Challenges of Replacing Fossil Heat with Biomass
Convenience
          The real advantage of heating oil, and to some extent natural gas, is its convenience as a fluid fuel. This enables very precise control, and this precision allows a great deal to be easily automated, and the consumer need only set their thermostat and pay their bills. The problem though is that those bills are only going up. The task of designing a biomass system that approaches this kind of convenience is a larger task than a Capstone class could take on, although the following discussion outlines some ideas of how it could be done.
          Ultimately this design project focused on just a portion of the problem, which is an affordable feeding mechanism that is precise enough to run an efficient woodstove remotely (without human supervision or concern).



Cost of Conversion

      The project began by investigating different designs that might be able to bring the cost of a woodstove down. The idea here would be to sacrifice a bit of convenience on the part of the user to bring down costs and provide a stable and secure heating source. It turns out that there are many cost-effective improvements that could be made for woodstoves- such as replacing cast iron with a cheaper insulating material- and these designs are outlined in more detail in the next section.
        But in reality, as much as this project tried to be practical, the wood stove designs remain academic. As mentioned on the "Home" page, an efficient wood burning system needs a heat exchanger. Even if the stove achieved complete combustion without heat exchanger a large amount of heat will simply flow up the chimney and out of the house. Heat exchangers are relatively expensive, and they must be hooked up to a heat distribution system to have any effect at all.
       There is hope for affordable retrofit though, because oil furnaces already do use heat exchangers. The focus of this project should have been, then, to design a retrofit of an oil furnace that changes as little as possible, but allows it to burn some sort of biomass instead of oil. The major challenge in this will probably be ash removal, and the other will be the retrofit of the fuel feeding mechanism- which will be considerably different for a solid fuel. Building some experience with the dynamics of biomass feeding systems became the final focus for the project- I decided to build a cheap feed auger prototype out of a milk jug, a drill and an optical sensor.

Uniformity

      There are obviously some inherent differences for designing for biomass compared to oil. As mentioned it being a solid fuel makes precise metering of the fuel difficult, and it also makes attaining uniformity of the fuel difficult. Life is extremely variable, and biomass reflects this variability. Liquid fuels are refined in separating columns where they stratify according to density, and this means that the chemical composition of the fuel can be known accurately. This is not the case with wood- its chemistry is extremely complex and all of its organic compounds are locked up together in solid matter. This can effect the efficiency of combustion, since adequate oxygen must be provided to achieve a clean burn, but at the same time every bit of air that is added carries more energy out with it up the chimney.
       A much greater difficulty with wood's heterogeneity is moisture content. The moisture content of wood can have a huge influence on the efficiency of combustion if the design is not robust enough to accommodate it, and then even if a clean burn is achieved the water leaves the chimney as water vapor- taking with it all the energy needed to boil it from a liquid to a gaseous state (heat of vaporization). One solution to this problem can be a preheated hopper, where the fuel sits in a high-temperature basin and pre-dries on its way in to the stove. By the ASTM (engineering standards used to get equivalent results in experiments) wood is considered bone-dry after two hours at 200 degrees farenheit, with a large enough hopper this could easily be achieved.
       However this leads to another problem, and that is auto-ignition. This is rare, but because wood contains oxygen within its chemical formula (fossil fuels have a little, but not much) the wood can actually self-ignite, without the presence of oxygen, if it gets too hot.
       I believe the solution to these challenges is robustness, one that can handle significant variation from its designed operation conditions without suffering huge inefficiencies, clogging or having other complete failures. This means a simple design that works with the fundamental nature of wood as a solid fuel, rather than trying to control it as if it were a liquid.
       This is the reason that the Capstone has settled on the feeding mechanism, as this is typically a high-price component which could be made more simple and possibly more robust.
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Deliver-ability and Storage

        Another challenge is the logistics of a constant fuel supply. This is related to convenience, but is a large enough challenge that it merits its own mention. Heating oil can simply be pumped in to a tank, which then feeds into the furnace. Acquiring, storing, and loading wood is not this easy. In fact all the long walks through winter's snow from the family room out to the wood pile by the garage may have been what started me on the path to being an engineer in the first place, thinking "there has got to be an easier way to do this!".
       It's a tricky problem though. Pellets are easier, because they are at least a fairly uniform size and shape, however they are not ideal because they require pre-processing to put them into that uniform size and shape. Wood chips are a bit better, because they can be made locally by anyone, but they are also a lot harder to deal with because of their lack of uniformity.
       Still I feel that the more Mainers can do for themselves the better-off we'll be, and so the focus of this design project is to make a hopper which can be metered effectively while using wood chips. As mentioned at the bottom of the "Home" section, an even better solution could be the use of a softer biomass (such as straw and other agricultural residues). As the biomass gets less rigid it becomes a more efficient fuel source (if you consider the energy that goes into growing/harvesting it) but it also is less dense, and becomes economically unfeasible to transport. This means that if straw and agricultural residues are going to fuel our homes, they will have to be local (which is a good thing for nutrient cycles anyways)