An on-site power plant in the US was running at close to its steam limits. A new combined heat and power plant aims to increase steam production, reliability and flexibility, reports Drew Robb
|The Eight Flags Energy site Credit: Rayonier|
There are several ways to implement combined heat and power (CHP). For smaller sites, gas engines can work well. But the preferred option for larger facilities is often a gas turbine, which can achieve high levels of efficiency when used in combined cycle mode in conjunction 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. The steam can be also used as part of industrial processes, as well as for heating and cooling.
A good example of this concept is a new CHP facility that is being built on Amelia Island in northeastern Florida to service Rayonier Advanced Materials, a supplier of high purity, cellulose specialty products. These cellulose specialties are custom-made to meet the specifications of automotive manufacturers. High-strength viscose products manufactured at Rayonier’s Fernandina Beach facility are also utilized to add strength to tires, as well as by food manufacturers to provide rigidity and strength to hot dog and sausage casings.
While the Fernandina plant produces 160,000 metric tons of softwood cellulose specialties per year, it was constructed in 1937 and its facilities have been undergoing upgrades. Further, the growth of the company necessitated that more steam be added in order to expand its various production processes. Accordingly, a new CHP plant known as Eight Flags Energy (a subsidiary of Chesapeake Utilities Corporation) is in the closing stages of construction.
In addition to the leasing the land to Eight Flags for the project, Rayonier Advanced Materials entered into a 20-year agreement to purchase low-cost steam from the facility for use in its Fernandina plant.
A 20 MW Titan 250 gas turbine from Solar Turbines and a Rentech HRSG will serve as the core elements of Eight Flags Energy. The plant is due to come online in the third quarter of this year. C A McDonald, General Manager of Eight Flags Energy Center, said that the pilings for the foundation had been driven, the turbine has arrived on site and commissioning is about to begin.
Rayonier Advanced Materials owns five fibre facilities in Georgia. Its wood procurement team purchases logs from a variety of logging operations and timber companies. These logs are debarked and chipped into uniform dimensions. The chipped wood is delivered to Amelia Island. Two biomass boilers at the Fernandina plant site burn bark to provide energy for its operations.
‘We bring in logs and burn bark for 40% of our overall energy requirements,’ said McDonald. ‘We also burn wood chips and other material for the remaining 60% of our energy, as well as our steam needs.’
The new turbine and HRSG will augment existing on-site facilities in order to provide operational reliablility and flexibility. McDonald said the current plant has been running at close to its steam limits. Due to expansion, Rayonier’s need for steam and energy has been rising. This new building will satisfy those demands while also offering 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 important in terms of overall plant output. In the past, boiler maintenance meant a reduction in steam output and hence a drop in production at Rayonier. When the new plant opens, the shortfall of steam during boiler maintenance will be made up using steam from Eight Flags.
The HRSG normally recovers 70,000 pounds of steam per hour, but it has the capability to increase that amount when needed.
‘Rentech duct burners between the gas turbine and the HRSG can increase production to 125,000 pounds per hour of process steam,’ says McDonald. ‘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 provided by Rayonier will be channeled through a hot water economizer in the HRSG to increase the water temperature by approximately 70°F (21°C). This hot water will be returned to Rayonier for use in production processes.
‘Heating the feedwater for our existing boilers will help us save even more on energy,’ notes McDonald.
On-site power economics
Rayonier does all of its power generation on-site. As it is connected to the grid, it sells excess power to the local utility, Florida Public Utility (FPU – also part of Chesapeake Utilities). However, if the plant experiences any downtime, it has the ability to draw power from the grid until its systems are back online.
When Eight Flags Energy comes online, it will also supply electricity to FPU, as well as process steam and hot water to Rayonier. As a result, approximately 50% of Amelia Island’s electricity requirements will be generated on the island. This will help to reduce line losses and risk due to the island only having a single transmission line.
Initial engineering and feasibility analysis included both natural gas-fired turbines and reciprocating engines from a variety of manufacturers. A wide range of information was assembled such as hourly thermal load projections from Rayonier, hourly electrical load from FPU, as well as local temperatures and local ambient conditions in order to isolate the right equipment to fit existing needs.
This analysis indicated that the Solar Titan 250 equipped with a Rentech HRSG would be the most efficient and cost-effective design.
‘We used intensive computer modeling to determine how much extra heat the various types of equipment could generate, and how much would it be worth to us,’ McDonald explains. ‘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.’
He stressed that the economic case for the new plant had to be clear for both parties: the overall price to Rayonier turned out to be less than it would be to receive electricity from the local grid. Further, the arrangement offers FPU electricity at a rate it considers viable. But none of this would have been feasible without CHP using an HRSG.
‘Operating a gas turbine in simple cycle mode would have given us lower efficiency so there would have been no project without the Rentech HRSG,’ says McDonald.
As a result of this economic analysis, Eight Flags Energy has already been certified as a Qualifying Facility (QF) by the Federal Energy Regulatory Committee (FERC) and is expected to have greater than 80% efficiency. As part of the Public Utility Regulatory Policies Act of 1978 (PURPA), QFs are awarded special rates and regulatory treatment provided, in the case of CHP, to a plant that produces electricity and another form of useful thermal energy in a way that is more efficient than the separate production of both forms of energy.
FPU is also happy with its end of the deal.
‘The two main drivers of this project were the high wholesale power cost being paid by FPU and the desire by Chesapeake to invest in an economically viable energy project,’ said Mark Cutshaw, Director, Business Development and Generation at FPU. ‘The CHP facility will produce electricity at a much lower cost than the current wholesale power cost. In addition, the industrial customer had its thermal needs met at much lower cost while gaining an additional source of steam.’
He added that Chesapeake Utilities was only willing to invest in a facility where it could see a clear-cut return on its investment over the long term, while also benefitting FPU customers and the local community. The first question to answer was whether to go with gas turbines or reciprocating engines.
‘A gas turbine has lower efficiency but higher electrical and thermal output than a reciprocating engine,’ said Cutshaw. ‘You have to analyze the electrical and thermal load and match it with the appropriate CHP technology.’
‘Recips work well for smaller sites,’ he adds. But beyond a particular size, the economics of gas turbines become more attractive. CHP via a combined-cycle gas turbine coupled to an HRSG was the solution that ticked all the boxes for Eight Flags.
‘Most every CHP application is different and there is no one right solution for all cases,’ said Cutshaw. ‘When the system requirements are determined, it is important to look at as many different technologies and options as possible in order to design the most efficient and cost-effective facility.’ For the Eight Flags site, he said that gas turbines achieve their highest levels of efficiency when combined with an HRSG.
The company first evaluated the various gas turbine models available in the 25 MW or below range. A study of the options on the market led to the conclusion that a natural gas-fired Solar Titan 250 would be the best fit for the requirements of Amelia Island. Similarly, a review of the various boiler and HRSG vendors came up with the face that the Rentech solution would be the best match for the site.
When The Solar Titan 250 is placed into operation later this year, it will run continuously (except for maintenance outages) and supply FPU with approximately 20 MW of electricity which will be used to provide electricity to 16,000 customers on Amelia Island.
‘In our situation, natural gas was the obvious choice and Solar was the most responsive vendor with equipment alternatives to match our requirements,’ said Cutshaw. ‘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.’
Solar Turbines, in fact, is finding heavy demand for CHP these days. Arnaud Van Der Haegen, Northeastern Sales Manager at the company, said that the bulk of his company’s business now comes from CHP. He added that Solar has more than 46,000 units operating in CHP facilities.
Working with partners
Cutshaw’s advice to others seeking to implement CHP is to partner with a well-respected engineering firm with experience in many types of CHP projects using different types of equipment. Work with them to determine what you are trying to accomplish in your situation. Once system requirements are figured out, look at as many technologies as possible. He adds that it is vital to ensure that all constituents benefit from the project.
‘Chesapeake Utilities Corporation, FPU, Rayonier Performance Fibers, electric customers on Amelia Island and the local community will all benefit from the CHP facility,’ says Cutshaw. ‘Allow the system requirements to drive the design and utilize the best equipment for your situation.’
Drew Robb is a US-based freelance journalist specializing in engineering and technology