Marshalltown Combined Cycle-Alliant

Alert 1123       Alliant Energy’s 600 MW Marshalltown Combined Cycle Project moving   Forward

Alliant Energy Corp.’s Interstate Power and Light subsidiary said it has reached agreement with the Iowa Office of Consumer Advocate (OCA) regarding construction of a new $700 million 600 MW natural gas-fired power plant in Marshalltown. The agreement, which must be approved by the Iowa Utilities Board, includes an OCA-recommended return on equity of 11 percent for the project and a return on equity of 10.3 percent used to calculate the allowance for funds used during construction. The Iowa Utilities Board set a May 21 hearing to consider Interstate Power’s request for a generation certificate for the Marshalltown station and to consider the utility’s proposed rate-making principles. “The 600 MW combined cycle natural gas Marshalltown Generating Station is a key piece of our long-term energy resource plan,” said Tom Aller, Interstate Power president. “Reaching a fair and reasonable settlement agreement with the OCA is a significant step forward in executing our plan.”

If approved, the power plant could be put in service in spring 2017.

https://efs.iowa.gov/cs/groups/external/documents/docket/mdaw/mtuz/~edisp/130753.pdf

The MGS will be an intermediate load, baseload-capable, combined-cycle

facility comprising of three electric power generating units. The MGS design will

be based on an average of 260 starts per year. This will allow load dispatchers

to place the MGS generating units in and out of service as required by

transmission grid load and system reliability requirements. IPL expects the

annual dispatch of the MGS will vary between a 10 and 50 percent capacity

factor, dependent upon system requirements, MISO market considerations, and

economic dispatch criteria. The three MGS generating units will also be capable

of baseload operations and capacity factors in excess of 70 percent as market

conditions and transmission grid load profiles change over the life of the MGS.

Individual loading of on-line generating units may be varied up or down in

response to variations in real-time energy and reactive power needs. This will

provide system load dispatchers with enhanced control for matching generation

output with load requirements from the transmission system.

The MGS’s three electric power generating units will be configured in what

is referred to as a 2x1 (two-on-one) combined-cycle configuration. This

configuration includes two CTG units, two heat recovery steam generators

(HRSG), and one STG. The final size and detailed configuration of the

generating equipment will not be known until after the procurement process.

Therefore, all of the following performance, output, fuel consumption and

efficiency information are based on heat balances for a range of commercially

available combined cycle plant equipment. At the conclusion of the procurement

process, IPL will update the Board regarding the selected equipment

performance.

As an integrated system, the MGS will be capable of generating 580 to 630

megawatts (MW) during summer peak conditions of 90 degrees Fahrenheit (F)

and 68 percent relative humidity (RH). The net heat rate attainable under the

same ambient conditions will be 6,100 to 6,200 BtuU/kWh kilowatt hours (kWh)

(Lower Heating Value, or LHV) at full load conditions. This configuration

represents a net thermal efficiency of greater than 55 percent. All of the

aforementioned output and performance values are preliminary in nature and are

subject to change as a result of the engineering/procurement/construction (EPC)

contractor’s design approved by IPL. These values represent a range of output

and performance that can be obtained with current CTG technology from various

manufacturers, some of which require HRSG duct firing to obtain the required

output.

The MGS will be fired primarily on natural gas. IPL is soliciting bids for the

project with a dual-fuel option. However, at this time, IPL does not see the need

for a backup fuel source. IPL is still soliciting bids for the project with the dualfuel

option because it can foresee two changes in circumstance that may require

an alternative fuel source in the future.

The high-temperature exhaust of each CTG will be directed to a three pressure, drum type, single reheating, natural circulation, horizontal HRSG. Each CTG will have its own HRSG. Consideration for duct firing will be evaluated during the procurement and detailed design phase of the project as a means to achieve the required power production in the STG through the raising of HRSG steam pressure and temperature.

The MGS steam cycle will be very similar to a conventional fossil fuel-fired

steam power station. Steam powers the STG and exhausts under vacuum to a

condenser. Condenser heat rejection will occur using an evaporative cooling

tower. To optimize the number of times cooling water can be cycled through this

system and reduce overall MGS water usage, city water will be utilized to supply

makeup water to the cooling tower. Water supplied by the Marshalltown Water

Works (MWW) originates from nearby groundwater wells and follows a series of

filtration and clarifying treatment processes to remove certain minerals present in

the raw water source. Using treated water from the MWW results in higher

quality replacement water (makeup) for the circulating water system that will

enable a higher cycle of concentration to be achieved in the cooling tower. The

evaporative cooling system and tower will also supply cooling water to other

MGS equipment such as oil coolers and generator coolers. The condensed

steam exiting the STG as condensate will then be pumped/recycled back to the HRSGs as feed water for the continuous generation of steam. The steam condensate cycle from HRSGs to STG to condenser and back to HRSG will essentially be a closed-loop process.

Water purity will be critical to the reliable operation of the MGS steam

cycle by reducing scaling and corrosion. Scaling and corrosion are undesirable,

and would result in a loss of plant efficiency, reduced reliability and availability,

premature equipment failure and increased cost of operation. The MGS design

will include further water purification and treatment systems to generate

demineralized water for steam makeup requirements and to purify recycled

condensate. The demineralizer system will be essential to ensure steam cycle

fluids do not cause unacceptable scaling or corrosion of the MGS equipment.

The CTG units will be equipped with evaporative inlet air coolers to reduce

the inlet air temperature during hot ambient conditions, resulting in increased

electric generation output.

The MGS will be designed for combined-cycle operation only and the

facility will include these other key components, as described in Table 1.3.1-1

below.

Table 1.3.1-1 – Key Components and Their Functions

Component Function

Auxiliary Boiler For lower pressure steam generation and supply to the

MGS used during off-line status and to enable a faster start-up.

Cooling System Equipment and Plant Auxiliary Equipment

Designed to operate on city water and possibly well water to allow for maximum flexibility in water source supply.

Water PurificationEquipment

Including chemical feed systems for production of high purity demineralized water for use in the steam cycle and for control of scaling and corrosion in the steam cycle, cooling tower system and other closed-loop cooling systems.

Storage and Feed System for Selective Catalytic Reduction (SCR) Operation.

Control Systems For system operations and operator control room.

Pumps and Fans For fluid handling and cooling.

The MGS cooling systems will require a source of makeup water to

replace water lost by evaporation, drift and blowdown. The cooling system will

be designed to utilize makeup water supplied by the MWW. IPL is also exploring

the potential for designing to accommodate an on-site treated well, but has no

definitive plans at this time to incorporate this option. Wastewater discharge from

the MGS, such as cooling tower blowdown and sanitary wastewater, will be

routed back to the Marshalltown Water Pollution Control Plant (MWPCP) for

treatment. The location of the MWPCP lines and expected routing of related

linear facilities will be developed during detailed design of the MGS, and will be

provided to the Board as soon as it is available.

The CTGs will use dry low emission (DLE) burners to achieve a low level

of Nitrogen Oxide (NOx) and Carbon Monoxide (CO) emissions. NOx emissions

will be further reduced by a SCR system in its associated HRSG. The SCR

catalyst will enhance the reaction of exhaust gas NOx with ammonia feed stream

to yield nitrogen and water vapor. A second oxidation catalyst will also be

included to further reduce CO emissions through the oxidation of CO into CO2

(Carbon Dioxide). The oxidation catalyst will also lower Volatile Organic

Compound (VOC) emissions. After treatment and heat recovery in each HRSG,

the CTG exhaust gases will discharge to the atmosphere. This facility will not be

capable of simple cycle operation and, therefore, its exhaust gases will always be

treated by the SCR and CO catalyst.