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Emission and Compliance Data

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SO2 Emission Reductions | SO2 Compliance | NOx Emission Reductions | NOx Compliance

The Acid Rain Program (ARP), established under Title IV of the 1990 Clean Air Act (CAA) Amendments, requires major emission reductions of sulfur dioxide (SO2) and nitrogen oxides (NOx), the primary precursors of acid rain, from the electric power industry. The SO2 program sets a permanent cap on the total amount of SO2 that may be emitted by electric generating units (EGUs) in the contiguous United States. The program is phased in, with the final 2010 SO2 cap set at 8.95 million tons, a level of about one-half of the emissions from the power sector in 1980. NOx reductions under the ARP are achieved through a program that applies to a subset of coal-fired EGUs and is closer to a traditional, rate-based regulatory system. Since the program began in 1995, the ARP has achieved significant emission reductions.

Table 1 shows a large reduction in SO2 and NOx emissions between 2005 and 2009. This reduction is proportionally much larger than the drop in heat input (a surrogate measure of electricity generation). Most of the reductions since 2005 are from early reduction incentives and stricter emission limits under the Clean Air Interstate Rule (CAIR), which covers most eastern states.


The Clean Air Interstate Rule (CAIR) was designed to reduce the amount of ozone and fine particulate matter (PM2.5) pollution that crosses state boundaries. To achieve these reductions CAIR uses three separate cap and trade programs to limit emissions of NOx and SO2, which contribute to the formation of ozone and PM2.5. The CAIR NOx ozone season program controls ozone formation under a summer-only NOx reduction program, while the CAIR NOx annual and CAIR SO2 annual programs control the formation of PM2.5 year round. The CAIR NOx ozone season and annual programs began in 2009, while required reductions under the CAIR SO2 annual program began in 2010.

Over the next several months, EPA will release a series of reports summarizing progress under the ARP. This first report presents 2009 data on emission reductions and compliance results. Future reports will evaluate progress under the ARP in 2009 by analyzing emission reductions, reviewing compliance results and market activity, and comparing changes in emissions to changes in acid deposition and surface water chemistry. For more information on the ARP, please visit the program's website.

Table 1: SO2, NOx, and Heat Input Trends in Acid Rain Program Units, by Fuel Type

Fuel Type 2005 2006 2007 2008 2009
Coal 9,835 3,356 20.77 9,243 3,208 20.44 8,767 3,069 20.75 7,517 2,817 20.25 5,656 1,824 18.03
Gas 34 142 5.35 7 131 5.70 10 141 6.33 7 129 6.21 5 125 6.51
Oil 350 129 0.99 135 63 0.58 149 68 0.61 84 46 0.48 55 32 0.37
Other 4 6 0.03 7 7 0.05 7 5 0.06 9 5 0.06 8 3 0.06
Total 10,223 3,633 27.14 9,393 3,409 26.77 8,934 3,284 27.74 7,616 2,997 27.01 5,724 1,984 24.97


Source: EPA, 2010

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SO2 Emission Reductions

The SO2 requirements under the ARP apply to EGUs, fossil fuel-fired combustors that serve a generator that provides electricity for sale, with an output of greater than 25 megawatts. The vast majority of ARP SO2 emissions result from coal-fired EGUs, although the program also applies to oil and gas units. As Figure 1 shows, ARP units have reduced annual SO2 emissions by 67 percent compared with 1980 levels and 64 percent compared with 1990 levels. Sources emitted 5.7 million tons of SO2 in 2009, well below the current annual emission cap of 9.5 million tons, and already below the statutory annual cap set for compliance in 2010.

Figure 1: SO2 Emissions from Acid Rain Program Sources, 1980–2009

Figure 1: SO2 Emissions from Acid Rain Program Sources, 1980 - 2009

Source: EPA, 2010

The states with the highest emitting sources in 1990 have generally seen the greatest SO2 reductions under the ARP (see Figure 2). Most of these states are upwind of the areas the ARP was designed to protect, and reductions have resulted in important environmental and health benefits over a large region. In addition, from 2008 to 2009, reductions in SO2 emissions from ARP units in 45 states totaled about 1.89 million tons, or about 25 percent for the year. Six states (Georgia, Indiana, North Carolina, Ohio, Pennsylvania, and West Virginia) accounted for most of the one-year reductions from 2008 to 2009, with reductions ranging from 108,758 to 258,296 tons of SO2 in each of these states.

From 1990 to 2009, annual SO2 emissions in 41 states and the District of Columbia fell by a total of approximately 10 million tons. In contrast, annual SO2 emissions increased by a total of just 39,512 tons in seven states from 1990 to 2009. The seven states with the greatest reductions in annual emissions since 1990 include Ohio, which decreased emissions by over 1.5 million tons, Indiana, which reduced its emissions by over one million tons, and Illinois, Kentucky, Pennsylvania, Tennessee, and West Virginia, each of which reduced total emissions during this time period by more than 500,000 tons.

Figure 2: State-by-State SO2 Emission Levels for Acid Rain Program Sources, 1990–2009

Figure 2: State-by-State SO2 Emission Levels for Acid Rain Program Sources, 1990-2009

Note: States shaded in blue are controlled for fine particles under the CAIR Annual NOx and SO2 programs.

Scale: Largest bar equals 2.2 million tons of SO2 emissions in Ohio, 1990.

Source: EPA, 2010

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SO2 Program Compliance

For 2009, EPA allocated 9.5 million SO2 allowances under the ARP. Together with 8.5 million unused allowances carried over (or banked) from prior years, there were 18 million allowances available for use in 2009. ARP sources emitted approximately 5.7 million tons of SO2 in 2009, less than the allowances allocated for the year, and far less than the total allowances available (see Figure 3). In 2010, the total number of Title IV allowances allocated annually will drop to 8.95 million and remain statutorily fixed at that annual level.

Figure 3: SO2 Emissions and the Allowance Bank, 1995–2009

Figure 3: SO2 Emissions and the Allowance Bank, 1995-2009

Source: EPA, 2010

Table 2 explains in more detail the origin of the allowances that were available in 2009, and Table 3 shows how those allowances were used. Approximately 5.7 million allowances were deducted from sources' accounts in 2009 to cover emissions, leaving 12.3 million banked allowances after the reconciliation process (see Table 3). In 2009, there were 3,572 EGUs subject to the SO2 program. All ARP facilities complied with the requirement to hold enough allowances to cover SO2 emissions.

Table 2: Origin of Acid Rain Program 2009 Allowable SO2 Emission Levels

Type of Allowance Allocation Number of Allowances Explanation of Allowance Allocation Type
Initial Allocation 9,191,897 Initial allocation is the number of allowances granted to units based on the product of their historical utilization and emission rates specified in the Clean Air Act.
Substitution Payback (2,577) A settlement agreement in Phase I of the program allowed some Phase I substitution units to keep Phase I allowances in exchange for surrendering future year allowances.
Allowance Auction 250,000 The allowance auction provides allowances to the market that were set aside in a Special Allowance Reserve when the initial allowance allocation was made.
Opt-in Allowances 109,407 Opt-in allowances are provided to units entering the program voluntarily. There were 20 opt-in units in 2009.
Total 2009 Allocation 9,548,727  
Total Banked Allowances 8,468,465 Banked allowances are those allowances accrued in a facility’s account from previous years, which can be used for compliance in 2009 or any future year.
Total 2009 Allowable Emissions 18,017,192  

Notes: Total banked allowances are adjusted from the 2008 Progress Report to account for additional allowance deductions made after the 2008 reconciliation was completed.

Source: EPA, 2010

Table 3: SO2 Allowance Reconciliation Summary, 2009

Total Allowances Held (1995–2009 vintages) 18,017,192
Affected Facility Accounts 12,077,465
Other (General and Non-Affected Facility Accounts) 5,939,727
Allowances Deducted* 5,727,132
Penalty Allowance Deductions ( 2010 Vintage) 0
Banked Allowances 12,290,060
Affected Facility Accounts 6,350,333
Other (General and Non-Affected Facility Accounts) 5,939,727

* Includes 5,833 allowances deducted from opt-in sources for reduced utilization.

Source: EPA, 2010

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NOx Emission Reductions

Title IV requires NOx emission reductions for certain coal-fired EGUs by limiting the NOx emission rate (expressed in lb/mmBtu). Congress applied these rate-based emission limits based on a unit's boiler type (see Table 4). The goal of the NOx program is to limit NOx emission levels from the affected coal-fired boilers so that their emissions are at least two million tons less than the projected level for the year 2000 without implementation of Title IV.

Table 4: NOx-Affected Title IV Units by Boiler Type and NOx Emission Limit, 2009

Coal-Fired Boiler Type Title IV Standard NOx Emission Limits (lb/mmBtu) Number of Units
Phase I Group 1 Tangentially Fired 0.45 132
Phase I Group 1 Dry Bottom, Wall-fired 0.50 107
Phase II Group 1 Tangentially Fired 0.40 296
Phase II Group 1 Dry Bottom, Wall-fired 0.46 290
Cell Burners 0.68 37
Cyclones > 155 MW 0.86 54
Wet Bottom > 65 MW 0.84 20
Vertically Fired 0.80 24
Total All Units   960

Source: EPA, 2009

Figure 4 shows that NOx emissions from all ARP sources were 2.0 million tons in 2009. This level is 6.1 million tons less than the projected level in 2000 without the ARP, or more than triple the Title IV NOx emission reduction objective. While the ARP was responsible for a large portion of these annual NOx reductions, other programs — such as CAIR, the NOx Budget Program under EPA's NOx State Implementation Plan (SIP) Call, the Ozone Transport Commission (OTC), and other regional and state NOx emission control programs — also contributed significantly to the NOx reductions achieved by sources in 2009. The significant drop from 2008 to 2009 is the largest single year decline in the history of the program. This large reduction in emissions is primarily attributable to compliance with the annual and ozone season CAIR programs for NOx beginning in 2009.

Figure 4: NOx Emission Trends for All Acid Rain Program Units, 1990–2009

Figure 4: NOx Emission Trends for All Acid Rain Program Units, 1990 - 2009

Source: EPA, 2010

From 1995 to 2009, annual NOx emissions from ARP units dropped by about 4.1 million tons, a net decrease of 67 percent. Forty-four states and the District of Columbia reduced NOx emissions during this period versus six states that accounted for only about 5,542 tons of increased NOx emissions during the same period (see Figure 5).

Figure 5: State-by-State NOX Emission Levels for Acid Rain Program Sources, 1990–2009

Figure 5: State-by-State NOX Emission Levels for Acid Rain Program Sources, 1990 - 2009

Scale: Largest bar equals 500,000 tons of NOX emissions in Ohio, 1990.

Source: EPA, 2010

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NOx Compliance

The ARP NOx Program does not impose a cap on NOx emissions and does not rely on allowance trading. The program allows affected sources to comply either by meeting a unit-specific emission rate or by including two or more units in an emission rate averaging plan. These options provide affected sources with the flexibility to meet the NOx emission reduction requirements in a cost-effective manner. In 2009, all 960 units that were subject to the ARP NOx Program achieved compliance.

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