The Florida Department of Environmental Protection on July 14 issued a draft approval on an application by Tampa Electric for air permitting needed to unload more limestone from ships at the coal-fired, scrubber-equipped Big Bend plant on Tampa Bay.
Big Bend is a nominal 1,892-MW facility. Units 1 through 4 have a combined electrical generation output of 1,821 MW. Units 1 through 3 each have a design electrical generating capacity of 445 MW. Unit 4 has a design electrical generating capacity of 486 MW. The fuel fired in all four units consists of coal, or a coal/petroleum coke blend containing a maximum of 20% petroleum coke by weight, or coal blended with coal residual generated from the Polk IGCC unit, or a coal/petroleum coke blend further blended with coal residual generated from the Polk station, and on-site generated fly ash.
Tampa Electric intends to transload limestone from ocean vessels to a temporary storage pile in the fuel yard. The limestone will then be transloaded via trucks to the limestone handling building or to new segregated permanent storage piles located within the existing east and south gypsum storage areas. These activities will create particulate emissions due to the transfer of limestone (i.e., truck loading and unloading operations/unloading, truck traffic on paved facility roads, front-end loader storage pile operations, and storage pile windblown dust). This project also includes the increased handling of limestone.
In February 2014, a permit was issued for Tampa Electric to temporarily authorize the unloading of various types of limestone transported to the facility by railcar and/or ocean vessels. The purpose of that project was to explore a higher quality of limestone from new sources to increase the performance of the Flue Gas Desulphurization (FGD) systems. Prior to this permit, the facility only received limestone via truck unloading. Based on a maximum amount of limestone being received at the facility via railcar/ocean vessel of 105,000 tons per year (TPY), the estimated PM/PM10 emissions from that project were less than 5 TPY.
The Big Bend Station is currently authorized to transload up to 105,000 TPY of limestone from railcars or ocean vessels using the existing solid fuel unloading and conveying systems to unload and store limestone in the fuel yard area. Through this new project, Tampa Electric is requesting to increase the high quality limestone handling to a transfer rate of 200,000 TPY from ocean vessels only.
Tampa Electric proposes to continue to utilize the existing solid fuel yard handling system to unload high quality limestone from the ocean vessels onto a temporary limestone storage pile located in the fuel yard. Trucks will haul the high quality limestone from the temporary storage pile to the east or south permanent storage areas, where the limestone will be unloaded and stored in areas segregated from the stored gypsum. The empty trucks will travel back to the fuel yard to reload until the temporary limestone pile in the fuel yard area is depleted. Heavy equipment (e.g. front end loaders) will stack and manage the limestone piles in the east or south storage areas. When the high quality limestone is needed for the FGD system, it will be loaded into trucks and hauled to the FGD unloading area. Also, limestone may be transferred between the east and south storage area to account for operational changes.
Tampa also permitting changes related to gas igniters
The department on July 14 also issued a draft air permit approval for Tampa Electric to make changes in the gas igniters used at Units 1-4 at Big Bend. Each boiler is equipped with a natural gas igniter system to control the combustion process. These igniter systems are capable of firing concurrently while firing coal or alone to provide heat for the generation of steam at lower loads. A prior air construction permit authorized construction of the natural gas igniter systems on Units 1-4, demolition of the No. 2 fuel oil ignition systems and gas pipeline.
The gas igniter systems for Units 1, 2 and 3 were designed with 24 igniters with up to 70 MMBtu/hour each for a total heat capacity of 1,680 MMBtu/hour. Unit 4 was designed with 16 igniters and 4 warmup igniter guns for a total heat input capacity of 1,920 MMBtu/hour. The original combined design heat input capacity for Units 1 to 4 was 6,960 MMBtu/hour when operating at a gas delivery pressure of 35 psi. Upon installation of the gas igniter systems, it was discovered that the igniters could theoretically be operated at higher gas delivery pressures than 35 psi, which could quickly raise the combustion gas temperature to the necessary level to place the plants selective catalytic reduction (SCR) systems into service. This provides the possibility of starting up the units solely on natural gas and placing the SCR systems into service prior to blending coal into the fuel mix.
With the opportunity for shorter start up periods and the ability to operate the SCR systems under all normal operating conditions (past start up) while firing gas, either alone or while co-firing solid fuels, there will be no periods of uncontrolled normal operation. Therefore, operating on natural gas as a replacement for solid fuel with the SCR systems in operation results in emissions reductions.
Based on current information, Tampa Electric claims that the existing system is capable of firing natural gas at 50 psig up to an equivalent heat input of approximately 9,900 MMBtu/hour without any physical modifications, and likely even higher. However, the natural gas distribution system is expected to physically limit the total operation of the gas igniters and process heaters to a maximum of 12,000 MMBtu/hour. The more gas firing that is exchanged for solid fuel-firing, the greater the reduction in actual emissions should be. The department is in favor of the utility testing the gas igniter systems to determine the maximum possible firing rate of the installed equipment and utilizing as much natural gas as possible. However, if Tampa Electric wishes to fire more than 12,000 MMBtu/hour on gas, an air construction permit will need to be obtained in order to modify the natural gas delivery system and to install additional and/or larger gas igniters.
Tampa Electric is also requesting the authority to evaporate excess recycle water when firing gas. Co-firing coal and natural gas or natural gas alone at lower loads typically reduces the amount of recycle water that is evaporated in the FGD system. This creates a surplus of recycle water at Big Bend Station. Authorization is requested to evaporate up to a maximum of 730 million gallons/year of recycle water to maintain the water balance at Bend Station. The recycle evaporation and igniter modification projects are considered one contiguous project for emission purposes since the igniter project impacts the recycle water balance. The recycle water will be injected into each boiler using a series of lances in the lower furnace.
The total dissolved solids (TDS) in the recycle water will be released as particulate matter (PM) in the boiler. The electrostatic precipitator (ESP) is expected to remove more than 99% PM through the collection of fly ash. The FGD system is also expected to remove nearly 50% PM through the scrubbing fluid. Approximately 19 tons/year of PM will be released to the atmosphere as a result of evaporating the recycle water in the boilers; however, this increase is more than offset by the lower PM emissions associated with firing natural gas. The recycle water will be evaporated in a controlled manner to maintain the same temperature profile throughout the system, including each wet stack.