Number 9 coal, often written as No. 9 coal, refers to a specific coal seam found in the Illinois Basin, a geological region spanning western Kentucky, southern Illinois, and southwestern Indiana. It is a bituminous (hard) coal with a heating value around 12,200 Btu per pound, making it a medium-energy fuel historically important to power generation and industry in the eastern United States.
What the “Number 9” Actually Means
Coal seams are layered deposits buried at different depths underground, and geologists number them to keep track of their position in the rock sequence. The No. 9 designation identifies a particular seam within the Illinois Basin’s stratigraphic column. In Kentucky, it is commonly called the Western Kentucky No. 9 seam. In Illinois and Indiana, the same seam is often referred to as the Springfield Coal, named after Springfield, Illinois, where it was first formally described.
This naming convention exists because different states historically surveyed their coal deposits independently, assigning their own numbering systems. So “No. 9” is a Kentucky label for a seam that has equivalent names across state lines. Regardless of the name used, it is one of the most extensively mined coal seams in the Illinois Basin.
Physical and Energy Characteristics
No. 9 coal is classified as high-volatile bituminous coal. In practical terms, that means it is a relatively energy-dense black coal that ignites and burns more readily than harder grades like anthracite. Testing by the U.S. Department of Energy measured its heating value at approximately 12,200 Btu per pound, which places it in the middle range of bituminous coals. For comparison, premium Appalachian bituminous coals can reach 13,000 to 14,000 Btu per pound, while lower-rank coals like lignite deliver only 6,000 to 8,000 Btu per pound.
The seam itself typically runs about 60 to 66 inches thick, according to data from the Mine Safety and Health Administration. That thickness made it attractive for underground mining because miners and equipment could operate within the seam without needing to cut into the surrounding rock. Thinner seams are harder and more expensive to extract, so a five-foot-plus seam represented an economically favorable target.
Where It Was Mined
The heart of No. 9 coal production was in western Kentucky, particularly in counties like Hopkins, Muhlenberg, and Webster. The seam also extends into parts of southern Illinois. Both underground and surface mining methods have been used to extract it, depending on how deep the seam sits at a given location.
Underground operations in the No. 9 seam primarily used conventional mining and room-and-pillar techniques. In room-and-pillar mining, crews cut a grid of tunnels (“rooms”) through the coal seam, leaving pillars of unmined coal standing to support the roof. The 60- to 66-inch seam height allowed equipment to move through these tunnels without excessive roof cutting, though conditions still varied across different mines. Surface mining was used in areas where the seam lay close enough to the ground that removing the overlying rock (called overburden) was cost-effective.
How It Was Used
No. 9 coal was burned primarily in coal-fired power plants and industrial boilers. Its moderate energy content and availability made it a staple fuel for electricity generation across the Midwest and Ohio Valley. Utilities blended it with other coal grades or burned it alone, depending on the plant’s design and emission requirements.
One factor that influenced how No. 9 coal performed in power plants was its sulfur content. Illinois Basin coals, including the No. 9 seam, tend to have higher sulfur levels than coals from the Powder River Basin in Wyoming or certain Appalachian seams. When burned, that sulfur converts to sulfur dioxide, a regulated air pollutant. The passage of the Clean Air Act amendments in 1990 pushed many utilities to either install scrubbing equipment or switch to lower-sulfur western coals, which reduced demand for Illinois Basin coal over the following decades.
Environmental Concerns From Burning No. 9 Coal
Beyond sulfur, burning any bituminous coal releases trace elements that raise environmental and health concerns. Hard coals like No. 9 contain an average of about 340 parts per million of chlorine and roughly 0.10 parts per million of mercury, based on global averages for bituminous coal. Individual seams can vary widely. Chlorine concentrations in U.S. coals range from as low as 10 parts per million to as high as 5,000, depending on the specific deposit.
Chlorine content matters more than it might seem. When coal burns, the chlorine in the fuel interacts with mercury vapor in the exhaust gas, converting it into a form that pollution control equipment can capture more easily. Coals with higher chlorine levels actually tend to produce flue gas where a larger fraction of mercury is in this capturable form. That relationship was confirmed in early surveys of mercury emissions from U.S. coal-fired plants, which found a clear correlation between chlorine in the fuel and the proportion of mercury that could be removed before it reached the atmosphere.
Ash disposal, water contamination from mine runoff, and carbon dioxide emissions are additional environmental issues tied to all coal combustion, and No. 9 coal is no exception. These concerns, combined with the economics of competing fuels like natural gas and renewables, have contributed to the decline of coal-fired generation across the region where No. 9 coal was once a dominant energy source.
No. 9 Coal’s Place in U.S. Energy History
The No. 9 seam played a significant role in industrializing the Midwest during the 20th century. Western Kentucky’s coalfields powered factories, heated homes, and fueled the expansion of the electric grid across a wide swath of the country. Towns in the region grew up around mining operations, and coal revenue shaped local economies for generations.
Production from the seam has dropped substantially since its peak, driven by tighter emission regulations, competition from cheaper and cleaner fuels, and the broader contraction of the U.S. coal industry. Many of the underground mines that once worked the No. 9 seam have closed. The coal that remains in the ground is still cataloged as a recoverable resource, but the economic and regulatory landscape makes large-scale extraction unlikely to return to historic levels.