chemical Looping Air Separation

9

(I)CLAS = (Integrated) Chemical Looping Air Separation

10

CLC = Chemical Looping Combustion

11

CLOC = Chemical Looping Oxy Combustor

12

CLOU = Chemical Looping Oxygen Uncoupling

13

CPU = Carbon Capture Unit

14

FDB = Fluidized Bed

15 16


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1

Energy & Fuels

REFERENCES

2 3 4

[1] Working Group III, IPCC special report on carbon dioxide capture and storage; Intergovernmental Panel on Climate Change: Cambridge, UK and New York, USA, 2005, 1-442

5 6

[2] Li, Z.S.; Zhang, T.; Cai, N. S., Experimental study of O2-CO2 production for the oxyfuel

7 8

combustion using a Co-based oxygen carrier. Ind. Eng. Chem. Res. 2008, 47 (19), 7147-7153.

9 10

[3] Wall, T., Combustion processes for carbon capture. Proceedings of the Combustion Institute 2007 31 (1), 31-47.

11 12 13

[4] Belo, L.P.; Sporl, R.; Shah, K.; Elliott, L.; Stanger, R.; Maier, J.; Wall, T., Sulfur capture by fly ash in air and oxy-fuel pulverized fuel combustion. Energy & Fuels 2014, 28(8), 5472-5479.

14 15 16

[5] Moghtaderi, B., Application of chemical looping concept for air separation at high temperatures. Energy Fuels 2010, 24, 190-198.

17 18 19

[6] Shah, K.; Moghtaderi, B.; Wall, T., Effect of flue gas impurities on the performance of a chemical looping based air separation process for oxy-fuel combustion. Fuel 2013, 103, 932-942.

20 21 22

[7] Shah, K.; Moghtaderi, B.; Wall, T., Selection of suitable oxygen carriers for chemical looping air separation: a thermodynamic approach. Energy Fuels 2012, 26(4), 2038-2045.

23 24 25 26 27 28

[8] Shah, K.; Moghtaderi, B.; Zanganeh, J.; Wall, T., Integration options for novel chemical looping air separation (ICLAS) process for oxygen production in oxy-fuel coal fired power plants. Fuel 2013, 107, 356-370. [9] Habib, M. A.; Badr, H. M.; Ahmed, S. F.; Ben-Mansour, R.; Mezghani, K.; Imashuku, S.; la O’, G.

29 30 31 32 33 34

J.; Shao-Horn, Y.; Mancini, N. D.; Mitsos, A.; Kirchen, P.; Ghoneim, A. F., A review of recent developments in carbon capture utilizing oxy-fuel combustion in conventional and ion transport membrane systems. Int. J. Energy Res. 2011, 35 (9), 741−764.

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173−182.

[10] Song, H.; Shah, K.; Doroodchi, E.; Wall, T.; Moghtaderi, B. Analysis on chemical reaction kinetics of CuO/SiO2 oxygen carriers for chemical looping air separation. Energy Fuels 2014, 28,


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[11] Song, H.; Shah, K.; Doroodchi, E.; Moghtaderi, B. Development of a Cu−Mg-based oxygen carrier with SiO2 as a support for chemical looping air separation. Energy Fuels 2014, 28, 163−172.

3 4 5

[12] Mattisson, T.; Lyngfelt, A.; Leion, H., Chemical-looping with oxygen uncoupling for combustion of solid fuels. Int. J. Greenhouse Gas Control 2009, 3(1), 11-19.

6 7 8

[13] Cao, Y.; Pan, W. P., Investigation of chemical looping combustion by solid fuels. 1. Process analysis. Energy & Fuels 2006, 20(5), 1836-1844.

9 10

[14] Orth, M.; Ströhle, J.; Epple, B., Design and Operation of a 1 MWth Chemical Looping Plant.

11

In Proc. 2nd Int. Conf. Chemical Looping, Sept, 2012, Darmstadt, Germany.

12 13 14 15

[15] Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L. F.; Adánez, J.; Abad, A., Development of CuO-based oxygen-carrier materials suitable for Chemical-Looping with Oxygen Uncoupling (CLOU) process. Energy Procedia 2011, 4, 417-424.

16 17 18

[16] Gayán, P.; Adánez-Rubio, I.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Adánez, J., Development of Cu-based oxygen carriers for Chemical-Looping with Oxygen Uncoupling (CLOU)

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process. Fuel 2012, 96, 226-238.

22 23 24 25 26 27 28

Identification of operational regions in the Chemical-Looping with Oxygen Uncoupling (CLOU) process with a Cu-based oxygen carrier. Fuel 2012, 102, 634-645.

29 30 31 32

[19] Abad, A.; Adánez-Rubio, I.; Gayán, P.; García-Labiano, F.; de Diego, L. F.; Adánez, J., Demonstration of chemical-looping with oxygen uncoupling (CLOU) process in a 1.5 kWth continuously operating unit using a Cu-based oxygen-carrier. Int. J. Greenhouse Gas Control 2012, 6, 189-200.

33 34 35 36

[20] Arjmand, M.; Azad, A.-M.; Leion, H.; Lyngfelt, A.; Mattisson, T., Prospects of Al2O3 and MgAl2O4-Supported CuO Oxygen Carriers in Chemical-Looping Combustion (CLC) and ChemicalLooping with Oxygen Uncoupling (CLOU). Energy Fuels 2011, 25(11), 5493-5502.

[17] Gayán, P.; Adánez-Rubio, I.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Adánez, J.,

[18] Adánez-Rubio, I.; Gayán, P.; Abad, A.; de Diego, L. F.; García-Labiano, F.; Adánez, J., Evaluation of a spray-dried CuO/MgAl2O4 oxygen carrier for the chemical looping with oxygen uncoupling process. Energy Fuels 2012, 26(5), 3069-3081.

37


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[21] Wen, Y.Y.; Li, Z.S.; Xu, L.; Cai, N.S., Experimental Study of Natural Cu Ore Particles as Oxygen Carriers in Chemical Looping with Oxygen Uncoupling (CLOU). Energy Fuels 2012, 26 (6), 3919-

3

3927.

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[22] Zhang, T.; Li, Z. S.; Cai, N. S., Continuous O2-CO2 production using a Co-based oxygen carrier in two parallel fixed-bed reactors. Korean J. Chem. Eng. 2009, 26, (3), 845-849.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

[23] Adanez, J.; Dueso, C.; de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Abad, A., Methane Combustion in a 500 Wth Chemical-Looping Combustion System Using an Impregnated Ni-Based Oxygen Carrier. Energy Fuels 2009, 23(1), 130-142. [24] Sadegh, S. K.; Pallarès, D; Normann, F; Johnsson, F., Progress of Combustion in an Oxy-fuel Circulating Fluidized-Bed Furnace: Measurements and Modeling in a 4 MWth Boiler. Energy & Fuels 2012, 27(10), 6222-6230. [25] Retrofitting CO2 Capture to Existing Power Plants: IEAGHG Report, Report 2011/02, May 2011 [26] Khalel, Z.; Rabah, A.; Barakat, T.A., A new cryogenic air separation process with flash separator, ISRN Thermodynamics, 2013, ID: 253437 [27] Pfaff, I.; Kather, A., Comparative thermodynamic analysis and integration issues of CCS steam power plants based on oxy-combustion with cryogenic or membrane based air separation, Energy Procedia 2009: 1 (1), 495-502. [28] Smith, A. R.; Klosek, J., A review of air separation technologies and their integration with energy

26 27 28

conversion processes. Fuel Process. Technol. 2001, 70 (2), 115-134.

29 30

oxy-coal power plants through an exergy-based process integration methodology. Energy 2014, 69, 272-284.

31 32 33 34

[30] Gou, C.; Cai, R.; Hong, H., A novel hybrid oxy-fuel power cycle utilizing solar thermal energy. Energy 2007, 32 (9), 1707-1714.

[29] Hagi, H.; Le Moullec, Y.; Nemer, M.; Bouallou, C., Performance assessment of first generation

35 36


Energy & Fuels

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1

Page 28 of 47

Table 1: Process simulation cases

Cases

Case no.

Description

Base case

Case 0

Conventional 200 MWe air-firing coal power plant using CFB boiler

Case 1

Coal-fired power station retrofit to oxy-firing using conventional CASU (99% O2 purity)

Case 2

Coal-fired power station retrofit to oxy-firing using conventional CASU (95% O2 purity)

Case 3

Coal-fired power station retrofit to oxy-firing using advanced CASU (99% O2 purity)

Case 4

Coal-fired power station retrofit to oxy-firing using advanced CASU (95% O2 purity)

Case 5-1

Coal-fired power station retrofit to oxy-firing using CLOC process with Cubased oxygen carriers and direct CH4 integration

Case 5-2

Coal-fired power station retrofit to oxy-firing using CLOC process with Cubased oxygen carriers and indirect CH4 integration

Case 5-3

Coal-fired power station retrofit to oxy-firing using CLOC process with Mnbased oxygen carriers and direct CH4 integration

Case 5-4

Coal-fired power station retrofit to oxy-firing using CLOC process with Mnbased oxygen carriers and indirect CH4 integration

Case 5-5

Coal-fired power station retrofit to oxy-firing using CLOC process with Cobased oxygen carriers and solar integration

Case 6

Coal-fired power station retrofit to oxy-firing using CLOU process with Cubased oxygen carriers

Conventional CASU

Advanced CASU

CLOC

CLOU 2 3 4 5 6 7 8 9 10 11 12 13 14


Page 29 of 47

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1

Energy & Fuels

Table 2: 200 MWe air-firing CFB plant data Overall plant performances Gross output Plant thermal efficiency Capacity factor Auxiliary power Coal consumption rate Coal heating value Fuel composition Moisture Ash Carbon Hydrogen Nitrogen Sulphur Oxygen Calorific value Environmental emissions Carbon dioxide Sulphur

211 MW 33.8% 88% 11 MW 28 kg/s 591,795 kWt As received basis 8.8% 30.3% 49.7% 3.3% 1.1% 0.5% 6.3% 20.90 MJ/kg 1.822 0.010

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 29 ACS Paragon Plus Environment

kg/kg of fuel kg/kg of fuel

Energy & Fuels

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1

Page 30 of 47

Table 3: Specifications of different unit operation blocks used in ASPEN Plus Unit operation

Reactor type

Conditions

Function

P= 1 bar, T = 25oC P =1 bar

Decomposition of coal to conventional components Combustion Calculate the combustion products and properties

Oxy-CFB Decomposition

RYield

Burn

RGibbs

Combustion

Calculator

Steam Generator APH1 APH2 FF

Heat exchanger

HSOT = 350-600oC

To generate steam

Heat exchanger Heat exchanger FabFl

To pre-heat the recycle stream To remove solids (fly ash)

FGD

RStoic

CSOT = 130oC CSOT = 200oC Eff. = 99.9% P = 1 bar, T = 150oC Eff. = 94%

CLAS/CLOU Oxidation reactor Reduction reactor CASU Compressor Distillation columns

RGibbs RGibbs

P = 1 bar, T = 800-950oC P = 1 bar, T = 900-1042oC

To remove SOx

For oxidation For reduction

Compr

P = 6, 7.5 bars

For air compression

RedFrac

T= -177 to -193 oC

For separation of oxygen

2 3 4 5 6 7 8 9 10 11 30 ACS Paragon Plus Environment

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Energy & Fuels

1

Table 4: Comparison of CLOC process using various oxygen carriers Case no.

5-1 CuO direct CH4 integratio n (52% RFG, with FWH* and NSC**)

5-2 CuO indirect CH4 integratio n (61% RFG, with FWH* and NSC**)

5-3

5-4

Mn2O3 direct CH4 integratio n (65% RFG, with FWH* and NSC**)

Mn2O3 indirect CH4 integration CoO (42% solar RFG, with integration FWH* and NSC**)

kW kW kW

58,536 86,686 69,899

58,536 86,686 69,899

58,536 86,686 69,899

58,536 86,686 69,899

58,536 86,686 69,899

kW

273,865

274,712

165,933

159,911

182,939

kW

6,056

21,512

3,312

13,899

-

kW

4,566

4,566

4,566

4,566

4,566

kW kW kW kW kW kW

102 278 201 777 1,000 8,021

102 402 304 858 1,000 8,021

102 317 229 1,024 1,000 8,021

102 404 306 862 1,000 8,021

102 297 265 568 1,000 8,021

kWt kWt

315,132 94,116

315,132 110,940

315,132 69,284

315,132 61,110

315,132 -

kWt

37,646

44,376

27,714

24,444

-

kWt kWt

684,662 0

686,779 0

414,833 0

399,778 0

524,034 0

0.4

0.4

0.4

0.4

0.4

kWt

770,826

785,461

468,047

431,757

507,304

kg/s

15.40

15.69

9.35

9.04

-

kg/s

800

802

587

566

216

kPa

25

25

25

25

25

kPa kW C

25 23,617 950

25 30,741 950

25 20,346 810

25 23,231 810

25 7,977 845

Description

Power production HP Turbine IP Turbine LP Turbine Power generation of the NSC** Additional power recovery via FWH* Auxiliary power consumption Boiler feed pump (turbinedriven) Condensate pump Primary Air Fans Forced Draft Fans Induced Draft Fans Spray Dryer FGD Miscellaneous Heating/cooling requirements Cooling tower duty Steam condensation process Waste heat recovery (40%) for heating/cooling application Oxidation reactor Reduction reactor Thermal efficiency of the NSC** External heat input from solar/CH4 Methane flow requirement Oxygen plant Air flow Pressure difference over the blower Pressure drop of FDB Blower power consumption Oxidation reactor operating


5-5

Energy & Fuels

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

temperature Equilibrium oxygen partial pressure (OXI) Reduction reactor operating temperature Equilibrium oxygen partial pressure (RED) Oxygen carrier flow Sweep gas flow Sweep gas temperature Sweep gas pressure CPU plant Specific power requirement for CPU25 CO2 for CPU plant CPU power demand*** Overall plant performances Coal consumption rate Coal heating value Gross power production Coal plant parasitic loads Oxygen requirement Oxygen plant power requirement Oxygen plant specific power requirement26-30

Page 32 of 47

0.04

0.04

0.04

0.04

0.04

1,025

1,042

900

915

894

0.20

0.26

0.21

0.26

0.26

1,050 111 326 1.25

1,053 383 251 1.25

2,427 158 433 1.25

2,350 283 306 1.25

640 143 165 1.25

135

135

135

135

135

88 38,501

94 40,973

74 32,544

76 33,157

49 21,433

kg/s kWt kW kW kg/s

28 591,795 490,475 10,379 106

28 591,795 506,778 10,687 106

28 591,795 379,799 10,693 82

28 591,795 384,365 10,695 79

28 591,795 393,494 10,253 44

kW

23,617

30,741

20,346

23,231

7,977

kWh per tonne O2 kW %

62

81

69

81

50

C

kg/s kg/s C Bar kWh per tonne CO2 kg/s kW

Net power production 417,978 424,378 316,217 317,282 353,832 Plant thermal efficiency 31.55 31.84 30.64 31.76 32.19 Total energy penalty 2.3 2.0 3.2 2.0 1.6 % (reference to base case) Efficiency gain (reference to 65.9 67.4 61.1 67.0 69.3 % Case 1) 1 * FWH: feedwater heating; ** NSC: New steam cycle; ***: considering 90% capture level of the plant 2 CO2 emission


Page 33 of 47

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

Energy & Fuels

Table 5: Comparison of CLOC and CLOU processes with air-firing and oxy-firing CFB plants with CASU operating at the typical operating conditions Cases

Base case

Conventional CASU

Advanced CASU (not yet available)

Case no.

0

1 (ref#)

2

3

4

Description

Airfiring CFB

99% O2 purity

95% O2 purity

99% O2 purity

95% O2 purity

kW kW kW kW kW

58,536 86,686 69,899 -

58,536 86,686 69,899 -

58,536 86,686 69,899 -

58,536 86,686 69,899 -

58,536 86,686 69,899 -

58,536 86,686 69,899 273,865 6,056

25,945 38,422 30,981 121,695 -

kW kW kW kW kW kW kW

4,566 102 374 283 747 1,000 8,021

4,566 102 547 414 907 1,000 8,021

4,566 102 547 414 907 1,000 8,021

4,566 102 547 414 907 1,000 8,021

4,566 102 547 414 907 1,000 8,021

4,566 102 278 201 777 1,000 8,021

2,024 45 0 0 571 443 3,555

kWt kWt

268,960 -

268,960 -

268,960 -

268,960 -

268,960 -

315,132 94,116

119,211 -

kWt

-

-

-

-

-

37,646

-

Power production HP Turbine IP Turbine LP Turbine Power generation of the NSC** Additional power recovery via FWH* Auxiliary power consumption Boiler feed pump (turbine-driven) Condensate pump Primary air fans Forced draft fans Induced draft fans Spray dryer FGD Miscellaneous Heating/cooling requirements Cooling tower duty Steam condensation process Waste heat recovery (40%) for heating/cooling application

33


CLOC

CLOU

5-1 CuO - direct CH4 integration (52% RFG, with FWH* and NSC**)

6 CuO - (70% RFG, with NSC**)

Energy & Fuels

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

Oxidation reactor Reduction reactor Energy associated with exhaust from reduction reactor Thermal efficiency of the NSC** External heat input from solar/CH4 Methane flow requirement Oxygen plant Air flow Pressure difference over the blower Pressure drop of FDB Blower power consumption Oxidation reactor operating temperature Equilibrium oxygen partial pressure (OXI) Reduction reactor operating temperature Equilibrium oxygen partial pressure (RED) Oxygen carrier flow Sweep gas flow Sweep gas temperature Sweep gas pressure CPU plant Specific power requirement for CPU25 CO2 for CPU plant CPU power demand*** Overall plant performances Coal consumption rate Coal heating value

kWt kWt

kWt kg/s kg/s kPa kPa kW C C kg/s kg/s C bar kWh per tonne CO2 kg/s kW kg/s kWt

Page 34 of 47

-

-

-

-

-

684,662 0

304,237 15,899

-

-

-

-

-

-

246,367

-

-

-

-

-

0.400 770,826 15.40

0.400 -

218 25 -

191 25 -

201 25 -

191 25 -

201 25 -

800 25 25 23,617 950 0.04 1,025 0.20 1,050 111 326 1.25

345 25 25 12,056 950 0.04 985 0.09 447 151 148 1.25

-

135

135

135

135

135

135

-

49 21,349

49 21,349

49 21,349

49 21,349

88 38,501

52 22,546

28 591,795

28 591,795

28 591,795

28 591,795

28 591,795

28 591,795

28 591,795

34


Page 35 of 47

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

Gross power production Coal plant parasitic loads Oxygen requirement Oxygen plant power requirement Oxygen plant specific power requirement26-30

Energy & Fuels

kW kW kg/s kW kWh per tonne O2 kW % % %

210,554 10,528 -

210,554 10,990 44 65,662

210,554 10,990 46 54,961

210,554 10,990 44 39,555

210,554 10,990 46 33,310

490,475 10,379 106 23,617

215,019 4,615 45 12,056

-

415

330

250

200

62

74

112,553 19.02 14.8 -

123,254 20.83 13.0 9.5

138,660 23.43 10.4 23.2

144,905 24.49 9.3 28.7

417,978 31.55 2.3 65.9

175,803 29.71 4.1 56.2

Net power production 200,027 Plant thermal efficiency 33.80 Total energy penalty (reference to base case) Efficiency gain (reference to Case 1) Additional notes: * FWH: feedwater heating ** NSC: new steam cycle ***: considering 90% capture level of the plant CO2 emission

#. Justification for the reference case: The CLOC process should be compared against the CASU with oxygen purity of 99%, because in the case of 95% O2 purity, the 5% impurities such as N2 and Ar will have to be compressed or removed in the compression process as per the sequestration site specifications, which may incur additional costs. However, ICLAS system in the CLOC process is expected to produce high purity 99% pure oxygen stream and eliminate the impurities such as N2 and Ar in the air reactor. Therefore, there will not be any additional energy requirement for N2 and Ar removal in the back end of oxy-fuel processes.

35


Energy & Fuels

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

Page 36 of 47

Table 6: 200 MWe CLOC plant performances using different oxygen carriers Case no.

0

5-1

Description

Air-firing CFB plant

CuO CuO direct CH4 indirect CH4 integration integration

Mn2O3 Mn2O3 direct CH4 indirect CH4 integration integration

kg/s kWt kWt kg/s kW kW kg/s

28 591,795 210,554 10,528 -

14 283,170 368,836 7 234,689 4,966 51

13 278,900 370,170 7 238,833 5,037 50

18 374,297 296,029 6 240,214 6,763 52

18 373,040 272,160 6 242,286 6,741 50

kW

-

11,301

14,487

12,868

14,644

kg/s kW kW %

200,000 -

42 18,423 200,000 0.50

44 19,309 200,000 0.54

47 20,583 200,000 0.36

48 20,901 200,000 0.36

Overall plant performances Coal consumption rate Coal heat input Methane heat input Methane flow requirement Gross power production Coal plant parasitic loads Oxygen requirement Oxygen plant power requirement CO2 emission CPU power demand Net power production Coal saving

5-2

36 Environment ACS Paragon Plus

5-3

5-4

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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

Energy & Fuels

Figure 1: Schematic diagram of indirect chemical looping combustion (indirect CLC)

Figure 2: Schematic diagram of direct chemical looping combustion (direct CLC)


Energy & Fuels

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

(a)

(b) Figure 3: Schematic diagram of CLOC process with (a) direct or (b) indirect methane firing.


Page 38 of 47

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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

Energy & Fuels

Figure 4: Aspen process model for CLOC (i.e. Oxy-CFB with ICLAS)

Figure 5: Aspen process model for the ICLAS block in CLOC process


Energy & Fuels

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

Figure 6: Aspen process model for oxy-firing CFB plant with CASU

Figure 7: Aspen process model for the conventional CASU unit


Page 40 of 47

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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

Energy & Fuels

Figure 8: Aspen process model for the CLOU

Figure 9: Equilibrium oxygen concentration at the outlet of reduction reactor


Energy & Fuels

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

Figure 10: CLOC process diagram using Co-based oxygen carriers with solar heat integration.

Figure 11: Plant thermal efficiency of CLOC using different oxygen carriers compared with that of the air-firing base case


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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

Energy & Fuels

Figure 12: Total energy penalty of oxy-firing CFB combustion using various oxygen plants

Figure 13: Specific power requirement of various oxygen plants


Energy & Fuels

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

Figure 14: Plant thermal efficiency comparison

Figure 15: Efficiency gain of oxy-firing CFB combustion using various oxygen plants compared to that using the conventional CASU


Page 44 of 47

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Energy & Fuels

Figure 16: Equilibrium oxygen concentration and heat generation/requirement in the oxidation (OXI) and reduction (RED) reactors as a function of reactor operating temperature.

Figure 17: Specific power requirement of the oxygen plant using ICLAS as a function of oxidation reactor temperature compared to those of the CASU cases.


Energy & Fuels

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

Figure 18: Plant thermal efficiency of CLOC process as a function of oxidation reactor temperature, compared to those of the air-firing base plant and oxy-firing CFB plant with CASU.

(a)


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Energy & Fuels

(b) Figure 19: (a) Specific power requirement of the oxygen plant, and (b) plant thermal efficiency of CLOC as a function of pressure drop of FDB, compared with those of the conventional CASU and/or air-firing base plant.

Figure 20: Plant thermal efficiency of CLOC as a function of NSC thermal efficiency, compared with those of the oxy-firing CFB plant with conventional CASU and air-firing base plant.


A Novel Hybrid Chemical-Looping Oxy Combustor Process for the

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