In the discussion of gasification, little is said about the waste that results from biomass, and therefore little appreciation of the fact, that some Authorities may consider them to be toxic.

The amount and type of waste is dependent on the nature of the biomass, so it is important to establish for yourself, if it's disposal will constitute a threat to humans and the environment. It is not possible to discuss the many variations that could exist, but most of this information could be used as a guide for most systems.

There are usually three sources of waste, course charcoal and ash from the bottom of the gasifier, fine soot (carbon) and ash from the cyclones and filters, and aqueous condensate as a result of gas cooling.

Course charcoal removed during the operation of a down draft gasifier, has had the incandescent CO2 produced in the oxidation zone, pass through it in the reduction phase, and the charcoal becomes low grade activated carbon. It is important to remember, that the degree of activation will be related to the temperature of the gas, and how it flows through the bottom char, as it can be quite uneven in some designs. If it is of a size that can be utilised, it should first be sieved to remove the ash and fines.

Dust from the cyclones and filters must be handled with caution. Cyclone dust is very much like talcum powder (about 10 micron) and filter dust cake can be even finer. If removing manually, the dust can easily become entrained in disturbed air, so a spray of water, or adding water to make a slurry makes it safer to handle. This dust is very good to add to soils or compost heaps, but location determines it's ultimate disposal.

Aqueous condensate from a correctly working gasifier, consists of water with suspended carbon solids, dissolved ash, and usually has a pH 8.2, or close to that figure. If the gasifier is malfunctioning, or incorrect fuels are trying to be gasified, the condensate becomes a toxic black liquor, and as such is a hazardous waste. It is very unpleasant to handle, and it's sticky tar scum will create a servicing problem within the cooler/condensers.

When the condensate is just dirty water, the course activated charcoal can be used to filter the solids and suspended colour out of the condensate before disposal down drainage systems. A simple test by spraying on a small patch of grass or weeds, can quickly show if it is detrimental to plant life, and if it is, then alternative disposal has to be considered.

The following photographs were taken during gasification fuel trials in June 2005.

Course activated charcoal as removed from the gasifier.
This is the same course charcoal to show size, but if wood chips, or other fine fuel is gasified, the charcoal will be smaller. This charcoal is from Medium Density Fibre Board. (MDF).
This is the dust typically found in cyclones. Note flocculated soot formations on the sides of the container, caused by reversion of the gas to soot and CO2 at temperatures over 500C. Also note small craters where larger particles of char have impacted into the finer surface dust.
IMG_1261.jpg Cyclone dust shows a mixture of particle sizes when disturbed.
Aqueous condensate, the bottle on the left is as drained but been left to stand over night, and the bottle on the right has been filtered through the coarse activated charcoal.
These condensate samples show the condensate straight out of the gasifier on the left, and how the same sample looked after settling over night.
The condensate puddle on the white surface, is from the condensate sample allowed to settle over night. The colour is just perceivable.