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Design and characterisation of an 'open source' pyrolyser for biochar production : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Chemical and Bioprocess Engineering at Massey University, Palmerston North, New Zealand
An 'open source' field-scale batch pyrolyser was designed and constructed to produce
biochar, which is the solid residue formed when biomass thermally decomposes in the
absence of oxygen. The design approach was focused on simplicity for the intended target
user, a hobby farmer. This is achieved in a batch process, where temperature ramp rates,
gas flows and the end-point are controlled. Solids handling is only required at either end of
the process. LPG is used as the initial heating source and later as the ignition source when
pyrolysis gases are recycled.
A mathematical model formulation of the process was developed to predict the proportions
of products produced as well as the time taken to achieve complete pyrolysis. Reaction
kinetics are complex and not fully understood. In this model, simplifications were taken to
provide guidelines for the reactor design as well as the effects of moisture on the process
The quality performance of the 'open source' pyrolyser was determined by comparing its
biochar to that produced in a lab scale gas fired drum pyrolyser. Parameters varied on the
lab drum pyrolyser were highest treatment temperature in the range 300 to 700 °C, sample
size, moisture content and grain direction for Pinus radiata. The properties that were
investigated are elemental composition (C, H, N, S), proximate analysis (moisture, volatile
matter and fixed carbon) and char yield (% wt/wt). The ash content was determined by
residue on ignition. For the lab scale experiments, it was found with increasing peak
temperature that yield, volatile matter and hydrogen to carbon ratio decrease. Yield was
unaffected by moisture, size and grain direction.
The design of the pilot reactor followed the principle observed with particle size that, in order
to get maximum residence time of the vapour and tar in the reactor, the reactor was
designed with a perforated core so that the vapours have a tortuous path of travel. This
design also meant that heat and mass transfer occurred in the same direction, from the
outer wall to the perforated core. In comparison to the lab scale pyrolyser, the same trends
were observed in regards to temperature. High yields of 29.7 wt % and 28.8 wt % were
obtained from wood with an initial moisture content of 21.9 wt % and 60.4 wt % respectively,
confirming yield is unaffected by moisture.
Mass and energy balances were conducted on both the lab scale and pilot scale pyrolysers.
For every kilogram of carbon in LPG used on the lab scale pyrolyser, an average of 0.25
kilograms of carbon is produced at 700 °C. Based on the optimum run for the pilot scale, for
every kilogram of carbon in LPG used, 2.6 kilograms of carbon is produced at 700 °C.