Combustion of Pyrolysis Char with a Fluidized Bed Reactor for Generating Process Heat

Portable biomass fast pyrolysis processes require combustion of byproducts to provide heat to the reactor and to dry feedstocks. Heat integration and byproduct utilization are especially important for small-scale, distributed processes where heat losses become more problematic. Pyrolysis char produced with a 1 ton per day (tpd) pilot reactor from pine woodchips was combusted with a pilot-scale fluidized bed reactor (FBR) to demonstrate the feasibility of process heat generation. The char product was characterized prior to combustion to determine carbon content, moisture, ash content, and heating value. Particle size distributions (PSD) of the char feed and collected ash material were measured. The fluidizing gas flow rate, gas inlet temperature, excess air percentage, and reactor bed temperature were varied to determine the effects on emissions and combustion efficiency. Gaseous emissions were continuously monitored for a range of fluidizing gas velocities and reactor bed temperatures. The effects of excess air on gaseous emissions were determined along with the inlet temperature of the fluidizing gas. The effects of reactor conditions on CO, SO₂, and NO_x were assessed using the FBR with a variable feed rate of 0.3 to 0.4 kg/h. Based on the experimental results, increasing the excess air and reactor bed temperature reduced CO emissions. Increasing the fluidizing gas flow rate decreases the residence time resulting in increased CO. Particulate emissions were determined after each run by measuring the ash blowout from the cyclone separator. Total fine particulates were collected within a baghouse filter after 16 hours of operation. Based on an energy balance over the entire system, the thermal efficiency and heat output were also determined. Reactor performance data using the pyrolysis char fuel will be used to scale-up the FBR design to a 7 kg/h system, right-sized for a nominally 1-tpd distributed pyrolysis process.