U.S.-FocUSed Biochar report - BioEnergy Lists

(vapour-phase) pyrolytic reactions in the formation of charcoal 88 years ago; today, many researchers still assumethat charcoal is solely a product of primary (solid-phase) pyrolytic reactions. In reality, charcoal contains both“primary” charcoal and “secondary” charcoal that is a coke derived from the decomposition of the organic vapors(“tars”) onto the solid carbonaceous solid.”Many factors influence secondary char formation, including the biomass particle size, the pressure of the pyrolysisreactor, and the relative composition of the vapors within the reactor. As such, there can be a range of resultingchar yields and biochar properties produced by Slow Pyrolysis technologies at exactly the same pyrolysistemperature, depending on the extent of secondary char formation. For this reason alone, all Slow Pyrolysis charsneed to have the adsorption capacity measured to establish the actual biochar quality. This variation of the charquality is a function of how much vapor leaves the pore spaces of the solid versus how much vapor stays withinthe adsorption sites and recondenses into those unique spaces. The presence or absence of those adsorbingspaces is a key element in the value of the biochar solid.While slow pyrolysis chars vary over a relatively small range of yields and properties, depending on specificreaction conditions at a given temperature, the greatest changes in char properties occurs when one modifiesthe reaction conditions and exits the unifying envelope of “Slow Pyrolysis” conditions. The principal alternatepyrolysis regimes are called “Gasification” and “Fast Pyrolysis”.“Gasification” is pyrolysis under conditions that a portion of the char is further reacted with oxygen and combustedto either carbon dioxide or carbon monoxide and ash. Biomass, especially wood, burns in a two-stepsequence of reactions. The first reaction is the conversion of wood to char, and is called carbonization with woodgasification. This is basically the slow pyrolysis conversion discussed above. Once the char is formed, there is asecond, hotter reaction, where the char is converted to ash, called char gasification.Campfires do a nice job of displaying the various gasification reactions. Initially, when the fire is first lit, there isonly wood and no char, so the only possible reaction is wood gasification, coupled with combustion of the woodgases. This is the yellow flames that can reach high above the burning wood. Over time, the wood converts to charand the red embers form at the bottom of the fire – this is where char gasification is occurring. If one keeps addingwood to the fire, both reactions are ongoing. At the end, when no additional wood is added, the campfire settlesinto a bed of embers and just char gasification is occurring. At the end of char gasification, and in the morning,only ash is left.The key to whether a gasification conditions are occurring during biomass pyrolysis is the presence of the red towhite-hot zone of char gasification. The key to the impact of gasifying conditions on the char is to examine the effectof the gasification temperatures on the residual char. Char gasification occurs at much higher temperaturesthan wood gasification, because there are no volatiles being formed to evaporate and cool the remaining solids.Char gasification is a direct reaction of gaseous oxygen with the char solids, yielding carbon dioxide and carbonmonoxide and thermal energy. The energy released heats the char until the reaction is proceeding as fast as thereis available oxygen. The rate of char gasification is controlled by controlling the amount of available oxygen. Assuch, wood gasifies to char as a function of temperature and heat transfer, while char gasifies to ash as a functionof the amount of available oxygen.If there is an excess of oxygen, virtually all the char is consumed and only ash remains. If there is a limitedamount of oxygen, then only a portion of the char is consumed and some remains to exit the bottom of the gasifier.However, the char that does avoid complete oxidization has been altered due to the conditions within thegasifier. One effect is the direct loss of some of the organic portion of the remaining char, which increases therelative portion of ash. A second effect is that the residual char has been exposed to high temperatures, so anyresidual volatiles have been driven off. As such, most gasifier chars have a high portion of fixed carbon and littleremaining volatile matter.In addition, the high gasification temperatures often convert ash carbonates to corresponding oxides by drivingoff carbon dioxide. The combination of higher relative ash content and higher ash oxides can result in gasifierBiochar and energy linkages in: Biochar and Energy Co-products 23