(30g) A Partial-Combustion Pyrolysis Model for an Energy + Biochar Reactor Design

The simultaneous production of biochar and thermal energy through slow pyrolysis is a promising process for efficient biomass utilization.  Partial combustion allows the reactor to be energy self-sufficient. The addition of oxygen (air) to the pyrolyzer, however, alters the reactor conditions, thus affecting the pyrolysis reaction kinetics and, ultimately, the biochar properties. The goals of this study are to develop a partial combustion reaction model to allow the design of a continuous, energy self-sufficient slow pyrolysis system that produces appreciable amounts of biochar for soil application, and thermal and electrical energy to operate a water desalination unit. This water desalination unit will use multiple effect distillation (MED), a thermal desalination method, to treat brackish groundwater. The overall system will be used in rural to convert agricultural residues to water suitable for irrigation or human consumption, and biochar to improve agricultural soil fertility and water holding capacity.

An auger reactor will be used for the pyrolysis and partial combustion of biomass. Often, such reactors are characterized by (a) biomass residence times that may be two to three orders of magnitude longer than the residence times of gas, and (b) very high solid to gas mass ratios. These conditions allow for significant simplification of the equations governing the transient operation of the reactor. A network of parallel and sequential reactions was used to describe the kinetics of biomass pyrolysis.  Since the feed gas to the reactor will also contain some oxygen, partial combustion of the produced char and volatiles was also considered to provide the energy needed for the desalination process. The kinetic constants of all reactions were estimated using thermogravimetry and differential scanning calorimetry. Assuming plug flow for the solid and gas phases, transient mass balances were developed for all the gaseous and solid species, using one-dimensional partial differential equations for the solids and two-dimensional equations with convection and dispersion terms for the gases.  Finally, two-dimensional PDEs were used to describe the transient energy balances for the solid and gas phases, with heat exchange through the reactor wall.  The system of coupled equations describing the operation of the reactor was solved using finite differences to determine the key parameters for optimal operation of our system: biomass feed rate and residence time, maximum pyrolysis temperature and fraction of air in the gas feed. A parametric study was also carried out to determine how feedstock properties (composition, particle size, moisture content etc.) affect the net energy ratio of our reactor, as well as the amount and the properties of produced biochar.  The net energy ratio of the reactor was defined as: (Enthalpy of exit gases + Energy content of produced biochar) /  (Energy content of biomass + Energy used to heat the reactor).