Gasification offers coal plants a renewable future5 June 2002
Circulating fluidised bed (CFB) gasification shows promise as a way of using biomass for co-firing at existing coal-fired power plants. The first commercial project is under construction in Belgium.
Among the renewables, biomass has potentially a very large role to play. And with legislation now in place both in Europe and elsewhere to drive up the contribution of renewables, power generators are increasingly interested in ways to exploit biomass. One particularly interesting line of development is the introduction of biomass co-firing at existing fossil fuel plants.
In Europe, it has been estimated that renewable solid fuels with a thermal potential of 30-150 MW are typically available within 50 km of a given power plant, enough to gasify and utilise directly in mid-or large-size coal-fired boilers.
Thus, a power plant with a gasifier that is connected to a large conventional boiler with a high-efficiency steam cycle offers an attractive and efficient way of using local renewable sources in energy production.
This is an opportunity that has not lost on Belgian power generator Electrabel, which is currently in the process of building the first commercial CFB biomass gasification plant, at its Ruien coal-fired plant.
The Foster Wheeler supplied CFB gasification plant will produce gas for co-firing with coal in an existing boiler.
With about 58 per cent of the country's electricity already coming from a CO2 free source, nuclear energy, Belgium's official renewables target is modest. By 2010, according to the European Council Directive on promoting renewables, Belgium's "indicative target" for the percentage of electricity generated from renewables is 6 per cent. But this represents a sharp increase over current levels (around 2 per cent in 2002) and a system of "green certificates" (along the lines of those being adopted in the UK and Italy) is being introduced.
Looking beyond 2010, there is estimated to be the potential for 10 TWh of Belgian annual electricity generation (less than 10 per cent) to come from renewables by 2020, according to the AMPERE commission. Wind is seen as the biggest potential contributor (5.4 TWh in 2020), but biomass is also expected to be a major player (contributing 3.5 TWh in 2020, compared with about 0.7 TWh in 1999).
Building on Lahti experience
Electrabel's Ruien biomass gasification project builds on four years of good operating experience with biomass gasification at the Kymijärvi power plant of Lahden Lämpövoima Oy in Finland (the Lahti project).
The aim of the Lahti project was to test the feasibility of using a CFB biomass gasifier as a fuel pre-treatment unit. And the project has indeed demonstrated commercial-scale direct gasification of wet biofuel and the use of hot, raw and very low calorific gas directly in an existing coal-fired boiler. Industrial and household wastes with high heating value have also been gasified.
The project has confirmed the anticipated advantages of biofuel gasification, eg reduced emissions, low investment and operation costs, and the utilisation of existing power plants.
The atmospheric CFB gasification system used at Lahti, and to be employed at Ruien, 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 of these components are entirely refractory lined. After the cyclone, the hot product gas flows into the air preheater, which is located below the cyclone. The gasification air, blown with a high-pressure air fan, is fed to the bottom of the reactor via an air distribution grid. When the gasification air enters the gasifier below the solid bed, the gas velocity is high enough to convey some of the bed particles out of the reactor and into the cyclone. In the uniflow 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, a difference compared with a conventional cyclone.
The fuel is fed into the lower part of the gasifier above the air distribution grid. The operating temperature in the reactor is typically 800-1000°C depending on the fuel and the application. When entering the reactor, the fuel particles start to dry rapidly, and a primary stage of reaction, namely pyrolysis, occurs. During this reaction, fuel converts to gases, charcoal, and tars. Part of the charcoal flows to the bottom of the bed and is oxidised to CO and CO2, generating heat. After the rest of these components flow upwards in the reactor, a secondary stage of reactions takes place. These reactions can be divided into heterogeneous reactions where char is one ingredient in the reactions, and homogeneous reactions where all the reacting components are in the gas phase. A combustible gas is produced from these and other reactions, which then enters the uniflow cyclone and escapes 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 reactor. These solids contain char which is combusted with the fluidising air introduced through the grid nozzles to fluidise 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 stabilises the temperatures in the process. The coarse ash accumulates in the gasifier and is removed from the bottom of the unit with a water-cooled bottom ash screw.
The Lahti gasifier was built to act as a fuel pre-treatment unit for the old PC boiler at Kymijärvi, which is Benson-type, once-through. Originally, the plant was heavy-oil fired, but was modified for coal firing in 1982.
The plant supplies electricity (167 MWe) and district heat (240 MWt). The steam data are 125 kg.s-1/540°C/170bar/540°C/40bar. In 1986, a gas turbine (49 MWe) was connected to the heat exchanger, preheating the boiler feed water.
The boiler uses 180 000 ton/a of coal and about 800 GWh/a of natural gas. The boiler is not equipped with a sulphur removal system, the coal being low sulphur, 0.3-0.5 per cent S, but the burners have flue gas circulation and staged combustion to reduce NOx.
The Lahti gasifier went commercial in March 1998 and initially used biofuels such as bark, wood chips, sawdust and uncontaminated wood waste. Other fuels have also been tested subsequently, including combustible refuse (REF), as well as railway sleepers (chipped on site) and shredded tyres.
Generally the plant has operated well, with availabilities in the high 90s. The fuels used are shown in table above.
The effect of gasification on the main boiler emissions has been as follows: NOx, decrease by 10 mg/MJ (= 5 to 10 per cent); SOx, decrease by 20-25 mg/MJ; CO, no change; HCl, increase by 5 mg/MJ (base level low); particulates, decrease by 15 mg/Nm3; heavy metals, increase in some (base level low); dioxins, etc, no change.
No signs of abnormal deposit formation on the boiler heat transfer surfaces have been detected.
The Ruien gasification project
Against the background of the promising results obtained from Lahti, a decision was made to go ahead with a commercial project, at Ruien, the largest fossil fuel-fired power station in Belgium. The gasifier will be connected to one of the existing boilers at Ruien.
The Ruien gasifier is expected to be delivered by the end of 2002 and will start operation in January 2003.
The Ruien project is a major breakthrough in the utilisation of recovered fuels with a high heating value, and several other similar projects are under consideration.
The approach adopted at Ruien and Lahti can easily be taken if there is existing capacity for combusting fossil fuels, as no new combustion or energy recovery equipment is needed. The produced gas can be used for replacing part of the fossil fuel with a renewable energy source. As demonstrated at Lahti, the operation of the existing unit will benefit from reduced emissions.
The concept of the gasification of recycled fuels can be developed further by adding a product gas cooling and cleaning unit prior to combustion. With this approach, clean fuel gas can be produced by treating wastes which normally cannot be burned very efficiently.
TablesFuel used at Lahti 1998 1999 2000 2001