Timo Eriksson, Foster Wheeler Energia Oy, Varkaus, Finland
Since the 1980s, Foster Wheeler has been steadily developing its CFB gasification technology and recently completed a unit for Electrabel in Belgium. Another unit, this time developed in cooperation with new technology partners, is now under development in Finland.
During the 1990s, a gasification process producing raw gas from a variety of biomass and recycled fuels for co-combustion in a pulverized coal (PC) boiler was developed. Three commercial atmospheric circulating fluidized bed (CFB)/BFB gasifiers, with fuel inputs ranging between 40 MW and 70 MW, were delivered between 1997 and 2003.
Based on the success of these projects and an evaluation of future needs, Foster Wheeler Energia Oy has been developing a gasification concept for converting difficult, mainly waste-derived solid fuels into clean gas to be co-combusted in existing boilers. The first project using this concept – an 80 MW gasifier, hot gas cleaning equipment, and involving co-combustion in a PC boiler – is in the design phase.
Figure 1. Electrabel’s Ruien power plant in Oudenaarde, Belgium
Fluidized bed gasification enables cheap and potentially CO2-neutral solid fuels to be converted into combustible gas for replacing expensive oil, natural gas, or coal. Locally available low-grade fuels, such as wood chips, wood waste, bark, demolition wood, straw, and waste such as REF (in-origin classified recycled fuel) and RDF (refuse-derived fuel) have all been gasified successfully.
A CFB gasifier is a true multi-fuel unit, offering good fuel flexibility, capable of handling a range of fuels in a single unit, with heat output varying according to the heat value of the fuel input. By connecting a gasifier to a PC boiler, cheap solid fuels can be converted into power and heat at high efficiency compared to stand-alone units using the same fuel input. The gasifier-PC combination also offers low capital and operating costs; requires only minor modifications to an existing main boiler; and provides high plant availability and reduced boiler emissions.
Figure 2. The Ruien gasifier is connected to the tangentially fired once-through boiler of Unit 5 of Electrabel’s Ruien power plant in Belgium
The atmospheric CFB gasification system developed by Foster Wheeler is relatively simple. It consists of a reactor, where gasification takes place; a cyclone to separate the circulating bed material from the gas; and a return pipe for returning the circulating material to the bottom part of the gasifier. All these components are entirely refractory-lined.
From the cyclone, the hot product gas flows into an air preheater located below the cyclone. Using a high-pressure fan, gasification air is blown into the bottom of the reactor, below a bed of particles through an air distribution grid. Air enters the gasifier at a velocity high enough to fluidize the particles in the bed and convey some of them out of the reactor and into the cyclone. In the cyclone, the gas and circulating solid material flow in the same direction – downwards – and both the gas and solids are extracted from the bottom of the cyclone.
The fuel is fed into the lower part of the gasifier, above the air distribution grid. Incoming biofuel typically contains 20-60 per cent water, 40-80 per cent combustibles, and one to two per cent of ash. Depending on the fuel and application, the operating temperature in the reactor is between 800 and 1000°C. When entering the reactor, the biofuel particles start to dry rapidly, and primary pyrolysis occurs, when the fuel is converted into gases, charcoal, and tars. Part of the charcoal flows to the bottom of the bed and is oxidized, generating heat. The products flow upwards in the reactor, and a secondary stage of reactions takes place.
These reactions can be divided into heterogeneous reactions, where char is one ingredient; and homogeneous reactions, where all the reacting components are in the gas phase. A combustible gas produced from these and other reactions then enters the cyclone and exits the system, together with some fine dust.
Most of the solids in the system are separated in the cyclone and returned to the lower part of the gasifier. These solids contain char, which is combusted with the fluidizing air introduced through the grid nozzles to fluidize the bed. This combustion process generates the heat required for the pyrolysis process and subsequent, mostly endothermic reactions. The circulating bed material serves as a heat carrier and stabilizes the temperatures in the process. Coarse ash accumulates in the gasifier and is removed from the bottom with a water-cooled bottom ash screw.
The first CFB gasifier connected to a PC boiler was constructed in 1997 at the Kymijärvi power plant of Lahden Lämpövoima Oy, which generates electricity (167 MWe) and district heat (240 MW) for the city of Lahti, Finland. The Lahti gasifier was connected to a 20-year-old Benson-type, once-through boiler normally fired on coal.
The Lahti gasifier started commercial operation in 1998 and initially used biofuels, such as bark, wood chips, sawdust, and uncontaminated wood waste. Other fuels have also been tested subsequently, including REF, railway sleepers, and shredded tyres. As a result of availability and price changes, the share of REF has gradually increased at the expense of cleaner biofuels. The gasifier has operated well with varying fuel mixes, with availabilities of 96 per cent or higher.
The effects of gasification on the main boiler were studied comprehensively during a one-year monitoring programme. The results are shown in Table 1.
The basic concept of combusting raw, uncleaned product gas in a PC boiler is therefore simple, and works very well with relatively clean biofuels. However, such fuels are not always abundantly available. Fuels with high alkalinity and chlorine content, such as straw, can also be utilized, but the product gas has to be cleaned of these harmful components before it is burned in an integrated boiler.
Straw gasification technology, including gas cooling and cleaning and straw feeder technology, was developed further and found technically feasible by Foster Wheeler and Denmark’s Energi E2 and TK-Energy between 1999 and 2001. Combustion studies focused on the conditions for gasification, gas cooling, dust separation, and gas combustion.
Several test runs with pelletized and loose straw were carried out at the
3 MWth CFB gasification pilot plant at Foster Wheeler’s R&D Centre in Karhula, Finland. In the first test series, pelletized straw was found to guarantee trouble-free fuel feeding and steady gasification, despite its high alkali content and the low melting point of the ash.
In the first test series, a secondary cyclone located after the gas cooler was used for particle separation from the syngas before it was combusted; subsequently, a high-temperature baghouse filter was installed in the syngas line after the gas cooler. As the tar content of the syngas was high, in the range of 10-20 g/m3n, baghouse operation temperature was selected in the range of 300-400°C to avoid filter blinding caused by condensing tars.
In the second gasification test series, loose straw was gasified. A newly developed straw feeding system was also tested, and the syngas was cleaned in the new baghouse filter before combustion. These tests confirmed that loose straw can also be successfully fed into the gasifier and gasified to produce a steady flow of combustible gas.
The third series of straw gasification tests, supported by the European Commission, focused on investigating the capabilities of the new feeding system to feed loose straw into the gasifier, the optimal process conditions and additives for gasifying loose straw, and the capability of the selected gas cleaning system to make the gas suitable for co-combustion in large CHP plants.
During the three test campaigns, more than 220 t of pelletized and loose straw have been gasified during over 400 hours of operation, and have enabled the following conclusions to be drawn:
- Gasification of loose straw is technically feasible
- Smooth, stable operation with high-alkaline fuel is possible
- Wood and straw can be gasified together, with trouble-free operation
- Carbon conversion is in the range of 95-97 per cent
- The gas cooler can be kept clean using soot blowing and spring hammering
- The hot gas filter operates well without tar blinding and removes alkalis and chlorides at 350-370°C from the syngas
- PAHs were formed, but not dioxins or furans.
Figure 3. The main components of a CFB gasifier
Gasifying recycled fuels
Foster Wheeler has been continuing to develop gasification-based solutions for turning waste fuels into energy, in the light of the growing pressure to recover energy from industrial and municipal waste and the upcoming ban in Europe on landfilling materials that can be composted or contain combustible fractions. More efficient recycling and energy recovery favours technologies utilizing combustible fractions operating at the highest efficiency.
In addition to pilot plant tests on gas cleaning, long-term testing has been carried out using slipstream equipment, in the form of a Foster Wheeler gas cooler and a commercial hot gas filtration system installed at the Lahti gasification plant. Some 1500 operating hours have been logged to date with product gas, and testing is continuing on this project led by Foster Wheeler and co-funded by TEKES (Finland’s National Technology Agency), in cooperation with Lahti Energia Oy and Energi E2.
In a further development this year, Foster Wheeler joined forces with Powest, a subsidiary of utility Pohjolan Voima Oy; Finnish biofuel, power, and heat producer Vapo Oy; and the Technical Research Centre of Finland (VTT) to develop the partners’ various waste treatment, gasification, gas cleaning, and co-combustion technologies. The three other partners had already been working together on developing Vapo’s mechanical-biological treatment (MBT) waste pre-treatment concept and various gas cleaning technologies, and had carried out various tests on pilot-scale test rigs at VTT’s laboratories.
The plan is to demonstrate the combined technology at a full-scale plant to be built alongside Vantaa Energia Oy’s Martinlaakso power plant in Greater Helsinki. The gasifier will be an 80 MW unit fired on 100 000 to 120 000 t of REF annually. The plant is in the design phase and given finalization of permitting and other approvals, construction is scheduled to start in summer 2004.