Construction of a model for the design of a rotary kiln for slow pyrolysis of biomass – GreenCarbon project

Abstract

Biomass pyrolysis is one of the most promising renewable energy technologies. This research aims to create a model which automatically designs a biomass slow pyrolysis reactor from the user inputs and provides mechanical specifications, kinetic properties and process parameters. From literature, the rotary kiln was identified as the most promising slow pyrolysis technology. This research divides the complex reactor design into three main parts, related to the transformation of the feedstock into the product: bed of solids, heat transfer and kinetics. The three aspects were integrated, and the reactor divided into physically identical slices for more straightforward implementation and to overcome model divergences. - The bed of solids defines and describes the behaviour of the solids inside the reactor mathematically. The length of the reactor must be calculated, and the contact areas to transfer heat from the wall (heat source). - Heat transfer is critical within pyrolysis. The model must ensure that all the particles reach the target temperature. The heat is indirectly transmitted to the solids and gases through conduction, convection and radiation. - Pyrolysis kinetics are challenging due to the complex mixture of substances in the feedstock and products. Isoconversional methods are used to estimate the value of the pre-exponential factor and activation energy for the conversion of each feedstock into the products. Additionally, the reaction orders are optimised through an innovative method that compares the error between different values. The result is a thorough methodology and comprehensive model for the design of a rotary kiln. This methodology included assumptions to achieve a compromise between accuracy, computational time and power. The finished model allows users to investigate custom reactor designs, as they can specify key variables such as feedstock and scale. It is a very powerful tool that can be used in industry or by potential investors to obtain a first approximation of the dimensions of a pyrolysis reactor. For research, it provides a robust methodology for the design of slow pyrolysis rotary kilns. It is acknowledged that the reactor model is not perfect, and some parts could be improved, but it provides a valuable step forward in the development of slow pyrolysis reactor design.