Autothermal Process Intensification: Overcoming Heat Transfer Bottlenecks in Non-Equilibrium Chemical Processes

Heat transfer is the bottleneck in many chemical processes, limiting throughput for a reactor even when chemical reaction and mass transfer are very fast. Heat transfer can be enhanced through a number of strategies, often adding significantly to the capital cost of a project. However, as long as heat transfer is rate limiting, reactor capacity only scales as the square of reactor diameter compared to the cube of reactor diameter for volumetric chemical rate-limited reactors. This not only constrains throughput as reactors are built larger, but substantially eliminates the economic advantage of building them larger.

The heat transfer bottleneck can be overcome through autothermal chemical processing, which balances the energy demand of endothermic chemical reactions with energy supply from exothermic chemical reactions within an adiabatic chemical reactor. Although autothermal chemical reactors have been previously demonstrated for equilibrium chemical processes (prominent examples include methane steam reformers and biomass/coal gasifiers), we are developing it for inherently non-equilibrium processes. Identifying complementary endothermic and exothermic reactions that can occur simultaneously while preserving the same products of an externally heated (or cooled) chemical process is not trivial. However, the gains in process intensification would be substantial, increasing throughput and reducing capital and operating costs of a chemical process.

We have applied the principle of non-equilibrium autothermal chemical processing to the pyrolysis of biomass. Although the enthalpy for pyrolysis is relatively small (comparable to the enthalpy of evaporation for methanol) and operates at 500°C, throughput for a pyrolyzer is constrained by the rate that thermal energy can be transported into the reactor. This energy is conventionally supplied from hot flue gas conveyed through heat exchanger tubes or hot granular media like sand or steel shot mixed directly with the pyrolyzing biomass. Although these heat transfer systems add tremendous complexity to the pyrolyzers, they can sustain only modest thermal fluxes in practical pyrolysis systems, limiting the commercial prospects for pyrolysis energy systems.

Autothermal pyrolysis is achieved by admitting a small amount of oxygen to a fluidized bed reactor, which partially oxidizes biomass reactant or pyrolysis products to provide the enthalpy of pyrolysis. Experiments performed in a 15.4 cm dia. fluidized bed reactor capable of operating as either a conventional or autothermal pyrolyzer found no significant loss in bio-oil yield or quality for these two modes of operation at comparable biomass throughput. More importantly, by removing the heat transfer bottleneck of conventional pyrolysis, operation in autothermal mode allowed biomass throughput to be dramatically increased, reaching five times the capacity of the conventionally operated pyrolyzer. Our analysis indicates oxidation of non-condensable gas species released during pyrolysis provided about half the energy for pyrolysis while the remainder of the energy is thought to come from partial oxidation of lignin in the biomass and char in the pyrolysis products.