Biomass gasification is the most efficient method to convert organic material into heat. Additionally, this gas may be used to fuel both internal and external combustion engines thereby creating the potential to do work, whether that is generation of electricity or to provide transportation. Gasification of biomass is a process whereby heat is applied to the fuel in an oxygen deprived atmosphere giving incomplete combustion. The gaseous products are tars, carbon monoxide, hydrogen, methane and carbon dioxide. All but the last are capable of supporting further combustion which usually occurs in a separate location or chamber. The gases may be transported over distance similarly to natural gas. Note that tars would need to be scrubbed from the mix if the gas is to be used in an internal combustion engine otherwise the engine will seize. The following will be the record of the design and build of a homebuilt wood gasification unit to provide heat in Northern Ontario Canada. You may need to adapt the design based on resources available in your area.
The gasification process comprises several separate thermal processes conducted in a controlled manner. These processes are drying, pyrolysis, reduction and oxidation. Drying is where free moisture and cell-bound water are removed from the biomass by evaporation. These processes should ideally take place at a temperature of up to about 160ºC using waste heat from the conversion process. In pyrolysis volatile gases are released from the dry biomass at temperatures ranging up to about 700ºC. These gases are non-condensable vapours (e.g. methane, carbon-monoxide) and condensable vapours (various tar compounds. The residuum from this process will be mainly activated carbon. Reduction is where the activated carbon reacts with water vapour and carbon dioxide to form combustible gases such as hydrogen and carbon-monoxide. The reduction (or gasification) process is carried out in temperatures ranging up to about 1100ºC. Finally is the process of oxidation where part of the carbon is burned to provide heat for the previously described processes.
Background[edit | edit source]
The 1st stage of the project is gathering information on past designs. Three basic designs were apparent from review of literature and the internet: updraft, downdraft and fluidized bed. The last is an industrial process requiring material and energy not readily available to all communities so will not be investigated in the context of a personal energy supply. Other designs do exist but will not be investigated here for similar reasons. Updraft technology, see Figure 1 (to be created), has both the fuel added and gas withdrawn from the top of the gasification chamber while air is supplied from the bottom. As a result the pyrolysis of the fuel also occurs in the upper portion of the fuel charge and moves downward consuming the fuel. The design is simple and robust but has 1 major drawback for a homebuilt unit, when the lid is opened to add more fuel the operator is exposed to the gases, of which a significant portion is Carbon Monoxide (CO), a deadly poison. As the gases are drawn up the tars are not cracked by passing through the reduction zone and so this gasifier design is not a good choice for supplying gas to an internal combustion (IC) engine. Downdraft technology, see figure has combustion and the gas drawn off the bottom and air drawn in through the top. This design has the advantages of pulling tars through the reduction zone thereby cracking them making a better fuel for engines and the fuel hopper may be opened to add fuel with less risk of CO poisoning as both air and gas are being drawn down. The challenges with this design though are that it is technically more complicated requiring a fan or engine to draw off the gases and as fuel feeds down to the combustion zone there is always the possibility of bridging. Fuel bridging is where the wood pieces jam against the sides and each other creating a blockage. This will result in reduced gasification or if sufficiently prolonged the process will stop entirely. Fuel agitation, through vibration or a mixer, overcomes this but again adds to the complexity of the system.
Based on this research it was decided to pursue an updraft design due to its relative simplicity.
Homebuilt Gasifier Design[edit | edit source]
Having decided in general the design it is now possible to research that design in greater depth. One of the principles of success I have garnered from experience is the KISS rule, Keep It Simple Stupid, and I've found that not following it usually means I'm the "stupid" part of that equation. Thus I looked for small scale applications of gasification. I was surprised that many of the smallest scale units were DIY cookstoves for camping made out of tin cans. Some included battery driven fans for ventillation but the simplest were made of 3 cans. This is commonly called the midge stove (Modified Inverted Downdraft Gasifier Experiment).
A bit more research has indicated that all refractory stick gasifiers have been tried and that the insulating properties have lead to explosive problems (Richard C. Hill, Design, Construction and Performance of Stick-Wood Fired Furnace for Residential and Small Commercial Application, Oct 1979, U.S. Dept. of Energy Contract EC 77-S-02-45). It was solved by use of a cooling jacket. This may or may not be necessary as the unit was bottom fired and the gas was taken off the bottom horizontally to a 2nd chamber. I intend the gas to rise, thereby excluding oxygen and to introduce outside air only in the combustion chamber and at the bottom of the gasification chamber. Rising Heat and gasses should take any additional O2 up the chimney and not pose a problem. This will be tested thoroughly.
After looking at a year's heating bills I am using about 6.16 cubic meters of natural gas daily during our 5 month heating period. This equates to about 230,000 btu's daily or 57.96 Mcal. At delivered efficiency of 80%, target rate, that means 72.45 Mcal daily. I am using calories as it is intended to have a 2 day heat reservoir in the form of a water tank and it is easy to calculate storage capacity in calories. A cubic meter, 1000 L, of water would have a storage capacity of 55 Mcal given an starting temperature of 90 celsius and a lower temperature of 35 celsius. This indicates a 3000 L tank would be required, 2m x 1m x 1.5m (L x W x H). Liquid - air heat exchangers are readily available and homebuilt ones are certainly possible using automotive radiators. Indeed by keeping the 12V DC fan and a dc circulating pump this could even supply heat during power blackouts with a spare battery and a manual charger (bicycle powered perhaps).