Using the Right Boiler to Make Power Plant Economics Work

Today’s Boiler
the official magazine of the American Boiler Manufacturers Association
Fall 2015

It’s tough to make the economics of a power plant work these days. Coal plants are largely out of the question due to regulatory restrictions that require the installation of very expensive equipment to reduce emissions. Even gas-fired plants can be a tough sell despite the low cost of natural gas in the U.S.

But the financial equation becomes viable once gas turbines are harnessed in combined cycle mode along with a heat recovery steam generator (HRSG). Waste heat from the exhaust can then be captured in the HRSG and used to make steam to drive a steam turbine that generates more electricity. This results in some of the highest levels of efficiency of any form of generation. Alternatively, the steam produced in the HRSG can be used as part of industrial processes, as well as for heating and cooling. When the steam is used in ways other than to produce more power, this is known as combined heat and power (CHP) or cogeneration.

FLORIDA: A SUNNY FORECAST FOR CHP

One such CHP facility is under construction on Amelia Island in northeast Florida to service Rayonier Performance Fibers. Rayonier, a supplier of cellulose specialty products, is partnering with Florida Power Utilities (FPU) to create Eight Flags Energy. It will twin up a 20 MW Titan 250 gas turbine by Solar Turbines and an HRSG by RENTECH Boiler Systems of Abilene, Texas, to provide the options of either operating solely as a combined cycle power plant or as a CHP plan to serve the steam and hot water needs of Rayonier.

Eight Flags Energy is expected to come online in the second half of 2016. This new facility will augment two biomass boilers on Rayonier’s current site on the island that burns bark, wood chips, and other material to satisfy existing steam and power requirements. However, with that plant running at close to its steam limits, expansion plans called for more power and more steam. Additionally, this new building offers the company the ability to take a boiler down for maintenance and switch the HRSG from supplying power to a steam turbine to providing steam to the factory. This is made possible due to the fact that the HRSG is built to recover 70,000 lbs of steam per hour. But it has the capability to increase that amount when needed by turning on duct burners.

“RENTECH duct burners between the gas turbine and the HRSG can increase production to 125,000 pounds per hour of process steam,” C.A. McDonald, general manager of Eight Flags Energy Center, said. “We can take advantage of these additional steam capabilities whenever we have a boiler down.”

In normal operation, boiler feed water will be converted into steam which will be returned to Rayonier for use in cellulose manufacturing. In addition, de-mineralized water will be channeled through a hot water economizer in the HRSG to increase the water temperature by approximately 70F degrees. This hot water will be returned to Rayonier for use in production processes. Rayonier accomplishes all of its power generation onsite. As it is connected to the grid, it sells excess power to the local utility Florida Public Utility (FPU). In the event of downtime, it can draw power from the grid. FPU, in turn, will use the power it produces at Eight Flags Energy to supply about half of Amelia Island’s electric requirements.

“The cost of the plant and the pricing arrangement for excess power we have made with FPU made it attractive to go with natural- gas fired CHP versus other incremental sources and fuels,” McDonald said. “Operating a gas turbine in simple cycle mode would have given us much lower efficiency so there would have been no project without the RENTECH HRSG.”

The Solar Titan 250 will run continuously (except for maintenance outages) and supply FPU with approximately 20 MWs of electricity for use locally on the island. Along with Rayonier, FPU conducted an exhaustive analysis of many different types of facility including a variety of gas turbines, reciprocating engines, boilers and HRSGs.

“The Solar Titan 250 turbine has a proven track record of reliable and efficient performance, and when used along with a RENTECH HRSG, fit well into the plan to operate the facility for many years to come,” Mark Cutshaw, director of business development and generation at FPU, said. “The CHP facility will produce electricity at a much lower cost than the current wholesale power cost.”