For the past several years, clean-burning natural gas has been getting the headlines as the energy source that is abundant and cheap, thanks to breakthroughs in technology. New fourth-generation nuclear plants are emerging as an increasingly safe and cost-efficient alternative. But now, coal is beginning to get a second look as an option that will meet our long-term energy needs and goals.
Coal has always had two key characteristics that have made its use attractive:
- It is readily available.
- It is low in cost.
The downside is that burning coal produces high levels of carbon dioxide emissions, as well as pollutants like mercury and sulfur dioxide.
Of the 30 billion tons of carbon dioxide that human activity causes to be released into the atmosphere each year, the burning of coal ranks second, behind only petroleum-fueled vehicles.
However, there has recently been a renewed interest in building more coal-fired electrical plants due to the engineering of new technologies that:
- Dramatically reduce carbon dioxide emissions.
- Reduce emissions of other pollutants.
- Radically increase fuel efficiencies.
- Create by-products that can actually help pay for the new technology.
Specifically, two new coal-burning technologies are now at an advanced pre-commercial stage of development, and both utilize pure oxygen in the burning of coal. The benefits of a pure oxygen combustion cycle have long been appreciated.
But the cost of separating oxygen and nitrogen has, until recently, presented a major barrier to commercialization. However, the increased efficiencies of these new technologies, coupled with reduced capital costs in other areas of the plant, have finally made this solution attractive.
The first promising approach is being developed by a consortium consisting of NET Power, Exelon, Toshiba, and The Shaw Group. Designed to run on “gasified coal,” this system improves on today’s most efficient power plants, which use a gas turbine plus a steam turbine that runs off the gas turbine’s exhaust heat.1 The NET Power technology eliminates the steam turbine by directing part of the carbon dioxide in the exhaust stream back into the gas turbine.
The result is a coal plant that runs at more than 50 percent efficiency, compared to an efficiency of only 30 percent for a traditional gas turbine system. This increased efficiency more than pays for the added cost of the oxygen separation equipment.
This approach is called a “high-pressure, super-critical carbon dioxide, oxyfuel power cycle.” One of the side benefits is that there are no air emissions.2 Whereas other power generation technologies do emit exhaust gases or employ expensive add-on carbon capture systems, the NET Power technology creates a primary by-product of pipeline-quality, high-pressure carbon dioxide.
In this pressurized state, the by-product is relatively pure compared to exhaust from a conventional plant. As such, it can be used in industrial processes, including making polycarbonate polymers that are then used to make plastic bottles or piped to algae farms producing huge quantities of super-cheap animal feed.
This pressurized carbon dioxide is also suitable for underground sequestration. Of the underground storage options, deep saline aquifers offer the most promise. Situated one to three kilometers underground, below the depth of freshwater aquifers, these potential storage areas are sealed beneath a layer of impermeable rock called caprock.
According to a recent modeling study by MIT researchers, the deep saline aquifers found in the United States have the capacity to hold at least 100 years’ worth of carbon dioxide that would be produced by the nation’s coal-fired power plants.3
While there are concerns that this sequestered carbon dioxide could be released by seismic activity, this simply means that site selection can’t be random.4 Storage sites under caprock layers are considered the most secure; in most cases, the caprock wouldn’t allow carbon dioxide to escape.
An example of a prime storage choice is the Mount Simon Sandstone repository in the Illinois Basin, which lies beneath at least three caprock layers.
The second promising new technology that utilizes pure oxygen is being developed by Unity Power Alliance, a partnership between ThermoEnergy of Worcester, Massachusetts, and the major Italian engineering firm Itea.5
This method pressurizes oxygen, causing combustion to take place at a high temperature, allowing the coal to burn cleaner and more efficiently. This simplifies the clean-up of flue gases.
Similar to the NET Power technology, this technology also produces a stream of high-pressure carbon dioxide that, along with other polluting gases, is easily captured. Because of this, extensive flue gas recycling treatment that is necessary for atmospheric oxy-combustion systems is not required.
This eliminates significant capital and operating costs. Additional savings can be realized because lower-cost, low-ranking coals, such as sub-bituminous and lignite, can be used, which leads to a cheaper, more efficient process.
Another distinct advantage of this approach is its ability to adjust plant output from 10 percent to 100 percent in just 30 minutes. This large a change takes several hours for a conventional coal-fired plant. The benefit is much more flexible power production that can be better integrated with intermittent power sources, such as wind turbines and solar panels.
Given this trend, we offer the following forecasts:
First, one of the most important uses for pressurized carbon dioxide derived from oxyfuel power plants will be in the oil and gas industry.
A significant part of the winning equation for the NET Power and Unity Power Alliance technologies is the market demand for pressurized carbon dioxide. While demand for CO2 to make plastics or grow algae may be relatively limited, a much bigger demand lies in extracting “stranded oil” from mature oil fields. This method, called “enhanced oil recovery” (EOR) has been used successfully for decades. According to U.S. Department of Energy estimates, nearly 84 billion barrels of oil could be recovered using EOR in the U.S., and between 500 billion and 1 trillion barrels could be recovered worldwide. The hurdle to widespread use of this technique has been the lack of a cost-effective method for producing EOR-ready carbon dioxide ? which is exactly what both the NET Power and Unity Power Alliance technologies produce. By seeing the bigger picture, it will be possible to create a mutually beneficial solution, where one problem will be solved by the availability of affordable pressurized carbon dioxide, and the storage question of carbon dioxide will be answered by sequestering large quantities of it below ground in old oil fields.
Second, these new clean technologies will not only eliminate a huge perceived environmental problem, but actually contribute positively to economic growth by reducing energy costs.
The driving force will be an increase in efficiency. Since the same power demands will be satisfied from less coal, emissions will be cut in direct proportion to the increase in efficiency. This is a clear example of how advances in technology can sustain and improve our lifestyles, negating the argument that as a nation we need to accept a step backward in energy consumption, along with the associated drop in our standard of living.
References List : 1. MIT Technology Review, June 22, 2012, "Novel Power Plants Could Clean Up Coal," by Kevin Bullis. ⓒ Copyright 2012 by the Massachusetts Institute of Technology. All rights reserved. http://www.technologyreview.com 2. For more information about next-generation power technology, visit the Shaw Group website at: http://ir.shawgrp.com 3. MIT Technology Review, June 18, 2012, "Researchers Say Earthquakes Would Let Stored CO2 Escape," by Mike Orcutt. ⓒ Copyright 2012 by the Massachusetts Institute of Technology. All rights reserved. http://www.technologyreview.com 4. Proceedings of the National Academy of Sciences, April 3, 2012, "Lifetime of Carbon Capture and Storage as a Climate-Change Mitigation Technology," by Michael L. Szulczewski, Christopher W. MacMinn, Howard J. Herzog, and Ruben Juanes. ⓒ Copyright 2012 by the National Academy of Sciences. All rights reserved. http://www.pnas.org 5. For more information about using pressurized pure oxygen in the development of clean coal power, visit the ThermoEnergy Corporation website at: http://www.thermoenergy.com
