Waste to Energy – Gasification of Wood Overview
This is another article in the series Waste to Energy (WtE).
Our quest to extract energy from waste has led us to investigate the gasification of waste wood to produce a gas for use in power generation.
Gasification of materials containing carbon is nothing new. During the second world war when petrol was in short supply many vehicles were run on gasified wood. I have seen pictures of London busses with the huge gas bag carried on the roof and I can still remember the gasworks in Bangor, where I grew up using coal gasification to produce town gas (and I am not that old!).
Gasification of wood for large power generation is still in the development stage, but this technology is not far out there from now. Already there are several good compact gasifying units in operation producing up 1000kW. The process used in these units to produce gas as an end product will likely be similar for a large plant.
That notwithstanding, there are a number of large power plants using Syngas produced from fossil fuels (usually coal). Syngas is used as fuel for gas turbines in power plants. Some of these plants are dual fuel and combined heat and power plants. These are very efficient.
Gasification of Wood Process
However getting back to wood gasification, the process starts with the chipping of the wood into small pieces, which are then conveyed into a heater which is usually fired with the end product gas. Wood has a very high water content, so the heater is used to remove the moisture content of the wood to below 2%.
There are complicated chemical equations which show the gasification of wood process, but I shall endeavor to simplify a really complex process.
Basically from the heater the dry wood chips are conveyed to a gasifier, which is a vertical vessel. There the chips are subjected to high temperature in virtually anaerobic conditions. This is the pyrolysis stage producing tar, char, and volatiles in this first process.
As the char and volatiles rise up the gasifier, they are then subjected to further intensified heating in the combustion stage. In this second stage oxygen is added to the char and the reactions form carbon monoxide, carbon dioxide, and steam which are then combusted. This provides more heat causing the gasification of the char, which is the third stage; it reacts with the carbon dioxide and steam to form the gasses of hydrogen and carbon monoxide, which happens about three quarters way up the gasifier to the top. This gas then exits at the top the gasifier where a portion of it is bled off for use in the drying and processing stages. It then passes through a tar extractor, is cooled, scrubbed and filtered, with the end product being known as producer gas or wood gas.
Power Generation from Wood Gas
The gas is now ready to be used as fuel in an internal combustion engine (or the very latest innovation- a mini gas turbine) which can be used to drive an electrical power generator having an output of up to 1000 kW. The gas can be used as it is straight from the storage vessel for spark ignition engines, but the engines will need a few slight modifications to the carburetor. Diesel engines will not require any modifications, but the gas will need to have at least a 20% mix with diesel to induce combustion.
There are very compact wood gasifying units incorporating the wood chipper, heater, gasifier, cooler, filters, engine and generator now on the market; these can be used to supply power to the grid, dwellings, or outback homesteads not connected to a mains electricity supply.