R.E. Burger Plant, Shadyside, Ohio

Dr. Robert Peltier, PE

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Coal is still king in the realm of power generation, but the palace is under siege by new environmental rules with bigger teeth. Emerging air emissions regulations—in the form of the Clean Air Interstate Rule (CAIR), the Clean Air Mercury Rule (CAMR), and a patchwork of state rules—make plant environmental upgrades more expensive and take a bigger bite out of the industry's investment dollar.

To be sure, the return on that investment will be a tangible improvement in the air we breathe. The gains in air quality attributable to the installation and upgrades of air quality control systems (AQCS) at power plants since passage of the original Clean Air Act in 1970 are certainly remarkable. But the days of inexpensive retrofits to capture "low-hanging fruit" are long gone. We'll leave the discussion of "How low can you go?" for another time.

Most AQCS designs are evolutionary, and at some point all of the technologies on which they are based have been mixed and matched at some plant in the U.S. Removing specific constituents of the exhaust gas stream—first NO_x, SO₂, and particulate matter (PM), and now mercury (Hg)—is so dependent on a plant's design, location, and fuel type that dozens of alternative reduction approaches have been tried over the past 35+ years.

NO_x control began with burner upgrades or replacements and was followed by selective catalytic reduction (SCR). SO₂removal at a 95% or higher level has become common since the first utility-scale scrubber entered service in 1988. Many coal plants delayed spending a bundle on a scrubber by switching to low-sulfur Powder River Basin (PRB) coal in the 1990s, but they encountered combustion problems as a consequence. Although compliance with PM regulations typically has been addressed by adding an electrostatic precipitator (ESP) or baghouse, the new PM_2.5 regulations may make existing units obsolete.

One AQCS for four pollutants

Retrofits to operating coal plants have become more expensive than anyone predicted. A recent survey by the EUCG (www.eucg.com) of 48 flue gas desulfurization (FGD) projects by 12 utilities pegged the average capital cost of a scrubber for a unit larger than 300 MW at about $300/kW (POWER, July 2007, "What's that scrubber going to cost?"). Other sources estimate the cost of an SCR system at more than $100/kW. This June, a Midwestern utility announced that it would be spending $750 million to add scrubbers to two 600-MW units now under construction at a cost of $2.3 billion.

Thankfully, new emissions-reduction alternatives promise greater cost-effectiveness than mixing and matching AQCS. The one perhaps furthest up the evolutionary ladder can control multiple pollutants simultaneously. The technology—which has been under continuous development by Powerspan Corp. (www.powerspan.com) since late 1996 and supported by FirstEnergy Corp. at its R.E. Burger plant since 1998—has now reached the point of commercialization (see box). R.E. Burger Plant (Figure 1) has two coal-fired baseloaded units, one coal-fired peaking unit, and three oil-fired peakers, for a total capacity of 427 MW.

1. Host with the most. In 2004, FirstEnergy's R.E. Burger Plant began operating, for demonstration purposes, a 50-MW (equivalent) version of Powerspan's Electro-Catalytic Oxidation (ECO) multipollutant removal process. Courtesy: FirstEnergy

Inside the ECO process
The unique feature of Powerspan's Electro-Catalytic Oxidation (ECO) technology (Figure 2) is its ability to simultaneously remove SO₂, NO_x, PM_2.5, acid gases, Hg, and other metals from the exhaust of a coal-fired power plant. ECO converts NO in flue gas to more-soluble NO_x compounds, which it then absorbs, along with SO₂. The process captures oxidized Hg in the exhaust using a wet electrostatic precipitator (ESP), which also collects aerosols and fine particles in the flue gas. The by-product of the ECO process, ammonium sulfate, can be marketed as a fertilizer.

2. Deceptively simple. An ECO process flow diagram illustrates how the multipollutant control system integrates with an existing plant. Source: Powerspan

In a typical plant, the ECO system is installed downstream of the plant's existing ESP or fabric filter. The multipollutant treatment occurs in three distinct steps:

1. A reactor oxidizes gaseous pollutant(s) using a dielectric barrier discharge. When the voltage across the electrodes of the reactor reaches the breakdown voltage of the flue gas, micro-discharges are formed and oxygen and water molecules in the flue gas are converted to the atomic oxygen and hydroxyl radicals that lead to the oxidation reactions.
2. An ammonia scrubber removes the NO_x compounds produced by the reactor and the SO₂ that is not oxidized. The scrubber has two liquid loops: a lower one for quenching the gas to its saturation temperature and an upper one to absorb the SO₂ and NO_x.
3. A wet ESP captures the aerosols produced in the first two steps, fine particulate matter, and oxidized mercury. The wet ESP and the ammonia scrubber are housed in a single absorber tower.
Liquid bled from the absorption loop and material captured in the WESP are collected in the lower quench loop. Unlike typical scrubber slurry, the only solids in the liquid are from the captured ash. This is because all of the salts are dissolved at a concentration just below the level at which they begin to crystallize. A slipstream from the quench loop then is pumped to the co-product processing system. Filters and absorbent beds remove any fine particles and Hg from the co-product slipstream, which can then be used as a commercially valuable liquid fertilizer containing 8% nitrogen and 9% sulfur or further processed to form fertilizer crystals. The 50-MW commercial demonstration unit tested at the R.E. Burger Plant in 2005 produced approximately 3,000 tons per month of liquid ammonium sulfate over a six-month continuous run. A larger unit would use a separate crystallizer to produce a more valuable dry co-product.
Another unique feature of the ECO process is its zero liquid discharge. Waste products are limited to small amounts of ash that enter the ECO system and are filtered from the liquid, along with a small amount of spent sorbent used to remove Hg from the co-product.

After successful pilot testing funded and hosted by FirstEnergy, in February 2004, Powerspan installed a 50-MW (equivalent) commercial demonstration unit (CDU) of its Electro-Catalytic Oxidation (ECO) process (Figure 3) at the Burger plant and began testing it as an integrated system. The CDU worked on an 110,000-scfm (standard cubic ft/min) gas slipstream from the ductwork of Unit 4 or 5, downstream of the plant's existing electrostatic precipitator (Figure 4). The treated gas was returned to the ductwork just upstream of the stack.

3. Proof of performance. In 2005 the ECO commercial demonstration unit was operated continuously for six months to confirm the design and reliability of the system. Although the demo ended two years ago, the ECO unit remains in service. Source: Powerspan

4. Hooked on gas. A 50-MW (equivalent) gas stream was taken from the stack gas, treated in the ECO system, and then returned to the ductwork just ahead of the stack. Source: Powerspan

A 180-day reliability run of the CDU that ended in September 2005 demonstrated that ECO meets all current performance requirements for NO_x, SO₂, and particulate control, and can remove 85% of the Hg in flue gas to boot. The table summarizes the results of the reliability run, during which the CDU was more than 98% available.

The demonstration project was funded by Powerspan, FirstEnergy, and the Ohio Coal Development Office of the Ohio Air Quality Development Authority (www.ohioairquality.org). In addition to the first two companies, the project team included the Siemens Power Generation subsidiary Wheelabrator Air Pollution Control Inc. (www.wapc.com), Fluor Power (www.fluor.com), and The Andersons Inc. (www.andersonsinc.com), which handled fertilizer co-product removal, marketing, and sales. The ECO CDU remains in service today.

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