GEK Wiki / Biomass Feed Stock Preparation
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Biomass Feed Stock Preparation

Page history last edited by jim mason 6 years, 10 months ago

return to: Build the GEK Gasifier

Guide to Biomass Feed Stocks

for the

GEK Gasifier

version 1-4.x




Feedstock selection and preparation is the most important variable between you and successful operation of your GEK or Power Pallet.   Biomass fuel does not come in standard forms like we are accustomed to with gasoline or diesel.  Shape, size, moisture levels and mineral contents vary greatly between locales and use scenarios.  Understanding the impacts of these variables on gasifier operation, and preparing your fuel accordingly, is your first and most important assignment for operating your new system.  Here below is our guide through the woods.  


Moisture Content

The GEK TOTTI and related Power Pallet will operate with up to 30% moisture content biomass.  20% is usually about ideal.  Depending on your typical load, higher or lower moisture contents will help you keep temps in the desired range, and thus tar levels to a minimum. 

  • Methods to measuring the moisture content of your feed stock:
    •  Moisture Meter
      • Moisture meters come with the GEK kits. These will give you an estimate of the moisture content of the wood. 
    • Microwave Method

1. Select a random sample of your biomass.

2. Weigh the sample (m_wet) and record.

3. Place the sample in a microwave and microwave for 15-60 seconds.

4. Record the new weight of the sample.

5. Repeat steps 3-4 until measured weights stabilize (within the noise of measurements).

6. Average the last three (stabilized) measurements (m_dry).

7. Report the percentage moisture content on a dry basis ([m_wet-m_dry]/m_dry * 100) note that it is reported dry basis.

Note: you can also use an oven, just be sure not to burn the sample. 

  • Too much moisture?
    • Increase air flow and increase heat around the chips. Spreading the chips onto a black tarp in the sun for several hours will bring the moisture content down.  


Particle Size

Up to GEK versions 4.x the particle size needs to be between 0.5-1.5 inches (13-38mm) at length. 

Particles below this size have less void space and prevent the flow of gasses through the packed bed of the gasifier. Sawdust and other fine material is best in fluidized bed gasifiers. 

Lengths of feed stock above this tend to cause material handling issues such as bridging in the hopper and across the reduction bell in the reactor as well as jamming of the auger. Avoid any long slivers of feed stock. 



After chipping, it is possible that the bulk feed stock might need to be passed through two screens to exclude the particle sizes outside of the needed range. A simple shaker on a framed screen hung at an angle makes a great automatic sifter for this purpose. 


Fixed Carbon to Volatile Ratio

Biomass is composed of two basic categories of material: fixed carbon and volatiles.  Fixed carbon is the carbon to carbon structural backbone of the biomass.  The remaining diverse cocktail of other molecular forms are collectively called volatiles.  Typical biomass fixed carbon to volatile ratios are about 20:80.


When biomass is heated in pyrolysis, the volatiles break off as gasses and liquids, usually called "tars".  What remains after pyrolysis is the fixed carbon, which we now call "charcoal".  Both charcoal and tars are important for gasification.    Tars are combusted and reduced over charcoal to make our desired product gasses of H2 and CO.  Tars are also thermally cracked directly to H2 and CO in the combustion zone.


High volatile content biomass can overwhelm cracking in the combustion zone, produce less than ideal amounts of charcoal for reduction, and generally lead to high amounts of tar in the gas stream.   We encourage you to not use fuels with a fixed carbon to volatile ratio of less than 20:80, or biomass known to have high amounts of resin or sap.


Ash Content

Feed stocks with high ash content increases the potential for clinker formation. Grasses and rice tend to have very high silica content that can also have this problem. Clinkers are fused silica and ash created at high heat that become problematic by immobilizing the mass flow through the reactor. These problems get worse at higher loads and/or lower moisture contents. It is highly suggested to conduct a few test runs with your feed stock if you are unsure about clinker formation. 


How much feed stock will I need?

First, assess the demand that will be placed on the system in kW or m3/hr of wood gas. Your demand should fall within the range of the 10kW (3'' bell) and the 20kW (4'' bell). The system ranges are as follows:


  10kW (3'' bell) 
20kW (4'' bell) 
electrical output 2 - 10kW
4 - 20kW
gas flow output 5 - 27m3/hr
10 - 54 m3/hr
feed stock consumption range (dry weight) 2.5 - 14 kg/hr   5 - 27 kg/hr


To find the volume of feed stock you will need, measure the bulk density of the feed stock and convert to volume. To find the bulk density, weight a known volume of your bulk feed stock.


Gas Quality and Feed stock

Gas quality can change across characteristic differences in the feed stock bulk. It is advised to test the tar content of the gas with your given feed stock before running an engine with the GEK. We offer a Research Experimenters Kit that has extra tools for tar testing, gas flow monitoring, extra thermocouples, and flow control as well as some other tools that are useful for investigating the operation of the system and gas quality. 

For an example of tar testing across multiple feed stocks check out the Multi Fuel Comparison page:


More information about pre processing and equipment on our forum:  






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