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

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 coalbased 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-toelectricity 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: [email protected] J.R. Heberle: [email protected] 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). 220 240 260 280 300 320 1.6 1.8 2 2.2 2.4 2.6 2.8 3 P total (atm) O 2 a d d e d ( k m o l/ 1 0 0 k g o f c o a l) 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 220 240 260 280 300 320 50 100 150 200 250 300 P total (atm) P H 2 O ( a tm ) Solids Loading = 0.150 Solids Loading = 0.200 Solids Loading = 0.300 Solids Loading = 0.400 (Syngas HV)/(Coal HV) = HV Ratio = 0.97