Atypical Thermal Applications
Waste wood, municipal solid waste, landfill gas and other biomass sources are commonly used by utilities, industrial plants and public institutions such as schools and hospitals to generate power and heat, whether used onsite or plugged into the grid. The appeal of biomass as a fuel to meet thermal loads isn’t limited to traditional users, however. From brick kilns to blacksmith studios to paint plants, it is being realized by a unique mix of consumers. For the Metrolina greenhouse in Huntersville, N.C., and Honeywell’s Hopewell, Va. chemical plant, the pieces fell perfectly into place when considering biomass as a fossil fuel replacement.
For Metrolina, heating the 162-acre greenhouse is a challenge, but biomass has helped ease the burden. Now finishing up its third biomass heating season—and also the harshest winter since the system’s installation—Jeff Woolsey, Metrolina boiler and systems engineer, says that the greenhouse will have used 85 to 90 percent biomass, though during milder winters, that number has reached 95 percent. “With the artic blasts this year, it’s gotten to be a little more challenging,” Woolsey says. “The interesting thing about greenhouses is that the coldest part of the year is when we tend to be emptiest in terms of product, because we’re gearing up for spring. Toward the end of winter and beginning of spring we’re jammed full—right now (mid-March) we’re at 100 percent.”
The weather, and how full the greenhouse is with product, significantly influences the facility’s heat load, according to Woolsey. Temperature throughout the greenhouse varies widely, with the warmest areas—about 72 degrees Fahrenheit—being present where plugs or baby plants are located, and the coolest where the finished crops are located, with an overall average temperature of about 65 degrees.
So how are different temperatures in different regions of the greenhouse accomplished? In the main blocks of the greenhouse are four Argus climate control computers that contain customized climate models for each zone. “We have four weather stations, and the computer looks at those, outdoor temperature, light, wind and other things, and comes up with a hot water target for each zone. “We’re able to hit our targets right on the money,” Woolsey says.
Behind the heating magic are four 8-MW Vyncke boilers, each rated at 29.5 MMBtu per hour. The boilers replaced 12 natural gas boilers that are still in place and operable, but used only as supplemental or back-up heat. “We try to never use them, but sometimes we’re forced to,” Woolsey says.
What makes Metrolina’s current heating system so ideally suited to its needs is, from Woolsey’s perspective, its current storage set up, which includes 3 million gallons of hot water storage via five tanks of various sizes. “It allows us to store about 415 MW of heat,” Woolsey says. “It’s ideal for us because on a summer day, greenhouses typically use zero heat. It could be 20 degrees outside, but we’re not going to use any heat if there’s sun—a greenhouse is a giant solar collector. We use sunlight hours as recharging periods.”
Interestingly, the 32 MW of capacity the four boilers supply is not nearly enough heat for the greenhouse, if supplied in real time. “So having anywhere from eight to 12 hours of daylight, we can run those boilers at full output, and just store the heat away inside of pipes and tanks,” Woolsey explains. “At night, we’re able to draw on the full 32 MW of boilers, plus storage—it’s like a giant battery. If we didn’t have storage, we’d never make it through the night. We would have to have twice as many boilers.”
In order to accommodate the biomass system when it was installed three years ago, a new building was constructed on the side of the property to house the boilers, ash conveyors and multicyclone air filters. A nearby storage house can hold several hundred trailer loads of fuel, which is delivered every weekday, multiple times per day, and sometimes on weekends. “At max, we burn up to 12 tractor tailor loads per day,” Woolsey says, adding that fuel is sourced from four main vendors and is mostly the result of land clearing being done for other purposes, and recycled wood pallets.
Once delivered on site, there are two ways the fuel may get to the boilers, Woolsey explains. The first is a tractor trailer backing up to moving floors and dumping the fuel, which is delivered to the boilers once it is screened. Once the floors that feed fuel to the boilers are full, remaining fuel is put into storage. “When that’s needed we use a front-end loader, and then it’s lifted up to the conveyor and sent into the boilers.”
On challenges with using biomass for heat, Woolsey says there’s more maintenance involved. “I would say any solid fuel requires more maintenance because of the conveyors, screens and moving floors; it’s just a lot more sophisticated in terms of mechanical structures,” Woolsey says. “For our gas boilers, the factory comes over every couple years to tune them up, and they don’t require much maintenance. We knew going in we’d be spending more on electrical energy and maintenance man hours, but because of the abundance of recycled wood and wood fuels, we pay about half of the price for our biomass fuel per dekatherm than we paid for gas, even at historic lows. That far offsets any extra maintenance costs.”
A greenhouse heating situation is unique in many ways, Woolsey adds. “Our load is highly variable, and doesn’t apply to most users. But I can tell you that the management here is extremely satisfied and happy—the whole system has performed above and beyond their expectations.”
While Honeywell’s massive caprolactum plant in Hopewell, Va., requires a much more consistent heat load than Metrolina, it, too, is an unconventional user of biomass, in the form of landfill gas.
Honeywell’s landfill gas-to-heat system is nothing short of extraordinary. The landfill gas that reaches the Hopewell plant as its final destination travels through 22 miles of 18-inch pipeline to reach the facility, which is the largest producer of caprolactum, a precursor to nylon. The system has been in place for about 10 years and is currently receiving 3,000 to 4,000 standard cubic feet per minute (scfm) of landfill gas, but it has plenty of room to grow, according to plant manager Kevin Keller, as the Waverly, Va., landfill is still young relative to the lifespan of a typical landfill.
When the landfill was built, it was intended to become one of the largest landfills on the East Coast, and would emit a very large volume of gas over its 50-year lifespan. “In 2001, the company that was hired to manage the gas approached Honeywell at the Hopewell site, recognizing that the site was a relatively large consumer of natural gas and that there’d be room for that plant to accommodate the volumes of landfill gas produced,” Keller says. “A deal was struck in 2002, that’s when construction of the pipeline started.”
Pipeline ownership is transferred to Honeywell once it reaches the Hopewell plant’s property. Once the pipeline enters the plant’s fence line, it emerges above ground, where the gas is analyzed for oxygen content. “Oxygen is important, because we want to assure it doesn’t reach a level that is dangerous,” Keller explains. The gas continues down an above-ground pipeline right to the boiler, where it is combusted to generate steam and represents about 20 to 30 percent of the boiler’s capacity.
During construction of the lengthy pipeline, multiple municipalities had to be consulted with, as well as the Virginia Department of Transportation, as much of the pipeline runs through its right of way.
Additionally, private property was purchased to facilitate installing the pipeline in a section outside of the right of way, Keller says. Due to unusually rainy weather, it took longer than anticipated to get the pipeline completed—about two years—with the first delivery of landfill gas occurring in early 2004.
The pipeline requires little maintenance as it is made of 18-inch-thick, high-density polyethylene, The Virginia State Corporation Commission in Virginia regulates and monitors the pipeline very similar to how it does utility gas pipelines, Keller says. Compressors at the landfill require the most maintenance, in order to ensure reliable flow of landfill gas to the Hopewell plant.
The system onsite is designed to take up to 5,000 scfm, so it hasn’t reached capacity yet. But since the intent is to consume whatever quantity of landfill gas is available, if that number ever exceeds what the plant can consume, an expansion may be considered. That time, however, is many years away, Keller adds. “Initial flows, back in 2004, were around half of what they are today. It’s a linear growth curve, and it grows 200 scfm each year. It’s a function of how the landfill management company is expanding the landfill, so it’s their schedule that dictates the future availability.”
Author: Anna Simet
Managing Editor, Biomass Magazine