Carbon Sequestration

Management of Greenhouse Gases Through Carbon Sequestration: TVA/DOE/EPRI Collaborative Projects

Introduction

The level of carbon dioxide (CO2) in the earth's atmosphere has increased by 31 percent since 1750. Many scientists now believe that most of the global warming observed over the past 50 years can be attributed to human activities. To address worldwide concerns that greenhouse gas increases may cause significant changes in global climate, the current Administration announced the Global Climate Change Initiative (GCCI) in February 2002.

The GCCI is designed to slow the growth of greenhouse gas emissions for the United States over the next 10 years. Based on current projections for 2012, U.S. greenhouse gas emissions will have to be reduced 4.6 percent below the levels projected to meet GCCI goals.

In accordance with this Initiative, the U S. Department of Energy (DOE) is striving to provide economically competitive and environmentally safe options to offset the projected growth in U.S. emissions of greenhouse gases, which include carbon dioxide (CO2), methane, nitrous oxide, and ozone. TVA currently is conducting three DOE-funded research projects aimed at developing and evaluating new technologies for the removal and storage of atmospheric carbon dioxide. The three DOE-funded projects are collaborative efforts of TVA and EPRI.

Two of the three projects are testing methods of capturing or "sequestering" CO2 and storing it in forms that would prevent its re-entrance into the atmosphere. The methods being tested in both projects are expected to provide benefits in addition to sequestering carbon, such as disposing of power plant byproducts in a safe and beneficial manner, creating other products or sources of energy, and/or enhancing land use. The third project is an economic comparison that will evaluate major technologies under development for CO2 sequestration.

Carbon Capture and Water Emissions Treatment System (CCWESTRS)

The CCWESTRS project is both a research and a demonstration effort aimed at assessing the cost-effectiveness of using power plant byproducts to enhance the growth of trees planted on coal mine spoil. The planting of 100 acres (40 hectares) of mine-spoil land adjacent to TVA's Paradise Fossil Plant (PAF) in Kentucky to sycamore and sweetgum trees is estimated to have the potential to sequester 70 to 90 metric tons of C per year over a 20-year period (1.5 to 2 tons per hectare per year).

One unique aspect of this study is the testing of both flue gas desulfurization (FGD) gypsum byproducts and FGD water emissions as soil amendments to increase the growth of trees planted. Whereas both of these byproducts normally must be disposed of to avoid potential environmental impacts, the system being investigated is designed to integrate these byproducts with improved terrestrial carbon sequestration.

Another unique aspect of the study at PAF is the presence of a Selective Catalytic Reduction unit upstream of the FGD system. In the process of converting nitrogen oxides to ammonia, selective catalytic reduction occasionally allows ammonia to pass through (slip) and enter the FGD system. Thus, ammonia levels in the FGD effluent may become high enough to require limits on discharge. The effluent also may contain criteria metals that would have to be removed before discharge. However, using the effluent as irrigation water for the trees should add not only needed moisture but also nitrogen to stimulate tree growth. It is expected that any metals present will not adversely impact the growth of the trees. Aqueous pollutants should be removed through plant uptake and soil adsorption.

Similarly, the FGD gypsum byproduct has proven to be an effective mulch for tree plantings, preventing soil-water evaporation and curbing weed competition with the trees. Potential benefits of the project are: (1) development of technology for passive water treatment of criteria pollutants, (2) use of power plant byproducts to increase carbon sequestration, (3) conversion of coal mine spoil land into wildlife habitat and attractive green space, and (4) production of wood products.

Chemical Fixation of CO2 in Coal Combustion Byproducts and Recycling Through Algal Biosystems

Each year in the United States, about 22 million tons of fly ash and similar FGD byproducts from power plants are stored in ash ponds and landfills. This fly ash and FGD waste has the potential to be used as a growth medium for algae that, in turn, could be used to sequester CO2 produced by gas turbine generators.

A method to transfer CO2 to an algal growth solution has been developed. Normally, the reaction of CO2 in an algal growth medium is slow. Also, because of low pH, the solution will de-gas significantly on exposure to the atmosphere. However, when CO2 is bubbled through a column packed with fly ash, almost all the gas goes into solution, and the pH remains higher than it ordinarily would for the same level of bicarbonate in solution.

The mass transfer rate of CO2 gas to carbonate solution is the rate-limiting step in producing carbon-enriched solutions. A co-current reactor containing fly ash has been compared to a similar reactor containing glass beads. The uptake of CO2 in the fly ash column was 5 to 9 times greater than the rate for the glass bead column. The fly ash also had the capacity to buffer the solution to a pH more suitable for biological systems.

This project, using fly ash as a catalyst, will create a carbon-enriched coal combustion product (CCP) liquid biosystem that will serve as an algal growth medium. CO2-rich stack gas will be diverted into the CCP solution in which the algae are expected to extract and utilize the carbon compounds for rapid growth. The algal biomass then will be extracted and converted to: (1) hydrogen gas via a gasifier or (2) CO2 and/or methane by anaerobic digestion for use in a gas turbine generator. In the methane system, the solid biomass residue can be used as fuel stock for the gasifier and the liquid residue can be used to provide nutrients to perpetuate the algal biosystem.

The CCP biosystem, because it is solar driven and makes use of potential waste products from combustion, requires minimal inputs of energy or materials. Based on optimum laboratory conditions, the area required for a 1,000-MW gas turbine plant is projected to be about 500 acres (200 hectares). However, because of the various factors that can be encountered in actual application, the area required for a passive system could be as high as 6,200 acres (2,500 hectares). An algal facility could produce in excess of 150 metric tons of biomass (dry weight) per hectare per year. The time required for the biosystem to attain steady-state production of liquid CO2, hydrogen, algae, and/or methane has not yet been determined but likely will be a primary factor in limiting output.

Economic Evaluation of Carbon Sequestration Technologies

A wide variety of technological approaches are being evaluated, both nationally and internationally, for the removal of CO2 from the atmosphere and for storage of carbon elsewhere on the planet. Carbon storage options being considered include active oil reservoirs, coal beds, depleted oil and gas reservoirs, deep aquifers, and even the world's oceans. Also, being explored is the enhancement of natural biological sinks, such as forests and croplands.

Determining which approaches are most viable naturally will depend on economic feasibility. However, obtaining accurate, consistent, and unbiased data for these technologies and performing consistent and logical economic comparisons will be difficult. This project aims to provide just such a set of economic comparisons applicable to the wide variety of carbon sequestration technologies.

A collaborative TVA-EPRI endeavor that involves a team of international experts—economists, scientists, and engineers from the Massachusetts Institute of Technology, Parsons Infrastructure and Technology, SFA Pacific, the International Energy Agency's Greenhouse Gas Programme, the University of Tennessee, and TVA—are developing and refining the framework for the evaluation. Economic parameters have been identified for various technological processes. And soon, process designs and economic models for the many approaches to carbon capture and storage will be developed, including the evaluation of forests and cropland as carbon sinks.

Summary

U.S. Department of Energy has funded TVA to perform three research projects to provide economically competitive and environmentally safe options to offset projected growth in U.S. emissions of carbon dioxide.
The Carbon Capture and Water Emissions Treatment System (CCWESTRS) is being evaluated to ascertain the cost effectiveness of using coal-fired power plant byproducts that are potential disposal problems—FGD gypsum sludge and FGD water emissions—as soil amendments on coal mine spoil land to potentially enhance the sequestration of carbon in planted trees.
Another project is testing a technique for precipitating CO2 produced by gas turbine generators as carbonates, using fly ash as a catalyst. The precipitated carbonates are being tested as a liquid biosystem for cultivating algae that potentially can be converted to usable fuels--hydrogen gas, liquid CO2, and/or methane.
A third collaborative effort is performing a consistent economic evaluation that compares a wide variety of technological approaches being tested for the capture and storage of CO2.

Information Contacts

Edward A. Stephens, Jr., (423) 751-7474, eastephens@tva.gov
Gregory A. Brodie, (423) 751-2784, gabrodie@tva.gov

Last updated on 9-16-2002.

Inquiries and comments should be sent to wjparkhurst@tva.gov.