(474a) Utilizing Thermochemical Process Streams to Upgrade Bio-Char Into Value Added Activated Carbon Through Physical Activation

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).  Current 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 syngas and flue gas 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.  Future 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.

Chemical and physical activation methods both introduce an activating agent to the carbon precursor and typically perform the reaction at very high temperatures in the range of 750°C – 900°C.  This study focuses on the 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.  The syngas from fast pyrolysis could be used directly as the activating agent or burned to produce flue gas components and process heat, both necessary for the activation reaction.  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 (FBR) and a plug flow reactor (PFR) 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 simulate syngas and flue gas streams.  This project investigated the influence of process parameters such as temperature, reaction time, flow rates, and gas composition to produce highly porous activated carbons.  The response variables include burn off %, BET surface area, total pore volume, and measurements of methylene blue adsorption capacity and iodine number.

The burn off % is calculated by measuring the bio-char weight before and after activation.  The surface area and total pore volume were measured using a Micromeritics TriStar 3000 automated gas adsorption analyzer.  Samples were first degassed using a Micromeritics FlowPrep 060 unit at 200°C for a minimum of 5 hours to ensure any moisture was removed prior to nitrogen isotherm analyses.  Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) model for the nitrogen isotherm.  Methylene blue adsorption capacity was measured following the American Standard Test Method (ASTM D3860) – Determination of Adsorptive Capacity of Activated Carbon by Aqueous Phase Isotherm Technique.  The iodine number was measured following ASTM D4607 – Determination of Iodine Number of Activated Carbon.

The physical activation experiments exposed dry bio-char to gas mixtures for 20, 40, 60, and 80 minute durations and temperature’s influence was studied from 800°C, 850°C, and 900°C.  An intermediate temperature of 850°C generally produced better activated carbons with higher surface area.  The activated carbon reached a maximum surface area at 50% burn off after 40 minutes.  After this time, it is believed that pore destruction becomes more prevalent during later stages of the reaction than the pore creation typically found in the beginning stages.

The activating agent’s total flow was varied from 1.3 liters per minute (Lpm) to 0.7 Lpm and 0.25 Lpm, and its influence on the activation was studied.  It was found that a lower gas flow rate produced activated carbons with higher surface areas.  Linares-Solano et al. discussed similar results in their paper, Porosity Development during CO₂ and Steam Activation in a Fluidized Bed Reactor.  They explained that higher flow rates provided a very reactive environment for the carbon and resulted with underdeveloped porosity from external surface burning.

The amount of CO₂ in the gas mixture was diluted with inert N₂ from 95% to 60% and 30% (volume %) while the total flow was held constant.  Linares-Solano et al. explains the key to well-developed porosity is linked to the CO₂ gasification kinetics.  During chemical reaction controlled kinetics, the reaction is limited by CO intermediates occupying reactive sites and the reaction occurs uniformly on the carbon (bio-char) particles.  During diffusion controlled kinetics, the reaction readily reacts on the particle surface and results in an unreacted core with poorly developed porosity.  Results from experiments with varying amounts of CO₂ are still under review.

The influence of syngas composition with varying ratios of CO₂:CO is studied to determine the optimum and tolerable amount of CO in the activating agent.  The syngas process stream possesses a large fraction of CO, so it may prove to be an important parameter since Linares-Solano et al. demonstrated that CO intermediates help facilitate the porosity development.  Results from these experiments are currently under review.

The activated carbon materials produced have similar pore characteristics, surface area, methylene blue adsorption capacity, and iodine number that are 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 products can be produced more economically using a synergistic combination of thermochemical, physical activation, and heat recovery processes.