Pilot scale pyrolyser : compliance and mechanistic modeling : a thesis presented in partial fulfillment of the requirement for the degree of Master of Engineering in Chemical and Process Engineering at Massey University, Palmerston North, New Zealand
"A pyrolysis reactor was built in a previous project by Bridges et al (2013).The reactor is cylindrical in geometry, with a height of 1000 mm and an internal diameter of 750 mm, it stands vertically. There is a 900 mm tall and 100 mm in diameter perforated core in the center of the reactor. At the base, a combustion chamber provides the hot gases required for heating. The hot gases produced travel up and around the reactor through an annulus region of 11 mm. Heat from the gases is transferred to the reactor wall and then to the wood-chips inside. As drying and pyrolysis reactions occur, gases flow in the same direction as the heat towards the perforated core at the center. Hot pyrolysis gases then flow downwards towards the combustion chamber where they are partially combusted before flowing around the reactor and out the flue stack. This project aimed at mathematically modeling this reactor and also improving the way emissions are released so that it complies with EPA air quality standards.
A mathematical model of an ‘open source’ pilot-scale pyrolysis reactor was produced to predict the product yield, carbon foot-print, biochar quality and the time taken to achieve complete pyrolysis. A non-equilibrium thermodynamic approach was used which allowed for the use of COMSOL Multi-Physics to solve the model. The Finite Element Method (FEM) was used to solve the system of equations. Pyrolysis kinetics are complex and no single model has yet been widely accepted, therefore simplifications were necessary in this model so that a reasonable solution time could be achieved while producing acceptable results. The model profile of the centre temperature closely followed that of the experimental results and thus the model was considered valid.
In addition, modifications were made to the original design of the pyrolyser in order to improve emissions compliance and improve operations of the pyrolysis. It was important to manage fugitive emissions and completely combust any volatile vapours that would be released into the atmosphere while controlling the operating parameters. In order to achieve this, the following were implemented:
1) The combustion chamber was sealed completely so that no fugitive emissions can escape while limiting the ingress of oxygen.
2) A secondary blower was installed in order to better control the oxygen supply to the burners.
3) The original steel lid, which warped during pyrolysis runs resulting in gaseous leaks, was replaced with a more rigid ceramic lid that doesn’t effectively expand when heated.
4) Two 3.4 kW burners were added to the single 3.4 kW burner flare. This gives a total power of 10.2 kW, which is estimated to be enough to completely burn all gaseous products leaving the system"--Preface