Coal Energy Conversion with Aquifer-Based Carbon Sequestration: An Approach to Electric Power Generation with Zero Matter Release to the Atmosphere

Transcription

1 Coal Energy Conversion with Aquifer-Based Carbon Sequestration: An Approach to Electric Power Generation with Zero Matter Release to the Atmosphere Investigators Reginald E. Mitchell (PI), Associate Professor, Mechanical Engineering; Christopher Edwards, Associate Professor, Mechanical Engineering; Scott Fendorf, Professor, Department of Environmental Earth System Sciences; Ben Kocar, Post-doctoral Student; Adam Berger, Eli Goldstein, John (J.R.) Heberle, BumJick Kim, Andrew Lee, and Paul Mobley, Graduate Researchers, Stanford University Abstract The overall objective of this project was to provide the information needed to develop a coal-based electric power generation scheme that takes water from a deep saline aquifer, desalinates and uses it to process coal at supercritical water conditions, and returns the water, which will contain dissolved CO 2, back to the aquifer along with the salts that were removed during desalination. The sensible energy of the hot conversion products is transferred to the working fluid of a heat engine for power generation. The only process effluents are supercritical water containing dissolved coal conversion products and solids, composed of ash and salts that precipitate from the supercritical water. In efforts to date, we have constructed a system-level model of the proposed plant and used it to evaluate the viability of the overall concept in terms of efficiency. Calculations indicate that efficiencies as high as 42%, on a lower heating value basis, can be obtained when account is made the energy penalties associated with oxygen separation, non-ideal pumps, compressors and turbines, and carbon dioxide sequestration. We have also completed the development of a software library for the thermodynamic properties of CO 2 and H 2 O, gases that behave non-ideally at the temperatures and pressures employed in the process units. In order to determine the thermodynamic properties of mixtures, a mixing model was also developed that models the departure from an ideal Helmholtz solution using a modified corresponding-states approach. The model has been used to determine thermodynamic properties when predicting the composition of the synthesis gas produced in the coal reformer and the composition of the combustion products. Wyodak coal, a low-ash, low-sulfur coal, was selected as the base-case coal for study. This is a sub-bituminous coal from Wyoming and is typical of the coals from the Powder River Basin that are currently being used for electric power generation. We have already determined the reactivity of the coal char to oxygen and carbon dioxide, and tests to determine the char reactivity to water have begun. The experimental facility being built to determine coal conversion rates under supercritical water conditions is complete as is the experimental facility being built to demonstrate stable combustion in supercritical water. In addition, geochemical models have been used to assess the fates of potentially toxic trace elements in the coal that may be dissolved in the water being returned to the aquifer. Results suggest that relatively high concentrations of As, Cr, Cu, Hg, Pb, Zn,

2 and Cd in the brine being returned to the aquifer are likely to be mitigated by secondary precipitation/adsorption reactions that occur within aquifers. The extent of these reactions will depend on the mineralogical makeup of the aquifer. Introduction This project had the objective of providing the information needed to develop a coalbased electric power generation process that involves coal conversion in supercritical water with CO 2 capture and storage in an inherently stable form. The only process effluents are supercritical water, containing dissolved coal conversion products, and solids, composed of fly ash and salts that precipitated from the water. Having no gaseous emissions, such a system eliminates the need for costly gas cleanup equipment. In addition, it provides a carbon dioxide sequestration option that may be more acceptable to the public. Coal-fired power plants have the potential to emit undesirable substances into the environment such as nitrogen and sulfur oxides, particulate matter, mercury, arsenic, lead, and uranium. Clean coal technologies that have been developed to remove these substances from flue gases before they are emitted into the atmosphere have significantly increased the cost of coal-derived energy, reducing the economic attractiveness of an otherwise inexpensive fuel. The economic benefit is further reduced when CO 2 must be removed from the flue gases and sequestered because of its potential impact on climate change. Constructing power plants that have no gaseous emissions at all but instead, that sequester the entire effluent stream would end the expensive cycle of continually identifying and managing the next-most-harmful coal conversion product. Deep saline aquifers have been recognized as suitable locations for the storage of CO 2. In the United States, the sites identified have the potential capacity to store about 86 years of CO 2 generated in coal-fired power plants at the rate of coal consumption for electric power generation in Preliminary estimates indicate that a 30-m thick aquifer having 1-Darcy permeability and 20% porosity can store the effluent of a 500 MW e power plant over its nominal 40-year lifetime. The proposed scheme for electric power generation under investigation produces CO 2 that is in equilibrium with the aquifer environment, eliminating the possibility of it migrating back to the surface through fissures or well bores once injected into the aquifer. The stream that will be returned to the aquifer will be in thermal and mechanical and close to chemical equilibrium with the water already in the aquifer. This injected stream will be less buoyant than liquid CO 2 at reservoir conditions, allaying any concerns about selective CO 2 release. The advantages of this aquifer-based coal-fired power plant relative to current and other proposed power generation systems include (i) maximally efficient power production while storing CO 2 products in indefinitely stable forms, (ii) zero traditional air pollutant emission and stack elimination, and (iii) size reduction of reactor vessel (compared to pulverized-coal systems). Although the thrust of the project is directed to clean coal utilization, the process being developed applies in general to all stationary thermal processors (gasifiers and combustors) using all types of fuel (coal, natural gas, oil, biomass, waste, etc.). Our investigation aims to lay the foundation for an efficient coal energy option with no matter release to the atmosphere and in which all fluid combustion products, particularly carbon dioxide, are pre-equilibrated in aquifer water before injection into the subsurface.

3 Overview of the Proposed Process In the proposed energy conversion scheme, a coal slurry, oxygen and water obtained from a deep saline aquifer are fed to a gasification reactor (a reformer) that is maintained at conditions above the critical temperature and pressure of water (T c,h2o = 647 K, P c,h2o = 221 bar). Due to the solvation properties of supercritical water (SCW), the small polar and nonpolar organic compounds released during coal extraction and devolatilization are dissolved in the water and the larger ones are hydrolyzed, yielding dissolved H 2, CO, CO 2, and low molecular weight hydrocarbons, without tar, soot or polyaromatic hydrocarbon formation. In essence, a synthesis gas dissolved in SCW is produced in the reformer. The syngas is sent through a solids separator in which the solids are returned to the reactor and the syngas is directed to a combustor where it reacts with oxygen to produce combustion products dissolved in supercritical water. The process stream that exits the combustor is a single-phase, supercritical solution of hot combustion products. The stream is passed through a heat exchanger, transferring its sensible energy to the working fluid of a heat engine that produces electrical power in a combined cycle scheme. The cooled water stream, saturated with combustion products, is returned to the same or nearby aquifer. Since inorganic salts are insoluble in supercritical water, the aquifer water must be desalinated before use. If not, the ability of the water to absorb coal conversion products would be reduced and the salts would precipitate in the gasification reactor, mixing with the ash. The salts would have to be separated from the ash if the ash were to be used, for instance, as aggregate material. Sulfur, nitrogen and many of the trace elements in coals are converted to insoluble salts in SCW. The salts formed will precipitate from the fluid mixture in the reformer, and will be removed from the system with the coal ash. In this coal conversion scheme, all trace species introduced with the coal (such as mercury, arsenic, etc.), as well as all coal conversion products, are sequestered in the aquifer along with the CO 2. The only matter not directed to the aquifer is the solid material consisting of the ash and precipitated inorganic salts. Although the proposed system still has an energy cost of air separation, the trade-off of carbon separation for air separation is advantageous since in the process all fluid coal combustion products are sequestered, including sulfur and trace metals, not just carbon dioxide. Research Objectives and Tasks The overall objective of this research project was to provide the information needed to design and develop the key process units in the proposed aquifer-based coal-toelectricity power plant with CO 2 capture and sequestration. The project was divided into four research areas - Area 1: Systems Analysis, Area 2: Supercritical Coal Reforming, Area 3: Synthesis Fluid Oxidation and Heat Extraction, and Area 4: Aquifer Interactions. In Area 1, thermodynamic analysis of the scheme provided fully qualified cycle efficiency and process analyses. These analyses were used to determine the design choices made in investigating component requirements in the proposed scheme. Research efforts in Area 2 (Supercritical Coal Reforming) were aimed at determining the supercritical water conditions that maximize the amount of chemical energy from the coal in the synthesis fluid. Defining the optimum amount of oxygen required to drive the gasification reactions and at the same time yield a high energy-content synthesis fluid as

4 a function of coal composition in the SCW environment was one of the goals of this task. In concert with this was the goal of developing models that can predict accurately coal conversion rates to synthesis fluid under SCW conditions. This required obtaining the data needed to characterize coal extraction and pyrolysis rates and char gasification and oxidation rates in supercritical water environments as functions of temperature, pressure and properties of the coal and its char. The research efforts associated with Area 3 (Synthesis Fluid Oxidation and Heat Extraction) were focused on the design of the oxidation reactor and transfer of the energy released to a heat engine in order to extract work. The stream exiting the oxidation reactor, entering the heat exchanger of the heat engine needs to operate as close as possible to material thermal limits to maximize heat engine efficiency. Thus, a primary goal of the oxidation reactor design effort was the distancing of oxidation zones from reactor walls. An additional requirement was control of reducing and oxidizing streams to avoid liner corrosion. Under consideration was the design of a combustor in which hydrodynamics and water injection were used to control reaction, mixing, and wall interactions. Research activities associated with Area 4 (Aquifer Interactions) were concerned with characterizing the impact of dissolved constituents in the water being returned to the aquifer on aquifer ecology. Of interests were the fates of contaminants prevalent within coal, such as arsenic, mercury, and lead. Geochemical conditions in the deep subsurface are likely to lead to the partitioning of elements such as As and Hg to the solid phase. These elements are subject to migration should physical isolation be disturbed. Another concern was the possible oxygenation of the aquifer, potentially destabilizing the sulfidic minerals. A third concern was the potential to develop dramatic fluctuations in ph resulting from variations in CO 2 content, possibly destabilizing aquifer solids and inducing dissolution or colloidal transport. Geochemical constraints are expected to diminish the risk imposed by heavy metal discharge into the physically isolated deep brines but in the research efforts, a combination of equilibrium based predictions and spectroscopic/microscopic characterization of the energy system products were performed to verify reaction end-points. Materials degradation represents one of the most critical issues in the development of the proposed process. The simultaneous presence of oxygen and ions in the supercritical fluid forms an aggressively corrosive environment. Consequently, reactor designs must minimize the contact between walls and SCW that contains oxygen. In our approach, a perforated liner was considered for use inside the combustor that permits cooling flows of pure water to protect the combustor surfaces from the hot oxygen-containing SCW. The sections below focus on the significant research activities undertaken during the final year that were directed towards meeting the overall project objectives. Project Status Area 1: Systems Analysis (Edwards) Development of Plant Concept. The construct of a system-level thermodynamic model of the proposed plant has been completed. The details of the model were presented in last year s annual status report, and will be included in the final project report. Model results indicate that for a

5 O 2 added (kmol/100kg of coal) combustor exit temperature of 1650 K, overall system efficiencies as high as 42% (on a lower heating value basis) can be expected, even when energy penalties associated with non-ideal components, operating an ASU to deliver oxygen to the system, and carbon sequestration are taken into account. As expected, the overall efficiency increases as the temperature of the fluid leaving the combustor increases since the product stream is the heat source for the combined cycle heat engine, and the net output of the combined cycle dominates the overall efficiency. The model also indicates that the amount of aquifer water that must be circulated for total dissolution of the CO 2 is not unreasonably high. For a 500 MW coal plant and an aquifer salinity of 20,000 ppm (as sodium chloride), required water flow rates range from 2,000 to 10,000 kg/s, depending upon the aquifer temperature and pressure. For reference, the amount of cooling water required by a traditional 500 MW coal plant is around 12,000 to 13,000 kg/s. Contacts (Systems Analysis) Christopher F. Edwards: cfe@stanford.edu J.R. Heberle: jrh@stanford.edu Area 2: Supercritical coal reforming (Mitchell) Autothermal Reformer Operation. The thermodynamic model indicates that the residual sensible energy in the stream leaving the heat exchanger where energy is transferred to the heat engine can be used to preheat the water fed to the reformer to a temperature as high as 700 K. By preheating the reactants for gasification, the amount of oxygen required for autothermal operation of the reformer is reduced and the heating value of the synthesis gas is increased. Shown in Fig. 1 is the required amount of oxygen per 100 kg of coal to operate an autothermal reformer at K as a function of total pressure when the water is preheated to 700 K. The required amount of oxygen is relatively insensitive to the total pressure for a given solids loading (the coal weight fraction in the slurry) Solids Loading = Solids Loading = Solids Loading = Solids Loading = P total (atm) Figure 1: Required oxygen for autothermal operation of the reformer at 647.3K without and with preheating of the reformer feed water.

6 P H2 O (atm) At any selected reformer operating pressure, as the solids loading is decreased, more slurry water needs to be heated. With preheated water, for a specified reformer operating pressure, the amount of oxygen required to keep the gasification temperature at K by partial oxidation of the coal decreases as the solids loading decreases. By preheating the water supplied to the reformer, less energy needs to be produced from combustion of coal and volatilized species; a higher concentration of combustible species exists in the synthesis gas compared to the no-preheat case. Also, the lower the coal fraction in the slurry, the less oxygen required for autothermal gasification. Consequently with preheated water, the lower the solids loading the higher the heating value of the synthesis gas. This is demonstrated in Figs. 2, in which are shown calculated values of the water partial pressure in the reformer as a function of the total reformer pressure and solids loading. The ratio of the heating value of the synthesis gas to the heating value of the coal (the HV ratio) is indicated for each solids loading curve (Syngas HV)/(Coal HV) = HV Ratio = HV Ratio = HV Ratio = 0.84 HV Ratio = 0.87 Solids Loading = Solids Loading = Solids Loading = Solids Loading = P total (atm) Figure 2: Heating values of synthesis fuel at 647.3K without and with water preheating. When evaluating the energy requirements of the reformer, real gas properties were used for all species for which data are available (H 2 O, O 2, N 2, CO 2, CH 4 ) when determining the equilibrium composition of the synthesis gas. For species for which real gas data are not available (H 2 S, HCl, Cl 2, CO, COS, C 2 H 2, C 2 H 4, C 2 H 6, NH 3, NO, NO 2, SO 2 ) the Peng-Robinson equation of state was used to describe their P-v-T behavior. Experimental Setup. An experimental facility was designed, built and assembled that permits the acquisition of data needed to develop a coal/biomass gasification mechanism suitable for supercritical water conditions (P > 218 atm and T > 647K). The focus point of the facility is an entrained flow reactor that can be pressurized up to 340 atm at temperatures up to 1000 K. The reactor is 15 meters long, sufficient for residence times up to 15 minutes when the coal-water slurry feed contains 20% solids and the total mass flow rate (slurry plus supercritical water (SCW) flow rates) is 20 g/min. The residence time in the

7 reactor is varied by controlling the solids content of the slurry and the flow rate of the water supplied. The feed water is pressurized and preheated to the test conditions before the solids slurry and any oxygen are admitted. Oxygen is added so that the heat release from partial oxidation of the solids can supply energy for autothermal gasification. The stream leaving the flow reactor is quenched and dumped into a water bath where the solids are collected and analyzed to determine the extent of conversion. A sidestream is directed to a gas analyzer, purchased from Rosemount Analytic, which measures in real time the concentrations of four species of interest: CO, CO 2, CH 4, and H 2. The side-stream can also be directed to the sampling loop of our gas chromatograph to permit the measurement of the concentrations of such other species as N 2, O 2, H 2 S, and NH 3. A schematic of the experimental facility is shown below in Fig. 3. A picture is shown in Fig, 4. Figure 3: Experimental setup for coal gasification under supercritical conditions. The facility consists of seven main systems: 1. Two water-pumping lines a. Pure water supply line: Directed to the preheater for high-pressure water b. Slurry supply line: Connected to a piston accumulator for slurry injection 2. Oxygen supply line 3. Coil-shaped, Inconel 625 reactor in a sand bath 4. Cool-down chamber 5. Liquid/solid-gas separation chamber 6. Gas analyzer 7. System control and data acquisition box Details of each of these systems are discussed in the sections that follow.

8 Figure 4: A picture of the experimental facility. 1. High-pressure water supply lines Two water supply lines were developed. one for pure water supply and the other for the coal-water slurry supply. The pure water line is preheated before it is injected into the reactor. A water pump, made by SC Hydraulic, is used to pressurize and supply water to the system. This pump is driven by air at 120 psi (8.17 atm) supplied from highpressure air tank. The air to drive this pump is regulated by a FESTO air controller to control the water pressure. The water pump and air controller are shown in Fig. 5. Since a flowmeter to measure the flow rate of high-pressure water at pressures as high as 340 atm could not be found, we measure the pressure difference between two pressure transducers (from DJ Instruments) across a flow restrictor (a 30.0 inch (762 mm) long stainless steel tubing with 1/16 inch ( mm) outer diameter and 0.01 inch (0.254 mm) inner diameter). Calibration indicates that with this restrictor, a 500 psi (34 atm) pressure drop corresponds to 20 grams/min of water flow and a 1500 psi (102 atm)

9 pressure drop corresponds to 40 grams/min of water flow. The flow restrictor is shown in Fig. 6. Figure. 5: The SC Hydraulic water pump (left). The FESTO air controller (right). Figure 6: The flow restrictor between two pressure transducers. The water in this line is preheated using three Chromalox MaxiZone cartridge heaters (0.685 inch (17.40 mm) OD, 20.5 inch (521 mm) length, 240 VAC and 1600 watts) surrounded by aluminum powder and calcium silicate insulation. The heater is shown in Fig. 7. The other high-pressure line is used for the coal-water slurry. The water pump for this line is made by Haskel and the pump-driving air is controlled by another FESTO air controller. A picture of the high-pressure water pump is shown Fig. 8.

10 Figure 7: The high-pressure water preheater. Figure 8: A Haskel high-pressure water pump. Due to its high viscosity, slurry cannot flow through the high-pressure water pump and flow restrictor for high-pressure water flowrate measurements. Therefore, the slurry is loaded on one side of a piston accumulator by a peristaltic pump before experiments, and pure high-pressure water from the Haskel pump supplies the loaded slurry to the system by moving the piston from the pure water side toward the slurry side of the accumulator. The high-pressure piston accumulator was purchased from High Pressure Equipment Company. For slurry loadings at low pressures, a Vector peristaltic slurry pump is used instead of the Haskel pump. The piston accumulator and the Vector peristaltic slurry pump are shown in Fig. 9.

11 Figure 9: The high-pressure piston accumulator (left). The Vector slurry pump (right). The water pumps only have a small volume per stroke compared to the overall amount of water required for experiments consequently, the pumps need to stroke frequently to supply the needed flow rates of pressurized water. The fluctuations due to these pump strokes need to be dampened. For each water supply line, a pulse dampening accumulator was set up between the water pump and pressure transducers. The accumulator contains a bladder inside the steel housing, and the bladder is filled with nitrogen. By expanding and contracting the bladder, the water pressure fluctuations can be dampened. Pressure gauges were attached so that the nitrogen pressure profiles in the bladder could be observed. The bladder accumulators were purchased from HyDac; one is shown in Fig. 10. Figure 10: A HyDac bladder accumulator.

12 2. Oxygen supply Oxygen is pressurized using a SC Hydraulic gas booster (shown in Fig. 11). This booster is driven by air from a high-pressure air tank at 150 psi (10.2 atm). The pressurized oxygen is stored in a stainless steel cylinder and flows through a highpressure oxygen flow controller made by Bronkhorst High-Tech. The controller controls the oxygen mass flow rate up to 10 slpm (14.3 grams/min). Figure 11: The SC Hydraulic oxygen booster. The preheated pure water and the slurry from the piston accumulator are mixed with high-pressure oxygen at a mixing junction, located at the starting point of the coiled reactor. 3. Reactor The coil-shaped reactor, shown in Fig. 12, is made of Inconel 625 to endure highpressure, high-temperature environments. The reactor is 15 meter long, with inch ( mm) ID and inch ( mm) OD. It is submerged in a bath filled with sand (aluminum oxide, 120 grit (=116 microns)) and sixteen Watlow Firerod cartridge heaters (0.375 inch (9.525 mm) OD, 7 inch (118 mm) length, 240VAC and 300 watts). The heaters are used to heat the air used to circulate the sand particles in order to maintain a uniform sand bath temperature. These heaters are capable of maintaining the sand bath at temperatures up to 1000 K. A two-channel heating system control station was developed to control temperatures of the preheater and the reactor sand bath. This station contains two temperature controllers, two over-temperature controllers, 40 amp solid state relays and high speed fuses.

13 Figure 12: The 15-m coiled tubular reactor. 4. Cool-down chamber The flow leaving the flow reactor needs to be quenched as rapidly as possible to prevent further reactions. To decrease the cool-down time, the tube diameter at the reactor exit is decreased to 0.06 inch (1.524 mm) ID to increase the velocity of the flow and improve the overall heat transfer by convection. Cold water flows around the cooldown tubing in the chamber. Our calculations indicate that a 1 meter long cool-down section is sufficiently long to freeze all the reactions in less than 20 seconds. The cooldown tube is pictured in Fig. 13 along with the cold water chamber. Figure 13: The cool-down tube and cold water chamber.

14 5. Liquid/Solid-Gas separator The flow contains liquid, gas products and solid particles, which must be separated. The gas is separated from the liquids and solids in the separation chamber, a highpressure cylinder made by High Pressure Equipment. At this stage, the flow is still at a high-pressure but it is at a low temperature after being cooled in the cooling chamber. Due to the limited volume of the separation chamber, during warm-up the flow bypasses the separator. The flow is directed through the separator only during actual tests. Highpressure HIPCO valves are used to direct the flow. The separation chamber is shown in Fig. 14. Figure 14: Separation chamber (left) and HIPCO control valves (right). The entire system pressure is controlled by a Tescom back-pressure-regulator. A Marsh Bellofram air controller supplies air at pressures up to150 psi (0~10.2 atm) to the Tescom back-pressure-regulator, sufficient to maintain system pressures up to 5500 psi (374 atm). The Tescom back-pressure regulator and Marsh Bellofram air controller are pictured in Fig Gas analyzer The gas leaving the separation chamber is directed to an analyzer where the gas composition is measured. Gas analysis is made using an Emerson Process Management X-STREAM gas analyzer that has four channels: CO (0~40%), CO 2 (0~40%), CH 4 (0~20%), and H 2 (0~50%). For measurements of gaseous products other than these four products, our Varian gas chromatograph is used. The analyzer is pictured in Fig. 16.

15 Figure 15: The Tescom back-pressure regulator (left). A Marsh Bellofram air controller (right). Figure 16: An Emerson Process Management X-STREAM gas analyzer. 7. System control box Air supplies to drive the SC Hydraulic water pump, the Haskel water pump, the SC Hydraulic oxygen booster, and the HIPCO valves are controlled by ASCO solenoid valves, which are operated by OPTO22 modules on our data acquisition box. The data acquisition/system control box was developed by combining a data acquisition board (DaqScan 2001) with extension boards (DBK 207/CJC, DBK 208 and DBK 11A) for analog inputs/outputs and digital inputs/outputs from IOtech. Signals from the Bronkhorst oxygen flow controller, the FESTO air controllers, and the thermocouples are also acquired on these boards. All inputs and outputs are managed remotely from a dedicated computer. The automated system control and data measurement programs are written in LabVIEW (National Instruments). The ASCO solenoid valve and system

16 control box are shown in Fig. 17. A picture of the automated system control panel is shown in Fig. 18. Figure 17: ASCO solenoid valves (left). The system control box (right). Figure 18: The automated system control program.

17 Continuing Research (Supercritical water reforming) Coal conversion tests in supercritical water environments are currently underway. Extents of coal conversion are being measured at selected residence times in various SCW environments. The data are being used to develop a model of char gasification in supercritical water. In order to help define the rates of the key reaction pathways, reactivity tests are also being performed in our pressurized thermogravimetric analyzer. References (Supercritical water reforming) 1. Churchill, S.W., and M. Bernstein, J. Heat Transfer, 99, 300, Contacts (Supercritical water reforming) 1. Reginald E. Mitchell: remitche@stanford.edu 2. B. J. Kim: bjkim@stanford.edu Area 3: Synthesis Fluid Oxidation and Heat Extraction (Edwards) Supercritical Combustor Development The next task of the project is the development of a supercritical water combustor. Our goal is to demonstrate stable combustion in supercritical water with nearstoichiometric amounts of fuel and oxidizer and with high volumetric firing rate. These conditions are desirable for a combustor integrated into the plant described in the Introduction. Design and Layout of Experimental Apparatus Wellig has identified six tasks that must be performed by a SCWO experimental apparatus: reactant storage, pressurization, preheating, reaction, pressure let down, and effluent discharge 1. Due to the firing rates we intend to use and the amount of fluid we will preheat, cooling of effluent is also a concern, since otherwise the ambient temperature in the laboratory could rise to an unacceptable level. Figure 19 is a schematic of the major hardware for the supercritical combustor experiments. At left are components to store and deliver pressurized fuel, oxygen, and water. In the center there is an electric resistance heater ahead of the reactor vessel. Combustion occurs above the heater in the lower section of the high-pressure stack. The upper section of the stack is a dilution cooler, where ambient temperature water is introduced to begin cooling the combustor effluent. At right is a condenser to complete cooling of products and reject thermal energy from the lab. Each subsystem is discussed in detail below. The fuel system is shown in Fig. 20. Prior to running the combustor, a low-pressure, air-powered pump transfers fuel into a bladder accumulator in the laboratory. This accumulator is sized to hold enough fuel for a four hour run at a maximum firing rate of 50 kw. During operation, the gas side of the bladder is pressurized with nitrogen to feed fuel into the combustor. This strategy avoids the use of a high-pressure fuel pump. The nitrogen is supplied from a six-pack of gas cylinders, pressurized with a two-stage, airpowered booster, and regulated to a pressure above the combustor operating pressure. Most of the (known) pressure drop from the accumulator pressure to the combustor pressure occurs across a metering valve. This needle valve is operated remotely with a stepper motor, so that fine changes in the valve s flow coefficient allow for adjustment of the fuel flow rate. The fuel line also has a normally-closed solenoid valve for positive

18 fuel shut-off. The fuel then flows through a spur gear flow meter and a check valve. Next, the fuel mixes with an adjustable amount of water so that pyrolysis products do not plug the fuel line through the heaters. Figure 19: Diagram of the supercritical combustor experiment. Reactant storage and pressurization components are shown to the left of the vertically oriented high pressure stack. At right are components to remove waste heat from the lab and separate the effluent components for disposal. Figure 20: Liquid fuel delivery system.

19 Figure 21 shows the oxygen system. Oxygen is fed from a six-pack of cylinders in the oxidizer galley. Like the fuel bladder, this quantity of oxygen was chosen since it has capacity for a four hour run. The oxygen is regulated to 600 psi and flows into the lab, where it is pressurized by a two-stage booster similar to the nitrogen booster in the fuel system. To provide a steady flow of gas, the output of the booster is connected to a plenum and a regulator. By pressurizing this plenum above delivery pressure, the regulator can deliver oxygen at an approximately constant pressure while the piston in the booster cycles. The tube after the regulator is fitted with a cooling jacket. The jacket is used to reduce temperature variations in the oxygen as the plenum blows down, facilitating repeatable metering and flow measurement downstream. Next are valves for manually isolating and bleeding different parts of the system. The remaining components are similar in function to those in the fuel system. Figure 21: Oxygen delivery system. There is a remote solenoid valve for positive oxygen shut-off, followed by a stepperdriven needle valve to control the flow rate. A thermocouple is located just upstream of the flow meter so that the latter s readings may be corrected for temperature variations. The flow meter consists of a 0.5μm filter in parallel with a differential pressure transducer. This arrangement is similar in operation to a laminar flow element. Finally, there is a check valve and a mixing tee. While water will almost always be mixed with the fuel, initial combustion experiments will involve injection of neat oxygen. The tee gives the option of mixing water with the oxygen later, which is a way to affect mixing in the combustor.

20 The low-pressure water system can be seen in Fig. 22. At the top left of the diagram, water enters from a domestic cold water supply and proceeds through a 20 micron particle filter. The line goes to a manual three-way valve that accepts either fresh water from the supply or recycled water from the drain tank. This valve directs the water to a needle valve and rotameter to ensure that the flow rate through the deionizer is not more than 2 gpm. This flow rate is the recommended limit for effective operation of the deionizer. Deionization is used to reduce corrosion in the heaters and high-pressure stack. The other line that feeds into the manual three-way valve before the deionizer is for direct recycling while the system is running water through the heaters and stack for warm-up and cool-down. This flow rate, averaging about 0.5 gpm, is controlled elsewhere and thus does not need to run through the needle valve and rotameter before entering the deionizer. After the water is treated in the deionizer, it enters a 400 gallon supply tank. This tank is filled before each set of combustion experiments and holds enough water for a four hour experiment. Figure 22: Low-pressure water handling system. The supply tank contains a submersible pump that sends water up into the head tank with a flow rate higher than is necessary for the experiment. The excess water overflows back into the supply tank, while water for the experiment is gravity fed through a 20 micron particle filter and valve before entering a triplex plunger pump rated for 5000 psi (345 bar). This positive-displacement pump provides all of the high-pressure water for the system. From the pump, the water goes to the high-pressure water manifold shown in Fig. 23. Much like the head tank, the water manifold is supplied by the triplex plunger pump with more water than is necessary for the experiment. Excess water returns to the supply tank from the manifold back pressure regulator (BPR) described later. There is a liquid level switch in the supply tank that controls the submersible pump to keep it from running dry as the water level drops in the supply tank. The head tank also has a liquid-

21 level switch that controls the motor driving the triplex plunger pump to keep it from running dry. The outlet stream from the high-pressure apparatus re-enters the low pressure system in the middle of Fig. 22 after depressurization in the stack BPR. The main components of the fluid here will range from water at ambient temperature during start-up and shutdown to a mixture of saturated steam (maximum vapor fraction of 0.5) and carbon dioxide during combustion. The flow will pass through the main condenser to cool the stream to near ambient temperature. The main and residual condensers will use process cooling water (PCW) from the building at 15 C and 30 gpm. The CO 2 and water vapor pass into the residual condenser to cool them below room temperature. This prevents water vapor from condensing in the exhaust line running to the roof. There is also a condensate drain in the vent line in case there is any condensation due, for example, to cold outdoor temperatures. Thermocouples in both the gas and liquid lines exiting the condensers are used to make sure that the condensers are operating correctly. The lines connecting the two condensers are arranged to form a p-trap and maintain a high water level in the main condenser for efficient heat transfer. The bottom outlets from the two condensers send cooled water through a 5 micron particle filter and two hydrocarbon filters in series. The hydrocarbon filters separate out any liquid hydrocarbons that are not pyrolyzed or oxidized in the system. Typically, unburned hydrocarbons are not present since the combustion efficiency of supercritical water oxidation is very high. The filtered water then enters a solenoid three-way valve to send the water to the drain tank during combustion and also allowing the water to be recycled to the supply tank during warm-up and cool-down. The drain tank has a centrifugal pump connected to its outlet with a liquid-level switch to make sure that it does not run dry. The outlet of the pump passes through a check valve and a manual three-way valve to send the water out the lab drain, or back through the deionizer and into the supply tank to be used again. The system for handling high-pressure water is shown in Fig. 23. These components take filtered water from the low-pressure supply system, pressurize it, and control and measure the flow rates of the several water streams that enter the high-pressure stack. After the triplex pump described above, the water passes by a bladder accumulator. Here an accumulator is employed in its usual role for pulsation dampening only, rather than as both a reservoir and dampener as in the fuel system. The water then enters a manifold, where it branches out into six lines. The manifold has a globe valve for each outlet so that any lines that are not needed may be isolated. Excess flow from the pump leaves through the manifold back pressure regulator (BPR) and returns to the supply tank. The BPR is operated by an embedded PID controller that tracks the manifold pressure with a transducer. This setup allows the BPR to keep a constant, set pressure in the manifold regardless of changes in flow rate through each output line. This configuration is intended to decouple the flow control in the six lines.

22 Figure 23: High-pressure water handling system. After the manifold the water lines each have a stepper-driven needle valve, flow rate sensor, and check valve. Unlike the fuel and oxygen systems, each water line does not have a solenoid shut-off valve. These are not needed since a small amount of water flow past a mostly closed metering valve is not a problem in any of the six lines. Furthermore, the needle valve selected for most of the lines is rated for repeated shut-off by the manufacturer. After the check valves, two of the lines reach the mixing tees described earlier for fuel and oxygen. The fuel and oxygen solutions and three more water lines flow through the heaters and to the combustor. The last line is for dilution cooling water, which goes directly to the cooling section of the high-pressure stack. There is a single outlet from the stack at the end of the dilution cooling section. It runs to a second BPR. This BPR is responsible for holding the desired operating pressure inside the combustor. Just ahead of it is a pressure transducer for feedback to the BPR controller and a thermocouple to ensure that the fluid is below the rated maximum temperature of the BPR (650 F or 617 K). The manual three-way valve after the BPR is for calibrating the water flow sensors. With the heaters off and the stack cold, cold water will be run through the pressurized system and out this valve. By measuring the mass of water output in a known time at several rates, the true mass flow rate will be correlated with the sensor output. The heaters, shown assembled in Fig. 24, were designed with operational flexibility in mind. They must heat at least five separate streams above the critical temperature of water. A computer model was constructed to predict the necessary length of each line to heat its stream to 700 K. The model operates under a constant wall temperature assumption. This assumption is justified by the use of cast aluminum as the medium between the electric heaters and tubes to be heated. The aluminum has a high thermal conductivity and low spatial temperature variation has been observed in a similar heater unit in use for another project in the research group. The model was modified for the use of variable properties and assumes that the fuel/water line is entirely water. However, the

23 heat transfer model used could only give a rough estimate because it cannot account for pseudoboiling in the tubes. Pseudoboiling occurs when the tube wall temperature is above the pseudocritical point (the temperature of maximum specific heat at a given pressure above the critical pressure) while the bulk of the flow is below the pseudocritical temperature. At this condition the flow has a low-density supercritical boundary layer and a high-density liquid core. Therefore, this regime operates very much like film boiling at subcritical pressures. The supercritical layer does not conduct heat as well into the core of flow, so deteriorated heat transfer occurs. The computer model was run twice with different properties to give best- and worst-case scenarios. To work between these two bounds, it was decided to construct the heaters with a large number of relatively short tube passes. This design allows the total passage length for each of the five flows to be varied between experimental runs by connecting together different numbers of tubes. Figure 24: Assembled heater units. Aluminum filler is visible through the casting holes in the shells. At the end of each unit, six cartridge heating elements and their wires are visible, along with inner and outer circles of Inconel 625 fluid tubes. The total power output of the heaters was chosen based on the enthalpy required to preheat enough water, oxygen, and fuel to fire the combustor at rates up to 50 kw and with mixed mean outlet temperatures from 1200 to 1800 K. About 40 ft. of electric

24 resistance cartridges of the chosen model are required to provide the desired 72 kw of heating. For ease of fabrication, transport, and use, it was decided to use a larger number of shorter cartridges. This choice works well with the choice of many short fluid tubes. Due to the large numbers of heaters and tubes, two identical heater units were designed and constructed. This geometry provides even heat transfer and convenient fluid tube connections. Each unit is independently controlled. This arrangement allows for the possibility of operating streams at different temperatures to simulate various operating conditions in later experiments. Each unit is comprised of a steel shell with machined plates welded to each end. The end plates have holes for six 6 kw electric cartridges, 40 in. long, and twelve ¼ OD Inconel 625 fluid tubes. The heaters are inserted into thin-walled steel sleeves. After insertion of the tubes and heaters, the shells were filled with cast aluminum. The aluminum transfers heat from the heater cartridges to the fluid tubes. The tubes are 60 in. long 20 in. longer than the heaters to allow for tubes to be shortened when worn connections need to be cut and replaced. (The tubes are not replaceable since they are cast in the aluminum.) The tube ends were offset axially so they could be placed close together without interference between adjacent fittings. Calcium silicate was selected as the insulation for the heaters. Two inch thick insulation wraps around the heaters along with 2 in. thick end caps drilled for the tubes to pass through. Any gaps in the hard insulation will be filled with fiberglass insulation. The expected heat loss to the environment from the heaters is less than 2%, based on a finite element model of the heaters using a free convection heat transfer coefficient at the outer insulation boundary. A cut-away view of the high-pressure stack is shown in Fig. 25. The stack has three major sections: the combustor, an elbow, and the dilution cooler. These sections are connected with Grayloc-style flanges (in blue), which eliminates the need for welding traditional flanges to the 2 in. thick vessel walls. They also allow the two flanges to be placed close together without interference. The body of each section is machined from solid, forged Inconel 625. All fluid, thermocouple, and pressure fittings are cone-andthread high-pressure fittings to ensure proper sealing at elevated temperatures. The combustor (the lower, vertical section in Fig. 25) has two functional zones. The lower portion is the header, with inlet ports for the fuel/water solution, oxygen, and water. These streams flow up through three coaxial passages formed by the outer vessel and two nozzles. Nozzles of varying diameter may be used to change the relationships between mass flow rates, fluid velocities, and Reynolds numbers at the burner. Swirlers may also be inserted to affect flame stabilization. Near the center of the combustor are two opposed sapphire windows for observation of the flame. In this drawing the nozzles end just above the bottom of the window so that flame attachment at the burner can be observed. Shorter nozzles could be substituted to observe flame lengths longer than the window aperture. Above the windows there is a liner (shown in red). This perforated metal liner functions like that in a gas turbine combustor, allowing relatively cool fluid (supercritical water in this case) to enter from the outer annular passage, shielding the

25 liner and the pressure wall from the hot combustion products in the core flow. This liner extends beyond the combustor, through the flanged joint and into the elbow. Figure 25: Section view of high pressure stack design, showing fluid ports, burner, axial viewing window, and liners. The elbow section turns the flow and holds a third sapphire window to provide an axial view of the flame in the combustor below. The window design (visible in this view) is identical to that of the two windows in the combustor. The bolted flange holding the window allows the window seals to be preloaded so they maintain a seal at the target operating conditions of 700 K and 250 bar. There is an inlet near the window for injection of 700 K water to prevent impingement of very hot (1800 K or more) combustion products on the window.

26 The final section of the high-pressure stack is the dilution cooler. This section houses another perforated liner and fluid inlets. Here, high-pressure, ambient temperature water is mixed with the combustion products to cool them below the stack BPR maximum temperature of 617 K. The high-pressure stack is shown installed in the lab in Fig. 26. The vessel is insulated from the black supporting stand by Macor ceramic parts. The stand is bolted to an optical table so that optical diagnostics may be added easily. Figure 26: The high-pressure stack shown without and with insulation. Construction Progress At the beginning of this project, the allocated lab space did not have adequate utilities to run the experiment. Domestic cold water supply, lab drain, and process cooling water lines have been installed. A 200 A, 480 V, 3 phase service was installed for the electric heaters and triplex water pump motor. All necessary utilities upgrades are now complete. All of the systems described in the preceding section have been installed. All pressure and flow rate sensors have been calibrated. The feed systems have been run at ambient temperature and nominal operating pressure long enough that pressure and flow control are well understood. Several attempts have been made at heating the system while pressurized in preparation for flame experiments. All of these tests were stopped after a few hours of heating when steam was observed leaking from the high-pressure stack. It was determined that the metal c-seals between the sections of the vessel were defective. The manufacturer has provided a replacement set of seals. These new seals are being installed and testing will resume. Thus far, temperatures of 450 K have been reached before leaks occurred. As soon as the vessel can be heated to and held at 700 K without

27 the appearance of steam leaks, injection of fuel and oxygen will begin in attempts to produce an autoignited flame. Publications 1. Heberle, J.R., Edwards, C.F. Coal Energy Conversion with Carbon Sequestration via Combustion in Supercritical Saline Aquifer Water. International Journal of Greenhouse Gas Control. Accepted for publication. References (Supercritical Combustor Development) 1. Wellig, B., Transpiring-wall reactor for supercritical water oxidation. Doctoral thesis no. 15,038, Swiss Federal Institute of Technology (ETH) Zurich. Available Contacts (Supercritical Combustor Development) Christopher F. Edwards: J.R. Heberle: Paul D. Mobley: Area 4: Aquifer Interactions. (Fendorf) Fate of Trace Elements in Aquifer Brine Solutions. Within this task, we assess the fate of potentially toxic trace elements in CO 2 - equilibrated brine solutions under elevated pressure and temperature. To accomplish this objective, geochemical models, including EQ3/6, PHREEQC, and MIN3P (1-3), were used to simulate post coal-combustion geochemistry of brine solutions delivered to deep aquifers. Bulk and Trace Element Composition of Simulated Brine and Coal Multiple factors will dictate the geochemistry of trace elements within CO 2 - equilibrated brines, including 1) the original composition of the brine solution, 2) the composition of the combusted coal, 3) the ratio of combusted coal:brine solution, 4) aquifer redox status, and 5) trace element removal during the coal gasification/combustion process. To establish a base-case scenario of trace element speciation and fate within a deep aquifer, factors 1-4 (above) are explored with geochemical simulations; factor 5 is excluded until trace elements are measured in solids (ash) removed during the combustion process (a future task). The composition of brine solutions may vary considerably, and are dictated by several factors including i) the composition of porewater at the time of burial, ii) the net effect(s) of diagenesis on porewater composition as influenced by ambient Table 1. Aqueous Constituents in Simulated Brine Constitutent mmol/kg H 2 O Sodium 1000 Chloride 1000 Lithium 3 Potassium 4 Magnesium 30 Calcium 80 Bromide 2 Barium 2.5 Alkalinity 10 ph 7.1 solids and other proximal porewaters, and iii) physical transport of materials to and from a the system via advective flow and solute mixing (4). While different combinations of these factors may result and yield brines with disparate chemistries, sedimentary brines typically possess salinities greater than 100 g/l and chloride concentrations greater than 10 g/l (4-6). Alkalinity may also vary considerably and range from 10 to 1000 mg/l.