August 15, 2007

MidAmerican's Walter Scott, Jr. Energy Center Unit 4 earns POWER's highest honor

Dr. Robert Peltier, PE

Keeping the air clean

When it selected an air quality control system, WSEC Unit 4 checked every box on the dealer's list of options (Table 2). The unit incorporates state-of-the-art pollution controls (Figure 5) to keep NO_x, SO₂, and particulates in check.

Table 2. Winning specs. These are the key air pollution controls and permit limits for Walter Scott, Jr. Energy Center Unit 4. Source: Hitachi America Ltd.

5. All the add-ons. Air quality control systems include a selective catalytic reduction unit, three spray dryer-absorbers, a pulse-jet filter train, and a tall stack. Courtesy: Hitachi America Ltd.

Hitachi supplied the selective catalytic reduction (SCR) system that reduces NO_x emissions immediately downstream of the boiler (Figure 6). The PRB coal contains high calcium and high catalyst poisons, and the dust easily sticks to the catalyst. This SCR system uses a Hitachi plate-type catalyst that has a higher resistance to dust plugging and has been modified to achieve higher durability in PRB-fired flue gas. The catalyst reactor is compact, with special flue-gas mixers upstream of the reactors. This mixer accelerates NH₃ mixing with flue gas during a short residence time using the U2A system from Wahlco Inc. (www.wahlco.com). In this process, the urea is diluted to a 40% urea/water solution, which then is hydrolyzed into NH₃.

6. Take a slice. A cross-section view of WSEC Unit 4's boiler and other major components. Sliding-pressure operation improves plant efficiency at partial loads. Source: Hitachi America Ltd.

Next in line downstream of the SCR system (Figure 7) are three Babcock & Wilcox (www.babcock.com) dry lime-injected spray dryer-absorbers (SDAs) for SO_x reduction and a pulse-jet bag filter train to control particulates. In each SDA, SO₂-laden hot flue gases mix with a finely atomized spray of fresh lime and recycled ash slurry to produce a dry waste that is easier to dispose of than the waste produced by wet flue gas desulfurization (FGD) systems.

7. Clearing the air. A schematic of the air quality control systems downstream of the boiler illustrates how NOx and SOx are removed. Source: Hitachi America Ltd.

A rotary atomizer with a 1,000-hp motor is an integral part of the SDA vessel. As the slurry droplets evaporate, they absorb SO₂, which reacts with dissolved and suspended alkaline material. The atomizer also sprays water to provide temperature control. The amount of water used is carefully controlled to avoid completely saturating the flue gas, which would impair performance by enabling wet solids to adhere to the surfaces of the absorber vessel water and the baghouse. However, the nearer the system comes to saturating the flue gas, the higher the level of SO₂ removal. The SDA outlet gas temperature is kept at about 17 degrees C above the adiabatic saturation (dew point) temperature. Typical of most FGD systems, the sorbent is delivered in aqueous form to a dedicated absorber vessel.

Gas leaving the SDAs immediately enters the filter trains, which are equipped with fabric bags to separate the solids (flyash and calcium/sulfur compounds) entrained in the flue gas. Each bag has 16 compartments. Cleaning is initiated either by a pressure drop or at a preset time interval. Each compartment is isolated by closing its outlet damper when broken bags are detected.

The reagent preparation system consists of two independent systems for the lime and recycled slurry. Pebble lime from the storage silo is fed to lime slakers, which hydrate it. The solids are collected on the filter bags, which contain unreacted calcium hydroxide; the solids can be used as recycled slurry to react and absorb SO₂ from the flue gas.

Powdered activated carbon injection equipment is available for mercury control, although the final type and quantities of reagent will be determined during future optimization tests. A specific Hg emission rate is not included by the plant's air quality control permit.