U.S.-FocUSed Biochar report - BioEnergy Lists

allowed to consolidate into typical slow pyrolysis chars, since that would result in additional water vapor beingrejected into the vapor phase and deteriorate the bio-oil condensate. As a result, most Fast Pyrolysis chars havebeen constrained to relatively low temperatures, in order to retain as much hydrogen and oxygen in the char. Additionalevidence of the low effective processing temperatures is the high level of volatiles present in many FastPyrolysis chars.As seen in the case of Gasification, Fast Pyrolysis conditions will concentrate the ash constituents in the residualchar, as a result of directing the carbon atoms of the biomass into the vapor phase and leaving the ash behind.However, the organic matter left in the char is generally devoid of carbonization char structures, including significantdevelopment of graphitic domains necessary for adsorption capacity i.e. not many pore spaces.A reasonable model of fast pyrolysis chars is a torrefied residual biomass organic fraction with elevated ash level,dictated by the relative partitioning of organics from the starting biomass into the bio-oil phase and the residualchar solids. Because the residual char has not been subjected to sufficient temperatures necessary to developinternal surface area, as depicted in Figure 1, and because fast pyrolysis char have been depleted of available carbonatoms to enhance the properties of the associated bio-oil, the development of favorable biochar properties inthe residual char is severely inhibited. The extent that Fast Pyrolysis chars exhibit favorable biochar properties isgenerally a measure of the extent within a particular process that some biomass avoids Fast Pyrolysis conditionsand/or is inadvertently converted to a better biochar via Slow Pyrolysis conditions present somewhere within theFast Pyrolysis reactor.Process-dependent Energy Co-product propertiesWhen choosing between slow pyrolysis, gasification or fast pyrolysis, it really comes down to the value of theproducts and byproducts. In general terms, all pyrolysis processes produce a combination of char and gaseousproducts that have relatively low fuel value, compared to traditional fossil fuels, principally due to the low energydensity of the starting biomass.A typical slow pyrolysis, optimized for biochar production, might produce 25 pounds of biochar and 75 poundsof gaseous products from 100 pounds of ash-free dry wood. Assuming the starting wood had an energy contentof 8,000 Btu/lb and the char has an energy content of 12,000 Btu/lb, one can see that the char contains 37.5% ofthe fuel value of the wood. This leaves 62.5% of the biomass energy in the gaseous products, for an energy densityof just 6,667 Btu/lb for the 75# of wood gas. By comparison, methane has a lower heating value over 21,400 Btu/lb – or more than 3 times the energy density of wood gas.For the case of gasification, the goal is to make gaseous products. The theoretical limit of gasification is to gasifyall the entire available organic portion of the biomass, which means 100 pounds of ash-free dry wood is convertedinto 100 pounds of gaseous products. Since energy cannot be created, the gaseous products have an energy densityof 8,000 Btu/lb, which is 20% better than slow pyrolysis, but still 2.67 lower than natural gas.By similar analysis, for fast pyrolysis, the theoretical limit for the bio-oil is 8,000 Btu/lb, which is about 40% ofthe typical energy density of diesel fuel, which is in the range of 20,000 Btu/lb. Bio-oil has one additional issue,which is that it is not actually an “oil”. Oils can be mixed with other oils, and bio-oil only mixes with water. Oilsare predominately hydrocarbons, composed of carbon and hydrogen, whereas bio-oils are almost identically thechemical composition of the original wood. At some level, the label “Bio-oil” may be misleading and a more accuratedescriptor, such as “Fast Pyrolysis Condensates” or even “Liquid Wood” may convey a more accurate imageof the material properties.Biochar and energy linkages in: Biochar and Energy Co-products 25