What is microwave-assisted pyrolysis?
Pyrolysis is a process of chemically decomposing organic materials at elevated temperatures in the absence of oxygen. The process typically occurs at temperatures above 430 °C (800 °F) and under pressure. It simultaneously involves the change of physical phase and chemical composition and is an irreversible process.
Pyrolysis is commonly used to convert organic materials into a solid residue containing ash and carbon, small quantities of liquid and gases.
Types of Pyrolysis Reactions
There are three types of pyrolytic reactions differentiated by the processing time and temperature of the biomass.
Slow pyrolysis is characterized by lengthy solids and gas residence times, low temperatures and slow biomass heating rates.
Flash pyrolysis occurs at rapid heating rates and moderate temperatures between 400 and 600 °C.
This process is primarily used to produce bio-oil and gas. During the process, biomass is rapidly heated to temperatures of 650 to 1000 °C depending on the desired amount of bio-oil or gas products.
Microwave‐assisted pyrolysis (MAP) is a new thermochemical process that converts biomass to bio‐oil. Compared with the conventional electrical heating pyrolysis, MAP is more rapid, efficient, selective, controllable, and flexible.
For conventional electrical heating methods, heat is transferred from high‐temperature gas to the fuel particle surface through convection mechanism and it is then further transferred from the outside surface to the inside core through conduction mechanism. A temperature gradient from outside to inside of the feedstock particle is formed because of the poor thermal conductivity of the feedstock material, and the released volatile diffuses from the inside core to the outside surface through a higher temperature region.
For the microwave heating method, microwave penetrates the feedstock particle and microwave energy is transformed into heat inside the particle. Because of the heat loss effect of particle surface, heat constantly accumulates inside the feedstock and is transferred outwards. A temperature gradient from inside to outside of feedstock particle is formed also because of the poor thermal conductivity of the feedstock material, and the released volatile diffuses from the inside core to the outside surface through a lower temperature region.
Advantages of MicrowaveAssisted Pyrolysis
The Microwave Assisted Pyrolysis is, compared to incineration, easily controllable. Incineration is complicated and expensive to control, and will usually create harmful or toxic components that have to be removed from the flue gas.
The pyrolysis is a process with no oxygen present. Consequently oxides cannot be formed. For the same reason dioxins cannot occur, as the formation of dioxins is dependent of the presence of oxygen.
The process is completely enclosed, and all products are collected and duly treated without any emissions to the environment, called dry distillation process. As no oxygen is added to the process, the produced gas will be a concentrated fuel gas with high calorific value.
The feedstock is brought into the pyrolysis reactor through air locks purged with inert gas to prevent oxygen to enter the reactor. It is then heated by means of microwaves to a temperature where the bonds between the solids and the volatiles in the material are broken. The volatile fraction consists of a vapor that is separated into gases and fluids by condensation.