Effects of Pressure and Particle Size on the Carbonization of a Packed Bed of Biomass

Abstract

A relatively new technique for charcoal production has been developed called Flash CarbonizationTM and has proven to be efficient and most importantly fast. The technique begins with a packed bed of biomass pressurized at 1-2 MPa. A fire is ignited at the bottom of the bed while air is introduced through the top. The flame travels up the bed converting the biomass to bio carbon otherwise known as charcoal. One of the most important metrics a charcoal has is its Fixed Carbon Yield (yfc) or the percentage of carbon left in it after it is carbonized. Many factors affect the yfc of charcoal three in particular were investigated in this thesis; feedstock, pressure, and particle size. Each feedstock that was used in the FC process and analyzed in this thesis was sent out to a lab to discover its elemental composition. Using this data calculations were performed to discover the highest theoretical yfc each feedstock could produce. A yfc between 33 and 36% was discovered which is not surprising since all feedstocks were various wood types. In experimentation it was shown that each of these feedstocks had similar yfc for identical operational conditions. The pressure at which an experiment was performed had a much more significant affect on yfc. Low pressure resulted in low yfc and as pressure increased so did yfc. Experimentation was limited to 2.17 MPa and was not able to see if this trend continued indefinitely. Increasing pressure beyond certain thresholds can also be very dangerous and have caused runaway reactions causing pressures and temperatures to spike in a matter of seconds. Particle size also has a great affect on yfc very similar to pressure, as particle size increases so does yfc. The one difference is that once a feedstock is larger than sawdust's the yfc levels off. This can prove beneficial since most material will need little preparation work in order to achieve the highest yfc.