This article describes the best available technology for straw-fired boilers for power production, using a case study focusing on the Sleaford plant in Lincolnshire, UK. The technology has been developed by Burmeister & Wain Energy A/S in Denmark.
Burmeister & Wain Energy A/S (BWE A/S) has been a pioneer in the field of straw-fired boilers for power production since the late 1980s. Based on more than three decades of experience in the field, BWE A/S has developed the best available technology (BAT) on the market, which is optimised to deliver high boiler efficiency and steam parameters, low maintenance and operation costs, and high availability and a long lifetime.
Today, BAT has become a standard request to key technology providers from renewable energy project stakeholders including authorities, politicians, power plant owners and investors. In an effort to secure market share, some technology providers choose to follow industry trends, whereas others are front runners and push new technologies into the marketplace.
There is significant demand for sustainable energy and renewable sources such as photovoltaic, wind and hydro power have grown substantially under government-subsidised schemes; however, alternative types of power supply are vital to ensure grid stability and supply security. The use of fossil fuels must be reduced to a minimum, meaning biomass becomes the obvious solution.
Straw is one of the most sustainable energy resources available in Europe. Unlike other types of biomass such as imported wood pellets that require certification, straw can be utilised without a major impact on the environment.
The Sleaford Renewable Energy Plant is being built for a heat input of up to 120MW - equivalent to a straw consumption of 30t an hour or 240,000t a year - and will generate up 38.5MW of power, enough to supply approximately 65,000 households and businesses. By replacing coal, the Sleaford plant reduces CO2 emissions by more than 150,000t a year.
The core of the plant is a vibration-grate-fired, drum-type boiler. Straw is probably the most difficult biomass fuel to handle and presents a challenge for other technologies such as circulating fluidised-bed (CFB) and travel grate boilers. The water-cooled vibration grate represents the BAT for straw combustion and provides a reliable operation that requires minimum maintenance. Straw contains very high amounts of chlorine and alkaline (potassium and sodium), resulting in a very low ash deformation temperature. The boiler is therefore carefully designed with three passes, slagging super heaters and a large super heater pitch made out of austenitic material grade tubes.
The fuel is supplied to the site as rectangular bales (Heston or Claas) and stored in two 3200m2 straw barns. Unattended cranes feed the bales onto the boiler's four fuel feeding lines through a receiving table. Here, the weight, length and moisture content of each individual bale is measured in order to control and optimise the combustion. The strings are cut and the bales pass through the BWE-developed bale openers, the loose straw fed by the four stokers onto the grates. The compressed straw in the water-cooled stoker ducts forms airtight ploughs, which prevent backfire and false air intake.
The grate is an integrated part of the evaporator system of the natural circulated boiler. The grate is inclined to a low angle, but is still allowed to be a part of the evaporator system without the risk of steam build-up and membrane overheating. The tube metal temperature at the grate surface is approximately 30-40°C above the water temperature.
The water-cooled grate is designed as a membrane wall with air nozzles in the fins. Primary air, which comprises roughly 30% of the total air content, is distributed based on a predesigned and optimised whole pattern in the grate surface. The main combustion air is introduced through a number of air injection nozzles above the grate that are connected to the furnace noses. The vibrations enable the transport of the fuel down the grate from the reception/heating/drying zone to the area where the main combustion takes place, then on to the zone where the coke glows out and finally to the cooling zone for the ashes before falling into the slag hopper. The combustion temperature varies along the grate and can reach 1,400°C.
The furnace and nose design as well as the distribution and orientation of the air nozzles ensures the mixing of the air and the combustible gasses. With optimised operation, the air excess will be in the region of 1.30-1.45, while the flue gas leaves the furnace at a temperature of 800-1,100°C depending on the fuel characteristics and load conditions.
Operating with fuels that have a low initial ash deformation temperature (down to 750°C) results in a build-up of slag on the super heater above the furnace and the final super heater in the top of the second pass. Slag builds up into a layer whose surface melts and drips down - the layer of ash protects the super heaters against corrosion.
The water-cooled vibrating grates offer very low maintenance costs, and provide high availability - typically, the grate membrane has an operating lifetime of more than ten years - and low power consumption.
The combustion air is pre-heated using a simple heat transfer system. Feed water is used to preheat the air in a fin tube air heater. From here, the cooled feed water is used for flue gas cooling in a number of flue gas cooler sections after the economiser. The flue gas is hereby cooled to 120-130°C and feed water is heated to the original temperature, typically around 200°C. The system ensures that any surface in the flue gas coolers is above the critical temperature that would otherwise result in aggressive acid corrosion caused by fuel chlorine content.
The plant's proven technology is designed for the clean and efficient combustion of straw, the by-product of wheat production, supplied mainly by farms within a 30-mile radius of Sleaford. Ash produced by the plant will be recycled as crop fertiliser. The Sleaford plant is equipped with flue gas cleaning system that meets the required emission limit values (IED ELV) for NOX, SO2 and particulate.
As well as generating 38.5MW of electrical power, the surplus heat generated by the plant will be used to heat a public swimming pool, bowling centre, football club and district council offices around Sleaford. Long-term fuel supply contracts with local farmers and the creation of 80 jobs support the local economy to the tune of approximately £10 million a year.
Developer: Eco2 Ltd
Plant owner: Eco2 Lincs Ltd (owned by Glennmont Partners)
Lenders: four international banks
EPC contractor: BWSC A/S and BWE A/S in consortium
O&M contractor: BWSC A/S - 12 years
Contract start: December 2011
Delivery time: 27 months.
Steam parameter: 540 °C @ 112 bar
Boiler type: drum, three pass, bottom supported
Fuel: rectangular straw bales (New Holland, Heston or Claas-type)
Additional fuel: 20% wood chips
Start-up fuel: combined LDO/gas burner
Boiler efficiency: 92.5% (LHV, EN12952-15)
Load range: 40-100% load
Load change rate: 3 %/min (4%/min with support fuel)
Combustion: four screw stokers firing on water-cooled vibrating grates.
Boilers of this type can be built to cater for 132MW of heat input and include a single reheat cycle. A wide variety of biomass fuels such as wood chips, sun flower cakes and oil seed rape can be used.
