Generally just lurk here, but some subjects I just can't leave alone. FWIW, I have a degree in Wood Science & Technology and also considerable experience in the pelleting process. Lake Girl has the best info thus far on the difference between heat output of softwood vs. hardwood. The resins in softwood can contribute to a higher Btu value for softwoods (amount depends on the actual softwood species). Some of these resins are destroyed in the furnish drying process, and some more in the extrusion process, but some remain in the pellets. These resins offgas terpenes and various other volatile organic compounds which is what gives pine its characteristic smell.
Otherwise, the major differences in hardwood vs. softwood are mostly in the vascular structures that transport water and sap. Most other differences (weight, color, density etc.) may be true in most cases, but there are usually exceptions for individual species on both sides. So, SOME species of softwood pellets may produce more heat than SOME species of hardwood pellet, but it is far from a certainty. The VERY BEST softwood pellet will likely produce more Btu/lb than the best hardwood pellet, but the difference will likely be 5% or less.
How much heat you get from your stove is an equation with far more variables than just the Btu value of your pellets. Moisture content is certainly a factor, but the pelleting process itself removes a fairly consistent amount of water, and generally results in pellets coming to you in a pretty narrow and consistent range of moisture content. (Thus you see the generalization on bags of pellets "Less than 5% moisture"). By the way - know what you are looking at with the claimed Btu value that is printed on bags or available on manufacturers websites. Btu value/lb may be figured in two ways - dry value or actual value. Because some heat is necessary to drive off the remaining (< 5%) moisture in a pellet, the Btu value of the pellet as it comes to you is somewhat less than the Btu value figured based on the pellet being absolutely dry. Since your pellets will never be absolutely dry (0% moisture) the dry value Btu rating is essentially worthless. Because Btu value is a selling point, however, some manufacturers like to trumpet the dry basis number as a selling point - a questionable practice at best. I have seen any number of posts here where folks have been tripped up (deceived, actually) by this practice.
Density is important, to a point, as is pellet geometry. I can densify pellets to the point they are nearly bulletproof - they will be extremely hard, super shiny, and darker colored due to the greater heat produced in the extrusion process Better pellet? Not to many folks - they can be harder to ignite and will burn slower. A given weight of wood will require a given amount of air to combust it properly. Therefore, more air is required for a denser pellet to burn properly and completely. The end result for many is incompletely burned pellets (invariably referred to as "ash") being pushed out of the burn pot and accumulating elsewhere. Without an air adjustment, the flame will look lazy, glass will darken quickly, heat output will be low and the stove will require frequent cleaning. Those in the know can adjust their stove and enjoy the benefits of the denser pellet....others will condemn the pellets as "trash"and "full of bark". Which way would you produce them - as good as they can be, or ?
Pellet geometry, especially length, also affects perceived heat output. Your feed auger, working at a set speed, will deliver significantly differing amounts of longer vs. shorter pellets to your burn pot. How different will depend on your particular stove's auger design - diameter and flighting design being especially important. The greater weight of pellets delivered per revolution will obviously produce more heat (again, if and only if, the proper amount of combustion air is provided).
Learning all you can about how your stove operates will be of considerable more value to you quibbling about Btu value of hardwood vs. softwood pellets!
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