Supercritical Water Coal Conversion with Aquifer-Based Sequestration of CO 2

Transcription

1 Supercritical Water Coal Conversion with Aquifer-Based Sequestration of CO 2 Profs. Reginald Mitchell, 1 Christopher Edwards 1 and Scott Fendorf 2 1 Mechanical Engineering Department 2 Department of Geological and Environmental Sciences Stanford University Stanford, CA GCEP Research Symposium 2008 October 1-3,

2 Projected World Electricity Generation by Fuel Type and World Carbon Dioxide Emissions: Electricity Generation by Fuel Type Carbon Emissions From Energy Information Adinistration (EIA), International Energy Annual 2005 (June-October 2007)

3 Annual Greenhouse Gas Emissions by Sector Emissions from fuel burning (coal, oil, natural gas, and wood) have altered the natural carbon cycle by increasing the concentration of CO 2 in the atmosphere. There is almost 37% more CO 2 in the atmosphere today (~383 ppm CO 2 ) than there was at the end of the 18th century. Environmentally benign use of coal as an energy resource requires CO 2 capture and sequestration.

4 Research Overview Fundamental studies are being undertaken to gain the understanding and information needed to design and develop a supercritical water coal-to-electricity conversion scheme with carbon dioxide capture and sequestration in an aquifer. 4

5 Deep Saline Aquifers CO2 capacity estimates in U.S. saline formations range from 1000 to 3700 billion tons of CO2, sufficient for 86 to 318 years of CO2 storage at the rate that coal was used for electric power generation worldwide in Taken from: Carbon Sequestration Atlas of the United States and Canada, NETL Dissolved solids must be removed before the aquifer water is suitable for dissolving the coal conversion products.

6 Why Coal Conversion in Supercritical Water? Nonpolar organic compounds and oxygen are miscible in all proportions in water above its critical point (647.3 K, 218 atm) and many salts are insoluble. Consequently, Small polar and nonpolar organic compounds released during coal extraction and devolatilization are completely miscible in SCW. Large organic compounds released during coal devolatilization hydrolyze in SCW yielding H 2, CO, CO 2, and low molecular weight hydrocarbons, without tar, soot or PAH formation. Sulfur, nitrogen and many trace elements in coals are oxidized to form insoluble salts in SCW. Coal gasification products will be dissolved in supercritical water, and any salts can be precipitated from the fluid mixture and removed from the system with the coal ash. There are no gaseous emissions. 6

7 System Schematic of Integrated Coal Conversion and Aquifer Sequestration additives Aquifer brine Brine pretreat Brine preheater SC Brine Salt separator Brine Salt mixer Injectant Water coal Coal slurry plant coal/water slurry Gasifier/ Reformer Ash Oxidizer syngas ASU Combustor Compressor Heat Exchanger Helium Turbine Work Processing occurs at a constant pressure near the critical point. air Pump Heat Exchanger H2O Turbine Work Condenser 7

8 Overall Project Objective and Tasks The overall project objective is to provide the information needed to develop the proposed coal-to-electricity scheme with in situ CO 2 sequestration. Task I. Supercritical Water Coal Reforming Research efforts are aimed at characterizing coal conversion rates under SCW conditions and determining the conditions that maximize the amount of chemical energy from the coal in the synthesis fluid. Task II. Supercritical Water Combustion of Synthesis Fluid Research efforts center around the design and operation of the oxidation reactor and the heat exchanger needed to transfer energy to the heat engine. Systems analysis is also part of this task. Task III. Aquifer Interactions Research activities are concerned with characterizing the impact of dissolved constituents in the exhaust water being returned to the aquifer on aquifer ecology. 8

9 Task I. Supercritical Coal Reforming Experimental activities will be undertaken to characterize the rates of extraction, devolatilization, gasification, and oxidation of coals in SCW. Models for coal conversion in SCW environments will be developed that take into account real gas thermodynamic and transport properties. O2 at 290 atm Water at 290 atm Pump Coal/water Slurry Back-Pressure Regulator High Pressure Tank Booster Oxygen High Pressure Tank Pump Water Air Water Heater Fluidized Sand Bath Ice Bath Schematic of experimental SCW coal reforming facility Analyzers Exhaust

10 Autothermal Gasifier Operation Sufficient oxygen is added to the gasifier to maintain a temperature of 647 K. To maintain SCW conditions, the partial pressure of water vapor in the gasiifier syngas mixture must be equal to (or exceed) its critical pressure of 218 atm. Coal composition, by wt. : 79.7% C, 4.2% H, 3,0% O, 1.5%N, 1.5%S Required oxygen for autothermal operation as a function of slurry solids loading (fraction of solids in water) when no reactant preheating. Water partial pressure, P H2O, in syngas mixture as a function of total pressure and slurry solids loading when no preheating.

11 Gasifier Synthesis Gas: Real Gas Effects The Peng-Robinson equation of state is used to describe the variations in pressure with specific volume and temperature. T = 647 K real gas ideal gas data Fugacity is used as a measure of the nonideality of a real gas. H 2 O Fugacity Coefficients, f/p T = 647 K, P = 290 atm H 2 S H 2 O O HCl CH SO N C 2 H Cl CO NH COS H C 2 H NO CO C 2 H NO Coal composition, by wt. 79.7% C, 4.2% H, 3,0% O, 1.5%N, 1.5%S, 0.01% Cl Gasification conditions: T = 647 K, P = 290 atm Slurry solids fraction: 0.2

12 Task II. Supercritical Water Combustion of Synthesis Fluid The stream 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 goals of the combustor design effort is the distancing of oxidation zones from reactor walls and the control of flow streams to avoid liner corrosion. Under consideration is the design of a combined reactorheat exchanger in which hydrodynamics and water injection are used to control reaction, mixing and wall interactions. 12

13 Schematic of Experimental SCW Oxidation Facility

14 Systems Analysis 500 MW Power Plant * SCWO system consists of the coal reformer and combustor units Component Brayton Cycle Power (MW) Compressor Turbine Net Rankine Cycle Condensate Pump Feed Pump Turbine Net ASU Water Pumps Overall Plant Fuel Heat Rate (LHV*m fuel ) Overall Efficiency 42.1% * SCWO system outlet temperature = 1600 K Brayton cycle compressor inlet pressure = 79.5 atm

15 Task III. Aquifer Interactions Activities to characterizing the impact of dissolved constituents in the water being returned to the aquifer on aquifer ecology will be undertaken. Of concern are: The fates of such contaminants in coals as arsenic, mercury and lead, which are subject to migration should physical isolation be disturbed. Oxygenation of the aquifer, potentially destabilizing the sulfidic minerals. 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. 15

16 Fates of Coal Trace Elements in Aquifer Environments Simulations have been conducted to assess the dissolved concentrations and fates of coal trace elements in brine fluids. Elements of interest include As, Cr, Cu, Hg, Pb, Zn, and Cd. In the simulations, trace elements in coal were reacted incrementally with 1-L brine water under oxic and anoxic conditions. The calculations use both the electrolyte theory of Pitzer and the extended Debye-Hückel formulation to estimate activity coefficients. The chemical equilibrium program EQ3/6 (and its associated thermodynamic database) was employed to perform the calculations.

17 Brine Effluent Equilibration Molality Molality Molality Molality Cr Zn As Cu Hg

18 Constituent Sorption to Precipitated Goethite Goethite (FeOOH) is the most common constituent of many forms of natural rust. It is a phase expected to form in the brine fluid and regulate dissolved metal(loid) concentrations. Molality Molality As As noted in the simulations, arsenic (As) is the primary component retained on the FeOOH solid.

19 Conclusions This investigation aims to lay the foundation for an efficient coal energy option with no matter release to the atmosphere. The CO 2 and other combustion products are pre-equilibrated in the water being injected into a deep saline aquifer, allaying any concerns about selective CO 2 release. Advantages of the proposed aquifer-based supercritical water coal conversion scheme for electric power generation relative to current and other proposed power generation systems include: maximally efficient power production while storing CO 2 products in indefinitely stable forms (mitigating public and technical concerns with potential gas releases from gas-only aquifer injection schemes), near-zero traditional air pollutant emissions, and size reduction of reactor vessel (compared to conventional pulverized coal-fired systems).

20 Posters Integration of Coal Energy Conversion with Aquifer-Based Carbon Sequestration Coal Energy Conversion via Combustion in Supercritical Aquifer Water: An Approach to Electric Power Generation without Atmospheric Emissions 20