The Sleaford Renewable Energy Plant, a straw-fired combined heat and power plant in predominantly agricultural Lincolnshire, UK, provides environmental, financial, and social benefits to its community and to the country, strengthening the ties between the power industry and those it serves.
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.
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 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.
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.”
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.
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.
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.
“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.”
Jim Wechsler is a technical editor with WSP | Parsons Brinckerhoff.