Wood chip power
Public Service of New Hampshire has made the
move to renewable energy—and significantly reduced emissions—with
the $75 million conversion of a coal-fired power plant to a biomass
power plant that is fired with wood chips.
By Tony Kryzanowski
The potential nose-dive in nitrogen oxides, mercury and sulfur
dioxide emissions is not the only reason why Public Service of New
Hampshire (PSNH)—the last remaining regulated power utility in New
England to still own generation facilities—has invested $75 million
to convert a coal-fired power plant to wood chips. But it was a good
one.
The Northern Wood Power project has cut nitrogen oxide (NOx)
emissions by 70 percent, mercury emissions by 90 percent, and sulfur
dioxide (SO2) emissions by 95 percent, through the conversion of one
of PSNH's three power generation units at its Schiller Station
facility from burning coal to burning wood chips. The Schiller
Station at the coastal city of Portsmouth had operated three
50-megawatt coal-fired steam boilers since the 1950s.
The Environmental Protection Agency has set a NOx emission limit of
.15 pounds per million BTUs of energy produced. The wood-fired
boiler at Schiller Station is operating at .075 pounds per million
BTUs. "We've cut the limit number in half," says Dick Despins,
station manager.
It would appear that new legislation brought in by several New
England states is having a positive impact by encouraging new power
plant construction and plant conversions in the region based on
using renewable fuels.
Despins says legislation in neighboring Massachusetts requiring
local utilities to provide a portion of their power from renewable
energy sources was definitely a primary driver behind the decision
by PSNH to convert one of the three power generators. The
legislation helped to create a market for what are called Renewable
Energy Certificates (RECs). Rather than investing in new "green
power" facilities themselves, conventional power suppliers have the
option of purchasing RECs from green power producers that qualify
under the rules established by state legislation. This has
particularly benefited states like New Hampshire and Maine, which
have a surplus of wood biomass because of vibrant local forest
industries, but lack the local energy demand to justify investment
in wood-fired power production.
Selling RECs will help to pay for the plant conversion at
Portsmouth, and PSNH estimates that between 300,000 and 400,000 RECs
will be generated annually by the converted power plant.
"According to the guidelines in Massachusetts, if you install a
fluidized-bed boiler and burn wood, you may qualify (as a REC
marketer)," says Despins. "That was the basis for us to go forward.
It sparked what we have today."
The company's investment in a wood chip-fired boiler will also help
it meet new renewable energy portfolio standards legislated this
year by New Hampshire. Beginning in 2008, a small percentage of the
power producer's energy production must come from renewable energy
sources.
The $75 million invested by PSNH at Schiller Station was used to
construct a new, 110-foot high, fluidized-bed boiler; emissions
control systems; a covered wood chip storage facility capable of
holding 10,000 tons of wood chips; a wood unloading and handling
system; and a covered conveyor to deliver the chips to the boiler.
The power utility concluded that retrofitting an existing facility
would be more cost effective than building and staffing a new
facility. The Schiller Station is also located near important major
transportation routes.
The decision by PSNH to construct the Northern Wood Power project
was also good news for the local economy considering that the new
boiler is expected to consume more than 400,000 tons of wood
annually. It created a new market for residual wood, which had been
on the downswing.
In addition to building a new fluidized-bed boiler and emissions
control system, PSNH's capital investment at its Schiller Station
went towards a covered wood chip storage facility capable of holding
10,000 tons of wood chips (above) and a covered conveyor to deliver
the chips to the boiler.
In many cases, trees and wood debris removed during development and
thought to be unsuitable for producing forest products ended up in
landfills or were disposed of by land spreading. A significant
amount of debris is also generated from conventional logging
operations. It comes as no surprise, therefore, that the New
Hampshire Timberland Owners Association was an early supporter of
the project.
"It's not a market where the loggers will earn their living," says
Despins. "It just adds value to the whole process."
Forest companies could produce marketable products from sawlogs and
smaller pulp logs. However, there was a significant amount of
branches, treetops and general residual wood that wasn't finding a
market. Also, not all tree species are suitable for producing wood
products. The demand created by the power plant conversion has
helped to create a market for that wood fibre.
"Being a New Hampshire company, we started to look at what role we
could play to help that declining sector of the timber industry,"
says Despins. "It's a fairly large component of the state economy."
Other groups supporting the project included the New Hampshire
Audubon Society and the Society for the Protection of New Hampshire
Forests. This northern New England state is the second most forested
state in the U.S., next to Maine, with 84 percent of its landmass
covered with trees.
What makes the emissions reduction possible at the Northern Woods
Power plant is the fluidized-bed boiler technology. PSNH explains
that unlike typical "stoker grate" boilers where wood chips are
simply burned as they pass along a rolling grate, the fluidized-bed
boiler circulates the wood chips and burns them while they are
suspended in air within the combustion chamber. This process burns
fuel more completely, dramatically limits the production of nitrogen
oxides and other emissions, and captures any unburned carbon
compounds for further combustion.
Public Service of New Hampshire's investment in a wood chip-fired
boiler at its Schiller Station facility (right) will help the
company meet new renewable energy portfolio standards legislated
by New Hampshire.
The new boiler system more efficiently converts water to
superheated, high-pressure steam, which in turns spins the turbines
to generate electricity.
While the environmental and economic dividends to the local economy
are obvious, with an estimate that the project will contribute $20
million to the local economy, Despins says using wood chips as the
fuel source is definitely more challenging. There are more variables
to take into consideration with wood, as compared to coal. For
example, because the power plant has 65 wood chip purchase
agreements, many of which fit within the "Mom and Pop" supplier
category, the attributes of one load versus another can be quite
variable. The moisture content can vary from one source to another
by as much as 10 percent, whereas moisture content in coal doesn't
change much beyond two percent. Hardwood chips also produce more
BTUs than softwood chips. So depending on the wood chip mix, the BTU
output from the feedstock mixture can also vary.
The wood chip size is also important. The boiler can accept a chip
up to 2.5 inches in size and screens generally ensure that only
chips up to this size are burned. However, occasionally a larger
size piece of wood manages to fit into a screen opening, which can
disrupt fuel flow.
Fuel cleanliness is important. Suppliers are expected to ensure that
any pieces of debris, particularly metal, are removed from the chip
load. The power plant also uses magnets to catch any metal pieces
before they enter the production flow.
Plant personnel received special training in the operation of the
Northern Wood Power plant to get familiar with the fluidized-bed
technology and with the state-of-the-art distributing control
system.
Despins is pleased with how well the wood burning technology has
worked and how well operators have progressed along the learning
curve since the plant came on line in December 2006. The 50
megawatts produced by the plant will continue to provide power to
about 50,000 New Hampshire homes.
The combustion system is capable of burning bark, but at this early
stage of operation, PSNH has opted to stick with clean wood chips.
"We're burning mostly whole tree chips just to try to maintain a
little bit of consistency with the fuel for a period of time until
we get comfortable with the variables of using wood," says Despins.
About one percent of the clean wood products the plant has consumed
have been chips produced from used pallets.
Before the power producer committed to converting one of its
coal-fired boilers, it had to ensure that it could access an
affordable and long-term supply of wood fibre. The North American
forest industry itself has been experiencing significant competitive
pressures from around the world, resulting in a number of facility
closures. However, PSNH evaluated how well the industry has adapted
in the past and felt confident that the wood chip supply would
remain adequate into the foreseeable future.
"Pulp and paper is not as prominent as it once was but the chip
industry has shown an ability to weather various changes in the
market with wood values," says Despins. Based on the company's
evaluation, it discovered that the state grows more trees than it
harvests in a year.
PSNH has signed one-year contracts with its suppliers, which provide
it with short-term security of supply. Suppliers are also content to
not be locked into a price over the long term.
While it is unlikely that more conversions will occur at Schiller
Station, because of its location on the seacoast and the limited
geographical area from where it can draw wood chips, Despins says
the company is on record stating that it is very interested in
constructing one and possibly two more wood-fired power generating
plants. However, current state law prevents PSNH from expanding its
generation portfolio, so legislators would have to change the law
for that to occur.
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