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Clean Coal

Coal with Carbon Capture and Sequestration Resources

The United States has the largest demonstrated reserve base of coal in the world. U.S. coal reserves will last 200 years, based on current production levels (EIA 2012a). Although 90% of the confirmed coal reserves are concentrated in 10 states, coal is currently mined in 27 states. About 72% of current production originates in five states: Wyoming, West Virginia, Kentucky, Pennsylvania, and Montana (EIA 2012a). There are roughly 1,300 coal mines in the United States. About 60% of the coal is mined at the surface, and the remaining 40% is mined underground (EIA 2012b).

All coal is composed primarily of carbon, with variable quantities of other elements, including hydrogen, sulfur, oxygen, and nitrogen. When coal is burned, carbon monoxide (CO), carbon dioxide (CO2), sulfur oxides (SOx), nitrogen oxides (NOx), particulate matter (PM), and trace metals (including mercury) are released to the atmosphere. Burning coal accounted for 37% of total U.S. CO2 emissions 2010 (EIA 2011). The combustion of coal produces larger amounts of CO2 per unit of energy than does the combustion of oil or natural gas. When carbon capture and sequestration (CCS) technology (also referred to as carbon capture and storage) is used, CO2 emissions can be greatly reduced.

Coal with Carbon Capture and Sequestration Technologies

Three types of clean coal power plant technologies are (1) pulverized coal (PC) oxycombustion, (2) integrated gasification combined cycle (IGCC), and (3) coal fluidized bed (CFB). An additional option is to retrofit existing coal plants with CCS technology.

In a PC power plant, raw coal is milled into a fine powder in a pulverizer. This increases the surface area for combustion and allows the coal to burn faster. The powdered coal is blown into a boiler and burnt at a high temperature. Water circulating in tubes in the boiler is converted into high-pressure steam, which subsequently turns a turbine shaft at high speeds. The turbine shaft is connected to a generator, which produces electricity (World Coal Association 2008).

A typical PC plant with clean technologies uses limestone to capture the SOx. Selective catalytic reduction reduces NOx, and fabric filters, or electrostatic precipitators control particulates and assist in the control of mercury (NETL 2008). A PC oxycombustion power plant uses oxygen diluted with recycled flue gases in the combustion process. This produces a flue gas stream of CO2 and water. The water can be removed, leaving a nearly pure stream of CO2. The CO2 is compressed and transported via a pipeline to a geological formation where it is stored (NETL 2008). The geological formations that are typically considered for storage are deep saline formations and partially depleted oil fields. Figure 1 is a schematic of a basic PC plant.

Figure 1. Basic Advanced, Low-emissions PC Plant
Figure 1. Basic Advanced Low-emissions PC Plant. (Source: Katzer 2007)

In an IGCC power plant, coal is gasified in a high-pressure, high-temperature reactor called a gasifier. The raw synthetic gas (syngas) is cooled and cleaned of particulates, SOx, and NOx. CO in the raw syngas is converted to CO2. The CO2 is separated from the raw syngas, pressurized, and sequestered, in a manner similar to the PC plant process used prior to the combustion activity. The raw syngas is typically burned locally as is or it may be methanized into substitute natural gas and subsequently fired in a gas turbine.

The heat of the gas turbine exhaust is used to create steam to run a steam turbine in a combined-cycle process that is more efficient than a PC plant. Plant efficiencies of up to 60% are possible for IGCC plants; compare these to efficiencies of less than 40% for PC plants. By removing emission-forming constituents from the syngas before burning, the SOx and NOx emissions are greatly reduced (DOE 1999). A basic IGCC power plant process schematic is shown in Figure 2.

Figure 2. Basic IGCC Plant, Coal and By-product Flow
Figure 2. Basic IGCC Plant, Coal, and By-product Flow (Source: Katzer 2007)

In a CFB power plant, a limestone or dolomite sorbent is mixed with the coal to capture the sulfur released by the coal combustion process. The removal of PM and NOx emissions is similar to that done in a PC plant. Air suspends the mixture of burning coal and sorbents in a combustion chamber so that the fluidized mixture can be circulated. The hot combustion gases with entrained solids exit at the top of the combustion chamber into a hot cyclone. The cyclone separates the solids from the combustion gases and returns the solids, including any unburned solid fuel, through a nonmechanical loop seal to the combustion chamber, where they mix with incoming fresh fuel. The long residence time of solids at the combustion temperature and the continuous recirculation of the solids ensure high combustion efficiencies and sulfur capture. The heat of the combustion process is used to create superheated steam to run a steam turbine that has a higher efficiency than does a PC plant and that has one-tenth of the SOx and NOx emissions. The CO2 emissions can be sequestered similar to the PC plant process (DOE 1992). A basic CFB plant schematic is shown in Figure 3.

Figure 3. Basic CFB Plant
Figure 3. Basic CFB Plant (Source: JEA 2003)

Another option that may reduce CO2 emissions is retrofitting an existing PC power plant with control and sequestration equipment. The carbon scrubbing and control equipment will have a substantial footprint on the plant site, which may preclude some sites from consideration. Also, a net loss in power to the grid occurs to account for the parasitic loads associated with operating the emission control and sequestration equipment. This eliminates small coal plants from being considered for retrofit. In addition, the age or remaining life of an existing coal plant must be considered when making any decision to retrofit a plant.

References

DOE (U.S. Department of Energy), 1992, Clean Coal Reference Plants: Atmospheric CFB. DOE/MC/25177-3307. Office of Fossil Energy. June.

DOE, 1999, Integrated Gasification Combined Cycle. Office of Fossil Energy.

EIA (Energy Information Administration), 2012a, Energy in Brief: What Is the Role of Coal in the United States? U.S. Department of Energy. July 18.

EIA, 2012b, Coal: Annual Coal Report. Revised December 12.

JEA, 2003, Clean Coal Technology, the JEA Large-Scale CFB Combustion Demonstration Project, Topical Report Number 22, a report on a project conducted jointly under a Cooperative Agreement between The U.S. Department of Energy/NETL and JEA, March.

Katzer, J., et. al, 2007, The Future of Coal, An Interdisciplinary MIT Study, ISBN 978-0-615-14092-6, http://web.mit.edu/coal/The_Future_of_Coal.pdf. Accessed March 2013.

NETL (National Energy Technology Laboratory), 2008, Pulverized Coal Oxycombustion Power Plants. Report DOE/NETL-2007/1291. Prepared for U.S. Department of Energy. Aug.

World Coal Association. (circa 2008). Coal Electricity, Coal Power Plants.