Tampa Electric is pursuing an air permit from the Florida Department of Environmental Protection that covers an upgrade project at Unit 3 of its coal-fired Big Bend power plant that will take care of a long-running slagging problem brought on by the addition over the years of new emissions control equipment.
The Big Bend plant is in Hillsborough County, on Tampa Bay. Tampa Electric, a unit of TECO Energy (NYSE: TE), applied to the department on March 26 for an air construction permit to authorize physical changes to Unit 3 to address slagging and fouling problems and to upgrade the electrostatic precipitator (ESP) on Unit 3. The department went out for public comment on the draft permit on July 2.
The facility consist of four steam boilers (Units 1-4) fired with solid fuels and controlled by individual ESP and selective catalytic reduction systems and two flue gas desulfurization systems serving all four steam units. There are also at this location three simple-cycle combustion turbines (CT Nos. 1-3) fired with No. 2 fuel oil.
Big Bend Unit 3 was constructed prior to the New Source Performance Standards (NSPS), having begun commercial operation in 1976. Unit 3 is a Riley Stoker Turbo-Furnace wet bottom design with a generator nameplate rating of 445 MW. It was equipped originally with an ESP, with the later addition of low NOX burners, selective catalytic reduction (SCR) and a cutover into the existing Unit 4 FGD.
“These types of boilers (sometimes called slag tap furnaces) are much more compact than pulverized coal boilers used by most large utility generating stations,” DEP noted. “They can burn a wide range of fuels and generate a higher proportion of bottom ash than fly ash (50 to 80% bottom ash vs. 15 to 20% bottom ash for pulverized coal boilers). With wet-bottom boilers, the molten ash is withdrawn from the boiler and allowed to flow into quenching water. The rapid cooling of the slag causes it to immediately crystallize into a black, dense, fine-grained glassy mass that fractures into angular particles, which can be crushed and screened for several uses.”
The agency added: “The changes to Unit 3 emissions controls over the past 15 years have exacerbated a wet slagging problem in the radiant areas and fouling in the convective pass of the boiler and consequently have lowered steam temperatures in the superheater, reheater, and economizer sections of the furnace. The two main types of deposits in boilers are slagging and fouling.”
The low NOx burners have been a special problem when it comes to slagging
Historically, Tampa Electric was able to operate at higher excess air levels in Unit 3 to minimize slagging in the boiler but when the low NOX burners were installed and other back end pollution controls were added, it limited this option for higher excess air levels. The proposed changes to the boiler’s radiant and convective heat transfer sections are necessary in order to address boiler slagging and fouling that have become worse with add-on pollution controls, especially the low NOX burners.
In this project, Tampa Electric has proposed modifications to the furnace for Unit 3 to address the slagging problem by improving the heat transfer sections and by changes to the nose area at the top of the furnace. The nose extension will increase the velocity of the exhaust gases and thereby reduce fouling while also reducing radiant heating of the ash to help keep ash temperatures in the upper furnace below the ash fusion temperature where slagging can occur. These changes include adding additional boiler surface area to increase the outlet steam temperature of the superheater, reheater and economizer sections. The utility will replace the original high temperature superheater, reheater, and economizer as well as the radiant superheater and the nose arch as part of this project.
For this project the utility has not requested an increase in the value of the heat input listed in the current permit for Unit 3, the original design value and what the company believes is the current maximum heat input of the boiler today, the nameplate rating of 4,115 MMBtu/hr. In the application, TECO stated that the boiler will operate at the existing low NOX condition, using 10% excess air as designed for the low NOX burners, and the past actual and future actual emissions are projected to be identical.
It is apparent to the department that the physical changes proposed (replacements of radiant super heater, reheater, economizer, and nose arch) to the Unit 3 furnace are modifications which have the potential to result in higher capacity factors or consistently higher heat inputs and therefore may lead to increases in annual emissions, said the DEP. To keep an eye on this, Tampa Electric will be required to evaluate the following pollutants from Unit 3 for which CEMS data is recorded: particulate matter, NOx, SO2 and carbon monoxide.