(296i) Effects of Light Gas Addition on Biomass Pyrolysis in Bubbling Fluidized Beds

In order to reduce the use of environmentally harmful fossil fuels, it is necessary that new, economically viable methods of producing renewable fuels be developed. One method which shows promise for producing affordable renewable fuels is the fast pyrolysis of biomass. A popular version of this process uses a fluidized bed of heated sand. Biomass is fed into the bed, it quickly pyrolyzes, and converts to bio-oil vapors, gases and char. As the bio-oil vapors remain in the reactor, they continue to undergo secondary reactions, converting to less useful products. It is therefore necessary for reactors to be designed for optimal residence times of both biomass and oil vapors. One method for accomplishing this balance is by controlling the flowrate and properties of the fluidizing gas, affecting both the mixing in the fluidized bed as well as the elutriation of pyrolysis products from the reactor. While nitrogen and steam are most often used as the fluidizing gas, the addition of light gasses such as hydrogen into the fluidizing gas can provide additional degrees of freedom for altering the hydrodynamics of the bed.

To examine the effects of incorporating light gasses into the fluidizing gas stream, low and high order computational methods were used to model pyrolysis of woody biomass particles in bubbling fluidized bed reactors. Simulations were performed of two separate designs, a 1” and 2” diameter cylindrical bench-scale reactor. For both designs, the numerical models were validated using experimental data at identical conditions. High order simulations were performed using the open source MFiX CFD software package developed and maintained by the National Energy Technology Laboratory (NETL). The MFiX Two-Fluid model was used for all simulations. This model approximates the solids phase as a continuum, instead of simulating the motion of individual particles. This significantly reduces the computational effort. A CSTR-in-series model was used for the low order modeling of the reactor. The number of reactors used to model the system were calculated from the mean and standard deviation of the reactor’s residence time distribution (RTD) curves, obtained from the high-order MFiX simulations.

Using these methods, the effect of introducing hydrogen into the nitrogen fluidizing stream was investigated. The mass fraction of hydrogen in the fluidizing gas was varied between 0% and 100% to examine how variation in the gas properties influences the product yields at the outlet of the reactor. For each gas composition studied, two flow conditions were considered. The first utilized a constant mass flow rate at the inlet, while the second increased the flow rate as H2 increased in order to maintain a constant ratio of fluid velocity to Umf. This ratio strongly affects the number and size of bubbles produced in the fluidized bed, ensuring the bed maintains similar characteristics across the range of gas compositions. On the other hand, a constant flow rate produces different behaviors in the fluidized bed. Results from this investigation provide useful insights into how the addition of light gasses influence the hydrodynamics and yield of pyrolysis products in bubbling fluidized bed pyrolyzers.