Wood pellets are used for both large scale biomass power projects and for smaller institutional and commercial heating.
Industrial Power Generators
Wood pellets are commonly used around the world, especially in Europe and Asia, as a substitute for coal in pulverized coal power plants either by blending coal and biomass together in a process known as biomass co-firing, or by replacing coal entirely with wood pellets. This is known as dedicated biomass firing.
Whether used for co-firing or for dedicated firing, wood pellets are handled similarly to coal. Pellets are ground into dust using modified coal mills or hammer mills. The dust is then mixed with air and blown through pipes which are connected to burners within a pulverized coal boiler. The dust-air mixture is ignited, acting much like a gas. Water is circulated inside the boiler in thousands of small tubes. The continuous flame boils the water within the tubes, creating steam. The steam, under pressure causes a turbine and generator to rotate. The generator uses a rotating magnetic field within a coil to create electricity which is fed to the electrical grid. Spent steam is passed through a condenser – essentially just pipes passing through cold water – and converted back into water which is circulated back into the boiler in a continuous process known as the Rankine Cycle.
Pellets are an easy substitute for coal. They can be easily ground into dust. They have low moisture content, approximately 6%, they handle easily, and are comparable to coal with energy content of around 18 gigajoules per tonne.
Unlike coal, wood pellets may not be stored outside because they will disintegrate if exposed to rain. Pellets must be stored inside using silos, domes or warehouse buildings. This means that for pulverized coal power plants to use wood pellets, the plants must be modified to add such storage, as well as separate conveying systems to transporting pellets from the storage to the milling process.
Click here for more information on biomass firing and co-firing
Converted pulverized coal boilers are not the only option for creating electricity from wood pellets at an industrial scale. Wood pellets are also used to create electricity by fuelling purpose-built circulating fluidized bed (CFB) boilers. There are many CFB boilers in operation in Japan. At the bottom of the boiler furnace there is a bed of inert material, usually sand. The bed is where the wood pellets spread. Incoming air from nozzles under the bed lifts the bed material and wood pellets, keeping them in suspension. Combustion takes place in this suspended condition. Heat is used to raise the temperature of water in the boiler tubes and the remaining process creates electricity using the Rankine Cycle. The main advantage of CFB boilers is that they can use a wider range of biomass fuels – most commonly a mix of wood pellets, wood chips and palm kernel shells.
Global power producers are now in a race to commercialize carbon capture and storage (CCS) technologies. It is apparent that, coupled with CCS, biomass firing and co-firing provides one of the very few means of removing substantial quantities of carbon dioxide from the atmosphere.
Canadian wood pellets are exported around the world for industrial use in large-scale electric power plants. Our industrial markets include the United Kingdom, the Netherlands, Belgium, Denmark, Japan and South Korea.
Institutional and Commercial Use
Canadian wood pellets are used for institutional and commercial heating using modern biomass boiler systems that are every bit as automated and convenient as natural gas and oil heating systems. Modern biomass heating systems are designed to require little maintenance beyond periodic ash disposal, can be refueled just once or twice a year, and can be controlled remotely by using a smart phone app.
Watch the Video: Heating up the Grand Falls Hospital with Pellets: A Success Story.
Modern wood pellet boilers are commonly used to replace heating oil resulting in lower operating cost and extremely low particulate emissions. Oil boilers emit around 50 mg/MJ while wood pellet boilers using emit closer to20 mg/MJ or less. Theuse of high efficiency modern filters, such as ceramicfilters can ensure that particulate emissions are kept extremely low (3 in the flue gas) at all times. Today, in Canada, wood pellets are used to heat schools, universities, government buildings, hospitals, churches, bakeries, greenhouses and for many other applications.
Modern wood pellet boilers make use of hydronic (water-based) heating. Wood pellets are delivered in bulk and stored in a silo, bunker or storge bin. Automated controls from the boiler will activate an auger that will transport pellets to the boiler combustion chamber. Water is heated through a heat exchanger and transported to a hot water buffer tank, which in turn will distribute hot water to a domestic hot water tank or through pipes to in-floor heating or wall-mounted radiators. Temperature is automatically controlled by a thermostat. Wood pellets can also be for traditional forced-air heating where pellets are combusted in a furnace and a blower is used to distribute hot air through heating ducts similar to the way natural gas and oil furnaces work.
Click here for a list of institutional biomass boilers in Canada.