September 2015: Optimizing Power Plant Emissions

Pollution reduction is above all else, a balancing act. Adding controls to reduce certain pollutants like sulfur dioxide (SO2) and nitrogen oxides (NOx) can affect power plant efficiency and, thus, increase carbon dioxide (CO2) emissions to the atmosphere.

There is a movement in some northeastern states to require utilities in the Midwest to minimize NOx emissions. The northeastern states believe that this NOx emission reduction will lead to lower ozone levels in the northeastern U.S. At the same time, the U.S. Environmental Protection Agency (U.S. EPA) has just released the Clean Power Plan, which calls for states to operate their utilities to minimize emissions of CO2 in order to meet targets for each state.

The problem is that you cannot minimize these two pollutants at the same time without affecting efficiency and the ability to meet energy demand. To illustrate the problem, I looked at utilities in Indiana in 2014 and the air pollutants generated producing 10,000 megawatts (MW) of energy for an hour. I looked at two scenarios: for the first, I assumed 9,500 MW generated from coal combustion with 500 MW supplemented by solar and wind energy; for the second, I assumed no supplemental solar or wind energy was available. Finally, I looked at how much of each pollutant would be generated if the utility optimized its units to minimize emissions for each of three different pollutants: sulfur dioxide (SO2), nitrogen oxides (NOx) and carbon dioxide (CO2).

Looking first at SO2, if we select to optimize efficiency for the lowest SO2 emissions per unit of energy, we will generate 3,630.9 pounds per hour (lb/hr) of SO2 in producing 10,000 MW of energy for one hour (assuming 500 MW from solar and wind energy). Along with this, we get 10,100.1 lb/hr of NOx and 8,044.8 tons per hour (T/hr) of CO2. If wind and solar energy are not available, the SO2 increases to 4,248.2 lb/hr (up 17.0 percent). Meanwhile NOx emissions increase to 10,652.0 lb/hr (up 5.5 percent) and CO2 emissions increase to 8,518.7 T/hr (up 5.9 percent).

If we select to optimize those units that produce the least NOx per unit of energy, we will produce 7,399 pounds per hour of NOX. Along with this we get 13,075.2 lb/hr of SO2 and 8,116.8 T/hr of CO2. If wind and solar energy are not available, the NOx emissions increase to 8,167.2 lb/hr (up 10.4 percent). Meanwhile SO2 emissions increase to 13,716.8 lb/hr (up 4.9 percent) and CO2 emissions increase to 8,596.5 T/hr (up 5.9 percent).

If we select those units that produce the least CO2 per unit of energy, we can meet the requirement producing 7,289.9 T/hr of CO2. Along with this we get 35,424.8 lb/hr of SO2 and 12,118.1 lb/hr of NOx emissions. If solar and wind energy are not available, CO2 emissions increase to 7,700.5 T/hr (up 5.6 percent). Meanwhile, SO2 emissions increase to 36,244.0 lb/hr (up 2.3 percent) and NOx emissions increase to 12,787.8 lb/hr (up 5.5 percent).

Table 1: Emissions for Various Scenarios
Pollutant to Minimize Wind/Solar MW SO2 (lb/hr) NOx (lb/hr) CO2 (T/hr)
SO2 500 3,630.9 10,100.1 8,044.8
SO2 0 4,248.2 10,652.0 8,518.7
NOx 500 13,075.2 7,399.9 8,116.8
NOx 0 13,716.8 8,167.2 8,596.5
CO2 500 35,424.8 12,118.1 7,289.9
CO2 0 36,244.0 12,787.8 7,700.5

The different scenarios for each pollutant are summarized in Table 1, making it easy to see if you optimize for one pollutant, another pollutant may increase significantly. It is impossible to optimize for all pollutants simultaneously. Since criteria pollutants (SO2 and NO2) have immediate health effects, it is risky to optimize for CO2 emissions at the expense of these pollutants.

This balancing act of units is very difficult to accomplish in real time. A particular unit may not be available when called upon. In the case of Indiana two of the utilities available in 2014 have closed in early 2015. Other utilities are expected to close in the next year or two or to switch fuels.

For much of the Midwestern U.S., this utility energy balancing is done by the Midwest Independent System Operator (MISO). In real time, they balance energy needs with availability. MISO currently is charged with minimizing costs, not emissions. The Clean Power Plan rule may impact how MISO and other similar organizations operate.

Coal units operate best as base load (stable operation). If they are moved to peaking status, the amount of pollutant emitted per unit of energy may increase. In addition, it may be difficult to keep control devices up to speed if the unit is in peaking status much of the time.

The world will become more complicated moving forward.