I have never seen this principle mentioned as somebody’s rule or law but it is accepted as such in the air pollution control community. In particular: as the control efficiency increases the control cost per ton reduced increases exponentially.
I will illustrate how this works using the example of particulate control on a coal-fired boiler. I used to work for Niagara Mohawk Power Company and they built and operated the Dunkirk Generating Station in western New York. I will describe the history of particulate control there up to the point it was retired several years ago. When the four coal-fired boilers were built in the 1950’s the only particulate pollution control equipment installed was a cyclone. A cyclone is basically ductwork that uses the principle of inertia to remove particles from the flue gas.
In the late 60’s, I believe but am not positive, New York State ordered Niagara Mohawk to do a better job cleaning the flue gas. To its credit, the company installed a hot-side electrostatic precipitator (ESP) that was over-sized and reduced the particulate levels well below the standard in effect even forty years later.
Despite the fact that the particulate levels were below emission standards there was a persistent problem with opacity. This is a measure of how opaque the plume is and before there was pollution monitoring equipment this was used to determine how well a boiler was operated. If the boiler is running efficiently there should be very little smoke visible but a dark opaque plume is a sure sign that it is not being run well. The standard methodology (which is still in use today) is for a trained “smoke reader” to observe the plume in six-minute intervals. An excess opacity emission event means smoke emissions of one or more six minute periods in which the average opacity exceeds 20%, except that one event in every hour may be excluded if the average opacity during the six-minute period does not exceed 27%. When continuous opacity monitoring systems were installed there was an opacity issue that the New York State Department of Environmental Conservation used as an excuse to fine the owners of the plant (NRG Energy after 1999) millions of dollars for the scourge of an aesthetic violation (remember the particulate limits were not exceeded). As a result, NRG installed a baghouse to eliminate the problem which worked extremely well until the station was retired.
For an order of magnitude estimate of costs I used the ESP and baghouse examples in the EPA Air Pollution Control Handbook. In the handbook the example ESP cost was $1,840,000 and the baghouse cost was $569,000. I assume that a cyclone would only cost $100,000.
For my example assume that the boiler generates 100 tons of particulate. A cyclone has an efficiency around 35% so it reduced emissions by 35 tons at a cost per ton of $2,857. The ESP has an efficiency of 98% would reduce the 65 remaining tons by 98% at a cost per ton of $28,885. In order to get the final two tons removed the baghouse cost per ton is $284,500 simply because the additional controls removed so little. Clearly this is an exponential increase in costs for the last little bit of emissions.
This relationship is a primary driver in greenhouse gas control costs. Consider energy efficiency at your home. The first bit of insulation in the attic does not cost much but gets a good reduction in energy loss. As the homeowner progressively adds insulation to the walls, upgrades the windows and doors and audits the last little bit of air infiltration the energy reductions get smaller and smaller so even if the control cost themselves stays the same the cost per efficiency increase goes up. It is true in every control instance.
Ultimately this rule simply quantifies the low-hanging fruit analogy. It is easy to pick the low hanging fruit but the higher you go the more it costs.