Experiments and discussions in the first half of 2023, regarding small-scale biochar reactors have made me realize that the pyrolysis of biomass and evaporation of biomass residual moisture are endothermic, but few practical measures of energy input requirements have been made. The concept of burning fossil fuel to provide energy for pyrolysis is ludicrous in the context of CDR, and the concept of burning biomass for fuel to provide energy, while perhaps practical for relatively primitive efforts, is problematic in the context of CDR for two reasons: it is inefficient - some of the carbon in the biomass is consumed in combustion and released as CO2, and it is wasteful of a potentially valuable resource - the pyrolysis gas has potential value as a replacement chemical feedstock as fossil fuel uses are deprecated worldwide. A solar-heated pyrolysis device would be useful, but what energy input is required? An electric-fired biochar reactor can provide data for the design of such a unit.
I sought to measure heat transfer to biochar reactors with external heating, and have realized that accurately measuring all the elements of heat transfer involved is an effort that exceeds my DIY capabilities. However, it may be possible to accurately determine heat input by placing the heat source inside the biochar reactor. An electric heating element inside a well-insulated reactor can be monitored simply - by observing the reactor temperature with a thermocouple, and by recording the electrical power consumption with an ammeter.
Reactor D Design
The reactor vessel will be a one-gallon paint can. The can will be insulated by coating it first with a layer of clay-plaster mix, then with additional plaster-sand mix. The clay-plaster layer will give rise to air voids if the plaster is degraded by heat. The total thickness of the insulating layer will be greater than 3cm.
The reactor vessel will have a lid that is removable, that forms a tight seal when closed, that is well insulated, and that has a vent for the escape of pyrolysis gas. A paint can lid can make a good seal to the paint can, but it requires some pressure to seal, and then some potential deformation to pry open. This is not compatible with clay and plaster insulation. Therefore, I anticipate forming a gasket in the seal-canal of the paint can, made from silicone rubber which is stable to about 300C but may degrade at higher pyrolysis temperatures. Alternative seal materials may be necessary. The paint can lid will be covered by a plaster cap similar to the reactor walls and base.
The reactor heater will be a terracotta rim of a flower-pot wrapped with ni-chrome resistance wire and connected to 120 volt AC power. The power will be regulated by a relay switch.
The temperature sensor will be a Type-K thermocouple mounted through the wall of the reactor vessel approximately 5cm below the top of the reactor
The controller will be an Arduino Uno PLC. It will monitor the output of the thermocouple and control the relay switch that provides power to the reactor heater. The controller will have three modes of control. At temperatures well below a target value, the relay will be engaged a high fraction of the time, possibly 100%. At temperatures near the target value, the relay will be engaged a lower fraction of the time but sufficient to cause continued temperature increase. At temperatures above the target value, the relay will be engaged infrequently to maintain the target temperature.
Components of Reactor D, clockwise from top left:
SSD1306 0.96 inch OLED display
Arduino Uno
modified flower pot, 14cm outside diameter, wrapped with nichrome wire R = 29.9 ohms
relay module - this module has ground and 5VDC terminal which proved to be active; it will power the arduino via the 5V bus. This is NOT a recommended way to power an Arduino, so this terminal should not be connected.
MAX6675 thermocouple and amplifier - the amplifer failed prior to any experiments.
The modified flower pot fits into the one-gallon paint can (I checked!). Biomass will be placed into a metal food can in the center of the modified flower pot so it does not touch the nichrome wires - the remnant base of the flower pot, not shown may be used to insure that the can and the nichrome wires remain isloated.