reactor morphology
the overall heat transfer coefficient encompasses many aspects of morphology, so it is important that all experiments are conducted with the same equipment setup, otherwise measurement differences can be due as much to difference in setup as to difference in experimental conditions of test material. For these experiments so far, the morphology has been a 1-gallon paint can modified with a ¼-inch copper vent tube in the can lid, the lid firmly seated so it does not leak gas. The heat source has been propane burning on a particular burner, with two sheet-metal wind shields - one around the burner base so the flame is uniform, and another on one side of the paint can and higher than the can, so the heat convection from the burner is relatively uniform around the can. Others may devise different morphologies, but whatever is devised must be replicated every time.
measure heat transfer
the heating response of water is well known. So by timing the boiling of water from a known initial temperature, a precise measure of absorbed heat can be calculated.
heat transfer by conduction, such as through a metal can bottom and sides, is dependent on the temperature difference between the heat source and the can contents. The temperature differential during heating water is different than during pyrolysis, but an accurate heat transfer coefficient accounts for this difference. The heat added during a pyrolysis experiment can be calculated from the measured temperature of the can.
However, there are additional barriers to heat transfer during pyrolysis that should be considered. While water has a high heat conductivity and heat capacity, and can circulate freely inside the reactor, during pyrolysis, heat transfer mechanisms can be completely different: the gases in the reactor have relatively low heat capacity and low thermal conductivity. The disposition of the biomass in the reactor can affect convection of these gases in unpredictable and variable ways. Therefore, heat transfer between the flame and the can is only one “stage” of the overall heat transfer.
experimental yield
Accurate measurement of initial and final material mass is necessary. cellulosic materials are approximately 44% carbon by dry weight, but some carbon is lost in pyrolysis gas, and the experimental feedstock material is generally not completely dry. Evaporating water requires considerable energy. Where possible, experiments should be conducted with dry material. Methods of drying and records of weight loss during drying are beneficial data.
How to assess pyrolysis completion? Cardboard pyrolysis is not complete when the product material retains structural strength (pieces of product remain intact and flexible, despite being blackened).
product biochar - this is valuable material for further experiments: do not discard it. We need to develop protocols for experimenting with agricultural use of biochar. I will add a procedure for characterizing soil (jar testing), and mixing varying quantities of biochar to reference soils and growing plants - to assess plant health and soil moisture retention.