Coal mining: Efficiency
How much of a seam ever reaches a buyer
What this tab covers How much of the energy available is actually captured or extracted, in per cent: theoretical limits, laboratory records, typical new equipment and in-service averages, each with exactly what is being measured.Here efficiency means the share of coal in the ground that ends up as saleable product. Surface strip mines take out over 90% of a seam, longwall mines about 78% and room and pillar mines about 62%, and washing then rejects about a fifth. The energy returned is roughly 50 to 100 times the energy spent.
Chart: Efficiency ranges
In serviceEfficiency by measure
| Measure | Range | Type | What is measured |
|---|---|---|---|
| Contour strip (surface) mining recovery | 92%1 | In service | Share of minable coal in place taken out of the ground, before washing (Appalachian study area) |
| Longwall (underground) mining recovery | 78%1 | In service | Share of minable coal in place taken out, before washing |
| Continuous miner, room and pillar (underground) recovery | 62%1 | In service | Share of minable coal in place taken out; pillars are left to hold up the roof |
| Auger mining (from a surface highwall) recovery | 30%1 | In service | Share of minable coal in place taken out |
| Coal washing (preparation plant) yield | 80 to 82%1 | In service | Share of mined material kept as clean coal after washing |
| Whole study area: finished coal as share of coal in place | 50%1 | In service | All methods combined, after restrictions (towns, roads, thin seams) and washing |
| Energy return on energy invested (US coal) | 50 to 100 ratio (energy out per unit in)2 | In service | Thermal-equivalent EROI of US coal mining, 1960s to 1987; about four times lower if fuels are weighted by quality |
Where the energy is lost
| Step | Loss |
|---|---|
| Pillars of coal left to support the roof in room and pillar mines | about 38% of the coal in room and pillar sections (calculated from 62% recovery)1 |
| Rock and dirt removed at the preparation plant | about 18-20% of mined tonnage1 |
| Coal sterilised under towns, roads and in thin seams | about 12% of coal in place (restricted resources)1 |
There is no thermodynamic limit here: the question is geometry and safety. Surface mines strip away the overburden and can take almost the whole seam. Underground mines must leave coal to hold up the roof unless, as in longwall mining, the roof is allowed to collapse behind a moving steel canopy. In a 1993 US Bureau of Mines study of one Appalachian seam, only about half of the coal in place ended up as finished product once restricted areas, mining losses and washing were counted.
Mining uses little energy relative to what it yields, so coal's energy return is among the highest of any fuel; it falls as seams get thinner and deeper.
Sources
- Coal Resource Recoverability: A Methodology (Information Circular 9368; Rohrbacher, Teeters, Sullivan and Osmonson), Table 12 US Bureau of Mines (hosted by USGS), 1993
- A Review of the Past and Current State of EROI Data (Gupta and Hall, Sustainability 3(10)) MDPI, 2011