So what is a biomass gasifier? Basically is a chemical reactor that converts wood, or other biomass substances, into a combustible gas that can be burned for heating, cooking, or for running an internal combustion engine. Gasifiers are an old, but generally overlooked alternative energy technology. Few people these days realize that gasifiers were used extensively by both sides during WWll to power cars, trucks and buses during fuel shortages. Gasifier technology rapidly evolved and matured during the war.
Gasification is achieved by partially combusting the biomass in the reactor, and using the heat generated to pyrolyse or thermally break down the rest of the material into volatile gasses. A well built gasifier will convert wood or other cellulosic biomass into the flammable gases Carbon Monoxide and Hydrogen.
My goals here were to build a gasifier using easy to obtain materials, that would run on readily available fuels, and would produce enough gas to at least run a small generator or other machine powered by an internal combustion engine. In this instructable I am presenting the finished product (so far) that has resulted from many months of experimentation and modification. To see the entire long and winding road I went down to get here, please visit the gasifier section of my web site at http://www.mdpub.com/gasifier/.
Step 1: How Does The Gasifier Work?
A word of warning here. This project is dangerous. The operation of a biomass gasifier produces lots of heat, also lots of flammable and poisonous gases. Never operate the gasifier indoors. The gases produced are flammable and potentially explosive if allowed to accumulate in an enclosed space, like a building. Also, the Carbon Monoxide the gasifier produces is lethal! Only operate the gasifier outdoors and try to stay up wind of the unit when it is running. Treat the gas coming out of the gasifier with the same respect as you would for the natural gas that you may have piped into your house. It is just as potentially explosive and deadly.
As I said in step 1, a biomass gasifier is a chemical reactor that converts wood, or other biomass substances, into a combustible gas. The formula is simple. Biomass + Heat = Pyrolysis Byproducts. Pyrolysis is a fancy-pants word that chemists use to describe the process of heat breaking down big molecules into smaller ones. In the gasifier we want to break big biomass molecules (mainly cellulose) down into smaller ones like Hydrogen and Carbon Monoxide.
Where does the heat come from? We get heat by partially combusting some of the biomass with a limited supply of Oxygen. The heat produced by the combustion then drives the pyrolysis reaction. A well built reactor will also convert combustion byproducts like CO2 and water vapor into flammable CO and H2 by passing them over a bed of hot charcoal, left over from the partial combustion, where they will get reduced.
Thus the gasifier converts most of the mass of the wood (or other biomass feedstock) into flammable gases with only some ash and unburned charcoal (bio-char) residue. That is the theory anyway. This is an extreme over-simplification of how the gasifier really works. Wood and other biomass is made of incredibly complex macro-molecules like Cellulose and Lignin that break down into hundreds or thousands of different smaller molecules as the reaction proceeds. There are thousands of different complex chemical reactions going on inside the reactor. The overall result though, if the gasifier is working well, is lots of clean, flammable gas.
Ideally, the gasifier would break down biomass into nothing but Hydrogen and Carbon Monoxide. Here in the real world though, things rarely work ideally. The dirty (literally) little secret about biomass gasification is tar production. Above I said that the macro-molecules that make up biomass get broken down into smaller molecules. Some of those smaller molecules are still pretty big though. If the gasifier is working well, these big breakdown by-products will be further "cracked" into smaller molecules. If the gasifier isn't working so well, these big molecules will wind up in the gas being produced. They will condense out of the gas as a thick, sticky, black, semi-liquid that very closely resembles roofing or road tar, but is even stinkier. Even a well-built gasifier produces a small amount of tar. Most real-world applications (like engines) can't handle much, or even any, tar. My struggle to design and build a working biomass gasifier could actually be accurately described as an ongoing battle to reduce tar production. The first few iterations of this gasifier produced more tar than gas. The complete history of this design can be found on my web site at http://www.mdpub.com/gasifier/. Below is the most important of all chemical reactions a novice gasifier builder needs to know.
Biomass + Poorly Designed Gasifier = Tar!
I strongly recommend that anyone interested in gasifier technology do some research and read up on it. There is a lot more to this technology than I can present in an instructable.