Middlebury College has installed their own biomass gasification plant on campus. Unlike similar projects carried out by other universities and colleges, Middlebury have made the plant a focus point with glass walls, so people can view into the facility. As this technology is so important to our fuel independent green energy future, its good to see the technology proudly presented, instead of been hidden away.
The biomass gasification plant uses wood chips from the local area, which could include 2-3 loads a day on full operation. The wood chip has to be of a certain size, and so has to be graded, and larger pieces put through a mill. It is also crucial that the moisture content of the wood chip be no more than 50%, as the efficiency of the gasifier is set-up with this tolerance.
The wood chips are feed into the gasifier on demand. On one side of the gasifier the wood chips are heated to a very high temperature in a low oxygen environment. The wood then smoulders and produces wood gas ‘syngas’. The syngas is then feed to the back of the boiler where oxygen is added for combustion, producing heat in excess of 1100 degrees Fahrenheit. The steam produced is used throughout the campus for heating, cooling, hot water and cooking. The surplus steam is used to drive turbines, which produce around 20% of the energy the campus needs.
The ash from the boiler is then collected and sent through a series of cyclone separators, which remove ash and particulates from the exhaust gases. The ash and particulates are then given to a local fertilizer company, to use as a soil amendment for agricultural land. The remaining heat from the exhaust gases is then extracted, to preheat water going to the boiler. The gases are then filtered through a series of filters with 99.7% efficiency. Therefore the largest emission from the stack is water vapour and CO2. And since the fuel was wood, this is therefore a carbon neutral process.
Middlebury should be applauded for designing such a highly efficient process. Combined heat and power units are regarded as been more efficient than simply heat or electricity plants. However Middlebury have gone further and extracted as much possible heat from the exhaust gases and filtered out the particulates. They have also made use of the particulates and ash as soil amendments.
Biomass Pellets and Gasification
Even though the raw material maybe the same, for example wood chips and wood pellets, there are crucial differences which can change the operating parameters of the process and make it easier to operation on an even smaller scale.
Biomass Pellet Size and Density
As mentioned in the Middlebury biomass gasification plant, the wood chip has to be of a certain size and graded and milled to create a consistent graded for the auger system to feed the boiler.
Pellets are a consistent size; therefore a grading and milling system would not be required. Also due to the smaller size of pellets and high-density smaller fuel transportation systems are required to feed the boiler the same quantity of raw material. The higher density of pellets also means the size of the burn chamber can be reduced as pellets produce a higher heat value in a much smaller space. So in general a much smaller set-up could operate on pellets to produce the same amount of energy.
Biomass Pellet Moisture Content
The Middlebury biomass gasifier could operate on wood chips up to 50%. This is an advantage in one sense as it means unprocessed virgin wood chip could be used. However it does reduce how flexible the process is. To operate biomass gasification CHP process on a small scale it is much easier and practical to generate electricity by using the wood gas ‘syngas’ in an internal combustion engine (petrol or diesel) to power a generator. However the high percentage of water vapour in the Middlebury process would make the gas unsuitable.
Biomass pellets on the other hand have much lower moisture content below 10%, and therefore wood gas ‘syngas’ produced from biomass pellets can be used in an internal combustion engine.
The PelHeat Mobile Pelletizer can process biomass including wood into fuel pellets. If you would be interested in registering your interest in the PelHeat Mobile Pelletizer and receiving updates on our progress and informed when the pelletizer is on sale please send us an email to:
Contact @ PelHeat . Com
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