Small muni co-op achieves big NO subscript x reductions The Texas Municipal Power Agency is two years ahead of schedule to comply with a state mandate to reduce NO subscript x emissions from its Gibbons Creek power plant to 0.165 lbs/mmBtu or less. Two things are remarkable about TMPA's achievement: NO subscript x was reduced by modifying only the plant's combustion and fuel-delivery systems, and the initiative wasn't structured as a typical EPC project--because the Texas regulator wouldn't allow it.

Power, Oct 2003 by Partlow, Bruce, Marz, Pat, Kaltenbach, Robert, Grusha, John, Corp, Foster Wheeler Energy

The cities of Bryan, Denton, Garland, and Greenville created the Texas Municipal Power Agency (TMPA) in 1977 to pool their resources in order to better cope with a perceived rise in natural gas prices. TMPA was the first entity of its kind in Texas to take advantage of a then-new state law allowing municipalities to own power plants.

Today, TMPA owns and operates the Gibbons Creek Electric Generating Station, which has one 480-MW, tangentially fired, pulverized coal unit. The unit became operational in 1983 and originally fired a locally mined, low-grade Texas lignite (Figure 1). Gibbons Creek is about 18 miles east of Bryan/College Station and roughly 90 miles north of Houston.

In 1996, TMPA management switched exclusively to PRB coal to improve the economics of Gibbons Creek and to lower SO subscript 2 emissions. The switch required relatively minor changes to the physical plant and its control systems. Only one of the original eight mills was removed from service, although only five mills are needed for full-load operation using PRB coal as fuel.

Gibbons Creek is located in an ozone attainment area of east Texas between the Dallas-Fort Worth and Houston-Galveston nonattainment regions. A few years ago, the Texas Natural Resource Conservation Commission (now the Texas Commission on Environmental Quality, TCEQ) decided that emissions from facilities located in nearby attainment regions can affect the environmental compliance of plants in nonattainment areas. As a result of Texas Legislature Senate Bill 7, TCEQ established a NO subscript x emission limit of 0.165 lb/mmBtu for plants in such areas. Staying within that limit will become mandatory in 2005 for TMPA and the Gibbons Creek plant.

At the time of the fuel switch to PRB coal, NO subscript x emissions from Gibbons Creek were limited to 0.60 lb/mmBtu. TMPA initially had trouble developing a response to the new NO subscript x regulations due to staffing and budget limitations. Adding to this difficulty was a complicating factor: The state would not allow management of TMPA (or any other Texas muni) to structure the necessary emissions-reduction initiative as an engineer, procure, and construct (EPC) project. This requirement, which typically forces vendors to bid only on portions of a project rather than as a turnkey job, serves to limit project flexibility and the potential for cost savings.

Hatching a plan

TMPA management assigned two people to work on the project with air quality scientists from Burns & McDonnell Engineering (Kansas City, Mo.). The team's primary goal was to reduce NO subscript x emissions without resorting to selective catalytic reduction (SCR) and at the same time correct long-term shortcomings resulting from the 1996 fuel switch. The team estimated that the project would take a minimum of three years to complete, with major installation and modification activities scheduled to take advantage of the unit's annual spring outages.

Burns & McDonnell acted as TMPA's engineer in a support/auditing role by creating specifications for major portions of the work and by assisting TMPA personnel in quality control and drawing support. Using a "first-principles" technique, the team developed a phased approach to maximize guaranteed NO subscript x reductions, minimize adverse operational changes, and complete the project within a relatively tight budget. Initial expectation of a 42% reduction in NO subscript x to a level of 0.20 lb/mmBtu seemed optimistic but nonetheless was made the initial goal.

The team envisioned a three-phase plan covering seven areas of plant operation. Its initial assumption was that no vendor would provide meaningful NO subscript x emissions guarantees unless the vendor could be assured that coal with certain specifications, including required particle fineness, would be delivered to the boiler. Phase I therefore focused on optimizing fuel preparation and transport to the boiler. Phase II focused on modifications that integrated combustion improvements with NO subscript x reduction. Phase III, which is ongoing, addresses additional NO subscript x reduction possibilities and fine-tuning of controls.

Tackling fuel supply first

The project kicked off with the examination of a variety of fuel-delivery problems. One was that the coal piping at Gibbons Creek was too large to handle PRB coal properly. Another was that the plant's existing system for controlling primary air to individual mills was adequate for safe combustion and temperature control but too variable for precise control and measurement. The project team corrected the fuel line-sizing problem by replacing the original, 26-inch-diameter piping with 4,000 feet of 24-inch piping designed and supplied by BendTec Inc. (Duluth, Minn.). TEI Constructors (Mobile, Ala.) had no problem installing the new pipe in 21 days (Figures 2 and 3).

The project team solved the primary-air problem by replacing the existing control system's venturi flow meters with modern airflow instrumentation in the form of Raleigh, N.C.-based Air Monitors' combustion air monitoring system (CAMS). As part of this effort, all of the plant's original damper drives were replaced with more rugged Beck drives. Several original dampers were replaced as well.