CHP gains momentum in the industrial sector
At a time when utilities and independent power producers are struggling to bring new combined cycle power plants online, combined heat and power (CHP) represents an area of relative success
Bruce Hedman, managing director and CHP systems accelerator coordinator at Entropy Research, said, “There are now more than 80 GW of installed combined heat and power (CHP) in the U.S. at more than 4,600 industrial and commercial facilities. That adds up to more than 7% of the nation’s generation capacity and 12% of its total generation.” Most of these installations operate on natural gas.
In some cases, the generation capacity is there largely for backup power. But in many more cases, it powers all or part of facility operations, as well as providing process steam and other services. In more than a few cases, the CHP system provides an extra revenue stream by feeding excess power into the local grid.
“Resilience is a key driver for CHP,” said Hedman. “There are many examples of CHP keeping facilities running through blackouts, hurricanes and other disasters.”
Consider the case of Tate & Lyle, a global provider of food and beverage ingredients. Its primary products business produces sweeteners, industrial starches used in paper and packaging and products used for animal nutrition. The company has around 4,400 employees in more than 60 locations across 30 countries.
Tate & Lyle has been a long-time user of CHP at many of its industrial locations. Two of its corn wet mills in Loudon, Tenn. and Lafayette South, Ind., have been awarded the U.S. Environmental Protection Agency’s (EPA’s) Energy Star certification, having met strict energy efficiency performance levels set by the EPA. It is the only corn refining plants in the U.S. to earn this accolade. Energy Star was introduced by EPA in 1992 as a voluntary, market-based partnership to reduce greenhouse gas (GHG) emissions through energy efficiency.
“This reflects the dedication and hard work of our operations team, especially our technicians who are committed to running these mills as efficiently as possible,” said Nick Waibel, global energy lead at Tate & Lyle.
Demanding corn milling processes
Tate and Lyle processes around 2% of the U.S. annual corn crop in several plants. The company takes shelled corn (maize), separates the kernels into their core components (starch, oil, protein and fiber) and processes them to create a range of products such as specialty sweeteners and bulk ingredients, which are distributed globally.
Corn wet milling uses all the corn. First, the corn is shelled and cleaned, then soaked in water at 50˚C between 20 and 30 hours. Sulphur dioxide is added to prevent excessive bacterial growth. As the corn swells and softens, the mildly acidic steepwater starts to loosen the gluten bonds with the corn, and to release the starch. The corn goes on to be coarsely milled to separate out the germ from the rest of the components. In a form of slurry, the corn flows to separators to separate out the corn germ, which contains about 85% of the corn’s oil. This part is washed, dried and sold for further processing to recover the oil.
The remaining slurry undergoes fine grinding to release the starch and gluten from the fiber. The starch-gluten suspension is sent to the starch separators. The collected fiber is dried for use in pet and animal feed. The starch-gluten suspension passes through a centrifuge where the lower density gluten is spun out. The gluten is also dried and used in animal feed. The starch, which still has a small percentage of protein remaining, is washed to remove the last traces of protein and leave a 99.5% pure starch. The starch can either be dried and sold as corn starch, or it can be modified to turn it into other products such as corn sweeteners, corn syrups, dextrose and fructose.
When converting to syrup, the starch is liquefied in the presence of an enzyme to convert it into a low-dextrose solution. Another enzyme is added to continue the conversion process. At any time, the enzyme treatment can be halted to produce the right mixture of sugars (like dextrose or maltose) for syrups to meet different needs. The syrup is refined in filters, centrifuges and ion-exchange columns and excess water is evaporated. Dextrose is most used for fermentation, although other sugars can be used. Dextrose is sent to the fermentation facilities to be converted into ethanol by traditional yeast fermentation, or into other bioproducts through either yeast or bacterial fermentation.
Such complex processing requires equipment such as boilers, tanks and pumps, distillation columns, milling equipment, separation screens and evaporators. Every step of the way, steam from the CHP plant plays a vital role in these processes as well as in the heating of the facility.
Cutting CO2 emissions
The facilities in Lafayette South and Loudon export ingredients to many countries around the world. In 2017, the company installed a CHP system at its Loudon facility, which generates electricity and steam to power and heat the facility, resulting in a year over year reduction of around 35% in greenhouse gas emissions at the site (see Figure 1). The addition of that one CHP plant slashed companywide CO2 emissions by around 10%, as well as leading to improved energy and operational efficiency.
The Tate & Lyle corn milling facility in Loudon, Tenn. has two Siemens Energy SGT-700 gas turbines and two heat recovery steam generators (HRSG) with supplemental duct firing provided by Rentech Boiler Systems. This replaced aging coal boilers. The Rentech HRSG recovers energy from the exhaust stream of a turbine. As supplemental duct burners are included, Tate & Lyle can run them regularly to boost steam production when required.
In addition to the obvious environmental drivers, economics played a part. The steadily rising price of coal in tandem with stricter emissions requirements would necessitate the addition of expensive pollution control equipment on the coal boilers. Instead of investing in flue gas desulfurization (FGD) and selective catalytic reduction (SCR) technology to stay within boiler maximum achievable control technology (MACT) limits, the company decided to spend a little more to revamp and expand its steam and electricity production capabilities — and establish an environmentally responsible pathway for the future.
“The facility will reduce emissions by more than 2,000 tons per year, satisfy our steam needs and enable us to realize big saving by switching from grid power to generating its own electricity,” said the plant manager at that time. “As boiler efficiency will rise sharply, fuel costs will also be considerably lower.”
One CHP plant leads to another
The return on investment and environmental benefits demonstrated by switching from coal boilers to natural gas-based CHP at the Loudon site encouraged the company to complete a similar upgrade at the Lafayette South food ingredient processing plant in Indiana. It chose to replace its aging coal boiler with a similar configuration as Loudon: two Siemens Energy SGT-700s and two Rentech HRSGs.
“Natural gas fired cogeneration brings clearer energy online and helps us move away from the use of coal boilers,” said Waibel. “We have set a goal of eliminating coal from all our sites by 2025. Our Decatur, Ill. facility, is also in the midst of replacing coal boilers with natural gas.”
The Siemens Energy units are designed for high output/efficiency and lower emissions with a wide fuel range capability. Due to high exhaust heat, it is well suited to CHP. It includes a third-generation dual fuel dry low emissions (DLE) combustion system which keeps NOx emission levels low.
“With an eye to the future, our Siemens Energy gas turbines will be able to eventually operate on hydrogen,” said Waibel. “In these dynamic times, we had to ensure our investment will serve us in the near term and the long term. It’s good to know our turbines can make the transition to hydrogen when that supply comes online in the future.”
The results of this plant upgrade were positive: Improved grid resiliency, greater process efficiency, carbon emissions slashed by more than 30% and a GHG emissions drop of around 40%. By replacing aging coal boilers, generating electricity on-site and recovering heat, overall energy efficiency has been boosted while substantially reducing energy costs and cutting water usage by 5%. The plant manager also reports greater process efficiency and power resiliency.
“We take our responsibility to the environment seriously and are committed to doing more to care for the planet, including lowering our greenhouse gas emissions by using cleaner energy,” said Travis Montoya, Lafayette South plant manager for Tate & Lyle.
Far from being projects executed at specific plants, this represents a company-wide target by the company for 2030 to deliver: a 30% reduction in CO2 emissions, all its waste to be beneficially used, a 15% drop in water use and the elimination of coal from its operations. Getting rid of the coal boilers at Loudon and Lafayette is a major step in achieving the latter goal.
“The Lafayette South plant project not only helps meet our sustainability goals in reducing greenhouse gas emissions and water use, it’s also a project that saves us money as a company, so it was really a win-win for us,” Waibel said. “Eliminating coal and going into a clean-burning fuel is the right next step for our plant.”
Boiler to CHP upgrades
Projects such as this are becoming more commonplace. Factoring in the price of coal, the growing number of emissions restrictions and the rising price of grid-based electricity (as well as the rising incidence of power outages), CHP using natural gas-fueled turbines is being looked upon as a viable investment at production and manufacturing facilities.
“Producing power, heat and cooling close to where it is used gives greater control over the energy supply, gaining the dual benefits of security and independence,” said Anders Stuxberg, power plant process integration specialist at Siemens Energy.