U.S.-FocUSed Biochar report - BioEnergy Lists

chars acting more like lime and raising pH when added to soils. For acidic soils, this is a beneficial impact, but foralkaline soils, elevating soil pH may result in lower crop compatibility and plant growth inhibition.One final impact of the high temperatures associated with gasifier operations is a portion of the char has beenexposed to excessively high temperatures such that adsorption capacity is deteriorated. As discussed earlier,chars typically exhibit an increasing adsorption capacity with increasing heat treatment temperature up to atfairly high temperature, after which the property “collapses” with increasing temperature.A parting consideration for gasifier chars is most current gasifiers have been designed to fundamentally produceheat or synthesis gas, and any residual char is a byproduct. As such, the properties of the gas tend to dictate thegasifier operating conditions, and the subsequent char quality is “what it is”. The end result is that gasifier charsmay range from reasonably appropriate biochars for addition to compatible soils to thinly disguised wood ashwith sparingly elevated portions of residual fixed carbon.An additional concern is gasifier chars often represent a diverse mixture of chars, resulting from the sheer size ofthe gasifier operation and the variability of bottom products exiting the process. Some portions are fully oxidized,some sparingly pyrolysed, and other bits may possess very attractive biochar characteristics. As such, individualsamples may not accurately characterize the actual distribution of chars exiting from the entire gasifier processand providing a consistent biochar product may prove problematic with gasifier chars.If gasifier chars are basically slow pyrolysis char that have been subjected to additional char gasification and theassociated effects of additional oxidation and high temperature, then “Fast Pyrolysis” chars are slow pyrolysischars that are formed too fast to allow the conventional slow pyrolysis reactions. The essence of Fast Pyrolysis isthat it happens fast enough that new characteristics are imparted in both the vapors and the residual char, characteristicsthat would not be present if the pyrolysis occurred at a slower rate.The goal in Fast Pyrolysis is to shift the destination of the fuel value, primarily associated with the carbon atomsin the biomass, from the residual char to the vapor phase, where it can subsequently be isolated by condensingthe vapor into a liquid known as “bio-oil”. If one examines Figure 1 and focuses on the yield curve, which tracksthe properties of the residual char, and infers the composite properties of the exiting vapors, one realizes that ifthe residual char carbon content goes up from the starting biomass, then the average carbon content of the exitingvapors must be less than the starting biomass by a corresponding amount.In Slow Pyrolysis, the solid gets the carbon atoms and the vapors get the higher portion of hydrogen and oxygen,much of it in the form of water vapor. The conventional condensate of the vapor phase of slow pyrolysis is calledpyroligneous acid or wood vinegar, and is a fairly well characterized liquid of minimal fuel value, but a historicsource of small oxidized organics such as methanol and acetic acid. The goal of Fast Pyrolysis is to modify thepyroligneous acid condensate into something new, called “bio-oil”.Since the goal in Fast Pyrolysis is to increase the carbon content of the bio-oil, then by conservation of mass andchemical species, there will be less carbon available for the residual fast pyrolysis char. In Fast Pyrolysis, the heattransfer rate is increased to an extent that the carbon atoms are swept into the vapor phase, then condensed tocapture them as bio-oil. The technique improves the relative percentage of carbon atoms removed from the solidphase, but cannot improve on the initial stoichiometry of the starting biomass – since it is not possible to leavebehind a more water-rich solid than the starting biomass.Most Fast Pyrolysis processes strive to direct as many carbon atoms into the vapor phase as possible and distinguishthemselves by creating fractions that isolate excess water in one condensate and other cuts with greaterpotential fuel value. To the extent this approach yields a bio-oil product of actual market value is up to the consumersof bio-oil to validate. However, the impact on the remaining biochar is predictable.Fast Pyrolysis char has incrementally lower portion of carbon than slow pyrolysis chars, since the objective wasto drive the carbon atoms into the vapor phase. The carbon atoms that remain in the solid phase have not been24U.S.-Focused Biochar Report:Assessment of Biochar’s Benefits for the United States of America