Jim Wechsler,
The Sleaford Renewable Energy Plant
(REP), a 38.5-MW straw-fired biomass combined heat and power facility,
exemplifies the virtues of locally sourced power and the mutually
beneficial relationship it creates between provider and customer. The plant, located in Kirkby-la-Thorpe, near
Sleaford, in predominantly agricultural Lincolnshire, England, is owned
by Glennmont Partners and managed by Eco2 Ltd. It generates enough power
for 65,000 homes and will provide free heat and hot water to five local
public buildings for 25 years under a district energy scheme.
Project History
In May 2012, Parsons Brinckerhoff was
appointed as the owner's engineer of the £165 million [US $249 million]
project with responsibility for technical oversight of the EPC
(engineer-procure-construct) contractor during the execution phase,
including design review, site inspections, supervision of construction
and commissioning activities, and quality assurance.
The Sleaford Renewable Energy Plant in
Lincolnshire, England, uses locally sourced straw to produce electricity
for 65,000 homes as well as heat and hot water for five public
buildings under a district energy scheme. Credit: Glennmont Partners.
The plant began exporting electricity to the UK
national power grid in January 2014 and entered commercial service in
September 2014.
How It Works
Sleaford REP is designed to use wheat and
barley straw, by-products of grain production, as its principal fuel.
The plant consumes 1,000 bales of straw per day at an average rate of
one bale per minute. It will burn approximately 240,000 tons of straw
each year.
The bulk of this straw is purchased during the
seasonal harvest and stored off-site until it is needed. The plant
maintains a three-day supply of straw in a warehouse on the premises. As fuel straw is consumed by the boiler, gantry
cranes transfer a new layer of bales - 12 at a time - onto the main
conveyor system, which transfers them to one of four identical feeder
lines. Each line consists of three subsystems - transport, dosing and
fuel feeding - that work together to ensure that the fuel straw supply
to the boiler is constant and consistent. Maintaining a continuous rate
of supply is essential to producing a reliable power output, so the
feeder lines are designed to adjust dynamically to the variations of
bales provided by the assorted suppliers.
The transport subsystem begins with a weighing
conveyor, which measures the length, weight and moisture content of each
bale. These values are used to calculate the calorific value of the
bale and the necessary feed rate. The bales then pass into a sealed compartment with
interlocked gates at each end. The entry and exit gates cannot be opened
at the same time. This chamber acts as a fire safety mechanism,
preventing possible fire spread between the enclosed and exposed
sections of the conveyor system. A bale arriving here is held until the
preceding bale passes through the exit gate. From that point, a supply
conveyor transfers the bale to the dosing subsystem.
The dosing subsystem controls the flow of fuel to
the feeding subsystem and, consequently, to the boiler. The first
segment, the adjustment conveyor, can detect gaps between bales and
compensate for them by accelerating a subsequent bale so that it
"catches up" to the prior bale. Next, the dosing conveyor varies its
speed according to current fuel demand and accounts for any adjustments
required based on the calculations taken at the weighing conveyor and
the desired plant electrical output.
The Sleaford REP air-cooled condenser system. Steam rises up the
branching steam ducts. As it cools and condenses, the water then drips
down a series of sloping narrow tubes and flows into the condensate tank
in the lower left corner of the image. Credit: Tadgh O'Connor / Parsons Brinckerhoff.
The bale twines are then cut, and the straw is
spread into a continuous stream. This loose straw then falls by gravity
down a chute into the feeding subsystem, which consists of a twin-screw
stoker feeder. The variable speed screws rotate individually. The
rotation of the screws pushes the straw forward through a duct that
leads to the furnace grate. The duct is lined with sensors to detect
back burning within the feeding system.
As the stoker screws push the straw into the duct,
the straw itself acts as a plug to create an air seal. Isolation dampers
before and after the stoker screws allow the system to be shut off as
needed. Burning straw in the boiler creates high-pressure
steam, which drives the rotor of a steam turbine at 5,500 rpm. The
output of the generator is 11 kV at 50 Hz. The voltage is increased to
132 KV at the step up transformer for transmission on the national power
grid. The spent steam is condensed, and returned to the boiler for
re-use.
"The electrical output of the plant follows the
boiler," said Parsons Brinckerhoff's Project Manager Tadhg O'Connor.
"Ideally, the boiler should maintain a constant pressure of 111 bar
(1600 psi). As the boiler pressure drops, the demand for steam
increases. The dosing conveyor responds by increasing the feed rate and
the combustion air."
Environmental Measures
Immediately exiting the boiler, the flue
gas passes through selective non-catalytic reduction treatment to reduce
nitrogen oxide emissions. Lime, in powder form, is then precisely dosed
to neutralize acids. The flue gas is then filtered through 1,800
6-meter (20-foot) fabric filter bags to reduce particulate emissions and
capture the lighter fly ash. The induced draft fan blows the clean flue
gas through a 60-meter (180-foot) exhaust stack to the atmosphere. The
exhaust stack is fitted with a continuous emission monitoring system
ensuring that faults do not result in excessive harmful emissions. The
exhaust stack is also fitted with selective catalytic reduction to
enable further reduction of nitrogen oxides emissions should this be
required in the future.
Straw from farms in Lincolnshire is burned in the Sleaford
Renewable Energy Plant. Most of the plant's straw is sourced from within
a 50-kilometer (30-mile) radius of the plant, seen in the background. Credit: Tadgh O'Connor / Parsons Brinckerhoff.
Ash produced during the burn is recycled for
revenue. Bottom ash can be sold to the construction industry as
aggregate, while phosphate- and potassium-rich fly ash can be sold to
local farmers to fertilize their crops.
The plant also captures some of the heat produced
during the generation of electricity and directs it to a district
heating system. This type of closed system uses energy more efficiently -
saving an estimated £2 million [US $3 million] in energy costs -and
releases less "waste" heat into the environment.
Local Boon
Sleaford REP sources all of its straw
from within an 80-kilometer (50-mile) radius; most of it from fewer than
50 kilometers (30 miles) away. This proximity reduces both the
operational cost and the environmental toll of acquiring and
transporting fuel to the plant.
The arrangement also provides a boon to area
farmers, who have signed long-term contracts to provide straw to the
plant. Such fuel purchases are expected to pump about £5.6 million [US
$8.5 million] into the local economy on an annual basis.
Stoker screws in the feeding subsystem push the straw into the boiler. Credit: Tadgh O'Connor / Parsons Brinckerhoff.
"Glennmont has created a real synergy here," said
O'Connor. "The plant feeds revenue to the local farmers by purchasing
their straw. Energy from straw creates further diversity in the energy
market, reducing the dependency on fossil fuels. The burned straw, in
turn, produces recyclable byproducts like heat and ash. Farmers can buy
the ash to fertilize their crops for the next season, and then the whole
cycle starts over Everybody wins."
http://www.renewableenergyworld.com/articles/print/volume-18/issue-3/features/bioenergy/biomass-plant-generates-electricity-and-opportunity-for-uk-community.html