G Blount

Ph.D M Gorensek

Ph.D L Hamm

; K O'neil

Ph.D ; C Kervévan

M.-H Beddelem

Pi-CO 2 Aqueous Post-Combustion CO 2 Capture: Proof of Concept through Thermodynamic, Hydrodynamic, and Gas-Lift Pump Modeling

Keywords: Carbon Capture, Enhanced Oil Recovery, EOR, Greenhouse Gas, GHG, Carbon Dioxide, CO2, Dissolved CO2, Industrial Emission

Partnering in Innovation, Inc. (Pi-Innovation) introduces an aqueous post-combustion carbon dioxide (CO2) capture system (Pi-CO2) that offers high market value by directly addressing the primary constraints limiting beneficial re-use markets (lowering parasitic energy costs, reducing delivered cost of capture, eliminating the need for special solvents, etc.). A highly experienced team has completed initial design, modeling, manufacturing verification, and financial analysis for commercial market entry. Coupled thermodynamic and thermal-hydraulic mass transfer modeling results fully support proof of concept. Pi-CO2 has the potential to lower total cost and risk to levels sufficient to stimulate global demand for CO2 from local industrial sources.

Introduction

Partnering in Innovation, Inc. (Pi-Innovation) introduces Pi-CO2, a novel post-combustion aqueous-based carbon dioxide (CO2) capture system. Pi-CO2 is a patented method that uses well known processes and standard components to offer post-combustion CO2 capture with low parasitic energy, low delivered CO2 product pricing for the enhanced oil recovery (EOR) market, and no hazardous degradation by-products.

Although the Pi-CO2 system was initially designed for Enhanced Oil Recovery (EOR) applications, Pi-Innovation is also part of the CO2-DISSOLVED project team, funded by the ANR (French National Research Agency). The CO2-DISSOLVED project examines an integration of CO2 capture, geothermal heat production, and local CO2 storage for low to medium CO2 concentration industrial emissions [START_REF] Kervévan | CO2-DISSOLVED: A Novel Approach to Combining CCS and Geothermal Heat Recovery[END_REF]. In separate papers [START_REF] Kervévan | CO2-DISSOLVED: a novel concept coupling geological storage of dissolved CO2 and geothermal heat recovery -Part 1: Assessment of the integration of an innovative low-cost, water-based CO2 capture technology[END_REF][START_REF] Castillo | CO2-DISSOLVED: a novel concept coupling geological storage of dissolved CO2 and geothermal heat recovery -Part 2: Assessment of the potential industrial applicability in France, Germany, and the[END_REF] we assess the feasibility of integrating the Pi-CO2 technology as part of the CO2-DISSOLVED concept, exploring specific modeling results in the context of low emission concentrations.

After a short description of the general configuration of the system, this paper presents thermodynamic, hydrodynamic, and gas-lift pump modeling results demonstrating the viability of the Pi-CO2 concept in the context of EOR applications.

Process description

The system uses water as a physical solvent for the capture of CO2 from flue gas. The water is circulated through a closed loop absorber and pressure swing desorber that operate in a large diameter, deep, water filled shaft that is sealed on the bottom and sides. The shaft functions as a very deep tank to create a range of hydrostatic pressure; from atmospheric pressure to approximately 60 bar. The higher hydrostatic pressures support maximized CO2 solubility in water, and lower pressures promote pressure swing desorption. Figure 1 and the following process step discussion illustrate the general configuration of the system. The primary process steps are as follows:

1. Flue gas is compressed and introduced into a counter-current absorber located in a sealed deep large diameter shaft (under 25 -60 bar hydrostatic pressure).

2. Hydrostatic pressure significantly increases the solubility of gases in water; CO2 is preferentially transferred to the water (physical solvent). The non-dissolved separated N2 rich gas fraction is diverted to the surface.

3. Heat from compression is transferred to the pressurized N2 rich gas stream from the absorber to recover compression energy with turbo-machinery.

4. CO2 effervesces from the water in the riser causing gas lift circulation to the surface, CO2 is extracted, and water is returned to the tank; CO2 product is compressed for delivery to the oil reservoir for enhanced oil recovery (EOR) via pipeline.

A simplified schematic of the configuration of the system in a large diameter deep shaft is shown in Figure 2. The figure shows the closed loop nature of the circulation, and the general position of the absorber and the riser pipe. The entry of compressed flue gas at approximately 60 bar (~600 meters depth), and the exit of the N2 rich stream for heating and energy recovery are also shown on the diagram.

Process Modeling

The process was modeled with Aspen Plus TM to quantify energy consumers, heat transfer from compression to energy recovery, and energy producers. Energy consumers consist of flue gas compression and product compression, energy producers consist of expander turbines. The capture system is configured for energy conservation by linking consumers with producers. Pinch analysis was performed to for best use of the compression heat by expanders. The latest process flow of Pi-CO 2 includes integration of drive systems and additional energy recovery that further offset the energy and cost of CO 2 capture. Figure 3 is a simplified summary of an early version of the Pi-CO2 process showing energy demand in megawatts electricity (MW e ) for consumers and energy offsets by producers in MW e for a 100MWe coal fired power plant with 82.2 metric tons/hour captured (MT/hr). Cooling targets for intercoolers and after coolers for compression are also shown as low pressure (LP), medium pressure (MP), and high pressure steam (HP) as megawatts thermal energy (MW th ). These cooling targets are for excess heat not recoverable by producers.

Mass Transfer Modeling

The transfer of CO2 from the compressed flue gas to the water phase is performed in a patented multistage counter current cascading absorber that has an entry gas pressure of ~60bar. Figure 4 illustrates a simplified single absorber stage; showing water and flue gas entry and CO2 enriched water and CO2 depleted gas exit. The CO2 depleted gas is directed down the absorber cascade. Houghton et al. [START_REF] Houghton | Absorption of Carbon Dioxide in Water under Pressures using a Gas -Bubble Column[END_REF] studied the transfer of CO2 to water under pressure in bubble columns. The absorber throughput for the Pi-CO2 system is far greater than most studies due to its large scale (diameter and length). Hills [START_REF] Hills | The operation of a bubble column at high throughputs: Gas holdup measurements[END_REF] investigated gas holdup behavior of bubble columns with high throughputs, and demonstrated that high gas velocities are achievable.

Drift flux modeling was used to evaluate the overall performance of the absorber for water velocity, void fraction, bubble diameter, bubble velocity, absorber dimensions, cascade attributes, water mass, CO 2 captured, and other parameters. A thermal-hydraulic drift-flux modeling approach is recognized in the literature as applicable for absorbers (Zuber and Findlay [6]; Wallis [START_REF] Wallis | One-Dimensional Two-Phase Flow[END_REF]; Lahey and Moody [START_REF] Lahey | The Thermal-Hydraulics of a Boiling Water Reactor[END_REF]; Collier [START_REF] Collier | Convective Boiling and Condensation[END_REF]; Todreas and Kazimi [START_REF] Todreas | Nuclear System I: Thermal Hydraulic Fundamentals[END_REF]). Driftflux modeling indicates that a single multistage cascading counter current absorber in a large diameter shaft is capable of capturing ~100 metric tons/hour CO 2 with greater than 90% capture efficiency. The system is scalable with the addition or removal of stages. Pi-CO2 can be efficiently scaled to capture very high volumes of CO2. The step wise absorption of CO 2 in the stage and cascade operating line with comparison to equilibrium conditions is shown below (Figure 5). CO2 concentration in the depleting flue gas is also illustrated with respect to the absorber cascade.

Simple Gas-Lift Pumping Modeling

Water circulation in the Pi-CO2 capture process is closed loop and uses gas-lift pumping to move the water. The compression energy of the gases that dissolve in the water provides the pumping energy through gas effervescence as the water rises up the return line in lower pressure conditions. The effervescing gas eliminates the need for a gas bubbler which is used in typical gas-lift pumping, and improves pump efficiency due to the uniform distribution of the effervescence.

Fan et. al. [START_REF] Fan | Experimental study on the performance of an air -lift pump for artificial upwelling[END_REF] performed a series of relatively large scale gas-lift pump tests in a freshwater lake in China. The tests were performed with a range of bubbler designs, they maintained relatively high gas flow rates, pumped large quantities of water, and maintained a high submergence factor. The general configuration of these tests is similar to pumping conditions for the Pi-CO2 capture process except: 1) the scale is smaller in the test, 2) a bubbler was used, 3) the test system had relatively high pipe boundary effects, 4) the length of the riser pipe is shorter, 5) the Fan et al tests only operated under turbulent flow conditions. The highest pumping capacities are achieved under slug flow conditions common to gas lift pumps. Figure 6 is a graph showing a curve fit for some of the data derived by Fan et al. [START_REF] Fan | Experimental study on the performance of an air -lift pump for artificial upwelling[END_REF].

Figure 6 Fan et al. [START_REF] Fan | Experimental study on the performance of an air -lift pump for artificial upwelling[END_REF] data with natural log curve fit When the empirical data from Fan et al. [START_REF] Fan | Experimental study on the performance of an air -lift pump for artificial upwelling[END_REF] is scaled for higher gas flow with the natural log curve (y=814.7ln(x)-636.51; R2=0.98) it is possible to generate a range of pumping capacities based upon metric tons of CO2/hour that would be effervescing from the water. Figure 7 is a graph of effervescing gas versus capacity of the pumped water to capture CO2. The figure illustrates that a volume of 20 metric tons of effervescing CO2 per hour can pump a water volume capable of capturing approximately 100 metric tons of CO2 per hour. The efficiency of gas-lift pumps decrease when too much gas is introduced. This situation is illustrated in Figure 7 where the water pumping capacity for 100 metric tons per hour of effervescing CO2 is only 125 metric tons per hour.

A simplistic gas-lift pump modeling using empirical data derived from large scale pump tests indicate that it is possible to circulate sufficient volumes of water for capture of over 100 mt/hr. The modeling effort also indicates that it will be necessary to remove the excessive gas effervescing from the rising water to maintain optimum gas-lift pumping that circulates water in the CO2 capture system.

Conclusions

Process modeling with Aspen Plus TM , mass transfer modeling using a thermal-hydraulic drift-flux approach, and simple gas-lift modeling, indicates that a low total parasitic energy demand and high capture efficiency can be achieved with the Pi-CO2 aqueous CO2 capture system. The modeling also indicates that it is possible to construct a system with a capacity of approximately 100 metric tons per hour. Multiple shafts with cascading absorber systems and gas-lift pumping can be configured to achieve capture capacities compatible with CO2 capture on a very large anthropogenic release scale. The large potential capture capacity is consistent with the size of the EOR market. Further modeling with the most current Pi-CO2 design demonstrates that a high purity product of greater than 95% CO2 can be produced. Financial analysis indicates that a CO2 product can be captured, purified, compressed, and pipelined to reservoirs for EOR at a price equal to approximately 2.5% of crude oil value per 1000ft2 CO2 (crude oil at $85/barrel).

Aspen Plus TM modeling of the system indicates that a product of greater than 95% CO2 can be produced with a parasitic energy demand of less than 25% while recovering approximately 90% of the CO2 from a coal based flue gas. The energy demand for system operation can be satisfied with a range of sources (electricity, steam, natural gas, process gas, etc.). Waste heat from a natural gas or process gas driven process can be recovered to significantly reduce overall energy cost.

Based on initial hydrodynamic modeling, mass transfer of CO2 from the flue gas was shown to be feasible in a series of moderately sized absorbers. In coal combustion settings, detailed Drift Flux modeling of the patented Mass Transfer System (MTS) confirms that its unique configuration can provide high efficiency transfer of CO2 to the water on a production scale. This aqueous process is easily scalable using commercially available equipment for the aboveground components. Analysis of the gas-lift pumping capacity of the MTS indicates that the volumetric flow of off-gassing CO2 is many times greater than that required to circulate the large volumes of water needed to achieve this production scale. Thus confirming gas lift pump efficiencies and confirming the assumption that additional energy and mechanical pumping are not needed for circulation.

The non-degradable nature of the physical solvent provides an opportunity to capture CO2 from flue gases associated with coal combustion, refining, smelting, and cement manufacture. Desulfurization common to existing flue gas sources is required to limit the contact of sulfur with the water. Compression of the flue gas prior to entry into the MTS offers the opportunity to condense reduced mercury that would otherwise be released to the atmosphere. This offers a unique differentiator and additional positive cost-benefit for coal-fired power plant operations.

In summary, Pi-CO2 uses well known processes and standard components in a novel configuration to offer CO2 capture with low parasitic energy, low cost, no hazardous constituents, no greenhouse gas (GHG) solvent lifecycle costs, and multiple ancillary benefits. Modeling results fully support proof of concept and demonstrate excellent prediction for CO2 and trace gas solubility; confirmation of energy demand and recovery potential; confirmation of step-wise mass transfer of the CO2 into the water and ex-solution purification steps; gas lift pump efficiencies; maximized gas contact, retention time, and collection and discharge of non-dissolved gases for energy recovery; and achievement of equilibrium in the column with 90% recovery using the current system design.

Figure 1

 1 Figure 1 Generalized process steps for the Pi-CO2 system

Figure 2

 2 Figure 2 Simplified configuration of system

Figure 3

 3 Figure 3 Early version of Pi-CO2 energy summary for a hypothetical 100MWe coal fired power plant

Figure 4

 4 Figure 4 Simplified absorber diagram showing entries and exits

Figure 5

 5 Figure 5 Modeled CO2 absorption behavior in water using a multistage cascading absorber

Figure 7

 7 Figure 7 Effervescing gas versus capacity of the pumped water to capture CO2


A Largeteau

S Oree\

C L Marzat

G Demazeau

MICROWAVE HEATING IN HIGH PRESSURE REACTION VESSELS

Keywords: Solvothermal, Microwaves, Synthesis, Powders, Pressure vesse!, Interna! heating

We report the use ofmicrowave-hydrothermal processing to synthesire various ceramic powders. Microwave-hydrothermal processing is compared with conventional hydro thennal processing in the crystallisation of MoO 2 • The presence of microwave fi eld leads to accelerated kinetics of crystallization of the finely divided molybdenum dioxide particles. Existing microwave heated pressure vessels for chemical synthesis cannot be used above 250 MPa and 270 °C because they contain parts made of polymeric materials. The objective ofthis work is to associate a microwave source to a high pressure vesse! in a way such that it might be used to carry out reactions in acqueous media at pressures around IOOMPa and temperatures above the critical point of water.

INTRODUCTION

functioning up to 400 MPa pressure and 700 °C. It has an internai diameter of 28.6 mm and an extemal diameter of 60 mm. At both ends of the cylinder, are found two high pressure plugs fitted with sapphire windows. A detailed view of the window mounting, similar in prin ciple to that proposed by Poulter [START_REF] Chervin | Mechanical strength of sap phire windows under pressure[END_REF], is given in Figure 2. An im portant feature of the closure element is that the window rests on an optically fiat ()./2) metallic seat and that no sealing material is used. Moreover, the cap is only meant to hold the sapphire window in place without subjecting it to any pre-pressure.

Bach high pressure plug is assembled to the ends of the pressure vessel by a cone closure mechanism as shown in Figure 3. lnlet and outlet holes enable the autoclave to be linked to a high pressure pump and the associated pressure control instrumentation. The entire de vice was tested up to 180 MPa at room temperature and remained leakproof in this range.

Mlcrowave Coupllng

In high pressure technology, it is highly recommended that the window aperture be minimised for safety reasons [START_REF] Chervin | Mechanical strength of sap phire windows under pressure[END_REF]. As a consequence, we have adopted a sapphire window having a diameter of 25.4 mm with sapphire merallic cap an unsupported diameter of 13.5 mm and a thickness of 6.35 mm. At such reduced dimensions and in order to avoid eut-off effects, excitation of the pressure vesse} is conveniently done through a coaxial, the window having the same role as an iris. In addition to its exceptional mechanical resistance under compression, the use of sapphire for the windows is also justified because its high permittivity (e� = 9.41 and tano=0.0001) allows a significant reduction in win dow size. Most high power generators at 2.45 GHz deliver microwave energy from a rectangular guide. We have therefore constructed a crossbar transition for converting the TE 01 mode wave propagation of the rectangular guide into the TEM mode of a coaxial line. The end of the centre conductor of the coaxial is in close contact with the high pressure window (Fig. 3). In these conditions, TM 01 mode is pre dominantly excited inside the autoclave. When water, ethanol or water-ethanol mixtures are present in the reaction vessel, the re flected power is less than 30%. This reflection can be further reduced to a few percent by using an impedance matching device.

The heating characteristics of water inside the vessel can be meas ured by a high pressure thermocouple placed in a region of the cavity where the electric field is low. Higher heating rates can be obtained by insulating the inner walls of the vessel with a closely fitting borosilicate glass tube. For example, in the absence of insulation, water initially at room temperature attains a temperature of 65 °C at the end of S minutes microwave heating at 450W: when an insulat ing inner tube is used, a temperature of 160 °C can be reached under equivalent conditions.

SOLVOTHERMALSYNTHESIS

Molybdenum dioxide MoO 2 is a black compound conventionally prepared by the reduction of the trioxide or ignition of ammonium molybdate. However, the particle size of the powder obtained in this manner is not fine enough and varies greatly. Our objective is to explore the formation of finely divided MoO 2 by the microwave solvothermal technique.

A few grammes of 99.8% pure ammonium molybdate crystals (NH 4 ) 2 MoO 4 from Aldrich, in a small glass tube, was introduced in the reaction vessel containing a mixture of equal volumes of water and ethanol. The contents of the autoclave were then microwave-heated at 400 to 1200 W while the pressure was maintained between 40 and 50 MPa. When the temperature of the reaction medium reached 345 °C after 28 minutes treatment, there was a rapid rise in temperature to around 410 °C followed by a marked increase in refl.ected power which led to an equally rapid cooling to a stable temperature of 280 °C. This phenomenon will be due to the decreases of microwave absorption of the reaction medium following the complete conversion of the sol uble ammonium molybdate phase into an insoluble phase. The total treatment time did not exceed 35 minutes.

RESULTS

On cooling, the reaction vessel was disassembled and a black slurry was collected. After separation of the precipitate from the liquid phase and drying, a finely divided powder was obtained. X-ray diffraction confirmed that the powder was highly crystalline MoO 2 • Figure 4 shows the X-ray diffraction pattern obtained.

FIGURE 1

 1 FIGURE 1 View of the parts of the disassembled high pressure cell. The hoops have been moved upwards for clarity in the drawing.

FIGURE 2 AFIGURE 3

 23 FIGURE 2 A two-dimensional view of the high pressure plug.

  FIGURE 4 X-ray diffraction pattern of the Mooz powder obtained by the microwave-solvothed method.

Scanning electron microscopy photographs reveal two types of particles present in the powder:

(i) monoclinic crystals of MoO2 of size 1 to 4 µm representing Jess than 20% of the powder. lt is through that formation of these particles occurs when the rate oftemperature rise is slow (Fig. 5a); (ii) small agglomerates of submicronic particles. These particles are supposed to nucleate when the rate of temperature rise is rapid (Fig. 5b).

In comparison, particles produced at 400 °C and 44 MPa by the conventional hydrothermal method (Fig. 5d) are usually of spherical shape having a diameter of 1 to 2microns.

CONCLUSION

The synthesis of a highly crystalline fonn of molybdenum dioxide using microwave heating has been demonstrated at about 400 °C and 45 MPa. The powders produced have a different microstructure compared to the powders produced by conventional solvothennal synthesis. More experimentation is required to understand the role of the factors (temperature, reaction medium composition, time and pressure) goveming the nucleation of the particles.