Fluidized bed This system’s forte is dealing with two or three different fuel sources at once. Consider the current and future fuel sources carefully when choosing a technology. If your supply may change, if you require mixed supplies to meet your volume demands at times, or if you may be forced to accept varying qualities of fuels in a sellers’ market, consider that flu- idized beds are very efficient with mixed fuel supplies. Fluidized beds are typically typically wet. Biomass also contains less energy by weight than almost any other type of fuel currently used. This in turn creates significant challenges for transport, storage, handling, and more. Even when bone dry, wood typically has half the energy content of coal by weight and one third that of natural gas. On top of that, wood’s fuel density is very low compared to coal (although higher than straw). cylinders (or square units at the larger scales) that are filled with sand to a small portion of their height (three feet of a 20-ft high cylinder like those used at Can- metENERGY’s Ottawa labs). The bottom is perforated with holes slightly smaller than the sand (so you don’t lose your sand of course), through which air is blown upward at just the right velocity to suspend the sand much like the numbered balls in a lottery draw. When various fuel types are added, they are thus mixed very well during combustion. Aside from the ability to mix fuels, fluidized beds handle wet fuels bet- You have to buy a combustion unit for the fuel that you are going to burn – A unit designed for coal will not work as well with wood chips, and certain- ly won’t work as well with agricultural residues. Combine half the energy content with five or six times the bulk and up ter. Since the moving sand particles are all pre-heated to about 800 degrees, their motion through the incoming fuel will serve to mix, heat, and effec- tively combust even wet fuels like sludge. In comparison, wet fuel added to a grate furnace will typically sit there as a lump. That same bucket of sludge in a fluidized bed will be mixed and treated to a much higher heat transfer rate. The technology also offers good NOx reduction and a very high efficiency, typically between 75 and 85% thermal efficiency. But there are tradeoffs, such as: • Fluidized beds are very expensive compared to grate furnaces of the same capacity. • They are not widely available at smaller scales, and are not yet cost efficient below 10 MW (th). Not coincidently, the largest biomass plant in the world, Alholmens Kraft’s 550 MW (th) plant in Fin- land, is a fluidized bed burning a mix of biomass, peat and coal. • Higher power consumption to keep it running, as you will need a compressor to blow the air through the sand. • Some sizing of the fuel is required. • More training required than a grate furnace. This last point is worth a word or two on its own though, since operator training is an issue even with grate furnaces. As these boiler systems move into greenhouses, sawmills or other industrial clients, we’re seeing more issues with insufficient training. Suppliers in some cases deliver the system, show the clients how to turn it on or give basic mechanical instructions, and then leave. Then if there are problems, there is often a lot of trial and error involved in solving them, a lot of frustration, and delays in getting the return from investment. Operator training is an issue, as there is not really a source of training in Canada to shorten that learning curve. CanmetENERGY is discussing this need with several colleges, to encourage the establishment of a renewable energy technologist program. still challeNges Yet even though direct combustion technologies like the above are proven, there are still some inherent challenges to using biomass. For starters, biomass other than special cases such as planer mill residues is CanadianBIOMASS 17 to 60% MC in fresh biomass, and you’re moving a lot of air and water with your energy. As a result, cost effective haul distances for fresh biomass will likely be well shy of 200 km, limiting the size of plant that can be built in any one location. Storage and handling systems will also need to be larger than those for conventional fuels. Here are some factors to consider: • Do what you can to get dry biomass or dry it prior to combustion. Boiler efficiency falls dramatically as moisture content increases, from say over 80% at bone dry to under 60% at over 55% MC. You’re
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