Gasifier Flow vs. Reaction Area

[ben]

Another topic we covered at the workshop is the balance of air/gas flow and reaction area in a gasifier. This is one of the key points to making a consistent quality and volume of wood gas and it's rarely talked about.

Flow is the amount of gas that can pass through your gasifier.

Reaction area is amount of surface area of the char.

Fatal Example #1: You use very large chunks of wood in a small gasifier. The gasifier isn't able to break them down rapidly enough to shrink them by the time they get deep into the combustion zone. What happens is that there are very large spaces between the chunks. This means there is almost no restriction on flow, but there is also very little reaction area to decompose tars and to aid in the endothermic reactions that make hydrogen and burnable carbon gases. In fact, there are large cavities where unrefined gases pass and cause engine buildup.

Fatal Example #2: You have very small chips of wood in your gasifier. The gasifier breaks them down so rapidly that they become tiny, some even dust, and they plug up the reduction zone. You have an incredible amount of reaction area, but you don't have adequate flow to ensure enough gas output and high enough combustion temperatures.

The key to being successful is in having a good balance of flow and reaction area. If fuel is too large, consider making it smaller. If fines are plugging the reduction consider a change in grate that will pass the char to the ash bin to keep steady gas flow.

With proper understanding of fuel quality, hearth dimensioning and "flow vs. reaction" area you are 90% of the way to making high quality gas. The last 10% is just taking the time to operate a gasifier get the hours under your belt. Happy gasifying!

[mike hartsgrove]

Thank you again ...Ben ,i really wish i could attend one of your workshops...