Modeling Energy Flow in an Integrated Pollutant Removal (IPR

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Modeling Energy Flow in an Integrated Pollutant Removal (IPR) System with CO2 Capture Integrated with Oxy-fuel Combustion Sivaram Harendra,* Danylo Oryshcyhn, Stephen Gerdemann, Thomas Ochs, and John Clark Process Development Division, National Energy Technology Laboratory (NETL), United States Department of Energy (DOE), Albany, Oregon 97321, United States ABSTRACT: Oxy-coal combustion is one of the technical solutions for mitigating CO2 in thermal power plants. Many processes have been evolved in past the decade to capture CO2 from process industries. Researchers at the National Energy Technology Laboratory (NETL) have patented a process, integrated pollutant removal (IPR), that uses off the shelf technology to produce a sequestration-ready CO2 stream from an oxy-combustion power plant. The IPR process as it is realized at the Jupiter Oxygen Burner Test Facility is a spray tower (direct-contact heat exchanger) followed by four stages of compression with intercooling. To study the energy flows of the oxy-combustion process, a 15 MWth oxy-combustion pulverized-coal-fired plant integrated with the IPR system was simulated and analyzed using ASPEN Plus and ASPEN energy analyzer. This paper discusses flue-gas recycle, energy flow, recovery, and optimization of IPR systems. ASPEN models of heat- and mass-transfer processes in a flue-gas-condensing heat-exchanger system were developed to predict the heat transferred from flue gas to cooling water. The flue-gas exit temperature, cooling water outlet temperature, and energy flows of IPR streams were computed using ASPEN models. Pinch principles are deployed for targeting design and operation-guiding purposes and balancing the heat and mass transfer in the IPR system. The results are expected to support sophistication of the IPR system design, improving its application in a variety of settings. They open the door for valuable IPR efficiency improvements and generalization of methodology for simultaneous management of energy resources.

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INTRODUCTION There is an increasing worldwide interest in finding better ways to use finite fossil energy resources while balancing the often conflicting goals of increasing efficiency and reducing the environmental impact. Currently, more than 85% of the energy that drives modern economies comes from fossil fuels, and this has stimulated research and development into more sustainable alternative energy sources. However, fossil fuels are predicted to remain the major source of energy for the foreseeable future.1 Burning large quantities of carbon-based fossil fuels inevitably produces large amounts of CO2, which has been linked to global climate change. Consequently, cost-effective and efficient use of finite fossil fuels that also allows for carbon capture has become an active field of research. Oxy-combustion is one possible way to burn fossil fuels while producing a flue gas of relatively pure CO2. In this process, oxygen replaces air as the oxidant. Explicit control of the oxygen concentration in the combustion process allows heat transfer and flame temperature to be optimized, primarily through employing recirculated flue gas as a diluent. Experimenters have shown that the temperature history seen through the radiant and convective passes of an air-fired boiler can be approached by recycling approximately 70% of the flue gas to dilute the pure oxygen to approximately 30%.2 The resulting exiting flue gas is primarily CO2, water, and excess O2. Other species, such as SO2, various nitrogen compounds, HCl, and Hg, are also present in quantities dependent upon the fossil fuel composition and the amount of air that leaks into the boiler. Removing the water by condensation, which also removes some of the water-soluble components, results in a product gas that is primarily CO2. It is estimated that oxy-combustion will reduce the energy efficiency of a power plant by about 10%.3−5 Thus, © 2012 American Chemical Society

there is a need to run the process as efficiently as possibly by recovering any energy lost to the environment, thus minimizing energy losses. Because the oxy-fuel combustion technology is still under development and many studies merely focus on its application in a power plant for CO2 capture, some studies on the flue gas have been performed to investigate the flue-gas recycle effect in an oxy-fuel combustion.6−10 However, the amount of recycled flue gas is one of the key parameters that has a significant effect on combustion characteristics, such as adiabatic flame temperature, thermal radiation, and thermal capacity. For example, to attain a similar adiabatic flame temperature to air-fired combustion, the O2 concentration of the gases passing through the burner is higher (typically 30% higher than that of air), necessitating that about two-thirds of the flue gas be recycled;10 the concentration of triatomic gas molecules (CO2 and H2O) in the flue gas increases and changes the emissivity of the gases, resulting in an increase of the thermal capacity of the gases. One of the objectives of this paper is the influence of recycle flue gas on boiler and stream parameters.10 A large number of industrial processes covering most industrial sectors use significant amounts of energy in the form of heat, which is rarely used efficiently. There are many studies in progress targeting different aspects of the oxy-combustion process, such as the operating conditions, CO2 removal, and various process configurations.11−13 There is considerable scope for the use of heat exchangers and other forms of heat equipment to enable waste heat to be recovered.11 The optimization of heat-exchanger Received: May 15, 2012 Revised: October 5, 2012 Published: October 13, 2012 6930

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Figure 1. Schematic diagram of the boiler with recycled flue gas.

networks (HENs) has a high potential to reduce this wasted energy penalty.12 It may be possible to harness energy that is commonly rejected in the form of pressure, temperature of waste gases and other products, or unburnt fuel from one process and use it to reduce the energy needs of the same or some other process. Understanding how the energy flows through the process as well as where it is lost is necessary before recovery of energy in the form of preheat of charge, combustion air, fuel, steam, or unburnt fuel can be maximized. Jupiter Oxygen Corporation, with support from the National Energy Technology Laboratory (NETL), has constructed a multi-fuel, air- or oxygen-fired burner test facility with a capacity of up to 15 MWth. Researchers at the NETL have patented a process, integrated pollutant removal (IPR), that uses off the shelf technology to produce a sequestration-ready CO2 stream from an oxy-combustion power plant. IPR is a staged approach to CO2 capture that integrates compression and condensation with heat recovery to produce a supercritical CO2/ tramp gas stream. This CO2 stream would then be suitable for sequestration or further purification as required. Within IPR, heat recovered from combustion products can be applied to the steam cycle via feedwater heaters or used as a source of process heat, minimizing the energy penalty to operate these systems.13 The NETL has installed a small, slipstream IPR system for use in conjunction with the oxy-fired burner test facility built at Jupiter Oxygen Corporation in Hammond, IN. A model of that system is discussed in this paper. Data from the installed system are used to verify the model results. With touch point model results verified, the model is then examined for system results not as readily measurable in the field. There is great potential to capture heat from IPR gaseous streams. The gaseous streams within IPR represent the largest and most readily exploited source of recoverable heat.14 Most of the heat (heat remaining after transfer to the boiler tubes) is captured from flue gas in the absorption column at the IPR inlet. The remaining heat is captured at the compression stages (both sensible and latent heat). This process involves direct heat transfer between the hot gas stream and a cooler circulating water stream accompanied by dehumidification of the gas. Water condensed from the flue gas can be recovered to offset the need for water intake from the environment. However, condensation in flue gas is a complicated phenomenon because heat and mass transfer of water vapor and various acid vapors simultaneously occur in the presence of noncondensable gases.15 Recovery of low-potential energy of flue gases in the

Table 1. Typical Composition of Coal Analysis components

as received (mass %)

dry (mass %)

moisture ash volatile matter fixed carbon sulfur gross calorific value (kJ/kg) carbon hydrogen nitrogen oxygen

4.12 12.47 34.62 48.79 3.48 28084 66.67 4.53 1.37 7.36

0 13.01 36.11 50.88 3.63 29291 69.53 4.73 1.43 7.67

absorption column has become one of the problems of interest in research.15 Effective condensing heat exchange requires that the coolant leaves the column at a certain temperature “ΔT” degrees lower than the wet bulb temperature of the gas at all times. This allows for thermal-driving forces and prevents fogging conditions in the tower. However, the maximum coolant exit temperature is attained by minimizing ΔT. “ΔT” values as low as 2.5 °C are feasible for packed-bed columns and have been previously demonstrated in the literature.15 In this paper, energy flow and possible energy recovery within the IPR system is discussed in detail. To study the energy flow and energy recovery in the IPR system, flue gas is simulated using ASPEN Plus. Grand composite curve, composite curve, and heatexchange network are used to study the energy flow and energy extraction of the IPR system. The grand composite curve is a graphical representation of the excess heat available to a process within each temperature interval. In intervals where a net heat surplus exists, that heat is cascaded to lower temperature intervals. Once the demand for heat is satisfied at lower temperature intervals, cooling utilities are applied to remove the remaining heat. In intervals where a net deficit of heat exists, the excess heat is used first from higher temperature intervals. Only after exhausting heat surpluses from higher temperature intervals are heating utilities applied. Composite curves are typical temperature−enthalpy diagrams of either cooling or heating loads at given time intervals.15 The objective of this study is to analyze the (i) energy flow though IPR systems, (ii) potential energy recovery in the IPR system, and (iii) optimization of the heat-exchange network of IPR systems. To that end, a computer simulation model in ASPEN Plus has been developed to simulate the coal-fired oxy-combustion and IPR systems. 6931

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provides the possibility of applying a low-dust load downstream of electrostatic precipitators (ESPs), as shown in Figure 1. The parameters used for simulation are shown in Table 2, and the simulated results are shown in Table 3. Boiler. The boiler is modeled using RYield and RGibbs blocks. The RGibbs block was used to model the reactions occurring at the later stages of IPR, which uses Gibbs free energy minimization with phase splitting to calculate equilibrium. This approach does not require specifying the reaction stoichiometry. Heat Exchanger/Cooler. IPR heat exchangers were modeled using HeatX block. HeatX can perform a full-zone analysis with the heat-transfer coefficient and pressure-drop estimation for single- and two-phase streams. This block has correlations to estimate the sensible heat, nucleate boiling, and condensation film coefficients. The heat-transfer coefficients and other important parameters used in the simulations are taken from manuals of heat exchangers, which are installed at the pilot-scale IPR system. Flash. In flash separation, a gaseous mixture is partially liquefied. Vapor reaches the equilibrium condition with the liquid phase, and the two phases are separated, with the gas phase richer in more volatile components. IPR flash operations are modeled using Flash2 block. This block performs vapor−liquid equilibrium calculations and determines the thermal and phase conditions of a mixture of one or more inlet streams. Absorber. The absorber column is modeled using the RadFrac block. RadFrac is a rigorous model for simulating all types of multistage vapor−liquid fractionation operations. It can model columns in which chemical reactions are occurring. Reactions can have fixed conversions, or they can be equilibrium, rate-controlled, or electrolytic. Simulation of the Compressor. Compressors, turbines, and fans can all be simulated in ASPEN Plus by a block called COMPR. COMPR models polytropic and positive displacement compressors, isentropic compressors and turbines, and fans. COMPR calculates the power required (or produced) given the pressure ratio, isentropic, polytropic, and mechanical efficiencies, and for positive displacement compressors, the clearance volume. The accuracy of the results depends upon the efficiencies specified. Filter. The filter block is used for the filtering operation.

MODELING BOILER FLUE-GAS SYSTEM, IPR, AND ENERGY COMPONENTS

A process flow diagram for the oxy-coal combustion is shown in Figure 1. Initially, the coal is dried using nitrogen. The composition of coal is shown in Table 1. The dried coal stream is fed into the boiler for combustion. As occurs in air firing, the recycled flue gas serves as a coal motivator and (with O2 added) as a coal comburent. These make up the primary and secondary gas streams, respectively, serving the burner of a PC boiler. To carry coal moisture as vapor at a relatively low temperature and to avoid the risk of explosion if the oxygen concentration is too high and the problem of corrosion resulting from acid gas components (SO3, HCl, etc.), the flue gas must be dried and recycled after all flue-gas cleaning units. Removal of particles as the first step

Table 2. ASPEN Plus Unit Operation Block Description for Boiler and Recycled Flue Gas block name (ASPEN block)

block parameters

dry reactor (Rstoic)

discharge pressure 1 atm dry flash (Flash2) P = 1 atm (14.7 psi) boiler (RYield, RGibbs) P = 1 atm (14.7 psi) cooler (Heater) P = 1 atm (14.7 psi) cyclone (Cyclone) separation effieciency 0.8 filter (FabFI) pressure drop 0.1 psi electrostatic separation precipitator (Fsplit) effieciency 0.01 flash separator (Flash2) split fraction 0.2 heat exchanger (HeatX)

exchange area 0.5 m2

description dry the coal using nitrogen separating N2 after use burning coal cools the flue gas removes fly ash particles compresses the flue gas into 136.09 atm removes remaining flash ash particles flash the flue gas stream and resend it to the boiler exchange heat with flue gas

Table 3. Simulated Boiler and Flue-Gas Results coal fed into the boiler mole flow (kmol/h) mass flow (kg/h) volume flow (m3/h) temperature (°C) pressure (atm) molar enthalpy (kJ/kmol) mass enthalpy (kJ/kg) enthalpy flow (kJ/h) molar entropy (kJ kmol−1 K−1) mass entropy (kJ kg−1 K−1) molar density (kmol/m3) mass density (kg/m3) average molecular weight component mole fraction H2O N2 O2 NO2 NO SO3 H2 C CO2 SO2 N2O

22.67 199.96 1.00027 −3417.6 −77509.9

1413.47

0 0 0 0 0 0 0 0 0 0 0

flue gas after the boiler

recycled flue gas (split ratio = 0.4)

flue gas to IPR (split ratio = 0.4)

10−5 10−6 10−5 10−3

2.42 77.07 595.69 2987.86 1.088 −1.28 × 105 −4031.025 −3.11 × 105 117.6605

0.96 30.82 38.40 204.44 0.98 −2.69 × 105 −8474.462 −2.61 × 105 25.36567

1.45 46.24 57.61 204.44 0.98 −2.69 × 105 −8474.462 −3.92 × 105 25.36567

−7.16 × 10−5 0.040 1.30 31.99

3.70 4.07 × 10−3 0.12 31.79

0.79 0.025 0.80 31.79

0.79 0.025 0.80 31.79

4.34 × 10−3 5.04 × 10−3 2.04 × 10−6 2.32 × 10−11 1.75 × 10−5 7.20 × 10−9 0.2895996 0 0.6938284 7.17 × 10−3 7.12 × 10−11

4.34 × 10−3 5.04 × 10−3 2.04 × 10−6 2.32 × 10−11 1.75 × 10−5 7.20 × 10−9 0.2895996 0 0.6938284 7.17 × 10−3 7.12 × 10−11

4.34 × 10−3 5.04 × 10−3 2.04 × 10−6 2.32 × 10−11 1.75 × 10−5 7.20 × 10−9 0.2895996 0 0.6938284 7.17 × 10−3 7.12 × 10−11

oxygen fed into the boiler 1.13 36.28 27.73 25 1.00027 −5.52 × −1.72 × −6.26 × −2.29 ×

0.00 0.00 1.00 0 0 0 0 0.00 0.00 0 0

× 100 × 100 × 100

× 100 × 100

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Splitting Streams. To simulate the stream bleeds from the various points in the IPR system, Fsplit blocks are used. The Fsplit block is used to split the flow into different branches, as shown below.

statements and can be used to calculate and set input variables based on special user inputs. Model variables are specified in components and streams with written FORTRAN codes, and the sequence is set in which the block must be executed during the flow sheet calculations. Cyclone and ESP. Both blocks are used for removing fly ash particles. The cyclone block separates an inlet gas stream containing solids into a solids stream and a gas stream carrying the residual solids. The remaining solids particles are separated using the ESP block. They are vertically mounted, collecting plates with discharging wires. The wires are parallel and positioned midway between the plates. Reactors. The RGibbs block was used to model the reactions occurring at the later stages of IPR, which uses Gibbs free energy minimization with phase splitting to calculate equilibrium, and it also does not require specification of the reaction stoichiometry. Combustion processes were used in RYield and RStoic models. Because coal is a nonconventional component, it is decomposed into constituent elements by the RYield block before it is sent to the RStoic block. The RStoic block can model reactions occurring simultaneously or sequentially. The following reactions were considered in the simulation:

mi = ximt Calculator Block. The calculator block is used to write codes for the boiler system. They provide a mechanism to incorporate FORTRAN

Figure 2. Influence of the recycled flue gas on the burner temperature.

C + O2 → CO

(1)

C + O2 → CO2

(2)

coal N + O2 → NO

(3)

coal S + O2 → SO2

(4)

coal H + O2 → H 2O

(5)

Pinch Analysis. The implementation of pinch principles for analysis of energy resources in the mid-1980s had a big impact on systematic energy management. The success of pinch technology in hundreds of different applications had provoked the implementation of the same principles for environmental protection. Thermal pinch analysis is a HEN optimization algorithm used for reducing energy consumption in processes by setting feasible energy targets and achieving them through optimizing the heat recovery systems, energy supply methods, and process operating conditions for energy reduction. Pinch analysis has proven to be a useful tool for the design of efficient HEN. From the simple addition of cold and hot streams in a temperature− enthalpy diagram, the best way to exchange heat from the hot to the cold streams can be found through a network with either the minimum number of components or the minimum amount of investment.14 The pinch analysis software used in this study is the ASPEN energy analyzer. The ASPEN Plus simulation file is exported to the ASPEN energy analyzer. It helps maximize the reuse of heating and cooling duties within the system, with an optimal HEN. The duties that are not satisfied by heat recovery then require external utilities. This approach

Figure 3. Influence of the flashed vapor on boiler parameters.

Figure 4. Schematic diagram of the ASPEN IPR model. 6933

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The influence of the recycled flue-gas stream on the burner flame temperature is shown in Figure 2. When there is no recycle, the burner temperature is around 3900 °C. When the recycle split ratio increased, the burner temperature decreases and, at the full flue-gas recycle, the burner temperature is around 1480 °C. Diatomic gases, such as CO2, H2O, and SO2, have the ability to absorb more heat. When the flue-gas recycle ratio is increased, more diatomic components are introduced at the combustion stage, which reduces the flame temperature. By adding the possibility of radically changing the O2 concentration in the comburent, oxy-fuel combustion adds a new engineering dimension for design purposes. Influence of Flashed Vapor on Boiler Parameters. The recycled flue gas consists of water vapor and corrosive components, such as SOx and HCl. The extent to which these are removed impacts the corrosivity and heat capacity of the comburent gases. When the component conditions and inlet parameters were kept constant, as in the general simulation case, the vapor fraction flashed in the vessel varied from 0.7 to 1 with a step change of 0.01. The results are shown in Figure 3. According to the ASPEN sensitivity analysis, if all of the flue-gas stream is flashed back to the boiler (100%), then there is no condensation of SO2 and H2O. At least a minimum of 2% liquid fraction condensation should occur in the flash vessel to condense corrosive components, such SO2 and HCl, along with water vapor. It happens at 49.19 atm (723 psia) flash pressure. IPR Modeled in ASPEN. Figure 4 gives a schematic description of the IPR process as it exists at Jupiter Oxygen Corporation in Hammond, IN. In this system, oxy-combustion flue gas enters from a tap on the flue-gas recirculation (FGR) duct. This gas enters IPR at approximately 200 °C and >1 atm and flows into a glass pipe spray tower. In the tower, the gas rises countercurrently to a spray stream of recirculated flue-gas water. This water can be buffered by injecting a sodium carbonate solution. This water stream enters the spray tower (TWR), where it cools, condenses, and cleans this ash-laden gas. The water is then recirculated, except for the volume removed to balance condensed water from flue gas as well as buffer solution injected into the system. The balance of the water leaving the tower (that which is not recirculated for use as tower spray) is removed from the process for treatment outside IPR. A blower (BLW) moves the spray-cleaned gas through at heat exchanger and set of H-BLW filters, ensuring cleaner and cooler gas enters the first compression stage (STG1). The first compression stage increases the pressure to ∼3 atm. The heat of compression is recovered via a counterflow indirect-contact heat exchanger. The heat exchange also condenses water vapor, which is collected into a vessel (V4) for analysis. Compression/cooling/liquid collection is repeated in three more stages to 14, 40, and 136 atm, resulting in a dry CO2 product, which is captured for analysis. The main reactions in the absorber are

Table 4. ASPEN Plus Unit Operation Block Description for the IPR System block name (ASPEN block)

block parameters

ABSORPER stages 2 (RadFrac) BLOWER (Pump) hp 0.01 COMPRE-1 (Compr) COMPRE-2 (Compr) COMPRE-3 (Compr) COMPRE-4 (Compr) FSPLIT-1 (Fsplit) FSPLIT-2 (Fsplit) FSPLIT-3 (Fsplit) FSPLIT-4 (Fsplit) FSPLIT-5 (Fsplit) FLASH-1 (Flash2)

discharge pressure 3.06 atm discharge pressure 13.61 atm discharge pressure 40.82 atm discharge pressure 136.09 atm split fraction 0.1 split fraction 0.1 split fraction 0.1 split fraction 0.1 split fraction 0.1 P = 1 atm (14.7 psi)

FLASH-2 (Flash2) P = 3.06 atm (45 psia) FLASH-3 (Flash2) P = 13.61 atm (200 psia) FLASH-4 (Flash2) P = 40.82 atm (600 psia) HEAFL1 (Flash 2) P = 3.062 atm (45 psia) HEAFL2 (Flash 2) P = 13.61 atm (200 psia) HEAFL3 (Flash 2) P = 40.82 atm (600 psia) HEAFL4 (Flash 2) P = 136.09 atm (2000 psia) HEATEX-1 exchange area 0.46 m3 (HeatX) HEATEX-2 exchange area 0.46 m3 (HeatX) HEATEX-3 exchange area 0.46 m3 (HeatX) HEATEX-4 exchange area 0.46 m3 (HeatX) HEATEX-5 exchange area 0.46 m3 (HeatX) RG1 (RGibbs) P = 3.06 atm, T = 240 °F

SEPER-1 (Sep)

P = 13.61 atm, T = 220 °F P = 40.82 atm, T = 210 °F P = 1 atm

WATSEP (Sep)

P = 13.61 atm

RG2 (RGibbs) RG3 (RGibbs)

description absorbs products from flue gas into water boost flue gas flow into next stages compresses the flue gas into 3.06 atm compresses the flue gas into 13.61 atm compresses the flue gas into 40.82 atm compresses the flue gas into 136.09 atm split the stream into exit split the stream into flash 1 split the stream into flash 2 split the stream into flash 3 split the stream into flash 4 flash part of the stream into COMPRE-1 flash part of the stream into COMPRE-2 flash part of the stream into COMPRE-3 flash part of the stream into COMPRE-4 removes condensate from heat exchanger 2 removes condensate from heat exchanger 3 removes condensate from heat exchanger 4 removes condensate from heat exchanger 5 hot condensed water and cooling water COMPRE-1 flue gas and cooling water COMPRE-2 flue gas and cooling water COMPRE-3 flue gas and cooling water COMPRE-4 flue gas and cooling water reactions occurring in HEATEX-2 reactions occurring in HEATEX-3 reactions occurring in HEATEX-4 condensation of water (representing filter) condensation of water (representing filter)

minimizes the external utility requirements, conserving energy and capital at a lower environmental impact.

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RESULTS AND DISCUSSION Influence of Recycled Flue Gas on the Burner Temperature. After ESP, the flue gas is directed to the Fsplit block where some of the flue gas is recycled back to the boiler. Here, the split ratio is defined as

Na 2CO3 + SO2 → Na 2SO3 + CO2

(6)

2NO + H 2O + 1.5O2 → 2HNO3

(7)

The low-pressure (LP)-IPR system compresses the combustion gas from the burner test facility to a nominal pressure of 13.6 atm (175 psi). The high-pressure side of IPR further compresses the gas to around 136.1 atm (2000 psi). Operating conditions of the IPR plant were built and simulated using ASPEN Plus to investigate the influence of different parameters.

split ratio=mass of flue gas recycled/mass of flue gas present after ESP 6934

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Table 5. Simulated IPR ASPEN Model with Results in Mole Fractions after Each Stage mole flow (kmol/h) mass flow (kg/h) volume flow (m3/h) temperature (°C) pressure (atm) vapor fraction liquid fraction solid fraction molar enthalpy (J/kmol) mass enthalpy (J/kg) enthalpy flow (W) molar entropy (J kmol−1 K−1) mass entropy (J kg−1 K−1) molar density (kmol/m3) mass density (kg/m3) average molecular weight component mole fraction H2O N2 O2 NO2 NO SO3 H2 Cl2 HCl C CO CO2 SO2 S N2O Na2CO3 Na2SO3 HNO3 Ar

inlet

stage 1

stage 2

stage 3

stage 4

1.32 45.35 43.03 160 1.088 1 0 0 −2.98 × 108 −8.68 × 106 −1.09 × 105 9133.866 266.1295 0.030708 1.053919 34.32113

0.78 32.52 6.48 37.51 3.062 1 0 0 −3.24 × 108 −7.86 × 106 −71040.9 −720.177 −17.4521 0.121482 5.013054 41.26586

0.776429 32.24 1.43 48.49 13.60 1 0 0 −3.25 × 108 −7.83 × 106 −70114 −12246.7 −294.861 0.539743 22.41749 41.53365

0.766647 31.95 0.39 33.46 40.82 1 0 0 −3.27 × 108 −7.84 × 106 −69579.4 −25225.4 −605.281 1.938815 80.80109 41.6755

0.766583 31.94 0.086 54.46 136.09 1 0 0 −3.29 × 108 −7.90 × 106 −70139.9 −40661.7 −975.666 8.891016 370.5406 41.67584

0.327554 0.058098 0.032455 3.01 × 10−6 5.61 × 10−4 0 0 0 5.51 × 10−5 0 3.01 × 10−4 0.560949 9.92 × 10−3 0 9.02 × 10−5 0 0 0 0.010017

0.018514 0.097326 0.053443 5.22 × 10−11 2.13 × 10−16 7.12 × 10−5 0 4.03 × 10−5 1.00 × 10−6 0 0 0.813973 3.40 × 10−16 0 8.57 × 10−20 0 0 3.20 × 10−10 0.016631

6.89 × 10−3 0.098802 0.054247 1.89 × 10−10 5.73 × 10−16 7.11 × 10−5 0 4.10 × 10−5 5.91 × 10−7 0 0 0.823062 5.77 × 10−16 0 5.77 × 10−19 0 0 1.19 × 10−9 0.016882

3.18 × 10−4 0.100044 0.054909 2.45 × 10−10 1.42 × 10−16 5.89 × 10−5 0 4.14 × 10−5 4.84 × 10−8 0 0 0.827537 4.09 × 10−17 0 1.16 × 10−19 0 0 1.89 × 10−9 0.017092

2.98 × 10−4 0.100052 0.054913 2.45 × 10−10 0 5.88 × 10−5 0 4.14 × 10−5 4.84 × 10−8 0 0 0.827544 0 0 0 0 0 1.84 × 10−9 0.017093

Figure 5. Composite curves of IPR systems.

Figure 6. Grand composite curve of IPR systems.

The model focused on five sections, which are shown in Figure 4: (i) modeling the absorption column and (ii) modeling four compression stages at low- and high-pressure stages of the IPR system. The coal used in this study is Illinois No. 6 bituminous coal, the proximate and ultimate analyses of which are listed in Table 1. After the system diagram is built, the sections and components are defined, the chemical properties are chosen, and the values needed are provided. To study the energy flow and energy recovery in the IPR system, 45.4 kg/h (100 lb/h) of

flue gas is simulated using ASPEN Plus. Parameters used for simulation are shown in Table 4, and the simulated results are shown in Table 5. The results show that around 83% of CO2 is found at the final stage. A significant amount of heat energy from combustion (about 25% in a subcritical boiler) is not available for transfer to steam in actual practice; some of this energy is recovered in traditional comburent heating and feedwater heating. In actual practice, the cooling load of IPR will be linked to feedwater heating as well, decreasing its parasitic load. This model (as well as the experiment installed at 6935

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Figure 7. HEN of IPR system.

temperature requirements at process outlets. In intervals where a net deficit of heat exists, the excess heat is used first from higher temperature intervals. Only after exhausting heat surpluses from higher temperature intervals are heating utilities applied. Composite curves are typical temperature−enthalpy diagrams o2f either cooling loads or heating loads at each temperature interval. The composite curve, grand composite curve, and HEN of IPR systems are shown in Figures 5, 6, and 7, respectively. From the simulated results and enthalpy/heat calculations, 21 652.7 kJ/h (20 522.86 Btu/h) of energy can be recovered at heat exchanger 1. It is noted that most of the energy recovery is achieved at heat exchanger 1 in the current IPR setup. Table 6 shows the possible energy recoveries of all heat exchangers. If the IPR is scaled up to treat 4540 kg/h (10 000 lb/h) flue gas, 1.10 MW (3.76 MMBtu/h) of heat can be recovered. The first law of thermodynamics is conventionally used to analyze the energy use, but it is unable to account for the quality aspect of energy. That is where exergy analysis becomes relevant. Exergy is the consequent of the second law of thermodynamics. It is a property that enables us to determine the useful work potential of a given amount of energy at some specified state. Work is considered pure exergy; therefore, the value given by ASPEN Plus is used directly as the exergy of each work stream, and its sign is used to identify if the work exits or enters the piece of equipment considering ASPEN Plus sign criteria. The + sign is used when the work stream enters the piece of equipment (consumption), and the − sign is used when the work stream exits the piece of equipment (generation). All of the heat and work streams in the process shall be drawn in the ASPEN Plus simulation as outputs, to force their calculation by ASPEN Plus and show their values. The exergy of a heat flow is given by

Table 6. Energy Recovery in Heat Exchangers heat exchanger ID heat heat heat heat heat

exchanger exchanger exchanger exchanger exchanger

1 2 3 4 5

temperature (°C)

energy recovery (kJ/h)

52.53 130.28 155.36 165.44 160.82

21652.7 3644.5 4722.3 3895.4 5732.8

Figure 8. Exergy and energy analyses on heat exchanger 1.

the JHBTF) recovers all available heat in IPR, mainly at the first wet heat exchanger. Heat recovery from flue gas can be expressed as Q r = mfg CpΔTfg

⎛ T⎞ E = ⎜1 − 0 ⎟Q ⎝ T⎠

(8)

where mfg is the mass flow rate of flue gases for heat recovery, Cp is the specific heat of flue gas, and ΔTp is the temperature drop of the flue gases. The grand composite curve is a graphical representation of the excess heat available to a process within each temperature interval. In intervals where a net heat surplus exists, heat is cascaded to lower temperature intervals. Once the demand for heat is satisfied at lower temperature intervals, cooling utilities are applied to remove the remaining heat to meet user-input

(9)

where heat Q and exergy E can have different signs. In this case, the exergy sign will always be considered positive, considering its physically appropriate direction to make this happen. According to the simulated results, high energy recovery can be extracted in heat exchanger 1. The exergy and energy analyses of heat exchanger 1 were shown in Figure 8. It was 6936

dx.doi.org/10.1021/ef301500g | Energy Fuels 2012, 26, 6930−6937

Energy & Fuels

Article

Table 7. Comparison of Model and Field Data CO2 H2O O2 SO2 NO2 NO N2O CO HCl N2/Ar

IPR inlet field

model

2 stage field

56 32.7 3.24 0.99 3.00 × 10−4 0.056 0.009 0.03 0.03 6.78

56 32.7 3.24 0.99 3.00 × 10−4 0.056 0.009 0.03 0.03 6.78

82 0.11 3.5 0.043 7.00 × 10−4 0.038 0.004 0.05 ND 14.55

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found that, with increase in the water flow rate at the cold side of heat exchanger 1 from 0 to 250 kg/h, the exergy reduced drastically from 215 200 to 26 400 J/kmol, whereas the enthalpy content reduced from 3 558 600 to 1 438 700 J/ kmol. The difference between exergy and energy at a particular flow rate increased with an increase in the water flow rate. Therefore, it is better to operate at a low water flow rate to recovery more useful energy (exergy). Model results were compared to field IPR data collected in December 2009 at Jupiter Oxygen Corporation in Hammond, IN. The model and field data are shown in Table 7. The major species present in the gaseous streams, such as CO2, H2O, and O2, are in good agreement with operating data. Model results show the species such as SOx and NOx are more fully removed in the model compared to field results. Some of these disagreements between experimental findings and the model predictions are attributed to equilibrium being used to predict a non-equilibrium process (note that interactions within the IPR process may be kinetically controlled). Also, the IPR simulation was performed in a steady-state mode, whereas the process measured in the field was not, generally, at steady state.

ND 4.08 × 10−9 ND ND ND 6.660 × 10−5 5.85 × 100

final stage field 89 0.06 4.34 0.003 0 4.00 × 10−10 0.005 0.06 ND 6.85

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CONCLUSION The boiler and IPR system were simulated using ASPEN Plus, and the flue-gas recycle of the model, energy flow of the IPR system, and potential energy recovery of the IPR system were analyzed. According to the simulated results, (i) at least 2% of flue-gas stream should be condensed to condense water vapor along with other corrosive products and (ii) there is a big potential of energy recovery from flue gas in the IPR system. The main heat recovery occurs at heat exchanger 1, which is just after the absorption column. Modeling the behavior of IPR systems for power plants shows that energy recovery and pollution removal can be achieved from flue-gas streams.

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model 82 0.4 5.3

AUTHOR INFORMATION

Corresponding Author

*Telephone: 541-918-8093. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This study was supported by the Oak Ridge Institute for Science and Education (ORISE) at the National Energy Technology Laboratory (NETL) with funding from the Department of Energy (DOE). 6937

dx.doi.org/10.1021/ef301500g | Energy Fuels 2012, 26, 6930−6937