Charcoal Gasifier No 1

Charcoal Gasifier No 1.
Doug Williams, Fluidyne, July 21, 2007

Hi Gasification Colleagues,

Back in June, I promised to give you some basic lessons on how to make a charcoal gasifier, and to explain some terminology. This is for those who understand nothing about gasification in a practical sense, so should kick start those with less ability or resources. It does not cover gasifiers that use raw biomass.

Before you start this project, understand one thing clearly.


TERMINOLOGY. It is not necessary to invent new words, but better to understand the meaning of words already accepted to accurately describe the process, or phenomena that we are creating.

OXIDATION. This is the zone of fire that creates the exothermic heat, to
power the gas making process. It does not form a uniform layer in gasifiers that have one air nozzle. Charcoal gasifiers function best with an air nozzle.

AIR NOZZLE. The air is admitted into the gasifier through an air nozzle, and is the best option for charcoal gasifiers. The bore size determines two things.The velocity of the air to create the temperature, and enough air at this temperature to make the quantity of gas you need. It can be a simple pipe, or be a pipe within a pipe to form a water jacket to make steam for addition to the incoming air to make hydrogen.

STEAM JACKET. This is an addition to the air nozzle to make steam, not boil the water. A water tank is usually welded to the upper outer case to preheat the water, and fed to the steam jacket, via a drip controlled by a simple tap valve.

OXIDATION LOBE. This is the area in front of the air nozzle where the oxygen in the air is consumed. It has a thin pear shape depending on the air entry velocity. The resulting high temperature of this lobe will melt any steel that it touches. Think of it as a thermal lance. Char is consumed to produce this heat, and you will get ash formation on the gravitational side or bottom of the lobe. Around the boundary of the lobe, only carbon dioxide
(CO2) is present, and the point at which reduction to CO begins.

REDUCTION. At the boundary of the oxidation lobe, the CO2 gas is so hot it should glow, and as it passes through the char around the lobe, it gives up this heat to the char making the carbon reactive to the CO2. This reaction continues into the surrounding char to a point where the temperature drops to below about 850C, because reduction is a heat consuming process (Endothermic). This is pear shaped around the oxidation lobe, and forms the reduction zone. The reduction char gives up molecules of carbon to the CO2, to become CO, the gas that will combust for our need.

HYDROGEN. A charcoal gasifier makes only CO if the charcoal is correctly prepared. Hydrogen will only form in these types of gasifiers if you add steam to the air inlet in some proportion, which is best set for each individuals situation and application. Moisture in wet charcoal will only be driven off as steam to form a abnormal condensate problem. Charcoal gasifiers do not have condensate to drain, as these problems are removed by the charcoal making process.

CARBON MONOXIDE. Very poisonous to humans, but will create many symptoms resulting in illness without killing you. Although we say that CO stops forming at 850C, it continues to be made if any air leakage enters the gasifier on shut-down, so at the molecular level, a tiny spark will have the capability to create a micro amount of CO. Always have a free air flow around the gasifier to prevent any accumulation of CO in the surrounding area.

INSULATION. Traditionally charcoal gasifiers were not insulated on the outsides, because the oxidation lobe is buried in char, and reduction consumes the higher temperature, so the remaining char becomes an insulator to the inside walls. Heat is only transferred to the char via the gas pathway towards the gas outlet, which will form a straight line from the air nozzle to the outlet. Within a distance of 2"away from the gas flow, the charcoal become a heat insulating barrier. The outer case does become hotter as the char is consumed exposing the bare internal walls. Outer insulation, or mesh grills, may be required as a protection from burns.

CHARCOAL. All charcoal is not equal in quality, and species should be taken into consideration for the best sizing option. Hard wood make the best charcoal, and need a size of about 1" nuggets, and soft woods, about 1.5".
Charcoals need to be sieved to size, and fines discarded. Very small gasifiers can utilise finer charcoal.

I have covered most of the terms that will be used in the discussion of these charcoal gasifiers, but may find the need to add others as we go. My next Posting No 2.will be discussing the options of how to build the gasifier, but please do not ask me questions until you have the whole story to complete the picture.

Doug Williams,
Fluidyne Gasification.