November 15, 2010 | General

District Energy From Woody Biomass

BioCycle November 2010, Vol. 51, No. 11, p. 42
Utilizing local, sustainably sourced woody biomass, instead of fossil fuels to run heating systems, reduces greenhouse gas emissions and keeps jobs as well as dollars in the local community.
Diane Greer

SMALL district energy systems combine heat loads across multiple buildings, allowing projects to use larger, more efficient and cleaner burning wood-fired systems. Utilizing local, sustainably sourced woody biomass – instead of fossil fuels to run these systems – reduces greenhouse gas emissions and keeps jobs as well as dollars in the local community. Operating costs can be shared and management and maintenance consolidated to save money.
Pricing for woody biomass feedstock is also more predictable, determined by local markets and not dependent on volatile world energy markets. Project economics, especially when replacing fuel oil with woody biomass, are generally favorable with reasonable payback periods.
In Boulder County, Colorado, wood from forest thinning operations has been heating a five-building county complex. The wood is supplied from the 30,000-acres of forest managed by Boulder County Parks and Open Space (BCPOS). “Our forests are out of whack because they have been fire-suppressed for so many years,” explains Therese Glowacki, BCPOS’s resource management manager. “We have been thinning them to improve forest health and for wildfire mitigation.”
Finding an outlet for the small diameter trees removed during thinning operations has been a challenge for BCPOS. Some wood has value for fencing and firewood, but the vast majority was either burned in slash piles or chipped and put back on the ground.
When Glowacki learned about new technologies using wood chips for heat at a conference in 2002, she immediately saw an opportunity. Boulder County was planning to construct an open space and a transportation complex. “We could use the excess material that we are taking off the forest to generate energy for the new buildings,” she explains.
To determine if the technology made sense for Boulder County, facility staff visited sites in New England using wood-fired heating systems. They found it to be proven technology that was simple to operate. A feasibility study concluded that locally available wood supplies were adequate to fuel the system. Initial costs would be slightly higher than a natural gas system, with payback estimated between 7 to 20 years depending on natural gas prices.
Boulder County decided to move forward with the $1.5 million project, selecting a Messersmith boiler to heat the 5-building, 95,000-sq.ft. complex. It was funded by the county and installed in 2005. “We tried to keep it simple, a no frills hot water heating system for five buildings that makes great economic and environmental sense,” Glowacki says.
The boiler sits atop a large fire-box. An underground system of pipes distributes hot water to buildings in the complex and returns the cooled water to the boiler. The heating plant consumes 600 to 700 tons of wood each heating season. Between 80 to 200 acres are thinned annually, producing 20 to 25 tons/ acre of woody biomass. The wood is primarily small diameter pines and trees killed by the mountain pine beetle infestation. “We have burned a lot of beetle kill wood and there is going to be so much more of it,” Glowacki adds.
Initially, the project ran into problems with wood chip quality. Chips stockpiled on the ground became contaminated with sand, dirt and rocks, causing problems with the augers and ripping the conveyors moving chips to the boiler. Now the chips never hit the ground. Thinned trees are cut, hauled to the chipper and chipped directly into a roll-off truck. They are delivered to outside storage bays or the system’s fuel bin. A traveling auger moves the chips onto a conveyor belt, which transports and drops them into a hopper. A metering auger feeds the chips into the firebox.
Emission controls were not required on the system due to control technologies that result in more complete combustion, thereby lowering emissions. Since the system was installed, the US EPA has issued more stringent regulations for controlling very small particulates. The existing system likely will be grandfathered under the old regulations. A second system planned for the Boulder County jail will require additional emission controls in the form of an electrostatic precipitator (ESP).
“We are thrilled that we are able to use a sustainable resource coming off our land in a productive way,” Glowacki says. “The fact that we are expanding shows we have enough support and the project is economically viable.”
The U.S National Renewable Laboratory (NREL) recently installed a wood-fired system to heat eight buildings on the its South Table Mountain Campus in Golden, Colorado. The project is part of the laboratory’s strategy to move toward net-zero carbon emissions from its facilities. “NREL wants to use renewable energy on a meaningful basis in large-scale projects, not just demonstration projects,” explains Chris Gaul, NREL energy engineer.
The 9-MBtu/hr system burns 3,500 tons/year of wood at full fire to heat the 500,000-sq.-ft. campus. The wood comes from local forest thinning operations removing overcrowded and beetle-killed stands.
Instead of purchasing and installing a system, NREL hired Ameresco, an Energy Service Performance Contractor (ESCO), to bear the project’s $3.2 million costs, provide the fuel and guarantee its operation. NREL pays for the hot water produced by the system at rates set for the next 23 years. Ameresco also is responsible for major maintenance tasks.
NREL specified efficiency requirements and a high degree of automation for the system. “A lot of moderate size systems require a person to come in with a rake and shovel and clean out the boiler at the end of the shift,” Gaul says. “Our labor is expensive so we were not going to do that.”
Based on NREL’s requirements, Ameresco selected a system from Advanced Recycling Equipment (ARE) in St. Mary’s, Pennsylvania, for its high degree of automation, which includes automatic ash removal and an interface with the laboratory’s building automation system. ARE provided a complete system package, from the fuel handling equipment to the technology required at the top of the smoke stack. “All the ESCO had to do was put a building around it and integrate it with our district heating system, which looked like a good idea,” Gaul says.
An extended combustion chamber causes the gases to travel an extra 15 to 20 feet before reaching the heat exchange tubes in the fire tube boiler. The extra distance gives the volatile organic gases (VOC) more time to fully combust, Gaul explains.
Wood is supplied by two companies – one that supplies chips for animal bedding and landscape mulch, and another that produces wood pellets. “An intermediary step in producing pellets generates wood chips,” Gaul explains. “It is a nice, clean consistent fuel.” He adds that fuel cleanliness makes or breaks a project and recommends a simple test that starts by putting a couple gallons of dried chips in a five-gallon bucket and filling it with water. Chips are sloshed around in the bucket and then removed by hand. What remains at the bottom of the bucket are contaminates, such as rocks, sand and staples.
Everything has not gone according to plan with the two-year old system. “The augers and conveyance system were too light duty for the application,” he says. “They break and wear out.”
Slag and clinkers – masses of incombustible matter fused together – have caused problems inside the boiler. “The operators are still trying to figure out the perfect mix of fuel, fuel moisture content and air and firing rate to avoid forming clinkers,” Gaul explains.
Ameresco is providing the extra maintenance and labor to keep the machinery running. One of the benefits of using an ESCO on the project is that they assume the risk if things don’t go according to plan, he adds.

An interesting project in Superior, Montana, is teaming the Mineral Community Hospital (MCH) with the local school district to build a wood-fired district heating system serving the hospital complex and the adjacent elementary and high schools. Each entity initially considered separate wood-fired systems. The school district looked into a woody biomass heating system when the high school’s 50-year-old fuel-oil boiler went down but it was cost-prohibitive, says Wayne Stanley, Superior School’s superintendent. “The payback would have been 35 to 40 years.”
Meanwhile, the hospital’s newly hired director of business development, George Bailey, was considering a biomass heating plant to replace MCH’s aging and inefficient fuel oil and propane heating systems. “Heating with fuel oil is extremely expensive,” he says. “I knew we had to do something.” Discussions between Bailey, Stanley and the MCH’s CEO Steve Carty about biomass heating plants got everyone thinking about building one combined system. “It really made sense to work together on a larger boiler system that we could share and save everyone money in the long run,” Stanley explains.
Based on the experience of other facilities in Montana using wood-fired heating systems, the total time required to operate and maintain the system is expected to be less than the time currently spent on the older fuel oil and propane systems, he adds. The project recently was awarded a $175,000 grant from Montana’s Department of Natural Resources and Conservation’s (MDNRC) Fuel for Schools and Beyond program. Carty believes there is a good possibility of obtaining additional grants through two other programs for biomass heating systems.
MCH hired a consulting firm to evaluate project economics and technologies for various centralized system alternatives. The assessment found a positive accumulated cash flow (ACF) in 11 years with a $700,000 subsidy for a centralized plant providing heat to the hospital complex, comprised of the existing hospital, a medical clinic, assisted living facility and a future hospital and wellness center, and the elementary and high schools. (ACF is a measure of payback taking into account financing and fuel escalation costs.) The ACF over 30 years shows a savings between $4 million to $4.5 million.
Wood chips to fuel the system, estimated at 1,150 tons/year, will be sourced from local forest land and include small diameter trees and trees killed by the beetle infestation. A company that already supplies wood for numerous Fuels for School’s projects around the state is located just 3 miles from the proposed plant.
Expanding the local wood products industry will have a positive effect on the community, creating jobs and keeping money previously spent on fossil fuel sources in the community. “The town once had a booming timber industry, which has been on the decline for several years,” explains Julie Kies, Montana Department of Natural Resources and Conservation program manager. “There is still one fairly strong mill, so they are really excited to make these connections with the local forest economy.”
The next step is the design and engineering of the systems. Three technologies are under consideration – steam turbines, reciprocating steam engines and an organic rankine cycle (closed circuit steam system). Analysis will also determine if using combined heat and power (CHP) makes sense for the project. Construction of the new system is expected to begin in the summer or fall of 2011.

Diane Greer is a Contributing Editor to BioCycle.

p. 44 Sidebar

THE City of Montpelier, Vermont, has committed to reduce greenhouse gas emissions and fossil fuel consumption by at least 80 percent by 2030. To help reach its goal, the city is hoping to replace the antiquated central heating plant serving 16 buildings in the State Capital complex with a 41 MMBtu combined heat and power (CHP) district energy system fueled with wood chips. The existing facility, operated by the State of Vermont since 1946, produces up to 26.1 MMbtus of heat using a wood boiler and two oil-fired boilers. Two of the boilers are over 60 years old.
In January, Montpelier received an $8 million grant funded under the American Recovery and Reinvestment Act to build the new facility with sufficient capacity to heat 180 buildings encompassing 1.8-million-sq.ft. In addition to the Capital Complex, the system will provide heat to city-owned facilities and 156 privately owned buildings in the downtown district. City officials believe the expanded system will spur economic development in Montpelier’s downtown and provide additional economic and security benefits by increasing reliance on locally harvested wood instead of fossil fuels.
A request for proposals, issued in August, provides general specifications for a CHP system satisfying an estimated heat load of 41 MMBtus. Electricity production, not specified, is intended to increase system efficiency and enhance project economics, explains Gwendolyn Hallsmith, Montpelier’s director of planning and community development. Construction of the underground hot water distribution system and wood chip storage facilities is included in the RFP.
Wood to fuel the new facility will come from the 750,000-acres of managed timberland within a 25-mile radius of Montpelier.
Feasibility studies found ample supplies of low-grade wood suitable for wood chip production in the immediate area. Obtaining sustainably harvested wood is an essential element of the project. “Only a small fraction of forests in Vermont are third party certified,” Hallsmith explains. “We are trying to establish standards for harvesting so that we can ensure that wood used for the plant is harvested in a sustainable way.”
Getting the project built will require coordination and cooperation between the city, which has wanted to develop a district heating plant for many years, the state, which owns the current facility, the State Legislature, which must ultimately approve the project and the voters, who will need to approve a bond to fund the system. The current drop in fuel oil prices is complicating project economics. “If fuel oil was $4/gallon like it was two years ago, this would be a slam dunk,” Hallsmith says. The $8 million federal grant will probably cover about one-third of the costs. Bids on the project were due in late October.

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