Thermodynamic assessment of heat recovery from a fluidized-bed

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Thermodynamic assessment of heat recovery from a fluidized-bed ventilation air methane abatement unit Francis Nadaraju, Andrew Maddocks, Jafar Zanganeh, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03197 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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Thermodynamic assessment of heat recovery from a fluidized-bed ventilation air methane abatement unit Francis Nadaraju, Andrew Maddocks, Jafar Zanganeh and Behdad Moghtaderi* Priority Research Centre for Frontier Energy Technologies & Utilisation, The University of Newcastle, University Drive, Callaghan, NSW, 2308, Australia; * Email: [email protected] Keywords ventilation air methane, Brayton cycle, power generation, greenhouse gas abatement, modeling, Aspen Plus®

Abstract

Methane, a greenhouse gas, is the second largest contributor to global warming after carbon dioxide; and is 25 times more effective at trapping heat in the atmosphere than carbon dioxide. In 2015, fugitive emissions of methane from Australian underground coal mines was reported at 25 million tonnes of carbon dioxide equivalent. Ventilation air methane (VAM) is present in low concentrations (below 1.0 vol %) and its abatement and use as an energy source is a challenge 1

ACS Paragon Plus Environment

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for the coal mining industry. This paper examines the recovery of heat from a fluidized-bed VAM abatement unit and utilisation in power generation via the Brayton cycle. The objective of the study was to determine the minimum methane concentration required to maintain autothermal operations and produce sufficient power to operate a fluidized bed plant without supplementary power or fuel. Four configurations were studied and simulated using Aspen Plus® software. For direct heat recovery, the minimum methane concentration increased with an increase in both the reactor outlet temperature and compressor outlet pressure. The minimum methane concentration for the indirect heat recovery configurations decreased when both the reactor outlet temperature and compressor outlet pressure increased. For all configurations the minimum methane concentration was limited by the maximum reactor inlet temperature of 600 °C (to prevent autoignition of the methane upstream of the reactor).

Introduction Fugitive emissions of methane from underground coal mines in Australia were 25.4 million tonnes of carbon dioxide equivalent in 2015 1. This was 74 % of the total reported fugitive methane emissions in Australia or 5 % of Australia’s national greenhouse gas emissions 1

. Methane, a greenhouse gas, traps radiation within the earth’s atmosphere 25 times more than

carbon dioxide over a 100-year period 2.

The air that exits an underground coal mine contains methane in the range 0.1-1.0 vol % 3 and is termed ventilation air methane (VAM). VAM is a challenge to mitigate due to its presence in ultra-low concentrations coupled with a variable flow rate and concentration 4.

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VAM abatement processes have been developed and are classified into principal and ancillary uses of VAM. Ancillary uses replace combustion air with VAM while principal uses consider the VAM as a primary fuel source 3. Technologies for VAM abatement include thermal flow reversal reactors (TFRR) 5, catalytic flow-reversal reactors (CFRR)

5

, chemical looping

combustion 6, fluidized bed reactors 7, lean fuel gas turbines 8, porous burners 9 and biofiltration 10

. Commercial installations of the VAM abatement technology are limited to thermal flow

reversal reactors. The other technologies are in various phases of development from laboratoryscale experiments to mine-site demonstrations.

The TFRR operates above 1000 °C

11

while the CFRR operates above 850 °C

12

. The high

operating temperatures are necessary to ensure that the low concentration methane is converted to carbon dioxide and water. However, these temperatures are significantly greater than the autoignition temperature of methane mixtures and pose a safety risk for the connection of an abatement plant directly to a ventilation shaft at an underground coal mine. The development of catalytic and fluid-bed technologies is aimed at reducing the operating temperature of an abatement plant to below the autoignition temperature and reduce the risk of a mine fire or explosion.

The TFRR and CFRR were shown to abate VAM at minimum concentrations of 0.22 vol % 13 and 0.2 vol %

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respectively. At these methane concentrations and the stated operating

temperatures, sufficient heat is available to preheat the ventilation air to the autoignition temperature of methane and ensure stable (auto-thermal) reactor operation. However, the

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recovery of heat for power generation requires additional fuel and hence the minimum concentrations will increase when power is generated. The high temperature, energy intensive VAM abatement processes are suitable to recover part of the heat from the reactor gases directly via hot gas withdrawal and indirectly by a centrally located heat exchanger

14

. Gosiewski, et al.

13

developed a demonstration TFRR plant that

recovered the heat directly by withdrawing part of the hot reactor gases. Autothermal operation was noted at 0.22 vol % while stable heat recovery took place at 0.43 vol %. Li, et al.

12

investigated the withdrawal of the hot gases from a CFRR. Autothermal operation was noted at a methane concentration 0.2 vol %. Extraction of heat from the hot gases was investigated at 0.3, 0.5 and 0.7 vol % methane but stable reactor conditions was not obtained at these concentrations.

Li, et al. 11 studied the indirect recovery of heat from a TFRR plant installed at a coal mine in China to produce superheated steam to drive a turbine. Auto-thermal operation was observed at 0.25 vol % methane while stable heat recovery was noted at a minimum methane concentration of 0.6 vol %. However, to ensure continuous power production, drainage gas was mixed with the VAM to give an average methane concentration of 1.0 vol % compared with an average VAM concentration of 0.23 vol % from the mine. The study of heat recovery from a CFRR using a central heat exchanger was undertaken by Wang, et al.

15

. Stable heat recovery in this case was

noted at a methane concentration of 0.4 vol % while autothermal operating conditions without heat recovery could not be achieved.

The TFRR and CFRR are fixed-bed reactors of inert material and catalyst respectively. Fluidized bed combustion provides a number of advantages compared with fixed-bed reactors 4


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including lower operating temperatures and smaller footprint. Fluid-beds, however, have a greater pressure drop than fixed-bed reactors and therefore require more power to operate. This increases the operating costs of a fluid-bed abatement plant compared with a fixed-bed abatement plant unless sufficient power can be generated from the combustion process to supply all the power requirements.

Combustion of VAM in a fluidized-bed was studied by Yang, et al. 7. The bed materials were limestone mixed with commercial 0.5 wt % palladium supported on alumina catalyst. The laboratory-scale fluidized-bed height was 200 mm, had a total mass of 2.7 kg and the limestone and catalyst was mixed in a ratio of 40:1. The methane concentrations between 0.15 to 3.0 vol % were investigated, however, the minimum methane concentration for auto-thermal operation was not determined. Heat recovery from the hot flue gases was not investigated.

Chemical looping combustion has been proposed for VAM abatement which employs two interconnected reactors. Metal oxide particles are circulated between the fuel and air reactor where reduction and oxidation takes place respectively. VAM would be supplied to the fuel reactor while regeneration of the reduced metal oxide takes place in the air reactor 16.

Zhang, et al. 4 studied the oxidation of ultra-low methane in VAM using metal oxide carriers in a chemical looping arrangement. Stone dust looping has been investigated by Shah, et al.

6

for

VAM abatement. Stone dust (predominantly calcium carbonate, CaCO3) is decomposed to calcium oxide (CaO) in the calciner reactor. The calcium oxide and VAM are supplied to the carbonator reactor while CaO and CaCO3 are cycled between the calciner and carbonator 5


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reactors. Heat recovery would be possible in both studies but this has not been investigated. Also, recovery of heat from a fluidized-bed for VAM abatement has not been studied. Furthermore, no literature is available on heat recovery from a VAM abatement process via a Brayton power cycle.

This study examines the recovery of the heat from a VAM abatement plant consisting of a single fluidized bed. The heat was recovered using the Brayton power cycle by simulating the process thermodynamically in commercially available simulation software, Aspen Plus®. The objective of this study was to determine the minimum methane concentration required to maintain autothermal operations and produce sufficient power to operate a fluidized bed plant without supplementary power or fuel.

Brayton cycle The ideal Brayton cycle consists of four processes and is represented in Figure 1: isentropic compression (1-2), isobaric heat addition (2-3), isentropic expansion (3-4) and isobaric heat removal (4-1).

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(a)

(b)

Figure 1 Representations of the ideal Brayton cycle on (a) pressure-specific volume (P-V) diagram and (b) temperature-specific entropy (T-S) diagram (after Boyce 17) The Brayton cycle uses the equipment shown in Figure 2. The gas is compressed (process 1-2) followed by heat addition by a heat exchanger (process 2-3). The heated gas then expands through the turbine (process 3-4). A closed-loop cycle results when the gas leaving the turbine rejects heat to a second heat exchanger (Qout) to return the gas to its initial state (point 1).

Figure 2 Schematic of equipment used to achieve the open-loop (solid lines) and closed-loop (solid- and dashed-lines) Brayton cycle (after Boyce 17) The energy changes in the cycle can be understood by invoking the first law of thermodynamics. For a steady-state process having a single inlet (A) and outlet (B) stream and constant gas flow rate, the first law is stated as:   u 2 − u A2 Q& − W& = m&  H B − H A + B + g ( z B − z A ) 2  

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(1)

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where Q& is the net rate of heat transfer to the system; W& is the work done by the system; m& is the mass flow rate; H and u are the specific enthalpy and velocity respectively; g is the acceleration due to gravity and z is the height above a reference point.

For the reversible Brayton cycle, heat transfer takes place at constant pressure while the compression and expansion processes are isentropic. The potential energy term can be neglected due to a small change in elevation between the turbomachine inlet and outlet nozzles. The kinetic energy term is omitted due to negligible heat transfer in a properly designed compressor or turbine. Moreover, the inlet and outlet pipes are sized to make fluid velocities roughly equal 18

. Equation (1) therefore becomes: compressor:

W&c = m& (H1 − H 2 )

(2)

turbine:

W&t = m& (H 3 − H 4 )

(3)

heat added:

Q& 23 = m& (H 3 − H 2 )

(4)

The overall thermal efficiency is defined as: W&

η cyc = &cyc = Q 23

W& t + W& c Q&

(5)

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Thermodynamic analysis The process to recover the heat from the fluidized-bed VAM abatement unit was simulated in Aspen Plus®. Three variables were investigated, namely, compressor outlet pressure, reactor temperature and methane concentration of the inlet VAM. The pressure was varied between 1.5 and 4.0 bar for direct turbine configurations and between 1.5 and 10 bar for indirect turbine 8


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configurations. The simulations catered for the deviation of the gas mixture from an ideal gas at high pressures by the Peng-Robinson (PR) equation of state 19. A total system pressure drop of 15 kPa was used for the modelling and was considered appropriate based on the experience of the research group in operating fluid bed VAM abatement plants. Modelling was undertaken for reactor temperatures between 500 to 800 °C on the basis that stone dust oxidised VAM between 500 to 650 °C 6 and iron oxide achieved high conversions of VAM between 600 to 800 °C 16. The maximum reactor inlet temperature of the VAM was limited to 600 °C to prevent autoignition of the methane upstream of the reactor. A fixed value was selected to ensure safe operation of the VAM abatement unit and to cater for fluctuations in the methane concentration present in the ventilation air at an underground coal mine. The inlet methane concentration in ventilation air was varied from 0.1 to 1.0 vol %. Combustion of methane takes place in the fluidized bed, modelled as an “RGibbs” reactor. This reactor model employs a Gibbs Free Energy Minimisation method to perform the relevant chemical equilibrium calculations for a given set of operating conditions. To cater for an overall methane conversion of 99 %, a bypass stream was used to model the unreacted methane that would be present in the reactor product stream. The recuperator and air heater were modelled using the “Heatx” heat exchanger block. This model calculates the duty and area of the heat exchanger between a single hot stream and a single cold stream. Further details on the assumptions and model parameters are listed in Table 1. The compressor and turbine were modelled as isentropic with an efficiency of 90 %. 9


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Table 1 Assumptions and operating parameters Unit

Parameter

Assumptions and operating parameters

Fluid bed temperature

500-800 °C

Methane conversion

99 %

Maximum fluid bed inlet

600 °C

operation VAM abatement reactor

temperature

Compressor

System pressure drop

15 kPa

Heat loss

Negligible

Cyclone efficiency

100 %

Discharge pressure

1.5 to 4.0 bar for direct heat recovery 1.5 to 10 bar for indirect heat recovery

Turbine

Isentropic efficiency

90 %

Mechanical efficiency

90 %

Flow rate

20 m3/s for direct heat recovery (VAM)

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4.0 to 15 kg/s for indirect heat recovery (air) Discharge pressure

1.0 bar

Isentropic efficiency

90 %

Mechanical efficiency

90 %

The sequence of unit operations (recuperator for preheating the inlet VAM stream; the VAM abatement reactor, compressor and turbine) was varied to give four configurations (Table 2). The configurations are presented schematically from Figure 3 to Figure 6. Options 1 and 2 recover the heat directly from the flue gases leaving the reactor while Options 3 and 4 utilise the heat indirectly via a second heat exchanger. In Option 1 (Figure 3), VAM at ambient conditions (1.0 bar, 30 °C) was compressed before being supplied to the recuperator. The flue gases leaving the reactor were then supplied to the turbine followed by the recuperator.

In Option 2 (Figure 4) the VAM is compressed and

preheated upstream of the reactor. Downstream of the reactor, the high temperature flue gases are supplied to the recuperator first then to the turbine.

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Figure 3 Option 1 – direct Brayton cycle with compressed VAM and high temperature flue gas expansion

Figure 4 Option 2 – direct Brayton cycle with compressed VAM and low temperature flue gas expansion

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Indirect heat transfer of the flue gases to air takes place in a second heat exchanger (Options 3 and 4). VAM was preheated and supplied to the reactor. In Option 3 (Figure 5), the high temperature flue gases downstream of the reactor are supplied to the second heat exchanger first then the recuperator. In Option 4, the flue gases leaving the reactor are first supplied to the recuperator then the second heat exchanger (Figure 6).

The mass flow rate of air through the air heater was varied to balance the output work of the turbine with the combined input work of the fan and compressor. In this arrangement the plant would be self-sustaining i.e. the power produced by the turbine would be sufficient to cater for the power requirements of the fan and compressor.

Figure 5 Option 3 – indirect Brayton cycle with preheated VAM and high temperature flue gas heat exchanger

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Figure 6 Option 4 – indirect Brayton cycle with preheated VAM and low temperature flue gas heat exchanger

Table 2 Summary of investigated options Option

Heat recovery

Compression

Flue gas expansion

1

Direct

Ambient VAM (1.0 bar, 30 °C)

Reactor outlet

2

Direct

Ambient VAM (1.0 bar, 30 °C)

Recuperator outlet

3

Indirect

Ambient air (1.0 bar, 30 °C)

Reactor outlet

4

Indirect

Ambient air (1.0 bar, 30 °C)

Recuperator outlet

Discussion

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The discussion of the modelling outcomes is limited to Options 1 and 3. The minimum methane concentration under any condition was 0.88 vol % for Option 2 and 0.8 vol % for Option 4 and few conditions reported a minimum methane concentration less than 1.0 vol%. This was an expected result as the temperature of the flue gas stream for heating the turbine working fluid was a maximum of 450 °C, which is unsuitable for the Brayton cycle. An Organic Rankine cycle would be more suitable for power generation in Options 2 and 4.

The influence of temperature, pressure, turbine working fluid flow rate and plant configuration, on the minimum methane concentration is discussed in the following sections.

Influence of fluid bed reactor temperature and compressor outlet pressure

The minimum methane concentration as a function of the reactor temperature for autothermal operation and to produce sufficient power for Option 1 is shown in Figure 7. The minimum methane concentration increased with both reactor temperature and compressor outlet pressure. At low compressor outlet pressures, the minimum methane concentration increased marginally up to a reactor temperature of 650 °C before a step change in the minimum methane concentration. The step change in minimum methane concentration was due to the restriction of the reactor inlet temperature to 600 °C. As the difference between the reactor inlet temperature and the reactor temperature increases, the methane required to heat the ventilation air from 600 °C to the reactor temperature also increases. At greater pressures, the step change was less pronounced as the minimum methane concentration is more dependent on the compressor power requirement and less dependent on the temperature difference between the inlet and reactor

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temperatures. At a compressor outlet pressure of 1.5 bar and reactor temperature of 500 °C, the minimum methane concentration was 0.21 vol %.

Figure 7 Minimum operating methane concentrations for Option 1 as a function of pressure at different reactor temperatures Figure 8 (a) shows the net power produced in Option 1 at a methane concentration of 1.0 vol %.

As the pressure increased, the power reached a maximum at any investigated

temperature. The shape of the curves was related to the PR equation of state for the gas mixture with the first derivative of Equation (9) resulting in a parabola. The power produced above a temperature of 650 °C was limited by the maximum investigated methane concentration of 1.0 vol %. At higher reactor temperatures more power was produced by the cycle due to the reactor flue gases entering the turbine at a higher temperature.

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Figure 8 (b) shows that less power was produced at a methane concentration of 0.5 vol % due to less methane in the ventilation air. The minimum methane concentration to produce power at 750 °C and 800 °C was 0.62 vol % and 0.83 vol % respectively, hence there are no results shown on Figure 8 (b) for these temperatures.

(a)

(b)

Figure 8 Power produced by the cycle in Option 1 as a function of pressure at different reactor temperatures at (a) 1.0 vol % methane and (b) 0.5 vol % methane

The minimum methane concentration as a function of compressor outlet pressure is presented in Figure 9 for Option 3. The minimum methane concentrations in Figure 9 refer to the concentrations required to produce sufficient power to meet the power requirements of both the air compressor and the fan supplying VAM through the circuit under auto-thermal operation. Minimum values for the methane concentration were reached at 650 °C and 700 °C with increased pressure.

At temperatures of 750 °C and 800 °C, as the pressure increased the

minimum methane requirement initially decreased and then remained constant at the concentration required to heat the ventilation air from 600 °C to the reactor temperature. The 17


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minimum air mass flow rate through the turbine decreased with increasing temperature. Less heat was transferred at low reactor temperatures which required a larger air flow rate through the turbine. The minimum methane concentration was 0.49 vol % at 700 °C and 3.5 bar. The air flow rate requirement at these conditions was 7.0 kg/s.

Figure 9 Minimum operating methane concentrations for Option 3 as a function of pressure at different reactor outlet temperatures (air mass flow rates shown for reactor temperatures 700 °C and 800 °C)

Plant configuration

The power required for the direct turbine configurations was greater than the corresponding indirect turbine configurations due to an inlet fan being required for the indirect options. The 18


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additional power requirements for indirect turbine configurations resulted in a greater minimum methane concentration for indirect configurations compared with direct turbine configurations. For example, the minimum methane concentration in Option 1 was 0.21 vol % at 500 °C while in Option 3 the minimum methane concentration was 0.49 vol % at 700 °C. Indirect turbine configurations will require an operating temperatures greater than direct turbine configurations to achieve auto-thermal operations and produce sufficient power for plant operations. The greater temperatures for indirect turbine configurations contribute to the greater minimum methane concentrations as more methane is required to heat the ventilation to the reactor temperature.

Although the minimum methane concentrations for indirect turbine configurations are greater than direct turbine configurations, there would be several advantages from a constructability perspective. In the direct turbine configurations, particles from the fluid bed would carry-over into the flue gases due to cyclone efficiencies being less than unity and damaging the turbine internals, unless high temperature filters were installed. This would not occur in the indirect turbine configurations. Installation of high temperature filters for direct turbine configurations would increase the pressure drop, consequently increasing the minimum methane concentration due to the additional power requirements. Indirect turbine configurations allow the working fluid flow rate to be independent of the ventilation air flow rate, allowing for easier operation and optimization of the turbine. Ventilation air flow rates from underground coal mines fluctuate, and these fluctuations would need to be considered in direct turbine configurations as the turbine working fluid is the ventilation air post methane oxidation.

Conclusion 19


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The recovery of heat from a single fluidized-bed VAM abatement unit was investigated thermodynamically using Aspen Plus® software. Four plant layouts were identified for the VAM abatement process that consisted of the fluidized-bed reactor, recuperator, compressor and turbine. At a ventilation air flow rate of 20 m3/s (1.0 bar, 30 °C) power was produced at the investigated compressor pressures, reactor temperatures and methane concentrations. Based on the modelling undertaken, it would be possible for a fluidized-bed abatement plant to operate auto-thermally and produce sufficient power to operate the plant without supplementary fuel or power at VAM concentrations that are typical at underground coal mines. Option 1 produced power at a minimum methane concentration of 0.21 vol % at 500 °C and 1.5 bar while Option 3 produced power at a minimum methane concentration of 0.49 vol % at 700 °C and 3.5 bar. Options 2 and 4 produced power at minimum methane concentrations of 0.88 vol % and 0.8 vol % respectively. An Organic Rankine cycle would be more suitable to produce power for these options.

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Acknowledgements The authors wish to acknowledge the financial support received from The University of Newcastle and the Priority Research Centre for Frontier Energy Technologies & Utilisation for the work presented in this paper.

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13. Gosiewski, K.; Pawlaczyk, A.; Jaschik, M., Energy recovery from ventilation air methane via reverse-flow reactors. Energy 2015, 92, 13-23. 14. Gosiewski, K.; Warmuzinski, K., Effect of the mode of heat withdrawal on the asymmetry of temperature profiles in reverse-flow reactors. Catalytic combustion of methane as a test case. Chemical Engineering Science 2007, 62, 2679-2689. 15. Wang, S.; Gao, D.; Wang, S., Steady and Transient Characteristics of Catalytic Flow Reverse Reactor Integrated with Central Heat Exchanger. Industrial & Engineering Chemistry Research 2014, 53, 12644−12654. 16. Zhang, Y.; Doroodchi, E.; Moghtaderi, B., Chemical looping combustion of ultra low concentration of methane with Fe2O3/Al2O3 and CuO/SiO2. Applied Energy 2014, 113, 19161923. 17. Boyce, M. P., Gas Turbine Engineering Handbook. Fourth ed.; Elsevier: 2012. 18. Smith, J. M.; Van Ness, H. C.; Abbott, M. M., Introduction to Chemical Engineering Thermodynamics. Sixth ed.; McGraw-Hill: New York, 2001. 19. Pratt, R. M., Thermodynamic properties involving derivatives: using the Peng-Robinson Equation of State. Chemical Engineering Education 2001, 1, 112-115.

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(a)

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(b)

Figure 1 Representations of the ideal Brayton cycle on (a) pressure-specific volume (P-V) diagram and (b) temperature-specific entropy (T-S) diagram (after Boyce [1])

Figure 2 Schematic of equipment used to achieve the open-loop (solid lines) and closed-loop (solid- and dashed-lines) Brayton cycle (after Boyce [1])

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Energy & Fuels

Figure 3 Option 1 – direct Brayton cycle with compressed VAM and high temperature flue gas expansion

Figure 4 Option 2 – direct Brayton cycle with compressed VAM and low temperature flue gas expansion

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Figure 5 Option 3 – indirect Brayton cycle with preheated VAM and high temperature flue gas heat exchanger

Figure 6 Option 4 – indirect Brayton cycle with preheated VAM and low temperature flue gas heat exchanger

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Energy & Fuels

Figure 7 Minimum operating methane concentrations for Option 1 as a function of pressure at different reactor temperatures

(a)

(b)

Figure 8 Power produced by the cycle in Option 1 as a function of pressure at different reactor temperatures at (a) 1.0 vol % methane and (b) 0.5 vol % methane

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Figure 9 Minimum operating methane concentrations for Option 3 as a function of pressure at different reactor outlet temperatures (air mass flow rates shown for reactor temperatures 700 °C and 800 °C)

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Thermodynamic assessment of heat recovery from a fluidized-bed

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