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 CO2, 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 CO2 and H2O, 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,

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 coal-

based electric power generation process that involves coal conversion in supercritical

water with CO2 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 CO2 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

CO2. In the United States, the sites identified have the potential capacity to store about

86 years of CO2 generated in coal-fired power plants at the rate of coal consumption for

electric power generation in 2005. Preliminary estimates indicate that a 30-m thick

aquifer having 1-Darcy permeability and 20% porosity can store the effluent of a 500

MWe power plant over its nominal 40-year lifetime. The proposed scheme for electric

power generation under investigation produces CO2 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 CO2 at

reservoir conditions, allaying any concerns about selective CO2 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 CO2 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.

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 (Tc,H2O = 647 K, Pc,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 H2, CO,

CO2, 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 CO2. 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-to-

electricity power plant with CO2 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

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 CO2 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

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 CO2 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 647.3 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).

220 240 260 280 300 3201.6

1.8

2

2.2

2.4

2.6

2.8

3

Ptotal

(atm)

O2 a

dd

ed

(km

ol/100kg

of

co

al)

Solids Loading = 0.150

Solids Loading = 0.200

Solids Loading = 0.300

Solids Loading = 0.400

Figure 1: Required oxygen for autothermal operation of the reformer at

647.3K without and with preheating of the reformer feed water.

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 647.3 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.

When evaluating the energy requirements of the reformer, real gas properties were

used for all species for which data are available (H2O, O2, N2, CO2, CH4) when

determining the equilibrium composition of the synthesis gas. For species for which real

gas data are not available (H2S, HCl, Cl2, CO, COS, C2H2, C2H4, C2H6, NH3, NO, NO2,

SO2) 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

220 240 260 280 300 32050

100

150

200

250

300

Ptotal

(atm)

PH

2O (

atm

)

Solids Loading = 0.150

Solids Loading = 0.200

Solids Loading = 0.300

Solids Loading = 0.400

(Syngas HV)/(Coal HV) = HV Ratio = 0.97

HV Ratio = 0.92

HV Ratio = 0.87

HV Ratio = 0.84

Figure 2: Heating values of synthesis fuel at 647.3K without and

with water preheating.

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 side-

stream is directed to a gas analyzer, purchased from Rosemount Analytic, which

measures in real time the concentrations of four species of interest: CO, CO2, CH4, and

H2. 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 N2, O2, H2S, and

NH3.

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.

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 high-

pressure 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 (1.5875 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)

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.

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.

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.

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 high-

pressure 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 high-

pressure, high-temperature environments. The reactor is 15 meter long, with 0.3125 inch

(7.9375 mm) ID and 0.5450 inch (13.843 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.

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 cool-

down 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 cool-

down tube is pictured in Fig. 13 along with the cold water chamber.

Figure 13: The cool-down tube and cold water chamber.

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 high-

pressure 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. High-

pressure 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. 15.

6. 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%), CO2 (0~40%), CH4

(0~20%), and H2 (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.

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

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.

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, 1977.

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 near-

stoichiometric 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 discharge1. 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, air-

powered 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

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 20: Liquid fuel delivery system.

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 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.

There is a remote solenoid valve for positive oxygen shut-off, followed by a stepper-

driven 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.

Figure 21: Oxygen delivery system.

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.

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-

Figure 22: Low-pressure water handling system.

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 shut-

down 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 CO2 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.

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

Figure 23: High-pressure water handling system.

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.

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

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.

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-and-

thread 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

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.

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.

Figure 25: Section view of high pressure stack design, showing fluid

ports, burner, axial viewing window, and liners.

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.

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

Figure 26: The high-pressure stack shown without and with

insulation.

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., 2003. Transpiring-wall reactor for supercritical water oxidation. Doctoral thesis no.

15,038, Swiss Federal Institute of Technology (ETH) Zurich. Available http://www.e-

collection.ethz.ch.

Contacts (Supercritical Combustor Development) Christopher F. Edwards: cfe@stanford.edu

J.R. Heberle: jrh@stanford.edu

Paul D. Mobley: pdmobley@stanford.edu

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 CO2-

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 CO2-

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

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.

Table 1. Aqueous Constituents in

Simulated Brine

Constitutent mmol/kg H2O

Sodium 1000

Chloride 1000

Lithium 3

Potassium 4

Magnesium 30

Calcium 80

Bromide 2

Barium 2.5

Alkalinity 10

pH 7.1

For modeling purposes, a circumneutral brine solution was assumed as possessing ~1000

mmol/kg chloride and 10 mmol/kg alkalinity, along with multiple trace salts (Table 1).

Despite substantial variation, generalities can be made regarding the quantity of

potentially toxic trace elements in sub-bituminous coal. For example, trace quantities of

lead and cadmium, and to a lesser extent, arsenic and mercury, are often found. The high

coal throughput for modern power plants, however,

results in large quantities of these trace elements

accruing in various combustion products, including

fly ash, slag, and gaseous emissions (7, 8). In our

geochemical models, a suite of trace elements are

stipulated at concentrations representative of those

found within coal combusted at conventional plants

(refs 7, 8; Table 2). Furthermore, coal used in the

simulations is assumed to possess ~67 moles of

carbon/kg dry weight, which will dictate the

quantity of CO2 equilibrated with brine.

Simulating the fate of trace elements in CO2-

equilibrated brine solutions

The geochemical modeling software package

PHREEQC in conjunction with PITZER and

WATEQ databases were used to simulate trace

elements post-combustion in brine solutions (2).

PHREEQC was written to simulate the equilibrium

chemistry of aqueous solutions interacting with

minerals and mineral surfaces and gas dissolution/exosolution, and using the PITZER

database allows for calculating the activity of charged aqueous species under high ionic

strength conditions (e.g. > 1M) representative of saline brines. For geochemical species

absent from the PITZER database, the WATEQ database was used with an extended

Debye-Huckle formulation to calculate activity coefficients.

To demonstrate potential trace elements concentrations within post-combustion brine

solutions, stoichiometric concentrations of coal constituents were added incrementally to

one kg of brine, thus representing various ratios of coal to brine within the combustion

process. Reaction constituents were then free to react with the aqueous milieu, and a

variety of geochemical parameters were calculated, including the formation of aqueous

species (inclusive of ion pairs), solid phases (based on saturation indices), and gases.

Intuitively, as trace element mixture is added to one kg of brine, aqueous concentrations

of all trace elements increase (Fig. 27). However, when solid phases are allowed to

precipitate, the activity of multiple trace elements is repressed. For example, when

anoxic, low redox potential (Eh) aquifer conditions are specified, sulfate reduction is

thermodynamically favored, leading to the precipitation of trace element-bearing sulfide

minerals such as realgar, covellite, cinnabar, and galena (Fig. 28).

Table 2. Solid phase constituents

in simulated coal combustion.

Constituent mmol/kg (dw)

Carbon 66666

Arsenic 0.6

Cadmium 4.7

Chromium 0.8

Copper 0.4

Mercury 0.08

Nickel 0.5

Lead 1.5

Antimony 0.04

Selenium 0.05

Zinc 0.7

Calcium 350

Magnesium 50

Iron 1000

Titanium 50

Potassium 216

Sodium 600

Thus, anoxic conditions may lower trace element concentrations (Fig. 29) owing to the

sink provided by sulfidic minerals (9).

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

SI

-5

0

5

10

15

20

Realgar (AsS)

CdS

Covellite (CuS)

Cinnabar (HgS)

Galena (PbS)

Eskolaite (Cr2O3)

Anoxic Mineral Saturation Indicies

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

SI

-15

-10

-5

0

5

10

15

20

25

Delafossite (CuFeO2)

Calomel (HgCl)

Pyromorphite (Pb5(Cl(PO4)3)

Chromite (Fe,Mg)Cr2O4

Oxic Mineral Saturation Indicies

Mineral Saturation Indicies

Coal:Brine Ratio Coal:Brine RatioReaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

SI

-5

0

5

10

15

20

Realgar (AsS)

CdS

Covellite (CuS)

Cinnabar (HgS)

Galena (PbS)

Eskolaite (Cr2O3)

Anoxic Mineral Saturation Indicies

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

SI

-15

-10

-5

0

5

10

15

20

25

Delafossite (CuFeO2)

Calomel (HgCl)

Pyromorphite (Pb5(Cl(PO4)3)

Chromite (Fe,Mg)Cr2O4

Oxic Mineral Saturation Indicies

Mineral Saturation Indicies

Coal:Brine Ratio Coal:Brine Ratio

Figure 28: Mineral saturation indices for trace element-bearing minerals under anoxic (left)

and oxic (right) conditions (values exceeding 0 suggest precipitation). Arrows represent the

nominal coal:brine used with the reactor design presented in this report.

Figure 27: PHREEQC simulations of aqueous trace elements

after coal combustion and equilibration with brine. “Coal:Brine

Ratio” represents fractions of coal (kg) and associated trace

elements used for equilibration with 1 kg solution.

Precipitation of solid phases not allowed.

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Oxic Brine/Effluent Equilibration

Coal:Brine Ratio

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Oxic Brine/Effluent Equilibration

Coal:Brine Ratio

Minerals may also precipitate under oxic conditions and are capable of scavenging

trace elements (e.g., delafossite, Cu-bearing phase, and chromite, a Cr-bearing solid)

(Fig. 30). Additinally, brines that become oxygenated will favor the precipitation of

Fe(III) bearing minerals such as goethite (FeOOH), which act as strong adsorbents

(scavengers) of various trace elements (10). Alternatively, Fe-oxides such as goethite

may form in-situ if aerated solutions are injected into anoxic aquifers that have an

abundance of Fe(II). Either of these scenarios will result in a decrease in trace element

concentrations due to sequestration reactions on the iron oxide surfaces (Fig. 31). In

particular, arsenic is predicted to adsorb strongly to Fe(III) (hydr)oxides, resulting in a

dramatic drop in its aqueous concentration.

Figure 29: Aqueous concentrations of trace elements as a function of

coal:brine under anoxic conditions without solid phase precipitation (left) and

with trace element-bearing solid phase precipitation allowed (right).

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Anoxic Brine/Effluent Equilibration

Coal:Brine Ratio Coal:Brine Ratio

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Anoxic Brine/Effluent Equilibration

Coal:Brine Ratio Coal:Brine Ratio

Figure 30: Aqueous trace element concentrations as a function of coal:brine

under oxic conditions without solid phase precipitation (left) and with trace

element-bearing solid phase precipitation allowed (right).

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Oxic Brine/Effluent Equilibration

Coal:Brine Ratio Coal:Brine Ratio

Mineral Precipitation Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

No Solid Phases

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-3

1e-6

1e-9

1e-12

1e-15

1e-18

1e-21

1e-24

1e-27

As Cr Cu Hg Pb Zn Cd

Oxic Brine/Effluent Equilibration

Coal:Brine Ratio Coal:Brine Ratio

Summary

High concentrations of problematic trace elements present within CO2-equilibrated brine

solutions are likely partially mitigated by secondary precipitation/adsorption reactions

occurring within the aquifer. The extent of these reactions will depend on the redox

status (aeration) of the delivered brine, as well as the redox status and mineralogical

makeup of the receiving aquifer. Future work will involve bench-scale testing of coal

combustion product equilibration with artificial brines in representative aquifer materials,

such as calcite or pyrite-bearing sandstones or shales.

References (Aquifer Interactions) 1. Mayer, K. U.; Frind, E. O.; Blowes, D. W., Multicomponent reactive transport modeling in

variably saturated porous media using a generalized formulation for kinetically controlled

reactions. Water Resour. Res 2002, 38, (9), 1174.

2. Parkhurst, D. L.; Appelo, C. A. J., User's guide to PHREEQC (version 2)-a computer program for

speciation, reaction-path, 1D-transport, and inverse geochemial calculations. U.S. Geol. Surv.

Water Resour. Inv. Rep. 1999, 99, 99-4259.

3. Wolery, T. J. EQ3/6, A Software Package for Geochemical Modeling of Aqueous Systems:

Package Overview and Installation Guide (Version 7.0); UCRL-MA-110662-PT-I; Lawrence

Livermore National Laboratory, Livermore, California: 1992.

4. Hanor, J. S., Physical and Chemical Controls on the Composition of Waters in Sedimentary

Basins. Marine and Petroleum Geology 1994, 11, (1), 31-45.

5. Martel, A. T.; Gibling, M. R.; Nguyen, M., Brines in the Carboniferous Sydney Coalfield, Atlantic

Canada. Applied Geochemistry 2001, 16, (1), 35-55.

6. Fontes, J. C.; Matray, J. M., Geochemistry and Origin of Formation Brines from the Paris Basin,

France .1. Brines Associated with Triassic Salts. Chemical Geology 1993, 109, (1-4), 149-175.

7. Klein, D. H.; Andren, A. W.; Carter, J. A.; Emery, J. F.; Feldman, C.; Fulkerson, W.; Lyon, W. S.;

Ogle, J. C.; Talmi, Y.; Vanhook, R. I.; Bolton, N., Pathways of 37 Trace-Elements through Coal-

Fired Power-Plant. Environmental Science & Technology 1975, 9, (10), 973-979.

Figure 31: Aqueous concentrations of trace element contaminants

as a function of coal:brine under aerated brine conditions without

(left) and with adsorption to the iron oxide mineral goethite (right).

No Sorption Reactions

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

As Cr Cu Hg Pb Zn Cd

Sorption to Goethite Allowed

Reaction Progress

0.000 0.005 0.010 0.015 0.020 0.025 0.030

(M)

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

Reaction Progress

Constituent Sorption to Precipitated Goethite

8. Querol, X.; Fernandezturiel, J. L.; Lopezsoler, A., Trace-Elements in Coal and Their Behavior

During Combustion in a Large Power-Station. Fuel 1995, 74, (3), 331-343.

9. Huertadiaz, M. A.; Morse, J. W., Pyritization of Trace-Metals in Anoxic Marine-Sediments.

Geochimica Et Cosmochimica Acta 1992, 56, (7), 2681-2702.

10. Brown, G. E.; Parks, G. A., Sorption of trace elements on mineral surfaces: Modern perspectives

from spectroscopic studies, and comments on sorption in the marine environment. International

Geology Review 2001, 43, (11), 963-1073.

Contacts (Aquifer Interactions)

Scott Fendorf: sfendorf@stanford.edu

Ben Kocar: kocar@stanford.edu