(567d) Upgrading Bio-Char Into Activated Carbon Using Thermochemical Process Streams: Flue Gas and Syngas

Fast pyrolysis involves the thermochemical conversion of biomass at 400C – 600C in the absence of oxygen to produce three main products:  liquid bio-oil, solid bio-char, and synthesis gas (syngas).  Most biofuel research focuses on upgrading the bio-oil further into renewable transportation fuels, while the co-products bio-char and syngas are considered less valuable and generally undesired.  Bio-char possesses many potential low value end-uses such as a soil amendment to improve soil quality, as a carbon sequestration method to reduce environmental greenhouse gases, and as a solid fuel for producing additional process heat.  Syngas comprising a large percentage of carbon monoxide could also be burned to supply process heat.  Using a heat recovery combustion unit to provide the heat required for the endothermic fast pyrolysis reaction results in flue gas combustion products, carbon dioxide and water vapor.  This project aims to upgrade thermochemical bio-char into higher value activated carbon using flue gas and syngas process streams as activating agents.

Activated carbon materials have many high value commercial uses including adsorbents for gas or liquid purification processes, catalyst supports for catalytic chemical processes, and advanced energy storage materials.  Water treatment and purification of potable water is a significant use for activated carbon materials.  The increasing demand for clean water in highly populated developing countries is expected to increase the global market demand for activated carbon materials for water purification.  Traditional feed stocks for activated carbon include coal, lignite, wood, and coconut shells, but these processes conventionally use slow pyrolysis to obtain larger yields of charcoal for activation.  Future fast pyrolysis biorefineries using herbaceous or woody biomass feed stocks will produce large quantities of bio-char that may need to be upgraded differently, depending on their composition and characteristics.

This study focuses on physical activation process and chose carbon dioxide (CO₂), carbon monoxide (CO), and steam (H₂O) as the activating agents to simulate syngas and flue gas process streams.  These activating agents would be very economical since they are inexpensive and readily available gas streams from the pyrolysis unit and heat recovery combustion process.  These syngas and flue gas atmospheres are investigated for upgrading the solid bio-char into higher value activated carbon.  Physical activation experiments were conducted using a fixed bed reactor and a rotary kiln reactor coupled with a gas flow system for metering the activating agent.  Mixtures of high purity N₂, CO₂, CO, and H₂O were used as model compounds to replicate syngas and flue gas streams.  This project investigated the influence of process parameters such as temperature, reaction time, flow rates, and gas composition on the response variables including burn off %, BET surface area and total pore volume.

The activated carbon materials produced have similar porosity and surface area comparable to conventional activated carbon materials and previously reported results.  Another value added co-product improves the economic potential of thermochemical conversion pathways for producing renewable transportation fuels and purification materials for water treatment.  These materials can be produced economically using a synergistic combination of thermochemical, physical activation, and heat recovery processes.